Tough, Transparent, 3D-Printable, and Self-Healing Poly(ethylene glycol)-Gel (PEGgel)

Facile fabrication of degradable, serrated polyethylene diacrylate microneedles using stereolithography

Microneedles have the potential for minimally invasive drug delivery. However, they are constrained by absence of rapid, scalable fabrication methods to produce intricate arrays and serrations for enhanced adhesion. 3D printing techniques like stereolithography (SLA) are fast, scalable modalities but SLAs require non-degradable and stiff resins. This work attempts to overcome this limitation by utilizing a poly (ethylene glycol diacrylate) (PEGDA, F3) resin and demonstrating its compatibility with a commercial SLA printer. FESEM images showed high printing efficiency of customized bioinks (F3) similar to commercial resins using SLA 3D printer. Mechanical endurance tests of whole MNA showed that MNs array printed from F3 resin (485 ± 5.73 N) required considerably less force than commercial F1 resin (880 ± 32.4 N). Penetration performance of F1 and F3 was found to be 10.8 ± 2.06 N and 0.705 ± 0.03 N. In-vitro degradation study in PBS showed that MNs fabricated from F3 resin exhibited degradation after 7 days, which was not observed with the commercial F1 resin provided by the manufacturer. The histology of porcine skin exhibited formation of triangular pores with pore length of 548 μm and efficient penetration into the deeper dermal layer. In conclusion, PEGDA can be used as for fabricating degradable, serrated solid MNs over commercial resin.

Sterically Hindered Organogels with Self-Healing, Impact Response, and High Damping Properties

Organogel materials are vital for impact or shock resistance because of their highly tailored dynamic properties. However, concurrently achieving excellent anti-impact and damping performances, high stability, and self-healing properties is challenging. Herein, a novel intelligent protective organogel (IPO) comprising a dynamic boronic ester containing poly(urethane-urea) as the network skeleton with a matching mesh size is synthesized, the network precisely entraps the hyperbranched fluid used as the bulky solvent via steric hindrance. The IPO exhibits self-healing ability, excellent impact responsiveness (a 1950-fold increase in flow stress under various impact speeds), and energy dissipation (the loss factor >0.8 from 10-4  to 104 Hz). The IPO maintains its dynamic mechanical properties during hot pressing and hydrolysis, exhibiting  high stability. Furthermore, the IPO exhibits omnibearing protection. When used as a protective coating, the IPO dissipates the impact force by 87% and 89% of control upon passive and active impact, respectively. When used as a shock pad, it attenuates 91% of the amplitude in the high-frequency vibrations. This study offers a novel perspective on the synthesis of tailored sterically hindered organogel and provides valuable insights into the development of next-generation intelligent protective materials that exhibit impact and vibration resistance.

Chitosan quaternary ammonium salt-oxidized sodium alginate-glycerol-calcium ion biobased self-healing hydrogels with excellent spontaneous repair performance

Self-healing hydrogels have attracted wide attention because of their potential applications in various fields. However, the complex processes, environmental requirements, and insufficient functionality limit their practical application. Herein, we synthesized a chitosan quaternary ammonium salt-oxidized sodium alginate-glycerol-calcium ion (HACC-OSA-Gly-Ca2+) biobased hydrogel with a multi-network structure that exhibits excellent self-healing abilities. This was achieved by utilizing reversible dynamic imine bonding, electrostatic interactions, Ca2+ ions as crosslinking points, and hydrogen bonding. The oxidation of sodium alginate (SA) with sodium periodate was carried out to obtain oxidized sodium alginate (OSA) with varying oxidation degrees. The resulting OSAs were then introduced into a glycerol-water solvent system containing chitosan quaternary ammonium salt (HACC) and calcium chloride, and this reaction successfully prepared the biobased eco-friendly self-healing hydrogel. The impacts of the oxidation degree (OD) of OSA on the microscopic morphology, mechanical properties, viscoelastic properties, swelling properties, and self-healing properties of the corresponding synthetic hydrogels were investigated. The outcomes indicated that the optimal HACC-OSA-Gly-Ca2+ hydrogel possessed good mechanical properties, with a tensile stress of 0.0132 MPa and elongation at break of 551.38%. Furthermore, the multiple bond interactions led to a high self-healing ratio (100%), with an elongation at break of about 614.29%, and excellent adhesion ability (average peel strength of 6.38 kN m-1) on various substrates. Additionally, the composite hydrogels exhibited excellent water retention, thermal stability, and resilience, making them promising for various potential applications. Moreover, the properties of the composite hydrogels could be facilely and finely tuned by varying the oxidation degree of OSA and ratio of each component. Thus, the presented strategy could enrich the construction as well as application of biopolymer-based self-healing hydrogels.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Hofmeister effect driven dynamic-bond cross-linked dialdehyde xylan hydrogels with rapid response and robust mechanical properties for expanding stent

