The ability of molecules to serve as computer switches has been a major area of research since the middle of the last century. Molecular switches, if become a reality, will offer appreciable reduction in hardware size, since these are very small. One can then imagine bimolecular computer about 1/20th the size of present day semiconductor-based computers.
Although, still a distant dream, the use of a hybrid technology in which the molecules and semiconductors combine and share duty could be possible in the near future. Such technology would improve the size of computers. Scientists have already sharpened their skills and are now trying to apply their knowledge to bring out the very best in this arena.
Several biological molecules are being considered for use in computers, but the bacterial protein- Bacteriorhodopsin (bR) has generated much interest among scientists.
In the past few decades, a lot of research was carried out in several laboratories in North America, Europe, and Japan, and scientists became successful in building prototype parallel-processing devices, three-dimensional memories, and protein-based neural networks.
Bacteriorhodopsin, a light harvesting bacterial protein, is the basic unit of protein memory, and is the key protein in halo bacterial photosynthesis. It functions like a light-driven photo pump.
Under exposure to light, it transports photons from the hollow bacterial cell to another medium, changes its mode of operation from photosynthesis to respiration, and converts light energy to chemical energy, thus, can be utilized to frame protein memories.
It grows in salt marshes where temperatures can exceed 150 degree Fahrenheit for an extended time period, and the salt concentration is approximately six times that of seawater. Survival in such an environment implies that this protein can resist thermal and photochemical damages.
Upon absorption of light, it generates a chemical and osmotic potential that serves as an energy source. It also has the ability to form thin films that exhibit excellent optical characteristics, and long-term stability. The protein generates photoelectrical signals upon conversion, and can be used as optical memory. It can be prepared in mass quantities.
Interest in this bacterial protein dates back to the early seventies, when Walther Stockenius of the University of California, and Dieter Osterhelt of Max Plank institute of Biochemistry, discovered that this protein exhibited unusual properties upon exposure to light, and soon scientists realized its potential for use in computers.
This was followed by a team of Soviet scientists headed by Yuri A. Oschinivhove of Semyakin Institute of Bioorganic Chemistry, which took interest in projects on this protein termed 'Project Rhodopsin', intended only for military applications.
Details of these project's achievements remain yet to be revealed. However, the Soviet military was able to make microfiche films out of Bacteriorhodopsin, known as 'Biochrome'.
Photo Cycle of Bacteriorhodopsin
It comprises a light absorbing component known as 'chromophore', that absorbs light energy and triggers a series of complex internal structural changes to alter the protein's optical and electrical characteristics. This phenomenon is known as photo cycle.
The initial resting state of the molecule is known as 'bR'. Green light transforms the initial 'bR' state to the intermediate state 'K'. Next 'K' relaxes, forms another intermediate state 'M', and then 'O' converts to another intermediate state 'P', which then relaxes to a more stable state 'Q'.
Blue light converts 'Q' back to the initial state 'bR'. Here, the idea is to assign any two long-lasting states to the binary values of '0' and '1', to store the required information.
Many of the erstwhile memory devices based on Bacteriorhodopsin could operate only at extreme cold temperatures of liquid nitrogen, where light-induced switching between 'bR' and the intermediate state 'K' could be controlled.
These devices were much faster than conventional semiconductor-based devices, as these exhibited the speed of a few trillionths of a second. Today, most Bacteriorhodopsin based devices function even at room temperature, switching between 'bR' and another intermediate stable state 'M'.
If a number of Bacteriorhodopsin molecules are arranged in a three-dimensional fashion, high-speed, high-density, low-cost memories with vast capacities that can handle large volumes of data can be realized. Such memories offer over 300-fold improvement in storage capacity over their two-dimensional counterparts.
Read/write operations on these can be performed with the help of colored lasers that are fixed at such points as to direct the beams through the required points in the plane of the cube.
Such memory cubes must be extremely uniform in their composition, and must be homogeneous to ensure good results, since excess of defect of molecules in one particular region tends to distort the stored information and render the memory cube useless.
The entire process of data storage or retrieval can be carried out in few milliseconds. The speed of these memories depends on the number of cubes operating in parallel.
Retrieval of stored data is carried out in a manner similar to storing the information, except that a detector images the light passing through the memory cube and senses 1s and 0s.
Here, the property of selective absorption of red light by the intermediate state 'O' is relied upon. The detector senses the luminescent power falling upon it and converts the variations of optical power into a correspondingly varying electric current.
An associative memory device that builds on holographic properties of thin films of Bacteriorhodopsin has been developed. Associative memories take images of data blocks as input, scan the entire memory independently of a central processor for data block that matches the input, and returns the closest match.
Such holographic thin films allow multiple images to be stored in the same segment of memory, thereby permitting simultaneous analysis of large sets of data. However, holograms based on this protein are erasable.
With fast random access capability, good reliability, and transportability, protein memories enhance the multimedia capabilities of computers to a great extent. Also, the advantages of optical data storage accrue to such memories.
Enormous access to information, manipulation, and storage of data in minimal time add to their reliability. Unlike disk memories, where physical contact with the magnetic head is required to read/write information, protein memories use laser beams, which improves their life with reduction in wear and tear.
Researchers are now closely following the way human brain stores, retrieves, and acts on the information to build a biological computer. They are trying to duplicate the capability of information retrieval by inputting a part of it, or any related aspect, instead of specifying the address of the memory location.
Though a group of researchers headed by Robert Birge of Suracuse University, USA, has succeeded in developing similar ones, much work is still required to make a fully-operational computer with memory that mimics the human brain.