Finding Bio-inspiration in Slug-Made Hydrogels

With a background in comparative biomechanics, Andrew Smith’s research on adhesive biomaterials has become more interdisciplinary over the course of his research career, intermingling biochemistry and mechanical assays to investigate sticky organisms through an engineering lens. In his laboratory at Ithaca College, Smith takes inspiration from a particularly unique slug, Arion subfuscus, which oozes an abundance of glue-like material as a defense mechanism. The slug’s unique hydrogel production serves as a remarkable model for designing synthetic adhesives with medical potential.

What are the properties of good medical adhesives?

The classic medical adhesive that people use has the same chemistry as super glue. It is strong but it is a hard plastic that is not super flexible and is not suited for internal use in the human body. Ideally, a medical adhesive would be made of a material that is more flexible to match the properties of the skin, like a tough hydrogel that is strong and flexible.

Another drawback to super glue-type adhesives is that if the injury is not sterile underneath, bacteria can be trapped under the adhesive and cause a bad infection. In an emergency situation, you may want an adhesive that can easily spread over an injury, set and seal to stop the bleeding, and then can be removed at the hospital. That is the most recent thing that we figured out from the slug, how to get a gel that is tough, sets fast, and has a reversible adhesion.

The slug secretes a material that, within seconds, turns into this rubbery, elastic mass that sticks everything together. Some slugs can climb walls and they have a little bit of stickiness, but Arion subfuscus secretes roughly five to 10 percent of its body mass in glue at a time, which spreads, sticks, and sets quickly. But if you soak the glue in a high salt solution, it will peel away.

How do you study slug hydrogels?

We used classic and modern biochemistry and molecular biology to determine the glue components, including spectroscopy and enzymatic digestion assays to determine the important ingredients, such as proteins, carbohydrates, and metal ions. We sequenced all of the slug’s mRNA transcripts and matched every protein in the glue with its transcript.

We identified which glue proteins could stick to different surfaces. They are all modified lectins, which is a common type of protein that typically binds to carbohydrates and can be modified to bind different molecules.¹ They have many aromatic side chains, which stick to wet surfaces well. We think these lectins are involved in adhesion through both hydrophobic and charge-based interactions. That means they stick to just about anything, but high salt concentrations wash most of them off.

How do the slugs create tough hydrogels?

Usually, tough hydrogels take a long time to set because the toughening mechanisms are a multi-step process during ingredient mixing. We investigated the slug’s secretion glands to see how the components of the glue could mix and set so quickly.

We identified two main glands involved in secretion, but the real shock was when we looked at the gland that was producing the protein components.² We were confident that the glue’s stiff properties came from proteins and that the long carbohydrates provided stretchiness. When we looked at the protein glands, they were not homogeneous. In parts of the gland, the material looked very granular like little spots, and then in another part it looked solid. We realized that the glue was setting before it was released. That gets around the problem of making a tough hydrogel, because the slug makes microscopic bits of gel in advance and assembles them afterwards, gluing them together like bricks and mortar. It is a complex mechanism and it is hard to duplicate. But if scientists can duplicate it, it will be easy to make a tough hydrogel that will adhere to anything.

What are the future applications for bio-inspired adhesives?

I would love to make a medical adhesive that could replace stitches. I have studied gastropods enough to see that snails and slugs can produce different forms of these gels with different properties―they have a whole toolkit. The long-term goal is to make designer gels with whatever properties we want; fast setting, slow setting, more reversible, less reversible, tougher, matching the mechanics of different tissues. If you make a really cool material, who knows how it could be used.

This interview has been condensed and edited for clarity.