Biostable Segmented Thermoplastic Polyurethane Shape Memory Polymers for Smart Biomedical Applications

The exceptional tunability of chemical, mechanical, and shape memory properties of segmented thermoplastic polyurethanes (STPUs) makes them promising materials in a wide range of biomedical applications. STPU-based shape memory polymers (SMPs) that stably maintain their chemical, thermomechanical, and shape memory properties even after implantation could provide a reliable platform for controlled environmental response. For example, materials could be actuated at set time points to deliver bioactive agents, or cleavable compounds could be incorporated into a stable SMP that responds to wound signals for use in diagnosis. To this end, a library of STPUs with varying ratios of hard to soft segments was synthesized and characterized. It was found that using polypropylene glycol as the soft segment and triethylene glycol as the chain extender with hexamethylene diisocyanate induced sufficient phase separation in STPUs to provide a shape memory system. The polymers had more than 90% shape recovery ability and high enough transition temperatures (56–58 °C under dry and 44–51 °C under wet conditions) to enable maintenance of their temporary shape at body temperature. We tuned chemical, mechanical, thermal, and shape memory properties by changing the composition and characterized their stability in degradation media (more than 94% mass remaining at 40 days under accelerated conditions for polymers with higher hard segment ratios). The cytocompatible STPUs were highly stable (>90%) in their primary and secondary geometries in in vitro degradation media, indicating that they would stay intact after implantation. Then, the materials were actuated by heating at user-defined time points, upon which they returned to their primary shapes. As a proof of concept, we incorporated magnetic nanoparticles into this system to provide a magnetically actuated SMP that induced 44% shape recovery only after 5 min of exposure to a magnetic field. This system provides a platform for future generations of STPUs that include other cleavable or environmentally responsive components so that wound or clinician-controlled signals can act as external stimuli to cause shape and morphological changes.