Powerful, soft combustion actuators for insect-scale robots.

Insects perform feats of strength and endurance that belie their small stature. Insect-scale robots-although subject to the same scaling laws-demonstrate reduced performance because existing microactuator technologies are driven by low-energy density power sources and produce small forces and/or displacements. The use of high-energy density chemical fuels to power small, soft actuators represents a possible solution. We demonstrate a 325-milligram soft combustion microactuator that can achieve displacements of 140%, operate at frequencies >100 hertz, and generate forces >9.5 newtons. With these actuators, we powered an insect-scale quadrupedal robot, which demonstrated a variety of gait patterns, directional control, and a payload capacity 22 times its body weight. These features enabled locomotion through uneven terrain and over obstacles.

Multistable Metafluid based Energy Harvesting and Storage.

The thermodynamic properties of fluids play a crucial role in many engineering applications, particularly in the context of energy. Fluids with multistable thermodynamic properties may offer new paths for harvesting and storing energy via transitions between equilibria states. Such artificial multistable fluids can be created using the approach employed in metamaterials, which controls macro-properties through micro-structure composition. In this work, the dynamics of such "metafluids" is examined for a configuration of calorically-perfect compressible gas contained within multistable elastic capsules flowing in a fluid-filled tube. The velocity-, pressure-, and temperature-fields of multistable compressible metafluids is studied by both analytically and experimentally, focusing on transitions between different equilibria. The dynamics of a single capsule is first examine, which may move or change equilibrium state, due to fluidic forces. The interaction and motion of multiple capsules within a fluid-filled tube is then studied. It shows that such a system can be used to harvest energy from external temperature variations in either time or space. Thus, fluidic multistability allows specific quanta of energy to be captured and stored indefinitely as well as transported as a fluid, via tubes, at standard atmospheric conditions without the need for thermal isolation.

A metafluid with multistable density and internal energy states.

Investigating and tailoring the thermodynamic properties of different fluids is crucial to many fields. For example, the efficiency, operation range, and environmental safety of applications in energy and refrigeration cycles are highly affected by the properties of the respective available fluids. Here, we suggest combining gas, liquid and multistable elastic capsules to create an artificial fluid with a multitude of stable states. We study, theoretically and experimentally, the suspension's internal energy, equilibrium pressure-density relations, and their stability for both adiabatic and isothermal processes. We show that the elastic multistability of the capsules endows the fluid with multistable thermodynamic properties, including the ability of capturing and storing energy at standard atmospheric conditions, not found in naturally available fluids.

Fluid-Driven Traveling Waves in Soft Robots.

Many marine creatures, gastropods, and earthworms generate continuous traveling waves in their bodies for locomotion within marine environments, complex surfaces, and inside narrow gaps. In this work, we study theoretically and experimentally the use of embedded pneumatic networks as a mechanism to mimic nature and generate bidirectional traveling waves in soft robots. We apply long-wave approximation to theoretically calculate the required distribution of pneumatic network and inlet pressure oscillations needed to create desired moving wave patterns. We then fabricate soft robots with internal pneumatic network geometry based on these analytical results. The experimental results agree well with our model and demonstrate the propagation of moving waves in soft robots, along with locomotion capabilities. The presented results allow fabricating soft robots capable of continuous moving waves using the common approach of embedded pneumatic networks and requiring only two input controls.

Valveless microliter combustion for densely packed arrays of powerful soft actuators.

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane-oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

Microscale Hydrodynamic Cloaking and Shielding via Electro-Osmosis.

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both "cloaking" conditions, where the flow field outside the cloaking region is unaffected by the object, and "shielding" conditions, where the hydrodynamic forces on the object are eliminated. Using field-effect electro-osmosis as a mechanism for injection of momentum, we present a theoretical framework and analytical solutions for a range of geometrical shapes, validate these both numerically and experimentally, and demonstrate the ability to dynamically switch between the different states.

Fluid Mechanics of Pneumatic Soft Robots.

Pressurization of gas within embedded channels and cavities is a popular method for actuating soft robots. Various previous works examined the effects of internal fluid mechanics on this actuation approach, as well as on leveraging viscous effects to extend the capabilities of soft robots. However, no existing works studied the combined effects of fluid viscosity and compressibility, relevant to miniaturized configurations, which is the aim of the current work. We derive a general model for compressible viscous flow in an elastic media representing a simplified miniaturized soft robot. We illustrate applying this model to periodic configurations, simplifying it via a long-wave approximation. Steady, and time-dependent solutions are obtained, allowing to model the flow and to provide insight into the actuation dynamics of miniaturized pneumatic soft robots.

Underactuated fluidic control of a continuous multistable membrane.

This work addresses the challenge of underactuated pattern generation in continuous multistable structures. The examined configuration is a slender membrane which can concurrently sustain two different equilibria states, separated by transition regions, and is actuated by a viscous fluid. We first demonstrate the formation and motion of a single transition region and then sequencing of several such moving transition regions to achieve arbitrary patterns by controlling the inlet pressure of the actuating fluid. Finally, we show that nonuniform membrane properties, along with transient dynamics of the fluid, can be leveraged to directly snap through any segment of the membrane.

