Fluctuating Curvature and Actuation in 4D Printed Asymmetric Networks by Frontal Photopolymerization

We investigate the spatiotemporal response of asymmetric polymer networks fabricated by frontal photopolymerization (FPP), a directional solidification process characterized by the emergence of conversion gradients and traveling waves, previously shown to support origami assembly. Employing a model system of UV cross-linking poly(ethylene glycol) diacrylate, we examine the frontal network conversion, the chemical exchanges during solvent development, and ensuing removal during drying. We find that the coupling of diffusion-evaporation and swelling-shrinkage processes gives rise to the formation of asymmetric ‘skin’ layers resulting in dynamic curvature fluctuations in otherwise planar beams, even in the absence of spatial patterning employed in FPP origami. Building on these findings, we demonstrate the fabrication of autonomous bistable switches and self-propulsion via a snapping instability that harness the environmental response of such ubiquitous asymmetric polymer networks.