Cellular hydrogels based on pH-responsive chitosan-hydroxyapatite system

In vitro degradation, swelling, and bioactivity performances of in situ forming injectable chitosan-matrixed hydrogels for bone regeneration and drug delivery

Injectable, tissue mimetic, bioactive, and biodegradable hydrogels offer less invasive regeneration and repair of tissues. The monitoring swelling and in vitro degradation capacities of hydrogels are highly important for drug delivery and tissue regeneration processes. Bioactivity of bone tissue engineered constructs in terms of mineralized apatite formation capacity is also pivotal. We have previously reported in situ forming chitosan-based injectable hydrogels integrated with hydroxyapatite and heparin for bone regeneration, promoting angiogenesis. These hydrogels were functionalized by glycerol and pH to improve their mechano-structural properties. In the present study, functionalized hybrid hydrogels were investigated for their swelling, in vitro degradation, and bioactivity performances. Hydrogels have degraded gradually in phosphate-buffered saline (PBS) with and without lysozyme enzyme. The percentage weight loss of hydrogels and their morphological and chemical properties, and pH of media were analyzed. The swelling ratio of hydrogels (55%-68%(wt), 6 h of equilibrium) indicated a high degree of cross-linking, can be suitable for controlled drug release. Hydrogels have gradually degraded reaching to 60%-70% (wt%) in 42 days in the presence and absence of lysozyme, respectively. Simulated body fluid (SBF)-treated hydrogels containing hydroxyapatite-induced needle-like carbonated-apatite mineralization was further enhanced by heparin content significantly.

Harnessing Macromolecular Chemistry to Design Hydrogel Micro- and Macro-Environments

Cell encapsulation within three-dimensional hydrogels is a promising approach to mimic tissues. However, true biomimicry of the intricate microenvironment, biophysical and biochemical gradients, and the macroscale hierarchical spatial organizations of native tissues is an unmet challenge within tissue engineering. This review provides an overview of the macromolecular chemistries that have been applied toward the design of cell-friendly hydrogels, as well as their application toward controlling biophysical and biochemical bulk and gradient properties of the microenvironment. Furthermore, biofabrication technologies provide the opportunity to simultaneously replicate macroscale features of native tissues. Biofabrication strategies are reviewed in detail with a particular focus on the compatibility of these strategies with the current macromolecular toolkit described for hydrogel design and the challenges associated with their clinical translation. This review identifies that the convergence of the ever-expanding macromolecular toolkit and technological advancements within the field of biofabrication, along with an improved biological understanding, represents a promising strategy toward the successful tissue regeneration.

Human mesenchymal stromal cells-laden crosslinked hyaluronic acid-alginate bioink for 3D bioprinting applications in tissue engineering

Three-dimensional (3D) bioprinting is considered one of the most advanced tools to build up materials for tissue engineering. The aim of this work was the design, development and characterization of a bioink composed of human mesenchymal stromal cells (hMSC) for extrusion through nozzles to create these 3D structures that might potentially be apply to replace the function of damaged natural tissue. In this study, we focused on the advantages and the wide potential of biocompatible biomaterials, such as hyaluronic acid and alginate for the inclusion of hMSC. The bioink was characterized for its physical (pH, osmolality, degradation, swelling, porosity, surface electrical properties, conductivity, and surface structure), mechanical (rheology and printability) and biological (viability and proliferation) properties. The developed bioink showed high porosity and high swelling capacity, while the degradation rate was dependent on the temperature. The bioink also showed negative electrical surface and appropriate rheological properties required for bioprinting. Moreover, stress-stability studies did not show any sign of physical instability. The developed bioink provided an excellent environment for the promotion of the viability and growth of hMSC cells. Our work reports the first-time study of the effect of storage temperature on the cell viability of bioinks, besides showing that our bioink promoted a high cell viability after being extruded by the bioprinter. These results support the suggestion that the developed hMSC-composed bioink fulfills all the requirements for tissue engineering and can be proposed as a biological tool with potential applications in regenerative medicine and tissue engineering.

Stimuli-responsive Biomaterials for Tissue Engineering Applications

The use of ''smart materials,'' or ''stimulus responsive'' materials, has proven useful in a variety of fields, including tissue engineering and medication delivery. Many factors, including temperature, pH, redox state, light, and magnetic fields, are being studied for their potential to affect a material's properties, interactions, structure, and/or dimensions. New tissue engineering and drug delivery methods are made possible by the ability of living systems to respond to both external stimuli and their own internal signals) for example, materials composed of stimuliresponsive polymers that self assemble or undergo phase transitions or morphology transformation. The researcher examines the potential of smart materials as controlled drug release vehicles in tissue engineering, aiming to enable the localized regeneration of injured tissue by delivering precisely dosed drugs at precisely timed intervals.

