Amoeba-like self-oscillating polymeric fluids with autonomous sol-gel transition

Sustainable Supramolecular Polymers

Polymer waste is a pressing issue that requires innovative solutions from the scientific community. As a beacon of hope in addressing this challenge, the concept of sustainable supramolecular polymers (SSPs) emerges. This article discusses challenges and efforts in fabricating SSPs. Addressing the trade-offs between mechanical performance and sustainability, the ultra-tough and multi-recyclable supramolecular polymers are fabricated via tailoring mismatched supramolecular interactions. Additionally, the healing of kinetically inert polymer materials is realized through transient regulation of the interfacial reactivity. Furthermore, a possible development trajectory for SSPs is proposed, and the transient materials can be regarded as the next generation in this field. The evolution of SSPs promises to be a pivotal stride towards a regenerative economy, sparking further exploration and innovation in the realm of sustainable materials.

Direct Conversion of Phase-Transition Entropy into Electrochemical Thermopower and the Peltier Effect

A thermocell generates thermopower from a temperature difference (ΔT) between two electrodes. The converse process of thermocells is an electrochemical Peltier effect, which creates a ΔT on the electrodes by applying an external current. The Seebeck coefficient (Se ) of the electrochemical system is proportional to the entropy change of the redox reaction; therefore, a redox system having a significant entropy change is expected to increase the Se . In this study, a thermoresponsive polymer having a redox-active moiety, poly(N-isopropyl acrylamide-co-N-(2-acrylamide ethyl)-N'-n-propylviologen) (PNV), is used as the redox species of a thermocell. PNV2+ dication undergoes the coil-globule phase transition upon the reduction to PNV+ cation radical, and a large entropy change is introduced because water molecules are freed from the polymer chains. The Se of PNV thermocell drastically increased to +2.1 mV K-1 at the lower critical solution temperature (LCST) of PNV. The entropy change calculated from the increment of Se agrees with the value evaluated by differential scanning calorimetry. Moreover, the electrochemical Peltier effect is observed when the device temperature is increased above the LCST. This study shows that the large entropy change associated with the coil-globule phase transition can be used in electrochemical thermal management and refrigeration technologies.

Engineering metabolic cycle-inspired hydrogels with enzyme-fueled programmable transient volume changes

An enzyme-fueled transient volume phase transition (TVPT) of hydrogels under out-of-equilibrium conditions is reported. The approach takes inspiration from the metabolic cycle, comprising nutrient intake and anabolism/catabolism followed by waste excretion. The incorporation of methacrylic acid and acrylated trypsin in a polymeric hydrogel allowed the TVPT of the gel to be fueled by lysozyme. With the intake of lysozyme as fuel, the construction/destruction of electrostatic cross-linkages induced transient shrinkage/swelling of the gel accompanied by the depletion of lysozyme activity. The system's transient response could be flexibly programmed by adjusting not only the fuel concentration but the chemical composition of materials. The lysozyme-fueled TVPT of the gel could be exploited to transient changes in the mechanical properties of the gel. Our work opens a route toward a new class of stimuli-responsive hydrogels for biomedical applications.

CO2 -Fueled Transient Breathing Nanogels that Couple Nonequilibrium Catalytic Polymerization

Here we present a "breathing" nanogel that is fueled by CO₂ gas to perform temporally programmable catalytic polymerization. The nanogel is composed of common frustrated Lewis pair polymers (FLPs). Dynamic CO₂ -FLP gas-bridging bonds endow the nanogel with a transient volume contraction, and the resulting proximal effect of bound FLPs unlocks its catalytic capacity toward CO₂ . Reverse gas depletion via a CO₂ -participated polymerization can induce a reverse nanogel expansion, which shuts down the catalytic activity. Control of external factors (fuel level, temperature or additives) can regulate the breathing period, amplitude and lifecycle, so as to affect the catalytic polymerization. Moreover, editing the nanogel breathing procedure can sequentially evoke the copolymerization of CO₂ with different epoxide monomers preloaded therein, which allows to obtain block-tunable copolycarbonates that are unachievable by other methods. This synthetic dissipative system would be function as a prototype of gas-driven nanosynthesizer.

