Rationale for Discrete Light Treatment Approaches in Wound Care

Abstract

Wound healing is a multifaceted and sequential process influenced by both local and systemic conditions. Chronic wounds can lead to functional impairments, persistent pain, and reduced quality of life posing a significant burden on the healthcare system. In the US, approximately 6.5 million patients suffer from chronic wounds annually, costing the healthcare system over $25 billion. Given these substantial costs, there is an urgent need for innovative and effective wound management approaches. Historically, light therapy has been utilized to treat various skin diseases. There has been tremendous recent progress in light treatment approaches. This chapter outlines the fundamentals of wound healing and examines how different types of light can modulate specific stages of wound healing. These treatments can be broadly categorized based on their biological tissue interactions as photothermal therapy (PTT), photodynamic therapy (PDT), and photobiomodulation (PBM). Each treatment has a discrete mechanism of action evoking directed biological responses to promote wound healing. Additionally, appreciating the fundamental premise of each approach enables rationalized combinations for optimal therapeutic clinical benefits. Light treatments offer an additional innovative approach to effective wound management.

1. Introduction

Wound healing is an extraordinarily dynamic and complex process. There are three overlapping phases of wound healing: inflammation, proliferation, and remodeling (maturation). A wound is defined as a break in the skin or other body tissues, usually due to mechanical injury. As most wound healing processes are examined post-injury, the earliest stage of healing usually involves hemostasis, which is often considered the initial phase of a four-stage process [1, 2]. There are other wound scenarios where the bleeding phase may not be discretely apparent, such as contusions, pressure or crush injuries, chemotherapeutic agents, and ionizing radiation-induced injuries, among others. The individual phases occur in a concurrent, organized manner, and the duration of each stage can vary depending on several extrinsic (presence of biofilms, moisture, etc.) and intrinsic (nutrition, blood flow, etc.) factors. Early stage post-injury, extravasation of blood that fills the injured area with plasma and cellular elements, mainly platelets. This initial scaffold serves as a provisional matrix necessary for cell migration and a reservoir of cytokines and growth factors [1, 3].

The inflammatory response, therefore, starts with vasodilation and increased vascular permeability, promoting the migration of neutrophils to the wound site [4, 5, 6]. The inflammatory cells namely, neutrophils, macrophages and lymphocytes are recruited by chemotactic substances released by platelets that adhere to the endothelial-lined vessel walls and transmigrate to the site of injury [7]. The activated macrophage is a key player responsible for removing microbes and damaged tissues [8, 9, 10]. These cells also secrete cytokines and growth factors that contribute to angiogenesis, fibroplasia, and extracellular matrix synthesis, including initial granulation tissue. The next stage is the proliferative phase, which involves new tissue formation and wound contraction involving concurrent proliferation of the epithelial, fibroblasts, and endothelial cells [11, 12]. Concurrent co-migration of fibroblasts and endothelial cells contributes to angiogenesis and fibroplasia. The new tissue forms a new network of blood vessels called the granulation tissue, providing increased perfusion and nutrition to the healing tissues. The leakiness of these early vessels is responsible for the clear wound fluid and creates a fluid microenvironment for cells to migrate and reorganize. Angiogenesis is characterized by migrating endothelial cells buddings, and maturing into tubular capillaries driven by platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF) and transforming growth factor-beta (TGF-β) that also promotes matrix secretion, wound contraction, and angiogenesis [7, 11, 13].

The final phase of wound healing is remodeling and resolution, where the anatomical structures are functionally restored. During this phase, the wound contracts and becomes stronger, and collagen deposition occurs that is initially thinner (Type III) and oriented parallel to the skin. This is subsequently remodeled into thicker (Type I) collagen and reoriented along the lines of tension, increasing tensile strength. Successful completion of the healing process necessitates a balance between the synthesis of the new matrix and turnover of provisional healing matrix [7, 11, 14]. Excessive collagen synthesis due to an imbalance of either mechanical forces or biochemical factors, such as persistent inflammation, can result in scars such as keloids (skin or mucosa) or adhesions (internal organs).

2. Factors affecting wound chronicity

The return of injured tissue to its normal anatomical structure and function within a reasonable period is considered successful wound healing. However, some wounds do not heal in a timely and orderly manner. Clinically, a chronic wound is defined as a full-thickness skin defect that fails to heal after 3 months of standard care [15]. Various factors (systemic and local) may impede the finely balanced repair processes, resulting in chronic, non-healing wounds [16]. The differential diagnosis of chronic wounds is broad, with the main causes being chronic venous insufficiency of the lower limbs, peripheral vascular and neuropathic changes associated with diabetes mellitus, and related ulcers with pressure mechanisms. This may result from lower mitogenic activity, premature aging of fibroblasts, and greater activity of matrix metalloproteinases (MMP), leading to greater extracellular matrix (ECM) degradation and persistent inflammatory mechanisms [15, 17]. As an example, diabetic hyperglycemia contributes to a diverse systemic complication, causing an array of local pathologies manifesting within the wound microenvironment, including chronic inflammation, dysregulated angiogenesis, and hypoxia-induced oxidative stress, among others. As a result, diabetic patients develop wounds characterized by impaired healing, prolonged inflammation, and reduced epithelization. Around 15% of patients suffering from type II diabetes will develop ulcers localized on the lower limbs, named diabetic foot ulcers (DFUs), a severe form of diabetic wounds that may lead to lower limb amputation or death [15]. Wound chronicity implies important functional limitations, decreased quality of life, chronic pain, and associated complications that have psychosocial repercussions on patients, as well as a significant increase in health care costs [17]. Managing non-healing wounds, specifically modulating individual healing phases, represent a significant wound management strategy [14].

3. Fundamental therapeutic premise of laser-biological tissue interactions

Biological responses to laser energy is based on photon interaction with biomolecules [18]. Laser energy is a coherent, collimated, and monochromatic energy source [18]. Incident laser energy can be transformed depending on the rate and total dose into biokinetic (thermal), biophysical (conformational), biochemical (fluorescence, redox) or their combinations within the exposed tissue environment (Figure 1) [20]. Incident laser energy is absorbed by the characteristic tissue chromophores defined as a chemical group that absorbs light, and if present in the visible region of the electromagnetic spectrum, imparts color to the substance [21]. Another form of energy transfer, especially with low-dose laser energy, in non-homogenous biological tissues is inelastic scattering. In ablative high-power laser-tissue interaction, incident laser energy is absorbed by the chromophores, which eventually raises the temperature, resulting in a process known as photothermolysis or photoablation [22]. The laser energy can be confined temporally by pulsing the beam into micro- to femtoseconds. This results in the development of high electromagnetic fields (plasma) around the interaction, eventually resulting in tissue degradation or vaporization [23]. Lower powers can be employed for photocoagulation or hypothermia that have additional benefits in wound care. As the major outcome of these interactions is based on heat generation, we will refer to the former high-power as ablative PTT and the latter as non-ablative Photothermal Therapy (PTT). In contrast to these high-dose photon interactions, lower-dose photons can incite non-thermal photochemical responses employed by discrete forms of light treatments, namely photodynamic therapy (PDT, destructive) and photobiomodulation (PBM) [19]. The remaining chapter outlines the utility of each of these discrete approaches in wound management.

