The effect of oxidation degree and volume ratio of components on properties and applications of in situ cross-linking hydrogels based on chitosan and hyaluronic acid

Fast Wound Healing with a New Functional Hyaluronic Acid Dual Network Hydrogel

As dressings for moist wound healing, hyaluronic acid hydrogels play a significant role in maintaining moisture and promoting wound healing. However, existing hydrogel dressings are inadequate in terms of slow gelation time, weak mechanical performance, and fast degradation, which increases the risk of secondary infections during treatment. Therefore, we developed a hyaluronic acid double network hydrogel (DNH). Compared to single-network hydrogels (hydrazone and Diels-Alder), DNH shows a short gelation time (25 s) and strong mechanical properties (Young's modulus = 82 kPa). These advantages enable DNH to immediately fill the irregular shape of the wound after gelation and remain intact after being squeezed. Swelling tests indicated that DNH had a suitable swelling ratio and maintained its structural integrity after swelling. We evaluated the use of DNH as a moist dressing for full-thickness wound healing in vivo. DNH-treated wounds healed faster, with enhanced blood vessel formation and macrophage polarization than gauze-treated wounds. These findings suggest that DNH not only accelerates wound healing but also improves tissue regeneration. Therefore, DNH may be a suitable moist dressing for wound healing.

Crosslinker-free in situ hydrogel induces self-aggregation of human dental pulp stem cells with enhanced antibacterial activity

Recently, injectable hydrogels have garnered significant attention in tissue engineering due to their controlled flowability, strong plasticity, adaptability, and good biocompatibility. However, research on readily injectable in situ-forming hydrogels capable of forming functional three-dimensional (3D) tissue condensations remains limited. This study explores the development and evaluation of a carboxymethyl chitosan (CMCTS)/oxidized hyaluronic acid (oHA) hydrogel incorporated with silver sulfadiazine (AgSD) for tissue engineering applications with inherent antibacterial activity. Through physicochemical analysis, the optimal formulation of CMCTS/oHA hydrogels was established. The hydrogel demonstrated excellent injectability, enabling minimally invasive in situ delivery. In vitro cytotoxicity assays identified 0.1 % AgSD as the optimal concentration, supporting cell proliferation while exhibiting antimicrobial efficacy against S. mutans and E. faecalis. In vivo studies revealed complete hydrogel degradation and good biocompatibility, with no adverse tissue reactions. The hydrogel's ability to form 3D cell aggregates and support tissue regeneration underscores its potential for future 3D tissue engineering applications. Consequently, the injectable CMCTS/oHA/AgSD hydrogel developed in this study holds significant potential for application in a wide range of bioengineering fields, including antibacterial substance delivery systems and 3D tissue engineering, indicating potential for future clinical application.

Design of tunable hyaluronic acid and O'-carboxymethyl chitosan formulations for the minimally invasive delivery of multifunctional therapies targeting rheumatoid arthritis

The development of injectable, dual-component formulations based on natural-based polysaccharides is a promising strategy for the localized treatment of rheumatoid arthritis (RA). In the present study, biomimetic formulations consisting of aldehyde-functionalized hyaluronic acid (AHA) and O-carboxymethyl chitosan (OCC) were developed, presenting rapid in situ gelation rates and finely tunable physicochemical properties. These two properties allowed for the controlled delivery of anti-inflammatory, antioxidant, and pro-regenerative agents (i.e., strontium-methotrexate (SrMTX) and europium-tannic acid nanocomplexes (EuTA NCs), making them suitable for application in in vivo RA-models. Biological analyses demonstrated the system's cytocompatibility and its ability to modulate the activity of human articular chondrocytes at the secretome level and scavenge nitric oxide (NO). Moreover, the loaded cargoes not only extended the anti-inflammatory properties of the formulation but also the radiolabeling of EuTA NCs with 68Ga allowed the visualization of the gel by positron emission tomography (PET). Overall, this work presents the design and in vitro evaluation of an easily modulable polymeric system that allows the in situ release of a multifunctional therapy with promising perspectives for RA treatment.

A chitosan/gelatin/aldehyde hyaluronic acid hydrogel coating releasing calcium ions and vancomycin in pH response to prevent the formation of bacterial biofilm

Osteomyelitis is a refractory disease of orthopedics, part of which is caused by medical implants. The main difficulties in treatment are the barrier effect after the formation of bacterial biofilm, and the difficulty in achieving sustained antibiotic intervention. In view of this situation, we studied a hydrogel coating that can release CaCl2 and vancomycin in pH-responsive manner. We used nano-TiO2 to modify Chitosan/ Gelatin/Aldehyde Hyaluronic Acid (CS/Gel/AHA) hydrogel, and combined with the dip-coating technique, prepared a coating with good mechanical strength. The hydrogel-loaded zeolitic imidazolate framework (ZIF) decomposes under acidic conditions, and the released Ca2+ act on the bacterial Bap protein to inhibit the formation of biofilm, and the released vancomycin kills free bacteria. The antibacterial coating achieved good bactericidal effect in both in vitro experiments and rat subcutaneous implant model. These results not only provide a new way to enhance the strength of hydrogels to prepare coatings, but also utilize a new approach to responsively inhibit the formation of biofilms, showing the promising application prospects of the coating in antibacterial treatment of medical implants.

The oxidized hyaluronic acid hydrogels containing paeoniflorin microspheres regulates the polarization of M1/M2 macrophages to promote wound healing

Controlling excessive inflammation of acute wound is an effective means to shorten the healing time. Therefore, targeted control of the inflammatory response of the wound is a promising therapeutic strategy. In this study, paeoniflorin (Pae) was encapsulated in microspheres and combined with oxidized hyaluronic acid hydrogels to prepare the hydrogel loaded with Pae microspheres (Pae-MPs@OHA) to promote the healing of acute wounds in rats. The results demonstrated that the particle size of the Pae-MPs was 6.84 ± 0.51 μm, and the positive charge was 26.87 ± 1.51 mV. The uniform spherical structure of the Pae-MPs was observed by TEM. The Pae-MPs@OHA can maintain colloidal state in the range of 0.1-3.16 Hz. FTIR suggested that Pae could be effectively wrapped in MPs, and SEM indicated that the Pae-MPs@OHA had a uniform network pore structure. The Pae-MPs@OHA can realize the sustained release of Pae for 96 h. Biocompatibility experiments showed that the Pae-MPs@OHA hydrogels were safe and available. The Pae-MPs@OHA hydrogels can accelerate wound healing in rats. HE and masson staining suggested that the Pae-MPs@OHA could reduce inflammatory cell infiltration, promote re-epithelialization and collagen formation. The Pae-MPs@OHA could decrease the number of M1 and increase the number of M2 in macrophages, thus regulating the release of inflammatory factor TNF-α and IL-1β. The results of molecular docking and western blot results also confirmed that the Pae-MPs@OHA could reduce the expression of NF-κB, pNF-κB, NLRP3, ASC and pro-caspase-1. These findings suggest that the Pae-MPs@OHA has great potential for application in the treatment of inflammatory wound.

