Since the first practical demonstration of a laser by Theodore Maiman in 1960 , the range of applications has heavily increased. With improvements in production as well as performance, diode lasers also became increasingly attractive. Due to direct electrical pumping, diode lasers are by far the most efficient light sources currently available [2, 3]. Being based on chip technology, they can be manufactured in high numbers and at low cost. Their dimensions of only a few mm3 enable very compact laser systems. All these features increase their application potential, including biomedical applications. Applications range from imaging and diagnostics, e.g., optical coherence tomography , fluorescence lifetime imaging , diffuse optical imaging , THz imaging , laser Doppler imaging  or Raman spectroscopy, to direct treatment such as photocoagulation , photo-dynamic therapy  or biomodulation and bioactivation .
Compared to lasers limited to specific atomic transitions, diode lasers cover a much wider spectral range. Depending on the used compound semiconductors and their composition, the emission wavelengths of typical III-V compound semiconductors range from blue to near-infrared (400 nm – 2 µm, ). Although spectral side modes are sufficiently suppressed at higher currents , the application of diode lasers may be limited by their spectral characteristics. In these cases, the emission bandwidth can be narrowed, e.g., by intrinsic  or external feedback . The latter also enables single-mode emission tunable over several tens of nanometers , in addition to the tunability obtained by adjusting the injection current or the laser temperature.
Despite the number of wavelengths that can be accessed with diode lasers, the output power might not be sufficient. In addition, other wavelengths, especially in the visible range, may not be achievable due to lack of available laser structures. One option also to achieve these wavelengths, or to increase the output power at a certain spectral region, is nonlinear frequency conversion , as discussed in this article. Other options are optically pumped semiconductor lasers  or solid-state lasers , although not covered in the present review. Due to the optical excitation these lasers show reduced optical efficiencies compared to electrically pumped diode lasers .
The output power and the beam propagation parameters (M2) of the diode lasers strongly depend on the design of the semiconductor structures. Nearly diffraction-limited beams are obtained with ridge-waveguide (RW) and tapered diode lasers. While the output power of RW lasers is limited to 1–2 W , more than 10 W are achieved with tapered lasers . High-power emission is also obtained with broad area (BA) diode lasers  or diode laser bars and stacks . However, these devices typically show reduced beam qualities that may be improved by additional feedback .
All these devices are edge emitting diode lasers, i.e., the propagation of the generated laser emission is in plane with the substrate surface . In surface emitting diode lasers, also known as vertical cavity surface emitting lasers (VCSEL), the propagation direction is normal to the substrate surface. Their optical cavities are short and the facet reflectivities are high, resulting in low threshold currents . The output power is typically in the milliwatt range but can be increased significantly by optical pumping in so-called vertical external cavity surface emitting lasers (VECSEL) . In comparison to edge-emitting diode lasers described in this article, the challenging factor towards high-power, near diffraction-limited emission from VCSELs is the proper heat removal from the active region .
In addition to continuous wave (CW) emission, diode lasers may also be operated in pulsed mode. Pulsed emission is achieved by mode-locking , Q-switching  or in a more direct manner by gain-switching . These techniques enable the generation of pico- to femtosecond pulses with repetition rates in the GHz range. Compared to other mode-locked or Q-switched lasers, the lower upper-state lifetime of nanoseconds  reduces the obtained pulse energies of diode lasers . However, generated pulses with up to 50 W peak power  are more than sufficient for applications such as fluorescence measurements, which will be discussed in the present review.
The above-mentioned characteristics, i.e., output power, beam properties, wavelength spectral coverage and tunability, compactness and low cost, make diode laser technology versatile and increasingly applicable in the biomedical field, in particular. In this review, we provide an overview of state-of-the-art edge emitting diode lasers and their use within key biomedical applications. At the end we give an outlook on the future perspective of diode lasers for emerging applications within the biomedical field.
2. Edge-emitting diode lasers
Two key advantages of diode lasers are their capability of covering a wide spectral range and the possibility of realizing different layouts with individual features. In this section we introduce the required material structures and explain how diode lasers are built up. At the end we focus on the performance of diode lasers with respect to the applications discussed in the article.
2.1. Material structures and fabrication of diode lasers
Several compound semiconductors have to be applied in order to cover the above mentioned spectral range between 400 nm and 2 µm. A coarse selection of the laser wavelength λ can be performed by adjusting the composition of the material, later given as molfraction x, y, z. An overview on the available group III-V-compound materials is shown in Fig. 1. In the blue to green spectral range InxAlyGa1–x–yN material is used [36, 37]. Red emitting diodes between 615 nm and 750 nm are based on InxAlyGa1-x-yP. Between 670 nm and 890 nm AlxGa1-xAs1-yPy is used as an active layer. Longer wavelengths up to 1.2 µm can be reached using InxGa1-xAsyP1-y or In1-x-yAlxGayAs, grown on GaAs [38, 39]. Grown on InP substrates, the latter materials can cover the range up to 2.3 µm. Even longer wavelengths can be addressed by using antimonide based layer structures, lead salt lasers or more recently quantum cascade lasers.
The basic design of typical vertical layer structures is given in Fig. 2. The active layer consists of one or more quantum wells or quantum dots. This layer is embedded into a p- and n-side waveguide, which is surrounded by cladding layers. The p-side is completed by a highly doped contact layer. The layer structures are typically grown by metal organic vapour phase epitaxy (MOVPE) [40–44] or molecular beam epitaxy (MBE) [45, 46] on different substrates with a diameter between 2–4 inch.
The wafers are processed into laser devices applying different lithographic, etching and plating technologies. Typically, a first lithography defines the stripe width along which the light is guided through the device. Two mechanisms can contribute to this guiding, index-guiding and gain-guiding. In order to achieve index-guiding, a ridge can be etched into the p-side waveguide layer as shown in Fig. 3a. The deep etching causes a step in the refractive index leading to lateral confinement. Typical RW lasers provide nearly diffraction-limited beam quality at output powers in the lower watt-range. For gain-guiding, typical for BA lasers, a conductive stripe is defined in the contact layer (Fig. 3b). This is done by destroying the conductivity outside the stripe using ion implantation or by etching a low MESA structure. Hereby the carrier injection and laser emission are limited to this area. The stripe width can be in the range from some 10 µm up to 800 µm. Such BA lasers reach significantly higher output powers up to some 10 W, but suffer from poor beam qualities with M2 values typically in the range 10–50. To reach even higher output power several emitters can be combined within one laser bar (Fig. 3c), which reaches CW output powers of several 100 W.
In order to obtain high output power emission with good beam quality, one of the most promising concepts is the tapered laser. Within one chip, the diffraction-limited radiation of a RW section is coupled into a flared section (Fig. 3d), which can be realized index-guided or typically gain-guided. This section acts ideally as a passive amplifier [47–52]. In the flared section the mode-area is slowly broadened while the single-transverse-mode profile is mostly maintained.
The emission linewidth of a diode laser can be stabilized and narrowed by introducing an internal grating into the resonator . In the case of distributed feedback lasers (DFB), the grating spans over the entire resonator length [54–56, 57–59]. Alternatively, it is possible to implement the grating as distributed Bragg reflector (DBR), acting as a wavelength-selective resonator mirror [60–63].
Having defined and fabricated these structures, an appropriate metallization of the p-side of the device is performed, followed by a thinning of the substrate and the rear side metallization.
The processed wafer is cleaved according to the desired resonator length and facet coated. In order to achieve high output power, special care has to be given to the cleaning and passivation of the laser facets [64–66]. This process step is followed by an optical coating of the facets. For laser devices, the rear facet is coated with a high reflectivity Rr ≈ 96%, whereas the front facet is anti-reflection coated with Rf ≤ 30%. Using the devices as a gain medium in external cavity configurations, one side of the device is anti-reflection coated with an extremely low reflectivity Rf < 5 × 10−4. For devices used as amplifiers both sides have this low reflectivity.