Figure 1.

Outlines the three main categories of phototherapy and their subcategories based on the mechanisms of action and effects. Photothermal therapy is divided into ablative techniques like debridement and disinfection, which involve thermal effects above 45°C and are destructive/surgical, versus non-ablative techniques like photocoagulation and hyperthermia, which are non-surgical. Photodynamic therapy involves disinfection through non-thermal (below 45°C) and destructive but non-surgical means. Photobiomodulation therapy encompasses non-thermal (below 45°C) and non-destructive, non-surgical applications such as relieving pain, reducing inflammation, modulating the immune system, promoting healing, and facilitating tissue regeneration. Modified from Arany [19].

4. Photothermal therapy

The primary use of surgical lasers is to surgically remove damaged or necrotic tissues while additionally assisting with dysbiotic biofilm disinfection. Lasers can also be used for hemostasis, such as its use after surgical excisions or debridement. In contrast, using light devices, especially near-infrared lamps, offers discrete non-ablative benefits in promoting wound healing. This latter field is more popularly termed hyperthermia therapy, where body temperature is elevated in a controlled manner. Both approaches are further discussed in the following sections.

4.1 High-power, ablative PTT (A-PTT) for surgical wound debridement and disinfection

The spatial precision of a laser, perhaps best evidenced with LASIK in ophthalmology with its depth and width control, offers exquisite advantages over surgical curettes and blades. Further, the concurrent photocoagulation (bloodless surgical field) provides superior field visualization, reducing inadvertent tissue damage. The use of lasers for wound debridement has been examined in various contexts, including burns, Sulfur mustard injuries, chronic wounds, infected wounds, and diabetic wounds [24, 25, 26, 27, 28, 29]. Research indicates that serial debridement of wounds results in a decreased infection rate and faster healing compared to infrequent or no debridement [30]. These effects have been ascribed to debridement reducing bacterial bioburden, relieving excessive pressure, promoting cytokine release, and facilitating drainage. Laser debridement is a largely unexplored wound debridement method limited to burn wounds and oral, periodontal contexts [31, 32, 33]. Despite restricted usage, it possesses numerous advantages, such as accuracy and consistency of tissue ablation (thereby minimizing injury to the wound site), enhanced patient comfort, and potential facilitation of wound healing. Particularly, the development of the erbium-doped yttrium-aluminum-garnet (Er: YAG) laser (2940 nm) and the erbium, chromium-doped yttrium-scandium-gallium-garnet (Er, Cr: YSGG) laser (2780 nm), which can be used on hard tissues with minimal thermal effect when combined with water cooling, make treatment of periodontal tissues (such as gingiva, tooth roots and bone tissue), as well as titanium implant, surfaces possible with lasers [34].

The efficacy and safety of using lasers for the treatment of chronic wounds has been clearly demonstrated by a few recent studies, one of which investigated these aspects in fully ablative CO₂ laser debridement and compared it to conventional surgical debridement when treating chronic wounds [26]. Guan et al. observed in a retrospective matched cohort trial that fully ablative CO₂ laser debridement had several advantages over routine sharp surgical debridement. Superior improvement in the wound’s condition and reduced wound area were observed; patients treated with CO₂ laser debridement also showed reduced healing time for chronic wounds without any adverse events. Similar results were observed by Gaur et al. when using a diode laser in conjugation with conventional surgical biopsy for the treatment and sterilization of the biopsy site [35]. Overall, the efficacy of laser debridement has shown to be more conservative, less painful, reduced microbial loads, and improved perfusion and healing. This approach has been advocated in refractory or complex wounds with dehiscence or bone exposures [36, 37, 38]. Some investigators have examined initial full-field ablation versus fractal resurfacing for maintenance [39]. This direct ablative approach must be distinguished from the laser-induced shockwaves for agitation of antimicrobial solutions that improve wound disinfection [40, 41].

4.2 Lower-power, non-ablative PTT (NA-PTT) or hypothermia treatments

This non-invasive treatment strategy utilizes either light alone or additional photothermal transduction agents (PTAs) to convert increased kinetic photonic energy into thermal energy. This therapy aims to achieve a controlled temperature elevation from 37°C to 39.5°C or 40.5°C (range 41.8–44°C) [42]. PTAs absorb photon energy under the effect of near-infrared radiation and induce conditions of local hyperthermia. As most of these agents also generate photochemical reactions, NA-PTT with PTAs are discussed under the PDT section. This part only elaborates on the light-alone approach that generates non-ablative heating in target tissues. Among all the wavelengths typically used, near-infrared radiation (650–900 nm) is favored for its simplicity of application, capacity for precise targeting, and minimal absorption by neighboring tissues and skin, facilitating noninvasive penetration to relatively deeper tissue layers [43]. NA-PTT can be performed locally, regionally or whole body with light devices, ultrasounds, shockwaves, radiofrequency, and magnetic fields. NA-PTT has several advantages: low drug resistance, high efficacy, limited complications, and non-invasiveness.

Exposure to mild local heat (41–43°C) can assist in cell proliferation, angiogenesis, wound healing, and bone regeneration [44]. Moderate heat ranging from 45°C to 50°C can rapidly destroy tumor cells while minimal damage to normal cells. Hyperthermia temperatures (>50°C) have been shown to be effective in inhibiting bacterial proliferation. Rapid cell death due to protein denaturation and damage to the cell membrane is induced using relatively high light intensity at sub-coagulative (43–55°C) or coagulative (55–100°C) temperatures. Alternatively, a state of hyperthermia can be induced using milder temperatures (41–43°C); however, these temperature settings do not result in cell death unless applied for a prolonged duration (>1 h) [45]. Cell death due to NA-PTT can result from two different signaling pathways in cells exposed to temperatures greater than 42°C: apoptosis and necrosis. While necrosis has been reported as the conventional cellular response for NA-PTT, a few studies highlight apoptosis as the principal cell death mechanism for specific irradiation conditions [46]. Typically, it has been seen that high-energy irradiation induces necrosis while low-energy irradiation promotes apoptosis. Duration of treatment and presence of external photoabsorbers (such as nanoparticles) also influence cell death. Additionally, necroptosis is a cell death mechanism that has been recently reported in NA-PTT [47]. Heat-induced protein denaturation is the central mechanism of NA-PTT cytotoxicity, where higher temperatures result in faster and more effective cell death. Exposure to temperatures over 42°C results in irreversible tissue damage, further leading to necroptosis and apoptosis at 46°C [47]. Heating tissue at temperatures over 49°C causes disruption of the cell membrane and results in leakage of cytoplasmic matter, leading to inflammation and necrosis [48].

