Chitosan-Based Biomimetically Mineralized Composite Materials is a kind of organic-inorganic composite materials fabricated by biomimetic mineralization technology using chitosan as a organic scaffold or template.
Bone and teeth are the two main hard tissues in the human body. Bone is a mineralized inorganic-organic composite, which is mainly composed of carbonated hydroxyapatite (HAP) and type I collagen. Dentin, cementum, and enamel are hard structures of teeth. Dentin and cementum are both collagenous composites similar to bone, with apatite as the mineral phase. However, enamel is quite special, as it is acellular, non-collagenous, and composed of 95–97% mineral by weight, with less than 1% organic material. While there are differences in the composition and structure of these hard tissues, they are all organic-inorganic composites formed through biomineralization processes regulated by a series of cells and organic matrices (proteins, polysaccharides, etc.).
Bone tissue engineering is now a popular and promising method for repairing bone defects due to the large-scale destruction or loss of bone tissue caused by events, such as trauma, infection, and tumor . It is a technique involving the provision of three-dimensional scaffolds that act as artificial extracellular matrices, allowing cells to proliferate and maintain their specific functions, and serve as a template for new bone formations . Numerous biomimetic scaffolds of different biomaterials have been applied in bone tissue engineering. Similarly, when dental caries, trauma, or erosion cause defects of dental hard tissue, current clinical treatments cannot restore the original structure and properties of teeth. Some biomimetic materials and strategies that have appeared in recent years may be promising ways to fabricate enamel-like or dentin-like structures.
Chitosan, a natural cationic polysaccharide, has a similar chemical structure and biological behaviors to the components of the extracellular matrix (ECM) of bone and teeth. Chitosan has many biological properties, such as biocompatibility, biodegradability, polyelectrolyte action, etc., which make it a suitable organic scaffold or template for the fabrication of organic-inorganic composites. Unlike simply mixing chitosan and inorganic minerals to construct composite materials, biomimetic mineralization technology, which is inspired by the biomineralization process, can deposit minerals in situ on chitosan organic templates, thereby constructing composite materials with closer structures and functions to those of bone or teeth. In recent years, chitosan-based composite materials fabricated by the biomimetic mineralization technique have been widely used in the field of bone tissue engineering and enamel or dentin biomimetic repair. Comparing with the artificial materials currently used to repair human hard tissues in clinic, such as ceramics, alloys, etc., chitosan-based materials have reduced costs and improved biocompatibility, with low possibility of causing allergic and inflammatory reactions in human body. Besides, the bioactivities and mechanical properties of chitosan-based materials can also be improved with the addition of inorganic minerals.
Chitosan is a natural cationic polysaccharide that is obtained by the N-deacetylation of chitin, which is the second most ubiquitous polymer-after cellulose-on earth. It is a linear copolymer composed of D-glucosamine (GlcN) and N-acetyl-d-glucosamine (GlcNAc), which are linked by the β-1, 4-glycosidic bond, with molecular weight ranges from 10 to over 1000 kDa . The chemical structure of chitosan is similar to that of glycosaminoglycan, the main component of the extracellular matrix (ECM). Deacetylation degree (DD), molecular mass, solubility, viscosity, crystallinity, flexibility, porosity, tensile strength, and conductivity are frequently evaluated physicochemical properties of chitosan. Among them, DD and molecular mass are two of the most important physical characteristics that affect both the chemical and biological properties of chitosan. In recent years, chitosan preparations with various DDs, molecular masses, and molecular derivatization patterns have attracted much attention because of their potentially beneficial biological properties. Chitosan has various outstanding biological properties, including a good polyelectrolyte action, biodegradability, biocompatibility, bioactivity, antimicrobial property, anticancer property, antioxidant property, cell adhesion properties, non-toxicity, and high flexibility for chemical functionalization. However, as chitosan is only highly soluble in most diluted acidic solutions at a pH below 6.5 and has a poor solubility in water or most organic solvents, its application field is severely limited. Therefore, improving the solubility of chitosan is a crucial step in extending its scope of application. Deacetylation, chemical modification by adding hydrophilic biomolecules to amino or hydroxyl groups (acylation, carboxylation, alkylation, quaternization, sulfonation, and phosphorylation), crosslinking and chemical or enzymatical depolymerization or degradation are available methods for improving the solubility of chitosan and also optimizing its biological properties. Because of the diverse properties of chitosan and its derivatives, they have been extensively applied in the medical and pharmaceutical fields, for example, they have been used in drug delivery, tissue engineering, wound management, gene and cancer therapy, antibiofilm drugs, etc.
