Researchers at Empa's Cellulose & Wood Materials lab together with Woo Soo Kim from the Simon Fraser University in Burnaby, Canada, have produced a flexible sensor that lies on the skin surface and is biocompatible. This is possible because it is made of nanocellulose - an inexpensive, renewable raw material, obtained in the form of crystals and fibers such as wood. Sources of the material include trees, bacteria, algae or residues from agricultural production, which means nanocellulose is relatively easy and sustainable to obtain. New composite materials based on nanocellulose can be developed and used as surface coatings, transparent packaging films or even to produce everyday objects like beverage bottles.
Researchers are also focusing on the biocompatibility of nanocellulose because it is particularly suitable for biomedical research. With the aim of producing biocompatible sensors that can measure important metabolic values, the researchers used nanocellulose as an "ink" in 3D printing processes. To make the sensors electrically conductive, the ink was mixed with silver nanowires. The researchers determined the exact ratio of nanocellulose and silver threads so that a three-dimensional network could form because cellulose nanofibers are better suited than cellulose nanocrystals to produce a cross-linked matrix with the tiny silver wires. "Cellulose nanofibers are flexible similar to cooked spaghetti, but with a diameter of only about 20 nanometers and a length of just a few micrometers," says Empa researcher Gilberto Siqueira.
The team developed sensors that measure medically-relevant metabolic parameters such as the concentration of calcium, potassium and ammonium ions. The electrochemical skin sensor sends its results wirelessly to a computer for further data processing. The tiny biochemistry lab on the skin is only half a millimeter thin. While the tiny biochemistry lab on the skin is capable of determining ion concentrations specifically and reliably, the researchers are already working on an updated version. "In the future, we want to replace the silver particles with another conductive material, for example on the basis of carbon compounds," Siqueira added. This would make the medical nanocellulose sensor not only biocompatible, but also completely biodegradable.
Stretchable Stopwatch Lights Up Human Skin
Researchers have also developed a stretchable light-emitting device that operates at low voltages and is safe for human skin. Before this discovery, alternating-current electroluminescent (ACEL) displays could be stuck on skin or other surfaces like a temporary tattoo, but required relatively high voltages to achieve sufficient brightness, which created safety concerns.
The goal was to develop an ACEL that could operate at lower voltages and be safer for human skin. Researchers sandwiched an electroluminescent layer - made of light-emitting microparticles dispersed in a stretchable dielectric material - between two flexible silver nanowire electrodes. The device contained a new type of dielectric material, in the form of ceramic nanoparticles embedded in a rubbery polymer, that increased the brightness compared with existing ACEL displays.
This display could find a broad range of applications in smart wearables, soft robotics and human-machine interfaces. They used this material to make a four-digit stopwatch display, which they mounted onto a volunteer's hand. At low voltages, the stretchable display was sufficiently bright to be seen under indoor lighting. The findings appeared in ACS Materials Letters.
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With over 30 years of writing and editing experience for newspapers, magazines and corporate communications, Kevin Kerfoot writes about natural health, nutrition, skincare and oral hygiene for Trusted Health Products’ natural health blog and newsletters.
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