Oxidized ionic polysaccharide hydrogels: Review on derived scaffolds characteristics and tissue engineering applications

Polysaccharide-based hydrogels have gained prominence due to their non-toxicity, biocompatibility, and structural adaptability for constructing tissue engineering scaffolds. Polysaccharide crosslinking is necessary for hydrogel stability in vivo. The periodate oxidation enables the modification of native polysaccharide characteristics for wound healing and tissue engineering applications. It produces dialdehydes, which are used to crosslink biocompatible amine-containing macromolecules such as chitosan, gelatin, adipic acid dihydrazide, silk fibroin, and peptides via imine/hydrazone linkages. Crosslinked oxidized ionic polysaccharide hydrogels have been studied for wound healing, cardiac and liver tissue engineering, bone, cartilage, corneal tissue regeneration, abdominal wall repair, nucleus pulposus regeneration, and osteoarthritis. Several modified hydrogel systems have been synthesized using antibiotics and inorganic substances to improve porosity, mechanical and viscoelastic properties, desired swelling propensity, and antibacterial efficacy. Thus, the injectable hydrogels provide a host-tissue-mimetic environment with high cell adhesion and viability, making them appropriate for scarless wound healing and tissue engineering applications. This review describes the oxidation procedure for alginate, hyaluronic acid, gellan gum, pectin, xanthan gum and chitosan, as well as the characteristics of the resulting materials. Furthermore, a critical review of scientific advances in wound healing and tissue engineering applications has been provided.

Polycaprolactone Hybrid Scaffold Loaded With N,O-Carboxymethyl Chitosan/Aldehyde Hyaluronic Acid/Hydroxyapatite Hydrogel for Bone Regeneration

Hydrogels have emerged as potential materials for bone grafting, thanks to their biocompatibility, biodegradation, and flexibility in filling irregular bone defects. In this study, we fabricated a novel NAH hydrogel system, composed of N,O-carboxymethyl chitosan (NOCC), aldehyde hyaluronic acid (AHA), and hydroxyapatite (HAp). To improve the mechanical strength of the fabricated hydrogel, a porous polycaprolactone (PCL) matrix was synthesized and used as a three-dimensional (3D) support template for NAH hydrogel loading, forming a novel PCL/NAH hybrid scaffold. A mixture of monosodium glutamate (M) and sucrose (S) at varied weight ratios (5M:5S, 7M:3S, and 9M:1S) was used for the fabrication of 3D PCL matrices. The morphology, interconnectivity, and water resistance of the porous PCL scaffolds were investigated for optimal hydrogel loading efficiency. The results demonstrated that PCL scaffolds with porogen ratios of 7M:3S and 9M:1S possessed better interconnectivity than 5M:5S ratio. The compressive strength of the PCL/NAH hybrid scaffolds with 9M:1S (561.6 ± 6.1 kPa) and 7M:3S (623.8 ± 6.8 kPa) ratios are similar to cancellous bone and all hybrid scaffolds were biocompatible. Rabbit models with tibial defects were implanted with the PCL/NAH scaffolds to assess the wound healing capability. The results suggest that the PCL/NAH hybrid scaffolds, specifically those with porogen ratio of 7M:3S, exhibit promising bone healing effects.

Injectable Self-Healing Antibacterial Hydrogels with Tailored Functions by Loading Peptide Nanofiber-Biomimetic Silver Nanoparticles

Polymer hydrogels find extensive application in biomedicine, serving specific purposes such as drug delivery, biosensing, bioimaging, cancer therapy, tissue engineering, and others. In response to the growing threat of bacterial infections and the escalating resistance to conventional antibiotics, this research introduces a novel injectable, self-healing antimicrobial hydrogel comprising bioactive aldolized hyaluronic acid (AHA) and quaternized chitosan (QCS). This designed QCS/AHA hydrogel incorporates self-assembling peptide nanofibers (PNFs) and small-sized silver nanoparticles (AgNPs) for tailored functionality. The resulting hybrid QCS/AHA/PNF/AgNPs hydrogel demonstrates impressive rheological characteristics, broad-spectrum antimicrobial efficacy, and high biocompatibility. Notably, its antimicrobial effectiveness against Escherichia coli and S. aureus surpasses 99.9%, underscoring its potential for treating infectious wounds. Moreover, the rheological analysis confirms its excellent shear-thinning and self-healing properties, enabling it to conform closely to irregular wound surfaces. Furthermore, the cytotoxicity assessment reveals its compatibility with human umbilical vein endothelial cells, exhibiting no significant adverse effects. The combined attributes of this bioactive QCS/AHA/PNF/AgNPs hydrogel position it as a promising candidate for antimicrobial applications and wound healing.

Development, Evaluation, and Molecular Dynamics Study of Ampicillin-Loaded Chitosan-Hyaluronic Acid Films as a Drug Delivery System

Periodontitis is an inflammatory periodontal disease defined by the progressive loss of tissues surrounding the tooth. Ampicillin is an antibiotic for managing and treating specific bacterial infections, including periodontitis. Periodontal pockets occur due to periodontal disease progression and act as a natural reservoir that is easily reachable for the insertion of a delivery system, and the amount of drug to be released has a major role in the efficiency of treatment of the disease. Polyelectrolyte complexes (PECs), particularly those based on chitosan and hyaluronic acid combinations, offer a promising avenue to overcome the challenges associated with drug delivery. These complexes are both biodegradable and biocompatible, making them an optimal choice for enabling targeted drug delivery. This study centers on developing and assessing the structure and dynamic attributes of a drug-PEC system encompassing ampicillin and chitosan-hyaluronic acid components, which represents a targeted drug delivery system to better alleviate the periodontitis. To achieve this goal, we conducted experiments including weight and drug content uniformity, swelling ındex, drug release %, FT-IR and SEM analyses, and atomistic molecular dynamics simulations on the drug PECs loaded with ampicillin with varying amounts of hyaluronic acid. All simulations and the experimental analysis suggested that increased HA amount resulted in an increase in drug release % and swelling index. The simulation outcomes provide insights into the nature of the drug and PEC interactions alongside transport properties such as drug diffusion coefficients. These coefficients offer valuable insights into the molecular behavior of ampicillin-PEC drug delivery systems, particularly in the context of their application in periodontitis treatment.

