Synthesis and characterization of a biocompatible chitosan–based hydrogel cross–linked via ‘click’ chemistry for controlled drug release

Synthesis and characterization of gellan gum-based hydrogels for drug delivery applications

In this study, gellan gum (Gel) derivatives were allowed to interact via aqueous Diels-Alder chemistry without the need for initiators, producing a crosslinked hydrogel network that exhibited good potential as a drug carrier using tramadol as a drug model. Hydrogel conjugation was achieved by the synthesis of a maleimide and furan-functionalized Gel, and the pre- and post-gelation chemical structure of the resulting hydrogel precursors was fully investigated. Potential uses of the developed hydrogel in the pharmaceutical industry were also evaluated by looking at its gelation duration, temperature, morphologies, swelling, biodegradation, and mechanical characteristics. The Gel-FM hydrogels were safe, showed good antimicrobial activity, and had a low storage modulus, which meant that they could be used in many biochemical fields. The encapsulation and release of tramadol from the hydrogel system in phosphate-buffered saline (PBS) at 37 °C were investigated under acidic and slightly alkaline conditions, replicating the stomach and intestinal tracts, respectively. The in-vitro release profile showed promising results for drug encapsulation, revealing that the drug could safely be well-encapsulated in acidic stomach environments and released more quickly in slightly alkaline intestinal environments. This implies that the hydrogels produced could work well as polymers for specifically delivering medication to the colon.

Poly(ethylene Glycol) Methyl Ether Methacrylate-Based Injectable Hydrogels: Swelling, Rheological, and In Vitro Biocompatibility Properties with ATDC5 Chondrogenic Lineage

Here, we present the synthesis of a series of chemical homopolymeric and copolymeric injectable hydrogels based on polyethylene glycol methyl ether methacrylate (PEGMEM) alone or with 2-dimethylamino ethyl methacrylate (DMAEM). The objective of this study was to investigate how the modification of hydrogel components influences the swelling, rheological attributes, and in vitro biocompatibility of the hydrogels. The hydrogels' networks were formed via free radical polymerization, as assured by 1H nuclear magnetic resonance spectroscopy (1H NMR). The swelling of the hydrogels directly correlated with the monomer and the catalyst amounts, in addition to the molecular weight of the monomer. Rheological analysis revealed that most of the synthesized hydrogels had viscoelastic and shear-thinning properties. The storage modulus and the viscosity increased by increasing the monomer and the crosslinker fraction but decreased by increasing the catalyst. MTT analysis showed no potential toxicity of the homopolymeric hydrogels, whereas the copolymeric hydrogels were toxic only at high DMEAM concentrations. The crosslinker polyethylene glycol dimethacrylate (PEGDMA) induced inflammation in ATDC5 cells, as detected by the significant increase in nitric oxide synthase type II activity. The results suggest a range of highly tunable homopolymeric and copolymeric hydrogels as candidates for cartilage regeneration.

Strategies for the Preparation of Chitosan Derivatives for Antimicrobial, Drug Delivery, and Agricultural Applications: A Review

Chitosan has received much attention for its role in designing and developing novel derivatives as well as its applications across a broad spectrum of biological and physiological activities, owing to its desirable characteristics such as being biodegradable, being a biopolymer, and its overall eco-friendliness. The main objective of this review is to explore the recent chemical modifications of chitosan that have been achieved through various synthetic methods. These chitosan derivatives are categorized based on their synthetic pathways or the presence of common functional groups, which include alkylated, acylated, Schiff base, quaternary ammonia, guanidine, and heterocyclic rings. We have also described the recent applications of chitosan and its derivatives, along with nanomaterials, their mechanisms, and prospective challenges, especially in areas such as antimicrobial activities, targeted drug delivery for various diseases, and plant agricultural domains. The accumulation of these recent findings has the potential to offer insight not only into innovative approaches for the preparation of chitosan derivatives but also into their diverse applications. These insights may spark novel ideas for drug development or drug carriers, particularly in the antimicrobial, medicinal, and plant agricultural fields.

