**5.2.2 Two-phase olive mill wastewaters**

Anaerobic treatment of this wastewater is very promising and beneficial. The production of biogas enables the process to generate or recover energy instead of just saving energy. This reduces operational costs as compared with other processes such as the physical, physicochemical or biological aerobic treatments previously mentioned (Wheatley, 1990). A kinetic study of the anaerobic digestion of wastewaters derived from the washing of virgin olive oil was previously reported (Borja et al., 1993). The study was carried out in a completely mixed reactor with biomass immobilized on sepiolite at a concentration of 10.8 g VSS L-1 operating at 35°C. COD removal efficiencies of more than 89% were achieved. Olive oil mills are usually small enterprises. Therefore, complex waste treatment systems are usually difficult to implement. Energy recovery from the generated wastewater with a single unit like the previously explained MFC (section 4.3) is very promising. Preliminary studies of the treatment of 2POMWW have been reported by Fermoso et al. (2011).


*3.4. Signature Verification*

In this stage, the user key has been verified. The verification process is performed by extracting the bit level information of the received key. If the key size is 16 bits, then a 16 bits keying mechanism is used; otherwise, an 8 bits keying mechanism is used (see Algorithm 4).


Cholera

*Edited by Sivakumar Joghi Thatha Gowder*

**CHOLERA**

Edited by **Sivakumar Joghi Thatha Gowder**

## Edited by **Sivakumar Joghi Thatha Gowder**

http://dx.doi.org/10.5772/2102 Edited by Sivakumar Joghi Thatha Gowder

#### **Contributors**

Davy Vanden Broeck, Albert Lagrou, Marc De Wolf, Anisia J. Silva-Benitez, Jorge A. Benitez, Chan Yean Yean, Choo Yee Yu, Geik Yong Ang, Frank Osei, Marzia Lazzerini, Ricardo Martin Gomez, Noelia Olivera, Maia Cédola, Satoshi Nakamura, Yutaka Midorikawa, Masami Nakatsu, Toru Watanabe, Rattanaphone Phethsouvanh, Phengta Vongphrachanh, Kongsap Akkhavong, Paul Brey, Crtomir Podlipnik, Jose Juan Reina Martin, Karl E. Klose, Anastasia R Rugel-Stahl

#### **© The Editor(s) and the Author(s) 2012**

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First published in Croatia, 2012 by INTECH d.o.o. eBook (PDF) Published by IN TECH d.o.o. Place and year of publication of eBook (PDF): Rijeka, 2019. IntechOpen is the global imprint of IN TECH d.o.o. Printed in Croatia

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**3. 3D printing technologies for tissue engineering**

A range of 3D printing methods has been developed in the recent years. According to their technique characteristic, printing methods are classified into four categories, which are reviewed in the following sections, respectively.

#### **3.1. Powder-based 3D printing**

Powder-based 3D printing is characterized by using a powder bed to provide raw material, and binding powders together by polymer glue or other thermal fusion methods. It is invented in 1993 by MIT, an extra z-axis was introduced into a commercial printer by adding a height-adjustable platform, allowing printing 3D structures. In addition, the printer cartridge stored binder solution substituting original pigment. When this binder deposited on the powder bed, it can glue material together and form the desired shape. After decades of development, newer powder-based 3D printing methods, selective laser sintering (SLS), and binder jetting (BJ), are all based on this basic concept (**Figure 1**).

bone regeneration is bioactive glass (45S5) whose main components are silicon dioxide and calcium oxide [22]. Both of these biocompatible ceramics and glasses have the ability to form a hydroxyl carbonate apatite (HCA) layer, which is thought to be the mechanism for their

Except for titanium and its alloys [23], which have a high bioactivity and biocompatibility to human tissue, not too much progress has been gotten for metals used in tissue engineering due to their low biocompatibility. Because of the intrinsic high strength and toughness of titanium alloys [24, 25], they are mainly used in the area of bone tissue engineering implants.

Among the many factors need to be considered, mechanical properties of scaffolds should be tailored according to the specific site in host tissue. For example, the critical compressive strength of scaffolds used for cortical bone tissue is completely different with that for a cancellous bone tissue. For the application of segmental bone defects of cortical bone, scaffolds require compressive strength comparable to its prototype, ranging from 100 to 150 MPa along the axial direction [26, 27]. In contrast, cancellous bone has a comparatively loosen structure, which is in the range of 2.5–6.5 MPa [28]. Other mechanical properties, such as elastic stiffness, fracture toughness, and relaxation rate should also be modulated to keep consistent with original tissue [29, 30]. Because mechanical property mismatch between scaffolds and

host tissue may cause stress shielding [31], which eventually results in osteoporosis.

Except for mechanical property, to achieve the goal of tissue reconstruction, scaffolds must meet some specific requirement for its architecture and internal structure. It is crucial to have interconnected pore within the bulk scaffolds transferring nutrients and oxygen for cell vascularization and proliferation. Considering the tradeoff between printing cost and biological performance ideal pore size for scaffolds ranges from 200 to 500 with a porosity between 60 and 90% [32]. However, it should be kept in mind that large pore size can facilitate cell vascularization [33]. In addition, graded channel structure can significantly promote cell migration by a capillary effect [34]. Another relevant factor is surface morphology of scaffolds, which affects the cell adhesion, can be modified plasma etching to improve its bioactivity, as well as

A range of 3D printing methods has been developed in the recent years. According to their technique characteristic, printing methods are classified into four categories, which are

Powder-based 3D printing is characterized by using a powder bed to provide raw material, and binding powders together by polymer glue or other thermal fusion methods. It is

**2.2. Mechanical features consideration for tissue engineering**

reformed via other deposition methods [35, 36].

reviewed in the following sections, respectively.

**3.1. Powder-based 3D printing**

**3. 3D printing technologies for tissue engineering**

bioactive behavior.

140 3D Printing

In SLS, particles are locally fused together to form a solid structure by a high-powered laser. During the printing process, the motion of laser beam is controlled by a computer-aided platform according to the input computer-aided design (CAD) file. After one layer sintered, a scroll will spread a new layer of power on the top of the previous layer, and the cycle repeats itself until the whole structure is completed. Unused particles away from heat affect zone can recycle after removing the 3D object from the powder bed, which decrease the cost of this method. Abundant processing parameter of SLS, for example, particle size, laser power, scan speed, and binder fraction, can be used to control the final structure and mechanical property of products [38]. Types of biocompatible materials that can be processed by SLS are broadening recently, from polymers and ceramics to metals. This diversity of material choice makes it possible to synthesize artificial organ matching the mechanical property of human tissue from different positions. The advantage of SLS method comes from the fact that high resolution of the laser beam. The feature size in SLS is decided both by the diameter of the laser beam and particle size, ranging from 10 to 500 μm [37, 39]. In addition, unfused powders on powder bed act as supporting materials to hold the unconnected part, decreasing minor deformation during processing. Furthermore, SLS is a one-step method that post-processing procedure, such as thermal treatment or solvent evaporation, is unnecessary when printing ceramics and metals. Polymers are the most common materials used in SLS for tissue engineering owing to its low synthesizing temperature. As for ceramics and metals, high processing heat may deteriorate the cell or drug embedded inside the printing material. For these reasons, drugs or growth factors are introduced into SLS printed scaffolds after the printing process [40].

Binder jetting is another powder-based method, which employs liquid binder to glue particles together forming the desired structure. The printer head uses either a thermal or a

**Figure 1.** (a) Schematic of SLS method, (b) process of SLS method, and (c) printed products [37].

piezoelectric actuator to deposit binder onto the powder bed. With respect to thermal actuator, a heating element vaporizes fluid to the gas inside of the reservoir, and the increasing volume squeeze droplet out of the nozzle. The thermal method has a high-efficiency at low cost. However, its accuracy is limited due to the difficulty to control the size of the droplet, and residue thermal stress inside the binder may damage the local structure of the printed material. In the piezoelectric system, a high-accuracy piezoceramic is employed to generate pressure to the fluid reservoir. The shape and volume of the jetting droplet are more uniform compared with that in a thermal system.

Choosing suitable materials, including particle and binder, is crucial to both the mechanical property and biological property of the printed scaffolds [41]. Biocompatible ceramics and metals can be used in the binder jetting, such as hydroxyapatite and titanium dioxide. Particle size is a key factor in binder jetting. Finer particles have a smaller pore size distribution in the powder bed, which dramatically decrease the drop penetration time. However, Fine particles have higher mass transfer velocity, which contributes to the sintering efficiency. Therefore, choosing suitable particle size is a trade-off process between processing stage and thermal treatment stage. After choosing the appropriate particle materials, binder materials that used to stick particles together need to be selected as well. For the application in the medical area, the binder should not leave toxic residue when burning out, or it is nontoxic itself. Water-based binder system [42] (a water solution of an acrylic polymer) and water-soluble binder system [43] (polyvinyl acetate (PVA) or polyethylene glycol (PEG)) are two kinds of binder commonly used in ceramic casting as well as binder jetting. Binder material should have a suitable viscosity property to keep spreading from nozzles while having enough penetration ability [44]. The shaping principle of binder jetting is more relying on physical process rather than chemical reaction, which gives rise to the flexibility in the material choice of particle used in binder jetting. Compared with SLS method, binder jetting need an extra post-processing to densify the loosen green body, because polymer binder cannot provide enough strength for the scaffolds in most cases.

