Surface hydroxylation of styrene-butadiene-styrene block copolymers for biomaterials

Patency of heparinized SBS shunts at high shear rates

The patency of 50 cm long, 1.7 mm ID heparin-polyvinyl alcohol (PVA) coated SBS arterio-venous shunts in pigs at shear rates greater than 1000 s-1 was found to be not different from that of identical shunts coated with PVA but without heparin. This was attributed to the absence of any measureable effect of surface bound heparin on platelet related thrombus formation at high shear rates. On the other hand, platelet adhesion values determined in the absence of flow by the open static method decreased with increasing heparin content in heparin-PVA films. The low overall patency (average life of 170 minutes) of the PVA coated SBS shunts (with and without heparin) was presumed to be related to the absence of circulating heparin during surgery and the consequent presence of tissue thromboplastin or cellular debris during the immediate postoperative period. Alternative protocols are needed to test heparinized materials at low shear rates in the absence of systemic heparin to properly assess the potential thrombo-resistance of such materials.

Heparinized styrene-butadiene-styrene elastomers

A heparinized high-strength elastomer has been developed which is potentially useful as a nonthrombogenic vascular prosthesis. A surface hydroxylated styrene-butadiene-styrene (SBS) block copolymer with at least 40% extent of reaction after glow-discharge cleaning was coated with a 20% acetylated polyvinyl alcohol/heparin mixture containing glutaraldehyde and magnesium chloride. After curing at 80 degrees C for 100 min, the polyvinyl alcohol, heparin, and hydroxylated SBS were covalently bound to each other by acetal bridges. The effects of the various substrate and coating parameters were optimized to achieve very strong adhesion between the coating layer and the surface hydroxylated SBS. Heparin was not leached from the surface of the new material using 3M saline at pH 7.4 despite a detection limit of 10(-5) micrograms heparin/cm2 min. Prolonged partial thromboplastin times of greater than 1200 sec were observed (control: PTT = 120 sec). Preliminary ex vivo testing using a simple arteriovenous shunt in the leg of a rabbit showed good thromboresistance. The heparinized SBS shunt chamber remained patent for more than two hours without desorption of heparin. It was concluded that surface hydroxylated SBS heparinized by acetal coupling owed its thromboresistance to the heparin covalently bound to the surface and not to a microenvironment of heparin in solution at the blood/material interface.

The improving hematologic picture in long-term surviving calves with total artificial hearts

A coagulopathy associated with severe hemolysis was a limiting factor in obtaining long-term survivors among calves with total artificial hearts in 1969. Conversion of design from sac-type hearts to flexible diaphragm hearts, and from Silastic Dacron-fibril intimas to smooth polyurethane intimas, resulted in an abatement of the coagulopathy. In the most recent series of animals studied at this laboratory, platelet counts are normal and platelet survivals are half of normal. Plasma hemoglobins are normal. The coagulation system is still activated at specific loci within the total artificial heart, but is best compensated for in calves treated with antiplatelet drugs, having polyurethane hearts.