Mechanochromic Stretchable Electronics

Cephalopod-inspired polymer composites with mechanically tunable infrared properties

Cephalopods have evolved an all-soft skin that can rapidly display colors for protection, predation, or communication. Development of synthetic analogs to mimic such color-changing abilities in the infrared (IR) region is pivotal to a variety of technologies ranging from soft robotics, flexible displays, dynamic thermoregulatory systems, to adaptive IR disguise platforms. However, the integration of tissue-like mechanical properties and rapid IR modulation ability into smart materials remains challenging. Here, by drawing inspiration from cephalopod skin, we develop an all-soft adaptive IR composite that can dynamically change its IR appearance upon equiaxial stretching. The biomimetic composite is built entirely from soft materials of liquid metal droplets and elastic elastomer, which are analogs of chromatophores and dermal layer of cephalopod skin, respectively. Driven by externally applied strains, the liquid metal inclusions transition between a contracted droplet state with corrugated surface and an expanded platelet state with relatively smooth surface, enabling dynamic variations in the IR reflectance/emissivity of the composite and ultimately resulting in reversible IR adaption. Strain-actuated flexible IR displays and pneumatically-driven soft devices that can dynamically manipulate their IR appearance are demonstrated as examples of the applicability of this material in emerging adaptive soft electronics.

Reversible CO2 Capture and On-Demand Release by an Acidity-Matched Organic Photoswitch

Separation of carbon dioxide (CO2) from point sources or directly from the atmosphere can contribute crucially to climate change mitigation plans in the coming decades. A fundamental practical limitation for the current strategies is the considerable energy cost required to regenerate the sorbent and release the captured CO2 for storage or utilization. A directly photochemically driven system that demonstrates efficient passive capture and on-demand CO2 release triggered by sunlight as the sole external stimulus would provide an attractive alternative. However, little is known about the thermodynamic requirements for such a process or mechanisms for modulating the stability of CO2-derived dissolved species by using photoinduced metastable states. Here, we show that an organic photoswitchable molecule of precisely tuned effective acidity can repeatedly capture and release a near-stoichiometric quantity of CO2 according to dark-light cycles. The CO2-derived species rests as a solvent-separated ion pair, and key aspects of its excited-state dynamics that regulate the photorelease efficiency are characterized by transient absorption spectroscopy. The thermodynamic and kinetic concepts established herein will serve as guiding principles for the development of viable solar-powered negative emission technologies.

Mechanochromism and Strain-Induced Crystallization in Thiol-yne-Derived Stereoelastomers

Most elastomers undergo strain-induced crystallization (SIC) under tension; as individual chains are held rigidly in a fixed position by an applied strain, their alignment along the strain field results in a shift from strain-hardening (SH) to SIC. A similar degree of stretching is associated with the tension necessary to accelerate mechanically coupled, covalent chemical responses of mechanophores in overstretched chains, raising the possibility of an interplay between the macroscopic response of SIC and the molecular response of mechanophore activation. Here, thiol-yne-derived stereoelastomers doped covalently with a dipropiolate-derivatized spiropyran (SP) mechanophore (0.25-0.38 mol%) are reported. The material properties of SP-containing films are consistent with undoped controls, indicating that the SP is a reporter of the mechanical state of the polymer. Uniaxial tensile tests reveal correlations between mechanochromism and SIC, which are strain-rate-dependent. When mechanochromic films are stretched slowly to the point of mechanophore activation, the covalently tethered mechanophore remains trapped in a force-activated state, even after the applied stress is removed. Mechanophore reversion kinetics correlate with the applied strain rate, resulting in highly tunable decoloration rates. Because these polymers are not covalently crosslinked, they are recyclable by melt-pressing into new films, increasing their potential range of strain-sensing, morphology-sensing, and shape-memory applications.

