Shape memory polymers (SMP) with 3D geometries and tunable shape-shifting behavior can open up new opportunities in intelligent devices. Achieving both simultaneously is difficult for conventional approaches. 4D printing allows fabrication of complex 3D SMP geometries that can change shapes (i.e., the fourth dimension is time), but tuning the shape memory response is challenging because of the printing constraints.Here, we report a material and process concept that allows digital light fabrication of SMP with fine control of not only the geometries but also the shape memory characteristics, within a printing time of 30 s. Digital light modulation allows spatio-temporal tuning of the material properties including shape memory transition temperature, rubbery modulus, and maximum elongation (up to 250%). Consequently, the process allows producing multiple-SMP within a single material construct using the same printing precursor. We demonstrate that this unique attribute is beneficial in constructing unusual shape-shifting 3D nano-photonic and electronic devices. The simplicity and versatility of our approach facilitates its future expansion into a wide range of geometrically complex devices with advanced functions.

The ability to undergo bond exchange in a dynamic covalent polymer network has brought many benefits not offered by classical thermoplastic and thermoset polymers. Despite the bond exchangeability, the overall network topologies for existing dynamic networks typically cannot be altered, limiting their potential expansion into unexplored territories. By harnessing topological defects inherent in any real polymer network, we show herein a general design that allows a dynamic network to undergo rearrangement to distinctive topologies. The use of a light triggered catalyst further allows spatio-temporal regulation of the network topology, leading to an unusual opportunity to program polymer properties. Applying this strategy to functional shape memory networks yields custom designable multi-shape and reversible shape memory characteristics. This molecular principle expands the design versatility for network polymers, with broad implications in many other areas including soft robotics, flexible electronics, and medical devices.

This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1−3 nm) surface oxide "skin" that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)even on substrates to which it adheresbecause the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the "sac" formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated "sac" on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide "sac" collapses. Thus, static contact angles can be manipulated mechanically from 0°to >140°due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.