Thermosensitive injectable dual drug-loaded chitosan-based hybrid hydrogel for treatment of orthopedic implant infections

A myriad of therapeutic agents and drug delivery systems are available to the surgeons for treating orthopedic implant-associated infections (OIAI), but only very few have demonstrated their effectiveness in preventing bacteria colonization and biofilm formation due to challenges in the local and sustainable therapeutic release. To address this issue, in this work, a thermosensitive injectable hydrogel based on chitosan (CH)-integrated hydroxyapatite nanoparticles (HAP NPs) containing vancomycin (Van) and quercetin (QC)-loaded in F127 micelles (CH-HAP-FQ-Van hydrogel) was fabricated with potential application in the treatment of OIAI. This dual drug delivery system demonstrated a pH-sensitive drug release pattern. In addition, 100 % growth inhibition of Staphylococcus aureus for a duration of 14 days was observed. Apart from the strong antioxidant activities owing to the co-administration of QC even after 432 h, this composite hydrogel revealed 95.88 ± 2.8 % S. aureus biofilm eradication. By consideration of degradation stability (53.52 ± 4.24 %) during 60 days along with smart gelation within 10 min at 37 °C and easy injectability, CH-HAP-FQ-Van hydrogel could be used as a promising ideal local drug delivery system for implant-related infections.

Copper-zinc/chitosan complex hydrogels: Rheological, degradation and biological properties

Polymer hydrogels crosslinked by therapeutic metal ions have attracted increased interest in recent years due to their unique and versatile properties. Chitosan hydrogels are widely investigated for various biomedical applications such as tissue engineering and drug delivery. Copper and zinc ions are considered as therapeutic metal ions, that have important roles in bone regeneration. The aim of this study was to investigate the physicochemical and biological properties of bimetallic-chitosan complex hydrogels with different cupric and zinc ions content. Scanning electron microscopy (SEM) revealed changes in the morphology from the microstructure with larger, tubular pores for aerogels with higher Zn content, to the sheets-like structure with long pores for samples with higher Cu content. FTIR analysis indicated the formation of bimetallic-chitosan aerogels. The obtained X-ray diffraction patterns showed a broadening of chitosan's characteristic diffraction maximum, while characterization of physical properties showed decreased swelling ability and increased shear modulus with higher Cu content. ICP-MS results showed a negligible amount of copper and zinc ions released under physiological conditions during 24 h indicating a strong physical crosslink between metal ions and chitosan chains. Furthermore, accelerated in vitro degradation showed that hydrogels maintained good stability during four weeks of lysozyme activity. The MTT assay indicated that the cytotoxicity of Cu2+-Zn2+/chitosan complexes could be adjusted by the amount of cupric ions. All results imply that Cu2+ and Zn2+ ions act as physical crosslinkers of the polymer network. Also, results are in agreement with the prediction of density functional theory (DFT) which indicated stronger chitosan-Cu tetrahedral aqua complex interactions in comparison to the chitosan-[Zn(H2O)4]2+ interactions.

β-Tricalcium Phosphate-Loaded Chitosan-Based Thermosensitive Hydrogel for Periodontal Regeneration

The current treatment for periodontitis is aimed at resolving gingival inflammation, whilst complete periodontal tissue regeneration is not predictable, and it represents a therapeutic challenge. Injectable biomaterials hold tremendous potential in dental tissue regeneration. This study aimed to investigate the ability of an injectable thermosensitive β-tricalcium phosphate (β-TCP) and chitosan-based hydrogel to carry cells and promote periodontal tissue regeneration. In this study, different concentrations of β-TCP-loaded chitosan hydrogels were prepared (0%, 2%, 4%, or 6% β-TCP, 10% β-glycerol phosphate, and 1.5% chitosan). The characteristics of the hydrogels were tested using rheology, a scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), degradation, and biological analyses. The new biomaterial showed a sol-gel transformation ability at body temperature and exhibited excellent chemical and physical characteristics, whilst the existence of β-TCP enhanced the structure and the properties of the hydrogels. The SEM confirmed the three-dimensional networks of the hydrogels, and the typical rheological properties of strong gel were observed. The EDX and XRD validated the successful incorporation of β-TCP, and similar patterns between different groups were found in terms of the FTIR spectra. The stable structure of the hydrogels under 100 °C was confirmed via DSC. Biological tests such as Alamar Blue assay and Live/Dead staining confirmed the remarkable biocompatibility of the hydrogels with pre-osteoblast MC3T3-E1 and human gingival fibroblast (HGF) cells for 14 days, and the results were validated with confocal imaging. This preliminary study shows great promise for the application of the β-TCP-loaded thermosensitive chitosan hydrogels as a scaffold in periodontal bone and soft tissue repair.

Polyvinylpyrrolidone-Alginate-Carbonate Hydroxyapatite Porous Composites for Dental Applications

An alternative approach for the currently used replacement therapy in dentistry is to apply materials that restore tooth tissue. Among them, composites, based on biopolymers with calcium phosphates, and cells can be applied. In the present work, a composite based on polyvinylpyrrolidone (PVP) and alginate (Alg) with carbonate hydroxyapatite (CHA) was prepared and characterized. The composite was investigated by X-ray diffraction, infrared spectroscopy, electron paramagnetic resonance (EPR) and scanning electron microscopy methods, and the microstructure, porosity, and swelling properties of the material were described. In vitro studies included the MTT test using mouse fibroblasts, and adhesion and survivability tests with human dental pulp stem cells (DPSC). The mineral component of the composite corresponded to CHA with an admixture of amorphous calcium phosphate. The presence of a bond between the polymer matrix and CHA particles was shown by EPR. The structure of the material was represented by micro- (30-190 μm) and nano-pores (average 8.71 ± 4.15 nm). The swelling measurements attested that CHA addition increased the polymer matrix hydrophilicity by 200%. In vitro studies demonstrated the biocompatibility of PVP-Alg-CHA (95 ± 5% cell viability), and DPSC located inside the pores. It was concluded that the PVP-Alg-CHA porous composite is promising for dentistry applications.

