LEDs used as an alternative to lasers are finding increasing application in wound healing. Early research by NASA has demonstrated that near infrared LEDs in the 630 to 880 nm range promote cell growth and thus stimulate faster healing for burns, fractures, radiation tissue damage and skin grafts, as well as for treating muscle and bone atrophy, a condition prevalent in astronauts on long space missions.
This article will review some of the research in this field and highlight the types of LEDs now being incorporated into LED light therapy products for promoting wound healing and skin care. The most popular wavelengths are 660 nm (surface healing) and 880 nm (deeper wound healing). Therapeutic products and medical equipment often combine more than one wavelength in a single unit.
Figure 1: Portable devices emitting visible red light, such as the Warp 10 from Quantum, have been designed to provide immediate first aid treatment for armed forces suffering minor wounds.
Such visible red LEDs and near-infrared emitters are readily available in these wavelengths from companies including Lumex, Marktech Optoelectronics and Osram Semiconductor.
It has been well documented that in space, particularly on long space missions, astronauts suffer from muscle and bone atrophy, due to long periods of weightlessness. In addition, minor injuries, such as cuts and grazes do not heal easily in a microgravity environment, or in submarine atmospheres, which are low in oxygen, high in carbon dioxide and lacking sunlight. NASA initially determined that LEDs originally developed for plant growth experiments were effective in the treatment of wounds, accelerating the healing process. Further research has more finely tuned the optimum wavelengths, and found that their use alone, in conjunction with hyperbaric oxygen, produces the best results.
The application of LED light therapy for the treatment of wound healing and muscle and bone atrophy is expected to generate considerable interest, not only in space applications and military combat casualty care, but also in civilian medical care. Market research1 forecasts a fast growing market for LEDs used in biophotonic and medical devices, with predictions for 2019 increasing five-fold to $324.7 million over the 2012 figure of $64.5 million.
Phototherapy and cell regeneration make up only a minority segment of this market. However, the marketers have identified that red LEDs, including near-infrared and IR devices make up at least half of the devices sold in life science type of applications. Further, the consultants have noted that the impressive growth predictions do not reflect the true market expansion, as unit prices continue to fall and volume usage expands.
Red is the color
Researchers working with NASA2 have found that light therapy using near-infrared LEDs operates by activating color-sensitive chemicals in body tissues, stimulating the process in a cell’s mitochondria. Light wavelengths from 680 nm to 880 nm have been found to travel through skin and muscle tissue, to prompt tissue and deep wound healing. Light penetration depends on power and wavelength, with higher wavelength LEDs more likely to penetrate further.
Clinical trials of a variety of single- and mixed-wavelength LED lamps have been taking place in hospitals in the US and elsewhere. Results indicate that near-infrared light therapy accelerates cell growth at 150 to 200% compared to non-treated cells. Target wavelengths are 630 to 680 nm, 730 nm and 850 to 880 nm, although research is continuing to determine which are most effective at stimulating cell growth.
In Europe, research at the Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, Vienna, Austria, has built on the initial NASA findings into near infrared, and extended its remit to study 470 nm blue light.3 As part of the Austrian cluster for tissue regeneration research work, the Institute has found that blue light can significantly and positively influence biological systems. Comparisons between red and blue light showed that both light sources substantially promote wound healing when compared to no light therapy; the key difference between them is their effect on keratin expression.
Extensive research is constantly finding new and expanding application areas for LED light therapy. In addition to accelerating the healing of wounds that military personnel may encounter during training or in action, such as burns, fractures, radiation tissue damage and skin grafts, medical researchers have found that the technology can also help heal and even prevent oral sores caused by chemotherapy and radiation treatments, accelerate skin/tissue healing after cosmetic surgery and laser ablative resurfacing, and help speed the healing of diabetic skin ulcers.
Further specialist research is underway to verify the effectiveness of specific wavelength LED light to treat eye disorders such as ‘lazy eye’, diabetic macular degeneration and retinopathy. NASA and other researchers have successfully used red LED probes to treat brain tumors. Other trials are taking place to use LEDs to destroy some cancer cells in conjunction with other cancer treatments. Deeply-penetrating wavelengths are thought to be effective in treating cranial traumas, and potentially, to help dementia sufferers.
