An Overview of the Mechanisms of Fractional CO₂ Laser in Scar Treatment

Abstract

This review aims to investigate the mechanisms underlying the application of ablative fractional CO₂ laser in scar treatment and analyze its effects on inflammatory responses, cell proliferation and collagen remodeling during wound healing. It focuses on how the fractional CO₂ laser promotes skin repair by modulating molecular mechanisms, thereby effectively improving scar appearance. This review summarizes numerous clinical and experimental studies on ablative fractional CO₂ laser treatment, focusing particularly on its regulatory effects on immune cells, fibroblasts, matrix metalloproteinases (MMPs), transforming growth factor-β (TGF-β), heat shock proteins (HSPs), microRNAs (miRNAs), and basic fibroblast growth factor (bFGF) at various stages of repair. The findings demonstrate that ablative fractional CO₂ laser significantly improves scars through precise thermal damage and modulation of the tissue repair. Specifically, by precisely regulating key factors such as MMPs, TGF-β, HSPs, miRNAs, and bFGF during the tissue repair phases, it promotes re-epithelialization, neovascularization, and the synthesis and orderly deposition of collagen, thereby balancing antifibrotic and pro-repair effects to improve scar texture and function. The fractional CO₂ laser is an effective modality for scar treatment that promotes scar resolution and wound healing through dynamic regulation. It demonstrates significant clinical efficacy for both atrophic and hypertrophic scars, offering new perspectives and approaches for future scar management.

Background

Scars are persistent and distressing sequelae that impair patients’ quality of life. On the basis of their morphological and pathological features, scars can be classified into various types, with atrophic and hypertrophic scars being the two most common and significant forms [1]. Atrophic scars often occur following inflammatory skin conditions, such as cystic acne or chickenpox [2]. The mechanism of their formation is primarily associated with an imbalance in the synthesis and degradation of collagen and elastic fibers caused by abnormal inflammatory responses [3, 4]. In contrast, hypertrophic scars are more commonly observed following skin trauma, burns, or surgical incisions, typically characterized by excessive activation of fibroblasts, overproduction and reduced degradation of collagen, leading to abnormal deposition of the extracellular matrix (ECM). Factors such as prolonged inflammation, excessive angiogenesis, and delayed re-epithelialization can induce the release of profibrotic signals, further promoting the formation of hypertrophic scars [5]. Therefore, effectively regulating these processes in the early stage of wound repair is crucial for preventing or mitigating the development of hypertrophic scars.

Currently, the dermatological lasers commonly used in clinical practice are primarily divided into two categories: ablative lasers and non-ablative lasers [1]. Traditional ablative laser resurfacing achieves skin remodeling through complete skin resurfacing. Although effective, it is often accompanied by adverse reactions such as edema, exudation, crusting, pigmentary changes, and scarring. In contrast, non-ablative laser treatments have milder side effects but often require multiple sessions and yield limited efficacy [6–8]. Ablative fractional resurfacing technology overcomes some limitations of these two traditional modalities and has been widely applied in cosmetic dermatology due to its advantages of rapid recovery, fewer adverse effects, and high efficacy. Ablative fractional resurfacing technology acts on the skin via fractional photothermolysis, selectively damaging tissue and forming microscopic treatment zones (MTZs) surrounded by undamaged tissue on the skin surface, which promotes faster skin repair while maintaining the integrity of surrounding tissues [1, 6, 9–11].

The fractional CO₂ laser is a key component in ablative fractional resurfacing technology. Its mechanism relies primarily on the selective thermal effect generated when its light, with a wavelength of 10,600 nm, is highly absorbed by water molecules within the tissue [12, 13]. Laser energy is concentrated in water-rich structures, such as collagen fibers, blood vessels, and keratinocytes, inducing immediate tissue contraction and vaporization within the MTZs and resulting in the formation of microcolumnar vaporization zones and zones of thermal coagulation necrosis (Fig. 1) [7, 14, 15]. These controllable micro-injuries can stimulate the release of various growth factors, initiating an orderly wound healing process.

Fig. 1.

