Polymeric surface modification has become an important technique over the last few decades in many areas, including medicine. It provides improved surface properties for specific applications without altering the bulk of the material. Graft copolymerisation is one technique which allows an existing polymer to covalently immobilise different monomers with or without new functional groups. Some polymeric implants are used as hard tissue substitutes for bone. However, the surface of the implant is often deficient in binding with hard tissue. Since phosphates are one of the building blocks of hydroxyapatite [Ca10(PO4)6(OH)2] or HAP, which is the main inorganic composition of bone, phosphate groups may be utilised on the surface of polymeric implants. This has the possible effect of directing bone formation at the material/tissue interface. The radiation-induced grafting of MOEP (methacryloyloxyethyl phosphate) onto ePTFE (expanded PTFE) has been carried out by a simultaneous irradiation technique in the presence of solvents. Two solvents and one mixed solvent system (in this study: methanol, MEK and a mixture of methanol, water and DCM) were used to dilute MOEP under various monomer concentrations and experiments were carried out using two different dose rates. The modified surfaces were characterised using the weight increase, XPS, FTIR-ATR, SEM, and contact angle measurement. The degree of grafting was found to be proportional to the monomer concentration but did not depend on the dose rates used. However, the grafting yield was strongly dependent upon the type of solvent used. The grafting decreased in the order: mixed solvent system ยป MEK > methanol. In addition, different morphologies of grafting were observed from SEM images depending on the solvent used. The samples prepared in methanol had a smooth morphology, whereas the samples obtained in MEK with a monomer concentration of greater than or equal to 10% w/v and with a mixed solvent system, had globular morphologies. A combination of in vitro tests onto three types of grafted membranes was performed (in this study: simulated body fluid (SBF), protein and cell attachment tests). Structure, composition and morphology of the calcium phosphate growth on the samples after 7 days of immersion in SBF were studied by SEM/EDX and FTIR-ATR. It was found that the growth of CaP onto the sample with low surface grafting (24%) and with a smooth morphology, was carbonated HAP. With the samples with higher grafting and globular morphologies, the inorganic minerals formed were less and had not HAP stoichiometric composition but were presumably Brushite and Octacalcium phosphate (OCP). The amount of protein adhesion in BSA solution (with 24 hours of immersion) and serum solution (with 1 hour of immersion) were determined gravimetrically and by XPS. In addition, the protein layer was investigated using FTIR-ATR. It was found that protein adsorption was highest on the surface with high grafting (100%) possibly due to the electrostatic interaction. Human osteoblast-like cell attachment and cell morphology (both after 3 hours) were examined by cellular protein synthesis assays and SEM. Cell attachment and morphology were also better on the samples with high grafting yield compared to that of low grafting, which follows the results of protein adsorption. The different in vitro tests do not agree, however, in all cases better bioactivity was found for grafted samples than that for untreated samples. This research emphasises the importance of using a series of in vitro tests to evaluate the bioactivity of materials. Although it is not clear as to what the optimum grafting conditions are at this stage, grafting of phosphate-containing monomer onto ePTFE has significantly improved the bioactivity in vitro.
| Identifer | oai:union.ndltd.org:ADTP/264883 |
| Date | January 2003 |
| Creators | Suzuki, Shuko |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Shuko Suzuki |
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