The biomedical field urgently needs for programmable stent materials with nontoxic, autonomous self-healing, injectability, and suitable mechanical strength, especially self-expanding characteristics. However, such materials are still lacking. Herein, based on gelatin and dialdehyde-functionalized xylan, we synthesized 3D-printable, autonomous, self-healing, and mechanically robust hydrogels with a reversible Schiff base crosslink network. The hydrogels exhibited excellent mechanical properties and automatic healing properties at room temperature. The solid mechanical properties originate from the Schiff base, hydrogen bonding interactions, and xylan nanoparticle reinforcement of the polymer networks. As a proof of concept, the Hofmeister effect enabled the hydrogel to contract in highly concentrated salt solutions. In contrast, the same hydrogel expanded and relaxed in dilute salt solutions (quick response within 10 s), showing ionic stimulus-response and excellent shape memory characteristics, which demonstrated that the prepared hydrogel could be used as self-expanding artificial vascular stents. In particular, good biocompatibility was confirmed by cytotoxicity and compatibility tests, and ex vivo arterial experiments further indicated the feasibility of these artificial vascular scaffolds (the expansion force reached 1.51 N). Combined with its ionic stimuli-responsive shape memory ability, the strong mechanical, self-healing, 3D-printable, and biocompatibility properties make this hydrogel a promising material for artificial stents in various biomedical applications.

Innovative Strategies in 3D Bioprinting for Spinal Cord Injury Repair

Spinal cord injury (SCI) is a catastrophic condition that disrupts neurons within the spinal cord, leading to severe motor and sensory deficits. While current treatments can alleviate pain, they do not promote neural regeneration or functional recovery. Three-dimensional (3D) bioprinting offers promising solutions for SCI repair by enabling the creation of complex neural tissue constructs. This review provides a comprehensive overview of 3D bioprinting techniques, bioinks, and stem cell applications in SCI repair. Additionally, it highlights recent advancements in 3D bioprinted scaffolds, including the integration of conductive materials, the incorporation of bioactive molecules like neurotrophic factors, drugs, and exosomes, and the design of innovative structures such as multi-channel and axial scaffolds. These innovative strategies in 3D bioprinting can offer a comprehensive approach to optimizing the spinal cord microenvironment, advancing SCI repair. This review highlights a comprehensive understanding of the current state of 3D bioprinting in SCI repair, offering insights into future directions in the field of regenerative medicine.

Muscle-Inspired Formable Wood-Based Phase Change Materials

Phase change materials (PCMs) are crucial for sustainable thermal management in energy-efficient construction and cold chain logistics, as they can store and release renewable thermal energy. However, traditional PCMs suffer from leakage and a loss of formability above their phase change temperatures, limiting their shape stability and versatility. Inspired by the muscle structure, formable PCMs with a hierarchical structure and solvent-responsive supramolecular networks based on polyvinyl alcohol (PVA)/wood composites are developed. The material, in its hydrated state, demonstrates low stiffness and pliability due to the weak hydrogen bonding between aligned wood fibers and PVA molecules. Through treatment of poly(ethylene glycol) (PEG) into the PVA/wood PEG gel (PEG/PVA/W) with strengthened hydrogen bonds, the resulting wood-based PCMs in the hard and melting states elevate the tensile stress from 10.14 to 80.86 MPa and the stiffness from 420 MPa to 4.8 GPa, making it 530 times stiffer than the PEG/PVA counterpart. Capable of morphing in response to solvent changes, these formable PCMs enable intricate designs for thermal management. Furthermore, supported by a comprehensive life cycle assessment, these shape-adaptable, recyclable, and biodegradable PCMs with lower environmental footprint present a sustainable alternative to conventional plastics and thermal management materials.

Nature-Inspired Molecular-Crowding Enabling Wide-Humidity Range Applicable, Anti-Freezing, and Robust Zwitterionic Hydrogels for On-Skin Electronics

Hydrogels are currently in the limelight for applications in soft electronics but they suffer from the tendency to lose water or freeze when exposed to dry environments or low temperatures. Molecular crowding is a prevalent occurrence in living cells, in which molecular crowding agents modify the hydrogen bonding structure, causing a significant reduction in water activity. Here, a wide-humidity range applicable, anti-freezing, and robust hydrogel is developed through the incorporation of natural amino acid proline (Pro) and conductive MXene into polyvinyl alcohol (PVA) hydrogel networks. Theoretical calculations reveal that Pro can transform "free water" into "locked water" via the molecular-crowding effect, thereby suppressing water evaporation and ice forming. Accordingly, the prepared hydrogel exhibits high water retention capability, with 77% and 55% being preserved after exposure to 20 °C, 28% relative humidity (RH) and 35 °C, 90% RH for 12 h. Meanwhile, Pro lowers the freezing temperature of the hydrogel to 34 °C and enhances its stretchability and strength. Finally, the PVA/Pro/MXene hydrogels are assembled as multifunctional on-skin strain sensors and conductive electrodes to monitor human motions and detect tiny electrophysiological signals. Collectively, this work provides a molecular crowding strategy that will motivate researchers to develop more advanced hydrogels for versatile applications.

