Reversible Self-Assembly of 3D Architectures Actuated by Responsive Polymers

Origami-inspired highly stretchable and breathable 3D wearable sensors for in-situ and online monitoring of plant growth and microclimate

The emerging wearable plant sensors demonstrate the capability of in-situ measurement of physiological and micro-environmental information of plants. However, the stretchability and breathability of current wearable plant sensors are restricted mainly due to their 2D planar structures, which interfere with plant growth and development. Here, origami-inspired 3D wearable sensors have been developed for plant growth and microclimate monitoring. Unlike 2D counterparts, the 3D sensors demonstrate theoretically infinitely high stretchability and breathability derived from the structure rather than the material. They are adjusted to 100% and 111.55 mg cm-2·h-1 in the optimized design. In addition to stretchability and breathability, the structural parameters are also used to control the strain distribution of the 3D sensors to enhance sensitivity and minimize interference. After integrating with corresponding sensing materials, electrodes, data acquisition and transmission circuits, and a mobile App, a miniaturized sensing system is produced with the capability of in-situ and online monitoring of plant elongation and microclimate. As a demonstration, the 3D sensors are worn on pumpkin leaves, which can accurately monitor the leaf elongation and microclimate with negligible hindrance to plant growth. Finally, the effects of the microclimate on the plant growth is resolved by analyzing the monitored data. This study would significantly promote the development of wearable plant sensors and their applications in the fields of plant phenomics, plant-environment interface, and smart agriculture.

Review on Microscale Sensors with 3D Engineered Structures: Fabrication and Applications

The intelligence of modern technologies relies on perceptual systems based on microscale sensors. However, because of the traditional top-down fabrication approaches performed on planar silicon wafers, a large proportion of existing microscale sensors have 2D structures, which severely restricts their sensing capabilities. To overcome these restrictions, over the past few decades, increasing efforts have been devoted to developing new fabrication methods for microscale sensors with 3D engineered structures, from bulk chemical etching and 3D printing to molding and stress-induced assembly. Herein, the authors systematically review these fabrication methods based on the applications of the resulting 3D sensors and discuss their advantages compared to their 2D counterparts. This is followed by a perspective on the remaining challenges and possible opportunities.

Laser direct write of heteroatom-doped graphene on molecularly controlled polyimides for electrochemical biosensors with nanomolar sensitivity

Fabrication of heteroatom-doped graphene electrodes remains a challenging endeavor, especially on flexible substrates. Precise chemical and morphological control is even more challenging for patterned microelectrodes. We herein demonstrate a scalable process for directly generating micropatterns of heteroatom-doped porous graphene on polyimide with different backbones using a continuous-wave infrared laser. Conventional two-step polycondensation of 4,4'-oxydianiline with three different tetracarboxylic dianhydrides enabled the fabrication of fully aromatic polyimides with various internal linkages such as phenylene, trifluoromethyl or sulfone groups. Accordingly, we leverage this laser-induced polymer-to-doped-graphene conversion for fabricating electrically conductive microelectrodes with efficient utilization of heteroatoms (N-doped, F-doped, and S-doped). Tuning laser fluence enabled achieving electrical resistivity lower than ~13 Ω sq^-1 for F-doped and N-doped graphene. Finally, our microelectrodes exhibit superior performance for electrochemical sensing of dopamine, one of the important neurotransmitters in the brain. Compared with carbon fiber microelectrodes, the gold standard in electrochemical dopamine sensing, our F-doped high surface area graphene microelectrodes demonstrated 3 order of magnitude higher sensitivity per unit area, detecting dopamine concentrations as low as 10 nM with excellent reproducibility. Hence, our approach is promising for facile fabrication of microelectrodes with superior capabilities for various electrochemical and sensing applications including early diagnosis of neurological disorders.

Laser reprogramming magnetic anisotropy in soft composites for reconfigurable 3D shaping

Responsive soft materials capable of exhibiting various three-dimensional (3D) shapes under the same stimulus are desirable for promising applications including adaptive and reconfigurable soft robots. Here, we report a laser rewritable magnetic composite film, whose responsive shape-morphing behaviors induced by a magnetic field can be digitally and repeatedly reprogrammed by a facile method of direct laser writing. The composite film is made from an elastomer and magnetic particles encapsulated by a phase change polymer. Once the phase change polymer is temporarily melted by transient laser heating, the orientation of the magnetic particles can be re-aligned upon change of a programming magnetic field. By the digital laser writing on selective areas, magnetic anisotropies can be encoded in the composite film and then reprogrammed by repeating the same procedure, thus leading to multimodal 3D shaping under the same actuation magnetic field. Furthermore, we demonstrated their functional applications in assembling multistate 3D structures driven by the magnetic force-induced buckling, fabricating multistate electrical switches for electronics, and constructing reconfigurable magnetic soft robots with locomotion modes of peristalsis, crawling, and rolling.

