Phase separation control of PDMS/PNIPAAm IPNs and the effect on drug release

Poly (dimethyl siloxane) (PDMS) possesses good mechanical properties, oxygen permeability and optical transparency, making it an excellent candidate biomaterial, including for ophthalmic purposes. PDMS, however, is a hydrophobic polymer with poor water uptake and surface wettability, limiting its overall usefulness. Therefore, combination with one or more hydrophilic polymers, in which the immiscible materials form a stable interpenetrating polymer network (IPN), has the potential to improve the utility of PDMS in biomaterials applications. In the current work, IPNs consisting of PDMS and poly(N-isopropylacrylamide) (PNIPAAm) were synthesized. Three separate approaches were employed to generate IPNs possessing controllable and tunable phase separation, interfacial properties, and drug release kinetics. First, IPNs were synthesized with long chain PDMS prepolymers (LC-IPN), to create a looser host network capable of supporting the integration of a more interconnected PNIPAAm guest phase. In the second approach, NIPAAm was copolymerized with Tris(trimethylsiloxy) silyl-propyl methacrylate (TRIS) to compatibilize the silicone and hydrogel interface. Finally, a methacrylate-containing silicone-coupling agent, methacryloxypropyltriethoxysilane (MATO), was used to generate a PDMS host network that was more compatible with the guest PNIPAAm phase. LC-IPNs yielded materials with more pronounced temperature-induced phase transition properties compared with IPNs previously prepared. TRIS copolymerization and MATO crosslinking generated IPNs with decreased domain sizes and increased transparency. Release of the model drugs dexamethasone, timolol maleate, riboflavin and lysozyme were examined from the various IPNs. The results demonstrated that controlled release properties were possible, depending on the nature of the drug. Therefore, by varying the host and guest compositions to balance drug solubility and diffusion characteristics, it is possible to tune IPN release properties to meet the demands of the desired application.