Reversibly Sticking Metals and Graphite to Hydrogels and Tissues

We have discovered that hard, electrical conductors (e.g., metals or graphite) can be adhered to soft, aqueous materials (e.g., hydrogels, fruit, or animal tissue) without the use of an adhesive. The adhesion is induced by a low DC electric field. As an example, when 5 V DC is applied to graphite slabs spanning a tall cylindrical gel of acrylamide (AAm), a strong adhesion develops between the anode (+) and the gel in about 3 min. This adhesion endures after the field is removed, and we term it as hard–soft electroadhesion or EA[HS]. Depending on the material, adhesion occurs at the anode (+), cathode (−), or both electrodes. In many cases, EA[HS] can be reversed by reapplying the field with reversed polarity. Adhesion via EA[HS] to AAm gels follows the electrochemical series: e.g., it occurs with copper, lead, and tin but not nickel, iron, or zinc. We show that EA[HS] arises via electrochemical reactions that generate chemical bonds between the electrode and the polymers in the gel. EA[HS] can create new hybrid materials, thus enabling applications in robotics, energy storage, and biomedical implants. Interestingly, EA[HS] can even be achieved underwater, where typical adhesives cannot be used.

The experiments are very simple. For example, two graphite slabs are placed on either side of a cylindrical hydrogel (5 cm tall), and 5 V DC is applied across the combination for ∼3 min. After this period, one of the graphite slabs is found to be strongly stuck to the hydrogel (see Figure 1). This adhesion endures long after the field is removed (gel-graphite pairs have remained adhered for months). We term this phenomenon as hard–soft electroadhesion or EA[HS], and we emphasize that it is conceptually different from all previous uses of the term “electroadhesion”. Adhesion can be achieved in just a few seconds if the gel has high ionic conductivity. The adhesion is very strong: the adhesion strength is limited mostly by the strength of the gel and is shown to exceed 150 kPa.

Adhesion fails if the gel dries out.