In order to operate the devices, they are mounted on special heat sinks, providing an efficient heat removal. The most common approach is to mount the diodes p-side down, reducing the thermal resistance . Long lasers with low thermal resistance can also be mounted p-side up, lowering the mounting induced stress . First the laser is soldered on a submount. Depending on the laser devices different materials can be used. If the devices are mounted without any significant strain between the semiconductor and the mount, a submount material with a comparable thermal expansion coefficient might be selected, such as CuW, BN  or BeO [69, 70]. If heat removal is crucial and the devices are tolerant against strain, submount materials such as chemical vapor deposition (CVD) diamond  can be used. Alternatively, AlN can be applied; a relatively cheap material and easy to handle. As solder AuSn is often used, which guarantees a reproducible soldering process. Finally the diode laser submount is mounted on a copper block of different geometries. These copper blocks can be cooled passively (i.e. conductively) or actively using micro-channel coolers. The fabricated laser diodes exhibit very long lifetimes up to several 10,000 h. Examples of such tests and the analysis of failures are reported in [72, 73].
2.2. Performance characteristics of diode lasers
All biomedical laser applications require certain parameters to be fulfilled. These can be, for example, wavelength, output power, beam quality, size and cost-efficiency of the laser systems. Diode lasers have proven their superior performance in these aspects. An overview on achieved maximum CW output powers at wavelengths between the blue and near-infrared spectral region is given in Fig. 4.
It is evident from Fig. 4 that diode lasers cover a large spectral range with increasing output power towards the near-infrared. Up to 25 W were obtained with broad area devices between 800–1000 nm . This wavelength range coincides with a local maximum in the absorption spectra of blood. Even though the beam quality is rather poor, these lasers are extensively used in dermatology, because output power and wavelength rather than beam quality are the key parameters, as explained in Section 'Direct application of high-power diode lasers in dermatology'.
In the red spectral range up to 5.6 W were reported for BA lasers . Around 1 W was achieved with tapered devices . The red to near-infrared wavelength region is preferred for diffuse spectroscopy and imaging, discussed in Section 'Diffuse near-infrared spectroscopy and imaging using diode lasers'. Due to their size, efficiency, and power requirements in the order of milliwatts, diode lasers are preferably applied in these applications.
As Fig. 4 shows, obtaining high-power laser emission at shorter wavelengths in the visible range is still challenging. In the green spectral range up to 170 mW were demonstrated using ridge waveguide lasers . With high-power green light being of high importance, for example, in dermatology and direct pumping of ultrashort pulsed lasers, frequency conversion represents a solution to increase the power at these wavelengths, as described in Chapter 5. Up to 12 W with near diffraction-limited beams were reported with tapered lasers at 978 nm  and 1060 nm . Both types of devices were based on intrinsic DBR gratings as rear-end mirrors. Due to the high reflectivity of the intrinsic gratings, the rear facets of the lasers require antireflection coating. Therefore, spurious spectral modes are not reflected back into the tapered section and the spectral linewidth is significantly narrowed . Due to their output power and their excellent spatial and spectral characteristics these devices are ideal for frequency conversion into the blue-green spectral range. Combined with the large number of material compositions, frequency conversion of diode lasers also enables access to new wavelengths, currently not covered.
In the blue spectral range up to 1.6 W were shown with direct-emitting BA devices . These wavelengths are preferably applied, for example, in fluorescence measurements typically requiring low power emission. One major challenge is to obtain yellow emission. This is mainly due to missing material structures for edge-emitting diode lasers around 590 nm or 1180 nm.
For any given structure or wavelength, pulsed emission is obtained simply by modulating the diode injection current. This enables generating pulses with adjustable pulsewidths and repetition rates suitable for applications, such as fluorescence based imaging. As explained in Section 'Gain-switched diode lasers generating optical pulses down to the picosecond range', the obtained peak power may be reduced compared to other laser systems, but still sufficient with respect to the sensitivity of biological targets.
It is obvious that based on their performance diode lasers become increasingly applicable in the biomedical field, and the following sections emphasize their advantages within key biomedical applications.
3. Direct application of high-power diode lasers in dermatology
As pointed out earlier, diode lasers provide increased output power in the near-infrared range. In dermatology these wavelengths combined with the absorption by blood are used to treat different diseases, such as vascular malformations and hemangiomas. Due to reduced absorption and scattering coefficients in tissue, corresponding diode lasers allow for longer penetration depths and the treatment of deeper-lying vessels. In addition, diode lasers address the need for compact and efficient systems. Their flexibility in wavelengths and the direct control of laser emission enable optimizing treatment parameters with respect to specific chromophores, the treatment outcome and reduction of side-effects.
3.1. Short introduction of selective photothermolysis
The application of lasers in the biomedical field is strongly related to light-tissue interactions. Such interactions enable both imaging as well as direct treatments. Light-tissue interactions can mainly be described and quantified by four different parameters: the refractive index, the scattering coefficient, the scattering phase function, and the absorption coefficient , respectively. While the scattering coefficient defines the probability of photon scattering events, the absorption coefficient provides information about the amount of energy being extracted from an incident light wave. Their wavelength dependence  and the ratio between the scattering coefficient and the sum of the scattering and absorption coefficients, called the albedo , determine the penetration depth and therefore the optimum wavelengths for different applications.
In the visible range (400–600 nm) the absorption is dominated by oxy- and deoxy-hemoglobin, and melanin (Fig. 5). Above 1300 nm water is the main absorber. Within that window (≈ 600–1300 nm) the absorption coefficients are reduced by 1–4 orders of magnitude.
The tissue response depends on the heat generated by absorption. Anderson and Parrish introduced selective photothermolysis, suggesting that selective tissue absorption within the so-called thermal relaxation time of the tissue leads to selective destruction of the target . The thermal relaxation time is the time in which the targeted tissue dissipates 50% of the generated heat and it scales with the square of the target diameter. It therefore depends on the absorption coefficient and size of the target, as well as on the laser wavelength and pulse duration. The optimum pulse duration should be equal to or slightly less than the thermal relaxation time, in order to avoid damaging the surrounding tissue. For each target there is a critical temperature. Temperatures exceeding that value will lead to coagulation, vaporization, and finally ablation of the tissue, respectively .
3.2. Diode lasers for photocoagulation
In dermatology, selective photothermolysis is chosen for applications, such as hair removal , skin rejuvenation , or photocoagulation , respectively. The latter is based on absorption of photon energy by blood and shall be the main application discussed in this section. Considering Fig. 5 the most obvious wavelengths for photocoagulation are in the green-yellow spectral range. Possible choices of lasers are, e.g., frequency doubled solid-state lasers, providing > 100 W of output power in CW or pulsed mode [105, 106]. These lasers tend to be bulky and expensive and thus alternative solutions are required. In addition, the very high absorption of blood in the green-yellow spectral range limits the penetration depth and the size of the vessels treated. Lasers with lower absorption are preferred to enhance volume heating of deeper-lying, larger vessels. In addition, a lower absorption in melanin has the potential to cause less damage to the skin. The main attenuation stems from the light scattering, which is reduced inversely proportional with wavelength. Hence, increasing the wavelength enhances penetration depth.
