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Colour changing inks: the future of health monitoring?

Researchers at Tufts University’s Silklab have developed a silk-based ink with the ability to detect physiological and environmental changes. Many people use wearable monitors and biosensors to look after their health, but current wearable monitors are often large, inconvenient devices that are not comfortable for the user. A silk-based ink to monitor health is an exciting development. 

This biomaterial based responsive ink expands on previous bioactive silk inks used for the detection of bacterial contamination in laboratories, via petri dishes or laboratory gloves. The new ink aims to enable biochemical sensing of large, conformal surfaces, such as clothing, through screen-printing (using mesh to transfer ink onto a material). 

The ink is made using regenerated silk fibroin (a protein from the silk of various insects that has been chemically processed) with a thickener (sodium alginate) and a plasticiser (glycerol). From here, a large variety of responsive molecules, such as pH indicators and enzymes, can be added to produce an ink that responds to, and indicates changes in, its environment by changing colour. The ink can then be printed onto a variety of materials, depending on its function. 

Testing has been conducted on t-shirts using a range of responsive inks, screen-printed in a uniform pattern, to detect pH changes at different location. The t-shirts were worn during exercise so that a topographical map could be produced relating to the varying pH levels (acidity) of the body, as well as indicating the distribution of sweat. Different inks can also be used to detect molecules and ions in the sweat, such as Na+ and K+, in order to determine the hydration levels of the individual and check for potential health problems. It could also be possible to find issues with a person’s diet and lifestyle, through detection of certain chemicals in their sweat. For example, the ink could indicate someone who is lacking sodium in their diet. 

Another experiment used the test molecule lactase oxidase, selected for its relevance to human exercise. Lactase oxidase is an enzyme capable of detecting lactate in the body. During exercise, muscles use up oxygen in order to produce the energy to function. Initially, exercise is aerobic as there is a sufficient supply of oxygen in the blood for efficient energy production. Once oxygen levels diminish, the body switches to anaerobic energy production, during which there is the production of lactate. Measuring lactate can show a person’s fitness levels; the faster the body produces lactate (respires anaerobically) the less efficient their body is. Usually, lactate is measured using enzymatic assays (laboratory-based methods to study enzymes) however, there are issues with long term storage of dried proteins. This issue is resolved using the ink as it is more stable. Additionally, the ink gives a more uniform colour change and would show a result much faster than performing an assay.

Responsive inks also have many applications in other areas, such as in the armed forces on tents or clothing to ensure individuals are healthy, or indicate when areas are exposed to chemicals in the air. Similarly, people working in hospitals or laboratories could wear clothing incorporated with the ink to ensure they are not carrying potentially harmful bacteria. 

The main advantage of the ink is the incorporation into fabrics, allowing comfortable, hassle-free detection, without the usual large, obstructive devices. It allows human physiology and performance to be monitored non-invasively using biological fluids (sweat, tears, saliva, breath etc.), and is often much faster than standard techniques. The colour changes can even be seen by the naked eye, although scientists can use camera-analysing techniques to find a more detailed colour variation. 

One issue to be addressed is how other external environmental factors might affect the colour changes. More testing is required to account for varying temperatures, light exposures, and other scenarios. 

Nevertheless, colour changing ink is a promising avenue for future health and environmental monitoring.  It is more wearable, stable, and faster than current methods.

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