## 1 Introduction  
Skin aging is classified into intrinsic aging, which occurs with age, and extrinsic aging due to the influence of ultraviolet B (UV-B) rays, tobacco use, and environmental pollution factors. Photoaging is caused by extrinsic aging, and UV-B irradiation is one of the important causes of photoaging [1]. Wrinkle formation is a major sign of photoaging [2]. Although the mechanism of wrinkle production is not completely known, one cause is an imbalance in collagen production within the skin tissue.  
The wavelength range of UV-B is between 280 and 320 nm, which allows UV-B to penetrate the epidermis and UV-B-induced damage to be made to the dermis [3, 4]. UV-B-induced damage causes photoaging and internal and external changes in the skin [5, 6]. The internal changes associated with UV-B-induced damage include the inhibition of collagen production; the external changes include wrinkles, pigmentation, and dryness [7, 8]. Wrinkle formation in the skin is the main characteristic of photoaging, and the reduction in collagen type 1 (COL1) expression and the increase in matrix metalloprotease-1 (MMP-1) expression are key mechanisms of wrinkle formation [9]. According to a study of aberrant collagen synthesis in the skin caused by photoaging, UV-B irradiation induces the activation of MMPs, which act to break down elastic collagen in the dermis and connective tissue components [10; 11]. MMP-1 activity plays a key role in collagen degradation, and collagen synthesis is inhibited by fragments of decomposed collagen. This reaction is a continuous process in photoaging conditions and is a major cause of wrinkles due to the decrease in collagen content in skin tissue [12].  
Multiple strategies have been developed to delay the aging process, including filler implants, botulinum toxin injections, and laser treatments [13; 14; 15]. Rhytidectomy, or facelift surgery, has been a recognized procedure since the early 20 th century [16]. The incision facelift method has disadvantages such as high cost, long surgery and recovery times, deep scars, and patient anxiety [17]. Using anti-wrinkle drugs such as retinoids [18] can avoid the disadvantages of surgery, but it takes a long time for the wrinkle improvement to take effect. Therefore, other face-lifting surgical techniques that can overcome the disadvantages are being developed. Face-lifting using lifting threads made of Polydioxanone (PDO) material is particularly safe and effective [19]. The recently developed PDO lifting thread is designed with a barb-like \"cog\", and the surgical approach involves implanting this thread directly above the superficial muscular aponeurotic system (SMAS) of the facial skin [20]. The PDO lifting thread is a biocompatible, absorbable thread, and recent research has shown that this thread decreases wrinkle formation through both a physical lifting effect and the promotion of collagen synthesis [21].  
HanDam (Twist) is a PDO lifting thread with a cog shape. Common lifting threads do not involve twisting, but HanDam (Twist) applies a unique and characteristic Twist technique. We expect that lifting threads with a Twist shape will further increase the positive effect of the cog shape. Previous research has been conducted on the improvement in materials and cog shape [22; 23]. However, the effectiveness of Twist-type lifting threads has not been studied in vivo. Thus, we examined the beneficial effects of HanDam (Twist) for the promotion of collagen synthesis and relief from wrinkles in a UV-induced photoaging hairless mouse model.