at high temperatures, have not been attained. Hydrated perfluorosulfonic polymer shows superior performance in fuel cells operating at moderate temperature (<90 ◦C), however, the properties of such polymer membranes are insufficient at higher temperatures. This puts new demands on the development of alternative polymeric proton exchange membranes [12]. Based on this concept, the use of ionic liquids appears to be promising with respect to high ion conductivity in polymers. Due to an ionic liquid’s ability to facilitate electron or ion motion, they are now enabling electroactive devices. Commercially available conductive membranes are swollen with ionic liquids to enhance their conductivity; alternatively, conductive membranes are synthesized from novel ionic liquid monomers, also termed polymerizable ionic liquids. The imidazole ring has gained much attention for its ability to tune the properties of the resulting ionic liquid. The imidazole ring is a very versatile scaffold for ionic liquids. The ring is easily ionized upon quaternization of the tertiary nitrogen atom, resulting in a permanent positive charge. A unique combination of various alkyl substituents and counteranions enables tuning of the physical properties of the liquid such as the melting point, the boiling point, and the viscosity to meet the demands of the application. The structure is uniquely tunable because of the inherent amphoteric behavior, i.e. the imidazole ring both accepts and donates protons. Finally, the imidazolium cation is associated with a mobile counteranion, which can be exchanged to further tune solubility and conductivity [13].
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BIODEGRADABLE POLYMERS FOR TISSUE ENGINEERING APPLICATIONS
Tissue engineering is an interdisciplinary field that blends classical engineering and the life sciences to repair or replace damaged tissues. The most common strategy to achieve this goal is to culture of patient’s own cell onto a three dimensional support matrix, so called scaffold, and then implant this construct to the patient. The function of a degradable scaffold is to act as a temporary support matrix for transplanted or host cells so as to restore, maintain, or improve tissue. The design of a polymeric scaffold plays a significant role in proper cell growth. Therefore, several important properties must be considered: fabrication, structure, biocompatibility, biodegradability, and mechanical strength.
Scaffolds may be created from various types of materials, including polymers. There are two sources of polymers used in tissue engineering: synthetic and natural. The main biodegradable synthetic polymers include polyesters, polyanhydrides, polyorthoesters, polycaprolactone, polycarbonate, and polyfumarate, while the natural origin polymers include collagen, alginate, agarose, hyaluronic acid derivatives, and chitosan. Among the man-made polymers, polyglycolide, polylactides, poly(caprolactone), and poly(dioxanone) constitute the major polymer groups that have been studied as matrix materials.
Natural polymers are typically biocompatible and enzymatically biodegradable. The main advantage for using natural polymers is that they contain bio-functional molecules that aid the attachment, proliferation, and differentiation of cells. However, disadvantages of natural polymers do exist. Depending upon the application, the previously mentioned enzymatic degradation may inhibit function. Further, the rate of this degradation may not be easily controlled. Since the enzymatic activity varies between hosts, so will the degradation rate. Therefore it may be difficult to determine the lifespan of natural polymers in vivo. Additionally, natural polymers are often weak in terms of mechanical strength but cross-linking these polymers have shown to enhance their structural stability.
Polymers that are chemically synthesized offer several notable advantages over natural-origin polymers. A major advantage of synthetic polymers is that they can be tailored to suit specific functions and thus exhibit controllable properties. Furthermore, since many synthetic polymers undergo hydrolytic degradation, a scaffold’s degradation rate should not vary significantly between hosts. A significant disadvantage for using synthetic polymers is that some degrade into unfavorable products, often acids. At high concentrations of these degradation products, local acidity may increase, resulting in adverse responses such as inflammation or fibrous encapsulation.
There are several methods to produce 3D scaffolds from polymers, such as particulate leaching combined with compression moulding or solvent casting, freezing drying, fiber bonding, electrospinning and rapid prototyping, etc.
Particulate leaching is an inexpensive method based on dispersing certain size particles within a polymeric solution or fine polymer powder and then moulding this mixture by solvent casting or compression moulding techniques. The final porous stucture is achieved by removal of porogen from polymeric construct.
Freeze-drying is the most common and simple method to produce scaffolds, especially from natural polymers. The scaffolds, with different pore size and porosity, can be formed by the simple procedure of freezing a polymer solution in a suitable mould and subsequently lyophilizing the frozen structure. The freezing process provides the nucleation of ice crystals from solution and further growth along the lines of thermal gradients. Ice removal by lyophilization generates a porous material.
Fiber-based scaffolds can be obtain with fiber bonding followed by commercial fiber production methods, such as melt spinning, dry spinning and wet spinning.
Electrospinning is a relatively simple and efficient method to produce polymeric fibers on a nano scale. It has been used in polymer processing technology for more than 70 years and recently had much attention from the biomedical field, particularly in tissue engineering due to the structural properties of fabricated fibrous structures having diameters in the range close to the collagen fibers found in the natural extracellular matrix of about 30–130 nm.
Rapid prototyping is a common name for a group of techniques, such as fused deposition modeling (FDM), laminated object manufacturing (LOM), three-dimensional printing (3DP), multiphase jet solidification (MJS) and 3D plotting, that can generate a physical model directly from computer aided design data. It is an additive process in which each part is constructed in a layer-by-layer manner. This technology allows one to produce a complex 3D structure of scaffolds with controlled architecture which means desired pore size, porosity and pore distribution.
POLYMER/CLAY NANOCOMPOSITES BY IN SITU METHODS
Polymer/clay
