Dr. Giles Humpston, Applications Manager
In recent years, LEDs have transmogrified from simple indicator lights on electronic devices to the light source of choice in many consumer, horticultural and industrial applications. This growth then poses the question “what’s next for LEDs”?
Predictions by several well-respected forecasting companies suggest that the next ‘big thing’ for LEDs will be sterilization and disinfection – industry analysts Yole Développement expect it to be a $610 million market by 2021.
Of course, generally speaking, we’re talking about the sterilization of viruses and bacteria – it has long been known that short-wavelength ultraviolet radiation, generally referred to as UVC, is the nearest thing yet invented to a death ray as far as many bacteria are concerned. Certain wavelengths are also mutagenic to viruses. A place where there are no bacteria or viruses is a sterile environment.
By using light, sterilization can be achieved without nasty chemicals, high temperature or expensive barrier materials. What’s more, the potential portability of UVC approaches to sterilization opens up a range of new use cases, from the treatment of drugs to on-the-fly sterilization of water in hard-to-reach areas of the developing world.
Historically the use of UVC for sterilization has not been widely deployed as the lamps contain high levels of mercury. Owing to the hazard of mercury vapor, these lamps require double containment, robust, industrial housings, and sophisticated power supplies to match. Not the sort of technology that could be incorporated easily into a cell phone, or other small or low-cost devices.
LEDs can be manufactured to produce UVC radiation. However, there are currently two caveats. First, the efficiency of commercially available, short-wavelength LEDs is very low at around 5 . Put another way, 95 percent of the electrical energy is converted to heat so UVC LEDs have major challenges with thermal management. The second problem, UVC radiation will degrade all organic materials. This means the LED package, optics and PCB can only be made from inorganic materials. No glues, injection mouldings or solder masks are permitted.
Single UVC LEDs often resemble early transistor technology with window TO-cans used for packaging low-power devices. These are fine for use in scientific instruments, but industry needs high-intensity sources for processes like water or surface sterilisation. Small arrays commonly use high temperature-co-fired ceramic substrates with a silicone cover/optic, but where brute photons are required, the thermal problem becomes so extreme that direct water cooling is the only viable approach. Arrays of LEDs are attached to a PCB of exceptional thermal conductivity. The PCB is then soldered in a water-cooled housing having a quartz cover. Due to the restriction on organic materials and the need for very low thermal resistance, the choice of PCB material is currently pretty much limited to aluminium nitride or nanoceramic-coated aluminium. Both are well established as substrates for conventional high-brightness LEDs so can be easily repositioned in this new role.
Given the high cost of short-wavelength LEDs and available packaging solutions, significant technological innovation is likely to accompany development of a plethora of new and exciting products ranging from personal hygiene to food preservation and ‘dry’ cleaning of fabrics.
As the ‘next big thing’ the sterilization market is an interesting (and potentially very profitable) new market. And if they address the unique thermal challenges of this application appropriately many LED companies will have the opportunity to clean up.
About the Author
Dr. Giles Humpston is a metallurgist by profession and has a doctorate in alloy phase equilibria. He is a cited inventor on more than 250 patents and has co-authored over 150 papers as well as several text books. Dr Humpston currently works as the Field Applications Manager for Cambridge Nanotherm on thermal substrate technologies.