composition, an ideal rheology property of inks is more difficult to achieve. Inhomogeneous cell distribution may result in an increase in viscosity locally, which causes nozzle jamming. Cell-laden hydrogels, including chemical cross-linking [47], and molecular physical gels [48], are preferred when printing by DIW method. In bio fabrication, the selection for hydrogels is limited to the biocompatible and biodegradable. A wide range of biopolymers has been examined their viability in the medical application, such as alginate, chitosan, collagen, gelatin, and silk. Among these materials, alginate is one of the most frequently used natural biopolymers for tissue repair, wound healing and drug delivery due to its prominent biocompatibility, and the ability to differentiate cells in culture. Controllable degradation property of alginate was achieved by varying oxidation percentages of alginate hydrogel [48]. In this research, cells behavior was investigated under different concentration ranging from 1 to 20%, as well as different oxidation percentages ranging from 0 to 10%. A certain combination of these two parameters (5% of oxidation and 15% of concentration) was favored by cells since they can form a hydrogel with suitable density to hold cells homogeneously. Silk is another kind of natural polymers produced by insects such as spiders or Lepidoptera. Being highly biocompatible and degradable, silk is of interest for a number of industrial applications as well as biomedical applications. Group of Lewis leads the research in DIW area. They designed a high-resolution scaffolds, which can be used for cartilage employing silk fibroin as bioink [49]. Cell compatibility of this scaffold benefited from the mild processing temperature and avoidance of toxic polymer binders. A two-level hierarchical silk structure was created using a template method by removing micro PCL particles after printed [50]. The morphology of resulting pores and its

3D Printing of Scaffolds for Tissue Engineering http://dx.doi.org/10.5772/intechopen.78145 143

**Figure 2.** Ink-based 3D printing method (a) schematic of FDM method (b) schematic of DIW method [46].

corresponding porosity were both determined by the sacrificial PCL particles.

Fused deposition modeling (FDM) also relies on nozzle and moving platform to construct the 3D structure. However, unlike DIW, of which raw material is under liquid state, FDM need an extra heater to soften material firstly. In addition, a fan is located at the end of the nozzle,

#### **3.2. Ink-based 3D printing**

The ink-based method is a process that deposits fluidic materials continually or discretely out of a nozzle to a 3D platform layer by layer. It is one of the most suitable ways for processing tissue materials since it can directly print bioinks, which mixture living cells or growth factors with the liquefied material. Several 3D printing methods use this approach, including direct ink writing and fused deposition modeling (FDM).

Indirect ink writing (DIW) method, viscoelastic inks are squeezing out of the nozzle by the pressure from a piston, a screw, or pneumatic force as shown in **Figure 2**. Utilizing an easy setup, pneumatic force system has the ability to adjust the pressure in a wide range making it the most applied method in DIW. Screw system has a complicated feed module compared with other methods; however, it can provide the largest driving force that suitable for high-viscosity materials [45]. Critical to this technology is the design of the fluid property of inks. They should possess an obvious shear thinning property that allows passing through micro-size nozzle easily while recovering adequate shear strength to maintain the desired shape after inks dispensing onto the substrates. If cells are introduced into the inks as part of

**Figure 2.** Ink-based 3D printing method (a) schematic of FDM method (b) schematic of DIW method [46].

piezoelectric actuator to deposit binder onto the powder bed. With respect to thermal actuator, a heating element vaporizes fluid to the gas inside of the reservoir, and the increasing volume squeeze droplet out of the nozzle. The thermal method has a high-efficiency at low cost. However, its accuracy is limited due to the difficulty to control the size of the droplet, and residue thermal stress inside the binder may damage the local structure of the printed material. In the piezoelectric system, a high-accuracy piezoceramic is employed to generate pressure to the fluid reservoir. The shape and volume of the jetting droplet are more uniform

Choosing suitable materials, including particle and binder, is crucial to both the mechanical property and biological property of the printed scaffolds [41]. Biocompatible ceramics and metals can be used in the binder jetting, such as hydroxyapatite and titanium dioxide. Particle size is a key factor in binder jetting. Finer particles have a smaller pore size distribution in the powder bed, which dramatically decrease the drop penetration time. However, Fine particles have higher mass transfer velocity, which contributes to the sintering efficiency. Therefore, choosing suitable particle size is a trade-off process between processing stage and thermal treatment stage. After choosing the appropriate particle materials, binder materials that used to stick particles together need to be selected as well. For the application in the medical area, the binder should not leave toxic residue when burning out, or it is nontoxic itself. Water-based binder system [42] (a water solution of an acrylic polymer) and water-soluble binder system [43] (polyvinyl acetate (PVA) or polyethylene glycol (PEG)) are two kinds of binder commonly used in ceramic casting as well as binder jetting. Binder material should have a suitable viscosity property to keep spreading from nozzles while having enough penetration ability [44]. The shaping principle of binder jetting is more relying on physical process rather than chemical reaction, which gives rise to the flexibility in the material choice of particle used in binder jetting. Compared with SLS method, binder jetting need an extra post-processing to densify the loosen green body, because polymer binder

The ink-based method is a process that deposits fluidic materials continually or discretely out of a nozzle to a 3D platform layer by layer. It is one of the most suitable ways for processing tissue materials since it can directly print bioinks, which mixture living cells or growth factors with the liquefied material. Several 3D printing methods use this approach, including direct

Indirect ink writing (DIW) method, viscoelastic inks are squeezing out of the nozzle by the pressure from a piston, a screw, or pneumatic force as shown in **Figure 2**. Utilizing an easy setup, pneumatic force system has the ability to adjust the pressure in a wide range making it the most applied method in DIW. Screw system has a complicated feed module compared with other methods; however, it can provide the largest driving force that suitable for high-viscosity materials [45]. Critical to this technology is the design of the fluid property of inks. They should possess an obvious shear thinning property that allows passing through micro-size nozzle easily while recovering adequate shear strength to maintain the desired shape after inks dispensing onto the substrates. If cells are introduced into the inks as part of

compared with that in a thermal system.

142 3D Printing

cannot provide enough strength for the scaffolds in most cases.

ink writing and fused deposition modeling (FDM).

**3.2. Ink-based 3D printing**

composition, an ideal rheology property of inks is more difficult to achieve. Inhomogeneous cell distribution may result in an increase in viscosity locally, which causes nozzle jamming.

Cell-laden hydrogels, including chemical cross-linking [47], and molecular physical gels [48], are preferred when printing by DIW method. In bio fabrication, the selection for hydrogels is limited to the biocompatible and biodegradable. A wide range of biopolymers has been examined their viability in the medical application, such as alginate, chitosan, collagen, gelatin, and silk. Among these materials, alginate is one of the most frequently used natural biopolymers for tissue repair, wound healing and drug delivery due to its prominent biocompatibility, and the ability to differentiate cells in culture. Controllable degradation property of alginate was achieved by varying oxidation percentages of alginate hydrogel [48]. In this research, cells behavior was investigated under different concentration ranging from 1 to 20%, as well as different oxidation percentages ranging from 0 to 10%. A certain combination of these two parameters (5% of oxidation and 15% of concentration) was favored by cells since they can form a hydrogel with suitable density to hold cells homogeneously. Silk is another kind of natural polymers produced by insects such as spiders or Lepidoptera. Being highly biocompatible and degradable, silk is of interest for a number of industrial applications as well as biomedical applications. Group of Lewis leads the research in DIW area. They designed a high-resolution scaffolds, which can be used for cartilage employing silk fibroin as bioink [49]. Cell compatibility of this scaffold benefited from the mild processing temperature and avoidance of toxic polymer binders. A two-level hierarchical silk structure was created using a template method by removing micro PCL particles after printed [50]. The morphology of resulting pores and its corresponding porosity were both determined by the sacrificial PCL particles.

Fused deposition modeling (FDM) also relies on nozzle and moving platform to construct the 3D structure. However, unlike DIW, of which raw material is under liquid state, FDM need an extra heater to soften material firstly. In addition, a fan is located at the end of the nozzle, which controls the solidifying velocity of the molten material. Biocompatible polymers such as a Polylactic acid (PLA), PCL, and poly (lactic-co-glycolic acid) (PLGA) are most frequently used materials in FDM method. PCL has been widely used in dental devices and wound repair because it has high printability and excellent interactive ability with tissue.

UV light curing photocurable polymers. Recent decades, new techniques such as two-photon polymerization (2PP) and projection micro-stereolithography (PμSL) (**Figure 4**), also called digital projection lithography (DPL), are developed toward a more precise and effective direction. In two-photon polymerization (2PP), a long wavelength near-infrared laser beam can be focused inside of the transparent resin rather than being restricted on the surface of resin [57]. Therefore, a real 3D structure can be constructed by controlling the focal point of the laser beam. The advantage of this method is the excitation volume in 2PP is far less than other laser methods, which gives it the best resolution beyond polymerization-based 3D printing. However, the continuous processing character of 2PP confines it to be a micro-size manufacture method. Gelatin modified with methacrylamide moieties (GelMA) shows a wide range of benefits for application in tissue engineering, such as low toxicity, non-immunogenic, and tunable physicochemical properties [58], which can be used as a polymeric precursor in the 2PP method. Laura Brigo et al. successfully processed scaffolds with feature size at submicron level [59] targeting biological use. They synthesized a highly effective reaction initiator, benzylidene cycloketone-based two-photon initiator (P2CK), providing a wide processing window for photon excitation. Larger post-deformation was observed in the woodpile structure synthesized by lower laser power, which derived from the low crosslinking degree. This loose structure property is more suitable for human BJ (hBJ) foreskin fibroblasts accommodation

3D Printing of Scaffolds for Tissue Engineering http://dx.doi.org/10.5772/intechopen.78145 145

**Figure 4.** Hierarchical structure printed by micro-stereolithography method (a) polymer metamaterial template (b) large-area, high-resolution additive manufacturing of hierarchical metamaterials (c)–(e) optical microscope images of

bulk hierarchical lattice material with a network of hierarchical stretch-dominated octet unit cells [56].

since these cells are easily penetrating into its bulk structure.