Accordingly, in Fig. 5 a trade-off solution to this problem is shown. The hemoglobin absorption curves also exhibit a local maximum in the range between 800–1000 nm and, simultaneously, the absorption in melanin is reduced. At these wavelengths, light experiences less scattering , increasing the penetration. However, the absorption coefficient of hemoglobin is reduced by more than one order of magnitude but still sufficient to obtain the effect. Available diode lasers are capable of emitting multiple tens of watts [81, 82] and can easily be coupled into multimode fibers for direct delivery. By providing sufficient optical energy in the most efficient and compact manner, whilst the beam propagation parameters not being crucial, these high-power, near-infrared diode lasers have become very attractive light sources for photocoagulation.
3.3. Treatment of vascular malformations and hemangiomas with diode lasers
3.3.1. Endovenous laser treatment of vascular malformations
Vascular malformations are disorders of blood or lymphatic vessels causing reddish or bluish lesions underneath the skin . For example, venous malformations are common disorders where the valves within the veins are unable to prevent the reflux of blood causing swelling, pain and muscle cramps. The surgical treatment of choice is ligation and stripping of the veins leading to complications such as trauma, bleeding and scars, as well as increased hospital costs and long recovery times . The non-surgical procedure is sclerotherapy, which can also cause pigment changes and scarring .
An alternative method is endovenous laser treatment (EVLT), a minimally invasive method introduced for the treatment for varicose veins . The heat generated by absorption diffuses through the blood and vessel walls initiating the development of steam bubbles that cause thermal injury and vessel occlusion .
The light energy of a high-power, long-pulsed, fiber-coupled laser is delivered directly into the vein through the fiber and guided by ultrasound imaging. The light pulses are initiated while the fiber is slowly withdrawn causing vessel closure. Compared to sclerotherapy, EVLT enables a more precise control of vein wall damage, lowering the recanalization rates of the closed vessels.
Diode lasers are the lasers of choice for EVLT. They provide the necessary power level and wavelengths in fiber-coupled packages enabling compact and cost-efficient laser systems for the treatment. Furthermore, the amount of energy can be precisely controlled directly by the laser current. The first demonstration of a diode laser EVLT was carried out using a 14 W, 810 nm laser . The actual procedure was carried out with 3–4 W delivered in 1–2 second pulses, required due to the blood flow dissipating the heat. The treated veins had mean diameters of 5 mm and lengths of 20 cm. The immediate results indicated an excellent closure rate of 100% comparing favorably to other minimally invasive techniques. These results were confirmed by other groups [107, 110-113]. A study of the short-term efficacy of EVLT showed that 99% out of 90 cases still showed vessel closures after 9 months follow-up . The patients were instructed to walk immediately after the procedure and continue their normal daily activities, indicating the viability of the procedure. The risk factors for nonocclusion are not only related to laser parameters, such as fluence (energy per cm2) or irradiation time, but also to physiological parameters such as the vein diameter and the distance of the thrombus to a larger vessel after the procedure . Therefore, accurate diagnosis is of paramount importance in determining the proper laser and its parameters, in order to optimize the outcome of these treatments and minimize side-effects.
3.3.2. Treatments of vascular malformations applied externally
While EVLT requires the light to be delivered through a fiber via minimally invasive surgery, other procedures, such as the treatment of port-wine stains or telangiectasia , rely on the energy being delivered directly through the skin. The success of these treatments relies on the combination of light absorption and penetration depth. For small vessel sizes, green-yellow lasers like solid state lasers or dye lasers are chosen . For larger vessels, deeper penetration is required. As discussed above, deeper penetration is achieved at longer wavelengths, obtained, for example, with near-infrared diode lasers. However, diode lasers are typically not preferred for treatments of these mostly superficial malformations. Nevertheless, a few groups did examine their capability in that field [117–121].
In one example, vascular abnormalities were treated with 150 ms pulses of a 980 nm laser at 300–500 J/cm2 . As mentioned above, longer wavelengths and short pulses increase the potential causing less damage to the skin. In another study laser therapy was combined with radiofrequency. In that case, the absorption of 250 ms laser pulses preheated the blood vessel and created conditions for selective radiofrequency applications . As a consequence, this combination allowed reducing the laser fluence (80–100 J/cm2), lowering the risk of possible damages to the epidermal layer even further. The overall response of the patients in that study was excellent showing 75–100% lesion clearance.
3.3.3. Treatment of hemangioma by diode laser surgery
In comparison to vascular malformations, hemangiomas are vascular tumors developing after birth and regressing after a couple of years . However, in case of symptoms such as bleeding, pain or functional compromise, treatment is strongly recommended. One preferred treatment is endolesional diode laser surgery . As mentioned above, diode lasers provide sufficient output power in fiber-coupled packages, enable compact and cost-efficient laser systems, and the amount of energy at the desired wavelength can be controlled by the laser injection current.
Using a 980 nm diode laser delivering 3–12 watts in continuous or long-pulsed mode, 160 pediatric patients were treated with head and neck hemangioma up to 7 cm in size. The results showed that diode laser treatment improves individual results for lesions up to 5 cm. A similar laser was used performing soft tissue surgery of oral hemangioma . The diode laser was chosen due to its ability to cut with high ablation capacity and reduced bleeding rates, while simultaneously coagulating soft tissue [124, 125]. It was noted that the removed specimens can have a size ≤ 5 mm to still enable a reliable histopathological diagnosis . The diode laser emission led to a sufficient hemostasis and precise incision margins without the need for suturing after surgery . Compared to competing lasers the same group concluded that diode lasers enabled cutting comparable to CO2 lasers and coagulation similar to Nd:YAG lasers . All these results confirm that diode lasers are competitive choices in soft tissue surgery.
Based on their advantages high-power diode lasers are increasingly preferred within applications in dermatology. The range of wavelengths that are accessed with diode lasers open a range of new opportunities, compared to competing systems. Combined, these wavelengths, the resulting penetration depths and the obtained output powers enable addressing individual treatment parameters in a highly efficient manner, while satisfying the need for compact, portable and low-cost laser systems. These advantages combined with the continuous work in diode laser technology will increase the number of direct diode laser applications in the biomedical field even further.
4. Wavelength-swept diode laser systems for optical coherence tomography
Optical coherence tomography (OCT) is an interferometric technique that generates cross-sectional images of scattering material with a typical depth resolution of a few micrometers . Rapidly wavelength-swept laser light sources, or simply swept sources, make ultra-fast OCT image acquisition possible. Semiconductor diodes are ideal gain media for these swept sources, as they permit broadband wavelength tuning at very high speed.
4.1. Optical coherence tomography
Due to the unique ability to image the morphology of biologic tissues non-invasively (Fig. 6, left), OCT has become a well-established tool for biomedical research and clinical diagnostics . It is used on regular basis for early detection of retinal pathologies and for monitoring treatment of those. Another clinical application is examining atherosclerotic plaques and coronary stents in cardiac blood vessels with endoscopic OCT systems. OCT is being used in many other fields of medical and biologic research, but also for technical purposes, such as non-destructive material testing or contact-free metrology .
An OCT system probes the sample with a beam of light (typically near-infrared), and obtains a depth-resolved reflectivity profile from the backscattered fraction. One such measurement is called an A-scan in analogy to ultrasound imaging. By scanning the beam laterally over the sample, a two- or three-dimensional image can be assembled from a number of adjacent A-scans. Most state-of-the-art OCT systems acquire A-scans in the frequency domain, i.e. by detecting the spectrum of the backscattered light after interference with a reference beam. They employ either broadband illumination and a spectrometer or a tunable narrowband light source and a fast photodetector [131, 132]. In the latter scheme (Fig. 6, right), the light source performs rapid sweeps over a broad wavelength range [133, 134], hence this method is termed swept-source OCT (SS-OCT). While SS-OCT requires more complex light sources than spectrometer-based OCT, it offers a number of advantages, such as longer imaging depth range , lower susceptibility to artifacts caused by sample motion , and the possibility of ultra-high speed image acquisition [137, 138].
