Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hysteresis-Free, Elastic, and Tough Hydrogel with Stretch-Rate Independence and High Stability in Physiological Conditions

Many existing synthetic hydrogels are inappropriate for repetitive motions because of large hysteresis, and their mechanical properties in warm and saline physiological conditions remain understudied. In this study, a stretch-rate-independent, hysteresis-free, elastic, and tough nanocomposite hydrogel that can maintain its mechanical properties in phosphate-buffered saline of 37 °C similar to warm and saline conditions of the human body is developed. The strength, stiffness, and toughness of the hydrogel are simultaneously reinforced by biomimetic silica nanoparticles with a surface of embedded circular polyamine chains. Such distinctive surfaces form robust interfacial interactions by local topological folding/entanglement with the polymer chains of the matrix. Load transfer from the soft polymer matrix to stiff nanoparticles, along with the elastic sliding/unfolding/disentanglement of polymer chains, overcomes the traditional trade-off between strength/stiffness and toughness and allows for hysteresis-free, strain-rate-independent, and elastic behavior. This robust reinforcement is sustained in warm phosphate-buffered saline. These properties demonstrate the application potential of the developed hydrogel as a soft, elastic, and tough bio-strain sensor that can detect dynamic motions across various deformation speeds and ranges. The findings provide a simple yet effective approach to developing practical hydrogels with a desirable combination of strength/stiffness and toughness, in a fully swollen and equilibrated state.

Strong, Tough, and Biocompatible Poly(vinyl alcohol)-Poly(vinylpyrrolidone) Multiscale Network Hydrogels Reinforced by Aramid Nanofibers

Poly(vinyl alcohol) (PVA) hydrogels are water-rich, three-dimensional (3D) network materials that are similar to the tissue structure of living organisms. This feature gives hydrogels a wide range of potential applications, including drug delivery systems, articular cartilage regeneration, and tissue engineering. Due to the large amount of water contained in hydrogels, achieving hydrogels with comprehensive properties remains a major challenge, especially for isotropic hydrogels. This study innovatively prepares a multiscale-reinforced PVA hydrogel from molecular-level coupling to nanoscale enhancement by chemically cross-linking poly(vinylpyrrolidone) (PVP) and in situ assembled aromatic polyamide nanofibers (ANFs). The optimized ANFs-PVA-PVP (APP) hydrogels have a tensile strength of ≈9.7 MPa, an elongation at break of ≈585%, a toughness of ≈31.84 MJ/m3, a compressive strength of ≈10.6 MPa, and a high-water content of ≈80%. It is excellent among all reported PVA hydrogels and even comparable to some anisotropic hydrogels. System characterizations show that those performances are attributed to the particular multiscale load-bearing structure and multiple interactions between ANFs and PVA. Moreover, APP hydrogels exhibit excellent biocompatibility and a low friction coefficient (≈0.4). These valuable performances pave the way for broad potential in many advanced applications such as biological tissue replacement, flexible wearable devices, electronic skin, and in vivo sensors.

Designed modular protein hydrogels for biofabrication

Designing proteins that fold and assemble over different length scales provides a way to tailor the mechanical properties and biological performance of hydrogels. In this study, we designed modular proteins that self-assemble into fibrillar networks and, as a result, form hydrogel materials with novel properties. We incorporated distinct functionalities by connecting separate self-assembling (A block) and cell-binding (B block) domains into single macromolecules. The number of self-assembling domains affects the rigidity of the fibers and the final storage modulus G' of the materials. The mechanical properties of the hydrogels could be tuned over a broad range (G' = 0.1 - 10 kPa), making them suitable for the cultivation and differentiation of multiple cell types, including cortical neurons and human mesenchymal stem cells. Moreover, we confirmed the bioavailability of cell attachment domains in the hydrogels that can be further tailored for specific cell types or other biological applications. Finally, we demonstrate the versatility of the designed proteins for application in biofabrication as 3D scaffolds that support cell growth and guide their function. STATEMENT OF SIGNIFICANCE: Designed proteins that enable the decoupling of biophysical and biochemical properties within the final material could enable modular biomaterial engineering. In this context, we present a designed modular protein platform that integrates self-assembling domains (A blocks) and cell-binding domains (B blocks) within a single biopolymer. The linking of assembly domains and cell-binding domains this way provided independent tuning of mechanical properties and inclusion of biofunctional domains. We demonstrate the use of this platform for biofabrication, including neural cell culture and 3D printing of scaffolds for mesenchymal stem cell culture and differentiation. Overall, this work highlights how informed design of biopolymer sequences can enable the modular design of protein-based hydrogels with independently tunable biophysical and biochemical properties.

Nanocellulose hydrogels formed via crystalline transformation from cellulose I to II and subsequent freeze cross-linking reaction

We describe nanocellulose (NC) hydrogels formed from chemically unmodified NC by cellulose crystalline transformation and subsequent freeze cross-linking reaction. The freeze cross-linked NC hydrogel with macropores (~100 μm) was prepared by freezing a mixture of NC and NaOH (0.2 mol L-1), adding citric acid to the frozen mixture, and thawing it. Using NaOH and freezing together induced the crystalline transformation of NC from cellulose I to II via freeze concentration. After the crystalline transformation, cross-linking between the NC and CA in the freeze concentration layer provided a strong NC network structure, forming NC hydrogels with high mechanical strength. The structural changes in NC caused by NaOH, freezing, and freeze cross-linking on the angstrom to micrometer scale were investigated with FT-IR, SAXS, PXRD, and SEM. The freeze cross-linked NC hydrogel easily retained powder adsorbents in its inner space by mixing the NC-NaOH sol and the powder, and the hydrogel showed high removal efficiency for heavy metals. The results highlight the versatility of chemically unmodified celluloses in developing functional materials and suggest possible practical applications. This study also provides new insights into the efficient use of chemical reactions of cellulose under freezing conditions.

