Heat-induced changes in the mechanics of a collagenous tissue: pseudoelastic behavior at 37 degrees C

The outcome of many clinical heat therapies depends on the post-treatment structural integrity of the tissue. Unfortunately, there are few data on heat-induced changes in the mechanical properties of tissues due to temperature levels commonly achieved in laser, microwave, and radio-frequency-based modalities. Without such information, one cannot design optimal clinical protocols. Hence, we present new findings on the uniaxial stress strain behavior at 37 degrees C of a model collagenous tissue (chordae tendineae) both before and after thermal damage. This damage was induced via a variety of different thermo-mechanical loads: isothermal heatings at temperatures from 65 degrees C to 90 degrees C, durations of heating from 120 to 3600 s, and isotonic loads during heating from 0 to 0.65 MPa. Our data reveal that chordae exhibit pseudoelastic responses both before and after heating, but the extensibility, hysteresis, and compliance all increase with increased thermal damage. Fortunately, these complex heat-induced changes in behavior can be parameterized using a single measure of the prior thermal damage. This will clearly simplify the requisite constitutive formulations.