Simultaneous Enhancement of Stiffness and Toughness in Hybrid Double-Network Hydrogels via the First, Physically Linked Network

Natural polymer starch-based materials for flexible electronic sensor development: A review of recent progress

In response to the burgeoning interest in the development of highly conformable and resilient flexible electronic sensors capable of transducing diverse physical stimuli, this review investigates the pivotal role of natural polymers, specifically those derived from starch, in crafting sustainable and biocompatible sensing materials. Expounding on cutting-edge research, the exploration delves into innovative strategies employed to leverage the distinctive attributes of starch in conjunction with other polymers for the fabrication of advanced sensors. The comprehensive discussion encompasses a spectrum of starch-based materials, spanning all-starch-based gels to starch-based soft composites, meticulously scrutinizing their applications in constructing resistive, capacitive, piezoelectric, and triboelectric sensors. These intricately designed sensors exhibit proficiency in detecting an array of stimuli, including strain, temperature, humidity, liquids, and enzymes, thereby playing a pivotal role in the continuous and non-invasive monitoring of human body motions, physiological signals, and environmental conditions. The review highlights the intricate interplay between material properties, sensor design, and sensing performance, emphasizing the unique advantages conferred by starch-based materials, such as self-adhesiveness, self-healability, and re-processibility facilitated by dynamic bonding. In conclusion, the paper outlines current challenges and future research opportunities in this evolving field, offering valuable insights for prospective investigations.

A co-assembly process for high strength and injectable dual network gels with sustained doxorubicin release performance

Adopting a non-covalent co-assembly strategy shows great potential in loading drugs efficiently and safely in drug delivery systems. However, finding an efficient method for developing high strength gels with thixotropic characteristics is still challenging. In this work, by hybridizing the low molecular weight gelator fluorenylmethyloxycarbonyl-phenylalanine (Fmoc-F) (first single network, 1st SN) and alginate (second single network, 2nd SN) into a dual network (DN) gel, gels with high strength as well as thixotropy were prepared efficiently. The DN gels showed high strength (103 Pa in SN gels and 105 Pa in DN gels) and thixotropic characteristics (yield strain <25%; recovery ratio >85% within 100 seconds). The application performance was verified by loading doxorubicin (DOX), showing better encapsulation capacity (77.06% in 1st SN, 59.11% in 2nd SN and 96.71% in DN) and sustained release performance (lasting one week under physiological conditions) than single network gels. Experimental and DFT results allowed the elaboration of the specific non-covalent co-assembly mechanism for DN gel formation and DOX loading. The DN gels were formed by co-assembly driven by H-bond and π-π stacking interactions and then strengthened by Ca2+-coupling. Most DOX molecules co-assembled with Fmoc-F and alginate through π-π stacking and H-bond interactions (DOX-I), with a few free DOX molecules (DOX-II) left. Proven by the release dynamics test, DOX was released through a diffusion-erosion process, in an order of DOX-I first and then DOX-II. This work suggests that non-covalent co-assembly is a useful technique for effective material strengthening and drug delivery.

An Innovative High-Strength Double-Network Hydrogel for Use as a Drilling Fluid Plugging Agent

The problem of wellbore leakage is a key challenge in the petroleum industry, limiting drilling progress and increasing drilling costs. Plugging agents play a role in repairing leaks and fractures; however, traditional plugging materials generally have low mechanical strength, poor adaptability to permeable strata, limited water absorption and expansion capabilities, and poor temperature and salt resistance. To address these limitations, a pioneering polyacrylic acid-polyacrylamide (PAA/PAM) double-network hydrogel was synthesized through aqueous solution polymerization in this study. Its strength, water absorption, expansion, temperature resistance, salt resistance, and plugging effectiveness were comprehensively evaluated. The results demonstrate that good mechanical performance is exhibited by the synthesized hydrogel, capable of withstanding a maximum stress of approximately 3.5 MPa at a 90% strain. Excellent water absorption and expansion are observed in the synthesized double-network hydrogel, with a maximum expansion of 6 times within 30 min and 8 times after 2 h. Test results show that the hydrogel had good temperature resistance and salt resistance, maintaining a strength grade E within the experimental range. The simulated evaluation of the plugging experiment indicates that, under conditions of 130 °C and 6 MPa, the leakage rate of the drilling fluid is maintained below 5 mL/min when the double-network hydrogel is utilized. From the above experimental results, it can be illustrated that excellent mechanical properties, impressive water absorption, and expansion capabilities are exhibited by the synthesized double-network hydrogel. Furthermore, the high-temperature resistance and salt resistance of the double-network hydrogel were also demonstrated. Therefore, In comparison to traditional plugging materials, significant promise is held by this newly synthesized double-network hydrogel material as a plugging agent in drilling fluids.

