Tough interpenetrating Pluronic F127/polyacrylic acid hydrogels

Effect of small amounts of akaganeite (β-FeOOH) nanorods on the gelation, phase behaviour and injectability of thermoresponsive Pluronic F127

Pluronic F127 (PF127) is a copolymer with an amphiphilic nature and can self-assemble to form micelles and, beyond 20% (w/v), form a thermoresponsive physical gel state. However, they are mechanically weak and easily dissolve in physiological environments, which limits their use in load-bearing in specific biomedical applications. Therefore, we propose a pluronic-based hydrogel with enhanced stability by incorporating small amounts of paramagnetic nanorods, akaganeite (β-FeOOH) nanorods (NRs) of aspect ratio ∼7, with PF127. Due to their weak magnetic properties, β-FeOOH NRs have been used as a precursor for preparing stable iron-oxide states (e.g., hematite and magnetite), and the studies on β-FeOOH NRs to be used as a primary component in hydrogels are at the nascent stage. Here we report a method to synthesize β-FeOOH NRs on a gram scale using a simple sol-gel process and characterize the NRs with various techniques. A phase diagram and thermoresponsive behaviour based on rheological experiments and visual observations are proposed for 20% (w/v) PF127 with low concentrations (0.1-1.0% (w/v)) of β-FeOOH NRs. We observe a unique non-monotonous behaviour in the gel network represented by various rheological parameters like storage modulus, yield stress, fragility, high-frequency modulus plateau, and characteristic relaxation time as a function of nanorod concentration. A plausible physical mechanism is proposed to fundamentally understand the observed phase behaviour in the composite gels. These gels show thermoresponsiveness and enhanced injectability, and could find applications in tissue engineering and drug delivery.

Enhanced Copolymer Gel Modified by Dual Surfactants for Flexible Zinc-Air Batteries

Zinc-air batteries using gels as carriers for electrolyte absorption have attracted extensive attention due to their flexibility, deformability, and high specific capacity. However, traditional mono-polymer gel electrolytes display poor mechanical properties and low ionic conductivity at wide-window temperatures. Here, the enhanced gel polymer (PAM-F/G) modified by dual surfactants is present by way of pluronic F127 and layered graphene oxide introduced into the polyacrylamide (PAM) matrix. The gel electrolyte procured by absorbing 6 M KOH exhibits improved mechanical characteristics, temperature adaptability, and a satisfactory ionic conductivity (276 mS cm^-1). The results demonstrate that a flexible zinc-air battery assembled by PAM-F/G electrolyte outputs a high power density (155 mW cm^-2) and can even operate reliably (>40 h) at -20 °C. These findings are available for promoting the research and popularization of flexible zinc-air batteries with high performance.

Fabrication of pH responsive hydrogel blends of chondroitin sulfate/pluronic F-127 for the controlled release of ketorolac: its characterization and acute oral toxicity study

OBJECTIVE

Ketorolac tromethamine (KT), selected as a model drug, is used in management of moderate to severe acute pain. It has a short half-life (∼5.5 h) and requires frequent dose administration when needed for longer period of time. In our current project, we designed pH responsive hydrogel blends of chondroitin sulfate/pluronic F-127 (CS/Pl) for the controlled release of ketorolac.

METHODS

Hydrogel blends were fabricated using free radical polymerization reaction technique utilizing different ratios of chondroitin sulfate (CS) (polymer) and pluronic F-127 (polymer), acrylic acid (monomer), N,N'-methyl-bisacrylamide (MBA) (cross-linker), initiator ammonium persulfate (APS) and tween-80 (surfactant). The fabricated hydrogel blends were studied and evaluated for pH responsiveness, swelling, water absorbency, in vitro drug release, and morphological characteristics such as SEM, XRD, FTIR, and TGA/DSC. Acute toxicity study was performed on rabbits.

RESULTS

Maximum swelling and water absorbency were shown by CS/Pl blends being significantly greater at 7.4 (basic pH) than in 1.2 (acidic pH). In vitro dissolution demonstrated pH responsive controlled KT release following zero order at higher pH (7.4) medium up to 36 h. FTIR studies confirmed the structures of our blends; SEM results showed porous framework; thermal studies revealed higher stability of hydrogels than the individual polymers; and XRD confirmed the nature of our blends. Toxicity study revealed the nontoxic nature of the hydrogel blends.

