Poster V16 - Reversibly Sticking Metals and Graphite to Tissues: A Potential Route to Anchor Biomedical Implants in the Body

Introduction: Adhesion between soft and hard solids has attracted great attention in recent years. Common examples of soft solids are hydrogels, which are three-dimensional networks of polymer chains swollen with water. Other examples of soft, aqueous materials include plant products (fruits and vegetables), aquatic animals, and the tissues in our body. Indeed, typical biological cells are soft and gel-like, with a water content around 70%. Yet, the architecture of vertebrate animals, including humans, shows the need to combine soft tissues with hard structural elements, i.e., bones, vertebrae, and the skeleton. The hard elements are needed to support the weight of the animal, provide structural integrity, and transmit force. The need to combine soft and hard elements also comes into the fore when designing soft robots or actuators. In this talk, we will show that a low DC voltage can trigger adhesion between soft, ionic-conductive materials and hard, electronic-conductive materials without the need for any adhesives. We believe that this phenomenon could be useful in various applications, such as implantation of hard structural materials into the human body.

Materials and

Methods: The hard materials used in this study include graphite, copper, lead, tin, nickel, iron, zinc, steel and titanium. The soft materials include various hydrogels (including both synthetic gels like polyacrylamide and biopolymer gels like alginate and gelatin), animal tissues and plant tissues (fruits and vegetables). Two slabs of the hard materials are placed in contact with the soft material and a DC voltage of typically 5 V is applied for 3 to 15 min.

Results, Conclusions, and Discussions: A low DC voltage induces adhesion between various hard-soft pairs [1]. 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 graphite anode (+) and the AAm gel in ~ 3 min. This adhesion endures after the field is removed and we term this phenomenon as hard-soft electroadhesion or EA[HS]. The adhesion is very strong: the adhesion strength measured by pull-off testing exceeds 150 kPa. EA[HS] works with chemical gels like AAm, physical gels like gelatin and alginate, and even soft, everyday objects like fruit (bananas, apples) and animal tissue (beef, pork). Depending on the gel chemistry, adhesion occurs at the anode (+), cathode (–), both electrodes, or neither. If adhesion is observed only to one electrode, it can typically be reversed by switching the polarity of the electrodes and re-applying the field. Hard objects that can be adhered via EA[HS] to AAm gels follow the electrochemical series: i.e., their reduction potential exceeds a critical value. For example, adhesion 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 hard material (electrode) and the polymers in the gel network. EA[HS] can be used to induce adhesion in contexts that are relevant for biomedical applications. For instance, stainless steel and titanium are two hard materials that are frequently used as implants in the body. We will show that both these metals can be adhered to a variety of tissues by this technique. Thus, EA[HS] could be used to anchor such implants in a specific location in the body.
[1] W. Xu, F. A. Burni and S. R. Raghavan, “Reversibly sticking metals and graphite to hydrogels and tissues.” ACS Central Science, 10, 695-707 (2024).

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