4.2. Special properties of swept sources
Most swept sources are tunable lasers in highly specialized configurations that meet the requirements for OCT. State-of-the-art swept sources feature sweep repetition rates ranging from 100 kHz up to several MHz. The tuning bandwidth can be well above 100 nm, which is desirable since the OCT depth resolution improves proportionally with the bandwidth . On the other hand, the dynamic linewidth, i.e. the instantaneous width of the narrowband spectrum while it is being tuned, is rather broad compared to classical CW laser lines. Up to several 10 GHz may be acceptable, which results in an OCT imaging depth range of a few millimeters [134, 139, 140]. In recent years, however, considerable efforts went into the development of swept sources with narrower dynamic linewidth in order to increase the imaging depth range [141-144].
The very high tuning speeds of 107–108 nm/s can only be realized using semiconductor laser gain media, which feature a short excited-state lifetime on the order of nanoseconds. Other swept source configurations based upon doped crystals or fibers did not show good performance at high sweep rates [134, 145, 146].
Semiconductor gain media have also a number of other advantages. They are available for many different wavelength ranges and offer broad gain bandwidths as well as unmatched flexibility for tailoring the gain spectrum. Due to direct electrical pumping, light sources can become very efficient and compact. It also permits straight-forward arbitrary shaping of the light source spectrum, which is useful for optimizing the OCT signal acquisition [147-149] and allows to correct for spectrally dependent transmittance of optical media in the probing beam path .
4.3. Swept source technology
Today, most swept sources in practical applications are external cavity tunable lasers (ECTLs) using a semiconductor optical amplifier (SOA) – i.e. a diode with single-mode waveguide and anti-reflection coated facets – as gain medium and a tunable band-pass filter in either a free-space or fiber-based setup. Free-space resonators can be very compact , especially in conjunction with a tunable filter based upon micro-electro-mechanical systems (MEMS) [152, 153].
Fiber-based setups (Fig. 7), on the other hand, which offer uncomplicated implementation of stable, alignment-free light sources, are preferred by the research community [139, 140, 154]. Furthermore, by using a long fiber resonator (several 100 to 1000 meters) one can synchronize the sweep rate of the tunable filter with the resonator roundtrip frequency . Using this technique, called Fourier domain mode-locking (FDML), one can overcome the tuning speed limitation given by the time required to build up laser light from spontaneous emission. Whereas ECTLs with short resonators have been demonstrated with sweep rates up to 400 kHz , more than 5 MHz could be achieved with an FDML laser .
Low-level laser (light) therapy (LLLT) in skin: stimulating, healing, restoring
Pinar Avci, MD,1,2Asheesh Gupta, PhD,1,2,3Magesh Sadasivam, MTech,1,2,5Daniela Vecchio, PhD,1,2Zeev Pam, MD,4Nadav Pam, MD,4 and Michael R Hamblin, PhD1,2,5,*
1Wellman Center for Photomedicine, Massachusetts General Hospital, Boston MA
2Department of Dermatology, Harvard Medical School, Boston MA
3Defence Institute of Physiology & Allied Sciences, Delhi, India
4Aripam Medical Center, Ashdod, Israel
5Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA
Correspondence Author: Michael R. Hamblin, PhD, Department of Dermatology Harvard Medical School, BAR 414 Wellman Center for Photomedicine Massachusetts General Hospital 40 Blossom Street Boston MA 02114 USA. Tel.: +1 617 726 6182; Fax: +1 617 726 8566. ude.dravrah.hgm.xileh@nilbmah (M. R. Hamblin)
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The publisher's final edited version of this article is available at Semin Cutan Med Surg
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Low-level laser (light) therapy (LLLT) is a fast-growing technology used to treat a multitude of conditions that require stimulation of healing, relief of pain and inflammation, and restoration of function. Although the skin is the organ that is naturally exposed to light more than any other organ, it still responds well to red and near-infrared wavelengths. The photons are absorbed by mitochondrial chromophores in skin cells. Consequently electron transport, adenosine triphosphate (ATP) nitric oxide release, blood flow, reactive oxygen species increase and diverse signaling pathways get activated. Stem cells can be activated allowing increased tissue repair and healing. In dermatology, LLLT has beneficial effects on wrinkles, acne scars, hypertrophic scars, and healing of burns. LLLT can reduce UV damage both as a treatment and as a prophylaxis. In pigmentary disorders such as vitiligo, LLLT can increase pigmentation by stimulating melanocyte proliferation and reduce depigmentation by inhibiting autoimmunity. Inflammatory diseases such as psoriasis and acne can also benefit. The non-invasive nature and almost complete absence of side-effects encourages further testing in dermatology.
Keywords: Acne, Dermatology, Herpes, Laser, LLLT, Low level laser therapy, Phototherapy, Skin disease, Skin Rejuvenation, Pigmentation, Vitiligo
Increasingly, non-invasive therapies for skin disease and skin rejuvenation are used, especially in Western countries where relatively high disposable incomes are combined with the desire for an ideal appearance fostered by societal pressures. Although the skin is the organ that is naturally most exposed to light, it still responds well to red and near-infrared wavelengths delivered at the correct parameters with therapeutic intent. Low-level laser therapy (LLLT) was discovered in the late 1960s, but only in recent times has it been widely applied in dermatology. The introduction of light emitting diode (LED) devices has reduced many of the concerns formerly associated with lasers, such as expense, safety concerns and the need for trained personnel to operate them. In fact, many LED devices are designed for home use and are widely sold on the internet. This review will cover the use of LLLT as possibly the ultimate non-invasive approach to treating the skin.
Low-Level Laser (Light) Therapy and Its Mechanism of Action
LLLT, phototherapy or photobiomodulation refers to the use of photons at a non-thermal irradiance to alter biological activity. LLLT uses either coherent light sources (lasers) or non-coherent light sources consisting of filtered lamps or light-emitting diodes (LED) or, on occasion, a combination of both. The main medical applications of LLLT are reducing pain and inflammation, augmenting tissue repair and promoting regeneration of different tissues and nerves, and preventing tissue damage in situations where it is likely to occur.1,2 In the last few decades, non-ablative laser therapies have been used increasingly for the aesthetic treatment of fine wrinkles, photoaged skin and scars, a process known as photorejuvenation (Table 1). More recently, this approach has also been used for inflammatory acne (Table 1).3 LLLT involves exposing cells or tissue to low-levels of red and near infrared (NIR) light. This process is referred to as ‘low-level’ because the energy or power densities employed are low compared to other forms of laser therapy such as ablation, cutting, and thermally coagulating tissue. Recently, medical treatment with LLLT at various intensities has been found to stimulate or inhibit an assortment of cellular processes.4
Examples of LLLT Devices for Dermatological Applications
The mechanism associated with the cellular photobiostimulation by LLLT is not yet fully understood. From observation, it appears that LLLT has a wide range of effects at the molecular, cellular, and tissue levels. The basic biological mechanism behind the effects of LLLT is thought to be through absorption of red and NIR light by mitochondrial chromophores, in particular cytochrome c oxidase (CCO) which is contained in the respiratory chain located within the mitochondria,5–7 and perhaps also by photoacceptors in the plasma membrane of cells. Consequently a cascade of events occur in the mitochondria, leading to biostimulation of various processes (Figure 1).8 Absorption spectra obtained for CCO in different oxidation states were recorded and found to be very similar to the action spectra for biological responses to the light.5 It is hypothesized that this absorption of light energy may cause photodissociation of inhibitory nitric oxide from CCO9 leading to enhancement of enzyme activity,10 electron transport,11 mitochondrial respiration and adenosine triphosphate (ATP) production (Figure 1).12–14 In turn, LLLT alters the cellular redox state which induces the activation of numerous intracellular signaling pathways, and alters the affinity of transcription factors concerned with cell proliferation, survival, tissue repair and regeneration (Figure 1).2,5,6,15,16
Although LLLT is now used to treat a wide variety of ailments, it remains somewhat controversial as a therapy for 2 principle reasons. First, there are uncertainties about the fundamental molecular and cellular mechanisms responsible for transducing signals from the photons incident on the cells to the biological effects that take place in the irradiated tissue. Second, there are significant variations in terms of dosimetry parameters: wavelength, irradiance or power density, pulse structure, coherence, polarization, energy, fluence, irradiation time, contact vs non-contact application, and repetition regimen. Lower dosimetric parameters can result in reduced effectiveness of the treatment and higher ones can lead to tissue damage.1 This illustrates the concept of the biphasic dose response that has been reported to operate in LLLT 1,18,19. Many of the published studies on LLLT include negative results. It is possibly because of an inappropriate choice of light source and dosage. It may also be due to inappropriate preparation of the patient’s skin before application of LLLT, such as: lack of removal of makeup and oily debris, which can interfere with the penetration of the light source, and failure to account for skin pigmentation.17 Inappropriate maintenance of the LLLT equipment can reduce its performance and interfere with clinical results as well. It is important to consider that there is an optimal dose of light for any particular application.