Characterization and Optimization of Injectable In Situ Crosslinked Chitosan-Genipin Hydrogels

In recent years, there has been an increased interest in injectable, in situ crosslinking hydrogels due to their minimally invasive application and ability to conform to their environment. Current in situ crosslinking chitosan hydrogels are either mechanically robust with poor biocompatibility and limited biodegradation due to toxic crosslinking agents or the hydrogels are mechanically weak and undergo biodegradation too rapidly due to insufficient crosslinking. Herein, the authors developed and characterized a thermally-driven, injectable chitosan-genipin hydrogel capable of in situ crosslinking at 37 °C that is mechanically robust, biodegradable, and maintain high biocompatibility. The natural crosslinker genipin is utilized as a thermally-driven, non-toxic crosslinking agent. The chitosan-genipin hydrogel's crosslinking kinetics, injectability, viscoelasticity, swelling and pH response, and biocompatibility against human keratinocyte cells are characterized. The developed chitosan-genipin hydrogels are successfully crosslinked at 37 °C, demonstrating temperature sensitivity. The hydrogels maintained a high percentage of swelling over several weeks before degrading in biologically relevant environments, demonstrating mechanical stability while remaining biodegradable. Long-term cell viability studies demonstrated that chitosan-genipin hydrogels have excellent biocompatibility over 7 days, including during the hydrogel crosslinking phase. Overall, these findings support the development of an injectable, in situ crosslinking chitosan-genipin hydrogel for minimally invasive biomedical applications.

Hydrogel scaffolds in the treatment of spinal cord injury: a review

Spinal cord injury (SCI) is a disease of the central nervous system often caused by accidents, and its prognosis is unsatisfactory, with long-term adverse effects on patients' lives. The key to its treatment lies in the improvement of the microenvironment at the injury and the reconstruction of axons, and tissue repair is a promising therapeutic strategy. Hydrogel is a three-dimensional mesh structure with high water content, which has the advantages of biocompatibility, degradability, and adjustability, and can be used to fill pathological defects by injectable flowing hydrophilic material in situ to accurately adapt to the size and shape of the injury. Hydrogels mimic the natural extracellular matrix for cell colonization, guide axon extension, and act as a biological scaffold, which can be used as an excellent carrier to participate in the treatment of SCI. The addition of different materials to make composite hydrogel scaffolds can further enhance their performance in all aspects. In this paper, we introduce several typical composite hydrogels and review the research progress of hydrogel for SCI to provide a reference for the clinical application of hydrogel therapy for SCI.

Applications of Stimuli-Responsive Hydrogels in Bone and Cartilage Regeneration

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

Recent Advances in Chitosan-Based Applications-A Review

Chitosan derived from chitin gas gathered much interest as a biopolymer due to its known and possible broad applications. Chitin is a nitrogen-enriched polymer abundantly present in the exoskeletons of arthropods, cell walls of fungi, green algae, and microorganisms, radulae and beaks of molluscs and cephalopods, etc. Chitosan is a promising candidate for a wide variety of applications due to its macromolecular structure and its unique biological and physiological properties, including solubility, biocompatibility, biodegradability, and reactivity. Chitosan and its derivatives have been known to be applicable in medicine, pharmaceuticals, food, cosmetics, agriculture, the textile and paper industries, the energy industry, and industrial sustainability. More specifically, their use in drug delivery, dentistry, ophthalmology, wound dressing, cell encapsulation, bioimaging, tissue engineering, food packaging, gelling and coating, food additives and preservatives, active biopolymeric nanofilms, nutraceuticals, skin and hair care, preventing abiotic stress in flora, increasing water availability in plants, controlled release fertilizers, dye-sensitised solar cells, wastewater and sludge treatment, and metal extraction. The merits and demerits associated with the use of chitosan derivatives in the above applications are elucidated, and finally, the key challenges and future perspectives are discussed in detail.

Reduced graphene oxide-enriched chitosan hydrogel/cellulose acetate-based nanofibers application in mild hyperthermia and skin regeneration

Asymmetric wound dressings have captured researchers' attention due to their ability to reproduce the structural and functional properties of the skin layers. Furthermore, recent studies also report the benefits of using near-infrared (NIR) radiation-activated photothermal therapies in treating infections and chronic wounds. Herein, a chitosan (CS) and reduced graphene oxide (rGO) hydrogel (CS_rGO) was combined with a polycaprolactone (PCL) and cellulose acetate (CA) electrospun membrane (PCL_CA) to create a new NIR-responsive asymmetric wound dressing. The rGO incorporation in the hydrogel increased the NIR absorption capacity and allowed a mild hyperthermy effect, a temperature increase of 12.4 °C when irradiated with a NIR laser. Moreover, the PCL_CA membrane presented a low porosity and hydrophobic nature, whereas the CS_rGO hydrogel showed the ability to provide a moist environment, prevent exudate accumulation and allow gaseous exchanges. Furthermore, the in vitro data demonstrate the capacity of the asymmetric structure to act as a barrier against bacteria penetration as well as mediating a NIR-triggered antibacterial effect. Additionally, human fibroblasts were able to adhere and proliferate in the CS_rGO hydrogel, even under NIR laser irradiation, presenting cellular viabilities superior to 90 %. Altogether, our data support the application of the NIR-responsive asymmetric wound dressings for skin regeneration.

