Multifunctional Gels Research Articles

Multifunctional organohydrogels enabling sensitive strain sensing and self-powered triboelectricity

Hydrogels have broad application prospects in portable strain sensors and triboelectric nanogenerators (TENG), and have attracted great attention in the field of wearable electronic devices and energy transfer devices. However, the development of multifunctional gels that can meet a variety of application scenarios is still a challenging problem. In this study, a multifunctional poly(acrylamide-co-2-acrylamide-2-methylpropanesulfonic acid)/lignosulfonate/Zr4+ organohydrogel noted as “P(AM-co-AMPS)/LS/Zr4+” was fabricated based on dynamic metal coordination bonds and hydrogen bonds, which could be used as flexible strain sensor and TENG electrode material. The organohydrogel based strain sensor could monitor both large and tiny human movements due to the high sensitivity (GF = 4.37, up to 200 %) and circulation stability. The assembled organohydrogel-based triboelectric nanogenerator (O-TENG) could generate an open-circuit voltage of 70.47 V and a short-circuit current of 25.21 μA. It is worth mentioning that the O-TENG could light up 44 LED bulbs by tapping with the palm at −20 °C, and the electronic output performance of the O-TENG after self-healing was unaffected. Additionally, the O-TENG exhibited enormous potentials in the biomechanical energy harvesting and self-powered motion sensors in the areas of human movements detection, human–computer interaction, electronic skin and self-powered devices.

Adhesive injectable cellulose-based hydrogels with rapid self-healing and sustained drug release capability for promoting wound healing

Injectable biocompatible hydrogels with multiple functions, including self-healing, adhesion, antibacterial activity, and suitable mechanical properties, are highly desirable for enhancing wound healing. In this study, a new class of multi-functional injectable self-healing cellulose-based hydrogels was synthesised using dynamic covalent acylhydrazone linkages for wound dressing. The carboxymethyl cellulose-graft-adipic dihydrazide (CMC-ADH)/4-Formylbenzoic acid-terminated poly(ethylene glycol) (PEG-FBA) (CMC-ADH/PEG-FBA) hydrogels have adjustable gelation time and excellent self-healing ability. In addition, drug release and in vitro antibacterial activities against Gram-positive and Gram-negative bacteria confirmed the sustained drug-release capacity of the hydrogels. Moreover, haemostasis and wound-healing effects were investigated using an in vivo haemorrhaging liver mouse model and a full-thickness skin defect model, and the results indicated that they not only promoted the wound-healing process but also presented excellent haemostatic effects. The CMC-ADH/PEG-FBA gels also exhibited good adhesion to irregular wounds and significantly enhanced angiogenic ability in vivo. This excellent wound-healing performance occurs because hydrogels can quickly stop bleeding, provide a moist and closed environment for the wound to prevent bacterial invasion, release ciprofloxacin (CIP), reduce inflammatory reactions, and promote wound tissue regeneration. In summary, the synthesised multi-functional gels are ideal candidates for treating haemorrhages and irregular wounds.

Simple preparation of redox induced shape memory and self-healable ionic liquid based gels for flexible strain sensors

The aim of this work is to fabricate the stretchable, redox-driven shape memory, and self-healable gels sensors using one-step self-assembling method at ambient temperature. The mixed aqueous solution of poly (vinyl alcohol) (PVA), ionic liquid (IL, 1-butyl-3-methylimidazolium tetrafluoroborate), and phytic acid (PA) can transfer to PVA/IL/PA gels due to the multiple hydrogen bonds and electrostatic interaction among the three different molecules. The gel exhibits outstanding redox-driven shape memory fixity in trivalent iron ion solution and reversible recovery in ascorbic acid solution, respectively. The reversible breakage and recombination of hydrogen bonds and electrostatic interaction endows the PVA/IL/PA gels excellent mechanical properties and self-healing effect. The optimum tensile strength and elongation at break (EB) of gels are 1.56 MPa and 368%, respectively. The EB and self-healing efficiency of the healed gel can reach up to 161.91% and 74%, which are competitive in ion gels. Moreover, the gel can be used as flexible strain sensors, which exhibit good performance in monitoring human activities and voice recognition. This study will provide one green and universal approach in fabricating multi-functional gels, which show promising applications in the field of tissue engineering, sensors and actuators, and soft electronic materials etc.

