Calcium phosphate (CaP)-based biomaterials have superior properties and have been widely used for bone defect repair, due to their similarities to the inorganic components of human bones. The biological performance of CaPs, as a determining factor for their applications, are dependent on their physicochemical properties. Hydroxyapatite (HAP) as the most thermally stable crystalline phase of CaP is mostly used in the form of ceramics or composites scaffolds with polymers. Nanostructured CaPs with large surface areas are suitable for drug/gene delivery systems. CaP is not only a family of natural minerals, but also includes biominerals in humans, which are the main inorganic component of hard tissue (bone and teeth).
1. Introduction
Bone as a mineralized tissue has an irreplaceable role in supporting and protecting the body of human beings. Defects of bone caused by trauma, aging, inflammation, infection, and tumors seriously affect people’s health and normal life
[1]. A critical bone defect, which refers to a defect greater than 2 cm in length or greater than 50 percent of the circumference of the defect, cannot completely regenerate by self-growth and requires the use of biomaterials to guide its repair
[2]. Millions of bone grafting operations are performed every year in the world for the treatment of critical bone defects, resulting in a huge economic burden to the families of patients and the whole of society
[3]. Therefore, the development of high-performance biomaterials for bone repair is of great scientific significance and clinical application value.
At present, the most commonly used methods for treating bone defects include bone transplantation, membrane-guided regeneration, Ilizarov technology, and bone tissue engineering
[4][5][6][7][8][4,5,6,7,8]. However, these methods are insufficient in meeting the requirements for clinical treatment of bone defects. Autologous bone grafting, the clinical gold standard for treating bone defects, is usually limited by the quantity of available tissue and the risk of secondary surgery and infection, blood loss and operation time
[3]. Allografts and xenografts are important alternative options of autografts for orthopaedic applications in terms of osteogenic, osteoinductive, and osteoconductive properties, and there are two main categories, including cellular bone matrices (CBM) and peptide enhanced xeno-hybrid bone grafts developed as commercial products for clinical use. Cellular bone matrices have four necessary and beneficial elements for bone growth and healing: osteoinduction, osteoconduction, osteogenic activity and angiogenic activity
[9]. However, CBM have several challenges with respect to intrinsic biological characteristics, such as viable cell sources, donor age at the time of graft harvest, and cell survival after transplantation, which may cause differences in expected outcomes due to different batches of the same product. Xeno-hybrid bone matrices are appealing, innovative, osteoconductive and osteoinductive bone substitutes to autografts. The compatibility of xeno-hybrid bone matrices in favoring cellular attachment, osseointegration, bone remodeling and satisfactory mechanical performance has been attested by numerous clinical studies
[9]. However, further independent clinical studies are required to confirm these promising results and to promote their application. It is worth mentioning that although there are potential risks of infection when using allografts, the allografts are procured, processed, and distributed only by Tissue Banks, which operate under strict guidelines and sterile conditions in Class A environments, which helps to minimize the abovementioned issues
[10]. The technology of tissue engineering represents an emerging strategy for repair of bone defect
[11]. However, it is still a big challenge to construct functional bone tissue in vitro, due to the proliferation and differentiation of seeding cells, bioactivity of growth factors and physicochemical and biological properties of scaffolds
[12][13][12,13]. Recently, in-situ tissue engineering has been proposed for autologous tissue regeneration, which is based on tissue-specific scaffolds, by regulating the microenvironment and in vivo recruiting stem and progenitor cells
[14][15][14,15]. Therefore, the preparation of functional scaffolds with ideal biocompatibility, bioactivity and biodegradability is the critical factor that limits the rapid development of in-situ tissue engineering for bone defect repair.
Synthetic bone scaffolds have been increasingly applied in the field of bone repair. Compared with the autologous bone grafts, although there are some poor properties of osteinductive and osteogenic activities, synthetic bone scaffolds with abundant sources provide a wide variety of choices in structure, chemical/mechanical properties and biological functions to meet specific requirements in bone regeneration
[15]. Considering the limitations above-mentioned, artificial bone substitutes have attracted tremendous attention and have been rapidly developed. Among the varied biomaterials used in bone repair, calcium phosphate (CaP)-based biomaterials occupy a particular position as a result of their resemblance to the chemical components and structures of natural bone tissue. CaP is not a specific material but represents a big family of materials that are compounds formed by the reaction of calcium ions and phosphate ions. The apatite reported by Werner in 1788 was the earliest discovered member of CaP
[16]. By 1926, Jong revealed the relationship between apatite and the inorganic minerals of bone
[17]. Therefore, CaP-based biomaterials (CaPs) were proposed for use as therapeutic agents for bone regeneration
[18]. In 1971, Monroe was the first to report the use of CaP ceramics, which are white translucent polycrystalline ceramics that contain hydroxyapatite (HAP)
[19]. Since that time, CaP ceramics have been developed greatly for the application of bone repair
[20][21][20,21]. A CaP bone cement (CPC) was created by the hydrolysis of TCP and was used for the first time in the early 1920s as a treatment for bone repair
[18]. Since then, CPC has been prepared with many different chemical formulas and application as described by Cama
[22]. Mineralized collagen with orderly, organized collagen and HAP is the basic unit of natural bone tissues and is involved in building complex biomineralized systems with hierarchical structures
[23]. Hence, researchers in the field of biomaterials are interested in biomimicking mineralized collagen for developing bone substitute materials by utilizing the biomimetics strategy
[24][25][24,25]. In 2003, biomimetic mineralized collagen nanofibrils were designed and prepared by Cui et al., which are similar in both composition and structure to natural bone
[26]. As of now, various methods have been developed for preparing mineralized collagen, and these products have excellent bioabsorbability and osteoconductive properties, leading to their potential in promoting bone regeneration
[27].
