Decoupled pH- and Thermo-Responsive Injectable Chitosan/PNIPAM Hydrogel via Thiol-Ene Click Chemistry for Potential Applications in Tissue Engineering

Near-Infrared Responsive Properties of Bone Repair Scaffolds Facilitated by Specific Osteoinductive Photothermal Converters for Highly Efficient Bone Repair

The effective repair of bone defects has long been a major challenge in clinical practice. Currently, research efforts mostly focus on achieving sufficiently good bone repair, with little attention paid to achieving both good and fast repair. However, achieving highly efficient (H-efficient) bone repair, which is both good and fast, can shorten the treatment cycle and facilitate rapid patient recovery. Therefore, the development of a H-efficient bone repair material is of significant importance. This study incorporated the previously developed osteoinductive photothermal agent (PTA) BPICT into printing paste to prepare a near-infrared (NIR)-responsive BPICT scaffold. Subsequently, the effects of photothermal therapy (PTT) on bone repair and drug release were assessed in vitro. To further validate the H-efficient bone repair properties of the BPICT scaffold, the scaffold was implanted into bone defects and its ability to promote bone repair in vivo was evaluated through radiology and histopathological analysis. The results indicated that compared to scaffolds containing only Icaritin (ICT), the BPICT scaffold can achieve PTT to promote bone repair through NIR irradiation, while also enabling the controlled release of ICT from the scaffold to enhance bone repair. Within the same observation period, the BPICT scaffold achieves more efficient bone repair than the ICT scaffold, significantly shortening the bone repair cycle while ensuring the effectiveness of bone repair. Therefore, the NIR-responsive scaffold based on PTT-mediated controlled release of bone growth factors represents a feasible solution for promoting H-efficient bone repair in the area of bone defects.

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

Bioengineering approaches for the endometrial research and application

The endometrium undergoes a series of precise monthly changes under the regulation of dynamic levels of ovarian hormones that are characterized by repeated shedding and subsequent regeneration without scarring. This provides the potential for wound healing during endometrial injuries. Bioengineering materials highlight the faithful replication of constitutive cells and the extracellular matrix that simulates the physical and biomechanical properties of the endometrium to a larger extent. Significant progress has been made in this field, and functional endometrial tissue bioengineering allows an in-depth investigation of regulatory factors for endometrial and myometrial defects in vitro and provides highly therapeutic methods to alleviate obstetric and gynecological complications. However, much remains to be learned about the latest progress in the application of bioengineering technologies to the human endometrium. Here, we summarize the existing developments in biomaterials and bioengineering models for endometrial regeneration and improving the female reproductive potential.

Co-delivery of carboplatin and doxorubicin using ZIF-8 coated chitosan-poly(N-isopropyl acrylamide) nanoparticles through a dual pH/thermo responsive strategy to breast cancer cells

A dual pH/temperature sensitive core-shell nanoformulation has been developed based on ZIF-8 coated with chitosan-poly(N-isopropyl acrylamide) (CS-PNIPAAm) for co-delivery of doxorubicin (DOX) and carboplatin (CBP) in breast cancer cells. The resulting nanoparticles (NPs) had particle sizes around 200 nm and a zeta potential of about +30 mV. The CBP and DOX loading contents in the final NPs were 11.6 % and 55.54 %, respectively. NPs showed a pH and thermoresponsive drug release profile with a sustained prolonged release under physiological conditions. The in vitro cytotoxicity experiments showed a significant synergism of CBP and DOX to induce the IC50 of 1.96 μg/mL in MCF-7 cells and 4.54 μg/mL in MDA-MB-231 cells. Also, the final NPs were safer than free DOX and CBP on normal cells. The in vitro study confirmed the higher potency of the designed NPs in combination therapy against breast cancer cells with lower side effects than free drugs.

Fabrication and characteristics of multifunctional hydrogel dressings using dopamine modified hyaluronic acid and phenylboronic acid modified chitosan

The healing of damaged skin is a complex and dynamic process, and the multi-functional hydrogel dressings could promote skin tissue healing. This study, therefore, explored the development of a composite multifunctional hydrogel (HDCP) by incorporating the dopamine modified hyaluronic acid (HA-DA) and phenylboronic acid modified chitosan (CS-PBA) crosslinked using boric acid ester bonds. The integration of HA-DA and CS-PBA could be confirmed using the Fourier transform infrared spectrometer and 1H nuclear magnetic resonance analyses. The fabricated HDCP hydrogels exhibited porous structure, elastic solid behavior, shear-thinning, and adhesion properties. Furthermore, the HDCP hydrogels exhibited antibacterial efficacy against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). Subsequently, the cytocompatibility of the HDCP hydrogels was verified through CCK-8 assay and fluorescent image analysis following co-cultivation with NIH-3T3 cells. This research presents an innovative multifunctional hydrogel that holds promise as a wound dressing for various applications within the realm of wound healing.

Advances in Stimuli-Responsive Chitosan Hydrogels for Drug Delivery Systems

Sustainable and controllable drug transport is one of the most efficient ways of disease treatment. Due to high biocompatibility, good biodegradability, and low costs, chitosan and its derivatives are widely used in biomedical fields. Specifically, chitosan hydrogel enables drugs to pass through biological barriers because of their abundant amino and hydroxyl groups that can interact with human tissues. Moreover, the multi-responsive nature (pH, temperature, ions strength, and magnetic field, etc.) of chitosan hydrogels makes precise drug release a possibility. Here, the synthesis methods, modification strategies, stimuli-responsive mechanisms of chitosan-based hydrogels, and their recent progress in drug delivery are summarized. Chitosan hydrogels that carry and release drugs through subcutaneous (dealing with wound dressing), oral (dealing with gastrointestinal tract), and facial (dealing with ophthalmic, ear, and brain) are reviewed. Finally, challenges toward clinic application and the future prospects of stimuli-responsive chitosan-based hydrogels are indicated.

