Injectable and Conductive Granular Hydrogels for 3D Printing and Electroactive Tissue Support

Wet tissue adhesive polymeric powder hydrogels for skeletal muscle regeneration

Volumetric muscle loss (VML) frequently results from traumatic incidents and can lead to severe functional disabilities. Hydrogels have been widely employed for VML tissue regeneration, which are unfortunately ineffective because of the lack of intimate contact with injured tissue for structural and mechanical support. Adhesive hydrogels allow for strong tissue connections for wound closure. Nevertheless, conventional adhesive hydrogels exhibit poor tissue adhesion in moist, bleeding wounds due to the hydration layer at the tissue-hydrogel interfaces, resulting in insufficient performance. In this study, we developed a novel, biocompatible, wet tissue adhesive powder hydrogel consisting of dextran-aldehyde (dex-ald) and gelatin for the regeneration of VML. This powder absorbs the interfacial tissue fluid and buffer solution on the tissue, spontaneously forms a hydrogel, and strongly adheres to the tissue via various molecular interactions, including the Schiff base reaction. In particular, the powder composition with a 1:4 ratio of dex-ald to gelatin exhibited optimal characteristics with an appropriate gelation time (258 s), strong tissue adhesion (14.5 kPa), and stability. Dex-ald/gelatin powder hydrogels presented strong adhesion to various organs and excellent hemostasis compared to other wet hydrogels and fibrin glue. A mouse VML injury model revealed that the dex-ald/gelatin powder hydrogel significantly improved muscle regeneration, reduced fibrosis, enhanced vascularization, and decreased inflammation. Consequently, our wet-adhesive powder hydrogel can serve as an effective platform for repairing various tissues, including the heart, muscle, and nerve tissues.

Stimuli-responsive microcarriers and their application in tissue repair: A review of magnetic and electroactive microcarrier

Microcarrier applications have made great advances in tissue engineering in recent years, which can load cells, drugs, and bioactive factors. These microcarriers can be minimally injected into the defect to help reconstruct a good microenvironment for tissue repair. In order to achieve more ideal performance and face more complex tissue damage, an increasing amount of effort has been focused on microcarriers that can actively respond to external stimuli. These microcarriers have the functions of directional movement, targeted enrichment, material release control, and providing signals conducive to tissue repair. Given the high controllability and designability of magnetic and electroactive microcarriers, the research progress of these microcarriers is highlighted in this review. Their structure, function and applications, potential tissue repair mechanisms, and challenges are discussed. In summary, through the design with clinical translation ability, meaningful and comprehensive experimental characterization, and in-depth study and application of tissue repair mechanisms, stimuli-responsive microcarriers have great potential in tissue repair.

Design of PEG-based hydrogels as soft ionic conductors

Conductive hydrogels have gained interest in biomedical applications and soft electronics. To tackle the challenge of ionic hydrogels falling short of desired mechanical properties in previous studies, our investigation aimed to understand the pivotal structural factors that impact the conductivity and mechanical behavior of polyethylene glycol (PEG)-based hydrogels with ionic conductivity. Polyether urethane diacrylamide (PEUDAm), a functionalized long-chain macromer based on PEG, was used to synthesize hydrogels with ionic conductivity conferred by incorporating ions into the liquid phase of hydrogel. The impact of salt concentration, water content, temperature, and gel formation on both mechanical properties and conductivity was characterized to establish parameters for tuning hydrogel properties. To further expand the range of conductivity available in these ionic hydrogels, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was incorporated as a single copolymer network or double network configuration. As expected, conductivity in these ionic gels was primarily driven by ion diffusivity and charge density, which was dependent on hydrogel network formation and swelling. Copolymer network structure had minimal effect on the conductivity which was primarily driven by counter-ion equilibrium; however, the mechanical properties and equilibrium swelling was strongly dependent on network structure. The structure-property relationships elucidated here enables the rationale design of this new double network hydrogel to achieve target properties for a broad range of applications.

Granular Biphasic Colloidal Hydrogels for 3D Bioprinting

Granular hydrogels composed of hydrogel microparticles are promising candidates for 3D bioprinting due to their ability to protect encapsulated cells. However, to achieve high print fidelity, hydrogel microparticles need to jam to exhibit shear-thinning characteristics, which is crucial for 3D printing. Unfortunately, this overpacking can significantly impact cell viability, thereby negating the primary advantage of using hydrogel microparticles to shield cells from shear forces. To overcome this challenge, a novel solution: a biphasic, granular colloidal bioink designed to optimize cell viability and printing fidelity is introduced. The biphasic ink consists of cell-laden polyethylene glycol (PEG) hydrogel microparticles embedded in a continuous gelatin methacryloyl (GelMA)-nanosilicate colloidal network. Here, it is demonstrated that this biphasic bioink offers outstanding rheological properties, print fidelity, and structural stability. Furthermore, its utility for engineering complex tissues with multiple cell types and heterogeneous microenvironments is demonstrated, by incorporating β-islet cells into the PEG microparticles and endothelial cells in the GelMA-nanosilicate colloidal network. Using this approach, it is possible to induce cell patterning, enhance vascularization, and direct cellular function. The proposed biphasic bioink holds significant potential for numerous emerging biomedical applications, including tissue engineering and disease modeling.

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.

Injectable and Self-Curing Single-Component Hydrogel for Stem Cell Encapsulation and In Vivo Bone Regeneration

An ideal hydrogel for stem cell therapy would be injectable and efficiently promote stem cell proliferation and differentiation in body. Herein, an injectable, single-component hydrogel with hyaluronic acid (HA) modified with phenylboronic acid (PBA) and spermidine (SM) is introduced. The resulting HAps (HA-PBA-SM) hydrogel is based on the reversible crosslinking between the diol and the ionized PBA, which is stabilized by the SM. It has a shear-thinning property, enabling its injection through a syringe to form a stable hydrogel inside the body. In addition, HAps hydrogel undergoes a post-injection "self-curing," which stiffens the hydrogel over time. This property allows the HAps hydrogel to meet the physical requirements for stem cell therapy in rigid tissues, such as bone, while maintaining injectability. The hydrogel enabled favorable proliferation of human mesenchymal stem cells (hMSCs) and promoted their differentiation and mineralization. After the injection of hMSCs-containing HAps into a rat femoral defect model, efficient osteogenic differentiation of hMSCs and bone regeneration is observed. The study demonstrates that simple cationic modification of PBA-based hydrogel enabled efficient gelation with shear-thinning and self-curing properties, and it would be highly useful for stem cell therapy and in vivo bone regeneration.

