The HNBR-based vulcanisates demonstrated good shape recovery performance, with a shape recovery percentage in the range of 91-100% (Fig. 11). The greatest percentage recovery was obtained for the vulcanisates with the highest ionic crosslink content. Moreover, the most considerable reduction of the ionic crosslink number during the thermal shrinking process was observed for these vulcanisates. This result confirms the assumption that the decomposition of ionic crosslinks is the most important reason for the shrinkability and shape recovery of HNBR vulcanisates. It can be concluded that the ionic crosslinks formed in the elastomer by the coagents serve as memory points in the elastomer network.

**Figure 11.** Percentage recovery of HNBR vulcanisates

#### *3.2.3. Dynamic mechanical properties of HNBR vulcanisates*

Dynamic - mechanical analysis was performed to confirm the existence of ionic clusters in the elastomer network. The loss factor (tanδ) of the HNBR vulcanisates with coagents is presented as a function of temperature in Figs. 12 and 13 as an example. The values of the glass transition temperature (Tg) are given in Table 4.

**Figure 12.** Tan δ versus temperature for HNBR vulcanisates before thermal treatment

**Figure 13.** Tan δ versus temperature for HNBR vulcanisates after thermal shrinking

**Table 4.** Glass transition temperature of HNBR vulcanisates

glass transition temperature (Tg) are given in Table 4.

*3.2.3. Dynamic mechanical properties of HNBR vulcanisates*

Dynamic - mechanical analysis was performed to confirm the existence of ionic clusters in the elastomer network. The loss factor (tanδ) of the HNBR vulcanisates with coagents is presented as a function of temperature in Figs. 12 and 13 as an example. The values of the

**Figure 12.** Tan δ versus temperature for HNBR vulcanisates before thermal treatment

**Figure 13.** Tan δ versus temperature for HNBR vulcanisates after thermal shrinking

The DMA analysis revealed the biphasic structure of the HNBR crosslinked in the presence of coagents based on nanosized CaO and MgO grafted with UCA. Two phase transitions were observed. The glass transition of the HNBR, which occurs at Tg, was observed in the range from (-21.2oC) to (-19.1oC) for vulcanisates with CaO and from (-20,6oC) to (-17.8oC) for vulcanisates containing MgO. The determined Tg values were correlated with the crosslink density of the vulcanisates. A fuzzy low-intensity peak was observed in the temperature range of (40-100oC) (corresponding to the ionic transition) because of the occurrence of a hard phase arising from the existence of the ionic aggregates.

Heating to the ionic transition temperature decreased the glass transition temperature of the vulcanisates by decomposing ionic crosslinks, reducing the crosslink density of the vulcanisates. The greatest decrease of the Tg value occurred in the case of the vulcanisates for which the greatest number of ionic crosslinks decomposed during heating (CaO-OA, CaO-LA, MgO-IA, MgO-OA). The fuzzy peak corresponding to the ionic transition disappeared; consequently, it can be concluded that the shape recovery of the HNBR vulcanisates was caused by the decomposition of the ionic crosslinks and changes in the biphasic structure of the crosslinked elastomer, similarly to XNBR.

### *3.2.4. Mechanical properties of HNBR vulcanisates*

Having established the ability of the HNBR vulcanisates to shrink upon exposure to the ionic degradation temperature and return to their original shape, we then examined their mechanical properties.

It is known that the formation of coagent bridges, which are labile ionic crosslinks inside the elastomer network formed during vulcanisation, improved the tensile properties of the vulcanisates (Przybyszewska & Zaborski 2008, 2009). However, it is reasonable to investigate the effect of the heat treatment of the vulcanisates on their mechanical properties.

As could be supposed, the thermal treatment of the vulcanisates during their shrinkage deteriorated their tensile strength as a result of the decomposition of their ionic crosslinks. Their tensile strength was reduced by 5% to 30% in comparison with that of the vulcanisates before thermal treatment (Fig. 14). The vulcanisates subjected to shrinking at elevated temperature also exhibited a lower elongation at break (Fig. 15).

**Figure 14.** Tensile strength of HNBR vulcanisates

**Figure 15.** Elongation at break of HNBR vulcanisates

#### **4. Conclusions**

The thermo-shrinkable properties of carboxylated (XNBR) and hydrogenated (HNBR) acrylonitrile butadiene elastomer were studied. XNBR was crosslinked with nanosized calcium, magnesium and zinc oxide to ensure the formation of ionic crosslinks in the elastomer matrix, which can serve as memory points and enhance its heat shrinkability. Similarly, HNBR was crosslinked in the presence of coagents based on nanosized calcium and magnesium oxides grafted with unsaturated carboxylic acids to achieve ionic crosslinks. The examined samples were allowed to shrink at a temperature above the ionic transition temperature. The XNBR vulcanisates revealed thermo-shrinkability in the range of 41% for MgO to 50% for ZnO, whereas the percentage of shape recovery was in the range of 90- 100%. Good shape recovery performance was also observed for the HNBR vulcanisates. The greatest shrinkage upon heating (approximately 35%) was achieved for the vulcanisates containing CaO-OA, CaO-LA and MgO-IA coagents. The percentage of shape recovery of these vulcanisates was in the range of 91-100%.

The thermo-shrinkability value and shape recovery ratio were strongly correlated with the ionic crosslink content of the elastomer network, and the number of ionic crosslinks was reduced by heating the samples above their ionic transition temperature. Greater ionic crosslink contents and their more significant decomposition corresponded to vulcanisates with greater thermo-shrinkability. DMA measurements confirmed the presence of ionic crosslinks and the existence of a biphasic structure in both the XNBR and HNBR elastomers. A fuzzy low-intensity peak was observed in the (tan δ) curve, in the temperature range of (50-100oC), which corresponds to the ionic transition that occurs at high temperatures as a result of the occurrence of a hard phase arising from ionic aggregates. This peak disappeared when the samples were heated above their ionic transition temperature. Therefore, it could be concluded that the thermo-shrinkability of the XNBR and HNBR vulcanisates and their shape recovery was a result of the decomposition of ionic crosslinks and changes in or the disappearance of the biphasic structure of the crosslinked elastomer.

### **Author details**

198 Advanced Elastomers – Technology, Properties and Applications

**Figure 14.** Tensile strength of HNBR vulcanisates

**Figure 15.** Elongation at break of HNBR vulcanisates

**4. Conclusions**

temperature also exhibited a lower elongation at break (Fig. 15).

Their tensile strength was reduced by 5% to 30% in comparison with that of the vulcanisates before thermal treatment (Fig. 14). The vulcanisates subjected to shrinking at elevated

The thermo-shrinkable properties of carboxylated (XNBR) and hydrogenated (HNBR) acrylonitrile butadiene elastomer were studied. XNBR was crosslinked with nanosized calcium, magnesium and zinc oxide to ensure the formation of ionic crosslinks in the elastomer matrix, which can serve as memory points and enhance its heat shrinkability. Similarly, HNBR was crosslinked in the presence of coagents based on nanosized calcium Magdalena Maciejewska and Alicja Krzywania-Kaliszewska *Technical University of Lodz; Institute of Polymer and Dye Technology, Poland*

### **Acknowledgement**

The authors wish to acknowledge the Polish Ministry of Science and Higher Education as well as the National Centre of Research and Development for supporting this study.

### **5. References**

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200 Advanced Elastomers – Technology, Properties and Applications

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### **Chapter 8**

## **Elastomer Application in Microsystem and Microfluidics**

Shuang (Jake) Yang and Kunqiang Jiang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48121

### **1. Introduction**

Elastomer has been widely used in academic and industry since it was invented in the nineteen century 1. By definition, elastomer is a polymer which is viscoelastic and able to regain its shape after deformation. Compared to other materials, elastomer has notably low Young's modulus and high yield strain. In general, amorphous elastomer is made of carbon, hydrogen, oxygen and/or silicon whose glass transition temperature is well below the application temperature, e.g., room temperature. As a result, it has excellent properties in elasticity 2, transparency 3, permeability 4, and insulation 5.

One of the most commonly used elastomers is silicone elastomer such as poly(dimethylsiloxane) (PDMS), which has been widely used in electronics and microfluidics due to its elasticity, optical transparency, UV transmission, permeability, biocompatibility, and availability. PDMS generally consists of repeat unit dimethylsiloxane. The repeat methyl is modifiable using functional groups such as urea or bis-urea for development of high mechanical strength of elastomer. The modified PDMS presents both a high mechanical strength and elasticity at room temperature due to functionality of the cross-linking domains and intrinsic reversibility of the supramolecular chains. PDMS is optically transparent and is usually inert, non-toxic and non-flammable 6. PDMS is viscoelastic and is normally measured using dynamic mechanical analysis (DMA), in which a specialized instrument is used to determine the material's flow characteristics over a wide range of temperature, flow rates, and deformations. In general, the shear modulus of PDMS is in the range of 100 kPa to 3 MPa, varied with the preparation conditions.

Characterization of elastomer by dynamic-mechanical testing includes temperature sweep, frequency sweep, strain sweep, damping and friction properties. The dynamic mechanical measurement is particularly useful to characterize conductive particles in elastomer for

© 2012 Yang and Jiang, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

understanding the particles effect on overall elastomer mechanical properties. It is very common to measure damping factor (tan) change with temperature for understanding the elastomer degree of crosslinking 7. The dynamic mechanical testing provides information on both uncrosslinked and crosslinked phases of the elastomer, as well as viscoelasticity. To this end, nanoindentation plays a significant role on viscoelastic characterization of elastomers for their creep and relaxation properties 8, 9.

Elastomer is an excellent matrix which can mix with other materials such as conductive particles and thermoplastics to enhance mechanical properties. For example, silver particles are added into silicone elastomer matrix to formation of conductive elastomer for highdensity electrical contacts in microelectronics. Polypropylene and ethylene-propylene copolymer elastomer significantly improves the dynamic mechanical, tensile and impact properties and even changes the modes of failure 10. The size, shape and spatial packing of elastomer inclusions are the most significant factors in controlling the mechanical behavior of blends. Since PDMS can be cured with different ratio of dimethylsiloxane and curing reagent, it is very useful to fabricate microfluidic devices for a variety of applications, including cell culture 11, ELISA assay 12, capillary electrophoresis 13, DNA sequencing 14.

In this chapter, we briefly discussed elastomer elastic and viscoelastic properties, as well as permeability. Elastomer is widely used in microelectronics and microfluidics due to all or part of these characteristics. Then we discussed several applications in microelectronics and microfluidics using elastomer as substrate. Finally, we introduce potential development of elastomer.

### **2. Elastomer property and its applications**

Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent 15. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the applied force is removed. The elasticity of elastomer makes it extremely useful in microelectronics and microfluidics. For example, the Intel CPU uses land grid array sockets (LGA), in which the socket has over thousands of elastomer contacts that consist of silver particles in elastomer matrix 16, 17. When socket, the CPU and printed circuit board (PCB) are assembled by compressing force, the silver-embedded elastomer is compressed to increase contact area between socket and the CPU, and between socket and the PCB. The excellent elastic deformation of elastomer can keep the electrically tight contact for minimum contact resistance. Elasticity of PDMS minimizes its stress relaxation and creep effect, which is severe in other polymer with low elasticity. The excellent elasticity of PDMS allows to release from microfluidic masters without damaging PDMS device 18, 19 and significant deflection to occur when a pressure difference develops 20. Therefore, it is important to characterize PDMS elasticity and understand how to improve such property for various applications.

