Objective To study whether the polyacrylamide hydrogel,as a soft tissue filler,will under go structural changes in the physiological pH environment.Methods Different solutions with different pH values were prepard,imitat- ing the human tissies,that is 37℃ thermo—stable environment.Samples were taken form time to time for test.Samples were taken 17and 36 months after operation,respectively.Results Within the common temperature of (37±2)℃,no hydrolysis were found in different solutions,except when pH=9 and pH...