Crosslinked chitosan—preparation and characterization

Progress on green crosslinking of polysaccharide hydrogels for drug delivery and tissue engineering applications

Natural polysaccharides are being studied for their biocompatibility, biodegradability, low toxicity, and low cost in the fabrication of various hydrogel devices. However, due to their insufficient physicochemical and mechanical qualities, polysaccharide hydrogels alone are not acceptable for biological applications. Various synthetic crosslinkers have been tested to overcome the drawbacks of standalone polysaccharide hydrogels; however, the presence of toxic residual crosslinkers, the generation of toxic by-products following biodegradation, and the requirement of toxic organic solvents for processing pose challenges in achieving the desired non-toxic biomaterials. Natural crosslinkers such as citric acid, tannic acid, vanillin, gallic acid, ferulic acid, proanthocyanidins, phytic acid, squaric acid, and epigallocatechin have been used to generate polysaccharide-based hydrogels in recent years. Various polysaccharides, including cellulose, alginate, pectin, hyaluronic acid, and chitosan, have been hydrogelized and investigated for their potential in drug delivery and tissue engineering applications using natural crosslinkers. We attempted to provide an overview of the synthesis of polysaccharide-based hydrogel systems (films, complex nanoparticles, microspheres, and porous scaffolds) based on green crosslinkers, as well as a description of the mechanism of crosslinking and properties with a special emphasis on drug delivery, and tissue engineering applications.

Biomaterials / bioinks and extrusion bioprinting

Bioinks are formulations of biomaterials and living cells, sometimes with growth factors or other biomolecules, while extrusion bioprinting is an emerging technique to apply or deposit these bioinks or biomaterial solutions to create three-dimensional (3D) constructs with architectures and mechanical/biological properties that mimic those of native human tissue or organs. Printed constructs have found wide applications in tissue engineering for repairing or treating tissue/organ injuries, as well as in vitro tissue modelling for testing or validating newly developed therapeutics and vaccines prior to their use in humans. Successful printing of constructs and their subsequent applications rely on the properties of the formulated bioinks, including the rheological, mechanical, and biological properties, as well as the printing process. This article critically reviews the latest developments in bioinks and biomaterial solutions for extrusion bioprinting, focusing on bioink synthesis and characterization, as well as the influence of bioink properties on the printing process. Key issues and challenges are also discussed along with recommendations for future research.

Chitosan and Its Carboxymethyl-Based Membranes Produced by Crosslinking with Magnesium Phytate

Membranes produced by crosslinking chitosan with magnesium phytate were prepared using highly deacetylated chitosan and its N-carboxymethyl, O-carboxymethyl and N,O-carboxymethyl derivatives. The conditions of the membrane production were described. IR, Raman, electron absorption and emission spectra were measured and analyzed for all the substrates. It was found that O-carboxymethyl chitosan derivative is the most effectively crosslinked by magnesium phytate, and the films formed on this substrate exhibit good mechanical parameters of strength, resistance and stability. Strong O-H···O hydrogen bonds proved to be responsible for an effective crosslinking process. Newly discovered membrane types produced from chitosan and magnesium phytate were characterized as morphologically homogenous and uniform by scanning electron microscopy (SEM) and IR measurements. Due to their good covering properties, they do not have pores or channels and are proposed as packaging materials.

Chitosan-Grafted Carbon Oxynitride Nanoparticles: Investigation of Photocatalytic Degradation and Antibacterial Activity

In this work, a series of chitosan (CS)-grafted carbon oxynitride (OCN) nanoparticles (denoted as CS-OCN) were successfully synthesized for the first time by thermal polycondensation and subsequent esterification. The structure and photocatalytic performance of CS-OCN nanoparticles were investigated. The XPS spectra of CS-OCN-3 showed the presence of amino bonds. The optimal photocatalytic degradation efficiency of the synthesized CS-OCN-3 could reach 94.3% within 390 min, while the photocurrent response intensity was about 150% more than that of pure OCN. The improved photocatalytic performance may be mainly attributed to the enhanced photogenerated carrier's separation and transportation and stronger visible light response after CS grafting. In addition, the inhibition diameter of CS-OCN-3 reached 23 mm against E. coli within 24 h under visible light irradiation, exhibiting excellent photocatalytic bactericidal ability. The results of bacterial inhibition were supported by absorbance measurements (OD600) studies of E. coli. In a word, this work provided a rational design of an efficient novel metal-free photocatalyst to remove bacterial contamination and accelerate the degradation of organic dyes.

