Self‐Folded Gripper‐Like Architectures from Stimuli‐Responsive Bilayers

Abstract

Self-folding microgrippers are an emerging class of smart structures that have widespread applications in medicine and micro/nanomanipulation. To achieve their functionalities, these architectures rely on spatially patterned hinges to transform into 3D configurations in response to an external stimulus. Incorporating hinges into the devices requires the processing of multiple layers which eventually increases the fabrication costs and actuation complexities. The goal of this work is to demonstrate that it is possible to achieve gripper-like configurations in an on-demand manner from simple planar bilayers that do not require hinges for their actuation. Finite element modeling of bilayers is performed to understand the mechanics behind their stimuli-responsive shape transformation behavior. The model predictions are then experimentally validated and axisymmetric gripper-like shapes are realized using millimeter-scale poly(dimethylsiloxane) bilayers that undergo differential swelling in organic solvents. Owing to the nature of the computational scheme which is independent of length scales and material properties, the guidelines reported here would be applicable to a diverse array of gripping systems and functional devices. Thus, this work not only demonstrates a simple route to fabricate functional microgrippers but also contributes to self-assembly in general.

Kirigami‐Inspired Self‐Assembly of 3D Structures

“…Bifurcation is a structural instability mechanism solely arising from the nonlinear geometric behavior of thin shell structures that eventually minimizes the mid‐section stretching at high mismatch strains. [40,41,49] In our case, the squares with Kirigami cuts transitioned from quasi‐axisymmetric doubly curved shapes to asymmetric singly curved ones at critical strains. The effects of their initial shapes on bifurcation strains are shown in Figure 3b.…”

“…As our computational model is independent of material selection, the design principles reported here would be applicable to other stimuli‐responsive material systems. Hence, polymers and hydrogels responsive to functionally relevant stimuli such as heat (liquid‐crystal elastomer (LCE),[60] shape‐memory polymer,[61] and pNIPAM[62]), chemical/solvent diffusion (poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)[63] and poly(ethylene glycol) diacrylate[64]), light (shape‐memory polymer[65] and graphene oxide/ polypeptide composite[66]), biochemical enzymes (gelatin and carboxymethylcellulose[67]), salt concentration (poly(methacryloxyethyltrimethylammonium chloride)[68]), pH (acrylic acid and 2‐(dimethylamino) ethyl methacrylate[69]), and magnetic fields (polypyrrole embedded with Ni/Au[70] and PDMS embedded with Fe[49]) could be used to self‐assemble bilayered Kirigami sheets. As a possible application of our work, we demonstrated a dynamically tunable optical system realized from self‐assembled Kirigami sheets.…”

“…A previously reported protocol was followed to perform the bilayer shape transforming experiments in solvent mixtures of 1‐propanol and xylene (Fisher scientific). [41,49] ImageJ[77] was used for postprocessing the figure presented in this paper.…”

“…In our computations, 4-node general purpose shell elements with throughthickness variation in material properties (the element size was determined from separate mesh refinement studies) and appropriate boundary conditions were used. While simulating the bifurcation behavior, small imperfections [40,41,49] were incorporated for reliably capturing the bilayer deformation beyond critical strains. Hyperelastic material properties (bulk and shear moduli are 962 and 0.18 MPa for the weakly cross-linked layer and 1214.8 and 32.2 MPa for the densely cross-linked layer) were used in our model.…”

Thermoresponsive Three‐Stage Optical Modulation of a Self‐Healing Composite Hydrogel

“…Smart materials, especially hydrogels, that can respond to external environmental stimulus open new access to applications including soft robotics, smart actuators, and wound dressing . All of these require the materials to have intrinsically tunable optical or mechanical properties while facing external stimuli such as light, pH, ionic strength, and temperature.…”