MTZ formation induced by fractional CO₂ laser

To precisely control the extent of thermal damage and optimize the healing process in clinical practice, regulating laser parameters is crucial. Firstly, the wavelength determines the target chromophore and the depth of tissue penetration, which is the fundamental basis for distinguishing ablative from non-ablative laser systems. Additionally, the energy density (fluence) (J/cm²) controls the intensity of thermal damage and directly affects the depth of MTZs. The pulse duration modulates the extent of thermal diffusion; a shorter pulse width helps to precisely confine the injury to the target area, avoiding unnecessary thermal accumulation in surrounding tissues. Furthermore, the spot size and the density of the treatment area (MTZs/cm²) collectively determine the proportion of the epidermal area covered during a single treatment. Balancing these variables is essential for inducing sufficient therapeutic necrosis for tissue remodeling, achieving an optimal balance between recovery speed and therapeutic outcome [16].

The fractional CO₂ laser has demonstrated effective therapeutic results for both atrophic and hypertrophic scars. Researchers indicate that fractional CO₂ laser significantly improves most atrophic acne scars and has become the gold standard for treating atrophic scars [14, 17]. In the treatment of hypertrophic scars, fractional CO₂ laser significantly improves the thickness, vascular distribution, elasticity, and aesthetic appearance of scars, with long-lasting efficacy and better outcomes than other single-laser treatments [18, 19].

Tissue Repair Process Following Fractional CO₂ Laser Treatment

The wound healing process following fractional CO₂ laser treatment can be divided into three phases: the inflammatory phase, the proliferative phase, and the remodeling phase. These three phases are not independent steps that occur linearly; rather, they overlap spatiotemporally and can even occur concurrently [20].

Inflammatory Phase

Following fractional CO₂ laser treatment, the skin initially enters the inflammatory phase, which is characterized by a transient inflammatory response and the rapid recruitment of immune cells. On the first day post-treatment, high expression of stress-related S100A8/A9 proteins and the upregulation of heat shock proteins (HSPs) can be detected in normal mouse skin models, rapidly initiating downstream immune responses [21].

During this phase, various pro-inflammatory cytokines and chemokines construct a complex signaling network. Studies have shown that the mRNA expression levels of IL-1β and IL-6 rapidly increase in normal mouse skin following laser treatment. Moreover, the upregulation of factors such as CXCL12 and CCL8 (in vitro 3D skin models) and CXCL2, CXCL5 and LCN2 (normal mouse skin models) promotes the accumulation of immune cells, such as neutrophils and macrophages [12, 21–24]. Additionally, in trials involving photoaged human skin, a rapid increase in the expression levels of antimicrobial peptides, cathelicidin and HBD2 (β-defensin 2), was observed, effectively preventing wound infection and coordinating innate immune responses [23].

After treatment, neutrophils are rapidly recruited in large numbers around the necrotic zones. They support tissue repair by phagocytosing cellular debris, undergoing degranulation, and secreting matrix-degrading enzymes and other factors. However, excessive neutrophil infiltration and the formation of neutrophil extracellular traps (NETs) may exert an inhibitory effect on subsequent dermal matrix remodeling [21, 24]. Meanwhile, macrophages exhibit significant infiltration on the first day post-laser treatment and present dynamic phenotypic switching. A study on normal mouse skin demonstrated that Arg1-positive macrophages increase while MHC II-positive macrophages decrease on the fifth day post-treatment. This switch from a pro-inflammatory to an anti-inflammatory, pro-reparative phenotype suggests that fractional CO₂ laser therapy can induce the immune microenvironment toward a direction conducive to wound healing [23, 24].

On day 1 following fractional CO₂ laser treatment of photoaged human skin, the number of neutrophils and macrophages increases significantly on day 1 but decreases markedly by day 7 [23, 24], indicating the transient and self-limiting nature of the inflammatory response. Concurrently, the columns of necrotic debris formed in the damaged dermal tissue are gradually discharged through the epidermis, signaling the resolution of the inflammatory response and the transition of the healing process to the proliferative phase [15, 21].

Proliferative phase

During the proliferative phase following fractional CO₂ laser treatment, human skin repairs wounds through re-epithelialization, vascular network reconstruction, collagen synthesis and ECM formation [21].

Re-epithelialization is a key process in this phase, primarily restoring the integrity and barrier function of the epidermis by the migration and proliferation of keratinocytes [25–27]. In normal human skin treated with a fractional CO₂ laser, re-epithelialization is rapidly completed within 24 h postoperatively, evidenced by all wounds being populated with migrating keratinocytes [28]. As the epidermal barrier gradually forms, the tissue enters a robust proliferative phase, characterized by significantly enhanced cellular proliferative activity around the microthermal injury zones. Immunofluorescence analysis indicates that in normal mouse skin on day 7 post-treatment, proliferating cell nuclear antigen (PCNA)-positive cells in the basal layer around the micro-injury zones increase significantly, leading to a temporary increase in epidermal thickness that reaches its peak before gradually returning to baseline on the 15th day [21].