Cell Differentiation-Inspired, Salt-Induced Multifunctional Gels for an Intelligent Soft Robot with an Artificial Reflex Arc

To make soft robotics intelligent, dazzling artificial skin and actuators have been created. However, compared to rigid commercial robots, the sophisticated demands of raw materials become a key challenge for autonomous soft actuators to realize manufacturing repeatability and reproducibility. Inspired by the stem cell, which has the potential to differentiate into multifunctional cells with the same original compositions, a potential multifunctional gel is presented. With well-designed polymer chains, the gel has a salt-induced regulating module, conductivity, and other bionic properties. Making use of this advantage, the gel acts as a double-duty electrode for both the actuator and sensor. An artificial reflex arc is therefore formed by their tight integration via an e-brain: a computing unit that specifically responds to organism intervention. This efficient strategy to obtain diverse components with minimal raw materials is promising for effortlessly fabricating fully soft robotics.

Programmable Multifunctional Gels with On-Demand Patterning Capability toward Hierarchical and Multi-Dimensional Encryption and Anti-Counterfeiting.

Hydrogels capable of optical switching have recently become one of the most celebrated materials for information encryption and anti-counterfeiting. However, challenges still remain for developing versatile gel-based platforms with on-demand multistage patterning and multi-dimensional encryption capacities as well as long-term stability. Herein, elaborately designed programmable and multifunctional gels with fascinating anti-swelling (swelling ratios < 0.1%), anti-freezing (below -70 °C), and anti-dehydration (over 3 months) abilities, solvent-induced reversible transparence variations, adjustable fluorescence, self-healing (86% in stress and 94% in strain), Fe3+-ethylenediaminetetraacetic acid disodium (EDTA·2Na)-induced reversible shape memory, and fluorescence off/on switch capabilities are facilely fabricated based on glycidyl methacrylate functionalized graphene quantum dots and Al3+ cross-linked gelatin and polyacrylic acid. Employing a simple mask photopolymerization or welding technique, various patterns can be readily and hierarchically encrypted on-demand into a single gel label, which can be further fixed into complex multi-dimensional architectures while quenching fluorescence after the treatment with Fe3+ to achieve high-security-level information encryption originating from the synergistic effects of the above multifunctions. The encrypted multi-level information can only be stepwise decrypted by an authorized individual who has mastered all decryption keys. Therefore, the creative design strategy for programing multifunctional gels opens up the possibility for hierarchical and multi-dimensional information encryption and anti-counterfeiting.

Temperature sensitive polyMOF hydrogel formed by in situ open-ring polymerization for infected chronic wound treatment

Chronic wounds, especially those caused by drug-resistant bacteria, remain mostly intractable to existing treatments. Photothermal therapy, apart from the additional inflammation it may cause, exhibits potent ability to efficiently destroy drug-resistant bacteria. A system capable of thermal therapy together with active regulation of the repairing process could be a potentially effective treatment for these wounds. Here we developed a highly integrated hydrogel made of curcumin loaded copper-tetrakis(4-carboxyphenyl)porphyrin metal–organic framework and poly (ethylene glycol)-poly(ε-caprolactone-co-lactide) hybrid by in situ ring-opening polymerization. It combines photothermal effect, immune modulatory, hemostasis and temperature-sensitive sol–gel transition capacity. This multifunctional polymer-metal–organic framework (polyMOF) hydrogel dressing showed the ability to completely heal drug-resistant bacteria infected wounds within 14 days and could be an effective and biocompatible material for chemo-photothermal therapy against chronic wounds. Moreover, our strategy to prepare the polyMOF hybrid may provide a universal approach for the development of multifunctional gels with great significance in clinical application.

