The increased use of heating in many medical specialties is driven by the availability of new devices, not an understanding of the underlying physics. Whereas most prior studies have quantified material response characteristics before and after thermal damage, the goal of this work is to describe changing response characteristics during a particular class of thermal damage tests-biaxial isometric tests on collagenous biomembranes. Evolution equations are derived/postulated that provide a good fit to data but more generally provide direction for future work.