4.3 Non-ablative PTT (NA-PTT) or hypothermia to promote wound healing

Several challenges exist that can affect normal wound healing, including bacterial infection, chronic wounds, and excessive wound healing [2]. To combat these, NA-PTT has been proposed as a treatment that imparts negligible damage to normal tissues and selectively eliminates bacteria to promote wound healing. While the efficiency of therapy can be countered by the elevation of heat shock proteins that assist cell survival (leading to cellular thermal resistance), numerous studies have affirmed that mild hyperthermia can increase the susceptibility of cancer cells and bacteria to therapeutic methods used in conjugation with NA-PTT [49]. Light (typically within the near-infrared region) is absorbed by the agent, which is stimulated to an excited singlet state. It then undergoes nonradiative vibrational relaxation to return to its ground state, which results in a collision between the excited PTA and surrounding molecules, heating up the environment due to increased kinetic energy to achieve local photocoagulation [45, 50]. Among the various PTAs available, organic nanomaterials have shown good biocompatibility and biodegradability; however, low photothermal conversion efficiency, poor photothermal stability, and complicated synthesis limit their widespread usage. Contrastingly, inorganic nanomaterials exhibit excellent near-infrared light absorbance, high photothermal efficiency, and photostability owing to their intrinsic optical properties. However, their non-biodegradable nature and potential long-term toxicity are issues of concern [51]. It has been reported that NA-PTT using gold nanoparticles has been shown to induce programmed cell death via protein Bid, which induces the oligomerization of proteins Bak and Bax that facilitate pore formation and result in the release of Cytochrome C from the intermembrane space. Cytochrome C interacts with other molecules and forms an apoptosome, which signals the activation of various caspases that lead to apoptotic cell death [52].

It has been observed that mild hyperthermia plays a beneficial role in wound repair, particularly for burn wounds [53]. As demonstrated by a study conducted by Shahabi et al. on BALB/c mice, wounds treated with mild hyperthermia exhibited significantly smaller size compared to untreated wounds evaluated on the same day [54]. Furthermore, treated wounds showed substantially reduced acute inflammation along with increased collagen formation and epithelialization compared with untreated wounds. With the progression of treatment, they also noticed a significant increase in neovascularization in the treated wounds. Studies also show that local hyperthermia can increase local perfusion and oxygenation at the wound site. Rabkin et al. observed increased subcutaneous tissue oxygen tension on the application of heat and a threefold increase in local perfusion, highlighting the role of hyperthermia in treating contaminated wounds and mitigating infections [55]. They found a linear relation between change in oxygen tension and reducing microbial burden.

Besides promoting cell proliferation by mild hyperthermia to accelerate wound healing and tissue regeneration, hydrogels with photothermal properties have been implicated in remotely controlled drug release. Hydrogels can absorb inflammation exudate and serve as a physical barrier by protecting the wound from bacteria. These hydrogels are formulated to resemble natural tissues and extracellular matrix, which helps them integrate with the wound and stimulate tissue regeneration [56]. Indocyanine green is an FDA-approved photothermal therapeutic agent with exceptional photothermal conversion efficacy [57]. Li et al. developed a hydrogel loaded with indocyanine green and rare-earth nanoparticles that effectively released Adriamycin upon near-infrared (NIR) activation [58].

Overall, both ablative and non-ablative PTT have significant utility in wound care that can be routinely employed in comprehensive management. To ensure appropriate biological tissue reactions and reliable therapeutic clinical outcomes, the device parameters of the device and manner of delivery must be adopted suitably.

5. Photodynamic therapy

Addressing skin wounds represents a significant global public health challenge, particularly chronic wounds, which place considerable physical and financial strain on patients and significantly diminish their quality of life [59, 60]. Photodynamic therapy (PDT) utilizes light-activated drugs called photosensitizers to achieve therapeutic effects. Initially, a photosensitizer is administered to the patient, either locally or systemically [61]. After a given period to enable optimal photosensitizer distribution, the treatment site is exposed to a specific visible or near-infrared light wavelength. The photosensitizer absorbs this light, triggering two discrete photochemical reactions, either Type I or Type II. These generate cytotoxic biochemical intermediates that ultimately lead to the desired therapeutic outcome [62]. It is worth noting that the history of PDT dates back over a 100 years and even received the 1903 Nobel Prize for Dr. Finsen Rydberg’s seminal work. Dougherty and colleagues conducted the first substantial animal and human tumor studies in the late 1970s at Roswell Park, Buffalo marked the beginning of the modern PDT era [63].

PDT was originally developed for the local destruction of solid tumors and has been trialed for tumors in various sites. The first approved use was for refractory superficial bladder cancer, utilizing Photofrin as the photosensitizer and transurethral light delivery. Other approvals include the treatment of obstructive and early-stage bronchial cancers, where light is applied through a bronchoscope, and esophageal lesions for palliation or high-grade dysplasia in Barrett’s Esophagus. PDT has shown high efficacy in skin cancer, particularly basal cell carcinoma, but its role compared to other treatments, such as local excision, is still being established. PDT has been less effective for squamous cell skin tumors and melanoma, where light penetration is a limitation [64]. In malignant brain tumors, PDT has been studied as an adjunctive therapy, with evidence of improved survival at high light doses. Clinical trials for prostate cancer are ongoing, using interstitial fiber-optic light delivery to treat recurrent tumors or as primary therapy for focal tumors [63, 65, 66]. PDT has also been trialed for intraperitoneal disseminated ovarian cancer and mesothelioma, with partial success. Head and neck cancer trials have targeted widespread, unresectable, or recurrent tumors, early-stage oral cancers, and nasopharyngeal tumors using a specialized trans-nasal light delivery system. PDT has been used in different stages of cancer patient management, including palliation, surgical adjuvant, stand-alone modality for primary lesion cure, and prophylactic treatment of dysplasia. Other challenging tumors under investigation include biliary tree tumors, pancreatic cancer treated laparoscopically, and spinal metastases to debulk tumor mass before vertebroplasty. PDT has also been explored for purging tumors and stem cells in bone marrow transplantation. However, this is not yet approved, as maintaining a high anti-tumor effect while preserving normal cell survival remains challenging [63, 66, 67].

5.1 Biological targets in PDT

In PDT targeting mammalian tissues, the clinical effect can result from direct target cell death (e.g., tumor cells) through necrosis or apoptosis, vascular damage leading to tissue ischemia and subsequent cell death, immune modulation, or a combination of these mechanisms. The cytotoxic impact of PDT is primarily attributed to three distinct mechanisms: Firstly, PDT exerts direct cytotoxicity toward tumor cells, precipitating their necrosis. Secondly, the generation of reactive oxygen species (ROS) during PDT provokes apoptosis by instigating alterations in oxygen free radicals, resulting in irreversible cellular and microvascular damage (Figure 2). Thirdly, the apoptotic or necrotic cells induced by PDT elicit immunogenic cell death, thereby instigating a cascade of immune responses and subsequent implications in the latter stages of therapy [69]. One of the potential advantages of PDT is the ability to adjust the photosensitizer choice and treatment parameters, such as the drug-light time interval, total PDT “dose,” and light fluence rate, to selectively target primary biological processes and resulting responses (Table 1) [70, 71, 72]. For example, PDT for AMD exploits the vascular response, while bone marrow purging focuses on cellular effects. Tumor treatments often involve a combination of these biological targets and responses. Unlike radiation therapy, where DNA is the primary target, in PDT, photosensitizers typically localize in or on cell membranes, often mitochondrial membranes, resulting in somatic cell death rather than anti-proliferative effects [73]. This leads to very rapid tissue responses, sometimes detectable before treatment completion. Since PDT is usually delivered as a single, high dose rather than fractionated over many sessions, there is limited opportunity to adjust treatment parameters. Pre-treatment optimization is crucial given the potential heterogeneity of tissue responses within and between targets or patients.