The most common elasticity experiment for many decades involved mechanically blending a reinforcing filler such as carbon black into natural rubber, crosslinking the elastomer with sulfur, and characterizing it network structure by stress-strain measurements in simple extension, or by extents of equilibrium swelling 21. However, in most instances, such as thinfilm elastomer, land grid array elastomer contact, and PDMS microfluidics, these methods may not be applicable due to effects of supporting materials. To characterize elastomer in those applications, nanoindentation has been an emerging technology for elastomer characterization and is an indispensable tool in many instances where other techniques are inapplicable. Nanoindentation is such tool for characterization of polymer on its viscoelasticity. Indentation technique was first introduced as advent of indentation fracture mechanics 22 in 1975 and ever since it has been developed for characterization of elasticplastic materials using the Oliver-Pharr's method 23. No valid method was reported for polymer nanoindentation analysis based on stress-strain experiments until the viscoelastic model was proposed in 2004 and other emerging studies that followed 8, 24, 25. Because of viscoelasticity, elastic-plastic model cannot be directly used to derive modulus and hardness. Also these measurements are dependent on the experimental parameters, such as loading rate, maximum load, holding duration, unloading rate, and minimum unloading force 26. To minimize the effects by those experimental parameters during indentation, the experimental conditions were proposed in such a way that fast loading was applied for minimizing viscoelastic deformation, long dwell time for full viscoelastic deformation, and quick unloading rate. The fast loading and unloading showed that only elastic-plastic deformation was observed, while viscoelastic deformation was observed in the dwell duration. Also, it was observed that the viscoelastic deformation can be measured by keeping a minimum force on the indenter so that the instrument can record the deformation progress with time. The modulus and viscosity coefficient of elastomer can be derived using flat-ended punch-tip 8, the elastic-viscoelastic-viscous deformation is described by following equation,

204 Advanced Elastomers – Technology, Properties and Applications

elastomers for their creep and relaxation properties 8, 9.

**2. Elastomer property and its applications**

such property for various applications.

elastomer.

understanding the particles effect on overall elastomer mechanical properties. It is very common to measure damping factor (tan) change with temperature for understanding the elastomer degree of crosslinking 7. The dynamic mechanical testing provides information on both uncrosslinked and crosslinked phases of the elastomer, as well as viscoelasticity. To this end, nanoindentation plays a significant role on viscoelastic characterization of

Elastomer is an excellent matrix which can mix with other materials such as conductive particles and thermoplastics to enhance mechanical properties. For example, silver particles are added into silicone elastomer matrix to formation of conductive elastomer for highdensity electrical contacts in microelectronics. Polypropylene and ethylene-propylene copolymer elastomer significantly improves the dynamic mechanical, tensile and impact properties and even changes the modes of failure 10. The size, shape and spatial packing of elastomer inclusions are the most significant factors in controlling the mechanical behavior of blends. Since PDMS can be cured with different ratio of dimethylsiloxane and curing reagent, it is very useful to fabricate microfluidic devices for a variety of applications, including cell culture 11, ELISA assay 12, capillary electrophoresis 13, DNA sequencing 14.

In this chapter, we briefly discussed elastomer elastic and viscoelastic properties, as well as permeability. Elastomer is widely used in microelectronics and microfluidics due to all or part of these characteristics. Then we discussed several applications in microelectronics and microfluidics using elastomer as substrate. Finally, we introduce potential development of

Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent 15. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the applied force is removed. The elasticity of elastomer makes it extremely useful in microelectronics and microfluidics. For example, the Intel CPU uses land grid array sockets (LGA), in which the socket has over thousands of elastomer contacts that consist of silver particles in elastomer matrix 16, 17. When socket, the CPU and printed circuit board (PCB) are assembled by compressing force, the silver-embedded elastomer is compressed to increase contact area between socket and the CPU, and between socket and the PCB. The excellent elastic deformation of elastomer can keep the electrically tight contact for minimum contact resistance. Elasticity of PDMS minimizes its stress relaxation and creep effect, which is severe in other polymer with low elasticity. The excellent elasticity of PDMS allows to release from microfluidic masters without damaging PDMS device 18, 19 and significant deflection to occur when a pressure difference develops 20. Therefore, it is important to characterize PDMS elasticity and understand how to improve

The most common elasticity experiment for many decades involved mechanically blending a reinforcing filler such as carbon black into natural rubber, crosslinking the elastomer with sulfur, and characterizing it network structure by stress-strain measurements in simple

$$h = \frac{P\_0 h\_{ln}}{E\_0 A\_0} + \sum\_{1}^{n} \frac{P\_0 h\_{ln}}{E\_l A\_0} (1 - e^{-E\_l t}/\eta\_l) + \frac{h\_{ln}}{\eta\_o} t^{\frac{1}{2}}$$

where *P* represents load, *h* is deformation and subscript *e* is elastic, coefficient of viscosity, *E* modulus, *t* time. Elastic modulus is thus derived by *P0hin/A0he*. This model provides a means for characterization of elastomer, such as investigation of effect of crosslinking, work of adhesion and fluid environment on elastic modulus 24. Nanoindentation on PDMS indicated that it is capable of measuring several order of magnitudes of elastic modulus of PDMS, with expected increase of the elastic modulus at higher crosslinked PDMS (2.80 MPa at 10:1 ratio, and 0.88 MPa at 30:1 ratio; monomer:crosslinker, wt%) 24, 27.

Elastomer is permeable to solvents, gases, moisture or even some small molecules 4, 28-30. Permeability has been employed to fabricate non-contact pump for the manipulation of aqueous solutions within PDMS microfluidic devices 20 or supply oxygen for large-scale culture of hepatocytes 31. On the other hand, PDMS permeability limits its application to microfluidic technology, in which coating such as glass-like layer using sol-gel chemistry is developed on PDMS to minimize permeability 32. The permeability of a PDMS film or membrane to a penetrant (i.e., gas or solvent) is defined as 28, 33,

$$P\_A = \frac{N\_A l}{p\_{2A} - p\_{1A}}$$

Where *PA* is the permeability coefficient, *NA* is the steady-state penetrant flux through the film, *p1A* and *p2A* are the downstream and upstream partial pressures of component *A*, and l is the membrane or film thickness. In general, PDMS permeability depends strongly on the feeding gas/solvent composition and temperature: an increase in vapor concentration or liquid pressure leads to increased chain mobility at constant feeding pressure and temperature, resulting in high diffusivity and permeability. The increase of permeability with pressure is attributed to the swelling of PDMS.

Due to the permeability nature of PDMS, many researchers synthesized co-polymer which contained PDMS as a permeable medium for gas/solvent. For example, the 'smart' tricomponent amphiphilic membranes consisting of poly(ethylene glycol) (PEG), PDMS and polypentamethylcyclopentasiloxane (PD5) domains were synthesized for biological applications, including immunoisolation of cells 29, 30. The high oxygen permeability of PDMS makes it feasible for the application in microdevices in cell culture 34. Compared to large-scale system / platform, microfluidics provides suitable environments for sample interaction, such as cell culture, with the significant increase of surface-to-volume ratio while the fluidic behavior is similar to the environments in vivo 29. Integration of PDMS into microfluidic platform can implement studies on cell-to-cell interactions and understanding of cell behaviors in vitro, emulation of situations observed in vivo. Several studies have demonstrated that microfluidic devices consisted of two PDMS layers were capable for cell culture 35. They further fabricated a complex microfluidics which consisted of stacking ten PDMS layers, in which four cell culture chambers and one-oxygen chamber inserts between.

### **3. Elastomer as electrical contact matrix in microelectronics**

Insulation elastomer in microelectronics plays a significant role to prevent a short-circuit 36. Electrical insulating polymers have been introduced nearly one hundred years ago and proved to be excellent long-term physical and electrical properties, including weatherability, moisture-sealing, corrosion resistant and durability. The polymer insulating materials are used in a wide range of applications such as surge arresters, insulators, insulation enhancement, and bushings 5. The initial polymers used in insulating materials were mainly on radiation-crosslinked and semicrystalline polyolefin copolymers. With development of manufacturing and materials technologies, elastomer has been increasingly designed for insulating applications 5, 37.

The elastomer formulations have been optimized so that an exceptional electrical and weathering performance is achieved, which are equal or above that of polyolefin copolymer materials. The silicone elastomer has many advantages such as hydrophobicity, hydrophobic recovery, weathering resistance, processability, and elastomeric mechanical properties. These advantages make silicone elastomer suitable for outdoor insulating applications. For example, PDMS has been used for high-voltage (HV) outdoor insulation in the real world as a replacement over conventional porcelain and glass insulation. As one silicone elastomer, PDMS as HV insulating materials is light weight, has superior vandal resistance, and contains better contamination performance. However, many research works have been conducted on molecular modification in order to improve PDMS properties, including degradation under exposure to discharge and arcing, mechanical strength etc. It has been demonstrated that hydrophobicity of PDMS can be rendered by exposure to discharges, which can be recovered when exposure ceases. Their studies showed that the lowly crosslinked PDMS can readily reorient between hydrophilic and hydrophobic states, therefore improving its weathering capabilities 38.

206 Advanced Elastomers – Technology, Properties and Applications

with pressure is attributed to the swelling of PDMS.

insulating applications 5, 37.

Where *PA* is the permeability coefficient, *NA* is the steady-state penetrant flux through the film, *p1A* and *p2A* are the downstream and upstream partial pressures of component *A*, and l is the membrane or film thickness. In general, PDMS permeability depends strongly on the feeding gas/solvent composition and temperature: an increase in vapor concentration or liquid pressure leads to increased chain mobility at constant feeding pressure and temperature, resulting in high diffusivity and permeability. The increase of permeability

Due to the permeability nature of PDMS, many researchers synthesized co-polymer which contained PDMS as a permeable medium for gas/solvent. For example, the 'smart' tricomponent amphiphilic membranes consisting of poly(ethylene glycol) (PEG), PDMS and polypentamethylcyclopentasiloxane (PD5) domains were synthesized for biological applications, including immunoisolation of cells 29, 30. The high oxygen permeability of PDMS makes it feasible for the application in microdevices in cell culture 34. Compared to large-scale system / platform, microfluidics provides suitable environments for sample interaction, such as cell culture, with the significant increase of surface-to-volume ratio while the fluidic behavior is similar to the environments in vivo 29. Integration of PDMS into microfluidic platform can implement studies on cell-to-cell interactions and understanding of cell behaviors in vitro, emulation of situations observed in vivo. Several studies have demonstrated that microfluidic devices consisted of two PDMS layers were capable for cell culture 35. They further fabricated a complex microfluidics which consisted of stacking ten PDMS layers, in which four cell culture chambers and one-oxygen chamber inserts between.

**3. Elastomer as electrical contact matrix in microelectronics**

Insulation elastomer in microelectronics plays a significant role to prevent a short-circuit 36. Electrical insulating polymers have been introduced nearly one hundred years ago and proved to be excellent long-term physical and electrical properties, including weatherability, moisture-sealing, corrosion resistant and durability. The polymer insulating materials are used in a wide range of applications such as surge arresters, insulators, insulation enhancement, and bushings 5. The initial polymers used in insulating materials were mainly on radiation-crosslinked and semicrystalline polyolefin copolymers. With development of manufacturing and materials technologies, elastomer has been increasingly designed for

The elastomer formulations have been optimized so that an exceptional electrical and weathering performance is achieved, which are equal or above that of polyolefin copolymer materials. The silicone elastomer has many advantages such as hydrophobicity, hydrophobic recovery, weathering resistance, processability, and elastomeric mechanical properties. These advantages make silicone elastomer suitable for outdoor insulating applications. For example, PDMS has been used for high-voltage (HV) outdoor insulation in the real world as a replacement over conventional porcelain and glass insulation. As one silicone elastomer, PDMS as HV insulating materials is light weight, has superior vandal resistance, and contains better contamination performance. However, many research works Recently, PDMS has been micropatterned using a photoresist lift-off technique for selective electrical insulation in microelectrode array applications 39. The micropatterning technique is able to manufacture PDMS patterns with feature as small as 15 µm and various thicknesses down to 6 µm. With micropatterned PDMS insulation layer, the electrical resistance between adjacent electrodes is within the specification. Additionally, the micropatterned PDMS insulation can be applied to biosensing microdevices such as in an extensive neuronal network. Other researchers used PDMS to develop a replaceable insulator for a single-use planar microelectrode array (MEA) in the study of electrogenic tissues 40. They demonstrated applications using microstencils for rejuvenation of an old MEA and the fabrication of a single-use MEA.

