Biodegradation and Biocompatibility

Crystallized silicon is very nonreactive and requires extremely high temperatures to become reactive. It is also known to be a nonbiocompatible material with very poor hemocompatibility [9]. However, in 1995, Canham [10] demonstrated the bioactivity of pSi layers in simulated body fluids (SBFs). Here, the term bioactive refers to silicon as a biomaterial, which is deflned as a nonviable material intended to interact with biological systems when used in a medical device. As noted by Canham, the transition of silicon to a bioactive state via the introduction of pores is consistent with the fact that aU other natural biological materials are porous [77]. In Canham s study, 1 gm-thick pSi layers were incubated in various SBFs for periods ranging from 6h to 6 weeks. While the highly porous Si (porosity 70%) dissolved in aU SBFs tested, the silicon with medium porosity ( 70%) was slowly biodegradable. Similar to solid silicon, very low-porosity silicon was shown to be bioinert Thus, porosity is directly related to bioactivity. 

In another study, primary mammalian hepatocytes isolated from a three-month-old female Lewis rat were cultured on either untreated pSi, fetal bovine serum-treated pSi or coUagen-coated pSi [91 ]. After a 24 h incubation period, measurements of cell adhesion showed the collagen oated pSi to be by far the optimal substrate, but after five days the ceU viability in the presence of pSi was similar to that in the presence of polystyrene. The production of albumin and urea, which were respectively considered as markers of hepatocyte synthetic function and of intact nitrogen metabolic pathways, was monitored in the cell culture for 14 days and shown to be comparable to values observed in the presence of polystyrene. Taken together, these data suggest that pSi does not exhibit any significant cytotoxic effects towards primary mammalian cell lines. 

Although studies to determine in-vivo biocompatibility, biodegradation and biodistribution studies are currently under way, no results have yet been presented. As mentioned above, in vitro dissolution studies have shown that silicic acid 

As with all therapeutic applications, aseptic techniques are necessary for production. It has been shown, using in vitro cell cultures, that both bacteria and fungi will readily colonize pSi, thus establishing a need for steriHzation prior to their use in clinical appHcations [77]. Although some preliminary studies on pSi sterilization, including autoclaving, have been presented, few reported data are available. Current clinical applications, which are limited to BioSilicon carriers, use irradiated products and a sterile formulant [93]. 

The utility of genetically engineered pol3rmers for biomedical apphcations will depend, to a large extent, on their 

Resorption of subcutaneously implanted SELF films has been evaluated in rats, over the coxu se of 7 weeks (2). A collagen control and SELP-0, with a 4 1 elastin to silk ratio were both resorbed within Iweek. SELP-8 implants, with a 2 1 ratio of elastin to sfik, retained 18% of their initial mass after 7 weeks, while SELP-3 implants, with a 1 1 elastin to silk ratio retained 58% of their initial mass. SELP-4 and SELP-5, each containing eight sfik-fike blocks and 3 2 and 2 1 elastin to silk ratios, respectively, showed no evidence of resorption after 7 weeks. These studies demonstrate that the resorption of SELPs is controlled more by the length of the silk-like blocks (i.e., sequence) than the elastin to silk ratio (i.e., composition). 

Histological analysis of the implanted films generally showed a mild immune response up to Iweek, with some 

To investigate their biocompatibifity with woimded tissue, SELP-7 and SELP-5 fibrous meshes were appfied to porcine dermal wounds. No adverse effects were observed and the wounds (2x2 cm partial and fiill dermal thickness) were completely epithelialized after 14 days. Histological evaluation revealed that some SELF filaments had been incorporated into the healing tissue. Similar results were observed with SELF sponges (2). 

The immunogenicity of SLP-F and SELF copolymers has been evaluated in rabbits. Compared to hyperimmune positive control rabbit sera, the immunogenicity of all pol5rmers was found to be relatively low. In cases where an elevated antibody titer was observed (e.g., against SLP-F), the antibody response was found to be directed only at the silk-like blocks of the polymer. No response was detected against the elastin-like blocks or fibronectin cellular attachment sites (2). 

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The high level of interest in the ring-opening polymerization (ROP) of cyclic esters (lactones) stems from the biocompatibility and biodegradability of their polymers. Resorbable aliphatic... 

Although the biocompatibility and biodegradability of these materials were rapidly determined, the bioactivity of Si02-PCL hybrid materials was not studied until recently [99]. In order to provide bioactivity to Si02-PCL hybrid materials, Rhee prepared triethoxysilane end-capped poly(s-caprolactone) which was then cocondensed with tetraethyl orthosilicate and calcium nitrate via the sol-gel method. The Ca-containing PCL/silica hybrid so obtained showed in vitro bioactivity and biodegradability. The hybridization procedure between the a,co-hydroxyl PCL and silica phases was proposed to be as follows ... 

PPC has become an emerging material in the landscape of thermoplastic polymers. Most of its essential properties are known. It is biocompatible and biodegradable, which makes it attractive for packaging purposes. PPC is a material with unusual... 

More than a dozen biocompatible and biodegradable polymers have been described and studied for their potential use as carriers for therapeutic proteins (Table 13.5). However, some of the monomer building blocks such as acrylamide and its derivatives are neurotoxic. Incomplete polymerization or breakdown of the polymer may result in toxic monomer. Among the biopolymers, poly-lactide cofabricated with glycolide (PLG) is one of the most well studied and has been demonstrated to be both biocompatible and biodegradable [12]. PLG polymers are hydrolyzed in vivo and revert to the monomeric forms of glycolic and lactic acids, which are intermediates in the citric acid metabolic pathway. 

Biodegradable polymers, both synthetic and natural, have gained more attention as carriers because of their biocompatibility and biodegradability and therewith the low impact on the environment. Examples of biodegradable polymers are synthetic polymers, such as polyesters, poly(orfho-esters), polyanhydrides and polyphosphazenes, and natural polymers, like polysaccharides such as chitosan, hyaluronic acid and alginates. 

The properties of biocompatibility and biodegradation of fish atelo-collagen are suitable for the scaffold in regenerative medicine. However, these phenomena strongly depend on the procedures for cross-linking. 

Carothers and colleagues were the first to explore the ROP of lactones. Many research laboratories have now been involved in this research area. The ROP of lactones is the method of choice for the production of biocompatible and biodegradable polyesters. Lactones are ambidentate and the polymerization may proceed by either alkyl-oxygen or acyl-oxygen scission. Evidence in favor of both types of scission is reported in the literature. 

Biocompatible and biodegradable PLG nanoparticles (80-150 nm) have been prepared by following the nanoprecipitation technique [33]. The nanoparticles were coated with a 5-10 nm thick layer of PPO-PEO block copolymer or with tetrafunctional (PEO-PPO)2N-CH2-CH2-N(PPO-PEO)2 [33]. Such coats are bound to the core of the nanosphere by hydrophobic interactions of the PPO chains, while the PEO chains protrude into the surrounding medium and form... 

Clinical applications of thermosensitive hydrogels based on NIPAAm and its derivatives have limitations [121], The monomers and cross-linkers used in the synthesis of the hydrogels are still not known to be biocompatible and biodegradable. The observation that acrylamide-based polymers activate platelets upon contact with blood, together with the unclear metabolism of poly(NIPAAm), requires extensive toxicity studies before clinical applications can merge. 

Chitosan, a biocompatible and biodegradable polycationic polymer with low toxicity, is known for its swelling ability and permeation-enhancing properties and represents a polymer of choice for the preparation of microspheres intended for nasal administration [74],... 

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