Soft material for soft actuators — TEI-tagged view

Aslan MiriyevpersName, Kenneth StackpersName, Hod LipsonpersName (2017date). “Soft material for soft actuators.” Nature Communicationstitle·j.

Object of study — a silicone-ethanol composite as an electrically driven soft actuatorterm·key

Research question — how to make self-contained, electrically driven soft actuatorsterm·key with high strainterm·key density

Contribution — a cheap castable and 3D-printableterm·key composite reaching ~900% strainterm·key and 1.3 MPa stress at low density

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Soft material for soft actuatorsterm·key Abstract Inspired by natural muscle, a key challenge in soft roboticsterm·key is to develop self-contained electrically driven soft actuatorsterm·key with high strainterm·key density. Various characteristics of existing technologies, such as the high voltages required to trigger electroactive polymers ( > 1KV), low strainterm·key ( < 10%) of shape memory alloys and the need for external compressors and pressure-regulating components for hydraulic or pneumatic fluidicelastomer actuators, limit their practicality for untethered applications. Here we show a single self-contained soft robust composite material that combines the elastic properties of a polymeric matrix and the extreme volume change accompanying liquid–vapor transitionterm·key. The material combines a high strainterm·key (up to 900%) and correspondingly high stress (up to 1.3 MPa) with low density (0.84 g cm −3 ). Along with its extremely low cost (about 3 cent per gram), simplicity of fabrication and environment-friendliness, these properties could enable new kinds of electrically driven entirely soft robots.

Introduction Inspired by biology, researchers aim to develop soft-bodied programmable motion in order to combine natural compliance with controllable actuation. One of the long standing challenges has been the lack of easily processed robust soft actuatorsterm·key with high strainterm·key density 1 – 5 . Such actuators would be easy to produce and to mold, cut, and 3D print into a desired shape, yet would produce large macroscopic actuation at relatively low voltage and current. Today, soft actuation techniques are based on either electroactive polymers 6 – 12 , shape memory alloys and shape memory polymers 13 – 15 , or compressed air and pressurized fluids actuators 16 – 24 . However, the high voltages required to trigger electroactive polymers ( > 1KV) and low strainterm·key ( < 10%) of shape memory alloys, as well as the need for external compressors and pressure-regulating components for hydraulic 16 , 18 , 21 , 24 or pneumatic 16 , 17 , 19 , 20 , 22 , 23 fluidic elastomer actuators, limit their miniaturization 2 , 4 , 16 and practicality for untethered applications. Recent demonstrations of actuation based on combustion 25 are ideal for impact delivery, but are less suitable for controllable kinematics.

Phase change materialsterm·dom offer an attractive alternative to conventional electromechanical actuatorsterm·dom. Such materials rely on the mechanical force produced by the rapid expansionterm·dom that occurs at the phase transition temperature. One of the classic examples of phase change materialsterm·dom is paraffin, which thermomechanical properties were first utilized in early 1930s 26 for self-regulating vents in greenhouses.While paraffin-based actuators can deliver large forces, their strainterm·key remains in the order of 10% volumetric change 26 – 28 , a strainterm·key that is on par with shape memory alloys and too small for most robotics applications.

A significantly higher expansionterm·dom strainterm·key may be achieved by utilizing reversible liquid–vapor phase transition, but such material systems have been traditionally difficult to contain and control. A number of recent devices use entrapped liquid inside balloons or between thin films, to form expanding cavities 29 – 32 . Electrically triggered deformation of soft elastomer membranes, utilizing liquid–gas transitionterm·dom of liquid, was reported to show large area expansionterm·dom 33 . However, such devices are challenging to manufacture and to form into arbitrary shapes because of their intricate internal design. For example, it is difficult to directly cast or 3D-print any of these actuators.

Here we propose a single easily prepared soft robust material that combines the elastic properties of a polymeric matrix and the extreme volume change of a fluid upon liquid–vapor transitionterm·key. We show and characterize the soft composite materialterm·dom comprised of a silicone elastomer matrixterm·dom with ethanolterm·key distributed throughout it in micro-bubbles, exhibiting strainsterm·key up to 900%, and demonstrate its use as an actuator in a range of robotic applications.

