Effect of Metals on Underwater Adhesion of Gastropod Adhesive Mucus

Many gastropods release mucus hydrogels, which help them to remain attached to different substrates. Although not as strong as synthetic or biomimetic adhesives, some of these hydrogels have the ability to adhere to wet surfaces. These complex hydrogels mainly consist of proteins and carbohydrates, their natural cross-linking reactions being dependent on the presence of metals. In this paper, we investigated the role of metals in improving the underwater adhesive property of the mucus hydrogel from the slug Laevicaulis alte. We found that the strength and duration of attachment of two glass surfaces under water by the mucus hydrogel could be enhanced by its simple treatment with salts of metals, namely, Ca, Mg, Cu, or Zn. The degree of enhancement followed the order Ca²⁺ < Mg²⁺ < Zn²⁺ < Cu²⁺. The Cu²⁺-treated hydrogel kept two glass surfaces attached under water for about 20 days, while Zn²⁺ treatment caused attachment for about 15 days, as compared to the 3-5 days of attachment caused by the untreated gel. Treatment with both metals increased the underwater stability of the hydrogel almost threefold, presumably by strengthening its cross-linking. However, the Cu²⁺-treated hydrogel fell short of its adhesive function in the case of fast attachment within time scale of minutes, showing considerably low adhesive strength. From this study, we conclude that the treatment with Zn²⁺ is the best choice for improvement of the underwater adhesive property in terms of strength and stability. Overall, this work presents a novel biological underwater adhesive. The dynamic behavior of this multicomponent hydrogel in a versatile metal-rich environment may guide us toward designing new useful biomimetics.