Rewrite“The schematic diagram of SCPL for preparing 3D bone scaffolds is shown in Figure 8. To begin with, the polymer material is dissolved in an organic solvent, followed by a homogeneous dispersion of the BG powders and water-soluble pore-forming agents (mostly sodium chloride). Then the mixed solution is poured into a clean mold for curing, and after the organic solvent is completely evaporated, the cured product is transferred to an aqueous solution to leach off the salts therein. Ultimately, a 3D porous structure of bioactive glass-based bone scaffolds can be obtained. The most attractive advantage of this method is that the characteristics of the scaffold product such as porosity, pore size, degradation rate and mechanical properties can be readily modulated by varying the initial mass fraction and particle size of the salt. It has been reported that the porosity of scaffolds prepared by this method is approximately 50%-90%.
Freeze-drying is an environmentally friendly method for the preparation of 3D bone scaffolds. First, the bioactive glass powder is dispersed in a polymer solution to form a homogeneous solution, and then the solution is cast into a mold for low-temperature freezing. During the cooling process, the water becomes ice in the form of crystal nuclei, which separates from the polymer containing the bioactive glass powder, resulting in the formation of a solvent-rich phase and a polymer-rich phase in the system. Subsequently, the water phase is removed by drying, leaving a rough bone scaffold with interconnections, which is then sintered to obtain the porous bone scaffold. For the cooling curing step, the temperature is generally in the range of -20°C and -80°C. Freeze-drying is a traditional method for the preparation of bone scaffolds, and although it has been exposed to drawbacks such as irregular pore sizes (15 μm-35 μm), long process, and high energy consumption, it is still exploited by many scholars today due to its ability to keep the original chemical properties of the material from being damaged.
TIPS is a method proposed in the early 1980s for manufacturing interconnected porous 3D bone scaffolds. The process involves dissolving bioactive glass powders and polymers in a high boiling and low volatility solvent above the melting point of the polymer material to form a homogeneous solution, after which the temperature is varied (usually lowered) to allow for phase separation of the homogeneous solution, i.e., solid-liquid or liquid-liquid phase separation. Afterwards, the system forms a two-phase structure with the polymer containing the bioactive glass powder as the continuous phase and the solvent as the dispersed phase. At this point, the solvent is extracted out by utilizing an appropriate extractant, thereby receiving a 3D scaffold with a certain structural shape (Fig. 9). One of the most attractive advantages of scaffolds prepared by this method is that the porosity tends to be over 95%, which provides a favorable space for the growth and spread of new tissues and blood vessels, and also serves as a guide for protein deposition and cell attachment to some extent. Consequently, TIPS has a promising application in the preparation of bone tissue engineering scaffolds.
Electrospinning is a simple and maneuverable method for preparing bone scaffolds, whose application has a history of nearly 100 years. The electrospinning system mainly consists of three components, including a high voltage power supply, a small-diameter nozzle, and a metal collector screen (Figure 10). The jet is stretched along the direction of the electric field force, and meanwhile, cleavage refinement and jet whipping make it elongated, thereby ultimately solidifying and depositing onto a collection device with opposite polarity to form nanofibers. The nanofibers have the advantages of large specific surface area, high porosity and uniform fiber diameter, which can mimic the structure of natural extracellular matrix in terms of scale and morphology. Therefore, the 3D nanofiber scaffolds fabricated by Electrospinning technique hold good biomimetic properties.”in an academic way