Nacre-inspired, strong, tough silk fibroin hydrogels based on biomineralization and the layer-by-layer assembly of ordered silk fabric

Hydrogels are attractive materials with structures and functional properties similar to biological tissues and widely used in biomedical engineering. However, traditional synthetic hydrogels possess poor mechanical strength, and their applications are limited. Herein, a multidimensional material design method is developed; it includes the in situ gelation of silk fabric and nacre-inspired layer-by-layer assembly, which is used to prepare silk fibroin (SF) hydrogels. The in situ gelation method of silk fabric introduces a directionally ordered fabric network in a silk substrate, considerably enhancing the strength of hydrogels. Based on the nacre structure, the layer-by-layer assembly method enables silk hydrogels to break through the size limit and increase the thickness, realizing the longitudinal extension of the hydrogels. The application of the combined biomineralization and hot pressing method can effectively reduce interface defects and improve the interaction between organic and inorganic interfaces. The multidimensional material design method helps increase the strength (287.78 MPa), toughness (18.43 MJ m-3), and fracture energy (50.58 kJ m-2) of SF hydrogels; these hydrogels can weigh 2000 times their own weight. Therefore, SF hydrogels designed using the aforementioned combined method can realize the combination of strength and toughness and be used in biological tissue engineering and structural materials.

Poly(Vinyl Alcohol)/Bovine Serum Albumin Hybrid Hydrogels with Tunable Mechanical Properties

In this study, a new strategy was adopted for obtaining polymer/protein hybrid hydrogels with shape stability and tunable mechanical or rheological characteristics by using non-toxic procedures. A chemical network was created using a poly(vinyl alcohol)(PVA)/bovine serum albumin (BSA) mixture in aqueous solution in the presence of genipin and reduced glutathione (GSH). Then, a second physical network was formed through PVA after applying freezing/thawing cycles. In addition, the protein macromolecules formed intermolecular disulfide bridges in the presence of GSH. In these conditions, multiple crosslinked networks were obtained, determining the strengthening and stiffening into relatively tough porous hydrogels with tunable viscoelasticity and a self-healing ability. A SEM analysis evidenced the formation of networks with interconnected pores of sizes between 20 μm and 50 μm. The mechanical or rheological investigations showed that the hydrogels' strength and response in different conditions of deformation were influenced by the composition and crosslinking procedure. Thus, the dynamics of the hybrid hydrogels can be adjusted to mimic the viscoelastic properties of the native tissues. The dynamic water vapor-sorption ability, swelling behavior in an aqueous environment, and bioadhesive properties were also investigated and are discussed in this paper. The hybrid hydrogels with tunable viscoelasticity can be designed on request, and they are promising candidates for tissue engineering, bioinks, and wound dressing applications.

Advanced biomaterial agent from chitosan/poloxamer 407-based thermosensitive hydrogen containing biosynthesized silver nanoparticles using Eucalyptus camaldulensis leaf extract

CONTEXT

The emergence of multidrug-resistant (MDR) pathogens poses a significant challenge for global public health systems, increasing hospital morbidity and mortality and prolonged hospitalization.

OBJECTIVE

We evaluated the antimicrobial activity of a thermosensitive hydrogel containing bio-synthesized silver nanoparticles (bio-AgNPs) based on chitosan/poloxamer 407 using a leaf extract of Eucalyptus calmadulensis.

RESULTS

The thermosensitive hydrogel was prepared by a cold method after mixing the ingredients and left at 4°C overnight to ensure the complete solubilization of poloxamer 407. The stability of the hydrogel formulation was evaluated at room temperature for 3 months, and the absorption peak (420 nm) of the NPs remained unchanged. The hydrogel formulation demonstrated rapid gelation under physiological conditions, excellent water retention (85%), and broad-spectrum antimicrobial activity against MDR clinical isolates and ATCC strains. In this regard, minimum inhibitory concentration and minimum microbial concentration values of the bio-AgNPs ranged from 2-8 μg/mL to 8-128 μg/mL, respectively. Formulation at concentrations <64 μg/mL showed no cytotoxic effect on human-derived macrophages (THP-1 cells) with no induction of inflammation.

CONCLUSIONS

The formulated hydrogel could be used in biomedical applications as it possesses a broad antimicrobial spectrum and anti-inflammatory properties without toxic effects on human cells.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Rapid fabrication of physically robust hydrogels

Hydrogel materials show promise for diverse applications, particular as biocompatible materials due to their high water content. Despite advances in hydrogel technology in recent years, their application is often severely limited by inadequate mechanical properties and time-consuming fabrication processes. Here we report a rapid hydrogel preparation strategy that achieves the simultaneous photo-crosslinking and establishment of biomimetic soft-hard material interface microstructures. These biomimetic interfacial-bonding nanocomposite hydrogels are prepared within seconds and feature clearly separated phases but have a strongly bonded interface. Due to effective interphase load transfer, biomimetic interfacial-bonding nanocomposite gels achieve an ultrahigh toughness (138 MJ m-3) and exceptional tensile strength (15.31 MPa) while maintaining a structural stability that rivals or surpasses that of commonly used elastomer (non-hydrated) materials. Biomimetic interfacial-bonding nanocomposite gels can be fabricated into arbitrarily complex structures via three-dimensional printing with micrometre-level precision. Overall, this work presents a generalizable preparation strategy for hydrogel materials and acrylic elastomers that will foster potential advances in soft materials.

