Highly transparent, self-healing, injectable and self-adhesive chitosan/polyzwitterion-based double network hydrogel for potential 3D printing wearable strain sensor

Reinforced enzyme mineralized chitosan hydrogels with superior mechanical and osteogenic properties

As a natural cationic polymer material, the application of chitosan hydrogel for bone tissue engineering has been greatly limited due to its poor mechanical strength. Enzymatic mineralization has drawn increased attention to effectively improve the mechanical properties of hydrogels. In this study, carboxymethyl chitosan (CMCS) hydrogels cross-linked with different concentrations of genipin (2.5 %, 5 % and 10 %) were prepared and further mineralized through enzyme-induced biomimetic mineralization. The mechanical properties of the CMCS hydrogels were significantly increased as a result of mineralization, showing improvement of 1200-1500 % on storage moduli, and even exhibiting certain tensile behavior with the elongation rate of 30-35 %, likely due to the uniform formation and small size of mineralized products. Interestingly, the cationicity of chitosan also exerted an important modulation effect and the mineralization behavior and mechanical properties of mineralized hydrogels. In addition, the enzymatic mineralized hydrogels showed enhanced biocompatibility and osteogenic differentiation in-vitro, likely due to its superior mechanical properties and the introduction of calcium phosphate biominerals. In vivo experiments further suggest excellent bone-forming activity for the enzymatic mineralized hydrogels. Overall, tuning cationicity and enzymatic mineralization provide an effective approach for the preparation of chitosan hydrogels with superior mechanical and biological properties for bone tissue engineering application.

A "Janus" Zwitterionic Hydrogel Patch for Tissue Repair and Prevention of Post-Operative Adhesions

Anti-peritoneal adhesions (PA) are very important after abdominal surgery for that PA often leads to other medical problems and imposes a huge financial burden on the national healthcare system. In this work, a "Janus" zwitterionic hydrogel patch where one side can adhere firmly to the tissue, while the other side has anti-fouling properties and has little interaction with the surrounding tissue has been developed. The "Janus" hydrogel patch is prepared by in situ formation of a bonding polymer layer poly(acrylic-co-N-hydroxysuccinimide acrylate) on one side of zwitterionic hydrogel. The mechanical, swelling, adhesion, biodegradability and biocompatibility tests are performed to study the function of "Janus" hydrogel patch to prevent wound adhesion and rapid repair. It is found that the adhesive side of the hydrogel patch has stable adhesion to tissues, avoiding the slippage faced by many commercial anti-adhesion gels in the body. The other zwitterionic side can resist proteins and fibroblasts and prevent external interactions or adhesion with other tissues. This convenient and effective method provides a new idea for the design of postoperative anti-adhesion materials and broadens the application of hydrogels in the biomedical field.

Self-Polymerized Tough and High-Entanglement Zwitterionic Functional Hydrogels

Zwitterionic hydrogels exhibit great potential in biomedical applications due to their antifouling properties and biocompatibility. However, the single-network structure of pure zwitterionic hydrogels leads to a low toughness and strength, limiting their application in biomedical fields. In this work, a high entanglement sulfobetaine methacrylate-dopamine hydrogel (SBMA-DA-PE) with low cross-linker content and high monomer concentration is prepared by using a dopamine oxidative radical polymerization method. Compared to a regular zwitterionic hydrogel, the SBMA-DA-PE hydrogel exhibits a 5-fold increase in tensile fracture stress and a 10-fold increase in compressive fracture stress. The SBMA-DA-PE hydrogel possesses excellent mechanical properties (the maximum compressive stress ≥4.85 MPa, the maximum compressive strain ≥90%). Besides, the non-covalent interactions between catechol or ortho-quinones within the SBMA-DA-PE hydrogel, combined with strong intermolecular electrostatic interactions, endow the SBMA-DA-PE hydrogel with great self-healing capabilities and fatigue resistance. The SBMA-DA-PE hydrogel demonstrates low swellability and possesses good antifouling properties. Furthermore, the good printability and conductivity of the tough SBMA-DA-PE hydrogel endows it with new possibilities for developing biological 3D scaffolds and electronic devices. Overall, this work provides new insights into the preparation of zwitterionic hydrogels with high mechanical strength and multi-functionality for biomedical applications.

Highly conductive and sensitive alginate hydrogel strain sensors fabricated using near-field electrohydrodynamic direct-writing process

Hydrogel flexible sensors have attracted considerable attention because of their wearability, biocompatibility, and precision signal transmission capability. However, the hydrogel strain sensors fabricated by conventional printing or hand-injection methods have difficulty balancing their mechanical strength and sensing characteristics, limiting the application of hydrogel strain sensors. Herein, polyvinyl alcohol and polyacrylamide were loosely crosslinked with sodium alginate through chemical cross-linking. Subsequently, MXene nanosheets were introduced for doping, the crosslinked hydrogel conductive network was constructed, and the hydrogel strain sensors were fabricated using the electrohydrodynamic (EHD) printing method. The ions in the EHD-printed hydrogel undergo directional movement under an externally enhanced electric field, causing the formation of more uniform and dense porous conductive networks inside the hydrogel, and high electrical conductivity (0.49 S m-1) is obtained. These hydrogel strain sensors have excellent mechanical properties (tensile strength: 0.17 MPa at 787 % strain), high sensitivity (gauge factor: 1.54 at 0-100 % strain), and low detection limits (1 % strain). Furthermore, demonstrations of real-time Morse code tapping information transmission, handwriting recognition during writing, and human physiological behavior monitoring demonstrations using the fabricated sensors indicate that the EHD-printed hydrogel strain sensor method has significant potential for wearable devices and human-computer interaction applications.