Emergent microrobotic oscillators via asymmetry-induced order

Spontaneous oscillations on the order of several hertz are the drivers of many crucial processes in nature. From bacterial swimming to mammal gaits, converting static energy inputs into slowly oscillating power is key to the autonomy of organisms across scales. However, the fabrication of slow micrometre-scale oscillators remains a major roadblock towards fully-autonomous microrobots. Here, we study a low-frequency oscillator that emerges from a collective of active microparticles at the air-liquid interface of a hydrogen peroxide drop. Their interactions transduce ambient chemical energy into periodic mechanical motion and on-board electrical currents. Surprisingly, these oscillations persist at larger ensemble sizes only when a particle with modified reactivity is added to intentionally break permutation symmetry. We explain such emergent order through the discovery of a thermodynamic mechanism for asymmetry-induced order. The on-board power harvested from the stabilised oscillations enables the use of electronic components, which we demonstrate by cyclically and synchronously driving a microrobotic arm. This work highlights a new strategy for achieving low-frequency oscillations at the microscale, paving the way for future microrobotic autonomy.

Spontaneous low-frequency oscillations, which are a feature of biological systems, are challenging to engineer into microrobotic systems. The authors discover a mechanism for asymmetry-induced order and realise electrical and mechanical oscillations in a particle collective to power a microrobotic arm.

Biomacromolecule-Fueled Transient Volume Phase Transition of a Hydrogel

A metabolic cycle-inspired hydrogel which exhibits the biomacromolecule-fueled transient volume phase transition is reported. This hydrogel has the affinity and digestive capacity for a fuel α-poly-L-lysine by incorporating acrylic acid and trypsin. The hydrogel captured fuel and transiently shrank owing to the construction of electrostatic cross-linkages. This process was inherently connected with the digestion of these cross-linkages and the release of oligo-lysine as waste, which induced the reswelling of the hydrogel at equilibrium. The transient volume change of the hydrogel realized the fuel-stimulated transient release of a payload. This study provides a strategy for engineering materials with biomacromolecule-fueled dynamic functions under the out-of-equilibrium condition.

Transient chirality inversion during racemization of a helical cobalt(III) complex

SignificanceWe first observed a transient chirality inversion on a simple unimolecular platform during the racemization of a chiral helical complex [LCo₃A₆]³⁺, i.e., the helicity changed from P-rich (right-handed) to M-rich (left-handed), which then racemized to a P/M equimolar mixture in spite of the absence of a reagent that could induce the M helix. This transient chirality inversion was observed only in the forward reaction, whereas the reverse reaction showed a simple monotonic change with an induction time. Consequently, the M helicity appeared only in the forward reaction. These forward and reverse reactions constitute a hysteretic cycle. Compounds showing such unique time responses would be useful for developing time-programmable switchable materials that can control the physical/chemical properties in a time-dependent manner.

Shape-Changing Particles: From Materials Design and Mechanisms to Implementation

Demands for next-generation soft and responsive materials have sparked recent interest in the development of shape-changing particles and particle assemblies. Over the last two decades, a variety of mechanisms that drive shape change have been explored and integrated into particulate systems. Through a combination of top-down fabrication and bottom-up synthesis techniques, shape-morphing capabilities extend from the microscale to the nanoscale. Consequently, shape-morphing particles are rapidly emerging in a variety of contexts, including photonics, microfluidics, microrobotics, and biomedicine. Herein, the key mechanisms and materials that facilitate shape changes of microscale and nanoscale particles are discussed. Recent progress in the applications made possible by these particles is summarized, and perspectives on their promise and key open challenges in the field are discussed.

