Structure Optimization and Mechanical Model for Microgel-Reinforced Hydrogels with High Strength and Toughness

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Constructing triple-network cellulose nanofiber hydrogels with excellent strength, toughness and conductivity for real-time monitoring of human movements

In recent years, there has been a lot of interest in developing composite hydrogels with superior mechanical and conductive properties. In this study, triple-network (TN) cellulose nanofiber hydrogels were prepared by using cellulose nanofiber as the first network, isotropic poly(acrylamide-co-acrylic acid) as the second network, and polyvinyl alcohol as the third network via a cyclic freezing-thawing process. The strong (9.43 ± 0.14 MPa tensile strength, (445.5 ± 7.0)% elongation-at-break), tough (15.12 ± 0.14 MJ/m3 toughness), and conductive (0.0297 ± 0.00021 S/cm ionic conductivity) TN cellulose nanofiber hydrogels were effectively created after being pre-stretched in an external force field, cross-linked by Fe3+ and added Li+. The produced composite TN cellulose nanofiber hydrogels were successfully used as a flexible sensor for real-time monitoring and detecting human movements, highlighting their potential for wearable electronics, medical technology, and human-machine interaction. CHEMICAL COMPOUNDS STUDIED IN THIS ARTICLE: Acrylamide (PubChem CID: 6579); Acrylic acid (PubChem CID: 6581); Ammonium persulfate (PubChem CID: 6579); N, N'-methylene bisacrylamide (PubChem CID: 17956053); Sodium bromide (PubChem CID: 253881); Sodium hydroxide (PubChem CID: 14798); Sodium hypochlorite (PubChem CID: 23665760); Sodium chlorite (PubChem CID: 23668197); 2,2,6,6-tetramethylpiperidinyl-1-oxide (PubChem CID: 2724126); Polyvinyl alcohol (PubChem CID: 11199); Lithium chloride (PubChem CID: 433294); Iron nitrate nonahydrate (PubChem CID: 129774236).

3D printed microstructured ultra-sensitive pressure sensors based on microgel-reinforced double network hydrogels for biomechanical applications

Hydrogel-based wearable flexible pressure sensors have great promise in human health and motion monitoring. However, it remains a great challenge to significantly improve the toughness, sensitivity and stability of hydrogel sensors. Here, we demonstrate the fabrication of hierarchically structured hydrogel sensors by 3D printing microgel-reinforced double network (MRDN) hydrogels to achieve both very high sensitivity and mechanical toughness. Polyelectrolyte microgels are used as building blocks, which are interpenetrated with a second network, to construct super tough hydrogels. The obtained hydrogels show a tensile strength of 1.61 MPa, and a fracture toughness of 5.08 MJ m-3 with high water content. The MRDN hydrogel precursors exhibit reversible gel-sol transitions, and serve as ideal inks for 3D printing microstructured sensor arrays with high fidelity and precision. The microstructured hydrogel sensors show an ultra-high sensitivity of 0.925 kPa-1, more than 50 times that of plain hydrogel sensors. The hydrogel sensors are assembled as an array onto a shoe-pad to monitor foot biomechanics during gaiting. Moreover, a sensor array with a well-arranged spatial distribution of sensor pixels with different microstructures and sensitivities is fabricated to track the trajectory of a crawling tortoise. Such hydrogel sensors have promising application in flexible wearable electronic devices.

A hierarchical porous aerohydrogel for enhanced water evaporation

Natural solar-powered steam generation provides a promising strategy to deal with deteriorating water resources. However, the practical applications of this strategy are limited by the tedious manufacturing of structures at micro-nano levels to concentrate heat and transport water to heat-localized regions. Herein, this work reports the fabrication of hierarchically porous aerohydrogel with enhanced light absorption and thermal localization at the air-solid interface. This aerohydrogel steam generator is fabricated by a simple yet controllable micropore generation approach to assemble air and hydrogel into hierarchically porous gas-solid hybrids. The tunable micropore size in a wide range from 99±49µm to 316±58μm not only enables contrasting sunlight absorptance (0.2 - 2.5µm) by reducing the reflection of solar light but also harnesses water transportation to the heating region via a capillary force-driven liquid flow. Therefore, a solar-vapor conversion efficiency of 91.3% under one sun irradiation was achieved using this aerohydrogel evaporator, reaching a ready evaporation rate of 2.76kg m-2 h-1 and 3.71kg m-2 h-1 under one and two sun irradiations, respectively. Our work provides a versatile and scalable approach to engineering porous hydrogels for highly efficient steam generation and opens an avenue for other potential practical applications based on this aerohydrogel.

