Enhancing Electrical Conductivity of Stretchable Liquid Metal-Silver Composites through Direct Ink Writing.

Structure-property-process relationships are a controlling factor in the performance of materials. This offers opportunities in emerging areas, such as stretchable conductors, to control process conditions during printing to enhance performance. Herein, by systematically tuning direct ink write (DIW) process parameters, the electrical conductivity of multiphase liquid metal (LM)-silver stretchable conductors is increased by a maximum of 400% to over 1.06 × 106 S·m-1. This is achieved by modulating the DIW print velocity, which enables the in situ elongation, coalescence, and percolation of these multiphase inclusions during printing. These DIW printed filaments are conductive as fabricated and are soft (modulus as low as 1.1 MPa), stretchable (strain limit >800%), and show strain invariant conductivity up to 80% strain. These capabilities are demonstrated through a set of electromagnetic induction coils that can transfer power wirelessly through air and water, even under deformation. This work provides a methodology to program properties in stretchable conductors, where the combination of material composition and process parameters leads to greatly enhanced performance. This approach can find use in applications such as soft robots, soft electronics, and printed materials for deformable, yet highly functional devices.

Kuttsukigami: sticky sheet design.

Shaping 3D objects from 2D sheets enables form and function in diverse areas from art to engineering. Here we introduce kuttsukigami, which exploits sheet-sheet adhesion to create structure. The technique allows thin sheets to be sculpted without requiring sharp folds, enabling structure in a broad range of materials for a versatile and reconfigurable thin-sheet engineering design scheme. Simple closed structures from cylindrical loops to complex shapes like the Möbius loop are constructed and modeled through the balance between deformation and adhesion. Importantly, the balance can be used to create experimental measurements of elasticity in complex morphologies. More practically, kuttsukigami is demonstrated to encapsulate objects from the kitchen to micro scales and to build on-demand logic gates through sticky electronic sheets for truly reusable, reconfigurable devices.

Metamaterial adhesives for programmable adhesion through reverse crack propagation.

Adhesives are typically either strong and permanent or reversible with limited strength. However, current strategies to create strong yet reversible adhesives needed for wearable devices, robotics and material disassembly lack independent control of strength and release, require complex fabrication or only work in specific conditions. Here we report metamaterial adhesives that simultaneously achieve strong and releasable adhesion with spatially selectable adhesion strength through programmed cut architectures. Nonlinear cuts uniquely suppress crack propagation by forcing cracks to propagate backwards for 60× enhancement in adhesion, while allowing crack growth in the opposite direction for easy release and reusability. This mechanism functions in numerous adhesives on diverse substrates in wet and dry conditions and enables highly tunable adhesion with independently programmable adhesion strength in two directions simultaneously at any location. We create these multifunctional materials in a maskless, digital fabrication framework to rapidly customize adhesive characteristics with deterministic control for next-generation adhesives.

X-ray scattering as an effective tool for characterizing liquid metal composite morphology.

Quantitative analysis of particle size and size distribution is crucial in establishing structure-property relationships of composite materials. An emerging soft composite architecture involves dispersing droplets of liquid metal throughout an elastomer, enabling synergistic properties of metals and soft polymers. The structure of these materials is typically characterized through real-space microscopy and image analysis; however, these techniques rely on magnified images that may not represent the global-averaged size and distribution of the droplets. In this study, we utilize ultra-small angle X-ray scattering (USAXS) as a reciprocal-space characterization technique that yields global-averaged dimensions of eutectic gallium indium (EGaIn) alloy soft composites. The Unified fit and Monte Carlo scattering methods are applied to determine the particle size and size distributions of the liquid metal droplets in the composites and are shown to be in excellent agreement with results from real-space image analysis. Additionally, all methods indicate that the droplets are getting larger as they are introduced into composites, suggesting that the droplets are agglomerating or possibly coalescing during dispersion. This work demonstrates the viability of X-ray scattering to elucidate structural information about liquid metal droplets for material development for applications in soft robotics, soft electronics, and multifunctional materials.

Tough Bonding of Liquid Metal-Elastomer Composites for Multifunctional Adhesives.

