Artificial Soft Cilia with Asymmetric Beating Patterns for Biomimetic Low‐Reynolds‐Number Fluid Propulsion

Abstract

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...

Metachronal patterns in artificial cilia for low Reynolds number fluid propulsion

“…S1). The ciliary beat frequency ( f ) is set to 0.25 Hz, which corresponds to an optimal trade-off between actuation frequency and viscous drag, as described in (16) (33). Computational studies show that the spacing between cilia influences the attained fluid flow, with smaller interciliary distances leading to larger flows, depending on the direction of the traveling wave (14).…”

Mechanically Programmable Dip Molding of High Aspect Ratio Soft Actuator Arrays

“…High aspect ratio actuator arrays can be leveraged in systems that emulate ciliary movement for the purpose of fluid mixing, fluid propulsion, general fluid manipulation, and low Reynolds number swimming. Test systems ranging from micrometer to millimeter scales have been fabricated by direct molding of actuators, casting into micromachined molds, casting into a deep‐chemical etched silicon wafer molds, and casting into a multipart precision molds with the help of capillary action to create one and two degree of freedom cilia arrays . Passive arrays of branching structures have been formed with a three part mold for the purpose of passive locomotion and similar methods could be used to create artificial ciliary systems.…”

“…Most of the arrays mentioned above are either passive structures, indirectly actuated by deformations in supporting structure, or manipulated via changes in a local magnetic field. The arrays created by Gorissen et al and Milana et al, however, are pneumatically actuated and the cilia are individually controllable.…”

“…Test systems ranging from micrometer to millimeter scales have been fabricated by direct molding of actuators, casting into micromachined molds, [1] casting into a deepchemical etched silicon wafer molds, [2] and casting into a multipart precision molds with the help of capillary action to create one [3] and two degree of freedom cilia arrays. [4] Passive arrays of branching structures have been formed with a three part mold for the purpose of passive locomotion [5] and similar methods could be used to create artificial ciliary systems. Larger arrays of smaller scale cilia structures have been demonstrated using a variety of alternative fabrication strategies including molding in a sacrificial filter that is later dissolved, [6] self-assembly of magnetic particles, [7] magnetic manipulation of rubber with embedded magnetic particles, [8] and roll touch molding.…”

Shaping Soft Robotic Microactuators by Wire Electrical Discharge Grinding

“…The mould is composed of two micro-milled parts and a cylindrical microrod placed in between the two parts, where the shape of the microrod is replicated as the inflatable cavity. We used these devices for different applications such as artificial cilia [ 24 ] and flexible endoscopes [ 25 ]. However, given the simple morphology of the inflatable cavity, these microactuators have a limited operating range with a maximum bending angle of up to 45° [ 25 ].…”