Influence of the Degree of Swelling on the Stiffness and Toughness of Microgel-Reinforced Hydrogels

The stiffness and toughness of conventional hydrogels decrease with increasing degree of swelling. This behavior makes the stiffness-toughness compromise inherent to hydrogels even more limiting for fully swollen ones, especially for load-bearing applications. The stiffness-toughness compromise of hydrogels can be addressed by reinforcing them with hydrogel microparticles, microgels, which introduce the double network (DN) toughening effect into hydrogels. However, to what extent this toughening effect is maintained in fully swollen microgel-reinforced hydrogels (MRHs) is unknown. Herein, it is demonstrated that the initial volume fraction of microgels contained in MRHs determines their connectivity, which is closely yet nonlinearly related to the stiffness of fully swollen MRHs. Remarkably, if MRHs are reinforced with a high volume fraction of microgels, they stiffen upon swelling. By contrast, the fracture toughness linearly increases with the effective volume fraction of microgels present in the MRHs regardless of their degree of swelling. These findings provide a universal design rule for the fabrication of tough granular hydrogels that stiffen upon swelling and hence, open up new fields of use of these hydrogels.

A pH and Temperature Dual-Responsive Microgel-Embedded, Adhesive, and Tough Hydrogel for Drug Delivery and Wound Healing

Stimuli-responsive hydrogels have attracted much attention over the past decade for potential bioengineering applications such as wound dressing and drug delivery. In this work, a pH and temperature dual-responsive microgel-embedded hydrogel has been fabricated by incorporating poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-AAc) based microgel particles into polyacrylamide (PAAm)/chitosan (CS) semi-interpenetrating polymer network (semi-IPN), denoted as microgel@PAM/CS. The resultant hydrogel possesses excellent mechanical properties including stretchability, compressibility, and elasticity. In addition, the microgel@PAM/CS hydrogels can tightly adhere to the surfaces of a variety of tissues such as porcine skin, kidney, intestine, liver, and heart. Moreover, it shows controlled dual-drug release profile of both bovine serum albumin (BSA) (as a model protein) and sulfamethoxazole (SMZ), an antibiotic. Excellent antimicrobial properties are obtained for SMZ-loaded microgel@PAM/CS hydrogels. Compared with traditional drug administration methods such as by mouth, injection, and inhalation, the microgel@PAM/CS hydrogels possess advantages such as higher drug loading efficiency (by more than 80%) and controllable and sustained (over 48 h) release. The microgel@PAM/CS hydrogels can significantly enhance the wound healing process. This work provides a facile approach for the fabrication of multifunctional stimuli-responsive microparticle-embedded hydrogels with semi-IPN structures, and the as-prepared microgel@PAM/CS hydrogels have great potential for applications as smart wound dressing materials in biomedical engineering.

Ink Based on the Tunable Swollen Microsphere for a 3D Printing Hydrogel with Broad-Range Mechanical Properties

The development of the effective 3D printing strategy for diverse functional monomers is still challenging. Moreover, the conventional 3D printing hydrogels are usually soft and fragile due to the lack of an energy dissipation mechanism. Herein, a microsphere mediating ink preparation strategy is developed to provide tailored rheological behavior for various monomer direct ink writings. The chitosan microspheres are used as an exemplary material due to their tunable swelling ratio under the acid-drived electrostatic repulsion of the protonated amino groups. The rheological behaviors of the swollen chitosan microsphere (SCM) are independent on the monomer types, and various functional secondary polymers could be carried at a wide loading ratio by the acid driving. The SCM reinforces the hydrogel as the sacrificial bonds. With the adjustable composition, the 3D printing hydrogel mechanical properties are tunable in wide windows: strength (0.4-1.01 MPa), dissipated energy (0.11-3.25 MJ m^-3), and elongation at break (47-626%). With the excellent printing and mechanical properties, the SCM inks enable multi-functional integration for soft device production, such as 4D printing robots and wearable strain sensors. We anticipate that this microsphere mediating 3D printing strategy can inspire new possibilities for the design of the robust hydrogels with a broad range of functionalities and mechanical performances.

