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Arguably the most fundamental part of the structure of a living cell is its membrane, which is the first line of distinction between self and non-self, and prevents the molecules making up the cell from diffusing away into the environment.
All living cells use the bilayer self-assembly principle to build their membranes. All the cell has to do is to synthesize the lipid molecules, which then spontaneously assemble into sheets which actively resist damage. The membrane is not just used passively by the cell as a barrier to retain important molecules, but also actively as a support and a highway to transport materials. The membrane holds captive many proteins used by the cell, and the force that keeps these membrane-bound proteins firmly anchored arises from the same hydrophilic-hydrophobic interactions which are responsible for the membrane's stability. The different parts of the machinery for photosynthesis in plants are bound in this way. So are the ion channels required for the operation of nerve cells, and many more.
Strictly speaking, the above discussion only proves the biological
relevance of bilayers. However for a number of experimental
purposes, bilayers are quite inconvenient. They are always
surrounded by water, and thus hidden away from a large number
of surface-sensitive techniques. They float freely and hence
do not stay put for observation at molecular resolution.
There are many situations where it is highly desirable to
tether the bilayer. A monolayer is inherently tethered
by the interface to which it is attached. The idea that
you can look at a monolayer and get information about
a bilayer is accepted by many
researchers and is usually called the
Principle of Equivalent States[1,2].
Modern microelectronics technology involves the patterning of a wide variety of thin films deposited on the surface of a wafer. The thinner these films can be made, the better the performance of the resulting circuit, but all film deposition techniques have a limit at which the level of defects becomes unacceptably high. The LB technique allows the deposition of low-defect films just a few nanometers thick of a wide range of organic materials.
Although the performance of organic materials in electronic applications is widely considered not to be competitive with that of inorganics, the situation is rapidly evolving with recent research into polymer electroluminescence. The last three years have seen demonstrations of low-overvoltage injection of both electrons and holes, minority carrier effects and efficient radiative recombination, all in a range of materials. Macroscopic organic mobilities are still typically less than 1 cm2/volt.sec, three orders of magnitude less than inorganic values, but it is quite plausible that this is due to disorder and that higher values will pertain for intramolecular transport. In any case to suppress undesired coupling between adjacent functional elements at the nanometer level, high values of mobility in desired directions will become less important than low values in undesired directions, and high anisotropy has only been demonstrated in organics. In addition, switching speeds at the molecular scale will be limited not by carrier transit times but by heat dissipation (which even in e.g. the Pentium is becoming a significant factor)[3].
Just like any other circuitry, there are systems
constraints which dictate that each individual function
of a molecular electronic circuit
must be powered from a low-impedance voltage supply.
The LB technique is one of just a few techniques
giving this capability. Its demonstrated defect levels
are at least as low as that of Sagiv-type silanol layers and much
better than Whitesides-type thiol layers. As a result
of the molecular mobility in the water-surface phase
of LB fabrication, LB shows a greater potential for
improvement in layer quality[4].
There are a number of different sorts of order which are important for understanding molecular assemblies. Crystalline order is very important because of its application in structural investigations. Essentially all the detailed information we have about molecular sizes and shapes comes from measurements of X-ray diffraction intensities, which can only be completely analyzed for crystals with long-range translational order. Strictly speaking, this sort of order is only possible for three-dimensional aggregates. From a molecular viewpoint, these aggregates are vast, with endless repetitions of a motif, the 'unit cell'. As far as functionality is concerned, most of these cells are uselessly buried in the interior of the aggregate, inaccessible for either information or matter processing on an individual basis.
Functional systems sometimes contain sub-assemblies of a two-dimensional crystalline sort. For example data-storage memories often contain many identical copies of a storage cell, or a translationally-uniform tape or disc. However the interesting part of the memory, which makes it actually do useful things, is not crystalline in this sense. This is not to say that functional systems are not ordered. They certainly show order, and it is order in a much more everyday sense of the word. The 'order' required for a functional system is a homely sort of order, like the refrigerator being stocked with food and located not far from the cooker, a water tap being located above the hand basin in the bathroom rather than over the middle of the floor and with a cake of soap placed nearby. In other words, in this sort of order the neighbours of each element are exactly the ones it requires to carry out its operation satisfactorily. A dynamic RAM may certainly have very many repeats of essentially identical storage cells. But the translational equivalence is not critical for the memory function. A different layout might indeed be very untidy, but it could still work. The essential aspect, without which the memory would not be functional, is the interconnection of each cell to a specific part of a read/write decoder circuit .
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