Fabrication of Soft Transparent Patient-Specific Vascular Models with Stereolithographic 3D printing and Thiol-Based Photopolymerizable Coatings

An ideal vascular phantom should be anatomically accurate, have mechanical properties as close as possible to the tissue, and be sufficiently transparent for ease of visualization. However, materials that enable the convergence of these characteristics have remained elusive. The fabrication of patient-specific vascular phantoms with high anatomical fidelity, optical transparency, and mechanical properties close to those of vascular tissue is reported. These final properties are achieved by 3D printing patient-specific vascular models with commercial elastomeric acrylic-based resins before coating them with thiol-based photopolymerizable resins. Ternary thiol-ene-acrylate chemistry is found optimal. A PETMP/allyl glycerol ether (AGE)/polyethylene glycol diacrylate (PEGDA) coating with a 30/70% AGE/PEGDA ratio applied on a flexible resin yielded elastic modulus, UTS, and elongation of 3.41 MPa, 1.76 MPa, and 63.2%, respectively, in range with the human aortic wall. The PETMP/AGE/PEGDA coating doubled the optical transmission from 40% to 80%, approaching 88% of the benchmark silicone-based elastomer. Higher transparency correlates with a decrease in surface roughness from 2000 to 90 nm after coating. Coated 3D-printed anatomical replicas are showcased for pre-procedural planning and medical training with good radio-opacity and echogenicity. Thiol-click chemistry coatings, as a surface treatment for elastomeric stereolithographic 3D-printed objects, address inherent limitations of photopolymer-based additive manufacturing.

Synthesis of magnetic sodium lignosulfonate hydrogel(Fe3O4@LS) and its adsorption behavior for Cd2+ in wastewater

Heavy metal pollution is becoming increasingly serious. Heavy metal pollutants are nonbiodegradable and can be bioenriched through the food chain, and thus, they greatly threaten the environment and human health. Hydrogels, as an ideal adsorbent, have been widely used to treat heavy metal industrial wastewater. Sodium lignosulfonate hydrogel (LS) was prepared by free-radical grafting copolymerization, and nano-Fe3O4 particles were loaded in LS by an in-situ precipitation method (Fe3O4@LS). The magnetic properties and adsorption capacity of Fe3O4@LS are closely related to the load capacity of Fe3O4. XRD, FTIR, XPS, SEM, TEM, BET, and TGA analyses of the materials were performed. Subsequently, the removal effect of the typical pollutant Cd2+ in heavy metal-polluted water was studied with Fe3O4@LS as the adsorbent. The influences of the Fe3O4@LS dosage and initial pH were investigated, and the adsorption kinetics and thermodynamics were further explored and discussed. Finally, the adsorption mechanism of Fe3O4@LS on Cd2+ was obtained. Results show that Fe3O4@LS has a more stable spatial network structure than LS, and the pore size, specific surface area and active sites increase. The maximum adsorption capacity can reach 88.00 mg/g when pH = 6 and the dosage of Fe3O4@LS is 1000 mg/L. The adsorption of Cd2+ by Fe3O4@LS conforms to pseudosecond-order kinetics and the Temkin isothermal adsorption model. Further mechanistic investigations show that the sorption of Cd2+ on Fe3O4@LS is mainly attributed to surface complexation, electrostatic attraction and coprecipitation. The coexistence of cations in water will inhibit the adsorption of Fe3O4@LS. Fe3O4@LS has superparamagnetism and a good response to an external magnetic field. The adsorption rate can still reach >60 % after four elutions with NaCl as the eluent. This material can be reused and has good application potential.

Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS)

We report a new method to shape double-network (DN) hydrogels into customized 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermoreversible sol-gel κ-carrageenan with a suitable cross-linker and photoinitiators/absorbers is optimized. A new TOPS system is utilized to photopolymerize the primary acrylamide network into a 3D structure above the sol-gel transition of κ-carrageenan (80 °C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37 μm) and vertical (180 μm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400%, respectively, under tension and simultaneously exhibit a high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.

Specimen Geometry Effect on Experimental Tensile Mechanical Properties of Tough Hydrogels

Synthetic tough hydrogels have received attention because they could mimic the mechanical properties of natural hydrogels, such as muscle, ligament, tendon, and cartilage. Many recent studies suggest various approaches to enhance the mechanical properties of tough hydrogels. However, directly comparing each hydrogel property in different reports is challenging because various testing specimen shapes/sizes were employed, affecting the experimental mechanical property values. This study demonstrates how the specimen geometry-the lengths and width of the reduced section-of a tough double-network hydrogel causes differences in experimental tensile mechanical values. In particular, the elastic modulus was systemically compared using eleven specimens of different shapes and sizes that were tensile tested, including a rectangle, ASTM D412-C and D412-D, JIS K6251-7, and seven customized dumbbell shapes with various lengths and widths of the reduced section. Unlike the rectangular specimen, which showed an inconsistent measurement of mechanical properties due to a local load concentration near the grip, dumbbell-shaped specimens exhibited a stable fracture at the reduced section. The dumbbell-shaped specimen with a shorter gauge length resulted in a smaller elastic modulus. Moreover, a relationship between the specimen dimension and measured elastic modulus value was derived, which allowed for the prediction of the experimental elastic modulus of dumbbell-shaped tough hydrogels with different dimensions. This study conveys a message that reminds the apparent experimental dependence of specimen geometry on the stress-strain measurement and the need to standardize the measurement of of numerous tough hydrogels for a fair comparison.