Prolonged Antibacterial Activity in Tannic Acid-Iron Complexed Chitosan Films for Medical Device Applications

Healthcare-associated infections (HAIs) represent a global burden, leading to significant mortality and generating financial costs. One important cause of HAIs is the microbiological contamination of implantable medical devices. In this context, a novel antimicrobial drug-eluting system, based on chitosan and loaded with gentamicin, a broad-spectrum antibiotic, was developed. The effects of the addition of tannic acid and different FeSO₄ concentrations on the loaded antibiotic release were evaluated. The properties of the films were assessed in terms of thickness, swelling, mass loss and wettability. The films' surface composition was characterized by X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The antibiotic release in phosphate buffer saline was quantified by high-performance liquid chromatography-mass spectrometry, and the antibacterial activity was evaluated. Hemolysis and cytotoxicity were also assessed. The results showed that the addition of tannic acid and iron decreased the swelling degree and degradation due to strong interactions between the different components, thus impacting gentamicin release for up to 35 days. In conclusion, this study presents a novel strategy to produce low-cost and biocompatible antimicrobial drug-eluting systems with sustained and prolonged antibacterial activity over more than a month.

Covalent Cross-Linking as a Strategy to Prepare Water-Dispersible Chitosan Nanogels

Due to the environmental problems generated by petroleum derivative polymers as mentioned in Agenda 2030, the use of natural polymers is increasing. Among them, cellulose and chitin are the most widespread biopolymers available in nature. Chitosan, obtained from chitin, is a really good candidate to develop nanocarriers due to its polyelectrolyte nature and ease of chemical modification. However, chitosan presents a solubility drawback in an aqueous medium at physiological pH (pH = 7.4), which restricts its applicability in biomedicine. In this work, nanogels were successfully synthesized from chitosan systems with different water solubilities (chitosan, oligosaccharide chitosan, and quaternized chitosan) using the reverse microemulsion method and polyethylene glycol diacid (PEGBCOOH) as a covalent cross-linking agent. Cross-linking with PEGBCOOH was analyzed by proton nuclear magnetic resonance (¹H-NMR), which allowed for nanogels to be prepared whose size and swelling were comparatively studied by transmission electron microscopy (TEM) and dynamic light scattering (DLS) and zeta potential, respectively. The particle size of the swollen nanogels showed a different pH-responsive behavior that decreased for chitosan, increased for oligosaccharide chitosan, and remained constant for quaternized chitosan. Nevertheless, a drastic reduction was observed in all cases in the culture medium. Along the same line, the dispersibility of the synthesized nanogels in different media was comparatively evaluated, showing similar values for the nanogels prepared from soluble chitosans than for water insoluble chitosan as a consequence of the cross-linking with PEGBCOOH. After 6 months of storage of the dried nanogels, the water dispersibility values remained constant in all cases, demonstrating the stabilizing effect of the employed cross-linking agent and the potential use of synthesized nanogels as substrates for drug delivery.