Biomaterial engineering for cell transplantation

The current paradigm of medicine is mostly designed to block or prevent pathological events. Once the disease-led tissue damage occurs, the limited endogenous regeneration may lead to depletion or loss of function for cells in the tissues. Cell therapy is rapidly evolving and influencing the field of medicine, where in some instances attempts to address cell loss in the body. Due to their biological function, engineerability, and their responsiveness to stimuli, cells are ideal candidates for therapeutic applications in many cases. Such promise is yet to be fully obtained as delivery of cells that functionally integrate with the desired tissues upon transplantation is still a topic of scientific research and development. Main known impediments for cell therapy include mechanical insults, cell viability, host's immune response, and lack of required nutrients for the transplanted cells. These challenges could be divided into three different steps: 1) Prior to, 2) during the and 3) after the transplantation procedure. In this review, we attempt to briefly summarize published approaches employing biomaterials to mitigate the above technical challenges. Biomaterials are offering an engineerable platform that could be tuned for different classes of cell transplantation to potentially enhance and lengthen the pharmacodynamics of cell therapies.

Development of chitosan-hyaluronic acid based hydrogel for local delivery of doxycycline hyclate in an ex vivo skin infection model

Doxycycline hyclate (DOXY) is a tetracycline derivative known as the broad-spectrum bacteriostatic drug. DOXY has been suggested as the first-line antibiotic for diabetic foot ulcers (DFU). Unfortunately, the long-term availability of DOXY in both oral and conventional topical dosage forms reduces its therapeutic effectiveness, which is closely linked to gastrointestinal side effects and acute pain during therapy, as well as uncontrolled DOXY release at the wound site. To address these shortcomings, we present for the first time a DOXY hydrogel system (DHs) built on crosslinks between carboxymethyl chitosan (CMC) and aldehyde hyaluronic acid (AHA). Three formulations of DHs were developed with different ratios of CMC and AHA, consisting of F1 (3:7, w/w), F2 (5:5, w/w), and F3 (7:3, w/w). Viscosity, rheology, gel strength, pH, swelling, gel fraction, wettability, stability, in vitro drug release, ex vivo antibacterial, and dermatokinetic studies were used to evaluate the DHs. According to the in vitro release study, up to 85% of DOXY was released from DHs via the Fickian diffusion mechanism in the Korsmeyer-Peppas model (n < 0.45), which provides controlled drug delivery. Because of its excellent physicochemical characteristics, F2 was chosen as the best DHs formulation in this study. Essentially, the optimum DHs formulation could greatly improve DOXY's ex vivo dermatokinetic profile while also providing excellent antibacterial activity. As a consequence, this study had promising outcome as a proof of concept for increasing the efficacy of DOXY in clinical therapy. Further extensive in vivo studies are required to evaluate the efficacy of this approach.

Application of Coating Chitosan Derivatives (N,O-Carboxymethyl Chitosan/Chitosan Oligomer Saccharide) in Combination with Polyvinyl Alcohol Solutions to Preserve Fresh Ngoc Linh Ginseng Quality

The postharvest preservation of Ngoc Linh ginseng (NL ginseng) is essential to retain its quality and sensory values for prolonged storage. In this study, the efficacy of NL ginseng preservation by coating chitosan derivatives in combination with polyvinyl alcohol (PVA) solutions was investigated under refrigeration conditions (~3 °C; ~40% RH) for 56 days. The effect of the chitosan-based solutions, including N,O-carboxymethyl chitosan (NOCC), chitosan oligomer saccharide (COS), or chitosan (CS), and the blend solutions (NOCC-PVA or COS-PVA) on the coated NL ginsengs was observed during storage. The pH values, viscosity, and film-forming capability of the coating solutions were determined, while the visual appearance, morphology, and mechanical properties of the films formed on glass substrates as a ginseng model for coating were also observed. The appearance, skin lightness, weight loss, sensory evaluation, total saponin content (TSC), total polyphenol content (TPC), and total antioxidant capacity (TAC) of the coated NL ginsengs were evaluated. The findings showed that the observed values of the coated NL ginsengs were better than those of the non-coated samples, with the exception of the COS-coated samples, which had completely negative results. Furthermore, the NOCC-PVA solution exhibited a better preservation effect compared with the COS-PVA one based on the observed indices, except for TPC and TAC, which were not impacted by the coating. Notably, the optimal preservation time was determined to be 35 days. This study presents promising preservation technology using the coating solution of NOCC-PVA, harnessing the synergistic effect of pH 7.4 and the form-firming capacity, to maintain the shelf life, medicinal content, and sensory attributes of NL ginseng.

Hydrogels with Reversible Crosslinks for Improved Localised Stem Cell Retention: A Review

Successful stem cell applications could have a significant impact on the medical field, where many lives are at stake. However, the translation of stem cells to the clinic could be improved by overcoming challenges in stem cell transplantation and in vivo retention at the site of tissue damage. This review aims to showcase the most recent insights into developing hydrogels that can deliver, retain, and accommodate stem cells for tissue repair. Hydrogels can be used for tissue engineering, as their flexibility and water content makes them excellent substitutes for the native extracellular matrix. Moreover, the mechanical properties of hydrogels are highly tuneable, and recognition moieties to control cell behaviour and fate can quickly be introduced. This review covers the parameters necessary for the physicochemical design of adaptable hydrogels, the variety of (bio)materials that can be used in such hydrogels, their application in stem cell delivery and some recently developed chemistries for reversible crosslinking. Implementing physical and dynamic covalent chemistry has resulted in adaptable hydrogels that can mimic the dynamic nature of the extracellular matrix.