Biocompatible, Resilient, and Tough Nanocellulose Tunable Hydrogels

Hydrogels have been proposed as potential candidates for many different applications. However, many hydrogels exhibit poor mechanical properties, which limit their applications. Recently, various cellulose-derived nanomaterials have emerged as attractive candidates for nanocomposite-reinforcing agents due to their biocompatibility, abundance, and ease of chemical modification. Due to abundant hydroxyl groups throughout the cellulose chain, the grafting of acryl monomers onto the cellulose backbone by employing oxidizers such as cerium(IV) ammonium nitrate ([NH₄]₂[Ce(NO₃)₆], CAN) has proven a versatile and effective method. Moreover, acrylic monomers such as acrylamide (AM) may also polymerize by radical methods. In this work, cerium-initiated graft polymerization was applied to cellulose-derived nanomaterials, namely cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), in a polyacrylamide (PAAM) matrix to fabricate hydrogels that display high resilience (~92%), high tensile strength (~0.5 MPa), and toughness (~1.9 MJ/m³). We propose that by introducing mixtures of differing ratios of CNC and CNF, the composite's physical behavior can be fine-tuned across a wide range of mechanical and rheological properties. Moreover, the samples proved to be biocompatible when seeded with green fluorescent protein (GFP)-transfected mouse fibroblasts (3T3s), showing a significant increase in cell viability and proliferation compared to samples comprised of acrylamide alone.

High-Sensitivity Flexible Sensor Based on Biomimetic Strain-Stiffening Hydrogel

Recently, flexible wearable and implantable electronic devices have attracted enormous interest in biomedical applications. However, current bioelectronic systems have not solved the problem of mechanical mismatch of tissue-electrode interfaces. Therefore, the biomimetic hydrogel with tissue-like mechanical properties is highly desirable for flexible electronic devices. Herein, we propose a strategy to fabricate a biomimetic hydrogel with strain-stiffening property via regional chain entanglements. The strain-stiffening property of the biomimetic hydrogel is realized by embedding highly swollen poly(acrylate sodium) microgels to act as the microregions of dense entanglement in the soft polyacrylamide matrix. In addition, poly(acrylate sodium) microgels can release Na⁺ ions, endowing hydrogel with electrical signals to serve as strain sensors for detecting different human movements. The resultant sensors own a low Young's modulus (22.61-112.45 kPa), high nominal tensile strength (0.99 MPa), and high sensitivity with a gauge factor up to 6.77 at strain of 300%. Based on its simple manufacture process, well mechanical matching suitability, and high sensitivity, the as-prepared sensor might have great potential for a wide range of large-scale applications such as wearable and implantable electronic devices.

Microgel reinforced zwitterionic hydrogel coating for blood-contacting biomedical devices

Zwitterionic hydrogels exhibit eminent nonfouling and hemocompatibility. Several key challenges hinder their application as coating materials for blood-contacting biomedical devices, including weak mechanical strength and low adhesion to the substrate. Here, we report a poly(carboxybetaine) microgel reinforced poly(sulfobetaine) (pCBM/pSB) pure zwitterionic hydrogel with excellent mechanical robustness and anti-swelling properties. The pCBM/pSB hydrogel coating was bonded to the PVC substrate via the entanglement network between the pSB and PVC chain. Moreover, the pCBM/pSB hydrogel coating can maintain favorable stability even after 21 d PBS shearing, 0.5 h strong water flushing, 1000 underwater bends, and 100 sandpaper abrasions. Notably, the pCBM/pSB hydrogel coated PVC tubing can not only mitigate the foreign body response but also prevent thrombus formation ex vivo in rats and rabbits blood circulation without anticoagulants. This work provides new insights to guide the design of pure zwitterionic hydrogel coatings for biomedical devices.

