Biomedical Engineering Reference
In-Depth Information
methods described in this chapter include nanoparticles for fl uorescent biological
labeling and tagging, drug and gene delivery, biodetection of pathogens and
proteins, nanotubes and nanochannels for probing the structure of DNA, tissue
engineering, magnetic nanoparticles for tumor destruction via heating (hyper-
thermia) and magnetic resonance imaging contrast enhancement, separation and
purifi cation of biological molecules and cells, and many others that are still cur-
rently under investigation (Salata
2004
) .
The methods for synthesis and patterning for many of these nanostructures are
founded on basic, well-known techniques and their modifi cations, as is described in
this chapter. The nanofabrication processes can be divided into the two well-known
approaches: “top-down” and “bottom-up.” The “top-down” approach uses tradi-
tional methods to guide the synthesis of nanoscale materials. The paradigm proper
of its defi nition generally dictates that in the “top-down” approach it all begins from
a bulk piece of material, which is then gradually or step-by-step removed to form
objects in the nanometer-size regime (~1 billionth of a meter or 10
−9
m). Well-
known techniques, such as photo lithography and electron beam lithography, anod-
ization, and ion- and plasma etching (PE), that are later described, all belong to this
type of approach. The top-down approach for nanofabrication is the one fi rst sug-
gested by Feynman in his famous American Physical Society lecture in 1959, “There
is plenty of room at the bottom” (Feynman
1959
) :
Now comes the interesting question: How do we make such a tiny mechanism? I leave that
to you. However, let me suggest one weird possibility. You know, in the atomic energy
plants they have materials and machines that they can't handle directly because they have
become radioactive. To unscrew nuts and put on bolts and so on, they have a set of master
and slave hands, so that by operating a set of levers here, you control the “hands” there, and
can turn them this way and that so you can handle things quite nicely.
Most of these devices are actually made rather simply, in that there is a particular cable,
like a marionette string, that goes directly from the controls to the “hands.” But, of course,
things also have been made using servo motors, so that the connection between the one
thing and the other is electrical rather than mechanical. When you turn the levers, they turn
a servo motor, and it changes the electrical currents in the wires, which repositions a motor
at the other end.
Now, I want to build much the same device - a master-slave system which operates
electrically. But I want the slaves to be made especially carefully by modern large-scale
machinists so that they are one-fourth the scale of the “hands” that you ordinarily maneuver.
So you have a scheme by which you can do things at one-quarter scale anyway - the little
servo motors with little hands play with little nuts and bolts; they drill little holes; they are
four times smaller. Aha! So I manufacture a quarter-size lathe; I manufacture quarter-size
tools; and I make, at the one-quarter scale, still another set of hands again relatively one-
quarter size! This is one-sixteenth size, from my point of view. And after I fi nish doing this
I wire directly from my large-scale system, through transformers perhaps, to the one-six-
teenth-size servo motors. Thus I can now manipulate the one-sixteenth size hands. Well,
you get the principle from there on. It is rather a diffi cult program, but it is a possibility …
The “bottom-up” approach on the other hand takes the idea of “top-down” approach
and fl ips it right over. In this case, instead of starting with large materials and chipping
them away to reveal small bits of them, it all begins from atoms and molecules that get
rearranged and assembled to larger nanostructures. It is the new paradigm for synthesis
in the nanotechnology world as the “bottom-up” approach allows a creation of diverse