3D Printing of Interpenetrating Network Flexible Hydrogels with Enhancement of Adhesiveness

3D printing of hydrogels has been widely explored for the rapid fabrication of complex soft structures and devices. However, using 3D printing to customize hydrogels with both adequate adhesiveness and toughness remains a fundamental challenge. Here, we demonstrate mussel-inspired (polydopamine) PDA hydrogel through the incorporation of a classical double network (2-acrylamido-2-methylpropanesulfonic acid) PAMPS/(polyacrylamide) PAAm to achieve simultaneously tailored adhesiveness, toughness, and biocompatibility and validate the 3D printability of such a hydrogel into customized architectures. The strategy of combining PDA with PAMPS/PAAm hydrogels leads to favorable adhesion on either hydrophilic or hydrophobic surfaces. The hydrogel also shows excellent flexibility, which is attributed to the reversible cross-linking of PDA and PAMPS, together with the long-chain PAAm cross-linking network. Among them, the reversible cross-linking of PDA and PAMPS is capable of dissipating mechanical energy under deformation. Meanwhile, the long-chain PAAm network contributes to maintaining a high deformation capability. We establish a theoretical framework to quantify the contribution of the interpenetrating networks to the overall toughness of the hydrogel, which also provides guidance for the rational design of materials with the desired properties. Our work manifests a new paradigm of printing adhesive, tough, and biocompatible interpenetrating network hydrogels to meet the requirements of broad potential applications in biomedical engineering, soft robotics, and intelligent and superabsorbent devices.

Combination of Milk and Plant Proteins to Develop Novel Food Systems: What Are the Limits?

In the context of a diet transition from animal protein to plant protein, both for sustainable and healthy scopes, innovative plant-based foods are being developing. A combination with milk proteins has been proposed as a strategy to overcome the scarce functional and sensorial properties of plant proteins. Based on this mixture were designed several colloidal systems such as suspensions, gels, emulsions, and foams which can be found in many food products. This review aims to give profound scientific insights on the challenges and opportunities of developing such binary systems which could soon open a new market category in the food industry. The recent trends in the formulation of each colloidal system, as well as their limits and advantages are here considered. Lastly, new approaches to improve the coexistence of both milk and plant proteins and how they affect the sensorial profile of food products are discussed.

Development of chitosan-based hydrogels for healthcare: A review

Chitosan-based hydrogels (CSH) are promising materials for healthcare. Based on the relationship among structure, property and application, researches reported within last decade are chosen to elucidate the developing approaches and potential applications of target CSH. The applications of CSH are classified into the conventional biomedical fields, such as drug controlled release, tissue repair and monitoring, and the essential ones including food safety, water purification and air cleaning. The approaches focused on in this article are the reversible chemical and physical ones. Apart from describing the current status of the development, suggestions are presented as well.

Unveiling the Role of Capping Groups in Naphthalene N-Capped Dehydrodipeptide Hydrogels

Self-assembled peptide-based hydrogels are archetypical nanostructured materials with a plethora of foreseeable applications in nanomedicine and as biomaterials. N-protected di- and tri-peptides are effective minimalist (molecular) hydrogelators. Independent variation of the capping group, peptide sequence and side chain modifications allows a wide chemical space to be explored and hydrogel properties to be tuned. In this work, we report the synthesis of a focused library of dehydrodipeptides N-protected with 1-naphthoyl and 2-naphthylacetyl groups. The 2-naphthylacetyl group was extensively reported for preparation of peptide-based self-assembled hydrogels, whereas the 1-naphthaloyl group was largely overlooked, owing presumably to the lack of a methylene linker between the naphthalene aromatic ring and the peptide backbone. Interestingly, dehydrodipeptides N-capped with the 1-naphthyl moiety afford stronger gels, at lower concentrations, than the 2-naphthylacetyl-capped dehydrodipeptides. Fluorescence and circular dichroism spectroscopy showed that the self-assembly of the dehydrodipeptides is driven by intermolecular aromatic π-π stacking interactions. Molecular dynamics simulations revealed that the 1-naphthoyl group allows higher order aromatic π-π stacking of the peptide molecules than the 2-naphthylacetyl group, together with hydrogen bonding of the peptide scaffold. The nanostructure of the gel networks was studied by TEM and STEM microscopy and was found to correlate well with the elasticity of the gels. This study contributes to understanding the interplay between peptide and capping group structure on the formation of self-assembled low-molecular-weight peptide hydrogels. Moreover, the results presented here add the 1-naphthoyl group to the palette of capping groups available for the preparation of efficacious low-molecular-weight peptide-based hydrogels.

Cartilage-bioinspired, tough and lubricated hydrogel based on nanocomposite enhancement effect

The maintenance of high load-bearing tissues and joint lubrication is essential for suppressing osteoarthritis. The lubrication of natural joints is mainly attributed to the hydration lubrication mechanism of articular cartilage. Phospholipids on the cartilage surface attract water molecules to form a tough hydrated layer to reduce friction. In this work, inspired by the phosphatidylcholine lipids, we synthesized lubricated nanospheres by grafting hydrophilic polymer brushes and further synthesized a nanocomposite hydrogel. The addition of the lubricated nanospheres enhanced both the mechanical and lubricated properties of the hydrogel. The nanocomposite-lubricated hydrogel exhibited a friction coefficient 81.7% lower than the blank hydrogel because of grafting the polymer brushes. Also, the nanocomposite enhancement helped the hydrogel achieve high mechanical properties with a compressive strength of 6.63 MPa (50%). The nanocomposite hydrogel developed here could be a promising candidate material in bionic articular cartilage substitute materials.