Preparation and characterization of photosensitive methacrylate-grafted sodium carboxymethyl cellulose as an injectable material to fabricate hydrogels for biomedical applications

Injectable materials have attracted great attention in the manufacture of in situ forming hydrogels for biomedical applications. In this study, a facile method to prepare methacrylic anhydride (MA)-modified sodium carboxymethyl cellulose (CMC) as an injectable material for the fabrication of hydrogels with controllable properties is reported. The chemical structure of the series of MA-grafted CMC (CMCMAs) with different MA contents was confirmed by Fourier transform infrared and nuclear magnetic resonance spectroscopy, and the properties of CMCMAs were characterized. Then, the CMCMAs gel (CMCMAs-G) was fabricated by crosslinking of MA under blue light irradiation. The gelation performances, swelling behaviors, transmittance, surface porous structures and mechanical properties of CMCMAs-G can be controlled by varying the content of MA grafted on the CMC. The compressive strength of CMCMAs-G was measured by mechanical compressibility tests and up to 180 kPa. Furthermore, the in vitro cytocompatibility evaluation results suggest that the obtained CMCMAs-G exhibit good compatibility for cell proliferation. Hence, our strategy provides a facile approach for the preparation of light-sensitive and an injectable CMC-derived polymer to fabricate hydrogels for biomedical applications.

Gallic acid: design of a pyrogallol-containing hydrogel and its biomedical applications

Polyphenol hydrogels have garnered widespread attention due to their excellent adhesion, antioxidant, and antibacterial properties. Gallic acid (GA) is a typical derivative of pyrogallol that is used as a hydrogel crosslinker or bioactive additive and can be used to make multifunctional hydrogels with properties superior to those of widely studied catechol hydrogels. Furthermore, compared to polymeric tannic acid, gallic acid is more suitable for chemical modification, thus broadening its range of applications. This review focuses on multifunctional hydrogels containing GA, aiming to inspire researchers in future biomaterial design. We first revealed the interaction mechanisms between GA molecules and between GA and polymers, analyzed the characteristics GA imparts to hydrogels and compared GA hydrogels with hydrogels containing catechol. Subsequently, in this paper, various methods of integrating GA into hydrogels and the applications of GA in biomedicine are discussed, finally assessing the current limitations and future development potential of GA. In summary, GA, a natural small molecule polyphenol with excellent functionality and diverse interaction modes, has great potential in the field of biomedical hydrogels.

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.

Progress of Research on Conductive Hydrogels in Flexible Wearable Sensors

Conductive hydrogels, characterized by their excellent conductivity and flexibility, have attracted widespread attention and research in the field of flexible wearable sensors. This paper reviews the application progress, related challenges, and future prospects of conductive hydrogels in flexible wearable sensors. Initially, the basic properties and classifications of conductive hydrogels are introduced. Subsequently, this paper discusses in detail the specific applications of conductive hydrogels in different sensor applications, such as motion detection, medical diagnostics, electronic skin, and human-computer interactions. Finally, the application prospects and challenges are summarized. Overall, the exceptional performance and multifunctionality of conductive hydrogels make them one of the most important materials for future wearable technologies. However, further research and innovation are needed to overcome the challenges faced and to realize the wider application of conductive hydrogels in flexible sensors.

Tilapia-Derived Granular Hydrogel as a 3D Scaffold Promoting Rapid Wound Healing

The skin, a crucial organ that protects the body, is vulnerable to external damage. Traditional tissue regeneration methods, including bulk hydrogels, aim to facilitate wound healing by interacting with host cells and providing a conducive environment. However, the nanoscale porosity of conventional hydrogels limits cell penetration and tissue regeneration. To overcome this, hydrogels composed of microgels have emerged as promising alternatives. In this study, we propose a granular hydrogel using decellularized tilapia skin. The tilapia skin-based microgels are cost-effective, immune-friendly, and have a high collagen content. Microgels based on the decellularized extracellular matrix of tilapia were successfully fabricated by using microfluidics. Through the assembly of these microgels using adhesive hyaluronic acid-catechol, the resulting 3D granular hydrogel scaffold facilitated enhanced cell growth, accelerated cell differentiation, and successful healing of full-thickness wounds in a mouse model. This study reveals the potential of tilapia skin-based granular hydrogel assembly in wound healing, overcoming conventional hydrogel limits.

Microstructure of the silk fibroin-based hydrogel scaffolds derived from the orb-web spider Trichonephila clavata

Due to the unique properties of the silk fibroin (SF) made from silkworm, SF-based hydrogels have recently received significant attention for various biomedical applications. However, research on the SF-based hydrogels isolated from spider silks has been rtricted due to the limited collection and preparation of naïve silk materials. Therefore, this study focused on the microstructural characteristics of hydrogel scaffolds derived from two types of woven silk glands: the major ampullate gland (MAG) and the tubuliform gland (TG), in the orb-web spider Trichonephila clavate. We compared these spider glands with those of the silk fibroin (SF) hydrogel scaffold extracted from the cocoon of the insect silkworm Bombyx mori. Our FESEM analysis revealed that the SF hydrogel has high porosity, translucency, and a loose upper structure, with attached SF fibers providing stability. The MAG hydrogel displayed even higher porosity, as well as elongated fibrous structures, and improved mechanical properties: while the TG hydrogel showed increased porosity, ridge-like or wall-like structures, and stable biocapacity formed by physical crosslinking. Due to their powerful and versatile microstructural characteristics, the MAG and TG hydrogels can become tailored substrates, very effective for tissue engineering and regenerative medicine applications.

The microparticulate inks for bioprinting applications

Three-dimensional (3D) bioprinting has emerged as a groundbreaking technology for fabricating intricate and functional tissue constructs. Central to this technology are the bioinks, which provide structural support and mimic the extracellular environment, which is crucial for cellular executive function. This review summarizes the latest developments in microparticulate inks for 3D bioprinting and presents their inherent challenges. We categorize micro-particulate materials, including polymeric microparticles, tissue-derived microparticles, and bioactive inorganic microparticles, and introduce the microparticle ink formulations, including granular microparticles inks consisting of densely packed microparticles and composite microparticle inks comprising microparticles and interstitial matrix. The formulations of these microparticle inks are also delved into highlighting their capabilities as modular entities in 3D bioprinting. Finally, existing challenges and prospective research trajectories for advancing the design of microparticle inks for bioprinting are discussed.