Dipyrene-Terminated Oligosilanes Enable Ratiometric Fluorescence Response in Polymers toward Mechano- and Thermo-Stimuli

Developing fluorescent motifs capable of displaying mechano- and thermo-stimuli reversibly and ratiometrically is appealing for monitoring the deformation or temperature to which polymers are subjected. Here, a series of excimer-type chromophores Si_n-Py (n = 1-3) consisting of two pyrenes linked with oligosilanes of one to three silicon atoms is developed as the fluorescent motif incorporated in a polymer. The fluorescence of Si_n-Py is steered with the linker length where Si₂-Py and Si₃-Py with disilane and trisilane linkers display prominent excimer emission accompanied by pyrene monomer emission. Covalent incorporation of Si₂-Py and Si₃-Py in polyurethane gives fluorescent polymers PU-Si₂-Py and PU-Si₃-Py, respectively, where intramolecular pyrene excimers and corresponding combined emission of excimer and monomer are obtained. Polymer films of PU-Si₂-Py and PU-Si₃-Py display instant and reversible ratiometric fluorescence change during the uniaxial tensile test. The mechanochromic response arises from the reversible suppression of excimer formation during the mechanically induced separation of the pyrene moieties and relaxation. Furthermore, PU-Si₂-Py and PU-Si₃-Py show thermochromic response toward temperature, and the inflection point from the ratiometric emission as a function of temperature gives an indication of the glass transition temperature (T_g) of the polymers. The design of the excimer-based mechanophore with oligosilane provides a generally implementable way to develop mechano- and thermo-dual-responsive polymers.

A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

A dinoflagellate-inspired mechanochromic film for fast and reversible information encryption and display

Inspired by dinoflagellates, we developed a flexible film consisting of spiropyran-based soft polyacrylate and Zn(OTf)₂. The open-ring form of spiropyran coordinated with Zn(OTf)₂ under stretching to produce a visible fluorescent color change from colorless to yellow. The potential of this film was demonstrated for fast and reversible information encryption and decryption.

Gallium-Based Liquid Metal Materials for Antimicrobial Applications

The hazards caused by drug-resistant bacteria are rocketing along with the indiscriminate use of antibiotics. The development of new non-antibiotic antibacterial drugs is urgent. The excellent biocompatibility and diverse multifunctionalities of liquid metal have stimulated the studies of antibacterial application. Several gallium-based antimicrobial agents have been developed based on the mechanism that gallium (a type of liquid metal) ions disorder the normal metabolism of iron ions. Other emerging strategies, such as physical sterilization by directly using LM microparticles to destroy the biofilm of bacteria or thermal destruction via infrared laser irradiation, are gaining increasing attention. Different from traditional antibacterial agents of gallium compounds, the pronounced property of gallium-based liquid metal materials would bring innovation to the antibacterial field. Here, LM-based antimicrobial mechanisms, including iron metabolism disorder, production of reactive oxygen species, thermal injury, and mechanical destruction, are highlighted. Antimicrobial applications of LM-based materials are summarized and divided into five categories, including liquid metal motors, antibacterial fabrics, magnetic field-responsive microparticles, liquid metal films, and liquid metal polymer composites. In addition, future opportunities and challenges towards the development and application of LM-based antimicrobial materials are presented.

Polymer mechanochemistry for the release of small cargoes

The field of force-induced release of small cargoes within polymeric materials has experienced rapid growth over the past decade, not only including achieving diversified functional materials that report force, trigger degradation, activate drugs and release catalysts, but also involving investigations on the interesting force-coupled reactivity of mechanophores, such as ferrocenes. In this highlight article, we review the recent progress on polymer mechanochemistry that releases small cargoes, including small molecules and metal ions. Since mechanophores play a key role in force-responsive materials, we introduce the progress by discussing different types of mechanophores and their mechanochemical reactions for the release of acids, gases, fluorophores, drugs, iron ions, and so on. At the end, we provide our perspectives on the remaining challenges and future targets in this growing field.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

A triphenylamine derivative and its Cd(ii) complex with high-contrast mechanochromic luminescence and vapochromism

A triphenylamine derivative and its Cd(ii) complex exhibited predominant mechanochromism and vapochromism with high-contrast color and emission changes.

