Researchers have developed a wearable patch that warns users by changing colors when they’ve had too much sun exposure.

Striking a balance between too much and too little sun exposure can be challenging, according to researchers at the University of Southern California Viterbi School of Engineering. Too much sun can cause melanoma, which kills 1 person every 52 minutes in the U.S., according to the Skin Cancer Foundation. But sun exposure stimulates the production of Vitamin D, which is vital for good bone health.

The millimeter-thick patch notifies users by changing color depending on total UV exposure. The sensor turns orange when the user has reached the World Health Organization’s recommended dose of Vitamin D.

Andrea Armani, lead researcher of the study published in ACS Sensors, touts the wearable as solving some of the problems that other sensors face  – the new patch does not require a power source and will function when wet. It can also adjust its response when the user applies sunscreen. The patch is made of an FDA-approved, non-toxic blend of polymers intended for human use and food contact.

Moving forward, the team reported that they hope to optimize the patch for different skin types using additional coatings and tailoring the color-changing active layer.