Responsive soft materials which can exhibit various three-dimensional (3D) shapes under the same stimulus are desirable for applications in adaptive and reconfigurable soft robots. Here, the authors report a laser rewritable magnetic composite film, whose responsive shape-morphing behaviors induced by a magnetic field can be digitally and repeatedly reprogrammed by a facile method of direct laser writing.

Deterministic Self-Morphing of Soft-Stiff Hybridized Polymeric Films for Acoustic Metamaterials

We reported a soft-stiff hybridized polymeric film that can self-morph to dedicated three-dimensional (3D) structures for application in acoustic metamaterials. The hybridized film was fabricated by laterally adhering a soft and responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel to stiff and passive SU-8 patterns. Upon thermal stimulation, deformation of the tough PNIPAM hydrogel was locally constrained by the stiff SU-8 patterns, thereby causing laterally nonuniform strain to their interfaces for mechanically buckling the hybridized films to 3D structures. Combined with finite element analysis, we demonstrated that the stiff SU-8 patterns effectively alleviated the uncontrollability and uncertainty during the self-morphing process, which was caused by unexpected mutual deformation between the active and passive domains in the self-morphing materials. Therefore, deterministic self-buckling to dedicated 3D structures was physically realized such as a wave-shaped peak-valley structure, 3D checkerboard patterns, and Gaussian curved surfaces from the hybridized polymeric films. Finally, we demonstrated that the self-morphed 3D structures with predesigned patterns can be used as acoustic materials for subwavelength noise control. This transformative way of constructing 3D structures by self-morphing of the hybridized polymeric films will be a substantial progress in fabricating smart and multifunctional materials for widespread applications in metamaterials, soft robotics, and 3D electronics.

Reprogrammable 3D Shaping from Phase Change Microstructures in Elastic Composites

Herein, we demonstrate reprogrammable 3D structures that are assembled from elastic composite sheets made from elastic materials and phase change microparticles. By controlling the phase change of the microparticles by localized thermal patterning, anisotropic residual strain is generated in the pre-stretched composite sheets and then triggers 3D structure assembly when the composite sheets are released from the external stress. Modulation of the geometries and location of the thermal patterns leads to complex 2D-3D shaping behaviors such as bending, folding, buckling, and wrinkling. Because of the reversible phase change of the microparticles, these programmed 3D structures can later be recovered to 2D sheets once they are heated for reprogramming different 3D structures. To predict the 3D structures assembled from the 2D composite sheets, finite element modeling was employed, which showed reasonable agreement with the experiments. The demonstrated strategy of reversibly programming 3D shapes by controlling the phase change microstructures in the elastic composites offers unique capabilities in fabricating functional devices such as a rewritable "paper" and a shape reconfigurable pneumatic actuator.

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli-responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature-responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D-formed membrane materials, which are pre-defined and selectively bonded to the substrates, are then assembled to 3D structures. They include "tent," "tower," "two-floor pavilion," "dome," "basket," and "nested-cages" with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro-electromechanical systems, and others.

Advances in biomimetic stimuli responsive soft grippers

A variety of biomimetic stimuli-responsive soft grippers that can be utilized as intelligent actuators, sensors, or biomedical tools have been developed. This review covers stimuli-responsive materials, fabrication methods, and applications of soft grippers. This review specifically describes the current research progress in stimuli-responsive grippers composed of N-isopropylacrylamide hydrogel, thermal and light-responding liquid crystalline and/or pneumatic-driven shape-morphing elastomers. Furthermore, this article provides a brief overview of high-throughput assembly methods, such as photolithography and direct printing approaches, to create stimuli-responsive soft grippers. This review primarily focuses on stimuli-responsive soft gripping robots that can be utilized as tethered/untethered multiscale smart soft actuators, manipulators, or biomedical devices.

Programmable Self-Assembling 3D Architectures Generated by Patterning of Swellable MOF-Based Composite Films

The integration of swellable metal-organic frameworks (MOFs) into polymeric composite films is a straightforward strategy to develop soft materials that undergo reversible shape transformations derived from the intrinsic flexibility of MOF crystals. However, a crucial step toward their practical application relies on the ability to attain specific and programmable actuation, which enables the design of self-shaping objects on demand. Herein, a chemical etching method is demonstrated for the fabrication of patterned composite films showing tunable self-folding response, predictable and reversible 2D-to-3D shape transformations triggered by water adsorption/desorption. These films are fabricated by selective removal of swellable MOF crystals allowing control over their spatial distribution within the polymeric film. Upon exposure to moisture, various programmable 3D architectures, which include a mechanical gripper, a lift, and a unidirectional walking device, are generated. Remarkably, these 2D-to-3D shape transformations can be reversed by light-induced desorption. The reported strategy offers a platform for fabricating flexible MOF-based autonomous soft mechanical devices with functionalities for micromanipulation, automation, and robotics.