Self-Organizing Microdroplet Protocells Displaying Light-Driven Oscillatory and Morphological Evolution

The development of synthetic systems that enable the sustained active self-assembly of molecular blocks to mimic the complexity and dynamic behavior of living systems is of great interest in elucidating the origins of life, understanding the basic principles behind biological organization, and designing active materials. However, it remains a challenge to construct microsystems with dynamic behaviors and functions that are connected to molecular self-assembly processes driven by external energy. Here, an active self-assembly of microdroplet protocells with dynamic structure and high structural complexity through living radical polymerization under constant energy flux is reported. The active microdroplet protocells exhibit nonlinear behaviors including oscillatory growth and shrinkage. This relies on the transient stabilization of molecular assembly, which can channel the inflow of energy through noncovalent interactions of pure synthetic components. The intercommunication of microdroplet protocells through stochastic fusion leads to the formation of a variety of dynamic and higher-order biomimetic microstructures. This work constitutes an important step toward the realization of autonomous and dynamic microsystems and active materials with life-like properties.

Particle packing into loose networks for tough and sticky composite gels

Hydrogel is an attractive material, but its application is limited due to its low mechanical strength. In this study, a tough composite gel could be prepared by synthesizing polymer particles within a polymer network having relatively loose cross-linking. Since the polymer network acts as a dispersion stabilizer during the synthesis of the hydrophobic polymer particles, a large amount of particles could be introduced into the gel without agglomeration. It was suggested that the high level of toughness was induced by the adsorption and desorption of the polymer chains on the surface of the finely packed particles. By using a stimuli-responsive polymer network, elasticity and plasticity of composite gels could be controlled in response to external stimuli, and adhesion on the gel surface could also be modulated.

A Shapeshifting Ferrofluidic Robot

To create a miniature shapeshifting robot capable of controlled movement, subdivision, regeneration, passage through small channels, engulfment of particles, object manipulation, and flow manipulation, a droplet of magnetically responsive ferrofluid is used. The ferrofluidic robot can achieve the aforementioned functions when both its position and shape are controlled using a custom electromagnetic field generation system. It is demonstrated that the proposed robot can perform these functions with submillimeter and subdegree error. A robot having these capabilities can remotely perform medical and microassembly tasks requiring fine dexterity that are currently difficult or impossible.

Growing Polymer Vesicles Generated by Polymerization Induced Self-Assembly Coupled With a Living Chemical Reactor

Chemical oscillatory reactions have attracted intensive attention due to their autonomous, continuous, and periodic features. Herein, the radicals generated in Belousov-Zhabotinsky (BZ) oscillator was used to initiate RAFT (reversible addition-fragmentation chain transfer) polymerization of 2-methoxyethyl acrylate (MEA) extending from hydrophilic poly(ethylene glycol) chain transfer agent (PEG-CTA) to give amphiphilic block copolymer, which self-assembled into collective objects with a size ranging from sub-micron to micron. Small-to-giant polymer vesicles could be generated using the above-mentioned BZ-PISA technology, the encapsulation of active BZ recipe into the vesicles also endorses the vesicles with growing features with potential for drug delivery and biomedical applications.

Multifunctional Supramolecular Hydrogel for Prevention of Epidural Adhesion after Laminectomy

Postoperative epidural adhesion remains a clinically challenging problem in spine surgery. Currently there are no effective and safe antifibrotic and antiadhesion biomaterials that have been specifically developed for this complication in clinical practice. Herein we designed and engineered an advanced antiadhesion hydrogel with multiple functionalities, including temperature-responsive gelation, self-healing, tissue adhesiveness, antioxidation, anti-inflammation, and antifibrosis. This multifunctional supramolecular hydrogel can be facilely constructed by integrating three functional modules, i.e., a thermosensitive triblock copolymer, poloxamer 407 (PX); a reactive oxygen species-eliminating and anti-inflammatory nanoparticle (TPCD NP); and an adhesion-enhancing compound, tannic acid (TA). The optimal formulation (PXNT) was hierarchically screened based on in vitro properties and in vivo activities. Therapeutically, local treatment with PXNT hydrogel effectively prevented epidural fibrosis and adhesion after laminectomy in both rats and rabbits. Of note, PXNT hydrogel showed more beneficial efficacy than different control thermosensitive hydrogels and a commercially available barrier product, Interceed. Mechanistically, PXNT hydrogel significantly attenuated local oxidative stress, inhibited inflammatory responses, and reduced fibrotic tissue formation. Moreover, treatment with PXNT hydrogel did not cause systemic adverse effects and neurological symptoms. Consequently, PXNT hydrogel is a highly promising biomaterial for preventing postlaminectomy epidural adhesion and adhesions after other surgeries.