A Smart Stimulation-Deadhesion and Antimicrobial Hydrogel for Repairing Diabetic Wounds Infected with Methicillin-Resistant Staphylococcus aureus

The healing of chronic diabetic wounds is a common and significant challenge in the medical field. Despite extensive efforts, the development of hydrogel dressings with satisfactory functionality remains an ongoing concern. In this study, a multifunctional hydrogel wound dressing (PAN/Ag-PLG) with adhesion, antibacterial, hemostatic, and other properties, which can effectively repair diabetic wounds infected with methicillin-resistant Staphylococcus aureus (MRSA), is presented. The hydrogel dressing is composed of gallic acid (GA)-functionalized polylysine (PL)-reduced silver nanoparticles (Ag-PLG), oxidized hyaluronic acid (OHA), and cross-linked polyacrylic acid grafted with N-hydrosuccinimide ester. Notably, compared to most conventional wound dressing that lack adhesion or are difficult to remove, the prepared hydrogels exhibit excellent adhesion and mild stimulation-triggered detachment. In vitro and in vivo experiments reveal that the PAN/Ag-PLG hydrogel exhibits outstanding biocompatibility and antibacterial properties and promotes diabetic wound repair by reducing oxidative damage and promoting cell migration and angiogenesis. The smart PAN/Ag-PLG hydrogel reported in this study provides an approach for the potential clinical development of painless antibacterial dressings.

Embedded 3D Bioprinting of Collagen Inks into Microgel Baths to Control Hydrogel Microstructure and Cell Spreading

Microextrusion-based 3D bioprinting into support baths has emerged as a promising technique to pattern soft biomaterials into complex, macroscopic structures. It is hypothesized that interactions between inks and support baths, which are often composed of granular microgels, can be modulated to control the microscopic structure within these macroscopic-printed constructs. Using printed collagen bioinks crosslinked either through physical self-assembly or bioorthogonal covalent chemistry, it is demonstrated that microscopic porosity is introduced into collagen inks printed into microgel support baths but not bulk gel support baths. The overall porosity is governed by the ratio between the ink's shear viscosity and the microgel support bath's zero-shear viscosity. By adjusting the flow rate during extrusion, the ink's shear viscosity is modulated, thus controlling the extent of microscopic porosity independent of the ink composition. For covalently crosslinked collagen, printing into support baths comprised of gelatin microgels (15-50 µm) results in large pores (≈40 µm) that allow human corneal mesenchymal stromal cells (MSCs) to readily spread, while control samples of cast collagen or collagen printed in non-granular support baths do not allow cell spreading. Taken together, these data demonstrate a new method to impart controlled microscale porosity into 3D printed hydrogels using granular microgel support baths.

Conductive Microgel Annealed Scaffolds Enhance Myogenic Potential of Myoblastic Cells

Conductive biomaterials may capture native or exogenous bioelectric signaling, but incorporation of conductive moieties is limited by cytotoxicity, poor injectability, or insufficient stimulation. Microgel annealed scaffolds are promising as hydrogel-based materials due to their inherent void space that facilitates cell migration and proliferation better than nanoporous bulk hydrogels. Conductive microgels are generated from poly(ethylene) glycol (PEG and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) to explore the interplay of void volume and conductivity on myogenic differentiation. PEDOT: PSS increases microgel conductivity two-fold while maintaining stiffness, annealing strength, and viability of associated myoblastic cells. C2C12 myoblasts exhibit increases in the late-stage differentiation marker myosin heavy chain as a function of both porosity and conductivity. Myogenin, an earlier marker, is influenced only by porosity. Human skeletal muscle-derived cells exhibit increased Myod1, insulin like growth factor-1, and insulin-like growth factor binding protein 2 at earlier time points on conductive microgel scaffolds compared to non-conductive scaffolds. They also secrete more vascular endothelial growth factor at early time points and express factors that led to macrophage polarization patterns observe during muscle repair. These data indicate that conductivity aids myogenic differentiation of myogenic cell lines and primary cells, motivating the need for future translational studies to promote muscle repair.

A Shear-Thinning, Self-Healing, Dual-Cross Linked Hydrogel Based on Gelatin/Vanillin/Fe3+ /AGP-AgNPs: Synthesis, Antibacterial, and Wound-Healing Assessment

A shear-thinning and self-healing hydrogel based on a gelatin biopolymer is synthesized using vanillin and Fe3+ as dual crosslinking agents. Rheological studies indicate the formation of a strong gel found to be injectable and exhibit rapid self-healing (within 10 min). The hydrogels also exhibited a high degree of swelling, suggesting potential as wound dressings since the absorption of large amounts of wound exudate, and optimum moisture levels, lead to accelerated wound healing. Andrographolide, an anti-inflammatory natural product is used to fabricate silver nanoparticles, which are characterized and composited with the fabricated hydrogels to imbue them with anti-microbial activity. The nanoparticle/hydrogel composites exhibit activity against Escherichia coli, Staphylococcus aureus, and Burkholderia pseudomallei, the pathogen that causes melioidosis, a serious but neglected disease affecting southeast Asia and northern Australia. Finally, the nanoparticle/hydrogel composites are shown to enhance wound closure in animal models compared to the hydrogel alone, confirming that these hydrogel composites hold great potential in the biomedical field.

Versatile and User-Friendly Anti-infective Hydrogel for Effective Wound Healing

Wound dressings play a crucial role in facilitating optimal wound healing and protecting against microbial infections. However, existing commercial options often fall short in addressing chronic infections due to antibiotic resistance and the limited spectrum of activity against both Gram-positive and Gram-negative bacteria frequently encountered at wound sites. Additionally, complex fabrication processes and cumbersome administration strategies pose challenges for cost-effective wound dressing development. Consequently, there is a pressing need to explore easily engineered biocompatible biomaterials as alternative solutions to combat these challenging wound infections. In this study, we present the development of an anti-infective hydrogel, P-BAC (polymeric bactericidal hydrogel), which exhibits simple administration and promotes efficient wound healing. P-BAC is synthesized via a one-step fabrication method that involves the noncovalent cross-linking of poly(vinyl alcohol), N-(2-hydroxypropyl)-3-trimethylammonium chitosan chloride-AgCl nanocomposite, and proline. Remarkably, P-BAC demonstrates broad-spectrum antibacterial activity against both planktonic and stationary cells of clinically isolated Gram-positive and Gram-negative bacteria, resulting in a significant reduction of bacterial load (5-7 log reduction). Moreover, P-BAC exhibits excellent efficacy in eradicating bacterial cells within biofilm matrices (>95% reduction). In vivo experiments reveal that P-BAC accelerates wound healing by stimulating rapid collagen deposition at the wound site and effectively inactivates ∼95% of Pseudomonas aeruginosa cells. Importantly, the shear-thinning property of P-BAC simplifies the administration process, enhancing its practicality and usability. Taken together, our findings demonstrate the potential of this easily administrable hydrogel as a versatile solution for effective wound healing with potent anti-infective properties. The developed hydrogel holds promise for applications in diverse healthcare settings, addressing the critical need for improved wound dressing materials.

