Heterogeneous Polymer Composites for High Stiffness 4D Printed Electrically Controllable Multifunctional Structures

4D printing is a rapidly emerging field in which 3D printed stimuli responsive materials are used to produce morphing and multifunctional structures, with time being the fourth dimension. The materials used for 4D printing are generally soft (elastic modulus range 10^-4 – 10 MPa) when undergoing shape change, which limits the scalability, actuation stress, and load bearing capabilities of current applications. Here, to address these limitations, we introduce heterogeneous polymer composites as a new class of stiff 4D printed materials. These composites are comprised of an epoxy matrix with a tunable cross-link density and a plurality of isotropic and anisotropic nano and micro fillers. Using this as a platform, we generate a palette of inks that span a large range of negative and positive linear coefficient of thermal expansion (-19.1 ± 0.3 – 128.8 ± 1.2 ppm ^oC^-1) with an elastic modulus range that is four orders of magnitude larger than existing 4D printed materials (0.34 ± 0.1 – 38.6 ± 1.4 GPa), and tunable electrical conductivities (0.7 ± 0.1 – 2.0 x 10³ ± 0.2 x 10³ S m^-1) for integrated Joule heating actuation and sensing. Using electrically controllable bilayers as building blocks, we design and print a 3D self-standing lifting robot, with record lifting capabilities (~888 times its own weight) and actuation stress (~6 MPa) for 3D printed actuators and comparable performance for commercially available actuators. Moreover, we were able to implement a closed loop control to our lifting robot to achieve autoregulated actuation with response that lie within 4.8% and 0.8% overshoot and undershoot, respectively. Furthermore, we design and print flat surfaces that morph into different self-supporting complex 3D surfaces. Finally, we combined our inks to 4D print an electrically controlled crawling lattice robot that can carry up to 144 times its own weight.