Does the Size of Microgels Influence the Toughness of Microgel-Reinforced Hydrogels?

Rapid advances in the biomedical field increasingly often demand soft materials that can be processed into complex 3D shapes while being able to reliably bear significant loads. Granular hydrogels have the potential to serve as artificial tissues because they can be 3D printed into complex 3D shapes and their composition can be tuned over short length scales. Unfortunately, granular hydrogels are typically soft such that they cannot be used for load-bearing applications. To address this shortcoming, individual microgels can be connected through a percolating network, such that they introduce the double network toughening mechanism into granular hydrogels. However, the influence of the microgel size and concentration on the processing and toughness of microgel-reinforced hydrogels (MRHs) remains to be elucidated. Here, we demonstrate that processing and toughness depend on the inter-microgel connectivity, while the stress at break is solely dependent on the microgel size. These findings offer an in-depth understanding of how liquid- and paste-like precursors containing soft, deformable microgels can be processed into bulk microstructured soft materials and the effect of the size and concentration of these microgels on the mechanical properties of microgel reinforced hydrogels. This article is protected by copyright. All rights reserved.

Recycling of Load-Bearing 3D Printable Double Network Granular Hydrogels

Sustainable materials, such as recyclable polymers, become increasingly important as they are often environmentally friendlier than their one-time-use counterparts. In parallel, the trend toward more customized products demands for fast prototyping methods which allow processing materials into 3D objects that are often only used for a limited amount of time yet, that must be mechanically sufficiently robust to bear significant loads. Soft materials that satisfy the two rather contradictory needs remain to be shown. Here, the authors introduce a material that simultaneously fulfills both requirements, a 3D printable, recyclable double network granular hydrogel (rDNGH). This hydrogel is composed of poly(2-acrylamido-2-methylpropane sulfonic acid) microparticles that are covalently crosslinked through a disulfide-based percolating network. The possibility to independently degrade the percolating network, with no harm to the primary network contained within the microgels, renders the recovery of the microgels efficient. As a result, the recycled material pertains a stiffness and toughness that are similar to those of the pristine material. Importantly, this process can be extended to the fabrication of recyclable hard plastics made of, for example, dried rDNGHs. The authors envision this approach to serve as foundation for a paradigm shift in the design of new sustainable soft materials and plastics.

Muscle-inspired double-network hydrogels with robust mechanical property, biocompatibility and ionic conductivity

Inspired by muscle architectures, double network hydrogels with hierarchically aligned structures were fabricated, where cross-linked cellulose nanofiber (CNF)/chitosan hydrogel threads obtained by interfacial polyelectrolyte complexation spinning were collected in alignment as the first network, while isotropic poly(acrylamide-co-acrylic acid) (PAM-AA) served as the second network. After further cross-linking using Fe³⁺, the hydrogel showed an outstanding mechanical performance, owing to effective energy dissipation of the oriented asymmetric double networks. The average strength and elongation-at-break of PAM-AA/CNF/Fe³⁺ hydrogel were 11 MPa and 480 % respectively, which the strength was comparative to that of biological tissues. The aligned CNFs in the hydrogels provided probable ion transport channels, contributing to the high ionic conductivity, which was up to 0.022 S/cm when the content of LiCl was 1.5 %. Together with superior biocompatibility, the well-ordered hydrogel showed a promising potential in biological applications, such as artificial soft tissue materials and muscle-like sensors for human motion monitoring.

