The effects of calcification on the structure and properties of biomedical polyurethanes

The formation of calcium salt deposits has been shown to cause premature failure of biomedical implant devices in several clinical cases. Although several mechanistic theories exist regarding the causes of calcification, it was necessary to consider the nature of the interaction of the calcium salts with the implant material to eventually understand how to prevent calcification. This study examined the effects of blending calcium chloride into the biomedical polyurethanes Biomer (Ethicon, Inc.) and Pellethane (Dow Chemical Co.) on tensile mechanical and molecular orientation properties. In addition, the pure polyurethanes were exposed to an aqueous solution of calcium chloride in physiological concentrations. Fracture properties and molecular orientation associated with the fracture phenomenon were measured using FTIR dichroism and a specially designed tensile stretching device. This device allowed for simultaneous measurements of stress, strain and orientation. The results of this study indicated that calcium chloride salts caused reductions in fracture toughness and stress distributions surrounding cracks. The effects were attributed to salt saturation of the soft segment matrix with calcium chloride followed by disruption of the domain structure at higher salt concentrations. The uptake of calcium chloride from aqueous solution showed changes in mechanical properties which were mediated by the soft segment matrix. In mechanical strain orientation studies, the domain structure of Biomer appeared to have its long axis in a direction perpendicular to the chain axes. Upon drawing, the domains oriented in the draw direction, causing the chain axes to display a negative orientation. Pellethane domains did not display this behavior because of less extensive hydrogen bonding in the domains.