Is 3D Printing Promising for Osteochondral Tissue Regeneration?

Osteochondral tissue regeneration is quite difficult to achieve due to the complexity of its organization. In the design of these complex multilayer structures, a fabrication method, 3D printing, started to be employed, especially by using extrusion, stereolithography and inkjet printing approaches. In this paper, the designs are discussed including biphasic, triphasic, and gradient structures which aim to mimic the cartilage and the calcified cartilage and the whole osteochondral tissue closely. In the first section of the review paper, 3D printing of hydrogels including gelatin methacryloyl (GelMa), alginate, and polyethylene glycol diacrylate (PEGDA) are discussed. However, their physical and biological properties need to be augmented, and this generally is achieved by blending the hydrogel with other, more durable, less hydrophilic, polymers. These scaffolds are very suitable to carry growth factors, such as TGF-β1, to further stimulate chondrogenesis. The bone layer is mimicked by printing calcium phosphates (CaPs) or bioactive glasses together with the hydrogels or as a component of another polymer layer. The current research findings indicate that polyester (i.e. polycaprolactone (PCL), polylactic acid (PLA) and poly(lactide-co-glycolide) (PLGA)) reinforced hydrogels may more successfully mimic the complex structure of osteochondral tissue. Moreover, more recent printing methods such as melt electrowriting (MEW), are being used to integrate polyester fibers to enhance the mechanical properties of hydrogels. Additionally, polyester scaffolds that are 3D printed without hydrogels are discussed after the hydrogel-based scaffolds. In this review paper, the relevant studies are analyzed and discussed, and future work is recommended with support of tables of designed scaffolds. The outcome of the survey of the field is that 3D printing has significant potential to contribute to osteochondral tissue repair.

High Mechanical Strength and Multifunctional Microphase-Separated Supramolecular Hydrogels Fabricated by Liquid-Crystalline Block Copolymer

The development of multifunctional supramolecular hydrogels with high mechanical strength and multifunction is in high demand. In this work, the diblock copolymer poly(acrylamide-co-1-benzyl-3-vinylimidazolium bromide)-block-polyAzobenzene is synthesized through reversible addition-fragmentation chain transfer polymerization. The dynamic host-guest interactions between the host molecule cucurbit[8] uril and guest units are used to fabricate a 3D network of supramolecular hydrogels. Investigations on the properties of the supramolecular hydrogels show that the tensile stress of the sample is 1.46 MPa, eight times higher than that of hydrogel without liquid-crystalline block copolymer, and the self-healing efficiency of the supramolecular hydrogels at room temperature is 88.3% (fracture stress) and 100% (fracture strain) after 24 h. Results show that microphase-separated structure plays a key role in the high-strength hydrogel, whereas the host-guest interaction endows the hydrogel with self-healing properties. The supramolecular hydrogels with high mechanical strength, photo-responsivity, injectability, and biocompatibility can be used in various potential applications.

4D Printing of Body Temperature-Responsive Hydrogels Based on Poly(acrylic acid) with Shape-Memory and Self-Healing Abilities

Additive manufacturing of smart materials that can be dynamically programmed with external stimuli is known as 4D printing. Among the 4D printable materials, hydrogels are the most extensively studied materials in various biomedical areas because of their hierarchical structure, similarity to native human tissues, and supreme bioactivity. However, conventional smart hydrogels suffer from poor mechanical properties, slow actuation speed, and instability of actuated shape. Herein, we present 4D-printed hydrogels based on poly(acrylic acid) that can concurrently possess shape-memory and self-healing properties. The printing of the hydrogels is achieved by solvent-free copolymerization of the hydrophilic acrylic acid (AAc) and hydrophobic hexadecyl acrylate (C16A) monomers in the presence of TPO photoinitiator using a stereolithography-based commercial resin printer followed by swelling in water. The printed hydrogels undergo a reversible strong-to-weak gel transition below and above human body temperature due to the melting and crystallization of the hydrophobic C16A domains. In this way, the shape-memory and self-healing properties of the hydrogels can be magically actuated near the body temperature by adjusting the molar ratio of the monomers. Furthermore, the printed hydrogels display a high Young's modulus (up to ∼215 MPa) and high toughness (up to ∼7 MJ/m³), and their mechanical properties can be tuned from brittle to ductile by reducing the molar fraction of C16A, or the deformation speed. Overall, the developed 4D printable hydrogels have great potential for various biomedical applications.

An investigation on tribological behavior of methacrylated κ-carrageenan and gellan gum hydrogels as a candidate for chondral repair

Natural polysaccharides have recently attracted attention as structural biomaterials to replace focal chondral defects. In the present study, in-vitro tribological performance of methacrylated κ-carrageenan and gellan gum hydrogels (KA-MA and GG-MA) was evaluated under physiological conditions. Coefficient of friction (COF) was continuously recorded over testing whilst worn area was measured post-testing. The findings help improve our understanding of KA-MA-H and GG-MA-H tribological performance under various physiological conditions. The friction and wear performance of the hydrogels improved in bovine calf serum lubricant at lower applied loads. Adhesion was the dominant wear mechanism detected by SEM. Among the proposed hydrogels GG-MA-H found robust mechanical properties, increased wear resistance and considerably low COF, which may suggest its potential usage as a cartilage substitute.