Twistable Origami and Kirigami: from Structure-Guided Smartness to Mechanical Energy Storage

For achieving active shape transformable materials and structures, smart materials with shape memory effects along with deliberate structure design are generally used as the critical parameters in realizing structure transformation. Beyond such conventional approaches, here a novel structure-guided multimaterial three-dimensional (3D) printing strategy based on twistable origami structures is demonstrated to realize dynamic smart shape transformation. By thermally or photothermally triggering the prestored energy in the twisted structures, the 3D-printed integrated origami structures based on Miura and square-twist origami structures coupled with modifying by kirigami approaches are enabled to present a variable multistep transformable feature as well as a manipulatable stimulus-response behavior. Such shape transformation configuration allows the integrated origami and kirigami structures for constructing smart structures in delivering dynamic multifunction. More importantly, the shape transformation mechanism also suggests a unique capability in mechanical energy storage and release, promising a novel prototype of mechanical actuators. Implication of the results offers a great platform to construct smart and active structures using structure-guided strategies.

Design and Fabrication of Heterogeneous, Deformable Substrates for the Mechanically Guided 3D Assembly

Development of schemes to form complex three-dimensional (3D) mesostructures in functional materials is a topic of broad interest, thanks to the ubiquitous applications across a diversity of technologies. Recently established schemes in the mechanically guided 3D assembly allow deterministic transformation of two-dimensional structures into sophisticated 3D architectures by controlled compressive buckling resulted from strain release of prestretched elastomer substrates. Existing studies mostly exploited supporting substrates made of homogeneous elastomeric material with uniform thickness, which produces relatively uniform strain field to drive the 3D assembly, thus posing limitations to the geometric diversity of resultant 3D mesostructures. To offer nonuniform strains with desired spatial distributions in the 3D assembly, this paper introduces a versatile set of concepts in the design of engineered substrates with heterogeneous integration of materials of different moduli. Such heterogeneous, deformable substrates can achieve large strain gradients and efficient strain isolation/magnification, which are difficult to realize using the previously reported strategies. Theoretical and experimental studies on the underlying mechanics offer a viable route to the design of heterogeneous, deformable substrates to yield favorable strain fields. A broad collection of 3D mesostructures and associated heterogeneous substrates is fabricated and demonstrated, including examples that resemble windmills, scorpions, and manta rays and those that have application potentials in tunable inductors and vibrational microsystems.

Laser-Induced Graphene: From Discovery to Translation

Laser-induced graphene (LIG) is a 3D porous material prepared by direct laser writing with a CO₂ laser on carbon materials in ambient atmosphere. This technique combines 3D graphene preparation and patterning into a single step without the need for wet chemical steps. Since its discovery in 2014, LIG has attracted broad research interest, with several papers being published per month using this approach. These serve to delineate the mechanism of the LIG-forming process and to showcase the translation into many application areas. Herein, the strategies that have been developed to synthesize LIG are summarized, including the control of LIG properties such as porosity, composition, and surface characteristics, and the advancement in methodology to convert diverse carbon precursors into LIG. Taking advantage of the LIG properties, the applications of LIG in broad fields, such as microfluidics, sensors, and electrocatalysts, are highlighted. Finally, future development in biodegradable and biocompatible materials is briefly discussed.

Mechanically Guided Assembly of Monolithic Three-Dimensional Structures from Elastomer Composites

Mechanically guided assembly is considered a facile and scalable methodology for fabrication of three-dimensional (3D) structures. However, most of the previous methods require multistep processes for bonding bi- or multilayers and only result in non-freestanding 3D structures because of usage of a supporting elastomer substrate. Herein, we report a functional elastomer composite that can be transformed to a freestanding and monolithic 3D structure driven by the mechanically guided assembly. Photolithography can be used to selectively tune the mechanical properties of UV-exposed regions which exhibit enhanced ductility compared with the nonexposed regions. Thus, a gradient of the residual strain in the thickness direction makes the films assemble into 3D structures. These 3D structures are also predicted by our computational models using finite element simulations, which yields a reasonable agreement with the experiments. The systematically designed 2D structures with varied patterns can be transformed to various 3D structures with the control of the residual strain gradient, via key processing parameters including pre-strain, film thickness, and UV exposure time. By integrating different active electronic components on the fabricated 3D structures, potential applications of this 3D platform in electronics were demonstrated. This study offers a unique capability in constructing monolithic and freestanding 3D assembly, paving new routes to many applications such as wearable electronics, smart textiles, soft robotics, and structural health monitoring.