Liquid metal (LM) composites, which consist of LM droplets dispersed in highly deformable elastomers, have recently gained interest as a multifunctional material for soft robotics and electronics. The incorporation of LM into elastic solids allows for unique combinations of material properties such as high stretchability with thermal and electrical conductivity comparable to metals. However, it is currently a challenge to incorporate LM composites into integrated systems consisting of diverse materials and components due to a lack of adhesion control. Here, a chemical anchoring methodology to increase adhesion of LM composites to diverse substrates is presented. The fracture energy increases up to 100× relative to untreated surfaces, with values reaching up to 7800 J m-2 . Furthermore, the fracture energy, tensile modulus, and thermal conductivity can be tuned together by controlling the microstructure of LM composites. Finally, the bonding technique is used to integrate LM composites with functional electronic components without encapsulation or clamping, allowing for extreme deformations while maintaining exceptional thermal and electrical conductivity. These findings can accelerate the adoption of LM composites into complex soft robotic and electronic systems where strong, reliable bonding between diverse materials and components is required.

Octopus-inspired adhesive skins for intelligent and rapidly switchable underwater adhesion.

The octopus couples controllable adhesives with intricately embedded sensing, processing, and control to manipulate underwater objects. Current synthetic adhesive-based manipulators are typically manually operated without sensing or control and can be slow to activate and release adhesion, which limits system-level manipulation. Here, we couple switchable, octopus-inspired adhesives with embedded sensing, processing, and control for robust underwater manipulation. Adhesion strength is switched over 450× from the ON to OFF state in <50 ms over many cycles with an actively controlled membrane. Systematic design of adhesive geometry enables adherence to nonideal surfaces with low preload and independent control of adhesive strength and adhesive toughness for strong and reliable attachment and easy release. Our bio-inspired nervous system detects objects and autonomously triggers the switchable adhesives. This is implemented into a wearable glove where an array of adhesives and sensors creates a biomimetic adhesive skin to manipulate diverse underwater objects.

On-Demand Programming of Liquid Metal-Composite Microstructures through Direct Ink Write 3D Printing.

Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle-like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high-power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.

Shape morphing mechanical metamaterials through reversible plasticity.

Biological organisms such as the octopus can reconfigure their shape and properties to perform diverse tasks. However, soft machines struggle to achieve complex configurations, morph into shape to support loads, and go between multiple states reversibly. Here, we introduce a multifunctional shape-morphing material with reversible and rapid polymorphic reconfigurability. We couple elastomeric kirigami with an unconventional reversible plasticity mechanism in metal alloys to rapidly (<0.1 seconds) morph flat sheets into complex, load-bearing shapes, with reversibility and self-healing through phase change. This kirigami composite overcomes trade-offs in deformability and load-bearing capacity and eliminates power requirements to sustain reconfigured shapes. We demonstrate this material through integration with onboard control, motors, and power to create a soft robotic morphing drone, which autonomously transforms from a ground to air vehicle and an underwater morphing machine, which can be reversibly deployed to collect cargo.

Cancer cell migration in collagen-hyaluronan composite extracellular matrices.