Recent advances in soft functional materials: preparation, functions and applications

Synthetic materials and biomaterials with elastic moduli lower than 10 MPa are generally considered as soft materials. Research studies on soft materials have been boosted due to their intriguing features such as light-weight, low modulus, stretchability, and a diverse range of functions including sensing, actuating, insulating and transporting. They are ideal materials for applications in smart textiles, flexible devices and wearable electronics. On the other hand, benefiting from the advances in materials science and chemistry, novel soft materials with tailored properties and functions could be prepared to fulfil the specific requirements. In this review, the current progress of soft materials, ranging from materials design, preparation and application are critically summarized based on three categories, namely gels, foams and elastomers. The chemical, physical and electrical properties and the applications are elaborated. This review aims to provide a comprehensive overview of soft materials to researchers in different disciplines.

Stable and Prolonged Autonomous Oscillation in a Self-Oscillating Polymer Brush Prepared on a Porous Glass Substrate

We developed an autonomous functional surface, named a "self-oscillating polymer brush surface", which exhibits swelling-deswelling of the modified polymer chains synchronized with the Belousov-Zhabotinsky (BZ) reaction. The grafted polymer chain is a random copolymer composed of thermoresponsive N-isopropylacrylamide, N-(3-aminopropyl)methacrylamide, and ruthenium tris(2,2'-bipyridine) [Ru(bpy)₃]. To provide stable oscillations over a long period of time, suppression of the dilution of the BZ reactants inside the polymer surface and the increase in the amount of immobilized Ru(bpy)₃ are important. Here, we modified the self-oscillating polymer brush on a porous glass substrate and characterized its dynamic behavior. The increased surface area of the porous glass allowed for an efficient introduction of the metal catalyst, which resulted in a stable BZ reaction observable by optical microscopy. Compared with an aqueous BZ solution and the self-oscillating polymer modified on a glass coverslip, the wave velocity and diffusion coefficient were significantly lower for the porous glass system, which suggested that the reaction-diffusion of the reactants was markedly different than those of the other two systems. Moreover, the wave velocity was unchanged on the porous glass system for 1 h, whereas that of the solution dropped by 30 μm s^-1. Waveform analyses based on the Field-Körös-Noyes mechanism revealed that densely packed Ru(bpy)₃ in the porous glass system affects the duration of the key processes in the BZ reaction. These findings can help with understanding the dynamic behavior of the self-oscillating polymer brush on a porous glass substrate. Stable self-oscillations on the polymer brush-grafted porous glass substrate will aid future applications such as mass transport systems.

On the blistering of thermo-sensitive hydrogel: the volume phase transition and mechanical instability

This paper explores the physical mechanisms responsible for the appearance of small blisters on the surface of temperature sensitive hydrogels as they deswell rapidly during their volume phase transition. For this, we develop a numerical model that couples the processes of hydrogel deswelling and blister growth due to the existence of a thin quasi-impermeable layer on its surface. The model points out that blister inflation originates at defects point under the gel's surface, under the effect of the increasing osmotic pressure in the gel as it undergoes its phase transition. Due to their large deformation, these blisters often experience a mechanical instability that triggers a sudden increase in their growth rate at the expense of their closest neighbors. Using a simple computational model, we then show that blisters are able to communicate via internal pressure and that these interactions are mediated by two characteristic time scales related to solvent transport within and between adjacent blisters. Our study finally indicates that these mechanisms can be controlled by temperature and the gel's cross-link density to achieve diversity of blister patterns on the gel's surface. The proposed analysis provides predictions that agree well with experimental observations of NiPAm gels which deswell in various conditions.

