Thermoplastic elastomers for small-diameter compliant grafts

The development of satisfactory replacement prostheses for small diameter natural vessels (less than 7 mm) still appears to be an unsolved problem. It has been suggested that mismatch in compliance between the graft and the natural vessel is one of the most important causes of the occlusion of the graft itself. The authors' approach was to synthesize potentially hemocompatible thermoplastic elastomers whose elastic properties could be deliberately varied by modifying their chemical structure. For this purpose polyurethanes were synthesized with both traditional polyether or polyester blocks of different lengths and with new soft and hard blocks such as polyisobutylene and poly 7-oxabicyclo heptane. The authors have also synthesized new comb-like thermoplastic elastomers obtained by reacting styrene-butadiene two-block living polymeric chains with ethylene-maleic anhydride copolymer (EMAC). With such materials, grafts with diameters ranging from 2.5-5 mm were produced using traditional techniques described in the literature. The mechanical properties of these vascular grafts were compared with those of the natural vessels, using various techniques. Prostheses that posessed mechanical properties most similar to those of the natural vessels were implanted in the femoral artery of dogs. Patency was checked daily by means of Doppler measurements of the distal femoral pulses. Some of the implanted grafts remained patent until the autopsy of the animal (45 days) and others are still in place.