![]() ![]() However, a few exceptions exist to this general rule, depending on the degradation products. the amount of void space within the volume of the scaffold is expected to lead not only to a higher rate of clearance of the degradation products from the graft site, but also to a higher degradation rate, since a larger surface of the bulk polymer comes into contact with biological fluids ( Muschler et al., 2004 Dellinger et al., 2006). However, for highly porous implants, the effects of the degradation products might not be pronounced, since the polymer is only a small amount of the total volume of the scaffold, and the released products might be readily cleared by extracellular fluids ( Muschler et al.,2004). When this occurs, the bulk polymer is rapidly solubilized, causing a deep decrease in the local pH ( Muschler et al., 2004), before the degradation products can be further metabolized or excreted via normal physiological pathways (lactic and glycolic acids are eliminated from the body via Kreb’s cycle as CO2 and in urine as water ( Sinha and Trehan, 2003)). Indeed, since the process of hydrolysis starts eroding the bulk polymer in a random manner, the total mass of the scaffold remains essentially the same for a relatively long time, until the molecular weight of the fragments formed is small enough to make them soluble. Although their degradation rate can be easily adjusted and tailored to the specific application by changing their hydrophobicity and crystallinity degrees ( Yang et al., 2001), their use as scaffolds in the clinical practice remains controversial, due to their unfavourable combination of degradation process (burst or ‘bolus’ degradation) and release products at the implant site. Such polymers are degradable in vivo via hydrolysis of the ester linkage, and are approved by the US Food and Drug Administration (FDA) for use as sutures, surgical meshes and fixation components. Among the most widely employed synthetic polymers, several polyesters have been reported in the last two decades, including poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly- ε-caprolactone (PCL) and their copolymers ( Dellon and Mackinnon, 1988 den Dunnen et al., 1993 Aldini et al., 1996 Bryan et al., 2000). However, those properties for natural polymers can be adjusted to a certain extent by means of several cross-linking methods (Lee et al., 2001 Itoh et al., 2002 Harley et al., 2004 Pek et al., 2004). ![]() ![]() The degradation rate and mechanical properties of synthetic polymers are more easily controllable, compared to those of naturally derived biomaterials, which, on the contrary, suffer from large batch-to-batch variations. Several biomaterials have been investigated for the production of nerve guides or axially orientated luminal fillers. The scaffold performance is clearly affected by the nature of the degradation mechanism and the products that are released into the host site as resorption occurs, since those substances, even though not cytotoxic, might deeply change the local cell environment and have a negative effect on tissue regeneration. A true regenerative medicine approach thus aims at using biodegradable polymers only, either synthetic or natural, for the scaffold fabrication. Sannino, in Biomedical Foams for Tissue Engineering Applications, 2014 4.2 Materials for foam scaffold fabricationĪlthough non- biodegradable scaffolds might be useful to induce a modest tissue regeneration, as demonstrated in PNS for silicone-based nerve guides ( Lundborg et al., 1991, 1994), their sustained presence in the long term causes a foreign body reaction, which requires a second surgical intervention to remove the permanent scaffold ( Mackinnon et al., 1984 Merle et al., 1989). ![]()
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