May 16, 2022
The technology can be adapted to track other blood components such as glucose

Researchers have developed a subdermal functional tattoo that can inform people on how much oxygen they are using up while exercising, measure blood glucose levels, monitor a number of blood components, or identify exposure to environmental toxins. While all these applications are possible, the researchers have demonstrated it with a proof of concept tattoo using the gel form of proteins found in silk, known as fibroins, which glows brighter or dimmer when placed under a lamp, depending the amount of blood oxygen levels. The properties of fibroins make it particularly suitable as a subdermal implantable material, which is biocompatible as well.

The health and performance of the body can be understood by substances in the blood such as glucose, oxygen, lactate and electrolytes. In healthcare settings, they can be tracked by using bulky machines or extracting blood samples from the patients. The subdermal tattoo makes the process much simpler, and involves literally shining a light on the condition of the patient. Diabetics have to draw blood to monitor glucose levels, often on a daily basis, to decide what to eat when and to take medication. The subdermal tattoo makes the process easier.

When re-assembled into a gel or a film, the fibroins can be assembled into a structure that can last under the skin for a few weeks to a year. When the silk eventually breaks down, it is compatible with the body and unlikely to evoke a response from the immune system. Lead investigator of the study David Kaplan says, “Silk provides a remarkable confluence of many great properties. We can form it into films, sponges, gels and more. Not only is it biocompatible, but it can hold additives without changing their chemistry, and these additives can have sensing capabilities that detect molecules in their environment. The oxygen sensor is a proof of concept for a range of sensors we could create.”

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The silk based sensor turns purple on exposure to oxygen. (Image credit: Thomas Falucci/Tufts University)

The silk proteins can pick up and hold additives without changing their own properties. The researchers introduced an additive known as PdBMAP to the silk proteins, which glows with an intensity and duration proportional to the levels of oxygen in the surrounding environment, when exposed to light of certain wavelengths. PdBMAP is useful as it responds too light wavelengths that can penetrate the skin. The researchers rely more on the duration of the glow, as the depth, size of implant and skin colour can all influence the intensity of the glow. The higher the oxygen levels, the shorter is the period for which the implant glows. In experiments, the implant was able to accurately track high, low and normal oxygen levels in model animals.

Thomas Falcucci, who developed the tattoo sensor says, “We can envision many scenarios in which a tattoo-like sensor under the skin can be useful. That’s usually in situations where someone with a chronic condition needs to be monitored over a long period of time outside of a traditional clinical setting. We could potentially track multiple blood components using a sensor array under the skin.”

A paper describing the implant has been published in Advanced Functional Materials.

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