Hydroxyapatite (HAP) is the most important mineral form of calcium phosphate salt existent as inorganic phase in human classified tissues i.e., bone and teeth  and it is chemically represented as Ca10 (PO4)6(OH)2. The capability of the human body to redevelop bone components that are damaged or lost is restricted. For this purpose, materials are to be developed that can sufficiently change bone tissue, specially mineralized tissue includes teeth and bone. Therefore researches on the preparation of hydroxyapatite nanoparticles has grown with much importance currently as bone mineral contains hydroxyapatite in the form of spherical/rod like nanoparticles. The synthesized hydroxyapatite powders have excellent properties such as biocompatibility, osteoconductivity, bioactivity and non-inflammatory as well as non-toxicity behaviour [2–6]. Therefore, HAP is an appropriate material for biomedical applications and it has been commonly used for hard tissue regeneration and repair . HAP has also been used for several applications including drug and gene delivery, gas sensor, chromatography, water purification and photo catalytic [8–10] etc. Meanwhile the application of HAP truly depends on the morphology, precursor sources, particle size, crystallinity and phase purity of nanoparticles etc. 
Strontium can exist in the mineral phase of bone, mainly at regions of high metabolic turnover and its useful effect in the osteoporosis treatment [12, 13]. Accordingly, several methods are reported for the non-stoichiometric hydroxyapatite that can be prepared with incorporation of anionic and cationic substitution including Na+, Mn2+, Mg2+, Sr2+, Zn2+, F−, OH− etc., Among these ions, the strontium element has been established to develop vital function in growing and also bone repair . It can regulate the degradation, improve the mechanical strength of the hydroxyapatite , and positively control their bioactive properties including osteoconduction. Strontium is well-known to act an significant role in both the stimulation of bone growth in osteoporotic treatment and reduction of bone resorption during bone remodelling , which naturally exists in bone and teeth [17, 18]. The strontium and calcium share the properties of group 2A elements, without much difficulty. Furthermore, strontium substituted hydroxyapatite has attracted enormous attention because it is a bone seeking element that appears a beneficial influence on bone growth and its excellent physical, chemical and structural parallel to Calcium in hydroxyapatite .
There are many techniques present in the literature for the preparation of hydroxyapatite and substitute hydroxyapatite nanoparticles such as ultrasound system [20, 21], precipitation method , sol-gel method , hydrothermal system [24, 25], microemulsion technique , hydrothermal coupled microemulsion method etc. [27, 28]. Nevertheless, most of the methods suffer from achieving the spherical morphology, uniform size distribution, more agglomeration, uncontrolled long term aging and aggregation due to stringent processes. Amongst these methods, the hydrothermal coupled microemulsion method is suitable for the preparation of HAP nanoparticles. Hydrothermal methods provides well crystallinity, phase purity and improved control over stoichiometry but more agglomeration and a very big size distribution , while microemulsion method offers controls the particle size, uniform morphology and to avoid agglomeration, ease of preparation with produced nano-particles. Consequently, combining these two methods may offer better preparation method for hydroxyapatite nanoparticles.
The present work mostly focuses on the formation of hydroxyapatite with crystallized and controlled morphology of hydroxyapatite nanoparticles using PVA by hydrothermal coupled microemulsion method. Polyvinyl alcohol is a polymer, non-ionic surfactant with good biocompatibility, excellent bioactivities, non-toxicity, more elastic modulus and water content [30–32,] as well as biodegradable polymer . The polyvinyl alcohol hydrogel possesses high potential for their use in tissue engineering and it can also play as a feasible alternative of current artificial bone. Polyvinyl Alcohol (PVA) is widely used in biochemical, pharmaceutical and biomedical applications [34–37] including orthopaedic devices, wound dressings, drug delivery devices  and artificial organs . The polyvinyl alcohol is most effective templates because it controls the morphology and reduces the particle size with preventing agglomeration.
In the current work, we report that the preparation of polyvinyl alcohol assisted Sr substituted hydroxyapatite nanoparticles by hydrothermal coupled microemulsion method using DMSO/cyclohexane/water combination. The results have shown that, the preparation of strontium substituted HAP powder has well phase purity with crystallinity and do not show more agglomeration with impurity, uniform spherical morphology and smaller particle size was reached through hydrothermal coupled microemulsion method. The as-achieved hydroxyapatite powders were characterized by different analytical techniques.