Tough, permeable and biocompatible microfluidic devices formed through the buckling delamination of soft hydrogel films

Microchannels in soft materials play an important role in developing movable, deformable, and biocompatible fluidic systems for applications in various fields. Intensively investigated approaches to create microscale channel architectures use mechanical instability in soft materials, which can provide intricate yet ordered architectures with low cost and high throughput. Here, for microchannel fabrication, we demonstrate the use of swelling-driven buckle delamination of hydrogels, which is a mechanical instability pattern found in compressed film/substrate layer composites. By spatially controlling interfacial bonding between a thin polyacrylamide (PAAm) gel film and glass substrate, swelling-driven compressive stress induces buckle delamination at programmed positions, resulting in the formation of continuous hollow paths as microchannels. Connecting flow tubes with a 3D-printed connecter provides a deformable microfluidic device, enabling pressure-driven flows without leakage from the connecter and rupture of the channels. Furthermore, by stacking less-swellable bulk gels on the device, we obtained a tough, permeable, and biocompatible microfluidic device. Finally, we performed a cell culture on the device and chemical stimulation to cells through the diffusion of molecules from the microchannels. The results of this work shed light on designing pressure sensitive/resistant microfluidic systems based on diverse hydrogels with intricate 3D morphologies and will be useful for applications in the fields of bioanalysis, biomimetics, tissue engineering, and cell biology.

Graphene Oxide-Doped Gellan Gum-PEGDA Bilayered Hydrogel Mimicking the Mechanical and Lubrication Properties of Articular Cartilage

Articular cartilage (AC) is a specialized connective tissue able to provide a low-friction gliding surface supporting shock-absorption, reducing stresses, and guaranteeing wear-resistance thanks to its structure and mechanical and lubrication properties. Being an avascular tissue, AC has a limited ability to heal defects. Nowadays, conventional strategies show several limitations, which results in ineffective restoration of chondral defects. Several tissue engineering approaches have been proposed to restore the AC's native properties without reproducing its mechanical and lubrication properties yet. This work reports the fabrication of a bilayered structure made of gellan gum (GG) and poly (ethylene glycol) diacrylate (PEGDA), able to mimic the mechanical and lubrication features of both AC superficial and deep zones. Through appropriate combinations of GG and PEGDA, cartilage Young's modulus is effectively mimicked for both zones. Graphene oxide is used as a dopant agent for the superficial hydrogel layer, demonstrating a lower friction than the nondoped counterpart. The bilayered hydrogel's antiwear properties are confirmed by using a knee simulator, following ISO 14243. Finally, in vitro tests with human chondrocytes confirm the absence of cytotoxicity effects. The results shown in this paper open the way to a multilayered synthetic injectable or surgically implantable filler for restoring AC defects.

Engineering Hydrogel Adhesion for Biomedical Applications via Chemical Design of the Junction

Hydrogel adhesion inherently relies on engineering the contact surface at soft and hydrated interfaces. Upon contact, adhesion normally occurs through the formation of chemical or physical interactions between the disparate surfaces. The ability to form these adhesion junctions is challenging for hydrogels as the interfaces are wet and deformable and often contain low densities of functional groups. In this Review, we link the design of the binding chemistries or adhesion junctions, whether covalent, dynamic covalent, supramolecular, or physical, to the emergent adhesive properties of soft and hydrated interfaces. Wet adhesion is useful for bonding to or between tissues and implants for a range of biomedical applications. We highlight several recent and emerging adhesive hydrogels for use in biomedicine in the context of efficient junction design. The main focus is on engineering hydrogel adhesion through molecular design of the junctions to tailor the adhesion strength, reversibility, stability, and response to environmental stimuli.

Tough hybrid microgel-reinforced hydrogels dependent on the size and modulus of the microgels

Microgel-reinforced (MR) hydrogels are tough hydrogels with dispersed rigid microgels embedded in a continuous soft matrix. MR gels have the great potential to provide not only mechanical toughness but also the desired functional matrix by incorporation of various functional microgels. Understanding the toughening mechanism of the MR hydrogels is critical for the rational design of the desired functionally tough MR gels. However, our current knowledge of the toughening mechanism of MR gels mainly comes from the MR hydrogels with both chemically crosslinked dispersed microgels and a continuous matrix. Little is known about the hybrid MR gels with physically crosslinked microgels embedded in a chemically crosslinked matrix. Herein, we synthesize such hybrid MR hydrogels with the ionic crosslinked calcium alginate microgels incorporated into the chemically crosslinked polyacrylamide (PAAm) matrix. The alginate microgels show strong size and modulus effects on the toughening enhancement: the larger microgels could toughen the MR gels more than the small ones, and the microgels with medium modulus could maximize the toughness of the MR gels. By comparison of the mechanical performances of the MR and the corresponding double network (DN) hydrogels, we have proposed that the hybrid MR gels may have the same toughening mechanism as the bulk DN gel. This work tries to better understand the structure-property relationships of both MR and DN gels and help in the design of more functionally tough MR gels with the desired properties.