2. Chemical Properties of Calcium Phosphate
2.1. Species of Calcium Phosphate
CaP is not only a family of natural minerals, but also includes biominerals in humans, which are the main inorganic component of hard tissue (bone and teeth)
[28][31]. In the past decades, a wide variety of CaP-based biomaterials have been used in bone regeneration studies and clinical applications. As is well known, CaP biomaterials promote cell adhesion and growth, which then induces the formation of new bone minerals via their interaction with extracellular matrix proteins
[29][43]. In the application of bone regeneration, the bioactivity of CaPs is critical and usually varied depending on their species
[30][44]. The bioactive features of CaPs is related with to the degradation properties of CaP
[31][45]. Due to different Ca/P ratios, the different species of CaP biomaterials result in variations in in vitro and in vivo calcium and phosphate ion release. Consequently, the pH of the local microenvironment of bone is affected by the released calcium and phosphate ions, which then influence the viability of osteoblasts and osteoclasts
[32][33][46,47]. Moreover, the increased concentration of calcium and phosphate ions can promote the formation of bone minerals, as well as affect the expression of osteogenic differentiation-related genes (e.g., Col-I, ALP, OPN, OCN, RunX2 and BMPs) of bone cells
[28][34][31,48].
Calcium exists wildly in natural bone minerals and is a key ion in forming the bone matrix
[35][49]. Ca
2+ is also capable of forming and maturing bone tissue by calcification. Ca
2+ influence the bone cell maturation and bone tissue regeneration by regulating related cellular signaling pathways
[36][37][50,51]. For instance, Ca
2+ activating ERK1/2 causes pathway activation of osteoblastic-related bone formation
[38][52]. In addition, an increased life span of osteoblasts has been observed with the activation of the PI3K/Akt signal axis by Ca
2+ [39][53]. Meanwhile, phosphate ions degraded from CaP are present in large quantities in the human body, and can be utilized in various physiological systems, including construction of proteins, nucleic acids, and adenosine triphosphate
[40][54]. Approximately 80% of phosphate ions in the body occur with calcium ions in the form of CaP in bone, which affects the development of bone tissue
[41][55]. It is well known that the differentiation and growth of osteoblasts are regulated by phosphate ions by IGF-1, ERK1/2, BMP, and other pathways
[42][43][56,57].
The osteoconductivity and osteoinductivity of CaP materials are closely related to their physical and chemical characteristics, such as, solubility, stability, and mechanical strength
[28][31], are determined by the species of the CaP materials, Therefore, the selection of one kind of CaP biomaterial from their family according its characteristics is important in preparing biomaterials for the use of bone regeneration. A large number of CaP biomaterials have been used in bone regeneration and other biological research, including tricalcium phosphate (TCP), hydroxyapatite (HAP), amorphous calcium phosphate (ACP), octacalcium phosphate (OCP), dicalcium phosphate anhydrous (DCPA), dibasic calcium phosphate dihydrate (DCPD), and tetracalcium phosphate (TTCP)
[44][42]. Basic information concerning these CaP biomaterials are displayed in the
Table 12 [45][33]. The crystal phases of calcium phosphate were discovered before the 20th century (amorphous phases were discovered in the 1950s) and accurately characterized during the 20th century (OCP was defined in 1957
[46][58]). After that, no new crystal phases (non-doped, non-substituted, only Ca, P, O, H) have been reported. Dicalcium phosphate monohydrate (CaHPO
4·H
2O, DCPM) was obtained by controlling the transformation of a special amorphous calcium phosphate (CaHPO
4·xH
2O, ACHP) in a water-deficient environment (water/methanol mixed solvent, or in humid air) by Lu et al. in 2020
[47][59]. The discovery of DCPM brought a new member to the calcium phosphate family. However, applications of DCPM in bone repair, and even in biomedicine, has not yet been carried out.
Table 12.
The basic information of usual calcium phosphate materials.
Name |
Formula |
Ca/P |
Solubility at 25 °C (g/L) |
HAP |
Ca | 10 | (PO | 4 | ) | 6 | (OH) | 2 |
1.67 |
~0.0003 |
α-TCP |
α-Ca | 3 | (PO | 4 | ) | 2 |
1.5 |
~0.0025 |
β-TCP |
β-Ca | 3 | (PO | 4 | ) | 2 |
1.5 |
~0.0005 |
ACP |
Ca | x | H | y | (PO | 4 | ) | z | · | n | H | 2 | O, | n | = 3–4.5, 15–20% H | 2 | O |
1.2–2.0 |
/ |
OCP |
Ca | 8 | (HPO | 4 | ) | 2 | (PO | 4 | ) | 4 | ·5H | 2 | O |
1.33 |
~0.0081 |
DCPA |
CaHPO | 4 |
1.0 |
~0.048 |
DCPD |
CaHPO | 4 | ·2H | 2 | O |
1.0 |
~0.088 |
TTCP |
Ca | 4 | (PO | 4 | ) | 2 | O |
2.0 |
~0.0007 |