## 1 Introduction  
Biodegradable scaffolds are becoming extremely promising objects for bone substitution and repair [1; 2; 3; 4]. They can provide a temporary structural support for cell growth and migration, and act as an extracellular matrix to induce tissue regeneration [5; 6]. Calcium sulfate (CaSO4) is one of the most absorbable materials among all biocearamics. It possesses good biocompatibility and osteoconduction [7, 8]. However, CaSO\({}_{4}\) degrades too fast and decreases pH of the surrounding environment by releasing acidic products, resulting in an inflammatory reaction. In addition, CaSO\({}_{4}\) has poor bioactivity which cannot form an effective bone bond with the surrounding tissue [9, 10].  
Hydroxyapatite (HAp, Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)) is a necessary component of the bone tissue. It possesses numerous favorable properties like bioactivity, osteoinductivity, and nontoxicity [11, 12]. Besides, HAp has slow biodegradability and presents alkalescency, which can neutralize the acidic product and maintain pH stabilization of the surrounding environment [13]. Moreover, nanoscale HAp is more efficient when used for bone repair compared with the conventional method, because it can stimulate tissue regeneration at the bone-implant interface [14]. Meanwhile, nano-hydroxyapatite (nHAp) possesses a large specific area, which can help to provide a strong interfacial interaction between nHAp and matrix, and thus enhance the mechanical strength [15].  
In recent years, nHAp as a potential bioactive material and nano-reinforcing phase has been studied [16, 17, 18]. [PERSON] et al. added different contents of nHAp into magnesium and exhibited the ideal result of adjusting its degradation behavior and pH value during the immersion of samples in simulated body fluid (SBF) [19]. [PERSON] et al. investigated biological characteristics of nanoceramics and indicated that nHAp possessed the favorable ability to induce bone-like apatite formation, and promote protein adsorption and stem cell differentiation [20]. [PERSON] et al. reported that nHAp was employed to intensify chitosan scaffolds by the biomimetic method, and showed that the mechanical strength and densification degree of scaffolds were not increased [21].  
Selected laser sintering (SLS) is a novel processing technology in tissue engineering applications, which allows the sintering of powder materials in an exceedingly brief duration compared with the conventional sintering methods. Besides, it can easily construct a complex appearance and controllable porous structure of the scaffolds. Bone scaffolds formed using SLS possess an interconnected porous structure, which is beneficial for supporting cell adhesion, growth, and ingrowth of new bone tissue. In this study, nHAp was added to CaSO\({}_{4}\) and it was expected that tunable degradation rate and good bioactivity would be obtained. The CaSO\({}_{4}\)/nHAp scaffold was prepared by SLS. The effects of nHAp on degradability, pH change trend, and bioactivity were tested by immersing the scaffold into the SBF solution. The mechanical property tests were carried out using the compression and indentation method. Furthermore, the biocompatibility was assessed by investigating the adhesion and proliferation behavior of the osteoblast-like cell on the scaffold.