A Novel Chitosan Composite Biomaterial with Drug Eluting Capacity for Maxillary Bone Regeneration

Bone grafting is one of the most commonly performed treatments for bone healing or repair. Autografts, grafts from the same patient, are the most frequently used bone grafts because they can provide osteogenic cells and growth factors at the site of the implant with reduced risk of rejection or transfer of diseases. Nevertheless, this type of graft presents some drawbacks, such as pain, risk of infection, and limited availability. For this reason, synthetic bone grafts are among the main proposals in regenerative medicine. This branch of medicine is based on the development of new biomaterials with the goal of increasing bone healing capacity and, more specifically in dentistry, they aim at simultaneously preventing or eliminating bacterial infections. The use of fibers made of chitosan (CS) and hydroxyapatite (HA) loaded with an antibiotic (doxycycline, DX) and fabricated with the help of an injection pump is presented as a new strategy for improving maxillary bone regeneration. In vitro characterization of the DX controlled released from the fibers was quantified after mixing different amounts of HA (10-75%). The 1% CS concentration was stable, easy to manipulate and exhibited adequate cuttability and pH parameters. The hydroxyapatite concentration dictated the combined fast and controlled release profile of CSHA50DX. Our findings demonstrate that the CS-HA-DX complex may be a promising candidate graft material for enhancing bone tissue regeneration in dental clinical practice.

Chitosan-Based Biomaterials for Bone Tissue Engineering Applications: A Short Review

Natural bone tissue is composed of calcium-deficient carbonated hydroxyapatite as the inorganic phase and collagen type I as the main organic phase. The biomimetic approach of scaffold development for bone tissue engineering application is focused on mimicking complex bone characteristics. Calcium phosphates are used in numerous studies as bioactive phases to mimic natural bone mineral. In order to mimic the organic phase, synthetic (e.g., poly(ε-caprolactone), polylactic acid, poly(lactide-co-glycolide acid)) and natural (e.g., alginate, chitosan, collagen, gelatin, silk) biodegradable polymers are used. However, as materials obtained from natural sources are accepted better by the human organism, natural polymers have attracted increasing attention. Over the last three decades, chitosan was extensively studied as a natural polymer suitable for biomimetic scaffold development for bone tissue engineering applications. Different types of chitosan-based biomaterials (e.g., molded macroporous, fiber-based, hydrogel, microspheres and 3D-printed) with specific properties for different regenerative applications were developed due to chitosan's unique properties. This review summarizes the state-of-the-art of biomaterials for bone regeneration and relevant studies on chitosan-based materials and composites.

Advances of Stimulus-Responsive Hydrogels for Bone Defects Repair in Tissue Engineering

Bone defects, as one of the most urgent problems in the orthopedic clinic, have attracted much attention from the biomedical community and society. Hydrogels have been widely used in the biomedical field for tissue engineering research because of their excellent hydrophilicity, biocompatibility, and degradability. Stimulus-responsive hydrogels, as a new type of smart biomaterial, have more advantages in sensing external physical (light, temperature, pressure, electric field, magnetic field, etc.), chemical (pH, redox reaction, ions, etc.), biochemical (glucose, enzymes, etc.) and other different stimuli. They can respond to stimuli such as the characteristics of the 3D shape and solid-liquid phase state, and exhibit special properties (injection ability, self-repair, shape memory, etc.), thus becoming an ideal material to provide cell adhesion, proliferation, and differentiation, and achieve precise bone defect repair. This review is focused on the classification, design concepts, and research progress of stimulus-responsive hydrogels based on different types of external environmental stimuli, aiming at introducing new ideas and methods for repairing complex bone defects.

Injectable Natural Polymer Hydrogels for Treatment of Knee Osteoarthritis

Osteoarthritis (OA) is a serious chronic and degenerative disease that increasingly occurs in the aged population. Its current clinical treatments are limited to symptom relief and cannot regenerate cartilage. Although a better understanding of OA pathophysiology has been facilitating the development of novel therapeutic regimen, delivery of therapeutics to target sites with minimal invasiveness, high retention, and minimal side effects remains a challenge. Biocompatible hydrogels have been recognized to be highly promising for controlled delivery and release of therapeutics and biologics for tissue repair. In this review, the current approaches and the challenges in OA treatment, and unique properties of injectable natural polymer hydrogels as delivery system to overcome the challenges are presented. The common methods for fabrication of injectable polysaccharide-based hydrogels and the effects of their composition and properties on the OA treatment are detailed. The strategies of the use of hydrogels for loading and release cargos are also covered. Finally, recent efforts on the development of injectable polysaccharide-based hydrogels for OA treatment are highlighted, and their current limitations are discussed.

Injectable nanocomposite hydrogels as an emerging platform for biomedical applications: A review

Hydrogels have attracted much attention for biomedical and pharmaceutical applications due to the similarity of their biomimetic structure to the extracellular matrix of natural living tissues, tunable soft porous microarchitecture, superb biomechanical properties, proper biocompatibility, etc. Injectable hydrogels are an exciting type of hydrogels that can be easily injected into the target sites using needles or catheters in a minimally invasive manner. The more comfortable use, less pain, faster recovery period, lower costs, and fewer side effects make injectable hydrogels more attractive to both patients and clinicians in comparison to non-injectable hydrogels. However, it is difficult to achieve an ideal injectable hydrogel using just a single material (i.e., polymer). This challenge can be overcome by incorporating nanofillers into the polymeric matrix to engineer injectable nanocomposite hydrogels with combined or synergistic properties gained from the constituents. This work aims to critically review injectable nanocomposite hydrogels, their preparation methods, properties, functionalities, and versatile biomedical and pharmaceutical applications such as tissue engineering, drug delivery, and cancer labeling and therapy. The most common natural and synthetic polymers as matrices together with the most popular nanomaterials as reinforcements, including nanoceramics, carbon-based nanostructures, metallic nanomaterials, and various nanosized polymeric materials, are highlighted in this review.

