Dynamics of electrohydraulic soft actuators.

Nature has inspired the design of robots in which soft actuators enable tasks such as handling of fragile objects and adapting to unstructured environments. Those tasks are difficult for traditional robots, which predominantly consist of hard components. Electrohydraulic soft actuators are liquid-filled shells that deform upon the application of electric fields; they excel among soft actuators with muscle-like force outputs and actuation strains, and with actuation frequencies above 100 Hz. However, the fundamental physics that governs the dynamics of electrohydraulic soft actuators is unexplored. Here, we study the dynamics of electrohydraulic soft actuators using the Peano-HASEL (hydraulically amplified self-healing electrostatic) actuator as a model system. Using experiments and a scaling analysis, we discover two dynamic regimes: a regime in which viscous dissipation reduces the actuation speed and a regime governed by inertial effects in which high-speed actuation is possible. For each regime, we derive a timescale that describes the influence of geometry, materials system, and applied external loads on the actuation speed. We also derive a model to study the dynamic behavior of Peano-HASEL actuators in both regimes. Although this analysis focuses on the Peano-HASEL actuator, the presented results may readily be generalized to other electrohydraulic actuators. When designed to operate in the inertial regime, electrohydraulic actuators will enable bio-inspired robots with unprecedented speeds of motion.

Digital logic for soft devices.

Although soft devices (grippers, actuators, and elementary robots) are rapidly becoming an integral part of the broad field of robotics, autonomy for completely soft devices has only begun to be developed. Adaptation of conventional systems of control to soft devices requires hard valves and electronic controls. This paper describes completely soft pneumatic digital logic gates having a physical scale appropriate for use with current (macroscopic) soft actuators. Each digital logic gate utilizes a single bistable valve-the pneumatic equivalent of a Schmitt trigger-which relies on the snap-through instability of a hemispherical membrane to kink internal tubes and operates with binary high/low input and output pressures. Soft, pneumatic NOT, AND, and OR digital logic gates-which generate known pneumatic outputs as a function of one, or multiple, pneumatic inputs-allow fabrication of digital logic circuits for a set-reset latch, two-bit shift register, leading-edge detector, digital-to-analog converter (DAC), and toggle switch. The DAC and toggle switch, in turn, can control and power a soft actuator (demonstrated using a pneu-net gripper). These macroscale soft digital logic gates are scalable to high volumes of airflow, do not consume power at steady state, and can be reconfigured to achieve multiple functionalities from a single design (including configurations that receive inputs from the environment and from human users). This work represents a step toward a strategy to develop autonomous control-one not involving an electronic interface or hard components-for soft devices.

Characterizing Chelation at Surfaces by Charge Tunneling.

This paper describes a surface analysis technique that uses the "EGaIn junction" to measure tunneling current densities (J(V), amps/cm2) through self-assembled monolayers (SAMs) terminated in a chelating group and incorporating different transition metal ions. Comparisons of J(V) measurements between bare chelating groups and chelates are used to characterize the composition of the SAM and infer the dissociation constant (Kd, mol/L), as well as kinetic rate constants (koff, L/mol·s; kon, 1/s) of the reversible chelate-metal reaction. To demonstrate the concept, SAMs of 11-(4-methyl-2,2'-bipyrid-4'-yl (bpy))undecanethiol (HS(CH2)11bpy) were incubated within ethanol solutions of metal salts. After rinsing and drying the surface, measurements of current as a function of incubation time and concentration in solution are used to infer koff, kon, and Kd. X-ray photoelectron spectroscopy (XPS) provides an independent measure of surface composition to confirm inferences from J(V) measurements. Our experiments establish that (i) bound metal ions are stable to the rinsing step as long as the rinsing time, τrinse ≪ 1koff; (ii) the bound metal ions increase the current density at the negative bias and reduce the rectification observed with free bpy terminal groups; (iii) the current density as a function of the concentration of metal ions in solution follows a sigmoidal curve; and (iv) the values of Kd measured using J(V) are comparable to those measured using XPS, but larger than those measured in solution. The EGaIn junction, thus, provides a new tool for the analysis of the composition of the surfaces that undergo reversible chemical reactions with species in solution.