Three-Dimensional Printing of Double-Network Hydrogels: Recent Progress, Challenges, and Future Outlook

Hydrogels are widely used materials due to their biocompatibility, their ability to mimic the hydrated and porous extracellular microenvironment, as well as their ability to tune both mechanical and biochemical properties. However, most hydrogels lack mechanical toughness, and shaping them into complicated three-dimensional (3D) structures remains challenging. In the past decade, tough and stretchable double-network hydrogels (DN gels) were developed for tissue engineering, soft robotics, and applications that require a combination of high-energy dissipation and large deformations. Although DN gels were processed into simple shapes by using conventional casting and molding methods, new 3D printing methods have enabled the shaping of DN gels into structurally complex 3D geometries. This review will describe the state-of-art technologies for shaping tough and stretchable DN gels into custom geometries by using conventional molding and casting, extrusion, and optics-based 3D printing, as well as the key challenges and future outlook in this field.

Optimizing the fracture toughness of a dual cross-linked hydrogel via molecular dynamics simulation

In this work, a coarse-grained model is adopted to explore the fracture toughness of a dual cross-linked hydrogel which consists of a physically cross-linked network and a chemically cross-linked network. By calculating the fracture energy, the optimized fracture toughness of the hydrogel appears at the intermediate content of the chemical network. To understand it, the structure change of both the chemical network and the physical network is first characterized during the tensile process. For the chemical network, the fraction and rate of broken bonds gradually improve with increasing content of the chemical network while the strain range where the bond breakage occurs is reduced. For the physical network, the number of clusters and the interaction energy first increase and then decrease with increasing strain. This reflects the breakage and reformation of the physical network, which dissipates more energy and improves the fracture energy. Furthermore, by stress decomposition, the stress is mainly borne by the physical network at small strain and the chemical network at large strain, which proves their synergistic effect in enhancing the hydrogel. Then, the number of voids is calculated as a function of strain. It is found that the voids initiate in the weak region at small strain while in the position of the bond breakage at large strain. Moreover, the number of voids decreases with increasing content of the chemical network at small strain. Finally, the effect of the strength of the chemical network or the physical network on the fracture toughness is discussed. The optimized fracture toughness of hydrogel appears at the intermediate strength.

Photo-degradable, tough and highly stretchable hydrogels

We present for the first time highly stretchable and tough hydrogels with controlled light-triggered photodegradation. A double-network of alginate/polyacrylamide (PAAm) is formed by using covalently and ionically crosslinked subnetworks. The ionic Ca²⁺ alginate interpenetrates a PAAm network covalently crosslinked by a bifunctional acrylic crosslinker containing the photodegradable o-nitrobenzyl (ONB) core instead of the commonly used methylene bisacrylamide (MBAA). Remarkably, due to the developed protocol, the change of the crosslinker did not affect the hydrogel's mechanical properties. The incorporation of photosensitive components in hydrogels allows external temporal control of their properties and tuneable degradation. Cell viability and cell proliferation assays revealed that hydrogels and their photodegradation products are not cytotoxic to the NIH3T3 cell line. In one example of application, we used these hydrogels for bio-potential acquisition in wearable electrocardiography. Surprisingly, these hydrogels showed a lower skin-electrode impedance, compared to the common medical grade Ag/AgCl electrodes. This work lays the foundation for the next generation of tough and highly stretchable hydrogels that are environmentally friendly and can find applications in a variety of fields such as health, electronics, and energy, as they combine excellent mechanical properties with controlled degradation.

Double-Network Tough Hydrogels: A Brief Review on Achievements and Challenges

This brief review attempts to summarize research advances in the mechanical toughness and structures of double-network (DN) hydrogels. The focus is to provide a critical and concise discussion on the toughening mechanisms, damage recoverability, stress relaxation, and biomedical applications of tough DN hydrogel systems. Both conventional DN hydrogel with two covalently cross-linked networks and novel DN systems consisting of physical and reversible cross-links are discussed and compared. Covalently cross-linked hydrogels are tough but damage-irreversible. Physically cross-linked hydrogels are damage-recoverable but exhibit mechanical instability, as reflected by stress relaxation tests. This remains one significant challenge to be addressed by future research studies to realize the load-sustaining applications proposed for tough hydrogels. With their special structure and superior mechanical properties, DN hydrogels have great potential for biomedical applications, and many DN systems are now fabricated with 3D printing techniques.