Developing hydrogels for gene therapy and tissue engineering

Hydrogels are a class of highly absorbent and easily modified polymer materials suitable for use as slow-release carriers for drugs. Gene therapy is highly specific and can overcome the limitations of traditional tissue engineering techniques and has significant advantages in tissue repair. However, therapeutic genes are often affected by cellular barriers and enzyme sensitivity, and carrier loading of therapeutic genes is essential. Therapeutic gene hydrogels can well overcome these difficulties. Moreover, gene-therapeutic hydrogels have made considerable progress. This review summarizes the recent research on carrier gene hydrogels for the treatment of tissue damage through a summary of the most current research frontiers. We initially introduce the classification of hydrogels and their cross-linking methods, followed by a detailed overview of the types and modifications of therapeutic genes, a detailed discussion on the loading of therapeutic genes in hydrogels and their characterization features, a summary of the design of hydrogels for therapeutic gene release, and an overview of their applications in tissue engineering. Finally, we provide comments and look forward to the shortcomings and future directions of hydrogels for gene therapy. We hope that this article will provide researchers in related fields with more comprehensive and systematic strategies for tissue engineering repair and further promote the development of the field of hydrogels for gene therapy.

Injecting hope: chitosan hydrogels as bone regeneration innovators

Spontaneous bone regeneration encounters substantial restrictions in cases of bone defects, demanding external intervention to improve the repair and regeneration procedure. The field of bone tissue engineering (BTE), which embraces a range of disciplines, offers compelling replacements for conventional strategies like autografts, allografts, and xenografts. Among the diverse scaffolding materials utilized in BTE applications, hydrogels have demonstrated great promise as templates for the regeneration of bone owing to their resemblance to the innate extracellular matrix. In spite of the advancement of several biomaterials, chitosan (CS), a natural biopolymer, has garnered significant attention in recent years as a beneficial graft material for producing injectable hydrogels. Injectable hydrogels based on CS formulations provide numerous advantages, including their capacity to absorb and preserve a significant amount of water, their minimally invasive character, the existence of porous structures, and their capability to adapt accurately to irregular defects. Moreover, combining CS with other naturally derived or synthetic polymers and bioactive materials has displayed its effectiveness as a feasible substitute for traditional grafts. We aim to spotlight the composition, production, and physicochemical characteristics and practical utilization of CS-based injectable hydrogels, explicitly focusing on their potential implementations in bone regeneration. We consider this review a fundamental resource and a source of inspiration for future research attempts to pioneer the next era of tissue-engineering scaffold materials.

Evolution of Hybrid Hydrogels: Next-Generation Biomaterials for Drug Delivery and Tissue Engineering

Hydrogels, being hydrophilic polymer networks capable of absorbing and retaining aqueous fluids, hold significant promise in biomedical applications owing to their high water content, permeability, and structural similarity to the extracellular matrix. Recent chemical advancements have bolstered their versatility, facilitating the integration of the molecules guiding cellular activities and enabling their controlled activation under time constraints. However, conventional synthetic hydrogels suffer from inherent weaknesses such as heterogeneity and network imperfections, which adversely affect their mechanical properties, diffusion rates, and biological activity. In response to these challenges, hybrid hydrogels have emerged, aiming to enhance their strength, drug release efficiency, and therapeutic effectiveness. These hybrid hydrogels, featuring improved formulations, are tailored for controlled drug release and tissue regeneration across both soft and hard tissues. The scientific community has increasingly recognized the versatile characteristics of hybrid hydrogels, particularly in the biomedical sector. This comprehensive review delves into recent advancements in hybrid hydrogel systems, covering the diverse types, modification strategies, and the integration of nano/microstructures. The discussion includes innovative fabrication techniques such as click reactions, 3D printing, and photopatterning alongside the elucidation of the release mechanisms of bioactive molecules. By addressing challenges, the review underscores diverse biomedical applications and envisages a promising future for hybrid hydrogels across various domains in the biomedical field.

Glycosaminoglycan-mimetic infernan grafted with poly(N-isopropylacrylamide): Toward a thermosensitive polysaccharide

Glycosaminoglycans (GAGs) are essential constituents of the cell surface and extracellular matrix, where they are involved in several cellular processes through their interactions with various proteins. For successful tissue regeneration, developing an appropriate matrix supporting biological activities of cells in a similar manner than GAGs remains still challenging. In this context, this study aims to design a thermosensitive polysaccharide that could further be used as hydrogel for tissue engineering applications. For this purpose, infernan, a marine bacterial exopolysaccharide (EPS) endowed with GAG-mimetic properties was grafted with a thermosensitive polymer, poly(N-isopropylacrylamide) (pNIPAM). Eight grafted polysaccharides were obtained by varying EPS/pNIPAM molar ratio and the molecular weight of pNIPAM. Their physicochemical characteristics and their thermosensitive properties were determined using a multi-technique, experimental approach. In parallel, molecular dynamics and Monte Carlo simulations were applied at two different scales to elucidate, respectively, the molecular conformation of grafted infernan chain and their ability to form an infinite network undergoing a sol-gel transition near the percolation, a necessary condition in hydrogel formation. It comes out from this study that thermosensitive infernan was successfully developed and its potential use in tissue regeneration as a hydrogel scaffold will further be assessed.