A Review on Biphasic Calcium Phosphate Materials Derived from Fish Discards

This review summarizes the results reported on the production of biphasic calcium phosphate (BCP) materials derived from fish wastes (i.e., heads, bones, skins, and viscera), known as fish discards, and offers an in-depth discussion on their promising potential for various applications in many fields, especially the biomedical one. Thus, considerable scientific and technological efforts were recently focused on the capability of these sustainable materials to be transformed into economically attractive and highly valuable by-products. As a consequence of using these wastes, plenty of beneficial social effects, with both economic and environmental impact, will arise. In the biomedical field, there is a strong and continuous interest for the development of innovative solutions for healthcare improvement using alternative materials of biogenic origin. Thus, the orthopedic field has witnessed a significant development due to an increased demand for a large variety of implants, grafts, and/or scaffolds. This is mainly due to the increase of life expectancy and higher frequency of bone-associated injuries and diseases. As a consequence, the domain of bone-tissue engineering has expanded to be able to address a plethora of bone-related traumas and to deliver a viable and efficient substitute to allografts or autografts by combining bioactive materials and cells for bone-tissue ingrowth. Among biomaterials, calcium phosphate (CaP)-based bio-ceramics are widely used in medicine, in particular in orthopedics and dentistry, due to their excellent bioactive, osteoconductive, and osteointegrative characteristics. Recently, BCP materials (synthetic or natural), a class of CaP, which consist of a mixture of two phases, hydroxyapatite (HA) and beta tricalcium phosphate (β-TCP), in different concentrations, gained increased attention due to their superior overall performances as compared to single-phase formulations. Moreover, the exploitation of BCP materials from by-products of fish industry was reported to be a safe, cheap, and simple procedure. In the dedicated literature, there are many reviews on synthetic HA, β-TCP, or BCP materials, but to the best of our knowledge, this is the first collection of results on the effects of processing conditions on the morphological, compositional, structural, mechanical, and biological properties of the fish discard-derived BCPs along with the tailoring of their features for various applications.

A Chitosan-Agarose Polysaccharide-Based Hydrogel for Biomimetic Remineralization of Dental Enamel

Developing multifunctional systems for the biomimetic remineralization of human enamel is a challenging task, since hydroxyapatite (HAP) rod structures of tooth enamel are difficult to replicate artificially. The paper presents the first report on the simultaneous use of chitosan (CS) and agarose (A) in a biopolymer-based hydrogel for the biomimetic remineralization of an acid-etched native enamel surface during 4–10-day immersion in artificial saliva with or without (control group) fluoride. Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry, Fourier transform infrared and Raman spectroscopies, X-ray diffraction, and microhardness tests were applied to investigate the properties of the acid-etched and remineralized dental enamel layers under A and CS-A hydrogels. The results show that all biomimetic epitaxial reconstructed layers consist mostly of a similar hierarchical HAP structure to the native enamel from nano- to microscale. An analogous Ca/P ratio (1.64) to natural tooth enamel and microhardness recovery of 77.4% of the enamel-like layer are obtained by a 7-day remineralization process in artificial saliva under CS-A hydrogels. The CS component reduced carbonation and moderated the formation of HAP nanorods in addition to providing an extracellular matrix to support growing enamel-like structures. Such activity lacked in samples exposed to A-hydrogel only. These data suggest the potential of the CS-A hydrogel in guiding the formation of hard tissues as dental enamel.

Biopolymers Hybrid Particles Used in Dentistry

This literature review provides an overview of the fabrication and application of biopolymer hybrid particles in dentistry. A total of 95 articles have been included in this review. In the review paper, the common inorganic particles and biopolymers used in dentistry are discussed in general, and detailed examples of inorganic particles (i.e., hydroxyapatite, calcium phosphate, and bioactive glass) and biopolymers such as collagen, gelatin, and chitosan have been drawn from the scientific literature and practical work. Among the included studies, calcium phosphate including hydroxyapatite is the most widely applied for inorganic particles used in dentistry, but bioactive glass is more applicable and multifunctional than hydroxyapatite and is currently used in clinical practice. Today, biopolymer hybrid particles are receiving more attention as novel materials for several applications in dentistry, such as drug delivery systems, bone repair, and periodontal regeneration surgery. The literature published on the biopolymer gel-assisted synthesis of inorganic particles for dentistry is somewhat limited, and therefore, this article focuses on reviewing and discussing the biopolymer hybrid particles used in dentistry.

