Barocaloric Effects in Dialkylammonium Halide Salts.

Barocaloric effects─solid-state thermal changes induced by the application and removal of hydrostatic pressure─offer the potential for energy-efficient heating and cooling without relying on volatile refrigerants. Here, we report that dialkylammonium halides─organic salts featuring bilayers of alkyl chains templated through hydrogen bonds to halide anions─display large, reversible, and tunable barocaloric effects near ambient temperature. The conformational flexibility and soft nature of the weakly confined hydrocarbons give rise to order-disorder phase transitions in the solid state that are associated with substantial entropy changes (>200 J kg-1 K-1) and high sensitivity to pressure (>24 K kbar-1), the combination of which drives strong barocaloric effects at relatively low pressures. Through high-pressure calorimetry, X-ray diffraction, and Raman spectroscopy, we investigate the structural factors that influence pressure-induced phase transitions of select dialkylammonium halides and evaluate the magnitude and reversibility of their barocaloric effects. Furthermore, we characterize the cyclability of thin-film samples under aggressive conditions (heating rate of 3500 K s-1 and over 11,000 cycles) using nanocalorimetry. Taken together, these results establish dialkylammonium halides as a promising class of pressure-responsive thermal materials.

Dissipation and energy propagation across scales in an active cytoskeletal material.

Living systems are intrinsically nonequilibrium: They use metabolically derived chemical energy to power their emergent dynamics and self-organization. A crucial driver of these dynamics is the cellular cytoskeleton, a defining example of an active material where the energy injected by molecular motors cascades across length scales, allowing the material to break the constraints of thermodynamic equilibrium and display emergent nonequilibrium dynamics only possible due to the constant influx of energy. Notwithstanding recent experimental advances in the use of local probes to quantify entropy production and the breaking of detailed balance, little is known about the energetics of active materials or how energy propagates from the molecular to emergent length scales. Here, we use a recently developed picowatt calorimeter to experimentally measure the energetics of an active microtubule gel that displays emergent large-scale flows. We find that only approximately one-billionth of the system's total energy consumption contributes to these emergent flows. We develop a chemical kinetics model that quantitatively captures how the system's total thermal dissipation varies with ATP and microtubule concentrations but that breaks down at high motor concentration, signaling an interference between motors. Finally, we estimate how energy losses accumulate across scales. Taken together, these results highlight energetic efficiency as a key consideration for the engineering of active materials and are a powerful step toward developing a nonequilibrium thermodynamics of living systems.

A Micromachined Picocalorimeter Sensor for Liquid Samples with Application to Chemical Reactions and Biochemistry.

Calorimetry has long been used to probe the physical state of a system by measuring the heat exchanged with the environment as a result of chemical reactions or phase transitions. Application of calorimetry to microscale biological samples, however, is hampered by insufficient sensitivity and the difficulty of handling liquid samples at this scale. Here, a micromachined calorimeter sensor that is capable of resolving picowatt levels of power is described. The sensor consists of low-noise thermopiles on a thin silicon nitride membrane that allow direct differential temperature measurements between a sample and four coplanar references, which significantly reduces thermal drift. The partial pressure of water in the ambient around the sample is maintained at saturation level using a small hydrogel-lined enclosure. The materials used in the sensor and its geometry are optimized to minimize the noise equivalent power generated by the sensor in response to the temperature field that develops around a typical sample. The experimental response of the sensor is characterized as a function of thermopile dimensions and sample volume, and its capability is demonstrated by measuring the heat dissipated during an enzymatically catalyzed biochemical reaction in a microliter-sized liquid droplet. The sensor offers particular promise for quantitative measurements on biological systems.

Ultra-sensitive and resilient compliant strain gauges for soft machines.

