Chemistry Reference
In-Depth Information
a fundamental problem to the top-down concept. The large interfacial energy stored
in these systems directly drives transport phenomena which are likely to lead to
their destruction. Building devices with feature sizes even close to molecular scales
therefore appears prohibitively difficult in a top-down approach. Any successful
concept for exploiting the full scale down to molecular sizes would have to master the
balancing act of stepping far enough out of equilibrium to make sustained dynamical
functions possible, but staying close enough to equilibrium to avoid the destructive
force of large gradients in free energy.
Yet we know that such systems exist abundantly on earth: all living matter has
dynamic functional units on a hierarchy of length scales, down to molecular size.
This has been possible only because evolution has used more than three billion years
of genotype experience to master the key task posed above on the phenotype level.
The building blocks, chiefly lipids, proteins, and nucleic acids, are designed such that
they either self-assemble into the desired structure or function, or can be assembled at
the expense of only small amounts of free energy, such as in the case of chaperonins
assembling the 'correct' tertiary structure of proteins [
3
]. At the same time, chemical
energy supply keeps the system sufficiently far off equilibrium to give rise to complex
dynamic function, and to maintain structural components as traits of non-equilibrium
steady states [
4
]. If we are to build devices with building blocks of molecular size,
it therefore appears advisable to follow a similar path exploiting the self-assembly
concept [
5
-
8
].
It suggests itself to use components similar to those which have been so success-
fully 'tested' by nature for eons. Soft matter, such as the materials of living systems,
is governed by typical interaction energies on the thermal scale (
∼
few kT), such that
non-trivial functions are possible preferentially at room temperature. It exhibits high
molecular mobilities as compared to classical solid state materials, like semicon-
ductors, thus enabling sufficiently rapid self-assembly (and re-assembly) processes.
Furthermore, the dispersion interactions dominating the structure and dynamics of
soft matter systems are small compared to chemical binding energies. As a con-
sequence, all molecules taking actively part in the functional processes stay intact.
This opens an almost unlimited variety of building blocks. Finally, the use of building
blocks similar to those of living matter appears particularly promising for devices
to be interfaced with living organisms, such as for modern orthotic or prosthetic
technology.
However, we cannot rely entirely on self-assembly, since the goal of any design
is to reach a certain 'phenotype', the idea of which is at the start of the whole
endeavor. Coding the full complexity of the desired device into the molecular build-
ing blocks, such that the desired structure emerges completely out of self-assembly,
would require full control of non-equilibrium states out of their microscopic condi-
tions; this is a long-standing and so far unsolved problem. Even if a solution was at
hand, the task would be still tremendous, and probably impossible to complete. It
will thus be necessary to provide a sufficiently strict pre-selection of configurations
which are 'allowed' to the system for its assembly. A conceptually straightforward
implementation of such pre-selection is some solid scaffold, which provides enough