Lightweight light arrays
Commercially-available light therapy products typically use arrays of LEDs. As an example, the Warp 10 from Quantum Warp-Light Devices (Figure 2) is an array providing a 10 cm² treatment area with 50 mW/cm² power output.
Figure 2: The consumer version of the Warp 10 from Quantum Warp-Light Devices uses an array of 660 nm red LEDs to provide wound-healing light therapy.
With LEDs in the near infrared (670 nm center wavelength), it is ideal for treating facial abrasions and preventing mouth sores caused by radiation and chemotherapy.
Another vendor, Elixa, offers smaller handheld devices measuring just 5 cm x 10 cm, yet containing 120 LEDs in red (660 nm), near infrared (880 nm) or blue (470 nm). Output is quoted at approximately 1 joule/min.
Figure 3: This LED array therapy product from Elixa incorporates 120 high-intensity LEDs, densely packed to create a portable, handheld unit.
The red light version (Figure 3) is claimed to penetrate to a depth of about 25 mm, for healing skin wounds and infections, while the near-infrared array penetrates to up to 75 mm, to increase blood flow and release nitric oxide, to soothe and heal sore muscles and joints. Other products include two- and three-way combined red/infrared/blue switchable units and a triple array mounted on a stand.
There are many benefits of employing LEDs in light therapy devices, but two stand out. The first is that arrays can be easily and cheaply constructed to incorporate multiple devices. Depending on their application, they may incorporate multiple wavelength devices or single wavelength LEDs. Simple electronics allow the arrays to be controlled in terms of how many devices are illuminated and at which wavelengths.
Arrays of LEDs can be easily assembled into large, flat structures for treating large area wounds, such as burns. The target application will also determine the optimum size of the array, the power requirement and, therefore, whether LEDs need to be tightly packed such that surface-mount devices are critical, or whether through-hole packaged devices will suffice.
Another key benefit is that by using miniature, surface-mount devices, lightweight, hand or field portable, low-power units can also be built. This makes them ideal for military field hospitals or remote medical centers and for special military operations where lightly-equipped troops are deployed in risk areas where high mobility and optimal physical condition are essential. In the US, the Naval Special Warfare Group (SEALS) have been trialing a unit combining 670, 720 and 880 nm LEDs for musculoskeletal injuries sustained in training.
The depth that the light needs to penetrate depends on whether the product is tackling surface tissue wounds or deep muscle treatment, which will also be significant factors in the choice of LEDs in terms of wavelength, energy, and power. Brightness, as measured by some LED manufacturers in mcd, is not considered to be relevant in this type of application. Light intensity measured in mW/cm² is a more appropriate parameter.
Design considerations when selecting LEDs
Various research projects promote the benefits of different wavelengths for the treatment of certain conditions. It is generally agreed that wavelengths between 625 and 900 nm are the most effective for wound healing and allied conditions. At the lower end, 630 nm and 660 nm appear to be favored. At longer wavelengths, 850 nm and 880 nm are the most popular. Work at the 730 nm to 760 nm and 900 to 940 nm levels has also proved useful, but higher cost and less availability of appropriate LEDs are issues here.
Near-infrared wavelengths (typically 850 nm or 880 nm) have been reported as penetrating deeper through skin and bone, but for open wounds, 630 and 660 nm devices have been shown to be just as beneficial in accelerating healing. Skin depth varies around the body, and transmission can also be affected by skin color. Panels incorporating a mix of wavelengths continue to be used in experiments to gather data on the healing of various wound types, locations, and tissue content that the light has to penetrate.
Research has indicated that for some conditions, treatment doses of 4 to 6 Joules/cm² are optimum. About 6 J/cm² equates to applying LED devices with strength of 30 mW/cm² for a duration of 200 seconds. More powerful devices may mean that treatment times can be shortened. Other research has shown that applying more powerful devices for longer can result in deeper penetration of the healing light.
