occur spontaneously during other processing; the patterns are random in orientation and organization with imprecise or no control over feature geometry. the advantage to creating unordered topographies over ordered is that the processes are often less time-consuming, less expensive, and do not require great skill and experience. unordered topographies can be created by polymer demixing, colloidal lithography and chemical etching. in polymer demixing, polymer blends experience spontaneous phase separation; it often occurs during conditions such as spin casting onto silicon wafers. features that can be created by this method include nanoscale pits, islands, and ribbons, which can be controlled to an extent by adjusting the polymer ratio and concentration to change the feature shape and size, respectively. there is not much control in the horizontal direction, though the vertical direction of the features can be precisely controlled. because the pattern is very unordered horizontally, this method can only be used to study cell interactions with specific height nanotopographies. colloidal lithography is inexpensive and can be used to create surfaces with controlled heights and diameters. nanocolliods are used as an etch mask by spreading them along the material surface, and then ion beam bombardment or film evaporation is used to etch away around the nanocolliods, creating nanocolumns and nanopits, respectively. the final surface structure can be controlled by varying the area covered by colloids and the colloid size. the area covered by the colloids can be changed by modifying the ionic strength of the colloid solution. this technique is able to create large patterned surface areas, which is necessary for tissue engineering applications. chemical etching involves soaking the material surface in an etchant such as hydrofluoric acid (hf) or sodium hydroxide (naoh) until the surface is etched away to a desired roughness as created by pits and protrusions on the nanometer scale. longer etch times lead to rougher surfaces (i.e., smaller surface pits and protrusions). structures with specific geometry or organization cannot be created by this rudimentary method because at best it can be considered a surface treatment for changing the surface roughness. the significant advantages of this method are ease of use and low cost for creating a surface with nanotopographies. silicon wafers were etched using hf, and it was demonstrated that cell adhesion was enhanced only in a specified range of roughness (20–50 nm). chemical cues in addition to creating topography with physical cues, it can be created with chemical cues by selectively depositing polymer solution in patterns on the surface of a substrate. there are different methods for depositing the chemical cues. two methods for dispensing chemical solutions include stripe patterning and piezoelectric microdispensing. stripe-patterned polymer films can be formed on solid substrates by casting diluted polymer solution. this method is relatively easy, inexpensive, and has no restriction on the scaffold materials that can be used. the procedure involves horizontally overlapping glass plates while keeping them vertically separated by a narrow gap filled with a polymer solution. the upper plate is moved at a constant velocity between 60 and 100 µm/s. a thin liquid film of solution is continuously formed at the edge of the sliding glass following evaporation of the solvent. stripe patterns prepared at speeds of 60, 70, and 100 µm/s created width and groove spacings of 2.2 and 6.1 µm, 3.6 and 8.4 µm, and 4.3 and 12.7 µm, respectively; the range of heights for the ridges was 50–100 nm. tsuruma, tanaka et al. demonstrated that embryonic neural cells cultured on film coated with poly-l-lysine attached and elongated parallel to poly(ε-caprolactone)/chloroform solution (1g/l) stripes with narrow pattern width and spacing (width: 2.2 µm, spacing: 6.1 µm). however, the neurons grew across the axis of the patterns with wide width and spacing (width: 4.3 µm, spacing: 12.7 µm). on average, the neurons on the stripe-patterned films had less neurites per cell and longer neurites compared to the neurons on non-patterned films. thus, the stripe pattern parameters are able to determine the growth direction, the length of neurites, and the number of neurites per cell. microdispensing was used to create micropatterns on polystyrene culture dishes by dispensing droplets of adhesive laminin and non-adhesive bovine serum albumin (bsa) solutions. the microdispenser is a piezoelectric element attached to a push-bar on top of a channel etched in silicon, which has one inlet at each end and a nozzle in the middle. the piezoelectric element expands when voltage is applied, causing liquid to be dispensed through the nozzle. the microdispenser is moved using a computer-controlled x-y table. the micropattern resolution depends on many factors: dispensed liquid viscosity, drop pitch (the distance between the centre of two adjacent droplets in a line or array), and the substrate. with increasing viscosity the lines become thinner, but if the liquid viscosity is too high the liquid cannot be expelled. heating the solution creates more uniform protein lines. although some droplet overlap is necessary to create continuous lines, uneven evaporation may cause uneven protein concentration along the lines; this can be prevented through smoother evaporation by modifying the dispensed solution properties. for patterns containing 0.5 mg/ml laminin, a higher proportion of neurites grew on the microdispensed lines than between the lines. on 10 mg/ml and 1 mg/ml bsa protein patterns and fatty-acid free bsa protein patterns a significant number of neurites avoided the protein lines and grew between the lines. thus, the fatty-acid-containing bsa lines were just as non-permissive for neurite growth as lines containing bsa with fatty acids. because microdispensing does not require direct contact with the substrate surfaces, this technique can utilitze surfaces with delicate micro- or nanotopology that could be destroyed by contact. it is possible to vary the amount of protein deposited by dispensing more or less droplets. an advantage of microdispensing is that patterns can be created quickly in 5–10 minutes. because the piezoelectric microdispenser does not require heating, heat-sensitive proteins and fluids as well as living cells can be dispensed. scaffold material the selection of the scaffold material is perhaps the most important decision to be made. it must be biocompatible and biodegradable; in addition, it must be able to incorporate any physical, chemical, or biological cues desired, which in the case of some chemical cues means that it must have a site available for chemically linking peptides and other molecules. the scaffold materials chosen for nerve guidance conduits are almost