The mechanical characterisation of a novel biomimetic hydrogel polymer.
Soft contact lenses, produced from hydrogels, are the single largest application of biomaterials. However, a number of factors that reduce the effective wear time have been identified, most notably the biological response. The monomer 2methacryloyloxyethyl phosphorylcholine (MPC) has been shown to minimise this adverse biological response, however, its effect on the mechanical properties of the contact lenses has not been investigated. This has been addressed in the present work employing a number of copolymers, principally MPC with 2-hydroxyethyl methacrylate (HEMA).
A review of the relevant literature identified both tensile and tear properties as important to the in-service performance of contact lenses. A standard tensile test methodology was adapted to examine the behaviour of dumbell specimens derived from contact lenses. Additional methodologies, which were developed, involve two distinct tear tests and a novel ultra-micro indentation technique. The latter involved the measurement of the penetration of an indenter into dehydrated disc specimens of the copolymer up to a maximum force of 7.5mN. These methodologies enabled the influence of a number of factors to be examined.
Results indicated that the manufacturing route and equilibrium water content (EWC) affected the resultant mechanical properties of the polymers. Specimens from lenses manufactured by in situ polymerisation in appropriate shaped moulds exhibited an ultimate tensile strength (UTS) of 0.7 MPa, a 23% increase on those derived from polymerised buttons. The relative concentration of specific impurities in the HEMA employed was also found to be important. The tear data was successfully modelled using generalised fracture mechanics. The ultra-micro indentation methodology identified polymerisation temperature as an important factor, the smallest range of indentations (0.95 - 1.55 gm) corresponding to a temperature of 40°C. In general, an increase in EWC resulted in a decrease in tensile and tear properties. However, further results indicated that the addition of other co-monomers to MPC-HEMA copolymers could produce a significant increase in UTS at a high EWC (80%).