Drug-free and non-crosslinked chitosan/hyaluronic acid hybrid hydrogel for synergistic healing of infected diabetic wounds

The management of infected diabetic wounds remains a major challenge in clinical practice. Recently, multifunctional hydrogels have attracted much attention in the area of wound healing. Herein, we developed the drug-free and non-crosslinked chitosan (CS)/hyaluronic acid (HA) hybrid hydrogel, so as to combine the multiple functions of CS and HA for synergistic healing of the methicillin-resistant Staphylococcus aureus (MRSA)-infected diabetic wound. As a result, CS/HA hydrogel showed the broad-spectrum antibacterial activity, the great capacity for promoting fibroblasts proliferation and migration, the excellent reactive oxygen species (ROS) scavenging ability, and the great cell-protection effects under oxidative stress. In the MRSA-infected diabetic mouse wounds, CS/HA hydrogel significantly promoted the wound healing via eliminating MRSA infection and enhancing epidermal regeneration, collagen deposition and angiogenesis. Considering the drug-free feature, the ready availability, the great biocompatibility and the excellent wound healing efficacy, CS/HA hydrogel may have great potentials in clinical use for the management of chronic diabetic wounds.

From crosslinking strategies to biomedical applications of hyaluronic acid-based hydrogels: A review

Hyaluronic acid (HA) is not only a natural anionic polysaccharide with excellent biocompatibility, biodegradability, and moisturizing effect, but also an essential factor that can affect angiogenesis, inflammation, cell behavior, which has a wide range of applications in the biomedical field. Among them, HA-based hydrogels formed by various physical or chemical crosslinking strategies are particularly striking. They not only retain the physiological function of HA, but also have the skeleton function of hydrogel, which further expands the application of HA. However, HA-based natural hydrogels generally have problems such as insufficient mechanical strength and susceptibility to degradation by hyaluronidase, which limits their application to a certain extent. To solve such problems, researchers have prepared a variety of HA-based multifunctional hydrogels with remarkable properties in recent years by adopting various structural modification methods or novel crosslinking strategies, as well as introducing functionally reactive molecules or moieties, which have extended the application scope. This manuscript systematically introduced common crosslinking strategies of HA-based hydrogels and highlighted the development of novel HA-based hydrogels in anticancer drug delivery, cartilage repair, three-dimensional cell culture, skin dressing and other fields. We hope to provide some references for the subsequent development of HA-based hydrogels in the biomedical field.

Injectable and Self-Healing Hydrogel Based on Chitosan-Tannic Acid and Oxidized Hyaluronic Acid for Wound Healing

Self-healing performance plays an important role in the in situ microinvasive injection of hydrogels, which can reduce sudden drug release and prolong the service life of hydrogels. In this paper, a multifunctional injectable and self-healing hydrogel for wound healing was developed. Chitosan (CS) was modified with TA to achieve potential adhesion, anti-inflammatory properties, and slower degradation rate. The hydrogel was formed by Schiff base reaction based on amino groups in CS and aldehyde groups in oxidized hyaluronic acid (OHA). The gel formation process was quick and convenient in mild conditions without extra initiators. Due to the dynamically reversible covalent bonds, the hydrogel could self-heal within 2 min after injection. It also had good biocompatibility and hemostatic performance. With the addition of TA, the hydrogel acquired anti-inflammatory properties and promoted cell growth, effectively accelerating the wound-healing process in vivo. The CS-TA/OHA hydrogel is expected to be used for skin repair.

The Role of Polymeric Biomaterials in the Treatment of Articular Osteoarthritis

Osteoarthritis is a high-prevalence joint disease characterized by the degradation of cartilage, subchondral bone thickening, and synovitis. Due to the inability of cartilage to self-repair, regenerative medicine strategies have become highly relevant in the management of osteoarthritis. Despite the great advances in medical and pharmaceutical sciences, current therapies stay unfulfilled, due to the inability of cartilage to repair itself. Additionally, the multifactorial etiology of the disease, including endogenous genetic dysfunctions and exogenous factors in many cases, also limits the formation of new cartilage extracellular matrix or impairs the regular recruiting of chondroprogenitor cells. Hence, current strategies for osteoarthritis management involve not only analgesics, anti-inflammatory drugs, and/or viscosupplementation but also polymeric biomaterials that are able to drive native cells to heal and repair the damaged cartilage. This review updates the most relevant research on osteoarthritis management that employs polymeric biomaterials capable of restoring the viscoelastic properties of cartilage, reducing the symptomatology, and favoring adequate cartilage regeneration properties.

Recent achievements in nano-based technologies for ocular disease diagnosis and treatment, review and update

Ocular drug delivery is one of the most challenging endeavors among the various available drug delivery systems. Despite having suitable drugs for the treatment of ophthalmic disease, we have not yet succeeded in achieving a proper drug delivery approach with the least adverse effects. Nanotechnology offers great opportunities to overwhelm the restrictions of common ocular delivery systems, including low therapeutic effects and adverse effects because of invasive surgery or systemic exposure. The present review is dedicated to highlighting and updating the recent achievements of nano-based technologies for ocular disease diagnosis and treatment. While further effort remains, the progress illustrated here might pave the way to new and very useful ocular nanomedicines.

Dynamic and Self-Healable Chitosan/Hyaluronic Acid-Based In Situ-Forming Hydrogels

In situ-forming, biodegradable, and self-healing hydrogels, which maintain their integrity after damage, owing to dynamic interactions, are essential biomaterials for bioapplications, such as tissue engineering and drug delivery. This work aims to develop in situ, biodegradable and self-healable hydrogels based on dynamic covalent bonds between N-succinyl chitosan (S-CHI) and oxidized aldehyde hyaluronic acid (A-HA). A robust effect of the molar ratio of both S-CHI and A-HA was observed on the swelling, mechanical stability, rheological properties and biodegradation kinetics of these hydrogels, being the stoichiometric ratio that which leads to the lowest swelling factor (×12), highest compression modulus (1.1·10−3 MPa), and slowest degradation (9 days). Besides, a rapid (3 s) self-repairing ability was demonstrated in the macro scale as well as by rheology and mechanical tests. Finally, the potential of these biomaterials was evidenced by cytotoxicity essay (>85%).