Recent advances in fabricating injectable hydrogels via tunable molecular interactions for bio-applications

Hydrogels with three-dimensional structures have been widely applied in various applications because of their tunable structures, which can be easily tailored with desired functionalities. However, the application of hydrogel materials in bioengineering is still constrained by their limited dosage flexibility and the requirement of invasive surgical procedures. Compared to traditional hydrogels, injectable hydrogels, with shear-thinning and/or in situ formation properties, simplify the implantation process and reduce tissue invasion, which can be directly delivered to target sites using a syringe injection, offering distinct advantages over traditional hydrogels. These injectable hydrogels incorporate physically non-covalent and/or dynamic covalent bonds, granting them self-healing abilities to recover their structural integrity after injection. This review summarizes our recent progress in preparing injectable hydrogels and discusses their performance in various bioengineering applications. Moreover, the underlying molecular interaction mechanisms that govern the injectable and functional properties of hydrogels were characterized by using nanomechanical techniques such as surface forces apparatus (SFA) and atomic force microscopy (AFM). The remaining challenges and future perspectives on the design and application of injectable hydrogels are also discussed. This work provides useful insights and guides future research directions in the field of injectable hydrogels for bioengineering.

Fundamental properties of smart hydrogels for tissue engineering applications: A review

Tissue engineering is an advanced and potential biomedical approach to treat patients suffering from lost or failed an organ or tissue to repair and regenerate damaged tissues that increase life expectancy. The biopolymers have been used to fabricate smart hydrogels to repair damaged tissue as they imitate the extracellular matrix (ECM) with intricate structural and functional characteristics. These hydrogels offer desired and controllable qualities, such as tunable mechanical stiffness and strength, inherent adaptability and biocompatibility, swellability, and biodegradability, all crucial for tissue engineering. Smart hydrogels provide a superior cellular environment for tissue engineering, enabling the generation of cutting-edge synthetic tissues due to their special qualities, such as stimuli sensitivity and reactivity. Numerous review articles have presented the exceptional potential of hydrogels for various biomedical applications, including drug delivery, regenerative medicine, and tissue engineering. Still, it is essential to write a comprehensive review article on smart hydrogels that successfully addresses the essential challenging issues in tissue engineering. Hence, the recent development on smart hydrogel for state-of-the-art tissue engineering conferred progress, highlighting significant challenges and future perspectives. This review discusses recent advances in smart hydrogels fabricated from biological macromolecules and their use for advanced tissue engineering. It also provides critical insight, emphasizing future research directions and progress in tissue engineering.

A pH-responsive, injectable and self-healing chitosan-coumarin hydrogel based on Schiff base and hydrogen bonds

Smart hydrogels have shown great potential applications in disease treatment due to their controlled and local drug-release ability. Herein, a smart hydrogel with pH-responsive, injectable, and self-healing properties for controlled release of taxifolin (TFL) was prepared by freezing-thawing and photo-crosslinking methods. The crosslinking network of hydrogels (CS-CA hydrogels) was constructed by the hydrogen bonds, Schiff base bonds, and cyclobutane rings using chitosan (CS) and coumarin (CA) as raw materials. The CS-CA hydrogel demonstrated a compressive strength of 1.04 MPa, a self-healing efficiency of 99.9 %, and could maintain structural and functional integrity after injection. In addition, the drug release rate and shape of the CS-CA hydrogels were tunable due to its pH sensitivity. The TFL cumulative release reached 60 % within 12 h at pH = 4, and after equilibration, the cumulative release of TFL at pH = 4 (80 %) was significantly higher than at pH = 9.2 (50 %). The CCK8 experiment showed that the resulting hydrogel had no cytotoxicity. Meanwhile, subcutaneous implantation experiments in mice showed that the CS-CA hydrogels had favorable biodegradability and compatibility.

Nano-injectable pH/NIR-responsive hydrogel for chemo-photothermal synergistic drug delivery

Conventional cancer treatments are highly toxic and ineffective; therefore, it is essential to develop less toxic and minimally invasive treatment methods. A pH/Near Infra-red (NIR) dual-responsive, nano-injectable smart hydrogel was fabricated by incorporating CuS nanoparticles into the hydrogel networks formed by a random copolymer of N-isopropylacrylamide (NIPAM) and double-bond functionalized uracil. Microstructural characterizations of synthesized polymer and hydrogels were carried out using transmission electron microscope (TEM), scanning electron microscope (SEM), nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FT-IR). Multiple hydrogen bonding interactions between uracils function as physical cross-linking points to construct the network structure of the polymeric nanogel without the addition of additional cross-linking agents, ensuring the material's safety. The amino group on the structure of uracil gives the uracil-modified polymeric hydrogel excellent pH responsiveness. Notably, as a temperature-responsive material, poly (N-isopropylacrylamide) (PNIPAM) nanogel solution can achieve in situ gel formation (within 100 s at 37°C) above its lower critical solution temperature (LCST), granting injectability to polymeric solutions. Moreover, using a hierarchical construction strategy, the variable loading of DOX and CuS was achieved. First, a heterogeneous system was created by encapsulating doxorubicin (DOX) inside the nanogel via hydrophobic and π-π stacking interactions, followed by the introduction of CuS nanoparticles as photosensitizers outside of the nanogels. Due to the presence of CuS nanoparticles, the gel is able to convert NIR light into local heat to enhance the destruction of tumor cells while simultaneously achieving rapid in situ gel formation. The in situ-forming hydrogel showed promising tissue biocompatibility. The in vitro antitumor test demonstrated the capacity of the nanocomposite hydrogel for chemo-photothermal synergistic therapy. Therefore, this prepared platform has the potential to become a safe and effective, smart-responsive drug carrier for chemotherapy and PTT synergy, a minimally invasive material for tumor treatment.