Results Materials system and its principles of action Choosing a polymer matrix and a fluid for the composite meta-material system was guided by the desired mechanical properties of a polymer, boiling point and practical handling restrictions of a fluid, and chemical compatibility of the two. We aimed to synthesize a cheap, simple, user- and environment-friendly material comprised of food-safe and bio-compatible materials. We chose PDMS-based silicone elastomerterm·key, a non-hazardous elastomer widely used for soft roboticterm·key applications, as a matrix material, and ethanolterm·key, a widely used alcohol with boiling temperature 78.4 °C and matrix-compatibility, as the active fluid (Supplementary Fig. 1 and Supplementary Discussion for a discussion of the material components choice).

Ethanolterm·key, included inside tiny micro-bubbles embedded in the elastic silicone rubber matrix, boils upon reaching the liquid–gas transitionterm·dom temperature, accompanied by tremendous increase in volume, leading to significant expansionterm·dom of the whole soft composite materialterm·dom. This composite material may be quickly and easily prepared by mixing ethanolterm·key with silicone elastomerterm·key (Supplementary Movie 1 , Supplementary Software 1 ). The mixed material is both castable and 3D-printableterm·key (Supplementary Movies 2 , 4 ), and after preparation will solidify in room-temperature curing. We successfully mixed various amounts of ethanolterm·key (0–33 vol%) in the two-part platinum-catalyzed silicone elastomerterm·key (Supplementary Fig. 2 ). In total 20 vol% ethanolterm·key was chosen as optimal composition.

We show the material as an artificial muscleterm·key that can be electrically actuated using a thin resistive wire (Fig. 1a ) and low power characteristics (8 V, 1 A) to exhibit significant expansion-contraction ability (Fig. 1b ). Fig. 1 Soft artificial muscleterm·key. The muscle is composed of ethanolterm·key distributed throughout the solid silicone elastomer matrixterm·dom. a Electrically actuated muscle including thin resistive wire in a rest position on a human hand. b Expanded muscle actuated (8 V, 1 A) Due to mixing, ethanolterm·key is distributed throughout the silicone elastomer matrixterm·dom in bubble-shaped pores and forms local pressure equilibrium with its vapors. During curing, ethanolterm·key vapors occupy air pockets distributed throughout the material, create new pores and lower the developing internal vapor pressure by expanding the pores until the equilibrium with the external environment pressure is achieved. Ethanolterm·key wets both the silicone elastomerterm·key gel and the cured solid (Supplementary Fig. 1 ). Thus, ethanolterm·key spreads on the inner walls of the bubbles and the remainder of the space inside them, if any, is occupied by ethanolterm·key vapors and air (Fig. 2a ). Density of the material including 20 vol% ethanolterm·key was measured at 0.84 g cm −3 . Upon heating the composite to a temperature of 78.4 °C, ethanolterm·key boils and the local pressure inside the bubbles grows, forcing the elastic silicone elastomer matrixterm·dom to comply by expansionterm·dom in order to reduce the pressure (Fig. 2b ). Fig. 2 Structure and principle of operation of the soft composite materialterm·dom. a Microstructure: Illustration and a stereoscope image; scale bar is 1 mm. b Illustration of the expansionterm·dom process on example of a single ethanolterm·key bubble. c Micro-CT images of the material cross-section at room temperature and during heating. d Infra-red images of the material as the heating starts and during expansionterm·dom; the material is heated using Ni–Cr resistive wire Micro-CT scans of the composite material cross-section before and after activation illustrate the expansionterm·dom phenomena and provide insight into the material at room temperature and during the heating with a spiral-shaped resistive wire (Fig. 2c ). To facilitate the interpretation of the micro-CT scans, the material specimens were placed on top of a plastic container with liquid ethanolterm·key.

Liquid ethanolterm·key evaporates with temperature, giving rise to internal pressure inside the bubbles, which results in slightly expanded silicone elastomer matrixterm·dom. When ethanolterm·key passes the liquid–vapor phase transition, extreme volume change occurs and the silicone elastomer matrixterm·dom significantly expands. With growth in local pressure, the boiling temperature increases and thus, continued heating to temperatures slightly higher than 78.4 °C is required for further expansionterm·dom, until no liquid ethanolterm·key remains in the bubbles (Fig. 2b ). Infrared radiation images of the material at room temperature and during expansionterm·dom (using Ni–Cr spiral) are shown in Fig. 2d .

