by Hilary Gallagher published: 2011-06-08
Since Richard Feynman coined the phrase "swallow-the-surgeon", scientists have been attempting to create tiny devices that can do the work of the surgeon – but from inside the body. Such a device must be tiny but responsive – converting an external stimulus to a mechanical action, like a scalpel being wielded to make an incision.
Microfabrication offers us the means to create mechanical devices on the micro- and nanometer scale. However, control of the device when it is placed within a biological environment is not straightforward. Mechanical actuation on such a small scale is often effected by means of an electrical, pneumatic, or hydraulic signal. However, such a signal must reach the device via a wire or tether through which electricity, air, or liquid flows. Of course, the presence of this tether would limit the use of these devices in the complex 3D environment of the human body.
In the human body, a different method of effecting mechanical action is used: chemical signaling. ATP (adenosine triphosphate) is the energy molecule of the cell and is specifically cleaved by proteases in the generation of mechanical energy. Chemicals are ideal signaling agents: they can diffuse over large distances through a variety of media and are highly selective for their targets. Microchemomechanical systems (MCMS) use this model of mechanical actuation. Prof. David H. Gracias has been at the forefront of this expanding field and has written a new review exploring the development of microchemomechanical systems.
His team has produced many working examples of MCMS devices, including a microgripper that opens and closes in oxidative or reductive environments, respectively. The microgripper is fabricated with a metallic bilayer hinge; selective modification of the metals produces tensile stress that causes the hinges to bend or flatten. Heating the grippers in an oxidative environment generates a thin oxide layer on the surface of the copper, which produces a large tensile stress and causes the hinge to “open”. Conversely, heating the device in a reductive environment removes the oxide layer and the hinge closes once more.
The research team has also produced devices that can be selectively controlled by enzymes; for example, a gripper that closes due to the degradative action of cellulase on a layer of cellulose in polymeric hinges, but opens again due to the action of proteases on gelatin layers in a separate set of hinges. Because cellulose is not naturally found in the human body, this device could potentially be targeted to diseased sites. In addition, the incorporation of metallic layers into these devices means that they can be magnetically directed.
It may be a while before the swallowable surgeon begins to make his rounds, but the ingenious devices highlighted in this review suggest that the field of microchemomechanical systems is only beginning to make its mark.
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