However, in long-term clinicopathological studies of human hypertrophic scars, changes in epidermal thickness exhibit heterogeneity. A study by Keshk et al. observed a significant decrease in epidermal thickness 3 months after the final treatment, accompanied by the reappearance of rete ridges [19]; whereas Makboul et al. observed an opposite trend, with a statistically significant increase in epidermal thickness 6 months post-treatment [29]. Such discrepancies in epidermal structural changes may be attributed to differences in the initial pathological state of the scars, individual variations, detection time points, and laser parameter settings, highlighting the complexity of fractional CO₂ laser therapy in modulating epidermal cells.

Furthermore, studies have shown that loricrin, a key marker of terminal epidermal differentiation, exhibits a dynamic “rise-and-fall” expression pattern (increasing on day 1, decreasing on days 3–5) in 3D skin models following laser treatment [22]. This indicates that the newly formed epidermis, upon laser stimulation, rapidly initiates an orderly differentiation and maturation program, laying the molecular foundation for constructing a fully functional barrier. This early molecular-level regulation ultimately translates into significant morphological improvements: in their histopathological evaluation of human tissues, Makboul et al. clearly observed significant thinning of the stratum corneum in hypertrophic scars after fractional CO₂ laser therapy [29]. This histological alteration marks the effective alleviation of the hyperkeratotic state of the scar, providing a microstructural explanation for the clinically observed improvement in rough scar texture.

The increase in CD31-positive vascular density in the dermis of photoaged human skin at 3 and 5 weeks post-treatment indicates the reconstruction of the vascular network [23], which has also been corroborated in normal mouse skin models and human scar tissues [19, 21, 23]. Vascular endothelial growth factor (VEGF) plays an essential role in this process [7, 30].

Following the inflammatory phase, dermal fibroblasts are activated and begin to synthesize extracellular matrix components, such as type I and type III collagen, fibronectin, and hyaluronic acid [11, 22, 23]. A transcriptomic study on normal human skin provided cytological evidence for this process: in the early stages post-treatment, gene expressions associated with fibroblast apoptosis and fibrogenic pathways decreased, while those related to connective tissue cell motility increased, prompting fibroblasts to preferentially migrate toward MTZs rather than immediately commencing large-scale synthesis [27]. Notably, the regulation of collagen metabolism by laser therapy may exhibit a biphasic characteristic of initial downregulation followed by subsequent upregulation. A study on mature hypertrophic scars revealed the early anti-fibrotic mechanism of laser therapy under pathological conditions: 48 h post-treatment, the mRNA expression levels of type I and type III procollagen were drastically downregulated. This suggests that fractional CO₂ laser therapy induces the regression of hypertrophic scar tissue by reducing early collagen expression levels [31]. Interestingly, this early downregulation of collagen expression is not exclusive to scar tissue; in another experiment on photoaged human skin, a downregulation in the mRNA expression of type I and type III procollagen was also observed on day 1 post-treatment [23]. This suggests that the early collagen-inhibitory property of laser therapy may not be limited to hypertrophic scars; this stage might reserve space for the subsequent orderly deposition of newly synthesized matrix.

As the repair process advances, the tissue gradually transitions into an active matrix synthesis and remodeling phase. Research based on a 3D human skin model found that even in the absence of inflammatory cell involvement, fibroblasts post-laser treatment still exhibited robust remodeling activity; the expression of COL12A1, a gene closely related to tissue remodeling, was upregulated post-treatment, reaching 230% of the control group’s level on day 5 [22]. This active trend of collagen synthesis was further corroborated in in vivo models: in a mouse model, the mRNA expression of COL1A1 was also confirmed to rise significantly on day 7, and along with COL3A1, showed a statistically significant increase on day 15, validating the sustained activity of collagen synthesis during the mid-to-late proliferative phase [21]. Immunostaining data on photoaged human skin further demonstrated that the expressions of type I and type III procollagen were significantly upregulated at 2 weeks post-treatment and persisted until at least week 5. These newly synthesized collagens are initially secreted as soluble precursors, subsequently converted into insoluble collagen fibers through proteolytic processing, and ultimately accomplish the regeneration and remodeling of the dermal matrix [23].

