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The Latest Adhesive Bioinspiration: Sandcastle Worms

Producing bonds that remain robust upon long-term immersion in water is a challenge that most adhesives on todays market would fail in. However, along the Californian coast worms are secreting adhesives that do just that, and using them to build undersea colonies the size of cars, a grain of sand of a time. Now, researchers are beginning to understand how the worms adhesives work. They hope that replicating them will help to fill the largely neglected market need for adhesives that can repair bone in the watery human body.

Phragmatopoma californica, the sandcastle worm, builds itself a tube-shaped home using a self-produced natural adhesive, and together communities of these worms build large colonies. "They will not leave their tubes," explains Russell Stewart, associate professor of bioengineering at the University of Utah¹. Instead, the worms have dozens of tentacles that reach out to gather food and building materials. Tiny hairs brush material down the tentacles so they can be grabbed by the a pincer-like "building organ". The worm "secretes two little dabs of glue onto the particle," says Stewart. "The building organ puts it onto the end of the tube and holds it there for about 25 seconds, wiggling it a little to see if the glue is set, and then it lets go. The glue is designed to set up and harden within 30 seconds after the worm secretes it." This worm undertakes this process under cold seawater, where the adhesive hardens to a tough, leathery, consistency within hours.

Figure 1: The sandcastle worm makes a protective home out of beads of zirconium oxide in a lab. At the University of Utah, scientists have created a synthetic version of the glue used to bind the beads for possible use in repairing fractured bones.
Credit: Fred Hayes.

Stewart and his co-workers found that this adhesive comprises a mixture of one highly acidic and two highly basic proteins, plus magnesium and calcium ions, that form into 50-80 nm diameter foam spheres². One of the three major proteins in the spheres contained noticeable quantities of 3,4-dihydroxyphenyl-L-alanine, also known as dopa, in its sequence. Dopa is also an important feature of the natural adhesives produced by mussels, which have been studied since the 1980s.

The different proteins interact at the pHs seen in the worms body, forming into complex liquid structures called coacervates. When the overall pH of the proteins is adjusted towards net neutrality the coacervate, a single liquid phase of concentrated protein, forms and is surrounded by another more dilute phase. The coacervates behave rheologically like a viscous particle dispersion rather than a viscoelastic polymer solution.

Figure 1: This scanning electron microscope image shows two glass beads that were glued together by a sandcastle worm as it built its tube-shaped home using the beads, which it was given in the laboratory.
Credit: Russell Stewart

The pH dependency helps explain how the worms watery liquid glue binds wet mineral substrates without dispersing into the ocean. The rapid change in pH from around 5 inside the worm to 8.2 in seawater is likely to trigger the rapid change from coacervate to leathery adhesive.

To mimic this effect, the Utah team has produced adhesives from water-soluble polyacrylate copolymers, mixed with divalent magnesium and calcium cations. The proteins were simulated by sidechain selection: 3,4-dihydroxyphenol and phosphate sidechains for the acidic protein; and amines for the basic protein. The mixture formed a dense phase with the properties of a complex coacervate at pH 7.2 and 8.3.

The researchers then optimized their adhesive at pH 8.2, the same pH as seawater, and used it to bond wet cow bones. They recorded a bond strength that was 37% of what a commercial cyanoacrylate adhesive used on the same substrates could achieve, and approximately a third of that of actual sandcastle worm glue.

The bonds failed cohesively, leaving adhesive on both surfaces of the bone, reinforcing Stewarts assertion that sandcastle worm glue mimics will be well suited to mineral substrates. The surface of the bone’s hydroxyapatite mineral phase is an array of both positive and negative charges that can interact with the acidic and basic side chains. Catechol-containing molecules like dopa in particula are known to wet out hydroxyapatite well.

Since the first work studying how mussels, and more recently geckos, hold on to surfaces, these creatures have provided researchers with excellent material to help them understand and improve adhesion. Now they have been joined by the sandcastle worm, adding coacervates to the options for mimicking nature open to adhesive scientists.

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