Laser radiation or non-coherent light has a wavelength and radiant exposure dependent capability to alter cellular behavior in the absence of significant heating.20 Phototherapy employs light with wavelengths between 390–1,100 nm and can be continuous wave or pulsed. In normal circumstances, it uses relatively low fluences (0.04–50 J/cm2) and power densities (< 100 mW/cm2).21 Wavelengths in the range of 390 nm to 600 nm are used to treat superficial tissue, and longer wavelengths in the range of 600nm to 1,100nm, which penetrate further, are used to treat deeper-seated tissues (Figure 2).4 Wavelengths in the range 700 nm to 750 nm have been found to have limited biochemical activity and are therefore not often used.1 Various light sources used in LLLT include inert gas lasers and semiconductor laser diodes such as helium neon (HeNe; 633 nm), ruby (694 nm), argon (488 and 514 nm), krypton (521, 530, 568, 647 nm), gallium arsenide (GaAs; > 760 nm, with a common example of 904 nm), and gallium aluminum arsenide (GaAlAs; 612–870 nm).17 A wide range of LED semiconductors are available at lower wavelengths, whose medium contains the elements indium, phosphide and nitride. One question that has not yet been conclusively answered is whether there is any advantage to using coherent laser light over non-coherent LED light.22 While some medical practitioners treat deep tissue lesions using focused lasers in “points”, in dermatology the use of LEDs is becoming increasingly common due to the relatively large areas of tissue that require irradiation.
Tissue penetration depths of various wavelengths.
LLLT for Skin Rejuvenation
Skin starts showing its first signs of aging in the late 20s to early 30s and it usually presents with wrinkles, dyspigmentation, telangiectasia, and loss of elasticity. Common histologic and molecular-level features are reduction in the amount of collagen, fragmentation of collagen fibers, elastotic degeneration of elastic fibers, upregulation of matrix metalloproteinases (MMPs), especially MMP-1 and MMP-2, dilated and tortuous dermal vessels, and atrophy and disorientation of the epidermis.23,24 Both chronological and environmental influences are responsible for the aging process of skin; however photodamage seems to be one of the most important causes of these changes.
Several modalities have been developed in order to reverse the dermal and epidermal signs of photo- and chronological aging. The main concept of most of these modalities is removing the epidermis and inducing a controlled form of skin wounding in order to promote collagen biosynthesis and dermal matrix remodeling. The most commonly used interventions as of today are retinoic acid (a vitamin A derivative), dermabrasion, chemical peels, and ablative laser resurfacing with carbon dioxide (CO2) or erbium: yttrium-aluminum-garnet (Er:YAG) lasers or a combination of these wavelengths.25–27 However, these procedures require intensive post-treatment care, prolonged downtime and may lead to complications such as long-lasting erythema, pain, infection, bleedings, oozing, burns, hyper- or hypopigmentation and scarring.28,29 These limitations created a need for the development of alternative rejuvenation procedures that were safer, more effective, had fewer side effects and minimum postoperative care and downtime, which in turn led to the emergence of non-ablative rejuvenation technologies.30–32 Non-ablative skin rejuvenation aims to improve photoaged and aging skin without destroying the epidermis.31,32 Irregular pigmentation and telangiectasia can be treated with intense pulsed light sources (IPL), 532 nm potassium-titanyl-phosphate lasers (KTP), and high-dose 585/595 nm pulsed dye lasers (PDL)33. Wrinkle reduction and skin tightening through thermal injury to the dermis (photothermolysis) can be achieved by other IPL sources (ie, low-dose 589/595 nm PDLs, 1064 & 1320 nm neodymium:yttrium-aluminum-garnet lasers, (Nd:YAG) 1450 nm diode lasers, and 1540 nm erbium fiber lasers).33
LED which is a novel light source for non-thermal, non-ablative skin rejuvenation has been shown to be effective for improving wrinkles and skin laxity (Figure 3).34–40 It is not a new phenomenon since the first reports of LLLT effects on increased collagen go back to 1987. Studies by Abergel et al. and Yu et al. reported an increase in production of pro-collagen, collagen, basic fibroblast growth factors (bFGF) and proliferation of fibroblasts after exposure to low-energy laser irradiation in vitro and in vivo animal models (Figure 4).41,42 Furthermore, LLLT was already known to increase microcirculation, vascular perfusion in the skin, alter platelet-derived growth factor (PDGF), transforming growth factor (TGF-β1) and inhibit apoptosis (Figure 4).1,43,44 Lee et al. investigated the histologic and ultrastructural changes following a combination of 830 nm, 55 mW/cm2, 66 J/cm2 and 633 nm, 105 mW/cm2, 126 J/cm2 LED phototherapy and observed alteration in the status of MMPs and their tissue inhibitors (TIMPs).33 Furthermore, mRNA levels of IL-1β, TNF-α, ICAM-1, and connexin 43 (Cx43) were increased following LED phototherapy whereas IL-6 levels were decreased (Figure 4) 33. Finally, an increase in the amount of collagen was demonstrated in the post-treatment specimens 33. Pro-inflammatory cytokines IL-1β and TNF-α are thought to be recruited to heal the intentionally formed photothermally-mediated wounds associated with laser treatments, and this cascade of wound healing consequently contributes to new collagen synthesis.33 LED therapy may induce this wound healing process through non-thermal and atraumatic induction of a subclinical ‘quasi-wound’, even without any actual thermal damage which could cause complications as in some other laser treatments.33 TIMPs inhibit MMP activities, so another possible mechanism for the increased collagen could be through the induction of TIMPs (Figure 4). When these observations are put together, it is possible that increased production of IL-1β and TNF-α might have induced MMPs in the early response to LED therapy. This may clear the photodamaged collagen fragments to enable biosynthesis of new collagen fibers. Later on, an increase in the amount of TIMPs might protect the newly synthesized collagen from proteolytic degradation by MMPs.33 Furthermore, increased expression of Cx43 may possibly enhance cell-to-cell communication between dermal components, especially the fibroblasts, and enhance the cellular responses to the photobiostimulation effects from LED treatment, in order to produce new collagen in a larger area which even includes the non-irradiated regions.33 In a clinical study performed by Weiss et al., 300 patients received LED therapy (590 nm, 0.10 J/cm2) alone, and 600 patients received LED therapy in combination with a thermal-based photorejuvenation procedure. Among patients who received LED photorejuvenation alone, 90% reported that they observed a softening of skin texture and a reduction in roughness and fine lines ranging from a significant reduction to sometimes subtle changes.36 Moreover, patients receiving a thermal photorejuvenation laser with or without additional LED photomodulation (n = 152) reported a prominent reduction in post-treatment erythema and an overall impression of increased efficacy with the additional LED treatment.36,45 This reduction in post-treatment erythema could be attributed to anti-inflammatory effects of LLLT.40 Using different pulse sequence parameters, a multicenter clinical trial was conducted, with 90 patients receiving 8 LED treatments over 4 weeks.37,46–48 The outcome of this study showed very favorable results, with over 90% of patients improving by at least one Fitzpatrick photoaging category and 65% of patients demonstrating global improvement in facial texture, fine lines, background erythema, and pigmentation. The results peaked at 4 to 6 months following completion of 8 treatments. Markedly increased collagen in the papillary dermis and reduced MMP-1 were common findings. Barolet et al.’s study is also consistent with the previously mentioned studies. They used a 3-D model of tissue-engineered human reconstructed skin to investigate the potential of 660 nm, 50 mW/cm, 4 J/cm2 LED in modulating collagen and MMP-1 and results showed upregulation of collagen and down-regulation MMP-1 in vitro.40 A split-face, single-blinded clinical study was then carried out to assess the results of this light treatment on skin texture and appearance of individuals with aged/photoaged skin.40 Following 12 LED treatments, profilometry quantification demonstrated that while more than 90% of individuals had a reduction in rhytid depth and surface roughness, 87% of the individuals reported that they have experienced a reduction in the Fitzpatrick wrinkling severity score.40
Examples of LLLT devices in dermatology for home and clinical use.