A facile bioorthogonal chemistry-based reversible to irreversible strategy to surmount the dilemma between injectability and stability of hyaluronic acid hydrogels

Injectable and stable hydrogels have great promise for clinical applications. Fine-tuning the injectability and the stability of the hydrogels at different stages has been challenging due to the limited number of coupling reactions. A distinct "reversible to irreversible" concept using a thiazolidine-based bioorthogonal reaction between 1,2-aminothiols and aldehydes in physiological conditions to surmount the dilemma between injectability and stability is presented for the first time. Upon mixing aqueous solutions of aldehyde-functionalized hyaluronic acid (SA-HA) and cysteine-capped ethylenediamine (DI-Cys), SA-HA/DI-Cys hydrogels formed through reversible hemithioacetal crosslinking within 2 min. The reversible kinetic intermediate facilitated thiol-triggered gel-to-sol transition, shear-thinning and injectability of the SA-HA/DI-Cys hydrogel but then converted to the irreversible thermodynamic network after injection, thereby permitting the resulting gel with improved stability. As compared to the Schiff base hydrogels, the hydrogels generated from this simple, yet effective concept awarded improved protection to the embedded mesenchymal stem cells and fibroblast during injection, retained the cells homogeneously within the gel, and allowed them further proliferation in vitro and in vivo. There is potential for the proposed approach of "reversible to irreversible" based on thiazolidine chemistry to be applied as a general coupling technique for developing injectable and stable hydrogels for biomedical applications.

A Facile Synthetic Approach to UV-Degradable Hydrogels

Light-degradable hydrogels have a wide range of application prospects in the field of biomedicine. However, the provision of a facile synthetic approach to light-degradable hydrogels under mild conditions remains a challenge for researchers. To surmount this challenge, a facile synthetic approach to UV-degradable hydrogels is demonstrated in this manuscript. Initially, an UV-degradable crosslinker (UVDC) having o-nitrobenzyl ester groups was synthesized in a single step through the employment of the Passerini three-component reaction (P-3CR). Both 1H NMR and MS spectra indicated the successful synthesis of high-purity UVDC, and it was experimentally demonstrated that the synthesized UVDC was capable of degradation under 368 nm light. Furthermore, this UVDC was mixed with 8-arm PEG-thiol (sPEG20k-(SH)8) to promptly yield an UV-degradable hydrogel through a click reaction. The SEM image of the fabricated hydrogel exhibits the favorable crosslinking network of the hydrogel, proving the successful synthesis of the hydrogel. After continuous 368 nm irradiation, the hydrogel showed an obvious gel-sol transition, which demonstrates that the hydrogel possesses a desirable UV-degradable property. In summary, by utilizing solely a two-step reaction devoid of catalysts and hazardous raw materials, UV-degradable hydrogels can be obtained under ambient conditions, which greatly reduces the difficulty of synthesizing light-degradable hydrogels. This work extends the synthetic toolbox for light-degradable hydrogels, enabling their accelerated development.

Injectable hydrogels for cartilage and bone tissue regeneration: A review

Annually, millions of patients suffer from irreversible injury owing to the loss or failure of an organ or tissue caused by accident, aging, or disease. The combination of injectable hydrogels and the science of stem cells have emerged to address this persistent issue in society by generating minimally invasive treatments to augment tissue function. Hydrogels are composed of a cross-linked network of polymers that exhibit a high-water retention capacity, thereby mimicking the wet environment of native cells. Due to their inherent mechanical softness, hydrogels can be used as needle-injectable stem cell carrier materials to mend tissue defects. Hydrogels are made of different natural or synthetic polymers, displaying a broad portfolio of eligible properties, which include biocompatibility, low cytotoxicity, shear-thinning properties as well as tunable biological and physicochemical properties. Presently, novel ongoing developments and native-like hydrogels are increasingly being used broadly to improve the quality of life of those with disabling tissue-related diseases. The present review outlines various future and in-vitro applications of injectable hydrogel-based biomaterials, focusing on the newest ongoing developments of in-situ forming injectable hydrogels for bone and cartilage tissue engineering purposes.

Bioinspired Stretchable MXene Deformation-Insensitive Hydrogel Temperature Sensors for Plant and Skin Electronics

Temperature sensing is of high value in the wearable healthcare, robotics/prosthesis, and noncontact physiological monitoring. However, the common mechanic deformation, including pressing, bending, and stretching, usually causes undesirable feature size changes to the inner conductive network distribution of temperature sensors, which seriously influences the accuracy. Here, inspired by the transient receptor potential mechanism of biological thermoreceptors that could work precisely under various skin contortions, we propose an MXene/Clay/poly(N-isopropylacrylamide) (PNIPAM) (MCP) hydrogel with high stretchability, spike response, and deformation insensitivity. The dynamic spike response is triggered by the inner conductive network transformation from the 3-dimensional structure to the 2-dimensional surface after water being discharged at the threshold temperature. The water discharge is solely determined by the thermosensitivity of PNIPAM, which is free from mechanical deformation, so the MCP hydrogels can perform precise threshold temperature (32 °C) sensing under various deformation conditions, i.e., pressing and 15% stretching. As a proof of concept, we demonstrated the applications in plant electronics for the real-time surface temperature monitoring and skin electronics for communicating between human and machines. Our research opens venues for the accurate temperature-threshold sensation on the complicated surface and mechanical conditions.

