There is growing interest in voxelated matter that is designed and fabricated voxel by voxel1–4. Currently, inkjet-based three-dimensional (3D) printing is the only widely adopted method that is capable of creating 3D voxelated materials with high precision1–4, but the physics of droplet formation requires the use of low-viscosity inks to ensure successful printing5. By contrast, direct ink writing, an extrusion-based 3D printing method, is capable of patterning a much broader range of materials6–13. However, it is difficult to generate multimaterial voxelated matter by extruding monolithic cylindrical filaments in a layer-by-layer manner. Here we report the design and fabrication of voxelated soft matter using multimaterial multinozzle 3D (MM3D) printing, in which the composition, function and structure of the materials are programmed at the voxel scale. Our MM3D printheads exploit the diode-like behaviour that arises when multiple viscoelastic materials converge at a junction to enable seamless, high-frequency switching between up to eight different materials to create voxels with a volume approaching that of the nozzle diameter cubed. As exemplars, we fabricate a Miura origami pattern14 and a millipede-like soft robot that locomotes by co-printing multiple epoxy and silicone elastomer inks of stiffness varying by several orders of magnitude. Our method substantially broadens the palette of voxelated materials that can be designed and manufactured in complex motifs. Voxelated soft matter is designed and fabricated using multimaterial multinozzle three-dimensional printing, which switches between different viscoelastic inks along the same print filament to print multiple materials simultaneously.

Voxelated soft matter via multimaterial multinozzle 3D printing.

Soft robots have garnered interest for real-world applications because of their intrinsic safety embedded at the material level. These robots use deformable materials capable of shape and behavioral changes and allow conformable physical contact for manipulation. Yet, with the introduction of soft and stretchable materials to robotic systems comes a myriad of challenges for sensor integration, including multimodal sensing capable of stretching, embedment of high-resolution but large-area sensor arrays, and sensor fusion with an increasing volume of data. This Review explores the emerging confluence of e-skins and machine learning, with a focus on how roboticists can combine recent developments from the two fields to build autonomous, deployable soft robots, integrated with capabilities for informative touch and proprioception to stand up to the challenges of real-world environments.

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Electronic skins and machine learning for intelligent soft robots

Origami has been employed to build deployable mechanical metamaterials through folding and unfolding along the crease lines. Deployable metamaterials are usually flexible, particularly along their deploying and collapsing directions, which unfortunately in many cases leads to an unstable deployed state, i.e., small perturbations may collapse the structure along the same deployment path. Here we create an origami-inspired mechanical metamaterial with on-demand deployability and selective collapsibility through energy analysis. This metamaterial has autonomous deployability from the collapsed state and can be selectively collapsed along two different paths, embodying low stiffness for one path and substantially high stiffness for another path. The created mechanical metamaterial yields load-bearing capability in the deployed direction while possessing great deployability and collapsibility. The principle in this work can be utilized to design and create versatile origami-inspired mechanical metamaterials that can find many applications.

https://www.pnas.org/content/pnas/115/9/2032.full.pdf

Origami-inspired, on-demand deployable and collapsible mechanical metamaterials with tunable stiffness

Phononic crystals (PCs) and metamaterials (MMs) can exhibit abnormal properties, even far beyond those found in nature, through artificial design of the topology or ordered structure of unit cells. This emerging class of materials has diverse application potentials in many fields. Recently, the concept of tunable PCs or MMs has been proposed to manipulate a variety of wave functions on demand. In this review, we survey recent developments in tunable and active PCs and MMs, including bandgap and bandgap engineering, anomalous behaviors of wave propagation, as well as tunable manipulation of waves based on different regulation mechanisms: tunable mechanical reconfiguration and materials with multifield coupling. We conclude by outlining future directions in the emerging field.

Tunable and Active Phononic Crystals and Metamaterials

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

/pdf/functional-fibers-and-fabrics-for-soft-robotics-wearables-5czg98hs27.pdf

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface.

Materials with engineered thermal expansion, capable of achieving targeted area/volume changes in response to variations in temperature, are important for a number of aerospace, optical, energy, and microelectronic applications. While most of the proposed structures with engineered coefficient of thermal expansion consist of bi-material 2D or 3D lattices, here it is shown that origami metamaterials also provide a platform for the design of systems with a wide range of thermal expansion coefficients. Experiments and simulations are combined to demonstrate that by tuning the geometrical parameters of the origami structure and the arrangement of plates and creases, an extremely broad range of thermal expansion coefficients can be obtained. Differently from all previously reported systems, the proposed structure is tunable in situ and nonporous.

Origami Metamaterials for Tunable Thermal Expansion.

