A cross-linking method for collagen-based biomaterials was developed using the water-soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide hydrochloride (EN). Cross-linking using EDC involves the activation of carboxylic acid groups to give 0-acylisourea groups, which form cross-links after reaction with free amine groups. Treatment of dermal sheep collagen (DSC) with EDC (E-DSC) resulted in materials with an increased shrinkage temperature (T5) and a decreased free amine group content, showing that cross-linking occurred. Addition of N-hydroxysuccinimide to the EDC-containing cross-linking solution (E/N-DSC) increased the rate of cross-linking. Cross-linking increased the 7, of non-cross-linked DSC samples from 56 to 73°C for E-DSC and to 66°C for E/N-DSC samples, respectively. For both cross-linking methods a linear relation between the decrease in free amine group content and the increase in Ts was observed. The tensile strength and the high strain modulus of E/N-DSC samples decreased upon cross-linking from 18 to 15 MPa and from 26 to 16 MPa, respectively. The elongation at break of E/N-DSC increased upon cross-linking from 142 to 180%. Keywords:Collagen, cross-linking, carbodiimide Received 16 March 1995; accepted 15 June 1995The high enzymatic turnover rate of collagen in the body makes stabilization of collagen-based biomaterials by chemical cross-linking methods necessary to give materials that maintain the desired mechanical properties and stability during the desired implantation period. Several cross-linking methods have been reported and in principle they can be divided into two groups. First, bifunctional reagents can be used to bridge amine groups of lysine or hydroxylysine residues of different polypeptide chains by monomeric or oligomeric cross-links. Second, amide-type cross-links can be formed by activation of the carboxylic acid groups of glutamic and aspartic acid residues followed by reaction of these activated carboxylic acid groups with amine groups of another polypeptide chain.Based on the use a4 bifunctional reagents for crosslinking, glutaraldehyde (GA) has generally been applied for the stabilization of collagen-based materials'.The use of hexamethylene diisocyanate (HMDIC) as a cross-linking agent was introduced by Chvapil et al. '. A promising class of cross-linking agents for collagen :more recently described is the polyepoxy compounds3,4.GA cross-linking involves the formation of short (branched) aliphatic chains' and pyridinium compounds5V6, while -in HMDIC cross-linking aliphatic Correspondence to Professor Dr J. Feijen. chains containing urea bonds are introduced between two adjacent amine groups7. Both GA and HMDIC cross-linking may lead to the presence of unreacted functional groups (probably aldehyde or amine groups after hydrolysis of isocyanate groups) in the collagen matrix, which can result in a cytotoxic reaction upon degradation of the collagens9'. Furthermore, it has been reported that GA cross-linked collagen-based biomaterials releases toxic GA (related...

The formation of Schiff bases during crosslinking of dermal sheep collagen (DSC) with glutaraldehyde (GA), their stability and their reactivity towards GA was studied. All available free amine groups had reacted with GA to form a Schiff base within 5 rain after the start of the reaction under the conditions studied (0.5% (w/w) GA). Before crosslinks are formed the hydrolysable Schiff bases initially present were stabilized by further reaction with GA molecules. An increase in shrinkage temperature (Ts) from 56 °C for non-crosslinked DSC (N-DSC) to 78°C for GA crosslinked DSC (G-DSC) was achieved after crosslinking for 1 h. From the relationship between the free amine group content and the Ts during crosslinking it was concluded that higher GA concentrations and longer reaction times will result in the introduction of pendant-GA-related molecules rather than crosslinks. After 24 h crosslinking an average uptake of 3 GA molecules per reacted amine group was found. No increase in the tensile strength of the materials was observed after crosslinking, which may be a result of formation of crosslinks within the fibres rather than in between fibres. Aligning of the fibres by applying a pre-strain to the samples and subsequent crosslinking yielded materials with an increased tensile strength.

The in vitro interaction of human endothelial cells (HEC) and polymers with different wettabilities in culture medium containing serum was investigated. Optimal adhesion of HEC generally occurred onto moderately wettable polymers. Within a series of cellulose type of polymers the cell adhesion increased with increasing contact angle of the polymer surfaces. Proliferation of HEC occurred when adhesion was followed by progressive flattening of the cells.Our results suggest that moderately wettable polymers exhibit a serum and/or cellular protein adsorption pattern that is favourable for growth of HEC. When serum is present in the culture medium, cell adhesion to wettable substrates seems to be influenced by the adsorption of serum proteins onto these substrates5. *Hospital Ziekenzorg, Enschede, The Netherlands. If cell adhesion is studied in serum-free medium, the absorption of protein(s) originating from the cells onto wettable substrates may be of importance6.Like other cell types endothelial cells can be cultured on glass and wettable tissue culture polystyrene, which is a glow discharge treated polystyrene. Bovine endothelial cells have been reported to grow on polyacrylamide beads' and on sulphonated polystyrene beads'. Furthermore, these cells grow upon crosslinked dextran beads precoated with collageng. The presence of collagen is also needed for growth upon the very hydrophilic polyHEMA". Poor growth of bovine endothelial cells was found upon the polyurethane Biomer, unless Biomer was precoated with gelatin or an extracellular matrix' ' Little is known, however, about the behaviour of (human) endothelial cells and other materials. We have previously presented preliminary data about the interaction of cultured human endothelial cells (H EC) with several polymeric surfaces".In the present study, we have examined the in vitro adhesion and proliferation of HEC on(to) a number of polymers with different wettabilities in culture medium containing serum. MATERIALS AND METHODS Cell culture