Mechanical properties A maximal volume expansionterm·dom of about 915% was measured at the temperature of 90 °C during controlled heating in a wide water bath (unconstrained volumetric expansionterm·dom). We used an Instron machine to measure the blocked directional force and actuation stressterm·var characteristics of the material during its electrical actuation using resistive spiral-shaped wire at low power (15 V, 1 A). First, we aimed to demonstrate an ability of the composite to lift a weight 1000 times more than its own. The 6 g material showed an ability of repeated lifting of a weight of 6.1 kg, shown in Fig. 3a as 30 repeated cycles of loading to 60 N (blocked one-directional force). A detailed view of three loading–unloading cycles is shown in Fig. 3b . In a separate experimentterm·mth, we measured maximal unidirectional force obtained at various blocked-strain levels in the 0–100% range. The maximal force for unstrained specimen weighing 2 g was about 120 N, which is equivalent to the actuation stressterm·var of 1.3 MPa and a maximal ability of lifting weight 6000 times larger than its own. Figure 3c shows that the force decreases with strainterm·key down to 35 N at 100% blocked strainterm·key. An extrapolation of the trend suggests that the material will reach strainterm·key limit of 140%, when allowed to expand in only one direction, as opposed to about 900% volumetric expansionterm·dom when unconstrained. Both 3D-printed and cast specimens show similar actuation behavior (Supplementary Movie 4 ). Fig. 3 Force–strainterm·key characteristics of the material (15 V, 1 A). a 30 following cycles of loading to 60 N (blocked force; cylindrical specimen; diameter 15.1 mm, length 40 mm; weight 6 g). b Detailed view of three cycles in a . c Blocked force at various elongations for constrained cylindrical actuation; error bar relates to s.d. Specimen diameter 11.1 mm, length 25 mm; weight 2 g. Note that the actuation depends on heating and cooling rates Implementation in robotics We demonstrate the implementation of our composite material as an actuator in a variety of robotic applications (Supplementary Movie 2 ). First, we show a McKibben-type muscle. Our self-sufficient artificial muscleterm·key does not require any compressors or pressure-regulating equipment (Fig. 4a ), and is capable of lifting weight much larger than its own (for example, a 13 g actuator lifts 1 kg in Fig. 4b ). We demonstrate its use as a bicep, which contracts and pulls the lower arm up, causing it to bend at the elbow (Fig. 4c ). The actuator is comprised of the composite material placed inside a braided mesh sleeving, fixed at the edges (Supplementary Fig. 3 ). The actuation is electrically driven using a spiral-shaped resistive wire (powered at 30 V, 1.5 A) passing inside the actuator. During the actuation, the composite material expands radially and contacts longitudinally, mimicking natural muscle behavior. Fig. 4 Implementation of the soft composite materialterm·dom as an actuator. a McKibben-type artificial muscleterm·key (soft composite materialterm·dom inside braided mesh sleeving) shows displacement of about 25%. b 13 g artificial muscleterm·key lifts the weight of 1 kg. c Soft artificial muscleterm·key implemented as a biceps lifting skeleton’s arm to 90° position at elbow ( a – c : actuation powered at 45 W (30 V, 1.5 A)). d Design of the bi-morph bending actuator. e All-soft two-leg “worm” and its locomotion powered at 8 W (8 V, 1 A). f The sleigh robot and its locomotion powered at 8 W (8 V, 1 A). g Tetrahedral robot evolved and 3D-printed in 2000 34 with embedded electrical motor. h The same robot with the soft composite materialterm·dom as an actuator embedded instead of the electrical motor. i Soft gripper lifting an egg (sequence from left to right; 8 V, 1 A) In addition, we designed an actuator consisting of our composite material attached to a layer of pure (unactuated) silicone elastomerterm·key, to create bimorph bending (Fig. 4d ). The actuation is electrically driven using a resistive wire powered at 8 W (8 V, 1 A). We used this actuator in two models of robots: the all-soft two-leg “worm” (Fig. 4e ), and a “sleigh” (Fig. 4f ). During the actuation, the composite material expands and bends due to the constricting force at the vicinity of the passive silicone elastomerterm·key layer.

