## 1 Introduction  
Skin is the body's largest protective organ, covering the entire external surface and acting as a barrier between the external environment and internal body systems [1]. It serves several essential functions, including regulating body temperature, preventing dehydration, and providing protection against ultraviolet (UV) light, pathogens, trauma, microorganisms, and physical injuries [2, 3]. Structurally, it consists of three main layers: the epidermis, dermis, and hypodermis (see Figure 1). Skin aging is a complex biological process influenced by intrinsic and extrinsic factors. With aging, the three layers of the skin (epidermis, dermis, and subcutaneous tissue) undergo degenerative changes, with the most noticeable alterations occurring in the dermis. Collagen, the primary structural component of the dermis and the most abundant protein in the body, plays a critical role in providing strength and support to the skin [4; 5]. Over time, these structural proteins and key components deteriorate, resulting in visible signs of aging [6; 7]. In contrast to young skin, which contains abundant, tightly packed, and well-organized collagen fibrils, aging skin exhibits dermal and epidermal atrophy, disorganization, and a significant loss of collagen fibers. Additionally, the reticular ridges flatten as the skin ages [8; 9]. Figure 2 illustrates a comparison between young and aging skin, highlighting the primary differences in the distribution and organization of collagen fibrils. As individuals seek effective solutions to combat the visible signs of aging, non-invasive treatments have gained increasing popularity, with an increasing emphasis on maintaining a youthful skin appearance. This shift has intensified the focus on skin collagen  
Figure 1: Human skin layer. Figure 2: Difference between young and aged skin. regeneration, and, as a result, numerous skin rejuvenation treatments--both invasive and non-invasive--have been developed to address this issue [10]. Among these, High-Intensity Focused Ultrasound (HIFU) has emerged as a revolutionary technology in aesthetic dermatology. HIFU utilizes focused ultrasound energy to penetrate deep into the skin layers, stimulating collagen synthesis and tissue remodeling without damaging the surface. This process enhances skin tightening, reduces wrinkles, and improves overall skin texture, making HIFU a preferred alternative to surgical facelifts and other invasive procedures. Invasive skin procedures penetrate the deeper layers of the skin to enhance its appearance through techniques such as surgical facelifts, deep chemical peels, and dermat abrasion. In contrast, non-invasive methods use energy-based technologies as alternatives to traditional invasive treatments, aiming to minimize potential complications and reduce healing time. These techniques include radiofrequency, intense pulsed light (IPL), and fractional lasers [11; 12]. Recently, High-Intensity Focused Ultrasound (HIFU) has emerged as a non-invasive therapeutic and cosmetic option for treating skin laxity and facial wrinkles, providing a safer alternative to traditional invasive treatments [13; 14]. According to [PERSON] et al., the Food and Drug Administration approved HIFU for dermatologic aesthetic use in 2008 and for brow lifting in 2009 [15; 16]. Currently, HIFU is used for facial rejuvenation, tightening, lifting, and body contouring [17; 18; 19]. Multiple studies have also evaluated the effectiveness and safety of HIFU on the neck, face, and other body areas [20; 21; 22]. HIFU devices work by delivering targeted transdermal heat to the deeper layers of the dermis and subcutaneous tissue, resulting in precise microcoagulation within the deeper dermis and the superficial muscul-loaponeurotic system (SMAS) while sparing the epidermis from damage [23] (see Figure 3). Figure 3 illustrates the mechanism by which ultrasound energy is focused to form thermal injury zones (TIZs).