Hyaluronan (HA) is a key component in the tumor microenvironment (TME) that participates in cancer growth and invasiveness. While the molecular weight (MW) dependent properties of HA can cause tumor-promoting and -repressing effects, the elevated levels of HA in the TME impedes drug delivery. The degradation of HA using hyaluronidases (HYALs), resulting in fragments of HA, is a way to overcome this, but the consequences of changes in HA molecular weight and concentration is currently unknown. Therefore, it is critical to understand the MW-dependent biological effects of HA. Here we examine the influence of HA molecular weight on biophysical properties that regulate cell migration and extracellular matrix (ECM) remodeling. In our study, we used vLMW, LMW and HMW HA at different physiologically relevant concentrations, with a particular interest in correlating the mechanical and structural properties to different cell functions. The elastic modulus, collagen network pore size and collagen fiber diameter increased with increasing HA concentration. Although the collagen network pore size increased, these pores were filled with the bulky HA molecules. Consequently, cell migration decreased with increase in HA concentration due to multiple, long-lived and unproductive protrusions, suggesting the influence of steric factors. Surprisingly, even though elastic modulus increased with HA molecular weight and concentration, gel compaction assays showed an increased degree of ECM compaction among HMW HA gels at high concentrations (2 and 4 mg mL-1 [0.2 and 0.4%]). These results were not seen in collagen gels that lacked HA, but had similar stiffness. HA appears to have the effect of decreasing migration and increasing collagen network contraction, but only at high HA molecular weight. Consequently, changes in HA molecular weight can have relatively large effects on cancer cell behavior. STATEMENT OF SIGNIFICANCE: Hyaluronan (HA) is a critical component of the tumor microenvironment (TME). Overproduction of HA in the TME results in poor prognosis and collapse of blood vessels, inhibiting drug delivery. Hyaluronidases have been used to enhance drug delivery. However, they lead to low molecular weight (MW) HA, altering the mechanical and structural properties of the TME and cancer cell behavior. Understanding how HA degradation affects cancer cell behavior is critical for uncovering detrimental effects of this therapy. Very little is known about how HA MW affects cancer cell behavior in tumor-mimicking collagen-HA composite networks. Here we examine how MW and HA content in collagen-HA networks alter structural and mechanical properties to regulate cell migration and matrix remodeling in 3D TME-mimicking environments.

Deterministic control of adhesive crack propagation through jamming based switchable adhesives.

Controlling delamination across a material interface is a foundation of adhesive science and technology. This ranges from creating permanent, strong adhesives which limit crack propagation to reversible adhesives which initiate cracks for release. Methods which dynamically control cracks can lead to more robust adhesion, however specific control of crack initiation, propagation, and arresting is challenging because time scales of crack propagation are much faster than times scales of mechanisms to arrest cracks. Here we show the deterministic control of crack initiation, propagation, and arresting by integrating a granular jamming layer into adhesive films. This allows for controlled initiation of a propagating crack by reducing rigidity and then rapidly arresting the crack through jamming, with a rise in stiffness and an 11× enhancement in adhesion. This process is highly reversible and programmable, allowing for numerous crack initiation, propagation, and arresting cycles at arbitrary selectable locations in a peeling adhesive. We demonstrate this crack-control approach in single and multiple peel directions under fixed load conditions in response to diverse pressurization input signal profiles (i.e. time varying propagation and arresting scenarios).

All-graphene-based open fluidics for pumpless, small-scale fluid transport via laser-controlled wettability patterning.

Open microfluidics have emerged as a low-cost, pumpless alternative strategy to conventional microfluidics for delivery of fluid for a wide variety of applications including rapid biochemical analysis and medical diagnosis. However, creating open microfluidics by tuning the wettability of surfaces typically requires sophisticated cleanroom processes that are unamenable to scalable manufacturing. Herein, we present a simple approach to develop open microfluidic platforms by manipulating the surface wettability of spin-coated graphene ink films on flexible polyethylene terephthalate via laser-controlled patterning. Wedge-shaped hydrophilic tracks surrounded by superhydrophobic walls are created within the graphene films by scribing micron-sized grooves into the graphene with a CO2 laser. This scribing process is used to make superhydrophobic walls (water contact angle ∼160°) that delineate hydrophilic tracks (created through an oxygen plasma pretreatment) on the graphene for fluid transport. These all-graphene open microfluidic tracks are capable of transporting liquid droplets with a velocity of 20 mm s-1 on a level surface and uphill at elevation angles of 7° as well as transporting fluid in bifurcating cross and tree branches. The all-graphene open microfluidic manufacturing technique is rapid and amenable to scalable manufacturing, and consequently offers an alternative pumpless strategy to conventional microfluidics and creates possibilities for diverse applications in fluid transport.

Mechanically Cloaked Multiphase Magnetic Elastomer Soft Composites for Wearable Wireless Power Transfer.