Remodeling phase

The remodeling phase is a critical stage of skin repair encompassing both the degradation and regeneration of the dermal matrix [11]. Histological and immunohistochemical studies on human skin have confirmed that the collagen remodeling response can last for at least three months [15], indicating that dermal reconstruction after fractional CO₂ laser treatment is a long-term and dynamic process.

The early post-laser period (days 0–10) is characterized by the degradation phase of the dermal matrix. Laser-induced MMPs are highly expressed during this period, leading to the degradation of the collagen-rich extracellular matrix in the dermis [11, 22, 32]. The transient increase in MMP levels aids in clearing damaged tissue, creating conditions for subsequent matrix formation and reconstruction [22, 27].

Subsequently, the skin gradually enters the dermal regeneration stage. Studies based on in vitro 3D reconstructed skin models have found that the levels of dermal scaffold proteins begin to gradually increase from day 10, reach a peak around day 20 and then slowly decline. During this phase, changes in the expression of α-smooth muscle actin (α-SMA), a core marker of myofibroblasts, are particularly crucial. In different research models, the regulation of α-SMA by fractional lasers exhibits distinct characteristics. On the one hand, in an in vitro 3D reconstructed model of normal human skin, elevated α-SMA expression was observed on the second day post-treatment. Although this early upregulation helps initiate wound contraction, it also carries a potential risk of inducing dermal fibrosis [11]. On the other hand, different anti-fibrotic outcomes have been observed in studies focusing on clinical pathological scars. After undergoing a complete course of fractional laser treatment, patients with hypertrophic scars and keloids exhibited a significant downregulation of α-SMA, which originally showed strong positive expression in their scar tissues [33]. This indicates that within the scar microenvironment, fractional lasers can effectively reverse the hyperactive phenotype of myofibroblasts and downregulate the abnormal expression of α-SMA by modulating upstream core growth factors such as TGF-β and bFGF. This relieves the pathological contracture of the scar, providing an essential histological foundation for the subsequent orderly reconstruction of a normal collagen network.

As the remodeling process deepens, the microstructure of the dermal matrix undergoes significant alterations. A study on mature human burn scars revealed that after two months post-treatment, type III collagen increased while type I collagen decreased in the tissue [34]. This trend was further validated in other 6-month long-term studies, manifesting as a significantly reduced type I/III collagen ratio that approaches normal skin levels, a decreased proportion of collagen, and an increased proportion of elastin [19, 35]. In scar tissues treated with fractional CO₂ laser, the originally disorganized and thick collagen bundles are gradually replaced by orderly, parallel collagen fibrils, accompanied by an enhanced elastic fiber network and increased vascular density [19, 29, 35] This structural remodeling is precisely regulated by molecular signals such as MMPs, TGF-β, and bFGF.

Molecular Mechanisms of Fractional CO₂ Laser in Scar Treatment

MMPs

MMPs are a group of calcium-dependent zinc-containing enzymes that play a critical role in collagen degradation and matrix remodeling [32].

Matrix metalloproteinase-1 (MMP-1) is a key interstitial collagenase that initiates matrix degradation in damaged skin. Induced by keratinocytes at the wound margins, it binds to type I collagen via the α2β1 integrin. In scar tissue, it primarily targets the initial cleavage of excessively deposited and densely packed type I and type III collagens [31, 32]. Furthermore, MMP-1 attenuates the affinity between type I collagen and α2β1 integrin, facilitating the migration of keratinocytes over type I collagen, thereby promoting re-epithelialization [25, 32, 36]. In a study on photoaged human skin, MMP-1 expression was observed to remain high during the first week post-treatment and decline sharply in the second week [23]. Similarly, another study on mature human burn scars observed elevated MMP-1 expression 48 h postoperatively [31]. This suggests that MMP-1 likely plays a primary role in degrading scar tissue during the early stage following laser treatment.