Highly Conductive and Mechanically Robust Cellulose Nanocomposite Hydrogels with Antifreezing and Antidehydration Performances for Flexible Humidity Sensors

Conductive hydrogels are emerging as an appealing material platform for flexible electronic devices owing to their attractive mechanical flexibility and conductive properties. However, the conventional water-based conductive hydrogels tend to inevitably freeze at subzero temperature and suffer from continuous water evaporation under ambient conditions, leading to a decrease in their electrical conductivities and mechanical properties. Thus, it is extremely necessary, but generally challenging, to create an antifreezing and antidehydration conductive gel for maintaining high and stable performances in terms of electrical conductivity and mechanical properties. Herein, we fabricated a cellulose nanofibril (CNF)-reinforced and highly ion-conductive organogel featuring excellent antifreezing and antidehydration performances by immersing it in the CaCl₂/sorbitol solution for solvent displacement. The incorporation of a rigid CNF serving as a dynamic connected bridge provided a hierarchical honeycomb-like cellular structure for the obtained CS-nanocomposite (NC) organogel networks, facilitating significant mechanical reinforcement. The synergy effects of sorbitol and CaCl₂ allowed high-performance integration with excellent antifreezing tolerance, antidehydration ability, and ionic conductivity. Strong hydrogen bonds were formed between water molecules and sorbitol molecules to impede the formation of ice crystals and water evaporation, thereby imparting the CS-NC organogels with extreme-temperature tolerance as low as -50 °C and pre-eminent antidehydration performance with over 90% weight retention. Furthermore, this CS-NC organogel exhibited high humidity sensitivity in a wide humidity detection range (23∼97% relative humidity) because of the ready formation of hydrogen bonds between water molecules and numerous hydrophilic groups in the binary solvent and elaborated polymer chains, which can be assembled as a stretchable humidity sensor to monitor human respiration with a fast response. This work provides a new prospect for fabricating intrinsically stretchable and high-performance humidity sensors using cellulose-based humidity-responsive materials for the emerging wearable applications.

Self-Healable and Super-Tough Double-Network Hydrogel Fibers from Dynamic Acylhydrazone Bonding and Supramolecular Interactions

Macroscopic hydrogel fibers are highly desirable for smart textiles, but the fabrication of self-healable and super-tough covalent/physical double-network hydrogels is rarely reported. Herein, copolymers containing ketone groups were synthesized and prepared into a dynamic covalent hydrogel via acylhydrazone chemistry. Double-network hydrogels were constructed via the dynamic covalent crosslinking of copolymers and the supramolecular interactions of iota-carrageenan. Tensile tests on double-network and parental hydrogels revealed the successful construction of strong and tough hydrogels. The double-network hydrogel precursor was wet spun to obtain macroscopic fibers with controlled drawing ratios. The resultant fibers reached a high strength of 1.35 MPa or a large toughness of 1.22 MJ/m3. Highly efficient self-healing performances were observed in hydrogel fibers and their bulk specimens. Through the simultaneous healing of covalent and supramolecular networks under acidic and heated conditions, fibers achieved rapid and near-complete healing with 96% efficiency. Such self-healable and super-tough hydrogel fibers were applied as shape memory fibers for repetitive actuating in response to water, indicating their potential in intelligent fabrics.

Getting control of hydrogel networks with cross-linkable monomers

The structure of a hydrogel network determines its ability to dissipate stress upon deformation, as well as its ability to swell in water. By designing systems with cross-linkable thiol groups in the monomers, radical thiol-ene chemistry was used to form controlled networks for acrylamide monomers. The use of radical thiol-ene chemistry effectively suppressed homo-polymerization of the bis(acrylamide) monomer and resulted in networks of alternating thiol and acrylamide monomers. Additionally, if the stoichiometry between the monomers is controlled, the network should approach that of ideality. In the case of bis(acrylamide) monomers, the incorporation of hydrogen-bond donors into the network creates a single network hydrogel with the benefits of high strength and ductility from the simultaneous incorporation of chemical and physical cross-links. Additionally, this strategy suppresses the formation of homo-polymerization in the acrylamide monomer to achieve an alternating network, which is supported with NMR characterization of base-digested fragments. For three different monomer compositions, the resulting gels had high compressive strength (up to 40 MPa) and tunable mechanical properties. The high mechanical strength of the 1 : 1, thiol : ene gel composition is due to the uniform distribution of cross-links, which creates defect-free networks for efficient stress transfer. The present one-pot synthetic strategy toward controlled gel networks affords monomer versatility and synthetic ease, as well as the potential for mechanically robust materials.

