Facial trauma is currently affecting millions of people around the world and the incident rates are progressively increasing each year. Injury in the facial region has been associated with many factors, such as falls, assaults, and cancer. Despite the large range of options to treat facial trauma, such as autologous and allogenic tissue, xenogenic substances, and synthetic materials, the outcome nevertheless is still not ideal due to the complexity of the facial region. To date, autologous tissue remains the “gold standard” in treating facial injury; however the availability limits its use. Therefore, synthetic materials provide a promising alternative to the current problem. One of the materials that is currently being used in facial reconstruction surgery as a facial implant is expanded poly(tetrafluoroethylene) (ePTFE). However, to be used in facial implant applications, ePTFE should not only mimic the soft tissue properties but also provide a strong anchoring with the underlying bone on one side. However, current facial implants including ePTFE possess limited integration with the bone tissue, thus resulting in minimal fixation. This results from undesirable properties of ePTFE, such as being bioinert and highly hydrophobic, so lacking in biomimetic properties. Thus, the main focus in this study is to introduce functional groups onto ePTFE membranes so that it can induce hydroxyapatite (HA) formation which is a major component of bone, in order to mimic bone-tissue integration in vivo. This project is focused on the surface modification of ePTFE via radiation-induced grafting and plasma treatment-induced grafting with two different functional groups; i.e. carboxylate and phosphate groups. The goal is to optimise the grafting process with respect to improved surface properties but to maintaining the mechanical integrity of the ePTFE.
The first thorough investigation of gamma irradiation-induced grafting of acrylic acid (AA) onto ePTFE was carried out and reported in Chapter 3. Radiation-induced grafting of AA yielded graft yields of up to 40 % and resulted in reducing the hydrophobicity and increasing the water uptake, in part due to the impregnation phenomenon. Grafting of tert-butyl acrylate (tBA) was investigated, but low graft yields were obtained and this was shown not to be affected by the solvents used during the grafting process. Grafting of acrylic acid-itaconic acid (AA/ICA) monomer mixtures was investigated and yielded graft yields of up to 50 % and a high surface coverage of 84 % as determined by XPS.
The grafting of AA onto ePTFE by Ar plasma pre-treatment is described in Chapter 4. It was found that only the top surface of ePTFE was grafted leaving the bulk properties of ePTFE intact. The main contributions of the species generated in the Ar plasma to the surface modification of ePTFE were successfully elucidated and it was found that charged and neutral species played equally important roles in the introduction of functional groups onto the ePTFE surface, while other species such as vacuum ultraviolet (VUV) play only a minor role. The grafting of AA onto ePTFE yielded membranes which were highly hydrophilic and displayed a high water uptake. The process of Ar plasma-induced grafting was also applied to the phosphate containing monomers monoacryloxyethyl phosphate (MAEP) and methacryloyloxyethyl phosphate (MOEP), however, lower yields and surface coverage were obtained in these systems.
The performance of the grafted membranes under a series of mechanical test was evaluated; tensile test, half-compression test, and nanoindentation and the results of these studies can be found in Chapters 3 and 4. The mechanical properties of the gamma irradiation induced grafted membranes were shown to be affected by the radiation processes with a decreased Young’s modulus (E), ultimate tensile strength (UTS), and elongation at break (ε) compared to untreated ePTFE. The mechanical properties of the grafted membranes correlate well with chain scission of the base polymer, as well as rigidity imposed by the grafted AA. The mechanical performances were much improved for the membranes grafted by Ar plasma treatment with only minor changes in E and UTS observed. This was further supported by the unchanged compression modulus after grafting. Overall, Ar plasma treatment-induced grafting of AA onto ePTFE is the preferred option for surface modification of ePTFE membranes use as facial prosthesis as it does not compromise the mechanical properties dramatically.
In vitro studies were performed on AA grafted ePTFE membranes involving protein adsorption as detailed in Chapter 5, and mineralisation in simulated body fluid (SBF) is described in Chapter 6. Three proteins (lysozyme (Lys), bovine serum albumin (BSA) and bovine lactoferrin (Lf)) with different charges and sizes were chosen in order to determine the effect of the protein properties on adsorption onto the modified membranes. Only minor amounts of protein adsorption were observed on untreated ePTFE membranes and this was attributed to the hydrophobic nature of the membrane. All proteins were observed to adsorb to the modified membrane predominantly via ionic interactions. The protein adsorptions onto AA grafted ePTFE was influenced by various parameters including protein properties (i.e. surface charge, pI and conformation stability), AA grafting properties (graft yield, graft extent, and COOH/F), as well as the pH and media in which the test was conducted.
The mineralisation in SBF or 1.5×SBF of untreated and modified ePTFE produced with either grafting technique (gamma radiation and plasma treatment) was investigated by scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX), attenuated total reflectance infrared spectroscopy (ATR-FTIR) and XPS. Hydroxyapatite [Ca10(PO4)6(OH)2] (HA) is an inorganic component of bone. The formation of HA in SBF mimics the bone-tissue integration in vivo. ePTFE grafted with various monomers (AA, AA/ICA monomer mixtures, MOEP, and MAEP) were investigated in order to evaluate the capacity for HA mineralisation for each grafted copolymer, and in addition to determine the outcome of mineral formation for the two different grafting techniques. The mineral type and amount formed was observed to be highly dependent on the graft extent as well as the functional groups introduced. Carboxylate groups were able to induce HA and carbonated HA/mixed mineral deposit formation, whereas a surface functionalised with phosphate groups (i.e. MAEP) induced formation of a mineral with a much lower Ca/P ratio. Uniform mineral distribution was observed for carboxylate grafted samples with high graft extents, whereas heterogeneous coverage of the mineral deposit was observed on samples with a low graft extent. In addition, the formation of HA on the carboxylate grafted ePTFE of high graft extent was found to be similar to the structure reported on bioactive glass or glass ceramics. Thus, the growth of apatite on these modified ePTFE membranes indicates an improvement of bioactivity, which is a promising result for obtaining strong bone-implant integration of the grafted facial implants.
This research emphasis the successful modification of ePTFE by both simultaneous gamma irradiation and post Ar plasma treatment-induced grafting techniques. The grafting of AA significantly improved the bioactivity in vitro of the modified ePTFE membrane and the new materials show promise as facial prosthesis.