Effects of the Degree of Phenol Substitution on Molecular Structures and Properties of Chitosan-Phenol-Based Self-Healing Hydrogels

Click chemistry is commonly used to prepare hydrogels, and chitosan-phenol prepared by using a Schiff base has been widely employed in the field of biomaterials. Chitosan-phenol is a derivative of chitosan; the phenol groups can disrupt both the inter- and intramolecular hydrogen bonds in chitosan, thereby reducing its crystallinity and improving its water solubility. In addition, chitosan-phenol exhibits various beneficial physiological functions. However, it is still unclear whether the degree of phenol substitution in the chitosan main chain affects the molecular interactions and structural properties of the self-healing hydrogels. To explore this issue, we investigated the molecular structure and network of self-healing hydrogels composed of chitosan-phenol with varying degrees of phenol substitution and dibenzaldehyde poly(ethylene oxide) (DB-PEO) using molecular dynamics simulations. We observed that when the degree of phenol substitution in the self-healing hydrogel was less than 15%, an increase in the degree of phenol substitution led to an increase in the interactions between chitosan-phenol and DB-PEO, and it enhanced the dynamic covalent bond cross-linking generated through the Schiff base reaction. However, when the degree of phenol substitution exceeded 15%, excessive phenol groups caused excessive intramolecular interactions within chitosan-phenol molecules, which reduced the binding between chitosan-phenol and DB-PEO. Our results revealed the influence of the degree of phenol substitution on the molecular structure of the self-healing hydrogels and showed an optimal degree of phenol substitution. These findings provide important insights for the future design of self-healing hydrogels based on chitosan and should help in enhancing the applicability of hydrogels in the field of biomedicine.

Antimicrobial PVA Hydrogels with Tunable Mechanical Properties and Antimicrobial Release Profiles

Hydrogels are broadly employed in wound healing applications due to their high water content and tissue-mimicking mechanical properties. Healing is hindered by infection in many types of wound, including Crohn's fistulas, tunneling wounds that form between different portions of the digestive system in Crohn's disease patients. Owing to the rise of drug-resistant infections, alternate approaches are required to treat wound infections beyond traditional antibiotics. To address this clinical need, we designed a water-responsive shape memory polymer (SMP) hydrogel, with natural antimicrobials in the form of phenolic acids (PAs), for potential use in wound filling and healing. The shape memory properties could allow for implantation in a low-profile shape, followed by expansion and would filling, while the PAs provide localized delivery of antimicrobials. Here, we developed a urethane-crosslinked poly(vinyl alcohol) hydrogel with cinnamic (CA), p-coumaric (PCA), and caffeic (Ca-A) acid chemically or physically incorporated at varied concentrations. We examined the effects of incorporated PAs on antimicrobial, mechanical, and shape memory properties, and on cell viability. Materials with physically incorporated PAs showed improved antibacterial properties with lower biofilm formation on hydrogel surfaces. Both modulus and elongation at break could be increased simultaneously in hydrogels after both forms of PA incorporation. Cellular response in terms of initial viability and growth over time varied based on PA structure and concentration. Shape memory properties were not negatively affected by PA incorporation. These PA-containing hydrogels with antimicrobial properties could provide a new option for wound filling, infection control, and healing. Furthermore, PA content and structure provide novel tools for tuning material properties independently of network chemistry, which could be harnessed in a range of materials systems and biomedical applications.

Ultrasound-Responsive Hydrogels for On-Demand Protein Release

The development of tunable, ultrasound-responsive hydrogels that can deliver protein payload on-demand when exposed to focused ultrasound is described in this study. Reversible Diels-Alder linkers, which undergo a retro reaction when stimulated with ultrasound, were used to cross-link chitosan hydrogels with entrapped FITC-BSA as a model protein therapeutic payload. Two Diels-Alder linkage compositions with large differences in the reverse reaction energy barriers were compared to explore the influence of linker composition on ultrasound response. Selected physicochemical properties of the hydrogel construct, its basic degradation kinetics, and its cytocompatibility were measured with respect to Diels-Alder linkage composition. Focused ultrasound initiated the retro Diels-Alder reaction, controlling the release of the entrapped payload while also allowing for real-time visualization of the ongoing process. Additionally, increasing the focused ultrasound amplitude and time correlated with an increased rate of protein release, indicating stimuli responsive control.

Crosslinkers for polysaccharides and proteins: Synthesis conditions, mechanisms, and crosslinking efficiency, a review

Polysaccharides and proteins are important macromolecules for developing hydrogels devoted to biomedical applications. Chemical hydrogels offer chemical, mechanical, and dimensional stability than physical hydrogels due to the chemical bonds among the chains mediated by crosslinkers. There are many crosslinkers to synthesize polysaccharides and proteins based on hydrogels. In this review, we revisited the crosslinking reaction mechanisms between synthetic or natural crosslinkers and polysaccharides or proteins. The selected synthetic crosslinkers were glutaraldehyde, carbodiimide, boric acid, sodium trimetaphosphate, N,N'-methylene bisacrylamide, and polycarboxylic acid, whereas the selected natural crosslinkers included transglutaminase, tyrosinase, horseradish peroxidase, laccase, sortase A, genipin, vanillin, tannic acid, and phytic acid. No less important are the reactions involving click chemistry and the macromolecular crosslinkers for polysaccharides and proteins. Literature examples of polysaccharides or proteins crosslinked by the different strategies were presented along with the corresponding highlights. The general mechanism involved in chemical crosslinking mediated by gamma and UV radiation was discussed, with particular attention to materials commonly used in digital light processing. The evaluation of crosslinking efficiency by gravimetric measurements, rheology, and spectroscopic techniques was presented. Finally, we presented the challenges and opportunities to create safe chemical hydrogels for biomedical applications.

