Electroadhesion of Hydrogels to Biological Tissues: A Discovery that Could Enable Sutureless Surgery

Abstract

This study concerns the topic of electroadhesion (EA), which refers to adhesion induced by an electric field. Previous research had demonstrated that a DC electric field could be used to adhere a cationic hydrogel to an anionic hydrogel. Here, we extend this phenomenon to new systems. First, we adhere cationic gels to animal (bovine) tissues by simply applying a DC field of ~ 10 V across a gel-tissue pair for 10 to 20 s. This adhesion persists indefinitely after the electric field is removed. Moreover, if the field is re-applied with reversed polarity, the EA is eliminated, and the materials can be separated. Because tissues have anionic character, only cationic gels can be stuck to them by EA. We also show that gels can be stuck over cuts or tears in tissues using EA, which can enable tissue repair (surgery) to be performed without the need for sutures or staples. Electroadhesion goes far beyond just bovine tissues. We have found that gels can be adhered by EA to tissues from various animals, including mammals (e.g., cow, pig, mouse); birds (e.g., chicken); fish (e.g., salmon); reptiles (e.g., lizards); amphibians (e.g., frogs), as well as various invertebrates (e.g., shrimp, worms). In addition, gels can also be adhered to plant tissue, including fruits (e.g., plums) and vegetables (e.g.; carrot), and also to fungi (mushrooms). In mammals, EA is strong for certain tissue types, such as arteries, intestines, and cornea across a range of species. Conversely, weak or no adhesion is observed with other mammalian tissues such as adipose and brain. These differences reveal some common themes in regard to EA: for instance, the higher the fraction of anionic polymers (proteins and/or polysaccharides) in the biological material, the higher the EA strength. Interestingly also, because tissues often have anisotropic structure, adhesion by EA can be strong in one tissue orientation, but weak or non-existent in the perpendicular one. Lastly, we delve into the mechanism behind EA. The EA strength between a cationic gel and an anionic material (gel or tissue) can be systematically enhanced in several ways. These include increasing the polymer concentration in the cationic gel as well as the cationic charge density. We also conduct experiments to unravel the contributions to EA from the charged polymer chains and the counterions. When cationic and anionic gels are contacted in the EA orientation and a high voltage of ~ 100 V is applied, the gels undergo “zipping”, i.e., they rapidly lock into adhesion due to electrostatic interactions in a manner that resembles the closing of a zip. Our findings suggest the following sequence of events for EA between gels. First, the DC field pulls counterions away from the gel-gel interface, which strongly polarizes the cationic and anionic chains at the interface. These chains then form a dense electrostatic complex (ESC), leading to adhesion of the gels. When the field is turned off, the ESC persists because it is thermodynamically stable. This explains why the adhesion remains strong and can even be permanent. Future work will investigate the applicability of EA towards surgeries, first in animals, and then potentially in humans.