Soft actuators for real-world applications

Abstract

Inspired by physically adaptive, agile, reconfigurable and multifunctional soft-bodied animals and human muscles, soft actuators have been developed for a variety of applications, including soft grippers, artificial muscles, wearables, haptic devices and medical devices. However, the complex performance of biological systems cannot yet be fully replicated in synthetic designs. In this Review, we discuss new materials and structural designs for the engineering of soft actuators with physical intelligence and advanced properties, such as adaptability, multimodal locomotion, self-healing and multi-responsiveness. We examine how performance can be improved and multifunctionality implemented by using programmable soft materials, and highlight important real-world applications of soft actuators. Finally, we discuss the challenges and opportunities for next-generation soft actuators, including physical intelligence, adaptability, manufacturing scalability and reproducibility, extended lifetime and end-of-life strategies.

Fig. 1. Working principles of tethered soft actuators.

a | Soft robotic systems can be actuated through changes in the fluid or air pressure ΔP. b | Dielectric elastomers are actuated by an electric field. c | Electrothermal actuation of a single twisted fibre. d | Forces can be transmitted by tendons. T_C, cooling temperature; T_H, heating temperature.

Fig. 2. Working principles of untethered and biohybrid soft actuators.

a-e | Actuation mechanisms triggered by magnetic fields (panel a), light (panel b), acoustic waves (panel c), temperature increase (panel d, red colour indicates increased temperature) and other environmental stimuli (panel e). f | Biohybrid devices can be designed on the basis of skeletal or cardiac muscle tissues and living microorganisms, such as Escherichia coli. CTE, coefficient of thermal expansion.

Fig. 3. State-of-the-art soft actuators with potential industrial applications.

a | Dry microfibrillar adhesive-covered soft gripper, shown gripping various objects, that combines 3D surface conformability and high adhesion strength by controllable equal load sharing at the interface. b | Schematic of a linear hydraulically amplified self-healing electrostatic (HASEL) actuator. c | A high-resolution soft sensor with optical output, fabricated with elastomeric light guides, can be used to differentiate and quantify the location, amplitude and type of mechanical deformation, as demonstrated with a wireless, soft wearable glove. d | The softness of an object can be perceived by finger touch by optimizing the geometric and material properties of micropatterned pillars. SLIMS, stretchable lightguide for multimodal sensing; LED, light-emitting diode. Panel a reprinted with permission from REF., National Academy of Sciences. Panel c reprinted with permission from REF., AAAS. Panel d reprinted with permission from REF., AAAS.

Fig. 4. Soft actuators in biomedical applications.

a | A functionalized catheter can be wirelessly navigated in blood vessels. b | Balloon catheters integrated with conformal electronics can provide real-time sensing during surgeries. c | Small biopsy devices can locomote in hard-to-access body sites and take biopsy samples. The biopsy device is capsulated in ice for smoother oral ingestion. d | Assisting soft actuators can have safe contact with soft tissues and provide therapeutic functions. e | Targeted drug delivery can be realized using biohybrid soft actuators. f | Artificial soft actuators can avoid the detection by macrophages. anti-TER-119, Ter-119 antibody; DOX, doxorubicin; E. coli MG1655, Escherichia coli strain MG1655; PEG, polyethylene glycol; RBC, red blood cell; SPIONs, superparamagnetic iron oxide nanoparticles; ZW, zwitterionic polymer. Panel a adapted from REF., CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Panel b adapted from REF., Springer Nature Limited. Panel c adapted with permission from REF., copyright 2020, Mary Ann Liebert, Inc. Panel d adapted with permission from REF., AAAS. Panel e adapted with permission from REF., AAAS. Panel f reprinted from REF., CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

Fig. 5. Encoding physical intelligence in soft robot bodies.

a | Mechanical instabilities can be used to create digital logic gates for binary mechanical computation and human interaction. b | Introducing cuts (kirigami) allows the buckling-induced transformation of flat sheets into 3D textured surfaces, the directional frictional properties of which can enable efficient crawling gaits. c | Strategically developing unequal strains, through a difference of materials in adjacent layers of a composite structure, can achieve programmable deformations and shape morphing. Panel a adapted with permission from REF., National Academy of Sciences. Panel b adapted with permission from REF., AAAS. Panel c adapted with permission from REF., AAAS.