Effects of Enteromorpha prolifera sulfated polysaccharide and aluminium ion addition on the multifunctional property of conductive hydrogel for wearable strain sensing

Although introducing Enteromorpha prolifera sulfated polysaccharide (SPEP) enhances the mechanical properties of hydrogels significantly, little is known about the effects of polysaccharide and ion addition on morphological and physicochemical properties of conductive hydrogel. Therefore, the Poly (acrylic acid)/SPEPn/Al3+m (PAA/SPEPn/Al3+m) hydrogels with different SPEP and Al3+ addition were synthesized by simple one-pot method. The porosity, tensile strength, and swelling ration increased, while compressive strength, elongation at break, self-healing, self-adhesion properties increased first and then decreased as SPEP addition increased from 0 % to 3.80 %. The Al3+ addition increased from 0.08 % to 0.30 %, both tensile and compressive strength increased first and then decreased, while elongation at break kept increasing. Unexpectedly, both increasing SPEP and Al3+ addition reduced the electrical conductivity, while SPEP increased the gauge factor of hydrogel. The hydrogel exhibited optimal comprehensive properties when SPEP and Al3+ addition were 2.31 % and 0.24 %, respectively. The PAA/SPEP2.31%/Al3+0.24% hydrogel showed high tensile strength (107.60 kPa), elongation at break (2426.67 %), strain self-healing rate (81.87 %), adhesion strength (21.61 kPa), and conductivity (3.60 S/m). Overall, the properties of PAA/SPEPn/Al3+m hydrogels can be regulated through tailoring SPEP and Al3+ addition, which can be used as on-demand strategy to improve the performance of PAA/SPEPn/Al3+m hydrogels for each application.

Exploring the Potentials of Chitin and Chitosan-Based Bioinks for 3D-Printing of Flexible Electronics: The Future of Sustainable Bioelectronics

Chitin and chitosan-based bioink for 3D-printed flexible electronics have tremendous potential for innovation in healthcare, agriculture, the environment, and industry. This biomaterial is suitable for 3D printing because it is highly stretchable, super-flexible, affordable, ultrathin, and lightweight. Owing to its ease of use, on-demand manufacturing, accurate and regulated deposition, and versatility with flexible and soft functional materials, 3D printing has revolutionized free-form construction and end-user customization. This study examined the potential of employing chitin and chitosan-based bioinks to build 3D-printed flexible electronic devices and optimize bioink formulation, printing parameters, and postprocessing processes to improve mechanical and electrical properties. The exploration of 3D-printed chitin and chitosan-based flexible bioelectronics will open new avenues for new flexible materials for numerous industrial applications.

Polysaccharide hydrogels for skin wound healing

Advances in the development and utilization of polysaccharide materials are highly promising, offering prominent applications in the field of tissue engineering for addressing diverse clinical needs, including wound healing, bone regeneration, cartilage repair, and treatment of conditions such as arthritis. Novel polysaccharide materials are popular owing to their inherent stability, biocompatibility, and repeatability. This review presents an overview of the biomedical applications of natural polysaccharide hydrogels and their derivatives. Herein, we discuss the latest advancements in the fabrication, physicochemical properties, and biomedical applications of polysaccharide-based hydrogels, including chitosan, hyaluronic acid, alginate, and cellulose. Various processing techniques applicable to polysaccharide materials are explored, such as the transformation of polysaccharide hydrogels into electrospun nanofibers, microneedles, microspheres, and nanogels. Furthermore, the use of polysaccharide hydrogels in the context of wound-healing applications, including hemostatic effects, antimicrobial activities, anti-inflammatory properties, and promotion of angiogenesis, is presented. Finally, we address the challenges encountered in the development of polysaccharide hydrogels and outline the potential prospects in this evolving field.

Injectable Hydrogels: A Paradigm Tailored with Design, Characterization, and Multifaceted Approaches

Biomaterials denoting self-healing and versatile structural integrity are highly curious in the biomedicine segment. The injectable and/or printable 3D printing technology is explored in a few decades back, which can alter their dimensions temporarily under shear stress, showing potential healing/recovery tendency with patient-specific intervention toward the development of personalized medicine. Thus, self-healing injectable hydrogels (IHs) are stunning toward developing a paradigm for tissue regeneration. This review comprises the designing of IHs, rheological characterization and stability, several benchmark consequences for self-healing IHs, their translation into tissue regeneration of specific types, applications of IHs in biomedical such as anticancer and immunomodulation, wound healing and tissue/bone regeneration, antimicrobial potentials, drugs, gene and vaccine delivery, ocular delivery, 3D printing, cosmeceuticals, and photothermal therapy as well as in other allied avenues like agriculture, aerospace, electronic/electrical industries, coating approaches, patents associated with therapeutic/nontherapeutic avenues, and numerous futuristic challenges and solutions.

Injectable hydrogels based on biopolymers for the treatment of ocular diseases

Injectable hydrogels based on biopolymers, fabricated utilizing diverse chemical and physical methodologies, exhibit exceptional physical, chemical, and biological properties. They have multifaceted applications encompassing wound healing, tissue regeneration, and across diverse scientific realms. This review critically evaluates their largely uncharted potential in ophthalmology, elucidating their diverse applications across an array of ocular diseases. These conditions include glaucoma, cataracts, corneal disorders (spanning from age-related degeneration to trauma, infections, and underlying chronic illnesses), retina-associated ailments (such as diabetic retinopathy, retinitis pigmentosa, and age-related macular degeneration (AMD)), eyelid abnormalities, and uveal melanoma (UM). This study provides a thorough analysis of applications of injectable hydrogels based on biopolymers across these ocular disorders. Injectable hydrogels based on biopolymers can be customized to have specific physical, chemical, and biological properties that make them suitable as drug delivery vehicles, tissue scaffolds, and sealants in the eye. For example, they can be engineered to have optimum viscosity to be injected intravitreally and sustain drug release to treat retinal diseases. Their porous structure and biocompatibility promote cellular infiltration to regenerate diseased corneal tissue. By accentuating their indispensable role in ocular disease treatment, this review strives to present innovative and targeted approaches in this domain, thereby advancing ocular therapeutics.

