The proliferation and cytotoxicity of the SF/HAP composite hydrogels were determined using the standard MTS assay with hMSCs to evaluate the potential of these materials as a scaffold for bone regeneration. Figure 6 shows that the MTS assay revealed increased cell proliferation rate as the HAP concentration increased, which indicated that HAP supported the proliferation of hMSCs. However, there was no significant difference in proliferation between SF-2 and SF-3. After 6 day of culture, hMSCs were found to have attached and distributed evenly on all hydrogel samples and a small number of hMSCs filled the pores, and formed a continuous monolayer in all hydrogel samples (Fig. 7). The cell monolayer density was increased with increasing HAP NPs concentration. The hMSCs were stained with a Live-Dead™ kit after 4 days of culture, and then observed with confocal microscopy. Green color represents the live cells, while red color represents the dead cells . After 4 days culture, most cells presented green fluorescence, which indicated no significant cell death in the hydrogels under culture, as shown in Fig. 8. The SF/HAP composite hydrogels induced by γ-ray irradiation have noteworthy potential as bone tissue scaffolds, because they showed no significant cytotoxicity against hMSCs.Fig. 6Proliferation of human mesenchymal stem cells on the SF/HAP composite scaffolds evaluated by MTS assay at day 6 Fig. 7Representative FE-SEM images of hMSCs cultured on a SF only, b SF-1% HAP, c SF-2% HAP, and d SF-3% HAP scaffolds at day 6 Fig. 8Viability and cytotoxicity staining of cells cultured on a, e SF only, b, f SF-1% HAP, c, g SF-2% HAP, and d, h SF-3% HAP scaffolds at day 4