CONCLUSION

The prepared CS/Pl hydrogels demonstrated stimuli-controlled release with delivery of drug for prolonged period of time and thus can minimize dosing frequency, safe drug delivery, increased patient compliance and easiness.

Opportunities and Challenges of Switchable Materials for Pharmaceutical Use

Switchable polymeric materials, which can respond to triggering signals through changes in their properties, have become a major research focus for parenteral controlled delivery systems. They may enable externally induced drug release or delivery that is adaptive to in vivo stimuli. Despite the promise of new functionalities using switchable materials, several of these concepts may need to face challenges associated with clinical use. Accordingly, this review provides an overview of various types of switchable polymers responsive to different types of stimuli and addresses opportunities and challenges that may arise from their application in biomedicine.

Microneedles Assisted Controlled and Improved Transdermal Delivery of High Molecular Drugs via Insitu Forming Depot Thermoresponsive Poloxamers Gels in Skin Microchannels

Skin considered as an attractive route for variety of drug molecules administration. However it proved to be the main physical barrier for drug flux owing to their poor permeability and low bioavailability across stratum corneum layer. In current study novel approach has been used to enhance transdermal delivery via microporation through combination of poloxamers gels and microneedles arrays. The phase transition of poloxamers at various concentrations from sol-gel was evaluated using AR2000 rheometer to confirm microneedles-assisted insitu forming depots. Temperature test confirmed gelation between 32-37 °C. Curcumin was loaded in poloxamer formulations at variable concentrations and its effect showed reduction in critical gelation temperature (CGT) owing to its hydrophobic nature. Microneedles (MNs) arrays (600 µm) prepared from Gantrez S-97, PEG 10000 and Gelatin B using (19 × 19) laser-engineered silicone micromoulds showed high mechanical stability investigated via Texture analyzer. From insitu dissolution profile Gelatin 15% w/w based MNs displayed quicker dissolution rate in comparison to PG10000. VivoSight® OCT scanner and dye tracking confirmed that PG10000 MNs arrays pierced SC layer, infiltrate the epidermis and goes to dermis layer. From invitro permeation, it was concluded that 20% w/w PF127^® gel formulations containing (0.1% and 0.3%) curcumin displayed high curcumin permeation for comparatively longer time through microporated skin samples in comparison to non-microporated skin. The curcumin distribution in skin tissues with higher florescence intensity was noted in MNs treated skin samples by confocal microscopy. FTIR confirmed the structure formation of fabricated MNs, while TGA showed dry, brittle and rigid nature of Gelatin MNs.

Development and Evaluation of pH-Responsive Pluronic F 127 Co-Poly- (Acrylic Acid) Biodegradable Nanogels for Topical Delivery of Terbinafine HCL

Research aimed to develop and evaluate biodegradable, pH-responsive chemically cross-linked Pluronic F127 co-poly- (acrylic acid) nanogels for dermal delivery of Terbinafine HCL (TBH) to increase its permeability and as a new approach to treat skin fungal infections. TBH-loaded nanogels were successfully synthesized from acrylic acid (AA) and Pluronic F127 by free-radical copolymerization technique using N,N′-methylene bisacrylamide (MBA) as crosslinker and ammonium persulphate (APS) as initiator. Prepared nanogels exhibited 93.51% drug entrapment efficiency (DEE), 45 nm particle size, pH-dependent swelling and release behavior. Nanogels were characterized using different physicochemical techniques. The ex-vivo skin retention studies through rat skin showed about 42.34% drug retention from nanogels while 1% Lamisil cream (marketed product) showed about 26.56% drug retention. Moreover, skin irritation studies showed that nanogels were not irritating. Nanogels showed improved in-vitro antifungal activity against Candida albicans compared to commercial product. In-vivo studies on rats infected with Candida albicans confirmed superiority of nanogels over 1% Lamisil for eradication of fungal infection. This confirms that TBH loaded in Pluronic F127 co-poly-(acrylic acid) nanogels provided greater targetibility and cure rates of poorly soluble TBH in animal model and hence nanogels could be a potential carrier for effective topical delivery of TBH for skin fungal infection treatment.

