Mechanical behaviour of electron beam-irradiated bovine collagen gel under compressive and tensile loading

Connective tissues often exhibit tension-compression asymmetry (TCA), a mechanical asymmetry that results from the microstructural organisation and physicochemical interactions within networks composed of otherwise isotropic components such as collagen. This study investigated the influence of electron beam irradiation as a cross-linking treatment on the TCA of type I collagen gels (7 wt%). The gels were irradiated at doses of 1, 10 and 25 kGy and subjected to unconfined compression testing to evaluate their mechanical properties. The resulting mechanical characteristics were compared with those obtained from the uniaxial tensile tests conducted in one of our previous studies. Most compression stress-strain curves exhibited non-linear behaviour. The non-irradiated gel exhibited a linear or slightly concave trend, indicating strain softening. Increasing irradiation dose produced increasingly convex curves, corresponding to strain stiffening, that is, behaviour characteristic of native biological soft tissues. The results revealed that non-irradiated and low dose irradiated (1 kGy) gels exhibited significant TCA, whereas higher irradiation doses resulted in a more symmetrical mechanical response in tension and compression. These findings suggest that cross-linking reduces the ability of collagen fibrils to reorient during loading, thereby effectively reducing the TCA. The results further indicate that simple unconfined compression tests provide a suitable approach for characterising the effects of irradiation on the mechanical properties of collagen gels.