Super-stretchable and extreme temperature-tolerant supramolecular-polymer double-network eutectogels with ultrafast in situ adhesion and flexible electrochromic behaviour

The current tough and stretchable gels with various integrated functions are mainly based on polymer hydrogels. By introducing a non-covalent supramolecular self-assembled network into a covalently cross-linked polymer network in the presence of eco-friendly and cost-effective deep eutectic solvents (DESs), we developed a new small molecule-based supramolecular-polymer double-network (SP-DN) eutectogel platform. This exciting material exhibits high stretchability and toughness (>18 000% areal strain), spontaneous self-healing ability, ultrafast (∼5 s) in situ underwater and low-temperature (-80 °C) adhesion, and unusual boiling water-resistance, as well as strong base-, strong acid- (even aqua regia), ultra-low-temperature- (liquid nitrogen, -196 °C), and high-temperature- (200 °C) resistance. All these outstanding properties strongly recommend the SP-DN eutectogels as a quasi-solid electrolyte for soft electrochromic devices, which exhibited exceptional flexibility and consistent electrochromic behaviours in harsh mechanical or temperature environments. The experimental and simulation results uncovered the assembly mechanism of the SP-DN eutectogels. Unlike polymer hydrogels, the obtained SP-DN eutectogels showed high molecular design freedom and structural versatility. The findings of this work offer a promising strategy for developing the next generation of mechanically robust and functionally integrated soft materials with high environmental adaptability.

Recent Advances in Design Strategies for Tough and Stretchable Hydrogels

The development of multifunctional hydrogels with excellent stretchability and toughness is one of the most fascinating subjects in soft matter research. Numerous research efforts have focused on the design of new hydrogel systems with superior mechanical properties because of their potential applications in diverse fields. In this Minireview, we consider the most up-to-date mechanically strong hydrogels and summarize their design strategies based on the formation of double networks and dual physical crosslinking. Based on the synthetic approaches and different toughening mechanisms, double-network hydrogels can be further classified into three different categories, namely chemically crosslinked, hybrid physically-chemically crosslinked, and fully physically crosslinked. In addition to the above-mentioned methods, we also discuss few uniquely designed hydrogels with the intention of guiding the future development of these fascinating materials for superior mechanical performance.

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.

Bioinspired double network hydrogels: from covalent double network hydrogels via hybrid double network hydrogels to physical double network hydrogels

The design and synthesis of double network (DN) hydrogels that can mimic the properties and/or structure of natural tissue has flourished in recent years, overcoming the bottlenecks of mechanical performance of single network hydrogels and extending their potential applications in various fields. In recent years, such bioinspired DN hydrogels with extraordinary mechanical performance, excellent biocompatibility, and considerable strength have been demonstrated to be promising candidates for biomedical applications, such as tissue engineering and biomedicine. In this minireview, we provide an overview of the recent developments of bioinspired DN hydrogels defined as DN hydrogels that mimic the properties and/or structure of natural tissue, ranging from, e.g., anisotropically structured DN hydrogels, via ultratough energy dissipating DN hydrogels to dynamic, reshapable DN hydrogels. Furthermore, we discuss future perspectives of bioinspired DN hydrogels for biomedical applications.

Development of an Ultrastretchable Double-Network Hydrogel for Flexible Strain Sensors

The weak mechanical properties of hydrogels due to the inefficient dissipation of energy in the intrinsic structures limit their practical applications. Here, a double-network (DN) hydrogel has been developed by integrating an ionically cross-linked agar network, a covalently cross-linked acrylic acid (AAC) network, and the dynamic and reversible ionically cross-linked coordination between the AAC chains and Fe³⁺ ions. The proposed model reveals the mechanisms of the improved mechanical performances in the DN agar/AAC-Fe³⁺ hydrogel. The hydrogen-bond cross-linked double helices of agar and ionic-coordination interactions of AAC-Fe³⁺ can be temporarily sacrificed during large deformation to readily dissipate the energy, whereas the reversible AAC-Fe³⁺ interactions can be regenerated after stress relief, which greatly increases the material toughness. The developed DN hydrogel demonstrates a remarkable stretchability with a break strain up to 3174.3%, high strain sensitivity with the gauge factor being 0.83 under a strain of 1000%, and good 3D printability, making the material a desirable candidate for fabricating flexible strain sensors, electronic skin, and soft robots.

Dynamic cross-linking of an alginate-acrylamide tough hydrogel system: time-resolved in situ mapping of gel self-assembly

Hydrogels are a popular class of biomaterial that are used in a number of commercial applications (e.g.; contact lenses, drug delivery, and prophylactics). Alginate-based tough hydrogel systems, interpenetrated with acrylamide, reportedly form both ionic and covalent cross-links, giving rise to their remarkable mechanical properties. In this work, we explore the nature, onset and extent of such hybrid bonding interactions between the complementary networks in a model double-network alginate-acrylamide system, using a host of characterisation techniques (e.g.; FTIR, Raman, UV-vis, and fluorescence spectroscopies), in a time-resolved manner. Further, due to the similarity of bonding effects across many such complementary, interpenetrating hydrogel networks, the broad bonding interactions and mechanisms observed during gelation in this model system, are thought to be commonly replicated across alginate-based and broader double-network hydrogels, where both physical and chemical bonding effects are present. Analytical techniques followed real-time bond formation, environmental changes and re-organisational processes that occurred. Experiments broadly identified two phases of reaction; phase I where covalent interaction and physical entanglements predominate, and; phase II where ionic cross-linking effects are dominant. Contrary to past reports, ionic cross-linking occurred more favourably via mannuronate blocks of the alginate chain, initially. Evolution of such bonding interactions was also correlated with the developing tensile and compressive properties. These structure-property findings provide mechanistic insights and future synthetic intervention routes to manipulate the chemo-physico-mechanical properties of dynamically-forming tough hydrogel structures according to need (i.e.; durability, biocompatibility, adhesion, etc.), allowing expansion to a broader range of more physically and/or environmentally demanding biomaterials applications.