To improve the biomedical performance of the polymer scaffolds printed by FDM, both coating [14], and doping [52] were developed for scaffolds. A better cell proliferation was obtained after surface modification by plasma treating since the improved surface roughness can adhere more cells [53]. In addition, mechanical and bioactivity properties of biopolymers are tunable by doping biocompatible reinforced particles such as HA and β-tricalcium phosphate (β-TCP) [54]. Moreover, the high concentration of reinforced phase is beneficial to the cell growth and differentiation. Generally, it is hard to attain desired mechanical property and biomedical property simultaneously. Recently, multi-material printer is developed to overcome this limitation by using a dual-printer in a single construct, as displayed in **Figure 3** [51, 55]. Through this multi-material printing system, optimized carrier materials that embedded different nutrients and cell types are dispensed on discrete location in the 3D structure in one step.

However, limitation of FDM lies in the poor choice of printing materials. Only thermoplastic materials can be fabricated by this method. Moreover, the high-temperature, ranging from 120 to 300°C, is not suitable for embedding cells or drugs inside filament when preparing scaffolds.

#### **3.3. Polymerization-based 3D printing**

The polymerization-based method starts with a process that exposing liquid photopolymer to a laser beam, then this specific exposing area would be solidified through polymer chain reaction. After repeating this process layer by layer, the final complex 3D structure can be constructed. The earliest version of this technique is stereolithography (SLA), which utilizes a low-power

**Figure 3.** (a) schematic of multi-material DIW method (b) multi-material scaffold [51].

UV light curing photocurable polymers. Recent decades, new techniques such as two-photon polymerization (2PP) and projection micro-stereolithography (PμSL) (**Figure 4**), also called digital projection lithography (DPL), are developed toward a more precise and effective direction.

which controls the solidifying velocity of the molten material. Biocompatible polymers such as a Polylactic acid (PLA), PCL, and poly (lactic-co-glycolic acid) (PLGA) are most frequently used materials in FDM method. PCL has been widely used in dental devices and wound

To improve the biomedical performance of the polymer scaffolds printed by FDM, both coating [14], and doping [52] were developed for scaffolds. A better cell proliferation was obtained after surface modification by plasma treating since the improved surface roughness can adhere more cells [53]. In addition, mechanical and bioactivity properties of biopolymers are tunable by doping biocompatible reinforced particles such as HA and β-tricalcium phosphate (β-TCP) [54]. Moreover, the high concentration of reinforced phase is beneficial to the cell growth and differentiation. Generally, it is hard to attain desired mechanical property and biomedical property simultaneously. Recently, multi-material printer is developed to overcome this limitation by using a dual-printer in a single construct, as displayed in **Figure 3** [51, 55]. Through this multi-material printing system, optimized carrier materials that embedded different nutri-

repair because it has high printability and excellent interactive ability with tissue.

ents and cell types are dispensed on discrete location in the 3D structure in one step.

**Figure 3.** (a) schematic of multi-material DIW method (b) multi-material scaffold [51].

**3.3. Polymerization-based 3D printing**

144 3D Printing

However, limitation of FDM lies in the poor choice of printing materials. Only thermoplastic materials can be fabricated by this method. Moreover, the high-temperature, ranging from 120 to 300°C, is not suitable for embedding cells or drugs inside filament when preparing scaffolds.

The polymerization-based method starts with a process that exposing liquid photopolymer to a laser beam, then this specific exposing area would be solidified through polymer chain reaction. After repeating this process layer by layer, the final complex 3D structure can be constructed. The earliest version of this technique is stereolithography (SLA), which utilizes a low-power In two-photon polymerization (2PP), a long wavelength near-infrared laser beam can be focused inside of the transparent resin rather than being restricted on the surface of resin [57]. Therefore, a real 3D structure can be constructed by controlling the focal point of the laser beam. The advantage of this method is the excitation volume in 2PP is far less than other laser methods, which gives it the best resolution beyond polymerization-based 3D printing. However, the continuous processing character of 2PP confines it to be a micro-size manufacture method. Gelatin modified with methacrylamide moieties (GelMA) shows a wide range of benefits for application in tissue engineering, such as low toxicity, non-immunogenic, and tunable physicochemical properties [58], which can be used as a polymeric precursor in the 2PP method. Laura Brigo et al. successfully processed scaffolds with feature size at submicron level [59] targeting biological use. They synthesized a highly effective reaction initiator, benzylidene cycloketone-based two-photon initiator (P2CK), providing a wide processing window for photon excitation. Larger post-deformation was observed in the woodpile structure synthesized by lower laser power, which derived from the low crosslinking degree. This loose structure property is more suitable for human BJ (hBJ) foreskin fibroblasts accommodation since these cells are easily penetrating into its bulk structure.

**Figure 4.** Hierarchical structure printed by micro-stereolithography method (a) polymer metamaterial template (b) large-area, high-resolution additive manufacturing of hierarchical metamaterials (c)–(e) optical microscope images of bulk hierarchical lattice material with a network of hierarchical stretch-dominated octet unit cells [56].

PμSL utilizes a digital micromirror device (DMD) [60] substituting the physical masks used in lithography [61] or liquid crystal mask used in liquid crystal display (LCD) [62] method. The basic theory used in PμSL is similar with SLA and 2PP, but the dynamic mask generator can manipulate millions of pixels at the same time rather than just one focus point, which endows PμSL the ability to process a high-resolution, large-scalability material within several minutes. A real 3D extracellular matrix (ECM) was built by DMD method to assess the difference between two-dimensional (2D) and 3D cell culture system [63]. In this research, poly (ethylene glycol) diacrylate (PEGDA), a commonly used biomaterial, was selected as lithography material to synthesize microwell-array structure. The opening space of microwell is changing along the z-position, from 250 μm at the top to 160 μm at the middle. This exquisite structure design with varying feature size was believed to have the potential to manipulate cell proliferation and cell–cell interactions. Similarly, in the 2PP method, GelMA is also a popular biomaterial employed in PμSL method [64]. Considering the different optical source in these two methods, the selection of chemicals for hydrogel preparation, such as a photoinitiator, was changing from P2CK to Irgacure 2959.

the designed direction. This method is especially useful in cell-laden scaffolds [68]. First, a 2D thin microplate with flexible hinge was built by chemical vapor deposition (CVD) together with lithography, as presented in **Figure 5**. After that, cells were cultured on the thin parylene plate and thus cell traction force drove the plates folding automatically. As the lattice scaffolds can hold the cells firmly by its closed microstructure, issues with respect to how to adhere

3D Printing of Scaffolds for Tissue Engineering http://dx.doi.org/10.5772/intechopen.78145 147

Another successful application of 4D printing in tissue engineering is making tracheobronchial scaffolds for patients who suffered from tracheobronchomalacia (TBM) [72–74]. The processing procedure including three parts. Firstly, a digital 3D model of tracheobronchial tree of patients was constructed by image software using the MRI scan data. Then the patientspecific scaffold was processed by one of the previously introduced 3D printing technologies according to the constructed 3D model. After implanted, this airway splint expanded automatically under the thermal stimuli from the internal warm organ, which leaves growing

**Figure 5.** 4D printing for self-folding cell-laden scaffolds (a) the cells adhere and stretch across two microplates (b) the cells are cultured on micro-fabricated parylene microplates (c) various 3D cell-laden microstructures (d) schematic of the parylene microplates without a flexible joint (e) a fluorescent image merged with phase contrast image of NIH/3T3 cells patterned only on the microplates (f) schematic of the parylene microplates with a flexible joint to achieve precise 3D

configurations after folding (g) a SEM image of the microplates with the flexible joint [68].

cells onto scaffolds can be avoided by this method.

space for Malacia airway.

Natural structural materials, as in the case of man bone and tooth, are generally lightweight and possess a balanced combination of strength and toughness. However, synthesized bone graft materials for wound repair are relatively brittle and thus cannot match the performance of the natural part [65]. To address this challenge, a spectacular meta-structure with high tensile elasticity (>50%) was built by Xiaoyu Zheng et al. using the PμSL method [56]. This metamaterial has seven level of the hierarchy, ranging from 10 to 50 nm, and thus the mechanical property of it can match the natural materials. The high elasticity getting from the graded structure gives us the foresight to improve the mechanical property, especially the crack resistance of the synthesized biomaterials applied in the bone graft.

#### **3.4. Four-dimensional (4D) printing**

Four-dimensional (4D printing) is a recently appeared terminology in 2013 [66] and immediately attracts wide attention in different areas. 4D printing adds a new dimension, time, to ordinary 3D printed products, which allows materials responding to suitable stimuli or selftransform after possessing. It is not a totally new technique but derives from shape-morphing systems [67–69] and relies on the original 3D printing techniques. The definition of 4D printing is still in a controversy that whether the structure degradable effect can be classified into 4D process [70]. In this context, the degradation of printed material will not be discussed as 4D printing. Transformation code of 4D printed materials is hidden in the exquisite design of its structure and constituents. It offers great potential for customized medical devices given that the dynamic mechanical property of printed material accords with the behavior of living tissues [71]. In addition, the time-dependent property of 4D printing makes it suitable for long-term application embedded in human body.