Conductive polymers for cardiac tissue engineering and regeneration

Cardiovascular diseases, such as myocardial infarction, are considered a significant global burden and the leading cause of death. Given the inability of damaged cardiac tissue to self-repair, cell-based tissue engineering and regeneration may be the only viable option for restoring normal heart function. To maintain the normal excitation-contraction coupling function of cardiac tissue, uniform electronic and ionic conductance properties are required. To transport cells to damaged cardiac tissues, several techniques, including the incorporation of cells into conductive polymers (CPs) and biomaterials, have been utilized. Due to the complexity of cardiac tissues, the success of tissue engineering for the damaged heart is highly dependent on several variables, such as the cell source, growth factors, and scaffolds. In this review, we sought to provide a comprehensive overview of the electro CPs and biomaterials used in the engineering and regeneration of heart tissue.

Injection-on-Skin Granular Adhesive for Interactive Human-Machine Interface

Realization of interactive human-machine interfaces (iHMI) is improved with development of soft tissue-like strain sensors beyond hard robotic exosuits, potentially allowing cognitive behavior therapy and physical rehabilitation for patients with brain disorders. Here, this study reports on a strain-sensitive granular adhesive inspired by the core-shell architectures of natural basil seeds for iHMI as well as human-metaverse interfacing. The granular adhesive sensor consists of easily fragmented hydropellets as a core and tissue-adhesive catecholamine layers as a shell, satisfying great on-skin injectability, ionic-electrical conductivity, and sensitive resistance changes through reversible yet robust cohesion among the hydropellets. Particularly, it is found that the ionic-electrical self-doping of the catecholamine shell on hydrosurfaces leads to a compact ion density of the materials. Based on these physical and electrical properties of the sensor, it is demonstrated that successful iHMI integration with a robot arm in both real and virtual environments enables robotic control by finger gesture and haptic feedback. This study expresses benefits of using granular hydrogel-based strain sensors for implementing on-skin writable bioelectronics and their bridging into the metaverse world.

Copper Ion-Modified Germanium Phosphorus Nanosheets Integrated with an Electroactive and Biodegradable Hydrogel for Neuro-Vascularized Bone Regeneration

Severe bone defects accompanied by vascular and peripheral nerve injuries represent a huge orthopedic challenge and are often accompanied by the risk of infection. Thus, biomaterials with antibacterial and neurovascular regeneration properties are highly desirable. Here, a newly designed biohybrid biodegradable hydrogel (GelMA) containing copper ion-modified germanium-phosphorus (GeP) nanosheets, which act as neuro-vascular regeneration and antibacterial agents, is designed. The copper ion modification process serves to improve the stability of the GeP nanosheets and offers a platform for the sustained release of bioactive ions. Study findings show that GelMA/GeP@Cu has effective antibacterial properties. The integrated hydrogel can significantly boost the osteogenic differentiation of bone marrow mesenchymal stem cells, facilitate angiogenesis in human umbilical vein endothelial cells, and up-regulate neural differentiation-related proteins in neural stem cells in vitro. In vivo, in the rat calvarial bone defect mode, the GelMA/GeP@Cu hydrogel is found to enhance angiogenesis and neurogenesis, eventually contributing to bone regeneration. These findings indicate that in the field of bone tissue engineering, GelMA/GeP@Cu can serve as a valuable biomaterial for neuro-vascularized bone regeneration and infection prevention.

Recent advances in 3D printable conductive hydrogel inks for neural engineering

Recently, the 3D printing of conductive hydrogels has undergone remarkable advances in the fabrication of complex and functional structures. In the field of neural engineering, an increasing number of reports have been published on tissue engineering and bioelectronic approaches over the last few years. The convergence of 3D printing methods and electrically conducting hydrogels may create new clinical and therapeutic possibilities for precision regenerative medicine and implants. In this review, we summarize (i) advancements in preparation strategies for conductive materials, (ii) various printing techniques enabling the fabrication of electroconductive hydrogels, (iii) the required physicochemical properties of the printed constructs, (iv) their applications in bioelectronics and tissue regeneration for neural engineering, and (v) unconventional approaches and outlooks for the 3D printing of conductive hydrogels. This review provides technical insights into 3D printable conductive hydrogels and encompasses recent developments, specifically over the last few years of research in the neural engineering field.

Single versus dual microgel species for forming guest-host microporous annealed particle PEG-MAL hydrogel

Inter-particle secondary crosslinks allow microporous annealed particle (MAP) hydrogels to be formed. Methods to introduce secondary crosslinking networks in MAP hydrogels include particle jamming, annealing with covalent bonds, and reversible noncovalent interactions. Here, we investigate the effect of two different approaches to secondary crosslinking of polyethylene glycol (PEG) microgels via reversible guest-host interactions. We generated a dual-particle MAP-PEG hydrogel using two species of PEG microgels, one functionalized with the guest molecule, adamantane, and the other with the host molecule, β-cyclodextrin (Inter-MAP-PEG). In a different approach, a mono-particle MAP-PEG hydrogel was generated using one species of microgel functionalized with both guest and host molecules (Intra-MAP-PEG). The Intra-MAP-PEG formed a homogenous distribution due to the single type of microgels used. We then compared the mechanical properties of these two types of MAP-PEG hydrogels and found that Intra-MAP-PEG resulted in significantly softer gels with lower yield stress. We investigated the effect of intra-particle guest-host interactions through titrated weight percentage and the concentration of functional groups added to the hydrogel. We found that there was an ideal concentration of guest-host molecules that enables intra- and inter-particle guest-host interactions with sufficient covalent crosslinking. Based on these studies, Intra-MAP-PEG provides a homogeneous guest-host hydrogel that is shear-thinning with reversible secondary crosslinking.

