*3.6. Quantification of Inflammatory Cytokine*

Osteoblasts can produce bone-active cytokines such as tumor necrosis factor-α and IL-6 [21,38]. In bone metabolism, these cytokines act as potent factors for osteoclast formation and bone resorption because they can mediate the effects of many stimulators of bone resorption, i.e., parathyroid hormone and IL-1 [22,23,40]. Among these cytokines, IL-6—the most potent osteoclastogenic factor—promotes the activation of osteoclast precursors and their subsequent differentiation into mature osteoclasts, leading to the bone resorption. In addition, a significant release of IL-6 can suppress cell viability. Thus, we determined the expression of IL-6 in cell cultures to evaluate the bone-forming ability and cytocompatibility of the PVDF composite nanofibers.

It was found that PVDF and PO06 promoted the expression of the IL-6 markedly due to the contamination by LPS and subsequent macrophage activation (Figure 11). However, all composite nanofibers containing the POSS–EGCG conjugate exhibited considerably decreased levels of IL-6 expression compared with the pure PVDF nanofiber. In particular, PE06 exhibited only a very small amount of IL-6 secretion, suggesting that the POSS–EGCG conjugate can downregulate the production of bone-active cytokines and improve the cytocompatibility of the composite nanofibers via the anti-inflammatory effect of EGCG [22,23,40]. Therefore, the content of the POSS–EGCG conjugate present was an essential factor in decreasing IL-6 production on the PVDF composite nanofibers.

**Figure 11.** Levels of IL-6 expression by RAW 264.7 cells on the PVDF composite nanofibers (*n* = 5). Significant difference from the pure PVDF nanofiber at each time point was denoted as *p*\* < 0.05.