High-Resolution 3D Printing of Mechanically Tough Hydrogels Prepared by Thermo-Responsive Poloxamer Ink Platform

High-resolution 3D-printable hydrogels with high mechanical strength and biocompatibility are in great demand because of their potential applications in numerous fields. In this study, a material system comprising Pluronic F-127 dimethacrylate (FDMA) is developed to function as a direct ink writing (DIW) hydrogel for 3D printing. FDMA is a triblock copolymer that transforms into micelles at elevated temperatures. The transformation increases the viscosity of FDMA and preserves its structure during DIW 3D printing, whereupon the printed structure is solidified through photopolymerization. Because of this viscosity shift, various functionalities can be incorporated through the addition of other materials in the solution state. Acrylic acid is incorporated into the pregel solution to enhance the mechanical strength, because the carboxylate group of poly(acrylic acid) ionically crosslinks with Fe³⁺ , increasing the toughness of the DIW hydrogel 37 times to 2.46 MJ m^-3 . Tough conductive hydrogels are also 3D printed by homogenizing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate into the pregel solution. Furthermore, the FDMA platform developed herein uses DIW, which facilitates multicartridges 3D printing, and because all the materials included are biocompatible, the platform may be used to fabricate complex structures for biological applications.

3D printed nitric oxide-releasing poly(acrylic acid)/F127/cellulose nanocrystal hydrogels

Hydrogels have been used as matrices for the topical delivery of nitric oxide (NO) for achieving vasodilation, wound healing and analgesic actions. More recently, supramolecular hydrogels comprised of poly(acrylic acid) (PAA) and micellar Pluronic F127 (F127), prepared by thermal reaction, emerged as a suitable matrix for the incorporation of hydrophilic NO donors, such as S-nitrosoglutathione (GSNO). Herein, we describe an innovative method for the three-dimensional (3D) printing of cellulose nanocrystal (CNC)-containing and semi-interpenetrating PAA/F127 hydrogels by PAA photopolymerization via digital light processing (DLP), in the absence of organic solvents. Scanning electron microscopy showed that, differently from typical porous PAA-based hydrogels, the 3D printed PAA/F127/CNC hydrogels have dense morphology. By using transmission electron microscopy we confirmed for the first time the presence of F127 micelles in the printable resin, and their preservation after the photopolymerization process. The F127 micelles conferred compressive recoverability to the 3D printed PAA/F127/CNC hydrogels, widening their potential applications as soft biomaterials. PAA/F127/CNC hydrogels charged with GSNO are shown to release NO spontaneously upon hydration at initial rates that depend on the GSNO charge and are higher in the presence of CNC. As local NO release may exert cell proliferation action, 3D printed PAA/F127/CNC/GSNO hydrogels may serve as a versatile soft biomaterial for local NO delivery in regenerative medicine and other biomedical applications.

A difunctional Pluronic®127-based in situ formed injectable thermogels as prolonged and controlled curcumin depot, fabrication, in vitro characterization and in vivo safety evaluation

Curcumin has been reported to be used widely against many types of pathological conditions in clinics. However, due to its limitations such as poor solubility, poor oral absorption and low stability have limited its applications. In the current study, a series of novel chemically cross-linkable depot gel formulations were developed based on thermoresponsive micellar polymer (Pluronic^®127) with polyelectrolyte hydrophilic monomer, that is, 2-acrylamido-2-methylpropane sulfonic acid by cold and in situ grafting polymerization method. The formulations were aimed to deliver curcumin at controlled rate from in situ formed depot after administration through subcutaneous route in vivo. The sol-gel phase transitions of formulations were observed by rheological analysis, tube titling and optical transmittance measurements. Maximum swelling of gel formulations was observed at pH 7.4 and below CGT, that is, 25 °C. The in vitro release profile exhibits maximum drug release at pH 7.4 and 25 °C owing to relaxed gel state. In vitro degradation profile of gel formulations showed controlled degradation rate. Cell growth inhibition study confirmed the biocompatibility and safe nature of bare gel formulations against L929 cell lines. In vitro cytotoxic study showed that curcumin loaded in gel formulation has controlled pharmacological activity against HeLa and MCF-7 cancer cells as compared to free drug solution. The IC₅₀ values calculated for pure curcumin solution (30 ± 0.77 µg/ml for HeLa and 27 ± 0.39 µg/ml for MCF-7) were found higher in comparison to curcumin-loaded thermogels against HeLa (19 ± 0.28 µg/ml and 23 ± 0.81 µg/ml) and MCF-7 (22 ± 0.54 µg/ml and 21 ± 0.49 µg/ml). Histopathological and hematological analysis showed the biocompatible nature of hydrogels. Structural confirmation was done by Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy (¹H NMR). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) confirmed the thermal stability of the gel formulation. The porous structure of gel formulations was assessed by scanning electron microscopic (SEM) analysis. Results concluded that newly developed gel formulations have thermoresponsive behavior with phase transition at body temperature and can be used as in situ controlled drug depot.