Stimuli-responsive dynamic hydrogels: design, properties and tissue engineering applications

The field of tissue engineering and regenerative medicine has been evolving at a rapid pace with numerous novel and interesting biomaterials being reported. Hydrogels have come a long way in this regard and have been proven to be an excellent choice for tissue regeneration. This could be due to their innate properties such as water retention, and ability to carry and deliver a multitude of therapeutic and regenerative elements to aid in better outcomes. Over the past few decades, hydrogels have been developed into an active and attractive system that can respond to various stimuli, thereby presenting a wider control over the delivery of the therapeutic agents to the intended site in a spatiotemporal manner. Researchers have developed hydrogels that respond dynamically to a multitude of external as well as internal stimuli such as mechanics, thermal energy, light, electric field, ultrasonics, tissue pH, and enzyme levels, to name a few. This review gives a brief overview of the recent developments in such hydrogel systems which respond dynamically to various stimuli, some of the interesting fabrication strategies, and their application in cardiac, bone, and neural tissue engineering.

Conductive microgel annealed scaffolds enhance myogenic potential of myoblastic cells

Bioelectricity is an understudied phenomenon to guide tissue homeostasis and regeneration. Conductive biomaterials may capture native or exogenous bioelectric signaling, but incorporation of conductive moieties is limited by cytotoxicity, poor injectability, or insufficient stimulation. Microgel annealed scaffolds are promising as hydrogel-based materials due to their inherent void space that facilitates cell migration and proliferation better than nanoporous bulk hydrogels. We generated conductive microgels from poly(ethylene) glycol and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to explore the interplay of void volume and conductivity on myogenic differentiation. PEDOT:PSS increased microgel conductivity over 2-fold while maintaining stiffness, annealing strength, and viability of associated myoblastic cells. C2C12 myoblasts exhibited increases in the late-stage differentiation marker myosin heavy chain as a function of both porosity and conductivity. Myogenin, an earlier marker, was influenced only by porosity. Human skeletal muscle derived cells exhibited increased Myod1 , IGF-1, and IGFBP-2 at earlier timepoints on conductive microgel scaffolds compared to non-conductive scaffolds. They also secreted higher levels of VEGF at early timepoints and expressed factors that led to macrophage polarization patterns observed during muscle repair. These data indicate that conductivity aids myogenic differentiation of myogenic cell lines and primary cells, motivating the need for future translational studies to promote muscle repair.

Design, characterization and applications of nanocolloidal hydrogels

Nanocolloidal gels (NCGs) are an emerging class of soft matter, in which nanoparticles act as building blocks of the colloidal network. Chemical or physical crosslinking enables NCG synthesis and assembly from a broad range of nanoparticles, polymers, and low-molecular weight molecules. The synergistic properties of NCGs are governed by nanoparticle composition, dimensions and shape, the mechanism of nanoparticle bonding, and the NCG architecture, as well as the nature of molecular crosslinkers. Nanocolloidal gels find applications in soft robotics, bioengineering, optically active coatings and sensors, optoelectronic devices, and absorbents. This review summarizes currently scattered aspects of NCG formation, properties, characterization, and applications. We describe the diversity of NCG building blocks, discuss the mechanisms of NCG formation, review characterization techniques, outline NCG fabrication and processing methods, and highlight most common NCG applications. The review is concluded with the discussion of perspectives in the design and development of NCGs.

Engineering Multi-Scale Organization for Biotic and Organic Abiotic Electroactive Systems

Multi-scale organization of molecular and living components is one of the most critical parameters that regulate charge transport in electroactive systems-whether abiotic, biotic, or hybrid interfaces. In this article, an overview of the current state-of-the-art for controlling molecular order, nanoscale assembly, microstructure domains, and macroscale architectures of electroactive organic interfaces used for biomedical applications is provided. Discussed herein are the leading strategies and challenges to date for engineering the multi-scale organization of electroactive organic materials, including biomolecule-based materials, synthetic conjugated molecules, polymers, and their biohybrid analogs. Importantly, this review provides a unique discussion on how the dependence of conduction phenomena on structural organization is observed for electroactive organic materials, as well as for their living counterparts in electrogenic tissues and biotic-abiotic interfaces. Expansion of fabrication capabilities that enable higher resolution and throughput for the engineering of ordered, patterned, and architecture electroactive systems will significantly impact the future of bioelectronic technologies for medical devices, bioinspired harvesting platforms, and in vitro models of electroactive tissues. In summary, this article presents how ordering at multiple scales is important for modulating transport in both the electroactive organic, abiotic, and living components of bioelectronic systems.

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

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

Multifunctional conductive hyaluronic acid hydrogels for wound care and skin regeneration

Although the main function of skin is to act as a protective barrier against external factors, it is indeed an extremely vulnerable tissue. Skincare, regardless of the wound type, requires effective treatments to prevent bacterial infection and local inflammation. The complex biological roles displayed by hyaluronic acid (HA) during the wound healing process have made this multifaceted polysaccharide an alternative biomaterial to prepare wound dressings. Therefore, herein, we present the most advanced research undertaken to engineer conductive and interactive hydrogels based on HA as wound dressings that enhance skin tissue regeneration either through electrical stimulation (ES) or by displaying multifunctional performance. First, we briefly introduce to the reader the effect of ES on promoting wound healing and why HA has become a vogue as a wound healing agent. Then, a selection of systems, chosen according to their multifunctional relevance, is presented. Special care has been taken to highlight those recently reported works (mainly from the last 3 years) with enhanced scalability and biomimicry. By doing that, we have turned a critical eye on the field considering what major challenges must be overcome for these systems to have real commercial, clinical, or other translational impact.

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and β-cyclodextrin-modified hyaluronan

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of β-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and β-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

Conductive and Adhesive Granular Alginate Hydrogels for On-Tissue Writable Bioelectronics

Conductive hydrogels are promising materials in bioelectronics that ensure a tissue-like soft modulus and re-enact the electrophysiological function of damaged tissues. However, recent approaches to fabricating conductive hydrogels have proved difficult: fixing of the conductive hydrogels on the target tissues hydrogels requires the aids from other medical glues because of their weak tissue-adhesiveness. In this study, an intrinsically conductive and tissue-adhesive granular hydrogel consisting of a PEDOT:PSS conducting polymer and an adhesive catechol-conjugated alginate polymer was fabricated via an electrohydrodynamic spraying method. Because alginate-based polymers can be crosslinked by calcium ions, alginate-catechol polymers mixed with PEDOT:PSS granular hydrogels (ACP) were easily fabricated. The fabricated ACP exhibited not only adhesive and shear-thinning properties but also conductivity similar to that of muscle tissue. Additionally, the granular structure makes the hydrogel injectable through a syringe, enabling on-tissue printing. This multifunctional granular hydrogel can be applied to soft and flexible electronics to connect humans and machines.