Figure 2.

The photosensitizer’s (PS) electrons from the ground state (S0) will absorb energy and move to singlet-excited states (S1). Some of the absorbed energy will be released via intersystem crossing, and the promoted electron will more to a triplet-excited state (T1). This triplet state has a relatively long half-life, allowing energy to be transferred to nearby oxygen molecules. This generates ¹O₂ in most cases via the type II pathway, which can damage the cells in the surrounding area. Adapted from Hu et al. [68].

Photosensitizer characteristics
Toxicity	The metabolic processes involving the photosensitizer should be structured in a manner that prevents the generation of additional harmful byproducts.
Mutagenicity and carcinogenicity	The photosensitizer’s therapeutic action aimed at alleviating a particular ailment should not inadvertently lead to the onset or exacerbation of another health condition.
Biodistribution	A photosensitizer possessing the ability to specifically reach and gather in desired tissues holds potential advantages. This assumption relies on the accurate identification of the appropriate target for light exposure and activation. If the target includes intracellular sites, such as mitochondrial membranes, it can result in programmed cell death through apoptosis. On the other hand, targeting the cell membrane or extracellular regions may cause cell death via vessel collapse, resulting in necrosis. Necrosis triggers a systemic response through the cytokine family, leading to widespread effects throughout the body.

Table 1.

Guidelines for selection of photosensitizers to be used in photodynamic therapy (adapted from Refs. [70, 71, 72]).

The light must be efficiently delivered to the PS molecules to make PDT the most effective. Lasers and light-emitting diodes (LEDs) are currently the most common light sources used in PDT, though in some cases filtered white light can also be used. However, most PS classes absorb ultraviolet or visible regions of the electromagnetic spectrum that overlap with the absorption of the main tissue chromophores (e.g., melanin, hemoglobin, bilirubin) [74]. The short penetration depth of visible light into tissues (<5 mm) is very effective for superficial lesions and limits the damage from reaching adjacent healthy tissues but also makes it harder to bring PDT into clinical applications that require illumination of deep tissues or large tumors [75]. Current commercial alternatives require the use of endoscopes and/or multiple optical fibers that are highly expensive and require rather specialized training for clinicians. However, because of the rapid and noticeable tissue responses, real-time monitoring of tissue response (e.g., changes in blood flow, presence of a necrotic treatment boundary) can help adjust dosing to avoid under-treating the target or over-treating adjacent normal tissues [76]. Additionally, PDT can be repeated multiple times, as seen in skin tumor treatments, without apparent induction of resistance or exceeding tissue tolerance, likely due to the preservation of collagen and tissue architecture [77, 78, 79]. Following the selection of a photosensitizer (Table 2), additional considerations must be addressed for its application such as targeting specific cells, the depth within the tissue where the target cell is situated, the extent of PS absorption by the target cell, as well as the prescribed protocol, including the wavelength employed in PDT to optimize the maximal convergence of PS illumination.

Photosensitizers	Activation wavelength	Type of tumor
HPD (hematoporphyrin) derived	625–630 nm	Mesothelioma, high grade glioma, advanced gastrointestinal tumors, sub-foveal choroidal neovascularization
BPD (benzoporphyrin) derived	689 nm	Non-/facia port-wine stains
5-ALA (5-aminolevulinic acid)	635 nm	Malignant gliomas
5-ALA methyl ester	635 nm	Basal cell carcinoma
5-ALA benzyl ester	635 nm	Gastrointestinal tumors
HPPH (hexylether derivative of pyropheophorbide-A)	665 nm	Several Barrett’s esophagus, endobronchial recurrence from lung cancer
Phthalocyanines or naphthalocyanines	650–770 nm	Cutaneous tumors, endobronchial lesions, head and neck tumors
Lutetium texaphyrin	732 nm	Cervical, prostate and brain tumors
SNET2 (tin etiopurpurin dichloride)	664 nm	Cutaneous metastatic breast cancer, basal-cell carcinoma, Kaposis’s sarcoma, prostate cancer
Boronated protoporphyrin	630 nm	Brain tumors
M-THPC (meta-tetra (hydroxyphenyl) chlorin)	652 nm	Head and neck tumors

Table 2.

Types of photosensitizers used in photodynamic therapy for malignant diseases.

5.2 Photodynamic therapy in wound care

A study has demonstrated that low-dose ALA-induced PDT stimulates cell proliferation, angiogenesis, and skin homeostasis regulation, thereby enhancing wound healing through the activation of localized ROS production in an acute mouse skin excision model [80]. Studies using animal models have reported an accelerated initiation of wound re-epithelialization following PDT, characterized by the presence of active fibroblasts, fibrin, and granulation tissue [81]. Evidence suggests a robust cellular infiltrate response in the treated chronic wound as one of the various properties of PDT. Findings from a study indicate that chronic wounds subjected to ALA-PDT exhibit heightened populations of neurons containing mediators implicated in wound healing, along with an increase in the proportion of mast cells containing nerve growth factor (NGF) and vasoactive intestinal peptide (VIP).

5.2.1 PDT for disinfections

PDT has emerged as a treatment option for localized bacterial infections, driven by the increasing problem of antibiotic resistance. Various photosensitizers have been found effective against multi-drug-resistant bacterial strains. For instance, methylene blue PDT has been approved in Canada for treating periodontitis, where a gel formulation of the drug is applied to infected gum pockets, followed by light delivery through an optical fiber placed in each pocket [82, 83, 84]. Ongoing trials explore using PDT for sterilizing chronically infected wounds, such as diabetic ulcers, and the nares to eliminate bacteria that can cause peri-operative self-infection of surgical wounds [85]. PDT has also been tested as an anti-fungal agent, including against Candida, and off-label for acne treatment, targeting the bacterium Acne vulgaris. These applications involve direct application of the photosensitizer and light to the infected tissue [86]. Other potential uses of PDT include sterilizing in situ implanted devices like in-dwelling catheters and field sterilization, such as in operating rooms, to prevent bacterial growth. In the 1980s, there was interest in using PDT to purge blood of viral agents pre-transfusion, particularly for HIV and hepatitis. Although technically successful, concerns about the economics of the technique and re-injection of blood-containing photosensitizer led to its limited adoption. However, with the development of photosensitizers that can be used at lower concentrations and demonstrate high anti-viral activity, this technique may regain interest in the future [87].

While bacteria colonize all chronic wounds, infected wounds hinder effective wound healing. Antibiotics provide limited respite, especially against multidrug-resistant bacteria that undergo rapid mutation to adapt to extreme environmental conditions. Additionally, bacteria can exist in colonies that form aggregates on a surface, known as biofilm, which show increased antibacterial resistance and affect antibacterial efficacy. These factors make developing strategies to combat bacterial infection an urgent necessity. Recent years have seen a positive outlook toward using nanotechnology in PTT, owing to its broad-spectrum antibacterial activity, prevention of drug resistance, and reliable controllability. However, PTT alone is often insufficient to completely eliminate bacteria due to uneven heat distribution in bacterial biofilms. To counter this, numerous recent studies have used PTT in conjugation with other therapeutic methods, such as PDT, antimicrobial peptides, or antibiotics, for better reparative outcomes [88, 89].