One efficacious application of 4D printing is for the self-folding system [69, 70]. Two or more different kinds of materials with diverse response to outside stimuli are incorporated into an integrated structure by dual-head printers. Under the same external stimuli, the deformation difference aroused from each component will cause the structure bending or swelling toward the designed direction. This method is especially useful in cell-laden scaffolds [68]. First, a 2D thin microplate with flexible hinge was built by chemical vapor deposition (CVD) together with lithography, as presented in **Figure 5**. After that, cells were cultured on the thin parylene plate and thus cell traction force drove the plates folding automatically. As the lattice scaffolds can hold the cells firmly by its closed microstructure, issues with respect to how to adhere cells onto scaffolds can be avoided by this method.

PμSL utilizes a digital micromirror device (DMD) [60] substituting the physical masks used in lithography [61] or liquid crystal mask used in liquid crystal display (LCD) [62] method. The basic theory used in PμSL is similar with SLA and 2PP, but the dynamic mask generator can manipulate millions of pixels at the same time rather than just one focus point, which endows PμSL the ability to process a high-resolution, large-scalability material within several minutes. A real 3D extracellular matrix (ECM) was built by DMD method to assess the difference between two-dimensional (2D) and 3D cell culture system [63]. In this research, poly (ethylene glycol) diacrylate (PEGDA), a commonly used biomaterial, was selected as lithography material to synthesize microwell-array structure. The opening space of microwell is changing along the z-position, from 250 μm at the top to 160 μm at the middle. This exquisite structure design with varying feature size was believed to have the potential to manipulate cell proliferation and cell–cell interactions. Similarly, in the 2PP method, GelMA is also a popular biomaterial employed in PμSL method [64]. Considering the different optical source in these two methods, the selection of chemicals for hydrogel preparation, such as a photoinitiator,

Natural structural materials, as in the case of man bone and tooth, are generally lightweight and possess a balanced combination of strength and toughness. However, synthesized bone graft materials for wound repair are relatively brittle and thus cannot match the performance of the natural part [65]. To address this challenge, a spectacular meta-structure with high tensile elasticity (>50%) was built by Xiaoyu Zheng et al. using the PμSL method [56]. This metamaterial has seven level of the hierarchy, ranging from 10 to 50 nm, and thus the mechanical property of it can match the natural materials. The high elasticity getting from the graded structure gives us the foresight to improve the mechanical property, especially the crack resis-

Four-dimensional (4D printing) is a recently appeared terminology in 2013 [66] and immediately attracts wide attention in different areas. 4D printing adds a new dimension, time, to ordinary 3D printed products, which allows materials responding to suitable stimuli or selftransform after possessing. It is not a totally new technique but derives from shape-morphing systems [67–69] and relies on the original 3D printing techniques. The definition of 4D printing is still in a controversy that whether the structure degradable effect can be classified into 4D process [70]. In this context, the degradation of printed material will not be discussed as 4D printing. Transformation code of 4D printed materials is hidden in the exquisite design of its structure and constituents. It offers great potential for customized medical devices given that the dynamic mechanical property of printed material accords with the behavior of living tissues [71]. In addition, the time-dependent property of 4D printing makes it suitable for

One efficacious application of 4D printing is for the self-folding system [69, 70]. Two or more different kinds of materials with diverse response to outside stimuli are incorporated into an integrated structure by dual-head printers. Under the same external stimuli, the deformation difference aroused from each component will cause the structure bending or swelling toward

was changing from P2CK to Irgacure 2959.

146 3D Printing

**3.4. Four-dimensional (4D) printing**

long-term application embedded in human body.

tance of the synthesized biomaterials applied in the bone graft.

Another successful application of 4D printing in tissue engineering is making tracheobronchial scaffolds for patients who suffered from tracheobronchomalacia (TBM) [72–74]. The processing procedure including three parts. Firstly, a digital 3D model of tracheobronchial tree of patients was constructed by image software using the MRI scan data. Then the patientspecific scaffold was processed by one of the previously introduced 3D printing technologies according to the constructed 3D model. After implanted, this airway splint expanded automatically under the thermal stimuli from the internal warm organ, which leaves growing space for Malacia airway.

**Figure 5.** 4D printing for self-folding cell-laden scaffolds (a) the cells adhere and stretch across two microplates (b) the cells are cultured on micro-fabricated parylene microplates (c) various 3D cell-laden microstructures (d) schematic of the parylene microplates without a flexible joint (e) a fluorescent image merged with phase contrast image of NIH/3T3 cells patterned only on the microplates (f) schematic of the parylene microplates with a flexible joint to achieve precise 3D configurations after folding (g) a SEM image of the microplates with the flexible joint [68].

## **4. Conclusion**

In conclusion, it is clear from the results discussed in this review that there is a huge potential for applying 3D printing in tissue engineering. 3D printing offers unique advantages toward flexible manufacturing, which can be employed to fabricate scaffolds with complex shape and internal porous structure. To improve the biological performance of printed scaffolds, it is crucial to choose suitable biomaterials introduced in Section 2, and it is equally important to select an appropriate printing technology discussed in Section 3. Although we have got great progress in the processing technique, we are still a long way from printing functional artificial tissue to completely substitute human tissue. To the best of our knowledge, 3D printing cannot build a bulk scaffold over one centimeter while possessing feature size at nanoscale. The precise control of scaffold structure, surface morphology and pore size is still a huge challenge for current 3D printing methods. In addition, post-processing is inevitable for most 3D printing methods, which limit the development of in-situ printing method. Moreover, there is a need for a significant amount of research to be carried out in order to understand the bioactive reaction between host tissue and biomaterials. With increasing research efforts in this field, we believe that future developments of novel biomaterials and processing techniques will lead us to a biocompatible artificial tissue that is smart enough to detect an event and respond to it.

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**2. Wire antennas — Miniaturization techniques**

**2. Wire antennas – Miniaturization techniques**

all the antennas presented in this chapter, while showing their main settings.

#### **2.1. Classical wire antenna: The dipole antenna**

whole antenna is a half-wavelength long.

The dipole antenna has been developed by Heinrich Rudolph Hertz around 1886 and still remains the most widely used antenna (Figure 2). It owns two identical (same length) and symmetrical metal wires, and its feeding device is connected at the center of the dipole, i.e. connected to the two adjacent wires ends. The dipole working results of a standing wave phenomenon depending on its length. The antenna fundamental mode occurs when the whole antenna is a half-wavelength long. **2.1. Classical wire antenna: the dipole antenna**  The dipole antenna has been developed by Heinrich Rudolph Hertz around 1886 and still remains the most widely used antenna (Figure 2). It owns two identical (same length) and symmetrical metal wires, and its feeding device is connected at the center of the dipole, i.e. connected to the two adjacent wires ends. The dipole working results of a standing wave phenomenon depending on its length. The antenna fundamental mode occurs when the

The radiated field of the dipole antenna working on its fundamental mode has a linear **Figure 2.** Dipole antenna shape (a) and its 3D radiation shape (b)

Figure 2. Dipole antenna shape (a) and its 3D radiation shape (b)

dipole and drops off to zero on the dipole's axis. Its maximum directivity equals 2.15dBi. The impedance bandwidth of this kind of antenna is quite wide since it is between 10% and 20% (it depends on the wire's radius) [28]. **2.2. The monopole antenna**  The radiated field of the dipole antenna working on its fundamental mode has a linear polarization. As shown in Figure 2, its radiation pattern is maximum at right angles to the dipole and drops off to zero on the dipole's axis. Its maximum directivity equals 2.15dBi. The impedance bandwidth of this kind of antenna is quite wide since it is between 10% and 20% (it depends on the wire's radius) [28].

polarization. As shown in Figure 2, its radiation pattern is maximum at right angles to the

#### By adding a perpendicular ground plane at the center of the dipole antenna, its length can **2.2. The monopole antenna**

**•** Folded configurations [8-10]

**10-2**

**10-1**

**100**

**101**

**Quality factor Q**

**102**

**103**

**104**

**105**

2 Progress in Compact Antennas

**•** Shorting walls or pins [15-16]

bandwidth) [27].

**•** Surface etching techniques [11-14]

**•** The use of high dielectric constant materials or magneto-dielectric materials [17-22].

**•** Creation of hybrid modes with particular boundary conditions in dielectric resonator antennas. It allows choosing their resonance frequencies (for multiband or wide impedance

**0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1**

**antenna = 100% antenna = 50% antenna = 20% antenna = 2%**

**k.a**

We will start this chapter by detailing wire antennas. Indeed, after explaining the classical dipole antenna, we will show how to miniaturize this kind of antennas based on shape design such as bending, folding and meandering. The second part will detail planar antennas. We will see the impact of materials properties under the patch antenna hat, i.e. dielectric or magneto-dielectric materials. Then, planar miniature antennas will be shown, e.g. Planar Inverted F Antenna (PIFA) and monopolar wirepatch antenna. The third part will exhibit Dielectric Resonator Antennas and how to use this kind of antennas for low frequency band application while having compact sizes. Finally, the last part will summary all the antennas

**•** Loading the radiating element with active components [23-26].

**Figure 1.** Quality factor according to the antenna dimensions and efficiencies

presented in this chapter, while showing their main settings.

be divided by two: that is the monopole antenna. Theoretically, this ground plane is considered as an infinite Perfect Electric Conductor (PEC) plane. In this case, the current in the reflected image [29-30] has the same direction and phase as the current in the dipole By adding a perpendicular ground plane at the center of the dipole antenna, its length can be divided by two: that is the monopole antenna. Theoretically, this ground plane is considered as an infinite Perfect Electric Conductor (PEC) plane. In this case, the current in the reflected image [29-30] has the same direction and phase as the current in the dipole antenna. Thus the quarter-wavelength monopole and its image together form a half-wavelength dipole that radiates only in the upper half of space (see Figure 3).

resonance frequency is also the same. The radiation resistance is proportional to (*h* / *λ*)

is very low and does not exceed 1% [31-33].

