# Fibers by Electrospinning and Their Emerging Applications in Bone Tissue Engineering
## 4 Drug Delivery System  
Drugs (e.g., therapeutic agents, growth factors, non-viral genes, etc.) could promote new bone formation and prevent inflammation [150]. Electrospun fibers are regarded as drug carriers that direct cell adhesion and differentiation by delivering drugs. Hence, a variety of strategies were proposed to design drug delivery systems by functionalizing electrospun fibrous scaffolds. Owing to the adjustable degradation rate and interconnected porous structure, electrospun polymeric nanofibers have been employed to load drugs. By adding drugs in polymer solution, drugs are directly encapsulated in electrospun polymeric nanofibers. [PERSON] et al. [90] prepared electrospun PLLA/poly(3-hydroxybutyrate) (PHB) nanofibers incorporated with curcumin and investigated the curcumin release profile in vitro. Experimental results showed the continuous release of curcumin during 14 days and improvement of the expression level of osteogenic gene markers, which confirmed the effect of curcumin on the osteogenic differentiation of human adipose-derived stem cells (ADSCs). Moreover, some drugs are considered antibacterial agents for inhibiting implant related infections. For example, rifampicin (Rf) is a major drug widely used for treatment of bone infections, exhibiting broad-spectrum antibacterial activity [151; 152]. In a study by [PERSON] et al., Rf was loaded in electrospun PCL/HAP nanofibers by mixing with polymer solution for preparing antibacterial BTE nanofibers [55]. Their drug release study particularly revealed an initial burst release of Rf due to the porous structure, which could prevent early infection when nanofibers were implanted. The synergistic effect of multiple growth factors is required because of the complicated process of bone formation. Therefore, it is important to design a multiple delivery system for mimicking the natural microenvironment, effectively promoting bone regeneration and vascularization. Many groups reported their strategies to deliver two or more drugs. For example, [PERSON] et al. developed a tricomponent delivery scaffold incorporating recombinant human BMP-2 (rhBMP-2), recombinant human vascular endothelial growth factor (rhVEGF), and calcium phosphate nanoparticles by co-electrospinning (Figure 8) [46]. In vitro study revealed that rhBMP-2 and rhVEGF were sequentially released to promote both bone regeneration and vascularization. Furthermore, in their recent study, the os teogenesis and angiogenesis properties of co-electrospun tricomponent scaffolds were evaluated in vivo, which showed enhancement of new bone formation in mouse cranial defect model [153]. However, it was noticed that unsuitable doses and inappropriate kinetics of drugs adversely affect bone regeneration [154; 155]. Although many studies confirmed that drugs could be easily blend into electrospun fibers, the initial burst release and uncontrollable release may occur. Thus, the delivery of drugs should be appropriately controlled to match the physiological feature of bone regeneration. To solve this problem, drug carriers (e.g., nanoparticles, microsphere, etc.) are incorporated into electrospun scaffolds (Figure 9). [PERSON] et al. [27] loaded N-acetyl cysteine (NAC) onto mesoporous silica nanoparticles (MSNs) by simple immersing process and fabricated electrospun NAC@MSNs/PLGA scaffold. In vitro release and cellular studies showed that the fabricated delivery system provided a sustained release of NAC with high concentration, which affected the osteogenic differentiation of rat bone marrow-derived stroma cell (rBMSC). Furthermore, MSNs were utilized by [PERSON] et al. to load alendronate (ALN), a drug to inhibit osteocl bone resorption [71]. Silicate is the degradation product of MSNs. As the ALN-loaded MSNs was released from the electrospun fibers, silicate and ALN affected bone formation and bone resorption respectively, achieving a dual regulation of bone regeneration.