Conductive materials are used to produce shielding gaskets for military, aerospace, electronics, and communications. The conductive elastomers are designed to balance requirements for electrical conductivity, thermal management, and cost performance 41. Conductive elastomer materials are ideal for customer applications requiring both excellent electromagnetic interference shielding and environmental sealing across a wide range of temperature. The conductive elastomers could be blended polyacetylene with different types of thermoplastic elastomers: styrene-butadiene-styrene, styrene-isoprene-styrene, and styrene-ethylenebutylene-styrene tri-block copolymers 42. The polyacetylene is a highly conjugated polymer which contains high concentration of unsaturated sites. Those sites are easily attacked by ozone / UV light and reacted with elastomer, formation of conductive elastomer composites. Or insulation elastomers are coated with compliant electrode material on both sides of the elastomer film 43. The dielectric elastomers were actuated by means of electrostatic forces applied via compliant electrodes. The performance of conductive elastomers is dependent on many factors, e.g., elastomer crosslinking method and type of monomer. In an electrically conductive heterogeneous binary polymer blends, consisting of ethylene-propylene-diene-monomer (EPDM) and polyaniline (PAni), the performance was significantly affected by the crosslinking method 44. In the blend, PAni underwent doping upon exposure to a protonic acid and became electrically conductive. Several methods have been studied for monomer crosslinking, such as the use of a phenolic resin for unsaturated rubbers and high service temperature products, inter-chain C-C bonds formation after the reticulation reaction, electron beam induced crosslinking for 3-D network. Results indicated that electron beam irradiation crosslinking method was not affected by the presence of the acid necessary for doping the conductive elastomers.

Insulation elastomer has been used as a matrix to hold conductive particles in land grid array socket since it significantly improves contact resistance and increases interconnect density. The need for high density electronics has led to the development of semiconductor array packages, such as ball grid array (BGA) and land grid array (LGA) packages. However, soldering a package to a printed circuit board (PCB) is challenging for input/output (I/O) counts greater than a thousand, and solder joint defects can induce reliability problems in the application. These problems can be solved by using an LGA socket, which eliminates the soldering process, provides a separable interconnect between the component and the PCB, and also enables easy product rework, repair, and upgrade. The conductive particles in elastomer matrix has been introduced into electronics in 1980's 16, 45. One such conductive elastomer composite is called Z-direction anisotropically conductive materials which are made of the magnetic alignment of conductive particles in elastomers 46. The composites contain many vertically aligned but laterally isolated chains of ferromagnetic metal spheres, and the ends of which protrude from the surfaces for better electrical contact. These materials exhibit Z-direction only electrical conduction, in combination with the compliant nature of the materials, can be exploited advantageously for a variety of electronic applications, including fine-pitch, area-array, circuit interconnects, circuit-testing, heat sink interfacing, and sensor devices 47, 48. The schematic diagram of this design structure is shown in Figure 1. This design has been used in a tactile shear sensor for applications such as robotic skins or grippers, touch sensitive actuators, and finger operated controls 49.

**Figure 1.** Schematic diagram of a Z-direction only electrical conduction, in which the component is electrical connected to the printed circuit board through vertically aligned and laterally isolated conductive materials. The contact is made by external contact force. Reprint and permission from 16

An LGA socket assembly consists of component, socket, and board as shown in Figure 2. The contact force is applied through a fixture 16, 45. The socket design can vary in terms of target component, contact, and housing. One class of LGA sockets incorporates conductive particles in elastomer (polymer) as electrical interconnects for LGA. This class of socket has comparatively low cost, ease of assembly, and short signal path; the elastomer has a sealing effect. However, elastomer-based interconnection is susceptible to failure due to creep and stress relaxation, whenever a force or deformation is provided. A decrease in the contact force due to stress relaxation may lead to the degradation of contact resistance. Increase of elasticity can effectively mitigate creep and stress relaxation, in that crosslinked elastomer has instantaneous recovery after removal of stress.

208 Advanced Elastomers – Technology, Properties and Applications

controls 49.

array packages, such as ball grid array (BGA) and land grid array (LGA) packages. However, soldering a package to a printed circuit board (PCB) is challenging for input/output (I/O) counts greater than a thousand, and solder joint defects can induce reliability problems in the application. These problems can be solved by using an LGA socket, which eliminates the soldering process, provides a separable interconnect between the component and the PCB, and also enables easy product rework, repair, and upgrade. The conductive particles in elastomer matrix has been introduced into electronics in 1980's 16, 45. One such conductive elastomer composite is called Z-direction anisotropically conductive materials which are made of the magnetic alignment of conductive particles in elastomers 46. The composites contain many vertically aligned but laterally isolated chains of ferromagnetic metal spheres, and the ends of which protrude from the surfaces for better electrical contact. These materials exhibit Z-direction only electrical conduction, in combination with the compliant nature of the materials, can be exploited advantageously for a variety of electronic applications, including fine-pitch, area-array, circuit interconnects, circuit-testing, heat sink interfacing, and sensor devices 47, 48. The schematic diagram of this design structure is shown in Figure 1. This design has been used in a tactile shear sensor for applications such as robotic skins or grippers, touch sensitive actuators, and finger operated

**Figure 1.** Schematic diagram of a Z-direction only electrical conduction, in which the component is electrical connected to the printed circuit board through vertically aligned and laterally isolated conductive materials. The contact is made by external contact force. Reprint and permission from 16

An LGA socket assembly consists of component, socket, and board as shown in Figure 2. The contact force is applied through a fixture 16, 45. The socket design can vary in terms of target component, contact, and housing. One class of LGA sockets incorporates conductive particles in elastomer (polymer) as electrical interconnects for LGA. This class of socket has comparatively low cost, ease of assembly, and short signal path; the elastomer has a sealing effect. However, elastomer-based interconnection is susceptible to failure due to creep and stress relaxation, whenever a force or deformation is provided. A decrease in the contact force due to stress relaxation may lead to the degradation of contact resistance. Increase of

**Figure 2.** Schematic diagram of elastomer land grid socket assembly. The interconnection part, elastomer socket, can be replaced with different type of sockets, including metalized particle interconnect (MPI), PariPoser film, and conductive rubber sockets (CRS). Reprint and permission from 16

Several mechanisms can lead to the elastomer failure, such as electrochemical migration 50-52, stress relaxation and creep 45, 53 and corrosion 16. This section will extensively emphasize these mechanisms and use respective models to predict the lifecycle of the components. The metal electrochemical migration was first identified in the short-circuit failure of Sun Microsystems Sun Fire 6800. Investigation on the failed parts did not observe any dendrite to bridge adjacent components and/or contacts. In general, dendrites were considered to short the electrical circuit. This finding indicated that even without dendrite formation a short-circuit can happen. The design of the experiment, which mimicked the device environment with accelerated conditions, was conducted to identify the failure mechanism and electrochemical migration processes based on the failure products 50-52. The new failure mechanism in the metal-in-elastomer socket was proposed on the facts that surface insulation resistance has catastrophically decreased and unknown materials were formed on

surface after a certain period of time incubation in the temperature-humidity-bias conditions. The metal-in-elastomer socket consists of silver particles in elastomer matrix. Previous studies showed that silver can migrate in the humidity-temperature-bias conditions 54. However, those studies added the external conductive ions, e.g., contaminants or silver ions, into the system to initialize the migration process 55. In recent studies, several substantial results exploited different electrochemical migration, including silver electrochemical migration (ECM) processes, migration products, and failure criterion on surface insulation resistance (105 ohm).

The silver ECM could occur when bias was applied to silver-in-elastomer socket in the humidity-temperature. Initially, the water layer absorbed on the surface is electrolyzed to hydrogen and hydroxide ions, which further migrate to cathode and anode respectively. Silver oxides to silver ions by accepting electrons through electrolyzed water solution under bias, and silver ions are accumulated in absorbed water layer. The silver ions are also migrated to cathode through water layer. Overall, the surface insulation resistance (SIR) decreases when ions are generated in the absorbed water layer, or formation of conductive electrolyte. The short-circuit failure occurs when silver ion concentration increases to the specific value when the surface insulation resistance decreases to 105 ohm.

Both film and dendrites were observed on the film after permanent failure when the surface insulation resistance of the silver-in-elastomer was less than 1000 ohm. Studies on this regard showed that the film growth was developed in deionized water, while the dendrite growth was developed in high conductivity water. It was found that the film grew on the surface without distinctive direction while dendrites grew from cathode to anode. The analysis of film and dendrites from electron scanning spectrometry and X-ray photoelectron microscopy showed that the film consists of silver oxide and dendrites are silver only. The process of the silver electrochemical migration is progressed as follow: silver oxidation at anode to formation of silver ion; silver ion migration from anode to cathode through absorbed water layer (6-7 water monolayers); partial silver ion reduction to silver oxide with hydroxide from water electrolysis (film); other silver ion reduction to formation of silver at the cathode (dendrites) 50-52.

New failure criterion on surface insulation resistance was proposed. In the IPC standard (IPC-TR-476A), the failure was defined as the surface insulation resistance below 106 ohm. New experimental findings showed that the component didn't fail when resistance below the resistance, no electrochemical migration related products detected either 50-52. In addition, the surface insulation resistance immediately decreased to the order of 106 ohm when the metal-in-elastomer was conditioned in the highly accelerated stress testing (130C/85% RH/bias). Until SIR was below 105 ohm, the silver dendrites were observed. The resistance ultimately was within hundreds of ohms when the dendrites bridged the adjacent contacts.

The new ECM process was proposed as: presence of moisture, anodic metal dissolution or ion generation and ion migration to cathode (electrochemical reaction), ion accumulation, and metal dendritic growth. The ion accumulation was found to be the rate controlling step in which the surface insulation resistance degraded significantly when the ion concentration accumulated to a specific value. This added step explained the failure occurrence without dendritic formation/bridging and perfected the process. Time-to-failure (TTF) model considered these factors to estimate the failure time, including water adsorption, ion generation, ion migration, and ion accumulation. The dendritic growth was excluded from the model based on the experimental findings. Furthermore, incorporation of dendritic growth model with TTF model can be used to estimate time-to-permanent component failure, since the dendritic bridge creates a permanent conductive path between conductors.