Self-healable nanocellulose composite hydrogels combining multiple dynamic bonds for drug delivery

Herein, we developed two nanocomposite polysaccharide hydrogels TPP-CNC and TPP-CNF via simple mixing method, which were constructed with multiple dynamic bonds. The microstructural features, mechanical properties, rheological properties, healable ability and biocompatibility of the complex hydrogels were evaluated. The TPP-CNC and TPP-CNF complex hydrogels exhibited higher tensile strength than pure polysaccharide hydrogel, from ~259 KPa to ~890 KPa and ~910 KPa, respectively, that was attributed to the contribution of ionic crosslinked network and hydrogen bonds. In addition, the hydrogels indicated superior fatigue resistance and high energy dissipation ratio during loading-unloading tests because of the physical sacrifice bonds, which also decreased the self-healing time at room temperature (~15 min). More importantly, the drug loaded nanocomposite hydrogels showed sustained release, reduction burst release, increased release under acidic environment, and the drug release kinetics belonged to Fickian diffusion mechanism. Therefore, the nanocellulose polysaccharide hydrogels have the highly promising to explore as biomaterials for drug delivery.

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.

Changes of Volatile Flavor Compounds in Large Yellow Croaker (Larimichthys crocea) during Storage, as Evaluated by Headspace Gas Chromatography-Ion Mobility Spectrometry and Principal Component Analysis

The large yellow croaker is one of the most economically important fish in Zhoushan, Zhejiang Province, and is well known for its high protein and fat contents, fresh and tender meat, and soft taste. However, the mechanisms involved in its flavor changes during storage have yet to be revealed, although lipid oxidation has been considered to be one important process in determining such changes. Thus, to explore the changes in the flavor of large yellow croaker fish meat during different storage periods, the main physical and chemical characteristics of the fish meat, including the acid value, peroxide value, p-anisidine value, conjugated diene value, and identities of the various flavor substances, were investigated and analyzed by multivariable methods, including headspace gas chromatography-ion mobility spectrometry (GC-IMS) and principal component analysis (PCA). It was found that after 60 d storage, the types and contents of the aldehyde and ketone aroma components increased significantly, while after 120 d, the contents of ketones (2-butanone), alcohols (1-propanethiol), and aldehydes (n-nonanal) decreased significantly. More specifically, aldehyde components dominated over ketones and lipids, while the n-nonanal content showed a downward trend during storage, and the 3-methylbutanol (trimer), 3-methylbutanol (dimer, D), 3-pentanone (D), and 3-pentanone (monomer) contents increased, whereas these compounds were identified as the key components affecting the fish meat flavor. Furthermore, after 120 d storage, the number of different flavor components reached its highest value, thereby confirming that the storage time influences the flavor of large yellow croaker fish. In this context, it should be noted that many of these compounds form through the Maillard reaction to accelerate the deterioration of fish meat. It was also found that after storage for 120 d, the physical indices of large yellow croaker meat showed significant changes, and its physicochemical properties varied. These results therefore demonstrate that a combination of GC-IMS and PCA can be used to identify the differences in flavor components present in fish meat during storage. Our study provides useful knowledge for understanding the different flavors associated with fish meat products during and following storage.

An injectable hydrogel having proteoglycan-like hierarchical structure supports chondrocytes delivery and chondrogenesis

The ECM of cartilage is composed of proteoglycans (PG) that contain glycosaminoglycan (GAG), aggrecan, hyaluronic acid (HA) and other molecular components which play an important role in regulating chondrocyte functions via cell-matrix interactions, integrin-mediated signalling etc. Implantation of chondrocytes encapsulated in scaffolds that mimic the micro-architecture of proteoglycan, is expected to enhance cartilage repair. With an aim to create a hydrogel having macromolecular structure that resembles the cartilage-specific ECM, we constructed a hierarchal structure that mimic the PG. The bottle brush structure of the aggrecan was obtained using chondroitin sulphate and carboxymethyl cellulose which served as GAG and core protein mimic respectively. A proteoglycan-like structure was obtained by cross-linking it with modified chitosan that served as a HA substitute. The physico-chemical characteristics of the above cross-linked injectable hydrogel supported long term human articular chondrocyte subsistence and excellent post-injection viability. The chondrocytes encapsulated in the PMH expressed significant levels of articular cartilage specific markers like collagen II, aggrecan, GAGs etc., indicating the ability of the hydrogel to support chondrocyte differentiation. The biocompatibility and biodegradability of the hydrogels was confirmed using suitable in vivo studies. The results revealed that the PG-mimetic hydrogel could serve as a promising scaffold for chondrocyte implantation.

Anti-tumor effect of local injectable hydrogel-loaded endostatin alone and in combination with radiotherapy for lung cancer

Endostatin (ES) can effectively inhibit neovascularization in most solid tumors and has the potential to make oxygen delivery more efficient and increase the efficacy of radiotherapy (RT). With a short half-life, ES is mainly administered systemically, which leads to low intake in tumor tissue and often toxic systemic side effects. In this study, we used hyaluronic acid-tyramine as a carrier to synthesize an ES-loaded hydrogel drug (ES/HA-Tyr) that can be injected locally. ES/HA-Tyr has a longer half-life and fewer systemic toxic side effects, and it exerts a better anti-angiogenic effect and anti-tumor effect with RT. In vitro, ES/HA-Tyr showed sustained release in the release assay and a stronger ability to inhibit the proliferation of human umbilical vascular endothelial cells (HUVECs) in the MTT assay; it exhibited a more potent effect against HUVEC invasion and a stronger anti-angiogenic effect on HUVECs in tube formation. In vivo, ES/HA-Tyr increased local drug concentration, decreased blood drug concentration, and caused less systemic toxicity. Further, ES/HA-Tyr effectively reduced tumor microvessel density, increased tumor pericyte coverage, decreased tumor hypoxia, and increased RT response. ES/HA-Tyr + RT also had increased anti-tumor and anti-angiogenic effects in Lewis lung cancer (LLC) xenograft models. In conclusion, ES/HA-Tyr showed sustained release, lower systemic toxicity, and better anti-tumor effects than ES. In addition, ES/HA-Tyr + RT enhanced anti-angiogenic effects, reduced tumor hypoxia, and increased the efficacy of RT in LLC-bearing mice.

Promising Polymeric Drug Carriers for Local Delivery: The Case of in situ Gels

BACKGROUND

At present, the controlled local drug delivery is a very promising approach compared to systemic administration, since it mostly targets the affected tissue. In fact, various drug carriers for local delivery have been prepared with improved therapeutic efficacy.

OBJECTIVE

in situ polymer gels are drug delivery systems that not only present liquid characteristics before their administration in body, but once they are administered, form gels due to gelation. Their gelation mechanism is due to factors such as pH alteration, temperature change, ion activation or ultraviolet irradiation. in situ gels offer various advantages compared to conventional formulations due to their ability to release drugs in a sustainable and controllable manner. Most importantly, in situ gels can be used in local drug delivery applications for various diseases.