Evaluation of microneedles-assisted in situ depot forming poloxamer gels for sustained transdermal drug delivery

In this study, for the first time, we have reported a sustained transdermal drug delivery from thermoresponsive poloxamer depots formed within the skin micropores following microneedle (MN) application. Firstly, we have investigated the sol–gel phase transition characteristics of poloxamers (PF®127, P108, and P87) at physiological conditions. Rheological measurements were evaluated to confirm the critical gelation temperature (CGT) of the poloxamer formulations with or without fluorescein sodium (FS), as a model drug, at various concentrations. Optimized poloxamer formulations were subjected to in vitro release studies using a vial method. Secondly, polymeric MNs were fabricated using laser-engineered silicone micromolds from various biocompatible polymeric blends of Gantrez S-97, PEG 10000, PEG200, PVP K32, and PVP K90. The MN arrays were characterized for mechanical strength, insertion force determination, in situ dissolution kinetics, moisture content, and penetration depth. The optimized MN arrays with good mechanical strength and non-soluble nature were used to create micropores in the neonatal porcine skin. Microporation in neonatal porcine skin was confirmed by dye-binding study, skin integrity assessment, and histology study. Finally, the in vitro delivery of FS from optimized poloxamer formulations was conducted across non-porated vs microporated skin samples using vertical Franz diffusion cells. Results concluded that permeation of FS was sustained for 96 h across the MN-treated skin samples containing in situ forming depot poloxamer formulations compared to non-microporated skin which sustained the FS delivery for 72 h. Confocal microscopic images confirmed the distribution of higher florescence intensity of FS in skin tissues after permeation study in case of MN-treated skin samples vs intact skin samples.

Do the properties of gels constructed by interlinking triply-responsive microgels follow from those of the building blocks?

Microgels (MGs) are swellable crosslinked polymer colloids. They can also be used as the only building block to construct nanostructured hydrogels which are denoted as doubly crosslinked microgels (DX MGs). Here, new triply responsive DX MGs comprised of interlinked MGs of oligo(ethylene glycol)methacrylate (OEGMA), 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA), methacrylic acid (MAA) and a o-nitrobenzyl-based UV photocleavable crosslinker are investigated. The MGs swelled or collapsed in response to temperature and pH changes. These behaviours were rationalised with a generic model using Monte Carlo simulations. The MGs also degraded when UV irradiated due to photocleavage of nPh. DX MGs were assembled from the MGs to give injectable gels that were not cytotoxic to nucleus pulposus cells. Comparison of the responsive properties of the DX MGs and MGs showed that the temperature and pH responses of the former were mostly governed by the latter. However, two key differences were found. Firstly, whilst increasing the crosslinker mol% in the MG building blocks (x) did not change MG particle swelling, the compression modulus (E) and swelling of the DX MG gels were strongly affected by x. The E value for the gels was tuneable using x which is a potentially useful new observation for DX MGs. Secondly, UV irradiation of the DX MGs enhanced gel mechanical photostability in contrast to the behaviour of the MGs. We find that the properties of the DX MGs do not simply follow those of the parent MGs and propose mechanisms to account for the differences. The new family of multi-responsive DX MGs presented in this study have potential application for soft tissue repair as injectable gels or as gel implants which report sterilisation.

A hybrid genipin-crosslinked dual-sensitive hydrogel/nanostructured lipid carrier ocular drug delivery platform

The objective of this study was to develop a novel hybrid genipin-crosslinked dual-sensitive hydrogel/nanostructured lipid carrier (NLC) drug delivery platform. An ophthalmic anti-inflammatory drug, baicalin (BN) was chosen as the model drug. BN-NLC was prepared using melt-emulsification combined with ultra-sonication technique. Additionally, a dual pH- and thermo-sensitive hydrogel composed of carboxymethyl chitosan (CMCS) and poloxamer 407 (F127) was fabricated by a cross-linking reaction with a nontoxic crosslinker genipin (GP). GP-CMCS/F127 hydrogel was characterized by FTIR, NMR, XRD and SEM. The swelling studies showed GP-CMCS/F127 hydrogel was both pH- and thermo-sensitive. The results of in vitro release suggested BN-NLC gel can prolong the release of baicalin comparing with BN eye drops and BN-NLC. Ex vivo cornea permeation study was evaluated using Franz diffusion cells. The apparent permeability coefficient (P_app ) of BN-NLC gel was much higher (4.46-fold) than that of BN eye drops. Through the determination of corneal hydration levels, BN-NLC gel was confirmed that had no significant irritation to cornea. Ex vivo precorneal retention experiments were carried out by a flow-through approach. The results indicated that the NLC-based hydrogel can prolong precorneal residence time. In conclusion, the hybrid NLC-based hydrogel has a promising potential for application in ocular drug delivery.

