The nuts and bolts of low-level laser (light) therapy

The Nuts and Bolts of Low-Level Laser (Light) Therapy

Hoon Chung 1,2, Tianhong Dai1,2, Sulbha K Sharma1, Ying-Ying Huang1,2,3, James D Carroll4, Michael R Hamblin1,2,5

Author Information 

1 Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA, USA
2 Department of Dermatology, Harvard Medical School, Boston, MA, USA
3 Aesthetic and Plastic Center of Guangxi Medical University, Nanning, People’s Republic of China
4 Thor Photomedicine Ltd, 18A East Street, Chesham HP5 1HQ, UK
5 Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA, USA
Address correspondence to Michael R. Hamblin, Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA, USA. ude.dravrah.hgm.xileh@nilbmah

PMID: 22045511  DOI: 10.1007/s10439-011-0454-7

The publisher’s final edited version of this article is available at Ann Biomed Eng
See other articles in PMC that cite the published article.


Soon after the discovery of lasers in the 1960s, it was realized that laser therapy had the potential to improve wound healing and reduce pain, inflammation, and swelling. In recent years the field sometimes known as photobiomodulation has broadened to include light-emitting diodes and other light sources, and the range of wavelengths used now includes many in the red and near-infrared. The term “low-level laser therapy” or LLLT has become widely recognized and implies the existence of the biphasic dose-response or the Arndt-Schulz curve. This review will cover the mechanisms of action of LLLT at a cellular and at a tissular level and will summarize the various light sources and principles of dosimetry that are employed in clinical practice. The range of diseases, injuries, and conditions that can be benefited by LLLT will be summarized with an emphasis on those that have reported randomized controlled clinical trials. Serious life-threatening diseases such as stroke, heart attack, spinal cord injury, and traumatic brain injury may soon be amenable to LLLT therapy. 

Keywords: Low-level laser therapy, Photobiomodulation, Mitochondria, Tissue optics, Wound healing, Hair regrowth, Laser acupuncture

Introduction & History 

Low-level laser therapy (LLLT), also known as photobiomodulation, came into being in its modern form soon after the invention of the ruby laser in 1960, and the helium-neon (HeNe) laser in 1961. In 1967, Endre Mester, working at Semmelweis University in Budapest, Hungary, noticed that applying laser light to the backs of shaven mice could induce the shaved hair to grow back more quickly than in unshaved mice.72 He also demonstrated that the HeNe laser could stimulate wound healing in mice.70 Mester soon applied his findings to human patients, using lasers to treat patients with nonhealing skin ulcers.69,71 LLLT has now developed into a therapeutic procedure that is used in three main ways: to reduce inflammation, edema, and chronic joint disorders9,18,40; to promote healing of wounds, deeper tissues, and nerves24,87; and to treat neurological disorders and pain.17

LLLT involves exposing cells or tissue to low levels of red and near-infrared (NIR) light and is referred to as “low level” because of its use of light at energy densities that are low compared to other forms of laser therapy that are used for ablation, cutting, and thermally coagulating tissue. LLLT is also known as “cold laser” therapy as the power densities used are lower than those needed to produce heating of tissue. It was originally believed that LLLT or photobiomodulation required the use of coherent laser light, but more recently, light-emitting diodes (LEDs) have been proposed as a cheaper alternative. A great deal of debate remains over whether the two light sources differ in their clinical effects.

Although LLLT is now used to treat a wide variety of ailments, it remains controversial as a therapy for two principal reasons: first, its underlying biochemical mechanisms remain poorly understood, so its use is largely empirical. Second, a large number of parameters such as the wavelength, fluence, power density, pulse structure, and timing of the applied light must be chosen for each treatment. A less than optimal choice of parameters can result in reduced effectiveness of the treatment or even a negative therapeutic outcome. As a result, many of the published results on LLLT include negative results simply because of an inappropriate choice of light source and dosage. This choice is particularly important as there is an optimal dose of light for any particular application, and doses higher or lower than this optimal value may have no therapeutic effect. In fact, LLLT is characterized by a biphasic dose-response: lower doses of light are often more beneficial than high doses.38,85,105,108

