Solar Freckles: Long-Term Photochromic Tattoos for Intradermal Ultraviolet Radiometry

Abstract

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.

Tattoo Inks for Optical Biosensing in Interstitial Fluid

“…[ 119 ] Other different projects demonstrating the future capabilities of tattoo functionalization beyond ISF biosensing have been initiated to create an interactive platform with our skin where biosensors are no longer an external device, they are an extension of our body providing new capabilities. Butterfield et al demonstrate the use of UV‐sensitive photochromic nano‐capsule tattoo ink to indicate overexposure of UV light ( Figure A–E), and the need for sun protection, known as “solar freckles.” [ 18 ] The capsules are reversible since their color disappears rapidly after application of topical sunscreen. The major cause of skin cancer is UV exposure, these tattoos would increase preventative behaviors providing a visual reminder to protect the skin and would reduce the incidence of this pathology.…”

“…Diffusion of the sensor from the tattoo site into the ISF is a challenge that must be controlled by modifying the sensor size, [ 114 ] but is a difficult challenge since they should be small enough to avoid surgical procedures but should be large enough to be retained in the desired area, tattoo biosensors should mimic in size the traditional tattoo pigments. [ 18 ] The dimension of the implanted sensor also affects the degree of tissue damage. An example of the importance of size is the miniaturization of a CGM sensor using microbeads to coat and reduce the sensor's size.…”

“…Among the applications of functionalized tattoos are the monitoring of different physiological biomarkers such as, glucose, [ 3 ] sodium, [ 4 ] albumin, [ 5 ] oxygen, [ 6 ] cortisol, [ 7 ] lactate, [ 8 ] pH, [ 9 ] nucleic acids [ 10 ] but also external ones such as environmental contaminants [ 11 ] or toxic food additives. [ 12 ] Functionalized tattoos are possible to release drugs, [ 13 ] vaccinate, [ 14 ] and also monitor levels of alcohol ingested, [ 15 ] body temperature, [ 16 ] electrocardiogram measurements, [ 17 ] prevention of sun overexposure, [ 18 ] and even incorporate identity documents or credit card details into our skin. [ 19 ] Functionalized tattoos for ISFs sensing provide effective solutions to monitor and record the evolution of a patient's disease with the capability of multiplexing, a considerable advantage in metabolic diseases such as diabetes where more than one parameter is normally unbalanced, like pH and glucose.…”

“…The persistence of tattoos in the dermis is determined by the size of the pigment. The sensors should mimic the commercial tattoo pigments, [ 18 ] being small enough to avoid surgical procedures but large enough to avoid diffusion from the implantation site. Combining different techniques provides a complete measurement of the ink particles' size ( Table 1 ).…”

Slot‐Die Coated Organic UV Indicators and Filters Processed from Green Solvents

“…[20] These criteria are highly applicable to photochromic or photodegradable organic dyes/polymers as cost-effective and easy-processable UV active layers. [3,10,15,18,19,23,24] Photochromic dyes have been widely employed as sensitive active layers for various sensing applications such as pH, metal ions, gas and vapor, temperature, and UV light. [9] For UV sensing applications as the topic of the current study, Zheng et al reported a wearable UV indicator based on electrospun spiropyran photochromic dyed fibers and a step toward textile based wearable UV indicators.…”