Visual stress sensing using dynamic porosity in shape memory thermoplastic elastomers

Shape memory polymers (SMP) have been extensively implemented for applications ranging from biomedical devices and soft robotics to deployable structures. However, these materials mostly rely on heat for shape recovery and programming. Here, we introduce a new class of smart SMPs with pressure-responsive characteristics based on dynamic porosity-an ability to configure a pore size within a solid- that allows for optical monitoring of the stimuli. Introducing pores in a structure reduces the relative density of the material and introduces interfaces resulting in a reduction in optical transparency of the bulk through light scattering. We show that utilizing the dynamic porosity, a macroporous film transitions instantly from an optically opaque (25% transmittance) to a transparent (96% transmittance) state by applying an out-of-plane contact pressure of 3.8 MPa at room temperature. The new SMP also presents an anomalous `cold' shape recovery of the pressure-activated films through repeated in-plane stretching that renucleates the pores and transitions the material back to the initially opaque state. The presented concept ushers a new class of responsive materials that interlace optical sensitivity features with micro- and nanoscale material deformations, establishing new research opportunities in the field of SMPs.