Prosthetic hydrogel ureters, made from poly(2-hydroxyethyl methacrylate), pHEMA, have demonstrated resistance to calcification during long term implantation in the urinary tract of dogs (1). Recent data suggests that impurity quantities of an ionizable comonomer, methacrylic acid, MAA, incorporated into the structure of the gel, may play a critical role in the calcification resistance of these materials. The ionic constituent of MAA in pHEMA leads to complex physicochemical behavior of gels in environments where the pH, ionic strength, and other aspects of the electrochemical solvent composition may vary. Specifically, small changes of pH or electrolyte near a critical value can produce massive changes of physical properties, such as the swollen volume or water content of these gels. Urine exhibits a changeable composition, and importantly, the critical pH and electrolyte content of pHEMA/MAA copolymers is within the common range of urine compositions. The physiochemical changes of these copolymers were exploited in the design of a prosthetic material for use in the urinary tract.