Effect of water content on transparency, swelling, lidocaine diffusion in p-HEMA gels

The dependencies of transparency, swelling and lidocaine diffusion on the water content in the polymerizing mixture are investigated for p-HEMA gels. The water fraction in the polymerizing mixture (WCIPM) is varied from 20 to 60%. The results show that p-HEMA gels with WCIPM less than 50% stay transparent, while those with larger WCIPM lose transparency to an extent that is proportional to the water content in excess of 50%. Also the gels with WCIPM > 50% are transparent above a wavelength of about 1000 nm, which suggests that these gels contain roughly pores that are a few microns in size. The gels with excess water also show a porous morphology in SEM studies. The gels with WCIPM less than 50% show almost identical microstructure and similar swelling and drying behavior. The partition coefficient of lidocaine partition coefficient in p-HEMA gels depends strongly on the aqueous drug concentration, which suggests binding of the lidocaine molecules on the gel. The binding of the drug to the p-HEMA matrix can be described by Langmuir isotherm and a diffusion model that includes binding of the drug to the polymer accurately models the drug uptake dynamics. The results of the model show that the diffusivity of the drug in the gels is independent of the water content for WCIPM < 50% and becomes larger on crossing the critical WCIPM value of 50%.