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...