AUTOCLAVABLE HIGHLY CROSS-LINKED POLYURETHANE NETWORKS IN OPHTHALMOLOGY

Highly cross-linked aliphatic polyurethane networks have been prepared by the bulk step reaction of low molecular weight polyols and hexamet hylenediisocyanate (HDI). These polyurethane networks are optically tr ansparent, colourless and autoclavable amorphous glassy thermosets, wh ich are suited for use in ophthalmic applications such as intraocular lenses and keratoprostheses. The properties or these glassy polyuretha nes, obtained from the reaction of the low molecular weight polyols tr iisopropanolamine (TIPA) or tetrakis(2-hydroxypropyl)ethylenediamine ( Quadrol) and HDI in stoichiometric proportions, have been investigated in more detail. The glassy Quadrol/HDI-based polyurethane exhibits a reduction in ultimate glass transition temperature from 85 to 48 degre es C by uptake of 1% of water, and good ultimate mechanical properties (tensile strength 80-85 MPa, elongation at break ca 15%, modulus ca 1 .5 GPa). IR spectra of these hydrophobic polyurethane networks reveale d the absence of an isocyanate absorption, indicating that all isocyan ates, apparently, had reacted during the crosslinking reaction. The bi ocompatibility could be increased by grafting tethered polyacrylamide chains onto the surface during network formation. These transparent cr oss-linked polyurethanes did not transmit UV light up to 400 nm, by in corporation of a small amount of the UV absorbing chromophore Coumarin 102, and could be sterilized simply by autoclaving. They were implant ed in rabbit eyes, either in the form of small circular disks or in th e form of a keratoprosthesis (artificial cornea). It was shown that th e material was well tolerated by the rabbit eyes. Serious opacificatio n of the cornea, a direct result of an adverse reaction to the implant , was never seen. Even 1 yr after implantation of a polyurethane kerat oprosthesis the eye was still 'quiet'.