Naturally Derived Carbon Dots In Situ Confined Self-Healing and Breathable Hydrogel Monolith for Anomalous Diffusion-Driven Phytomedicine Release

Fluorescent nanocarbons are well-proficient nanomaterials because of their optical properties and surface engineering. Herein, Apium graveolens-derived carbon dots (ACDs) have been synthesized by a one-step hydrothermal process without using any surplus vigorous chemicals or ligands. ACDs were captured via an in situ gelation reaction to form a semi-interpenetrating polymer network system showing mechanical robustness, fluorescent behavior, and natural adhesivity. ACDs-reinforced hydrogels were tested against robust uniaxial stress, repeated mechanical stretching, thixotropy, low creep, and fast strain recovery, confirming their elastomeric sustainability. Moreover, the room-temperature self-healing behavior was observed for the ACDs-reinforced hydrogels, with a healing efficacy of more than 45%. Water imbibition through hydrogel surfaces was digitally monitored via "breathing" and "accelerated breathing" behaviors. The phytomedicine release from the hydrogels was tuned by the ACDs' microstructure regulatory activity, resulting in better control of the diffusion rate compared to conventional chemical hydrogels. Finally, the phytomedicine-loaded hydrogels were found to be excellent bactericidal materials eradicating more than 85% of Gram-positive and -negative bacteria. The delayed network rupturing, superstretchability, fluorescent self-healing, controlled release, and antibacterial behavior could make this material an excellent alternative to soft biomaterials and soft robotics.

Emerging polymeric material strategies for cartilage repair

Cartilage is found throughout the body, serving an array of essential functions. Owing to the limited healing capacity of cartilage, damage or degeneration is often permanent and so requires clinical intervention. Established surgical techniques generally rely on biological grafting. However, recent advances in polymeric materials provide an encouraging alternative to overcome limits of auto- and allografts. For regenerative engineering of cartilage, a polymeric scaffold ideally supports and instructs tissue regeneration while also providing mechanical integrity. Scaffolds direct regeneration via chemical and mechanical cues, as well as delivery and support of exogenous cells and bioactive factors. Advanced polymeric scaffolds aim to direct regeneration locally, replicating the heterogeneities of native tissues. Alternatively, new cartilage-mimetic hydrogels have potential to serve as synthetic cartilage replacements. Prepared as multi-network or composite hydrogels, the most promising candidates have simultaneously realized the hydration, mechanical, and tribological properties of native cartilage. Collectively, the recent rise in polymers for cartilage regeneration and replacement proposes a changing paradigm, with a new generation of materials paving the way for improved clinical outcomes.

Multi-energy dissipation mechanisms in supramolecular hydrogels with fast and slow relaxation modes

Reversible cross-links by non-covalent bonds have been widely used to produce supramolecular hydrogels that are both tough and functional. While various supramolecular hydrogels with several kinds of reversible cross-links have been designed for many years, a universal design that would allow control of mechanical and functional properties remains unavailable. The physical properties of reversible cross-links are usually quantified by thermodynamics, dynamics, and bond energies. Herein, we investigated the relationship between the molecular mobility and mechanical toughness of supramolecular hydrogels consisting of two kinetically distinct reversible cross-links via host-guest interactions. The molecular mobility was quantified as the second-order average relaxation time (〈τ〉_w) of the reversible cross-links. We discovered that hydrogels combining fast (〈τ〉_w = 1.8 or 18 s) and slowly (〈τ〉_w = 6.6 × 10³ or 9.5 × 10³ s) reversible cross-links showed increased toughness compared to hydrogels with only one type of cross-link because relaxation processes in the former occurred with wide timescales.

Alginate Fiber-Enhanced Poly(vinyl alcohol) Hydrogels with Superior Lubricating Property and Biocompatibility

The design of a novel interpenetrating network hydrogel inspired by the microscopic architecture of natural cartilage based on a supramolecular sodium alginate (SA) nanofibril network is reported in this paper. The mechanical strength and toughness of the poly(vinyl alcohol) (PVA) hydrogel were significantly improved after being incorporated with the alginate nanofibril network. The multiple hydrogen bonds between PVA chains and alginate fibers provided an efficient energy dissipation, thus leading to a significant increase in the mechanical strength of the PVA/SA/NaCl hydrogel. The PVA/SA/NaCl hydrogel demonstrated superior water-lubrication and load-bearing performance due to noncovalent interactions compared with pure PVA hydrogels. Moreover, the bioactivity of the PVA/SA/NaCl hydrogel was proved by the MC3T3 cell proliferation and viability assays over 7 days. Therefore, alginate fiber-enhanced hydrogels with high strength and low friction properties are expected to be used as novel biomimetic lubrication materials.

Cellulose Nanocrystals-Incorporated Thermosensitive Hydrogel for Controlled Release, 3D Printing, and Breast Cancer Treatment Applications