Polysaccharide-Based Biomaterials in Tissue Engineering: A Review

In addition to proteins and nucleic acids, polysaccharides are an important type of biomacromolecule widely distributed in plants, animals, and microorganisms. Polysaccharides are considered as promising biomaterials due to their significant bioactivities, natural abundance, immunoactivity, and chemical modifiability for tissue engineering (TE) applications. Due to the similarities of the biochemical properties of polysaccharides and the extracellular matrix of human bodies, polysaccharides are increasingly recognized and accepted. Furthermore, the degradation behavior of these macromolecules is generally nontoxic. Certain delicate properties, such as remarkable mechanical properties and tunable tissue response, can be obtained by modifying the functional groups on the surface of polysaccharide molecules. The applications of polysaccharide-based biomaterials in the TE field have been growing intensively in recent decades, for example, bone/cartilage regeneration, cardiac regeneration, neural regeneration, and skin regeneration. This review summarizes the main essential properties of polysaccharides, including their chemical properties, crosslinking mechanisms, and biological properties, and focuses on the association between their structures and properties. The recent progress in polysaccharide-based biomaterials in various TE applications is reviewed, and the prospects for future studies are addressed as well. We intend this review to offer a comprehensive understanding of and inspiration for the research and development of polysaccharide-based materials in TE.

Recent Advances of Chitosan-Based Injectable Hydrogels for Bone and Dental Tissue Regeneration

Traditional strategies of bone repair include autografts, allografts and surgical reconstructions, but they may bring about potential hazard of donor site morbidity, rejection, risk of disease transmission and repetitive surgery. Bone tissue engineering (BTE) is a multidisciplinary field that offers promising substitutes in biopharmaceutical applications, and chitosan (CS)-based bone reconstructions can be a potential candidate in regenerative tissue fields owing to its low immunogenicity, biodegradability, bioresorbable features, low-cost and economic nature. Formulations of CS-based injectable hydrogels with thermo/pH-response are advantageous in terms of their high-water imbibing capability, minimal invasiveness, porous networks, and ability to mold perfectly into an irregular defect. Additionally, CS combined with other naturally-derived or synthetic polymers and bioactive agents has proven to be an effective alternative to autologous bone and dental grafts. In this review, we will highlight the current progress in the development of preparation methods, physicochemical properties and applications of CS-based injectable hydrogels and their perspectives in bone and dental regeneration. We believe this review is intended as starting point and inspiration for future research effort to develop the next generation of tissue-engineering scaffold materials.

Designed sponges based on chitosan and cyclodextrin polymer for a local release of ciprofloxacin in diabetic foot infections

Diabetic foot infections are the most common complications requiring hospitalisation of patients with diabetes. They often result in amputation to extremities and are associated with high morbi-mortality rates, especially when bone is infected. Treatment of these complications is based on surgical procedures, nursing care and systemic antibiotic therapy for several weeks, with a significant risk of relapse. Due to low blood flow and damage caused by diabetic foot infection, blood supply is decreased, causing low antibiotic diffusion in the infected site and an increase of possible bacterial resistance, making this type of infection particularly difficult to treat. In this context, the aim of this work was to develop a medical device for local antibiotic release. The device is a lyophilized physical hydrogel, i.e a sponge based on two oppositely charged polyelectrolytes (chitosan and poly(cyclodextrin citrate)). Cyclodextrins, via inclusion complexes, increase drug bioavailability and allow an extended release. Using local release administration increases concentrations in the wound without risk of toxicity to the body and prevents the emergence of resistant bacteria. The hydrogel was characterised by rheology. After freeze-drying, a curing process was implemented. The swelling rate and cell viability were evaluated, and finally, the sponge was impregnated with a ciprofloxacin solution to evaluate its drug release profile and its antibacterial activity.

In vitro and in vivo investigation of osteogenic properties of self-contained phosphate-releasing injectable purine-crosslinked chitosan-hydroxyapatite constructs

Bone fracture repair is a multifaceted, coordinated physiological process that requires new bone formation and resorption, eventually returning the fractured bone to its original state. Currently, a variety of different approaches are pursued to accelerate the repair of defective bones, which include the use of 'gold standard' autologous bone grafts. However, such grafts may not be readily available, and procedural complications may result in undesired outcomes. Considering the ease of use and tremendous customization potentials, synthetic materials may become a more suitable alternative of bone grafts. In this study, we examined the osteogenic potential of guanosine 5′-diphosphate-crosslinked chitosan scaffolds with the incorporation of hydroxyapatite, with or without pyrophosphatase activity, both in vitro and in vivo. First, scaffolds embedded with cells were characterized for cell morphology, viability, and attachment. The cell-laden scaffolds were found to significantly enhance proliferation for up to threefold, double alkaline phosphatase activity and osterix expression, and increase calcium phosphate deposits in vitro. Next, chitosan scaffolds were implanted at the fracture site in a mouse model of intramedullary rod-fixed tibial fracture. Our results showed increased callus formation at the fracture site with the scaffold carrying both hydroxyapatite and pyrophosphatase in comparison to the control scaffolds lacking both pyrophosphatase and hydroxyapatite, or pyrophosphatase alone. These results indicate that the pyrophosphatase-hydroxyapatite composite scaffold has a promising capacity to facilitate bone fracture healing.

Stimuli-Responsive Materials for Tissue Engineering and Drug Delivery

Smart or stimuli-responsive materials are an emerging class of materials used for tissue engineering and drug delivery. A variety of stimuli (including temperature, pH, redox-state, light, and magnet fields) are being investigated for their potential to change a material's properties, interactions, structure, and/or dimensions. The specificity of stimuli response, and ability to respond to endogenous cues inherently present in living systems provide possibilities to develop novel tissue engineering and drug delivery strategies (for example materials composed of stimuli responsive polymers that self-assemble or undergo phase transitions or morphology transformations). Herein, smart materials as controlled drug release vehicles for tissue engineering are described, highlighting their potential for the delivery of precise quantities of drugs at specific locations and times promoting the controlled repair or remodeling of tissues.