Laser-Tissue Interactions 

Light is part of the spectrum of electromagnetic radiation (ER), which ranges from radio waves to gamma rays. ER has a dual nature as both particles and waves. As a wave which is crystallized in Maxwell’s Equations, light has amplitude, which is the brightness of the light, wavelength, which determines the color of the light, and an angle at which it is vibrating, called polarization. The wavelength (λ) of light is defined as the length of a full oscillation of the wave, such as shown in Fig. 1a. In terms of the modern quantum theory, ER consists of particles called photons, which are packets (“quanta”) of energy that move at the speed of light. In this particle view of light, the brightness of the light is the number of photons, the color of the light is the energy contained in each photon, and four numbers (X, Y, Z, and T) are the polarization, where X, Y, Z are the directions and T is the time.

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term “laser” originated as an acronym for light amplification by stimulated emission of radiation.65 The emitted laser light is notable for its high degree of spatial and temporal coherence.

Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called “pencil beam.” The laser can be launched into a beam of very low divergence to concentrate its power at a large distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. Lasers are employed in applications where the light of the required spatial or temporal coherence could not be produced using simpler technologies.

Quite often, the laser beam is described as though it had a uniform irradiance (the power of the laser divided by the spot size). Most often, the laser beam assumes a Gaussian shape (that of a normal distribution), as shown in Fig. 1b.118 There is a peak irradiance, and the irradiance decreases with distance from the center of the beam. This may be important in situations in which there are large variations in power. As power is increased, the irradiance in the tail of the Gaussian profile increases, and the distance of the critical threshold from the center of the beam becomes larger. For this type of profile, the spot size is often referred to as the 1/e2 radius, or diameter, of the beam; at this radial distance from the center of the beam, irradiation is lower by a factor of 0.135 (1/e2) relative to the peak irradiance. About 85% of the power of the laser beam is present within the 1/e2 diameter.

Light Emitting Diodes (LED)
A light-emitting diode (LED) is a semiconductor light source. Introduced as a practical electronic component in 1962 early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting diode is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern.78

Cellular and Tissular Mechanisms of LLLT

The precise biochemical mechanism underlying the therapeutic effects of LLLT is not yet well-established. From observation, it appears that LLLT has a wide range of effects at the molecular, cellular, and tissular levels. In addition, its specific modes of action may vary among different applications. Within the cell, there is strong evidence to suggest that LLLT acts on the mitochondria27 to increase adenosine triphosphate (ATP) production,43 modulations of reactive oxygen species (ROS), and the induction of transcription factors.15 Several transcription factors are regulated by changes in cellular redox state. Among them redox factor-1 (Ref-1) dependent activator protein-1 (AP-1) (a heterodimer of c-Fos and c-Jun), nuclear factor kappa B (NF-κB), p53, activating transcription factor/cAMP-response element-binding protein (ATF/CREB), hypoxia-inducible factor (HIF)-1, and HIF-like factor.15 These transcription factors then cause protein synthesis that triggers further effects down-stream, such as increased cell proliferation and migration, modulation in the levels of cytokines, growth factors and inflammatory mediators, and increased tissue oxygenation.45 Figure 2 shows the proposed cellular and molecular mechanisms of LLLT.

Immune cells, in particular, appear to be strongly affected by LLLT. Mast cells, which play a crucial role in the movement of leukocytes, are of considerable importance in inflammation. Specific wavelengths of light are able to trigger mast cell degranulation,22 which results in the release of the pro-inflammatory cytokine TNF-a from the cells.115 This leads to increased infiltration of the tissues by leukocytes. LLLT also enhances the proliferation, maturation, and motility of fibroblasts, and increases the production of basic fibroblast growth factor.31,67 Lymphocytes become activated and proliferate more rapidly, and epithelial cells become more motile, allowing wound sites to close more quickly. The ability of macrophages to act as phagocytes is also enhanced under the application of LLLT.

