Mechanochromic Polyurethane Shape Memory Polymer for Biomedical Applications

The incorporation of functional molecular switches into smart materials imparts dynamic material properties, gaining deeper insight into how molecular structure affects the functionality of these materials and aiding the development of novel sensor devices. To enable mechanochromic biomaterials capable of sensing shape changes, we explored the incorporation of spiropyran (SP) mechanophores into a polyurethane (PUR) shape memory polymer (SMP). SPs reversibly generate variations in fluorescence and visual colors due to conversion from inactivated SP to activated merocyanine (MC) in response to force. We hypothesized that SP-containing PUR (PUR-SP) could undergo simultaneous shape and color changes. Small quantities of SP were dissolved in control PUR solutions with different hard-to-soft segment ratios, and PUR-SP films were formed by solvent-casting. The effect of SP incorporation on material properties, including mechanical, shape memory, thermal, and cytocompatibility, was studied. Mechanochromic behavior was analyzed by straining the films and imaging using a camera and fluorescence microscopy. We also employed a previously developed bacterial protease-responsive PUR SMP to confirm that SP incorporation enables simultaneous shape and color changes in the presence of bacteria. Strained samples showed increased fluorescence (up to 56%, p < 0.05), which was reversed upon shape recovery. Mechanochromic behavior was affected by the hard-to-soft segment ratio of the PUR, SP concentration, and strain percentage. Bacteria-responsive PURs with SP showed reduction in fluorescence and complete biofilm removal after incubation with Staphylococcus aureus for 24 h, which conveyed the potential to use SP in PURs as a molecular force probe with color-based bacteria detection. This technology could be expanded to include a range of other stimuli-responsive functionalities in future work to enable shape and color changes based on environmental cues.