Microfluidic Tissue Engineering and Bio-Actuation

Bio-hybrid technologies aim to replicate the unique capabilities of biological systems that could surpass advanced artificial technologies. Soft bio-hybrid robots consist of synthetic and living materials and have the potential to self-assemble, regenerate, work autonomously, and interact safely with other species and the environment. Cells require a sufficient exchange of nutrients and gases, which is guaranteed by convection and diffusive transport through liquid media. The functional development and long-term survival of biological tissues in vitro can be improved by dynamic flow culture, but only microfluidic flow control can develop tissue with fine structuring and regulation at the microscale. Full control of tissue growth at the microscale will eventually lead to functional macroscale constructs, which are needed as the biological component of soft bio-hybrid technologies. This review summarizes recent progress in microfluidic techniques to engineer biological tissues, focusing on the use of muscle cells for robotic bio-actuation. Moreover, the instances in which bio-actuation technologies greatly benefit from fusion with microfluidics are highlighted, which include: the microfabrication of matrices, biomimicry of cell microenvironments, tissue maturation, perfusion, and vascularization.

Development and In Vitro Cytotoxicity of Citrus sinensis Oil-Loaded Chitosan Electrostatic Complexes

Electrostatic complexes based on chitosan, lecithin, and sodium tripolyphosphate were produced and evaluated with respect to their encapsulation capacity and cytotoxicity. Physical chemical properties were determined by zeta potential values and size distributions. For encapsulation assays, the emulsification method was followed, and Citrus senensis peel oil was utilized as volatile compound model. Morphology of complexes with oil incorporated was observed by scanning electron microscopy. The cytotoxicity of complexes was related to cell viability of zebrafish hepatocytes. The complexes produced presented positive Zeta potential values and size distributions dependent on the mass ratio between compounds. Higher concentrations of sodium tripolyphosphate promote significant changes (p < 0.05) in zeta values, which did not occur at smaller concentrations of the crosslinking agent. These complexes were able to encapsulate Citrus sinensis peel oil, with encapsulation efficiency higher than 50%. Cytotoxicity profiles showed that in a range of concentrations (0.1–100 μg/mL) studied, they did not promote cellular damage in zebrafish liver cells, being potential materials for food and pharmaceutical applications.

Self-assembled chitosan derived microparticles inhibit tumor angiogenesis and induce apoptosis in Ehrlich-ascites-tumor bearing mice

Self-assembled microparticles from chitosan (SAMC) was prepared by depolymerization induced by potassium persulfate. Particle size distribution data showed averaged around 5 μm size and SEM indicated the sequential formation of "RBC" shaped particles. Soluble SAMC consists of 'deacetylated' residues as revealed by ¹³C NMR. SAMC showed antitumor efficacy in human breast cancer cell lines through mitigation in cell proliferation, colony formation and cell migration. Anti-tumor and anti-angiogenic properties of SAMC was found in vivo Ehrlich ascites tumor (EAT) bearing mice model resulting in tumor growth inhibition (EAT control, 17.4 ml; SAMC treated, 6.8 ml) and improved survival potency (15 days). Moreover, the decrease in ascites VEGF secretion (EAT control, 1354 ng; SAMC treated, 351 ng) accompanied with reduction in neovessel formation. Apoptosis induction by SAMC was confirmed by DNA fragmentation, caspase activities and fluorescence staining methods respectively. SAMC may be a safe candidate for anti-tumor dietary supplement production in food industry.

A relative study on energy and exergy analysis between conventional single slope and novel stepped absorbable plate solar stills

The innovation of novel absorbing materials using composite materials and nanotechnology is of new trends for many researches. Here, the present study is concerning to enhance the distilled water productivity of a proposed solar still (PSS) using novel absorbing materials. The absorbing material is composed of chitosan (obtained from waste shrimp shells), ethylene diamine tetraacetic acid (EDTA), and Chrysopogon zizaniodes (Vetiver). The combination of these materials is coined as CHEDZ, and it acts as a super absorbent polymer that is coated on the stepped solar still. Evaporation rate increases due to this absorbent, which further increases the yield of the still. In this present study, the PSS is compared with the conventional solar still (CSS) for the use of assessing the yield of freshwater in the same atmospheric circumstance. The experimental setup was performed through the period from December to February 2020 in the Indian climatic condition. The freshwater productivity was improved to 3.05 L/day while the yield of the CSS is 2.47 L/day. The increase in efficiency obtained from a PSS is 39.71% more than the productivity attained from the CSS. The energy efficiency of the PSS is 18.34% and the exergy efficiency is 0.45%.

