Introduction: The ability to achieve adhesion ‘on command’ between two soft materials is both interesting and valuable. Imagine two solids that do not adhere upon contact, but when a stimulus is applied, strong and persistent adhesion is induced (switched on) between them. The stimulus of interest in this work is an electric field. We focus on the scenario where one solid is a hydrogel, a 3-D network of crosslinked polymer chains in water, and the other solid is a biological tissue. Could we adhere the gel to the tissue by applying an electric field? In this work, we will show that this is indeed possible, and moreover, that such electroadhesion (EA) is a universal phenomenon that extends across all kinds of soft tissues encountered in biology. We are currently exploring the use of EA for biomedical applications, i.e., to perform surgery without sutures - by simply electroadhering a gel patch.
Materials and
Methods: Cationic gels were made by mixing positively charged monomer, N,N’-dimethylaminoethyl methacrylate (QDM), with acrylamide (AAm), N,N’-methylenebis(acrylamide) (BIS), ammonium persulfate (APS), and N,N,N’,N’-tetramethylethylene-diamine (TEMED) in water. The mixture was poured into Petri dishes, polymerized under nitrogen for 1.5 h, and used within 24 h. A 10 V DC power source (Agilent E3612A) was used for electroadhesion experiments. The cationic gel was connected to the positive electrode, and the anionic material (gel, tissue, or plant) to the negative electrode. Adhesion times were 10 seconds for gel-gel, 30 seconds for gel-animal tissue, and 60 seconds for gel-plant tissue pairs. Reversal of electroadhesion was done by reversing the polarity and reapplying the voltage. Pull-off tests were performed using a TA Instruments DMA Q800. Samples were superglued to clamps and pulled apart at 0.1 N/min until failure. Rheological properties were measured using an AR2000 stress-controlled rheometer, with samples cut to a 1-2 mm thickness.
Results, Conclusions, and Discussions: In this work, we examine a range of biological tissues and test their adhesion to gels via electroadhesion (EA). We study tissues from various animals and plant-derived materials, including vegetables and fruits, covering species across the domains of life. Gels adhere by EA to tissues from various animals, including mammals (cow, pig, mouse, human), birds (chicken), fish (salmon), reptiles (lizards), amphibians (frogs), and invertebrates (shrimp, worms). Gels also adhere to plant tissues, such as fruit (plums) and vegetables (carrots). Only cationic gels adhere by EA, not anionic or nonionic gels. These cationic gels adhere strongly to certain tissues (blood vessels, muscles, cornea) but not others (brain, fat). We postulate that the higher the fraction of anionic polymers (proteins and/or polysaccharides) in the biological material, the higher the strength of EA to cationic gels. Our central finding is that EA adhesion of cationic gels is a universal property across all of biology, exhibited by numerous animal and plant tissues. We measured adhesion strength using pull-off testing and found consistent trends with regard to tissue type. For example, muscles from various animals (cow, chicken, mouse, fish) adhere to gels by EA with an adhesion strength around 15 kPa. Tissues with anisotropic structures adhere strongly in one orientation (transverse) but not in the perpendicular orientation (longitudinal). Surprisingly, gels can be adhered by EA not only to soft tissues like muscles but also to stiff ones like cartilage. These results provide insights into both the mechanism of EA and the structure of biological tissues. Lastly, we confirm the adhesion of cationic gels by EA to human cadaver tissue, suggesting potential biomedical applications. EA can also be safely achieved to tissues in live animals (mice) without causing damage due to the use of low voltage and current. The ability to affix a gel to tissue on command could revolutionize surgical procedures, offering a novel method to seal tears or cuts in tissues without sutures.
1. Borden, L. K.; et al. Nat. Commun. 2021, 12, 4419.
2. Borden, L. K.; et al. ACS Appl. Mater. Interfaces 2023, 15, 17070-17077.