Homo-Dyad with Outer Hydration Layer Approach for Developing NIR-II Chromophore of High Stability and Water-Solubility as Injectable and Sprayable Optical Probe

Dyes with extended conjugate structures are the focus of extensive design and synthesis efforts, aiming to confer unique and improved optical and electronic properties. Such advancements render these dyes applicable across a wide spectrum of uses, ranging from second-window near-infrared (NIR-II) bioimaging to organic photovoltaics. Nevertheless, the inherent benefits of long conjugation are often accompanied by persistent challenges like aggregation, fluorescence quenching, absorption blueshift, and low stability and poor water solubility. Herein, a unique structural design strategy termed "homo-dyad with outer hydration layer" is introduced to address these inherent problems, tailored for the development of imaging probes exhibiting long absorption/emission wavelengths. This approach involves bringing two heptamethine cyanines together through a flexible linker, forming a homo-dyad structure, while strategically attaching four polyethylene glycol (PEG9) chains to the terminal heterocycles. This approach imparts excellent water solubility, biocompatibility, and enhanced chemical, photo-, and spectral stability for the dyes. Utilizing this strategy, a biomarker-activatable probe (HD-FL-4PEG9-N) for NIR-II fluorescent and 3D multispectral optoacoustic tomography imaging is developed, and its effectiveness in disease visualization. It can not only serve as an injectable probe for acute kidney injury imaging due to its high water solubility, but also a sprayable probe for imaging bacterial-infected wounds.

3D printing of reactive macroporous polymers via thiol-ene chemistry and polymerization-induced phase separation

Using thiol-ene chemistry, polymerization-induced phase separation, and DLP 3D printing, we present a method for manufacturing reactive macroporous 3D structures. This approach enables the fabrication of structures with tunable physicochemical properties and compressibility. Moreover, it facilitates post-functionalization through thiol-Michael addition reactions, thereby expanding performance and application potential.

3D bioprinting approaches for spinal cord injury repair

Regenerative healing of spinal cord injury (SCI) poses an ongoing medical challenge by causing persistent neurological impairment and a significant socioeconomic burden. The complexity of spinal cord tissue presents hurdles to successful regeneration following injury, due to the difficulty of forming a biomimetic structure that faithfully replicates native tissue using conventional tissue engineering scaffolds. 3D bioprinting is a rapidly evolving technology with unmatched potential to create 3D biological tissues with complicated and hierarchical structure and composition. With the addition of biological additives such as cells and biomolecules, 3D bioprinting can fabricate preclinical implants, tissue or organ-like constructs, andin vitromodels through precise control over the deposition of biomaterials and other building blocks. This review highlights the characteristics and advantages of 3D bioprinting for scaffold fabrication to enable SCI repair, including bottom-up manufacturing, mechanical customization, and spatial heterogeneity. This review also critically discusses the impact of various fabrication parameters on the efficacy of spinal cord repair using 3D bioprinted scaffolds, including the choice of printing method, scaffold shape, biomaterials, and biological supplements such as cells and growth factors. High-quality preclinical studies are required to accelerate the translation of 3D bioprinting into clinical practice for spinal cord repair. Meanwhile, other technological advances will continue to improve the regenerative capability of bioprinted scaffolds, such as the incorporation of nanoscale biological particles and the development of 4D printing.

A Gellan Gum, Polyethylene Glycol, Hydroxyapatite Composite Scaffold with the Addition of Ginseng Derived Compound K with Possible Applications in Bone Regeneration

Engineered bone scaffolds should mimic the natural material to promote cell adhesion and regeneration. For this reason, natural biopolymers are becoming a gold standard in scaffold production. In this study, we proposed a hybrid scaffold produced using gellan gum, hydroxyapatite, and Poly (ethylene glycol) within the addition of the ginseng compound K (CK) as a candidate for bone regeneration. The fabricated scaffold was physiochemically characterized. The morphology studied by scanning electron microscopy (SEM) and image analysis revealed a pore distribution suitable for cells growth. The addition of CK further improved the biological activity of the hybrid scaffold as demonstrated by the MTT assay. The addition of CK influenced the scaffold morphology, decreasing the mean pore diameter. These findings can potentially help the development of a new generation of hybrid scaffolds to best mimic the natural tissue.

Multifunctional Self-Assembled Block Copolymer/Iron Oxide Nanocomposite Hydrogels Formed from Wormlike Micelles