Figure 4i shows an electrically driven soft gripper, comprised of the composite material with two soft legs at the edges for grasping. Upon actuation, bimorph bending occurs and the legs move towards the item (e.g., a raw egg) and enable its grasping and lifting.

In addition to soft body locomotion and grasping, we demonstrate the ability of the artificial muscleterm·key to substitute electrical motor in an existing robot. We used a biomimetic robot produced using evolutionary algorithms 34 . The original robot 34 (Fig. 4g ) was 3D-printed and contained a small electrical stepper motor embedded in the upper bar. We reprinted the original robot and replaced the electrical motor with a detachable 3D-printed unit (Supplementary Fig. 4 ) including our composite material as the active material (Fig. 4h ) embedded in a Teflon sleeve. We applied an electrical current (8 V, 1 A) through a resistive coil embedded inside the muscle, which exhibited directional expansionterm·dom and acted as a piston moving the upper part of the bar forward. We demonstrate the locomotion of each robot in Supplementary Movie 2 .

Discussion The proposed soft composite materialterm·dom demonstrates a combination of high strainterm·key (up to > 900%) and correspondingly high stress (up to 1.3 MPa) at low density (0.84 g cm −3 ). Even at 100% strainterm·key the material develops stress of 0.4 MPa and is capable of lifting weight about 1700 times greater than its own. These characteristics place this material in previously inaccessible region of the actuator stress–strainterm·key charts (Fig. 5a ). Our actuators are Pareto-undominated in specific actuation stressterm·var versus strainterm·key (Fig. 5b ). We suggest that the strainterm·key limit of our material is the maximal strainterm·key of the silicone elastomer matrixterm·dom (980%, according to the manufacturer). Along with its extremely low cost (laboratory cost of about 3 cent per gram), ease of fabrication, and environmental friendliness, these properties make this material an attractive solution where strainterm·key density is a critical factor. Fig. 5 Comparative stress-strain charts for electrically driven actuators. a Actuation stressterm·var plotted against strainterm·key. b Specific actuation stressterm·var (actuation stressterm·var divided by density of the material) plotted against strainterm·key. Abbreviations: DEA- dielectric elastomer actuator, FEA- fluidic elastomer actuator, IPMC- ionic polymer-metal composite, PAM- pneumatic artificial muscleterm·key (McKibben actuator), SMA- shape memory alloy. The proposed material is labeled “Soft Actuatorterm·key”. The ellipse designates a range of observed strainsterm·key spanning from constrained unidirectional expansionterm·dom of 140% to unconstrained volumetric expansionterm·dom of 900%. For Thermal Expansionterm·dom actuators, 10 and 100 K are the temperature change ranges in Kelvin degrees Efficiencyterm·key of the actuator heated by a resistive wire may be estimated as a ratio between the mechanical work produced per time, and the invested (consumed) electrical energy. Using data from Fig. 3 , the consumed electrical power is a product of the applied DC voltage and current: 15 V·1A = 15 W. According to Fig. 3b , the time it takes the actuator to reach a force of 60 N was 70 s. The strainterm·key level at 60 N force may be estimated from Fig. 3c as 70%. Accordingly, linear expansionterm·dom of the 40 mm long specimen was 0.7×40 mm = 28 mm = 0.028 m. Thus, the mechanical work done by the actuator may be calculated as a product of the force and the distance, namely 60 N·0.028 m = 1.68 J. The output power is 1.68 J/70 s = 0.024 W. Thus, the efficiencyterm·key of the actuator is (0.024 W/15 W)·100% ≈ 0.2%. This value corresponds to heating caused by a single-coil of the resistive wire. In Supplementary Discussion and Supplementary Fig. 5 we show that using a wire of the same resistance in different designs (single-, double-, and triple-coiled wire), heating times may vary significantly. For instance, using triple-coiled wire shortens the heating time by 40%, allowing to increase the efficiencyterm·key by this value. In this way, a simple change in a wire design, allowing more uniform distribution of the heat, may increase the efficiencyterm·key of our material to about 0.3%. This value is comparable to the values for other thermal expansionterm·dom actuators and for some shape memory alloys in the Ashby chart 35 . In Fig. 6 we show the efficiencyterm·key plotted against the actuation strainterm·key for most of the existing actuation methods relevant for soft actuation, including materials (SMA, piezoelectric materials, etc) and devices (hydraulic and pneumatic setups). Fig. 6 Maximal efficiencyterm·key plotted versus actuation strainterm·key for various actuating methods. Our material is labeled as “Soft Actuatorterm·key”. For Thermal Expansionterm·dom actuators, 10 and 100 K are the temperature change ranges in Kelvin degrees. The elliptical shape denotes the range of strainterm·key possible ranging from purely linear expansionterm·dom (140%) to full (unconstrained) volumetric expansionterm·dom (900%) It may be seen that actuation methods exhibiting high actuation strainterm·key (about 100%) along with high efficiencyterm·key are based on either hydraulic or pneumatic (FEA, about 87%; PAM, about 40%) devices. For DEAs 36 , depending on the mode of operation, the maximal efficiencyterm·key value of 26% was reported for the acrylic actuators (which are also superior to other DEAs in strainterm·key). However, such hydraulic and pneumatic solutions require external compressors and pressure-regulating equipment. In turn, DEAs require very high voltage for their operation. According to the chart, the next possible solution is SMAs; however, they show efficiencyterm·key of about 2% and strainsterm·key < 10%. This dichotomy highlights the state-of-the-art trade-off, in which a combination of reasonable efficiencyterm·key and strainterm·key cannot be achieved using easily-operated material-actuators based on low voltage, but only by devices requiring external system for power conversion (a compressor or high-voltage converter). In contrast, the proposed material offers high strainterm·key directly from low-voltage source, typically available in untethered applications. We suggest that the high actuation strainterm·key, along with low cost and simplicity of preparation of the proposed soft material, merit further exploration of methods to improve its efficiencyterm·key, which would then allow efficient material-actuator with very high strainterm·key.