**Figure 5.** Inverted L Antenna (ILA) shape

**Figure 6.** Inverted F Antenna (IFA) shape

low frequencies.

h the length of the vertical part (see Figure 5). Actually, the horizontal part occurs as a capacitive charge and this makes the antenna difficult to match on 50Ω. Therefore, the antenna bandwidth

Adding a ground wire on the horizontal part facilitates the ILA matching. This new antenna design is called Inverted F Antenna (IFA) (Figure 6). This wire is equivalent to a self-inductance in parallel with the capacitance of the horizontal wire. That involves a parallel resonance at

2 , with 5

Compact Antennas — An overview http://dx.doi.org/10.5772/58837

Figure 3. Monopole antenna shape (a) and its 3D radiation shape (b) **Figure 3.** Monopole antenna shape (a) and its 3D radiation shape (b)

proportional to 2 (*h* / ) , this latter is therefore decreasing the square of the antenna height h. Therefore it presents a 3 dB gain higher than the dipole antenna. The radiation resistance is proportional to (*h* / *λ*) 2 , this latter is therefore decreasing the square of the antenna height h.

Therefore it presents a 3 dB gain higher than the dipole antenna. The radiation resistance is

The first mobile phones were using this kind of antennas to receive the Global System for Mobile Communications (GSM) (see Figure 4). The first mobile phones were using this kind of antennas to receive the Global System for Mobile Communications (GSM) (see Figure 4).

To reduce the monopole antenna global dimensions, we can bend the wire to be parallel to the ground: that is the Inverted L Antenna (ILA) [31]. Its design is depicted Figure 5, there is **Figure 4.** Monopole antenna integrated inside a mobile phone

i.e. up to 20%.

both a vertical and a horizontal parts. Since its electrical length is the same than the In practice, the finite ground plane disturbs the radiation pattern and the maximum directivity is decreasing. The monopole antenna bandwidth is quite the same as the dipole, i.e. up to 20%.

#### **2.3. Inverted L and F antennas (ILA and IFA)**

To reduce the monopole antenna global dimensions, we can bend the wire to be parallel to the ground: that is the Inverted L Antenna (ILA) [31]. Its design is depicted Figure 5, there is both a vertical and a horizontal parts. Since its electrical length is the same than the monopole, its resonance frequency is also the same. The radiation resistance is proportional to (*h* / *λ*) 2 , with h the length of the vertical part (see Figure 5). Actually, the horizontal part occurs as a capacitive charge and this makes the antenna difficult to match on 50Ω. Therefore, the antenna bandwidth is very low and does not exceed 1% [31-33].

**Figure 5.** Inverted L Antenna (ILA) shape

(a) (b)

Therefore it presents a 3 dB gain higher than the dipole antenna. The radiation resistance is

The first mobile phones were using this kind of antennas to receive the Global System for

) , this latter is therefore decreasing the square of the antenna height

, this latter is therefore decreasing the square of the antenna height h.

Therefore it presents a 3 dB gain higher than the dipole antenna. The radiation resistance is

The first mobile phones were using this kind of antennas to receive the Global System for

In practice, the finite ground plane disturbs the radiation pattern and the maximum directivity is decreasing. The monopole antenna bandwidth is quite the same as the dipole,

To reduce the monopole antenna global dimensions, we can bend the wire to be parallel to the ground: that is the Inverted L Antenna (ILA) [31]. Its design is depicted Figure 5, there is both a vertical and a horizontal parts. Since its electrical length is the same than the

In practice, the finite ground plane disturbs the radiation pattern and the maximum directivity is decreasing. The monopole antenna bandwidth is quite the same as the dipole, i.e. up to 20%.

To reduce the monopole antenna global dimensions, we can bend the wire to be parallel to the ground: that is the Inverted L Antenna (ILA) [31]. Its design is depicted Figure 5, there is both a vertical and a horizontal parts. Since its electrical length is the same than the monopole, its

Figure 3. Monopole antenna shape (a) and its 3D radiation shape (b)

**Figure 3.** Monopole antenna shape (a) and its 3D radiation shape (b)

proportional to 2 (*h* /

i.e. up to 20%.

h.

proportional to (*h* / *λ*)

4 Progress in Compact Antennas

2

Mobile Communications (GSM) (see Figure 4).

Mobile Communications (GSM) (see Figure 4).

Figure 4. Monopole antenna integrated inside a mobile phone

**2.3. Inverted L and F antennas (ILA and IFA)**

**Figure 4.** Monopole antenna integrated inside a mobile phone

**2.3. Inverted L and F antennas (ILA and IFA)**

Adding a ground wire on the horizontal part facilitates the ILA matching. This new antenna design is called Inverted F Antenna (IFA) (Figure 6). This wire is equivalent to a self-inductance in parallel with the capacitance of the horizontal wire. That involves a parallel resonance at low frequencies.

**Figure 6.** Inverted F Antenna (IFA) shape

To fit the input impedance around 50Ω, we can adjust wire's parameters (radius, distance with the feeding point,…). However, typical impedance bandwidths are around 2% or 3% [34-35].

#### **2.4. The helical antenna**

This kind of antenna allows reducing the physical length of an antenna (Figure 7). Basically, its fundamental mode is due to a quarter wavelength resonance. However, its bending structure can involve some capacitive and/or inductive resonances. This antenna has been widely used in mobile phone devices.

impedance bandwidth is between 1% and 4% and depends on both the dielectric permittivity

Using a material allow the reduction of the guided wavelength and thus the physical length of an antenna. Indeed, the patch antenna length is directly proportional to the refractive index of the dielectric substrate *n* = *εr*.*μr*. Using a dielectric material is the most common method to reduce the antenna size [39-40]. In this sub-section, we will show on one hand the antenna size reduction for a planar antenna printed on a dielectric substrate and on the other hand on a

We consider the patch antenna presented Figure 8 with a 4cm-length and a 3mm-thickness.

The Table 1 summarizes main results for patch antennas with strictly same dimensions.

Patch antenna with ε*r* =1 3.7 GHz 3.8 GHz 4.1 % Patch antenna with ε*r* =9 1.3 GHz 1.32 GHz 0.98 %

**Table 1.** Comparison between two antenna patches with same dimensions printed on different substrates

**Resonance frequency Matching frequency Impedance bandwidth**

Compact Antennas — An overview http://dx.doi.org/10.5772/58837 7

To show the impact of a dielectric substrate we consider two different cases:

and the thickness of the substrate.

magneto-dielectric material substrate.

**•** Substrate with low dielectric permittivity *εr* =1.

**•** Substrate with higher dielectric permittivity *εr* =9.

**• Using a dielectric material**

*3.2.1. Use of materials*

**Figure 8.** Patch antenna

**3.2. Miniaturization techniques of planar antennas**

**Figure 7.** Helical antenna

Global dimensions of this kind of antennas are around λ0/10 for its height with a λ0/40 diameter. Its typical impedance bandwidth is up to 8%. Thus this antenna is covering the entire GSM band [36-37].

### **3. Planar antennas – Miniaturization techniques**

#### **3.1. Classical planar antenna: the patch antenna**

The patch antenna was introduced by John Q. Howell in 1972 [38]. This kind of antenna presents a metallic top hat mounted on a dielectric substrate. Its lower face is the ground plane and its feeding can be a coaxial probe (Figure 8), a microstrip line or a coplanar waveguide.

The two metal sheets together form a resonant part of a microstrip transmission line with a length equals to a half of wavelength. Thus, its higher dimension is equal to *λg* / 2, with *λg* the the guided wavelength. A simple patch antenna radiates a linearly polarized wave and its radiation can be regarded as a result of the current flowing on the patch and the ground plane. Thus its maximum gain is relative to the vertical axis of the patch and can reach 7 or 8 dB. The

**Figure 8.** Patch antenna

To fit the input impedance around 50Ω, we can adjust wire's parameters (radius, distance with the feeding point,…). However, typical impedance bandwidths are around 2% or 3% [34-35].

This kind of antenna allows reducing the physical length of an antenna (Figure 7). Basically, its fundamental mode is due to a quarter wavelength resonance. However, its bending structure can involve some capacitive and/or inductive resonances. This antenna has been

Global dimensions of this kind of antennas are around λ0/10 for its height with a λ0/40 diameter. Its typical impedance bandwidth is up to 8%. Thus this antenna is covering the entire GSM

The patch antenna was introduced by John Q. Howell in 1972 [38]. This kind of antenna presents a metallic top hat mounted on a dielectric substrate. Its lower face is the ground plane and its feeding can be a coaxial probe (Figure 8), a microstrip line or a coplanar waveguide.

