Shape memory in un-cross-linked biodegradable polymers

Shape memory is extremely useful in minimally-invasive deployment of medical devices, particularly stents used in the cardiovascular system. Current stents employ Nitinol, which exhibits shape memory on the basis of a phase transformation. Cross-linked polymers may also be made to exhibit shape-memory effects, but are sometimes precluded for use in stents for other reasons. Un-cross-linked poly(lactide-co-glycolide) (PLGA) and poly(L-lactic acid) (PLLA) have been used in biodegradable stent prototypes. Their shape memory polymers (SMPs) were investigated in this paper. The three important parameters in SMPs are strain fixity, strain recovery and permanent strain. The effects of deformation temperature, deformation strain level and creeping time on those parameters were evaluated in depth. High deformation temperatures will give high fixity, high recovery, but also high permanent deformation for both PLGA and PLLA. Generally, a lower stress applied for a longer duration (the 'creeping' method) is preferred to a higher stress applied 'instantly' for achieving shape memory. These effects are explained on the basis of molecular orientation and slippage effects.