Smart Tattoo Radiation Sensors
Tattoos that read UV and gamma dose from inside the skin.
Every wearable dosimeter shares one flaw: it can be taken off. Badges get forgotten, left in a locker, or removed by whoever least wants the reading. A sensor implanted in the skin cannot be misplaced, does not chafe, and is still there years later. A single one is a millimeter or two across. The particles that make it work are a thousand times smaller than that.
Three of them exist so far, each answering a different question.
Solar Freckles came first, and were the first intradermal nanosensors demonstrated in living human skin rather than a model. They are ultraviolet radiometers: dermally implanted colorimetric sensors that turn bluer the harder the sun is hitting you, in real time and readable by naked eye. The color fades again within minutes of stepping into shade or applying sunscreen, which makes them a nagging visual reminder rather than a number in an app. The implantation takes seconds and the sensors keep working for months to years. Tattooable in-skin sensing had been a science fiction premise since the 1990s; this was the version that worked in a person.
The gamma dosimeter takes the same idea somewhere more consequential. Radiation workers, first responders and patients need to know their ionizing dose, and that is precisely the setting where a removable badge is least trustworthy. We doped polystyrene nanoparticles with photoresponsive dithienylethenes to make a colorimetric gamma-ray dosimeter that is reversible: it darkens with dose and is reset with red light, so the same implant can be read and reused. It is a proof of concept, and it points at personal dosimetry that cannot be forgotten or tampered with.
The dosimeters answer the other half of the ultraviolet question. A radiometer tells you how strong the sun is now; a dosimeter counts what you have accumulated, which is what actually determines whether you got enough light for vitamin D or too much for your skin. A redesigned generation of UV-photochromic pigments gave us better tunability, stability and biocompatibility, and the same chemistry can be formulated to behave as a dosimeter or as a plain sunscreen-efficacy monitor. They stay functional and re-usable in the dermis for years, which is the part that makes daily use realistic where a disposable sticker is not.
Skin cancer is the most common malignancy in North America, Europe and Australia. The argument running through all three is the same: the information that helps you avoid it should live where you cannot leave it behind.
Carson J. Bruns, Associate Professor, ATLAS Institute and the Paul M. Rady Department of Mechanical Engineering, University of Colorado Boulder.