Tannic-Acid-Cross-Linked and TiO2-Nanoparticle-Reinforced Chitosan-Based Nanocomposite Film

A chitosan-based nanocomposite film with tannic acid (TA) as a cross-linker and titanium dioxide nanoparticles (TiO₂) as a reinforcing agent was developed with a solution casting technique. TA and TiO₂ are biocompatible with chitosan, and this paper studied the synergistic effect of the cross-linker and the reinforcing agent. The addition of TA enhanced the ultraviolet blocking and mechanical properties of the chitosan-based nanocomposite film. The reinforcement of TiO₂ in chitosan/TA further improved the nanocomposite film's mechanical properties compared to the neat chitosan or chitosan/TA film. The thermal stability of the chitosan-based nanocomposite film was slightly enhanced, whereas the swelling ratio decreased. Interestingly, its water vapor barrier property was also significantly increased. The developed chitosan-based nanocomposite film showed potent antioxidant activity, and it is promising for active food packaging.

High-performance pervaporation chitosan-based membranes: new insights and perspectives

Today, the need of replacing synthetic polymers in the membrane preparation for diverse pervaporation (PV) applications has been recognized collectively and scientifically. Chitosan (CS), a bio-polymer, has been studied and proposed to achieve this goal especially in specific azeotropic water-organic, organic-water, and organic-organic separations, as well as in assisting specific processes (e.g. seawater desalination and chemical reactions). Different concepts of CS-based membranes have been developed, which include material blending and composite and mixed matrix membranes which have been tested for different separations. Hereby, the goal of this review is to provide a critical overview of the ongoing CS-based membrane developments, paying a special attention to the most relevant findings and results in the field. Furthermore, future trends of CS-based membranes in PV technology are presented, as well as concluding remarks and suggested strategies for the new scientist in the field.

Preparation and characterization of thermally crosslinked films based on chitosan and poly(2-ethyl-2-oxasoline)

Сrosslinked films based on chitosan (СHI) and poly(2-ethyl-2-oxazoline) (POZ) were prepared by thermal crosslinking. The optimal synthesis conditions and the composition of the film compositions were determined. The highest yield of the gel fraction was observed for CHI:POZ (80:20) films with a crosslinking time of 4 h at a temperature of 100°С. The main physicochemical properties of films based on pure CHI and CHI:POZ have been studied. The film swelling ability was reduced with the increase of poly(2-ethyl-2-oxazolin) content. The formation of crosslinks between N,N’-methylene-bis-acrylamide and functional amine groups of chitosan was proposed based on IR-spectroscopy data.

Regenerable g-C3N4–chitosan beads with enhanced photocatalytic activity and stability

In this study, a series of regenerable graphitic carbon nitride–chitosan (g-C₃N₄–CS) beads were successfully synthesized via the blend crosslinking method. The prepared beads were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The structural characterization results indicate that the g-C₃N₄ granules were uniformly distributed on the surface of the chitosan matrix, and the structures of g-C₃N₄ and CS are maintained. In addition, the prepared g-C₃N₄–CS beads exhibited efficient MB degradation and stability. The optimum photocatalytic activity of our synthesized g-C₃N₄–CS beads was higher than that of the bulk g-C₃N₄ by a factor of 1.78 for MB. The improved photocatalytic activity was predominantly attributed to the synergistic effect between in situ adsorption and photocatalytic degradation. In addition, the reacted g-C₃N₄–CS beads can be regenerated by merely adding sodium hydroxide and hydrogen peroxide. Additionally, the regenerated g-C₃N₄–CS beads exhibit excellent stability after four runs, while the mass loss is less than 10%. This work might provide guidance for the design and fabrication of easily regenerated g-C₃N₄-based photocatalysts for environmental purification.

In this study, a series of regenerable graphitic carbon nitride–chitosan (g-C₃N₄–CS) beads were successfully synthesized via the blend crosslinking method.

