Soft Actuators Made of Discrete Grains

Abstract

microscale actuation. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions. To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. Phasechange actuators are capable of inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. [29][30][31][32] Accepting the advantages of phasechange actuators, we now turn our attention to granular media. Granular assemblies, consisting of discrete grains, can be tuned to accomplish a variety of different responses. For example, favorable self-assembly of granular media can be achieved via modulating interparticle interactions and external stimuli, [33][34][35][36] and granular assemblies with different packing configurations have been shown to yield different bulk properties. [36][37][38][39] Furthermore, granular media can exhibit optimized changes in stiffness during jamming transitions, enabling tunable moduli. [16,[40][41][42][43][44] When unjammed, the bulk material is compliant, and when jammed, the bulk material can achieve the stiffness of the grains. Finally, when mixed into carrier fluids, granular media can impart thixotropic behavior on the carrier fluid due to dynamic jamming, [45][46][47][48] enabling 3D printing of previously unprintable materials. With all the advantageous properties granular media has to offer, the technology has seen application in soft robotics, [49][50][51][52] medical devices, [53][54][55] flexible airfoils, [56] and programmable aggregate architecture. [57] Here, we bring together the unique advantages of phasechange soft actuators and granular media to introduce soft granular actuators made of discrete, volumetrically expanding grains. A single active grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force microscopic actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Furthermore, agglomerates of active grains can self-assemble from disordered arrangements to conform around objects and exhibit variable moduli. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packing...

Effects of particle size and oxide shell on variable stiffness performance of phase-changing materials

“…Field’s metal particles are manufactured via a process improved from Buckner et al 19 by modifying the thixotropic (shear-thinning) mixing techniques from Yup et al 20 and Eristoff et al 21 A 1000-mL beaker is filled with 200 mL of a thixotropic aqueous mixture, in this case a solution of deionized water and a confectionery sodium alginate powder (Modernist Pantry) with a loading of 2 wt%. This mixture nominally has a viscosity of ∼2.6 pa∙s (similar to room-temperature honey), but thins to ∼0.2 pa∙s (similar to maple syrup) when subjected to a shear rate of 1000/s.…”

“…This mixture nominally has a viscosity of ∼2.6 pa•s (similar to room-temperature honey), but thins to ∼0.2 pa•s (similar to maple syrup) when subjected to a shear rate of 1000/s. 21 The mixture is then warmed to approximately 70°C by placing the beaker inside of a heated silicone oil bath (95°C). Ingots of Field's metal (Roto144 F, RotoMetals, Inc.) with a total weight of 60 g are placed inside the heated medium and allowed to melt over the course of about 10 min.…”

Wireless Miniature Magnetic Phase‐Change Soft Actuators

“…However, the existing phase-change actuators, e.g., with a diameter of 16.9 mm and length of 63 mm, [ 12 , 13 ] are still too large for potential miniature robotic and device applications, [ 14 ] or unable to deploy with wireless actuation. [ 15 ] It would be beneficial if the actuator could wirelessly control the device’s rigid-body motion, besides its inflation and deflation, toward minimally invasive medical applications. [ 2b,h , 16 ]…”

“…However, the existing phase-change actuators, e.g., with a diameter of 16.9 mm and length of 63 mm, [12,13] are still too large for potential miniature robotic and device applications, [14] or unable to deploy with wireless actuation. [15] It would be beneficial if the actuator could wirelessly control the device's rigid-body motion, besides its inflation and deflation, toward minimally invasive medical applications. [2b,h,16] To tackle these challenges, here we propose a balloon-shaped inflatable wireless miniature soft actuator using a magnetic soft composite, which is composed of silicone rubber (Ecoflex) with embedded hard-magnetic NdFeB microparticles and iron-oxide Fe 3 O 4 nanoparticles (Fe 3 O 4 NPs).…”

Oscillating light engine realized by photothermal solvent evaporation

“…In this work, in order to obtain output work, a high actuation stress, a large morphing amplitude, as well as a high actuation frequency, are highly desired. To realize continuous work output of an actuator, a pulsed stimulus is generally required by periodically switching on/off the supply of moisture 26 , 27 , electricity 28 , 29 , light 30 – 34 , etc., or by using an alternating current electrical supply. However, alternatively switching on/off sunlight to realize oscillating actuation is not convenient.…”