The bacterio-rhodopsin protein
is one of the most promising organic memory materials. Seven helix-shaped
polymers form a membrane structure, which contains a molecule known as the
retinal chromophor. The chromophor absorbs light of a certain color and is
therefore able to switch to another stable state in addition to its original
state. Only blue light can change the molecule back to its original state.
There have been many methods
and proteins researched for use in computer applications in recent years.
However, among the most promising approaches, and the focus of this particular
web page, is 3-Dimensional Optical RAM storage using the light sensitive
protein bacterio-rhodopsin.
How
Does Protein Memory Work?
In a prototype memory system,
bacterio-rhodopsin stores data in a 3-D matrix. The matrix can be build by
placing the protein into a curvette (a transparent vessel) filled with a
polyacrylamide gel. The protein, which is in the bR state , gets fixed in by
the polymerization of the gel. A battery of Krypton lasers and a
charge-injection device (CID) array surround the curvette and are used to write
and read data.
While a molecule changes
states within microseconds, the combined steps to read or write operation take
about 10 milliseconds. However like the holographic storage, this device
obtains data pages in parallel, so a 10 Mbps is possible. This speed is similar
to to that of slow semiconductor memory.
Data Writing Technique:
Bacterio-rhodopsin, after
being initially exposed to light (in our case a laser beam), will change to
between photo isomers during the main photochemical event when it absorbs
energy from a second laser beam. This process is known as sequential one-photon
architecture, or two-photon absorption. While early efforts to make use of this
property were carried out at cryogenic temperatures (liquid nitrogen
temperatures), modern research has made use of the different states of
bacterio-rhodopsin to carry out these operations at room-temperature.
The process breaks down like
this:
Upon initially being struck
with light (a laser beam), the bacterio-rhodopsin alters it's structure from the
bR native state to a form we will call the O state. After a second pulse of
light, the O state then changes to a P form, which quickly reverts to
a very stable Q state, which is stable for long
periods of time (even up to several years).
The data writing technique
proposed by Dr. Birge involves the use of a three-dimensional data storage
system. In this case, a cube of bacterio-rhodopsin in a polymer gel is
surrounded by two arrays of laser beams placed at 90 degree angles from each
other. One array of lasers, all set to green (called "paging" beams),
activates the photocycle of the protein in any selected square plane, or page,
within the cube. After a few milliseconds, the number of intermediate O stages of bacterio-rhodopsin
reaches near maximum. Now the other set, or array, of lasers - this time of red
beams - is fired.
The second array is programmed to strike only the region of
the activated square where the data bits are to be written, switching molecules
there to the Pstructure. The P intermediate then
quickly relaxes to the highly stable Q state. We then
assign the initially-excited state, the O state, to a
binary value of 0, and theP and Q states are
assigned a binary value of 1. This process is now analogous to the binary
switching system which is used in existing semiconductor and magnetic memories.
However, because the laser array can activate molecules in various places
throughout the selected page or plane, multiple data locations (known as
"addresses") can be written simultaneously - or in other words, in
parallel.
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