Efficiencyterm·key and operation of the proposed actuator material highly depends on heating and cooling rates. For resistive heating, higher current or more distributed heating networks are likely to provide faster material expansionterm·dom. For cooling, an optimized design of the actuator geometry and the surface area may facilitate faster cooling rates. For example, a thin strip with large surface-to-volume ratio is likely to cool much faster than bulk material. In addition, active cooling solutions such as a Peltier Junction, or a liquid–flow cooling channels could also be used. In the latter, as silicone repels water (Supplementary Fig. 1 ), internal channels may be designed in the soft material for water-flow cooling. However, such solutions would require additional power and space on the potential robot/device.

Alternatively, we suggest a biology inspired solution using agonist-antagonist muscle pairsterm·mth. It is well known that during the contraction of biceps to bend the arm at the elbow, triceps are relaxed, and vice versa. This feature may be used to significantly reduce the de-actuation time of the proposed actuator. Once one actuator (biceps) will complete its work to bend the arm, the task of bringing the arm back into its initial open position may be done by triggering the second actuator (triceps), instead of waiting for the cooling of the biceps (Fig. 7 ). In the Supplementary Movie 3 we show that this method may increase the actuation/de-actuation time 2.4 times. Fig. 7 Agonist-antagonist soft actuatorterm·key pair (20 V, 1 A). a Initial position of biceps and triceps actuators; b Actuation (bending the arm) by biceps; c De-actuation (bringing the arm to its initial position) by triceps. Actuators size: 20 mm diameter, 100 mm length. This setup reduced actuation time by a factor of 2.4 compared with a single actuator To summarize, our work proposed a self-contained soft robust composite material, combining very high strainterm·key and reasonably high stress with low density, which is easily produced from bio-compatible components at a very low cost. This material-actuator may serve in a variety of applications, from traditional robotics to advanced bio-medical needs, and may enable a new kind of entirely soft robots.