Minimally Invasive Delivery of Percutaneous Ablation Agent via Magnetic Colloidal Hydrogel Injection for Treatment of Hepatocellular Carcinoma

Percutaneous thermotherapy, a minimally invasive operational procedure, is employed in the ablation of deep tumor lesions by means of target-delivering heat. Conventional thermal ablation methods, such as radiofrequency or microwave ablation, to a certain extent, are subjected to extended ablation time as well as biosafety risks of unwanted overheating. Given its effectiveness and safety, percutaneous thermotherapy gains a fresh perspective, thanks to magnetic hyperthermia. In this respect, an injectable- and magnetic-hydrogel-construct-based thermal ablation agent is likely to be a candidate for the aforementioned clinical translation. Adopting a simple and environment-friendly strategy, a magnetic colloidal hydrogel injection is introduced by a binary system comprising super-paramagnetic Fe3O4 nanoparticles and gelatin nanoparticles. The colloidal hydrogel constructs, unlike conventional bulk hydrogel, can be easily extruded through a percutaneous needle and then self-heal in a reversible manner owing to the unique electrostatic cross-linking. The introduction of magnetic building blocks is exhibited with a rapid magnetothermal response to an alternating magnetic field. Such hydrogel injection is capable of generating heat without limitation of deep penetration. The materials achieve outstanding therapeutic results in mouse and rabbit models. These findings constitute a new class of locoregional interventional thermal therapies with minimal collateral damages.

Fabrication of Microgel-Modified Hydrogel Flexible Strain Sensors Using Electrohydrodynamic Direct Printing Method

Hydrogel flexible strain sensors, renowned for their high stretchability, flexibility, and wearable comfort, have been employed in various applications in the field of human motion monitoring. However, the predominant method for fabricating hydrogels is the template method, which is particularly inefficient and costly for hydrogels with complex structural requirements, thereby limiting the development of flexible hydrogel electronic devices. Herein, we propose a novel method that involves using microgels to modify a hydrogel solution, printing the hydrogel ink using an electrohydrodynamic printing device, and subsequently forming the hydrogel under UV illumination. The resulting hydrogel exhibited a high tensile ratio (639.73%), high tensile strength (0.4243 MPa), and an ionic conductivity of 0.2256 S/m, along with excellent electrochemical properties. Moreover, its high linearity and sensitivity enabled the monitoring of a wide range of subtle changes in human movement. This novel approach offers a promising pathway for the development of high-performance, complexly structured hydrogel flexible sensors.

Three-Dimensional Printing of Hydrogels for Flexible Sensors: A Review

The remarkable flexibility and heightened sensitivity of flexible sensors have drawn significant attention, setting them apart from traditional sensor technology. Within this domain, hydrogels-3D crosslinked networks of hydrophilic polymers-emerge as a leading material for the new generation of flexible sensors, thanks to their unique material properties. These include structural versatility, which imparts traits like adhesiveness and self-healing capabilities. Traditional templating-based methods fall short of tailor-made applications in crafting flexible sensors. In contrast, 3D printing technology stands out with its superior fabrication precision, cost-effectiveness, and satisfactory production efficiency, making it a more suitable approach than templating-based strategies. This review spotlights the latest hydrogel-based flexible sensors developed through 3D printing. It begins by categorizing hydrogels and outlining various 3D-printing techniques. It then focuses on a range of flexible sensors-including those for strain, pressure, pH, temperature, and biosensors-detailing their fabrication methods and applications. Furthermore, it explores the sensing mechanisms and concludes with an analysis of existing challenges and prospects for future research breakthroughs in this field.

Multifunctional, Degradable Wearable Sensors Prepared with an Initiator and Crosslinker-Free Method

The present zwitterionic hydrogel-based wearable sensor exhibits various limitations, such as limited degradation capacity, unavoidable toxicity resulting from initiators, and poor mechanical properties that cannot satisfy practical demands. Herein, we present an initiator and crosslinker-free approach to prepare polyethylene glycol (PEG)@poly[2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) (PSBMA) interpenetrating polymer network (IPN) hydrogels that are self-polymerized via sunlight-induced and non-covalent crosslinking through electrostatic interaction and hydrogen bonding among polymer chains. The PEG@PSBMA IPN hydrogel possesses tissue-like softness, superior stretchability (∼2344.6% elongation), enhanced fracture strength (∼39.5 kPa), excellent biocompatibility, antibacterial property, reliable adhesion, and ionic conductivity. Furthermore, the sensor based on the IPN hydrogel demonstrates good sensitivity and cyclic stability, enabling effective real-time monitoring of human body activities. Moreover, it is worth noting that the excellent degradability in the saline solution within 8 h makes the prepared hydrogel-based wearable sensor free from the electronic device contamination. We believe that the proposed strategy for preparing physical zwitterionic hydrogels will pave the way for fabricating eco-friendly wearable devices.