Chitosan Hydrogels Based on the Diels-Alder Click Reaction: Rheological and Kinetic Study

The Diels–Alder reaction is recognized to generate highly selective and regiospecific cycloadducts. In this study, we carried out a rheological and kinetic study of N-furfuryl chitosan hydrogels based on the Diels–Alder click reaction with different poly(ethylene)glycol-maleimide derivatives in dilute aqueous acidic solutions. It was possible to prepare clear and transparent hydrogels with excellent mechanical properties. Applying the Winter and Chambon criterion the gel times were estimated at different temperatures, and the activation energy was calculated. The higher the temperature of gelation, the higher the reaction rate. The crosslinking density and the elastic properties seem to be controlled by the diffusion of the polymer segments, rather than by the kinetics of the reaction. An increase in the concentration of any of the two functional groups is accompanied by a higher crosslinking density regardless maleimide:furan molar ratio. The hydrogel showed an improvement in their mechanical properties as the temperature increases up to 70 °C. Above that, there is a drop in G’ values indicating that there is a process opposing to the Diels–Alder reaction, most likely the retro-Diels–Alder.

Polysaccharide hydrogels: Functionalization, construction and served as scaffold for tissue engineering

Polysaccharide hydrogels have been widely utilized in tissue engineering. They interact with the organismal environments, modulating the cargos release and realizing of long-term survival and activations of living cells. In this review, the potential strategies for modification of polysaccharides were introduced firstly. It is not only used to functionalize the polysaccharides for the consequent formation of hydrogels, but also used to introduce versatile side groups for the regulation of cell behavior. Then, techniques and underlying mechanisms in inducing the formation of hydrogels by polysaccharides or their derivatives are briefly summarized. Finally, the applications of polysaccharide hydrogels in vivo, mainly focus on the performance for alleviation of foreign-body response (FBR) and as cell scaffolds for tissue regeneration, are exemplified. In addition, the perspectives and challenges for further research are addressed. It aims to provide a comprehensive framework about the potentials and challenges that the polysaccharide hydrogels confronting in tissue engineering.

Review of Recent Advances and Their Improvement in the Effectiveness of Hydrogel-Based Targeted Drug Delivery: A Hope for Treating Cancer

Using hydrogels for delivering cancer therapeutics is advantageous in pharmaceutical usage as they have an edge over traditional delivery, which is tainted due to the risk of toxicity that it imbues. Hydrogel usage leads to the development of a more controlled drug release system owing to its amenability for structural metamorphosis, its higher porosity to seat the drug molecules, and its ability to shield the drug from denaturation. The thing that makes its utility even more enhanced is that they make themselves more recognizable to the body tissues and hence can stay inside the body for a longer time, enhancing the efficiency of the delivery, which otherwise is negatively affected since the drug is identified by the human immunity as a foreign substance, and thus, an attack of the immunity begins on the drug injected. A variety of hydrogels such as thermosensitive, pH-sensitive, and magnetism-responsive hydrogels have been included and their potential usage in drug delivery has been discussed in this review that aims to present recent studies on hydrogels that respond to alterations under a variety of circumstances in "reducing" situations that mimic the microenvironment of cancerous cells.

Design of drug-loaded 3D printing biomaterial inks and tailor-made pharmaceutical forms for controlled release

Curcumin and chloramphenicol are drugs with different solubility properties in physiological conditions due to their hydrophobic and hydrophilic structure, respectively. In this work, sodium alginate-cellulose nanofibers (SA-CNF) based inks loaded with curcumin and/or chloramphenicol have been developed for syringe extrusion 3D printing technology. Printability and shape fidelity of the drug-loaded inks were analyzed through rheological characterization. Suitable drug-loaded inks were 3D printed showing shape fidelity, and samples were either freeze-dried or crosslinked with Ca²⁺ and air-dried to achieve functional pharmaceutical forms with different morphological characteristics. In vitro drug delivery tests were carried out from the resulted forms and it was observed that the release performed faster in freeze-dried than in Ca²⁺ crosslinked/air-dried ones for all cases, resulting in two different methods for controlling drug delivery over time. The differences in aqueous solubility of the drugs, the different CNF content of the inks and the surface area of the samples also played an important role during drug delivery, involving strategies to control the release over an extended duration.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

pH-sensitive polymer-based carriers as a useful approach for oral delivery of therapeutic protein: A review