5.2.2 PDT for anti-angiogenesis

Photodynamic therapy has emerged as a highly effective treatment for age-related macular degeneration (AMD), the leading cause of blindness in the elderly in the Western world. The “wet” form of AMD, characterized by abnormal blood vessel growth in the choriocapillaris, can lead to central vision loss. PDT has been used in over 2 million cases to date, significantly impacting treatment outcomes. Before PDT, thermal laser coagulation was the only available treatment, but its effectiveness was limited. In PDT for AMD, a diode laser delivers light through a fundus camera to irradiate a 3 mm diameter spot shortly after intravenous injection of the photosensitizer VisudyneR while the drug is still in the vascular compartment [90]. This process targets and kills vascular endothelial cells, leading to thrombosis and vessel closure. While PDT does not restore lost vision, it can slow or halt further vision loss with several rounds of treatment over several months. Visudyne-PDT has become the standard treatment for AMD, although trials combining PDT with anti-VEGF therapy are currently underway [91]. The use of PDT in modulating angiogenesis in chronic wounds may be useful in accelerating the resolution of granulation tissue and promoting remodeling.

5.2.3 PDT for anti-inflammatory, immunomodulation, and skin remodeling

PDT has been extensively studied in dermatology, particularly for skin tumors. It has shown high efficacy in basal cell carcinoma, especially for basal cell nevus syndrome and difficult-to-treat sites like the eyelids. However, PDT has not been approved for basal cell carcinoma due to the high success rates of established alternatives like excision and cryosurgery [92, 93]. In dermatological PDT, 5-aminolevulinic acid (ALA) is used to synthesize protoporphyrin IX (PpIX) endogenously in target cells, making it effective for conditions like actinic keratosis or sun-damaged skin. ALA-PDT is approved for these indications and has led to off-label use in cosmetic dermatology for acne, hair removal, and skin remodeling [94, 95, 96]. PDT has also been studied for psoriasis, a condition characterized by uncontrolled keratinocyte proliferation leading to plaques, particularly on joints like the elbows and knees. While other forms of phototherapy, like UV-B treatment or PUVA, are options for psoriasis, they carry a risk of UV-induced skin cancer. PDT using visible light-activated photosensitizers presents a promising alternative, but its efficacy for psoriasis is still under investigation [97, 98, 99]. PDT has effectively treated specific skin conditions such as acne vulgaris, rosacea, genital warts, and superficial tumors [100]. PDT has been employed for the treatment of diverse conditions such as malignancies, microbial infections, and esthetic concerns such as actinic keratoses, squamous cell carcinoma, basal cell carcinoma, Bowen’s disease, mycosis fungoid, warts, acne, rejuvenation, and necrobiosis lipoidica besides other conditions [61, 101, 102, 103, 104, 105, 106, 107, 108]. These PDT applications suggest its immunomodulatory and anti-inflammatory responses could be utilized in chronic wound scenarios effectively, but require a more thorough investigation.

6. Photobiomodulation

The scientific study of the interaction of light in the form of non-ionizing radiation and living organisms is known as Photobiology. This includes phenomena such as photosynthesis and bioluminescence. An innovative approach to utilizing low-dose light treatments to modulate host biological responses is termed Photobiomodulation (PBM) therapy, which evokes potent therapeutic biological response [20]. PBM has gained widespread popularity and is commonly employed in clinical medicine due to its non-invasive nature and cost-effectiveness. PBM, also known as low-level laser therapy (LLLT) or light therapy, involves using low-level lasers or light-emitting diodes (LEDs) to stimulate cellular processes, promote tissue repair of skin injuries, reduce inflammation, and produce analgesia. Research with respect to PBM for wound healing has explored its effects on several types of wounds, including chronic wounds and surgical incisions. The therapeutic approaches include the PBM-induced effect in mobilizing immune cells into the bloodstream and the recruitment and modulation of their pro-regenerative activity into the wound site. PBM is performed with various wavelengths spanning visible to near-infrared and is increasingly appreciated as an energy transfer phenomenon based on absorption and inelastic scattering [20]. PBM is one of the most popular forms of light-based therapies currently due to significant recent advances in its demonstrated therapeutic benefits in supportive cancer care, where it has been noted to enhance resilience and minimize adverse side-effects of cancer treatments [109, 110].

6.1 Mechanism of action of photobiomodulation

There are over 300 terms for using this treatment such as low-level light/laser therapy, cold laser therapy, infrared or red-light therapy. This non-invasive technique utilizes low doses of light to stimulate cellular function and promote tissue repair [111]. Efficacy of PBM is dependent on several factors that include positioning of cells within the proximity of light exposure, cell type, molecular composition, cellular redox state, tissue microenvironment conditions, and various parameters related to the PBM technique itself, including wavelength, power density, delivery method (pulsed or continuous), size and shape of the light beam or spot are being used for irradiation purposes along with duration/frequency of exposure [109]. Extensive studies have been conducted to understand the mechanisms of action of PBM on biological tissues [111]. Various molecular targets and signaling pathways have been identified. These can be broadly categorized into three main compartments: the intracellular mitochondria (Cytochrome C Oxidase), the cell membrane (photosensitive transporters and receptors like TRPV-1, Opsin 2–4), and the extracellular milieu (Latent TGF-β1 activation) (Figure 3). Therapeutic benefits of PBM are observed owing to the direct biological effect of these discrete components. Cytochrome C Oxidase (CCO), the intracellular mitochondrial enzyme, demonstrates distinctive absorption characteristics within the non-ionizing, visible spectrum (blue and red) and near-infrared (NIR) wavelength, which is the most popular cited PBM mechanism. Direct absorption of light at specific wavelengths by CCO leads to the enhancement of mitochondrial activity, which eventually leads to an increase in ATP production, along with the generation of mild, transient reactive oxygen species (ROS) [112, 113, 114, 115]. Studies have demonstrated that the absorption spectrum of CCO spans from 580 to 850 nm, specifically with absorption bands present at the red region (660 nm) and the near-infrared region (810 nm) [116, 117]. The enhanced mitochondrial activity directly activates several signaling pathways, leading to higher ATP/ADP ratios and transitory reactive oxygen species. Both effects can mediate the activation of signaling pathways that further induce transcription factors associated with cell migration, proliferation, protein and nucleic acid synthesis, and apoptosis inhibition [115]. Such effects allow for accelerated tissue repair, modulated inflammatory processes, and pain signaling to enhance angiogenesis, collagen synthesis, and re-epithelization [114, 117].

Figure 3.

Currently outlines mechanism of photobiomodulation (PBM) therapy. Broadly there are three categories: intracellular involving cytochrome C oxidase, cell membrane receptors and transporters, and extracellular latent TGF-β1. It is being increasingly appreciated that multiple pathways and synergy among them integrate into biological responses that are ultimately responsible for the therapeutic clinical outcomes.

