Fabricating 3D Structures by Combining 2D Printing and Relaxation of Strain

Abstract

materials to achieve extreme overhangs, and are limited by trade-offs between the speed of printing and the minimum feature sizes and surface roughness. [8,9] In contrast, "2D printing" techniques (such as digital printing and screen printing) are rapid, relatively inexpensive, and routinely achieve sub-millimeter feature sizes.Nature has also evolved the ability to form complex 3D structures (e.g., leaves, flowers, and tendrils) from initially quasiplanar structures. [10][11][12][13][14][15] These 2D to 3D transformations have inspired the design of synthetic structures that display complex changes in shape when subjected to appropriate stimuli (surface or interfacial stress, edge stress, misfit strain, residual stress, differential growth, swelling, and mechanical instabilities). [16,17] These processes also suggest possible routes to micro-and nanoelectromechanical systems (MEMS and NEMS), [18,19] and to systems for drug delivery, [20] actuation, [19,21] microrobotics, [22,23] and new materials. [23,24] In this paper, we describe a technique to fabricate 3D structures from planar elastomeric bilayers with mismatched strain that is compatible with digital printing. We determine that a relatively simple set of primitive structures printed on a prestretched 2D sheet can result in a variety of complex 3D objects by producing both positive and negative curvature of whole parts, as well as "pop-up" structures on 2D or 3D sheets. This paper describes the fabrication of elastomeric three-dimensional (3D) structures starting from two-dimensional (2D) sheets using a combination of direct-ink printing and relaxation of strain. These structures are fabricated in a two-step process: first, elastomeric inks are deposited as 2D structures on a stretched elastomeric sheet, and second, after curing of the elastomeric inks, relaxation of strain in the 2D sheet causes it to deform into a 3D shape. To predict bending of elastomeric objects fabricated with this technique, a simple mechanical model is developed. The strategy of using initially 2D materials to fabricate 3D structures offers four new features that complement digital fabrication techniques. (i) It provides a simple route to create shapes with complex curves, suspended features, and internal cavities. (ii) It is a faster method of fabricating some types of shapes than "conventional" 3D printing, because the features are printed in 2D. (iii) It forms surfaces that can be both smoother, and structured in a way that is not compatible with layer-by-layer processing. (iv) It forms structures that can be deformed reversibly after fabrication by reapplying strain. This paper demonstrates these features by fabrication of helices, structures inspired by cubes and tables, "pop-up" structures, and soft grippers. 3D PrintingThe term "3D printing" encompasses additive manufacturing techniques that use one or more translational elements (stage and/or printing heads) that are controlled by a computer to move an ink deposition nozzle, or laser-writing optics, to fabricate a digitally ...

Leveraging Monostable and Bistable Pre‐Curved Bilayer Actuators for High‐Performance Multitask Soft Robots

“…This is in sharp contrast to the actuated monotonic bending of pre‐strained soft actuators without changing their bending directions reported in previous studies. [ 13–14 ] The switch from monotonic positive bending to flip to negative curvature with the increase of p are well captured by the corresponding FEA simulations (Figure 3a,b, right).…”

“…It shows that θ increases monotonically and nonlinearly with ε pre , which is consistent with previous studies on the bistrip soft actuators. [ 13–14 ] At the same applied ε pre , the actuators with a thicker pre‐stretched layer exhibit a larger bending angle than their counterparts due to their lower bending stiffness resistance of thin top layer. The disparity of θ between two counterparts increases dramatically with ε pre (Figure 2c), showing a dominant role of the bilayer thickness ratio h b / h t in manipulating the shape of pre‐curved 2D bending actuators.…”

“…Such a prestrain release strategy has been widely used in designing stretchable electronics [ 11 ] and self‐assembly of 3D structures from 2D sheets and stripes at all scales through buckling. [ 12 ] Very recently, it has been applied to fabricate 3D elastomeric structures [ 13 ] and bio‐inspired fast‐response soft machines from 2D pre‐strained bi‐strips. [ 14 ] However, most of these fabricated 3D structures are stable, exhibiting monotonic bending behavior under actuation.…”

Nonlinear elasticity of incompatible surface growth

“…Our analysis also highlights the possibility to manufacture non-Euclidean elastic solids artificially, not only with a tailored pre-stress, but also with a particular stiffness distribution. Such solids can be designed to undergo specific elastic instabilities and to exhibit specific patterning in the technologically relevant conditions [79][80][81][82][83][84].…”

Elastic Energy Storage Enables Rapid and Programmable Actuation in Soft Machines

“…Here, we present a design strategy to significantly increase the elastic strain energy stored by completely soft robots upon actuation, enabling their rapid and programmable actuation and recovery. Our work finds its foundations in previous approaches to create 3D structures using controlled buckling and selective bonding as a design principle and the work of Cafferty et al, who demonstrated the fabrication of soft grippers using a combination of direct‐ink printing and relaxation of strain. Complimentary to these works, we propose a strategy to exploit elastic energy storage to fabricate a variety of prestressed soft actuators (PSAs) by attaching a flexible but inextensible sheet (paper, textile, or plastic) to a prestressed elastomeric layer.…”