Surface copolyurethanes

The surface morphologies of these block copolyurethanes differ from their bulk morphologies (4-6). Because the surface controls the interaction of a vascular implant with blood, the surface structure and its relation to the bulk structure of the same material was determined also. Originally, ESCA was explored to study the surface structure because the depth of penetration was within the first 100 A. The low surface depth of penetration and subtle shifts in binding energies that result in peak splittings of the elemental spectra appeared to make this an attractive method to study the chemical and bonding environments of the elements (40). 

In summary, the surface chemical and morphological structures of block copolyether-urethane-ureas may be determined by ESCA and FTIR coupled with internal reflectance techniques to probe the surface and bulk structures. These ESCA and FTIR data are being used to model the domain-interface structure of these copolyurethanes and their interaction with blood protein. 

It has been found that the relationship shown in Figure 8.10(b) can be used to measure strain in a wide variety of situations. It is known that polydiacetylenes absorb visible light very strongly, and so for the bulk copolyurethane the spectrum is obtained only from material in the surface regions. Hence any strain measurements will be only for surface material. Moreover, since it is possible to focus the laser beam in the spectrometer to a spot of the order of 2 pm in di2uneter, it is possible to obtain considerable spatial resolution. 

Various examples of using these materials for surface strain mapping have been presented in a recent publication [69]. The determination of stress concentrations around defects such as holes or notches in a deformed plate of the copolyurethane is shown in Figure 8.11. A circular hole and a notch of predetermined dimensions were accurately machined into a 3 mm thick specimen of the copolymer. The specimen was deformed in tension in the Raman spec-... 

Silastic Rubber, fluoroelastomer, hydrocarbon rubbers and copolyurethane materials have been fabricated into vascular prostheses (27-35). The results with latex rubber. Silastic Rubber, fluorocarbon elastomer, and hydrocarbon rubber were not good. Failure in most instances occurred within 24 hours of implantation in dogs. However, these surfaces are relatively thrombogenic. In addition, these grafts were much less compliant than the natural artery. 

At the time of this writing, the compliant copolyurethane vascular prostheses are still patent at 15 months implantation. One prosthesis removed at this time showed minimal hypertrophy. Thus, to achieve success in small diameter vascular prostheses, one must balance the surface chemical and physical properties of the material for blood compatibility and the mechanical properties for matching the compliancy of the natural vessel. 

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