MMP-3, also known as stromelysin-1, is expressed by keratinocytes in the proximal proliferative population near the leading edge during wound healing [32]. It degrades partially damaged collagen, proteoglycans, elastin, laminin, and fibronectin [32, 36, 37]. In research involving fractional CO₂ laser treatment on photoaged human skin, the temporal expression pattern of MMP-3 was observed to be similar to that of MMP-1, remaining at high levels consistently for 7 days post-treatment and dropping sharply in the second week [23]. A similar expression pattern of MMP-3 was also observed in an in vitro 3D skin model, where MMP-3 expression increased on day 1 post-laser but decreased by day 5. The early upregulation of MMP-3 can promote fibroblast-mediated wound contraction and clear the damaged extracellular matrix, providing space for the formation of new tissue; whereas in the later remodeling phase, the downregulation of MMP-3 facilitates the deposition of new tissue [22]. MMP-3 knockdown experiments have shown that its absence leads to reduced keratinocyte proliferation and premature keratinization, ultimately delaying wound closure [37]. These findings underscore the role of MMP-3 in wound healing after therapy.

MMP-9 (also known as gelatinase B) is primarily responsible for degrading gelatin produced after initial cleavage by interstitial collagenases. It also possesses the ability to degrade various matrix components, including type IV and type VII collagen, elastin, and fibrillin [32]. This broad substrate degradation capability enables it to effectively clear matrix debris and relieve the physical constraints of the basement membrane on keratinocytes, thereby promoting epithelial regeneration and dermal remodeling. On day 1 after fractional CO₂ laser treatment of photoaged human skin, MMP-9 immunostaining was observed, primarily reflecting neutrophil infiltration. This is because early MMP-9 is mainly stored in neutrophil granules and is rapidly released upon infiltration. However, MMP-9 gene expression showed a delayed increase, maintaining high levels for 1–3 weeks post-treatment [23]. In a study using a red Duroc pig model of hypertrophic scarring, immunofluorescence results similarly observed a mild elevation in MMP-9 levels on day 35 post-laser treatment [38]. Compared to MMP-1 and MMP-3, MMP-9 exhibits a more sustained expression, allowing it to clear collagen fragments generated by previous matrix proteinase degradation, thereby preparing for subsequent tissue repair.

MMP-13 is also a potent, broad-spectrum collagenase capable of degrading almost all types of fibrillar collagen, and it can coordinate with MMP-9 in the re-epithelialization process [35, 37]. Following fractional CO₂ laser treatment, MMP-13 expression often exhibits a relatively delayed characteristic. For example, in post-treatment photoaged human skin, MMP-13 was observed to peak on day 14 [39]. This suggests that it may be more involved in the later stages of clearing residual collagen debris and orderly rearranging newly synthesized collagen. However, in another study on mature human burn scars, no statistically significant difference in MMP-13 expression levels was observed 48 h post-treatment compared to pre-treatment levels [31]. This could be due to the detection time preceding the MMP-13 expression peak, but it also implies that MMP-13 might not play a core role during fractional CO₂ laser treatment of hypertrophic scars.

MMP-12 (also known as macrophage metalloelastase) is primarily responsible for the specific degradation of elastin, while also possessing the capability to degrade matrix components [32]. In 3D skin models, a significant downregulation of MMP-12 can be observed on day 5 post-treatment [22]. Another study on photoaged human skin similarly observed that MMP-12 was immediately downregulated after fractional CO₂ laser treatment and remained in a downregulated state for an extended period [39]. This sustained low-expression state may suggest a delicate “selective remodeling” mechanism in fractional laser therapy: while other MMPs actively degrade old collagen, the tissue specifically protects the elastic fiber network from excessive destruction by downregulating MMP-12. This mechanism also provides a plausible molecular biological explanation for the clinically observed enhancement of the elastic fiber network in scar tissues following treatment [35].

Heat Shock Proteins

Following fractional CO₂ laser treatment, the expression of HSP family members is upregulated, which aids in cellular resistance to laser-induced damage and promotes tissue repair and wound healing [12].

HSP70 is a critical marker of thermal injury. Functioning as a molecular chaperone, it assists cells in handling abnormally folded proteins, prevents protein misfolding and aggregation, plays a vital role in cellular repair and wound healing, and can induce the expression of multiple growth factors such as TGF-β [12, 36]. A study using a skin explant model showed that HSP70 was significantly upregulated 1 h after fractional CO₂ laser treatment, peaked between 1 and 24 h, and then significantly declined within 7 days, with no notable differences across different energy settings [36]. In contrast, a study on normal human skin observed enhanced HSP70 staining starting at 4 h post-treatment, which remained at high levels for up to 3 months postoperatively, indicating a persistent molecular-level healing response [28]. These two distinct temporal patterns may stem from differences in the research models.

