Heat-Induced Changes in the Mechanics of a Collagenous Tissue: Isothermal, Isotonic Shrinkage

Abstract

We present data from isothermal, isotonic-shrinkage tests wherein bovine chordae tendineae were subjected to well-defined constant temperatures (from 65 to 90°C), durations of heating (from 180 to 3600 s), and isotonic uniaxial stresses during heating (from 100 to 650 kPa). Tissue response during heating and “recovery” at 37°C following heating was evaluated in terms of the axial shrinkage, a gross indicator of underlying heat-induced denaturation. There were three key findings. First, scaling the heating time via temperature and load-dependent characteristic times for the denaturation process collapsed all shrinkage data to a single curve, and thereby revealed a time-temperature-load equivalency. Second, the characteristic times exhibited an Arrhenius-type behavior with temperature wherein the slopes were nearly independent of applied load—this suggested that applied loads during heating affect the activation entropy, not energy. Third, all specimens exhibited a time-dependent, partial recovery when returned to 37°C following heating, but the degree of recovery decreased with increases in the load imposed during heating. These new findings on heat-induced changes in tissue behavior will aid in the design of improved clinical heating protocols and provide guidance for the requisite constitutive formulations.