Shape memory polymers (SMPs) and hydrogels are two major materials classes of stimuli-responsive materials that have been widely used in 4D printing. As such, additively manufactured responsive architected materials are made of either SMPs or hydrogels. SMP can memorize its original printed shape and restore it when needed, but its stimuli-responsive morphing is only one-way in that mechanical processing is always required for shape programming. Stimuli-responsive hydrogels can deform reversibly depending on various environmental changes, but they suffer from extremely slow time-scale for shape change due to a quadratic scaling law for the diffusive water molecule migration process and are not suitable for applications where surrounding water is not available. Recently, liquid crystal elastomers (LCEs), combining polymeric elasticity with liquid crystalline anisotropy, have received growing attention as a new class of a 4D printing material that can overcome the limitations of SMPs and hydrogels.