A Light‐Driven Microgel Rotor

Abstract

developed swimming strategies that do not apply to the macroscopic world due to the high viscous drag experienced by the body at a length scale where the inertial forces are negligible. [5,6] The ratio between these forces defines the Reynolds number Re, which is on the order of 10 −4 for microorganisms. [5] Two major mechanisms used by micro-organism to swim and stir surrounding fluids are the beating of the cilia and rotating of the helical flagella, which are filaments capable of active bending deformation or rotation. [5][6][7][8][9][10] These filaments driven by internally generated forces work with viscous and elastic forces to generate propulsive thrusts. [11][12][13] The physical balance between elastic forces and viscous drag determines the swimming speed, or pumping frequency and performance. [14] To our best knowledge, a microswimmer based on curling of a 2D spiral is yet to be found in nature, though helical-shaped micro-organisms are abundant, such as spirilla and spirochetes. Curling is a way to store elastic energy and is widely used in engineering, such as piezoelectric devices, [15] energy storage in clock springs, or 3D displacement in artificial muscles. [16] In this report, we present a novel type of microswimmer, i.e., a spiralshaped microgel that is capable of rotating by nonreciprocal deformations. The body of the microswimmer is ribbon-shaped and consists of a cross-linked poly(N-isopropylacrylamide) (PNIPAM) hydrogel laden with gold nanorods (AuNRs). AuNRs are engineered to absorb photon energy in the near infrared and generate localized heat that triggers volume changes of the surrounding hydrogel. [17][18][19] Coated on one surface with a thin metal layer, the bilayer hydrogel ribbon is able to swell and curl in water along its length to form a 2D spiral. [20][21][22] Upon heating, the solubility decrease of the polymer network causes the hydrogel layer to shrink, so that the spiral unwinds. We note here that the bending rigidity of the ribbon and its stiffness are greatly affected by the water content in it. The light-induced temperature-jumps give rise to mechanical response over a few hundred milliseconds, where the rate-limiting factor is the mass transport within the gel. [22][23][24][25][26][27][28] We hypothesize that photothermal heating creates a transient state, where an imbalance exists between the stresses defining the local and the mean curvature of the ribbon. [21] These elastic restoring forces of bending and stretching are counterbalanced by the viscous drag of the surrounding fluid, which sets the rotor in motion. Since the material is relatively soft, the curling dynamics depends strongly on the dissipation mechanisms. [21,29,30] Unlike previous studies that focused on the end equilibrium states of the hydrogel volume-phase transition, we study the response dynamics upon short-lived stimuli here, an aspect thatThe current understanding of motility through body shape deformation of micro-organisms and the knowledge of fluid flows at the microscale provides ample example...

Engineering Hydrogel‐Based Biomedical Photonics: Design, Fabrication, and Applications

“…This temperature increase causes the hydrogel to contract toward the light (positive phototaxis) ( Figure a). [ 183 ] Different variations of this strategy have been reported in the past year, such as gold nanopatterned‐disks that wrinkle at the air–water interface when exposed to light, [ 184 ] spiral‐shaped structures which move through light‐controlled spinning, [ 185 ] and photo‐responsive microcrawlers motioned by light‐modulated friction. [ 186 ] Researchers have recently combined these advances with 3D printing technology to print PNIPAAm/Gold nanorod actuators with designer shape and improved resolution.…”

Light‐Powered Microrobots: Challenges and Opportunities for Hard and Soft Responsive Microswimmers

“…Using a similar approach, a light‐driven rotor based on a photoresponsive 2D spiral attached to a microsphere was described. [ 128 ] Under pulsed irradiation, the system was able to rotate thanks to nonreciprocal curling deformations of the spiral tail. Changing the irradiation conditions (e.g., duration of pulses, power), the kinematics of the curling deformation can be adjusted to obtain bending and unbending cycles that follow distinct paths.…”

“…This system mimicking natural cilia is shown in Figure 6D and satisfies the requirement for net rotational thrust at low Reynolds numbers. [ 128 ]…”

“…Reproduced under the terms of the CC‐BY license. [ 128 ] Copyright 2019, The Authors, published by Wiley‐VCH GmbH. E) LCN‐based swimmer microrobots.…”

“…This system mimicking natural cilia is shown in Figure 6D and satisfies the requirement for net rotational thrust at low Reynolds numbers. [128] A different yet interesting case is that of crawling robots, where a reciprocal actuation, such as a simple contraction-expansion cycle, was shown to lead to efficient movement by exploiting a head-to-tail asymmetry of the surface friction on ratchet surfaces. [129] Microrobotic crawlers prepared by using photoresponsive PNIPAm with embedded gold nanoparticles arranged in a simple shape with a size of 100 Â 50 Â 30 μm 3 produce, when irradiated on one side, a contraction-expansion cycle Figure 6.…”