210 Advanced Elastomers – Technology, Properties and Applications

surface insulation resistance (105 ohm).

the cathode (dendrites) 50-52.

contacts.

surface after a certain period of time incubation in the temperature-humidity-bias conditions. The metal-in-elastomer socket consists of silver particles in elastomer matrix. Previous studies showed that silver can migrate in the humidity-temperature-bias conditions 54. However, those studies added the external conductive ions, e.g., contaminants or silver ions, into the system to initialize the migration process 55. In recent studies, several substantial results exploited different electrochemical migration, including silver electrochemical migration (ECM) processes, migration products, and failure criterion on

The silver ECM could occur when bias was applied to silver-in-elastomer socket in the humidity-temperature. Initially, the water layer absorbed on the surface is electrolyzed to hydrogen and hydroxide ions, which further migrate to cathode and anode respectively. Silver oxides to silver ions by accepting electrons through electrolyzed water solution under bias, and silver ions are accumulated in absorbed water layer. The silver ions are also migrated to cathode through water layer. Overall, the surface insulation resistance (SIR) decreases when ions are generated in the absorbed water layer, or formation of conductive electrolyte. The short-circuit failure occurs when silver ion concentration increases to the

Both film and dendrites were observed on the film after permanent failure when the surface insulation resistance of the silver-in-elastomer was less than 1000 ohm. Studies on this regard showed that the film growth was developed in deionized water, while the dendrite growth was developed in high conductivity water. It was found that the film grew on the surface without distinctive direction while dendrites grew from cathode to anode. The analysis of film and dendrites from electron scanning spectrometry and X-ray photoelectron microscopy showed that the film consists of silver oxide and dendrites are silver only. The process of the silver electrochemical migration is progressed as follow: silver oxidation at anode to formation of silver ion; silver ion migration from anode to cathode through absorbed water layer (6-7 water monolayers); partial silver ion reduction to silver oxide with hydroxide from water electrolysis (film); other silver ion reduction to formation of silver at

New failure criterion on surface insulation resistance was proposed. In the IPC standard (IPC-TR-476A), the failure was defined as the surface insulation resistance below 106 ohm. New experimental findings showed that the component didn't fail when resistance below the resistance, no electrochemical migration related products detected either 50-52. In addition, the surface insulation resistance immediately decreased to the order of 106 ohm when the metal-in-elastomer was conditioned in the highly accelerated stress testing (130C/85% RH/bias). Until SIR was below 105 ohm, the silver dendrites were observed. The resistance ultimately was within hundreds of ohms when the dendrites bridged the adjacent

The new ECM process was proposed as: presence of moisture, anodic metal dissolution or ion generation and ion migration to cathode (electrochemical reaction), ion accumulation, and metal dendritic growth. The ion accumulation was found to be the rate controlling step

specific value when the surface insulation resistance decreases to 105 ohm.

The ion-accumulation based model assumed that the silver discharged at the anode was directly proportional to the amount of electrical charge that passed through the electrode, or Faraday's Law, the total mass of metal dissolution equaled the total mass of metal ions discharged into the electrolyte. Unlike the bulk resistance of conductive particles in elastomer 56, by use of Ohm's law and Brunauer-Emmett-Teller (BET) model 57, the time-tofailure model was given by,

$$TTF = nF \times \frac{m\_0}{M} \times \beta \times \frac{1}{V} \times \frac{(1 - RH)[1 + (c - 1)RH]}{cRH} \times e^{\left(\frac{E\_\alpha}{RT}\right)t}$$

Where *<sup>s</sup>* is the surface insulation conductivity, *R* is the resistance, *m0* is the critical mass loss required to dissolve to reach the critical ion concentration when resistance decreases to value of failure criterion, *V* is potential, *RH* is relative humidity, *n* is chemical valence, *M* is the molecular weight, *F* and *R* are constant, , *c* and *E* are coefficient relevant to relative humidity, temperature and materials properties. Using this model, the time-to-failure is estimated for elastomer sockets. For silver in elastomer socket, the *TTF* can be simplified as,

$$TTF = 1.11 \times 10^{-5} \times \frac{1}{V} \times \frac{(1 - RH)}{RH} \times e^{\left(\frac{7846}{T}\right)t}$$

Based on this model, the silver-in-elastomer socket fails in 150 hours after incubation in 90C/90%/20 V conditions; while it can resist up to 27000 h in service at conditions of 25C/50%/110 V.

Another failure mechanism is called creep and stress relaxation due to viscoelastic elastomer as matrix in socket. As shown in Figure 2, the mechanical support fixed elastomer deformation as constant. As a result, the stress applied to the socket is expected to decrease gradually due to the stress relaxation of elastomer. To understand failure mechanism and its effect on contact resistance, the dynamic mechanical analyzer (DMA) was used to characterize elastomer stress relaxation. The chamber of DMA can control temperature by air gas and liquid nitrogen. Creep is performed by constant stress and stress relaxation by constant strain. The creep process is often categorized into three stages: primary creep (a stage of decreasing creep rate), secondary creep (a stage of constant creep rate), and tertiary creep (a stage of increasing creep rate). When the load is initially applied, there is an instantaneous elongation, a primary stage of a transient nature during which slip and work hardening occur in the most favorably oriented grains. Then there is a secondary stage of steady-state creep during which the deformation continues at an approximately constant rate. The third stage (tertiary stage) takes place when the stress is high enough that the creep

$$R\_c = \frac{\rho}{2} \sqrt{\frac{\pi H}{F\_0[1 - B \ln\left(\frac{t}{C} + 1\right)]}} + \frac{\sigma\_f H}{F\_0[1 - B \ln\left(\frac{t}{C} + 1\right)]}$$

generates micropatterns by contact printing and microstructures by embossing and replica molding, has been used to manufacture blazed grating optics, stamps for chemical patterning, and microfluidics devices 19, 62, 63. Different from photolithography technique 64, the soft lithography is not subject to the limitations set by optical diffraction and optical transparency, with feature size down to 10-100 nm. Soft lithography techniques is termed as 'multilayer soft lithography' and developed by Quake et al. 59 in 2000. Briefly, multilayer structures were formed by bonding PDMS layers from separated cast in a micromachining mold. Since each layer has excess of one of the components, the reactive molecules remained at the interface form permanent bonding after further curing. They used such technique for development of microfluidic large-scale integration chips that contain plumbing networks with thousands of micromechanical PDMS valves and hundreds of individually addressable chambers. Their chips can be used to construct the microfluidic analog of a comparator array and a microfluidic memory storage device similar to the behavior of random-access memory.

212 Advanced Elastomers – Technology, Properties and Applications

secondary stage was also a function of temperature.

�� <sup>=</sup> � 2 �

Where the *Rc* is contact resistance,

socket for scheduled socket replacement.

rate accelerates until fracture occurs 58. The results for the creep of the silicone elastomer are shown in Figure 3 (a). Both primary and secondary creeps are observed. The results show that the creep deformation increases with temperature. The constant creep rate during the

Stress relaxation refers to the time-dependent contact force decrease under a constant strain. Figure 3 (b) plotted behavior of stress relaxation of elastomer under various temperatures. The significance of stress relaxation on elastomer socket is that contact resistance increases at a decreased contact force. It is thus important to derive the relationship time-dependent contact resistance due to stress relaxation of elastomer. Combination of stress relaxation and

� + ���

is initial contact force, *t* is time, and *B*, *C* are temperature dependent parameters, given by,

The *Rc – t* can be used for estimation of contact resistance degradation of metal-in-elastomer

**Figure 3.** Elastomer creep (a) and stress relaxation (b) behavior at various temperatures. Dots:

Elastomer is also one of popular materials for microfluidics application as substrate, microvalves, pumps 59-61. Elastomer can be easily patterned by curing on a micromachining mold as an alternative microfabrication technique. The soft lithography technique, which

experimental data; solid line: fitted curve. Reprint and permission from 45

**4. Elastomer as substrate in microfluidics**

+

��� ���� � ��� ��

� + ���

� (for silicon elastomer)

is electrical resistivity (ohm.m), *H* is contact hardness, *F0*

contact resistance, the time-dependent contact resistance model is derived 45,

�� ���� � ��� ��

� = ��2�� � � �����, and � = ���2� � � ����������

PDMS microfluidics have great potential to be biomedical applications which only require small amounts of sample, routine operation by untrained personnel and low cost 65. One of such examples was developed on a simple, inexpensive PDMS microfluidic diagnostic device, which performs sandwich immunoassays for medicine and biological studies 12, 63. Screws are used in the device as virtual valves to on and off fluids. The low-cost on soft lithography microfabrication and raw materials makes this device great potential for portable healthcare delivery and monitoring. PDMS microfluidics can be used for reagent mixing in microscale channel 66. The challenge for flow mixing in a small dimension is mainly due to the difficult liquid turbulence inside chip. By use of PDMS as microfluidics substrate, a topological structure can be fabricated to exploit the laminarity of the flow to repeatedly fold the flow and double the lateral concentration gradient in a compact chip. Two different solutions are contained in a T-junction, and fluid flow is repeatedly split, rotated, and merged 67. The chip design was obtained by a rapid-prototyping master mold, and results showed that effective mixing can be achieved on short length scales.

PDMS microfluidics has applied to a variety of applications such as cell sorting, DNA sizing and sorting. DNA sequence is successfully performed in PDMS devices including sample preparation and electrophoresis analysis 14. PDMS has substantial advantages over glass or silicon dioxide substrates, since these latter materials were adhesive for the cells of interest. In addition, Chou et al. 68 developed a PDMS device for the sizing and sorting of restriction fragments of DNA based on single DNA molecule detection. The device sizes DNA on the basis of fluorescence intensity in which the DNA was labeled with fluorescence dye.

PDMS device can be easily fabricated by non-covalent bonding or plasma treated permanent bonding after replica by molding. The PDMS microfluidics for capillary electrophoresis was developed by Effenhauser et al. 69 in 1997. They used replica molding to obtain PDMS device and bonded reversibly the device with a flat piece of PDMS. The performance of PDMS CE is not ideal on unmodified surface, especially for proteins/peptides analysis, due to hydrophobicity of the unmodified PDMS surface. Later, Whitesides et al. developed PDMS microfluidics device by a rapid prototyping and irreversible bonding through plasma

oxidation19. Different PDMS devices were developed to demonstrate separation performance of fluorescently labeled amino acids and proteins using capillary zone electrophoresis (CZE) and gel electrophoresis (GE).

One-dimensional sodium dodecyl sulfate (SDS) capillary gel electrophoresis (CGE) can be also performed in PDMS microfluidics due to its versatile surface chemistry. PDMS surface is treated by UV-Ozone so that it could immobilize other hydrophilic molecules to prevent electroosmotic flow and protein/peptide non-specific binding. For complex cell lysate analysis, isoelectric focusing (IEF) can be interfaced with SDS-CGE as the first dimension separation. What makes PDMS special for IEF-SDS/CGE is because of conventional procedures for fabricating 3-D microfluidic channels 70, and the facility with which PDMSbased devices can be readily assembled and disassembled. For example, after IEF separation, the PDMS was disassembled and connected with a 3-D channel for SDS/CGE. Recent development on 2-D IEF-SDS/CGE showed that other thermoplastic would be better substrate of choice for integrated 2-D complex cell lysate analysis 71, 72.