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol).

This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiol-terminated derivatives of oligo(ethylene glycol) (HS(CH2CH2O)nCH3; HS(EG)nCH3); these SAMs are positioned between gold bottom electrodes and Ga2O3/EGaIn top electrodes. Comparison of the attenuation factor (ß of the simplified Simmons equation) across these SAMs with the corresponding value obtained with length-matched SAMs of oligophenyls (HS(Ph)nH) and n-alkanethiols (HS(CH2)nH) demonstrates that SAMs of oligo(ethylene glycol) have values of ß (ß(EG)n = 0.29 ± 0.02 natom-1 and ß = 0.24 ± 0.01 Å-1) indistinguishable from values for SAMs of oligophenyls (ß(Ph)n = 0.28 ± 0.03 Å-1), and significantly lower than those of SAMs of n-alkanethiolates (ß(CH2)n = 0.94 ± 0.02 natom-1 and 0.77 ± 0.03 Å-1). There are two possible origins for this low value of ß. The more probable involves hole tunneling by superexchange, which rationalizes the weak dependence of the rate of charge transport on the length of the molecules of HS(EG)nCH3 using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. Based on this mechanism, SAMs of oligo(ethylene glycol)s are good conductors (by hole tunneling) but good insulators (by electron and/or hole drift conduction). This observation suggests SAMs derived from these or electronically similar molecules are a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of ß for SAMs of S(EG)nCH3 rests on the possibility of disorder in the SAM and a systematic discrepancy between different estimates of the thickness of these SAMs.

Ultrafast, High-Strain, and Strong Uniaxial Hydrogel Actuators from Recyclable Nanofibril Networks.

Polymer hydrogels mimic biological tissues and are suitable for future lifelike machines. However, their actuation is isotropic, so they must be crosslinked or placed in a turgor membrane to achieve high actuation pressures, severely impeding their performance. Here, it is shown that organizing cellulose nanofibrils (CNFs) in anisotropic hydrogel sheets leads to mechanical in-plane reinforcement that generates a uniaxial, out-of-plane strain with performance far surpassing polymer hydrogels. These fibrillar hydrogel actuators expand uniaxially by 250 times with an initial rate of 100-130% s-1 , compared to <10 times and <1% s-1 in directional strain rate for isotropic hydrogels, respectively. The blocking pressure reaches 0.9 MPa, similar to turgor actuators, while the time to reach 90% of the maximum pressure is 1-2 min, compared to 10 min to hours for polymer hydrogel actuators. Uniaxial actuators that lift objects 120 000 times their weight and soft grippers are showcased. In addition, the hydrogels can be recycled without a loss in performance. The uniaxial swelling allows adding channels through the gel for local solvent delivery, further increasing the actuation rate and cyclability. Thus, fibrillar networks can overcome the major drawbacks of hydrogel actuators and is a significant advancement towards hydrogel-based lifelike machines.

Electrochemically Controlled Hydrogels with Electrotunable Permeability and Uniaxial Actuation.

The unique properties of hydrogels enable the design of life-like soft intelligent systems. However, stimuli-responsive hydrogels still suffer from limited actuation control. Direct electronic control of electronically conductive hydrogels can solve this challenge and allow direct integration with modern electronic systems. An electrochemically controlled nanowire composite hydrogel with high in-plane conductivity that stimulates a uniaxial electrochemical osmotic expansion is demonstrated. This materials system allows precisely controlled shape-morphing at only -1 V, where capacitive charging of the hydrogel bulk leads to a large uniaxial expansion of up to 300%, caused by the ingress of ≈700 water molecules per electron-ion pair. The material retains its state when turned off, which is ideal for electrotunable membranes as the inherent coupling between the expansion and mesoporosity enables electronic control of permeability for adaptive separation, fractionation, and distribution. Used as electrochemical osmotic hydrogel actuators, they achieve an electroactive pressure of up to 0.7 MPa (1.4 MPa vs dry) and a work density of ≈150 kJ m-3 (2 MJ m-3  vs dry). This new materials system paves the way to integrate actuation, sensing, and controlled permeation into advanced soft intelligent systems.

