The usual way to get biological tissues and materials like gels and metals to stick together is using sutures, adhesives or both. Although this generally works, it’s far from ideal, with adhesives forming a barrier layer between tissues and the hard or impossible to undo nature of these methods. A viable alternative might be electroadhesion using cation and anion pairs, which uses low-voltage DC to firmly attach the two sides, with polarity reversal loosening the connection with no permanent effects. This is what a group of researchers have been investigating for a few years now, with the most recent paper on the topic called Reversibly Sticking Metals and Graphite to Hydrogels and Tissues by [Wenhao Xu] and colleagues published this year in ACS Central Science.

This follows on the 2021 study published in Nature Communications by [Leah K. Borden] and colleagues titled Reversible electroadhesion of hydrogels to animal tissues for suture-less repair of cuts or tears. In this study a cationic hydrogel (quaternized dimethyl aminoethyl methacrylate, QDM) was reversibly bonded to bovine aorta and other tissues, with said tissues functioning as the anionic element. Despite demonstrated functionality, the exact mechanism which made the application of 3-10 VDC (80 – 125 mA) for under a minute (10+ seconds) cause both sides to bond so tightly, and reversibly, is still unknown. This is where the recent study provides a mechanism and expands the applications.

Rather than just hydrogels and soft issues, the researchers found that graphite and a range of metals could also be adhered in this way, including tin, lead and copper. Meanwhile across a range of biological tissues (chicken & cow muscle, tomato, banana, potato, etc.) it was found that these were either anionic or cationic, or sometimes both. As a possible explanation the researchers hypothesize that metal-gel adhesion (hard-soft electroadhesion) is the result of oxidation resulting in chemical bonds between the surfaces, depending on the metal’s reduction potential.

The researchers demonstrate potential uses for this technology in the form of grippers as a solid grip can be maintained on an object with a reversal of the voltage potential enabling a release. The hydrogel and biological tissue adhesion in the 2021 study was demonstrated to be a viable method for replacing sutures and adhesives, while being fully reversible in case of a mistake or after the patch has served its purpose. Although the exact mechanisms here still need to be fully elucidated, it’s an interesting glimpse at a potential future where low-voltage DC is all it takes to patch up organs and tissues, connect soft and hard surfaces in robotics in a durable fashion and generally banish adhesives and suturing from time-honored fields.

Thanks to [moerkedal] for the tip.