Molluscan adhesive gels

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2009

A unique glue formed by animals can stick to surfaces exposed to water.

KEY CONCEPTS
• Mollusks produce adhesive gels that are very difficult to remove, particularly from surfaces exposed to water.
• Studies indicate that metals present in minor concentrations in the adhesive gels are instrumental in their formation. Removal of metals leads to the collapse of the adhesive gels.
• The role of metals in stabilizing adhesive gels suggests that small concentrations of trace metals could be involved in the formation of varnishes in lubrication systems.

Varnish is formed by the thermal and oxidative stressing of a lubricant. Oxidation of the lubricant can generate byproducts that can eventually agglomerate or polymerize to form varnish.

One of the key properties of varnish is its ability to adhere to metal surfaces. Varnish is an amorphous material that is present in a variety of colors and consistencies. If left untreated, varnish can reduce lubricant flow, plug filters and create sticky valves. All of these factors lead to reduced lubricant life, which, as a consequence, causes higher friction and wear rates.

In a previous TLT article, the origin of varnish formation in gas turbine oils was discussed (1). One key element in creating the varnish was the role of metals in facilitating the auto-oxidation process.

Mollusks are a group of animals that generate adhesive gels enabling them to attach to surfaces so firmly that they are difficult to remove. Andrew Smith, associate professor of biology at Ithaca College, in Ithaca, N.Y., says, “The adhesive generated by mollusks literally sticks onto most surfaces including grass, metals and skin.”

Smith has measured the strength of molluscan adhesive gels. He says, “For most animals, the shear strength of the adhesive is 100 kilopascals or 1 kilogram/cm.2 The amount of force needed to detach the adhesive from a surface is 200 kilopascals or 2 kilograms/cm.²”

Smith indicates that the adhesives from these animals are not as strong as epoxy glue. But they show remarkable strength when considering that the main component in these adhesives is water. Smith adds, “Molluscan adhesives contain 97% water and display superior performance compared to synthetic glues when immersed in water.”

In studying these adhesives, Smith and his research group have focused on working with the slug, Arion subfuscus, which is native to the area near Ithaca College. Smith adds, “This slug secretes the adhesive gel more as a defensive mechanism. Once secreted it transforms chemically into a very strong glue.”

Figure 3 shows an image of the slug and the glue being stretched between two forceps. Smith explains, “In being stretched, the glue is being held by the forceps on the right but is literally just sticking to the forceps on the left. Once stretched to its maximum, the glue recoils to its original shape. This image shows that the glue formed by the slug is not just a typical slime that can be produced by other organisms.”


Figure 3. Mollusks such as the slug, Arion subfuscus, produce an adhesive gel that is very difficult to remove. Two forceps are used to stretch the glue to its maximum. Once reached, the glue recoils to its original shape. (Courtesy of Ithaca College)

One of the objectives of the research being carried out by Smith’s group is to determine how the glue is formed. The potential answer may have some ramifications for how varnish is produced.

ROLE OF METALS
The remaining 3% of the slug secretion is a combination of protein and carbohydrates. Smith says, “The protein identified has a molecular weight between 15,000 and 200,000 Dalton. The carbohydrate content varies by species and in some cases is a polysaccharide.” The concentration of protein in the slug secretion is higher.

One other factor that seems to be important in the formation of the glue is the metal content of the secretion. A significantly higher level of metals is detected in the secretion than in the environment. Smith indicated that three key metals present in the secretion are iron, zinc and calcium. He adds, “There is 10 times as much zinc as iron and 100 times as much calcium as iron in the secretion.”

The level of zinc was determined to be 46 +/-7ppm and 189+/-80 ppm in two sets of experiments. Levels of iron range between 2 and 7 ppm.

Smith has concrete evidence that the protein present in the secretion is essential in the formation of crosslinks that lead to the development of the glue. He says, “We have been able to purify the proteins that are unique to the glue and show that they have gel-stiffening activity (suggesting that they are crosslinkers). The proteins in the mollusk secretions needed for the development of crosslinks appear to be unique for specific species. In the case of the slug, the protein identified shows a strong affinity to iron. This could lead to iron being involved as a key participant in forming the crosslink.”

Smith speculates that iron can catalyze a free radical-driven oxidation of specific functional groups, converting them into groups that are more likely to form crosslinks. He adds, “Iron, calcium and zinc have been known to form stable structures with various species present in the protein. For example, iron and calcium can bind with carboxyl groups present in the proteins, and zinc can interact with the amino acid, histidine.”

In trying to determine if the metals are critical to the formation of the crosslinks, a series of metal chelating agents were added to the glue to see if it would fall apart. Extraction of these metals led to the collapse of the glue. Smith says, “We worked with a variety of chelating agents including EDTA (ethylenediamine tetraacetic acid) and defroxamine. The latter is supposed to have a strong affinity specifically for iron.”

Smith has not been able to find out which, if any, of the metals are critical to the collapse of the crosslinks. He says, “We thought that using defroxamine would indicate the role of iron in the crosslinks. Unfortunately, defroxamine has been found to be effective in chelating with other metals. It grabs zinc equally as well as iron.” One conclusion reached from the chelating study is that iron is not part of the final crosslink in the glue.

Attempts to remove all of the metals and then add them back one-by-one failed. Smith found that it was hard to mix the metals into the secretion uniformly. Future work will focus on trying to extract each of the metals separately to determine their impact on the structure of the glue.

While the glue produced by these animals may not seem to have much relevance to varnish, the work described may provide some indication for the role that trace levels of metals play in the formation of varnishes in lubrication systems, especially in the presence of small concentrations of water.

Further information can be found in two recent articles (2-3) or by contacting Smith at asmith@ithaca.edu. 

REFERENCES
1. Canter, N. (2007), “Spark Discharges and Varnish Formation,” TLT, 63 (11), pp. 8–9.
2. Smith, A., Robinson, T., Salt, M., Hamilton, K., Silvia, B. and Blasiak, R. (2007), “Robust Cross-Links in Molluscan Adhesive Gels: Testing for Contributions from Hydrophobic and Electrostatic Interactions,” Comparative Biochemistry and Physiology Part B, 152 (2), pp. 110–117.
3. Werneke, S., Swann, C., Farquharson, L, Hamilton, K. and Smith, A. (2007), “The Role of Metals in Molluscan Adhesive Gels,” The Journal of Experimental Biology, 210 (12), pp. 2137–2145.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.