**1. Introduction**

Bone is a natural inorganic–organic composite consisting of collagen fibrils containing well-arrayed apatite nanocrystals. Hydroxyapatite [HAp, Ca10(PO4)6(OH)2] is chemically similar to the inorganic component of the bone matrix, which has led to extensive research efforts to use this material as a bone substitute in both orthopedic and dental fields. Recently, HAp has been used in range of biomedical applications, such as matrices for drug release and bone tissue engineering scaffold materials.

HAp can be synthesized through a variety of well-developed techniques, such as the sol-gel process [1], the wet-chemical reaction [2], the solid-state reaction [3] and the chemical vapor deposition [4]. The synthesis of HAp using the sol-gel method consists of the molecular-level mixing of calcium and phosphorus precursors under significantly milder conditions. Hence, a sol-gel synthesis of HAp has recently attracted considerable attention.

The sol-gel synthesis technique is also used in conjunction with various spinning techniques to fabricate HAp fibers [5]. Electrospinning is an easy and simple method that utilizes electrical fields to fabricate nano- to microfibers from a polymer solution. The topological structure of the electrospun matrix closely mimics the dimensions of the natural extracellular matrix (ECM). The ECM plays a key role in triggering intracellular signaling cascades for tissue regeneration. Thus, the development of new ECM substitutes has attracted a wide attention as a scaffold for tissue engineering. Franco et al. successfully fabricated HAp nanofibers by combining electrospinning and a non-alkoxide sol-gel system [6]. Pasuri noted that electrospun HAp nanofibers do not activate macrophages in vitro and can be resorbed by human osteoclasts [7].

Recently, much attention has been attracted by HAp as a drug delivery carrier for the loading and delivery of therapeutic agents, such as proteins, growth factors, genes, and drugs [8–10]. Mesoporous HA nanoparticles possess a higher drug-loading capacity and drug-release efficiency than HAp nanoparticles as a result of their large surface areas and high pore volumes [11]. The synthesis of mesoporous HAp nanoparticles using template reagents, such as Pluronic P123 [12], cetyltrimethylammonium bromide (CTAB) [13], and Pluronic F127 [14], is the most well-known method. In the synthesis of mesoporous HAp, the self-assembly of inorganic HAp phases and template reagents followed by template removal can produce the mesoporous structure.

Münchow [15] incorporated CaO nanoparticles into electrospun matrices and demonstrated improved cell viability and osteogenic differentiation. Hence, the aims of this study were to fabricate and characterize a nanofibrous structure of porous hydroxyapatite-CaO (p-HApFs) by utilizing an electrospinning process based on a sol-gel precursor and to evaluate the release profiles and antibacterial activity of tetracycline from p-HApFs.