Construction of a Soft Antifouling PAA/PSBMA Hydrogel Coating with High Toughness and Low Swelling through the Dynamic Coordination Bonding Provided by Al(OH)3 Nanoparticles

Marine biofouling, resulting from the adhesion of marine organisms to ship surfaces, has long been a significant issue in the maritime industry. In this paper, we focused on utilizing soft and hydrophilic hydrogels as a potential approach for antifouling (AF) coatings. Acrylic acid (AA) with a polyelectrolyte effect and N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA) with an antipolyelectrolyte effect were selected as monomers. By adjusting the monomer ratio, we were able to create hydrogel coatings that exhibited low swelling ratio in both fresh water and seawater. The Al(OH)3 nanoparticle, as a physical cross-linker, provided better mechanical properties (higher tensile strength and larger elongation at break) than the chemical cross-linker through the dynamic coordination bonds and plentiful hydrogen bonds. Additionally, we incorporated trehalose into the hydrogel, enabling the repair of the hydrogel network through covalent-like hydrogen bonding. The zwitterion compound SBMA endowed the hydrogel with excellent AF performance. It was found that the highest SBMA content did not lead to the best antibacterial performance, as bacterial adhesion quantity was also influenced by the charge of the hydrogel. The hydrogel with appropriate SBMA content being close to electrical neutrality exhibits the strongest zwitterionic property of PSBMA chains, resulting in the best antibacterial adhesion performance. Furthermore, the pronounced hydrophilicity of SBMA enhanced the lubrication of the hydrogel surface, thereby reducing the friction resistance when applied to the hull surface during ship navigation.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Supramolecular hydrogels for wound repair and hemostasis

The unique network characteristics and stimuli responsiveness of supramolecular hydrogels have rendered them highly advantageous in the field of wound dressings, showcasing unprecedented potential. However, there are few reports on a comprehensive review of supramolecular hydrogel dressings for wound repair and hemostasis. This review first introduces the major cross-linking methods for supramolecular hydrogels, which includes hydrogen bonding, electrostatic interactions, hydrophobic interactions, host-guest interactions, metal ligand coordination and some other interactions. Then, we review the advanced materials reported in recent years and then summarize the basic principles of each cross-linking method. Next, we classify the network structures of supramolecular hydrogels before outlining their forming process and propose their potential future directions. Furthermore, we also discuss the raw materials, structural design principles, and material characteristics used to achieve the advanced functions of supramolecular hydrogels, such as antibacterial function, tissue adhesion, substance delivery, anti-inflammatory and antioxidant functions, cell behavior regulation, angiogenesis promotion, hemostasis and other innovative functions in recent years. Finally, the existing problems as well as future development directions of the cross-linking strategy, network design, and functions in wound repair and hemostasis of supramolecular hydrogels are discussed. This review is proposed to stimulate further exploration of supramolecular hydrogels on wound repair and hemostasis by researchers in the future.

Self-healing, ultra-stretchable, and highly sensitive conductive hydrogel reinforced by sulfate polysaccharide from Enteromorpha prolifera for human motion sensing

The synthesis of multifunctional conductive hydrogel has attracted extensive attention worldwide due to their integrated properties of stretchability, self-adhesion, self-healing, and high sensitivity, while it is still a challenge. Although various kinds of polysaccharides and their derivatives are used to achieve the aforementioned objective, there are few researches about hydrogel design introducing sulfated polysaccharide from Enteromorpha prolifera (SPE), which is rich in hydroxyl, sulfate, and carboxyl groups providing amounts of reaction sites for hydrogel synthesis. Herein, conductive hydrogel (PAA-Al3+-SPE3) reinforced by SPE was designed by simple one pot hot polymerization method. This hydrogel demonstrated charming extension ratio (up to 4027.40 %), strain stress (up to 59.94 kPa), compressive strength (19.71 Mpa), and high conductivity sensibility (GF 6.76, 300 % - 700 %). Additionally, PAA-Al3+-SPE3 showed good self-healing property (repaired autonomously after 60 s) and satisfied self-adhesion (31.11 kPa) due to the reversible hydrogen bonds and metal coordination interactions. Furthermore, the PAA-Al3+-SPE3 hydrogel showed great real-time sensing performance to monitor various motions. These findings suggest the potential of PAA-Al3+-SPE3 hydrogel as an affordable and reliable conductive sensing material. Meantime, the first utilization of SPE to construct flexible wearable sensors offers new route for the high-value application of Enteromorpha prolifera.

Self-Healing Polymeric Materials and Composites for Additive Manufacturing

Self-healing polymers have received widespread attention due to their ability to repair damage autonomously and increase material stability, reliability, and economy. However, the processability of self-healing materials has yet to be studied, limiting the application of rich self-healing mechanisms. Additive manufacturing effectively improves the shortcomings of conventional processing while increasing production speed, accuracy, and complexity, offering great promise for self-healing polymer applications. This article summarizes the current self-healing mechanisms of self-healing polymers and their corresponding additive manufacturing methods, and provides an outlook on future developments in the field.

Chitosan-Based Self-Healing Hydrogel: From Fabrication to Biomedical Application

Biocompatible self-healing hydrogels are new-generation smart soft materials that hold great promise in biomedical fields. Chitosan-based self-healing hydrogels, mainly prepared via dynamic imine bonds, have attracted broad attention due to their mild preparation conditions, excellent biocompatibility, and self-recovery ability under a physiological environment. In this review, we present a comprehensive overview of the design and fabrication of chitosan-based self-healing hydrogels, and summarize their biomedical applications in tissue regeneration, customized drug delivery, smart biosensors, and three/four dimensional (3D/4D) printing. Finally, we will discuss the challenges and future perspectives for the development of chitosan-based self-healing hydrogels in the biomedical field.

