Slip, Slop, Slap – Sense

Kiwi kids have grown up with the “slip, slop, slap” mantra of how to deal with the problems of exposure to the strong ultraviolet conditions of a New Zealand summer, and avoid the associated short-term sunburn and long-term melanoma risks. In future, slipping on a shirt, slopping on some sunscreen and slapping on a hat may be supplemented with sliding on a sensor, thanks to work by MacDiarmid Principal Investigator Dr Martin Allen, of the Department of Electrical and Computer Engineering at the University of Canterbury.

These electronic sensors use special ‘ultraviolet’ semiconductors to measure the parts of the electromagnetic spectrum that are responsible for skin damage. Work on these materials forms the basis for research under the nanofabrication and devices research theme sponsored by the MacDiarmid Institute, and related commercialisation efforts backed by MacDiarmid Institute Board funding.

“Metal oxides are a special class of ‘ultraviolet’ semiconductor that are pretty unusual – they don´t play by the same rules as conventional materials,” says Allen. His research covers both the fundamental properties of these unusual materials and how to engineer their functionality to deliver devices that work specifically in the ultraviolet part of the electromagnetic spectrum.

One such example is zinc oxide, which is already a common substance in sunscreens and cosmetics, where its ability to block UV rays is well  appreciated. However, many of the fundamental semiconductor properties of zinc oxide are not well understood and form the subject of considerable debate between theorists and experimentalists.

Allen is intrigued by the fact that zinc oxide appears to have different material properties depending on how you cut its crystal. With zinc oxide, one outer surface can consist entirely of zinc atoms, while another presents a face of oxygen atoms, resulting in significantly different behaviour.

“It´s quite remarkable…[and] something we´re trying to understand and harness.”

Our sun emits a range of ultraviolet light – UVA, UVB and UVC – but only the first two pass through the atmosphere to affect humans, animals and plants. UVA is involved in premature aging by attacking the elasticity of the collagen fibres in the skin, causing the leathery wrinkling seen on those who have spent a long time in the sun. It is also implicated in skin cancer, though its role is not well understood, and Allen expects that there will be increasing interest in monitoring UVA as a result.

UVB radiation has a higher energy, causing sunburn and directly breaking the bonds in DNA, which can lead to skin cancer. It also has a positive role, as UVB radiation is involved in the production of Vitamin D in the skin. Vitamin D is necessary for bone growth, insulin secretion and other important health aspects, and its deficiency is associated with osteoporosis, bowel cancer and influenza susceptibility.

While it is a concern that New Zealand has very high rates of skin cancer, there are also growing concerns about the increasing levels of Vitamin D deficiency, especially during winter.

“Too much sun is obviously bad for you; but too little sun probably isn’t great either,” says Allen.

Being able to directly measure and monitor levels of UV exposure will provide a means of discovering what exposure levels are beneficial versus detrimental.

Allen describes the technical effort to detect wavelengths in just the UV part of the solar spectrum as very challenging since the solar energy in the visible spectrum can overwhelm poorly designed sensors. The aim is to make a “visible-blind” detector with an identical response to human skin that is small, robust and of course cheap.

A range of new devices has already been trialled, in conjunction with both the Otago and Auckland Schools of Medicine and the National Institute for Water and Atmospheric Research (NIWA). Completed studies have investigated the UV exposure of skiers at Mt Hutt Skifield, primary schoolchildren at schools across New Zealand, and outdoors workers in Central Otago. Small badges, about the size and shape of a tea-light candle, have been able to provide exposure data which was then correlated against activity diaries. A couple of hundred sensors are being provided for further studies in the sunny climes of California, as part of collaboration with USC, and Allen notes that there is considerable interest worldwide.

Allen says that one of the surprises to come from those studies was the UV exposure some children received after school. Many schools are conscious of UV exposure during the day, often requiring students to wear hats outdoors or remain under shaded areas when eating lunch, but clearly there are other aspects that bear investigation.

Apart from the health implications, there is considerable interest in using the wristband or badge sensors as a teaching tool, providing a means for students to self-monitor exposure and also conduct experiments into the effectiveness of shade, sun screens, sunglasses and clothing. Such studies, it is hoped, will lead them to improve their own sun-smart strategies as a result.

“It is known that if children find things out for themselves, rather than being directly told, they´re more likely to modify their behaviour.”

Allen is now interested in finding ways to make UV detectors that can be tuned specifically to the wavelengths involved in the production of Vitamin D. The Vitamin D issue has changed the research landscape surrounding UV exposure and far more research is needed to get any new health messages completely right since New Zealand and Australia still lead the world in terms of per capita melanoma registrations and deaths.

It´s not all about health though – UVC detectors could be used to detect the emissions from rocket launches and fires.

Ultimately, what Allen would really like to do is to find a metal oxide semiconductor that could also emit UV efficiently and cheaply. Such a discovery could lead to replacing the world´s current mercury-based fluorescent lighting with a far better, more energy-efficient solution. This would have a major impact on carbon emissions and also provide huge commercial opportunities, underpinned by this fundamental research.