Wearable electronics allow for new and immersive experiences between technology and the human body, but conventional devices are made from rigid functional components that lack the necessary compliance to safely interact with human tissue. Recently, liquid inclusions have been incorporated into elastomer composites to produce functional materials with high extensibility and ultrasoft mechanical responses. While these materials have shown high thermal and electrical conductivity, there has been an absence of research into compliant magnetic materials through the incorporation of magnetic fluids. Compliant magnetic materials are important for applications in soft matter engineering including sensing, actuation, and power transfer for soft electronics and robotics. In this work, we establish a new class of highly functional soft materials with advanced magnetic and mechanical properties by dispersing magnetic colloidal suspensions as compliant fluid inclusions into soft elastomers. Significantly, the rigid magnetic particles are encapsulated by the fluid. This mechanically cloaks the solid particles and enables a fluid-like mechanical response while imparting high magnetic permeability to the composite. This microstructure reduces the modulus of the composite below that of the initial elastomer to <40 kPa while increasing the permeability by over 100% to greater than 2. We demonstrate the functionality of these materials through conformable magnetic backplanes, which enables a completely soft, coupled inductor system capable of transferring power up to 100% strain and wearable devices for wireless power transfer.

Tuning surface functionalization and collagen gel thickness to regulate cancer cell migration.

Cancer cells have a tremendous ability to sense and respond to extracellular matrix (ECM) stiffness, modulating invasion. The magnitude of the sensed stiffness can either promote or inhibit the migration of cancer cells out of the primary tumor into surrounding tissue. Work has been done on examining the role of stiffness in tuning cancer cell migration by controlling elastic modulus in the bulk. However, a powerful and complementary approach for controlling stiffness is to leverage interactions between stiff-soft (e.g. glass-hydrogel) interfaces. Unfortunately, most work in this area probes cells in 2D environments. Of the reports that probe 3D environments, none have assessed the role of mechanical linkage to the interface as a potential handle in controlling local stiffness and cell behavior. In this paper, we examine the migration of cancer cells embedded in a collagen fiber network between two flat plates. We examine the role of both surface attachment of the collagen network to the stiff interface as well as thickness (50-540 µm) of the collagen gel in driving collagen organization, cell morphology and cell migration. We find that surface attachment and thickness do not operate overlapping mechanisms, because they elicit different cell responses. While thickness and surface chemistry appear to control morphology, only thickness regulates collagen organization and cell migration speed. This suggests that surface attachment and thickness of the collagen gel control cell behavior through both collagen structure and local stiffness in confined fiber-forming networks.

Liquid Metal-Elastomer Soft Composites with Independently Controllable and Highly Tunable Droplet Size and Volume Loading.

Soft composites are critical for soft and flexible materials in energy harvesting, actuators, and multifunctional devices. One emerging approach to create multifunctional composites is through the incorporation of liquid metal (LM) droplets such as eutectic gallium indium (EGaIn) in highly deformable elastomers. The microstructure of such systems is critical to their performance; however, current materials lack control of particle size at diverse volume loadings. Here, we present a fabrication approach to create liquid metal-elastomer composites with independently controllable and highly tunable droplet size (100 nm ≤ D ≤ 80 µm) and volume loading (0 ≤ ϕ ≤ 80%). This is achieved through a combination of shear mixing and sonication of concentrated LM/elastomer emulsions to control droplet size and subsequent dilution and homogenization to tune LM volume loading. These materials are characterized utilizing dielectric spectroscopy supported by analytical modeling, which shows a high relative permittivity of 60 (16× the unfilled elastomer) in a composite with ϕ = 80%, a low tan δ of 0.02, and a significant dependence on ϕ and minor dependence on droplet size. Temperature response and stability are determined using dielectric spectroscopy through temperature and frequency sweeps with DSC. These results demonstrate a wide temperature stability of the liquid metal phase (crystallizing at <-85 °C for D < 20 µm). Additionally, all composites are electrically insulating across wide frequency (0.1 Hz-10 MHz) and temperature (-70 to 100 °C) ranges even up to ϕ = 80%. We highlight the benefit of LM microstructure control by creating all-soft-matter stretchable capacitive sensors with tunable sensitivity. These sensors are further integrated into a wearable sensing glove where we identify different objects during grasping motions. This work enables programmable LM composites for soft robotics and stretchable electronics where flexibility and tunable functional response are critical.