HSP47 is an endoplasmic reticulum (ER)-resident chaperone specific to collagen synthesis. It interacts with procollagen in the ER, assisting in the folding and assembly of procollagen α1(I) and α2(I) chains, thereby promoting the maturation and stable deposition of newly formed collagen. A deficiency of HSP47 impairs the maturation of type I and type IV collagen, affecting the formation of microfibrils and the basement membrane. Studies indicate that HSP47 expression in the dermis is positively correlated with the rate of collagen formation. In a study by Helbig et al. on photo-damaged human skin, a slight increase in the distribution and intensity of HSP47 was observed within 14 days across different energy groups, peaking at 3–14 days post-treatment with no significant differences among the energy groups [12]. Conversely, in another study on normal human skin, immunohistochemical results indicated that HSP47 expression increased on the fifth postoperative day and remained at a high level for at least 3 months [28]. This suggests that HSP47 is involved in long-term wound healing and collagen remodeling. Although existing data are mostly derived from non-scarred tissue, given the core regulatory role of HSP47 in collagen maturation, it is highly likely to play a critical role in guiding the orderly rearrangement of nascent collagen and determining the ultimate quality of dermal matrix remodeling within the microenvironment of fractional laser-remodeled pathological scars.

TGF-β

TGF-β is a crucial cytokine that regulates cell proliferation, migration and matrix remodeling throughout the process of cutaneous wound healing. It is primarily released by degranulated platelets and macrophages at the site of injury and influences multiple phases of skin repair [31]. During the inflammatory phase, TGF-β recruits macrophages and granulocytes via chemotaxis and mediates the release of proinflammatory cytokines, including interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) [20]. In the proliferative phase, it induces the expression of migration integrins in keratinocytes, promoting their migration toward the wound edge and thus accelerating re-epithelialization. During matrix formation and remodeling, TGF-β exhibits strong pro-fibrotic characteristics; it not only attracts and activates fibroblasts, inducing their differentiation into myofibroblasts with high α-SMA expression to stimulate collagen synthesis, but it also downregulates the expression of matrix metalloproteinases (MMPs) to inhibit collagen degradation. Consequently, the abnormal hyperactivity of the TGF-β signaling pathway is considered a key driver in the formation of pathological scars [4, 12, 20, 29, 36, 40, 41]. A study using a single fractional CO₂ laser treatment at 2.77 J/cm² on photoaged human skin demonstrated that TGF-β expression peaked 3 days post-treatment and then gradually declined until day 30 [20]. A similar biphasic, time-dependent response of initial upregulation followed by downregulation was also observed in a normal mouse skin model [21]. This early and transient elevation helps initiate tissue repair, while the subsequent timely decline relieves the inhibitory effect on MMPs, preventing the occurrence of excessive fibrosis.

The TGF-β family comprises three primary isoforms: TGF-β1, TGF-β2, and TGF-β3. Traditionally, TGF-β1 and TGF-β2 are believed to exhibit pro-fibrotic properties, whereas TGF-β3 possesses anti-fibrotic effects [31]. Among them, TGF-β1 is highly expressed in hypertrophic scars and plays a crucial role in their pathogenesis [29, 40]. In a study by Qu et al. on mature human burn scars, the expression of TGF-β1 showed no significant change 48 h post-laser irradiation. However, at this time point, the expressions of TGF-β2 and TGF-β3 were significantly downregulated, suggesting that within the microenvironment of mature burn scars, TGF-β3 may similarly exert a pro-fibrotic effect [31]. In a study by Makboul et al., immunohistochemical results revealed a marked decrease in TGF-β1 in human hypertrophic scar tissue 6 months after 4 sessions of fractional CO₂ laser treatment [29], a result similarly observed in another study on human scar tissues [35]. The downregulation of TGF-β1 inhibits the sustained activation of myofibroblasts, which also explains the subsequent downregulation of α-SMA expression in the treated scar tissue. Thus, it is evident that fractional CO₂ laser therapy may participate in scar resolution by downregulating TGF-β2 and TGF-β3 in the early stages and downregulating TGF-β1 in the later stages.