Force-modulated reductive elimination from platinum(ii) diaryl complexes

Coupled mechanical forces are known to drive a range of covalent chemical reactions, but the effect of mechanical force applied to a spectator ligand on transition metal reactivity is relatively unexplored. Here we quantify the rate of C(sp2)-C(sp2) reductive elimination from platinum(ii) diaryl complexes containing macrocyclic bis(phosphine) ligands as a function of mechanical force applied to these ligands. DFT computations reveal complex dependence of mechanochemical kinetics on the structure of the force-transducing ligand. We validated experimentally the computational finding for the most sensitive of the ligand designs, based on MeOBiphep, by coupling it to a macrocyclic force probe ligand. Consistent with the computations, compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ∼290 pN range of restoring forces. The calculated natural bite angle of the free macrocyclic ligand changes with force, but 31P NMR analysis and calculations strongly suggest no significant force-induced perturbation of ground state geometry within the first coordination sphere of the (P-P)PtAr2 complexes. Rather, the force/rate behavior observed across this range of forces is attributed to the coupling of force to the elongation of the O⋯O distance in the transition state for reductive elimination. The results suggest opportunities to experimentally map geometry changes associated with reactions in transition metal complexes and potential strategies for force-modulated catalysis.

Hydrochromic Visualization of a Keggin-Type Structure Triggered by Metallic Fluids for Liquid Displays, Reversible Writing, and Acidic Environment Detection

Hydrochromic visualization of a liquid interface shows vital potential applications in liquid displays, reversible writing, and acidic environmental detection, which offers a platform for detection and forewarning due to its intuitive and visual characteristics. Herein, we report a hydrochromic display due to the interfacial effect of liquid metal (LM)-triggered ammonium metatungstate (AMT) with instant dual-mode color switching. The double-electron-transfer reaction of the AMT on the surface of gallium-based LM caused the formation of heteropoly blue in the presence of acidic surroundings, resulting in a reversible color switching from being colorless to blue or blue to colorless. This visual interfacial discoloration phenomenon can be applied to the liquid display on diverse patterns of the LM surface. Furthermore, papers with a functional display were prepared, which can be used for writing up to eight times with dual-mode color switching. In addition, the reactive activity of acid triggering make it a potential candidate for use in visualizing an acidic environment with a detection range of pH = 1 to 0 (0.1-1.5 M). Briefly, this interfacial discoloration phenomenon enriches the interfacial engineering of LM and provides a unique prospective and wide-range platform for the application of LM.

Toward Mechanochromic Soft Material-Based Visual Feedback for Electronics-Free Surgical Effectors

A chromogenically reversible, mechanochromic pressure sensor is integrated into a mininvasive surgical grasper compatible with the da Vinci robotic surgical system. The sensorized effector, also featuring two soft-material jaws, encompasses a mechanochromic polymeric inset doped with functionalized spiropyran (SP) molecule, designed to activate mechanochromism at a chosen pressure and providing a reversible color change. Considering such tools are systematically in the visual field of the operator during surgery, color change of the mechanochromic effector can help avoid tissue damage. No electronics is required to control the devised visual feedback. SP-doping of polydimethylsiloxane (2.5:1 prepolymer/curing agent weight ratio) permits to modulate the mechanochromic activation pressure, with lower values around 1.17 MPa for a 2% wt. SP concentration, leading to a shorter chromogenic recovery time of 150 s at room temperature (25 °C) under green light illumination. Nearly three-times shorter recovery time is observed at body temperature (37 °C). To the best of knowledge, this study provides the first demonstration of mechanochromic materials in surgery, in particular to sensorize unpowered surgical effectors, by avoiding dramatic increases in tool complexity due to additional electronics, thus fostering their application. The proposed sensing strategy can be extended to further tools and scopes.

Hierarchical Tactile Sensation Integration from Prosthetic Fingertips Enables Multi-Texture Surface Recognition

Multifunctional flexible tactile sensors could be useful to improve the control of prosthetic hands. To that end, highly stretchable liquid metal tactile sensors (LMS) were designed, manufactured via photolithography, and incorporated into the fingertips of a prosthetic hand. Three novel contributions were made with the LMS. First, individual fingertips were used to distinguish between different speeds of sliding contact with different surfaces. Second, differences in surface textures were reliably detected during sliding contact. Third, the capacity for hierarchical tactile sensor integration was demonstrated by using four LMS signals simultaneously to distinguish between ten complex multi-textured surfaces. Four different machine learning algorithms were compared for their successful classification capabilities: K-nearest neighbor (KNN), support vector machine (SVM), random forest (RF), and neural network (NN). The time-frequency features of the LMSs were extracted to train and test the machine learning algorithms. The NN generally performed the best at the speed and texture detection with a single finger and had a 99.2 ± 0.8% accuracy to distinguish between ten different multi-textured surfaces using four LMSs from four fingers simultaneously. The capability for hierarchical multi-finger tactile sensation integration could be useful to provide a higher level of intelligence for artificial hands.

