Aug 8, 2007
Polymers get stuck into switchable adhesion
"Geckel" glue has been busy grabbing the limelight, but it's not the only nanoscale adhesive making the headlines. Scientists in the UK and Germany have come up with an easy-to-make switchable formulation that could find its way into drug-delivery systems, nano-actuators and even shampoo. Nanotechweb.org speaks with Mark Geoghegan of Sheffield University's polymer centre and co-author of the textbook: Nanoscale Science and Technology, to find out more.
How do you formulate your nano-adhesive and what makes it switchable?
Our system consists of two main components – a polymeric gel and a 20 nm thick brush layer. The gel is made by simply sticking a monomer into a solution with cross-linker and a catalyst. The brush layer is a little more complicated to put together and is synthesised by atom transfer radical polymerisation. In other words, chemically modifying the surface and then adding a monomer, a catalyst and a ligand to the solution and stirring. This allows a "brush-like" polymer layer to be grown with one end attached to the silicon substrate, although the methodology is not restricted to silicon.
Because we are dealing with charged polymers or polyelectrolytes, the adhesion can be switched "on" or "off" by simply changing the pH. In water, we have an attractive system that contains a positively charged brush layer and a negatively charged gel, which results in adhesion. At a pH of 2 the gel loses its charge and the plain Coulombic attraction goes, turning off the adhesion. Hydrogen bonding almost certainly contributes to the attractive force, but disentangling the roles of hydrogen bonding and Coulombic attractions is difficult.
The joker in the pack is the Velcro effect. Forcing the gel and brush-coated surface together improves the adhesion. At low pressures we see the adhesion as being purely interfacial. However, when you press the two surfaces together at high pressures, it's possible that the brush's "bristles" get caught up in the mesh of the gel, rather like Velcro behaves. We need to investigate this effect further.
Considering the strength, reversibility and scalability of your system, what do you see as the most promising uses for your polymer-based adhesive?
In the paper we refer to fluidic systems where channels can be opened or closed by gates adhering to each other and detaching depending on pH. Polyelectrolyte brushes have been touted as nanoactuators for several years. Drug delivery is another candidate. The easy example is for drugs targeting the colon that need to survive the stomach, which is acidic, and then let go of their cargo in a different pH environment. Our method gives medical researchers another tool to consider.
Personal care is another application that comes to mind. You would be surprised by the amount of technology that goes into shampoo and conditioner. Charged molecules play an important role here. The bottom line is that our technology is new and incredibly simple. I am convinced that it will find a use.
How does your formulation compare with other nano nano-adhesives, such as last month's "geckel" glue?
The "geckel" adhesive is a phenomenal example of what can be achieved with current technology. There have been other gecko-inspired adhesives, but the "geckel" version is nice because it works in water. The glue can be used thousands of times, although you need a means of physically removing it from the surface.
Many of the uses for the "geckel" formulation apply to our switchable adhesive, but the real advantage of our technology is that it is very straightforward and can be controlled in situ. It is also easy to scale up. So far we've covered centimetre-sized areas with our switchable adhesive.
The benefit of "geckel" glue over our technology is that it only makes requirements of one surface, whereas we need to control both components – the polyacid and the polybase.
What are the next steps for you and your team?
We need to improve and understand the technology. The pH differences are quite large – pH 1–2 is quite low for adhesion to be terminated – but this is something that we might be able to improve. Our next moves are scientific rather than technological as we need to investigate the role of the different bonding mechanisms. When we have a better feeling for the science, we'll be in a stronger position to direct the technology and gear-up for potential applications.
The researchers presented their work in Angewandte Chemie.
About the author
James Tyrrell is editor of nanotechweb.org.