Building block properties govern granular hydrogel mechanics through contact deformations

Granular hydrogels have been increasingly exploited in biomedical applications, including wound healing and cardiac repair. Despite their utility, design guidelines for engineering their macroscale properties remain limited, as we do not understand how the properties of granular hydrogels emerge from collective interactions of their microgel building blocks. In this work, we related building block features (stiffness and size) to the macroscale properties of granular hydrogels using contact mechanics. We investigated the mechanics of the microgel packings through dynamic oscillatory rheology. In addition, we modeled the system as a collection of two-body interactions and applied the Zwanzig and Mountain formula to calculate the plateau modulus and viscosity of the granular hydrogels. The calculations agreed with the dynamic mechanical measurements and described how microgel properties and contact deformations define the rheology of granular hydrogels. These results support a rational design framework for improved engineering of this fascinating class of materials.

Annealing High Aspect Ratio Microgels into Macroporous 3D Scaffolds Allows for Higher Porosities and Effective Cell Migration

Growing millimeter-scaled functional tissue remains a major challenge in the field of tissue engineering. Therefore, microporous annealed particles (MAPs) are emerging as promising porous biomaterials that are formed by assembly of microgel building blocks. To further vary the pore size and increase overall MAP porosity of mechanically stable scaffolds, rod-shaped microgels with high aspect ratios up to 20 are chemically interlinked into highly porous scaffolds. Polyethylene glycol based microgels (width 10 µm, lengths up to 200 µm) are produced via in-mold polymerization and covalently interlinked into stable 3D scaffolds via epoxy-amine chemistry. For the first time, MAP porosities can be enhanced by increasing the microgel aspect ratio (mean pore sizes ranging from 39 to 82 µm, porosities from 65 to 90%). These porosities are significantly higher compared to constructs made from spherical or lower aspect ratio rod-shaped microgels. Rapid filling of the pores by either murine or primary human fibroblasts is ensured as cells migrate and grow extensively into these scaffolds. Overall, this study demonstrates that highly porous, stable macroporous hydrogels can be achieved with a very low partial volume of synthetic, high aspect ratio microgels, leading to large empty volumes available for cell ingrowth and cell-cell interactions.

Matrix-Assisted In Situ Polymerization of a 3D Conductive Hydrogel Structure

It is challenging to fabricate 3D architectures of conductive hydrogels and impart uniform conductivity at the same time. Here, we demonstrate a one-step 3D printing technique for controlling the 3D structure of hydrogel materials while simultaneously conferring uniform conductivity. The core technology lies in the in situ polymerization of conductive polymers by the diffusion of monomers and redox initiators to an interface. An alginate ink containing ammonium peroxide as a redox initiator is printed in a silica nanoparticle matrix containing a pyrrole monomer. A 3D structure of conductive polypyrrole is uniformly fabricated on the surface of the alginate immediately after the printing. This simple process provides uniform electrical conductivity throughout the bulk structure.

Injectable Microporous Annealed Particle Hydrogel Based on Guest-Host-Interlinked Polyethylene Glycol Maleimide Microgels

Microporous annealed particle (MAP) hydrogels have emerged as a versatile biomaterial platform for regenerative medicine. MAP hydrogels have been used for the delivery of cells and organoids but often require annealing post injection by an external source. We engineered an injectable, self-annealing MAP hydrogel with reversible interparticle linkages based on guest-host functionalized polyethylene glycol maleimide (PEG-MAL) microgels. We evaluated the effect of guest-host linkages on different types of microgels fabricated by either batch emulsion or mechanical fragmentation methods. Batch emulsion generated small spherical microgels with controllable 10-100 μm diameters and mechanical fragmentation generated irregular microgels with larger diameters (100-200 μm). Spherical microgels (15 μm) showed self-healing behavior and completely recovered from high strain while fragmented microgels (133 μm) did not recover. Guest-host interactions significantly contributed to the mechanical properties of spherical microgels but had no effect on fragmented microgels. Spherical microgels were superior to the fragmented microgels for co-injection of immune cells and pancreatic islets due to their lower force of injection, demonstrating more homogeneously distributed cells and greater cell viability after injection. Based on these studies, the spherical guest-host MAP hydrogels provide a controllable, injectable scaffold for engineered microenvironments and cell delivery applications.

A Photoannealed Granular Hydrogel Facilitating Hyaline Cartilage Regeneration via Improving Chondrogenic Phenotype

Hydrogel-based chondrocyte implantation presents a promising tissue engineering strategy for cartilage repair. However, the widely used elastic hydrogels usually restrict cell volume expansion and induce the dedifferentiation of encapsulated chondrocytes. To address this limitation, a photoannealed granular hydrogel (GH) composed of hyaluronic acid, polyethylene glycol, and gelatin was formulated for cartilage regeneration in this study. The unannealed GH prepared by Diels-Alder cross-linked microgels could be mixed with chondrocytes and delivered to cartilage defects by injection, after which light was introduced to anneal the scaffold, leading to the formation of a stable and microporous chondrocyte deploying scaffold. The in vitro studies showed that GH could promote the volume expansion and morphology recovery of chondrocytes and significantly improve their chondrogenic phenotype compared to the nongranular hydrogel (nGH) with similar compositions. Further in vivo studies of subcutaneous culture and the rat full-thickness cartilage defect model proved that chondrocyte loaded GH could significantly stimulate hyaline cartilage matrix deposition and connection, therefore facilitating hyaline-like cartilage regeneration. Finally, the mechanistic study revealed that GH might improve chondrogenic phenotype via activating the AMP-activated protein kinase/glycolysis axis. This study proves the great feasibility of GHs as in situ chondrocyte deploying scaffolds for cartilage regeneration and brings new insights in designing hydrogel scaffold for cartilage tissue engineering.