Biodegradable electrohydraulic actuators for sustainable soft robots.

Combating environmental pollution demands a focus on sustainability, in particular from rapidly advancing technologies that are poised to be ubiquitous in modern societies. Among these, soft robotics promises to replace conventional rigid machines for applications requiring adaptability and dexterity. For key components of soft robots, such as soft actuators, it is thus important to explore sustainable options like bioderived and biodegradable materials. We introduce systematically determined compatible materials systems for the creation of fully biodegradable, high-performance electrohydraulic soft actuators, based on various biodegradable polymer films, ester-based liquid dielectric, and NaCl-infused gelatin hydrogel. We demonstrate that these biodegradable actuators reliably operate up to high electric fields of 200 V/µm, show performance comparable to nonbiodegradable counterparts, and survive more than 100,000 actuation cycles. Furthermore, we build a robotic gripper based on biodegradable soft actuators that is readily compatible with commercial robot arms, encouraging wider use of biodegradable materials systems in soft robotics.

HASEL Artificial Muscles for a New Generation of Lifelike Robots-Recent Progress and Future Opportunities.

Future robots and intelligent systems will autonomously navigate in unstructured environments and closely collaborate with humans; integrated with our bodies and minds, they will allow us to surpass our physical limitations. Traditional robots are mostly built from rigid, metallic components and electromagnetic motors, which make them heavy, expensive, unsafe near people, and ill-suited for unpredictable environments. By contrast, biological organisms make extensive use of soft materials and radically outperform robots in terms of dexterity, agility, and adaptability. Particularly, natural muscle-a masterpiece of evolution-has long inspired researchers to create "artificial muscles" in an attempt to replicate its versatility, seamless integration with sensing, and ability to self-heal. To date, natural muscle remains unmatched in all-round performance, but rapid advancements in soft robotics have brought viable alternatives closer than ever. Herein, the recent development of hydraulically amplified self-healing electrostatic (HASEL) actuators, a new class of high-performance, self-sensing artificial muscles that couple electrostatic and hydraulic forces to achieve diverse modes of actuation, is discussed; current designs match or exceed natural muscle in many metrics. Research on materials, designs, fabrication, modeling, and control systems for HASEL actuators is detailed. In each area, research opportunities are identified, which together lays out a roadmap for actuators with drastically improved performance. With their unique versatility and wide potential for further improvement, HASEL actuators are poised to play an important role in a paradigm shift that fundamentally challenges the current limitations of robotic hardware toward future intelligent systems that replicate the vast capabilities of biological organisms.

Liquid Crystal Elastomers with Enhanced Directional Actuation to Electric Fields.

The integration of soft, stimuli-responsive materials in robotic systems is a promising approach to introduce dexterous and delicate manipulation of objects. Electrical control of mechanical response offers many benefits in robotic systems including the availability of this energy input, the associated response time, magnitude of actuation, and opportunity for self-regulation. Here, a materials chemistry is detailed to prepare liquid crystal elastomers (LCEs) with a 14:1 modulus contrast and increase in dielectric constant to enhance electromechanical deformation. The inherent modulus contrast of these LCEs (when coated with compliant electrodes) directly convert an electric field to a directional expansion of 20%. The electromechanical response of LCE actuators is observed upon application of voltage ranging from 0.5 to 6 kV. The deformation of these materials is rapid, reaching strain rates of 18% s-1 . Upon removal of the electric field, little hysteresis is observed. Patterning the spatial orientation of the nematic director of the LCEs results in a 2D-3D shape transformation to a cone 8 mm in height. Individual and sequential addressing of an array of LCE actuators is demonstrated as a haptic surface.

Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints.