There are many proteins and enzymes in the human body, and their dysfunction can lead to disease. The use of proteins as a drug is common in various diseases such as diabetes. Proteins are hydrophilic molecules whose spatial structure is critical to their correct function. There are different ways to the administration of proteins. Protein structures are degraded by gastric acid and enzymes in the gastrointestinal tract and have a slight ability to permeation from the gastrointestinal epithelium due to their large hydrophilic nature. Therefore, their oral use has limitations. Since the oral use of drugs is one of the best and easiest routes for patients, many studies have been done to increase the stability, penetration and ultimately increase the bioavailability of proteins through oral administration. One of the studied strategies for oral delivery of protein is the use of pH-sensitive polymer-based carriers. These carriers use different pH-sensitive polymers such as eudragit®, chitosan, dextran, and alginate. The use of pH-sensitive polymer-based carriers by protecting the protein from stomach acid (low pH) and degrading enzymes, increasing permeability, and maintaining the spatial structure of the protein leads to increased bioavailability. In this review, we focus on the various polymers used to prepare pH-sensitive polymer-based carriers for the oral delivery of proteins.

3D printed alginate-cellulose nanofibers based patches for local curcumin administration

Alginate and nanocellulose are potential biomaterials to be employed as bioinks for three-dimensional (3D) printing. Alginate-cellulose nanofibers (A-CNF) formulations with CNF amounts up to 5 wt% were developed and rheologically characterized to evaluate their printability. Results showed that formulations with less than 3 wt% CNF did not present suitable characteristics to ensure shape fidelity after printing. Selected A-CNF bioinks were 3D printed and freeze-dried to obtain porous scaffolds. Morphological and mechanical analysis were performed, showing that CNF contributed to the reinforcement of the scaffolds and modulated their porosity. The applicability for drug delivery was evaluated by the addition of curcumin to printable A-CNF formulations. The curcumin loaded bioinks were successfully 3D printed in patches and the in vitro release tests showed that alginate and CNF played an important role in curcumin stabilization, whereas the CNF content and the disintegration of the scaffold were essential in the release kinetics.

One pot synthesis of new cross-linked chitosan-Schiff' base: Characterization, and anti-proliferative activities

Four novel chitosan hydrogels were successfully synthesized through the cross-linking reaction of chitosan with different concentrations of ethyl 5-(3,5-dihydroxy-1,4-dioxan-2-yl)-2-methylfuran-3-carboxylate. Their structures were confirmed by Fourier transform infrared spectroscopy (FT-IR), ¹³C Cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS ¹³C NMR), ultraviolet-visible spectroscopy, thermogravimetric analysis (TGA, DTA), and X-ray diffraction (XRD). Cytotoxicity on hepatocellular carcinoma (HepG-2) cell line and a normal African green monkey kidney (Vero) cell line were studied using the MTT assay. The resultant hydrogels showed a good inhibitory effect comparing to the un-modified parent; the hydrogels with the lowest degree cross-linking (0.125 and 0.25 mol cross-linker per one chitosan residue) showed potent anticancer activity in the HepG2 cells with IC₅₀ of 57.9 and 80.9 μg/ml, respectively. These results show that the newly synthesized cross-linked chitosan derivatives demonstrated more selectivity to the HepG2 than the Vero cells, indicating its potential for Investigation in the cure of hepatocellular carcinoma.

Fabrication of self-healing pectin/chitosan hybrid hydrogel via Diels-Alder reactions for drug delivery with high swelling property, pH-responsiveness, and cytocompatibility

Self-healing hydrogels with pH-responsiveness could protect loaded drugs from being destroyed till it arrives to the target. The pectin-based hydrogel is a candidate due to the health benefit, anti-inflammation, antineoplastic activity, nontoxicity, and biospecific degradation, et al. However, the abundant existence of water-soluble branched heteropolysaccharide chains influenced its performance resulting in limitation of the potential. In the present study, we prepared a series of self-healing pectin/chitosan hydrogels via the Diels-Alder reaction. Moreover, pectin/chitosan composite hydrogel was prepared as a contrast. By comparison, it can be seen that the Diels-Alder reaction greatly improved the cross-linking density of hydrogels. The self-healing experiments showed excellent self-healing performance. In different swelling mediums, significant transformation in the swelling ratio was shown, indicating well-swelling property, pH- and thermo-responsiveness. The drug loading and release studies presented high loading efficiency and sustained release performance. The cytotoxicity assay that showed a high cell proliferation ratio manifested great cytocompatibility.