Cationic Cellulose Nanocrystals-Based Nanocomposite Hydrogels: Achieving 3D Printable Capacitive Sensors with High Transparency and Mechanical Strength

Hydrogel ionotronics are intriguing soft materials that have been applied in wearable electronics and artificial muscles. These applications often require the hydrogels to be tough, transparent, and 3D printable. Renewable materials like cellulose nanocrystals (CNCs) with tunable surface chemistry provide a means to prepare tough nanocomposite hydrogels. Here, we designed ink for 3D printable sensors with cationic cellulose nanocrystals (CCNCs) and zwitterionic hydrogels. CCNCs were first dispersed in an aqueous solution of monomers to prepare the ink with a reversible physical network. Subsequent photopolymerization and the introduction of Al3+ ion led to strong hydrogels with multiple physical cross-links. When compared to the hydrogels using conventional CNCs, CCNCs formed a stronger physical network in water that greatly reduced the concentration of nanocrystals needed for reinforcing and 3D printing. In addition, the low concentration of nanofillers enhanced the transparency of the hydrogels for wearable electronics. We then assembled the CCNC-reinforced nanocomposite hydrogels with stretchable dielectrics into capacitive sensors for the monitoring of various human activities. 3D printing further enabled a facile design of tactile sensors with enhanced sensitivity. By harnessing the surface chemistry of the nanocrystals, our nanocomposite hydrogels simultaneously achieved good mechanical strength, high transparency, and 3D printability.

Effect of the constituent networks of double-network gels on their mechanical properties and energy dissipation process

Double-network (DN) gels, consisting of brittle first and ductile second networks, possess extraordinary strength, extensibility, and fracture toughness while maintaining a high solvent content. Herein, we prepare DN gels consisting of various concentrations of the first and second networks to investigate the effect of each network structure on the tensile and fracture properties of DN gels. The results showed that the tensile properties of DN gels before yielding are mainly dominated by the first network, serving as a skeleton, whereas the properties after necking are determined by both networks. Moreover, we found that the DN gels with significant energy dissipation capacities exhibit high fracture resistance. Thus, this study not only confirms the factors determining the mechanical characteristics of DN gels but also explains how the two networks concertedly improve the toughness of DN gels.

3D Printable Strain Sensors from Deep Eutectic Solvents and Cellulose Nanocrystals

Stretchable and conductive hydrogels have been intensively studied as wearable electronics to monitor the physiological activities of human bodies. However, it remains a challenge to fabricate robust hydrogels as sensors with complex 3D structures. Here, we designed a 3D printable ink from cellulose nanocrystals (CNCs), deep eutectic solvents (DESs), and ionically cross-linked polyacrylic acid (PAA). DESs composed of choline chloride and ethylene glycol served as a nonvolatile medium with high ionic conductivity. The dispersion of CNCs in a mixture of DESs, acrylic acid, and Al³⁺ ions formed ionogels with a reversible physical network for 3D printing. After the printing process, the ionogel was solidified by the photopolymerization of acrylic acid in the presence of Al³⁺ ions to form a second ionically cross-linked network. The first physical network of CNCs provides an energy-dissipating mechanism to make a strong and highly stretchable nanocomposite ionogel. When compared to hydrogels, we found that the DES/CNC nanocomposite ionogel was more stable in the air because of the low volatility of DESs. We further used the DES/CNC ink to 3D print an auxetic sensor with negative Poisson's ratios so that the sensor provided a conformal contact with the skin during large deformation. In addition, the auxetic sensor could continuously monitor and identify different motions of the human body by the change in resistance. These results demonstrate a simple and rapid strategy to fabricate stable and sensitive strain sensors from cheap and renewable feedstock.

Experimental study on the mechanical properties of biological hydrogels of different concentrations

BACKGROUND

Biological hydrogels provide a conducive three-dimensional extracellular matrix environment for encapsulating and cultivating living cells. Microenvironmental modulus of hydrogels dictates several characteristics of cell functions such as proliferation, adhesion, self-renewal, differentiation, migration, cell morphology and fate. Precise measurement of the mechanical properties of gels is necessary for investigating cellular mechanobiology in a variety of applications in tissue engineering. Elastic properties of gels are strongly influenced by the amount of crosslinking density.

OBJECTIVE

The main purpose of the present study was to determine the elastic modulus of two types of well-known biological hydrogels: Agarose and Gelatin Methacryloyl.

METHODS

Mechanical properties such as Young's modulus, fracture stress and failure strain of the prescribed gels with a wide range of concentrations were determined using tension and compression tests.