Possible mechanism of actions for LLLT’s effects on skin rejuvenation.
LLLT for Acne
Pathogenesis of acne vulgaris has not yet been clarified, however current consensus is that it involves four main events: follicular hyperconification, increased sebum secretion effected by the androgenic hormone secretions, colonization of Propionibacterium acnes and inflammation.49P. acnes plays a key role by acting on triglycerides and releasing its cytokines which in turn trigger inflammatory reactions and alter infundibular keratinization.49 Current treatments for acne vulgaris include topical and oral medications such as topical antibiotics, topical retinoids, benzoyl peroxide, alpha hydroxy acids, salicylic acid, or azaleic acid. In severe cases, systemic antibiotics such as tetracycline and doxycycline, oral retinoids, and some hormones are indicated.50 Medications work by counteracting microcomedone formation, sebum production, P. acnes, and inflammation.50 Despite many options that are available for treatment of acne vulgaris, many patients still respond inadequately to treatment or experience some adverse effects.
Phototherapy (light, lasers, and photodynamic therapy) has been proposed as an alternative therapeutic modality to treat acne vulgaris and was proposed to have less side effects compared to other treatment options.51 Exposure of sunlight was reported to be highly effective for treatment of acne with efficacy up to 70%.52 The sunlight decreases the androgenic hormones in the sebaceous gland, but the unwanted effect of exposure to UVA and UVB limits sunlight for the treatment of acne. Recently, phototherapy with visible light (mainly blue light, red light or combination of both) started being used in the treatment of acne (Figure 3).52 One mechanism of action of phototherapy for acne is through the absorption of light (specifically blue light) by porphyrins that have been produced by P. acnes as a part of its normal metabolism, and that act as endogenous photosensitizers. 49,53 This process causes a photochemical reaction and forms reactive free radicals and Singlet oxygen species which in turn lead to bacterial destruction (Figure 5).49,53 Red light is known to penetrate deeper in tissues when compared to blue light.50 It has been demonstrated that red light can affect the sebum secretion of sebaceous glands and change keratinocyte behavior.54 Furthermore, red light might also exert its effects by modulating cytokines from macrophages and other cells, which in turn could reduce inflammation.51,54
Illustration of acne treatment with red and blue light.
Several studies reported that LLLT in the red to near infrared spectral range (630–1000 nm) and nonthermal power (less than 200 mW) alone or in combination with other treatment modalities (mainly blue light), is effective for treatment of acne vulgaris.17,49,52,54,55 One of these studies demonstrated significant reduction in active acne lesions after 12 sessions of treatment using 630-nm red spectrum LLLT with a fluence of 12 J/cm2 twice a week for 12 sessions in conjunction with 2% topical clindamycin; however the same study showed no significant effects when a 890 nm laser was used.50 A few studies also showed that the combination of blue light and red light have synergistic effects in acne treatment.49,54–56 It is proposed that synergistic effects of mixed light is due to synergy between the anti-bacterial and anti-inflammatory effect of blue and red light respectively (Figure 5).49,56 It is also worth mentioning that in most studies improvement in inflammatory lesions were higher than the improvement in comedones.49,56
LLLT for Photoprotection
It is widely accepted that the UV range (< 400 nm) exposure is responsible for almost all damaging photo-induced effects on human skin.57–59 Some proposed mechanisms for UV induced skin damage are collagen breakdown, formation of free radicals, inhibition of DNA repair, and inhibition of the immune system.57–59 Existing solutions to prevent UV induced damaging effects are based on minimizing the amount of UV irradiation that reaches the skin, which is achieved by either avoidance of sun exposure or by use of sunscreens. However sometimes sun avoidance might be hard to implement, especially for the people involved in outdoor occupations or leisure activities. On the other hand, the photoprotective efficacy of topical sunscreens have their own limitations as well, which include decreased efficacy following water exposure or perspiration, spectral limitations, possible toxic effects of nanoparticles that are contained by most sunscreens,60 user allergies, and compliance.