In-Situ Forming pH and Thermosensitive Injectable Hydrogels to Stimulate Angiogenesis: Potential Candidates for Fast Bone Regeneration Applications

Biomaterials that promote angiogenesis are required for repair and regeneration of bone. In-situ formed injectable hydrogels functionalised with bioactive agents, facilitating angiogenesis have high demand for bone regeneration. In this study, pH and thermosensitive hydrogels based on chitosan (CS) and hydroxyapatite (HA) composite materials loaded with heparin (Hep) were investigated for their pro-angiogenic potential. Hydrogel formulations with varying Hep concentrations were prepared by sol-gel technique for these homogeneous solutions were neutralised with sodium bicarbonate (NaHCO3) at 4 °C. Solutions (CS/HA/Hep) constituted hydrogels setting at 37 °C which was initiated from surface in 5-10 minutes. Hydrogels were characterised by performing injectability, gelation, rheology, morphology, chemical and biological analyses. Hydrogel solutions facilitated manual dropwise injection from 21 Gauge which is highly used for orthopaedic and dental administrations, and the maximum injection force measured through 19 G needle (17.191 ± 2.296N) was convenient for manual injections. Angiogenesis tests were performed by an ex-ovo chick chorioallantoic membrane (CAM) assay by applying injectable solutions on CAM, which produced in situ hydrogels. Hydrogels induced microvascularity in CAM assay this was confirmed by histology analyses. Hydrogels with lower concentration of Hep showed more efficiency in pro-angiogenic response. Thereof, novel injectable hydrogels inducing angiogenesis (CS/HA/Hep) are potential candidates for bone regeneration and drug delivery applications.

Optimizing Nanohydroxyapatite Nanocomposites for Bone Tissue Engineering

Bone tissue engineering involves the combined use of materials with functional properties to regenerate bone. Nanohydroxyapatite (nHA) can influence the behavior of cells. The functional and structural properties of nHA can be controlled during nanoparticle synthesis. This review defines the relationship between the attributes of nHA nanoparticles and their biological effects, focusing on biocompatibility, surface-area-to-volume ratio, bonding chemistry, and substrate functionality. The paper explores how these aspects have been applied in the development of scaffolds for the repair of damaged bone or regeneration of missing bone.

Nanohydroxyapatite Reinforced Chitosan Composite Hydrogel with Tunable Mechanical and Biological Properties for Cartilage Regeneration

With the continuous quest of developing hydrogel for cartilage regeneration with superior mechanobiological properties are still becoming a challenge. Chitosan (CS) hydrogels are the promising implant materials due to an analogous character of the soft tissue; however, their low mechanical strength and durability together with its lack of integrity with surrounding tissues hinder the load-bearing application. This can be solved by developing a composite chitosan hydrogel reinforced with Hydroxyapatite Nanorods (HANr). The objective of this work is to develop and characterize (physically, chemically, mechanically and biologically) the composite hydrogels loaded with different concentration of hydroxyapatite nanorod. The concentration of hydroxyapatite in the composite hydrogel was optimized and it was found that, reinforcement modifies the hydrogel network by promoting the secondary crosslinking. The compression strength could reach 1.62 ± 0.02 MPa with a significant deformation of 32% and exhibits time-dependent, rapid self-recoverable and fatigue resistant behavior based on the cyclic loading-unloading compression test. The storage modulus value can reach nearly 10 kPa which is needed for the proposed application. Besides, composite hydrogels show an excellent antimicrobial activity against Escherichia coli, Staphylococcus aureus bacteria’s and Candida albicans fungi and their cytocompatibility towards L929 mouse fibroblasts provide a potential pathway to developing a composite hydrogel for cartilage regeneration.

Influence of the Soluble⁻Insoluble Ratios of Cyclodextrins Polymers on the Viscoelastic Properties of Injectable Chitosan⁻Based Hydrogels for Biomedical Application

Injectable pre-formed physical hydrogels provide many advantages for biomedical applications. Polyelectrolyte complexes (PEC) formed between cationic chitosan (CHT) and anionic polymers of cyclodextrin (PCD) render a hydrogel of great interest. Given the difference between water-soluble (PCDs) and water-insoluble PCD (PCDi) in the extension of polymerization, the present study aims to explore their impact on the formation and properties of CHT/PCD hydrogel obtained from the variable ratios of PCDi and PCDs in the formulation. Hydrogels CHT/PCDi/PCDs at weight ratios of 3:0:3, 3:1.5:1.5, and 3:3:0 were elaborated in a double–syringe system. The chemical composition, microstructure, viscoelastic properties, injectability, and structural integrity of the hydrogels were investigated. The cytotoxicity of the hydrogel was also evaluated by indirect contact with pre-osteoblast cells. Despite having similar shear–thinning and self-healing behaviors, the three hydrogels showed a marked difference in their rheological characteristics, injectability, structural stability, etc., depending on their PCDi and PCDs contents. Among the three, all the best above-mentioned properties, in addition to a high cytocompatibility, were found in the hydrogel 3:1.5:1.5. For the first time, we gained a deeper understanding of the role of the PCDi/PCDs in the injectable pre-formed hydrogels (CHT/PCDi/PCDs), which could be further fine-tuned to enhance their performance in biomedical applications.

