Liquid crystal elastomers (LCEs) are crosslinked polymers within which liquid crystal molecules are linked with elastomeric polymer chains. Various anisotropic materials properties of LCEs can be created through an orientational control of mesogens during fabrication process. In particular, LCEs exhibit reversible mechanical deformation along the direction of mesogen alignment in response to various external stimuli. Therefore, when properly associated with a suitable additive manufacturing process, LCEs can be fabricated to a 3D object that displays programmed stimuli-responsive deformation. Thus, LCEs offer a potential breakthrough in 4D printing as an alternative material solution that overcomes the limitations of typical 4D printing materials such as shape memory polymers and hydrogels. The orientational order of mesogens in LCEs can be controlled by a wide range of methods, including mechanical stretch, viscous shear flow, magnetic/electric-field, and surface treatment. We review herein the physical principles of the key methods for LCE molecular programming and recent advances in additive manufacturing processes that utilize these principles to enable 4D printing with LCEs. Various applications of additively manufactured LCEs are also highlighted. 

 Shape memory polymers are gaining significant interest as one of the major constituent materials for the emerging field of 4D printing. While 3D-printed metamaterials with shape memory polymers show unique thermomechanical behaviors, their thermal transport properties have received relatively little attention. Here, we show that thermal transport in 3D-printed shape memory polymers strongly depends on the shape, solid volume fraction, and temperature and that thermal radiation plays a critical role. Our infrared thermography measurements reveal thermal transport mechanisms of shape memory polymers in varying shapes from bulk to octet-truss and Kelvin-foam microlattices with volume fractions of 4%–7% and over a temperature range of 30–130 °C. The thermal conductivity of bulk shape memory polymers increases from 0.24 to 0.31 W m−1 K−1 around the glass transition temperature, in which the primary mechanism is the phase-dependent change in thermal conduction. On the contrary, thermal radiation dominates heat transfer in microlattices and its contribution to the Kelvin-foam structure ranges from 68% to 83% and to the octet-truss structure ranges from 59% to 76% over the same temperature range. We attribute this significant role of thermal radiation to the unique combination of a high infrared emissivity and a high surface-to-volume ratio in the shape memory polymer microlattices. Our work also presents an effective medium approach to explain the experimental results and model thermal transport properties with varying shapes, volume fractions, and temperatures. These findings provide new insights into understanding thermal transport mechanisms in 4D-printed shape memory polymers and exploring the design space of thermomechanical metamaterials. 

 Temperature is well known to have a significant effect on the overall mechanical performance of viscoelas-tomers. In this work, we investigate the thermo-mechanically coupled behavior of VHB 4910 using a combinedexperimental and modeling approach. We first characterize the material behavior by performing a set of largedeformation uniaxial experiments at different temperatures. We then model the observed thermo-mechanicalbehavior and calibrate that model to the uniaxial experiments. Lastly, the model is implemented as a usermaterial subroutine in a finite element package for validation purposes. A key finding of this work is that whileincreasing the temperature stiffens the elastic contribution, it concurrently reduces the viscoelastic contributionto the overall behavior of VHB.