This article reports the preparation of multifunctional magnetic nanocomposite hydrogels formed from wormlike micelles. Specifically, iron oxide nanoparticles were incorporated into a temperature responsive block copolymer, poly(glycerol monomethacrylate)-b-poly(2-hydroxypropyl methacrylate) (PGMA-b-PHPMA), and graphene oxide (GO) dispersion at a low temperature (∼2 °C) through high-speed mixing and returning the mixture to room temperature, resulting in the formation of nanocomposite gels. The optimal concentrations of iron oxide and GO enhanced the gel strength of the nanocomposite gels, which exhibited a strong magnetic response when a magnetic field was applied. These materials retained the thermoresponsiveness of the PGMA-PHPMA wormlike micelles allowing for a solid-to-liquid transition to occur when the temperature was reduced. The mechanical and rheological properties and performance of the nanocomposite gels were demonstrated to be adjustable, making them suitable for a wide range of potential applications. These nanocomposite worm gels were demonstrated to be relatively adhesive and to act as strain and temperature sensors, with the measured electrical resistance of the nanocomposite gels changing with applied strain and temperature sweeps. The nanocomposite gels were found to recover efficiently after the application of high shear with approximately 100% healing efficiency within seconds. Additionally, these nanocomposite worm gels were injectable, and the addition of GO and iron oxide nanomaterials seemed to have no significant adverse impact on the biocompatibility of the copolymer gels, making them suitable not only for 3D printing in nanocomposite engineering but also for potential utilization in various biomedical applications as an injectable magnetic responsive hydrogel.

Self-Healable and 4D Printable Hydrogel for Stretchable Electronics

Materials with high stretchability and conductivity are used to fabricate stretchable electronics. Self-healing capability and four-dimensional (4D) printability are becoming increasingly important for these materials to facilitate their recovery from damage and endow them with stimuli-response properties. However, it remains challenging to design a single material that combines these four strengths. Here, a dually crosslinked hydrogel is developed by combining a covalently crosslinked acrylic acid (AAC) network and Fe3+ ions through dynamic and reversible ionically crosslinked coordination. The remarkable electrical sensitivity (a gauge factor of 3.93 under a strain of 1500%), superior stretchability (a fracture strain up to 1700%), self-healing ability (a healing efficiency of 88% and 97% for the mechanical and electrical properties, respectively), and 4D printability of the hydrogel are demonstrated by constructing a strain sensor, a two-dimensional touch panel, and shape-morphing structures with water-responsive behavior. The hydrogel demonstrates vast potential for applications in stretchable electronics.

Two-Level Self-Thickening Mechanism of a Novel Acid Thickener with a Hydrophobic-Associated Structure during High-Temperature Acidification Processes

Two acid thickeners, ADMC and ADOM, were prepared by aqueous solution polymerization using acrylamide (AM) and methacryloyloxyethyl trimethyl ammonium chloride (DMC) as raw materials, with or without the introduction of octadecyl polyoxyethylene ether methacrylate (OEMA). It was characterized by FTIR, 1H NMR, and the fluorescence spectra of pyrene. The double-layer thickening mechanism of ADOM was proved by comparing the thickening and rheological properties of ADMC and ADOM tested by a six-speed rotary viscometer and a HAKKE MARSIV rheometer during the acidification process. The results showed that the synthetic product was the target product; the first stage of the self-thickening ADOM fresh acid solution during high-temperature acidification was mainly affected by Ca2+ concentration, and the second stage of self-thickening was mainly affected by temperature. The residual viscosity of the 0.8 wt% ADOM residual acid solution was 250, 201.5, and 61.3 mPa·s, respectively, after shearing at 90, 120, and 150 °C for 60 min at a shear rate of 170 s-1. The thickening acid ADOM with a hydrophobic association structure has good temperature resistance and shear resistance, which can be used for high-temperature deep-well acid fracturing. In addition, no metal crosslinking agent was introduced in the system to avoid damage to its formation, and ADOM exhibited good resistance to Ca2+, which could provide ideas for the reinjection of the acidizing flowback fluid. It also has certain advantages for environmental protection.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Effect of polyethylene glycol on polysaccharides: From molecular modification, composite matrixes, synergetic properties to embeddable application in food fields

Polyethylene glycol (PEG) is a flexible, water-soluble, non-immunogenic, as well as biocompatible polymer, and it could synergize with polysaccharides for food applications. The molecular modification strategies, including covalent bond interactions (amino groups, carboxyl groups, aldehyde groups, tosylate groups, etc.), and non-covalent bond interactions (hydrogen bonding, electrostatic interactions, etc.) on PEG molecular chains are discussed. Its versatile structure, group modifiability, and amphiphilic block buildability could improve the functions of polysaccharides (e.g., chitosan, cellulose, starch, alginate, etc.) and adjust the properties of combined PEG/polysaccharides with outstanding chain tunability and matrix processability owing to plasticizing effects, compatibilizing effects, steric stabilizing effects and excluded volume effects by PEG, for achieving the diverse performance targets. The synergetic properties of PEG/polysaccharides with remarkable architecture were summarized, including mechanical properties, antibacterial activity, antioxidant performance, self-healing properties, carrier and delivery characteristics. The PEG/polysaccharides with excellent combined properties and embeddable merits illustrate potential applications including food packaging, food intelligent indication/detection, food 3D printing and nutraceutical food absorption. Additionally, prospects (like food innovation and preferable nutrient utilization) and key challenges (like structure-effectiveness-applicability relationship) for PEG/polysaccharides are proposed and addressed for food fields.