Preparation, thermal response mechanisms and biomedical applications of thermosensitive hydrogels for drug delivery

INTRODUCTION

Drug treatment is one of the main ways of coping with disease today. For the disadvantages of drug management, thermosensitive hydrogel is used as a countermeasure, which can realize the simple sustained release of drugs and the controlled release of drugs in complex physiological environments.

AREAS COVERED

This paper talks about thermosensitive hydrogels that can be used as drug carriers. The common preparation materials, material forms, thermal response mechanisms, characteristics of thermosensitive hydrogels for drug release and main disease treatment applications are reviewed.

EXPERT OPINION

When thermosensitive hydrogels are used as drug loading and delivery platforms, desired drug release patterns and release profiles can be tailored by selecting raw materials, thermal response mechanisms, and material forms. The properties of hydrogels prepared from synthetic polymers will be more stable than natural polymers. Integrating multiple thermosensitive mechanisms or different kinds of thermosensitive mechanisms on the same hydrogel is expected to realize the spatiotemporal differential delivery of multiple drugs under temperature stimulation. The industrial transformation of thermosensitive hydrogels as drug delivery platforms needs to meet some important conditions.

Applications of Stimuli-Responsive Hydrogels in Bone and Cartilage Regeneration

Bone and cartilage regeneration is an area of tremendous interest and need in health care. Tissue engineering is a potential strategy for repairing and regenerating bone and cartilage defects. Hydrogels are among the most attractive biomaterials in bone and cartilage tissue engineering, mainly due to their moderate biocompatibility, hydrophilicity, and 3D network structure. Stimuli-responsive hydrogels have been a hot topic in recent decades. They can respond to external or internal stimulation and are used in the controlled delivery of drugs and tissue engineering. This review summarizes current progress in the use of stimuli-responsive hydrogels in bone and cartilage regeneration. The challenges, disadvantages, and future applications of stimuli-responsive hydrogels are briefly described.

Decellularized Extracellular Matrix for Remodeling Bioengineering Organoid's Microenvironment

Over the past decade, stem cell- and tumor-derived organoids are the most promising models in developmental biology and disease modeling, respectively. The matrix is one of three main elements in the construction of an organoid and the most important module of its extracellular microenvironment. However, the source of the currently available commercial matrix, Matrigel, limits the application of organoids in clinical medicine. It is worth investigating whether the original decellularized extracellular matrix (dECM) can be exploited as the matrix of organoids and improving organoid construction are very important. In this review, tissue decellularization protocols and the characteristics of decellularization methods, the mechanical support and biological cues of extraccellular matrix (ECM), methods for construction of multifunctional dECM and responsive dECM hydrogel, and the potential applications of functional dECM are summarized. In addition, some expectations are provided for dECM as the matrix of organoids in clinical applications.

Research Progress in Enzymatically Cross-Linked Hydrogels as Injectable Systems for Bioprinting and Tissue Engineering

Hydrogels have been developed for different biomedical applications such as in vitro culture platforms, drug delivery, bioprinting and tissue engineering. Enzymatic cross-linking has many advantages for its ability to form gels in situ while being injected into tissue, which facilitates minimally invasive surgery and adaptation to the shape of the defect. It is a highly biocompatible form of cross-linking, which permits the harmless encapsulation of cytokines and cells in contrast to chemically or photochemically induced cross-linking processes. The enzymatic cross-linking of synthetic and biogenic polymers also opens up their application as bioinks for engineering tissue and tumor models. This review first provides a general overview of the different cross-linking mechanisms, followed by a detailed survey of the enzymatic cross-linking mechanism applied to both natural and synthetic hydrogels. A detailed analysis of their specifications for bioprinting and tissue engineering applications is also included.

A novel bidirectional perfusion-like administered system for NIR-II fluorescence imaging precision diagnosis of bladder cancer

Intravesical instillation has been considered an efficient route for detecting bladder cancer. However, only a small fraction of administered dose permeates into tumor tissues, and insufficient retention time limits their application. In this work, a novel intravesical bidirectional perfusion-like administered mode was developed to improve diagnostic accuracy of bladder tumor imaging. Specifically, the ultrasmall AuPd-P-FA Nanoprobe exhibit excellent NIR-II FL imaging performance due to electronic structure perturbation. Benefiting from the size advantage for kidney metabolism and FA targeting specificity, AuPd-P-FA could effectively administration to bladder tumor. When AuPd-P-FA reached maximum enrichment at 1 h post-injection, the localized and mild thermal energy produced upon laser irradiation activated a phase transition. This thermo-sensitive characteristic could prolong the retention time in bladder and the fluorescence signal could be clearly observed at 6 h post-injection with high accuracy. This novel intravesical bidirectional perfusion-like administered mode is expected to achieve a non-invasive diagnosis of early bladder cancer.

Thiolated Polymeric Hydrogels for Biomedical Applications: A Review

Hydrogels are a three-dimensional (3D) network of hydrophilic polymers. The physical and chemical crosslinking of polymeric chains maintains the structure of the hydrogels even when they are swollen in water. They can be modified with thiol by thiol epoxy, thiol-ene, thiol-disulfide, or thiol-one reactions. Their application as a matrix for protein and drug delivery, cellular immobilization, regenerative medicine, and scaffolds for tissue engineering was initiated in the early 21st century. This review focuses on the ingredients, classification techniques, and applications of hydrogels, types of thiolation by different thiol-reducing agents, along with their mechanisms. In this study, different applications for polymers used in thiolated hydrogels, including dextran, gelatin, polyethylene glycol (PEG), cyclodextrins, chitosan, hyaluronic acid, alginate, poloxamer, polygalacturonic acid, pectin, carrageenan gum, arabinoxylan, carboxymethyl cellulose (CMC), gellan gum, and polyvinyl alcohol (PVA) are reviewed.

