A 4D printing method is developed where the shape of a 3D-printed part can change rapidly into a new permanent one by heating.

The creation of reversibly-actuating components that alter their shapes in a controllable manner in response to environmental stimuli is a grand challenge in active materials, structures, and robotics. Here we demonstrate a new reversible shape-changing component design concept enabled by 3D printing two stimuli responsive polymers—shape memory polymers and hydrogels—in prescribed 3D architectures. This approach uses the swelling of a hydrogel as the driving force for the shape change, and the temperature-dependent modulus of a shape memory polymer to regulate the time of such shape change. Controlling the temperature and aqueous environment allows switching between two stable configurations – the structures are relatively stiff and can carry load in each – without any mechanical loading and unloading. Specific shape changing scenarios, e.g., based on bending, or twisting in prescribed directions, are enabled via the controlled interplay between the active materials and the 3D printed architectures. The physical phenomena are complex and nonintuitive, and so to help understand the interplay of geometric, material, and environmental stimuli parameters we develop 3D nonlinear finite element models. Finally, we create several 2D and 3D shape changing components that demonstrate the role of key parameters and illustrate the broad application potential of the proposed approach.

Soft robots have the appealing advantages of being highly flexible and adaptive to complex environments. However, the low‐stiffness nature of the constituent materials makes soft robotic systems incompetent in tasks requiring relatively high load capacity. Despite recent attempts to develop stiffness‐tunable soft actuators by employing variable stiffness materials and structures, the reported stiffness‐tunable actuators generally suffer from limitations including slow responses, small deformations, and difficulties in fabrication with microfeatures. This work presents a paradigm to design and manufacture fast‐response, stiffness‐tunable (FRST) soft actuators via hybrid multimaterial 3D printing. The integration of a shape memory polymer layer into the fully printed actuator body enhances its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The printed Joule‐heating circuit and fluidic cooling microchannel enable fast heating and cooling rates and allow the FRST actuator to complete a softening–stiffening cycle within 32 s. Numerical simulations are used to optimize the load capacity and thermal rates. The high load capacity and shape adaptivity of the FRST actuator are finally demonstrated by a robotic gripper with three FRST actuators that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.

Recent research using 3D printing to create active structures has added an exciting new dimension to 3D printing technology. After being printed, these active, often composite, materials can change their shape over time; this has been termed as 4D printing. In this paper, we demonstrate the design and manufacture of active composites that can take multiple shapes, depending on the environmental temperature. This is achieved by 3D printing layered composite structures with multiple families of shape memory polymer (SMP) fibers – digital SMPs - with different glass transition temperatures (Tg) to control the transformation of the structure. After a simple single-step thermomechanical programming process, the fiber families can be sequentially activated to bend when the temperature is increased. By tuning the volume fraction of the fibers, bending deformation can be controlled. We develop a theoretical model to predict the deformation behavior for better understanding the phenomena and aiding the design. We also design and print several flat 2D structures that can be programmed to fold and open themselves when subjected to heat. With the advantages of an easy fabrication process and the controllable multi-shape memory effect, the printed SMP composites have a great potential in 4D printing applications.

In this work, we advance printed active composites by combining 3D printing, printed electronics, and liquid crystal elastomers (LCEs) to achieve soft actuators with free-standing two-way shape changing behaviors. Incorporated LCE strips are activated by Joule heating produced by printed conductive wires, while uniaxial deformation of the LCE strip is utilized as a driving force to achieve bending in the printed composite. The bending behavior of laminated hinges is first characterized in order to obtain a precise control of actuation, which is then exploited to actuate four demonstrative designs: a morphing airplane, a miura-ori structure, a cubic box, and a soft crawler. The soft morphing airplane and miura-ori structure are designed and fabricated with multiple laminated hinges to demonstrate the synergistic actions during actuation. The cubic box is constructed to show the capability of sequential folding by implementing multiple groups of conductive wires to achieve accurately addressable heating with temporal control. Finally, the two-way transformation is utilized as a driving force for the locomotion of a soft crawler stimulated by a periodic rectangular wave current. These examples show the great potential of using the hybrid 3D printing and pick-and-place method and using LCEs to achieve controllable shape change structures for a variety of potential practical applications.

Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer–based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high–water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.

A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage thermal-photo curing reaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.

Structures capable of reversible shape changes directly after 3D printing are highly desirable for myriad of applications ranging from soft robotics to implantable medical devices. Liquid crystal elastomers (LCEs) have been studied for their large, reversible mechanical actuations in response to temperature. Recently, several efforts were carried out to direct ink write (DIW) 3D print LCE, however, these methods require high temperatures for printing and actuation. Here, we present a novel LCE ink formulation which allows LCE to be printed at room temperature with a maximum actuation temperature of 75 °C. These advantages allow the 3D printed LCEs to be integrated with other 3D printing methods and materials to create more complex shape changes. By 3D printing LCE and conductive wires, which provide Joule heating, we demonstrated an active hinge, a reversibly opening and closing box, a soft robotic gripper for pick and place, and a printed hand for sign language.