Natural bone exhibits a hierarchical structure, mainly consisting of multilayered collagen fibers and the inorganic component, HAP. In consideration of events such as trauma, infection, and tumor, which cause the large-scale destruction or loss of bone tissue, exploring materials that can replace or even reconstruct bone structure is an urgent challenge in orthopedic clinical practice. Bone tissue engineering is currently a hot research field and aims to realize bone reconstruction and regeneration, focusing on scaffolds, cells, growth factors, and their interrelation in a microenvironment. For a bone tissue engineering scaffold to be successful, it must be highly porous, osteoconductive, biodegradable, biocompatible, mechanically strong, and capable of efficiently guiding new bone formation in the defect. Preparing organic-inorganic composite nanofibers to simulate the composition of the ECM is an effective strategy for providing bone tissue engineering scaffolds. According to the structure and composition of natural bone, natural macromolecule/HAP composite scaffolds synthesized by biomimetic mineralization with natural bioactive macromolecules are currently key research focuses in this field . Among these macromolecules, chitosan is a popular alternative because of its excellent biological properties
Different chitosan-based organic-inorganic composite materials using the biomimetic mineralization technique and their important properties in the field of bone tissue engineering are presented in Table 1. Using a wet chemical method, Doan et al. prepared chitosan/hydroxyapatite (CS/HAP) nanofibers with a homogeneous HAP deposit. The composite nanofibrous scaffold promoted osteogenic differentiation by inducing ossification and enhanced the expressions of collagen type I, alkaline phosphatase, osteocalcin, bone sialoprotein, and osterix, thus showing that it has considerable potential in bone tissue engineering applications. Compared with ordinary chitosan, carboxymethyl chitosan (CMCS) has a better water-solubility, biodegradability, and bioactivity, which allows CMCS to chelate Ca2+ and induce the deposition of apatite. HAP-coated electro-spun CMCS nanofibers prepared by biomimetic mineralization using 5 times simulated body fluid increased the ALP activity and the gene expression level of Runx2 and ALP and promoted new bone formation and maturation. In order to fabricate a hybrid nanostructured HAP-CS composite scaffold with HAP nanorods perpendicularly-oriented to CS fibers, Guo and his co-workers applied a two-stage preparation process using brushite (DCPD, CaHPO4·2H2O) as transitory precursors and mimicked the biomineralization process of the apatite in bone tissue. The process included the deposition of DCPD on the CS fiber porous scaffold using a dip-coating method and the formation of a hybrid nanostructured HAP-CS composite scaffold through the in-situ conversion of DCPD into HAP using a bioinspired mineralization process. The composite scaffold exhibited good mechanical properties and could support the adhesion and proliferation of hBMSCs. Moreover, it could promote the formation of new bone in rat calvarial defects. To further improve the mechanical and biological properties of chitosan-HAP composite scaffolds, graphene was also introduced into the composite scaffolds. Graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), are highly biocompatible and can easily be functionalized by various organic and inorganic compounds due to the presence of various functional side groups (hydroxyl, carboxyl, and epoxides) on its surface. It was found that extensive mineralization occurred in the CS-GO conjugate system because of strong electrostatic interactions between the functional groups (carboxyl groups of GO and amino groups) of CS and calcium ions in an SBF solution. The combination of a chitosan–graphene oxide conjugate and biomimetic mineralization was advantageous in favorably modulating cellular activity. It induced homogeneous spatial osteoblastic cell growth and increased mineralization . Another study showed that chitosan acted as an interfacial soft polymeric template on the surface of RGO, promoting an ordered growth of the hydroxyapatite particles. The three-component composite mineralized scaffold mimicked the structure and composition of natural bone and exhibited a relatively higher rate of cell proliferation, osteogenic differentiation, and osteoid matrix formation.
