Conventional connectors, such as buttons, Velcro and zips, typically rely on mechanical interactions and mate interlocking to join components together. In contrast, the new nanowire (NW) connectors, developed by Ali Javey and Ron Fearing of the University of California at Berkeley, utilize chemical interactions. Not only can these devices be reduced to nanoscale dimensions without losing their sticking power, they also have identical mates, which makes them "unisex". This is in contrast to conventional connector technologies and makes it easier to integrate them in a variety of applications.

Conventional adhesives, such as glue, tapes and synthetic gecko, also exploit chemical interactions that can readily be scaled down. The problem here, however, is that these adhesives are designed to bind to any surface rather than just specific sites, as is required for connector applications. "The challenge in our project was to develop connectors that utilize chemical interactions that are highly scalable and yet achieve selective binding," Javey told "The NW connectors technology addresses this challenge by using a novel nanofibrillar system."

The team, which includes researchers from the Lawrence Berkeley National Lab, employed germanium NW forests measuring around 20–30 nm in diameter and 30 µm in length as the backbone of the chemical connectors to make the nanofibrillar structures. Thanks to their high Young's modulus, the structures do not collapse or aggregate. The researchers also deposited a polymer shell onto the surface of the NWs to improve adhesion strength.

Strong shear binding
Because NWs are designed to be stiff, they have low van der Waals interactions and do not easily stick to flat surfaces. However, when they encounter surfaces like themselves (that is nanofibrillar surfaces with similar aspect ratios and dimensions), interpenetration of the NW forests results in the contact area being amplified by around 1000 times. This leads to strong shear binding of around 163 N/cm2, which is well above the shear adhesion strengths of most connector technologies (around 5–15 N/cm2).

"Not many natural or man-made objects have the dimensions of NW forests, so the connectors are highly specific, having minimal adhesion with most foreign surfaces, such as cloth or even Velcro," explained Javey. "This specific binding makes the NW forests an ideal connector technology."

The connectors can also be easily removed and reattached thanks to their superb anisotropic adhesion behaviour (shear to normal strength ratio of around 25), he adds.

According to the team, the NW connectors could be used in various applications, ranging from clothing, reconfigurable structures, such as toys, and in any device that requires reversible component assembly.

The scientists are now exploring connector technologies that can operate in different environments. "We are also looking at how to enhance their robustness and life-time," revealed Javey.

The results were reported in Nano Letters.