Methods Materials We used platinum-catalyzed two-part silicone rubber Ecoflex 00-50 (Smooth-On, PA, USA) as a matrix material and ethanolterm·key ≥ 99.5% (Sigma Aldrich, MO, USA) as an active phase change materialterm·dom. Properties of the silicone rubber are shown in Table 1 below. Material preparation involves thorough hand-mixing of 20 vol% of ethanolterm·key with silicone elastomerterm·key (first with part A for about 2 min, then mixed with part B for about 2 min). The material is ready-to-cast and ready-to-print after the preparation. Room temperature curing of the cast or 3D-printed part takes up to 3 h. A commercially available 0.25 mm diamter Ni-chrome resistive wire was used for electrically driven heating of the artificial muscleterm·key (i.e., for the actuation). To comply with the expansionterm·dom of the actuator material, a helical spiral shape was chosen for the Ni–Cr wire. The wire was hand-wound on an 8 mm screw driver shaft as shown in Supplementary Fig. 6 . Table 1 Properties of the silicone rubber Ecoflex 00-50 (manufacturer declared) Specific gravity (ASTM-D-1475) Mixed viscosity (ASTM-D-2393) Pot Life (ASTM-D-2471) Cure time Shore hardness (ASTM-D-2240) Tensile strength (ASTM-D-412) Elongation at break (ASTM-D-412) Ecoflex 00-50 1.07 g cm −3 8000 cps 18 min 3 h 00-50 315 psi 980% 3D-Printing Fabrication of the actuator using 3D-Printer was performed on a lab-made desktop 3D-printer capable of direct printing of two materials in one print. For optimal printing using 14 gauge syringe tip, the material was held in the mixing container for 15 min before printing. The maximum 3D printing resolution of 0.8 mm was achieved using 20 gauge syringe tip.

3D-printing of the robotic demonstrators was done on commercial FDM machines: Ultimaker 2 + , Ultimaker (Gendermalsen, Netherlands) for the sleigh robot (material: PLA), and uPrint, Stratasys (MN, USA) for tetragonal evolved robot (material: acrylonitrile butadiene styrene (ABS)).

Characterization Olympus SZ51 (Tokyo, Japan) stereoscope with analytic software was used for optical characterization of the actuator material. We used 30 × 10 × 3 mm size specimens and appropriate lighting conditions to obtain images with highest contrast. Micrometrix AccyPyc II1340 (GA, USA) pycnometer was used for density evaluation of the actuator material. Ramé-hart (NJ, USA) model 190CA goniometer was used for contact angle measurements. Headspace Gas Chromatography Mass Spectrometry (GC–MS) analysis was performed on the PerkinElmer Clarus SQ8C model (Waltham, MA, USA) apparatus. Micro-CT (Mediso, Boston, MA), a small-animal CTterm·mth scanner with an image resolution of 100 microns, was used for x-ray computer tomographyterm·mth (CTterm·mth) of the internal structure of the actuator material. Water bath experimentterm·mth was conducted for evaluation of the maximal expansionterm·dom of the actuator material. A desktop hot-plate was used to heat the water bath (borosilicate glass beaker) while the water temperature was continuously controlled by a K-type thermocouple connected to the Fluke 52-2 dual input digital thermometer ( ± 0.1 °C). The water level was monitored during the experimentterm·mth and the volume was constantly evaluated. An Instron 8841 (MA, USA) machine was used for mechanical properties evaluation. For the blocked force experimentterm·mth, the cylindrical muscle specimen (40 mm length, 15.1 mm diameter) was placed in a polytetrafluoroethyleneterm·dom (PTFEterm·dom) cylinder, and inserted into a hollow aluminum cylinder with a closed top. The latter was attached to the upper static part of the Instron machine, while the specimen at the bottom faced a smaller aluminum cylinder connected directly to the load cell. The muscle was actuated by heating, provided using a Ni–Cr resistive wire and low power (15 V, 1 A). At heating, the muscle expanded and pressed against the bottom cylinder connected to the load cell, which detected and recorded the force. The experimentterm·mth setup schematic is shown in Supplementary Fig. 7 . Displacement against force was measured in a similar experimentterm·mth, at which specimens with constant dimensions (25 mm length, 11.1 mm diameter) were placed in the PTFEterm·dom tubes with various dimensions to enable expansionterm·dom to a desired strainterm·key levels. Then blocked force was measured at the developed strainterm·key. Five specimens were tested in each testing set. The volume expansionterm·dom and stress–strainterm·key experimentsterm·mth were conducted 24 h after the curing of the specimens.

Data availability All data are available from the authors upon reasonable request.

Electronic supplementary material Supplementary Information Supplementary Movie 1 Supplementary Movie 2 Supplementary Movie 3 Supplementary Movie 4 Supplementary Software 1