Soft machines are a promising design paradigm for human-centric devices1,2 and systems required to interact gently with their environment3,4. To enable soft machines to respond intelligently to their surroundings, compliant sensory feedback mechanisms are needed. Specifically, soft alternatives to strain gauges-with high resolution at low strain (less than 5 per cent)-could unlock promising new capabilities in soft systems. However, currently available sensing mechanisms typically possess either high strain sensitivity or high mechanical resilience, but not both. The scarcity of resilient and compliant ultra-sensitive sensing mechanisms has confined their operation to laboratory settings, inhibiting their widespread deployment. Here we present a versatile and compliant transduction mechanism for high-sensitivity strain detection with high mechanical resilience, based on strain-mediated contact in anisotropically resistive structures (SCARS). The mechanism relies upon changes in Ohmic contact between stiff, micro-structured, anisotropically conductive meanders encapsulated by stretchable films. The mechanism achieves high sensitivity, with gauge factors greater than 85,000, while being adaptable for use with high-strength conductors, thus producing sensors resilient to adverse loading conditions. The sensing mechanism also exhibits high linearity, as well as insensitivity to bending and twisting deformations-features that are important for soft device applications. To demonstrate the potential impact of our technology, we construct a sensor-integrated, lightweight, textile-based arm sleeve that can recognize gestures without encumbering the hand. We demonstrate predictive tracking and classification of discrete gestures and continuous hand motions via detection of small muscle movements in the arm. The sleeve demonstration shows the potential of the SCARS technology for the development of unobtrusive, wearable biomechanical feedback systems and human-computer interfaces.

Chemically Coupled Interfacial Adhesion in Multimaterial Printing of Hydrogels and Elastomers.

Functional devices that use hydrogels as ionic conductors and elastomers as dielectrics have the advantage of being soft, stretchable, transparent, and biocompatible, making them ideal for biomedical applications. These devices are typically fabricated by manual assembly. Techniques for the manufacturing of soft materials have generally not looked at integrating multiple dissimilar materials. Silane coupling agents have recently shown promise for creating strong bonds between hydrogels and elastomers but have yet to be used in the extrusion printing of complex devices that integrate both hydrogels and elastomers. Here, we demonstrate the viability of silane coupling agents in a system with the rheology and functional composition necessary for three-dimensional (3D) extrusion printing of hydrogel-elastomer materials, specifically polyacrylamide (PAAm) hydrogel and poly(dimethylsiloxane) (PDMS) hydrophobic elastomer. By introducing a charge-neutral surfactant in the PDMS and adjusting silane concentrations in the PAAm, cast material samples demonstrate strong adhesion. We were also able to achieve an interfacial toughness of up to Γ = 193 ± 6.3 J/m2 for a fully extrusion printed PAAm hydrogel-on-PDMS bilayer. This result demonstrates that an integration strategy based on silane coupling agents makes it possible for extrusion printing of a wide variety of hydrogel and silicone elastomers.

Lightweight Highly Tunable Jamming-Based Composites.

Tunable-impedance mechanisms can improve the adaptivity, robustness, and efficiency of a vast array of engineering systems and soft robots. In this study, we introduce a tunable-stiffness mechanism called a "sandwich jamming structure," which fuses the exceptional stiffness range of state-of-the-art laminar jamming structures (also known as layer jamming structures) with the high stiffness-to-mass ratios of classical sandwich composites. We experimentally develop sandwich jamming structures with performance-to-mass ratios that are far greater than laminar jamming structures (e.g., a 550-fold increase in stiffness-to-mass ratio), while simultaneously achieving tunable behavior that standard sandwich composites inherently cannot achieve (e.g., a rapid and reversible 1800-fold increase in stiffness). Through theoretical and computational models, we then show that these ratios can be augmented by several orders of magnitude further, and we provide an optimization routine that allows designers to build the best possible sandwich jamming structures given arbitrary mass, volume, and material constraints. Finally, we demonstrate the utility of sandwich jamming structures by integrating them into a wearable soft robot (i.e., a tunable-stiffness wrist orthosis) that has negligible impact on the user in the off state, but can reduce muscle activation by an average of 41% in the on state. Through these theoretical and experimental investigations, we show that sandwich jamming structures are a lightweight highly tunable mechanism that can markedly extend the performance limits of existing structures and devices.