The design of elastomer microfluidics can be made based on these studies 71-73. The electrokinetic injection of defined sample plugs within single dimension microfluidic systems has been extensively studied, with the most widely used injector configurations including the cross, double-T, and triple-T topologies 74, 75, and extended configurations employing continuous sample injection leveraging flow switching techniques. Floating injection and pinched-injection methods 76, 77 have been employed in simple cross-injectors for the controlled definition of small sample plugs, while double- and triple-T designs are generally employed for the introduction of larger sample volumes. Sample leakage 78 was observed in a 2-D device during sample injection which was the central issue for separation performance as shown in Figure 4. After the desired sample plug has been transferred to the separation channel, additional samples enter the injection region due to diffusion and fringing of the electrical field during the electrokinetic transfer process. The excess sample tailing degrades separation performance. To solve this issue, backbiasing is a commonly used to eliminate sample leakage. Practically, bias voltages can be applied to sample inlet and waste reservoirs, electrokinetically pulling back excess sample from the injection zone. The effective of this approach depended on the accurate selection of backbiasing bias. This requires characterization of device geometry, medium in device, and buffer resistivity. A few groups 49, 72, 79 have proposed 2-D chip designs consisting of an array of second dimension separation channels aligned with an identical number of injection channels on the opposite side of the first dimension microchannel, a more efficient design for spatially multiplexed 2-D separation chips is shown in Figure 4 (a). The design consists of a single first dimension separation channel intersected by multiple second dimension separation channels on one side, and sample injection channels on the opposite side, with the injection channels staggered with respect to the separation channels to ensure complete and simultaneous sampling of species separated in the first dimension channel. In this staggered design, the parallel second dimension microchannels may be regarded as an array of double-T injectors operating in parallel. However, unlike the single-channel case, the double-T injectors are not electrically isolated in the multidimensional system.

electrophoresis (CZE) and gel electrophoresis (GE).

substrate of choice for integrated 2-D complex cell lysate analysis 71, 72.

oxidation19. Different PDMS devices were developed to demonstrate separation performance of fluorescently labeled amino acids and proteins using capillary zone

One-dimensional sodium dodecyl sulfate (SDS) capillary gel electrophoresis (CGE) can be also performed in PDMS microfluidics due to its versatile surface chemistry. PDMS surface is treated by UV-Ozone so that it could immobilize other hydrophilic molecules to prevent electroosmotic flow and protein/peptide non-specific binding. For complex cell lysate analysis, isoelectric focusing (IEF) can be interfaced with SDS-CGE as the first dimension separation. What makes PDMS special for IEF-SDS/CGE is because of conventional procedures for fabricating 3-D microfluidic channels 70, and the facility with which PDMSbased devices can be readily assembled and disassembled. For example, after IEF separation, the PDMS was disassembled and connected with a 3-D channel for SDS/CGE. Recent development on 2-D IEF-SDS/CGE showed that other thermoplastic would be better

The design of elastomer microfluidics can be made based on these studies 71-73. The electrokinetic injection of defined sample plugs within single dimension microfluidic systems has been extensively studied, with the most widely used injector configurations including the cross, double-T, and triple-T topologies 74, 75, and extended configurations employing continuous sample injection leveraging flow switching techniques. Floating injection and pinched-injection methods 76, 77 have been employed in simple cross-injectors for the controlled definition of small sample plugs, while double- and triple-T designs are generally employed for the introduction of larger sample volumes. Sample leakage 78 was observed in a 2-D device during sample injection which was the central issue for separation performance as shown in Figure 4. After the desired sample plug has been transferred to the separation channel, additional samples enter the injection region due to diffusion and fringing of the electrical field during the electrokinetic transfer process. The excess sample tailing degrades separation performance. To solve this issue, backbiasing is a commonly used to eliminate sample leakage. Practically, bias voltages can be applied to sample inlet and waste reservoirs, electrokinetically pulling back excess sample from the injection zone. The effective of this approach depended on the accurate selection of backbiasing bias. This requires characterization of device geometry, medium in device, and buffer resistivity. A few groups 49, 72, 79 have proposed 2-D chip designs consisting of an array of second dimension separation channels aligned with an identical number of injection channels on the opposite side of the first dimension microchannel, a more efficient design for spatially multiplexed 2-D separation chips is shown in Figure 4 (a). The design consists of a single first dimension separation channel intersected by multiple second dimension separation channels on one side, and sample injection channels on the opposite side, with the injection channels staggered with respect to the separation channels to ensure complete and simultaneous sampling of species separated in the first dimension channel. In this staggered design, the parallel second dimension microchannels may be regarded as an array of double-T injectors operating in parallel. However, unlike the single-channel case, the double-T injectors are not electrically isolated in the multidimensional system.

**Figure 4.** Schematic diagram of two-dimensional capillary electrophoresis microfluidics design with (a) simplified device of a spatially-multiplexed separation platform with five second dimension microchannels, and (b) image during the sample transfer process using a fabricated chip without backbias channels. Reprint and permission from 71

This interconnected design can result in significant variations in performance between the different injectors, as depicted in Figure 4 (b). In this image, the sample within the first dimension channel is being electrokinetically transferred into the second dimension channel array by applying a uniform bias voltage in the injection channel reservoirs while grounding the second dimension reservoirs. Three main features are evident in this injection process. The substantial tailing of sample occurs at the head of each second dimension channel, resulting in sample dispersion during transfer; in all but the center second dimension channel, the injection is highly asymmetric, reflecting a non-uniform electric field distribution within the first dimension channel; sample from the outermost regions of the first dimension channel

continually leaks into the second dimension array due to a combination of diffusion and electric field fringing, leading to additional tailing which can continue long after sample from the center region of the first dimension channel has been fully transferred separation devices .

The solution for unsymmetrical injection and tailing was proposed by adding backbias channel s parallel to the separation channels as shown in Figure 4 (b) 70. Analytical calculation indicated that two-fold of resistance in the backbias channel generated uniform current and potential distribution in the 2-D channel network, not only in 1-D channel, but also injection and separation channels. Additionally, the angled 1-D channel can level the electric field distribution in the local region of T intersection. As a result, the angled 1-D channel substantially reduced sample tailing with negligible effect on the first dimension separation. These design perspectives have been used for complex biological sample analysis.

Use of PDMS for liquid chromatography has been limited largely due to its low bonding strength 80 and high gas/liquid permeability 81. It was reported to fabricate column in a device using PDMS 82-84. Since PDMS is inexpensive and thus it can be fabricated as a disposable column, it can be used as a stationary phase in liquid chromatography columns. The siloxane monomer was added into a molding to formation of desired features. The featured PDMS was reversibly sealed with cover PDMS by placing the molded chip in contact with cover PDMS, or irreversibly sealed by plasma oxidation. The PDMS chip was treated with silanes to minimize sample non-specific absorption. In general, PDMS is not the good materials of choice for high performance liquid chromatography. In such instance, other thermoplastics such as polyimide (PI) 85, fuse silica capillary 75, cyclic olefin copolymer (CoC) 86, are recommended.

Droplets formed by mixing one fluid in another or emulsions are very useful in a wide range of applications including personal care products, foods, microbiology, cell biology, and drug delivery vehicles 87, 88. It has been recognized that a successful droplet device for applications is able to control over size and distribution in microscale and nanoscale emulsions 88. Droplet formation can be obtained by either turbulence to break apart an immiscible mixture or continuous flow. PDMS devices for micro-droplet formation are fabricated by soft lithography in a molding. The native PDMS surface is usually hydrophobic and excellent surface for water-in-oil emulsions. The hydrophobic PDMS surface ensures that the oil droplets encapsulation of water phase inside without being contacted the PDMS channel walls 89, 90; while hydrophilic PDMS surface can be used for oilin-water droplets and biological sample analysis 91, 92. Several surface modifications have been reported by the sequential layer-by-layer deposition of polyelectrolytes yielding hydrophilic microchannels. Others used oxygen-plasma and oxygen-C2F6 to modify PDMS surface in order to enhance hydrophilicity and reduce channel electroosmotic flow (EOF). Alternatively, use of chemical modification, e.g., 2-hydroxyethyl methacrylate (HEMA), can permanently enhance PDMS hydrophilicity 91. In layer-by-layer (LbL) surface modification approach, segments of sodium chloride, poly(allylamine hydrochloride), and poly(sodium 4-styrenesulfonate) solutions are separated by air plugs and loaded into a piece of tubing and sequentially flushed through the channel at a constant flow rate. This LbL modification provides long storage time up to 5 months without noticeable behavior change of droplet formation. The LbL modified PDMS device can be used for water-oil-water double emulsion system for applications such as pharmaceutical compound delivery.

An interesting PDMS device for static microdroplet arrays is fabricated to droplet trapping, incubation, and release of enzymatic and cell-based assays 93. They simply used a singlelayered PDMS microfluidic structure. The aqueous droplets are trapped en masse and optically monitored for extended periods of time. The array droplet approach is used to characterize droplet shrinkage, aggregation of encapsulated biological cells and enzymatic reactions. On the other hand, to optimize the droplet formation, a design which use feedback control loop is developed 88. They successfully demonstrated that the closed loop control system with negative feedback can produce the required droplet size with minimal noise and no systematic steady sate error. This close-loop design is utilized for a number of applications, including micro-chemistry and emulsion production.

## **5. Outlook of elastomer application**

216 Advanced Elastomers – Technology, Properties and Applications

continually leaks into the second dimension array due to a combination of diffusion and electric field fringing, leading to additional tailing which can continue long after sample from the center region of the first dimension channel has been fully transferred separation devices .

The solution for unsymmetrical injection and tailing was proposed by adding backbias channel s parallel to the separation channels as shown in Figure 4 (b) 70. Analytical calculation indicated that two-fold of resistance in the backbias channel generated uniform current and potential distribution in the 2-D channel network, not only in 1-D channel, but also injection and separation channels. Additionally, the angled 1-D channel can level the electric field distribution in the local region of T intersection. As a result, the angled 1-D channel substantially reduced sample tailing with negligible effect on the first dimension separation.

Use of PDMS for liquid chromatography has been limited largely due to its low bonding strength 80 and high gas/liquid permeability 81. It was reported to fabricate column in a device using PDMS 82-84. Since PDMS is inexpensive and thus it can be fabricated as a disposable column, it can be used as a stationary phase in liquid chromatography columns. The siloxane monomer was added into a molding to formation of desired features. The featured PDMS was reversibly sealed with cover PDMS by placing the molded chip in contact with cover PDMS, or irreversibly sealed by plasma oxidation. The PDMS chip was treated with silanes to minimize sample non-specific absorption. In general, PDMS is not the good materials of choice for high performance liquid chromatography. In such instance, other thermoplastics such as polyimide

Droplets formed by mixing one fluid in another or emulsions are very useful in a wide range of applications including personal care products, foods, microbiology, cell biology, and drug delivery vehicles 87, 88. It has been recognized that a successful droplet device for applications is able to control over size and distribution in microscale and nanoscale emulsions 88. Droplet formation can be obtained by either turbulence to break apart an immiscible mixture or continuous flow. PDMS devices for micro-droplet formation are fabricated by soft lithography in a molding. The native PDMS surface is usually hydrophobic and excellent surface for water-in-oil emulsions. The hydrophobic PDMS surface ensures that the oil droplets encapsulation of water phase inside without being contacted the PDMS channel walls 89, 90; while hydrophilic PDMS surface can be used for oilin-water droplets and biological sample analysis 91, 92. Several surface modifications have been reported by the sequential layer-by-layer deposition of polyelectrolytes yielding hydrophilic microchannels. Others used oxygen-plasma and oxygen-C2F6 to modify PDMS surface in order to enhance hydrophilicity and reduce channel electroosmotic flow (EOF). Alternatively, use of chemical modification, e.g., 2-hydroxyethyl methacrylate (HEMA), can permanently enhance PDMS hydrophilicity 91. In layer-by-layer (LbL) surface modification approach, segments of sodium chloride, poly(allylamine hydrochloride), and poly(sodium 4-styrenesulfonate) solutions are separated by air plugs and loaded into a piece of tubing and sequentially flushed through the channel at a constant flow rate. This LbL modification provides long storage time up to 5 months without noticeable behavior change of droplet formation. The LbL modified PDMS device can be used for water-oil-water double emulsion

These design perspectives have been used for complex biological sample analysis.

(PI) 85, fuse silica capillary 75, cyclic olefin copolymer (CoC) 86, are recommended.

system for applications such as pharmaceutical compound delivery.

The application of elastomer is being continuously exploited. The future of elastomer is closely related to the specific requirement on applications. One of the hot topics is development of nano- and microfluidics for assisted reproductive technologies 94. For example, the studies have demonstrated that thickness of PDMS layers at 100 µm in chips produced significantly fewer embryos due to drastically shift in osmolality of embryo-culture media. It could be the solution by development of a hybrid membrane consisting of PDMS-parylene-PDMS for maintaining a stable cell growth environment. Therefore, investigation of new type of PDMS materials or hybrid PDMS can provide means for such applications.

