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1. 2D material programming for 3D shaping

   https://doi.org/10.1038/s41467-021-20934-w  

   Nojoomi, Amirali ; Jeon, Junha ; Yum, Kyungsuk ( January 2021 , Nature Communications)

   Two-dimensional (2D) growth-induced 3D shaping enables shape-morphing materials for diverse applications. However, quantitative design of 2D growth for arbitrary 3D shapes remains challenging. Here we show a 2D material programming approach for 3D shaping, which prints hydrogel sheets encoded with spatially controlled in-plane growth (contraction) and transforms them to programmed 3D structures. We design 2D growth for target 3D shapes via conformal flattening. We introduce the concept of cone singularities to increase the accessible space of 3D shapes. For active shape selection, we encode shape-guiding modules in growth that direct shape morphing toward target shapes among isometric configurations. Our flexible 2D printing process enables the formation of multimaterial 3D structures. We demonstrate the ability to create 3D structures with a variety of morphologies, including automobiles, batoid fish, and real human face.

    

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2. Robotic surfaces with reversible, spatiotemporal control for shape morphing and object manipulation

   https://doi.org/10.1126/scirobotics.abf5116  

   Liu, Ke ; Hacker, Felix ; Daraio, Chiara ( April 2021 , Science Robotics)

   Continuous and controlled shape morphing is essential for soft machines to conform, grasp, and move while interacting safely with their surroundings. Shape morphing can be achieved with two-dimensional (2D) sheets that reconfigure into target 3D geometries, for example, using stimuli-responsive materials. However, most existing solutions lack the ability to reprogram their shape, face limitations on attainable geometries, or have insufficient mechanical stiffness to manipulate objects. Here, we develop a soft, robotic surface that allows for large, reprogrammable, and pliable shape morphing into smooth 3D geometries. The robotic surface consists of a layered design composed of two active networks serving as artificial muscles, one passive network serving as a skeleton, and cover scales serving as an artificial skin. The active network consists of a grid of strips made of heat-responsive liquid crystal elastomers (LCEs) containing stretchable heating coils. The magnitude and speed of contraction of the LCEs can be controlled by varying the input electric currents. The 1D contraction of the LCE strips activates in-plane and out-of-plane deformations; these deformations are both necessary to transform a flat surface into arbitrary 3D geometries. We characterize the fundamental deformation response of the layers and derive a control scheme for actuation. We demonstrate that the robotic surface provides sufficient mechanical stiffness and stability to manipulate other objects. This approach has potential to address the needs of a range of applications beyond shape changes, such as human-robot interactions and reconfigurable electronics.

    

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3. Boundary curvature guided programmable shape-morphing kirigami sheets

   https://doi.org/10.1038/s41467-022-28187-x  

   Hong, Yaoye ; Chi, Yinding ; Wu, Shuang ; Li, Yanbin ; Zhu, Yong ; Yin, Jie ( January 2022 , Nature Communications)

   Kirigami, a traditional paper cutting art, offers a promising strategy for 2D-to-3D shape morphing through cut-guided deformation. Existing kirigami designs for target 3D curved shapes rely on intricate cut patterns in thin sheets, making the inverse design challenging. Motivated by the Gauss-Bonnet theorem that correlates the geodesic curvature along the boundary with the Gaussian curvature, here, we exploit programming the curvature of cut boundaries rather than the complex cut patterns in kirigami sheets for target 3D curved morphologies through both forward and inverse designs. The strategy largely simplifies the inverse design. Leveraging this strategy, we demonstrate its potential applications as a universal and nondestructive gripper for delicate objects, including live fish, raw egg yolk, and a human hair, as well as dynamically conformable heaters for human knees. This study opens a new avenue to encode boundary curvatures for shape-programing materials with potential applications in soft robotics and wearable devices.

    

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4. Liquid Crystal Elastomer Lattices with Thermally Programmable Deformation via Multi‐Material 3D Printing

   https://doi.org/10.1002/adma.202310743  

   Kotikian, Arda ; Watkins, Audrey_A ; Bordiga, Giovanni ; Spielberg, Andrew ; Davidson, Zoey_S ; Bertoldi, Katia ; Lewis, Jennifer_A ( January 2024 , Advanced Materials)

   An integrated design, modeling, and multi‐material 3D printing platform for fabricating liquid crystal elastomer (LCE) lattices in both homogeneous and heterogeneous layouts with spatially programmable nematic director order and local composition is reported. Depending on their compositional topology, these lattices exhibit different reversible shape‐morphing transformations upon cycling above and below their respective nematic‐to‐isotropic transition temperatures. Further, it is shown that there is good agreement between their experimentally observed deformation response and model predictions for all LCE lattice designs evaluated. Lastly, an inverse design model is established and the ability to print LCE lattices with the predicted deformation behavior is demonstrated. This work opens new avenues for creating architected LCE lattices that may find potential application in energy‐dissipating structures, microfluidic pumping, mechanical logic, and soft robotics.

    

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5. Biasing Buckling Direction in Shape‐Programmable Hydrogel Sheets with Through‐Thickness Gradients

   https://doi.org/10.1002/adfm.201905273  

   Zhou, Ying ; Duque, Carlos M. ; Santangelo, Christian D. ; Hayward, Ryan C. ( September 2019 , Advanced Functional Materials)

   A photocrosslinkable poly(N,N′‐diethylacrylamide) copolymer allows for the photolithographic fabrication of hydrogel sheets with nonuniform crosslinking density and swelling ratio. Using this material system, different 3D shapes with nonzero Gaussian curvatureKare successfully programmed by prescribing a “metric” defined by in‐plane variations in swelling. However, this methodology does not control the direction of buckling adopted by each positive K feature, and therefore cannot controllably select between different isometric shapes defined by a single metric. Here, by introducing gradients in swelling through the thickness of the gel sheet by tuning the absorption of the UV‐light used for crosslinking, a preferential buckling direction is locally specified for each feature by the direction of UV exposure. By also controlling the strength of coupling between neighboring features, this is shown to be an effective method to program buckling direction of each unit within a canonical corrugated surface shape.

    

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