## 4 Discussion  
Skin photoaging induced by UV-B irradiation is characterized by wrinkle formation and the inhibition of collagen synthesis. We conducted an experiment using a photoaging model that induces wrinkles in irradiated mice through UV-B exposure. UV-B-induced photoaging inhibits collagen synthesis in the skin, and decreased collagen synthesis impedes skin elasticity, resulting in wrinkle formation [27]. In the current study, we confirmed that mice irradiated with UV-B developed wrinkles. The HanDam (non-Twist) and HanDam (Twist) groups treated with PDO thread showed wrinkle improvement to the normal state. Wrinkle improvement was maintained for up to 4 weeks after treatment (Figure 3). The observed wrinkle improvement is associated with the presence of cogs on PDO threads and is attributed to skin attraction induced by physical stimulation [28]. Wrinkles induced by UV-B irradiation are graded according to depth. The more severe the decline in collagen synthesis and the ease of skin folding, the deeper the wrinkles are  
Figure 6: Effects of HanDam (Twist) on activation of TGF-\(\beta\), COL1, and MMP-1 in UV-B irradiation-induced skin aging in hairless mice. Four weeks post-thread treatment, (**a**) the protein expression related to collagen synthesis through physical stimulation was evaluated using Western blot analysis, and (**b**) displays a histogram showing TGF-\(\beta\), COL1, and MMP-1 relative densities. (a–c) Mean values with different letters differed significantly (\(p<0.05\)) among groups, as shown by [PERSON]’s test. formed, and depth is used as an indicator of wrinkles [29]. We confirmed the depth of wrinkles created by UV irradiation. HanDam (non-Twist) and HanDam (Twist) reduced wrinkle depth to normal levels after treatment. In addition, the wrinkle depth increased at 6 weeks compared to immediately post-treatment in the thread-treated groups, and HanDam (Twist) improved wrinkle depth compared to HanDam (non-Twist) (Figure 4). The 6-week post-treatment results confirmed that HanDam (Twist) had an improved ability to maintain wrinkle depth reduction compared with HanDam (non-Twist). Photoaging induced by UV-B irradiation in the skin leads to lower collagen density in the dermis [23]. [PERSON]'s trichrome staining is a representative method to selectively stain collagen and is suitable for examining collagen density in histological analysis [30,31]. Histological analysis in the present study confirmed that the decrease in collagen density due to UV-B was recovered through HanDam (non-Twist) and HanDam (Twist) treatment. The HanDam (non-Twist) and HanDam (Twist) groups showed greater collagen production than the normal group collagen due to the physical stimulation of the skin by the lifting thread (Figure 5). The increase in TGF-\(\beta\) and the synthesis of COL1 induced by physical stimulation have been shown to be active at 4 weeks post-treatment [32]. Therefore, we evaluated the correlation between TGF-\(\beta\) and collagen production induced by physical stimulation through protein density analysis 4 weeks after PDO thread treatment. TGF-\(\beta\) plays a key role in the wound-healing process related to collagen synthesis caused by mechanical stimulation by PDO threads [33,34]. Our experimental results confirmed that TGF-\(\beta\) and COL1 increased equally in the HanDam (Twist) group and that compared to the HanDam (non-Twist) group, the production of these two proteins was higher in the HanDam (Twist) group (Figure 6). This result is based on the physical characteristics of the Twist form. The non-Twist form is a shape where the protrusions are arranged in a straight line, whereas the Twist form is a shape where the cogs are arranged in all directions. The PDO lifting thread is known to have a lifting effect due to its mechanical effect and collagen production by fibroblast stimulation by increasing the tension between tissues [35,36,37,38].