The 20th century's robotic systems have been made from stiff materials, and much of the developments have pursued ever more accurate and dynamic robots, which thrive in industrial automation, and will probably continue to do so for decades to come. However, the 21st century's robotic legacy may very well become that of soft robots. This emerging domain is characterized by continuous soft structures that simultaneously fulfill the role of robotic link and actuator, where prime focus is on design and fabrication of robotic hardware instead of software control. These robots are anticipated to take a prominent role in delicate tasks where classic robots fail, such as in minimally invasive surgery, active prosthetics, and automation tasks involving delicate irregular objects. Central to the development of these robots is the fabrication of soft actuators. This article reviews a particularly attractive type of soft actuators that are driven by pressurized fluids. These actuators have recently gained traction on the one hand due to the technology push from better simulation tools and new manufacturing technologies, and on the other hand by a market pull from applications. This paper provides an overview of the different advanced soft actuator configurations, their design, fabrication, and applications.

Fluidic soft actuators are enlarging the robotics toolbox by providing flexible elements that can display highly complex deformations. Although these actuators are adaptable and inherently safe, their actuation speed is typically slow because the influx of fluid is limited by viscous forces. To overcome this limitation and realize soft actuators capable of rapid movements, we focused on spherical caps that exhibit isochoric snapping when pressurized under volume-controlled conditions. First, we noted that this snap-through instability leads to both a sudden release of energy and a fast cap displacement. Inspired by these findings, we investigated the response of actuators that comprise such spherical caps as building blocks and observed the same isochoric snapping mechanism upon inflation. Last, we demonstrated that this instability can be exploited to make these actuators jump even when inflated at a slow rate. Our study provides the foundation for the design of an emerging class of fluidic soft devices that can convert a slow input signal into a fast output deformation.

From stadium covers to solar sails, we rely on deployability for the design of large-scale structures that can quickly compress to a fraction of their size (1-4). Historically, two main strategies have been pursued to design deployable systems. The first and most common approach involves mechanisms comprising interconnected bar elements, which can synchronously expand and retract (5-7), occasionally locking in place through bistable elements (8, 9). The second strategy instead, makes use of inflatable membranes that morph into target shapes by means of a single pressure input (10-12). Neither strategy however, can be readily used to provide an enclosed domain able to lock in place after deployment: the integration of protective covering in linkage-based constructions is challenging and pneumatic systems require a constant applied pressure to keep their expanded shape (13-15). Here, we draw inspiration from origami, the Japanese art of paper folding, to design rigid-walled deployable structures that are multistable and inflatable. Guided by geometric analyses and experiments, we create a library of bistable origami shapes that can be deployed through a single fluidic pressure input. We then combine these units to build functional structures at the meter-scale, such as arches and emergency shelters, providing a direct pathway for a new generation of large-scale inflatable systems that lock in place after deployment and offer a robust enclosure through their stiff faces. Origami | Multistability | Inflatable structures | Deployable structuresLarge, deployable structures should ideally (i) occupy the minimum possible volume when folded; (ii) be autonomous when deploying; (iii) lock in place after deployment; and (iv) provide a structurally robust shell (if they are designed to define a closed environment). To satisfy all these requirements, we here present a novel approach with roots in the Japanese art of paper folding: origami. Extensively used in robotics (16-20), metamaterials (21-25) and structures (26-30), origami principles have potential to lead to efficient large-scale deployable structures as they offer (i) a versatile crease-based approach to shape design (31-33); (ii) an easy actuation through inflation, if enclosed (34-36); (iii) self-locking capabilities when designed to support multiple energy wells (37-44); and (iv) the possibility to create a protective environment through their faces. While previous origami systems have explored inflatability and multistability separately (34-44), here we show that these two properties can coexist, unlocking an unprecedented design space of meter-scale inflatable structures that harness multistability to maintain their deployed shape without the need for continuous actuation (see schematics in Fig. 1a) Triangular facets as a platform for bistable and inflatable structures. To create inflatable and bistable origami structures, we start by considering a triangular building block ABC and denote with α and β the internal angles enclosed by the edges AB-AC and AB-BC, res...

Cilia are hair-like organelles, present in arrays that collectively beat to generate flow. Given their small size and consequent low Reynolds numbers, asymmetric motions are necessary to create a net flow. Here, we developed an array of six soft robotic cilia, which are individually addressable, to both mimic nature’s symmetry-breaking mechanisms and control asymmetries to study their influence on fluid propulsion. Our experimental tests are corroborated with fluid dynamics simulations, where we find a good agreement between both and show how the kymographs of the flow are related to the phase shift of the metachronal waves. Compared to synchronous beating, we report a 50% increase of net flow speed when cilia move in an antiplectic wave with phase shift of −π/3 and a decrease for symplectic waves. Furthermore, we observe the formation of traveling vortices in the direction of the wave when metachrony is applied.