Tiny, Battery-Free Wearable Device Records Exposure to Light Outdoors and During Phototherapy

Researchers at the Northwestern Medicine and McCormick School of Engineering have developed the world’s smallest wearable, battery-free device for measuring the exposure to light over several wavelengths, from the ultraviolet (UV) to visible and even infrared parts of the solar spectrum. The device has the ability to record up to three individual wavelengths of light at the same instant.

Miniaturized, battery-free wireless devices capable of accurately monitoring exposure to the sun or to lamps for phototherapy. (Imagine credit: John Rogers Group/Northwestern University)

The fundamental physics of the device and extensions of the platform to a wide range of clinical applications were described in a study reported in Science Translational Medicine. These foundational ideas are the basis of consumer devices launched in November 2018 to alert users to their UVA exposure, allowing them to take steps to protect their skin from sun damage.

Upon mounting the solar-powered, virtually indestructible device on human study participants, multiple forms of light exposure were recorded during outdoor activities, even in the water. The device monitored therapeutic UV light in clinical phototherapy booths for psoriasis and atopic dermatitis and also blue light phototherapy for newborns with jaundice in the neonatal intensive care unit. It also exhibited the potential to measure white light exposure for seasonal affective disorder.

In essence, it allows precision phototherapy for these health conditions and can monitor, accurately and separately, UVA and UVB exposure for people at high risk for melanoma, a deadly type of skin cancer. The sensor can help warn recreational users about impending sunburn.

The device was developed by a group of investigators in the team of John Rogers, PhD, the Louis Simpson and Kimberly Querrey Professor of Materials Science and Engineering, Biomedical Engineering in the McCormick School of Engineering and a professor of Neurological Surgery at Northwestern University Feinberg School of Medicine.

From the standpoint of the user, it couldn’t be easier to use—it’s always on yet never needs to be recharged. It weighs as much as a raindrop, has a diameter smaller than that of an M&M and the thickness of a credit card. You can mount it on your hat or glue it to your sunglasses or watch.

John Rogers, PhD, Professor of Neurological Surgery, Northwestern University Feinberg School of Medicine

It is also waterproof, robust, and avoids the need for a battery. “There are no switches or interfaces to wear out, and it is completely sealed in a thin layer of transparent plastic,” stated Rogers. “It interacts wirelessly with your phone. We think it will last forever.”

Rogers made attempts to break it. His students dunked devices in a simulated washing machine and in boiling water. It could still work.

Northwestern researchers are specifically excited about the use of the device for measuring the entire UV spectrum and accumulating total daily exposure.

There is a critical need for technologies that can accurately measure and promote safe UV exposure at a personalized level in natural environments,” stated co-senior author Steve Xu, MD, MSc, ’18 GME, an instructor of Dermatology and a Northwestern Medicine dermatologist.

We hope people with information about their UV exposure will develop healthier habits when out in the sun. UV light is ubiquitous and carcinogenic. Skin cancer is the most common type of cancer worldwide. Right now, people don’t know how much UV light they are actually getting. This device helps you maintain an awareness and for skin cancer survivors, could also keep their dermatologists informed.

Steve Xu, Dermatologist, Northwestern University Feinberg School of Medicine

The researchers stated that the interactions of the light wavelengths with the skin and body are in different ways.

Being able to split out and separately measure exposure to different wavelengths of light is really important. UVB is the shortest wavelength and the most dangerous in terms of developing cancer. A single photon of UVB light is 1,000 times more erythrogenic, or redness inducing, compared to a single photon of UVA.

John Rogers, PhD, Professor of Neurological Surgery, Northwestern University Feinberg School of Medicine

Moreover, the intensity of light’s biological effect varies continuously based on weather patterns, space, and time.

If you’re out in the sun at noon in the Caribbean, that sunlight energy is very different than noon on the same day in Chicago,” stated Xu.

In the United States, skin cancer is reaching epidemic proportions. Squamous cell carcinoma and basal cell carcinoma of the skin account for over 5.4 million cases per year at a cost of US$8.1 billion every year. It has been estimated that in 2018, there will be 178,000 new cases of melanoma, leading to 9000 deaths. One person dies of melanoma every hour.

First accurate dosing of phototherapy

At present, the amount of light actually received by patients from phototherapy is not measured. “We know that the lamps for phototherapy are not uniform in their output—a sensor like this can help target problem areas of the skin that aren’t getting better,” stated Xu. Physicians are not aware of the amount of blue light that is actually absorbed by a jaundiced newborn or the amount of white light received by a patient with seasonal affective disorder from a light box. For the first time, the new device will measure this and enable physicians to optimize the therapy by altering the position of the patient or the light source.

According to the researchers, since the device operates in an “always on” mode, its measurements are highly precise and accurate compared to any other light dosimeter currently available. Existing dosimeters sample light intensity only briefly at preset time intervals and assume the light intensity at times between those measurements to be constant, which is not necessarily the case, specifically in active, outdoor-use scenarios. They are also heavy, clunky, and expensive.

How the tiny sensor works

Light passing through a window in the sensor strikes a millimeter-scale semiconductor photodetector. This device generates a minuscule electrical current with a magnitude proportional to the light’s intensity. This current passes to an electronic component known as a capacitor in which the associated charge is absorbed and stored. The voltage across this capacitor is read by a communication chip embedded in the sensor, which passes the result digitally and wirelessly to the smartphone of the user. Simultaneously, it discharges the capacitor, thus resetting the device.

Multiple capacitors and detectors enable separate measurements of UVA and UVB exposure. The device communicates with the phone of the users to access information related to global UV index (the amount of light coming through the clouds) and weather. The user can combine this information and infer the time period for which they have been in the direct sun and out of shade. The phone of the user can then send an alert if they have been in the sun for a very long time and need to move to a shady place.

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