Bonding: Nature is Our Guide

The history of mankind provides so many examples of bonding applications throughout the different periods of time that it is tempting to consider bond­ing to be an invention of man. However, in truth it is nature that has shown us the way. The following examples of bonding from the plant and animal kingdoms demonstrate how man has learned from nature, so enabling us to develop the technology of bonding.

Example: Paper Wasps

Thinking of bonding and the world of insects, let’s consider for a moment the paper wasp that is native to Central Europe: It has pincers that enable it to break down wood mechanically, coarsely breaking down the long fibres of cellulose by means of a scraping motion. It then eats these fragments and mixes aqueous digestive juices with them. This further shortens the length of the cellulose fibres by chemical means. The adhesive for nest-building is now ready for use. On drying, the water evaporates from the mass, the cellulose fibres form a mat and the adhesive hardens. Paper wasps can build extremely strong nests using this technique. This technology has long been used by people for decorating their homes: The tackiness of wallpaper paste is based on the same principle.

Example: Rubber Tree

Water, a solvent and dispersing agent, can be prob­lematical for the long-term stability of bonds. Nature also provides a solution here, this time from the plant kingdom: Rubber milk from rubber tree foliage is a dispersion of polymers (natural latex) in water. Using a dispersion is hence a way of employ­ing the environmentally friendly solvent water and at the same time creating bonds having good long-term stability. This trick of nature has long been used by the wood processing industry.

Example: Honey Bees

In contrast to paper wasps which use an adhesive based on the solvent water, the adhesive used by honey bees for nest building contains no solvent ­namely wax, which is a liquid at a bee’s body tem­perature. Only on cooling does the adhesive solid­ify into its strong form. Bees’ wax hence meets the ideal requirements of modern adhesives (hotmelts): solvent-free but can be applied as a liquid.

Example: Barnacles

FEICA.barnacle

Barnacles are crustaceans that live in coastal waters.  The free-swimming larvae can bond to virtually all hard marine materials.  The bonding is achieved by means of a secretion from the so-called "cement glands".  This secretion is a 2-component reactive adhesive possessing high resistance to water and prodigious long-term stability.  The bonding is not all dependent on the composition of the base surface.  Even while the barnacle is growing and when its outer skin peels the barnacle remains firmly bonded to the base surface.  This is because there is constantly new secretion of adhesive to ensure the bond remains intact.

Example: Termites

About 150 million years ago the soldiers of primitive termites opssessed saber-like jaws to repulse enemies.  Some 30 million years later a nozzle-like structure developed above the pincers.  The highest developed form was reached 70 million years later; the jaw pincers had disappeared and only the nozzle remained, from which adhesive is sprayed to incapacitate attackers. The production of modern cars would be unimaginable without being able to apply adhesive in this way.

Example: Geckos

geckoThe gecko adheres to surfaces using only dispersion forces. Millions of tiny spatulae enable the gecko to make intimate contact with any surface. Although dispersion forces are small, the large number of interactions involved provides a total adhesive strength sufficient to support the weight of a gecko. The gecko's amazing climbing ability depends on weak molecular attractive forces called van der Waals forces, named after a Dutch physicist of the late 1800s. Van der Waals forces are weak electrodynamic forces that operate over very small distances but bond to nearly any material. It has been discovered that a gecko's ability to stick to surfaces depends on geometry — not chemistry.  The setae (microscopic hairs) on the bottom of a gecko's feet are only as long as two diameters of a human hair, or 100-millionths of a meter long. Each seta ends with 1,000 even tinier pads at the tip. These tips, called spatulae, are 200 billionths of a meter wide — below the wavelength of visible light. In 2002,  researchers at the University of California-Berkeley were able to produce two artificial hair tips, leading the way for the development of the first dry, adhesive microstructures. Possibilities for future applications of a dry, self-cleaning adhesive are enormous, ranging from nanosurgery to aerospace applications.