The two metal sheets together form a resonant part of a microstrip transmission line with a length equals to a half of wavelength. Thus, its higher dimension is equal to *λg* / 2, with *λg* the the guided wavelength. A simple patch antenna radiates a linearly polarized wave and its radiation can be regarded as a result of the current flowing on the patch and the ground plane. Thus its maximum gain is relative to the vertical axis of the patch and can reach 7 or 8 dB. The

**3. Planar antennas – Miniaturization techniques**

**3.1. Classical planar antenna: the patch antenna**

**2.4. The helical antenna**

6 Progress in Compact Antennas

**Figure 7.** Helical antenna

band [36-37].

widely used in mobile phone devices.

impedance bandwidth is between 1% and 4% and depends on both the dielectric permittivity and the thickness of the substrate.

#### **3.2. Miniaturization techniques of planar antennas**

#### *3.2.1. Use of materials*

Using a material allow the reduction of the guided wavelength and thus the physical length of an antenna. Indeed, the patch antenna length is directly proportional to the refractive index of the dielectric substrate *n* = *εr*.*μr*. Using a dielectric material is the most common method to reduce the antenna size [39-40]. In this sub-section, we will show on one hand the antenna size reduction for a planar antenna printed on a dielectric substrate and on the other hand on a magneto-dielectric material substrate.

#### **• Using a dielectric material**

We consider the patch antenna presented Figure 8 with a 4cm-length and a 3mm-thickness. To show the impact of a dielectric substrate we consider two different cases:

The Table 1 summarizes main results for patch antennas with strictly same dimensions.

**Table 1.** Comparison between two antenna patches with same dimensions printed on different substrates

As we can see in this table, the resonance frequency is divided by three with the increase of the dielectric permittivity value. Indeed, the miniaturization factor is close to the refractive index *εr*.*μr* = 9=3. However, the antenna miniaturization involves the reduction of its performances as its impedance bandwidth is divided by four.

Another solution is to use a magneto-dielectric material.

#### **• Using a magneto-dielectric material**

Hansen and Burke [41] have expressed the zero-order impedance bandwidth of a patch antenna printed on a t-thick magneto-dielectric material by the following equation:

$$BW = 96\sqrt{\frac{\mu}{\varepsilon}} \cdot \frac{t}{\lambda\_0} \bigg/ \sqrt{2} \left(4 + 17\sqrt{\varepsilon \cdot \mu}\right) \tag{1}$$

*3.2.2. Modification of the antenna shape*

Notches integration

this element (Figure 9).

patch antenna.

**• Meander antenna**

**Meander antenna**

**3.2.3. Short cut insertion**

and allows the increase of its impedance bandwidth.

**3.2.2. Modification of the antenna shape**

The integration of notches on the antenna top hat is often used. It allows to artificially increase the electrical length of the radiating element by extending the current "path" on this element

The integration of notches on the antenna top hat is often used. It allows to artificially increase the electrical length of the radiating element by extending the current "path" on

(a) (b)

The radiating element dimensions can be reduced up to 50% comparing with a classical

The radiating element dimensions can be reduced up to 50% comparing with a classical patch

As for the helical antenna, the meander antenna allows decreasing the physical length of a planar antenna. The advantage is that this antenna is planar and thus easy to integrate inside a mobile phone. We can present on the Figure 10 a widely used meander antenna for the GSM reception on mobile phones. It is printed on a 0.8mm-thick FR4 substrate and is

As for the helical antenna, the meander antenna allows decreasing the physical length of a planar antenna. The advantage is that this antenna is planar and thus easy to integrate inside a mobile phone. We can present on the Figure 10 a widely used meander antenna for the GSM reception on mobile phones. It is printed on a 0.8mm-thick FR4 substrate and is matched on

Figure 9. Integration of notches on antenna's top hat (a) Surface current lines (b)

**Figure 9.** Integration of notches on antenna's top hat (a) Surface current lines (b)

1%-bandwidth around 900 MHz with λ0/3 x λ0/5 dimensions.

**Figure 10.** Meander antenna integrated within a mobile phone for the GSM reception

matched on 1%-bandwidth around 900 MHz with λ0/3 x λ0/5 dimensions.

Figure 10. Meander antenna integrated within a mobile phone for the GSM reception

antenna printed on a magneto-dielectric material presents the same miniaturization factor

Compact Antennas — An overview http://dx.doi.org/10.5772/58837 9

**• Notches integration**

(Figure 9).

antenna.

Thus, compared to high dielectric permittivity, high permeability materials allow to reduce the size of a patch antenna without decreasing its relative impedance bandwidth. In [42], Niamien et al. investigates magneto-dielectric materials losses and provides expressions of antenna impedance bandwidth and efficiency according to both dielectric and magnetic losses for a patch antenna. They showed that both the radiation efficiency and the impedance bandwidth increase with the permeability.

Considering the previous patch antenna (with a 4cm-length and 3mm-thick) by changing the dielectric material by a magneto-dielectric material, we obtain the Table 2 results.

**Table 2.** Comparison between antenna patches with same dimensions printed on different substrates

This table compares patch antenna results with a same refractive index *n* = *εr*.*μr* =3. It should be noticed that all the materials are considered without any loss.

Therefore the comparison between the dielectric and magneto-dielectric materials shows that using the latter in a patch antenna allows increasing its impedance bandwidth. A patch antenna printed on a magneto-dielectric material presents the same miniaturization factor and allows the increase of its impedance bandwidth.

#### *3.2.2. Modification of the antenna shape* **3.2.2. Modification of the antenna shape**

and allows the increase of its impedance bandwidth.

#### **• Notches integration** Notches integration

As we can see in this table, the resonance frequency is divided by three with the increase of the dielectric permittivity value. Indeed, the miniaturization factor is close to the refractive index *εr*.*μr* = 9=3. However, the antenna miniaturization involves the reduction of its

Hansen and Burke [41] have expressed the zero-order impedance bandwidth of a patch

Thus, compared to high dielectric permittivity, high permeability materials allow to reduce the size of a patch antenna without decreasing its relative impedance bandwidth. In [42], Niamien et al. investigates magneto-dielectric materials losses and provides expressions of antenna impedance bandwidth and efficiency according to both dielectric and magnetic losses for a patch antenna. They showed that both the radiation efficiency and the impedance

Considering the previous patch antenna (with a 4cm-length and 3mm-thick) by changing the

dielectric material by a magneto-dielectric material, we obtain the Table 2 results.

Patch antenna with ε*r* =9 and μ*r* =1 1.3 GHz 1.32 GHz 0.98 % Patch antenna with ε*r* =4 and μ*r* =2.25 1.35 GHz 1.38 GHz 1.87 % Patch antenna with ε*r* =3 and μ*r* =3 1.37 GHz 1.41 GHz 2.82 % Patch antenna with ε*r* =2.25 and μ*r* =4 1.38 GHz 1.45 GHz 3.29 % Patch antenna with ε*r* =1 and μ*r* =9 1.31 GHz 1.65 GHz 4.66 %

**Table 2.** Comparison between antenna patches with same dimensions printed on different substrates

be noticed that all the materials are considered without any loss.

This table compares patch antenna results with a same refractive index *n* = *εr*.*μr* =3. It should

Therefore the comparison between the dielectric and magneto-dielectric materials shows that using the latter in a patch antenna allows increasing its impedance bandwidth. A patch antenna printed on a magneto-dielectric material presents the same miniaturization factor and allows

( )

e m

*t BW* (1)

Resonance frequency Matching frequency Impedance bandwidth

antenna printed on a t-thick magneto-dielectric material by the following equation:

0 96 . 2 4 17 .

= +

m

e l

performances as its impedance bandwidth is divided by four.

Another solution is to use a magneto-dielectric material.

**• Using a magneto-dielectric material**

8 Progress in Compact Antennas

bandwidth increase with the permeability.

the increase of its impedance bandwidth.

The integration of notches on the antenna top hat is often used. It allows to artificially increase the electrical length of the radiating element by extending the current "path" on this element (Figure 9). The integration of notches on the antenna top hat is often used. It allows to artificially increase the electrical length of the radiating element by extending the current "path" on this element (Figure 9).

**Figure 9.** Integration of notches on antenna's top hat (a) Surface current lines (b)

Figure 9. Integration of notches on antenna's top hat (a) Surface current lines (b)

patch antenna. **Meander antenna** As for the helical antenna, the meander antenna allows decreasing the physical length of a The radiating element dimensions can be reduced up to 50% comparing with a classical patch antenna.

The radiating element dimensions can be reduced up to 50% comparing with a classical

#### planar antenna. The advantage is that this antenna is planar and thus easy to integrate inside a mobile phone. We can present on the Figure 10 a widely used meander antenna for **• Meander antenna**

the GSM reception on mobile phones. It is printed on a 0.8mm-thick FR4 substrate and is matched on 1%-bandwidth around 900 MHz with λ0/3 x λ0/5 dimensions. As for the helical antenna, the meander antenna allows decreasing the physical length of a planar antenna. The advantage is that this antenna is planar and thus easy to integrate inside a mobile phone. We can present on the Figure 10 a widely used meander antenna for the GSM reception on mobile phones. It is printed on a 0.8mm-thick FR4 substrate and is matched on 1%-bandwidth around 900 MHz with λ0/3 x λ0/5 dimensions.

**Figure 10.** Meander antenna integrated within a mobile phone for the GSM reception

#### *3.2.3. Short cut insertion*

#### **• Planar Inverted F Antenna**

As for the dipole and the monopole, it is possible to integrate a metallic plate inside the patch antenna in order to divide its main dimension by two. Indeed, on the fundamental mode of the patch, we can integrate a short cut where the electric filed is null. To manage to match the antenna the metallic plate dimensions have to be optimised (Figure 11).

