Surface Modification of an Expanded Poly(tetrafluoroethylene) Implant

Facial reconstructive surgery involves prosthetic implants. The success of these implants depends on the mechanical, architectural, and surface properties of the implant material. The mechanical and architectural properties of the material must mimic the tissue of which the implant replaces. Materials with good bulk properties rarely possess the appropriate surface characteristics required for good biocompatibility. One problem that can occur at the interface between a biomaterial and the living tissue is the formation of fibrous connective tissue. When connective tissue forms it separates the implant from the bone; causing poor osseoinduction. The ultimate ramifications include pain, loosening of the implant, damage to the local tissue, and revision surgery. Surface grafting techniques are common in the electrochemical industry and separation science; however, it is only recently that these techniques have been investigated in the area of biomaterials. Changing the surface properties of a biomaterial has the potential to improve biocompatibility, bioactivity, and most importantly, healing time. Radiation-induced grafting of methacryloxyethyl phosphate (MOEP) and monoacryloxyethyl phosphate (MAEP) onto expanded poly(tetrafluoroethylene) (ePTFE) was achieved by simultaneous and post irradiation techniques in the presence of solvents and solvent mixtures. The modified surfaces were characterized using weight increase, XPS, ATR-FTIR, SEM, ToF-SIMS, and contact angle measurement. Grafting yield was found to be solvent dependent; displaying concentration dependent increases for grafting of MOEP in methanol and MOEP in MEK. For grafting of MAEP in various solvent systems low grafting yield by weight was observed. However, surface coverage of the copolymer was greatest for MAEP in water, followed by MOEP in MEK as determined by XPS. All grafting systems produced improved wettabilities, however, none of the modified surfaces produced were hydrophilic. Moreover, different solvent dependent grafting morphologies were observed from SEM micrograph images. The distribution of the graft co-polymer, both with respect to lateral distribution and depth into the membrane, was investigated using ToF-SIMS and MRI, respectively. The chemical nature of the homopolymers and graft copolymers were examined by ICP-AES, XPS, ATR-FTIR, and ToF-SIMS, concluding that the grafting process induces a number of events including: phosphate cleavage, cross-linking, and branching. Biocompatibility of the modified ePTFE was tested by in vitro protein adsorption and macrophage response studies. In vivo implantation of a biomaterial causes initially protein adsorption on the materials surface. Which proteins preferentially adsorb is dependent on the nature of the surface. It is the proteins on the surface to which cells respond: including macrophages that can signal either a normal healing response or a pro-inflammatory reaction. Proteins adsorbed onto surfaces incubated in foetal bovine serum solution were determined by surface-MALDI-ToF-MS. Two samples produced spectra with peaks of changed relative intensity and width compared to BSA at m/z = ~ 45 kDa. This peak potentially corresponds to four pro-inflammatory indicated proteins. Macrophage response was analysed by two methods using FACS analysis of RAW264.7/ELAM-eGFP cells and ELISA TNF-α assays. These two analytical methods correlated well and showed high macrophage activation for the MOEP grafted samples. In addition, SEM was used to examine macrophage cell morphology. One MAEP grafted sample was found to produce a very low macrophage response. Moreover, this sample was one of those identified in the protein adsorption studies as preferentially adsorbing ~ 45 kDa protein(s), thus making this a key sample for future investigations. This work has provided grafting methods that reproducibly modify the surface properties of a current ePTFE implant. Control over position of the graft-copolymers on the membrane has been demonstrated. A thorough description of the chemistry of the homopolymers and graft-copolymers has been achieved. Furthermore, this study has identified a graft copolymer system that has potential as biomaterial implant material owing to its good biocompatibity