The impressive locomotion and manipulation capabilities of spiders have led to a host of bioinspired robotic designs aiming to reproduce their functionalities; however, current actuation mechanisms are deficient in either speed, force output, displacement, or efficiency. Here-using inspiration from the hydraulic mechanism used in spider legs-soft-actuated joints are developed that use electrostatic forces to locally pressurize a hydraulic fluid, and cause flexion of a segmented structure. The result is a lightweight, low-profile articulating mechanism capable of fast operation, high forces, and large displacement; these devices are termed spider-inspired electrohydraulic soft-actuated (SES) joints. SES joints with rotation angles up to 70°, blocked torques up to 70 mN m, and specific torques up to 21 N m kg-1 are demonstrated. SES joints demonstrate high speed operation, with measured roll-off frequencies up to 24 Hz and specific power as high as 230 W kg-1 -similar to human muscle. The versatility of these devices is illustrated by combining SES joints to create a bidirectional joint, an artificial limb with independently addressable joints, and a compliant gripper. The lightweight, low-profile design, and high performance of these devices, makes them well-suited toward the development of articulating robotic systems that can rapidly maneuver.

A soft ring oscillator.

Periodic actuation of multiple soft, pneumatic actuators requires coordinated function of multiple, separate components. This work demonstrates a soft, pneumatic ring oscillator that induces temporally coordinated periodic motion in soft actuators using a single, constant-pressure source, without hard valves or electronic controls. The fundamental unit of this ring oscillator is a soft, pneumatic inverter (an inverting Schmitt trigger) that switches between its two states ("on" and "off") using two instabilities in elastomeric structures: buckling of internal tubing and snap-through of a hemispherical membrane. An odd number of these inverters connected in a loop produces the same number of periodically oscillating outputs, resulting from a third, system-level instability; the frequency of oscillation depends on three system parameters that can be adjusted. These oscillatory output pressures enable several applications, including undulating and rolling motions in soft robots, size-based particle separation, pneumatic mechanotherapy, and metering of fluids. The soft ring oscillator eliminates the need for hard valves and electronic controls in these applications.

A soft, bistable valve for autonomous control of soft actuators.

Almost all pneumatic and hydraulic actuators useful for mesoscale functions rely on hard valves for control. This article describes a soft, elastomeric valve that contains a bistable membrane, which acts as a mechanical "switch" to control air flow. A structural instability-often called "snap-through"-enables rapid transition between two stable states of the membrane. The snap-upward pressure, ΔP 1 (kilopascals), of the membrane differs from the snap-downward pressure, ΔP 2 (kilopascals). The values ΔP 1 and ΔP 2 can be designed by changing the geometry and the material of the membrane. The valve does not require power to remain in either "open" or "closed" states (although switching does require energy), can be designed to be bistable, and can remain in either state without further applied pressure. When integrated in a feedback pneumatic circuit, the valve functions as a pneumatic oscillator (between the pressures ΔP 1 and ΔP 2), generating periodic motion using air from a single source of constant pressure. The valve, as a component of pneumatic circuits, enables (i) a gripper to grasp a ball autonomously and (ii) autonomous earthworm-like locomotion using an air source of constant pressure. These valves are fabricated using straightforward molding and offer a way of integrating simple control and logic functions directly into soft actuators and robots.

Slit Tubes for Semisoft Pneumatic Actuators.

This article describes a new principle for designing soft or 'semisoft' pneumatic actuators: SLiT (for SLit-in-Tube) actuators. Inflating an elastomeric balloon, when enclosed by an external shell (a material with higher Young's modulus) containing slits of different directions and lengths, produces a variety of motions, including bending, twisting, contraction, and elongation. The requisite pressure for actuation depends on the length of the slits, and this dependence allows sequential actuation by controlling the applied pressure. Different actuators can also be controlled using external "sliders" that act as reprogrammable "on-off" switches. A pneumatic arm and a walker constructed from SLiT actuators demonstrate their ease of fabrication and the range of motions they can achieve.

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.

Stretchable, transparent, ionic conductors.

Existing stretchable, transparent conductors are mostly electronic conductors. They limit the performance of interconnects, sensors, and actuators as components of stretchable electronics and soft machines. We describe a class of devices enabled by ionic conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts. We demonstrate a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reaction. The ionic conductors have higher resistivity than many electronic conductors; however, when large stretchability and high transmittance are required, the ionic conductors have lower sheet resistance than all existing electronic conductors.