Mechanoactivation of Color and Autonomous Shape Change in 3D-Printed Ionic Polymer Networks

Stimuli-responsive materials can enhance the field of three-dimensional (3D) printing by generating objects that change shape in response to external cues. While temperature and pH are common inputs for initiating a response in a 3D-printed object, there are few examples of using a mechanical input to afford a response. Herein, we report a suite of mechanochromic ionic liquid gel inks that can be used to fabricate 3D-printed objects that use a single mechanoactivation event to elicit both a mechanochromic response and an autonomous shape change. Direct-ink write 3D printing was used to deposit ionic liquid gel inks to create multimaterial objects that underwent a predetermined mechanoactivated shape change (mechanomorphic) when the sample was pulled and then released. When spiropyran was incorporated into the inks, the onset of spiropyran isomerization into its purple merocyanine form occurred at strains dependent upon the particular ion gel ink formulation. We suggest that the color onset could be used as a simple indicator for when the strain required to achieve a predetermined change in shape has been reached, potentially serving as real-time visual cue for the user. Such morphing 3D-printed structures with integrated instructions have potential application to areas including stowable structures as well as multiresponsive autonomous components and sensors.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Mechanically Induced Bright Luminescence from 1,2-Dioxetane Containing PDMS Boosted by Fluoroboron Complex as an In-Chain Fluorophore

Improving mechanochemiluminescent (MCL) sensitivity of 1,2-dioxetane containing polymers is important for the applications of stress-reporting soft materials. Herein, a series of MCL poly(dimethylsiloxane) (PDMS) have been synthesized by simultaneously incorporating difluoroboron β-diketonate dye and 1,2-dioxetane as the co-crosslinkers to tune the energy transfer process across polymer chains. By covalently linked fluoroboron complex in PDMS network, the aggregation of the complex is overcome. Owing to its excellent opto-physical properties, this fluoroboron complex is shown to be an effective in-chain fluorophore to effectively enhance the chemiluminescence from polymeric 1,2-dioxetane that is broken either thermally or mechanically. Studies on the optomechanical properties of these PDMS show that MCL intensity is increased with the concentration of fluoroboron complex and the wavelength of the emission is shifted. The results of the present study appear to be broadly useful for designing elastomeric networks with chemiluminescent property not only attractive for optical technology, but also useful for damage self-reporting.

Onset of Mechanochromic Response in the High Strain Rate Uniaxial Compression of Spiropyran Embedded Silicone Elastomers

The molecular processes that accompany dynamic mechanical response to large deformations at high strain rate (≈1000 s^-1 or higher) underlie the early stages of damage in materials, but understanding of material response in this regime is typically limited to macroscopic constitutive equations. Here, spiropyran mechanophores are embedded in very short, stress-bearing strands in silicone elastomers, and their mechanochromic response to uniaxial compression is explored in a Split Hopkinson Pressure (or Kolsky) Bar. At strain rates of 1000 s^-1 , the onset of mechanochromism occurs at lower strains, but higher stresses, than in the same materials under quasi-static loading. Similar to quasi-static loading, however, a negligible effect of mechanophore structure on the critical strain for colorimetric onset is observed. The results suggest that nonequilibrium, inhomogeneous local tension distributions in the elastomers lead to greater stress in individual strands than at the same strains under equilibrium loading, but that within the regions of force concentration, mechanochromic onset is determined primarily by a limiting local strain threshold.