Fluidic soft actuators are enlarging the robotics toolbox by providing flexible elements that can display highly complex deformations. Although these actuators are adaptable and inherently safe, their actuation speed is typically slow because the influx of fluid is limited by viscous forces. To overcome this limitation and realize soft actuators capable of rapid movements, we focused on spherical caps that exhibit isochoric snapping when pressurized under volume-controlled conditions. First, we noted that this snap-through instability leads to both a sudden release of energy and a fast cap displacement. Inspired by these findings, we investigated the response of actuators that comprise such spherical caps as building blocks and observed the same isochoric snapping mechanism upon inflation. Last, we demonstrated that this instability can be exploited to make these actuators jump even when inflated at a slow rate. Our study provides the foundation for the design of an emerging class of fluidic soft devices that can convert a slow input signal into a fast output deformation.

/pdf/inflatable-soft-jumper-inspired-by-shell-snapping-40iiqb5vps.pdf

Inflatable soft jumper inspired by shell snapping.

Soft robots powered by pressurized fluid have recently enabled a variety of innovative applications in areas as diverse as space exploration, search and rescue systems, biomimetics, medical surgery, and rehabilitation. Although soft robots have been demonstrated to be capable of performing a number of different tasks, they typically require independent inflation of their constituent actuators, resulting in multiple input lines connected to separate pressure supplies and a complex actuation process. To circumvent this limitation, we embed the actuation sequencing in the system by connecting fluidic actuators with narrow tubes to exploit the effects of viscous flow. We developed modeling and optimization tools to identify optimal tube characteristics and we demonstrate the inverse design of fluidic soft robots capable of achieving a variety of complex target responses when inflated with a single pressure input. Our study opens avenues toward the design of a new generation of fluidic soft robots with embedded actuation control, in which a single input line is sufficient to achieve a wide range of functionalities.

Harnessing Viscous Flow to Simplify the Actuation of Fluidic Soft Robots

Compliant, continuum structures allow living creatures to perform complex tasks inaccessible to artificial rigid systems. Although advancements in hyper-elastic materials have spurred the development of synthetic soft structures (i.e., artificial muscles), these structures have yet to match the precise control and diversity of motions witnessed in living creatures. Cephalopods tentacles, for example, can undergo multiple trajectories using muscular hydrostat, a structure consisting of aggregated laminae of unidirectional muscle fibers. Here, we present a self-adhesive composite lamina inspired by the structural morphology of the muscular hydrostat, which adheres to any volumetrically expanding soft body to govern its motion trajectory. The composite lamina is stretchable only in one direction due to inextensible continuous fibers unidirectionally embedded within its hyper-elastic matrix. We showcase reconfiguration of inflation trajectories of two- and three-dimensional soft bodies by simply adhering laminae to their surfaces. Soft actuators are typically confined to a single, predetermined deformation trajectory. Here, the authors report on a bioinspired self-adhesive material which can be attached, detached, and re-attached to expanding soft bodies to render reconfigurable and controllable deformation trajectories.

/pdf/reconfigurable-soft-body-trajectories-using-unidirectionally-457oqnujiv.pdf

Reconfigurable soft body trajectories using unidirectionally stretchable composite laminae.

Multi-welled energy landscapes arising in shells with nonzero Gaussian curvature typically fade away as their thickness becomes larger because of the increased bending energy required for inversion. Motivated by this limitation, we propose a strategy to realize doubly curved shells that are bistable for any thickness. We then study the nonlinear dynamic response of one-dimensional (1D) arrays of our universally bistable shells when coupled by compressible fluid cavities. We find that the system supports the propagation of bidirectional transition waves whose characteristics can be tuned by varying both geometric parameters as well as the amount of energy supplied to initiate the waves. However, since our bistable shells have equal energy minima, the distance traveled by such waves is limited by dissipation. To overcome this limitation, we identify a strategy to realize thick bistable shells with tunable energy landscape and show that their strategic placement within the 1D array can extend the propagation distance of the supported bidirectional transition waves. Curved elastic shells have unique mechanical behavior and multiple stable configurations, but these properties fade when the shell thickness increases. Here the authors report a strategy to realize bistable doubly curved shells with arbitrary thickness, and how to optimize the dynamic response of one-dimensional connected arrays of such doubly-curved bistable shells.

/pdf/universally-bistable-shells-with-nonzero-gaussian-curvature-2e3ib66z02.pdf

Nikolaos Vasios

Papers

Origami Metamaterials for Tunable Thermal Expansion.

Inflatable soft jumper inspired by shell snapping.

Harnessing Viscous Flow to Simplify the Actuation of Fluidic Soft Robots

Reconfigurable soft body trajectories using unidirectionally stretchable composite laminae.

Universally bistable shells with nonzero Gaussian curvature for two-way transition waves.