Nerve-chip link closer

By Kimberly Patch, Technology Research News

Although human-machine hybrids are likely to remain in the realm of science fiction for decades, researchers are beginning to meld tissue and technology at the cellular level.

Bridging the wide communications gap between biology and electronics by connecting a cell to a semiconductor means both the cell and the electronic device can potentially take advantage of the best attributes of the other.

Researchers at the University of Texas at Austin have taken a step toward cell-semiconductor communications by soldering semiconductors to nerve cells. Key to the process is the solder -- a modified peptide molecule that binds to a human neuron protein on one side, while the other sticks to a microscopic particle of semiconductor material, or nanocrystal.

"We make particles in a solution," and stick the modified peptides onto the surface of the particles, said Brian Korgel, an assistant professor of chemical engineering at the University of Texas at Austin. "The particles [then] stick to the cells in specific locations."

The specialized peptide molecule connects a particle to a biological receptor that sticks out of the nerve cell membrane. The connection brings the particle to within 20 nanometers of the electrically-active cell membrane, which is closer than previous methods. Nerve cells can be grown on semiconductor materials, but do not stick as closely to the electronics; previous research efforts have grown nerves on electronics, but leave a 50-nanometer gap.

As cells go, nerve cells are relatively large -- the cells the researchers used were 60 microns in diameter, which is a little over 10 times the size of a red blood cell, and nearly the diameter of human hair. At 5 nanometers, the particles were more than 5,000 times smaller. A micron is one thousandth of a millimeter; a nanometer is one thousandth of a micron.

Although it may be difficult to get them together, nerve cells and electrical components do have something in common -- they both communicate using electrical signals.

Nerve cells use changes in their electrical fields to create specific nerve firing patterns. "This is one of the underlying properties that enable brain functions like memory," said Korgel. Electrical field-effect transistors turn on and off based on the amount of electricity flowing through a gate electrode.

This mutual responsiveness to electrical fields makes intermaterial communications possible, said Korgel. "The nerve cell can effectively function as a gate on a field-effect transistor if the two materials are interfaced properly," he said.

Nerve-semiconductor links could eventually be used to allow nerves to directly control prosthetics, said Christine Schmidt, an assistant professor of biomedical engineering at the University of Texas at Austin.

“Bioprosthetic devices [like] retinal implants [and] mechanical prosthetics could be connected to the nervous system and brain using semiconductor materials such as those we are investigating.” In addition, existing devices like cochlear implants may be improved using these materials, she said.

Cells and nanocrystals could also be combined to detect tiny quantities of chemicals that are toxic to cells, said Korgel. There's also potential for using nerve cells to boost computer memory devices, he said. "One idea that I find particularly exciting is the prospect of combining nanocrystals, nerves and conventional microelectronics to create nerve-cell memory devices," he said.

It is also theoretically possible to use optically-activated nanocrystals to probe cells to study their internal electro-chemical reactions, according to Korgel.

The researchers are currently looking into mechanisms that will allow the semiconductor nanocrystals to communicate with the nerve cells, said Korgel. "If we stimulate the nanocrystal with light... will the nerve feel the stimulus? Normally a nerve cell would not be affected by light, but with the nanocrystals attached, could we [change] the function of the nerve cells? These are the questions that we are currently trying to answer," he said.

The idea and methods are excellent, said Shuguang Zhang, associate director of the Center for Biomedical Engineering at the Massachusetts Institute of Technology. "Such direct linkage will likely find application in understanding the nerve connections and the strength of the connections through the fine adjustment of the electric input. This is a significant step forward to interface nerve cells with conducting and semiconducting materials," he said.

The method may eventually be useful in repairing damaged nerve systems; it could also serve to "interface the 40-year young semiconducting industry with biology that has evolved over billions of years. It is one step closer to... Star Trek," Zhang said. However, because dry computers and water-based cells are so inherently different "there still remains a big gap and challenge to be worked out," he said.

The researchers have extended the use of luminescent nanocrystals in biological applications, but it remains to be seen how useful the interface will be because the nanocrystals may still not be close enough to the membrane of the cell to interface with it electrically, said Peter Fromherz, a professor of biophysics at the Max Planck Institute of Biochemistry. "These particles are so far from the membrane that they feel little of the electrical field across the membrane," he said.

There are many hurdles to overcome before cells and semiconductor nanocrystals will combine in practical products, said Korgel. "This is really an unexplored area and we have much to learn," he said. Practical uses are probably a decade away, "but this is only a guess," he said.

Korgel and Schmidt’s research colleagues were Jessica O. Winter and Timothy Y. Liu. They published the research in the October 30, 2001 issue of Advanced Materials. The research was funded by the National Science Foundation (NSF), the Welsh Foundation, DuPont, the Petroleum Research Fund, the Gilson Longenbaugh Foundation and the Whittaker Foundation.

Timeline:   10 years
Funding:   Government, Corporate, Private
TRN Categories:  Biological, Chemical, DNA and Molecular Computing; Materials Science and Engineering; Semiconductors
Story Type:   News
Related Elements:  Technical paper, "Recognition Molecule Directed Interfacing between Semiconductor Quantum Dots and Nerve Cells," Advanced Materials, October 30, 2001.


December 5, 2001

Page One

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