Kinetics of thermal damage to a collagenous membrane under biaxial isotonic loading

Prior isothermal uniaxial isotonic tests on tendons reveal that higher temperatures hasten the rate of thermal denaturation whereas larger mechanical loads delay it; moreover, these findings suggest a time-temperature-load equivalency whereby similar levels of denaturation, as reflected by tissue shrinkage, can be attained via many combinations of heating time, temperature level, and mechanical loading. Yet, most tissues and organs experience multiaxial loads in vivo, and their microstructure differs significantly from that of tendons, thus, we must also evaluate the effects of multiaxial stresses on the kinetics of denaturation in other tissues. In this paper, we describe a new experimental approach for performing isothermal biaxial isotonic tests on thin sheet-like specimens and we report effects of various thermomechanical loads on the rate and amount of multiaxial shrinkage of bovine epicardium. Consistent with uniaxial studies, epicardial shrinkage generally increased sigmoidally with heating time, and a characteristic heating time revealed increases in the rate of shrinkage with higher temperature and decreases with larger biaxial loads. Although this characteristic time exhibited an Arrhenius-type character, time-temperature-load equivalency was not obtained when scaling time with this metric. General multiaxial thermomechanics is thus too complex to explain via a simple extension of uniaxial findings on tendons and there is a pressing need for more data and an appropriate theoretical framework.