The Role of Amide Groups in Vinyl Monomers Containing Siloxane Groups for Highly Oxygen Permeable Hydrogels

In order to improve the hydrophilicity of silicone-containing monomers, N-2-(N′,N′-dimethylcarbamoyl)ethyl-N-[3-tris(trimethylsiloxy)silylpropyl]acrylamide (SiDAAA) and N-2-(N′,N′-dimethylcarbamoyl)ethyl-N-{3-[methylbis(trimethylsiloxy)silyl]propyl}acrylamide (SiDAAA2) were designed. Although these two monomers had complicated side chain structures, these showed similar radical polymerizability (from 2.6 × 104 to 3.8 × 104 of Mn, and from 87 to 90% of conversion) as conventional silicone-containing monomer 3-tris(trimethylsiloxy)silylpropyl methacrylate (TRIS) due to the acrylamide backbone. Moreover, the additional amide group in those monomers contributed to improved miscibility during copolymerization with N-(hydroxyethyl)acrylamide (HEAA) and 2-hydroxyethyl methacrylate (HEMA), which was determined by light transmission observation at 88% transmittance using a SiDAAA/HEAA (50/50) hydrogel. In contrast, TRIS/HEAA was not miscible, and TRIS/HEMA (60/40) gave only 26% transmittance. The glass-transition temperatures of poly(SiDAAA) and poly(SiDAAA2) were 61 and 28 °C, which were much higher than that of poly(TRIS) at 0.8 °C. The oxygen permeability of the hydrogels varied with the copolymer composition, mainly due to the siloxanyl group contents. In particular, permeability coefficients of more than 100 Barrer (1 Barrer = 10−11 (cm3 O2) cm cm−2 s−1 mmHg−1) which is sufficient oxygen permeability for corneal safety in the case of contact lens applications, can be obtained at lower than 60% water content. The biocompatibility of these hydrogels is also discussed.