Cheap, light, flexible, yet robust circuit boards are critical for wearable electronics, among other applications. In the future, those electronics might be printed on flexible circuits made out of bacterial cultures used to make the popular fermented black tea drink called kombucha, according to a recent paper posted to the arXiv preprint server.<p>As we've reported previously, making kombucha merely requires combining tea and sugar with a kombucha culture known as a SCOBY (symbiotic culture of bacteria and yeast), aka the "mother"—also known as a tea mushroom, tea fungus, or a Manchurian mushroom. It's akin to a sourdough starter. A SCOBY is a firm, gel-like collection of cellulose fiber (biofilm), courtesy of the active bacteria in the culture creating the perfect breeding ground for the yeast and bacteria to flourish. Dissolve the sugar in non-chlorinated boiling water, then steep some tea leaves of your choice in the hot sugar-water before discarding them.<p>Once the tea cools, add the SCOBY and pour the whole thing into a sterilized beaker or jar. Then cover the beaker or jar with a paper towel or cheesecloth to keep out insects, let it sit for two to three weeks, and voila! You've got your own home-brewed kombucha. A new "daughter" SCOBY will be floating right at the top of the liquid (technically known in this form as a pellicle).<p>Beyond the popularity of the beverage, kombucha cultures hold promise as a useful biomaterial. For instance, in 2016, an Iowa State professor of apparel, merchandising, and design named Young-A Lee gained attention for her proof-of-concept research in using dried SCOBY as a sustainable leather substitute for biodegradable SCOBY-based clothing, shoes, or handbags. In 2021, scientists at Massachusetts Institute of Technology and Imperial College London created new kinds of tough "living materials" that could one day be used as biosensors, helping purify water or detect damage to "smart" packing materials. Experiments last year by researchers at Montana Technological University (MTU) and Arizona State University (ASU) showed that membranes grown from kombucha cultures were better at preventing the formation of biofilms—a significant challenge in water filtration—than current commercial membranes.<p>“Nowadays kombucha is emerging as a promising candidate to produce sustainable textiles to be used as eco-friendly bio wearables,” co-author Andrew Adamatzky, of the University of the West of England in Bristol, told New Scientist. “We will see that dried—and hopefully living—kombucha mats will be incorporated in smart wearables that extend the functionality of clothes and gadgets. We propose to develop smart eco-wearables which are a convergence of dead and alive biological matter.”<p>Adamatzky previously co-authored a 2021 paper demonstrating that living kombucha mats showed dynamic electrical activity and stimulating responses, as well as a paper last year describing the development of a bacterial reactive glove to serve as a living electronic sensing device. Inspired by the potential of kombucha mats for wearable electronics, he and his latest co-authors have now demonstrated that it's possible to print electronic circuits onto dried SCOBY mats.<p>The team used commercially sourced kombucha bacteria to grow their mats, then air-dried the cultures on plastic or paper at room temperature. The mats don't tear easily and are not easily destroyed, even when immersed in water for several days. One of the test mats even survived oven temperatures up to 200° C (392° F), although the mats will burn when exposed to an open flame. Adamatzky et al. were able to print conductive polymer circuits onto the dried kombucha mats with an aerosol jet printer and also successfully tested an alternative method of 3D printing a circuit out of a conductive polyester/copper mix. They could even attach small LEDs to the circuits with an epoxy adhesive spiked with silver, which were still functioning after repeatedly being bent and stretched.<p>According to Adamatzky et al., unlike the living kombucha mats he worked with previously, the dried SCOBY mats are non-conductive, confining the electrical current to the printed circuit. The mats are also lighter, cheaper, and more flexible than the ceramic or plastic alternatives. Potential applications include wearable heart rate monitors, for instance, and other kombucha-based devices. "Future research will be concerned with printing advanced functional circuits, capable for detecting—and maybe recognizing—mechanical, optical, and chemical stimuli," the authors concluded.