Review on recent progress in chitosan-based hydrogels for wastewater treatment application

Recently, chitosan has been used as a raw material for synthesis of hydrogels in a wide range of potential and practical applications like wastewater treatment, drug delivery, and tissue engineering. This review represents an overview of the application of chitosan-based hydrogels for wastewater treatment and helps researchers to better understand the potential of these adsorbents for wastewater treatment. It covers recently used and prospected methods for synthesis and modification of these hydrogels. Chitosan-based hydrogels are modified physically and chemically through crosslinking, grafting, impregnation, incorporating of hard fillers, blending, interpenetrating, and ion-imprinting methods to improve adsorption and mechanical properties. Understanding of these methods provides useful information in the design of efficient chitosan-based hydrogels and the select of appropriate pollutants for removal. This review provides a brief outlook on future prospects of chitosan-based hydrogels for wastewater application.

Injectable chitosan-hydroxyapatite hydrogels promote the osteogenic differentiation of mesenchymal stem cells

Injectable hydrogels have emerged as promising biomaterials for tissue engineering applications. The goal of this study was to evaluate the potential of a pH-responsive chitosan-hydroxyapatite hydrogel to be used as a three-dimensional support for encapsulated mesenchymal stem cells (MSCs) osteogenic differentiation. In vitro enzymatic degradation of the hydrogel, during 28 days of incubation, in simulated physiological condiditons, was characterized by swelling measurements, molecular weight determination and SEM analysis of hydrogel microstructure. Osteogenic differentiation of encapsulated MSCs was confirmed by osteogenic Runx2, collagen type I and osteocalcin immunostaining and alkaline phosphatase quantification. The deposition of late osteogenic markers (calcium phosphates) detected by Alizarin red and von Kossa staining indicated an extracellular matrix mineralization.

Development of a gallic acid-loaded chitosan and polyvinyl alcohol hydrogel composite: Release characteristics and antioxidant activity

The physico-chemical properties of a chitosan and polyvinyl alcohol (CS/PVA)-based hydrogel composite were investigated. Tetraethyl orthosilicate (TEOS) was employed as a crosslinking agent. The results indicated that the chitosan-based composite presented a thermal resistance up to 200°C. The structural properties, which were evaluated using FTIR and DSC, showed good miscibility between chitosan and polyvinyl alcohol. SEM presented a compact and homogeneous structure. The release profile of the chitosan-based hydrogel composite was investigated using gallic acid (GA). It showed high antioxidant activities, which were monitored using DPPH radical scavenging. Diffusion of water into the chitosan-based hydrogel was assumed to be pseudo-Fickian in PBS solution. The CS/PVA-based hydrogel composite exhibited good properties as a drug delivery system.

Multi-responsive hydrogels with UCST- and LCST-induced shrinking and controlled release behaviors of rhodamine B

By using a disulfide-functionalized crosslinker, a pH- and thermo-responsive 2-(dimethylamino) ethyl methacrylate (DMAEMA) monomer and a zwitterionic sulfobetaine methacrylate (SBMA) monomer were conjugated to fabricate a multi-responsive P(DMAEMA-SS-SBMA) copolymeric hydrogel. Apparent UCST and LCST volume transitions were observed in the P(DMAEMA-SS-SBMA) hydrogels with equivalent weight fractions of monomers. Different pore size and response sensitivity of shrunken structures below UCST and above LCST were visualized by SEM images. The hydrogel exhibited a highly swollen state with a swelling ratio of 17.8 and a pore size of 106μm at 45°C, they deswelled unequally at 5°C with a compact surface with pore size of 30μm and a loose bulk with pore size of 83μm, while they deswelled uniformly at 65°C with dense shrunken structure with small pore size of 12μm. The dual-thermoresponsive hydrogel was promising in controlled drug release. The initial drug release was predominantly controlled by diffusion, and the long-term release was influenced by the swelling ratio. Below UCST, the relatively hydrophilic shrunken structure and slow diffusion had a synergistic effect on the sustained release. Above LCST, the fast diffusion and the rapid "off" effect of hydrophobic skin layer resulted in a burst release. Additionally, pH-tunable swelling and redox-sensitive degradation were also observed.

Hydrogels for Biomedical Applications: Cellulose, Chitosan, and Protein/Peptide Derivatives

Hydrogels based on polysaccharide and protein natural polymers are of great interest in biomedical applications and more specifically for tissue regeneration and drug delivery. Cellulose, chitosan (a chitin derivative), and collagen are probably the most important components since they are the most abundant natural polymers on earth (cellulose and chitin) and in the human body (collagen). Peptides also merit attention because their self-assembling properties mimic the proteins that are present in the extracellular matrix. The present review is mainly focused on explaining the recent advances on hydrogels derived from the indicated polymers or their combinations. Attention has also been paid to the development of hydrogels for innovative biomedical uses. Therefore, smart materials displaying stimuli responsiveness and having shape memory properties are considered. The use of micro- and nanogels for drug delivery applications is also discussed, as well as the high potential of protein-based hydrogels in the production of bioactive matrices with recognition ability (molecular imprinting). Finally, mention is also given to the development of 3D bioprinting technologies.