Ramped versus stepwise thermoelastic testing of latex and elastic tissues

Thermoelastic testing assesses the elastic mechanisms of polymers through measurement of the retractive force (f) of constrained samples with increasing temperature (T). f contains an entropic (fs) and an internal energy component (fe), where f= fs +fe. The elastic mechanism is normally described by the energetic contribution (fe/f). We have produced a novel thermoelastic testing device capable of performing "stepwise" or "ramped" temperature profiles and have shown excellent agreement between these two techniques for both latex and bovine elastin. Experiments on latex produced an fe/f= 0.18 +/- 0.05 (mean +/-SD, n=15, ramped protocol) that was independent of extension ratio and temperature. These results demonstrate the highly entropic elastic mechanism in this well-defined material. In agreement with previous studies, thef-T curves for elastin were non-linear, leveling off above approximately 60 degrees C. Previous studies quote fe/f for elastin within the 50-70 degrees C range where volume changes (via loss of water) of elastin are thought to be negligible. While we observed a mean fe/f for elastin of 0.18 +/- 0.04 at 70 degrees C (not significantly different from that of latex), the fe/f values for elastin were highly temperature-dependent over the entire experimental temperature range (20-90 degrees C). These observations may reflect a continuous water loss with increasing temperature in our samples. However, since thermoelastic analysis assumes that force depends only on temperature, other complicating factors must also be considered: e.g. thermal transitions such as microfibril denaturation. These complications call into question the physical meaning of fe/f reported for elastin at any temperature.