METHODS

This review includes the basic knowledge and theory of in situ gels as well as their various applications according to their administration route.

RESULTS

Various natural, semisynthetic, and synthetic polymers can produce in situ polymeric gels. For example, natural polysaccharides such as alginic acid, chitosan, gellan gum, carrageenan etc. have been utilized as in situ gels for topical delivery. Besides the polysaccharides, poloxamers, poly(Nisopropylacrylamide), poly(ethyleneoxide)/ (lactic-co-glycolic acid), and thermosensitive liposome systems can be applied as in situ gels. In most cases, in situ polymeric gels could be applied via various administration routes such as oral, vaginal, ocular, intranasal and injectable.

CONCLUSION

To conclude, it can be revealed that in situ gels could be a promising alternative carrier for both chronic and immediate diseases.

Preparation and Application of Quaternized Chitosan- and AgNPs-Base Synergistic Antibacterial Hydrogel for Burn Wound Healing

Infection is the major reason that people die from burns; however, traditional medical dressings such as gauze cannot restrain bacterial growth and enhance the healing process. Herein, an organic- and inorganic-base hydrogel with antibacterial activities was designed and prepared to treat burn wounds. Oxidized dextran (ODex) and adipic dihydrazide grafted hyaluronic acid (HA-ADH) were prepared, mixed with quaternized chitosan (HACC) and silver nanoparticles to fabricate Ag@ODex/HA-ADH/HACC hydrogel. The hydrogel, composed of nature biomaterials, has a good cytocompatibility and biodegradability. Moreover, the hydrogel has an excellent antibacterial ability and presents fast healing for burn wounds compared with commercial Ag dressings. The Ag@ODex/HA-ADH/HACC hydrogel will be a promising wound dressing to repair burn wounds and will significantly decrease the possibility of bacterial infection.

Hyaluronic Acid-Based Scaffolds as Potential Bioactive Wound Dressings

The negative factors that result in delayed and prolonged wound healing process include microbial pathogens, excess wound exudates, underlying conditions, smoking, obesity, etc. Most of the currently used wound dressings demonstrate an inadequate capacity to treat wounds resulting from the factors mentioned above. The commonly used wound dressings include hydrogels, films, hydrocolloids, foams, fibers, sponges, dermal patches, bandages, etc. These wound dressings can be loaded with various types of bioactive agents (e.g., antibiotics, nanoparticles, anti-inflammatory drugs, etc.) to improve their therapeutic outcomes. Biopolymers offer interesting properties suitable for the design of wound dressings. This review article will be based on hyaluronic-acid-based scaffolds loaded with therapeutic agents for the treatment of wounds.

Feasibility study of oxidized hyaluronic acid cross-linking acellular bovine pericardium with potential application for abdominal wall repair

Bovine pericardium(BP)is one of the biological membranes with extensive application in tissue engineering. To fully investigate the potential clinical applications of this natural biological material, a suitable cross-linking reagent is hopefully adopted for modification. Glutaraldehyde (GA) is a clinically most common synthetic cross-linking reagent. In the study, oxidized hyaluronic acid (AHA) was developed to substitute GA to fix acellular bovine pericardium (ABP) for lower cytotoxicity, aiming to evaluate the feasibility of AHA as a cross-linking reagent and develop AHA-fixed ABP as a biological patch for abdominal wall repair. The AHA with the feeding ratio (1.8:1.0) has an appropriate molecular weight and oxidation degree, almost no cytotoxicity and good cross-linking effect. The critical cross-linking characteristics and cytocompatibility of AHA-fixed ABP were also investigated. The results demonstrated that 2.0% AHA-fixed ABP had the most suitable mechanical properties, thermal stability, resistance to enzymatic degradation and hydrophilicity. Moreover, 2.0% AHA-fixed samples exhibited an excellent cytocompatibility with human peritoneal mesothelial cells (HPMC) and low antigenicity. It also showed a prominent anti-calcification ability required for abdominal wall repair. Our data provided experimental basis for future research on AHA as a new cross-linking reagent and AHA-fixed ABP for abdominal wall repair.

Recent advances in composite hydrogels prepared solely from polysaccharides

The proliferating demand for sustainable, biodegradable, and biologically safe materials has triggered the development of polysaccharide-based hydrogels. The translation of research on single polysaccharide-based hydrogels into their desired clinical or industrial application is minimal. This is attributable to their lack of mechanical strength, inadequate stability, and constrained the possibility of their modulation to obtain the desired property. Polysaccharide-based composite hydrogels (PCHs) have proven their mantle to counteract this issue while expanding the horizons for their applications. PCHs can be fabricated by physical and/or chemical interlinking techniques, which entails the association of macromolecular chain linkages. The resulting composites can impart remarkably higher stability and elevate the suitability and efficiency of the system. Owing to these advantages, the research on PCHs has been gaining momentum. They are emerging as a lucrative alternative for the conventional molecules used for the fabrication of such materials. The review would initially focus on providing a detailed outlook for the various physical/chemical techniques involved in the preparation of PCHs. Subsequently, the characterization techniques used to understand the structural and chemical behavior of PCHs would be discussed. The article would also elaborate on the various fields of application and the possible areas for future research of PCHs.

In vitro Preparation and Evaluation of Sustained-Release Microcapsules of Salvianolic Acid

Objective

This study aims to investigate the preparation of sustained-release microcapsules of salvianolic acid.

Methods

The stability of salvianolic acid microcapsules was improved, and the time of action was prolonged in the present study. This was prepared using the spray-drying method, with chitosan as the carrier. In the preparation process, the prescription and process were optimized by L9 (34) using an orthogonal design, with yield and drug loading as indexes, in order to obtain optimum conditions.

Results

The optimal process and prescription for the preparation of salvianolic acid microcapsules were found to be as follows: mass concentration of chitosan, 1.5%; mass ratio of salvianolic acid to chitosan, 1:3; inlet air temperature, 190°C; and peristaltic pump speed, 300 mL·h-1. The surface of the microcapsules was round, the drug loading was 25.99% ± 2.14%, the yield was 51.88% ± 2.84%, the entrapment efficiency was 86.21% ± 2.89%, and the average particle size was 105.6 ± 2.56 nm. The microcapsules in vitro had certain sustained release characteristics. The internally fitted first-order release model equation was ln(1-Q) = -0.236 t + 4.591 7, r = 0.920. In addition, the results of differential scanning calorimetry show that the properties of salvianolic acid were not changed by the microcapsules.