Chitosan-based transparent and conductive hydrogel with highly stretchable, adhesive and self-healing as skin-like sensor

Hydrogel sensors attained increasing attention due to their excellent mechanical and sensing properties. However, it is still a big challenge to fabricate hydrogel sensors with multifunctional properties of transparent, high stretchability, self-adhesive and self-healing ability. In this study, chitosan as a natural polymer has been employed to construct a polyacrylamide-chitosan-Al³⁺ (PAM-CS-Al³⁺) double network (DN) hydrogel with high transparency (>90 % at 800 nm), good electrical conductivity (up to 5.01 S/m) and excellent mechanical properties (strain and toughness as high as 1040 % and 730 kJ/m³). Moreover, the dynamic ionic and hydrogen bond interaction between PAM and CS endowed the PAM-CS-Al³⁺ hydrogel good self-healing ability. In addition, the hydrogel possesses good self-adhesive ability on different substrates, including glass, wood, metal, plastic, paper, polytetrafluoroethylene (PTFE) and rubber. Most importantly, the prepared hydrogel could be assembled into transparent, flexible, self-adhesive, self-healing and high sensitive strain/pressure sensor for monitoring human body movement. This work may pave the way for fabricating the multifunctional chitosan-based hydrogels which has potential application in the fields of wearable sensor and soft electronic devices.

Injectable zwitterionic cryogels for accurate and sustained chemoimmunotherapy

Chemoimmunotherapy is an effective method to treat cancer, and thus various vehicles have been constructed to co-deliver immune agents and anticancer drugs. But the immune induction process in vivo is highly susceptible to the influence of the material itself. To avoid immune reactions by the materials of delivery systems, herein, a new kind of zwitterionic cryogels (SH cryogels) with extremely low immunogenicity was prepared for chemoimmunotherapy of cancer. Their macroporous structure enabled the SH cryogels to have good compressibility and be injected through a conventional syringe. The loaded chemotherapeutic drugs and immune adjuvants were accurately, locally and long-termly released in the vicinity of tumors, enhancing the outcome of tumor therapy and minimizing the damage caused by the chemotherapeutic drugs to other organ tissues. In vivo tumor treatment experiments indicated that chemoimmunotherapy using the SH cryogel platform could inhibit the growth of breast cancer tumors to the greatest extent. Furthermore, macropores of SH cryogels supported cells to move freely in the cryogels, which could promote the dendritic cells to capture the in situ produced tumor antigens and present them to T cells. The ability to act as cradles for cell infiltration made the SH cryogels promising for applications as vaccine platforms.

Highly Stretchable, Self-Adhesive, Antidrying Ionic Conductive Organohydrogels for Strain Sensors

As flexible wearable devices, hydrogel sensors have attracted extensive attention in the field of soft electronics. However, the application or long-term stability of conventional hydrogels at extreme temperatures remains a challenge due to the presence of water. Antifreezing and antidrying ionic conductive organohydrogels were prepared using cellulose nanocrystals and gelatin as raw materials, and the hydrogels were prepared in a water/glycerol binary solvent by a one-pot method. The prepared hydrogels were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The mechanical properties, electrical conductivity, and sensing properties of the hydrogels were studied by means of a universal material testing machine and LCR digital bridge. The results show that the ionic conductive hydrogel exhibits high stretchability (elongation at break, 584.35%) and firmness (up to 0.16 MPa). As the binary solvent easily forms strong hydrogen bonds with water molecules, experiments show that the organohydrogels exhibit excellent freezing and drying (7 days). The organohydrogels maintain conductivity and stable sensitivity at a temperature range (-50 °C-50 °C) and after long-term storage (7 days). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 6.47 (strain, 0-400%) could detect human motions. Therefore, multifunctional organohydrogel wearable sensors with antifreezing and antidrying properties have promising potential for human body monitoring under a broad range of environmental conditions.

A Rapidly and Highly Self-Healing Poly(Sulfobetaine Methacrylate) Hydrogel with Stretching Properties, Adhesive Properties, and Biocompatibility

This study focuses on the preparation of stretchable zwitterionic poly(sulfobetaine methacrylate) (PSBMA) hydrogels. To address the weak mechanical properties of chemically crosslinked PSBMA hydrogels, a physical crosslinking method utilizing hydrophobic interactions to crosslink hydrogels to approach tough properties is developed. Here, sodium dodecyl sulfate (SDS)-based micelle is used as a physical crosslinker to prepare physically crosslinked PSBMA (PSBMA_phy ) hydrogels, and ethylene glycol dimethylacrylate (EGDMA) is used to prepare a control group of chemically crosslinked PSBMA (PSBMA_chem ) hydrogels. The mechanical properties of the two hydrogels are compared, and PSBMA_phy hydrogels exhibit greater flexibility than the PSBMA_chem hydrogels. When the PSBMA_phy hydrogels are subjected to external forces, the micelles act as dynamic crosslinking sites, allowing the stress to disperse and prevent the hydrogel from breaking. In addition, the PSBMA_phy hydrogels have nearly 100% self-healing properties within 2.5 min. The PSBMA_phy hydrogels exhibit usable adhesive properties to porcine skin and subcutis. MTT and hemolysis tests show that the PSBMA_phy hydrogels have excellent biocompatibility and hemocompatibility. This study proposes that the multifunctional PSBMA_phy hydrogels with micelles will be potential to carry drugs for use in drug delivery systems in the future.

Wearable biosensors for human fatigue diagnosis: A review

Fatigue causes deleterious effects to physical and mental health of human being and may cause loss of lives. Therefore, the adverse effects of fatigue on individuals and the society are massive. With the ever-increasing frequency of overtraining among modern military and sports personnel, timely, portable and accurate fatigue diagnosis is essential to avoid fatigue-induced accidents. However, traditional detection methods require complex sample preparation and blood sampling processes, which cannot meet the timeliness and portability of fatigue diagnosis. With the development of flexible materials and biosensing technology, wearable biosensors have attracted increased attention to the researchers. Wearable biosensors collect biomarkers from noninvasive biofluids, such as sweat, saliva, and tears, followed by biosensing with the help of biosensing modules continuously and quantitatively. The detection signal can then be transmitted through wireless communication modules that constitute a method for real-time understanding of abnormality. Recent developments of wearable biosensors are focused on miniaturized wearable electrochemistry and optical biosensors for metabolites detection, of which, few have exhibited satisfactory results in medical diagnosis. However, detection performance limits the wide-range applicability of wearable fatigue diagnosis. In this article, the application of wearable biosensors in fatigue diagnosis has been discussed. In fact, exploration of the composition of different biofluids and their potential toward fatigue diagnosis have been discussed here for the very first time. Moreover, discussions regarding the current bottlenecks in wearable fatigue biosensors and the latest advancements in biochemical reaction and data communication modules have been incorporated herein. Finally, the main challenges and opportunities were discussed for wearable fatigue diagnosis in the future.