Ti3C2Tx MXene as a novel functional photo blocker for stereolithographic 3D printing of multifunctional gels via Continuous Liquid Interface Production

Stereolithographic 3D printing has been increasingly applied in the rapid fabrication of gels with specially-designed 3D structures. Herein, MXene (Ti 3 C 2 T x ) is reported for the first time as a versatile photo blocker for a photo-curable aqueous ink for a newly developed stereolithography technology (Continuous Liquid Interface Production, CLIP) to fabricate hydrogels with delicate geometries. As an effective photo blocker, the MXene nanosheets can significantly suppress the light scatting in the hydrogel precursor ink during CLIP printing, giving rise to printing quality. Subsequent solvent exchange with glycerin produces an organogel, which shows excellent adhesive properties towards various surfaces, making it a promising adhesive sensor to detect different body movements. Meanwhile, the CLIP-printed organogel demonstrates promise in atmospheric water harvesting taking advantage of the natural hygroscopicity of organogel and excellent light-to-heat conversion performance with MXene nanosheets. Furthermore, the CLIP-printed hollow organogel shows higher efficiency benefiting from larger surface area of hollow organogel. The results of this work strongly suggest that MXene can serve as an effective multifunctional additive for CLIP inks, which may open up new opportunities for the 3D printing of various advanced devices.

Magnetic Assembly of a Multifunctional Guidance Conduit for Peripheral Nerve Repair

AbstractNerve growth conduits are designed to support and promote axon regeneration following nerve injuries. Multifunctionalized conduits with combined physical and chemical cues, are a promising avenue aimed at overcoming current therapeutic barriers. However, the efficacious assembly of conduits that promote neuronal growth remains a challenge. Here, a biomimetic regenerative gel is developed, that integrates physical and chemical cues in a biocompatible “one pot reaction” strategy. The collagen gel is enriched with magnetic nanoparticles coated with nerve growth factor (NGF). Then, through a remote magnetic actuation, highly aligned fibrillar gel structure embedded with anisotropically distributed coated nanoparticles, combining multiple regenerating strategies, is obtained. The effects of the multifunctional gels are examined in vitro, and in vivo in a 10‐mm rat sciatic nerve injury model. The magneto‐based therapeutic conduits demonstrate oriented and directed axonal growth, and improve nerve regeneration in vivo. The study of multifunctional guidance scaffolds that can be implemented efficiently and remotely provides the foundation to a novel therapeutic approach to overcome current medical obstacles for nerve injuries.

Designing multifunctional gels with electrical conductivity, mechanical toughness and self-oscillating performance

Self-oscillating polymer gels driven by the Belousov–Zhabotinsky (BZ) oscillating chemical reaction are a new class of functional gels that have potential applications in autonomously functioning membranes and as artificial muscle actuators.

Multistimuli-Responsive Self-Healable and Moldable Nickel(II)-Based Gels for Reversible Gas Adsorption and Palladium Sequestration via Gel-to-Gel Transformation.

We report the in situ formation of Ni-based supramolecular organogel and organic-aqueous gels using amine appended triazole ligand, having varying morphology and rheological properties. These gels are self-healable and moldable or injectable respectively depending on the absence or presence of water in the gelation medium. Our studies reveal that the formation and rupture of hydrogen bonds assisted by the solvent movement is responsible for the self-healing nature of the gels. The porous structure of the gel has been observed from the migration of dye molecules on the self-healed gel. In addition, the gels show dual function of reversible adsorption of toxic gases and sequestration of heavy metal ions, especially palladium via gel-to-gel transformation. It is imperative to stress that such transformation is extremely rare for small molecule based metallogels. The dynamic nature of Ni-Ntriazole interactions has been utilized in achieving the reversible gas/vapor responsive behavior of the metallogels, which could be suitable in developing colorimetric probes for the detection of toxic gases and heavy metal ions. Such multifunctional gels are exceptional in contemporary literature and are expected to find utility in fabricating smart multistimuli-responsive gel-based materials in the future.