miRNAs

Recent studies have demonstrated that miRNAs also play a pivotal role in skin healing and scar remodeling induced by fractional CO₂ laser. In hypertrophic scar tissues, 152 miRNAs are differentially expressed, suggesting their active involvement in the regulation of fibrosis [42]. Furthermore, the miR-17–92 cluster can participate in the regulation of the fibrotic process by targeting various regulatory components within the TGF-β signaling pathway; notably, miR-18a exerts its effects by inhibiting the expression of Smad4 [31, 43]. The miR-17–92 cluster can also downregulate anti-angiogenic factors to promote angiogenesis, while miR-19a/b and miR-20 serve as inflammatory modulators during wound healing, effectively suppressing keratinocyte inflammation and promoting wound closure [43–45]. Following fractional CO2 laser treatment, expressions of miR-18a and miR-19a within the miR-17–92 cluster are significantly upregulated in human hypertrophic scar tissues [31]. These findings suggest that fractional CO₂ laser may selectively activate specific miRNAs to negatively regulate TGF-β signaling and inhibit local inflammation, thereby facilitating scar remodeling.

bFGF

bFGF plays a critical multi-phase role in the human wound healing process. As a broad-spectrum mitogen, bFGF can promote the growth and differentiation of various cell types, exhibit potent angiogenic and mitogenic activities, and participate in the entire process of the inflammatory phase, proliferative phase and remodeling phase [20, 31, 41, 46, 47]. A study utilizing a 2.77 J/cm² fractional CO₂ laser on photoaged human skin observed a gradual upregulation of bFGF expression levels over 30 days post-treatment; however, this trend was absent in the 2.07 J/cm² and 4.15 J/cm² treatment groups [20]. Conversely, in another study treating ten patients with mature burn scars, a significant decrease in endogenous bFGF expression was observed 48 h post-treatment [31]. These discrepancies in expression dynamics may be attributed to differences in baseline skin pathological states, observation time, and fluence settings.

During the early inflammatory phase of wound healing, bFGF can recruit leukocytes to the wound site to initiate healing. During the proliferative phase, it enhances the migration and proliferation of fibroblasts and keratinocytes, promotes re-epithelialization, stimulates balanced synthesis of ECM such as collagen and hyaluronic acid, induces angiogenesis to improve local oxygenation and nutrition and thus accelerates tissue regeneration. During remodeling, bFGF upregulates MMP-1 expression in HS-derived fibroblasts and inhibits the differentiation of myofibroblasts derived from fibroblasts and ESCs (epidermal stem cells). This mechanism prevents excessive collagen deposition and fibrosis, ultimately averting the formation of pathological scars [20, 31, 46, 48–50].

Given that fractional laser treatment of mature scars may lead to the early downregulation of endogenous bFGF expression, the clinical strategy of exogenous supplementation is essential. Clinical studies have confirmed that fractional CO₂ laser combined with rb-bFGF gel yields superior therapeutic efficacy in treating mature facial burn scars compared to laser monotherapy. The timely intervention of exogenous bFGF not only accelerates tissue regeneration and the healing process but also significantly reduces the incidence of postoperative complications, such as persistent erythema and PIH [46, 48].

Conclusion

This review demonstrates that fractional CO₂ laser therapy significantly improves scar quality through precisely controlled thermal injury-induced wound healing mechanisms (inflammation-proliferation-remodeling). Its core mechanisms involve the regulation of molecular pathways—including MMPs, TGF-β, bFGF, HSPs, and miRNAs (Fig. 2) (Table 1)—to stimulate orderly collagen remodeling. Its efficacy in improving scar elasticity, thickness, and symptoms has been well documented.

Fig. 2.

The core molecular mechanisms and cellular interactions during scar remodeling induced by fractional CO₂ laser

Table 1.

The temporal dynamic changes of key molecules (MMPs, HSP70, HSP47, TGF-β1, TGF-β2, TGF-β3, bFGF) in different models after fractional CO₂ laser therapy