A 3D printable, highly stretchable, self-healing hydrogel-based sensor based on polyvinyl alcohol/sodium tetraborate/sodium alginate for human motion monitoring

Self-healing hydrogels have great application potential in the field of bio-sensors due to their self-healing, flexibility and excellent tensile properties. However, most hydrogel-based sensors are processed by template method, which is unable to fabricate complex three-dimensional (3D) structures, and limits the development of hydrogel-based sensor devices. A simple yet efficient one-pot method was proposed to fabricate polyvinyl alcohol/sodium tetraborate/sodium alginate hydrogel inks (SPB), also a fabricating process of self-healing hydrogel based on 3D printing technology has been proposed. The SPB hydrogel rapidly healed (<30 s) at room temperature, while its mechanical properties and conductivity also recovered quickly after healing. Besides, it could be used as wearable strain sensors, whose high stretchability (>2800 % strain) and sensitivity (gauge factor: 18.56 at 2000 % strain) could not only detect very large stretch deformations, but also detect the tiny pressure changes in the human body, such as finger flexion, knee flexion, and respiration. This study provides a method for the rapid fabrication of complex-structured hydrogel-based sensors, which is helpful for the hydrogel-based sensor applications in human motion detection and wearable devices.

Manufacturing and post-engineering strategies of hydrogel actuators and sensors: From materials to interfaces

Living bodies are made of numerous bio-sensors and actuators for perceiving external stimuli and making movement. Hydrogels have been considered as ideal candidates for manufacturing bio-sensors and actuators because of their excellent biocompatibility, similar mechanical and electrical properties to that of living organs. The key point of manufacturing hydrogel sensors/actuators is that the materials should not only possess excellent mechanical and electrical properties but also form effective interfacial connections with various substrates. Traditional hydrogel normally shows high electrical resistance (~ MΩ•cm) with limited mechanical strength (<1 MPa), and it is prone to fatigue fracture during continuous loading-unloading cycles. Just like iron should be toughened and hardened into steel, manufacturing and post-treatment processes are necessary for modifying hydrogels. Besides, advanced design and manufacturing strategies can build effective interfaces between sensors/actuators and other substrates, thus enhancing the desired mechanical and electrical performances. Although various literatures have reviewed the manufacture or modification of hydrogels, the summary regarding the post-treatment strategies and the creation of effective electrical and mechanically sustainable interfaces are still lacking. This paper aims at providing an overview of the following topics: (i) the manufacturing and post-engineering treatment of hydrogel sensors and actuators; (ii) the processes of creating sensor(actuator)-substrate interfaces; (iii) the development and innovation of hydrogel manufacturing and interface creation. In the first section, the manufacturing processes and the principles for post-engineering treatments are discussed, and some typical examples are also presented. In the second section, the studies of interfaces between hydrogels and various substrates are reviewed. Lastly, we summarize the current manufacturing processes of hydrogels, and provide potential perspectives for hydrogel manufacturing and post-treatment methods.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Chitosan-based high-strength supramolecular hydrogels for 3D bioprinting

The loss of tissues and organs is a major challenge for biomedicine, and the emerging 3D bioprinting technology has brought the dawn for the development of tissue engineering and regenerative medicine. Chitosan-based supramolecular hydrogels, as novel biomaterials, are considered as ideal materials for 3D bioprinting due to their unique dynamic reversibility and fantastic biological properties. Although chitosan-based supramolecular hydrogels have wonderful biological properties, the mechanical properties are still under early exploration. This paper aims to provide some inspirations for researchers to further explore. In this review, common 3D bioprinting techniques and the properties required for bioink for 3D bioprinting are firstly described. Then, several strategies to enhance the mechanical properties of chitosan hydrogels are introduced from the perspectives of both materials and supramolecular binding motifs. Finally, current challenges and future opportunities in this field are discussed. The combination of chitosan-based supramolecular hydrogels and 3D bioprinting will hold promise for developing novel biomedical implants.

A Highly Mechanical, Conductive, and Cryophylactic Double Network Hydrogel for Flexible and Low-Temperature Tolerant Strain Sensors

Due to their stretchability, conductivity, and good biocompatibility, hydrogels have been recognized as potential materials for flexible sensors. However, it is still challenging for hydrogels to meet the conductivity, mechanical strength, and freeze-resistant requirements in practice. In this study, a chitosan-poly (acrylic acid-co-acrylamide) double network (DN) hydrogel was prepared by immersing the chitosan-poly (acrylic acid-co-acrylamide) composite hydrogel into Fe₂(SO₄)₃ solution. Due to the formation of an energy-dissipative chitosan physical network, the DN hydrogel possessed excellent tensile and compression properties. Moreover, the incorporation of the inorganic salt endowed the DN hydrogel with excellent conductivity and freeze-resistance. The strain sensor prepared using this DN hydrogel displayed remarkable sensitivity and reliability in detecting stretching and bending deformations. In addition, this DN hydrogel sensor also worked well at a lower temperature (−20 °C). The highly mechanical, conductive, and freeze-resistant DN hydrogel revealed a promising application in the field of wearable devices.