At the most basic level, LLLT acts by inducing a photochemical reaction in the cell, a process referred to as biostimulation or photobiomodulation. When a photon of light is absorbed by a chromophore in the treated cells, an electron in the chromophore can become excited and jump from a low-energy orbit to a higher-energy orbit.42,108 This stored energy can then be used by the system to perform various cellular tasks. There are several pieces of evidence that point to a chromophore within mitochondria being the initial target of LLLT. Radiation of tissue with light causes an increase in mitochondrial products such as ATP, NADH, protein, and RNA,83 as well as a reciprocal augmentation in oxygen consumption, and various in vitro experiments have confirmed that cellular respiration is upregulated when mitochondria are exposed to a HeNe laser or other forms of illumination.

The relevant chromophore can be identified by matching the action spectra for the biological response to light in the NIR range to the absorption spectra of the four membrane-bound complexes identified in mitochondria.42 This procedure indicates that complex IV, also known as cytochrome c oxidase (CCO), is the crucial chromophore in the cellular response to LLLT.44 CCO is a large transmembrane protein complex, consisting of two copper centers and two heme–iron centers, which is a component of the respiratory electron transport chain.10 The electron transport chain passes high-energy electrons from electron carriers through a series of transmembrane complexes (including CCO) to the final electron acceptor, generating a proton gradient that is used to produce ATP. Thus, the application of light directly influences ATP production by affecting one of the transmembrane complexes in the chain: in particular, LLLT results in increased ATP production and electron transport.47,84

The precise manner in which light affects CCO is not yet known. The observation that NO is released from cells during LLLT has led to speculation that CCO and NO release are linked by two possible pathways (Fig. 3). It is possible that LLLT may cause photodissociation of NO from CCO.46,52 Cellular respiration is downregulated by the production of NO by mitochondrial NO synthase (mtNOS, a NOS isoform specific to mitochondria), that binds to CCO and inhibits it. The NO displaces oxygen from CCO, inhibiting cellular respiration and thus decreasing the production of ATP.5 By dissociating NO from CCO, LLLT prevents this process from taking place and results in increased ATP production. An alternative or parallel mechanism to explain the biological activity of red or NIR light to release NO from cells or tissue is the following.61,127 A new explanation has been recently proposed for how light increases NO bioavailability. 88 CCO can act as a nitrite reductase enzyme (a one-electron reduction of nitrite gives NO) particularly when the oxygen partial pressure is low.6 Ball et al. showed 590 ± 14 nm LED light stimulated CCO/NO synthesis at physiological nitrite concentrations at hypoxia condition.6

The influence of LLLT on the electron transport chain extends far beyond simply increasing the levels of ATP produced by a cell. Oxygen acts as the final electron acceptor in the electron transport chain and is, in the process, converted to water. Part of the oxygen that is metabolized produces reactive oxygen species (ROS) as a natural by-product. ROS are chemically active molecules that play an important role in cell signaling, regulation of cell cycle progression, enzyme activation, and nucleic acid and protein synthesis. Because LLLT promotes the metabolism of oxygen, it also acts to increase ROS production. In turn, ROS activates transcription factors, which leads to the upregulation of various stimulatory and protective genes. These genes are most likely related to cellular proliferation,76 migration,32 and the production of cytokines and growth factors, which have all been shown to be stimulated by low-level light.125,128

The processes described above are almost certainly only part of the story needed to explain all the effects of LLLT. Among its many effects, LLLT has been shown to cause vasodilation by triggering the relaxation of smooth muscle associated with endothelium, which is highly relevant to the treatment of joint inflammation. This vasodilation increases the availability of oxygen to treated cells, and also allows for greater traffic of immune cells into the tissue. These two effects contribute to accelerated healing. NO is a potent vasodilator via its effect on cyclic guanine monophosphate production, and it has been hypothesized that LLLT may cause photodissociation of NO, not only from CCO but from intracellular stores such as nitrosylated forms of both hemoglobin and myoglobin, leading to vasodilation.61