Supramolecular poly(acrylic acid)/F127 hydrogel with hydration-controlled nitric oxide release for enhancing wound healing

Topical nitric oxide (NO) delivery has been shown to accelerate wound healing. However, delivering NO to wounds at appropriate rates and doses requires new biomaterial-based strategies. Here, we describe the development of supramolecular interpolymer complex hydrogels comprising PEO-PPO-PEO (F127) micelles embedded in a poly(acrylic acid) (PAA) matrix, with S-nitrosoglutathione (GSNO) molecules dissolved in the hydrophilic domain. We show that PAA:F127/GSNO hydrogels start releasing NO upon hydration at rates controlled by their rates of water absorption. SAXS measurements indicate that the supramolecular structure of the hydrogels retains long-range order domains of F127 micelles. The PAA/F1227 hydrogels displayed dense morphologies and reduced rates of hydration. The NO release rates remain constant over the first 200 min, are directly correlated with the hydration rates of the PAA:F127/GSNO hydrogels, and can be modulated in the range of 40 nmol/g h to 1.5 μmol/g h by changing the PAA:F127 mass ratio. Long-term NO-release profiles over 5 days are governed by the first-order exponential decay of GSNO, with half-lives in the range of 0.5-3.4 days. A preliminary in vivo study on full-thickness excisional wounds in mice showed that topical NO release from the PAA:F127/GSNO hydrogels is triggered by exudate absorption and leads to increased angiogenesis and collagen fiber organization, as well as TGF-β, IGF-1, SDF-1, and IL-10 gene expressions in the cicatricial tissue. In summary, these results suggest that hydration-controlled NO release from topical PAA:F127/GSNO hydrogels is a potential strategy for enhancing wound healing.

STATEMENT OF SIGNIFICANCE

The topical delivery of nitric oxide (NO) to wounds may provide significant beneficial results and represent a promising strategy to treat chronic wounds. However, wound dressings capable of releasing NO after application and allowing the modulation of NO release rates, demand new platforms. Here, we describe a novel strategy to overcome these challenges, based on the use of supramolecular poly(acrylic acid) (PAA):F127 hydrogels charged with the NO donor S-nitrosoglutathione (GSNO) from whereby the NO release can be triggered by exudate absorption and delivered to the wound at rates controlled by the PAA:F127 mass ratio. Preliminary in vivo results offer a proof of concept for this strategy by demonstrating increased angiogenesis; collagen fibers organization; and TGF-β, IGF-1, SDF-1, and IL-10 gene expressions in the cicatricial tissue after topical treatment with a PAA:F127/GSNO hydrogel.

Highly deformable hydrogels constructed by pH-triggered polyacid nanoparticle disassembly in aqueous dispersions

Most hydrogels are prepared using small-molecule monomers but unfortunately this approach may not be feasible for certain biomaterial applications. Consequently, alternative gel construction strategies have been established, which include using covalent inter-linking of preformed gel particles, or microgels (MGs). For example, covalently interlinking pH-responsive MGs can produce hydrogels comprising doubly crosslinked microgels (DX MGs). We hypothesised that the deformability of such DX MGs was limited by the presence of intra-MG crosslinking. Thus, in this study we designed new nanoparticle (NP)-based gels based on pH-swellable NPs that are not internally crosslinked. Two polyacid NPs were synthesised containing methacrylic acid (MAA) and either ethyl acrylate (EA) or methyl methacrylate (MMA). The PMAA-EA and PMAA-MMA NPs were subsequently vinyl-functionalised using glycidyl methacrylate (GMA) prior to gel formation via free-radical crosslinking. The NPs mostly disassembled on raising the solution pH but some self-crosslinking was nevertheless evident. The gels constructed from the EA- and MMA-based NPs had greater breaking strains than a control DX MG. The effect of varying the solution pH during curing on the morphology and mechanical properties of gels prepared using PMAA-MMA-GMA NPs was studied and both remarkable deformability and excellent recovery were observed. The gels were strongly pH-responsive and had tensile breaking strains of up to 420% with a compressive strain-at-break of more than 93%. An optimised formulation produced the most deformable and stretchable gel yet constructed using NPs or MGs as the only building block.

