Compressive behaviour of a physiological tissue

Mechanical response of reconstituted, freeze-dried collagen under compressive loads

The mechanical response under uniaxial compression of collagen discs, made from freeze-dried collagen reconstituted at acid pH, was investigated at 22, 29.5 and 35 degrees C. The pH during compression was maintained at the physiological level of 7.4. The fluid transport model developed by Bert (1970) was fitted to the data. Assumptions, made in the development of the model, were tested and the range of reliability of the model is discussed. The two fitting parameters of the model were found to be stronger functions of hydration than was previously reported. The compressive response of collagen was found to be only weakly influenced by temperature. At 35 degrees C there was evidence of thermal degradation. Some creep was noted at times greater than five half-times, but no attempt was made to quantify it.

Locust oviposition: stress softening of the extensible intersegmental membranes

The mechanical properties in simple extension of the extensible intersegmental membrane of the mature female locust have been studied. The testing machine is described. The main feature of biological importance is stress softening (the Mullins Effect) of the membrane. The filler to which this phenomenon is due is probably mainly the chitin fraction. Stress softening allows the locust to extend the membranes elastically and yet require a much smaller restoring force to retract the membranes. Consequently there is no need for the locust to hold on to the sides of the hole with her ovipositor valves, nor to increase the internal pressure, to maintain the membranes extended: the weight of the abdomen is sufficient. Other features resulting from stress softening are that successive egg pods are laid deeper, and that the membrane is much toughened. The subject of toughness in biological materials is enlarged upon, and the practice of ‘conditioning’ discussed briefly