In situ-gel-forming thermoresponsive copolymers have been widely exploited in controlled delivery applications because their critical gel temperature is similar to human body temperature. However, there are limitations to controlling the delivery of biologics from a hydrogel network because of the poor networking and reinforcement between the copolymer networks. This study developed an in situ-forming robust injectable and 3D printable hydrogel network based on cellulose nanocrystals (CNCs) incorporated amphiphilic copolymers, poly(ε-caprolactone-co-lactide)-b-poly(ethylene glycol)-b-poly(ε-caprolactone-co-lactide (PCLA). In addition, the physicochemical and mechanical properties of injectable hydrogels were controlled by physically incorporating CNCs with amphiphilic PCLA copolymers. CNCs played an unprecedented role in physically reinforcing the PCLA copolymers' micelle network via intermicellar bridges. Apart from that, the free-flowing closely packed rod-like CNCs incorporated PCLA micelle networks at low temperature transformed to a stable viscoelastic hydrogel network at physiological temperature. CNC incorporated PCLA copolymer sols effectively coordinated with hydrophobic doxorubicin and water-soluble lysozyme by a combination of hydrophobic and hydrogen bonding interaction and controlled the release of biologics. As shown by the 3D printing results, the biocompatible PCLA hydrogels continuously extruded during printing had good injectability and maintained high shape fidelity after printing without any secondary cross-linking steps. The interlayer bonding between the printed layers was high and formed stable 3D structures up to 10 layers. Subcutaneous injection of free-flowing CNC incorporated PCLA copolymer sols to BALB/c mice formed a hydrogel instantly and showed controlled biodegradation of the hydrogel depot without induction of toxicity at the implantation sites or surrounding tissues. At the same time, the in vivo antitumor effect on the MDA-MB-231 tumor xenograft model demonstrated that DOX-loaded hydrogel formulation significantly inhibited the tumor growth. In summary, the CNC incorporated biodegradable hydrogels developed in this study exhibit a prolonged release with special release kinetics for hydrophobic and hydrophilic biologics.

Recent Advances in Macroporous Hydrogels for Cell Behavior and Tissue Engineering

Hydrogels have been extensively used as scaffolds in tissue engineering for cell adhesion, proliferation, migration, and differentiation because of their high-water content and biocompatibility similarity to the extracellular matrix. However, submicron or nanosized pore networks within hydrogels severely limit cell survival and tissue regeneration. In recent years, the application of macroporous hydrogels in tissue engineering has received considerable attention. The macroporous structure not only facilitates nutrient transportation and metabolite discharge but also provides more space for cell behavior and tissue formation. Several strategies for creating and functionalizing macroporous hydrogels have been reported. This review began with an overview of the advantages and challenges of macroporous hydrogels in the regulation of cellular behavior. In addition, advanced methods for the preparation of macroporous hydrogels to modulate cellular behavior were discussed. Finally, future research in related fields was discussed.

Direct Electrochemical Signaling in Organic Electrochemical Transistors Comprising High-Conductivity Double-Network Hydrogels

In composite hydrogels, the high electrical performance of poly(3,4-ethylenedioxythiophene) complexed with poly(styrenesulfonate) (PEDOT:PSS) is integrated with complementary structural and electrochemical functions via a rationally designed poly(acrylamide) second network incorporating phenylboronic acid (PBA). Free-standing double-network hydrogels prepared by a simple one-pot radical polymerization exhibit state-of-the-art electrical conductivity (∼20 S cm^-1 in phosphate buffered saline) while retaining a degree of hydration similar to that of biological soft tissues. Low resistance contacts to Au electrodes are formed via facile thermo-mechanical annealing and demonstrate stability over a month of continuous immersion, thus enabling hydrogels to serve as channels of organic electrochemical transistors (OECTs). Despite thicknesses of ∼100 μm, gating of hydrogel OECTs is efficient with transconductances g_m ∼ 40 mS and on/off ratios of 10³ in saturation mode operation, whereas sufficiently high conductivity enables linear mode operation (g_m ∼ 1 mS at -10 mV drain bias). This drives a shift of sensing strategy toward detection of electrochemical signals originating within the bulky channel. A kinetic basis for glucose detection via diol esterification on PBA is identified as the coupling of PBA equilibrium to electrocatalyzed O₂ reduction occurring on PEDOT in cathodic potentials. Hydrogel OECTs inherently amplify this direct electrochemical signal, demonstrating the viability of a new class of soft, structural biosensors.

Recent Advances in Design Strategies of Tough Hydrogels

Hydrogels are a fascinating class of materials popular in numerous fields, including tissue engineering, drug delivery, soft robotics, and sensors, attributed to their 3D network porous structure containing a significant amount of water. However, traditional hydrogels exhibit poor mechanical strength, limiting their practical applications. Thus, many researchers have focused on the development of mechanically enhanced hydrogels. This review describes the design considerations for constructing tough hydrogels and some of the latest strategies in recent years. These tough hydrogels have an up-and-coming prospect and bring great hope to the fields of biomedicine and others. Nonetheless, it is still no small challenge to realize hydrogel materials that are tough, multifunctional, intelligent, and zero-defect. This article is protected by copyright. All rights reserved.

Hydrogel Polyester Scaffolds via Direct-Ink-Writing of Ad Hoc Designed Photocurable Macromonomer

Synthetic, degradable macromonomers have been developed to serve as ink for 3D printing technologies based on direct-ink-writing. The macromonomers are purposely designed to be cross-linkable under the radical mechanism, to impart hydrophilicity to the final material, and to have rheological properties matching the printer’s requirements. The suitable viscosity enables the ink to be printed at room temperature, in absence of organic solvents, and to be cross-linked to manufacture soft 3D scaffolds that show no indirect cytotoxicity and have a hydration capacity of up to 100% their mass and a compressive modulus in the range of 0.4–2 MPa.

