The biocompatibility and the biodurability of silicones is a result of various material properties such as hydrophobicity, low surface tension, high elasticity and chemical and thermal stability. A variety of biomedical implants employ an inflatable silicone rubber balloon filled up with a saline solution. Commercial examples of such a system are silicone rubber breast implants, tissue expanders and gastric bands for obesity control. Despite the advantages, saline filled silicones systems still have a certain set of challenges that need to be addressed in order to improve the functionality of these devices and validate their use as biomaterials. The central goal of this research is to identify these concerns, design solutions and provide a better understanding of the behavior of implantable silicones.
The first problem this research focuses on is the quantification and identification of the low molecular weight silicones that are inadequately crosslinked in the elastomeric matrix and therefore can be leached out by solvent extraction. We have developed an environmentally friendly pre-extraction technique using supercritical CO2 and also determined the exact nature of the extractables using Gas Chromatography.
We have also attempted to address the issue of an observed loss of pressure in the device during application by studying the relaxation behavior of silicone elastomer using Dynamic Mechanical Analysis and constructing long-term relaxation master curves. We have also developed a technique to develop highly hydrophobic fluorinated barrier layers for the silicone in order to prevent diffusion of water vapor across the walls of the implant. This involves a hybrid process consisting of surface modification by plasma technology followed by two different coating formulations. The first formulation employed UV curable fluorinated acrylate monomers for the coating process and the second was based on Atom Transfer Radical Polymerization (ATRP) to generate a fluorinated coating covalently grafted on the silicone surface in the form of dense polymer brushes.
The research also attempts to validate the use of sterilization of the implant with gamma irradiation by comprehensively reviewing the existing literature and then summarizing the effects of gamma irradiation on linear, cyclic and crosslinked silicones. We have predicted a model describing the effects of irradiation and supplemented that with data in the laboratory.
Finally we have investigated the use of biological enzymes as alternate catalyst systems for the synthesis of silicone copolymers. We have demonstrated the use of the enzyme Lipase (CALB), as a catalyst for the synthesis of fluorosilicone copolymers containing ester and amide linkages.