It has recently been suggested that infrared (IR) exposure might have protective effects against UV-induced skin damage mainly by triggering protective/repair responses to UV irradiation. In the natural environment, visible and IR solar wavelengths predominate in the morning and UVB and UVA are maximal around noon which suggest that mammalians already possess a natural mechanism which, in reaction to morning IR radiation, prepares the skin for upcoming potentially damaging UV radiation at noon.61 However, opposing views also exist, such as Krutmann’s study demonstrating IR-induced disturbance of the electron flow of the mitochondrial electron transport chain which leads to inadequate energy production in dermal fibroblasts.62 Schroeder’s report is another example stating that IR alters the collagen equilibrium of the dermal extracellular matrix by leading to an increased expression of the collagen-degrading enzyme MMP-1, and by decreasing the de novo synthesis of the collagen itself.59 As previously mentioned, the same light source may have opposite effects on the same tissue depending on the parameters used and these conflicting views are probably due to the biphasic effects of light.18,19
Menezes et al. demonstrated that non-coherent near infrared radiation (NIR) (700–2,000 nm) generated a strong cellular defense against solar UV cytotoxicity in the absence of rising skin temperature and it was assumed to be a long-lasting (at least 24 hours) and cumulative phenomenon.63 Following this study, Frank et al. proposed that IR irradiation prepares cells to resist UVB-induced damage by affecting the mitochondrial apoptotic pathway.64 IR pre-irradiation of human fibroblasts was shown to inhibit UVB activation of caspase-9 and -3, partially release of cytochrome c and Smac/Diablo, decrease pro-apoptotic (ie, Bax) and increase anti-apoptotic proteins (ie, Bcl-2 or Bcl-xL).64 The results suggested that IR inhibited UVB-induced apoptosis by modulating the Bcl2/Bax balance, pointing to a role of p53, a sensor of gene integrity involved in cell apoptosis and repair mechanisms. In a further study, Frank et al. studied more specifically the role of the p53 cell signaling pathway in the prevention of UVB toxicity.64 The response to IR irradiation was shown to be p53 dependent which further suggests that IR irradiation prepares cells to resist and/or to repair further UVB-induced DNA damage. Finally, the IR induction of defense mechanisms was supported by Applegate et al. who reported that the protective protein, ferritin, normally involved in skin repair (scavenger of Fe2+ otherwise available for oxidative reactions) was induced by IR radiation.65
In an in vitro study, it was reported that an increase dermal fibroblast procollagen secretion reduces metalloproteinases (MMP) or collagenase production following non-thermal non-coherent deep red visible LED exposures (660 nm, sequential pulsing mode).40 These results correlated with significant clinical improvement of rhytids in vivo.40 In a subsequent in vivo pilot study, effect of this wavelength in 3 healthy subjects using a minimal erythemal dose (MED) method adapted from sunscreen SPF determination has been investigated.61 The results showed that LED therapy was effective, achieving a significant response in the reduction of the erythema induced by UVB.61 Following this pilot study a further investigation has been performed to find out in vivo aspects of this phenomenon. Effects of non-thermal, non-coherent 660 nm LED pulsed treatments in providing enhanced skin resistance prior to upcoming UV damage was investigated in a group of subjects with normal fair skin and patients presenting polymorphous light eruption (PLE). Results suggested that LED based therapy prior to UV exposure provided significant dose-related protection against UVB induced erythema. A significant reduction in UVB induced erythema reaction was observed in at least one occasion in 85% of subjects as well as in the patients suffering from PLE. Furthermore, a sun protection factor SPF-15-like effect and a reduction in post-inflammatory hyperpigmentation were observed. An in vitro study by Yu et al. revealed that HeNe laser irradiation stimulated an increase in nerve growth factor (NGF) release from cultured keratinocytes and its gene expression.66 NGF is a major paracrine maintenance factor for melanocyte survival in skin.67 It was shown that NGF can protect melanocytes from UV-induced apoptosis by upregulating BCL-2 levels in the cells.68 Therefore, an increase in NGF production induced by HeNe laser treatment may provide another explanation for the photoprotective effects of LLLT.
LLLT for Herpes Virus Lesions
One of the most common virus infections is caused by herpes simplex virus (HSV). HSV is chronic and lasts one’s entire life. The exposure of the host to several kinds of physical or emotional stresses such as fever, exposure to UV light, and immune suppression causes virus reactivation and migration through sensory nerves to skin and mucosa, localizing particularly on the basal epithelium of the lips and the perioral area.69 Up to 60% of sufferers will experience a prodromic stage, after which the lesions develop through stages of erythema, papule, vesicle, ulcer and crust, until healing is achieved. It is accompanied by pain, burning, itching, or tingling at the site where the blisters form. Immune responses to HSV infection involve: macrophages, Langerhans cells, natural killer cells, lymphocyte-mediated delayed-type hypersensitivity, and cytotoxicity.70
While several anti-viral drugs such as acyclovir and valacyclovir are used to control recurrent herpes outbreaks, only limited reduction in the lesions’ healing time has been observed.69 Furthermore, development of drug-resistant HSV strains is of increasing significance especially in immunocompromised patients.70 Therefore, new therapy modalities that can shorten the recurrent episodes and cause prominent reduction of related pain and inflammation are necessary.
LLLT has been suggested as an alternative to current medications for accelerated healing, reducing symptoms and influencing the length of the recurrence period.69,71,72 Among 50 patients with recurrent perioral herpes simplex infection (at least once per month for more than 6 months), when LLLT (690 nm, 80 mW/cm2, 48 J/cm2) was applied daily for 2 weeks during recurrence-free periods it was shown to decrease the frequency of herpes labialis episodes.73 In another study with similar irradiation parameters (647 nm, 50 mW/cm2, 4.5 J/cm2), investigators achieved a significant prolongation of remission intervals from 30 to 73 days in patients with recurrent herpes simplex infection.74 Interestingly, patients with labial herpes infection showed better results than those with genital infection. However, irradiation did not effect established HSV latency in a murine model.75
Even though mechanism of action is still not clear, an indirect effect of LLLT on cellular and humoral components of the immune system involved in antiviral responses rather than a direct virus-inactivating effect was proposed.76 Inoue et al. investigated tuberculin reactions at the bilateral sites of the backs of sensitized guinea pigs. They applied a single low-power laser irradiation dose at a fluence of 3.6 J per cm2on one side and compared it to contralateral non-irradiated sites.77 Interestingly, following irradiation, tuberculin reaction was suppressed not only at the irradiated site but also at the contralateral nonirradiated site. It is worth to note that this phenomenon was observed when mononuclear cells were dominant in the perivascular cellular infiltration. Based on their results, they have suggested a possible systemic inhibitory effect of LLLT on delayed hypersensitivity reactions.77 Activation and proliferation of lymphocytes78–81 and macrophages82 as well as the synthesis and expression of cytokines83,84 following low intensities of red and NIR light have been reported by several investigators. The question of whether these effects of LLLT have any influence on HSV infection remains to be determined.
LLLT for Vitiligo
Vitiligo is an acquired pigmentary disorder characterized by depigmentation of the skin and hair. The underlying mechanism of how the functional melanocytes disappear from the involved skin is still under investigation. However, findings suggest that regardless of the pathogenetic mechanism involved, keratinocytes, fibroblasts, melanoblasts and melanocytes may all be involved in both the depigmentation and also the repigmentation processes of vitiligo.66,85–89 Therefore, stimulation of these epidermal and dermal cells may be a possible treatment option. Due to the obscure pathogenesis of the disease, treatment of vitiligo has generally been unsatisfactory. Current existing therapies that induce varying degrees of repigmentation in patients with vitiligo are topical corticosteroids, phototherapy, and photochemotherapy (PUVA).89 In 1982, a group of investigators found that low energy laser irradiation had effects on defective biosynthesis of catecholamine in certain dermatological conditions including scleroderma and vitiligo.90,91 Later on, one of the investigators from the same group reported that after 6–8 months of treating 18 vitiglio patients with low-energy HeNe laser (632 nm, 25 mW/cm2 ) therapy, marked repigmentation was observed in 64% of the patients and some follicular repigmentation was observed in the remaining 34%.91 Since then, LLLT has been suggested as an alternative effective treatment option for patients with vitiligo.66,88,89