Conclusion

Sustained-release microcapsules of salvianolic acid can be successfully prepared by adopting marine polysaccharide as a carrier.

Dextran hydrogels via disulfide-containing Schiff base formation: Synthesis, stimuli-sensitive degradation and release behaviors

Dextran hydrogels (Dex-SS) containing both disulfide and Schiff base bonds were developed via facile method based on the dextran oxidation and subsequent formation of Schiff base linkages between polyaldehyde dextran and cystamine, denoted as the disulfide-containing Schiff base reactions. Results of rheology, swelling and ¹³C CP/MAS NMR study indicated that cross-linking degree of Dex-SS hydrogels depended strongly on the molar ratio of -CHO/-NH₂. Acidic and reductive (GSH) environment sensitive degradation behaviors of Dex-SS hydrogels were then evidenced by SEM, rheology study and Ellman's assay. Moreover, doxorubicin (DOX) was loaded into the hydrogel matrix and pH/GSH-responsive release behaviors were demonstrated. Cytocompatibility of Dex-SS hydrogel and effective cell uptake of released DOX was finally proved by transwell assay with HepG2 cells. Take advantages of the abundance of vicinal hydroxyl on a variety of polysaccharides, the disulfide-containing Schiff base reactions is considered as versatile method to develop stimuli-sensitive hydrogels for local drug delivery.

Structural and biological investigation of chitosan/hyaluronic acid with silanized-hydroxypropyl methylcellulose as an injectable reinforced interpenetrating network hydrogel for cartilage tissue engineering

Cartilage damage continues to pose a threat to humans, but no treatment is currently available to fully restore cartilage function. In this study, a new class of composite hydrogels derived from water-soluble chitosan (CS)/hyaluronic acid (HA) and silanized-hydroxypropyl methylcellulose (Si-HPMC) (CS/HA/Si-HPMC) has been synthesized and tested as injectable hydrogels for cartilage tissue engineering when combined without the addition of a chemical crosslinking agent. Mechanical studies of CS/HA and CS/HA/Si-HPMC hydrogels showed that as Si-HPMC content increased, swelling rate and rheological properties were higher, compressive strength decreased and degradation was faster. Our results demonstrate that the CS and HA-based hydrogel scaffolds, especially the ones with 3.0% (w/v) Si-HPMC and 2.5/4.0% (w/v) CS/HA, have suitable physical performance and bioactive properties, thus provide a potential opportunity to be used for cartilage tissue engineering. In vitro studies of CS/HA and CS/HA/Si-HPMC hydrogels encapsulated in chondrocytes have shown that the proper amount of Si-HPMC increases the proliferation and deposition of the cartilage extracellular matrix. The regeneration rate of the CS/HA/Si-HPMC (3%) hydrogel reached about 79.5% at 21 days for long retention periods, indicating relatively good in vivo bone regeneration. These CS/HA/Si-HPMC hydrogels are promising candidates for tissue compatibility injectable scaffolds. The data provide proof of the principle that the resulting hydrogel has an excellent ability to repair joint cartilage using a tissue-engineered approach.

RESEARCH HIGHLIGHTS

An injectable hydrogel based on CS/HA/Si-HPMC composites was developed.

The CS/HA/Si-HPMC hydrogel displays the tunable rheological with mechanical properties.

The CS/HA/Si-HPMC hydrogel is highly porous with high swelling and degradation ratio.

Increasing concentration of Si-HPMC promote an organized network in CS/HA/Si-HPMC hydrogels.

Injectable CS/HA/Si-HPMC hydrogels have a high potential for cartilage tissue engineering.

Modeling of Association Properties of Cetylpyridinium Halides (Cl, Br) and their Influence on Liberation and Rheological and pH Balances of Hydrogels with Ibuprofen

The paper studies the influences of two cationic surfactants - cetylpyridinium chloride (CPCl) and cetylpyridinium bromide (CPBr) - at concentrations 10-times below and 10-times above the CMC - at 258C-408C on the release of ibuprofen from hydrogels, rheological properties and pH. The association of surfactants in the water solutions was studied by conductometry. It was found that the temperature dependences of the surfactants’ CMCs are not linear and the CMCs of CPCl are higher than that of CPBr. The concentrations of the polymer (chitosan) and the drug were kept constant. Amounts of the released drug increased with the increasing temperature. From the hydrogels with CPBr and CPCl, whose surfactant concentration was below the respective CMC, higher amounts of the active ingredient were released than from the hydrogels with surfactant concentrations above the respective CMC. It is shown that for increasing temperature the hydrogels exhibit gradual transition for pseudo plastic to thixotropic system, what is advantageous for their practical application. The hydrogels’ pH values were in the interval 5.10-5.46, which is ideal for their application on the skin and for the chitosan swelling. The presented topic is a precondition of synthesis of the on-line and off-line control algorithms of an optimal release kinetics, elaborated by the authors.

Sodium alginate/collagen hydrogel loaded with human umbilical cord mesenchymal stem cells promotes wound healing and skin remodeling

Stem cell transplantation is a promising therapy for wound healing, but the low retention and survival of transplanted stem cells limit their application. Injectable hydrogels exert beneficial effects in skin tissue engineering. In this study, an injectable hydrogel composed of sodium alginate (SA) and collagen type I (Col) was synthesized as a tissue scaffold to improve the efficacy of stem cells in a full-thickness excision wound model. Our results showed that SA/Col hydrogel was injectable, biodegradable, and exhibited low immunogenicity, which could promote the retention and survival of hUC-MSCs in vivo. SA/Col loaded with hUC-MSCs showed reduced wound size (p < 0.05). Histological and immunofluorescence results confirmed that SA/Col loaded with hUC-MSCs significantly promoted the formation of granulation, enhanced collagen deposition and angiogenesis, increased VEGF and TGF-β1 expression (p < 0.05), and mitigated inflammation evidenced by lower production of TNF-α and IL-1β and higher release of IL-4 and IL-10 (p < 0.05). Furthermore, SA/Col loaded with hUC-MSCs significantly lowered the expression of NLRP3 inflammasome-related proteins (p < 0.05). Taken together, our results suggest that SA/Col loaded with hUC-MSCs promotes skin wound healing via partly inhibiting NLRP3 pathway, which has potential to the treatment of skin wounds.

