the specification teaches thromboresistant compositions made of suitable biologically compatible plastic materials and a substance having a negative charge , e . g ., graphite , incorporated therein in the framework of the lattices of the plastic , for use in prosthetic vascular devices intended to be permanently grafted in body loci wherein use of a thromboresistant prosthetic is desirable . an exemplary combination is polytetrafluoroethylene and graphite made into suitable structures for use as prosthetic arteries , veins and heart valves is taught .