Antimicrobial Hybrid Coatings Combining Enhanced Biocidal Activity under Visible-Light Irradiation with Stimuli-Renewable Properties

Hybrid, organic-inorganic, biocidal films exhibiting polishing properties were developed as effective long-lasting antimicrobial surface coatings. The films were prepared using cationically modified chitosan, synthesized by the reaction with 3-bromo-N,N,N-trimethylpropan-1-aminium bromide, to introduce permanent biocidal quaternary ammonium salt (QAS) groups along the polymer backbone and were cross-linked by a novel, pH-cleavable acetal cross-linker, which allowed polishing the hybrid coatings with the solution pH. TiO₂ nanoparticles, modified with reduced graphene oxide (rGO) sheets, to narrow their band gap energy value and shift their photocatalytic activity in the visible light regime, were introduced within the polymer film to enhance its antibacterial activity. The hybrid coatings exhibited an effective biocidal activity in the dark (∼2 Log and ∼3 Log reduction for Gram-negative and Gram-positive bacteria, respectively), when only the QAS sites interacted with the bacteria membrane, and an excellent biocidal action upon visible-light irradiation (∼5 Log and ∼6 Log reduction for Gram-negative and Gram-positive bacteria, respectively) due to the synergistic antimicrobial effect of the QAS moieties and the rGO-modified TiO₂ nanoparticles. The gradual decrease in the film thickness, upon immersion of the coatings in mildly basic (pH 8), neutral (pH 7), and acidic (pH 6) media, reaching 10, 20, and 70% reduction, respectively, after 60 days of immersion time, confirmed the polishing behavior of the films, whereas their effective antimicrobial action was retained. The biocompatibility of the hybrid films was verified in human cell culture studies. The proposed approach enables the facile development of highly functional coatings, combining biocompatibility and bactericidal action with a "kill and self-clean" mechanism that allows the regeneration of the outer surface of the coating leading to a strong and prolonged antimicrobial action.

Click chemistry-based biopolymeric hydrogels for regenerative medicine

Click chemistry is not a single specific reaction, but describes ways of generating products which emulate examples in nature. Click reactions occur in one pot, are not disturbed by water, generate minimal and inoffensive byproducts, and are characterized by a high thermodynamic driving force, driving the reaction quickly and irreversibly towards a high yield of a single reaction product. As a result, over the past 15 years it has become a very useful bio-orthogonal method for the preparation of chemical cross-linked biopolymer-based hydrogel, in the presence of e.g. growth factors and live cells, or in-vivo. Biopolymers are renewable and non-toxic, providing a myriad of potential backbone toolboxes for hydrogel design. The goal of this review is to summarize recent advances in the development of click chemistry-based biopolymeric hydrogels, and their applications in regenerative medicine. In particular, various click chemistry approaches, including copper-catalyzed azide-alkyne cycloaddition reactions, copper-free click reactions (e.g. the Diels-Alder reactions, the strain-promoted azide-alkyne cycloaddition reactions, the radical mediated thiol-ene reactions, and the oxime-forming reactions), and pseudo-click reactions (e.g. the thiol-Michael addition reactions and the Schiff base reactions) are highlighted in the first section. In addition, numerous biopolymers, including proteins (e.g. collagen, gelatin, silk, and mucin), polysaccharides (e.g. hyaluronic acid, alginate, dextran, and chitosan) and polynucleotides (e.g. deoxyribonucleic acid), are discussed. Finally, we discuss biopolymeric hydrogels, cross-linked by click chemistry, intended for the regeneration of skin, bone, spinal cord, cartilage, and cornea. This article provides new insights for readers in terms of the design of regenerative medicine, and the use of biopolymeric hydrogels based on click chemistry reactions.

