Abstract
In search for appropriate materials of potential use to relieve injured articular cartilage, we explored copolymers from HEMA (2-hydroxy-methyl-methacrylate) and MMA (methyl-methacrylate). Such copolymers can be synthesized by thermal or photochemical induced polymerization reaction. The water uptake by swelling to homogeneous hydrogels can easily be controlled by varying the mixing ratio of the hydrophilic (HEMA) and hydrophobic (MMA) monomer, and the nature and amount of added crosslinker (typically EGDA, ethyleneglycol-dimethacrylate). Essentially the same variables strongly influence the mechanical properties, i.e. modulus (stiffness), relaxation response, as well as tribological behavior.
The polymer samples were engineered in molds from degassed formulations containing various amounts of HEMA and MMA, 10 % deionized water, and 0.01 % AIBN for thermal polymerization (12 h @ 70°C) or 0.5 % Darocur 1173 (2-hydroxy-2-methyl-1-phenyl-propane-1-one, for photopolymerization, 360 nm UV radiation, 5 to 7 min, sample thickness up to 5 mm). The samples were immersed in saline buffer after curing to allow free swelling to the equilibrium water content (EWC). Subsequently, samples were mechanically and tribologically tested. The mechanical moduli were determined at different strains and as a function of MMA content using a Zwicki Z5.0 (Zwick-Roell, Ulm, Germany). Tribological versus cartilage tissue was performed on an in-house-built pin-on-plate setup. Flat polymer samples were mounted and tested versus fresh porcine osteochondral grafts, harvested from humeral heads.
Mechanical testing revealed that the elastic modulus of pHEMA can be tuned as a function of MMA (0–50%) with 1 to 2 % bifunctional crosslinker to values ranging between 0.5 to 50 MPa, and corresponding water content of 40 to 10 % (decreasing with increasing MMA content). Friction measurements revealed a very low friction coefficient of around 0.02 for pHEMA-cartilage pairings. The values are 2–5 fold smaller than typical values of CoCrMo or UHMWPE versus cartilage.
Hydrogels from HEMA and MMA as main constituents are already rather well known for their biocompatibility. Knowledge of the dependence of e.g. the mechanical properties from chemical composition and polymer network structure makes this system ideal to design anisotropic specimen with controlled macrostructure to be used for temporal or permanent implants.