Tannic acid modified antifreezing gelatin organohydrogel for low modulus, high toughness, and sensitive flexible strain sensor

Current hydrogel strain sensors have met assorted essential requirements of wearing comfort, mechanical toughness, and strain sensitivity. However, an increment in the toughness of a hydrogel usually leads to an increase in elastic moduli that could be unfavorable for wearing comfort. In addition, traits of biofriendly and sustainability require synthesis of the hydrogels from natural polymer-based networks. We propose a novel strategy to fabricate an ionic conductive organohydrogel from natural biological macromolecule "gelatin" and polyacid "tannic acid" to resolve these challenges. Tannic acid modified the structure of the gelatin network in the ionic conductive organohydrogels, that not only led to an increase in toughness accompanying a decrease in elastic moduli but also headed to higher strain sensitivity and tunability. The proposed methodology exhibited tunable tensile modulus from 27 to 13 kPa, tensile strength from 287 to 325 kPa, elongation at fracture from 510 to 620%, toughness from 500 to 550 kJ/m³_, conductivity from 0.29 to 0.8 S/m, and strain sensitivity (GF = 1.4-6.5). Moreover, the proposed organohydrogel exhibited excellent freezing tolerance. This study provides a facile yet powerful strategy to tune the mechanical and electrical properties of organohydrogels which can be adapted to various wearable sensors.

Skin-Adaptable, Long-Lasting Moisture, and Temperature-Tolerant Hydrogel Dressings for Accelerating Burn Wound Healing without Secondary Damage

Developing multifunctional wound dressings, possessing not only skin-like mechanical properties and adaptability, long-lasting moisture, and temperature tolerance that maximally mimics the human skin but also on-demand adhesion without unnecessary bleeding and secondary damage upon peeling, is necessary but remains a challenge. Herein, a novel dual cross-linked and multifunctional hydrogel, termed PSNC hydrogel for polymerized sulfobetaine methacrylate (SBMA), N-(2-amino-2-oxyethyl)acrylamide (NAGA), and 1-carboxy-N-methyl-N-di(2-methacryloyloxy-ethyl)methanaminium inner salt (CBMAX), was fabricated as a wound dressing for burn injuries via one-pot radical polymerization in glycerine (GLY)/H₂O solvent. The dual cross-linked network of the PSNC hydrogel combined the double hydrogen bonding of N-(2-amino-2-oxyethyl)acrylamide (NAGA) with a covalently cross-linked zwitterionic network, endowing the hydrogel with skin-like mechanical properties with a high stretchability of 1613.8 ± 79.8%, a tensile strength of 77.5 ± 1.8 kPa, and a tensile modulus of 1.9 ± 0.1 kPa. Moreover, the hydrogel with well-developed adaptability can withstand skin deformation without breaking or debonding attributed to its good tissue adhesiveness and self-healing ability. Further, the utilization of the GLY/H₂O binary solvent effectively prevented the crystallization and evaporation of free water, endowing the hydrogel with not only long-lasting moisture but also excellent temperature tolerance in a wide range from -20 to 60 °C. More importantly, the PSNC hydrogel could effectively accelerate wound healing of burn injuries and could be easily removed on-demand with saline without causing secondary damage due to intense hydration. Such a novel PSNC zwitterionic hydrogel could be a promising candidate for the treatment of burn wounds and tissue regeneration.

A review on the features, performance and potential applications of hydrogel-based wearable strain/pressure sensors

Over the past few years, development of wearable devices has gained increasing momentum. Notably, the demand for stretchable strain sensors has significantly increased due to many potential and emerging applications such as human motion monitoring, prosthetics, robotic systems, and touch panels. Recently, hydrogels have been developed to overcome the drawbacks of the elastomer-based wearable strain sensors, caused by insufficient biocompatibility, brittle mechanical properties, complicated fabrication process, as the hydrogels can provide a combination of various exciting properties such as intrinsic electrical conductivity, suitable mechanical properties, and biocompatibility. There are numerous research works reported in the literature which consider various aspects as preparation approaches, design strategies, properties control, and applications of hydrogel-based strain sensors. This article aims to present a review on this exciting topic with a new insight on the hydrogel-based wearable strain sensors in terms of their features, strain sensory performance, and prospective applications. In this respect, we first briefly review recent advances related to designing the materials and the methods for promoting hydrogels' intrinsic features. Then, strain (both tensile and pressure) sensing performance of prepared hydrogels is critically studied, and alternative approaches for their high-performance sensing are proposed. Subsequently, this review provides several promising applications of hydrogel-based strain sensors, including bioapplications and human-machine interface devices. Finally, challenges and future outlooks of conductive and stretchable hydrogels employed in the wearable strain sensors are discussed.

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.

Advances in multifunctional chitosan-based self-healing hydrogels for biomedical applications

Multifunctional self-healing hydrogels have recently attracted considerable interest in biomedical applications owing to their diverse properties, including self-healing, adhesion, conduction, antibacterial, and stimulus-response, which can meet various application requirements, ranging from wound dressings and delivery vehicles to the production of scaffolds for tissue repair and regeneration. As a natural polycationic polysaccharide with good biocompatibility, chitosan is widely used in hydrogel formation as there are many amino and hydroxyl groups along the chains that can actively participate in various physical effects and chemical reactions, which enable it to construct self-healing hydrogels and fulfill multiple functions. In this review, the formation of chitosan-based self-healing hydrogels and the related self-healing mechanism are summarized, including Schiff base, metal coordination, ionic and hydrogen bonds, hydrophobic and host-guest interactions, with a focus on the strategies for their multi-functionalization. In the last section, the applications of the chitosan-based self-healing hydrogels in the fields of wound dressings, delivery vehicles, scaffolds, and biological sensors are discussed. Overall, it is highly expected that this review could provide an insight into the prospective development of multifunctional self-healing hydrogels for biomedical applications.