Biomedical polymer scaffolds mimicking bone marrow niches to advance in vitro expansion of hematopoietic stem cells

Hematopoietic stem cell (HSC) transplantation provides an effective platform for the treatment of hematological disorders. However, the donor shortage of HSCs and immune responses severely restrict the clinical applications of HSCs. Compared to allogeneic transplantation, autogenous transplantation poses less risk to the immune system, but the problem associated with insufficient HSCs remains a substantial challenge. A significant strategy for obtaining sufficient HSCs is to promote the expansion of HSCs. In vivo, a bone marrow microenvironment supports the survival and hematopoiesis of HSCs. Therefore, it is crucial to establish a platform that mimics the features of a bone marrow microenvironment for the in vitro expansion of HSCs. Three-dimensional (3D) scaffolds have emerged as the most powerful tools to mimic cellular microenvironments for the growth and proliferation of stem cells. Biomedical polymers have been widely utilized as cell scaffolds due to their advantageous features including favorable biocompatibility, biodegradability, as well as adjustable physical and chemical properties. This review focuses on recent advances in the study of biomedical polymer scaffolds that mimic bone marrow microenvironments for the in vitro expansion of HSCs. Bone marrow transplantation and microenvironments are first introduced. Then, biomedical polymer scaffolds for the expansion of HSCs and future prospects are summarized and discussed.

Injectable hydrogel with dual-sensitive behavior for targeted delivery of oncostatin M to improve cardiac restoration after myocardial infarction

Myocardial infarction (MI) is a common cardiovascular disease that seriously endangers human health and complex pathophysiology (e.g., coronary artery obstruction, myocardial apoptosis, necrosis, inflammation, fibrosis, etc.) is involved. Therein, the loss of cardiomyocytes after MI in adults leads to gradual heart failure, which probably brings irreparable damage to the patient. Unfortunately, due to a cluster of limitations, currently used MI repair approaches always exhibit simple functions, low efficiency, and can hardly match the myocardial ischemia environment and clinical needs. In this study, we selected oncostatin M (OSM), a pleiotropic cytokine belonging to the interleukin-6 family that possesses an important role in cardiomyocyte dedifferentiation, cell proliferation, and regulation of inflammatory processes. Moreover, an injectable hydrogel with pH- and temperature-responsive behavior that can react with the acidic microenvironment of the ischemic myocardium was developed to deliver OSM locally. The functional hydrogel (poly (chitosan-co-citric acid-co-N-isopropyl acrylamide), P(CS-CA-NIPAM)) was fabricated by the facile reversible addition-fragmentation chain transfer polymerization and can be injected into the lesion site directly. After the gelation in situ, the OSM-loaded hydrogel exhibited continuous and localized release of OSM in response to specific pH and changes in MI rats, thereby accelerating angiogenesis and proliferation of cardiomyocytes, inhibiting myocardial fibrosis and improving cardiac function effectively. This study may provide a new perspective for the application of dual-sensitive hydrogels clinically, especially in tissue engineering for MI repair and drug delivery.

Hydrogels for Tissue Engineering: Addressing Key Design Needs Toward Clinical Translation

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Stimuli-Responsive Antibacterial Materials: Molecular Structures, Design Principles, and Biomedical Applications

Infections are regarded as the most severe complication associated with human health, which are urgent to be solved. Stimuli-responsive materials are appealing therapeutic platforms for antibacterial treatments, which provide great potential for accurate theranostics. In this review, the advantages, the response mechanisms, and the key design principles of stimuli-responsive antibacterial materials are highlighted. The biomedical applications, the current challenges, and future directions of stimuli-responsive antibacterial materials are also discussed. First, the categories of stimuli-responsive antibacterial materials are comprehensively itemized based on different sources of stimuli, including external physical environmental stimuli (e.g., temperature, light, electricity, salt, etc.) and bacterial metabolites stimuli (e.g., acid, enzyme, redox, etc.). Second, structural characteristics, design principles, and biomedical applications of the responsive materials are discussed, and the underlying interrelationships are revealed. The molecular structures and design principles are closely related to the sources of stimuli. Finally, the challenging issues of stimuli-responsive materials are proposed. This review will provide scientific guidance to promote the clinical applications of stimuli-responsive antibacterial materials.

Poly(N-isopropylacrylamide) Based Electrically Conductive Hydrogels and Their Applications

Poly(N-isopropylacrylamide) (PNIPAM) based electrically conductive hydrogels (PNIPAM-ECHs) have been extensively studied in recent decades due to their thermal-responsive (leading to the volume change of hydrogels) and electrically conductive performance. The incorporation of conductive components into the PNIPAM hydrogel network makes it become conductive hydrogel, and as a result, the PNIPAM hydrogel could become sensitive to an electrical signal, greatly expanding its application. In addition, conductive components usually bring new stimuli-responsive properties of PNIPAM-based hydrogels, such as near-infrared light and stress/strain responsive properties. PNIPAM-ECHs display a wide range of applications in human motion detection, actuators, controlled drug release, wound dressings, etc. To summarize recent research advances and achievements related to PNIPAM-ECHs, this manuscript first reviews the design and structure of representative PNIPAM-ECHs according to their conductive components. Then, the applications of PNIPAM-ECHs have been classified and discussed. Finally, the remaining problems related to PNIPAM-ECHs have been summarized and a future research direction is proposed which is to fabricate PNIPAM-ECHs with integrated multifunctionality.