Strong and tough polysaccharide organohydrogels for strain, humidity and temperature sensors

To avoid the potential toxicity of monomer residues in synthetic polymer based organohydrogels, natural polysaccharide-based organohydrogels are expected to be used in multi-functional wearable sensory systems, but most of them have unsatisfactory stiffness, strength and fracture toughness. Herein, a cooking and soaking strategy is proposed to prepare novel natural polysaccharide-based organohydrogels possessing outstanding stiffness, strength, toughness, freezing resistance, heating resistance and long-term durability. The agar organohydrogel exhibits a fracture stress of 3.3 MPa, a Young's modulus of 2.26 MPa and a fracture toughness of 14.8 kJ m-2, the κ-carrageenan organohydrogel exhibits a fracture stress of 3.3 MPa, a Young's modulus of 4.34 MPa and a fracture toughness of 11.0 kJ m-2, and the gellan organohydrogel exhibits a fracture stress of 1.2 MPa, a Young's modulus of 2.81 MPa and a fracture toughness of 5.4 kJ m-2. Furthermore, the agar organohydrogels are assembled into multi-functional wearable sensors by introducing NaCl as a conducting agent exhibiting responses to strain (5-150%), temperature (-15 to 60 °C) and humidity (11-97%), and possessing exceptional multi-sensory capabilities. Therefore, the developed strategy has shown a new pathway towards strengthening polysaccharide-based organohydrogels with potential for application in wearable sensory systems.

Magnetic hydrogel applications in articular cartilage tissue engineering

Articular cartilage defects afflict millions of individuals worldwide, presenting a significant challenge due to the tissue's limited self-repair capability and anisotropic nature. Hydrogel-based biomaterials have emerged as promising candidates for scaffold production in artificial cartilage construction, owing to their water-rich composition, biocompatibility, and tunable properties. Nevertheless, conventional hydrogels typically lack the anisotropic structure inherent to natural cartilage, impeding their clinical and preclinical applications. Recent advancements in tissue engineering (TE) have introduced magnetically responsive hydrogels, a type of intelligent hydrogel that can be remotely controlled using an external magnetic field. These innovative materials offer a means to create the desired anisotropic architecture required for successful cartilage TE. In this review, we first explore conventional techniques employed for cartilage repair and subsequently delve into recent breakthroughs in the application and utilization of magnetic hydrogels across various aspects of articular cartilage TE.

Gum Arabic-based three-dimensional printed hydrogel for customizable sensors

Most three-dimensional (3D) printed hydrogel exhibit non-idealized rheological properties in the process of direct ink writing and complicated curing. Therefore, accurate writability and convenient curing for 3D printed hydrogel remain a challenge. In this paper, we developed a typical 3D printed hydrogel which realized direct ink writing (DIW) at temperatures similar to human body. Silicon dioxide (SiO2) and Gum Arabic (GA) formed the Bingham fluid to ensure shape stability. The rapid initiation system of potassium persulfat (KPS) and N,N,N',N' -tetramethylethylenediamine (TMEDA) allowed the 3D printed hydrogel precursor solution to transiently form a hydrophobic conjoined cross-linking network structure of acrylamide (AAM) and lauryl methacrylate (LMA) after printing, resulting in preferable mechanical properties. Hydrogel precursor solution showed better rheological properties with the nature of Bingham fluids, and achieved transient cross-linking at 30 °C for 10 s in the rheological test. A variety of 3D printed hydrogel with individual strain sensing properties are prepared as customizable sensor that could monitor significant strain signals within 0-20 % strain with high sensitivity. Moreover, they were discovered excellent temperature sensitivity over a wide operating range (0-80 °C). The 3D printing hydrogel sensors were expected to have broad application prospects in flexible wearable devices and medical monitoring.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Molecular Clogging Organogels with Excellent Solvent Maintenance, Adjustable Modulus, and Advanced Mechanics for Impact Protection

Inspired by mechanically interlocking supramolecular materials, exploiting the size difference between the bulky solvent and the cross-linked network mesh, a molecular clogging (MC) effect is developed to effectively inhibit solvent migration in organogels. A bulky solvent (branched citrate ester, BCE) with a molecular size above 1.4 nm is designed and synthesized. Series of MC-Gels are prepared by in situ polymerization of crosslinked polyurea with BCE as the gel solvent. The MC-Gels are colorless, transparent, and highly homogeneous, show significantly improved stability than gels prepared with small molecule solvents. As solvent migration is strongly inhibited by molecular clogging, the solvent content of the gels can be precisely controlled, resulting in a series of MC-Gels with continuously adjustable mechanics. In particular, the modulus of MC-Gel can be regulated from 1.3 GPa to 30 kPa, with a variation of 43 000 times. The molecular clogging effect also provides MC-Gels with unique high damping (maximum damping factor of 1.9), impact resistant mechanics (high impact toughness up to 40.68 MJ m-3 ). By applying shatter protection to items including eggs and ceramic armor plates, the potential of MC-Gels as high strength, high damping soft materials for a wide range of applications is well demonstrated.