Pattern Formation in Heterostructured Gel by the Ferrocyanide-Iodate-Sulfite Reaction

Pattern formation in the reaction-diffusion systems for the ferrocyanide-iodate-sulfite reaction has been investigated. Previous studies have been conducted in a uniform medium. However, in this study, we reported the pattern formation in heterostructured gels with different network densities. The chemical states of the gel depend on the diffusivity, which in turn depends on the network density of the gel. Consequently, a pH pattern reflecting the heterostructured gel emerged. Furthermore, adjusting the condition produces novel patterns in the heterostructured gel.

Bubble-Propelled Jellyfish-like Micromotors for DNA Sensing

A chemically powered jellyfish-like micromotor was proposed by using a multimetallic shell and a DNA assembly with catalase decorations modified on the concave surface to simulate the umbrella-shaped body and the muscle fibers on the inner umbrella of jellyfish. Relying on the catalytic generation of oxygen gas by catalase in H₂O₂ fuel, the jellyfish-like micromotor showed good bubble-propelled motion in different biomedia with speed exceeding 209 μm s^-1 in 1.5% H₂O₂. The jellyfish-like micromotors could also be applied for motion detection of DNA based on a displacement hybridization-triggered catalase release. The proposed jellyfish-like micromotors showed advantages of easy fabrication, good motion ability, sensitive motion detection of DNA, and good stability and reproducibility, indicating considerable promise for biological application.

A single-stranded coordination copolymer affords heterostructure observation and photoluminescence intensification

Few artificial systems can be exfoliated into, and observed as, single wires with lengths of more than several micrometers, and no previous example features a copolymer structure; this is in contrast with biopolymers such as single-strand DNAs. Here, we create a set of one-dimensional coordination copolymers featuring bis(dipyrrinato)zinc complex motifs in the main chain. A series of random copolymers is synthesized from two types of bridging dipyrrin proligand and zinc acetate, with various molar ratios between the proligands. Sonication of the bulk solid copolymer in organic solvent exfoliates single strands with lengths of 1.4 to 3.0 μm. Atomic force microscopy at ambient conditions visualizes the copolymer structure as height distributions. The copolymer structure improves its photoluminescence (up to 32%) relative to that of the corresponding homopolymers (3 and 10%). Numerical simulation based on a restricted random walk model reproduces the photoluminescence intensification, suggesting at the same time the existence of fast intrawire exciton hopping.

Chemomechanical Motion of a Self-Oscillating Gel in a Protic Ionic Liquid

An autonomous swelling-deswelling oscillation of polymer gels in a hydrated protic ionic liquid (PIL) as a proton source for the Belousov-Zhabotinsky (BZ) reaction is presented. Methylammonium hydrogen sulfate ([maH⁺ ][HSO₄ ^- ]) was employed as the PIL because it provides stable redox oscillation in the BZ reaction. Due to the significantly higher pK_a for [maH⁺ ][HSO₄ ^- ] than those for conventional proton sources for the BZ reaction, chemomechanical oscillation can be expected under weaker acidic conditions. The self-oscillating polymer was designed as a ternary random copolymer of N-isopropylacrylamide, N-(3-aminopropyl)methacrylamide, and the Ru(bpy)₃ moiety as a catalyst for the BZ reaction. The copolymer exhibited spontaneous soluble-insoluble oscillation in hydrated [maH⁺ ][HSO₄ ^- ] containing NaBrO₃ and malonic acid. Macroscopic swelling-deswelling oscillation of the porous bulk gel prepared by covalently connecting microgel particles was also observed.

Photo/thermoresponsive ABC triblock copolymer-based ion gels: photoinduced structural transitions

