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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.

Recent developments in soft materials and 3D printing are promoting the rapid development of novel technologies and concepts, such as 4D printing and soft machines, that in turn require new methods for fabricating conductive materials. Despite the ubiquity of silver nanoparticles (NPs) in the conducting electrodes of printed electronic devices, their potential use in stretchable conductors has not been fully explored in 4D printing and soft machines. This paper studies the effect of thermal cure conditions on conductivity and electro-mechanical behaviors of silver ink by the direct ink write (DIW) printing approach. We found that the electro-mechanical properties of silver wires can be tailored by controlling cure time and cure temperature to achieve conductivity as well as stretchability. For the silver NP ink we used in the experiments, silver wires cured at 80 °C for 10–30 min have conductivity >1% bulk silver, Young’s modulus <100 MPa, yield strain ∼9%, and can retain conductivity up to 300% strain. In addition, under stress controlled cyclic loading/unloading conditions, the resistance of these wires is only about 1.3 times the initial value after the 100th repeat cycle (7.6% maximum strain in the first cycle). Silver wires cured at 120 °C for 10–20 min are more sensitive to strain and have a yield strain of around 6%. These properties indicate that the silver ink can be used to fabricate stretchable electrodes and flex sensors. Using the DIW fabrication method, we printed silver ink patterns on the surface of 3D printed polymer parts, with the future goal of constructing fully 3D printed arbitrarily formed soft and stretchable devices and of applying them to 4D printing. We demonstrated a fully printed functional soft-matter sensor and a circuit element that can be stretched by as much as 45%.

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Recent developments in 3D/4D printing for functional materials and devices require embedding conductive wires within a 3D printed part. One common approach is to direct-ink-write conductive silver nanoparticle (NP) ink on a 3D printed structure and subsequently sinter the ink. Intense pulsed light (IPL) sintering is a promising choice due to its advantage of rapid sintering. Despite the extensive studies of IPL sintering metal NPs on plastic foil, its use for hybrid 3D/4D printing has not been fully explored. In this paper, we studied the multi-cycle sintering method that alternates between light exposure cycle and cooling cycle to IPL sintering silver NP ink on a widely used commercial 3D printed elastomeric dark material. We investigated how the light energy can affect the sintered conductive wire quality by varying light exposure cycle, cooling cycle and the distance from light source to the sample, with fixed multiple small pulses. Numerical analysis, in situ temperature measurements, in situ resistance measurements, and microstructure characterization results confirmed the effectiveness of the multi-cycle IPL sintering method in obtaining thick conductive wires on the light absorbing substrate. Peeling tests and electromechanical experiments were performed to evaluate the interfacial and electromechanical properties of the sintered wire. We reported the phenomenon of wrinkles on the substrate and analyzed the mechanism by using finite element method. The proposed method can be applied to other 3D printed materials with different conductive wire dimension and thus have the potential to be integrated with the printed stretchable device for hybrid 3D printing and 4D printing in the future.

Shape morphing materials have been extensively studied to control the formation of sophisticated three-dimensional (3D) structures and devices for a broad range of applications. Various methods, including the buckling of pre-strained bilayer composites, stimuli-responsive shape-shifting of shape memory polymers, and hydrogels, have been previously employed to transform 2D sheets to 3D structures and devices. However, the residual stress locked in these shape-shifting structures will drive them to gradually revert to their original layouts upon the removal of external stimuli or constrains. Here, we report a multistimuli-responsive vitrimer (m-vitrimer) bearing thermal- and photo-reversible disulfide bonds as shape programmable and healable materials for functional 3D devices. The mechanical properties and thermomechanical properties of vitrimer were tuned by altering the disulfide content and catalyst loading. Heat and light exposure induces effective stress relaxation and network rearrangement, enabling material shape programming and healing. We demonstrate that printed flexible smart electronics are fabricated using the m-vitrimer as a matrix and printed conductive silver nanoparticles as conductive wire. The printed electronics possess good electro-mechanical properties, strong interfacial bonding, and thermal- and photo-responsive shape programming. Moreover, the m-vitrimer can be healed upon damage by heat and light, which partially restores silver conductivity and protect the electronics from further damage. The converging of multi-stimuli-responsive polymers and printed electronics for functional 3D devices have the potential of finding broad applications in smart and morphing electronics, biomedical devices, and 4D printing.

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With the development of fully printed electronics, soft sensors are in demand in various fields, such as wearable electronics, soft machines, etc. Most soft resistive sensors are made of conductive...