Improved wound healing was among the very first clinical observations with PBM treatments [118]. A key PBM mechanism mediating this response is its ability to directly activate a potent pro-healing latent growth factor complex, TGF-β1 [119, 120]. The role of TGF-β1 spans a broad range of health and wellness, including processes such as development, immune responses, wound healing, and malignancies. These growth factors have been shown to act on several cell types in a context-dependent manner and appear to play a crucial role in the healing milieu [116, 119]. Our prior work with an in vivo human oral tooth extraction model and PBM treatments noted increased TGF-β1 expression immediately post-injury [120]. Additionally, immunohistochemistry analysis revealed an elevated expression of TGF-β3 14 days post-treatment. Further in vitro and in vivo studies using biochemical and molecular assays, transgenic mice, and chemical inhibitors have outlined the PBM-induced redox-activated mechanism for latent TGF-β1 activation that involved a single amino acid, a methionine at position 253 on its latency-associated peptide [119, 121]. In a more recent study, Khan et al. observed the effects of PBM therapy on a modified burn wound model for third-degree burns in C57BL/6 mice [122]. Using a dynamic treatment protocol consisting of an 810 nm laser at 70 mW/cm² for 5 minutes, or a total dose of 21 J/cm² PBM-treated wounds were observed to have significantly improved closure and healing responses 9 days post-treatment when compared to controls. In wounds subjected to PBM, activation of TGF-β1 signaling was evidenced through increased nuclear localization of phospho-Smad2/3 complexes [122]. Further, to examine the mechanism of PBM-promoted wound healing, a chimeric knock-in mice model with the β1 dimer in the TGF-β1 latency-associated peptide was substituted with β3 (TGF-β^L1β3/L1β3). In wild type TGF-β^L1β1/L1β1 mice, PBM was observed to expedite the burn wound healing response, whereas a partial response was observed in the heterozygous chimeric TGF-β^L1β1/L1β3 mice while poor healing was noted in the homozygous chimeric TGF-β^L1β1/L1β3 mice. Immunostaining revealed no significant changes in the inflammatory response, further implying the central role of TGF-β1 signaling in the healing process of PBM-treated burn wounds [122]. Thus, PBM treatments via TGF-β1 activation is the primary mechanism mediating its wound healing therapeutic responses.

6.2 Cell-type responses to photobiomodulation treatments

Recent studies have observed distinct impacts of PBM on diverse types of cells during the wound healing process. Studies have highlighted PBM treatments to promote the functions of keratinocytes, fibroblasts, macrophages, neutrophils, and endothelial cells [114, 116, 117, 123]. These functions include migration and proliferation of these cells, promoting inflammatory infiltration, macrophage phagocytoses, wound contraction, angiogenesis, fibroblast matrix synthesis, and keratinocyte maturation [111]. Furthermore, a study conducted by Otterço et al. examined the effects of PBM in the kinetics of wound healing responses [114]. The experiment involved 20 male Wistar rats, with 10 rats undergoing treatment utilizing a GaAlAs laser emitting at 670 nm with a continuous pulse and an energy density of 14.28 J/cm² for a consecutive period of 15 days. Histopathological and immunohistochemistry analyses were conducted on the 4th, 11th, and 16th days, whereas the histopathological results revealed significantly lower values of inflammatory infiltrate on the 16th day in the laser group compared to the control. The immunochemistry analysis revealed several mechanistic effects, including reduced levels of tumor necrosis factor TNF-α, increased expression of vascular endothelial growth factor, and higher levels of collagen type 1 fiber with improved organization and arrangement, all of which promoted wound healing in the PBM-treated Wistar rats [114, 124]. Similarly, PBM treatments have recorded modulation of platelet biology within a mouse model of hemophilia, including heightened mechanical rigidity and enhanced activity of clots via improved adhesion of coagulation factors [125]. Although there is a posed risk for thrombosis, the treatments promoted wound healing and presented no signs of disruption to protective coagulation responses [111, 114]. Additionally, it was observed that the combined application of red and infrared (650 nm and 830 nm, respectively) lasers had the most pronounced systemic impact on the healing of skin wounds in athymic mice [126]. Moreover, in cultured human fibroblasts, PBM using a diode laser (red light, 660 nm) was found to stimulate gene expression in various cell adhesion molecules (CAMs) and extracellular proteins in an in vitro wound model and reduced matrix degradation and positively regulated its synthesis [127, 128].

6.3 PBM for oral wounds

Outlined in the mechanism of action, PBM treatments have been documented to directly induce reactive oxygen species that further activate latent transforming growth factors, namely TFG-β1 and TFG-β3. These growth factors have been shown to act on several cell types in a context-dependent manner and appear to play a crucial role in the healing milieu [116, 119]. With an in vivo model of human tooth extraction and socket healing (Figure 4), Arany et al. [120], showcased an augmented expression of TGF-β1 immediately following irradiation with a Gallium Arsenide far-infrared laser at 904 nm, administered at an energy dosage of 3 J/cm². Additionally, immunohistochemistry analysis revealed an elevated expression of TGF-β3 14 days post-treatment. This observation implies that photobiomodulation therapy has the potential to activate latent transforming growth factors, thereby playing a pivotal role in facilitating a quicker healing response [120, 122].

Figure 4.

Clinical PBM treatment being performed with a dental laser on a patient for mucositis.

6.4 PBM for venous ulcers

In a recent double-blind, randomized, placebo-controlled human clinical study, Vitse et al. worked to analyze the efficacy of PBM treatments on chronic venous leg ulcers [129]. The study was designed with 24 test subjects randomized to treatments of PBM or placebo over a period of 12 weeks. The study aimed to compare ulcer size, pain reduction, and the incidence of complete wound closure between the placebo and PBM treatments. All subjects underwent standard wound healing care, encompassing necrotic tissue debridement, application of hydrating wound dressings, daily compression therapy, support stockings, and nutritional guidance. PBM treatments utilized a red laser at a wavelength of 635 nm, comprising three diodes, each with an output power of 17.5 mW. This irradiance was 2.46 mW/cm² during 20-minute sessions, resulting in a total fluence of 2.95 J/cm². Chronic leg ulcers were evaluated at 4 and 12 weeks via digital photography (laser planimetry), arterial blood flow measurements (laser Doppler), a record of pain medications, and an analog pain score (0–100) provided by the subjects [116]. The results revealed a significant reduction in mean ulcer size of 0.46 cm² and mean pain score of 43.54 points between the test and placebo groups [129]. Furthermore, the results indicated a significant reduction in ulcer pain at both 4 and 12 weeks after PBM treatment [116]. These positive outcomes suggest that photobiomodulation holds promise as a viable approach for the long-term treatment of chronic leg ulcers. It is imperative to explore the effects of dosage and wavelength in the modulated healing response to optimize PBM as a robust modality for venous ulcer care.