The degradation and the tissue response evoked by poly(1,3-trimethylene carbonate) [poly(TMC)] and copolymers of TMC with either 52 mol % D,L-lactide (DLLA) or 89 mol % epsilon-caprolactone (CL) were evaluated in vivo by subcutaneous implantation of polymer films in rats for periods up to one year. Poly(TMC) specimens were extensively degraded after 3 weeks and, as confirmed by histology, totally resorbed in less than a year. A fast linear decrease in thickness and mass without a change in molecular weight was observed. Initially an acute sterile inflammatory tissue reaction, caused by the implantation procedure, was observed, followed by a mild macrophage-mediated foreign body reaction that lasted during the resorption period of the polymer. It is concluded that in vivo, poly(TMC) is degraded via surface erosion involving cellular-mediated processes. The degradation of the copolymers was slower than that of poly(TMC), taking place via autocatalyzed bulk hydrolysis, preferentially of ester bonds. The TMC-DLLA copolymer degraded 20 times faster than the TMC-CL one. In both cases, the tissue reaction upon implantation resembled a sterile inflammatory reaction followed by a foreign body reaction that led to the polymer encapsulation. Significant mass loss was only observed for the TMC-DLLA copolymer, which underwent 96% mass loss in 1 year. When extensive mass loss started, a mild-to-moderate secondary foreign body reaction, related to clearance of the polymer fragments, was triggered. The results presented in this study demonstrate that poly(TMC) and both TMC copolymers are biodegradable and biocompatible materials, making these polymers attractive for the preparation of short- and long-term degradable devices for soft tissue engineering.

The adhesion of human endothelial cells (HEC) onto a series of well-characterized methacrylate polymer surfaces with varying wettabilities and surface charges was studied either in serum-containing (CMS) or in serum-free (CM) culture medium. HEC adhesion in CMS onto (co)polymers of hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) was found to be optimal on the moderately wettable copolymer (mol ratio 25 HEMA/75 MMA). Positively-charged copolymers of HEMA or MMA with trimethylaminoethyl methacrylate-HCl salt (TMAEMA-Cl), both with mol ratios of 85/15 and a negatively-charged copolymer of MMA with methacrylic acid (MAA), mol ratio 85/15, showed high numbers of adhering HEC. In CM, HEC adhered onto the three charged copolymers mentioned above, but neither onto the copolymer of HEMA and MAA (mol ratio 85/15) nor onto the HEMA/MMA co- and homopolymers. Complete cell spreading in CM was only observed on the positively-charged copolymers.

Bacterial collagenase was used to study the susceptibility of dermal sheep collagen (DSC) cross-linked with a mixture of the water-soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide hydrochloride and N-hydroxysuccinimide (E/N-DSC) towards enzymatic degradation. Contrary to non-cross-linked DSC (N-DSC), which had a rate of weight-loss of 18.1% per hour upon degradation, no weight loss was observed for E/N-DSC during a 24 h degradation period. The tensile strength of the E/N-DSC samples decreased during this time period, resulting in partially degraded samples having 80% of the initial tensile strength remaining. The susceptibility of E/N-DSC samples towards enzymatic degradation could be controlled by varying the degree of cross-linking of the samples. Ethylene oxide sterilization of E/N-DSC samples made the material more resistant against degradation compared with non-sterilized E/N-DSC samples. This may be explained by a decrease of the adsorption of bacterial collagenase onto the collagen owing to reaction of ethylene oxide with remaining free amine groups in the collagen matrix.

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The loading of biocompatible matrices with growth factors offers the opportunity to induce specific cell behavior. The attachment of heparan sulfate (HS) to these matrices may promote the binding, modulation, and sustained release of signaling molecules. In this study, basic fibroblast growth factor (bFGF) was bound to crosslinked collagenous matrices with and without covalently attached HS. The tissue response to these matrices was evaluated after subcutaneous implantation in rats. Attachment of HS to collagen matrices increased the bFGF binding capacity threefold and resulted in a more gradual and sustained release of the growth factor in vitro. bFGF primarily was located at the matrix margins. In vivo, the presence of HS without bFGF resulted in a transient vascularization, predominantly at the matrix periphery. Angiogenesis was further enhanced by combining HS with bFGF. In contrast to collagen-HS and collagen/bFGF matrices, collagen-HS/bFGF matrices remained highly vascularized throughout the matrix during the 10-week implantation period. In addition, these latter matrices revealed an intense and prolonged tissue response and considerably promoted the generation of new tissue. Foreign body reactions were only observed sporadically at this time interval. It is concluded that bFGF loading of collagen-HS matrices has additional value for those tissue-engineering applications that require enhanced angiogenesis and generation of new tissue.