Rational design of a supramolecular hydrogel with customizable pH-responsiveness on the basis of pH-induced ionization/protonation transition of BSA

Developing customizable pH-responsiveness for supramolecular hydrogels is of great significance and has drawn tremendous attention. Through systematic simulation analysis, we formulated a simple supramolecular hydrogel (i.e., poly(AAm-co-NaSS)/BSA on the basis of electrostatic interaction between the sulfonate groups of poly(AAm-co-NaSS) and the protonated side groups of BSA, and proposed a novel pH-responsive mode for it: changing the internal electric charge composition of the hydrogel through pH-induced ionization/protonation transition of BSA, thereby regulating the structural stability/shrinkage/extension of the supramolecular network. On basis of this theory, the pH-responsiveness of the poly(AAm-co-NaSS)/BSA hydrogel, in principle, could be pre-designed by adjusting the initial BSA/NaSS ratio. In this regard, we fabricated a poly(AAm-co-NaSS)/BSA hydrogel prototype with a BSA/NaSS ratio of 1/57 and investigated its rheological/swelling/disassembling behavior under different pH conditions (1.7, 4.7, 7.7, 10.7, and 13.7). In addition, we also prepared two capecitabine-loaded poly(AAm-co-NaSS)/BSA hydrogel prototypes with BSA/NaSS ratios of 1/57 and 1/102 respectively at pH 4.0, and compared their drug release behavior in SGF and SIF. Finally, the experimental results fitted well with our theoretical expectations, which testified the rationality of our assumption. Thus, we believed that the poly(AAm-co-NaSS)/BSA supramolecular hydrogel could find diverse applications in the future.

Multi-responsive and conductive bilayer hydrogel and its application in flexible devices

Multi-stimuli-responsive hydrogels are intelligent materials that present advantages for application in soft devices compared with conventional machines. In this paper, we prepared a bilayer hydrogel consisting of a poly(2-(dimethylamino)ethyl methacrylate) layer and a poly(N-isopropylacrylamide) layer. The hydrogel responded to temperature, pH, NaCl, and ethanol by undergoing bending deformation. At 40 °C, it only took 23 s for the hydrogel to bend nearly 300°. Carbon black was also introduced into the hydrogel network to render it conductive. Based on its multi-stimuli-responsive properties and conductivity, the hydrogel was used to construct a 4-arm gripper, thermistor, and finger movement monitor. The time required to grip and release an object was 141 s. The resistance changed with temperature, which affected the brightness of an LED. Finger motions were monitored, and the bending angle could be distinguished.

Click Chemistry Hydrogels for Extrusion Bioprinting: Progress, Challenges, and Opportunities

The emergence of 3D bioprinting has allowed a variety of hydrogel-based "bioinks" to be printed in the presence of cells to create precisely defined cell-loaded 3D scaffolds in a single step for advancing tissue engineering and/or regenerative medicine. While existing bioinks based primarily on ionic cross-linking, photo-cross-linking, or thermogelation have significantly advanced the field, they offer technical limitations in terms of the mechanics, degradation rates, and the cell viabilities achievable with the printed scaffolds, particularly in terms of aiming to match the wide range of mechanics and cellular microenvironments. Click chemistry offers an appealing solution to this challenge given that proper selection of the chemistry can enable precise tuning of both the gelation rate and the degradation rate, both key to successful tissue regeneration; simultaneously, the often bio-orthogonal nature of click chemistry is beneficial to maintain high cell viabilities within the scaffolds. However, to date, relatively few examples of 3D-printed click chemistry hydrogels have been reported, mostly due to the technical challenges of controlling mixing during the printing process to generate high-fidelity prints without clogging the printer. This review aims to showcase existing cross-linking modalities, characterize the advantages and disadvantages of different click chemistries reported, highlight current examples of click chemistry hydrogel bioinks, and discuss the design of mixing strategies to enable effective 3D extrusion bioprinting of click hydrogels.

Enzyme immobilization in hydrogels: A perfect liaison for efficient and sustainable biocatalysis

Biocatalysis is an established chemical synthesis technology that has by no means been restricted to research laboratories. The use of enzymes for organic synthesis has evolved greatly from early development to proof‐of‐concept – from small batch production to industrial scale. Different enzyme immobilization strategies contributed to this success story. Recently, the use of hydrogel materials for the immobilization of enzymes has been attracting great interest. Within this review, we pay special attention to recent developments in this key emerging field of research. Firstly, we will briefly introduce the concepts of both biocatalysis and hydrogel worlds. Then, we list recent interesting publications that link both concepts. Finally, we provide an outlook and comment on future perspectives of further exploration of enzyme immobilization strategies in hydrogels.

Recent Applications of Dual-Stimuli Responsive Chitosan Hydrogel Nanocomposites as Drug Delivery Tools

Polysaccharides are a versatile class of macromolecules that are involved in many biological interactions critical to life. They can be further modified for added functionality. Once derivatized, these polymers can exhibit new chemical properties that can be further optimized for applications in drug delivery, wound healing, sensor development and others. Chitosan, derived from the N-deacetylation of chitin, is one example of a polysaccharide that has been functionalized and used as a major component of polysaccharide biomaterials. In this brief review, we focus on one aspect of chitosan's utility, namely we discuss recent advances in dual-responsive chitosan hydrogel nanomaterials.