RESULTS

The elastic modulus, failure stress and strain were found to be strongly influenced when the amount of concentration in the hydrogels was changed. The elastic modulus for a lower level of concentration, not considered in this study, was also predicted using statistical analysis.

CONCLUSIONS

Closed matching of the mechanical properties of the gels revealed that the bulk tension and compression tests could be confidently used for assessing mechanical properties of delicate biological hydrogels.

Seawater-enhanced tough agar/poly(N-isopropylacrylamide)/clay hydrogel for anti-adhesion and oil/water separation

Hydrogels as typical hydrophilic materials are promising candidates for designing novel functional materials for anti-fouling, oil/water separation, wearable devices, tissue scaffolds, etc. However, it still remains a challenge to design stable and tough hydrogels for applications in complex environments of high stress, temperature, salt, and pH. Herein, we fabricate a novel seawater-enhanced Agar/Poly(N-isopropylacrylamide)/clay hydrogel (APNC gel) through a facile photo-initiated polymerization process. The APNC gel consists of fully interpenetrating double networks with negatively-charged clay serving as physical cross-linkers. The resulting gel exhibits tough mechanical strength (tensile strength of 0.85 MPa and compression strength of 1.68 MPa) and excellent stabilities for high temperature (100 °C) and high salt levels (20 wt% NaCl). Especially, the strength of the APNC gel is greatly enhanced (up to 5.04 MPa) by seawater, which contains numerous inorganic ions (Mg2+, Na+, K+, etc.). Meanwhile, the APNC gel presents excellent anti-adhesion performance due to the negatively-charged clay. Thus, a hydrogel-coated mesh with underwater superoleophobicity has been designed for oil/seawater separation. The resulting mesh can selectively remove oil from seawater with high separation efficiency (up to 99%). These characteristics demonstrate that the tough APNC gel will be an ideal material candidate for developing functional materials applied in a complex environment.

Role of Nanoparticle-Polymer Interactions on the Development of Double-Network Hydrogel Nanocomposites with High Mechanical Strength

Extensive experimental and theoretical research over the past several decades has pursued strategies to develop hydrogels with high mechanical strength. Our study investigated the effect of combining two approaches, addition of nanoparticles and crosslinking two different polymers (to create double-network hydrogels), on the mechanical properties of hydrogels. Our studies revealed that these orthogonal approaches may be combined to synthesize hydrogel composites with enhanced mechanical properties. However, the enhancement in double network hydrogel elastic modulus due to incorporation of nanoparticles is limited by the ability of the nanoparticles to strongly interact with the polymers in the network. Moreover, double-network hydrogel nanocomposites prepared using lower monomer concentrations showed higher enhancements in elastic moduli compared to those prepared using high monomer concentrations, thus indicating that the concentration of hydrogel monomers used for the preparation of the nanocomposites had a significant effect on the extent of nanoparticle-mediated enhancements. Collectively, these results demonstrate that the hypotheses previously developed to understand the role of nanoparticles on the mechanical properties of hydrogel nanocomposites may be extended to double-network hydrogel systems and guide the development of next-generation hydrogels with extraordinary mechanical properties through a combination of different approaches.

High-Resolution 3D Printing of Stretchable Hydrogel Structures Using Optical Projection Lithography

Double-network (DN) hydrogels, with their unique combination of mechanical strength and toughness, have emerged as promising materials for soft robotics and tissue engineering. In the past decade, significant effort has been devoted to synthesizing DN hydrogels with high stretchability and toughness; however, shaping the DN hydrogels into complex and often necessary user-defined two-dimensional (2D) and three-dimensional (3D) geometries remains a fabrication challenge. Here, we report a new fabrication method based on optical projection lithography to print DN hydrogels into customizable 2D and 3D structures within minutes. DN hydrogels were printed by first photo-crosslinking a single network structure via spatially modulated light patterns followed by immersing the printed structure in a calcium bath to induce ionic cross-linking. Results show that DN structures made by this method can stretch four times their original lengths. We show that strain and the elastic modulus of printed structures can be tuned based on the hydrogel composition, cross-linker and photoinitiator concentrations, and laser light intensity. To our knowledge, this is the first report demonstrating quick lithography and high-resolution printing of DN (covalent and ionic) hydrogels within minutes. The ability to shape tough and stretchable DN hydrogels in complex structures will be potentially useful in a broad range of applications.

