Glue on, glue off: Food sticks to graphite using electricity
Food is taboo in many laboratories, but not in the realm of Srinivasa Raghavan at the University of Maryland, USA. There, his team recently tested how well chicken, blueberries and bananas adhere to electrodes made of metals or graphite after they were briefly exposed to an electric field. Samples of artificial and natural hydrogels, such as those made from gelatin and alginates, were also tested. In short, it was about binding soft materials with a high water content to solid surfaces.
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The researchers themselves were surprised by the result. Not only that the gluing tests for numerous combinations were successful. The effect could also be reversed with a voltage of opposite polarity. The connections lasted at least months – “they still hold up to this day,” the study says – and they were waterproof. The team recently published the findings from the tests in the specialist journal ACS Central Science published, could be used in the future for robotics, medicine and as a new type of battery.
“It’s strange that this relatively simple phenomenon has only been discovered now,” says Raghavan. So far, apparently no one has investigated it systematically. In his team, however, the experiments were a further logical step. “We have been working with gels and electricity for a long time,” says the researcher. Among other things, his team discovered that hydrogels stick to biological tissue with electrical help. The discovery could lead to gel plasters in the future that help wound healing, as the researchers reported in the magazine in 2021 Nature reported.
Experiments with hydrogels, fruit and meat
For their new experiments, the researchers clamped samples of water-rich materials between two plates made of metal or graphite and applied a voltage. Hydrogels, peeled bananas, apples and grapes as well as tissue samples from chickens, pigs and cattle were used as test specimens. The researchers also tested several metals and observed the effects of different voltages and electrification times.
A cylindrical hydrogel sample based on acrylamide – height five centimeters, diameter two centimeters, weight 30 grams – adhered firmly to the positive pole made of graphite after just three minutes at five volts DC. The bond was so strong that the gel was torn when trying to separate it. However, if a voltage with the opposite sign was applied, the connection was released non-destructively. Without further intervention, the adhesive effect of the samples remained for months. The study says that the material should not dry out. Otherwise it would shrink in the air and the connection would loosen.
During the experiments, the team found some general connections. “The adhesion force increases with increasing voltage, time in the electric field and the ionic conductivity of the gel,” it reports. The latter can be increased by adding salt. Salts consist of oppositely charged ions and therefore charge carriers – sodium chloride, known as common salt, consists of positively charged sodium and negatively charged chloride ions.
Mixed findings
However, there was no general finding as to whether and if so, which soft, water-rich material sticks to which electrode. While tomatoes, beef and chicken stuck to the positive pole after being electrified, apples and pigs stuck to the negative pole. Banana, onion and potato stuck to both electrodes, as did a gelatin gel. These connections could then no longer be broken even by reversing the voltage. The adhesive effect did not occur at all with other test materials, for example with grapes, blueberries or cucumbers. The researchers suspect that the salt content is too low and therefore there are too few electrically conductive ions as the reason for this.
There were also differences in the metals. Copper, lead and tin, for example, stuck together with the acrylamide-based gel after a direct voltage was applied, but nickel, iron, zinc and titanium did not. The researchers write that this phenomenon must have something to do with electrochemical processes at the interfaces. The metals with the adhesive effect are nobler than the others and do not give off electrons as easily. The applied voltage then primarily has an oxidizing effect on the gel, which in turn creates the adhesive effect, according to the researchers’ hypothesis.
The phenomenon of so-called electroadhesion itself is not new. Around 1920, Danish engineers Frederik Alfred Johnsen and Knud Rahbek reported that some porous materials adhere to metals through electrical polarization. They used high electrical voltages for their experiments. As a result, the materials at the interfaces became electrically charged in opposite directions and then hung together like the north and south poles of a magnet. However, this electrostatic attraction disappears as soon as the electricity is switched off.
Colorful range of possible applications
It has recently been possible to achieve permanent adhesion between glass and hyrogel through electrification, but only with a very specific material combination, as Raghavan’s team notes in the study. The newly discovered effect is not universal, but it already offers a wide range of possible uses.
The group has already built prototypes for some applications: a type of electric gripper that lifts, sets down and releases a gel at the push of a button, a soft robotics system with gel “muscles” between two metal plates and a prototype for a gel battery Copper as a positive pole, zinc as a negative pole and two different gels in between. Perhaps the most important use case, however, is medicine, says Raghavan. He hopes that metal implants will be anchored even better in the tissue in the future thanks to the new adhesive effect.
(Older brother)
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