Tailoring the in situ conformation of bacterial cellulose-graphene oxide spherical nanocarriers

Bacterial cellulose (BC)/graphene oxide (GO) sphere-like hydrogels have been biosynthesized by in situ route in dynamic cultivation. The GO concentration during BC biosynthesis (0.01 and 0.05 mg mL^-1) was the determining factor for the conformation of the final hydrogels: encapsulation (BC/GO 0.01) or distribution through all the body of the spheres (BC/GO 0.05). The as-prepared sphere hydrogels were characterized in terms of physico-chemical properties, thermal stability, microstructure, and swelling capacity in different media. In addition, a chemical treatment with ascorbic acid was performed in order to obtain reduced graphene oxide (rGO) into the spheres (BC/rGO). After the chemical treatment, electrostatic force microscopy (EFM) revealed electrical interactions due to the presence of rGO inside the spheres and resistivity values in the range of semiconductive materials were obtained (10⁶ Ω·cm), making BC/rGO spheres promising for the development of electro-stimulated systems. The in vitro release study of ibuprofen (IB), showed that the reduction process led to an increase of 73 and 92% of drug release with respect to BC/GO 0.05 and BC/GO 0.01 spheres, respectively. Moreover, the encapsulation conformation showed more homogeneous porous structure and thus, a cumulative drug release of 63% was reached after 6 h.

Application status and technical analysis of chitosan-based medical dressings: a review

Chitosan has wide applications in the field of medical dressings due to its good biomedical properties. This review provides the application status and technical analysis of chitosan medical dressings. First, we introduce the source and chemical structure of chitosan. Then, we investigate the mechanism of chitosan showing different medical properties. We also show the application of supramolecular chitosan-based hydrogels in the dressing field and the formulation optimization and the preparation technology of chitosan dressings for fabricating chitosan-based dressings with various morphologies and medical functions. After that, we introduce the research process of the modification method of chitosan dressings including single modification, blending modification, crosslinking modification, etc. Finally, based on the study of the medical effects of chitosan dressings, we analyze the existing problems in the preparation process and propose corresponding solutions from the aspects of the morphology, clinical feedback effect, and future development trends. This paper can provide a reference for further studies of skin tissue engineering and the development of new chitosan medical dressings.

Chitosan has wide applications in the field of medical dressings due to its good biomedical properties.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

Functional Chitosan-based Materials for Biological Applications

Background:

Bio-based materials, as the plentiful and renewable resources for natural constituents which are essential for biomedical and pharmaceutical applications, have not been exploited adequately yet. Chitosan is a naturally occurring polysaccharide obtained from chitin, which has recently attracted widespread attention owing to its excellent activity. This review shows the methods of extraction and modification of chitosan and provides recent progress of synthesis and use of chitosan-based materials in biological applications.

Methods:

By consulting the research literature of the last decade, the recent progresses of functional chitosan-based materials for biological applications were summarized and divided into the methods of extraction chitosan, the chemical modification of chitosan, chitosan-based materials for biological applications were described and discussed.

Results:

Chemical modification of chitosan broadens its applications, leading to developing numerous forms of chitosan-based materials with excellent properties. The excellent bioactivity of chitosan-based material enables it serves potential applications in biomedical fields.

Conclusion:

Chitosan-based materials not only exhibit the excellent activities of chitosan but also show other appealing performance of combined materials, even give the good synergistic properties of chitosan and its composite materials. Further studies are needed to define the ideal physicochemical properties of chitosan for each type of biomedical applications. The development of various functional chitosan-based materials for biological applications will be an important field of research, and this kind of material has important commercial value.

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

Stimuli-responsive chitosan (CS) hydrogels exhibit great potential for drug delivery and tissue engineering; however, the structure of these stimuli-responsive CS hydrogels, such as dual pH- and thermo-responsive hydrogels, is difficult to control or needs additional crosslinking agents. Here, a new dual pH- and thermo-responsive hydrogel system is developed by combining pH-responsive C₆ -OH allyl-modified CS (OAL-CS) with thermo-responsive poly(N-isopropylacrylamide) (PNIPAM). The thiol groups in PNIPAM and the allyl groups in OAL-CS can rapidly form crosslinking hydrogel network by "thiol-ene" click chemistry under UV irradiation. As expected, the swelling ratio of the OAL-CS/PNIPAM hydrogel can be controlled by changing pH and temperature. Moreover, the hydrogel displays non-cytotoxic nature toward human bone marrow mesenchymal stem cells, and the histological analyses reveal the subcutaneous tissue with no signs of inflammation after 5 days of injection in vivo. The results indicate that the new OAL-CS/PNIPAM hydrogel has potential to serve as a smart injectable platform for application in drug delivery and tissue engineering.

