Abstract
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.
Highlights
Shape memory polymers (SMPs) are smart materials that can change their shape from a permanent state to a temporary one upon exposure to an external stimulus, such as temperature, pH variation, enzyme release, or strain
These results show that we were able to successfully tune the hard to soft segment ratios in synthesized segmented thermoplastic polyurethanes (STPUs)
We synthesized a library of STPUs and investigated the effect of the hard to soft segment ratio on the shape memory behavior, degradation rate, and thermomechanical properties
Summary
Shape memory polymers (SMPs) are smart materials that can change their shape from a permanent state to a temporary one upon exposure to an external stimulus, such as temperature, pH variation, enzyme release, or strain. Many SMPs have nonspecific shape change upon heating to body temperature.[9,11] A stable SMP system enables introduction of new cues, such as protease- or oxidatively cleavable groups, or magnetic or light-responsive components, which enable clinically defined shape changes as the healing environment changes or as a clinician applies an external stimulus at desired time points.
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