Design of a Versatile pH-Responsive Hydrogel for Potential Oral Delivery of Gastric-Sensitive Bioactives

A pH-responsive hydrogel system was prepared by free radical polymerization of acrylamide and methyl acrylic acid in the presence of N-N′-methylene bisacrylamide. Sodium bicarbonate was further applied as a blowing agent, which afforded a porous hydrogel structure. The hydrogel system achieved a constant super swelling rate within simulated intestinal buffer (~4%/min) and remained relatively static within simulated gastric buffer (~0.8%/min). The hydrogel system was able to achieve matrix resilience greater than 30% under a relatively high strain of 40%. In addition, the hydrogel system demonstrated significant swelling properties in response to simulated intestinal environmental over 24 h, with contrasting characteristics in simulated gastric buffer. The hydrogel demonstrated type IV isotherm porosity characteristics, with remarkable MRI and SEM variations in gastric and intestinal simulated fluids. Drug loading was observed to be greater than 98% using theophylline as a prototype drug, evaluating its controlled release kinetics over 24 h. The hydrogel exhibited substantial pH-responsive activity, which could be used as a versatile platform for targeted release of gastric-sensitive therapeutics to the small intestine.

Synthetic self-assembled homogeneous network hydrogels with high mechanical and recoverable properties for tissue replacement

Homogeneous network hydrogels with high mechanical and recoverable properties for tissue replacement are prepared by one-pot free radical polymerization.

Advances in interpenetrating polymer network hydrogels and their applications

Interpenetrating polymer network (IPN) hydrogels brought distinct benefits compared to single network hydrogels like more widely controllable physical properties, and (frequently) more efficient drug loading/release. However, IPN strategy is not sufficient to design hydrogels with enhanced mechanical properties required for regenerative medicine like replacement of natural cartilage or artificial cornea. Some of the novel techniques promoted last decade for the preparation of IPN hydrogels which fulfill these requirements are discussed in the review. Among them, “double network” strategy had a strong contribution in the development of a large variety of hydrogels with spectacular mechanical properties at water content up to 90 %. Using cryogelation in tandem with IPN strategy led to composite cryogels with high mechanical properties and high performances in separation processes of ionic species. Highly stretchable and extremely tough hydrogels have been obtained by combining a covalently cross-linked synthetic network with an ionically cross-linked alginate network. IPN hydrogels with tailored mesh size have been also reported.

Super Tough, Ultrastretchable, and Thermoresponsive Hydrogels with Functionalized Triblock Copolymer Micelles as Macro-Cross-Linkers

Vinyl-functionalized thermosensitive Pluronic F127 micelles have been used as multifunctional cross-links for the synthesis of super tough, highly resilient and thermoresponsive nanomicelle (NM) hydrogels. Pluronic F127 diacrylate (F127DA) with vinyl groups on both ends self-assembled in aqueous solution into micelles. Such micelles served as multifunctional macro-cross-links to copolymerize with acrylamide (AAm) monomers, generating novel NM hydrogels with extraordinary tensile and compressive properties, without using any chemical cross-linkers. Uniaxial tensile tests demonstrated a fracture strain above 2265%, an ultimate stress of 276 kPa, and a fracture energy of 2.34 MJ/m³. Under compression tests, these hydrogels did not fracture up to 98% strain and 62 MPa stress. Cyclic compressive loading-unloading tests at 90% strain showed no decay of the hyseteresis energy, indicating an unprecedented fatigue resistance.

Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks

As swollen polymer networks in water, hydrogels are usually brittle. However, hydrogels with high toughness play critical roles in many plant and animal tissues as well as in diverse engineering applications. Here we review the intrinsic mechanisms of a wide variety of tough hydrogels developed over the past few decades. We show that tough hydrogels generally possess mechanisms to dissipate substantial mechanical energy but still maintain high elasticity under deformation. The integrations and interactions of different mechanisms for dissipating energy and maintaining elasticity are essential to the design of tough hydrogels. A matrix that combines various mechanisms is constructed for the first time to guide the design of next-generation tough hydrogels. We further highlight that a particularly promising strategy for the design is to implement multiple mechanisms across multiple length scales into nano-, micro-, meso-, and macro-structures of hydrogels.