Adhesives form stronger bonds
An adhesive that becomes stronger rather than brittle as moisture levels fall could find application in low-moisture environments such as space.
A team at Kansas State University, USA, has created a product based on peptides – compounds containing two or more amino acids that link together – to form nanoscale threads called fibrils which become entangled to give a mechanical type of adhesion akin to Velcro.
Professor of Biochemistry John Tomich explains, ‘The [patented] adhesive was designed not to polymerise covalently, like most adhesives, but to undergo selfassembly above pH 9.0. Below that, it remains as monomers in an aqueous solution.
‘These self-assemble into nanofibrils that are stabilised by hydrogen bonds, and it is the entanglement of the individual fibrils with themselves, as well as a rough or porous surface, that produces the mechanical adhesion’.
Any potential hydrogen bond donor could influence adhesive strength, he says. Since water is one of the most abundant hydrogen bond donors, its presence at high concentrations will penetrate the adhesive and reduce the ability of the peptides to bond with each other as opposed to binding to water.
Different variants with different water-resistance properties have been developed, but none are completely water-resistant. Tomich also notes that the Velcro-like fibres generated here are on the nanoscale, unlike original Velco developed on the macro-scale and therefore, for the moment, are not likely to be entangled at the same level as Velcro.
The adhesive has so far been tested on materials such as wood and glass. To measure its strength, the shear was measured by pulling the ends of two glued wood strips. Tomich adds, ‘The strength of the adhesive is slightly above 4MPa. Wood failure occurs at around 6MPa. This peptide’s value is less than commerical adhesives yet its sensitivity to water that makes it unique.’
Commercialisation has begun, and Tomich says it could be manufactured synthetically or by using recombinant DNA technologies to have them produced in bacterial cells, the latter being more cost effective.
In real-world use, leaching of the adhesive should not be an issue, as the peptide’s self-assembly does not require a catalyst for polymerisation. ‘Should any peptide become free,’ says Tomich, ‘It should be biodegradable in the natural environment. This potential lack of toxicity could be another useful property but has yet to be examined.’
In regards to space applications in outer space, Tomich sees them as being on the International Space Station, rather than say the tiles on the Shuttle (or its successor), since the heat of re-entry would be likely to make the peptides degrade.
Steve McDaniels, Manager of NASA’s Failure Analysis and Materials Evaluation Branch at the Kennedy Space Center, comments, ‘Just as Velcro is currently used onboard the International Space Station, the utility of this material would likely depend on its unique properties – properties that have yet to be determined. For example, in the reduced gravity of low-Earth orbit and space, would t he mechanical aspects of the bond still function?’
Furthermore, Dave Gibbon, Chief Mechanical Engineer at Surrey Satellite Technology Ltd, Guildford, UK, says, ‘If it were to be used in space it would need a significant characterisation and qualification programme. For example, can it survive in the temperature extremes of space, and will it be effected by long-term radiation exposure?’
By contrast, however, its deterioration in the presence of water could offer other applications, such as in a timing or moisturedetection device. The team also envisages a system whereby the adhesive forms part of a circuit and, when the adhesive fails in the presence of moisture, it breaks the circuit and activates an alarm.