Due to the low bonding strength, PDMS has seldom used for high performance liquid chromatography application (HPLC). The HPLC can be directly interfaced with detection system such as electrospray ionization or mass spectrometry. As a result, it becomes research of interest to synthesize PDMS which has strong bonding strength (via covalent bond) and low permeability, while PDMS optical transparency and elasticity are largely remained. In summary, PDMS is becoming more and more popular as the materials of choice for biological analysis due to its good physical, mechanical, and chemical properties, as well as its excellent biocompatibility.

A thermoplastic rubber material has recently been developed that can completely mend itself when the fracture interfaces are rejoined and left to heal for a moderate time 95. This "smart" rubber is easy to synthesize and displays excellent mechanical properties. This material could be used for microelectronics, microfluidics and biology system.

### **Author details**

Shuang (Jake) Yang\* *Department of Pathology, Johns Hopkins University, Baltimore, USA*

Shuang (Jake) Yang and Kunqiang Jiang *Department of Mechanical Engineering, University of Maryland, USA Department of Chemistry and Biochemistry, University of Maryland, USA*

<sup>\*</sup> Corresponding Author

#### **6. References**

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218 Advanced Elastomers – Technology, Properties and Applications

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## **Elastomeric Electronics: A Microfluidic Approach**

Shi Cheng

222 Advanced Elastomers – Technology, Properties and Applications

[81] Kuncová-Kallio, J. & Kallio, P.J. 2486-2489 (IEEE, 2006).

molded in poly (dimethyl siloxane). 22, 3736-3743 (2001).

capillary electrophoresis devices. 21, 107-115 (2000).

plasma and chemical treatments. 83, 1277-1279 (2006).

in-oil-in-water double emulsions. 10, 1814-1819 (2010).

assisted reproduction. 57, 125-135 (2002).

1200, 55-61 (2008).

(2010).

(2002).

in microchannels. 82, 364 (2003).

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[82] Weng, X., Chon, C.H., Jiang, H. & Li, D. Rapid detection of formaldehyde concentration in food on a polydimethylsiloxane (PDMS) microfluidic chip. 114, 1079-1082 (2009). [83] Slentz, B.E., Penner, N.A., Lugowska, E. & Regnier, F. Nanoliter capillary electrochromatography columns based on collocated monolithic support structures

[84] Ocvirk, G. et al. Electrokinetic control of fluid flow in native poly (dimethylsiloxane)

[85] Levkin, P.A. et al. Monolithic porous polymer stationary phases in polyimide chips for the fast high-performance liquid chromatography separation of proteins and peptides.

[86] Mair, D.A., Geiger, E., Pisano, A.P., Fréchet, J.M.J. & Svec, F. Injection molded

[87] Anna, S.L., Bontoux, N. & Stone, H.A. Formation of dispersions using "flow focusing"

[88] Miller, E., Rotea, M. & Rothstein, J.P. Microfluidic device incorporating closed loop feedback control for uniform and tunable production of micro-droplets. 10, 1293-1301

[89] Link, D.R. et al. Electric control of droplets in microfluidic devices. 45, 2556-2560 (2006). [90] Fujii, T. PDMS-based microfluidic devices for biomedical applications. 61, 907-914

[91] Bodas, D. & Khan-Malek, C. Formation of more stable hydrophilic surfaces of PDMS by

[92] Bauer, W.A.C., Fischlechner, M., Abell, C. & Huck, W.T.S. Hydrophilic PDMS microchannels for high-throughput formation of oil-in-water microdroplets and water-

[93] Huebner, A. et al. Static microdroplet arrays: a microfluidic device for droplet trapping, incubation and release for enzymatic and cell-based assays. 9, 692-698 (2009). [94] Beebe, D., Wheeler, M., Zeringue, H., Walters, E. & Raty, S. Microfluidic technology for

[95] Wietor, J.L. & Sijbesma, R.P. A Self‐Healing Elastomer. 47, 8161-8163 (2008).

microfluidic chips featuring integrated interconnects. 6, 1346-1354 (2006).

protein separations in a plastic microfludic network. 76, 742-748 (2004).

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50208

### **1. Introduction**

Thousands of millions of electronic units have been produced since the first invention of electronics. These devices and systems, in which stiff, solid metals, insulators, and semiconductors have dominated for ages, are exclusively made in the rigid format, and usually retain a static shape once fabricated. Things have however changed recently. Researchers are currently attempting to shape a soft and rubbery future for electronics, where elastic materials like elastomers would be widely used to replace conventional rigid materials. The reason is rather simple. We as human beings are soft, just like most other living creatures in nature, but existing electronics are not. Shouldn't we create some kind of new electronics that resemble ourselves?

Extensive research activities on developing elastomeric electronics that may withstand severe bending, twisting, and straining, with maintained electronic functions, have been recently carried out. The mechanical characteristics of gold thin films on polydimethylsiloxane (PDMS) surfaces: wrinkles and cracks, have been investigated by Whitesides and Hutchinson for the first time in 1998 [1]. Subsequently, wrinkled electrodes on pre-stretched elastic substrates were employed to implement electroactive polymer actuators [2], and studies on wrinkled gold electrodes on PDMS substrates were conducted in the Wagner's and Suo's groups at Princeton [3],[4]. Micromechanics and manufacturing processes based on relaxed and pre-stressed PDMS slabs were developed in-depth. The most significant contributions were made by the Rogers's group at University of Illinois, Urbana-Champaign, where stretchable and foldable silicon integrated circuits (ICs) on "wavy" silicon ribbons (from Silicon On Insulator (SOI)) in PDMS were proposed for fully integrated stretchable electronics [5],[6]. Dozens of appealing devices based on stretchable silicon ICs, including electronic eye sensor [7], smart gloves/skins [8], implanted medical devices [9], and wearable ergonomic biomedical sensors [10], have been demonstrated. An alternative approach utilizing anisotropic etching of bulk wafers was also introduced [11],[12]. Later, thin meandered stretchable interconnects encased in silicone rubber substrates, operating at

© 2012 Cheng, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

different frequencies, were presented [13]-[15]. New electrical nanocomposite materials containing sliver nano-particles or carbon nanotubes/graphene that can flex and strain showed promising results as well [16],[17].

Microfluidics based stretchable electronics were initiated by Whitesides, who first studied low temperature metling solder filled microstructured elastomeric channels [18],[19]. Recently, this concept was extended to implement elastic direct current (DC) circuits, by incorporating eutectic gallium and indium alloy (EGaIn) into microfluidic channels in thin sheets of elastomers [20]-[22]. However, all these previously mentioned studies deal with either low-frequency ICs or relatively simple interconnects. Stretchable electronics at radio frequencies remained an unexploited field until early 2009, at which time Cheng demonstrated the first stretchable fluidic antennas, for enabling wireless communication and remote sensing [23],[24]. Similar work was reported afterwards using EGaIn alloy as conductors for the antennas instead of Galinstan [25]. Significantly enhanced elasticity of the resulting antennas could be achieved by introducing a different type of siloxane [26], and mechanically reconfigurable antennas could also be realized [27]. In 2010, Cheng proposed a hybrid integration strategy for the first demonstration of active microfluidic stretchable RF electronics (μFSRFEs), an integrated RF radiation sensor, *cf*. Fig. 1.a. [28]. Cheng further developed the μFSRFEs to the multi-layer configurations, and reported an integrated stretchable large-area wireless strain sensor, as seen in Fig. 1.b. [29].

**Figure 1.** Foldable, flexible, stretchable elastomeric electronic devices: a). Microfluidic stretchable radiation sensor. b). Elastomeric, reversibly stretchable, large-area wireless strain sensor.

The following sections in this chapter address the recent progresses in the emerging field of microfluidics based elastomeric electronics. Fabrication processes, integration techniques, application examples, as well as future perspectives are presented and discussed.

## **2. Fabrication of single-layer microfluidics based passive elastomeric electronic devices**

In brief, the manufacturing process steps can be summarized as follows: master construction, molding, liquid-alloy injection or filling, and encapsulation, *cf*. Fig. 2.

showed promising results as well [16],[17].

different frequencies, were presented [13]-[15]. New electrical nanocomposite materials containing sliver nano-particles or carbon nanotubes/graphene that can flex and strain

Microfluidics based stretchable electronics were initiated by Whitesides, who first studied low temperature metling solder filled microstructured elastomeric channels [18],[19]. Recently, this concept was extended to implement elastic direct current (DC) circuits, by incorporating eutectic gallium and indium alloy (EGaIn) into microfluidic channels in thin sheets of elastomers [20]-[22]. However, all these previously mentioned studies deal with either low-frequency ICs or relatively simple interconnects. Stretchable electronics at radio frequencies remained an unexploited field until early 2009, at which time Cheng demonstrated the first stretchable fluidic antennas, for enabling wireless communication and remote sensing [23],[24]. Similar work was reported afterwards using EGaIn alloy as conductors for the antennas instead of Galinstan [25]. Significantly enhanced elasticity of the resulting antennas could be achieved by introducing a different type of siloxane [26], and mechanically reconfigurable antennas could also be realized [27]. In 2010, Cheng proposed a hybrid integration strategy for the first demonstration of active microfluidic stretchable RF electronics (μFSRFEs), an integrated RF radiation sensor, *cf*. Fig. 1.a. [28]. Cheng further developed the μFSRFEs to the multi-layer configurations, and reported an integrated

stretchable large-area wireless strain sensor, as seen in Fig. 1.b. [29].

**Figure 1.** Foldable, flexible, stretchable elastomeric electronic devices: a). Microfluidic stretchable

The following sections in this chapter address the recent progresses in the emerging field of microfluidics based elastomeric electronics. Fabrication processes, integration techniques,

In brief, the manufacturing process steps can be summarized as follows: master

radiation sensor. b). Elastomeric, reversibly stretchable, large-area wireless strain sensor.

application examples, as well as future perspectives are presented and discussed.

construction, molding, liquid-alloy injection or filling, and encapsulation, *cf*. Fig. 2.

**electronic devices**

**2. Fabrication of single-layer microfluidics based passive elastomeric**

**Figure 2.** Manufacturing process steps of single-layer passive microfluidics based elastomeric electronics.

First of all, the design patterns are transferred to a 100 μm thick SU-8 100 (MicroChem, Newton, MA) layer on a silicon wafer with a standard soft lithography, and then developed and thermally stabilized at 150°C for 30 min to enhance the adhesion between SU-8 layer and silicon substrate. Subsequently, the PDMS prepolymer and cross linker (Elastosil RT601A and B, Wacker Chemie, Munich, Germany) are thoroughly mixed at a ratio of 9:1 (wt:wt) and poured onto the SU-8 master. After degassing, the PDMS mixture is cured at 70°C in an oven for 30 min. Then the cured thin PDMS replica is peeled off and a couple of holes are punched out for liquid alloy injection or filling. Meanwhile a thin blank PDMS lid is prepared using blank silicon wafer. Later, the PDMS replica and blank lid are bonded, using corona discharging (ETP, Chicago, IL, USA) activation. Through the punched holes in the PDMS, the liquid metal alloy (Galinstan, 68.5 % Ga, 21.5 % In, 10 % Sn, σ=3.46106 S/m) is manually injected into the channels. This alloy remains in liquid state from -19°C to 1300°C. The ventilation outlets are encapsulated with uncured PDMS mixture as mentioned above.