Experimental Models	Hypertrophic scar (human/red Duroc pig)	Photoaged human skin	Normal skin (human/mouse)/skin explant/in vitro 3D skin model	Sources
MMP-1	Human: Expression increased at 48 h post-treatment	Remained at high levels during the first week post-treatment and declined sharply in the second week		[23, 31]
MMP-3		Remained at high levels during the first week post-treatment and declined sharply in the second week	In vitro 3D skin model: Expression increased on day 1 post-treatment and decreased on day 5	[22, 23]
MMP-9	Red Duroc pig: Levels mildly elevated on day 35 post-treatment	Positive immunostaining on day 1 post-treatment Gene expression remained at relatively high levels for 1–3 weeks post-treatment		[23, 38]
MMP-12		Downregulated immediately after treatment and remained in a downregulated state for a prolonged period	In vitro 3D skin model: Expression significantly downregulated on day 5 post-treatment	[22, 39]
HSP70			Human: Enhanced staining was detected 4 h post-treatment and remained at high levels for up to 3 months postoperatively Skin explant: Significantly upregulated at 1 h post-treatment, peaked between 1 and 24 h, and significantly declined within the subsequent 7 days	[28, 36]
HSP47		Peaked at 3–14 days post-treatment	Human: Expression increased on the fifth postoperative day and remained at high levels for at least 3 months postoperatively	[12, 28]
TGF-β 1	Human: No significant change in TGF-β1 expression 48 h post-treatment TGF-β1 expression significantly decreased at 6 months post-treatment		Mouse: Expression significantly increased on the first postoperative day, subsequently declined, and showed an upward trend again from day 7 to day 13	[29–31]
TGF-β 2, TGF-β 3	Human: Expressions of TGF-β2 and TGF-β3 were significantly downregulated within 48 h post-laser irradiation			[31]
bFGF	Human: Significantly decreased 48 h post-treatment	2.77 J/cm² treatment group: bFGF expression levels gradually upregulated within 30 days post-treatment 2.07 J/cm² and 4.15 J/cm² groups: No significant changes observed		[20, 31]

However, as a narrative review, this study acknowledges certain limitations. On the one hand, a substantial body of current literature evidence is derived primarily from animal models of varying species or biopsies of normal and photoaged human skin, while studies directly investigating the molecular mechanisms in different types of human pathological scars remain critically scarce. Furthermore, there are vast discrepancies in laser parameter settings across different studies; this high degree of heterogeneity significantly limits the horizontal comparability of research findings and the generalizability of conclusions. Concurrently, existing studies largely rely on single-time-point observations, failing to comprehensively capture the complex and dynamic transcriptional landscape during the long-term scar remodeling process following laser treatment.

A profound understanding of these molecular mechanisms provides vital theoretical support for improving clinical practice and developing synergistic therapeutic strategies. Excessive or uncontrolled inflammatory responses induced by ablative lasers are key factors leading to severe adverse effects, particularly PIH, which presents a high incidence in individuals with darker skin phototypes. Although the triggering modalities differ, based on the abnormal activation mechanism of melanocytes via paracrine networks (such as the HGF/SCF axis) observed in ultraviolet-induced inflammation models, we postulate that the intense inflammatory cascade triggered by laser thermal injury may also induce PIH through analogous pathways.

Therefore, in clinical practice, physicians should shift from simple physical parameter adjustment strategies toward the exploration of combination therapies to compensate for the limitations of monotherapy. For instance, given the early downregulation of bFGF following fractional laser treatment, the concurrent application of exogenous rb-bFGF within a specific time window may effectively replenish the deficit of pro-healing signals. Additionally, the PBM therapy as an adjunctive modality has demonstrated tremendous potential. Recent research by Bueno et al. indicates that combined PBM therapy not only modulates the inflammatory response and reduces the occurrence of side effects but also optimizes the dermal remodeling structure by significantly increasing the diameter of type I collagen fibers and promoting the neogenesis of elastic fibers, thereby enhancing overall skin texture [13]. Another study, which depleted neutrophils using an anti-Ly6G antibody, also confirmed that targeted mitigation of early acute inflammation can substantially improve the ultimate outcomes of dermal remodeling [21].

Meanwhile, to achieve more precise clinical guidance, future research should be dedicated to bridging the gap between basic science and clinical application. This urgently requires the acquisition of more longitudinal, multi-time-point biopsy data from specific human scar tissues to elucidate the dose-response relationships among specific laser parameter modulations, the evolution of key molecular pathways, and the ultimate clinical outcomes. Building upon this foundation, multi-omics technologies, including spatial transcriptomics, should be further integrated to dissect the three-dimensional molecular regulatory networks of “laser-scar” interactions. This will not only help elucidate the heterogeneous responses of different scar types but also provide a robust theoretical basis for formulating personalized treatment strategies, maximizing therapeutic efficacy across diverse patient populations, and minimizing adverse reactions.

Author contributions

Zhang linjing wrote the main manuscript text. Liu Chenxi helped in collecting and organizing the literature. All authors reviewed the manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.

Clinical trial number

Not applicable.