Bone defects were one of the common diseases in orthopaedic surgery, which often caused functional disorders such as bone nonunion and delayed healing [1]. Therefore, a growing deal of attention has been paid to treating bone defects [2]. At present, a series of calcium phosphate bioceramics were the main materials for bone repair, but the traditional high-temperature sintered ceramics suffered from some shortcomings such as low biological activity and slow degradation. Compared with bioceramics, the degradable synthetic polymers had an edge in terms of controllable properties and simple process. Among them, a great interest has been given to the polylactic acid (PLA) due to adjustable degradability and mechanical properties by changing molecular weight. PLA possessed good biocampatibility, and the degradation products were also nontoxic, promoting its clinical studies owing to these merits [[3], [4], [5], [6]], especially in bone repair [[7], [8], [9]]. However, poor hydrophilicity, weak mineralization and osteogenic activity, which were not conducive to early cell adhesion, proliferation and new bone regeneration, inhibited the further application of PLA porous materials in bone repair [10,11].

To overcome these drawbacks, the construction of PLA porous materials with high surface activity by modification has become a research hotspot. Hydroxyapatite (HA), the mineral phase of bone tissues, which possessed biocompatibility and could stimulate bone growth, has been found to promote adhesion of tissues and increase the bioactivity of the materials [12]. Negrescu et al. [13] introduced HA into 3D-Printed PLA porous scaffolds. As expected, the addition of HA into the PLA matrix was beneficial to mimick bone extracellular matrix, leading to positive effects on the osteogenic differentiation of pre-osteoblasts. Wang et al. [14] applied PLA/HA scaffold to repair 5-mm-diameter femur defects of New Zealand white rabbits. At three months postoperatively, the new bone tissues adhered to the surface of the PLA/HA, while in the single PLA group, they were sporadically distributed, indicating that HA endowed PLA with a faster bone regeneration rate than the single PLA. Choudhary et al. [15] dipped hydroxyapatite (HA) coating on surface of PLA/Al2O3 to prepare PLA/Al2O3/HA porous composites. There were many cells on surface of PLA/Al2O3/HA, and the expression of cell activity was more obvious compared to that of PLA and PLA/Al2O3 after coculture with MG63 for 7 days. Besides, Ca nodule aggregation on surface of PLA porous composites containing HA was more evident than that of the single PLA after 14-day coculture with rat osteoblasts, illustrating that HA could promote the mineralization of extracellular matrix [16].

In addition to HA, polymer with good hydrophilicity could be also appropriate for modifying PLA porous materials. Gelatin, the hydrolyzed products of collagen, had superior hydrophilic properties that could play a positive role on cell adhesion, migration, and mineralization [17]. Yu et al. [18] prepared PLA/gelatin porous scaffold by electrospinning, which was used for repairing an articular cartilage defect rat model with a 2 mm diameter. After coculture with PLA/gelatin for 24 h, chondrocytes had good cell activity and grew well. There were almost no dead chondrocytes, indicating that the PLA/gelatin had good biological function in vitro. CT results showed that PLA/gelatin was basically integrated with the surrounding regenerated subchondral bones. Although the trabecular structure was incomplete with a loose structure, the defect was still smaller than the control group, illustrating that gelatin had an ability of promoting osteogenesis. Sampath et al. [19] prepared PLA/gelatin porous composites by dipping gelatin on surface of PLA porous matrix. The mineralization of depositions on surface of PLA/gelatin was more significant than that of PLA, characterized by the increase of Ca/P from 1.46 to 1.60 after 15-day immersion in SBF. After coculture with MSCs for 3 days, the PLA/gelatin porous composites showed high cellular activity and obvious osteogenic differentiation. Besides, Mostafa et al. [8] fabricated PLA/PCL/gelatin porous composites by dipping gelatin into the PLA/PCL porous matrix. After 8-week implantation into rat skull defects, the contents of OCN and bone mineralization in PLA/PCL/gelatin group were more obvious than those in the control group, indicating that gelatin had the ability to promote new bone regeneration. These mentioned results further demonstrated that the mineralization and biological activity of the PLA porous composites with gelatin were significantly increased in contrast to those of the single PLA.