Modified n-HA/PA66 scaffolds with chitosan coating for bone tissue engineering: cell stimulation and drug release

The dipping-drying procedure and cross-linking method were used to make drug-loaded chitosan (CS) coating on nano-hydroxyapatite/polyamide66 (nHA/PA66) composite porous scaffold, endowing the scaffold controlled drug release functionality. The prefabricated scaffold was immersed into an aqueous drug/CS solution in a vacuum condition and then crosslinked by vanillin. The structure, porosity, composition, compressive strength, swelling ratio, drug release and cytocompatibility of the pristine and coating scaffolds were investigated. After coating, the scaffold porosity and pore interconnection were slightly decreased. Cytocompatibility performance was observed through an in vitro experiment based on cell attachment and the MTT assay by MG63 cells which revealed positive cell viability and increasing proliferation over the 11-day period in vitro. The drug could effectively release from the coated scaffold in a controlled fashion and the release rate was sustained for a long period and highly dependent on coating swelling, suggesting the possibility of a controlled drug release. Our results demonstrate that the scaffold with drug-loaded crosslinked CS coating can be used as a simple technique to render the surfaces of synthetic scaffolds active, thus enabling them to be a promising high performance biomaterial in bone tissue engineering.

Enhanced degradation of chitosan by applying plasma treatment in combination with oxidizing agents for potential use as an anticancer agent

Solution plasma (SP) treatment in combination with oxidizing agents, i.e., hydrogen peroxide (H₂O₂), potassium persulfate (K₂S₂O₈) and sodium nitrite (NaNO₂) were adopted to chitosan degradation in order to achieve fast degradation rate, low chemicals used and high yield of low-molecular-weight chitosan and chitooligosaccharide (COS). Among the studied oxidizing agents, H₂O₂ was found to be the best choice in terms of appreciable molecular weight reduction without major change in chemical structure of the degraded products of chitosan. By the combination with SP treatment, dilute solution of H₂O₂ (4-60mM) was required for effective degradation of chitosan. The combination of SP treatment and dilute solution of H₂O₂ (60mM) resulted in the great reduction of molecular weight of chitosan and water-soluble chitosan was obtained as a major product. The resulting water-soluble chitosan was precipitated to obtain COS. An inhibitory effect against cervical cancer cell line (HeLa cells) of COS was also examined.

Diffusion in and around alginate and chitosan films with embedded sub-millimeter voids

Hydrogel scaffolds from biopolymers have potential use in the controlled release of drugs, and as 3-D structure for the formation of tissue matrix. This article describes the solute release behavior of alginate and chitosan films with embedded voids of sub-millimeter dimensions. Nitrogen gas was bubbled in a fluidic arrangement to generate bubbles, prior to the crosslinking. The crosslinked gel was dried in a vacuum oven, and subsequently, soaked in Vitamin B-12 solution. The dimensions of the voids immediately after the cross-linking of gel, and also after complete drying were obtained using a digital microscope and scanning electron microscope respectively. The porosity of the gel was measured gravimetrically. The release of Vitamin B-12 in PBS buffer on a shaker was studied. The release experiments were repeated at an elevated temperature of 37°C in the presence of lysozyme. The diffusion coefficient within the gel layer and the mass transfer coefficient at the interface with the bulk-liquid were estimated using a mathematical model. For comparison, the experiment was repeated with a film that does not have any embedded void. The enhancement in diffusion coefficient due to the presence of voids is discussed in this article.