Regulating Heterogeneous Catalysis of Gold Nanoparticles with Polymer Mechanochemistry

Polymer mechanochemistry has emerged as a unique approach to regulate homogeneous catalysis in chemical transformations. The utilization of polymer mechanochemistry to regulate heterogeneous catalysis, however, still remains to be investigated. In this study, using polymer-grafted gold nanoparticles as the model heterogeneous catalysts, we show that polymer chains can be mechanically ruptured from the surface of gold nanoparticles, and thus, the catalytic activity of gold nanoparticles can be accelerated under sonication. The mechanical activation of polymer-grafted gold nanoparticles only occurs when the grafted polymer chains exceed a threshold molecular weight. This mechanical behavior is similar to those mechanophore-linked polymers. More importantly, further characterizations reveal that the Au-Au bonds instead of the Au-S bonds are broken at the heterointerfaces of polymer chains and gold nanoparticles. Our study unveils an unprecedented characteristic of polymer-grafted metallic nanoparticles in response to external mechanical stress.

Soft, Stretchable, and Pneumatically Triggered Thermochromic Optical Filters with Embedded Phosphorescence

Phosphorescence is commonly used in nature to communicate using light. There are many ways to activate phosphorescence, including UV light, heat, and mechanical forces, but there are few methods to control phosphorescence once activated. Here, we present soft composite devices-which we call "optical filters"-for controlling the release of light by phosphorescence within a stretchable matrix. The filters consist of liquid metal wires, phosphorescent particles, and thermochromic pigments embedded in an elastomeric matrix. UV light initially activates the phosphorescence of rare-earth long-lasting luminescent particles. At room temperature, the thermochromic pigments block the phosphorescence from leaving the matrix. However, Joule heating of the liquid metal can change the opacity of the thermochromic pigments, which tunes the color, intensity, and wavelength of phosphorescence that exits the composite. In addition, the resistance of the liquid metal wires changes with physical deformation, thereby converting mechanical forces (strain, compression, and pneumatic inflation) into an optical response. Controlled phosphorescence, combined with the electrical conductivity of the liquid metal and the overall soft matrix, enables potential applications as an electronic skin for soft robotics, stretchable electronics, and prosthetics.

Liquid Metal-Based Soft Microfluidics

Motivated by the increasing demand of wearable and soft electronics, liquid metal (LM)-based microfluidics has been subjected to tremendous development in the past decade, especially in electronics, robotics, and related fields, due to the unique advantages of LMs that combines the conductivity and deformability all-in-one. LMs can be integrated as the core component into microfluidic systems in the form of either droplets/marbles or composites embedded by polymer materials with isotropic and anisotropic distribution. The LM microfluidic systems are found to have broad applications in deformable antennas, soft diodes, biomedical sensing chips, transient circuits, mechanically adaptive materials, etc. Herein, the recent progress in the development of LM-based microfluidics and their potential applications are summarized. The current challenges toward industrial applications and future research orientation of this field are also summarized and discussed.

Polymer mechanochemistry-enabled pericyclic reactions

Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

Reduced strain mechanochemical activation onset in microstructured materials

In this study, we show that mechanochemical activation in responsive materials with designed, periodic microstructures can be achieved at lower applied strains than their bulk counterparts.

Strain-Dependent Kinetics in the Cis-to-Trans Isomerization of Azobenzene in Bulk Elastomers

The cis-to-trans isomerization of azobenzene is accelerated in a bulk PDMS elastomer under uniaxial tension. The kinetics are cleanly described by a single-exponential first-order process (k = 2.7 × 10^-5 s^-1) in the absence of tension but become multiexponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (∼92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3-2.7 × 10^-5 s^-1), whereas the rate constant of the minority (∼8%) process increases from k = 10.1 × 10^-5 s^-1 at 40% strain to k = 21.3 × 10^-5 s^-1 at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 to 44 pN across strains of 40-90%.

Materials tactile logic via innervated soft thermochromic elastomers

Conventional machines rely on rigid, centralized electronic components to make decisions, which limits complexity and scaling. Here, we show that decision making can be realized on the material-level without relying on semiconductor-based logic. Inspired by the distributed decision making that exists in the arms of an octopus, we present a completely soft, stretchable silicone composite doped with thermochromic pigments and innervated with liquid metal. The ability to deform the liquid metal couples geometric changes to Joule heating, thus enabling tunable thermo-mechanochromic sensing of touch and strain. In more complex circuits, deformation of the metal can redistribute electrical energy to distal portions of the network in a way that converts analog tactile ‘inputs’ into digital colorimetric ‘outputs’. Using the material itself as the active player in the decision making process offers possibilities for creating entirely soft devices that respond locally to environmental interactions or act as embedded sensors for feedback loops.