Injectable Nanocomposite Hydrogels for Cancer Therapy

Hydrogel is a kind of 3D polymer network with strong swelling ability in water and appropriate mechanical and biological properties, which make it feasible to maintain bioactive substances and has promising applications in the fields of biomaterials, soft machines, and artificial tissues. Unfortunately, traditional hydrogels prepared by chemical crosslinking have poor mechanical properties and limited functions, which limit their further application. In recent years, with the continuous development of nanoparticle research, more and more studies have combined nanoparticles with hydrogels to make up for the shortcomings of traditional hydrogels. In this article, the types and functions of hydrogels and nanomaterials are introduced first, as well as the functions and applications of injectable nanocomposite hydrogels (INHs), then the latest progress of INHs for cancer treatment is reviewed, some existing problems are summarized, and the application prospect of NHs is prospected.

Synergy coordination of cellulose-based dialdehyde and carboxyl with Fe3+ recoverable conductive self-healing hydrogel for sensor

A novel dual ionic network cellulose-based composite conductive self-healing hydrogel was fabricated with high elongation, rapid recoverability, high conductive sensitivity, self-healing ability and good biocompatibility. The hydrogel was constructed by the synergistic complexations of new-fashioned bidentate aldehyde groups on dialdehyde cellulose nanofibers (DACNFs) and carboxyl groups of acrylic acid (AA) with Fe³⁺. The elongation (~1300%) of the hydrogel containing 1 wt% DACNFs was approximate 13-fold of the pure PAA hydrogel and can recover to original state within 2 min after 80% compression. The self-healing efficiency increased with the addition of DACNFs in the dual ionic network cellulose-based composite conductive self-healing hydrogel. The hydrogel configured for a wearable test and showed high stretching sensitivity with a gauge factor of 13.82 at strain within 1.6%. The gauge factor (GF) decrease with the incremental strain within 20%. GF were 0.696 between 20% and 300% strain, 0.837 within 300% and 500%. Meanwhile, the current had a good linear relationship with the bending angles of hydrogels and pressure on hydrogels, which may provide a great potential in monitor both minor variations and large movements.

Additive Manufacturing of Sensors for Military Monitoring Applications

The US Department of Defense (DoD) realizes the many uses of additive manufacturing (AM) as it has become a common fabrication technique for an extensive range of engineering components in several industrial sectors. 3D Printed (3DP) sensor technology offers high-performance features as a way to track individual warfighters on the battlefield, offering protection from threats such as weaponized toxins, bacteria or virus, with real-time monitoring of physiological events, advanced diagnostics, and connected feedback. Maximum protection of the warfighter gives a distinct advantage over adversaries by providing an enhanced awareness of situational threats on the battle field. There is a need to further explore aspects of AM such as higher printing resolution and efficiency, with faster print times and higher performance, sensitivity and optimized fabrication to ensure that soldiers are more safe and lethal to win our nation's wars and come home safely. A review and comparison of various 3DP techniques for sensor fabrication is presented.

Recent Advances in Injectable Dual Crosslinking Hydrogels for Biomedical Applications

Injectable dual crosslinking hydrogels hold great promise to improve therapeutic efficacy in minimally invasive surgery. Compared with prefabricated hydrogels, injectable hydrogels can be implanted more accurately into deeply enclosed sites and repair irregularly shaped lesions, showing great applicable potential. Here, the current fabrication considerations of injectable dual crosslinking hydrogels are reviewed. Besides, the progress of the hydrogels used in corresponding applications and emerging challenges are discussed, with detailed emphasis in the fields of bone and cartilage regeneration, wound dressings, sensors and other less mentioned applications for their more hopeful employments in clinic. It is envisioned that the further development of the injectable dual crosslinking hydrogels will catalyze their innovation and transformation in the biomedical field.

Wearable lignin-based hydrogel electronics: A mini-review

In recent years, various biomacromolecule-based hydrogels have been extensively and deeply studied in the field of wearable electronics. However, the application of lignin-based hydrogels in flexible devices is still in its infancy. This is mainly due to the significant differences in physical and chemical properties of industrially extracted lignin. In order to seek the universal applicability of diversified lignin in the preparation of hydrogel electronics, we mainly paid attention to the natural physical and chemical properties of lignin to discuss feasible solutions for functional gel design. These properties include chemical reactivity, UV shielding, antibacterial, bio-degradability, anti-oxidation, etc. Finally, in view of lignin's unique properties and the demand for high-quality flexible electronics, some insights are proposed regarding the future research and development directions of lignin-based hydrogel electronics.

A highly sensitive strain sensor based on a silica@polyaniline core-shell particle reinforced hydrogel with excellent flexibility, stretchability, toughness and conductivity

Hydrogel-based flexible strain sensors for personal health monitoring and human-machine interaction have attracted wide interest among researchers. In this paper, hydrophobic association and nanocomposite conductive hydrogels were successfully prepared by introducing polyaniline coated silica (SiO2@PANI) core-shell particles into an acrylamide-lauryl methacrylate (P(AM/LMA)) copolymer matrix. The hydrophobic interaction between the SiO2@PANI core-shell particles and the hydrophobic LMA in the P(AM/LMA) chains induced the hydrogels with outstanding mechanical properties. Furthermore, the polyaniline on the SiO2 surface and the inorganic salt formed a conductive network, which synergistically enhanced the conductivity of the hydrogels. The obtained hydrogels integrate high tensile strength (1398 kPa), ultra-stretchability (>1000%), wonderful strain sensitivity (gauge factor = 10.407 at 100-1100% strain), quick response (300 ms), and excellent durability (>300 cycles) due to the hydrophobic association and nanocomposite effect. The prepared SiO2@PANI-P(AM/LMA) hydrogel shows high stress sensitivity to detect human movements and displays a broad application prospect in flexible strain-sensor field.