### **3. Liquid metal, elastic, microfluidic unbalanced loop antenna**

The fabrication process described in the previous section has been successfully employed to implement a single-layer, liquid metal, stretchable, unbalanced loop antenna, which consists of a radiating element and a semi-circular ground plane, as depicted in Fig. 3 [23]. Several cylindrical reservoirs are aligned along the upper tube of the antenna prototype to ensure

good electrical connections while being flexed. The presence of these reservoirs introduces slight resonance frequency decrease of the antenna. The lower semi-circular area acts as a ground plane for the antenna. A number of posts are aligned to space the top and bottom PDMS membranes in the ground area, which has negligible influence on the electrical characteristics of the antenna.

**Figure 3.** Geometrical schematic of stretchable unbalanced loop antenna: *R*=18.1 mm, *R*sub=47.5 mm, *R*reservior=1 mm, *W*S=400 μm, *L*grid=1.5 mm, *W*grid=1.5 mm, *D*grid=1.9 mm, *h*=1.0 mm, *h*metal=100 μm, and *θ*=30°.

Mechanical properties of the antenna prototype were characterized experimentally. Fig. 4 shows that the resulting antenna is reversibly foldable, flexible, and stretchable. Severe bending and twisting in the tests did not cause any mechanical damages. In addition, the antenna can withstand extreme levels of strains of up to 40% along multiple axis. As a result of the uneven thickness of the PDMS substrate and the heterogeneous pattern of the antenna, slight mechanical inhomogeneity was observed while stressing along different orientations, *cf*. Figs. 4. b and c. In principle, elasticity of up to 100% should be in reach. Nevertheless, the openings in the membranes for electrical tests limit the mechanical deformability, reliability, and robustness, as the antenna can be easily torn from these openings.

Electrical properties of the antenna in its original state were obtained from numerical simulations. Experiments on port impedance and radiation characteristics of the antenna in both relaxed and flexed states were performed. Fig. 5 presents the simulated and measured reflection coefficients (*S*11). The non-strained antenna features good impedance matching at 2.4 GHz, with an input impedance of approximately 75+15j Ω, Fig. 5.a. Applying strains on the antenna introduces an increase of the length of the upper radiating loop so that leads to a decrease on its resonance frequency, Fig. 5.b. In the case of stressing along *y*-axis, the antenna input resistance decreases due to the stronger coupling between the upper antenna arm and the lower ground plane, whereas this resistances goes up while stretching along *x*-axis.

characteristics of the antenna.

good electrical connections while being flexed. The presence of these reservoirs introduces slight resonance frequency decrease of the antenna. The lower semi-circular area acts as a ground plane for the antenna. A number of posts are aligned to space the top and bottom PDMS membranes in the ground area, which has negligible influence on the electrical

**Figure 3.** Geometrical schematic of stretchable unbalanced loop antenna: *R*=18.1 mm, *R*sub=47.5 mm, *R*reservior=1 mm, *W*S=400 μm, *L*grid=1.5 mm, *W*grid=1.5 mm, *D*grid=1.9 mm, *h*=1.0 mm, *h*metal=100 μm, and *θ*=30°.

and robustness, as the antenna can be easily torn from these openings.

Mechanical properties of the antenna prototype were characterized experimentally. Fig. 4 shows that the resulting antenna is reversibly foldable, flexible, and stretchable. Severe bending and twisting in the tests did not cause any mechanical damages. In addition, the antenna can withstand extreme levels of strains of up to 40% along multiple axis. As a result of the uneven thickness of the PDMS substrate and the heterogeneous pattern of the antenna, slight mechanical inhomogeneity was observed while stressing along different orientations, *cf*. Figs. 4. b and c. In principle, elasticity of up to 100% should be in reach. Nevertheless, the openings in the membranes for electrical tests limit the mechanical deformability, reliability,

Electrical properties of the antenna in its original state were obtained from numerical simulations. Experiments on port impedance and radiation characteristics of the antenna in both relaxed and flexed states were performed. Fig. 5 presents the simulated and measured reflection coefficients (*S*11). The non-strained antenna features good impedance matching at 2.4 GHz, with an input impedance of approximately 75+15j Ω, Fig. 5.a. Applying strains on the antenna introduces an increase of the length of the upper radiating loop so that leads to a decrease on its resonance frequency, Fig. 5.b. In the case of stressing along *y*-axis, the antenna input resistance decreases due to the stronger coupling between the upper antenna arm and the lower ground plane, whereas this resistances goes up while stretching along *x*-axis.

**Figure 4.** Photographs of the elastomeric fluidic unbalanced loop antenna: a). relaxed state, b). with 40% *x*-axis elongation, c). with 40% *y*-axis elongation, d). with biaxial elongation, e). bent state, and f). twisted state.

**Figure 5.** Simulated and measured port impedance of the elastic unbalanced loop antenna operating between 2.0 and 2.8 GHz: a). simulated (solid) and measured (dashed) *S*11 of the relaxed antenna; b). measured *S*11 of the flexed antenna with 20% *x*-axis (solid), 40% *x*-axis (dashed) and 20% *y*-axis (dotted), 40% *y*-axis (dashdot) elongation. The *S*11 at 2.4 GHz is marked by a cross sign on each curve.

Fig. 6 shows simulated and measured radiation patterns of the antenna prototype at 2.4 GHz.

As expected, the relaxed antenna resembles conventional unbalanced loops that exhibit broad beam coverage, especially in the *yz*-plane where nearly perfect omnidirectionality is seen. The maximum antenna gain is around 2.7 dBi, and the measured cross-polarization (*Gφ* in the *xz*-plane and *Gθ* in the *yz*-plane) is approximately 15 dB lower than the corresponding co-polarization. Variations of the radiation patterns of the strained antenna can be found, *cf*. Fig. 6.b. The omnidirectionality of the *yz*-plane radiation patterns degrades, particularly while the antenna is stressed along *y*-axis. The increasing cable influence caused by the *y*-axis strain explains the ripples within the angle of 45o-135o on the measured *xz*plane radiation pattern. Also, the level of the measured cross-polarization of the stretched antenna slightly increases compared to that of the relaxed one.

**Figure 6.** Simulated and measured radiation patterns of the a). relaxed and b). flexed antenna at 2.4 GHz. The corresponding coordinate system is depicted in Fig. 3. The antenna gain along *φ* and *θ* is defined as *Gφ* and *Gθ*, respectively.

High conductivity of Galinstan and large cross dimensions of the microfluidic channels attribute to low conductive losses and high radiation efficiency of the antenna. The measured data indicate a resonance frequency decrease of up to 18% while stretching, but the radiation efficiency at 2.4 GHz remains greater than 80%. In the experiments, no significant radiation efficiency decrease can be seen. This fact implies that galvanic connections in the microstructured elastomeric channels are not interrupted by straining. Good electrical continuity of the liquid alloy owes to its excellent wettability on PDMS substrate surfaces. Though the presented unbalanced loop antenna achieves good radiation properties, its resonance frequency detuning introduced by stretching leads to relatively poor total efficiency at 2.4 GHz. Other antenna concepts with more robust port impedance and radiation characteristics in response to deformation would be feasible alternative solutions.

### **4. Bendable, stretchable, fluidic UWB antenna**

The concept of microfluidics based elastomeric electronics is then extended to realize a planar inverted cone antenna (PICA) for the ultrawideband (UWB) frequency range of 3.1- 10.6 GHz [24]. The reason for choosing the PICA is that its impedance matching and radiation characteristics are expected to be insensitive to its mechanical deformation. Also, its uniplanar configuration makes it a suitable antenna type for a design that should be bendable and stretchable. Fig. 7 shows the design schematically, with a leaf-shaped radiation and a large ground plane.

Resembling to the previously demonstrated elastic unbalanced loop, the implemented PICA prototype features excellent reversible deformability in the mechanical tests, as shown in Fig. 8. Extreme levels of straining of up to 40% along either *x*- or *y*-axis do not cause any mechanical failures, *cf*. Figs. 8.b and d. Moreover, the resulting prototype also withstands severe mechanical folding and twisting. After removal of applied mechanical forces, the antenna returns to its original state.

228 Advanced Elastomers – Technology, Properties and Applications

a). b).

defined as *Gφ* and *Gθ*, respectively.

radiation and a large ground plane.

**Figure 6.** Simulated and measured radiation patterns of the a). relaxed and b). flexed antenna at 2.4 GHz. The corresponding coordinate system is depicted in Fig. 3. The antenna gain along *φ* and *θ* is

in response to deformation would be feasible alternative solutions.

**4. Bendable, stretchable, fluidic UWB antenna**

High conductivity of Galinstan and large cross dimensions of the microfluidic channels attribute to low conductive losses and high radiation efficiency of the antenna. The measured data indicate a resonance frequency decrease of up to 18% while stretching, but the radiation efficiency at 2.4 GHz remains greater than 80%. In the experiments, no significant radiation efficiency decrease can be seen. This fact implies that galvanic connections in the microstructured elastomeric channels are not interrupted by straining. Good electrical continuity of the liquid alloy owes to its excellent wettability on PDMS substrate surfaces. Though the presented unbalanced loop antenna achieves good radiation properties, its resonance frequency detuning introduced by stretching leads to relatively poor total efficiency at 2.4 GHz. Other antenna concepts with more robust port impedance and radiation characteristics

The concept of microfluidics based elastomeric electronics is then extended to realize a planar inverted cone antenna (PICA) for the ultrawideband (UWB) frequency range of 3.1- 10.6 GHz [24]. The reason for choosing the PICA is that its impedance matching and radiation characteristics are expected to be insensitive to its mechanical deformation. Also, its uniplanar configuration makes it a suitable antenna type for a design that should be bendable and stretchable. Fig. 7 shows the design schematically, with a leaf-shaped

Resembling to the previously demonstrated elastic unbalanced loop, the implemented PICA prototype features excellent reversible deformability in the mechanical tests, as shown in Fig. 8. Extreme levels of straining of up to 40% along either *x*- or *y*-axis do not cause any mechanical failures, *cf*. Figs. 8.b and d. Moreover, the resulting prototype also withstands

**Figure 7.** Geometry of the 2-D stretchable PICA. Dimensions are: *R*=10 mm, *Rsub*=47.5 mm, *Lg*=25 mm, *Wg*=40 mm, *W1*=1.25 mm, *W2*=1.75 mm, *G*=300 μm, *h*=1 mm, and *hmetal*=100 μm.

**Figure 8.** Photographs of the bendable, flexible, stretchable PICA: a). and c). relaxed state, b). strained antenna with 40% *x*-axis elongation, d). strained antenna with 40% *y*-axis elongation, e). folded antenna, and f). twisted antenna. The corresponding coordinate system is presented in Fig. 7.

Figs. 9 and 10 present simulated and measured reflection coefficients of the elastic, microfluidic PICA. The relaxed antenna shows good impedance match (*S*11 < -10 dB), within 3-11 GHz, both in numerical simulations and experiments.

**Figure 9.** Simulated and measured *S*11 of the non-stretched antenna.

Applying strains along the *x*-axis results in the increased height of the antenna radiator so that its first resonance frequency decreases, *cf*. Fig. 10. Port impedance of the antenna is somewhat sensitive to its geometry, and consequently the antenna exhibits slightly varying impedance matching while stressed. However, good impedance match maintains at the entire UWB frequency band even if the antenna is strained to 40%.

**Figure 10.** Measured reflection coefficients of the flexed antenna.

Measured radiation patterns at 2.5 GHz of the antenna in its relaxed and flexed states are displayed in Figs. 11 and 12. Similar to conventional fat monopole antennas, the nonstrained antenna features broad coverage, especially in the *yz*-plane. The maximum antenna gain at 2.5 GHz was measured to be 2.2 dBi. The cross-polarization discrimination is very good. Numerical simulations are in line with the corresponding experimental data. Stretching the antenna along either *x*- or *y*-axis up to 40% introduces slight variations in the measured radiation patterns at 2.5 GHz, but without any significant gain reduction.

