Artificial skin with coffee to improve wearables for different skin tones

Currently, many optical medical and consumer devices based on light detection perform well if the wearer has a light skin tone. The more pigment, or melanin, the skin contains, the more the devices struggle because melanin significantly absorbs light of specific colours and can even block optical signals. For instance, optical devices for measuring blood oxygen might give inaccurate readings on patients with darker skin tones. This bias has raised concerns about diagnostic accuracy, equality and patient safety in healthcare.

Behind the problem lies optical device product development. Historically, it has given most attention to light skin. Lack of testing on darker skin and consequential inaccuracy in optical pulse oximeter readings raised public attention during the COVID-19 pandemic.

“Industrial players making optical wearables devices feel the pain, but they don't know how to cure it. Very few companies use optical phantoms, largely because they are not commercially available. Besides, their expertise and focus is in optical, electrical and biomedical engineering from the viewpoint of a device, not the object itself, such as human skin,” says Alexey Popov, Principal Scientist and the lead of VTT’s research team that has presented a groundbreaking solution in the Journal of Biomedical Optics. 

VTT’s research team decided to address the bias and risk of misdiagnosis by developing new optical skin-mimicking phantoms. The desired phantoms should be able to replicate the optical and mechanical properties of real skin across a wide range of skin tones, while exhibiting a similar reflectance spectrum.

The team designed and fabricated multilayered silicone phantoms containing the same main layers as real skin: the epidermis, the dermis with an integrated vascular structure and the subcutaneous fat. In the artificial vessels, blood-like liquid is flowing with the help of a micropump. This artificial cardiovascular system is essential for testing optical technology.

“We tested our phantoms with a wearable device also developed at VTT. It is like a smart watch with light emitting diodes and detectors that measure the circulating artificial blood in real time,” Popov says. 

For the top layer, the researchers aimed at using different skin colours matching Caucasian, South Asian and African populations. To imitate these colours in the phantoms’ skin tone, several materials were tested, such as commercial pigment mixtures – and coffee, which is a familiar choice in Finland, where coffee consumption is famously high.

“By far, the best and most practical material for mimicking melanin was instant coffee, with very interesting optical properties. Coffee has a melanin-like absorption spectrum. By changing the concentration of coffee, we were able to cover a significant portion of the skin tone colour range,” tells Alexey Popov.

Finally, the phantoms were evaluated using optical spectroscopy and hyperspectral imaging. The results were clear: VTT’s new phantoms validated their potential as valuable tools for improving the development of optical medical devices and consumer wearables for diverse populations. Even phantoms with tattoos and real ink could be developed to solve similar issues concerning millions of people.

“Another selling point is that our phantoms are also very durable. We tested them over nine months, but they last even longer. This is way better compared to phantoms that get spoiled in one-two days or a week – because our phantoms do not contain water or organic components like proteins, lipids and real blood,” says Popov. 

The solution also targeted the problem with product trials on humans. The trials are challenging as the test objects – humans and their body parts – are unstable, difficult to control, non-reproducible and not available around the clock. Phantoms close to reality could minimise or even eliminate the need for testing devices with real skin.

The research team will continue publishing results based on the use of phantoms. Topics include the diagnostics of the cardiovascular system, calibration curves adjustment of pulse oximeters for different skin tones and AI-enabled recovery of physiological parameters from skin reflectance spectra.

VTT’s team has showcased the phantoms in exhibitions and to companies, raising wide interest.

“This is a very multidisciplinary field. Very often, companies don't have the knowledge, models and specific tools to properly address problems related to skin tones. That is why they come to us and wish to rely on our expertise. We already have ongoing collaboration with prominent players in the field. In parallel, we are also pursuing public funding for implementing our ideas, which require more steps to become attractive to the industrial community,” tells Alexey Popov.

The optical skin-mimicking phantom study by VTT was funded by Business Finland's PhotonWear project and the Research Council of Finland's PREIN Flagship.