6.5 PBM for burn wounds

Khan et al. observed the effects of PBM therapy on a modified burn wound model for third-degree burns in C57BL/6 mice [122]. Using a dynamic treatment protocol consisting of an 810 nm laser at 70 mW/cm² for 5 minutes, or a total dose of 21 J/cm², PBM-treated wounds had significantly improved closure and healing responses 9 days post-treatment compared to controls. In wounds subjected to PBM, evident activation of TGF-β1 signaling was indicated through an augmented nuclear localization of phosphor-Smad2/3. This observation implies that the PBM-induced activation of TGF-β signaling may contribute to enhancing the healing and management of burn wounds. Further, they also examined the effects of PBM treatment on wild type TGF-β^L1β1/L1β1 mice and a chimeric knock-in mice model, featuring the replacement of the β1 dimer in the TGF-β1 latency-associated peptide with β3 (TGF-β^L1β1/L1β3). In wild type TGF-β^L1β1/L1β1 mice, PBM was observed to expedite the burn wound healing response, whereas a partial response was observed in the heterozygous chimeric TGF-β^L1β1/L1β3 mice. PBM treatments did not yield significant outcomes in the healing process in the heterozygous chimeric TGF-β^L1β1/L1β3 mice, and immunostaining revealed no significant changes in the inflammatory response, further implying the central role of TGF-β1 signaling in the healing process of PBM treated burn wounds. In summary, near-infrared and red-light treatments have been shown to be prominent biophysical therapeutic modalities. In another double-blind, placebo-controlled trial, Oliveira et al. used low-dose LED therapy (658 nm) was administered to address second and third-degree burn injuries [116]. Following treatment, all participants noted diminished pain and itching, along with reduced inflammatory exudate and fibrin, enhanced re-epithelialization, and well-organized granulation tissue at sites treated with photobiomodulation (PBM) therapy (peak power of 40 mW, beam size of 0.13 cm², and irradiance of 0.31 W/cm²), as compared to control sites on the opposite side.

6.6 PBM for pressure ulcers

Pressure ulcers, often arising from reduced blood flow due to applied pressure on the skin, are characterized by a localized injury to the skin and soft tissue. Quick and effective treatment is crucial to avert potential life-threatening complications, leading to the growing popularity of PBM therapy in managing these injuries. In a randomized, single-blind clinical study, Taradaj et al. worked to evaluate the effects of three common PBM wavelengths on the healing response of pressure ulcers [116]. Their study utilized a total of 71 subjects with stage II and III pressure ulcers (EPUAP scale), categorizing them into four groups: (1) 658 nm (red) laser, (2) 808 nm (NIR) laser, (3) 940 nm (NIR) laser, and (4) placebo. All patients underwent routine wound healing care, comprising daily wound irrigations using a 0.9% physiologic saline solution and application of 1% hydrophilic sulfadiazine cream. Furthermore, participants received adapted footwear, engaged in self-care practices, and preventive measures were implemented to mitigate the risk of disabilities. PBM treatments were performed daily, five times a week over a month, using a GaAlAs diode laser with an average dosage of 4 J/cm², spot size of 0.1 cm², maximum output power of 50 mW, and continuous radiation emission [130]. Ulcers were evaluated using planimetry with an infrared camera to obtain thermographic images and temperature profiles. At 3 months post-treatment, their findings indicated a notable increase in healing rates within the 658 nm treated group, showing a heightened rate of approximately 59%, compared to the 940, 808 nm, and placebo, exhibiting an estimated healing rate of 17% [130]. Furthermore, the 658 nm treated group demonstrated significant efficacy in facilitating wound closure, showcasing a 71% decrease in ulcer size 1-month post-treatment. On the other hand, the 808 nm and 940 nm treated groups did not exhibit a significant improvement in healing rates compared to the placebo (31%, 30%, and 28%, respectively) [116]. These findings suggest that PBM treatments within the red, visible spectrum contribute significantly to the healing process of pressure ulcers.

6.7 PBM in diabetes

Diabetes mellitus (DM) is a complex metabolic disease that compromises the metabolism of carbohydrates and lipids via an inadequate production or sensitivity to insulin [131, 132, 133]. Over the past 50 years, the prevalence of DM has risen at an unsettling rate, increasing from 108 million in 1980 to nearly 422 million in 2014 [133]. DM is associated with chronic hyperglycemia and leads to prolonged complications such as cardiovascular disease, kidney disease, peripheral and autonomic neuropathy, diabetic foot ulcers, and impaired white blood cell activity [131, 132, 133, 134, 135]. Among the complications outlined above, an estimated 25% of individuals with DM face the possibility of developing a diabetic foot ulcer (DFU), carrying an elevated risk for chronic wounds that may lead to infection or gangrene [135, 136]. Hyperglycemia plays a role in the impairment of white blood cells, affecting their ability to migrate and clot, ultimately leading to a delayed wound-healing process [132]. The prolonged healing process and a reoccurrence rate ranging from 40% to 65% have led to approximately 3% of DM patients requiring amputation of the lower limb, as well as a high mortality rate of 75.9% within 3 years [133, 137, 138].

Current approaches to treat and manage diabetic foot ulcers include patient education and prevention, systemic glycemic control, negative pressure therapy, topical growth factors, and local wound care [132, 133, 135]. Conventional wound care of DFUs involves a multidisciplinary team of healthcare professionals that work through a process of debridement, cleaning, antibiotic treatment, and dressing of the ulcer. Methods for debridement include autolytic, biochemical, osmotic, biological, and surgical removal of necrotic or infected tissue [135]. Nevertheless, all the aforementioned treatment methods are ineffective in delivering sufficient care as only a 50% healing rate is attained [135]. These individual treatments are focused on basic wound care and are empirical in nature.

Photobiomodulation has exhibited beneficial outcomes in treating chronic wounds and diabetic foot ulcers. In studies involving wound healing models, PBM (laser—InGaAlP) at 660 nm demonstrated the ability to expedite collagen production and increase the overall percentage of type III collagen in diabetic animals [139]. Numerous in vivo and in vitro studies have substantiated the promising effects of PBM, such as Maiya et al., who examined the impacts of PBM at a 632.8 nm visible wavelength with a fluence of 4.8 J/cm² administered 5 days per week on the healing of diabetic wounds in alloxan-induced diabetic animals [140]. Examination of the histopathological and biochemical characteristics of the wounds exhibited an accelerated and improved healing process from PBM treatments. On average, laser-treated groups were healed within 3 weeks, while the control groups required over 8 weeks to heal [140]. In a literature review of 13 studies conducted over the last 20 years, the most effective PBM features for healing DFUs include red light with wavelengths ranging from 630 to 660 nm, or infrared light at 850 to 890 nm, and an irradiance between 3 and 7 J/cm² [134]. In a challenging recent case of Fournier’s syndrome that involves necrotizing gangrene of perineum and external genitalia, adjunctive use of 660 and 810 nm laser PBM treatment was noted to be extremely effective in managing these severe wounds (Figure 5). It should be emphasized that in this particular case of necrotizing fasciitis, infection control is of paramount importance and the use of PBM not only improves host immune defenses (via Human βDefensin-2 secretion), but also promotes the surrounding epithelium and underlying connective tissue to be effectively stimulated to heal these wounds [141, 142, 143].

Figure 5.

