While tattooable nanotechnology for in-skin sensing and communication has been a popular concept in science fiction since the 1990s, the first tattooable intradermal nanosensors have only emerged in the past few years, and none have been demonstrated in human skin. We developed a photochromic tattoo that serves as an intradermal ultraviolet (UV) radiometer that provides naked-eye feedback about UV exposure in real time. These small tattoos, or “solar freckles”, comprise dermally implanted colorimetric UV sensors in the form of nanoencapsulated leuco dyes that become more blue in color with increasing UV irradiance. We demonstrate the tattoos’ functionality for both quantitative and naked-eye UV sensing in porcine skin ex vivo, as well as in human skin in vivo. Solar freckles offer an alternative and complementary approach to self-monitoring UV exposure for the sake of skin cancer prevention. Activated solar freckles provide a visual reminder to protect the skin, and their color disappears rapidly upon removal of UV exposure or application of topical sunscreen. The sensors are implanted in a minimally invasive procedure that lasts only a few seconds, yet remain functional for months to years. These semipermanent tattoos provide an early proof-of-concept for long-term intradermal sensing nanomaterials that provide users with biomedically relevant information in the form of an observable color change.

Personal dosimeters are critical tools to ensure environmental and occupational health and safety. Inherent to all wearable dosimeters is a compliance risk because they can be removed from a person accidentally or intentionally (and thus be misplaced, forgotten, or tampered with) or can cause discomfort. Intradermal smart tattoos are nanoscale implants that overcome these limitations because they can be permanent and comfortable within the skin. Here, we demonstrate a proof of concept for reversible intradermal colorimetric γ-ray dosimeters comprising polystyrene nanoparticles doped with photoresponsive dithienylethenes. We speculate that smart tattoos sensing dangerous radiation may improve convenience and compliance in personal dosimetry applications.