Preparations of Tough and Conductive PAMPS/PAA Double Network Hydrogels Containing Cellulose Nanofibers and Polypyrroles

To afford an intact double network (sample abbr.: DN) hydrogel, two-step crosslinking reactions of poly(2-acrylamido-2-methylpropanesulfonic acid) (i.e., PAMPS first network) and then poly(acrylic acid) (i.e., PAA second network) were conducted both in the presence of crosslinker (N,N'-methylenebisacrylamide (MBAA)). Similar to the two-step processes, different contents of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) oxidized cellulose nanofibers (TOCN: 1, 2, and 3 wt.%) were initially dispersed in the first network solutions and then crosslinked. The TOCN-containing PAMPS first networks subsequently soaked in AA and crosslinker and conducted the second network crosslinking reactions (TOCN was then abbreviated as T for DN samples). As the third step, various (T-)DN hydrogels were then treated with different concentrations of FeCl3(aq) solutions (5, 50, 100, and 200 mM). Through incorporations of ferric ions into (T-)DN hydrogels, notably, three purposes are targeted: (i) strengthen the (T-)DN hydrogels through ionic bonding, (ii) significantly render ionic conductivity of hydrogels, and (iii) serve as a catalyst for the forth step to proceed with in situ chemical oxidative polymerizations of pyrroles to afford polypyrrole-containing (sample abbr.: Py) hydrogels [i.e., (T-)Py-DN samples]. The characteristic functional groups of PAMPS, PAA, and Py were confirmed by FT-IR. Uniform microstructures were observed by cryo scanning electron microscopy (cryo-SEM). These results indicated that homogeneous composites of T-Py-DN hydrogels were obtained through the four-step process. All dry samples showed similar thermal degradation behaviors from the thermogravimetric analysis (TGA). The T2-Py5-DN sample (i.e., containing 2 wt.% TOCN with 5 mM FeCl3(aq) treatment) showed the best tensile strength and strain at breaking properties (i.e., σTb = 450 kPa and εTb = 106%). With the same compositions, a high conductivity of 3.34 × 10-3 S/cm was acquired. The tough T2-Py5-DN hydrogel displayed good conductive reversibility during several "stretching-and-releasing" cycles of 50-100-0%, demonstrating a promising candidate for bioelectronic or biomaterial applications.

Specialty Tough Hydrogels and Their Biomedical Applications

Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted. While challenges remain before these specialty tough hydrogels will be deemed translationally acceptable for clinical applications, promising preliminary results undoubtedly spur great hope in the potential impact this embryonic research field can have on the biomedical community.

Multiply Interpenetrating Polymer Networks: Preparation, Mechanical Properties, and Applications

This review summarizes work done on triply, or higher, interpenetrating polymer network materials prepared in order to widen the properties of double polymer network hydrogels (DN), doubly interpenetrating polymer networks with enhanced mechanical properties. The review will show that introduction of a third, or fourth, polymeric component in the DNs would further enhance the mechanical properties of the resulting materials, but may also introduce other useful functionalities, including electrical conductivity, low-friction coefficients, and (bio)degradability.