A photo/thermoresponsive ABC triblock copolymer-based ion gel exhibiting photoinduced structural transitions accompanied by significant rheological changes is newly developed. The ABC triblock copolymer comprises an ionic liquid (IL)-phobic A block, an IL-philic B block, and a photo/thermoresponsive C block containing azobenzene moieties. The IL-phobic A block forms a rigid micellar core in an IL over a wide temperature range and the photo/thermoresponsive C block undergoes upper critical solution temperature (UCST)-type phase transition in ILs. In concentrated polymer solution, the ABC triblock copolymer can form a percolated micellar network at low temperatures through aggregation of A and C blocks as physical crosslinks, bridged by IL-philic B blocks. In contrast, the ion gel undergoes structural transition to jammed micelles at high temperatures due to the disassembly of the thermoresponsive C block, resulting in significant softening of the ion gel. Importantly, the temperature dependences of the viscoelastic properties of the ion gel differ drastically depending on photo-irradiation conditions as the photoinduced isomerization of azobenzene moieties in the C block modulates the affinity between the polymer chain and IL. Utilizing this feature, photoinduced softening/hardening of the ion gel is realized at constant temperature. This study provides a promising strategy to control the rheological properties of nonvolatile soft materials via contactless light irradiation that could be exploited in various applications such as photoresponsive soft actuators and photo-healable soft materials.

Precisely Tunable Sol-Gel Transition Temperature by Blending Thermoresponsive ABC Triblock Terpolymers

Here, we report a facile methodology to control the sol-gel transition temperature (T_gel) of a physically cross-linked hydrogel by blending two kinds of ABC triblock terpolymers. Well-defined triblock terpolymers including thermosensitive N-isopropylacrylamide (NIPAAm), ABC1, and ABC2, were prepared by sequential reversible addition-fragmentation chain transfer polymerization. The chemical structure as well as the molecular weight of the A and B blocks for both polymers are identical, whereas the C blocks are different. The C block of ABC1 (C1) is a statistical copolymer of NIPAAm with hydrophobic n-butyl acrylate (BA), while that of ABC2 (C2) is a PNIPAAm homopolymer. Independently prepared ABC triblock terpolymer solutions exhibit well-defined sol-gel transitions. The T_gel of ABC1 is lower than that of ABC2 since hydrophobic BA is copolymerized into block C1. Remarkably, the T_gel varies linearly within this temperature range by simply blending the two polymers, while the resultant gel strength (∼G') remains almost unchanged. Therefore, the T_gel can be precisely adjusted by the mixing ratio of the two polymers. This method for straightforward manipulation of T_gel has great potential for various soft material applications such as biomaterials for tissue engineering, drug delivery systems, and injectable gels.

ATP-Driven Temporal Control over Structure Switching of Polymeric Micelles

An adenosine triphosphate (ATP)-fueled micellar system in the out-of-equilibrium state was constructed based on 4,5-diamino-1,3,5-triazine (DAT)-containing block copolymer. The block copolymer self-assembled into spherical micelles in equilibrium steady state at pH higher than its p K_a. The pendant DAT residues in protonated form acted as ATP catchers via hydrogen bonding and electrostatic interactions. Activated by ATP fuel, the polymeric micelles spontaneously disrupted into small aggregates of ATP/polymer hybrid complexes. The consumption of ATP energy via the enzymatic hydrolysis led to dissociation of the complexes and reversible formation of polymeric micelles. A transient self-assembly cycle, in which the assembly underwent autonomous division-fusion motion, was created using ATP fuel and enzyme; the switching of assembly structure was sustained by continuous supply of ATP fuel. This DAT-containing block copolymer have good biocompatibility, and drug-loaded micelles display ATP-responsive release behavior. It is expected that this ATP-fueled supramolecular assembly system will provide a functional platform for biomimic chemistry and therapeutic applications.

Design of localized spatiotemporal pH patterns by means of antagonistic chemical gradients

Spatially localized moving and stationary pH patterns are generated in two-side-fed reaction-diffusion systems. The patterns are sandwiched between two quiescent zones and positioned by the antagonistic gradients of the reactants of the self-activatory process. Spatial bistability, spatiotemporal oscillations, and formation of stationary Turing patterns have been predicted by numerical simulations and observed in experiments performed by using different hydrogen ion autocatalytic chemical systems. The formation of stationary patterns due to long-range inhibition is promoted by a large molecular weight hydrogen ion binding polymer.

Spatially localized moving and stationary pH patterns are generated in two-side-fed reaction-diffusion systems.

Electro-interconverted thermogelling and thermothinning polymer solutions

We report an intelligent electrothermal system exhibiting two remarkably different temperature response rheological behaviors: thermothinning versus thermogelling, which are controlled by voltage.