Light Sources & Dosimetry

Currently, one of the biggest sources of debate in the choice of light sources for LLLT is the choice between lasers and LEDs. LEDs have become wide-spread in LLLT devices. Most initial work in LLLT used the HeNe laser, which emits light of wavelength 632.8-nm, while nowadays semi-conductor diode lasers such as gallium arsenide (GaAs) lasers have increased in popularity. It was originally believed that the coherence of laser light was crucial to achieving the therapeutic effects of LLLT, but recently this notion has been challenged by the use of LEDs, which emit non-coherent light over a wider range of wavelengths than lasers. It has yet to be determined whether there is a real difference between laser and LED, and if it indeed exists, whether the difference results from the coherence or the monochromaticity of laser light, as opposed to the non-coherence and wider bandwidth of LED light.

Future development in LLLT devices will be the use of organic light-emitting diodes (OLEDs). These are LEDs in which the emissive electroluminescent layer is a film of organic compounds that emit light in response to an electric current.122 They operate in a similar manner to traditional semiconductor material whereby electrons and the holes recombine forming an exciton. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by the emission of radiation whose frequency is in the visible region.

The wavelengths of light used for LLLT fall into an “optical window” at red and NIR wavelengths (600–1070 nm) (Fig. 1d). Effective tissue penetration is maximized in this range, as the principal tissue chromophores (hemoglobin and melanin) have high absorption bands at wavelengths shorter than 600 nm. Wavelengths in the range 600–700 nm are used to treat superficial tissue, and longer wavelengths in the range 780–950 nm, which penetrate further, are used to treat deeper-seated tissues. Wavelengths in the range 700–770 nm have been found to have limited biochemical activity and are therefore not used. There are also reports of the effectiveness of wavelengths outside the range of absorption of NIR light by CCO. These wavelengths are in the near IR,36 the mid-IR region including carbon dioxide laser (10.6 μm)126, and also include broadband IR sources in the 10–50 μm range.39 The chromophore in these situations is almost certainly water, possibly present in biological membranes in some nanostructured form, that is different from bulk water allowing biological effects without gross heating of the tissue.94,95 It is at present not clear at which wavelength CCO absorption ceases and water absorption commences to be important.

The power of light used typically lies in the range of 1–1000 mW and varies widely depending on the particular application. There is evidence to suggest that the effectiveness of the treatment varies greatly on both the energy and power density used: there appear to be upper and lower thresholds for both parameters between which LLLT is effective. Outside these thresholds, the light is either too weak to have any effect, or so strong that its harmful effects outweigh its benefits.

Response to LLLT changes with wavelength, irradiance, time, pulses, and maybe even coherence and polarization, the treatment should cover an adequate area of the pathology, and then there is a matter of how long to irradiate for.

Dosimetry is best described in two parts,

  1. Irradiation parameters (“the medicine”) see Table 1
  2. Time/energy/fluence delivered (“the dose”) see Table 2

Dosimetry in LLLT is highly complicated. The large of a number of interrelated parameters (see Table 1) has meant that there has not yet been a comprehensive study reported that examined the effect of varying all the individual parameters one by one, and it must be pointed out that it is unlikely there will ever be such a study carried out. This considerable level of complexity has meant that the choice of parameters has often depended on the experimenter’s or the practitioner’s personal preference or experience rather than on a consensus statement by an authoritative body. Nevertheless, the World Association of Laser Therapy (WALT) has attempted to provide dosage guidelines (http://www.walt.nu/dosage-recommendations.html).

Survey of Conditions Treated with LLLT

LLLT is used for three main purposes: to promote wound healing, tissue repair, and the prevention of tissue death; to relieve inflammation and edema because of injuries or chronic diseases; and as an analgesic and a treatment for other neurological problems. These applications appear in a wide range of clinical settings, ranging from dentistry to dermatology, to rheumatology and physiotherapy. Table 3 summarizes some of the published studies in animal models of diseases and conditions treated with LLLT. Table 4 summarizes some of the published clinical trials of LLLT.