Segmental-type vitiligo is associated with a dysfunction of the sympathetic nerves in the affected skin and it is relatively resistant to conventional therapies.66 Based on the previous reports stating that HeNe laser irradiation leads to improvement in nerve injury92–94 and LLLT induces repigmentation responses,95,96 it was proposed that the HeNe laser might be a potential treatment modality for treatment of segmental type vitiligo. 66 When the HeNe laser light was administered locally (3 J/cm2, 1.0 mW, 632.8 nm), marked perilesional and perifollicular repigmentation (> 50%) was observed in 60% of patients with successive treatments. Both NGF and bFGF stimulate melanocyte migration and deficiencies of these mediators may participate in the development of vitiligo.86,97,98 In the same study, when cultured keratinocytes and fibroblasts were irradiated with 0.5–1.5 J per cm2 HeNe laser, significant increase in bFGF release both from keratinocytes and fibroblasts as well as a significant increase in NGF release from keratinocytes was reported. 66 Additionally, the medium from HeNe laser irradiated keratinocytes stimulated [3H]thymidine uptake and proliferation of cultured melanocytes. Another study by Lan et al. demonstrated that the HeNe laser (632.8 nm, 1 J/cm2 and 10 mW) stimulates melanocyte proliferation through enhanced α2β1 integrin expression88 and induces melanocyte growth through upregulation of the expression of phosphorylated cyclic-AMP response element binding protein (CREB) which is an important regulator of melanocyte growth.88 ECM molecules are also important elements of the pigmentation process due to their regulatory roles for physiological functions of pigment cells including morphology, migration, tyrosinase activity and proliferation.99–101 Type IV collagen is present in the basement membrane and is known to have an intricate relationship with melanocytes in the epidermis such as increasing melanocyte mobility.89 Following, HeNe irradiation, the attachment of melanocytes to type IV collagen was found to be significantly enhanced which also indicated modulation of melanocyte physiological function by HeNe laser irradiation.88 Furthermore, among various ECM proteins found in the dermis, fibronectin was shown to have significant effects on both differentiation and migration of cultured melanoblasts and melanocytes.102,103 In 1983, Gibson et al. demonstrated that the physical distribution of fibronectin in vivo was closely associated with the migration path undertaken by melanoblasts during the repigmentation process of vitiligo.104 Based on Lan at al.’s findings, an immature melanoblast cell line (NCCmelb4) showed significant decrease in the attachment to fibronectin following HeNe laser treatment while the attachment of a more differentiated melanoblast cell line (NCCmelan5) to fibronectin increased about 20% following 1 J/cm2, 10 mW HeNe laser treatment.89 Lastly, expression of integrin a5b1 which mediate locomotion of pigment cells was found to be enhanced on NCCmelb4 cells.89
LLLT for Producing Depigmentation
Most studies carried out for vitiligo show the stimulatory effects of LLLT on pigmentation; however in a previously mentioned study, while testing effects of blue and red laser for acne treatment, an interesting and unexpected result was found for the first time.49 Combining both blue (415 +−5 nm, irradiance 40 mW/cm2, 48 J/cm2 ) and red (633 +- 6 nm, 80 mW/cm2, 96 J/cm2 ) light produced an overall decrease in the melanin level. Instrumental measurement results showed that melanin level increased by 6.7 (the median of differences between the melanin level before and after one treatment session) after blue light irradiation without a statistical significance (P > .1), whereas it decreased by 15.5 with statistical significance (P < .005) after red light irradiation. This finding may have some relationship with the laser’s brightening effect of the skin tone, which 14 out of 24 patients spontaneously reported after the treatment period. However as of today, no other studies investigated or reported similar decrease in melanin levels following red light irradiation. Considering that different parameters are used for vitiligo and acne treatment, different effects of red light on the same tissue might be due to the biphasic effects of LLLT.18,19
LLLT for Hypertrophic Scars and Keloids
Hypertrophic scars and keloids are benign skin tumors that usually form following surgery, trauma, or acne and are difficult to eradicate. Fibroblastic proliferation and excess collagen deposits are the 2 main characteristics105 and imbalance between rates of collagen biosynthesis and degradation superimposed on the individual’s genetic predisposition have been implicated in their pathogenesis. A wide range of surgical (eg, cryotherapy, excision), non-surgical (e.g., pharmacological, mechanical pressure, silicone gel dressings), and laser therapies (CO2, pulsed dye, fractional ablative, and non-ablative lasers) have been tested with variable success, however until now, an optimal treatment of these lesions remains to be found.106–108 It has recently been proposed that poor regulation of interleukin (IL)-6 signaling pathways and transforming growth factor beta-I (TGF-βI) expression have a significant role in this process and thus inhibition of the IL-6 pathway and/or TGF-βI could be a potential therapeutic target.106,107,109–111 Based on the reports demonstrating the effects of LLLT on decreasing IL-6 mRNA levels,33 modulation of PDGF, TGF-β, interleukins such as IL-13 and IL- 15, MMPs, which are all also associated with abnormal wound healing,112,113 it was proposed to be an alternative therapy to existing treatment options. The use of LLLT as a prophylactic method to alter the wound healing process to avoid or attenuate the formation of hypertrophic scars or keloids has been investigated by Barolet and Boucher in 3 cases studies, where following scar revision by surgery or CO2 laser ablation on bilateral areas, a single scar was treated daily by the patient at home with NIR - LED 805 nm at 30 mW/cm2 and 27 J/cm2.112 The first patient had pre-auricular linear keloids bilaterally post-face lift procedure and surgical scar revision/excision had been performed. The second patient had hypertrophic scars on the chest bilaterally post-acne when the CO2 laser was used for resurfacing. The third patient had hypertrophic scars on the back bilaterally post-excision and again the CO2 laser was used for resurfacing. As a result, significant improvements on the NIR-LED treated vs the control scar were seen in all efficacy measures and moreover no significant treatment-related adverse effects were reported.112
LLLT for Burns
In a clinical study by Weiss et al. 10 patients received LED treatment (590 nm) for acute sunburn using a once-or twice-daily treatment regimen for 3 days, treating only half of the affected anatomic area.36 Decreased symptoms of burning, redness, swelling, and peeling were reported. One patient received LED treatment twice daily for 3 days only on half of his back, and other half was left untreated.36 When compared with the untreated side, decreased MMP-1 was demonstrated on the LED-treated side through immunofluorescence staining. Moreover, RT-PCR gene expression analysis showed a significant decrease in MMP-1 gene expression on the LED-treated side at both 4 and 24 hours post–UV injury compared with the untreated side. Other significant changes were also noted with LED treatment related to inflammation and dermal matrix composition 4 days post–ultraviolet (UV) exposure.36
One of the main complications of receiving laser treatment is burns which may be devastating for the patient. LED was suggested as a treatment modality for facilitating faster healing. A group of 9 patients who had a variety of second-degree burns from nonablative laser devices were given LED therapy once a day for 1 week and according to both the patient and the physician, healing occurred 50% faster.36 Also the same investigators conducted a pilot study, where one forearm was injured by a CO2 laser using a computer pattern generator to deliver the identical treatment to both test sites. Both sites received daily dressing changes using a non-stick dressing and Polysporin ointment, but one site also received additional LED treatment.36 As a result, when compared to the untreated control site, accelerated reepithelialization was observed in the LED-treated site 36.
LLLT for Psoriasis
More recently LLLT has been considered for treatment of plaque psoriasis. A recent preliminary study investigated the efficacy of a combination of 830 nm (near infrared) and 630 nm (visible red light) to treat recalcitrant psoriasis using LED irradiation. All patients with psoriasis resistant to conventional therapy were enrolled and were treated sequentially with 830 nm and 630 nm wavelengths in 2 20-min sessions with 48 hours between session for 4 or 5 weeks. The results showed no adverse side effects and a resolution of psoriasis.114 The limitation of this study was the small number of patients enrolled, however the results observed encourage future investigations for use of LLLT in treating psoriasis.
LLLT appears to have a wide range of applications of use in dermatology, especially in indications where stimulation of healing, reduction of inflammation, reduction of cell death and skin rejuvenation are required. The application of LLLT to disorders of pigmentation may work both ways by producing both repigmentation of vitiligo, and depigmentation of hyperpigmented lesions depending on the dosimetric parameters. The introduction of LED array-based devices has simplified the application to large areas of skin. There is no agreement as yet on several important parameters particularly whether red, NIR, or a combination of both wavelengths is optimal for any particular application. There is a credibility gap that needs to be overcome before LLLT is routinely applied in every dermatologist’s office.
This work was supported by the US NIH (R01AI050875 to MRH)
Conflict of Interest Disclosures: The authors have completed and submitted the ICMJE Form for Disclosure for Potential Conflicts of Interest and none were reported. Dr Gupta has received a grant from Boyscast Fellowship, Rolo-11, in India. All other authors have nothing to disclose.
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