Flexible thermoelectric generators with inkjet-printed bismuth telluride nanowires and liquid metal contacts.

Solution phase printing of nanomaterials is becoming increasingly important for the creation of scalable flexible electronics including those associated with biomedical and energy harvesting applications. However, the use of solution-phase printed thermoelectric energy generators (TEGs) has been minimally explored. Herein we report a highly flexible inkjet-printed TEG. Bismuth telluride (Bi2Te3) and bismuth antimony telluride (Bi0.5Sb1.5Te3) nanowires (NWs) are inkjet printed onto polyimide to form n-type and p-type legs for the TEGs. A post-print thermal annealing process is used to increase the thermoelectric performance of the printed NWs while eutectic gallium-indium (EGaIn) liquid metal contacts electrically connect the TEG legs in series. Annealing conditions for the combination of p/n legs are examined to maximize the thermoelectric efficiency of the TEG prototype. The maximum power factor was found to be 180 µW m-1 K-2 and 110 µW m-1 K-2 for the Bi2Te3 and Bi0.5Sb1.5Te3 nanowires respectively. A maximum power for the fully printed TEG device measured 127 nW at a 32.5 K temperature difference. The performance of the TEG device does not diminish even after multiple bending experiments (up to 50 times) around a tight radius of curvature (rod-dia. 11 mm). Hence this inkjet-printed flexible TEG is a step towards a fully functional wearable TEG device.

An autonomously electrically self-healing liquid metal-elastomer composite for robust soft-matter robotics and electronics.

Large-area stretchable electronics are critical for progress in wearable computing, soft robotics and inflatable structures. Recent efforts have focused on engineering electronics from soft materials-elastomers, polyelectrolyte gels and liquid metal. While these materials enable elastic compliance and deformability, they are vulnerable to tearing, puncture and other mechanical damage modes that cause electrical failure. Here, we introduce a material architecture for soft and highly deformable circuit interconnects that are electromechanically stable under typical loading conditions, while exhibiting uncompromising resilience to mechanical damage. The material is composed of liquid metal droplets suspended in a soft elastomer; when damaged, the droplets rupture to form new connections with neighbours and re-route electrical signals without interruption. Since self-healing occurs spontaneously, these materials do not require manual repair or external heat. We demonstrate this unprecedented electronic robustness in a self-repairing digital counter and self-healing soft robotic quadruped that continue to function after significant damage.

Extreme Toughening of Soft Materials with Liquid Metal.

Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various energy dissipation techniques. However, bio-materials exhibit extreme toughness by combining multi-scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic materials architecture that also exhibits multi-modal toughening, whereby embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J m-2 to 11,900 ± 2600 J m-2 ) over an unfilled polymer. For some LM-embedded elastomer (LMEE) compositions, the toughness is measured to be 33,500 ± 4300 J m-2 , which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, soft materials.

Tunable Mechanical Metamaterials through Hybrid Kirigami Structures.

Inspired by the art of paper cutting, kirigami provides intriguing tools to create materials with unconventional mechanical and morphological responses. This behavior is appealing in multiple applications such as stretchable electronics and soft robotics and presents a tractable platform to study structure-property relationships in material systems. However, mechanical response is typically controlled through a single or fractal cut type patterned across an entire kirigami sheet, limiting deformation modes and tunability. Here we show how hybrid patterns of major and minor cuts creates new opportunities to introduce boundary conditions and non-prismatic beams to enable highly tunable mechanical responses. This hybrid approach reduces stiffness by a factor of ~30 while increasing ultimate strain by a factor of 2 (up to 750% strain) relative to single incision patterns. We present analytical models and generate general design criteria that is in excellent agreement with experimental data from nanoscopic to macroscopic systems. These hybrid kirigami materials create new opportunities for multifunctional materials and structures, which we demonstrate with stretchable kirigami conductors with nearly constant electrical resistance up to >400% strain and magnetoactive actuators with extremely rapid response (>10,000% strain s-1) and high, repeatable elongation (>300% strain).