Design Molecular Topology for Wet-Dry Adhesion.

Recent innovations highlight the integration of diverse materials with synthetic and biological hydrogels. Examples include brain-machine interfaces, tissue regeneration, and soft ionic devices. Existing methods of strong adhesion mostly focus on the chemistry of bonds and the mechanics of dissipation but largely overlook the molecular topology of connection. Here, we highlight the significance of molecular topology by designing a specific bond-stitch topology. The bond-stitch topology achieves strong adhesion between preformed hydrogels and various materials, where the hydrogels have no functional groups for chemical coupling, and the adhered materials have functional groups on the surface. The adhesion principle requires a species of polymer chains to form a bond with a material through complementary functional groups and form a network in situ that stitches with the polymer network of a hydrogel. We study the physics and chemistry of this topology and describe its potential applications in medicine and engineering.

Author Correction: Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Adhesion between Hydrophobic Elastomer and Hydrogel through Hydrophilic Modification and Interfacial Segregation.

Recent progress in the printing of soft materials has made it possible to fabricate soft stretchable devices for a range of engineering applications. These devices tend to be heterogeneous systems, and their reliability depends to a large extent on the integrity of the interfaces between the various materials in the system. Previous studies on the printing of hydrogels have highlighted the need to investigate the adhesion between extrusion printable dielectric elastomers and hydrogels. Here we consider polydimethylsiloxane (PDMS) and a polyacrylamide hydrogel that contains lithium chloride and a nonionic rheological modifier. We show that the adhesion between oxygen plasma-treated PDMS and the hydrogel increases with time to reach a stable value of 15 J m-2 after ∼6 days. During that time, the contact angle of water on the PDMS interface remains constant at ∼30°, suggesting that hydrophobic recovery of plasma-treated PDMS is suppressed by the presence of the hydrogel. It is further observed that a thin viscous layer develops at the interface between PDMS and hydrogel, which results in energy dissipation upon debonding and which allows full recovery of the adhesion after debonding and rejoining. This viscous layer develops only in the presence of the rheological modifier in the hydrogel and the hydrophilic surface treatment of the PDMS.

Hydrogel bowls for cleaning oil spills on water.

We demonstrate a hydrogel bowl capable of selectively and rapidly collecting spilled oil while floating on water. The bowl has macroscopic openings in its sidewall, and its surface is first coated with octadecyltrichlorosilane (OTS) and then with diffusion pump oil, which imparts exceptional hydrophobic, oleophilic, and high oil wettability properties. The use of a hydrogel makes it possible to obtain surface hydrophobicity and oleophilicity, while also being inexpensive, eco-friendly, and easy to fabricate. Using a prototype of the bowl and a small pump system, we demonstrate that oils with a broad range of viscosities (2.7-2000.0 cSt at 20-40 °C) are more rapidly and efficiently collected from the surface of both pure water and seawater than with any other reported technique. The hydrogel bowl can collect oil for more than one month without losing its efficiency and can be stored in oil for reuse. Therefore, such hydrogel bowls represent a new alternative to conventional oil spill remediation techniques.

Highly Stretchable and Tough Hydrogels below Water Freezing Temperature.

Hydrogels consist of hydrophilic polymer networks dispersed in water. Many applications of hydrogels rely on their unique combination of solid-like mechanical behavior and water-like transport properties. If the temperature is lowered below 0 °C, however, hydrogels freeze and become rigid, brittle, and non-conductive. Here, a general class of hydrogels that do not freeze at temperatures far below 0 °C, while retaining high stretchability and fracture toughness, is demonstrated. These hydrogels are synthesized by adding a suitable amount of an ionic compound to the hydrogel. The present study focuses on tough polyacrylamide-alginate double network hydrogels equilibrated with aqueous solutions of calcium chloride. The resulting hydrogels can be cooled to temperatures as low as -57 °C without freezing. In this temperature range, the hydrogels can still be stretched more than four times their initial length and have a fracture toughness of 5000 J m-2 . It is anticipated that this new class of hydrogels will prove useful in developing new applications operating under a broad range of environmental and atmospheric conditions.