Tough, stretchable and compressive alginate-based hydrogels achieved by non-covalent interactions

In this study, two alginate-based hydrogels with good mechanical strength, toughness and resilience were synthesized by hydrophobic interaction and coordination bonding. Sodium alginate/poly(acrylamide) semi-interpenetrating network (NaAlg/PAM semi-IPN) hydrogels were first synthesized through the micelle copolymerization of acrylamide and stearyl methacrylate in the presence of sodium alginate, then calcium alginate/poly(acrylamide) double network (CaAlg/PAM DN) hydrogels were prepared by immersing the as-prepared NaAlg/PAM semi-IPN hydrogels in a CaCl₂ solution. FT-IR and XPS results revealed NaAlg/PAM semi-IPN hydrogels and CaAlg/PAM DN hydrogels were successfully synthesized through non-covalent interactions. The tensile strength of CaAlg/PAM DN hydrogels could reach 733.6 kPa, and their compressive strengths at 80% strain are significantly higher than those of the corresponding NaAlg/PAM semi-IPN hydrogels, which is attributed to the alginate network crosslinked by Ca²⁺. The dual physically crosslinked CaAlg/PAM DN hydrogels can achieve fast self-recovery, and good fatigue resistance, which is mainly assigned to energy dissipation through dynamic reversible non-covalent interactions in both networks. The self-healing ability, swelling behavior and morphology of the synthesized alginate-based hydrogels were also evaluated. This study offers a new avenue to design and construct hydrogels with high mechanical strength, high toughness and fast self-recovery properties, which broadens the current research and application of hydrogels.

Alginate-based hydrogels based on non-covalent interactions were synthesized, and exhibited good mechanical strength, toughness and resilience.

Self-healing stimuli-responsive cellulose nanocrystal hydrogels

A facile and universal approach to prepare cellulose nanocrystal reinforced functional hydrogels is proposed. An organic solvent-free and eco-friendly method was adopted, where both the modification and polymerization were conducted in an aqueous solution. Cellulose nanocrystal (CNC) and sodium alginate (SA) were first oxidized under mild conditions to introduce aldehyde groups. Subsequently, amine-containing vinyl functionalized monomers were introduced to the surface of CNC or backbone of oxidized SA via a dynamic Schiff-base reaction. The bio-based hydrogels were then prepared via a one-pot in-situ polymerization, where the functional CNC and SA served as novel macro-cross-linkers that contributed to the structural integrity and mechanical stability of the hydrogels. The hydrogels displayed uniform chemical and macroscopic structures and could self-heal within several hours at room temperature. The design of specific monomers will allow the introduction of stimuli-responsive properties to the functional hydrogels and a chemically robust thermally-triggered actuator was demonstrated. Due to its flexible design and practical approach, the hydrogels could find potential uses in agricultural and pharmaceutical products.

Locust bean gum/gellan gum double-network hydrogels with superior self-healing and pH-driven shape-memory properties

In this work, we prepared locust bean gum (LBG)/gellan gum (Gg) double network (DN) hydrogels based on pH-sensitive borate-ester bonds in the LBG network and hydrogen-bond-associated double-helix bundles in the Gg network by using two novel natural polysaccharide polymers. The DN hydrogels with optimized Gg and borax concentrations exhibit good mechanical properties (the fracture tensile stress is almost three times that of the LBG single network hydrogel). Because of their unique thermo- and pH-sensitive DN structure, the LBG/Gg DN hydrogels also show excellent self-healing, thermo-processability, and pH-driven shape memory properties. Such novel DN hydrogels demonstrate strong potentiality in many challenging applications such as biomedicine, soft robotics and other fields.

Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hydrogels are frequently used biomaterials due to their similarity in hydration and structure to biological tissues. However, their utility is limited by poor mechanical properties, namely, a lack of strength and stiffness that mimic that of tissues, particularly load-bearing tissues. Thus, numerous recent strategies have sought to enhance and tune these properties in hydrogels, including interpenetrating networks (IPNs), macromolecular cross-linking, composites, thermal conditioning, polyampholytes, and dual cross-linking. Individually, these approaches have achieved hydrogels with either high strength (σ f > 10 MPa), high stiffness (E > 1 MPa), or, less commonly, both high strength and stiffness (σ f > 10 MPa and E > 1 MPa). However, only certain unique combinations of these approaches have been able to synergistically achieve retention of a high, tissuelike water content as well as high strength and stiffness. Applying such methods to stimuli-responsive hydrogels has also produced robust, smart biomaterials. Overall, methods to achieve hydrogels that simultaneously mimic the hydration, strength, and stiffness of soft and load-bearing tissues have the potential to be used in a much broader range of biomedical applications.

Optimized Association of Short Alkyl Side Chains Enables Stiff, Self-Recoverable, and Durable Shape-Memory Hydrogel

This work reports a self-healing and shape-memory hydrogel integrating multiple mechanical properties. The network configuration is featured as entangled networks cross-linked by distributed association of very short alkyl chains (hexyl, six carbons). These cross-linking knots are interconnected by the long hydrophilic polyvinyl alcohol backbone. The optimal aggregation of hexyl side chains leads to the broadened distribution in bonding strength as verified by static and dynamic mechanical characterization. These structural features contribute to high strength, toughness, stiffness, and yet fast recoverability. Furthermore, the hydrophobic and supramolecular nature of aggregated alkyl chains offers high durability and solvent-assistant healing function. Finally, distributed association of hexyl side chains confers a broadened temperature-dependent modulus, allowing for encoding stepwise shape recovery from a temporary shape at different temperatures and/or times.