Sticking Together: Injectable Granular Hydrogels with Increased Functionality via Dynamic Covalent Inter-Particle Crosslinking

Granular hydrogels are an exciting class of microporous and injectable biomaterials that are being explored for many biomedical applications, including regenerative medicine, 3D printing, and drug delivery. Granular hydrogels often possess low mechanical moduli and lack structural integrity due to weak physical interactions between microgels. This has been addressed through covalent inter-particle crosslinking; however, covalent crosslinking often occurs through temporal enzymatic methods or photoinitiated reactions, which may limit injectability and material processing. To address this, a hyaluronic acid (HA) granular hydrogel is developed with dynamic covalent (hydrazone) inter-particle crosslinks. Extrusion fragmentation is used to fabricate microgels from photocrosslinkable norbornene-modified HA, additionally modified with either aldehyde or hydrazide groups. Aldehyde and hydrazide-containing microgels are mixed and jammed to form adhesive granular hydrogels. These granular hydrogels possess enhanced mechanical integrity and shape stability over controls due to the covalent inter-particle bonds, while maintaining injectability due to the dynamic hydrazone bonds. The adhesive granular hydrogels are applied to 3D printing, which allows the printing of structures that are stable without any further post-processing. Additionally, the authors demonstrate that adhesive granular hydrogels allow for cell invasion in vitro. Overall, this work demonstrates the use of dynamic covalent inter-particle crosslinking to enhance injectable granular hydrogels.

Controlled Covalent Self-Assembly of a Homopolymer for Multiscale Materials Engineering

Polymer self-assembly is a crucial process in materials engineering. Currently, almost all polymer self-assembly is limited to non-covalent bonding methods, even though these methods have drawbacks as they require complicated synthesis techniques and produce relatively unstable structures. Here, a novel mechanism of covalent polymer self-assembly is discovered and employed to address drawbacks of non-covalent polymer self-assembly. A simple ketone homopolymer is found to self-assemble into nano- to macroscale hydrogels during covalent crosslinking. In contrast to non-covalent self-assembly, the covalent self-assembly is independent of and unaffected by solvent conditions (e.g., polarity and ionic strength) and does not require additional agents, e.g., organic solvents and surfactants. The covalent polymer self-assembly is subjected to a new mechanism of control by tuning the covalent crosslinking rate. This leads to nanogels with an unprecedented and tightly controlled range of dimensions from less than 10 nm to above 100 nm. Moreover, the crosslinking rate also regulates the assembly behavior of microgels fabricated by microfluidics. The microgels self-assemble into granular fibers, which is 3D printed into stable porous scaffolds. The novel covalent polymer assembly method has enormous potential to revolutionize multiscale materials fabrication for applications in drug delivery, tissue engineering, and many other fields.

Mussel adhesion: A fundamental perspective on factors governing strong underwater adhesion

Protein-based underwater adhesives of marine organisms exhibit extraordinary binding strength in high salinity based on utilizing a variety of molecular interaction mechanisms. These include acid-base interactions, bidentate bindings or complex hydrogen bonding interactions, and electrochemical manipulation of interfacial bonding. In this Perspective, we briefly review recent progress in the field, and we discuss how interfacial electrochemistry can vary interfacial forces by concerted tuning of surface charging, hydration forces, and tuning of the interfacial ion concentration. We further discuss open questions, controversial findings, and new paths into understanding and utilizing redox-proteins and derived polymers for enhancing underwater adhesion in a complex salt environment.

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.

Microfluidics Fabrication of Micrometer-Sized Hydrogels with Precisely Controlled Geometries for Biomedical Applications

Micrometer-sized hydrogels are cross-linked three-dimensional network matrices with high-water contents and dimensions ranging from several to hundreds of micrometers. Due to their excellent biocompatibility and capability to mimic physiological microenvironments in vivo, micrometer-sized hydrogels have attracted much attention in the biomedical engineering field. Their biological properties and applications are primarily influenced by their chemical compositions and geometries. However, inhomogeneous morphologies and uncontrollable geometries limit traditional micrometer-sized hydrogels obtained by bulk mixing. In contrast, microfluidic technology holds great potential for the fabrication of micrometer-sized hydrogels since their geometries, sizes, structures, compositions, and physicochemical properties can be precisely manipulated on demand based on the excellent control over fluids. Therefore, micrometer-sized hydrogels fabricated by microfluidic technology have been applied in the biomedical field, including drug encapsulation, cell encapsulation, and tissue engineering. This review introduces micrometer-sized hydrogels with various geometries synthesized by different microfluidic devices, highlighting their advantages in various biomedical applications over those from traditional approaches. Overall, emerging microfluidic technologies enrich the geometries and morphologies of hydrogels and accelerate translation for industrial production and clinical applications.

Recent Advances in Microgels: From Biomolecules to Functionality

The emerging applications of hydrogel materials at different length scales, in areas ranging from sustainability to health, have driven the progress in the design and manufacturing of microgels. Microgels can provide miniaturized, monodisperse, and regulatable compartments, which can be spatially separated or interconnected. These microscopic materials provide novel opportunities for generating biomimetic cell culture environments and are thus key to the advances of modern biomedical research. The evolution of the physical and chemical properties has, furthermore, highlighted the potentials of microgels in the context of materials science and bioengineering. This review describes the recent research progress in the fabrication, characterization, and applications of microgels generated from biomolecular building blocks. A key enabling technology allowing the tailoring of the properties of microgels is their synthesis through microfluidic technologies, and this paper highlights recent advances in these areas and their impact on expanding the physicochemical parameter space accessible using microgels. This review finally discusses the emerging roles that microgels play in liquid-liquid phase separation, micromechanics, biosensors, and regenerative medicine.

Addressing the Shortcomings of Polyphenol-Derived Adhesives: Achievement of Long Shelf Life for Effective Hemostasis

For rapid and effective hemostasis of uncontrollable bleeding, versatile hemostatic agents have been emerging. Among them, polyphenol-derived adhesives have attracted those hemostatic materials due to instantaneous formation of sticky barriers by robust interactions between the material and the serum proteins from wound. However, a critical challenge in such phenolic materials lies in long-term storage due to spontaneous oxidation under humid environments, leading to changes in hemostatic capability and adhesive strength. Here, we report a transparent hemostatic film consisting of gallol-conjugated chitosan (CHI-G) for minimizing the phenolic oxidation even for 3 months and maintaining strong tissue adhesiveness and its hemostatic ability. The film undergoes a phase transition from solid to injectable hydrogels at physiological pH for efficiently stopping internal and external hemorrhage. Interestingly, the hemostatic capability of the CHI-G hydrogels after 3 month storage depends on (i) the folded microstructure of the polymer with optimal gallol modification and (ii) an initial phase of either a solution state or a solid film. When the hydrogels are originated from the dehydrated film, their successful hemostasis is observed in a liver bleeding model. Our finding would provide an insight for design rationale of hemostatic formulations with long shelf-life.

