Credit: © 2010 ACS

Chemical reactions can be used to change the physical properties of a material, and such processes are widely used in nature to drive movement. This form of chemical signalling is, however, rarely applied artificially — most micromechanical devices rely on electrical, hydraulic or pneumatic signals. The development of devices that operate in response to biological cues, such as a disease biomarker, would be of particular interest. Now, David Gracias and co-workers from Johns Hopkins University have fabricated miniaturized grippers that can collect and release biological material in response to being sequentially exposed to two different enzymes (pictured; J. Am. Chem. Soc. doi: 10.1021/ja106218s; 2010).

The miniature gripper consists of a petal-shaped assembly of three metals — nickel, chromium and gold. Each 'petal' contains rigid flat sections and two sections of pre-stressed metal that at equilibrium would cause the structure to bend. These two pre-stressed sections are, however, held flat by layers of crosslinked biopolymer — one by gelatin (a polypeptide) and one by carboxymethylcellulose (a polysaccharide). Adding a protease enzyme caused the layer of gelatin to degrade, thus making the first set of hinges bend and close the gripper. The action of a second enzyme — this time a cellulase — causes the second hinge to actuate. This hinge is designed to fold the structure in the opposite direction and amounts to opening the gripper.

Gracias and co-workers built a model liver, and were able to guide a gripper to the desired location within it using magnetic forces. Addition of the enzyme by injection caused the gripper to close and obtain a tissue sample.

The original version of this story first appeared on the Research Highlights section of the Nature Chemistry website.