Sticky stuff

Scientists have merged two of nature’s most elegant strategies for wet and dry adhesion to produce a synthetic material that one day could lead to more durable and longer-lasting bandages, patches, and surgical materials.

The scientists, supported by the National Institute of Dental and Craniofacial Research (NIDCR), part of the US National Institutes of Health, have designed a synthetic material that starts with the dry adhesive properties of the gecko lizard and supplements it with the underwater adhesive properties of a mussel.

The hybrid material, which they call a geckel nanoadhesive, proved in initial testing to be adherent under dry and wet conditions. It also adhered much longer under both extremes than previous gecko-based synthetic adhesives, a major issue in this area of research.

The researchers say that this is the first time that two polar opposite adhesion strategies in nature have been merged into a man-made reversible adhesive. 

'Our work represents a proof of principle that it can be done,' said Dr Phillip Messersmith, a scientist at Northwestern University.  'A great deal of research still must be done to refine the fabrication process and greatly reduce its cost.  There’s no reason to believe that these improvements can’t be achieved, but it’s going to take time.'
 
Dr. Messersmith said the inspiration for the geckel nanoadhesive came about two years ago when he noticed an article about the adhesive force of a single hair from the foot of gecko. As lizard fans have long marvelled, geckos climb walls and other dry, steep surfaces not by producing a glue-like substance but through a natural adaptation of the hairs that cover the soles of their feet.
 
Roughly one-tenth the thickness of a human hair, each gecko hair splits multiple times at the end. These split ends contain cup-like structures called spatulae that vastly increase the hair’s surface area. Whereas a human hair touches a surface just once, the gecko makes multiple contacts with the suction-like spatulae. With roughly a half million hairs on each foot, scientists estimate a gecko has a billion spatulae at work as it scampers up a wall.    
 
Messersmith knew that researchers have attempted for several years to produce synthetic adhesives based on the adherence strategy of the gecko. Unfortunately, gecko adhesion doesn’t work well in water. Messersmith, who studies the underwater adhesion of mussels, had an idea. What if each synthetic gecko-inspired polymer, called a pillar, was coated with a man-made adhesive protein inspired by the mussel? As Messersmith mused, nobody had ever tried it and, if successful, this hybrid approach might spawn a new and potentially superior direction in designing temporary adhesive materials.
 
Messersmith’s idea turned out to be correct. He and his colleagues designed a small nanopolymer array that mimicked the natural spatial patterns of the hair on the foot of a gecko. They then coated their creation with a thin layer of a synthetic compound. This unusual compound mimics the reversible bonding action of a mussel adhesive protein that Messersmith’s group has studied for the past several years.
 
In their initial experiments, which were led by graduate student Haeshin Lee, they found that the wet adhesive force of each pillar increased nearly 15 times when coated with the mussel mimetic and applied to titanium oxide, gold, and other surfaces. The dry adhesive force of the pillars also improved when coated with the compound. 
 
“That actually wasn’t so surprising to us,' said Lee.  'The mussel-inspired adhesive is extremely versatile in that it can bond reversibly to inorganic surfaces under wet and dry conditions.'
 
As Lee noted, the next research hurdle was whether their hybrid geckel nanoadhesive would continue to stick to surfaces after multiple contacts. This has been a major challenge with other gecko-based adhesives.  They typically stick well at first but lose their ability to adhere after a few cycles of contact with a tipless cantilever.

Using the cantilever and repeatedly touching it down, Lee developed a camera to visualise the process down to individual pillars. He found that the geckel hybrid maintained 85 percent of its adherence under wet conditions after 1,100 contacts with the tip.  Under dry conditions, the level of adherence was 98 percent.

'This isn’t quite a home run, but it’s somewhere in between a double and a triple,' said Lee, who devised on his own a special imaging device to visualise individual pillars during the experiments.

Messersmith said that while the results are extremely promising, his group still must tackle several practical problems before it can scale up its research. 'Any time that you fabricate an array of nano pillars of this type over large areas, you must have a very effective way of doing it without losing the efficacy of the approach,' said Messersmith. 'We’ll also need to reduce the fabrication costs to make geckel commercially viable.'
 
But Messersmith said he envisions great possibilities for geckel. 'Band aids already adhere well, except if you go swimming, take a shower, or somehow expose it to a lot of water,' said Messersmith.  'So I think the most important thing with this adhesive is the added value of resisting immersion in water.'
 
'I should add that the essential component of the wet adhesive polymer on the pillars contains a chemical that we have discovered last year adheres well to mucosal surfaces, such as those inside our mouth,' he noted.  'It may be possible to develop patches in the future that can be applied on the inside of the cheek to cover damaged tissue.'