Shape-morphing systems can be found in many areas, including smart textiles, autonomous robotics, biomedical devices, drug delivery and tissue engineering. The natural analogues of such systems are exemplified by nastic plant motions, where a variety of organs such as tendrils, bracts, leaves and flowers respond to environmental stimuli (such as humidity, light or touch) by varying internal turgor, which leads to dynamic conformations governed by the tissue composition and microstructural anisotropy of cell walls. Inspired by these botanical systems, we printed composite hydrogel architectures that are encoded with localized, anisotropic swelling behaviour controlled by the alignment of cellulose fibrils along prescribed four-dimensional printing pathways. When combined with a minimal theoretical framework that allows us to solve the inverse problem of designing the alignment patterns for prescribed target shapes, we can programmably fabricate plant-inspired architectures that change shape on immersion in water, yielding complex three-dimensional morphologies.

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Biomimetic 4D printing

Soft materials capable of transforming between three-dimensional (3D) shapes in response to stimuli such as light, heat, solvent, electric and magnetic fields have applications in diverse areas such as flexible electronics1,2, soft robotics3,4 and biomedicine5–7. In particular, magnetic fields offer a safe and effective manipulation method for biomedical applications, which typically require remote actuation in enclosed and confined spaces8–10. With advances in magnetic field control
 11
 , magnetically responsive soft materials have also evolved from embedding discrete magnets
 12
 or incorporating magnetic particles
 13
 into soft compounds to generating nonuniform magnetization profiles in polymeric sheets14,15. Here we report 3D printing of programmed ferromagnetic domains in soft materials that enable fast transformations between complex 3D shapes via magnetic actuation. Our approach is based on direct ink writing
 16
 of an elastomer composite containing ferromagnetic microparticles. By applying a magnetic field to the dispensing nozzle while printing
 17
 , we reorient particles along the applied field to impart patterned magnetic polarity to printed filaments. This method allows us to program ferromagnetic domains in complex 3D-printed soft materials, enabling a set of previously inaccessible modes of transformation, such as remotely controlled auxetic behaviours of mechanical metamaterials with negative Poisson’s ratios. The actuation speed and power density of our printed soft materials with programmed ferromagnetic domains are orders of magnitude greater than existing 3D-printed active materials. We further demonstrate diverse functions derived from complex shape changes, including reconfigurable soft electronics, a mechanical metamaterial that can jump and a soft robot that crawls, rolls, catches fast-moving objects and transports a pharmaceutical dose.

Printing ferromagnetic domains for untethered fast-transforming soft materials

This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

https://pubs.rsc.org/en/content/articlepdf/2017/PY/C6PY01585A

Stimuli-responsive polymers and their applications

This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead-in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors.

Photonic Crystals for Chemical Sensing and Biosensing

Self-folding microscale origami patterns are demonstrated in polymer films with control over mountain/valley assignments and fold angles using trilayers of photo-crosslinkable copolymers with a temperature-sensitive hydrogel as the middle layer. The characteristic size scale of the folds W = 30 μm and figure of merit A/ W (2) ≈ 5000, demonstrated here represent substantial advances in the fabrication of self-folding origami.

Programming Reversibly Self‐Folding Origami with Micropatterned Photo‐Crosslinkable Polymer Trilayers

Colorimetric temperature sensors are prepared from photo-crosslinkable polymers by sequentially spin-coating and crosslinking alternating layers of poly(N-isopropylacrylamide) and poly(para-methyl styrene). Layer thicknesses and copolymer chemistries are chosen to provide robust colorimetric temperature sensors that cover nearly the full visible spectrum.

Photonic Multilayer Sensors from Photo‐Crosslinkable Polymer Films

A series of copolymers containing covalently attached benzophenone (BP) photo-cross-linkers were synthesized, and their UV-induced gelation was monitored as a function of the extent of BP conversion. For poly(methyl methacrylate) copolymers, the recombination yield between BP- and aliphatic-centered radicals was estimated and compared to that for dimerization of each species, directly confirming that the high gelation efficiencies observed for these copolymers arise due to the additional cross-linking pathways provided by covalently incorporated BP, as compared to doping with a small-molecule cross-linker. The placement of the hydrogen species most susceptible to abstraction by triplet benzophenone is found to greatly influence gelation efficiency, since radical generation on the polymer backbone typically increases the probability of dislinking events, while hydrogen abstraction pendent to the copolymer backbone tends to enhance cross-linking. Finally, the presence of atmospheric oxygen during photo-cros...

Gelation of Copolymers with Pendent Benzophenone Photo-Cross-Linkers

A straightforward method to increase the refractive index of photocrosslinkable polymers by incorporation of high index inorganic nanoparticles is demonstrated and shown to enhance the reflection efficiency of thermochromic 1D photonic multilayers. The refractive index of spin-coated and UV-crosslinked films based on poly(para-methyl styrene) (PpMS) copolymers is increased from 1.57 for the copolymer alone to as high as 1.67 for nanocomposite samples with a volume fraction of 0.38 of ZrO2 nanoparticles. Thermochromic photonic multilayers consisting of alternating films of PpMS–ZrO2 with poly(N-isopropylacrylamide) (PNIPAM) copolymers shows the increases in reflectance as large as 2.5-fold compared to PpMS/PNIPAM multilayers lacking particles. In addition, ZrO2 nanoparticles are used to increase the refractive index of PNIPAM-based films up to 1.68 with a volume fraction of 0.49 of nanoparticles, enabling the fabrication of alternating PNIPAM–ZrO2/PNIPAM multilayers with a well-defined Bragg peak that shifts from 635 nm at 6 °C to 410 nm at 48 °C, and reflectance intensities as high as ≈0.30.

Photocrosslinkable Nanocomposite Multilayers for Responsive 1D Photonic Crystals

A radiation sensor includes a substrate and a polymer multilayer film including alternating layers of a high refractive index polymer and a low refractive index polymer. The high refractive index polymer and the low refractive index polymer each comprise repeat units derived from a photo-crosslinkable monomer. The radiation sensors are useful in preparing various articles, including wearable patches, packaging materials, labels, and window panes. Also described is a method of detecting radiation using the radiation sensors.

Maria C. Chiappelli

Papers

Programming Reversibly Self‐Folding Origami with Micropatterned Photo‐Crosslinkable Polymer Trilayers

Photonic Multilayer Sensors from Photo‐Crosslinkable Polymer Films

Gelation of Copolymers with Pendent Benzophenone Photo-Cross-Linkers

Photocrosslinkable Nanocomposite Multilayers for Responsive 1D Photonic Crystals

Photonic polymer multilayers for colorimetric radiation sensing