3D printable, thermo-responsive, self-healing, graphene oxide containing self-assembled hydrogels formed from block copolymer wormlike micelles

Graphene oxide (GO) containing block copolymer nanocomposite hydrogels formed from poly(glycerol monomethacrylate-block-hydroxypropyl methacrylate) (PGMA-PHPMA) wormlike micelles were prepared by either mixing GO and copolymer at low temperature or via in situ reversible addition-fragmentation chain-transfer (RAFT) polymerisation-induced self-assembly (PISA) of HPMA in the presence of a PGMA macromolecular chain-transfer agent and GO flakes. Hydrogels containing 15-25% w/w copolymer and 0 and 8% w/w GO, based on copolymer, were investigated and the maximum gel strength measured was ∼33 kPa for a 25% w/w copolymer gel prepared by in situ polymerisation and containing 2% w/w GO based on copolymer. This gel strength represents a fifteen-fold increase over the same copolymer gel without the addition of GO. The nanocomposite gels were found to recover efficiently after the application of high shear, with up to 98% healing efficiency within seconds. These gels are also 3D printable, self-healing, adhesive and temperature responsive on cooling and re-heating. The observed properties were both GO and copolymer concentration dependent, and tensile testing demonstrated that the nanocomposite gels had higher moduli, elongation at break and toughness than gels prepared without GO.

A liquid-free conducting ionoelastomer for 3D printable multifunctional self-healing electronic skin with tactile sensing capabilities

Conductive elastomers with both softness and conductivity are widely used in the field of flexible electronics. Nonetheless, conductive elastomers typically exhibit prominent problems such as solvent volatilization and leakage, and poor mechanical and conductive properties, which limit their applications in electronic skin (e-skin). In this work, a liquid-free conductive ionogel (LFCIg) with excellent performance was fabricated by utilizing the innovative double network design approach based on a deep eutectic solvent (DES). The double-network LFCIg is cross-linked by dynamic non-covalent bonds, which exhibit excellent mechanical properties (2100% strain while sustaining a fracture strength of 1.23 MPa) and >90% self-healing efficiency, and a superb electrical conductivity of 23.3 mS m-1 and 3D printability. Moreover, the conductive elastomer based on LFCIg has been developed into a stretchable strain sensor that achieves accurate response recognition, classification, and identification of different robot gestures. More impressively, an e-skin with tactile sensing functions is produced by in situ 3D printing of sensor arrays on flexible electrodes to detect light weight objects and recognize the resulting spatial pressure variations. Collectively, the results demonstrate that the designed LFCIg has unparalleled advantages and presents wide application potential in flexible robotics, e-skin and physiological signal monitoring.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Durable and Adjustable Interfacial Engineering of Polymeric Electrolytes for Both Stable Ni-Rich Cathodes and High-Energy Metal Anodes

Achieving stable cycling of high-voltage solid-state lithium metal batteries is crucial for next-generation rechargeable batteries with high energy density and high safety. However, the complicated interface problems in both cathode/anode electrodes preclude their practical applications hitherto. Herein, to simultaneously solve such interfacial limitations and obtain sufficient Li⁺ conductivity in the electrolyte, an ultrathin and adjustable interface is developed at the cathode side through a convenient surface in situ polymerization (SIP), achieving a durable high-voltage tolerance and Li-dendrite inhibition. The integrated interfacial engineering fabricates a homogeneous solid electrolyte with optimized interfacial interactions that contributes to tame the interfacial compatibility between LiNi_x Co_y Mn_z O₂ and polymeric electrolyte accompanied by anticorrosion of aluminum current collector. Further, the SIP enables a uniform adjustment of solid electrolyte composition by dissolving additives such as Na⁺ and K⁺ salts, which presents prominent cyclability in symmetric Li cells (>300 cycles at 5 mA cm^-2 ). The assembled LiNi_0.8 Co_0.1 Mn_0.1 O₂ (4.3 V)||Li batteries show excellent cycle life with high Coulombic efficiencies (>99%). This SIP strategy is also investigated and verified in sodium metal batteries. It opens a new frontier for solid electrolytes toward high-voltage and high-energy metal battery technologies.

Fibro-Gel: An All-Aqueous Hydrogel Consisting of Microfibers with Tunable Release Profile and its Application in Wound Healing

Injectable hydrogels are valuable tools in tissue engineering and regenerative medicine due to their unique advantages of injectability with minimal invasiveness and usability for irregularly shaped sites. However, it remains challenging to achieve scalable manufacturing together with matching physicochemical properties and on-demand drug release for a high level of control over biophysical and biomedical cues to direct endogenous cells. Here, the use of an injectable fibro-gel is demonstrated, a water-filled network of entangled hydrogel microfibers, whose physicochemical properties and drug release profiles can be tailored to overcome these shortcomings. This fibro-gel exhibits favorable in vitro biocompatibility and the capability to aid vascularization. The potential use of the fibro-gel for advancing tissue regeneration is explored with a mice excision skin model. Preliminary in vivo tests indicate that the fibro-gel promotes wound healing and new healthy tissue regeneration at a faster rate than a commercial gel. Moreover, it is demonstrated that the release of distinct drugs at different rates can further accelerate wound healing with higher efficiency, by using a two-layer fibro-gel model. The combination of injectability and tailorable properties of this fibro-gel offers a promising approach in biomedical fields such as therapeutic delivery, medical dressings, and 3D tissue scaffolds for tissue engineering.