Based the merits of HA and gelatin in terms of mineralization ability and biological activity, the gelatin/HA mixed coating was constructed on surface of PLA porous matrix to prepare PLA multicomponent composites (PLA-gH). It was anticipated that PLA-gH would possess favourable mineralization and osteogenic properties due to surface modification by the gelatin/HA mixed coating. Meanwhile, PLA-gH may be endowed with good degradation behaviour owing to the construction of gelatin/HA mixed coating.

Simulated body fluids (SBF) was often used to evaluate the biological activity of bone repair materials [20,21], which could predict the in vivo mineralization and bone integration of different materials, together with judging the bone regeneration based on the formation of apatites during SBF immersion. However, in recent years, the accuracy of such in vitro predictions has been questioned due to the discrepancy between the results of some in vitro and in vivo tests. Traditional SBF mainly mimicked the concentration of inorganic ions in body fluids, but ignored proteins. It was inevitable for contact between the implanted materials and proteins in body fluids of actual organisms. In many cases, some active proteins even had good affinity with the implanted materials, which could promote bone regeneration. Therefore, applying protein-deficient simulated body fluids for in vitro studies may result in the gaps between the in vitro experimental results and the actual situations [22]. To address these issues, the researchers began to apply simulated body fluids containing protein to assess the in vitro biological activity of bone repair materials [23]. As a model protein, bovine serum albumin (BSA) was often used to evaluate the interactions between proteins and the implanted materials [24]. BSA, comprised a single polypeptide chain, which contained 586 amino acid residues. The cross-linked 17 disulfide bridges of cysteine (Cys) amino acid residues stabilized the structure of this globular protein with a molecular weight of 66.8 kDa [25,26]. Nowadays, the studies of BSA applied in the field of bone tissue engineering and the in vitro immersion have been gradually increased. Yang et al. [27] and Raic et al. [28] both studied the in vitro osteogenic mineralization of BSA. It was worth noting that the calcium nodule content in BSA group was both higher compared to that in the control group, indicating that BSA increased ALP activity and effectively promoted the formation of mineralized calcium nodules. Although with much progress, the studies on BSA were still mainly focused on its effects on the degradation and mineralization of different materials in vitro, especially in biodegradable alloys. Hou et al. [29] analyzed the effects of proteins on magnesium degradation. The results revealed that BSA slightly reduced the degradation rate of Mg during in vitro immersion and promoted the mineralization of Ca/P salts in the degradation layer. Mardina et al. [30] studied the influence of BSA on the corrosion of zinc in Zahrina's simulated interstitial fluid. The resultant results indicated that BSA inhibited the corrosion rate of zinc, which was consistent with Zhang's report [31]. According to this literature, the maximum corrosion rate of pure zinc was 0.45 ± 0.02 mm·y−1 when no BSA was added into the artificial plasma, while in BSA, it decreased to 0.13 ± 0.08 mm·y−1. Besides, the mineralization of pure zinc in BSA was unobvious than that in the single artificial plasma. The available literatures have shown that the mineralization, degradation of the different materials varied when immersed in the media regardless of BSA content.

In conclusion, PLA-gH was prepared in order to improve the degradation, mineralization, and osteogenic properties of PLA. Besides, the effects of BSA on PLA matrix degradation and Ca-P depositions were mainly discussed, which were clarified by comparing the surface morphology, crystallinity and the mineralization behaviours of PLA-gH in SBF-BSA with those in SBF. The cell adhesion, proliferation, osteogenic activity and matrix mineralization of PLA-gH were also studied. In addition, PLA-gH was applied to repair skull defects of rats for further evaluating its biosafety and the in vivo osteogenic property. Furthermore, the reliability of applying SBF-BSA to predict the bioactivity and the osteogenic properties of PLA-gH was also analyzed according to the mineralization of PLA-gH in vitro and in vivo.