Polyether polyurethanes: biostable or not?

Certain polyether polyurethanes have been shown to be biostable in long-term implant studies. Others retain good bulk properties, but have been shown to develop cracks on their tissue contacting surfaces. Two cracking mechanisms have been identified, in vivo stress cracking and metal ion oxidation. Stress cracking is the result of an interaction between the in vivo mammalian environment and residual stress (strain) in the implanted polymer. Mild autooxidation can be initiated by stress cracking. More extensive autooxidation can be initiated and propagated by corrosion of metallic device components, especially the corrosion products of cobalt. Both mechanisms are controllable, thus, do not necessarily preclude the use of polyether polyurethanes in implantable devices.