Smart stimuli-responsive injectable gels and hydrogels for drug delivery and tissue engineering applications: A review

Hydrogels are widely used biomaterials in the delivery of therapeutic agents, including drugs, genes, proteins, etc., as well as tissue engineering, due to obvious properties such as biocompatibility and their similarity to natural body tissues. Some of these substances have the feature of injectability, which means that the substance is injected into the desired place in the solution state and then turns into the gel, which makes it possible to administer them from a way with a minimal amount of invasion and eliminate the need for surgery to implant pre-formed materials. Gelation can be caused by a stimulus and/or spontaneously. Suppose this induces due to the effect of one or many stimuli. In that case, the material in question is called stimuli-responsive because it responds to the surrounding conditions. In this context, we introduce the different stimuli that cause gelation and investigate the different mechanisms of the transformation of the solution into the gel in them. Also, we study special structures, such as nano gels or nanocomposite gels.

Polymer Gels: Classification and Recent Developments in Biomedical Applications

Polymer gels are a valuable class of polymeric materials that have recently attracted significant interest due to the exceptional properties such as versatility, soft-structure, flexibility and stimuli-responsive, biodegradability, and biocompatibility. Based on their properties, polymer gels can be used in a wide range of applications: food industry, agriculture, biomedical, and biosensors. The utilization of polymer gels in different medical and industrial applications requires a better understanding of the formation process, the factors which affect the gel's stability, and the structure-rheological properties relationship. The present review aims to give an overview of the polymer gels, the classification of polymer gels' materials to highlight their important features, and the recent development in biomedical applications. Several perspectives on future advancement of polymer hydrogel are offered.

A Mori-Zwanzig Dissipative Particle Dynamics Approach for Anisotropic Coarse Grained Molecular Dynamics

Coarse grained (CG) molecular dynamics simulations are widely used to accelerate atomistic simulations but generally lack a formalism to preserve the dynamics of the system. For spherical particles, the Mori-Zwanzig approach, while computationally complex, has ameliorated this problem. Here we present an anisotropic dissipative particle dynamics (ADPD) model as an extension of this approach, which accounts for the anisotropy for both conservative and nonconservative interactions. For a simple anisotropic system we parametrize the coarse grained force field representing ellipsoidal CG particles from the full-atomistic simulation. To represent the anisotropy of the system, both the conservative and dissipative terms are approximated using the Gay-Berne (GB) functional forms via a force-matching approach. We compare our model with other CG models and demonstrate that it yields better results in both static and dynamical properties. The inclusion of the anisotropic nonconservative force preserves the microscopic dynamical details, and hence the dynamical properties, such as diffusivity, can be better reproduced by the aspherical model. By generalizing the isotropic DPD model, this framework is effective and promising for the development of the CG model for polymers, macromolecules, and biological systems.

Self-Healing Polymeric Soft Actuators

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

Recent advances in bio-inspired ionic liquid-based interfacial materials from preparation to application

Natural creatures always display unique and charming functions, such as the adhesion of mussels and the lubrication of Nepenthes, to maintain their life activities. Bio-inspired interfacial materials infused with liquid, especially for ionic liquids (ILs), have been designed and prepared to meet the emerging and rising needs of human beings. In this review, we first summarize the recent development of bio-inspired IL-based interfacial materials (BILIMs), ranging from the synthesis strategy to the design principle. Then, we discuss the advanced applications of BILIMs from anti-adhesive aspects (e.g., anti-biofouling, anti-liquid fouling, and anti-solid fouling) to adhesive aspects (e.g., biological sensor, adhesive tape, and wound dressing). Finally, the current limitations and future prospects of BILIMs are provided to feed the actual needs.

Vascular smooth muscle-inspired architecture enables soft yet tough self-healing materials for durable capacitive strain-sensor

Catastrophically mechanical failure of soft self-healing materials is unavoidable due to their inherently poor resistance to crack propagation. Here, with a model system, i.e., soft self-healing polyurea, we present a biomimetic strategy of surpassing trade-off between soft self-healing and high fracture toughness, enabling the conversion of soft and weak into soft yet tough self-healing material. Such an achievement is inspired by vascular smooth muscles, where core-shell structured Galinstan micro-droplets are introduced through molecularly interfacial metal-coordinated assembly, resulting in an increased crack-resistant strain and fracture toughness of 12.2 and 34.9 times without sacrificing softness. The obtained fracture toughness is up to 111.16 ± 8.76 kJ/m², even higher than that of Al and Zn alloys. Moreover, the resultant composite delivers fast self-healing kinetics (1 min) upon local near-infrared irradiation, and possesses ultra-high dielectric constants (~14.57), thus being able to be fabricated into sensitive and self-healing capacitive strain-sensors tolerant towards cracks potentially evolved in service.