A novel dual crosslinked polysaccharide hydrogel with self-healing and stretchable properties

We synthesized oxidatively modified acetoacetyl cellulose OCAA, and then a double-network polysaccharide complex hydrogel was prepared. The hydrogel exhibited very good mechanical strength, self-healing behavior, and good biocompatibility.

A self-healing and injectable hydrogel based on water-soluble chitosan and hyaluronic acid for vitreous substitute

Vitreous, an essential dioptric medium for the human eyes, must be filled with artificial materials once damaged. Carboxymethyl chitosan (CMCTS) is one of the most important water-soluble chitosan derivatives with improved biocompatibility and biodegradability. In this study, oxidized hyaluronic acid (OHA) was prepared as crosslinking reagent. CMCTS and OHA were used to develop a biocompatible, self-repairing and in-situ injectable hydrogel for vitreous substitutes. Results showed the hydrogel with controllable swelling properties, high transparency, acceptable cytocompatibility on mouse fibroblast L929 and histocompatibility in vivo. Furthermore, hydrogel was injected in-situ into the vitreous cavity after vitrectomy on New Zealand Rabbits, no significant and persistent adverse effects were observed during the 90-day follow-up period. In addition, the hydrogel maintained intraocular pressure of the operated eyes and the inherent position of the retina. Collectively, this injectable, biodegradable, nontoxic hydrogel possessed enormous potential to become a vitreous substitute material.

Polysaccharide-Based Nanomaterials for Ocular Drug Delivery: A Perspective

Ocular drug delivery is one of the most challenging issues in ophthalmology because of the complex physiological structure of the eye. Polysaccharide-based nanomaterials have been extensively investigated in recent years as ideal carriers for enhancing the bioavailability of drugs in the ocular system because of their biocompatibility and drug solubilization. From this perspective, we discuss the structural instability of polysaccharides and its impact on the synthesis process; examine the potential for developing bioactive polysaccharide-based ocular drug nanocarriers; propose four strategies for designing novel drug delivery nanomaterials; and suggest reviewing the behavior of nanomaterials in ocular tissues.

Design of nanostructure chitosan hydrogels for carrying zinc phthalocyanine as a photosensitizer and difloxacin as an antibacterial agent

Here, we develop homogeneous hydrogel networks containing zinc tetraamino-phthalocyanine (ZnTAPc) as photosensitizer cross-linked with glutaraldehyde. The cross-linking process occurs via imine bond formation by reaction of NH₂ groups of ZnTAPc and chitosan with aldehyde groups of the glutaraldehyde. Insertion of ZnTAPc interestingly increases its solubility in water medium. 2.0% and 4% w/w of the ZnTAPc were used with respect to chitosan polymer to generate hydrogel photosensitizer. FT-IR and UV-Vis spectroscopy, scanning electron microscopy, and rheological measurements were applied to evaluate the properties of the prepared hydrogels. Finally, difloxacin HCl was selected as a fluoroquinolone drug for the assessment of the drug release features of the made hydrogels. The difloxacin release was affected by the amount of ZnTAPc and pH medium. The activity of the hydrogels in photosensitizing process was considered by computing the rate of 1,3-diphenylisobenzofuran adsorption reduction as a singlet oxygen chemical quencher.

3D Printing of a Reactive Hydrogel Bio-Ink Using a Static Mixing Tool

Hydrogel-based bio-inks have recently attracted more attention for 3D printing applications in tissue engineering due to their remarkable intrinsic properties, such as a cell supporting environment. However, their usually weak mechanical properties lead to poor printability and low stability of the obtained structures. To obtain good shape fidelity, current approaches based on extrusion printing use high viscosity solutions, which can compromise cell viability. This paper presents a novel bio-printing methodology based on a dual-syringe system with a static mixing tool that allows in situ crosslinking of a two-component hydrogel-based ink in the presence of living cells. The reactive hydrogel system consists of carboxymethyl chitosan (CMCh) and partially oxidized hyaluronic acid (HAox) that undergo fast self-covalent crosslinking via Schiff base formation. This new approach allows us to use low viscosity solutions since in situ gelation provides the appropriate structural integrity to maintain the printed shape. The proposed bio-ink formulation was optimized to match crosslinking kinetics with the printing process and multi-layered 3D bio-printed scaffolds were successfully obtained. Printed scaffolds showed moderate swelling, good biocompatibility with embedded cells, and were mechanically stable after 14 days of the cell culture. We envision that this straightforward, powerful, and generalizable printing approach can be used for a wide range of materials, growth factors, or cell types, to be employed for soft tissue regeneration.

Hyaluronic Acid: Redefining Its Role

The discovery of several unexpected complex biological roles of hyaluronic acid (HA) has promoted new research impetus for biologists and, the clinical interest in several fields of medicine, such as ophthalmology, articular pathologies, cutaneous repair, skin remodeling, vascular prosthesis, adipose tissue engineering, nerve reconstruction and cancer therapy. In addition, the great potential of HA in medicine has stimulated the interest of pharmaceutical companies which, by means of new technologies can produce HA and several new derivatives in order to increase both the residence time in a variety of human tissues and the anti-inflammatory properties. Minor chemical modifications of the molecule, such as the esterification with benzyl alcohol (Hyaff-11® biomaterials), have made possible the production of water-insoluble polymers that have been manufactured in various forms: membranes, gauzes, nonwoven meshes, gels, tubes. All these biomaterials are used as wound-covering, anti-adhesive devices and as scaffolds for tissue engineering, such as epidermis, dermis, micro-vascularized skin, cartilage and bone. In this review, the essential biological functions of HA and the applications of its derivatives for pharmaceutical and tissue regeneration purposes are reviewed.

Hydrogels Based on Schiff Base Linkages for Biomedical Applications

Schiff base, an important family of reaction in click chemistry, has received significant attention in the formation of self-healing hydrogels in recent years. Schiff base reversibly reacts even in mild conditions, which allows hydrogels with self-healing ability to recover their structures and functions after damages. Moreover, pH-sensitivity of the Schiff base offers the hydrogels response to biologically relevant stimuli. Different types of Schiff base can provide the hydrogels with tunable mechanical properties and chemical stabilities. In this review, we summarized the design and preparation of hydrogels based on various types of Schiff base linkages, as well as the biomedical applications of hydrogels in drug delivery, tissue regeneration, wound healing, tissue adhesives, bioprinting, and biosensors.