Highly compressive and stretchable poly(ethylene glycol) based hydrogels synthesised using pH-responsive nanogels without free-radical chemistry

Poly(ethylene glycol) (PEG) based hydrogels are amongst the most studied synthetic hydrogels. However, reports on PEG-based hydrogels with high mechanical strength are limited. Herein, a class of novel, well-defined PEG-based nanocomposite hydrogels with tunable mechanical strength are synthesised via ring-opening reactions of diglycidyl ethers with carboxylate ions. The pH responsive crosslinked polyacid nanogels (NG) in the dispersed phase act as high functionality crosslinkers which covalently bond to the poly(ethylene glycol) diglycidyl ethers (PEGDGE) as the continuous matrix. A series of NG-x-PEG-y-z gels are prepared where x, y and z are concentrations of NGs, PEGDGE and the PEGDGE molecular weight, respectively. The hydrogel compositions and nano-structural homogeneity of the NGs have strong impact on the enhancement of mechanical properties which enables property tuning. Based on this design, a highly compressive PEG-based nanocomposite hydrogel (NG-13-PEG-20-6000) exhibits a compressive stress of 24.2 MPa, compressive fracture strain greater than 98% and a fracture energy density as high as 1.88 MJ m-3. The tensile fracture strain is 230%. This is amongst one of the most compressive PEG-based hydrogels reported to-date. Our chemically crosslinked gels are resilient and show highly recoverable dissipative energy. The cytotoxicity test shows that human nucleus pulposus (NP) cells remained viable after 8 days of culture time. The overall results highlight their potential for applications as replacements for intervertebral discs or articular cartilages.

High-Strength and Tough Cellulose Hydrogels Chemically Dual Cross-Linked by Using Low- and High-Molecular-Weight Cross-Linkers

Hydrogels are the focus of extensive research interests due to their potential application in the fields of biomedical materials, biosensors, agriculture, and cosmetics. Natural polysaccharide is one of the good candidates of these hydrogels. However, weak mechanical properties of cellulose hydrogels greatly limit their practical application. Here, chemically dual-cross-linked cellulose hydrogels (DCHs) were constructed by sequential reaction of cellulose with low- and high-molecular-weight cross-linkers to obtain relatively short chains and long chains cross-linked networks. Both the distribution and density of the cross-linking domains in the hydrogel networks were monitored by three-dimensional Raman microscopic imaging technique. Interestingly, the ruptured stress of DCHs in tensile and compressive tests were 1.7 and 9.4 MPa, which were 26.3- and 83.9-fold larger than those of chemically single-cross-linked cellulose hydrogel. The reinforcement mechanism of DCH was proposed, as the breaking of the short-chain cross-linking in the networks effectively dissipated mechanical energy, and the extensibility of the relatively long-chain cross-linking maintained the elasticity of DCH. Therefore, both the strength and toughness of DCH was enhanced, and the dual networks consisting of short-chain and long-chain cross-linking played an important role in the improvement of the mechanical properties of the cellulose hydrogels. The application prospect of the robust cellulose hydrogels with bimodal network structure would be greatly broadened in the sustainable biopolymer fields.

Methods for producing microstructured hydrogels for targeted applications in biology

Hydrogels have been broadly studied for applications in clinically motivated fields such as tissue regeneration, drug delivery, and wound healing, as well as in a wide variety of consumer and industry uses. While the control of mechanical properties and network structures are important in all of these applications, for regenerative medicine applications in particular, matching the chemical, topographical and mechanical properties for the target use/tissue is critical. There have been multiple alternatives developed for fabricating materials with microstructures with goals of controlling the spatial location, phenotypic evolution, and signaling of cells. The commonly employed polymers such as poly(ethylene glycol) (PEG), polypeptides, and polysaccharides (as well as others) can be processed by various methods in order to control material heterogeneity and microscale structures. We review here the more commonly used polymers, chemistries, and methods for generating microstructures in biomaterials, highlighting the range of possible morphologies that can be produced, and the limitations of each method. With a focus in liquid-liquid phase separation, methods and chemistries well suited for stabilizing the interface and arresting the phase separation are covered. As the microstructures can affect cell behavior, examples of such effects are reviewed as well. STATEMENT OF SIGNIFICANCE: Heterogeneous hydrogels with enhanced matrix complexity have been studied for a variety of biomimetic materials. A range of materials based on poly(ethylene glycol), polypeptides, proteins, and/or polysaccharides, have been employed in the studies of materials that by virtue of their microstructure, can control the behaviors of cells. Methods including microfluidics, photolithography, gelation in the presence of porogens, and liquid-liquid phase separation, are presented as possible strategies for producing materials, and their relative advantages and disadvantages are discussed. We also describe in more detail the various processes involved in LLPS, and how they can be manipulated to alter the kinetics of phase separation and to yield different microstructured materials.