Tissue Adhesive, Conductive, and Injectable Cellulose Hydrogel Ink for On-Skin Direct Writing of Electronics

Flexible and soft bioelectronics used on skin tissue have attracted attention for the monitoring of human health. In addition to typical metal-based rigid electronics, soft polymeric materials, particularly conductive hydrogels, have been actively developed to fabricate biocompatible electrical circuits with a mechanical modulus similar to biological tissues. Although such conductive hydrogels can be wearable or implantable in vivo without any tissue damage, there are still challenges to directly writing complex circuits on the skin due to its low tissue adhesion and heterogeneous mechanical properties. Herein, we report cellulose-based conductive hydrogel inks exhibiting strong tissue adhesion and injectability for further on-skin direct printing. The hydrogels consisting of carboxymethyl cellulose, tannic acid, and metal ions (e.g., HAuCl4) were crosslinked via multiple hydrogen bonds between the cellulose backbone and tannic acid and metal-phenol coordinate network. Owing to this reversible non-covalent crosslinking, the hydrogels showed self-healing properties and reversible conductivity under cyclic strain from 0 to 400%, as well as printability on the skin tissue. In particular, the on-skin electronic circuit printed using the hydrogel ink maintained a continuous electrical flow under skin deformation, such as bending and twisting, and at high relative humidity of 90%. These printable and conductive hydrogels are promising for implementing structurally complicated bioelectronics and wearable textiles.

Recent Development of Conductive Hydrogels for Tissue Engineering: Review and Perspective

In recent years, tissue engineering techniques have been rapidly developed and offer a new therapeutic approach to organ or tissue damage repair. However, most of tissue engineering scaffolds are nonconductive and cannot establish effective electrical coupling with tissue for the electroactive tissues. Electroconductive hydrogels (ECHs) have received increasing attention in tissue engineering owing to their electroconductivity, biocompatibility and high water content. In vitro, ECHs can not only promote the communication of electrical signals between cells, but also mediate the adhesion, proliferation, migration, and differentiation of different kinds of cells. In vivo, ECHs can transmit the electric signal to electroactive tissues and activate bioelectrical signaling pathways to promote tissue repair. As a result, implanting ECHs into damaged tissues can effectively reconstruct physiological functions related to electrical conduction. In this review, we first present an overview about the classifications and the fabrication methods of ECHs. And then, the applications of ECHs in tissue engineering, including cardiac, nerve, skin and skeletal muscle tissue, are highlighted. At last, we provide some rational guidelines for designing ECHs towards clinical applications. This article is protected by copyright. All rights reserved.

Design and Fabrication of Polymeric Hydrogel Carrier for Nerve Repair

Nerve regeneration and repair still remain a huge challenge for both central nervous and peripheral nervous system. Although some therapeutic substances, including neuroprotective agents, clinical drugs and stem cells, as well as various growth factors, are found to be effective to promote nerve repair, a carrier system that possesses a sustainable release behavior, in order to ensure high on-site concentration during the whole repair and regeneration process, and high bioavailability is still highly desirable. Hydrogel, as an ideal delivery system, has an excellent loading capacity and sustainable release behavior, as well as tunable physical and chemical properties to adapt to various biomedical scenarios; thus, it is thought to be a suitable carrier system for nerve repair. This paper reviews the structure and classification of hydrogels and summarizes the fabrication and processing methods that can prepare a suitable hydrogel carrier with specific physical and chemical properties. Furthermore, the modulation of the physical and chemical properties of hydrogels is also discussed in detail in order to obtain a better therapeutic effect to promote nerve repair. Finally, the future perspectives of hydrogel microsphere carriers for stroke rehabilitation are highlighted.

Methods to Characterize Granular Hydrogel Rheological Properties, Porosity, and Cell Invasion

Granular hydrogels are formed through the packing of hydrogel microparticles and are emerging for various biomedical applications, including as inks for 3D printing, substrates to study cell-matrix interactions, and injectable scaffolds for tissue repair. Granular hydrogels are suited for these applications because of their unique properties including inherent porosity, shear-thinning and self-healing behavior, and tunable design. The characterization of their material properties and biological response involves technical considerations that are unique to modular systems like granular hydrogels. Here, we describe detailed methods that can be used to quantitatively characterize the rheological behavior and porosity of granular hydrogels using reagents, tools, and equipment that are typically available in biomedical engineering laboratories. In addition, we detail methods for 3D cell invasion assays using multicellular spheroids embedded within granular hydrogels and describe steps to quantify features of cell outgrowth (e.g., endothelial cell sprouting) using standard image processing software. To illustrate these methods, we provide examples where features of granular hydrogels such as the size of hydrogel microparticles and their extent of packing during granular hydrogel formation are modulated. Our intent with this resource is to increase accessibility to granular hydrogel technology and to facilitate the investigation of granular hydrogels for biomedical applications.

Assembling Microgels via Dynamic Cross-Linking Reaction Improves Printability, Microporosity, Tissue-Adhesion, and Self-Healing of Microgel Bioink for Extrusion Bioprinting

Extrusion bioprinting has been widely used to fabricate complicated and heterogeneous constructs for tissue engineering and regenerative medicine. Despite the remarkable progress acquired so far, the exploration of qualified bioinks is still challenging, mainly due to the conflicting requirements on the printability/shape-fidelity and cell viability. Herein, a new strategy is proposed to formulate a dynamic cross-linked microgel assembly (DC-MA) bioink, which can achieve both high printability/shape-fidelity and high cell viability by strengthening intermicrogel interactions through dynamic covalent bonds while still maintaining the relatively low mechanical modulus of microgels. As a proof-of-concept, microgels are prepared by cross-linking hyaluronic acid modified with methacrylate and phenylboric acid groups (HAMA-PBA) and methacrylated gelatin (GelMA) via droplet-based microfluidics, followed by assembling into DC-MA bioink with a dynamic cross-linker (dopamine-modified hyaluronic acid, HA-DA). As a result, 2D and 3D constructs with high shape-fidelity can be printed without post-treatment, and the encapsulated L929 cells exhibit high cell viability after extrusion. Moreover, the addition of the dynamic cross-linker (HA-DA) also improves the microporosity, tissue-adhesion, and self-healing of the DC-MA bioink, which is very beneficial for tissue engineering and regenerative medicine applications including wound healing. We believe the present work sheds a new light on designing new bioinks for extrusion bioprinting.