Credit: © 2010 ACS

Chemical reactions can be used to change the properties of a material, but despite this concept being widespread in nature it has yet to be widely applied artificially — the majority of 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 fabricated1 miniaturized grippers that can collect and release biological material in response to being sequentially exposed to two different enzymes.

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.