Designers of high intensity, high energy panels of densely packed LEDs may need to consider cooling requirements. For example, an array of twenty 850 nm LEDs in 5 mm packages, packed into a minimal 25 mm² space, each using 50 mA continuously, will overheat. As a rule of thumb, designers consider that 50 mW LEDs should be spaced at approximately 1.65 devices/cm². More powerful devices, at 100 mW would need to be spaced further apart. Some would consider that more than 25 mW/cm² using 830 nm or 850 nm chips might require a cooling fan. FDA requirements specify that 0.8 W per square inch is the maximum energy that can be applied to a device that touches the skin.
Another aspect of heat concerns safety. Experiments have shown that pale skin can barely feel any warmth from the application of a 30 mW/cm² LED in the 600 to 900 nm range after several seconds. Yet, 150 mW/cm² applied for 1 to 2 minutes to a darker skin tone may result in too much heat at the skin’s surface, as melanin can block more light from penetrating. Even 50 mW/cm² could cause 2nd degree burns if a lamp were used for a prolonged period of time in an environment where the heat cannot escape (i.e. wrapped tightly on the skin).
Lenses that focus an LED’s light output may be less relevant in many products for this type of application because the light is typically dispersed as it passes through the skin anyway. Similarly, a wide viewing angle is not always important if the product is going to be used in close proximity to the skin.
LED manufacturers, including Lumex and Osram Opto Semiconductors, offer LED arrays and custom build services as well as discrete devices for customers requiring a specific wavelength or combination of wavelengths. Marktech Optoelectronics has specialized in-chip on-board solutions integrating up to six mixed or same wavelength LED chips in small formats, such as TO5 cans or surface-mount packages. All three companies are familiar with the tough safety requirements of the medical sector and regularly supply standard and custom parts to OEM customers building approved medical devices.
When it comes to sourcing LED visible emitters in the 630 to 680 nm range, Marktech has one of the widest ranges available. The MTE6066N5-UR features a forward current (DC) of 50 mA and power dissipation of 120 mW. Delivered in a T1 ¾, 5 mm radial package, the device is claimed to provide high luminous intensity.
For 850 to 880 nm LED emitters, Osram Opto offers a wide range in its popular Oslon, Smartled and Midled families with a variety of surface-mount packaging options. Designers can choose from forward currents (DC) of 70 mA, 100 mA or 1 A. The SFH4050 has a forward current of 100 mA and a radiant intensity of 10 mW/sr at 100 mA. Power dissipation is 180 mW, forward voltage is 1.5 V and it is contained in a standard 0603 surface-mount package.
For the in-between wavelengths, centering at 770 nm, Marktech once again has a number of devices available. The MTE1077N1-R is specified for medical applications, featuring high reliability, narrow beam angle and packaged in a 5 mm 1 ¾ radial leaded package. Forward current is rated at 50 mA and power dissipation at 100 mW. It can be used in pulsed applications rated at 0.5 A. Forward voltage is typically 1.55 V.
Some specialist light therapy applications require devices in the 940 nm region, moving into the infrared region. A number of suppliers serve this sector with both through-hole and surface-mount devices. A typical example from Lumex is the OED-EL-1L2 contained in a standard 5 mm radial package. It features a forward current of 100 mA, radiant intensity of 60 mW/sr at 100 mA and a typical forward voltage of 1.2 V.
Building on early research by NASA, the range of healing applications for red and near-infrared light keeps on expanding. LEDs are proving the ideal light source, deriving light therapy product marketing opportunities beyond the military and aerospace sector into the civilian clinical/medical sector and into consumer oriented ‘well-being’ products. Light therapy devices for animals are also gaining in popularity.
The construction of LED arrays for these devices depends heavily on the target application. Parameters to consider include wavelength and the mix of wavelengths and the power/energy of the devices to determine the level of penetration through body tissue and the length of treatment time required. Packaging density and removing excess heat, together with meeting strict regulations for medical devices, are further aspects requiring consideration at the design stage.
Although a fair selection of suitable red and near-infrared LED emitters are readily available, it is important to study the data sheets to relate the device specifications provided with the operational parameters required for this type of application.