Preparation and characterization of vanillin-crosslinked chitosan therapeutic bioactive microcarriers

Chitosan microspheres with diameter of 14.3-48.5 μm were prepared by emulsion method and using natural vanillin as cross-linking agent. The surface morphology and microstructure of the microspheres were characterized by scanning electron microscopy, X-ray diffraction and Fourier-transform infrared spectroscopy, etc. The hollow microspheres showed a well-defined spherical shape with median diameter of 30.3 μm and possessed a uniform surface with micro-wrinkles, which is in favor of the drug release. Interpenetrating network cross-linking mechanism might result from the Schiff base reaction and the acetalization of hydroxyl and carbonyl between chitosan and vanillin. Berberine, as a model drug, was loaded in the microspheres and released in a sustainable manner. The drug loading ratio could change from 9.16% to 29.70% corresponding to the entrapment efficiency of 91.61% to 74.25%. In vitro cell culture study using MG63 cells and in vivo implantation clearly showed that the microspheres could provide favorable cell attachment and biocompatibility. The results confirm that the drug-loaded vanillin-crosslinked chitosan microspheres could be a worthy candidate either as carriers of drugs and cells, or as therapeutic matrix for bone repair and regeneration.

Temperature-sensitive hydrogels by graft polymerization of chitosan and N-isopropylacrylamide for drug release

Thermo-responsive polysaccharidic hydrogels were designed and synthesized by a free radical induced grafting procedure. Chitosan was chosen as biopolymer to impart biocompatibility and biodegradability to the macromolecular systems, while N-isopropylacrylamide (NIPAAm) was selected as co-monomer responsive for the thermo-sensitive properties. Ammonium persulfate was the initiator system and different polymeric networks have been synthesized by modulating the amount of NIPAAm in the polymerization feed. The resulting hydrogels were proposed as drug delivery devices and their performance was evaluated by using Diclofenac sodium salt as a model drug. Hydrogels were carefully characterized by FT-IR spectrophotometry, calorimetric analyses and swelling behavior in a temperature range of 15-45°C. Finally, to verify the suitability of these hydrogels as thermo-responsive devices, the drug release profiles were studied performing in vitro experiments around the swelling-shrinking transition temperatures of the macromolecular systems.

Application of spectroscopic methods for structural analysis of chitin and chitosan

Chitin, the second most important natural polymer in the world, and its N-deacetylated derivative chitosan, have been identified as versatile biopolymers for a broad range of applications in medicine, agriculture and the food industry. Two of the main reasons for this are firstly the unique chemical, physicochemical and biological properties of chitin and chitosan, and secondly the unlimited supply of raw materials for their production. These polymers exhibit widely differing physicochemical properties depending on the chitin source and the conditions of chitosan production. The presence of reactive functional groups as well as the polysaccharide nature of these biopolymers enables them to undergo diverse chemical modifications. A complete chemical and physicochemical characterization of chitin, chitosan and their derivatives is not possible without using spectroscopic techniques. This review focuses on the application of spectroscopic methods for the structural analysis of these compounds.

Free radical-induced chitosan depolymerized products protect calf thymus DNA from oxidative damage

Low molecular weight chitosan (LMWC) and chitooligosaccharides (COs), obtained by persulfate-induced depolymerization of chitosan showed scavenging of OH. and O2.- radicals and offered protection against calf thymus DNA damage. Over 85% inhibition of free radicals and DNA protection were observed. LMWC (0.05 micromol) showed a strong inhibitory activity compared to COs (3.6 micromol). Further, LMWC showed calf thymus DNA condensation reversibly giving stability, as evident from CD, TEM and melting curves (Tm). A fluorescence study suggests the binding of LMWC in the minor groove, forming H-bonds to the backbone phosphates without distorting the double helix structure.

Electrospun nano-fibre mats with antibacterial properties from quaternised chitosan and poly(vinyl alcohol)

Nano-fibres containing quaternised chitosan (QCh) have been successfully prepared by electrospinning of QCh solutions mixed with poly(vinyl alcohol) (PVA). The average fibre diameter is in the range of 60-200 nm. UV irradiation of the composite electrospun nano-fibrous mats containing triethylene glycol diacrylate as cross-linking agent has resulted in stabilising of the nano-fibres against disintegration in water or water vapours. Microbiological screening has demonstrated the antibacterial activity of the photo-cross-linked electrospun mats against Staphylococcus aureus and Escherichia coli. The obtained nano-fibrous electrospun mats are promising for wound-healing applications.