always hydrogels. the hydrogel may be composed of either biological or synthetic polymers. both biological and synthetic polymers have their strengths and weaknesses. it is important to note that the conduit material can cause inadequate recovery when (1) degradation and resorption rates do not match the tissue formation rate, (2) the stress-strain properties do not compare well to those of neural tissue, (3) when degrading swelling occurs, causing significant deformation, (4) a large inflammatory response is elicited, or (5) the material has low permeability. hydrogel hydrogels are a class of biomaterials that are chemically or physically cross-linked water-soluble polymers. they can be either degradable or non-degradable as determined by their chemistry, but degradable is more desirable whenever possible. there has been great interest in hydrogels for tissue engineering purposes, because they generally possess high biocompatibility, mechanical properties similar to soft tissue, and the ability to be injected as a liquid that gels. when hydrogels are physically cross-linked they must rely on phase separation for gelation; the phase separation is temperature-dependent and reversible. some other advantages of hydrogels are that they use only non-toxic aqueous solvents, allow infusion of nutrients and exit of waste products, and allow cells to assemble spontaneously. hydrogels have low interfacial tension, meaning cells can easily migrate across the tissue-implant boundary. however, with hydrogels it is difficult to form a broad range of mechanical properties or structures with controlled pore size. synthetic polymer a synthetic polymer may be non-degradable or degradable. for the purpose of neural tissue engineering degradable materials are preferred whenever possible, because long-term effects such as inflammation and scar could severely damage nerve function. the degradation rate is dependent on the molecular weight of the polymer, its crystallinity, and the ratio of glycolic acid to lactic acid subunits. because of a methyl group, lactic acid is more hydrophobic than glycolic acid causing its hydrolysis to be slower. synthetic polymers have more wieldy mechanical properties and degradation rates that can be controlled over a wide range, and they eliminate the concern for immunogenicity. there are many different synthetic polymers currently being used in neural tissue engineering. however, the drawbacks of many of these polymers include a lack of biocompatibility and bioactivity, which prevents these polymers from promoting cell attachment, proliferation, and differentiation. synthetic conduits have only been clinically successful for the repair of very short nerve lesion gaps less than 1–2 cm. furthermore, nerve regeneration with these conduits has yet to reach the level of functional recovery seen with nerve autografts. collagen-terpolymer collagen is a major component of the extracellular matrix, and it is found in the supporting tissues of peripheral nerves. a terpolymer (terp) was synthesized by free radical copolymerization of its three monomers and cross-linked with collagen, creating a hybrid biological-synthetic hydrogel scaffold. the terpolymer is based on poly(nipaam), which is known to be a cell friendly polymer. terp is used both as a cross-linker to increase hydrogel robustness and as a site for grafting of bioactive peptides or growth factors, by reacting some of its acryloxysuccinimide groups with the –nh2 groups on the peptides or growth factors. because the collagen-terpolymer (collagen-terp) hydrogel lacks a bioactive component, a study attached to it a common cell adhesion peptide found in laminin (yigsr) in order to enhance its cell adhesion properties. poly (lactic-co-glycolic acid) family the polymers in the plga family include poly (lactic acid) (pla), poly (glycolic acid) (pga), and their copolymer poly (lactic-co-glycolic acid) (plga). all three polymers have been approved by the food and drug administration for employment in various devices. these polymers are brittle and they do not have regions for permissible chemical modification; in addition, they degrade by bulk rather than by surface, which is not a smooth and ideal degradation process. in an attempt to overcome the lack of functionalities, free amines have been incorporated into their structures from which peptides can be tethered to control cell attachment and behavior. methacrylated dextran (dex-ma) copolymerized with aminoethyl methacrylate (aema) dextran is a polysaccharide derived from bacteria; it is usually produced by enzymes from certain strains of leuconostoc or streptococcus. it consists of α-1,6-linked d-glucopyranose residues. cross-linked dextran hydrogel beads have been widely used as low protein-binding matrices for column chromatography applications and for microcarrier cell culture technology. however, it has not been until recently that dextran hydrogels have been investigated in biomaterials applications and specifically as drug delivery vehicles. an advantage of using dextran in biomaterials applications include its resistance to protein adsorption and cell-adhesion, which allows specific cell adhesion to be determined by deliberately attached peptides from ecm components. aema was copolymerized with dex-ma in order to introduce primary amine groups to provide a site for attachment of ecm-derived peptides to promote cell adhesion. the peptides can be immobilized using sulfo-smmc coupling chemistry and cysteine-terminated peptides. copolymerization of dex-ma with aema allowed the macroporous geometry of the scaffolds to be preserved in addition to promoting cellular interactions. poly(glycerol sebacate) (pgs) a novel biodegradable, tough elastomer has been developed from poly(glycerol sebacate) (pgs) for use in creation of a nerve guidance conduit. pgs was originally developed for soft tissue engineering purposes to specifically mimic ecm mechanical properties. it is considered an elastomer because it is able to recover from deformation in mechanically dynamic environments and to effectively distribute stress evenly throughout regenerating tissues in the form of microstresses. pgs is synthesized by a polycondensation reaction of glycerol and sebacic acid, which can be melt processed or solvent processed into the desired shape. pgs has a young's modulus of 0.28 mpa and an ultimate tensile strength greater than 0.5 mpa. peripheral nerve has a young's modulus of approximately 0.45 mpa, which is very close to that of pgs. additionally, pgs experiences surface degradation, accompanied by losses in linear mass and strength during resorption. following implantation, the degradation half-life was determined to be 21 days; complete degradation occurred at day 60. pgs experiences minimal water absorption during degradation and does not have detectable swelling; swelling can cause