Clinical progression of an ulcer caused by Fournier’s syndrome treated with photobiomodulation therapy. The images chronologically depict the improvement in the ulcerated lesion over the course of 40 days of treatment, starting from day 1. Laser diodes at wavelengths of 660 and 880 nm were used to perform PBM three times per week at a fluence of 20 J/cm². A. Shows the initial presentation of the ulcer (yellow-brown) on day 1, prior to treatment initiation. B. to H. Demonstrate the gradual healing process, with a reduction in inflammation, wound size, and tissue damage observed over time with continued lase therapy sessions. The final image at day 40 exhibits significant healing and re-epithelialization of the affected areas, indicating the efficacy of the PBM in managing this case of Fournier’s syndrome, treatments are currently continuing.

Several studies have elucidated the mechanistic pathways and reactions associated with photobiomodulation in diabetic wounds. In an investigation involving a 660 nm laser at treatments of 5 J/cm² in diabetic wounded cells, a notable elevation in epidermal growth factor (EGF) and activation of its phosphorylated EGF receptor was observed. This activation was mediated through the Janus kinase (JAK)/Signal transducer and activators of the transcription (STAT) pathway. The activation of this pathway was identified to modulate cell migration and proliferation, indicating that PBM treatments directly affect cellular autocrine signaling [133]. Diabetic wounded fibroblast cells irradiated with a Helium-Neon laser at a wavelength of 632.8 nm with an energy density of 5 J/cm² corresponded to a rise in the levels of IL-6, an anti-inflammatory cytokine, and contributed to increased cellular proliferation and migration. In a follow-up study conducted by Houreld et al. diabetic wounded fibroblasts subjected to photobiomodulation irradiation at a dosage of 5 J/cm² and a wavelength of 830 nm exhibited a favorable impact on wound healing [144]. This effect was characterized by the reduced expression of pro-inflammatory cytokines IL-1β and TNF-α, as well as the stimulation of nitric oxide and reactive oxygen species [131, 133].

Among clinical trials conducted utilizing lasers within the red and near infrared wavelength range, Minatel et al. investigated the effects of 660 and 890 nm laser treatment in a double-blind randomized study with diabetic patients suffering from chronic leg ulcers [145]. PBM treatments were administered to the wounds twice a week, for a length of 90 days. The results have demonstrated that combined 660 and 890 nm light accelerated tissue granulation, improving the healing rate of diabetic foot ulcers that did not show improvement with other forms of intervention. A subsequent clinical trial conducted by Kaviani et al. involved 23 patients diagnosed with stages I and II chronic diabetic ulcers [146]. The ulcers were subjected to laser treatment at a wavelength of 685 nm with a dose of 10 J/cm². The findings indicated a notable reduction in ulcer size in 4 weeks post-treatment, with a substantial proportion of patients exhibiting complete wound healing. As already mentioned by this group, the initial benefits of PBM (low-dose light treatments), were primary noted in the promotion of wound healing. Subsequently, well-designed controlled human studies have provided increasing evidence supporting the effectiveness of this specific application [116], for example the use of PBM in the management of chronic diabetic foot ulcers (DFUs), which diminishes the inflammatory phase, enhances angiogenesis, boosts blood circulation, refines the synthesis and organization of the extracellular matrix, and alleviates pain and infection [147]. Furthermore, a study carried out with 68 patients with chronic diabetic foot ulcers (DFU) showed a significant reduction in the area (mm²) of ulcers in the treated group compared to the control, after 15 days of daily therapy [148]. Also, a meta-analysis which evaluated 13 randomized controlled trials (RCTs) and a total of 413 patients. In comparison with the control group, PBM significantly increased the complete healing rate, reduced the ulcer area, and shortened the mean healing time of patients with DFUs [149]. Nevertheless, although PBM shows promise as an effective adjuvant treatment for expediting diabetic wound healing, additional evidence from larger sample sizes and higher-quality RCTs is essential to establish the efficacy of PBM and determine the appropriate parameters for optimal wound healing.

Wounds arising from diabetes mellitus impose considerable socioeconomic burdens on affected individuals, contributing to heightened amputations, mortality rates and financial expenditures. Wound healing is a complex pathophysiological process that involves a variety of biological and chemical pathways. Conventional treatments adjoined with the use of low-level light therapy have shown promising efficacy in the therapeutic intervention of diabetic wounds. Photobiomodulation appears to be a viable, non-invasive, and safe alternative method for treating chronic diabetic foot ulcers. Several studies have illustrated evidence behind PBM and the effects it has on diabetic wounds. Continued exploration into the biochemical and cellular responses of photobiomodulation therapy can help translate this innovative technology into clinical applications for the effective management of diabetic lesions.

7. Conclusions

Non-healing wounds impose a substantial burden on both the healthcare system and the quality of life for affected patients. The pathogenesis of these wounds involves various factors, yet a common denominator is the consistent lack of adequate vascularization, leading to insufficient delivery of oxygen, nutrients, and growth factors to the site of the injury [150].

7.1 Sequence of different laser-based treatment modalities: PTT vs. PDT vs. PBM

A novel approach stemming from the confluence of several light-based treatment modalities termed as photoimmunotherapy is one of the recent advances focused on wound healing through the modulation of immune responses [19]. Utilizing near-infrared-induced hyperthermia to enhance the efficacy of antitumor treatments has been recognized for its ability to boost local perfusion and enhance immune reactivity, a concept also referred to as photoimmunotherapy. These approaches seem to leverage both photothermal effects and potentially photobiomodulation (nitric oxide generation), alongside non-thermal vasodilation at the tissue level, in order to alleviate hypoxia. This, in turn, improves ionizing radiation-induced tumor cytotoxicity or facilitates better tumor access to chemotherapy or immunotherapy agents [151]. Photoimmunotherapy can be accomplished through different light treatment approaches, encompassing photothermal destruction, nonspecific or targeted photodynamic responses, and Photobiomodulation therapy, which directly modulate host immune mechanisms following the antigenic stimulation induced by photothermal and photodynamic approaches. The distinct biological responses induced by light provide a strong mechanistic basis for their combined use, such as the sequential process of eliminating microbial or tumor cells followed by the stimulation of host immune responses [19].

Besides technical similarities, there are several clinically relevant differences between PTT and PDT. Firstly, due to photosensitizer accumulation and light targeting, PDT is considerably more selective than PTT. Furthermore, due to its non-thermal nature, PDT is harmless to surrounding collagenous structures and nerves. Nevertheless, in certain cases, the accumulation of ocular and cutaneous photosensitizers can lead to photosensitivity, posing challenges in treatment logistics and potentially compromising safety. In contrast to PDT, the selectivity of PTT is predominantly dependent upon localized light delivery, inevitably resulting in the formation of a temperature gradient. Another essential difference between PTT and PDT is the light source used. Additionally, oxygen requirement is a crucial point of distinction between PTT and PDT. Effective PDT requires functional vasculature and a constant supply of oxygen for the generation of reactive oxygen species. Contrastingly, PTT is essentially oxygen-independent, making it an appropriate option for treating hypoxic tumors [152].

Biophotonics seamlessly combines lasers, photonics, nanotechnology, and biotechnology, offering a novel perspective for theragnostic (diagnostic and therapeutic) applications [153, 154]. The various approaches outlined in this chapter highlight the significant utility in wound management. Synergistic and complementary modalities offer novel, cost-effective, and clinically efficacious treatments that could be employed actively.