Recent advances in electronic skins: material progress and applications

Electronic skins are currently in huge demand for health monitoring platforms and personalized medicine applications. To ensure safe monitoring for long-term periods, high-performance electronic skins that are softly interfaced with biological tissues are required. Stretchability, self-healing behavior, and biocompatibility of the materials will ensure the future application of electronic skins in biomedical engineering. This mini-review highlights recent advances in mechanically active materials and structural designs for electronic skins, which have been used successfully in these contexts. Firstly, the structural and biomechanical characteristics of biological skins are described and compared with those of artificial electronic skins. Thereafter, a wide variety of processing techniques for stretchable materials are reviewed, including geometric engineering and acquiring intrinsic stretchability. Then, different types of self-healing materials and their applications in electronic skins are critically assessed and compared. Finally, the mini-review is concluded with a discussion on remaining challenges and future opportunities for materials and biomedical research.

Tough, Self-Healing, and Conductive Elastomer ─Ionic PEGgel

Ionically conductive elastomers are necessary for realizing human-machine interfaces, bioelectronic applications, or durable wearable sensors. Current design strategies, however, often suffer from solvent leakage and evaporation, or from poor mechanical properties. Here, we report a strategy to fabricate ionic elastomers (IHPs) demonstrating high conductivity (0.04 S m-1), excellent electrochemical stability (>60,000 cycles), ultra-stretchability (up to 1400%), high toughness (7.16 MJ m-3), and fast self-healing properties, enabling the restoration of ionic conductivity within seconds, as well as no solvent leakage. The ionic elastomer is composed of in situ formed physically cross-linked poly(2-hydroxyethyl methacrylate) networks and poly(ethylene glycol) (PEG). The long molecular chains of PEG serve as a solvent for dissolving electrolytes, improve its long-term stability, reduce solvent leakage, and ensure the outstanding mechanical properties of the IHP. Surprisingly, the incorporation of ions into PEG simultaneously enhances the strength and toughness of the elastomer. The strengthening and toughening mechanisms were further revealed by molecular simulation. We demonstrate an application of the IHPs as (a) flexible sensors for strain or temperature sensing, (b) skin electrodes for recording electrocardiograms, and (c) a tough and sensing material for pneumatic artificial muscles. The proposed strategy is simple and easily scalable and can further inspire the design of novel ionic elastomers for ionotronics applications.

Digital Light Processing 3D Printing of Tough Supramolecular Hydrogels with Sophisticated Architectures as Impact-Absorption Elements

Processing tough hydrogels into sophisticated architectures is crucial for their applications as structural elements. However, Digital Light Processing (DLP) printing of tough hydrogels is challenging because of the low-speed gelation and toughening process. Described here is a simple yet versatile system suitable for DLP printing to form tough hydrogel architectures. The aqueous precursor consists of commercial photoinitiator, acrylic acid, and zirconium ion (Zr⁴⁺ ), readily forming tough metallo-supramolecular hydrogel under digital light because of in situ formation of carboxyl-Zr⁴⁺ coordination complexes. The high-stiffness and antiswelling properties of as-printed gel enable high-efficiency printing to form high-fidelity constructs. Furthermore, swelling-induced morphing of the gel is also achieved by encoding structure gradients during the printing with grayscale digital light. Mechanical properties of the printed hydrogels are further improved after incubation in water due to the variation of local pH and rearrangement of coordination complex. The swelling-enhanced stiffness affords the printed hydrogel with shape fixation ability after manual deformations, and thereby provides an additional avenue to form more complex configurations. These printed hydrogels are used to devise an impact-absorption element or a high-sensitivity pressure sensor as proof-of-concept examples. This work should merit engineering of other tough gels and extend their scope of applications in diverse fields.

Three-Dimensional Printable, Highly Conductive Ionic Elastomers for High-Sensitivity Iontronics

The development of hydrogels and ionic gels for applications in fields such as soft electronics and wearable sensors is limited by liquid evaporation or leakage. Ionic conductors without volatile liquids are better choices for flexible and transparent devices. Here, a liquid polymer electrolyte (LPE) is prepared from a mixture of lithium bis(trifluoromethane)sulfonimide and polyethylene glycol (PEG) above the melting point of PEG. A three-dimensional (3D) printable solvent-free ionic elastomer (IE) is introduced by photopolymerization of ethyl acrylate and hydroxyethyl acrylate in the prepared LPE. The conductivity is significantly improved by the presence of a high content of the lithium salt. Dynamic cross-linking networks improve the stretchability and resilience of the elastomer. The pattern design capability of the IE is provided by light-curing 3D printing. These features demonstrate that the IE has broad application prospects in flexible sensors, ion skins, and soft robots.

A robust and sensitive flexible strain sensor based on polyurethane cross-linked composite hydrogels for the detection of human motion

Polyurethane cross-linked composite hydrogel and wireless Bluetooth module were assembled for the detection of human motion.