Table 1. Applications of chitosan-based biomimetically mineralized composite materials in bone tissue engineering.
Hydroxyapatite (HAP); wet chemical method (WCM); simulated body fluid (SBF); alkaline phosphatase (ALP); dicalcium phosphate dihydrate (DCPD); graphene oxide (GO); reduced graphene oxide (RGO); amorphous calcium phosphate (ACP); polymer-induced liquid precursor (PILP); nanohydroxyapatite (HAP); layer-by-layer (LBL); poly(lactic acid) (PLA); poly(ethylene glycol) (PEG), calcium phosphate (CaP).
In addition to the abovementioned composite materials, which contain chitosan as the only organic template, in recent years, researchers have also combined chitosan with other polymers to prepare multi-component biomimetically mineralized scaffold materials. In the field of bone tissue engineering, collagen (Col) is usually a good natural polymer for forming a hybrid scaffold with chitosan. Wang et al. used CMC as a polyelectrolyte template to stabilize ACP in order to form nanocomplexes of CMC/ACP and then fabricated mineralized collagen scaffolds using a biomimetic method based on the polymer-induced liquid precursor process. They found that nanocomplexes of CMC/ACP significantly increased the modulus of the collagen scaffolds, and the scaffolds could better promote the regeneration of bone tissue in defects. Another similar study also prepared CMC/ACP nanocomplexes under acidic conditions (pH < 3.5) and realized biomimetic synchronous self-assembly/mineralization (SSM) of a collagen scaffold. Zou et al. compounded three matrix materials (CS, Col, and PLA) uniformly, with the assistance of sonication and amidation to regulate the in-situ crystallization of nHAP in order to fabricate CS/Col/PLA/nHAP scaffolds. The scaffolds improved the mechanical properties and the formation of crystals in the SBF, and it had a good biocompatibility and could maintain the cell growth. Moreover, gelatin, a protein derived from collagen with a similar structure to collagen, is a biodegradable biopolymer with a high biocompatibility, which has also been widely used in biomimetic composite scaffolds with chitosan. A gelatin-chitosan core-shell structured nanofibers mat with a three-dimensional porous structure was fabricated by a coaxial electrospinning technique. An arginine-glycine-aspartic acid (RGD)-like structure was formed to mimic the organic component of the natural bone extracellular matrix, and then homogeneous HAP was deposited on its surface using a wet chemical method. The biomimetic composite scaffolds could further enhance osteoblast cell proliferation. Heinemann et al. prepared organically modified hydroxyapatite (ormoHAP) in gelatin gels using the double migration technique and mineralized chitosan porous scaffolds created using the Net-Shape-Nonwoven (NSN) technique. The mineralized NSN-scaffolds improved the attachment, proliferation, and differentiation of hBMSC, presenting a remarkable application potential for bone tissue engineering. In addition to collagen and gelatin, other biodegradable polymers or proteins, such as cellulose, PLA, PEG, and silk fibroin, were also effective organic additives, acting as crosslinking agents or scaffolds of the chitosan-based composite materials. While various chitosan/calcium phosphates are the most common biomimetically mineralized composite materials used in bone tissue engineering, some chitosan/silica biomimetically mineralized scaffolds have also been applied due to their capability in inducing the formation of apatite and good potential for promoting new bone regeneration.