Step-growth Production of Nanogels for Use as Macromers with Dimethacrylate Monomers

A series of functional nanogels were synthesized by a step-growth mechanism that involved diisocyanate addition to a modest stoichiometric excess of multi-thiols. Nanogels with sizes less than 10 nm were obtained as room temperature liquids with residual thiol groups used to attach methacrylate functionality. Depending on nanogel structure, bulk nanogel properties varied widely, as did the properties of the nanogel-derived and nanogel-modified polymers. Photopolymerization of the reactive nanogels in combination with a dimethacrylate monomer showed dramatically enhanced reaction rate and conversion compared with the dimethacrylate homopolymer. Polymerization shrinkage/ stress as well as mechanical properties of the polymer networks were controlled by changing the ratio of nanogels and dimethacrylate monomers used in formulations. Thus, this study shows the potential of step-growth nanogels for beneficial changes in resin reactivity and application-based performance.

Design of photoinitiator-functionalized hydrophilic nanogels with uniform size and excellent biocompatibility

Three well-controlled biocompatible hydrophilic nanogels were synthesized, and they can effectively initiate photopolymerization and improve the mechanical properties of polymers.

Inorganic/Organic Double-Network Ion Gels with Partially Developed Silica-Particle Network

Tough inorganic/organic composite network gels consisting of a partially developed silica-particle network and a large amount of an ionic liquid, named micro-double-network (μ-DN) ion gel, are fabricated via two methods. One is a one-pot/one-step process conducted using a simultaneous network formation via sol-gel reaction of tetraethyl orthosilicate and free radical polymerization of N, N-dimethylacrylamide in an ionic liquid. When the network formation rates of the inorganic and organic networks are almost the same, the μ-DN structure is formed. The second method is simpler and involved the use of silica nanoparticles as the starting material. By controlling the dispersion state of the silica nanoparticles in an ionic liquid, the μ-DN structure is formed. In both μ-DN ion gels, silica nanoparticles partially aggregate and form network-like clusters. When a large deformation is induced in the μ-DN ion gels, the silica-particle clusters rupture and dissipate the loaded energy. The fracture stress and Young's modulus of the μ-DN ion gel increase as the size of the silica nanoparticles decreases. The increment in the mechanical strength would have been caused by the increase in the total van der Waals attraction forces and the total number of hydrogen bonding in the silica-particle networks.

Microstructured Elastomer-PEG Hydrogels via Kinetic Capture of Aqueous Liquid-Liquid Phase Separation

Heterogeneous hydrogels with desired matrix complexity are studied for a variety of biomimetic materials. Despite the range of such microstructured materials described, few methods permit independent control over microstructure and microscale mechanics by precisely controlled, single-step processing methods. Here, a phototriggered crosslinking methodology that traps microstructures in liquid-liquid phase-separated solutions of a highly elastomeric resilin-like polypeptide (RLP) and poly(ethylene glycol) (PEG) is reported. RLP-rich domains of various diameters can be trapped in a PEG continuous phase, with the kinetics of domain maturation dependent on the degree of acrylation. The chemical composition of both hydrogel phases over time is assessed via in situ hyperspectral coherent Raman microscopy, with equilibrium concentrations consistent with the compositions derived from NMR-measured coexistence curves. Atomic force microscopy reveals that the local mechanical properties of the two phases evolve over time, even as the bulk modulus of the material remains constant, showing that the strategy permits control of mechanical properties on micrometer length scales, of relevance in generating mechanically robust materials for a range of applications. As one example, the successful encapsulation, localization, and survival of primary cells are demonstrated and suggest the potential application of phase-separated RLP-PEG hydrogels in regenerative medicine applications.