Miniaturization has been a main theme in science for quite some time, and scientists envision that we'll eventually be able to make entire micromanufaturing and micromechanical systems. To operate these systems, it will be essential to have a variety of micro-tools available. One of the more commonly used tools in the macro-world is a gripper, which can pick up an object securely and release it when necessary. Chemical and biomolecular engineers at Johns Hopkins University wanted to create a version of this tool for micro-objects.
They were especially interested in using chemical triggers to control their microgripper, as these are used in biological machinery for various purposes, like activating macrophage function. Thus, they designed a microgripper that closed and opened in the presence of different chemicals.
They shaped the gripper similar to a hand with a hexagonal palm, six fingers, and six hinges. The palm and fingers are made out of gold-coated nickel. The hinges are formed out of three layers, with two metals and a polymer—in order from bottom to top, they are chromium, copper, and a stiff polymer. The chromium and copper form a stressed bilayer that wants to reduce its stress by bending, which would close the fingers. However, the polymer is very stiff, so it prevents the bending and closing of the fingers until triggered. The nickel enables the microgrippers to be moved around by a magnet, as it is ferromagnetic, while the gold helps the nickel layer form properly.
The stiff polymer dissolves in acetic acid, an organic acid that is responsible for the sourness and smell of vinegar. So, when acetic acid is added to water containing microgrippers, the fingers close as the polymer dissolves away. To reopen the microgripper, hydrogen peroxide is added to the solution to dissolve the copper layer. When there is only a chromium layer left at the hinges, the fingers straighten out. Obviously, this means that the hardware is one-time use only.
The researchers can make microgrippers that are as small as 700μm across when open and about 200μm when closed. The microgrippers could pick up glass beads, wires, and tubes. They remained securely closed while a magnet was used to move the items to a different location. Upon the addition of hydrogen peroxide, the microgrippers successfully released their load.
This is the first time that scientists have made this sort of micro-tool. The authors anticipate micro-mechanical applications for their invention, but they do not foresee medical uses among them, as their methods are not biologically compatible. For microgrippers that work in living organisms, they have another design, which will be unveiled soon in a separate publication.
Journal of the American Chemical Society, 2008. DOI: 10.1021/ja806961p
Image © American Chemical Society