Micro-Focused Ultrasound Technology for Remodeling of Skin Tissue Architecture in an Animal Model

Background: As a novel clinical therapeutic technique, micro-focused ultrasound (MFU) has garnered significant attention in the field of skin rejuvenation in recent years. Although clinical studies have demonstrated the marked efficacy of MFU in improving skin laxity, the specific histological and molecular mechanisms of its rejuvenation effects on the skin remain unclear. Methods: The abdominal skin of Bama minipigs was treated with MFU, specific treatment parameters employed both the 3 mm and 4.5 mm therapeutic head with the following settings: 25 Hz/1 min/6.63 W, 25 Hz/2.5 min/6.63 W, 25 Hz/1 min/1.32 W, and 10 Hz/1 min/6.63 W, with adjacent areas serving as controls. The thickness of the dermis, fat layer, and superficial musculoaponeurotic system (SMAS) layer was measured by ultrasound imaging at baseline, immediately after treatment, and at 14, 30, 90 days posttreatment. The histopathological, immunohistochemical, and transcriptomic changes of each time point were studied, to compare protein content and gene expression of matrix metalloproteinase (MMP) enzymes, transforming growth factor β (TGF-β), epidermal growth factor (EGF), Ki67, type I collagen, and elastin. Results: MFU induced dermal thickening, which was sustained up to 90 days. Immediately posttreatment (0 days), the parameters 4.5 mm, 10 Hz, 1 min, 6.63 W showed an average thickening of 0.16 mm, with a growth rate of 14.48 ± 3.64%, which was significantly higher than the 0% growth rate observed in the control group (p < 0.05). With the parameters of 3.0 mm, 25 Hz, 1 min, 1.32 W, the dermis thickened by an average of 0.69 mm at 90 days posttreatment, with a growth rate of 39.33 ± 14.34%. In contrast, the control group showed an increase of about 0.18 mm, with a growth rate of 0.92 ± 13.25% (p < 0.05). A prolonged treatment duration (2.5 min) and higher power levels (6.63 W) did not exhibit statistically different effects on the promotion of dermal thickness in this study. MFU also promoted collagen production in the SMAS layer in the long term. After treatment with parameters 3.0 mm, 25 Hz, 1 min, 1.32 W, the SMAS thickened by 0.12 mm at 30 days, while the control group showed an increase of 0.03 mm (p < 0.05). At 90 days, the SMAS thickened by approximately 0.17 mm, whereas the control group decreased by 0.04 mm (p < 0.05). Following treatment with parameters 3.0 mm, 10 Hz, 1 min, 6.63 W, the SMAS thickened by 0.11 mm at 30 days, with the control group showing an increase of 0.06 mm (p < 0.05). Immunofluorescence staining indicated a significant increase in type I and III collagen, MMP1, MMP3, TGF-β and Ki67 at 14 and 30 days after MFU treatment. The gene expression of MMP family members, TGF-β, EGF, type I collagen, and elastin fibers was markedly upregulated at 14, 30, and 90 days. Conclusions: MFU treatment can stimulate the proliferation of collagen fibers in the dermis. It can also promote the proliferation of collagen in the SMAS layer. However, the effect of MFU treatment on fat is minimal due to limited energy and depth penetration.