Photochemistry of nitric oxide and S-nitrosothiols in human skin

Nitric oxide (NO) is related to a wide range of physiological processes such as vasodilation, macrophages cytotoxicity and wound healing. The human skin contains NO precursors (NOx). Those are mainly composed of nitrite (NO2-), nitrate (NO3-), and S-nitrosothiols (RSNOs) which forms a large NO store. These NOx stores in human skin can mobilize NO to blood stream upon ultraviolet (UV) light exposure. The main purpose of this study was to evaluate the most effective UV light wavelength to generate NO and compare it to each NO precursor in aqueous solution. In addition, the UV light might change the RSNO content on human skin. First, we irradiated pure aqueous solutions of NO2- and NO3- and mixtures of NO2- and glutathione and NO3- and S-nitrosoglutathione (GSNO) to identify the NO release profile from those species alone. In sequence, we evaluated the NO generation profile on human skin slices. Human skin was acquired from redundant plastic surgical samples and the NO and RSNO measurements were performed using a selective NO electrochemical sensor. The data showed that UV light could trigger the NO generation in skin with a peak at 280-285 nm (UVB range). We also observed a significant RSNO formation in irradiated human skin, with a peak at 320 nm (UV region) and at 700 nm (visible region). Pre-treatment of the human skin slice using NO2- and thiol (RSHs) scavengers confirmed the important role of these molecules in RSNO formation. These findings have important implications for clinical trials with potential for new therapies.

Fabrication of Patterned Hydrogel Interfaces: Exploiting the Maleimide Group as a Dual Purpose Handle for Cross-Linking and Bioconjugation

Functional hydrogels that can be obtained through facile fabrication procedures and subsequently modified using straightforward reagent-free methods are indispensable materials for biomedical applications such as sensing and diagnostics. Herein a novel hydrogel platform is obtained using polymeric precursors containing the maleimide functional group as a side chain. The maleimide groups play a dual role in fabrication of functional hydrogels. They enable photochemical cross-linking of the polymers to yield bulk and patterned hydrogels. Moreover, the maleimide group can be used as a handle for efficient functionalization using the thiol-maleimide conjugation and Diels-Alder cycloaddition click reactions. Obtained hydrogels are characterized in terms of their morphology, water uptake capacity, and functionalization. Micropatterned hydrogels are obtained under UV-irradiation using a photomask to obtain reactive micropatterns, which undergo facile functionalization upon treatment with thiol-containing functional molecules such as fluorescent dyes and bioactive ligands. The maleimide group also undergoes conjugation through the Diels-Alder reaction, where the attached molecule can be released through thermal treatment via the retro Diels-Alder reaction. The antibiofouling nature of these hydrogel micropatterns enables efficient ligand-directed biomolecular immobilization, as demonstrated by attachment of streptavidin-coated quantum dots.

Functional DNA-based hydrogel intelligent materials for biomedical applications

Multifunctional intelligent DNA hydrogels have been reviewed for many biomedical applications.

Light-driven assembly of biocompatible fluorescent chitosan hydrogels with self-healing ability

Nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) was successfully used to cross-link complementary tetrazole and maleimide chitosan derivatives into hydrogel networks using irradiation.

The role of cellulose nanocrystals in biocompatible starch-based clicked nanocomposite hydrogels

Starch-based nanocomposite hydrogels were successfully prepared by the Diels-Alder click cross-linking reaction between furan-functionalized starch derivative and a water-soluble tetrafunctional maleimide compound, adding cellulose nanocrystals (CNC) as nanoreinforcement. The effect of increasing the CNC content on rheological and swelling properties as well as on the morphology of the hydrogels was analyzed. Besides, in order to evaluate the applicability of the as-prepared hydrogels as delivery systems, drug release measurements and in vitro cytotoxicity assays were also performed. It was found that the prepared nanocomposite hydrogels presented higher stiffness as the CNC content increased. The incorporation of the nanocrystals modified the internal porous microstructure of the hydrogels, affecting consequently both the swelling capacity and the drug-delivery kinetics. Moreover, the prepared nanocomposite hydrogels showed non-toxic behavior, demonstrating their potential applicability in the biomedical field, especially as sustained drug delivery systems.

Novel chitosan-ethylene glycol hydrogel for the removal of aqueous perfluorooctanoic acid

It is urgent to explore an effective removal method for perfluorooctanoic acid (PFOA) due to its recalcitrant nature. In this study, a novel chitosan-based hydrogel (CEGH) was prepared with a simple method using chitosan and ethylene glycol through a repeated freezing-thawing procedure. The adsorption of PFOA anions to CEGH agreed well to the Freundlich-Langmuir model with a maximum adsorption capacity as high as 1275.9 mg/g, which is higher than reported values of most adsorbents for PFOA. The adsorption was influenced by experimental conditions. Experimental results showed that the main removal mechanism was the ionic hydrogen bond interaction between carbonyl groups (COO^-) of PFOA and protonated amine (NH⁺) of the CEGH adsorbent. Therefore, CEGH is a very attractive adsorbent that can be used to remove PFOA from water in the future.

