## 4 Discussion  
Acne is a multifactorial inflammatory disease affecting pilosebaceous follicles of the skin. Acne may leave post-inflammatory hyperpigmentation, post-inflammatory erythema, and scars with a textural change. Post-inflammatory erythema is common after acne inflammation. It is cosmetically unacceptable without adequate treatment [10]. Post-inflammatory erythema is considered a low-grade scar, not a superficial erythematous or vascular lesion [11]. RF devices have been used to treat acne vulgaris and post-inflammatory erythema. They have a minimal post-treatment recovery time and a high safety profile, especially for darker skin types [12]. Non-ablative RF needles can cause thermal damage to sebaceous glands. They can help treat acne by lowering sebum excretion, and several studies have found a quantitative relationship between sebum production and acne. [13]. [PERSON] et al. have treated 22 acne patients with a non-ablative RF device that could deliver RF energy at 6 MHz [14]. They showed a 75% or better diminution of active acne lesion counts in 82% of their patients. Unlike monopolar RF, bipolar RF affects only the microneedle radiofrequency zone, allowing surrounding tissues to accelerate the wound healing process, which can result in a shorter recovery time [8]. A bipolar configuration provides a more controlled energy distribution and less pain than a monopolar RF. Moreover, newly developed pulsed-type RF differs from conventional continuous-type RF in that it transmits energy at very close intervals, thereby providing sufficient energy with less damage [15]. One study has demonstrated dramatic improvements in post-inflammatory erythema according to investigators' and patients' subjective assessments when using a bipolar pulsed RF device [8]. Additionally, after pulsed-type RF, in vivo mouse model histological specimens exhibited coagulation columns of thermal injury around each microneedle electrode in the dermis without noticeable tissue injury in the epidermis [16]. The first mechanism of bipolar pulsed RF for treating erythema is to create coagulation columns, a water-drop-shaped oval zone of thermal injury, at the tip of each electrode [16]. The electric current generates heat as it travels through the skin and meets tissue resistance. This heat causes collagen shrinkage and coagulation columns. Partial collagen denaturation occurs due to this heating, leading to collagen contraction and thickening, which can trigger neo-collagenosis and remodeling. Bipolar RF can not only cause dermal collagen remodeling but can also cause CD31-positive blood vessels to collapse and shrink in size due to coagulation [15]. Small, collapsed, slit-like CD31-positive blood vessels were noted throughout the dermis [15]. It could be one of the reasons why bipolar pulsed RF could reduce post-inflammatory erythema after acne. The second mechanism of bipolar pulsed RF improving erythema is associated with the downregulation of VEGF-A (vascular endothelial growth factor-A). Several studies  
Figure 4: Examples of improved clinical photos. **(a,d)** V1 (before the first session), **(b,e)** V3 (after the second session), and **(c,f)** V4 (after the third session). have suggested that post-inflammatory erythema due to acne is related to VEGF-mediated angiogenesis and lymphangiogenesis [17]. NF-kB, which activates IL-8 and VEGF expression, is chronically active in inflammatory diseases, including acne [10]. Histologically, IL-8, NF-kB, and VEGF staining intensity reductions are also observed eight weeks after the second session of RF treatment [10]. In other studies, downregulation of VEGF-A mRNA expression has been found in specimens from RF treatment groups compared with untreated control skin samples [15]. Additionally, Western blot analyses revealed gradual downregulation of the VEGF protein after treatment that was suppressed over the following 14 days in the RF treatment group, compared with untreated control skin [15].