**•** The radius of the feeding probe. The higher the radius is, the lower the input impedance

**•** The ground wire – feeding probe separation. The Q-factor is increasing when the length

As presented in [45-46], the use of a closed slot into the antenna top hat involves a significant reduction of the resonant frequency (Figure 13). Indeed, the introduction of a slot in the hat of the antenna changes the equivalent capacitance of the antenna short-circuited hat by increasing its value. As presented in [45-46], the use of a closed slot into the antenna top hat involves a significant reduction of the resonant frequency (Figure 13). Indeed, the introduction of a slot in the hat of the antenna changes the equivalent capacitance of the antenna short-circuited

(a) (b)

**-15**

**-10**

**|S11| (dB)**

**-5**

**0**

The longer the electrical length of the slot is, the lower the resonant frequency is. We can compare the |S11| parameters of the wire patch antenna presented in Figure 13 with and without the notch [47]. As expected, adding a notch inside the wire patch antenna top hat

**4. Dielectric resonator antennas (DRAs) – Miniaturization techniques**

A non-exhaustive list of main simple-shaped DRAs characteristics is described below:

magnetic constant of the material. In a dielectric material case, 1 *r*

of antennas because of minimal conductor losses associated with a DRA.

 and *r*

 The radiation efficiency of the DRA is highly depending on the material losses. In case of a low-loss dielectric material, DRAs allow to achieve better efficiency than other kind

.

**2 2.5 3 3.5 4 -20**

**Frequency (GHz)**

 where 0

are respectively the dielectric and the

is the free-space

**With notch Without notch**

Compact Antennas — An overview http://dx.doi.org/10.5772/58837 11

and the main

The main dimension of a DRA is proportional to 0 / .*r r*

wavelength at the resonant frequency, *r*

dimension of a DRA is proportional to 0 / *r*

The design of a DRA in any geometry must satisfy various specifications including: the resonant frequency, the impedance bandwidth, the field distribution inside the resonator and also the radiated field. The intent of this part is to provide an overview of main findings of investigations on simple-shaped DRAs. Then, it will deal with the different

Figure 13. Monopolar wire patch antenna with a notch (a) and corresponding |S11| parameters

allows decreasing the working frequency but also the impedance bandwidth.

**Figure 13.** Monopolar wire patch antenna with a notch (a) and corresponding |S11| parameters

between the ground wire and the feeding probe core is increasing.

imaginary part is.

**Figure 12.** Monopolar wire patch antenna

hat by increasing its value.

miniaturization techniques of DRAs.

**4.1. DRAs characteristics**

**Figure 11.** Planar Inverted F Antenna (PIFA)

In [43], a Planar Inverted Antenna with λ0/6 x λ0/8 x λ0/35 dimensions at 2.45GHz is matching over a 6.5% impedance bandwidth.

#### **• Monopolar wire patch antenna**

The design of a classical wire patch antenna is presented in Figure 12. It is composed by two metallizations etched on each face of a dielectric substrate. The lower metallic plate acts as ground and the upper metallic plate constitutes the antenna top hat. This kind of antenna is fed by a coaxial probe which is connected to the top hat through the ground plane and the dielectric substrate. The ground wire acts as a short-circuit to the capacitance of the antenna constituted by the top hat above the ground plane and allows achieving a new low-frequency parallel resonance. The resonance frequency is smaller than the classical antenna fundamental cavity mode [44]. It is primarily set by the size of the top hat, the height of the antenna, the permittivity of the substrate and the ground wire diameter.

The main antenna parameters to adjust the antenna impedance matching to 50Ω are:

**•** The ground wire radius. The smaller the radius is, the higher the maximum of the input impedance real part is.

**Figure 12.** Monopolar wire patch antenna

hat by increasing its value.

miniaturization techniques of DRAs.

**4.1. DRAs characteristics**

*3.2.3. Short cut insertion*

10 Progress in Compact Antennas

**• Planar Inverted F Antenna**

**Figure 11.** Planar Inverted F Antenna (PIFA)

over a 6.5% impedance bandwidth.

**• Monopolar wire patch antenna**

impedance real part is.

permittivity of the substrate and the ground wire diameter.

As for the dipole and the monopole, it is possible to integrate a metallic plate inside the patch antenna in order to divide its main dimension by two. Indeed, on the fundamental mode of the patch, we can integrate a short cut where the electric filed is null. To manage to match the

In [43], a Planar Inverted Antenna with λ0/6 x λ0/8 x λ0/35 dimensions at 2.45GHz is matching

The design of a classical wire patch antenna is presented in Figure 12. It is composed by two metallizations etched on each face of a dielectric substrate. The lower metallic plate acts as ground and the upper metallic plate constitutes the antenna top hat. This kind of antenna is fed by a coaxial probe which is connected to the top hat through the ground plane and the dielectric substrate. The ground wire acts as a short-circuit to the capacitance of the antenna constituted by the top hat above the ground plane and allows achieving a new low-frequency parallel resonance. The resonance frequency is smaller than the classical antenna fundamental cavity mode [44]. It is primarily set by the size of the top hat, the height of the antenna, the

The main antenna parameters to adjust the antenna impedance matching to 50Ω are:

**•** The ground wire radius. The smaller the radius is, the higher the maximum of the input

antenna the metallic plate dimensions have to be optimised (Figure 11).

As presented in [45-46], the use of a closed slot into the antenna top hat involves a significant reduction of the resonant frequency (Figure 13). Indeed, the introduction of a slot in the hat of the antenna changes the equivalent capacitance of the antenna short-circuited hat by increasing its value. As presented in [45-46], the use of a closed slot into the antenna top hat involves a

significant reduction of the resonant frequency (Figure 13). Indeed, the introduction of a slot in the hat of the antenna changes the equivalent capacitance of the antenna short-circuited

compare the |S11| parameters of the wire patch antenna presented in Figure 13 with and without the notch [47]. As expected, adding a notch inside the wire patch antenna top hat

**4. Dielectric resonator antennas (DRAs) – Miniaturization techniques**

A non-exhaustive list of main simple-shaped DRAs characteristics is described below:

magnetic constant of the material. In a dielectric material case, 1 *r*

of antennas because of minimal conductor losses associated with a DRA.

 and *r*

 The radiation efficiency of the DRA is highly depending on the material losses. In case of a low-loss dielectric material, DRAs allow to achieve better efficiency than other kind

.

 where 0

are respectively the dielectric and the

is the free-space

and the main

The main dimension of a DRA is proportional to 0 / .*r r*

wavelength at the resonant frequency, *r*

dimension of a DRA is proportional to 0 / *r*

The design of a DRA in any geometry must satisfy various specifications including: the resonant frequency, the impedance bandwidth, the field distribution inside the resonator and also the radiated field. The intent of this part is to provide an overview of main findings of investigations on simple-shaped DRAs. Then, it will deal with the different

The longer the electrical length of the slot is, the lower the resonant frequency is. We can **Figure 13.** Monopolar wire patch antenna with a notch (a) and corresponding |S11| parameters

allows decreasing the working frequency but also the impedance bandwidth.

Figure 13. Monopolar wire patch antenna with a notch (a) and corresponding |S11| parameters

The longer the electrical length of the slot is, the lower the resonant frequency is. We can compare the |S11| parameters of the wire patch antenna presented in Figure 13 with and without the notch [47]. As expected, adding a notch inside the wire patch antenna top hat allows decreasing the working frequency but also the impedance bandwidth.

A large number of DRA excitations are currently used, e.g. microstrip line, coaxial probe excitation, coplanar waveguide… The next subsection will deal with the most commonly used

This subsection examines techniques to design compact DRAs. Targeted applications are mobile handsets or wireless tablet. There are several techniques to make DRAs more compact. By adding metal plates, inserting a high permittivity layer (multisegment DRA) or removing

The rectangular DRA shape has been studied in the first part. The perfect metallic wall implies that electric fields are normal to this conductor, while magnetic fields are tangential. E and H fields presented Figure 14 assume that a metallic plate can be inserted in the middle of the DRA according to the y-component. The principle is detailed and explained by the Figure 14.

> H field E field

By applying the image theory, it is possible to insert a metal plate in the y=w/2 plane. The Table 3 extracted from [48] shows the influence of the metallic plate insertion on resonant frequency

εr w (cm) d (cm) h (cm) Metallization f0(GHz) Bandwidth

12 2.75 2.75 2.95 No 1.98 10% 12 2.75 2.75 2.95 Yes 1.24 5.6%

**Table 3.** Influence of the metallic plate insertion on both resonant frequency and impedance bandwidth

Groundplane

z y w w/2

Metallization

Compact Antennas — An overview http://dx.doi.org/10.5772/58837 13

portions of the DRA, a significant size reduction can be achieved.

excitations.

**4.2. DRAs miniaturization techniques**

**• Addition of a metallic plate on a DRA face**

It also shows the E and H fields of the TE111 mode.

h

**Figure 14.** Integration of a metallic plate

and impedance bandwidth.

### **4. Dielectric resonator antennas (DRAs) – Miniaturization techniques**

The design of a DRA in any geometry must satisfy various specifications including: the reso‐ nant frequency, the impedance bandwidth, the field distribution inside the resonator and al‐ so the radiated field. The intent of this part is to provide an overview of main findings of investigations on simple-shaped DRAs. Then, it will deal with the different miniaturization techniques of DRAs.