Marine polysaccharide-based composite hydrogels containing fucoidan: Preparation, physicochemical characterization, and biocompatible evaluation

Marine polysaccharide-based hydrogels have drawn much attention for diversified biomedical applications owing to their excellent (bio)physicochemical properties. In the present work, a series of marine polysaccharide-based hydrogels composed of chitosan, alginate, or fucoidan are prepared via a facile chemical cross-linking approach in an alkali/urea aqueous system. The prepared hydrogels possess tunable microporous architecture, swelling, and biodegradable properties by changing the components and proportions of marine polysaccharides. Importantly, the developed hydrogels are mechanically robust and the maximum compressive stress is up to 28.37 ± 4.63 kPa. Furthermore, the composite hydrogels exhibit excellent cytocompatibility, blood compatibility, and histocompatibility. When implanted subcutaneously in rats, the hydrogels containing fucoidan inhibit the inflammatory response of surrounding tissue. Thus, the designed composite hydrogels are promising bio-scaffolds in biomedical applications.

Dual pH-/thermo-responsive chitosan-based hydrogels prepared using "click" chemistry for colon-targeted drug delivery applications

A dual pH-/thermo-responsive hydrogel was designed based on a polyelectrolyte complex of polyacrylic acid (PAA) and norbornene-functionalized chitosan (CsNb), which was synergized with chemical crosslinking using bistetrazine-poly(N-isopropyl acrylamide) (bisTz-PNIPAM). The thermo-responsive polymeric crosslinker, bisTz-PNIPAM, was synthesized via reversible addition-fragmentation transfer polymerization of NIPAM. FTIR, XRD, rheological and morphological analyses demonstrated the successful formation of the polyelectrolyte network. The highly porous structure generated through the in-situ "click" reaction between Tz and Nb resulted in a higher drug loading (29.35 %). The hydrogel (COOH/NH₂ mole ratio of 3:1) exhibited limited drug release (8.5 %) of 5-ASA at a pH of 2.2, but it provided an almost complete release (92 %) at pH 7.4 and 37 °C within 48 h due to the pH responsiveness of PAA, hydrogel porosity, and shrinkage behavior of PNIPAM. The hydrogels were biodegradable and non-toxic against human fibroblast cells, suggesting their considerable potential for a colon-targeted drug delivery system.

Biomimetic anti-freezing polymeric hydrogels: keeping soft-wet materials active in cold environments

As one of the most outstanding materials, the analysis of the structure and function of hydrogels has been extensively carried out to tailor and adapt them to various fields of application. The high water content, which is beneficial for plenty of applications in the biomedical setting, prevents the adoption of hydrogels in flexible electronics and sensors in real life applications, because hydrogels lose their excellent properties, including conductivity, transparency, flexibility, etc., upon freezing at sub-zero temperatures. Therefore, depressing the liquid-solid phase transition temperature is a powerful means to expand the application scope of hydrogels, and will benefit the chemical engineering and materials science communities. This review summarizes the recent research progress of anti-freezing hydrogels. At first, approaches for the generation of anti-freezing (hydro)gels are introduced and their anti-freezing mechanisms and performances are briefly discussed. These approaches are either based on addition of salts, alcohols (cryoprotectants and organohydrogels), and ionic liquids (ionogels), modification of the polymer network or a combination of several techniques. Then, a concise overview of applications leveraged by the widened temperature resistance is provided and future research areas and developments are envisaged.

An injectable photothermally active antibacterial composite hydroxypropyl chitin hydrogel for promoting the wound healing process through photobiomodulation

Prevention of bacterial infection, acceleration of wound closure and promotion of skin regeneration are crucial in the wound healing process. In this work, the photothermal activity of an injectable thermosensitive composite hydrogel based on hydroxypropyl chitin (HPCH), tannic acid (TA) and ferric ions (Fe3+) was studied. It was found that the photothermal efficiency was enhanced when the molar ratio of Fe3+/TA increased up to 20. The composite hydrogel possessed good cytocompatibility and hemocompatibility with a low dosage of the antibacterial agent TA. In vitro and in vivo antibacterial tests showed that the HPCH/TA/Fe hydrogel possessed an effective and rapid bactericidal effect with 10 minutes of near-infrared laser irradiation. Furthermore, the combination of a low-level laser therapy with the hydrogel is conducive to the acceleration of wound closure and promotion of skin tissue repair. Thus, the injectable photothermally active antibacterial composite hydrogel has great potential for the infected skin wound regeneration in clinical applications.

The Scissors Effect in Action: The Fox-Flory Relationship between the Glass Transition Temperature of Crosslinked Poly(Methyl Methacrylate) and Mc in Nanophase Separated Poly(Methyl Methacrylate)-l-Polyisobutylene Conetworks

The glass transition temperature (Tg) is one of the most important properties of polymeric materials. In order to reveal whether the scissors effect, i.e., the Fox-Flory relationship between Tg and the average molecular weight between crosslinking points (Mc), reported only in one case for polymer conetworks so far, is more generally effective or valid only for a single case, a series of poly(methyl methacrylate)-l-polyisobutylene (PMMA-l-PIB) conetworks was prepared and investigated. Two Tgs were found for the conetworks by DSC. Fox-Flory type dependence between Tg and Mc of the PMMA component (Tg = Tg,∞ - K/Mc) was observed. The K constants for the PMMA homopolymer and for the PMMA in the conetworks were the same in the margin of error. AFM images indicated disordered bicontinuous, mutually nanoconfined morphology with average domain sizes of 5-20 nm, but the correlation between Tg and domain sizes was not found. These new results indicate that the macrocrosslinkers act like molecular scissors (scissors effect), and the Tg of PMMA depend exclusively on the Mc in the conetworks. Consequently, these findings mean that the scissors effect is presumably a general phenomenon in nanophase-separated polymer conetworks, and this finding could be utilized in designing, processing, and applications of these novel materials.