Cardiac microphysiological devices with flexible thin-film sensors for higher-throughput drug screening.

Microphysiological systems and organs-on-chips promise to accelerate biomedical and pharmaceutical research by providing accurate in vitro replicas of human tissue. Aside from addressing the physiological accuracy of the model tissues, there is a pressing need for improving the throughput of these platforms. To do so, scalable data acquisition strategies must be introduced. To this end, we here present an instrumented 24-well plate platform for higher-throughput studies of engineered human stem cell-derived cardiac muscle tissues that recapitulate the laminar structure of the native ventricle. In each well of the platform, an embedded flexible strain gauge provides continuous and non-invasive readout of the contractile stress and beat rate of an engineered cardiac tissue. The sensors are based on micro-cracked titanium-gold thin films, which ensure that the sensors are highly compliant and robust. We demonstrate the value of the platform for toxicology and drug-testing purposes by performing 12 complete dose-response studies of cardiac and cardiotoxic drugs. Additionally, we showcase the ability to couple the cardiac tissues with endothelial barriers. In these studies, which mimic the passage of drugs through the blood vessels to the musculature of the heart, we regulate the temporal onset of cardiac drug responses by modulating endothelial barrier permeability in vitro.

Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing.

The glass forming ability (GFA) of metallic glasses (MGs) is quantified by the critical cooling rate (R C). Despite its key role in MG research, experimental challenges have limited measured R C to a minute fraction of known glass formers. We present a combinatorial approach to directly measure R C for large compositional ranges. This is realized through the use of compositionally-graded alloy libraries, which were photo-thermally heated by scanning laser spike annealing of an absorbing layer, then melted and cooled at various rates. Coupled with X-ray diffraction mapping, GFA is determined from direct R C measurements. We exemplify this technique for the Au-Cu-Si system, where we identify Au56Cu27Si17 as the alloy with the highest GFA. In general, this method enables measurements of R C over large compositional areas, which is powerful for materials discovery and, when correlating with chemistry and other properties, for a deeper understanding of MG formation.

3D Printing of Transparent and Conductive Heterogeneous Hydrogel-Elastomer Systems.

A hydrogel-dielectric-elastomer system, polyacrylamide and poly(dimethylsiloxane) (PDMS), is adapted for extrusion printing for integrated device fabrication. A lithium-chloride-containing hydrogel printing ink is developed and printed onto treated PDMS with no visible signs of delamination and geometrically scaling resistance under moderate uniaxial tension and fatigue. A variety of designs are demonstrated, including a resistive strain gauge and an ionic cable.

Instrumented cardiac microphysiological devices via multimaterial three-dimensional printing.

Biomedical research has relied on animal studies and conventional cell cultures for decades. Recently, microphysiological systems (MPS), also known as organs-on-chips, that recapitulate the structure and function of native tissues in vitro, have emerged as a promising alternative. However, current MPS typically lack integrated sensors and their fabrication requires multi-step lithographic processes. Here, we introduce a facile route for fabricating a new class of instrumented cardiac microphysiological devices via multimaterial three-dimensional (3D) printing. Specifically, we designed six functional inks, based on piezo-resistive, high-conductance, and biocompatible soft materials that enable integration of soft strain gauge sensors within micro-architectures that guide the self-assembly of physio-mimetic laminar cardiac tissues. We validated that these embedded sensors provide non-invasive, electronic readouts of tissue contractile stresses inside cell incubator environments. We further applied these devices to study drug responses, as well as the contractile development of human stem cell-derived laminar cardiac tissues over four weeks.