Wound healing was one of the first applications of LLLT, when HeNe lasers were used by Mester et al. to treat skin ulcers.69-71 LLLT is believed to affect all three phases of wound healing111: the inflammatory phase, in which immune cells migrate to the wound, the proliferative phase, which results in increased production of fibroblasts and macrophages, and the remodeling phase, in which collagen deposition occurs at the wound site and the extra-cellular matrix is rebuilt.

LLLT is believed to promote wound healing by inducing the local release of cytokines, chemokines, and other biological response modifiers that reduce the time required for wound closure and increase the mean breaking strength of the wound.8,32,73 Proponents of LLLT speculate that this result is achieved by increasing the production and activity of fibroblasts and macrophages, improving the mobility of leukocytes, promoting collagen formation, and inducing neovascularization. 31,60,67,80,90,104

However, there is a lack of convincing clinical studies that either prove or disprove the efficacy of LLLT in wound healing. The results that are currently available are conflicting and do not lead to any clear conclusions. For example, Abergel et al. found that the 632.8 nm HeNe laser did not have any effect on the cellular proliferation of fibroblasts, while the 904 nm GaAs laser actually lowered fibroblasts proliferation.1 In contrast, other studies noted an increase in proliferation of human fibroblasts exposed to 904 nm GaAs lasers,85 rat myofibroblasts exposed to 670 nm GaAs lasers,67, and gingival fibroblasts exposed to diode lasers (670, 692, 780, and 786 nm).3 In vivo studies in both animal and human models show similar discrepancies. A study by Kana et al. claimed that the treatment of open wounds in rats with HeNe and argon lasers resulted in faster wound closure.41 Bisht et al. found a similar increase in granulation tissue and collagen expression in rats using the same treatment as Kana.7 However, Anneroth et al. failed to observe any beneficial effects after laser treatment in a comparable rat model.4 In human studies, Schindl et al. reported that application of a HeNe laser was beneficial in promoting wound healing in 3 patients,99 whereas Lundeberg et al. found no statistically significant difference between leg ulcer patients treated with a HeNe laser and those treated with a placebo.62

The scarcity of well-designed clinical trials makes it difficult to assess the impact of LLLT on wound healing. Our task is further complicated by the difficulty in comparing studies, because of the large number of factors involved. In addition to the multiple parameters that must be adjusted to apply LLLT, such as the wavelength and power of the light, the effectiveness of the treatment also depends on many factors such as the location and nature of the wound, and the physiologic state of the patient. For example, impaired wound healing is one of the major chronic complications of diabetes,25,89, and is thought to result from various factors, including decreased collagen production and impaired functionality of fibroblasts, leukocytes, and endothelial cells.25,106 It has therefore been hypothesized that LLLT could have beneficial effects in stimulating wound healing in diabetic patients.98,100,124 Thus, in order to obtain a convincing verdict on the impact of LLLT on wound healing, we will require several large, randomized, placebo-controlled, and double-blind trials that compare the effects of LLLT on wounds that are as similar as possible. A greater understanding of the cellular and biochemical mechanisms of LLLT would also be useful in assessing these studies, as it would enable us to pinpoint exactly what criteria to use in determining the effectiveness of the therapy.

There appears to be more firm evidence to support the success of LLLT in alleviating pain and treating chronic joint disorders, than in healing wounds. A review of 16 randomized clinical trials including a total of 820 patients found that LLLT reduces acute neck pain immediately after treatment, and up to 22 weeks after completion of treatment in patients with chronic neck pain.17 LLLT has also been shown to relieve pain because of cervical dentinal hypersensitivity,93, or from periodontal pain during orthodontic tooth movement. 114 A study of 88 randomized controlled trials indicated that LLLT can significantly reduce pain and improve health in chronic joint disorders such as osteoarthritis, patellofemoral pain syndrome, and mechanical spine disorders.9 However, the authors of the study urge caution in interpreting the results because of the wide range of patients, treatments, and trial designs involved.

