# Silk Protein-Based Membrane for Guided Bone Regeneration
## 3 Silk Fibroin Membrane
### Silk Fibroin Membrane with Other Components  
#### 3.2.1 4-Hexylresorcinol  
4-hexylresorcinol (4-HR) is natural phenolic compound that has antimicrobial, antioxidant, and antimutagenic activity [62]. It has been used as a food additive for preventing the enzymatic browning of fruit and seafood [63]. Medically, it has been used as an antiparasitic and antiseptic agent, and now, it is used as a topical antiseptic ingredient for skin and throat lozenges [64; 65]. 4-HR has an anticancer effect through the suppression of the transglutaminase and inhibition of the nuclear factor-\(\kappa\)B (NF-\(\kappa\)B) pathways [66]. The NF-\(\kappa\)B pathway is related to osteoclast differentiation, and the NF-\(\kappa\)B pathway inhibitor can impair the osteoclast activation [67; 68]. As a result, 4-HR can inhibit the osteoclast activation, and it may contribute to new bone regeneration. Figure 3: Micro- computerized tomography (CT) image at 8 weeks after operation [19]. SF membrane was applied on the rabbit calvariat defect. (**a**) Unfilled defect in the control group; (**b**) SF membrane group; (**c**) bone volume analysis of the micro-CT. New bone formation was significantly higher in the SF membrane group than the control (“_p_ < 0.05). (“_**_” means significant difference). Figure 2: Scanning electron microscopy (SEM) images of cell attachment on the surface of silk fibroin (SF) membranes. (**A**) SEM image shows the SF membrane surface for day 0; (**B**) the cells gradually filled across the fiber texture, covering approximately 10–20% of the SF membrane surface; (**C**) at 5 days, the gap between the SF membrane fibers was filled and covered by the cell attachment; (**D**) at 7 days, the surface of the SF membrane was nearly filled and covered with cells (reproduced from previous publication by permission) [17]. 4-HR has been used as an ingredient of bone graft material, and it has shown favorable results in new bone regeneration [69]. The 4-HR and HA combined graft material has shown higher initial bone regeneration in a rabbit calvarial defect [70]. 4-HR is coated on the titanium alloy surface and induces the osteogenic cell proliferation and contributes to the osseointegration of the implant. The 4-HR and HA coating titanium surface shows more rapid cell proliferation and higher osteoclastcin expression and ALP activity than the HA-only surface coating. The HA and 4-HR coating implant requires higher torque for removal and shows more bone contact area than the HA-only implant coating [71]. The antiseptic property and inhibition of osteoclastogenesis of 4-HR may contribute to the osteogenic marker expression and osseointegration of the implant. 4-HR is a component of silk-based membranes for the GBR technique, and it shows higher bone regeneration ability in bone defects [72]. 4-HR-incorporated silk membranes are used in the calvarial defect, and they show more new bone regeneration compared with silk-only membranes [73], and the 3% 4-HR-loaded silk membrane showed more new bone formation compared with the commercial collagen membrane in the rabbit calvarial defect (Figure 4) [72; 74]. In peri-implant defects, 4-HR stably maintains the space for new bone growth and effectively induces new bone regeneration around the implant (Figure 5) [18]. 4-HR has antiseptic and osteoclast inactivation activity and can be used as major component of the silk membrane for the GBR technique. Grafted biomaterial can induce the foreign body reaction and giant cell formation in the implanted site [75]. 4-HR inhibits the foreign body reaction through suppression of diacylglycerol kinase  
Figure 4: 3% 4-hexylresorcinol (4-HR) + silk membrane (**left side**) or collagen membrane (**right side**) was applied on the parietal bone defect of a rabbit [72]. Figure 5: Silk fibroin + 4-HR membrane was applied in a peri-implant defect [18]. Histological images at 8 weeks after operation. (**a**) Unfilled peri-implant defect in the control group; (**b**) silk fibroin + 4-HR group shows new bone regeneration in the peri-implant defect.