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.
Bacterio-rhodopsin is a protein found in the purple membranes of several species of bacteria, most notably Halobacterium halobium . This particular bacteria lives in salt marshes. Salt marshes have very high salinity and temperatures can reach 140 degrees Fahrenheit. Unlike most proteins, bacterio-rhodopsin does not break down at these high temperatures.
Early research in the field of protein-based memories yielded some serious problems with using proteins for practical computer applications. Among the most serious of the problems was the instability and unreliable nature of proteins, which are subject to thermal and photochemical degradation, making room-temperature or higher-temperature use impossible. Largely through trial and error, and thanks in part to nature's own natural selection process, scientists stumbled upon bacterio-rhodopsin , a light-harvesting protein that has certain properties which makes it a prime candidate for computer applications. While bacterio-rhodopsin can be used in any number of schemes to store memory, we will focus our attention on the use of bacterio-rhodopsin in 3-Dimensional Optical Memories.
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 P structure. 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 the P 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.