Synthetic Materials

One of the main advantages of synthetic materials is the ability to precisely control their physico-chemical properties, such as molecular weight of the polymer, strength, degradation time, mechanical properties, and hydrophobicity [10]. Amongst the most widely used polymers in tissue engineering are the poly (a-hydroxy acids) of aliphatic polyesters, such as PLA, PGA, and PLGA [11]. These synthetic polymers can be produced in numerous physical forms, including meshes, sponges, and films, and molded into many shapes, depending on the type of tissue one wishes to emulate (e.g., heart, kidney, ear). The rate of biodegradation ofPLGA scaffolds depends not only on the ratio oflactide and glycolide, but also on whether the polymer is mixed with polycaprolactone, collagen, or other synthetic polymers [12-14]. Cell attachment can be improved by covalently modifying the polymers, or by passively coating the scaffolds [15]. Growth factors can also be incorporated into the matrix in order to improve biocompatibility [16].

Since their discovery some 30 years ago, electrically conductive polymers - also known as "synthetic metals" - have been used in many areas of applied chemistry and physics, such as light-emitting diodes and batteries [17]. More recently, there has also been a growing interest in conductive polymers for diverse biomedical applications, specifically as conductive scaffolds for cardiac and neural tissue engineering. The rationale for using conductive polymers is based on the fact that the eukaryotic cell plasma membrane is charged and that, specifically in neurons and myocytes, a multitude of cell functions, such as attachment, proliferation, migration and differentiation could be modulated through electrical stimulation [18-22]. Common classes of organic conductive polymers include polyacetylene, polypyrrole (PPy), polythiophene, polyaniline (PANi), and poly(para-phenylene vinylene). Some of these conductive polymers (especially PPy) have found certain biomedical applications, such as for the immobilization of proteins [23, 24]. Christine Schmidt and her co-workers were the first to employ PPy for tissue engineering purposes [18, 25-28]. Interestingly, this group most recently described a novel 12-mer peptide (T59) that selective binds to conductive PPy and promotes cell attachment [29] This peptide may become useful for immobilizing a variety of bioactive molecules on PPy and other synthetic/conductive polymers, without altering their bulk properties.

Some recent studies have used PANi, another well-characterized organic conducting polymer, as an electroactive substrate for tissue engineering applications [30-33]. PANi is biocompatible in vitro and in long-term animal studies in vivo [31]. To date, most of these studies have investigated the biological properties of PANi solvent-cast into 2D films, rather than engineered into 3D (nano) fibrous scaffolds. A few years ago, Diaz et al. [34] reported that doped, conductive PANi blended with polystyrene (PS) and/or polyethylene oxide (PEO) could be electrospun into nanofibers. In extending these studies, we recently co-electrospun PANi with gelatin, for instance denatured collagen, to yield nanofibrous scaffolds which are both highly biocompatible and electroactive, and may be suitable for applications in cardiac and cardiovascular tissue engineering [35].

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