Designing bio-inspired computational features in soft systems without centralized processing remains challenging. Here, the authors propose a passive display based on thermochromic elastomers by leveraging Joule heating of embedded liquid metal wires by changing geometry in response to deformation.

Stretchable and reflective displays: materials, technologies and strategies

Displays play a significant role in delivering information and providing visual data across all media platforms. Among displays, the prominence of reflective displays is increasing, in the form of E-paper, which has features distinct from emissive displays. These unique features include high visibility under daylight conditions, reduced eye strain and low power consumption, which make them highly effective for outdoor use. Furthermore, such characteristics enable reflective displays to achieve high synergy in combination with wearable devices, which are frequently used for outdoor activities. However, as wearable devices must stretch to conform to the dynamic surfaces of the human body, the issue of how to fabricate stretchable reflective displays should be tackled prior to merging them with wearable devices. In this paper, we discuss stretchable and reflective displays. In particular, we focus on reflective displays that can be divided into two types, passive and active, according to their responses to stretching. Passive displays, which consist of dyes or pigments, exhibit consistent colors under stretching, while active displays, which are based on mechanochromic materials, change their color under the same conditions. We will provide a comprehensive overview of the materials and technologies for each display type, and present strategies for stretchable and reflective displays.

A Hierarchical Nanoparticle-in-Micropore Architecture for Enhanced Mechanosensitivity and Stretchability in Mechanochromic Electronic Skins

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Multimaterial actinic spatial control 3D and 4D printing

Production of objects with varied mechanical properties is challenging for current manufacturing methods. Additive manufacturing could make these multimaterial objects possible, but methods able to achieve multimaterial control along all three axes of printing are limited. Here we report a multi-wavelength method of vat photopolymerization that provides chemoselective wavelength-control over material composition utilizing multimaterial actinic spatial control (MASC) during additive manufacturing. The multicomponent photoresins include acrylate- and epoxide-based monomers with corresponding radical and cationic initiators. Under long wavelength (visible) irradiation, preferential curing of acrylate components is observed. Under short wavelength (UV) irradiation, a combination of acrylate and epoxide components are incorporated. This enables production of multimaterial parts containing stiff epoxide networks contrasted against soft hydrogels and organogels. Variation in MASC formulation drastically changes the mechanical properties of printed samples. Samples printed using different MASC formulations have spatially-controlled chemical heterogeneity, mechanical anisotropy, and spatially-controlled swelling that facilitates 4D printing.

Objects with varied mechanical properties can be produced by additive manufacturing, but multimaterial control along all three axes of printing is still limited. Here the authors use wavelength control during vat polymerization and demonstrate printing of objects with spatial control of the composition and stiffness.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

Spiropyrans have played a pivotal role in the emergence of the field of chromism following their discovery in the early 20th century, with almost ubiquitous use in materials applications especially since their photochromism was discovered in 1952.

Mechanochromism of a dumbbell D–π–A–π–D phenothiazine derivative

A dumbbell D–π–A–π–D phenothiazine derivative changes its fluorescence color from orange to red under force stimuli due to π-stacking conversion between H- and J-aggregates.

Self-encapsulation liquid metal materials for flexible and stretchable electrical conductors

A one-step strategy for fabricating flexible conductors via phase separation is proposed, wherein, the liquid metal was implanted into polydimethylsiloxane, whose viscosity was changed using hexane. Such self-encapsulating composite exhibited good electronic and mechanical stability under mechanical cycles with no significant leaking of droplets during the testing process.

A one-step strategy for fabricating flexible conductors via phase separation is proposed, wherein, the liquid metal was implanted into polydimethylsiloxane, whose viscosity was changed using hexane.

Mechanochromic luminescence based on a phthalonitrile-bridging salophen zinc(ii) complex

Here, we showcase the impressive stimuli-responsive properties of a luminescent zinc(ii)–salophen complex CN-Zn, highlighting a reversible mechanochromic property.

Mechanochromic composite elastomers for additive manufacturing and low strain mechanophore activation

Composite silicone inks provide access to 3D-printable elastomers that are mechanochemically active at lower strains that single component analogs.

Substituent Effects and Mechanism in a Mechanochemical Reaction

We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO₂ para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s^-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.