Super-Strong, Nonswellable, and Biocompatible Hydrogels Inspired by Human Tendons

Fabricating artificial materials that mimic the structures and properties of tendons is of great significance. Possessing a tensile stress of approximately 10.0 MPa and a water content of around 60%, human tendons exhibit excellent mechanical properties to support daily functions. In contrast to tendons, most synthetic hydrogels with similar water content typically exclude qualified strength, swelling resistance, and biocompatibility. Herein, a facile strategy based on poly(vinyl alcohol) (PVA) and tannic acid (TA) is demonstrated to tackle this problem via a combination of sequential steps including freezing-thawing PVA aqueous solutions to form crystalline regions, prestretching and air drying in confined conditions to induce anisotropic structures, soaking in TA solutions to form multiple hydrogen bondings between PVA and TA, and finally dialyzing against water for the removal of residual TA molecules and the rearrangements and homogenization of multiple hydrogen bonds. The obtained PVA hydrogels possess hierarchically anisotropic structures, where the alignment of PVA bundles promotes high modulus, while the hydrogen bonding between PVA and TA endows them with an energy dissipation mechanism. Benefitting from the synergy of material composition and structural engineering, the obtained hydrogel displays super-strong mechanics (a tensile stress of 19.3 MPa and a toughness of 32.1 MJ/m³), outperforming most tough hydrogels. Remarkably, this hydrogel demonstrates excellent swelling resistance. It barely expands after immersion in deionized water, phosphate-buffered saline (PBS), and SBF aqueous solutions for 7 days with the strength and volume nearly the same as their initial values. All of the features, combined with excellent cytocompatibility, make it an ideal material for biotechnological and biomedical applications.

Polyoxazoline Hydrogels fabricated by Stereolithography

The development of hydrogel materials in additive manufacturing displaying stiff and strong mechanical properties while maintaining high water uptake, remains a great challenge. Taking advantage of the versatility of poly(oxazoline)...

The Importance of Interfaces in Multi-Material Biofabricated Tissue Structures

Biofabrication exploits additive manufacturing techniques for creating 3D structures with a precise geometry that aim to mimic a physiological cellular environment and to develop the growth of native tissues. The most recent approaches of 3D biofabrication integrate multiple technologies into a single biofabrication platform combining different materials within different length scales to achieve improved construct functionality. However, the importance of interfaces between the different material phases, has not been adequately explored. This is known to determine material's interaction and ultimately mechanical and biological performance of biofabricated parts. In this review, this gap is bridged by critically examining the interface between different material phases in (bio)fabricated structures, with a particular focus on how interfacial interactions can compromise or define the mechanical (and biological) properties of the engineered structures. It is believed that the importance of interfacial properties between the different constituents of a composite material, deserves particular attention in its role in modulating the final characteristics of 3D tissue-like structures.

Anisotropic Chitosan Scaffolds Generated by Electrostatic Flocking Combined with Alginate Hydrogel Support Chondrogenic Differentiation

The replacement of damaged or degenerated articular cartilage tissue remains a challenge, as this non-vascularized tissue has a very limited self-healing capacity. Therefore, tissue engineering (TE) of cartilage is a promising treatment option. Although significant progress has been made in recent years, there is still a lack of scaffolds that ensure the formation of functional cartilage tissue while meeting the mechanical requirements for chondrogenic TE. In this article, we report the application of flock technology, a common process in the modern textile industry, to produce flock scaffolds made of chitosan (a biodegradable and biocompatible biopolymer) for chondrogenic TE. By combining an alginate hydrogel with a chitosan flock scaffold (CFS+ALG), a fiber-reinforced hydrogel with anisotropic properties was developed to support chondrogenic differentiation of embedded human chondrocytes. Pure alginate hydrogels (ALG) and pure chitosan flock scaffolds (CFS) were studied as controls. Morphology of primary human chondrocytes analyzed by cLSM and SEM showed a round, chondrogenic phenotype in CFS+ALG and ALG after 21 days of differentiation, whereas chondrocytes on CFS formed spheroids. The compressive strength of CFS+ALG was higher than the compressive strength of ALG and CFS alone. Chondrocytes embedded in CFS+ALG showed gene expression of chondrogenic markers (COL II, COMP, ACAN), the highest collagen II/I ratio, and production of the typical extracellular matrix such as sGAG and collagen II. The combination of alginate hydrogel with chitosan flock scaffolds resulted in a scaffold with anisotropic structure, good mechanical properties, elasticity, and porosity that supported chondrogenic differentiation of inserted human chondrocytes and expression of chondrogenic markers and typical extracellular matrix.

Fabrication of Cationic Poly(vinyl alcohol) Films Cross-Linked Using Copolymers Containing Quaternary Ammonium Cations, Benzoxaborole, and Carboxy Groups

Water-insoluble cationic poly(vinyl alcohol) (PVA) films were fabricated using a mixed aqueous solution of PVA and poly([2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC)-co-methacrylic acid (MAAc)-co-5-methacrylamido-1,2-benzoxaborole (MAAmBO)) copolymer (3D). The surface of the PVA film is typically negatively charged, and simple fabrication methods for water-insoluble PVA films with cationic surface charges are required to expand their application fields. METAC, which has a permanent positive charge owing to the presence of a quaternary ammonium cation, was selected as the cationic unit. The MAAc and MAAmBO units were used as two types of cross-linking structures for the thermal cross-linking of the hydroxy and carboxy groups of the MAAc unit (covalent bonding) as well as the diol and benzoxaborole groups of the MAAmBO unit (dynamic covalent bonding). The films were thermally cross-linked at 135 °C for 4 h without the addition of materials. After immersion in surplus water at 80 °C for 3 h, the cross-linked PVA/3D films retained almost 100% of their weights. The ζ-potential of the water-insoluble PVA/3D film was 9.4 ± 0.8 mV. The PVA/3D film was strongly dyed using anionic acid red 1 (AR1) because of its positively charged surface. Interestingly, it could also be slightly dyed using cationic methylene blue (MB) and became transparent (original state) after immersion in water for 2 days. These results suggested that positive and negative charges coexisted in the PVA/3D film, and the surface properties were positively inclined. Moreover, the degree of hemolysis of the PVA/3D films was similar to that of the negative control, which showed high blood compatibility. To our knowledge, this is the first report on the fabrication of water-insoluble cationic PVA films using two types of cross-linking structures containing carboxy and benzoxaborole groups. The cross-linked PVA films were analyzed using Fourier transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), and contact angle (CA) and ζ-potential measurement, as well as by determining the mechanical properties, adsorption of charged molecules, and biocompatibility. These readily fabricated water-insoluble PVA films with positive charges can show potential applications in sensors, adsorption systems, and antimicrobial materials.