Nonuniform Electro-osmotic Flow Drives Fluid-Structure Instability.

We demonstrate the existence of a fluid-structure instability arising from the interaction of electro-osmotic flow with an elastic substrate. Considering the case of flow within a soft fluidic chamber, we show that above a certain electric field threshold, negative gauge pressure induced by electro-osmotic flow causes the collapse of its elastic walls. We combine experiments and theoretical analysis to elucidate the underlying mechanism for instability and identify several distinct dynamic regimes. The understanding of this instability is important for the design of electrokinetic systems containing soft elements.

Single-Input Control of Multiple Fluid-Driven Elastic Actuators via Interaction Between Bistability and Viscosity.

A leading concept in soft robotics actuation, as well as in microfluidics applications such as valves in lab-on-a-chip devices, is applying pressurized flow in cavities embedded within elastic bodies. Generating complex deformation patterns typically requires control of several inputs, which greatly complicates the system's operation. In this study, we present a novel method for single-input control of a serial chain of bistable elastic chambers connected by thin tubes. Controlling a single flow rate at the chain's inlet, we induce an irreversible sequence of transitions that can reach any desired state combination of all bistable elements. Mathematical formulation and analysis of the system's dynamics reveal that these transitions are enabled, thanks to bistability combined with pressure lag induced by viscous resistance. The results are demonstrated through numerical simulations combined with experiments for chains of up to five chambers, using water-diluted glycerol as the injected fluid. The proposed technique has a promising potential for development of sophisticated soft actuators with minimalistic control.

Leveraging Viscous Peeling to Create and Activate Soft Actuators and Microfluidic Devices.

The research fields of microfluidics and soft robotics both involve complex small-scale internal channel networks, embedded within a solid structure. This study examines leveraging viscous peeling as a mechanism to create and activate soft actuators and microchannel networks, including complex elements such as valves, without the need for fabrication of structures with micron-scale internal cavities. We consider configurations composed of an internal slender structure embedded within another elastic solid. Pressurized viscous fluid is introduced into the interface between the two solids, thus peeling the two elastic structures and creating internal cavities. Since the gap between the solids is determined by the externally applied pressure, the characteristic size of the fluid network may vary with time and be much smaller than the resolution of the fabrication method. This study presents a model for the highly nonlinear elastic-viscous dynamics governing the flow and deformation of such configurations. Fabrication and experimental demonstrations of micron-scale valves and channel networks created from millimeter scale structures are presented, as well as the transient dynamics of viscous peeling-based soft actuators. The experimental data are compared with the suggested model, showing very good agreement.

Leveraging Internal Viscous Flow to Extend the Capabilities of Beam-Shaped Soft Robotic Actuators.

Elastic deformation of beam-shaped structures due to embedded fluidic networks (EFNs) is mainly studied in the context of soft actuators and soft robotic applications. Currently, the effects of viscosity are not examined in such configurations. In this work, we introduce an internal viscous flow and present the extended range of actuation modes enabled by viscosity. We analyze the interaction between elastic deflection of a slender beam and viscous flow in a long serpentine channel embedded within the beam. The embedded network is positioned asymmetrically with regard to the neutral plane and thus pressure within the channel creates a local moment deforming the beam. Under assumptions of creeping flow and small deflections, we obtain a fourth-order integro-differential equation governing the time-dependent deflection field. This relation enables the design of complex time-varying deformation patterns of beams with EFNs. Leveraging viscosity allows to extend the capabilities of beam-shaped actuators such as creation of inertia-like standing and moving wave solutions in configurations with negligible inertia and limiting deformation to a small section of the actuator. The results are illustrated experimentally.

Dynamics of Elastic Beams with Embedded Fluid-Filled Parallel-Channel Networks.

A pressurized fluid-filled parallel-channel network embedded in an elastic beam, asymmetrically to the neutral plane, will create a deformation field within the beam. Deformation due to embedded fluidic networks is currently studied in the context of soft actuators and soft-robotic applications. Expanding on this concept, configurations can be designed so that the pressure in the channel network is created directly from external forces acting on the beam, and thus can be viewed as passive solid-fluid composite structures. We approximate the deformation of such structures and relate the fluid pressure and geometry of the network to a continuous deformation-field function. This enables the design of networks creating steady arbitrary deformation fields as well as to eliminate deformation created by external time-varying forces, thus increasing the effective rigidity of the beam. In addition, by including the effects of the deformation created by the channel network on the beam inertia, we can modify the response of the beam to external time-varying forces. We present a scheme to design channel networks that create predefined oscillating deformation patterns in response to external oscillating forces. The ability to include inertial effects is relevant to the design of dynamic soft robots and soft actuators. Our results are illustrated and validated by numerical computations.