Biomaterials used as implants and in various devices must exhibit long-term (years) compatibility with the physiological environment, including blood, and additionally must also remain stable to perform mechanical functions, excepting applications where biodegradation is required. This paper focuses on problems and challenges of polymeric materials in contact with blood in the following categories: (1) artificial heart valves, (2) cardiovascular assist devices and artificial hearts, (3) vascular prostheses, and (4) the biological evaluation of materials prior to their human use, especially with respect to species related hematological differences of experimental animals. Besides thrombosis (which is the most obvious consequence of incompatibility), the calcification of chemically treated tissue prostheses as well as synthetic elastomers used in many cardiovascular devices is discussed in terms of biochemical and physico-chemical parameters together with its significance in long-term (years) implant applications. Complement activation brought about by contact of blood with foreign surfaces has received less than deserved attention in the evaluation of biomaterials and devices, despite the potentially serious problems. Relative ignorance in selecting appropriate animals for the biological evaluation of biomaterials whose hematological profiles and behavior of platelets, red and white cells to trauma and response to foreign surfaces differ decisively from those of humans, often leads to less than meaningful predictions for eventual clinical uses. The state-of-art realities are examined in conjunction with medical, societal, ethical, and economic boundaries.
The long-term biocompatibility and physical performance of polymeric materials in the physiological environment depend both on adsorption and absorption processes. While the former has received significant attention in the literature, the latter has not been sufficiently appreciated. Accelerated testing of prosthetic devices in the wrong media and temperatures yield misleading information as exemplified by pumping bladders of heart assist devices and heart valves. Although glutaraldehyde-treated porcine heterograft heart valves performed better than expected in humans over a period of several years, physical degradations that have been observed may be associated with the breakdown of the cross-links. Appropriately selected smooth-surfaced biomaterials and hydrogels are far better suited for temporary blood contacting prosthetic applications such as left ventricular heart assist devices (LVADs) than polyester flocked fibril surfaces that result in the deposition of a thick layer of fibrin/cellular mesh with clot-like morphology with inherent dangers of loss of polyester fibrils, and the shedding of clots as the result of cyclic flexing of the pump bladders. With the proliferation of various medical devices, the possibility of increased litigations with secondary and tertiary liabilities involving not only physicians, surgeons, and device manufacturers, but also suppliers of materials and components, it is essential to select pertinent rather than complex test procedures.
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