Similar simulations and experiments are also performed at 5 GHz, where ripples and slight asymmetry occur in the radiation patterns caused by disturbance from feed cable. The presence of higher order modes at 5 GHz together with the increasing cable influence degrades the cross-polarization discrimination. Compared to the experimental data at 2.5 GHz, larger variations on the measured radiation patterns at 5 GHz are observed while the antenna is in its strained states, particularly in the *yz*-plane.

**Figure 9.** Simulated and measured *S*11 of the non-stretched antenna.

entire UWB frequency band even if the antenna is strained to 40%.

**Figure 10.** Measured reflection coefficients of the flexed antenna.

antenna is in its strained states, particularly in the *yz*-plane.

Applying strains along the *x*-axis results in the increased height of the antenna radiator so that its first resonance frequency decreases, *cf*. Fig. 10. Port impedance of the antenna is somewhat sensitive to its geometry, and consequently the antenna exhibits slightly varying impedance matching while stressed. However, good impedance match maintains at the

Measured radiation patterns at 2.5 GHz of the antenna in its relaxed and flexed states are displayed in Figs. 11 and 12. Similar to conventional fat monopole antennas, the nonstrained antenna features broad coverage, especially in the *yz*-plane. The maximum antenna gain at 2.5 GHz was measured to be 2.2 dBi. The cross-polarization discrimination is very good. Numerical simulations are in line with the corresponding experimental data. Stretching the antenna along either *x*- or *y*-axis up to 40% introduces slight variations in the

Similar simulations and experiments are also performed at 5 GHz, where ripples and slight asymmetry occur in the radiation patterns caused by disturbance from feed cable. The presence of higher order modes at 5 GHz together with the increasing cable influence degrades the cross-polarization discrimination. Compared to the experimental data at 2.5 GHz, larger variations on the measured radiation patterns at 5 GHz are observed while the

measured radiation patterns at 2.5 GHz, but without any significant gain reduction.

**Figure 11.** Measured a). *xz*- and b). *yz*-plane (according to the coordinate system in Fig. 7) radiation patterns at 2.5 GHz of the non-stretched and stretched antenna with 40% *x*-axis elongation.

**Figure 12.** Measured a). *xz*- and b). *yz*-plane (according to the coordinate system in Fig. 7) radiation patterns at 2.5 GHz of the non-stretched and stretched antenna with 40% *y*-axis elongation.

The measured radiation efficiency within the frequency range of 3-10 GHz is shown in Fig. 13. Although the radiation efficiency at the lower end of the frequency range decreases when the antenna is stretched, it is still greater than 70%. This, together with the experimental data on port impedance, indicates that the PICA prototype achieves good total antenna efficiency regardless of stretching.

Much work however remains. First of all, comprehensive environmental reliability and durability tests, e.g. vibration, temperature cycling, and aging, are needed, since it is indeed a new way of fabricating antennas, with new materials. Electrical characteristics of the antenna at extreme temperature condition, e.g. below the melting point of the liquid alloy, should be evaluated when a special measurement setup is established. Studies on antenna radio interfaces are also necessary. Development of fully integrated stretchable wireless electronic systems consisting of flexible thin embedded active chips, stretchable interconnects, and highly efficient stretchable antennas are the end objective, with a need for much research efforts.

**Figure 13.** Experimental results on the radiation efficiency of the relaxed and flexed antennas.

### **5. Single-layer stretchable elastomeric integrated active RF electronics**

The concept of microfluidics based passive stretchable elastomeric electronics has been further developed to the integrated device level utilizing localized stiff cells (LSCs), as illustrated in Fig. 14 [28]. A microfluidic, elastic, large-area antenna is realized in the same manner as the previous stretchable antennas, by incorporating liquid alloy into microstructured elastomeric channels. Established IC chips associated with passive components are assembled on small flexible laminates. Subsequently, a few tin-plated contact pins resembling cantilevers are soldered to the flexible circuits, and a semi-spherical solder ball is then mounted on the bottom surface of each contact pin on the other end to improve galvanic connection to the liquid fluid, *cf*. Fig. 14.e. Whereafter, the flexible circuits are embedded into the antenna substrate, with each contact pin immersing in the liquid alloy. In the end, uncured PDMS mixture droplets are deposited on top of the flexible circuits to locally enhance the stiffness of the elastomeric substrate and encapsulate the flexible circuits as well as the flex-to-stretch interfaces. The LSCs with enhanced stiffness than surrounding areas ensures that nearly zero stress and displacement between the rigid and the elastic structures would arise inside the LSCs when applying strains to the heterogeneously integrated device. The overall elasticity of the hybrid device is degraded compared to the standalone stretchable antennas due to the presence of the LSCs. But the reliability of the integrated device is greatly improved. Also, good wetting of the liquid metal alloy on tin-plated pins and solder balls ensures reliable electrical connections between contact pins and the antenna.

The proposed hybrid integration strategy was employed to implement a 900 MHz microfluidic stretchable RF radiation sensor, as seen in Fig. 15. The resulting sensor prototype is capable of performing real-time monitoring on the human exposure level to electromagnetic fields (EMF). Once the exposure level to EMFs exceeds the threshold power, the integrated light emission diode (LED) will be switched on. This sensor is of importance to human health as more and more EMFs are generated by today's telecom networks and mobile terminals, and might cause health issues. The integrated radiation sensor contains three sub-modules fully encased in a large-area elastomeric substrate, including a stretchable unbalanced loop antenna for capturing RF radiation from ambient environments, an RF power detection unit for converting received RF energy to the corresponding DC voltages, and a LED indicator for visualizing.

between contact pins and the antenna.

and a LED indicator for visualizing.

**Figure 13.** Experimental results on the radiation efficiency of the relaxed and flexed antennas.

**5. Single-layer stretchable elastomeric integrated active RF electronics**

The concept of microfluidics based passive stretchable elastomeric electronics has been further developed to the integrated device level utilizing localized stiff cells (LSCs), as illustrated in Fig. 14 [28]. A microfluidic, elastic, large-area antenna is realized in the same manner as the previous stretchable antennas, by incorporating liquid alloy into microstructured elastomeric channels. Established IC chips associated with passive components are assembled on small flexible laminates. Subsequently, a few tin-plated contact pins resembling cantilevers are soldered to the flexible circuits, and a semi-spherical solder ball is then mounted on the bottom surface of each contact pin on the other end to improve galvanic connection to the liquid fluid, *cf*. Fig. 14.e. Whereafter, the flexible circuits are embedded into the antenna substrate, with each contact pin immersing in the liquid alloy. In the end, uncured PDMS mixture droplets are deposited on top of the flexible circuits to locally enhance the stiffness of the elastomeric substrate and encapsulate the flexible circuits as well as the flex-to-stretch interfaces. The LSCs with enhanced stiffness than surrounding areas ensures that nearly zero stress and displacement between the rigid and the elastic structures would arise inside the LSCs when applying strains to the heterogeneously integrated device. The overall elasticity of the hybrid device is degraded compared to the standalone stretchable antennas due to the presence of the LSCs. But the reliability of the integrated device is greatly improved. Also, good wetting of the liquid metal alloy on tin-plated pins and solder balls ensures reliable electrical connections

The proposed hybrid integration strategy was employed to implement a 900 MHz microfluidic stretchable RF radiation sensor, as seen in Fig. 15. The resulting sensor prototype is capable of performing real-time monitoring on the human exposure level to electromagnetic fields (EMF). Once the exposure level to EMFs exceeds the threshold power, the integrated light emission diode (LED) will be switched on. This sensor is of importance to human health as more and more EMFs are generated by today's telecom networks and mobile terminals, and might cause health issues. The integrated radiation sensor contains three sub-modules fully encased in a large-area elastomeric substrate, including a stretchable unbalanced loop antenna for capturing RF radiation from ambient environments, an RF power detection unit for converting received RF energy to the corresponding DC voltages,

**Figure 14.** Schematic of the hybrid integration of the single-layer stretchable elastomeric RF electronics.

**Figure 15.** Photograph of the stretchable elastomeric RF radiation sensor prototype.

Prior to the hybrid integration, experimental evaluations on electrical characteristics of the standalone antenna and the RF power detection sub-module assembled on a flex foil are conducted. The implemented stretchable fluidic antenna exhibits similar mechanical and electrical performance as the previously presented unbalanced loop, but operates at a lower frequency of approximately 900 MHz. The power conversion sub-module in the integrated RF radiation sensor involves an off-the-shelf power detector IC chipset (Linear Technology, LT 5534), two decoupling and one coupling capacitors, an inductor for impedance matching at the RF input, and a green LED indicator, all assembled on a small flex foil with a size of 10 mm × 18 mm, as seen in Fig. 15. When the input RF energy at the detector exceeds the threshold power of the integrated device, the LED indicator is switched on, and vice versa. The entire integrated power detection unit can be powered by four serially connected AA rechargeable batteries with a DC supply voltage of 5.23 V. The power conversion behavior of the integrated device is first characterized using a signal generator and a digital

multimeter. Fig. 16 presents the measured DC voltages at the output of the power detection unit in response to varying input RF power at 900 MHz. A dynamic range of -60 dBm to 0 dBm can be achieved, and the threshold power for turning on/off the LED indicator is found to be slightly higher than -30 dBm.

**Figure 16.** Measured output DC voltages of the RF power detection sub-module and the LED on/off states versus varying input RF power.

The integrated elastic radiation sensor device is tested in a demonstration setup in Fig. 17. The RF radiation source consists of an RF signal generator and a horn antenna placed 5 m away from the sensor device in the line-of-sight.

**Figure 17.** Schematic illustration of the RF radiation sensing demonstration setup.

Experiments presented in Fig. 18 verify that the resulting microfluidics based elastomeric integrated device maintains its radiation sensing capabilities even if it is strained along multiple axis. Extreme levels of twisting do not cause any failures in its operation either.

to be slightly higher than -30 dBm.

states versus varying input RF power.

either.

away from the sensor device in the line-of-sight.

multimeter. Fig. 16 presents the measured DC voltages at the output of the power detection unit in response to varying input RF power at 900 MHz. A dynamic range of -60 dBm to 0 dBm can be achieved, and the threshold power for turning on/off the LED indicator is found

**Figure 16.** Measured output DC voltages of the RF power detection sub-module and the LED on/off

**Figure 17.** Schematic illustration of the RF radiation sensing demonstration setup.

The integrated elastic radiation sensor device is tested in a demonstration setup in Fig. 17. The RF radiation source consists of an RF signal generator and a horn antenna placed 5 m

Experiments presented in Fig. 18 verify that the resulting microfluidics based elastomeric integrated device maintains its radiation sensing capabilities even if it is strained along multiple axis. Extreme levels of twisting do not cause any failures in its operation

**Figure 18.** Photographs of the elastomeric RF radiation sensor device operating in ordinary office environment: a). relaxed state, b). with 15% strain applied along *y*-axis, e). with manually applied strain along multiple directions, and f). with severe twisting. The sensor was directly illuminated by a radiation source 5 m away. The coordinate system is shown in Fig. 15.

### **6. Multi-layer stretchable elastomeric integrated active RF electronics**

Very recently, the emerging field of microfluidics based elastomeric electronics has been further advanced to multi-layer μFSRFEs, with the demonstration of a microfluidic, reversibly stretchable, large-area wireless strain sensor [29]. The manufacturing process for multi-layer elastomeric passive components resembles the single-layer fabrication, but with a few minor modifications and process steps added, *cf*. Fig. 19.