In order to disinfect the protective clothing of the medical staff at the Huoshenshan Hospital in Wuhan, China so as to prevent the SARS-CoV-2 virus from spreading outside of the clinic, a disinfection tent with UVC LEDs was set up as a novel approach. In the 1.5m × 0.75m × 2m room with layered synthetic fabric walls, UVC emitters from American supplier Bolb are fitted to the reflective surfaces of the ceiling, walls and flooring. During the 30-second disinfection process, the UVC LEDs provide a dosage of 6 mJ/cm2 with a consistent brightness of 200 μW/cm2. The light wavelength of 265 to 280nm destroys genetic information, ensuring that the virus can no longer spread or infect other cells.
Artificial UV Sources
For a long time, ultraviolet light was generated using mercury-based radiation sources, for example using low and medium-pressure mercury vapor (Hg) lamps that generate UV light in a spectrum of 185 to 405nm by means of gas discharges. UV light can also be generated using UV cold cathode tubes (UV-CCL or UV lamps) in a spectrum of between 185 and 405nm by means of glow discharge.
UV LEDs emit UV rays in a spectrum of between 227 and 405nm by means of electroluminescence. The wavelengths are particularly short when using UVC LEDs-between 260 and 270nm-which provides the greatest sterilization effect. Figure 1 shows this using the example of cryptosporidium, a parasite that spreads in particular through unpurified drinking water. Other pathogens, bacteria and viruses exhibit very similar characteristics.
LEDs also offer a compellingly stable spectral output under specific temperature conditions and an almost limitless number of switching cycles, which makes them ideal for mobile solutions that need to provide full light output without any delay.
A Multitude of Weapons
UV rays are invisible to the human eye throughout their entire wave range of 100 to 400nm. Their frequencies are divided into UVA, UVB and UVC bands, and these have different effects on living organisms.
LEDs allow us to pretty much choose the wavelength at our discretion. UVA LEDs with a wavelength of 315 to 400nm offer greater penetration in dispersed biological tissue such as human skin compared to UVB and UVC rays. UVA LEDs are used in fields such as dentistry and cosmetics, for example in tanning studios and nail studios. In the industrial sector, UVA LEDs are used to cure resins, adhesives and paints.
With a wavelength of 280 to 315nm, UVB LED rays have comparatively little penetrating power when it comes to dispersed biological tissue, but they are subject to more scatter. UVB rays encourage the formation of vitamin D in the human body, which is why UVB LEDs are mainly used in medicine for phototherapy and dermatological treatments.
No Defense Against UVC Rays
The high-energy light from UVC LEDs is subject to ever more scatter in biological tissue. With a wavelength of 100 to 280nm, these rays do not penetrate tissue especially deeply, but they can burn unprotected skin. Because the ozone layer in Earth's atmosphere absorbs the natural UVC radiation of sunlight, no earthly organism has developed defense mechanisms against UVC rays, and this is also true of viruses and bacteria. This vulnerability makes irradiation with artificial UVC light an especially effective method of sterilization and disinfection.
UVC LEDs in Practical Applications
Each micro-organism reacts differently to UVC radiation, which is why the intensity of the radiation should be geared towards the desired reduction rate, which is the number of killed micro-organisms. The intensity of UV radiation is inversely proportional to the square of the distance, meaning that as the distance from the radiation source increases, the UV radiation very quickly loses its effectiveness, which is why the object to be disinfected should be as close as possible to the emitter.
Viruses, including the SARS-CoV-2 virus, are commonly spread by air, so the use of UVC LEDs in air conditioning systems seems advisable. In addition to the required reduction rate, air flow rates and air flow geometry also have to be considered.
UV light with a wavelength of 254nm has proven to be especially effective at killing micro-organisms, although when directly applied, it can be harmful to the skin and eyes. On the other hand, "far UVC" light (207 to 222nm) also broadly disables most pathogens in the air without damaging exposed human tissue.
Other viruses and bacteria are also transferred via surfaces, including influenza, noroviruses, rotaviruses, streptococcus and salmonella. For sterilizing larger surfaces, a suitable product would be for example the low-power PU35CL1.0 UVC LED from Lextar with output of 2-4mW and 20mA, which can also be used to pasteurize drinks, package antimicrobial foods, and sterilize toothbrushes.
For smaller-scale installations, Bolb has introduced the compact mid-power S3535-DR100-W272-P40 UVC LED with dimensions of 3.5 x 3.5 x 0.9mm³. With DC power consumption of40mW and a current of just 100mA, it stands out as having the lowest energy consumption in the world with the lowest heat output.
In the high-power segment, Bolb has introduced the S6060-DR250-W272-P100 UVC LED into its range, the most powerful component with DC power consumption of 100mW at 250mA.
Bolb's UVC LEDs are especially well-suited for the treatment of drinking water and disinfection of water in pools or RVs, as well as applications involving stricter irradiation intensity requirements (W/m²) as applicable in fields such as industrial filter systems, air purifiers, medical disinfection boxes and vacuum cleaners.
Selection Criteria for UV LEDs
A key selection criteria for UV LEDs is the beam angle, with certain beam angles being required in certain applications. Bolb's UVC LEDs have a beam angle of 150°, which can be further focused using lenses from Ledil as necessary. Because this reduces the irradiated surface, it also increases the radiation energy per square meter, meaning that the exposure time required is reduced when applying the same energy output. Different UV lenses with compatible glasses allow the irradiation output to be easily scaled for different purposes. For its UV lenses, Ledil uses a special grade of silicone that is especially compatible with UVC wavelengths, as well as aluminum reflectors that are highly reflective of all UV wavelengths, making them especially suitable for disinfection applications.
Other selection criteria for UV LEDs include national UV standards, reflectivity on different materials (Figure 3), heat management, drivers, power consumption and the inverse-square law, which dictates how the intensity of a beam diminishes with increasing distance from the light source.
Bolb's Blazar surface emitter satisfies many of these criteria. This UVC module with 25 LEDs (5×5) and a 55° reflector achieves an effective output of 2W with power consumption of just 1.25A.
Multi-UV LEDs are currently in development. With a dual wavelength chip, they can provide multiple UV wavelengths, for example UVA and UVC, essentially making them an all-purpose weapon in the struggle against viruses, bacteria and other pathogens.
A Strategic Approach to Fighting Pathogens
UVC LEDs were originally developed to fight multidrug-resistant micro-organisms such as Methicillin-resistant Staphylococcus aureus (MRSA). Testing is currently underway to determine how UVC LEDs can also be used to fight viruses. Viruses can only reproduce with the aid of a host. They infest a cell and reprogram it using the cell's own ribonucleic acid (RNA). As the newly produced viruses infect other cells, the host cell is destroyed by this reproduction process. High-energy, short-wave UVC light is absorbed by the RNA of the virus, which causes the genetic information to be destroyed, making the virus unable to spread and infect even more cells.
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