Soft-elasticity in monodomain liquid crystal elastomers (LCEs) is promising for impact-absorbing applications where strain energy is ideally absorbed at constant stress. Conventionally, compressive and impact studies on LCEs have not been performed given the notorious difficulty synthesizing sufficiently large monodomain devices. Here, we use direct-ink writing 3D printing to fabricate bulk (>cm³) monodomain LCE devices and study their compressive soft-elasticity over 8 decades of strain rate. At quasi-static rates, the monodomain soft-elastic LCE dissipated 45% of strain energy while comparator materials dissipated less than 20%. At strain rates up to 3000 s⁻¹, our soft-elastic monodomain LCE consistently performed closest to an ideal-impact absorber. Drop testing reveals soft-elasticity as a likely mechanism for effectively reducing the severity of impacts – with soft elastic LCEs offering a Gadd Severity Index 40% lower than a comparable isotropic elastomer. Lastly, we demonstrate tailoring deformation and buckling behavior in monodomain LCEs via the printed director orientation.

Auxetic materials have negative Poisson’s ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions. The auxetics community has long sought synthetic molecular auxetics – non-porous, inherently auxetic materials which are simple to fabricate and avoid porosity-related weakening. Here, we report, synthetic molecular auxeticity for a non-porous liquid crystal elastomer. For strains above ~0.8 applied perpendicular to the liquid crystal director, the liquid crystal elastomer becomes auxetic with the maximum negative Poisson’s ratio measured to date being -0.74 ± 0.03 – larger than most values seen in naturally occurring molecular auxetics. The emergence of auxeticity coincides with the liquid crystal elastomer backbone adopting a negative order parameter, Q_B = -0.41 ± 0.01 – further implying negative liquid crystal ordering. The reported behaviours consistently agree with theoretical predictions from Warner and Terentjev liquid crystal elastomer theory. Our results open the door for the design of synthetic molecular auxetics.

Liquid crystal elastomers (LCEs) have long been celebrated for their exceptional shape actuation and mechanical properties. For much of the last half century, a major focus for the field has been the development of LCE chemistries and how to process the so-called “monodomain” configurations. This foundation work has now led to a plethora of materials and processes that are enabling the demonstration of devices that are close to real-world applicability as responsive and reprocessable actuators. In this Perspective, we review and discuss the key recent developments in the processing of actuating LCE devices. We consider how processing has been used to increase the practicality of electrical, thermal, and photo stimulation of LCE shape actuation; how dynamic chemistries are enhancing the functionality and sustainability of LCE devices; and how new additive manufacturing technologies are overcoming the processing barriers that once confined LCE actuators to thin film devices. In our outlook, we consider all these factors together and discuss what developments over the coming years will finally lead to the realization of commercial shape actuating LCE technologies.