Rewrite“Compared with Mg and its alloys, CaP based bioceramics exhibit more prominent advantages. Firstly, the composition of calcium phosphorus-based bioceramics is extremely similar to the inorganic components of natural bone tissue, and therefore, calcium phosphorus-based bioceramics tend to possess fantastic histocompatibility, bioactivity, and osteoconductivity in living organisms. Secondly, calcium phosphorus-based bioceramics have excellent processability and they can be machined into a porous three-dimensional (3D) structure, which allows for the growth of seed cells and capillaries as well as the input of nutrients. What's more, the final degradation products of these bioceramics in vivo (e.g., calcium ions, phosphorus ions) happen to be the nutrients required for cellular activities, and they are reused by tissue cells to promote cellular osteogenic differentiation. Also, the excellent high temperature resistance, corrosion resistance, oxidation resistance and mechanical properties are noteworthy advantages. However, calcium-phosphorus based bioceramics have the intrinsic characteristics of high brittleness and low toughness, which greatly limit their application in the bone reconstruction field, especially in bone bearing area occasions. Nowadays, the most extensively investigated calcium-phosphorus based bioceramics in bone tissue engineering mainly involve HA and tricalcium phosphate (TCP).
HA is the main inorganic phase component of natural bone (~ 65%), with a Ca/P atomic ratio of approximately 1.67. Considerable studies have revealed that hydroxyapatite occupies a very important position in the field of research on new bone substitute materials due to its good biocompatibility, excellent osteoconductivity, and high biological safety. The first application of hydroxyapatite in the field of bone regeneration and repair can be traced back to the 1930s. Researchers found that hydroxyapatite filled in the body could form chemical bonds with human bone tissue under the influence of the internal environment, and grow new bone tissue at the junction between the HA implant and the natural bone. Subsequently, the new bone then climbs and grows along the interconnected pores inside the implant, ultimately transforming into natural bone tissue. Unlike bioactive glasses, hydroxyapatite does not require the formation of an intermediate bonding zone when combined with bone. After HA is filled into a bone defect, osteoblasts differentiate directly on its surface to form a bone matrix, after which an amorphous electron density band of 3-5 μm in width is generated. Next, the collagen cell bundles will grow into this region, while inter bone crystallization occurs in this amorphous band. After the mineralization matures, the amorphous band narrows to 0.05-0.2 μm, and the bonding between the HA implant and bone is accomplished through this very narrow bonding band.
TCP is likewise a widely used bone regeneration and repair material in tissue engineering, which integrates good biocompatibility and bioactivity. It can actively fuse with bone tissue after implantation in the human body without inflammation and toxic side effects. Based on the crystalline phase, TCP tends to be further subdivided into α-TCP and β-TCP. They are chemically similar, but their crystallographic characteristics are distinguished. Generally, α-TCP has relatively slightly higher firing temperature, crystallinity and mechanical strength than β-TCP, while its biological properties are significantly inferior to those of β-TCP. As a consequence, β-TCP enjoys priority in the clinical application and basic research fields of bone defect repair. Furthermore, β-TCP has a lower interfacial energy compared with HA, which enables it to degrade rapidly in vivo (almost 10-20 times that of HA) and release large amounts of calcium and phosphate ions, thus accelerating the mineralization process of osteoblasts.
The promising results achieved by the calcium-phosphorus based bioceramic/BG composites strongly indicate that the combination of the two is successful and feasible. In their composite system, the original excellent degradation properties, mineralization characteristics and osteogenic activity of bioactive glass are still preserved, and meanwhile, the mechanical properties are greatly improved due to the intervention and benign combination of calcium-phosphorus based bioceramics. The effective combination of calcium-phosphorus based bioceramics and bioactive glass in a sense breaks through the constraint that bioactive glass only works in non-load-bearing and low load-bearing areas, thus making it possible to act as a bone substitute material in the load-bearing area.”in an academic way