Fluid-driven origami-inspired artificial muscles

Abstract

Artificial muscles hold promise for safe and powerful actuation for myriad common machines and robots. However, the design, fabrication, and implementation of artificial muscles are often limited by their material costs, operating principle, scalability, and single-degree-of-freedom contractile actuation motions. Here we propose an architecture for fluid-driven origami-inspired artificial muscles. This concept requires only a compressible skeleton, a flexible skin, and a fluid medium. A mechanical model is developed to explain the interaction of the three components. A fabrication method is introduced to rapidly manufacture low-cost artificial muscles using various materials and at multiple scales. The artificial muscles can be programed to achieve multiaxial motions including contraction, bending, and torsion. These motions can be aggregated into systems with multiple degrees of freedom, which are able to produce controllable motions at different rates. Our artificial muscles can be driven by fluids at negative pressures (relative to ambient). This feature makes actuation safer than most other fluidic artificial muscles that operate with positive pressures. Experiments reveal that these muscles can contract over 90% of their initial lengths, generate stresses of ∼600 kPa, and produce peak power densities over 2 kW/kg-all equal to, or in excess of, natural muscle. This architecture for artificial muscles opens the door to rapid design and low-cost fabrication of actuation systems for numerous applications at multiple scales, ranging from miniature medical devices to wearable robotic exoskeletons to large deployable structures for space exploration.

Keywords: actuator; artificial muscle; origami; robotics; soft robotics.

Fig. 1.

Design, fabrication, and resulting multiscale actuators. (A) Miniature linear actuators use polyether ether ketone (PEEK) zigzag origami structures as the skeletons and PVC films as the skins. These biocompatible materials make the actuators suitable for medical and wearable applications. (B) A large-scale high-power actuator is assembled using a zigzag skeleton composed of nylon plates (fold width = 10 cm). The skin is made of thermoplastic polyurethane (TPU)-coated nylon fabric. A car wheel (diameter ≈75 cm, weight ≈22 kg) is lifted to 20 cm within 30 s (Movie S3). (C) Principle of operation of the actuators. Contraction is mainly driven by the tension force of the skin. This force is produced by the pressure difference between the internal and external fluids. Removing fluid from the actuator will temporarily decrease the internal pressure. (D) Fabrication process. A standard actuator can be quickly fabricated in three simple steps: (step 1) skeleton construction using any of a number of techniques, (step 2) skin preparation, and (step 3) fluid-tight sealing.

Fig. 2.

Linear zigzag actuators made of a variety of materials using different fabrication methods. (A) A transparent actuator lifts a clear acrylic plate. Skeleton material ($SkeM$): 0.254 mm transparent polyester sheet. Fabrication method ($FM$): laser cutting and manual folding. Skin material ($SkiM$): 0.102 mm transparent PVC film (Vinyl). Driving fluid ($DF$): air. (B) A soft linear actuator contracts well even when it is confined into a metal screw nut. $SkeM$: silicone rubber (M4601). $FM$: casting. $SkiM$: 0.24 mm TPU film. $DF$: air. (C) A vacuum-driven water-soluble actuator is dissolved in hot water (≈ 70 °C) within 5 min. $SkeM$: polyvinyl alcohol (PVA). $FM$: 3D printing. $SkiM$: 0.025 mm PVA film. $DF$: air. (D) A water pump-driven hydraulic actuator pulls an underwater object for 3.5 cm in 20 s. $SkeM$: 0.254 mm stainless steel (316). $FM$: manual forming. $SkiM$: 0.24 mm TPU film. $DF$: water.

Fig. 3.

Various basic actuation motions and programmable pseudosequential actuation with multiple degrees of freedom. (A) A 19 cm-long linear zigzag actuator contracts to a compressed structure shorter than 2 cm. The 1D contraction ratio is ∼90%. (B) A 2D origami skeleton using the Miura-ori patterns (area: $11\times 10$ cm²) can contract to a dense bar-shaped structure (area: $9\times 1$ cm²). The 2D area contraction ratio approaches 92%. (C) A 3D “magic-ball” origami using the water-bomb pattern (radius: 3.5 cm) contracts to a compacted cylindrical structure (radius: 0.9 cm; height: 6.5 cm). The 3D volume decreases 91% after this contraction. (D) Bending motion can be achieved by using an asymmetrical beam structure as the skeleton. (E) Using a flasher origami pattern as the skeleton, the actuator rotates more than 90 degrees around its center, and its 2D surface contracts by 54% simultaneously. (F) A complex out-of-plane motion combining torsion and contraction can be programed through a 2D Miura-ori origami pattern with select folds weakened. (G) Three fingers on a robotic hand are actuated at different rates using a single control of the internal air pressure. The skeletal structure of this robotic hand is 3D printed from nylon. Different hinge strengths inside the structural voids are designed for these three fingers, which produce significantly different bending stiffnesses: $k_{s1}(red)<k_{s2}(green)<k_{s3}(blue)$. The bending stiffness of each finger determines its own bending angle at a certain internal pressure level. (H) A bottle of water is gripped, lifted, and twisted by a single-channel vacuum-driven robotic arm. The robotic arm has a modular structure including a cup-shaped gripper and a cylindrical lifter. The gripper uses a polyester magic-ball origami as its skeleton, while a much stiffer compression spring (302 stainless steel) is used as the lifter’s skeleton. When the internal pressure decreases smoothly, the gripping motion will always start first, then the lifting and twisting motions start later as the internal pressure reduces further.

Fig. 4.

Dynamic characterization of a lightweight actuator. (A) A miniature linear zigzag actuator (weight ≈ 2.6 g, volume ≈ 32 cm³) and a common ping-pong ball (weight ≈ 2.5 g, volume ≈ 33.5 cm³). The actuator is primarily made of polyester sheets (skeleton thickness: 0.254 mm; skin thickness: 0.038 mm). (B) It can lift objects several orders of magnitude more massive using a negative internal air pressure (−80 kPa). (C and D) Dynamic performance in load-lifting tests. The actuator can lift a 1 kg load to 5.5 cm within 0.2 s (C). This indicates an average power density of ∼1.04 kW/kg. A peak power density over 2 kW/kg was obtained during the 2 kg load-lifting tests (D).