a thermoplastic polyurethane having prolonged biostability has tertiary amino groups in the polymer chain . protonation of the amino groups with an aqueous acid causes the polymer to absorb up to 1 , 700 % by weight of water and swell . the protonated amino groups form a complex with an antithrombogenic agent to give an antithrombogenic polyurethane . the invention includes a shaped article of the polyurethane .