Spectral descriptors for bulk metallic glasses based on the thermodynamics of competing crystalline phases.

Metallic glasses attract considerable interest due to their unique combination of superb properties and processability. Predicting their formation from known alloy parameters remains the major hindrance to the discovery of new systems. Here, we propose a descriptor based on the heuristics that structural and energetic 'confusion' obstructs crystalline growth, and demonstrate its validity by experiments on two well-known glass-forming alloy systems. We then develop a robust model for predicting glass formation ability based on the geometrical and energetic features of crystalline phases calculated ab initio in the AFLOW framework. Our findings indicate that the formation of metallic glass phases could be much more common than currently thought, with more than 17% of binary alloy systems potential glass formers. Our approach pinpoints favourable compositions and demonstrates that smart descriptors, based solely on alloy properties available in online repositories, offer the sought-after key for accelerated discovery of metallic glasses.

Extremely Stretchable and Fast Self-Healing Hydrogels.

Dynamic crosslinking of extremely stretchable hydrogels with rapid self-healing ability is described. Using this new strategy, the obtained hydrogels are able to elongate 100 times compared to their initial length and to completely self-heal within 30 s without external energy input.

First-Principles Analysis on the Catalytic Role of Additives in Low-Temperature Synthesis of Transition Metal Diborides Using Nanolaminates.

Carbon is known to significantly accelerate the formation reaction of zirconium diboride in reactive nanolaminates, although the detailed mechanism remains unclear. Here we investigate the catalytic effect of both C and N on the synthesis of ZrB2 using a first-principles theoretical approach. We show that the strong interactions of C and N with Zr at the B/Zr interfaces of the nanolaminate enhance the solid-state amorphization of the Zr lattice. Amorphization of the Zr, in turn, accelerates intermixing of the constituent layers of the reactive nanolaminate. On the basis of these results we propose that the addition of elements with strong binding energies to transition metals may facilitate low-temperature synthesis of transition metal diborides using reactive nanolaminates.

Adhesion between highly stretchable materials.

Recently developed high-speed ionic devices require adherent laminates of stretchable and dissimilar materials, such as gels and elastomers. Adhesion between stretchable and dissimilar materials also plays important roles in medicine, stretchable electronics, and soft robots. Here we develop a method to characterize adhesion between materials capable of large, elastic deformation. We apply the method to measure the debond energy of elastomer-hydrogel bilayers. The debond energy between an acrylic elastomer and a polyacrylamide hydrogel is found to be about 0.5 J m(-2), independent of the thickness and the crosslink density of the hydrogel. This low debond energy, however, allows the bilayer to be adherent and highly stretchable, provided that the hydrogel is thin and compliant. Furthermore, we demonstrate that nanoparticles applied at the interface can improve adhesion between the elastomer and the hydrogel.

Fire-Resistant Hydrogel-Fabric Laminates: A Simple Concept That May Save Lives.

There is a large demand for fabrics that can survive high-temperature fires for an extended period of time, and protect the skin from burn injuries. Even though fire-resistant polymer fabrics are commercially available, many of these fabrics are expensive, decompose rapidly, and/or become very hot when exposed to high temperatures. We have developed a new class of fire-retarding materials by laminating a hydrogel and a fabric. The hydrogel contains around 90% water, which has a large heat capacity and enthalpy of vaporization. When the laminate is exposed to fire, a large amount of energy is absorbed as water heats up and evaporates. The temperature of the hydrogel cannot exceed 100 °C until it is fully dehydrated. The fabric has a low thermal conductivity and maintains the temperature gradient between the hydrogel and the skin. The laminates are fabricated using a recently developed tough hydrogel to ensure integrity of the laminate during processing and use. A thermal model predicts the performance of the laminates and shows that they have excellent heat resistance in good agreement with experiments, making them viable candidates in life saving applications such as fire-resistant blankets or apparel.