LLLT is also being considered as a viable treatment for serious neurological conditions such as traumatic brain injury (TBI), stroke, spinal cord injury, and degenerative central nervous system disease.

Although traumatic brain injury is a severe health concern, the search for better therapies in recent years has not been successful. This has led to an interest in more radical alternatives to existing procedures, such as LLLT. LLLT is hypothesized to be beneficial in the treatment of TBI. In addition to its effects in increasing mitochondrial activity and activating transcription factors, LLLT could benefit TBI patients by inhibiting apoptosis, stimulating angiogenesis, and increasing neurogenesis.29 Experiments carried out with two mouse models indicated that LLLT could reduce the brain-damaged area at 3 days after treatment, and treatment with a 665 nm and 810 nm laser could lead to a statistically significant difference in the Neurological Severity Score (NSS) of mice that had been injured by a weight being dropped onto the exposed skull.121

Transcranial LLLT has also been shown to have a noticeable effect on acute human stroke patients, with significantly greater improvement being seen in patients 5 days after LLLT treatment compared to sham treatment (p < 0.05, National Institutes of Health Stroke Severity Scale.)51 This difference persisted up to 90 days after the stroke, with 70% of patients treated with LLLT having a successful outcome compared to 51% of control patients. The improvement in the functional outcome because of applying transcranial LLLT after a stroke has been confirmed by studies in rat and rabbit models.54,81

Further experiments have tried to pinpoint the mechanism underlying these results. As expected, increased mitochondrial activity has been found in brain cells irradiated with LLLT,54 indicating that the increased respiration and ATP production that usually follows laser therapy is at least partly responsible for the improvement shown in stroke patients. However, there is still the possibility that LLLT has other effects specific to the brain. Several groups have suggested that improvements in patient outcomes are because of the promotion of neurogenesis, and the migration of neurons.81 This hypothesis is supported by the fact that the benefits of LLLT following a stroke may take 2–4 weeks to manifest, reflecting the time necessary for new neurons to form and gather at the damaged site in the brain.21,101 However, the exact processes underlying the effects of LLLT in a stroke patient are still poorly understood.

LLLT has also been considered as a candidate for treating degenerative brain disorders such as familial amyotrophic lateral sclerosis (FALS), Alzheimer’s disease, and Parkinson’s disease (PD).75,129 Although only preliminary studies have been carried out, there are encouraging indications that merit further investigation. Michalikova et al. found that LLLT could reverse memory degradation and induce improved cognitive performance in middle-aged mice,74 and Trimmer et al. found that motor function was significantly improved in human patients treated with LLLT in an early stage of FALS.112

Conclusion & Outlook 

Advances in the design and manufacturing of LLLT devices in the years to come will continue to widen the acceptability and increase adoption of the therapy among the medical profession, physical therapists, and the general public. While the body of evidence for LLLT and its mechanisms is still weighted in favor of lasers and directly comparative studies are scarce, ongoing work using non-laser irradiation sources is encouraging and provides support for growth in the manufacture and marketing of affordable home-use LED devices. The almost complete lack of reports of side effects or adverse events associated with LLLT gives security for issues of safety that will be required.

We believe that LLLT will steadily progress to be better accepted by both the medical profession and the general public at large. The number of published negative reports will continue to decline as the optimum LLLT parameters become better understood, and as reviewers and editors of journals become aware of LLLT as a scientifically-based therapy. On the clinical side, the public’s distrust of big pharmaceutical companies and their products is also likely to continue to grow. This may be a powerful force for the adoption of therapies that once were considered as “alternative and complementary,” but now are becoming more scientifically accepted. LLLT is not the only example of this type of therapy, but needle acupuncture, transcranial magnetic stimulation, and microcurrent therapy also fall into this class. The day may not be far off when most homes will have a light source (most likely a LED device) to be used for aches, pains, cuts, bruises, joints, and which can also be applied to the hair and even transcranially to the brain.