Fundamentals of composites containing fibrous materials and hydrogels: A review on design and development for food applications

The combination of fiber and hydrogel in a system can provide substantial benefits for both components, including the development of three-dimensional structures for the fiber, followed by modifications in the rheological and mechanical properties of the hydrogel. Despite a large increase in the use of fiber-hydrogel composites (FHCs) in various sciences and industries such as biomedicine, tissue engineering, cosmetics, automotive, textile, and agriculture, there is limited information about FHCs in the realm of food application. In this regard, this study reviews the mechanism of FHCs. The force transmission between fiber and hydrogel, which depends on the interactions between them during loading, is the main reason to enhance the mechanical properties of FHCs. Moreover, articles about such FHCs that have the potential for foods or food industries have been described. Additionally, the information gaps about edible FHCs were highlighted for further research. Finally, the methods of fiber formation have been summarized.

Double network laminarin-boronic/alginate dynamic bioink for 3D bioprinting cell-laden constructs

The design of dynamically crosslinked hydrogel bioinks for three-dimensional (3D) bioprinting is emerging as a valuable strategy to advance the fabrication of mechanically tuneable cell-laden constructs for 3Din vitrodisease modelling and tissue engineering applications. Herein, a dynamic bioink comprising boronic acid-functionalised laminarin and alginate is explored for bioprinting 3D constructs under physiologically relevant conditions. The formulated bioink takes advantage of a double crosslinked network that combines covalent but reversible boronate ester bonds and ionic gelation via divalent cations. Moreover, it exhibits suitable rheological properties and improved mechanical features owing to its modular crosslinking chemistry, yielding stable constructs with user-programmable architecture. We explored such dynamic bioink as a supporting matrix for different cell classes, namely osteoblast precursors, fibroblasts and breast cancer cells. The resulting cell-laden bioprinted hydrogels display a homogeneous cell distribution post-printing and exceptional cell viability (>90%) that can be maintained for prolonged time periods in culture (14 days) for all cell lines. This simple and chemically versatile approach is envisaged to accelerate the development of multifunctional bioinks and contribute towards the fabrication of biomimetic 3D scaffolds with applicability in a wide range of predictive or exploratory biomedical platforms.

Implicit rule on the elastic function of a swollen polyacrylamide hydrogel

A full understanding of the elastic properties of hydrogels under swelling is required for their practical application in the chemical and biomedical engineering fields. This is because hydrogels are expected to retain water during mechanical use in moist atmospheres. In the present study, we investigated the relationship between the elastic modulus and the swelling ratio in a specific type of hydrogel (a polyacrylamide gel). The elasticity and swelling data revealed that these two parameters are proportionally related in hydrogels comprising adequate amounts of monomers and crosslinkers. We also demonstrated that this proportional relationship inherently conforms to the linear elastic behaviour predicted by the Flory-Rehner free energy function (the F-R model). The implicit rule is established by the extended F-R model with two scaling exponents. The extended model is capable of representing the irregular elasticity of swollen gels formed from low- or high-molecular-weight polymers.

Processing, mechanical properties and bio-applications of silk fibroin-based high-strength hydrogels

Hydrogels are an attractive class of materials that possess similar structural and functional characteristics to wet biological tissues and demonstrate a diversity of applications in biomedical engineering. Silk fibroin (SF) is a unique natural polymer due to its fibrous protein nature, versatile formats, biocompatibility, tunable biodegradation and is thus a good hydrogel candidate for bio-applications. Compared to synthetic polymer hydrogels, poor mechanical performance is still a fatal drawback that hinders the application of SF hydrogels as structural materials. Researchers have attempted to develop strategies to construct silk fibroin-based high-strength hydrogels (SF-HSHs). Herein, we firstly provide an overview of the approaches of processing SF-HSHs with a focus on the physical/non-covalent crosslinking mechanisms. The examples of SF-HSHs are discussed in detail under four categories, including physical-crosslinked, dual-crosslinked, double network and composite hydrogels respectively. A brief section follows to elucidate on the gelation mechanisms of SF-HSHs before a description of the utility of SF-HSHs in biomedicine and devices is presented. Finally, the potential challenges and future development of SF-HSHs are briefly discussed. This review aims to enhance our understanding of the structure-mechanical property-function relationships of soft materials made from natural polymers and guide future research of silk fibroin-based hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: Silk fibroin (SF) extracted from silk fibres is increasingly applied in the biomedical field, and SF hydrogel has been an emerging area for frontier bio-research. Since SF biopolymer has an intrinsic tendency to form regular β-sheet stacks, it can be processed into purely physically crosslinked hydrogels, thus avoiding the use of chemical crosslinkers. Nevertheless, akin to other natural polymers, lab-produced SF is variable (i.e. the molecular weight and distribution), and the gelation of SF hydrogel is challenging to control. In addition, hydrogels made from SF are usually weak and brittle, which hinders the wide use of this biofriendly and biodegradable hydrogel. Recently, there is a pressing need for high strength hydrogels from natural polymers for biomedical applications, and SF is proposed as a strong candidate. Therefore, we have studied the literature in the past 10 years and would like to focus on the gelation mechanism and mechanical strength of SF hydrogels for the review.