Conversion and temperature profiles during the photoinitiated polymerization of thick orthopaedic biomaterials

Polymerization of a tetrafunctional monomer was investigated under a variety of photoinitiation conditions to assess the ability to form thick materials in situ for orthopaedic applications. The major biological concerns include local cell and tissue necrosis due to the polymerization exotherm and low conversions at greater depths due to light attenuation through thick samples. Experimental results indicate that depth of cure and temperature rises are controllable by altering the photoinitiator concentration, initiating light intensity, and type of photoinitiator. For example, no measurable conversion was detected at a 1.0 cm depth when polymerization was initiated with 1.0 wt% DMPA and 100 mW/cm2 ultraviolet light, whereas 40% conversion was obtained when the initiator concentration was lowered to 0.1 wt%. This conversion was further increased to 55% when a photobleaching initiator system was employed. At the highest rate of initiation studied (i.e., 1.0 wt% DMPA irradiated with 100 mW/cm2 ultraviolet light), a maximum temperature of 49°C was reached at the sample surface; however, this temperature dramatically decreased to 33°C when the light intensity was decreased to 25 mW/cm2. Finally, dual initiating systems that synergistically combine the advantages of photoinitiation and thermal initiation were investigated.