Magnetoelasticity of gels

Magnetoelastic gel is an active material that is used widely these days. The behavior of these multifunctional gels is derived from a polymer matrix and magnetoresponsive inclusions. The polymer matrix provides structural integrity as well as load bearing capacity to the magnetoelastic gel. The magnetic behavior of the magnetoelastic gel is attributed to a large number of nano-to-micron-sized magnetic particles disbursed in the polymer matrix. The magnetoelastic gel is said to be diluted if the interparticle interactions are negligible/small or concentrated if there are strong interparticle interactions. We consider strong interparticle interactions in the magnetoelastic gel. When the magnetic field is applied to the magnetoelastic gel, the disbursed magnetic particles tend to translate and rotate to a new deformed configuration. Due to these translations and rotations of the many magnetoelastic particles, the polymer matrix around each particle deforms. These micro-deformations then coalesce and lead to the overall macroscopic deformation of the magnetoelastic gel. Both magnetization and mechanical strain characterize the magnetoelastic behavior of the magnetoelastic gel. In this article, an energy minimization approach is followed to find the equilibrium magnetization and strain. We formulate the total energy of the magnetoelastic gel on multiple-length scales and minimize it to obtain these equilibrium magnetization and mechanical strain. We also investigate the effect of particle size and polarization under the framework of energy minimization.

Investigation of mechanical properties and internal structure of novel ionic double-nework gels and comparison with conventional hydrogels

Gels are an important class of soft and wet materials having superior properties with diverse applications in tissue engineering, bio-medical engineering, and electrochemistry. In order to accelerate the development of gels, it is required to synthesize multi-functional gels of high mechanical strength, ultra low surface friction and suitable elastic modulus with a variety of methods and new materials and investigate their internal structure to ensure the best implementation to any suitable fields. Among many types of gels, ionic gels made from ionic liquids (ILs) will be possibly used for diverse applications in electrochemical devices as sensors, actuators, batteries and in the field of tribology. ILs are characterized by many unique physico-chemical properties which make them a potential candidate for gel materials. In the present work a novel approach for preparing Ionic double-network gels (iDN gels) using IL have been synthesized utilizing photo-polymerization process. A hydrophobic monomer methyl methacrylate has been used as a first network and a hydrophobic IL monomer, N,N-diethyl-N-(2-mthacryloylethyl)-N-methylammonium bistrifluoromethylsulfonyl)imide has been used as a second network. The resulting iDN gels show transparency, flexibility and good mechanical toughness. An attempt to determine the mesh size of the iDN gels has been made for the first time to understand the internal structure of the ionic liquid based gels by scanning microscopic, light scattering. Three types of crosslinking densities of the iDN gels were experimentally determined from the size of internal structure, solvent content and Young's modulus. By comparing the three mesh densities, the relation between the network structure and mechanical properties of the gels is discussed. We compared the crosslinking densities of the iDN gels and conventional hydrogels to understand the network structure of the iDN gels and discussed its difference of structural behavior from that of hydrogels.

Agarose based multifunctional materials: Evaluation of thixotropy, self-healability and stretchability

This paper reports a microwave assisted one pot facile synthesis of ester derivatives of agarose (Agr-GAEst) through chemical reaction of agarose (Agr) with gallic acid (GA), an organic acid found in many plants employing carbodiimide chemistry. Agr-GAEst was characterised by FT-IR, 13C NMR spectroscopy, thermogravimetric analysis (TGA), DMA measurements, scanning electron microscopy (SEM), UV–vis spectrophotometry and rheometry. The native agarose was insoluble in ethylene glycol, but Agr-GAEst (obtained at an optimized molar ratio of 1: 0.5) formed good quality gel (tanδ∼0.1) at 4% (w/v) concentration. The gel thus obtained exhibited substantial degree of thixotropy (hysteresis loop area=38.73%), rapid self-healing ability (12min) upon complete cleavage of the gel and excellent stretching ability (>20 times of its original length). These types of multifunctional gels would find applications in food and personal health care industries.

