Hypertrophic scar (HS) is a fibroproliferative disease that frequently occurs after burns, trauma, surgery, etc. It has been reported that the incidence of HS can be as high as 70% after burns [1]. Patients with HS are often accompanied by itching, joint contracture, and other symptoms, which significantly affect the quality of life [2]. The exact pathogenesis of HS is still elusive, but it is generally accepted that HS is the consequence of dysregulated wound healing. One of the main reasons for HS formation is the excessive production of collagen at the end of wound healing, which may be triggered by either excessive proliferation of fibroblasts, or over-abundant collagen deposition by residual fibroblasts. At the end of normal wound healing, the number of fibroblasts reduces via apoptosis. Blockage of this process leads to an increased number of fibroblasts and redundant collagen deposition, which ultimately causes HS [3]. In addition, studies have found that increased expression of transforming growth factor-β1 (TGF-β1) often exists in HS tissues, which results in an enhanced secretion of collagen from fibroblasts and the over-differentiation of normal fibroblasts into myofibroblasts [4]. Furthermore, excessive vascularization in the late stage of wound healing is another important reason for the formation of HS. Studies have found that the number of blood vessels in HS tissues is significantly increased and the diameter of vessels in HS tissues is larger than that in normal skin tissues, indicating the essential roles of vascularization in HS formation [5].

Due to the complexity of HS formation, the therapeutic effects of current treatment targeting one single pathological process are limited and often accompanied by various side effects. For example, 5-fluorouracil (5-FU) is often used in the treatment of HS due to its apoptosis-inducing ability [6], however it may cause erythema, ulcer, dyspigmentation, or even tissue necrosis [7]. In addition, collagenase injection has been found to attenuate HS by dissolving the residual collagen fibers and inhibiting the collagen synthesis of fibroblasts. However, it may cause swelling, tenderness, and ulcers [8]. Furthermore, pulsed dye lasers (PDL) have been proven to limit the growth of HS by damaging microvascular structures [9], while side effects such as burns on the skin, pigmentation, and skin purpura are often observed after use [10], [11]. Combination therapy is also used in clinic to further improve the therapeutic effects. For example, injection of 5-FU combined with triamcinolone acetonide (TAC) has been recommended as a promising alternative for the treatment of HS [12], however, combined therapy increases the complexity of the pharmacological mechanisms, and its safety needs to be further evaluated. To date, modalities for HS prevention are comparatively limited. Silicone gels or patches are often used after wound closure to prevent the formation of HS. However, this is a time-consuming strategy that requires a minimum of 12 hours of treatment per day and lasts for at least 2 months [13].

Studies on natural products offer a potential solution to address the above issue as they often possess diverse biological activities, allowing the use of one molecule to regulate multiple pathological processes simultaneously. Based on the literature, we found that Protocatechuic aldehyde (C7H6O3, PA) holds various bioactivities, such as inducing apoptosis, reducing collagen synthesis, and inhibiting angiogenesis. For example, Deng et al. (2020) reported that PA inhibits the growth and migration of breast cancer cells [14], and the inhibition of PA on tumor cells is achieved by triggering apoptosis [15]. In addition, it has been found that PA significantly attenuates isoproterenol (ISO)-induced myocardial fibrosis and collagen deposition by directly mediating the conformation dynamics of collagen [16]. Furthermore, PA has also been shown to inhibit angiogenesis by regulating the functions of human umbilical vein endothelial cells (HUVECs) and vascular smooth muscle cells [17], [18], [19]. Based on this, we therefore hypothesized that PA holds inhibitory effects on HS by simultaneously inducing hypertrophic scar-derived fibroblasts (HSF) apoptosis, inhibiting HSF collagen production, and suppressing excessive angiogenesis (Fig. 1).

In addition, improving drug delivery efficiency also benefits HS prevention and treatment. Except for physical therapy, most of the current anti-scarring drugs are topically applied to the lesions, however, poor drug delivery efficiency leads to unsatisfactory prevention or treatment outcomes [20]. The emergence of microneedle (MN) technology provides a potential solution to this issue. Studies have demonstrated that MN transdermal drug delivery strategy is not only more effective than other drug delivery approaches but also reduces systemic toxicity. Compared to traditional transdermal drug delivery methods, the MN drug delivery system is painless, minimally invasive, and does not require expensive equipment. Indeed, MN itself can reduce the stress in the tissue microenvironment, thus reducing the formation of HS [21]. These demonstrate the advantages of the MN technique in the application of scar management.

In this study, we evaluated the apoptosis-inducing, collagen-inhibiting, and angiogenesis-suppressing abilities of PA in vitro. Additionally, hyaluronic acid MN loaded with PA (HA-PA-MN) or HA mixed with gelatin loaded with PA (HA/gelatin-PA-MN) were fabricated and examined for their potential for HS prevention or treatment in vivo using a rabbit ear HS model.