Thermoelectric generators built entirely from food-grade hydrogels and vanillin harvest heat from hot meals to power edible displays that change color when food reaches safe eating temperature.
(Nanowerk Spotlight) Power has been the limiting factor for electronics designed to be swallowed. Any device that enters the digestive tract must operate without toxic materials, rigid components, or bulky energy storage, yet most power sources fail on at least one of those counts. Edible batteries made from food-safe ingredients can run circuits briefly, but their low energy density makes them impractical for sustained use. An alternative is to generate electricity where the device already is.
Thermoelectric generators offer that possibility by converting heat gradients into voltage through the Seebeck effect, as charge carriers drift from warmer regions toward cooler ones. Ingestible electronics encounter one such gradient immediately when exposed to hot food, which releases a large amount of thermal energy before digestion begins. Conventional thermoelectrics, however, rely on materials such as lead telluride or silicon–germanium alloys that are incompatible with ingestion.
Hydrogel-based thermoelectrics replace electrons with mobile ions and can bend, stretch, and function in wet environments, but most still depend on synthetic polymers, toxic crosslinking agents, or plastic additives. These constraints have left truly edible thermoelectric power sources out of reach.
A paper published in Advanced Functional Materials (“From Food to Power: Hydrogel Thermoelectrics for Ingestible Electronics”) now reports the first fully edible thermoelectric system that harvests heat from food and converts it into a visible signal. Researchers at the École Polytechnique Fédérale de Lausanne built thermoelectric generators from chitosan and alginate hydrogels, crosslinked with vanillin and loaded with potassium chloride.
These generators power an edible electrochromic display made from gelatin and anthocyanins extracted from red cabbage. Place the assembly beneath a hot chocolate cake, and the display shifts from purple to blue as the dessert cools, indicating when it is safe to eat.
Vanillin-crosslinked chitosan. a) Schematic illustration and chemical structures of chitosan, vanillin, used as a crosslinker, and the resulting hydrogel used as an edible thermoelectric generator. b) Time-dependent storage modulus (green) and loss modulus (blue) of an aqueous solution containing 5 wt% chitosan and 1.75 mol% vanillin, obtained from oscillatory rheological measurements. The inset shows a zoom-in of the initial 700 s. c) FTIR spectra of chitosan (yellow) and chitosan crosslinked with vanillin (orange). The region highlighted in blue represents the peak transitions associated with hydrogen bond formation, while the region highlighted in green corresponds to the formation of Schiff base linkages upon the addition of vanillin to chitosan. d) Stress–strain curves of chitosan that has been crosslinked with 1 mol% (yellow), 1.5 mol% (orange), and 1.75 mol% (red) vanillin. (Image: Reproduced from DOI:10.1002/adfm.202525982, CC BY) (click on image to enlarge)
The generators work through ionic thermoelectricity rather than electron flow. Charged polymer networks trap certain ions while letting others move freely, and a temperature gradient drives the mobile ions from hot to cold, creating voltage.
Chitosan, a polysaccharide derived from crustacean shells, carries a positive charge. Add potassium chloride, and the chitosan grabs onto negatively charged chloride ions, slowing them down. Potassium ions, repelled by the positive polymer, move easily. Apply heat to one side, and potassium races toward the cold end while chloride lags behind. That charge separation generates voltage, making chitosan a p-type thermoelectric material.
Turning chitosan into a sturdy hydrogel without toxic chemicals posed a key challenge. Standard crosslinkers like glutaraldehyde are poisonous. The researchers used vanillin instead, the molecule that gives vanilla its smell. Vanillin bonds covalently to chitosan’s amino groups through a Schiff base reaction. At 1.75 mol%, it produced hydrogels that stretched to 188% strain before breaking, flexible and robust enough for practical handling.
For the complementary n-type material, the team chose alginate, a negatively charged polysaccharide. Alginate traps potassium ions and releases chloride ions, reversing which species moves faster under a thermal gradient. Wire the two materials in series and their voltages add up.
The team then optimized performance. Raising salt concentration from 1 wt% to 7.5 wt% pushed thermopower to roughly 62 mV/K. Expanding the contact area between hydrogel and electrode from 1 to 5 cm² lifted thermopower from 5 to 25 mV/K. Thicker gels sustained steeper temperature gradients when the hot side sat at 60 °C. Non-edible ionic thermoelectrics typically produce 1–27 mV/K, placing this edible system at the high end of comparable performance.
One thermoelectric element alone could not power anything useful. Connecting six alternating chitosan and alginate units in series delivered about 1 V across a 20 K temperature difference, producing 21 µW. The devices held up for at least an hour, long enough for a meal.
The display used gelatin infused with anthocyanins, pigments that change color depending on their chemical environment. At rest the film looked purple; applying 1 V shifted it to blue within 15 min by altering the protonation state of the pigment molecules. Spectroscopy confirmed the color change through an absorption peak shift from 374 nm to 280 nm.
For a real-world test, the team placed six thermoelectric units and a display beneath a commercial chocolate lava cake heated to 60 °C. Edible gold electrodes deposited onto starch-based paper connected the components. Instructions on such cakes typically warn consumers to wait about 10 min before eating to avoid burns. The researchers sized the display so it turned fully blue within that window, signaling a safe temperature. Intermediate yellow-green hues at the edges traced the gradual spread of ion diffusion and the multichromatic behavior of anthocyanins.
After eating, the electronics must vanish. Immersed in simulated gastric fluid at 37 °C, the generators and display lost 90% of their mass within 6 h and dissolved completely within 24 h.
This work offers a proof of concept for self-powered edible electronics that need no battery. Beyond alerting diners to hot food, the platform might enable ingestible medical sensors that disappear after doing their job, smart packaging that monitors freshness, or environmental monitors designed to biodegrade. The study shows that food-grade materials can harvest thermal energy and drive a functional display, opening a route toward electronics safe enough to swallow.
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