Biomimetic Multifunctional Porous Chalcogels as Solar Fuel Catalysts

Biological systems that can capture and store solar energy are rich in a variety of chemical functionalities, incorporating light-harvesting components, electron-transfer cofactors, and redox-active catalysts into one supramolecule. Any artificial mimic of such systems designed for solar fuels production will require the integration of complex subunits into a larger architecture. We present porous chalcogenide frameworks that can contain both immobilized redox-active Fe(4)S(4) clusters and light-harvesting photoredox dye molecules in close proximity. These multifunctional gels are shown to electrocatalytically reduce protons and carbon disulfide. In addition, incorporation of a photoredox agent into the chalcogels is shown to photochemically produce hydrogen. The gels have a high degree of synthetic flexibility, which should allow for a wide range of light-driven processes relevant to the production of solar fuels.

Engineering of multifunctional gels integrating highly efficient growth factor delivery with endothelial cell transplantation

Transplantation of Bcl-2-transduced human umbilical vein endothelial cells (ECs) in protein gels into the gastrocnemius muscle improves local reperfusion in immunodeficient mouse hosts with induced hind limb ischemia. We tested the hypothesis that incorporation of local, sustained growth factor delivery could enhance and accelerate this effect. Tissue engineering scaffolds often use synthetic polymers to enable controlled release of proteins, but most synthetic delivery systems have major limitations, most notably hydrophobicity and inefficient protein loading. Here, we report the development of a novel alginate-based delivery system for vascular endothelial growth factor-A(165) (VEGF) that exhibits superior loading efficiency and physical properties to previous systems in vitro. In vivo, VEGF released from alginate microparticles within protein gels was biologically active and, when combined with EC transplantation, led to increased survival of transplanted cells at 28 days. The composite graft described also improved early (14 days) tissue perfusion and late (28 days) muscle myoglobin expression, a sign of recovery from ischemia, compared with EC transplantation and VEGF delivery separately. We conclude that our improved approach to sustained VEGF delivery in tissue engineering is useful in vivo and that the integration of high efficiency protein delivery enhances the therapeutic effect of protein gel-based EC transplantation.

Multifunctional hydrogels that promote osteogenic hMSC differentiation through stimulation and sequestering of BMP2.

The extracellular environment controls many cellular activities thereby linking external material cues to internal cell function. By better understanding these processes, synthetic extracellular material niches can be tailored to present cells with highly regulated physical and/or chemical cues that promote or suppress selected cell functions. Here, poly(ethylene glycol) (PEG) hydrogels were functionalized with fluvastatin-releasing grafts and growth factor binding heparin domains to enable the dynamic exchange of information between the material and cells from the outside-in and inside-out (i.e., bidirectional signaling). By incorporating a fluvastatin-releasing graft and carefully controlling the dose and temporal release, materials were designed to promote bone morphogenic protein (BMP2) and alkaline phosphatase (ALP) production by human mesenchymal stem cells (hMSCs). When the release of fluvastatin was controlled to occur over 2 weeks, BMP2 and ALP production was increased 2.2-fold and 1.7-fold, respectively, at day 28 compared to hMSCs cultured in the absence of fluvastatin. By introducing a heparin functionality into the gel to sequester and localize the hMSC-produced BMP2, the osteogenic differentiation of hMSCs was further augmented over fluvastatin delivery alone. Osteopontin and core binding factor α1 gene expression was 6-fold and 4-fold greater for hMSCs exposed to fluvastatin in the presence of the heparin functionalities, respectively. These results demonstrate how multifunctional gels that interact with cells in a bidirectional manner can efficiently promote selected cell functions, such as the osteogenic differentiation of hMSCs.