Synthesis and behavior of click cross-linked alginate hydrogels: Effect of cross-linker length and functionality

Various bismaleimides and trismaleimides of varying molar masses, chemical architectures and functionalities were explored as cross-linkers for furan-modified alginate chains via Diels-Alder click reactions. An environmentally friendly approach is described for the preparation of hydrogels based on naturally occurring biomacromolecules, without catalysts. The behavior of the resulting polysaccharides-based hydrogels was analyzed in terms of swelling, rheological properties and drug-release efficiency, in connection with potential biomedical applications. The use of the different cross-linkers allows tuning the mechanical properties as well as the pulsatile swelling behavior of the hydrogels. When using trifunctional cross-linkers stiffer hydrogels were formed with high storage modulus whereas the chain length and the composition of the cross-linker clearly influence the swelling of the hydrogel network. In connection with drug delivery applications, release of vanillin as a traceable aromatic biobased model drug was also monitored as a function of hydrogel composition. To the best of our knowledge, for the first-time furan-modified alginates were reacted and studied with polyethylene glycol-based bis or trismaleimides with different molar masses and architectures, resulting in advanced hydrogels with different behavior.

A review on applications of chitosan-based Schiff bases

Biopolymers have become very attractive as they are degradable, biocompatible, non-toxic and renewable. Due to the intrinsic reactive amino groups, chitosan is vibrant in the midst of other biopolymers. Using the versatility of these amino groups, various structural modifications have been accomplished on chitosan through certain chemical reactions. Chemical modification of chitosan via imine functionalization (RR'CNR″; R: alkyl/aryl, R': H/alkyl/aryl and R″: chitosan ring) is significant as it recommends the resultant chitosan-based Schiff bases (CSBs) for the important applications in the fields like biology, catalysis, sensors, water treatment, etc. CSBs are usually synthesized by the Schiff condensation reaction between chitosan's amino groups and carbonyl compounds with the removal of water molecules. In this review, we first introduce the available synthetic approaches for the preparation of CSBs. Then, we discuss the biological applications of CSBs including antimicrobial activity, anticancer activity, drug carrier ability, antioxidant activity and tissue engineering capacity. Successively, the applications of CSBs in other fields such as catalysis, adsorption and sensors are demonstrated.

Entrapment of chitosan, pectin or κ-carrageenan within methacrylate based hydrogels: Effect on swelling and mechanical properties

Composite hydrogels were obtained by the entrapment of chitosan, pectin or κ-carrageenan within methacrylate-based hydrogels to improve their swelling and the mechanical properties. The results indicated that the water uptake (WU) of κ-carrageenan and chitosan hydrogels were until 3.5 and 2.2 times higher than the WU of the synthetic hydrogel, respectively. The surface morphologies of the hydrogels showed that the pectin and κ-carrageenan favors the formation of larger and more defined pores. The mechanical properties indicated that the pectin increased slightly the mechanical properties and the κ-carrageenan improves the mechanical properties of the synthetic hydrogel reaching up 400 N of compression load. Therefore, the entrapment of κ-carrageenan within synthetic hydrogels improved both the swelling and the mechanical properties. The biocompatibility of the hydrogels was evaluated with in vitro cytotoxicity assays and the results indicated that they could be considered as candidates for biomedical use.

Enhanced Antibacterial Activity of Silver Doped Titanium Dioxide-Chitosan Composites under Visible Light

Nano titanium dioxide (TiO₂) with photocatalytic activity was firstly modified by diethanolamine, and it was then doped with broad spectrum antibacterial silver (Ag) by in situ method. Further, both Ag doped TiO₂-chitosan (STC) and TiO₂-chitosan (TC) composites were prepared by the inverse emulsion cross-linking reaction. The antibacterial activities of STC composites were studied and their antibacterial mechanisms under visible light were investigated. The results show that in situ doping and inverse emulsion method led to good dispersion of Ag and TiO₂ nanoparticles on the cross-linked chitosan microsphere. The STC with regular particle size of 1–10 μm exhibited excellent antibacterial activity against E. coli, P. aeruginosa and S. aureus under visible light. It is believed that STC with particle size of 1–10 μm has large specific surface area to contact with bacterial cell wall. The increased antibacterial activity was attributed to the enhancement of both electron-hole separations at the surface of nano-TiO₂ by the silver ions under the visible light, and the synergetic and sustained release of strong oxidizing hydroxyl radicals of nano-TiO₂, together with silver ions against bacteria. Thus, STC composites have great potential applications as antibacterial agents in the water treatment field.