#### **4.1. DRAs characteristics**

A non-exhaustive list of main simple-shaped DRAs characteristics is described below:

A large number of DRA excitations are currently used, e.g. microstrip line, coaxial probe excitation, coplanar waveguide… The next subsection will deal with the most commonly used excitations.

#### **4.2. DRAs miniaturization techniques**

The longer the electrical length of the slot is, the lower the resonant frequency is. We can compare the |S11| parameters of the wire patch antenna presented in Figure 13 with and without the notch [47]. As expected, adding a notch inside the wire patch antenna top hat

**4. Dielectric resonator antennas (DRAs) – Miniaturization techniques**

A non-exhaustive list of main simple-shaped DRAs characteristics is described below:

**•** The main dimension of a DRA is proportional to *λ*0 / *εr*.*μr* where *λ*0 is the free-space wavelength at the resonant frequency, *εr* and *μr* are respectively the dielectric and the magnetic constant of the material. In a dielectric material case, *μr* =1 and the main dimension

**•** The radiation efficiency of the DRA is highly depending on the material losses. In case of a low-loss dielectric material, DRAs allow to achieve better efficiency than other kind of

**•** For a given dielectric constant, both resonant frequency and radiated Q-factor are defined according to the resonator dimensions. That allows having a great flexibility and some

**•** Another degree of freedom is the large spectrum of available dielectric materials. That allows doing the best trade-off between dimensions and impedance bandwidth according

**•** A number of modes can be excited within the DRA, many of them provide dipolar-like

**•** The most common targeted frequencies presented by the research literatures are ranging

**•** For a given DRA geometry, the radiation patterns can be made to change by exciting

antennas because of minimal conductor losses associated with a DRA.

The design of a DRA in any geometry must satisfy various specifications including: the reso‐ nant frequency, the impedance bandwidth, the field distribution inside the resonator and al‐ so the radiated field. The intent of this part is to provide an overview of main findings of investigations on simple-shaped DRAs. Then, it will deal with the different miniaturization

allows decreasing the working frequency but also the impedance bandwidth.

techniques of DRAs.

12 Progress in Compact Antennas

**4.1. DRAs characteristics**

of a DRA is proportional to *λ*0 / *εr*.

to the intended application.

radiation characteristics.

from 1GHz to 40 GHz.

different resonant modes.

degrees of freedom to design such an antenna.

This subsection examines techniques to design compact DRAs. Targeted applications are mobile handsets or wireless tablet. There are several techniques to make DRAs more compact. By adding metal plates, inserting a high permittivity layer (multisegment DRA) or removing portions of the DRA, a significant size reduction can be achieved.

#### **• Addition of a metallic plate on a DRA face**

The rectangular DRA shape has been studied in the first part. The perfect metallic wall implies that electric fields are normal to this conductor, while magnetic fields are tangential. E and H fields presented Figure 14 assume that a metallic plate can be inserted in the middle of the DRA according to the y-component. The principle is detailed and explained by the Figure 14. It also shows the E and H fields of the TE111 mode.

**Figure 14.** Integration of a metallic plate

By applying the image theory, it is possible to insert a metal plate in the y=w/2 plane. The Table 3 extracted from [48] shows the influence of the metallic plate insertion on resonant frequency and impedance bandwidth.

**Table 3.** Influence of the metallic plate insertion on both resonant frequency and impedance bandwidth

Thus, the metal plate insertion allows dividing by two the DRA size, while reducing the resonant frequency. However, as pointed by the Table 3, the metallic plate insertion involves also the decrease of the impedance bandwidth.

**5. Summary of compact antennas performances**

**Substrate Dimensions Impedance**

presented antennas.

In this part, as a first conclusion we can summary the main performances of the previous

**bandwidth**

Dipole Air λ0/2 10% à 20% 2.1 99%

Monopole Air λ0/4 10% à 20% 5.1 99%

ILA Air λ0/20 x λ0/4 1% 7 98%

IFA Air λ0/20 x λ0/4 2% 4 98%

Helical Air λ0/10 x λ0/40 7% 6 98%

**Radiation Directivity**

**(dBi)**

Compact Antennas — An overview http://dx.doi.org/10.5772/58837

> **Total efficiency**

15

#### **• Multisegment DRA**

Another way to decrease the DRA size is to insert different substrate layers as illustrated Figure 15.

**Figure 15.** Multisegment DRA

It allows achieving strong coupling when the first insertion has a relatively high dielectric permittivity. This technique is detailed in [48] and [49]. The Table 4 summarizes a parametrical study done in [49] for one layer inserted (Figure 15) with w=7.875 mm, d=2 mm, h=3.175 and εr=10. It is mounted on a 0.762 mm height substrate of permittivity εs=3. The TE111 mode of the DRA is excited with a 50Ω microstrip line.

**Table 4.** A parametrical study done in [31] for one layer inserted

Thus, a thin layer insertion allows improving the coupling of modes inside the DRA while decreasing the resonant frequency thanks to the decrease of the effective dielectric permittivity of the DRA. As the previous technique, the downside is the decrease of the impedance bandwidth.

## **5. Summary of compact antennas performances**

Thus, the metal plate insertion allows dividing by two the DRA size, while reducing the resonant frequency. However, as pointed by the Table 3, the metallic plate insertion involves

Another way to decrease the DRA size is to insert different substrate layers as illustrated Figure

It allows achieving strong coupling when the first insertion has a relatively high dielectric permittivity. This technique is detailed in [48] and [49]. The Table 4 summarizes a parametrical study done in [49] for one layer inserted (Figure 15) with w=7.875 mm, d=2 mm, h=3.175 and εr=10. It is mounted on a 0.762 mm height substrate of permittivity εs=3. The TE111 mode of the

t (mm) εi Measured f0(GHz) Bandwidth 0 - 15.2 21% 0.25 20 14.7 18% 0.635 20 14.5 18% 1 20 13.9 16% 0.25 40 14.7 20% 0.635 40 13.7 13% 1 40 12.9 5% 0.25 100 14.7 16% 0.635 100 13.1 7% 1 100 10.8 5%

Thus, a thin layer insertion allows improving the coupling of modes inside the DRA while decreasing the resonant frequency thanks to the decrease of the effective dielectric permittivity of the DRA. As the previous technique, the downside is the decrease of the impedance

also the decrease of the impedance bandwidth.

**• Multisegment DRA**

14 Progress in Compact Antennas

**Figure 15.** Multisegment DRA

bandwidth.

DRA is excited with a 50Ω microstrip line.

**Table 4.** A parametrical study done in [31] for one layer inserted

15.

In this part, as a first conclusion we can summary the main performances of the previous presented antennas.

**Substrate Dimensions Impedance**

DRA (2 layers)

**6. Conclusion**

**Author details**

**References**

L. Huitema and T. Monediere

University of Limoges, Xlim Laboratory, France

no.2, pp. 170-182, Feb. 1981

*Transactions on*, vol.12, no.1, pp. 23-27, Jan 1964

pp.672, May 1996

εlayer1=100 εlayer2=10 (tanδε=5.10-4) **bandwidth**

To conclude, an overview of classical antennas with their miniaturization techniques has been presented and detailed in this chapter while mentioning a lot of literature references. Classical wire antennas as monopoles present good impedance bandwidth, but they remain too large to be integrated inside last generations of mobile devices. Planar antennas have the advantage to be generally low profiles and thus easier to be integrate. However, patch antennas or planar inverted F antennas have maximum gains relative to the vertical axis. Thus, wire patch antenna presents a good alternative since it radiates as a dipole antenna and is significantly smaller. Concerning the dielectric resonator antennas, they can be miniature and can resonate and be

[1] Hansen, R.C.;, "Fundamental limitations in antennas," *Proceedings of the IEEE*, vol.69,

[2] McLean, J. S.;, "A re-examination of the fundamental limits on the radiation Q of elec‐ trically small antennas," *Antennas and Propagation, IEEE Transactions on*, vol.44, no.5,

[3] Collin, R.; Rothschild, S.;, "Evaluation of antenna Q," *Antennas and Propagation, IEEE*

matched on different frequency by creating some partial boundary condition [50].

λ0/4 x λ0/14 x λ0/7 5% 6.4 95%

**Radiation Directivity**

**(dBi)**

Compact Antennas — An overview http://dx.doi.org/10.5772/58837

> **Total efficiency**

17


**Author details**

*Heat Transfer - Design, Experimentation and Applications*

from the graph representing the lowest temperature of 310.01 K for the concentration of 0.1 vol. % graphene-iron oxide system for 0.5 mm fin spacing while for water it is 317.05 K. These hybrid coolants because of their high thermal conductivity are capable of removing more heat when used in microchannels. The percentage reduc-

tion in base temperature when using graphene-iron oxide is 1.96%.

found to be 0.994 which validates the model.

*Simulated base temperature versus flow rate - graphene-iron oxide.*

**5. Comparison of simulated and experimental base temperatures**

**Figure 11** shows the comparison of simulated and experimental base temperatures for 0.1 volume % graphene – iron oxide and the correlation coefficient was

The present work indicates the usage of graphene as one of the components in the synthesis of hybrid nanofluids for CPU cooling by microchannel cooling

*Validation of experimental and simulated values of T base – 0.1 volume % graphene-iron oxide.*

**116**

**6. Conclusion**

**Figure 11.**

**Figure 10.**

Chitra Boobalan1 , Sudha Ganesh2 and Parthiban Rangaswamy1 \*

1 Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam, Tamilnadu, India

2 Department of Chemical Engineering, Sri Venkateswara College of Engineering, Pennalur, Tamilnadu, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. 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.