Fundamental modes of swimming correspond to fundamental modes of shape: engineering I-, U-, and S-shaped swimmers

Hydrogels have received increased attention due to their biocompatible material properties, adjustable porosity, ease of functionalization, tuneable shape, and Young's moduli. Initial work has recognized the potential that conferring out‐of‐equilibrium properties to these on the microscale holds and envisions a broad range of biomedical applications. Herein, a simple strategy to integrate multiple swimming modes into catalase‐propelled hydrogel bodies, produced via stop‐flow lithography (SFL), is presented and the different dynamics that result from bubble expulsion are studied. It is found that for “Saturn” filaments, with active poles and an inert midpiece, the fundamental swimming modes correspond to the first three fundamental shape modes that can be obtained by buckling elastic filaments, namely, I, U, and S‐shapes

Friction-directed self-assembly of Janus lithographic microgels into anisotropic 2D structures

We present a method for creating ordered 2D structures with material anisotropy from self-assembling micro-sized hydrogel particles (microgels). Microgel platelets of polygonal shapes (hexagon, square, and rhombus), obtained by a continuous scalable lithographic process, are suspended in an aqueous environment and sediment on an inclined plane. As a consequence of gravitational pull, they slide over the plane. Each half of the microgel is composed of a different type of hydrogel [poly(N-isopropylacrylamide) (PNIPAM), and poly(ethylene glycol) diacrylate (PEGDA), respectively] which exhibit different frictional coefficients when sheared over a substrate. Hence the microgels self-orientate as they slide, and the side with the lower frictional coefficient positions in the direction of sliding. The self-oriented microgels concentrate at the bottom of the tilted plane. Here they form densely packed structures with translational as well as orientational order

Light-responsive hydrogel microcrawlers, powered and steered with spatially homogeneous illumination

Sub-millimeter untethered locomoting robots hold promise to radically change multiple areas of human activity such as microfabrication/assembly or health care. To overcome the associated hurdles of such a degree of robot miniaturization, radically new approaches are being adopted, often relying on soft actuating polymeric materials. Here, we present light-driven, crawling microrobots that locomote by a single degree of freedom actuation of their light-responsive tail section. The direction of locomotion is dictated by the robot body design and independent of the spatial modulation of the light stimuli, allowing simultaneous multidirectional motion of multiple robots. Moreover, we present a method for steering such robots by reversibly deforming their front section, using ultraviolet (UV) light as a trigger. The deformation dictates the robot locomotion, performing right- or left-hand turning when the UV is turned on or off respectively. The robots' motion and navigation are not coupled to the position of the light sources, which enables simultaneous locomotion of multiple robots, steering of robots and brings about flexibility with the methods to deliver the light to the place of robot operation

Fundamental modes of swimming correspond to fundamental modes of shape : engineering I-, U-, and S-shaped swimmers

Hydrogels have received increased attention due to their biocompatible material properties, adjustable porosity, ease of functionalization, tuneable shape, and Young's moduli. Initial work has recognized the potential that conferring out-of-equilibrium properties to these on the microscale holds and envisions a broad range of biomedical applications. Herein, a simple strategy to integrate multiple swimming modes into catalase-propelled hydrogel bodies, produced via stop-flow lithography (SFL), is presented and the different dynamics that result from bubble expulsion are studied. It is found that for “Saturn” filaments, with active poles and an inert midpiece, the fundamental swimming modes correspond to the first three fundamental shape modes that can be obtained by buckling elastic filaments, namely, I, U, and S-shapes

A new class of single-material, non-reciprocal microactuators

A crucial component in designing soft actuating structures with controllable shape changes is programming internal, mismatching stresses. In this work, a new paradigm for achieving anisotropic dynamics between isotropic end-states—yielding a non-reciprocal shrinking/swelling response over a full actuation cycle—in a microscale actuator made of a single material, purely through microscale design is demonstrated. Anisotropic dynamics is achieved by incorporating micro-sized pores into certain segments of the structures; by arranging porous and non-porous segments (specifically, struts) into a 2D hexagonally-shaped microscopic poly(N-isopropyl acrylamide) hydrogel particle, the rate of isotropic shrinking/swelling in the structure is locally modulated, generating global anisotropic, non-reciprocal, dynamics. A simple mathematical model is introduced that reveals the physics that underlies these dynamics. This design has the potential to be used as a foundational tool for inducing non-reciprocal actuation cycles with a single material structure, and enables new possibilities in producing customized soft actuators and modular anisotropic metamaterials for a range of real-world applications, such as artificial cilia

An automated platform for assembling light-powered hydrogel microrobots and their subsequent chemical binding

This paper presents light powered hydrogel microrobots (100 μm), that are directed to specific locations in their environment by an automated platform. The microrobots are actuated by focused laser light and crawl in aqueous environments by periodic volumetric changes of a section of their bodies. The platform consists of a stage, manipulated by stepper drivers and controlled by a Raspberry PI 4. This positions the laser light in the desired locations to move microrobots towards a goal location. The microrobots are localized via a microscope camera and repetitive usage of an algorithm based on Hough Gradient Method. The optimal position for the laser is chosen before every step so that the disk reaches the goal as fast as possible. Multiple disks are moved to form a formation of predefined geometry. An algorithm for finding the optimum sequence of disk movements to suitable positions is introduced. Subsequently, the disks are bound together chemically, using local UV illumination as the binding trigger. The bound formation can perform useful tasks, such as pushing and depositing a cargo at a target location