Shape-shifting structured lattices via multimaterial 4D printing.

Shape-morphing structured materials have the ability to transform a range of applications. However, their design and fabrication remain challenging due to the difficulty of controlling the underlying metric tensor in space and time. Here, we exploit a combination of multiple materials, geometry, and 4-dimensional (4D) printing to create structured heterogeneous lattices that overcome this problem. Our printable inks are composed of elastomeric matrices with tunable cross-link density and anisotropic filler that enable precise control of their elastic modulus (E) and coefficient of thermal expansion [Formula: see text] The inks are printed in the form of lattices with curved bilayer ribs whose geometry is individually programmed to achieve local control over the metric tensor. For independent control of extrinsic curvature, we created multiplexed bilayer ribs composed of 4 materials, which enables us to encode a wide range of 3-dimensional (3D) shape changes in response to temperature. As exemplars, we designed and printed planar lattices that morph into frequency-shifting antennae and a human face, demonstrating functionality and geometric complexity, respectively. Our inverse geometric design and multimaterial 4D printing method can be readily extended to other stimuli-responsive materials and different 2-dimensional (2D) and 3D cell designs to create scalable, reversible, shape-shifting structures with unprecedented complexity.

Effect of microtextured surface topography on the wetting behavior of eutectic gallium-indium alloys.

Liquid-embedded elastomer electronics have recently attracted much attention as key elements of highly deformable and "soft" electromechanical systems. Many of these fluid-elastomer composites utilize liquid metal alloys because of their high conductivities and inherent compliance. Understanding how these alloys interface with surfaces of various composition and texture is critical to the development of parallel processing technology, which is needed to create more complex and low-cost systems. In this work, we explore the wetting behaviors between droplets of gallium-indium alloys and thin metal films, with an emphasis on tin and indium substrates. We find that metallic droplets reactively wet thin metal foils, but the wettability of the foils may be tuned by the surface texture (produced by sputtering). The effects of both composition and texture of the substrate on wetting dynamics are quantified by measuring contact angle and droplet contact diameter as a function of time. Finally, we apply the Cassie-Baxter model to the sputtered and native substrates to gain insight into the behavior of liquid metals and the role of the oxide formation during interfacial processes.

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

4D printing is an emerging field where 3D printing techniques are used to pattern stimuli-responsive materials to create morphing structures, with time serving as the fourth dimension. However, current materials utilized for 4D printing are typically soft, exhibiting an elastic modulus (E) range of 10-4 to 10 MPa during shape change. This restricts the scalability, actuation stress, and load-bearing capabilities of the resulting structures. To overcome these limitations, multiscale heterogeneous polymer composites are introduced as a novel category of stiff, thermally responsive 4D printed materials. These inks exhibit an E that is four orders of magnitude greater than that of existing 4D printed materials and offer tunable electrical conductivities for simultaneous Joule heating actuation and self-sensing capabilities. Utilizing electrically controllable bilayers as building blocks, a flat geometry that morphs into a 3D self-standing lifting robot is designed and printed, setting new records for weight-normalized load lifted and actuation stress when compared to other 3D printed actuators. Furthermore, this ink palette is employed to create and print planar lattice structures that transform into various self-supporting complex 3D shapes. Finally these inks are integrated into a 4D printed electrically controlled multigait crawling robotic lattice structure that can carry 144 times its own weight.

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

4D printing is an emerging field where 3D printing techniques are used to pattern stimuli-responsive materials to create morphing structures, with time serving as the fourth dimension. However, current materials utilized for 4D printing are typically soft, exhibiting an elastic modulus (E) range of 10-4 to 10 MPa during shape change. This restricts the scalability, actuation stress, and load-bearing capabilities of the resulting structures. To overcome these limitations, multiscale heterogeneous polymer composites are introduced as a novel category of stiff, thermally responsive 4D printed materials. These inks exhibit an E that is four orders of magnitude greater than that of existing 4D printed materials and offer tunable electrical conductivities for simultaneous Joule heating actuation and self-sensing capabilities. Utilizing electrically controllable bilayers as building blocks, a flat geometry is designed and printed that morphs into a 3D self-standing lifting robot, setting new records for weight-normalized load lifted and actuation stress when compared to other 3D printed actuators. Furthermore, the ink palette is employed to create and print planar lattice structures that transform into various self-supporting complex 3D shapes. These contributions are integrated into a 4D printed electrically controlled multigait crawling robotic lattice structure that can carry 144 times its own weight.

Innervated, Self-Sensing Liquid Crystal Elastomer Actuators with Closed Loop Control.

The programmable assembly of innervated LCE actuators (iLCEs) with prescribed contractile actuation, self-sensing, and closed loop control via core-shell 3D printing is reported. This extrusion-based direct ink writing method enables coaxial filamentary features composed of pure LM core surrounded by an LCE shell, whose director is aligned along the print path. Specifically, the thermal response of the iLCE fiber-type actuators is programmed, measured, and modeled during Joule heating, including quantifying the concomitant changes in fiber length and resistance that arise during simultaneous heating and self-sensing. Due to their reversible, high-energy actuation and their resistive feedback, it is also demonstrated that iLCEs can be regulated with closed loop control even when perturbed with large bias loads. Finally, iLCE architectures capable of programmed, self-sensing 3D shape change with closed loop control are fabricated.

Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics.

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigated through comparison with focused ion beam sintering and by studying the effects of thermal propagation in sintered films. The effects of laser fluence, nanoparticle size, film thickness, and substrate material on resistance of the sintered films are evaluated. Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under flexing, multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser manufacturing with the material benefits of liquid conductors on elastic substrates to demonstrate decisive progress toward commercial-scale manufacturing of soft electronics.

High-Operating-Temperature Direct Ink Writing of Mesoscale Eutectic Architectures.

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 µm, enabling the manipulation of light in the visible and infrared range.

Hybrid Self-Assembly during Evaporation Enables Drop-on-Demand Thin Film Devices.

We propose and demonstrate a hybrid self-assembly process as the mechanism for producing strikingly uniform deposits from evaporating drops composed of cosolvents. This assembly process leverages both particle-fluid interactions to carry the particles to the drop surface and particle-interface interactions to assemble the particles into a uniform film. We anchor our results in a cosolvent evaporation model that agrees with our experimental observations. We further employ the process to produce thin film devices such as flexible broadband neutral density filters and semitransparent mirrors. Our observations suggest that this assembly process is free of particle-substrate interactions, which indicates that the results should be transferable across a multitude of material/substrate systems.

Label-free detection of Staphylococcus aureus captured on immutable ligand arrays.

The rapid capture and label-free detection of Staphylococcus aureus , an opportunistic bacterium that can infect upon contact, can be performed using periodic microarrays of ligand-protein conjugates created by noncontact (inkjet) printing, darkfield imaging conditions, and a FFT-based readout method. Ink solutes were prepared using bovine serum albumin (BSA) conjugated to a glycan with high affinity for bacterial adhesins and printed as dot-matrix arrays with periodicities of 80-120 µm using a thermal injection method. Upon exposing the glycan-BSA microarrays to live strains of S. aureus , patterns emerge that can be detected under optical darkfield conditions. These patterns can be decoded by fast Fourier transform (FFT) analysis to generate fault-tolerant readout signals that correspond to the capture of S. aureus, with a limit of detection between 10(2) and 10(3) cfu/mL. Inkjet printing provides independent control over array periodicity, enabling FFT signals to be assigned to specific frequencies in reciprocal k-space.