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.

Autonomous robots are comprised of actuation, energy, sensory, and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, humans and other animals that robots strive to emulate contain highly sophisticated and interconnected systems at the cellular, tissue, and organ levels, which allow multiple functions to be performed simultaneously. Here, we examine how nature builds to establish a new paradigm for autonomous robots with Embodied Energy. Currently, most untethered robots use batteries to store energy and power their operation. To extend their operating time, additional battery blocks must be added in tandem with supporting structures, increasing their weight and reducing their efficiency. Recent advancements in energy storage techniques enable chemical or electrical energy sources to be embodied directly within the materials and mechanical systems used to create robots. This perspective highlights emerging examples of Embodied Energy, focusing on the design and fabrication of enduring autonomous robots.

In nature, liquid propulsion in low-Reynolds-number regimes is often achieved by arrays of beating cilia with various forms of motion asymmetry. In particular, spatial asymmetry, where the cilia follow a different trajectory in their effective and recovery strokes, is an efficient way of generating flow in low Reynolds regimes. However, this type of asymmetry is difficult to mimic and control artificially. In this paper, an artificial soft cilium that comprises two pneumatic actuators that can be controlled individually is developed. These two independent degrees of freedom allow for the first time adjustment and study of spatial asymmetry in the cilium's beating pattern. Using low-Reynolds-number flow measurements, it is confirmed that spatial asymmetry allows for the generation of fluid propulsion. These twodegree-of-freedom soft cilia provide a platform to study ciliary fluid transport mechanisms and to mimic biologic viscous propulsion.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201900462. of a single cilium as well as for whole cilia arrays. For a single cilium, orientational, temporal, and spatial asymmetry can be distinguished, [6] where the latter has the highest impact on low Reynolds fluid propulsion. Spatial asymmetry, where the cilium tip describes a different path during the effective and recovery stroke, is quantified by the swept area; the larger the area, the higher the net flow. [7] At the array level, an additional type of asymmetry has been observed, metachronal asymmetry, which is characterized by a phase difference between neighboring cilia [8] that gives rise to a global wave-like movement.With the development of microsystem technology, artificial cilia can now be fabricated and are foreseen to find applications in microrobotic devices, such as microswimmers, [9,10] microsensors, [11] micropumps, [12][13][14] and micromixers. [15][16][17][18] The vast majority of artificial cilia consist of microactuators that are incorporated in silicone rubber pillars or plate-like flexible structures in order to mimic the biological hair-like design. Current actuation methods include electric fields, [15] magnetic fields, [19][20][21][22][23] vibrations, [24,25] mechanical forces, [26] or pressurized fluids. [27,28] However, asymmetric motion remains the most challenging feature to mimic in artificial cilia systems. In nature, nonreciprocal beating is achieved by a change in bending stiffness between the effective and recovery stroke: [29] A higher stiffness is observed during the fast effective stroke, where the cilium does not deform significantly, whereas in the slower recovery phase the cilium has a lower stiffness and tends to be deformed by the drag forces. To mimic such an asymmetric motion, the artificial cilium needs at least two deformation modes that need to be sequentially addressed. This can be achieved through elastic instabilities, [7] the interaction between elastic, viscous, and actuation forces, [26,30] or a multise...

Soft robots are an interesting alternative for classic rigid robots in applications requiring interaction with organisms or delicate objects. Elastic inflatable actuators are one of the preferred actuation mechanisms for soft robots since they are intrinsically safe and soft. However, these pneumatic actuators each require a dedicated pressure supply and valve to drive and control their actuation sequence. Because of the relatively large size of pressure supplies and valves compared to electrical leads and electronic controllers, tethering pneumatic soft robots with multiple degrees of freedom is bulky and unpractical. Here, a new approach is described to embed hardware intelligence in soft robots where multiple actuators are attached to the same pressure supply, and their actuation sequence is programmed by the interaction between nonlinear actuators and passive flow restrictions. How to model this hardware sequencing is discussed, and it is demonstrated on an 8‐degree‐of‐freedom walking robot where each limb comprises two actuators with a sequence embedded in their hardware. The robot is able to carry pay loads of 800 g in addition to its own weight and is able to walk at travel speeds of 3 body lengths per minute, without the need for complex on‐board valves or bulky tethers.

Soft robotic systems typically follow conventional control schemes, where actuators are supplied with dedicated inputs that are regulated through software. However, in recent years an alternative trend is being explored, where the control architecture can be simplified by harnessing the passive mechanical characteristics of the soft robotic system. This approach is named “morphological control”, and it can be used to decrease the number of components (tubing, valves and regulators) required by the controller. In this paper, we demonstrate morphological control of bio-inspired asymmetric motions for systems of soft bending actuators that are interconnected with passive flow restrictors. We introduce bending actuators consisting out of a cylindrical latex balloon in a flexible PVC shell. By tuning the radii of the tube and the shell, we obtain a nonlinear relation between internal pressure and volume in the actuator with a peak and valley in pressure. Because of the nonlinear characteristics of the actuators, they can be assembled in a system with a single pressure input where they bend in a discrete, preprogrammed sequence. We design and analyze two such systems inspired by the asymmetric movements of biological cilia. The first replicates the swept area of individual cilia, having a different forward and backward stroke, and the second generates a travelling wave across an array of cilia.

Elastic pneumatic actuators are fuelling new devices and applications in soft robotics. Actuator miniaturisation is critical to enable soft microsystems for applications in microfluidics and micromanipulation. This work proposes a fabrication technique to make out-of-plane bending microactuators entirely by soft lithography. The only bonding step required is to seal the embedded fluidic channels, assuring the structural integrity of the microactuators. The process consists of fabricating two SU8 mold halves using different lithographic layers.Polydimethilsiloxane (PDMS) is poured on the bottom mold, which is subsequently aligned and assembled with the top mould. The process allows for out-of-plane actuators with a diameter of 300 µm and for fabricating arrays of up to 36 actuators that are row-addressable.These active micropillars have an aspect ratio of 1:1.5 and when pressurized at 1 bar, show a bending angle of approximately 30°.

No abstract

No abstract