What can electronic plasters and adhesive tattoos do?

We usually use plasters to protect wounds from dirt. But the adhesive strips can serve completely different purposes on the skin: Plasters can be equipped to warn us, for example, when we drink too much alcohol, when we have a fever, or when our blood sugar is too low. Some adhesive tattoos are so intelligent that they also release active ingredients when the body needs them. But how does that work? And what modern options are already available?

Researchers developed the first "electronic patches" as long as ten years ago. Behind them are mostly transparent, air-permeable plastic films to which tiny circuits, sensors, LEDs, solar cells, antennas or other electronic systems are attached. These so-called "electronic epidermal systems" can be applied to the human skin in the form of a wafer-thin patch - no glue or needles are required to pierce the skin, and there is no need for cables to connect the power supply.

For the person wearing the electronics, it is hardly noticeable because the patch usually weighs less than a gram. Some of these systems are even so small that they can be integrated into a stick-on tattoo. Both patches and tattoos can remain stuck to the skin for longer periods of time, because they fold, bend and stretch along with the skin as it moves. This makes them versatile: Depending on the intended use, electric patches and adhesive tattoos can be stuck to the arm, neck or chest, for example.

Plasters that indicate the state of wound healing.

This development may make it easier to care for chronic wounds, for example. Normally, it is customary for such wounds to be checked regularly to check that they are not infected with pathogens. Until now, this required changing the dressing, but this irritated the wound area and increased the risk of pathogens entering the wound from the outside.

Thanks to modern plasters, this is no longer absolutely necessary: instead of having to remove the dressing again and again, these technically "upgraded" adhesive strips have special sensors that fluoresce in color when illuminated with UV lamps. The color indicates whether the wound is healing normally or whether an infection is developing.

This is possible because the sensor molecules react to the biochemical changes that occur in a wound during the healing process. For example, depending on the phase of healing, the amount of oxygen increases or decreases and the pH value also changes. If the wound heals normally, for example, the pH rises to a value of eight, then falls to a value of five to six. However, if a wound does not close, the pH value remains between seven and eight.

Modern thermometer

But it's not just wound healing that can be monitored more easily: In the future, plasters equipped with sensors could also indicate the temperature of the skin's surface. To do this, for example, electrodes and circuits made of very thin gold wires are attached to stretchable films, which are encased in a special plastic that insulates them and protects them from moisture. These sensors measure skin temperature variations as well as expensive infrared cameras commonly used in medicine.

Continuous monitoring of skin temperature can be important for physicians because it gives them clues about the patient's blood flow: If the blood vessels are dilated and there is a lot of blood flow, our skin becomes warm. If the vessels contract, it cools down again. If the blood flow is disturbed, the patient may have heart problems or suffer from a disease of other important organs. In the case of wounds, an elevated temperature may be a sign of infection.

However, these thermometer patches still exist only as prototypes. Until they are launched on the market, scientists need to further optimize their wireless power supply in particular.

Warning against dehydration or too much alcohol

The sensor patches could also be suitable as warning systems. For example, adhesive strips with electrical systems that monitor a person's water balance are under development. If the wearer of this patch has drunk too little, the sensors register this and send a warning message to a wristband or to the smartphone of the person affected or a relative.

For example, people who are particularly at risk from a lack of water, such as dementia patients who forget to drink, or athletes, can always keep an eye on their water balance and thus prevent damage to their health, such as overheating of the body and subsequent dizziness, circulatory problems or even kidney failure.

The same applies to drinking alcohol - except that the warning is not for too little, but for too much: Newly developed sensor patches on the skin can detect even the smallest traces of alcohol in sweat. To do this, a certain active ingredient on the adhesive tattoos first stimulates the skin to sweat, and another substance reacts with the alcohol in the sweat. This electrochemical reaction in turn generates a signal in the measuring electrodes, which then transmit it to the smartphone.

In this way, the intelligent patch reveals whether we are still fit to drive after a party, for example, and creates an overview of how much alcohol we are drinking, even if we don't keep track of it at the party itself. It could also be of help in the future for people who want to reduce their alcohol consumption.

Check and regulate blood sugar

And it's not just alcohol that can be measured through sweat: Blood sugar can also be tested using patches that examine body fluid. In the future, this could spare diabetics the painful prick in the fingertip with which they take a drop of blood to check their blood sugar. This is because they have to check their blood glucose levels regularly because either not enough insulin is produced or the blood glucose hormone is no longer effective enough. As a result, the blood sugar is no longer transported from the blood into the cells, which can lead to hyperglycemia. To prevent this, those affected have to take medication or inject themselves with insulin.

In the future, such patches could even take over the supply of insulin in the event of hyperglycemia. Researchers are already working on intelligent patches that could replace the unpleasant and, if the dosage is incorrect, life-threatening insulin injections. In addition to the blood glucose sensor, the patch is to be equipped with tiny needles that administer the exact amount of medication into the skin needed to lower blood glucose levels.

To do this, the tiny needles are coated with a barrier that normally prevents the drug from being released. But when the sensors register an elevated blood glucose level, a built-in heating element begins to heat the needles. At around 41 degrees Celsius, the barrier material evaporates, the protective layer disappears and the drug thus enters the body. At least in experiments with mice, such a patch has already been shown to work successfully: It even kept blood glucose levels stable longer than conventional injections. Such patches with active ingredient reservoirs and microneedles are also being developed and tested as drug dispensers for other diseases. Even some vaccinations could be administered with such patches instead of needles in the future.

Research on patches continues

And research is still being conducted on other electronic patches: For example, scientists at the Fraunhofer Institute for Reliability and Microintegration have been trying since last year to develop a sensory patch for athletes that analyzes sweat and immediately sends data on the wearer's cardiovascular system, such as blood pressure and pulse, to a smartphone, for example. Adhesive strips that measure certain minerals such as potassium and sodium or, for example, uric acid in sweat are also already being tested.

In addition, the electronic patches are being further optimized. In the meantime, there are adhesive sensors that are only paper-thin films. They are made of water-soluble microfibers that wash off after being applied to the skin, leaving only a mesh of tiny gold wires that adheres so tightly to the skin that it adapts to all movements and does not come off. Even after being stretched thousands of times, the electrical conductivity of the nanowires remains intact. The gaps between the wires are so large that they barely block sweat glands in the skin. In addition, the wire mesh is very skin-friendly.

Other researchers have also succeeded in producing a patch with a wire net made of iron and a film of transparent plastic made of a certain cellulose extracted from plants. Thus, it is neither harmful to the environment nor toxic to humans. Moreover, these patches not only conform well to the skin and can be worn for days, but can be peeled off after use and simply dissolved in vinegar without leaving any toxic ingredients behind.

And research on the smart adhesive strips is still ongoing. For example, experts are still looking for ways to make the production and materials of the patches so inexpensive that they can be used by patients in countries that have little money for health care.