Probes made of silicon currently are used to study brain function and disease but may one day be used to apply electrical signals that restore damaged areas of the brain. A major drawback to these probes, however, is that they cause the body to produce scar tissue that eventually accumulates and prevents the devices from making good electrical contact with brain cells called neurons, said Thomas Webster, an assistant professor of biomedical engineering.
New findings showed that the nanotubes not only caused less scar tissue but also stimulated neurons to grow 60 percent more fingerlike extensions, called neurites, which are needed to regenerate brain activity in damaged regions, Webster said.
The findings are detailed in a paper appearing this month in the journal Nanotechnology, published by the Institute of Physics in the United Kingdom. The paper was written by Webster, Purdue doctoral students Janice L. McKenzie and Rachel L. Price, former postdoctoral fellow Jeremiah U. Ejiofor and visiting undergraduate student Michael C. Waid from the University of Nebraska.
The nanotubes were specially designed so that their surfaces contained tiny bumps measured in nanometers, or billionths of a meter. Conventional silicon probes do not contain the nanometer-scale surface features, causing the body to regard them as foreign invaders and surround them with scar tissue. Because the nanometer-scale features mimic those found on the surfaces of natural brain proteins and tissues, the nanotubes induce the formation of less scar tissue.
The scar tissue is produced by cells called astrocytes, which attach to the probes. The Purdue researchers discovered that about half as many astrocytes attach to the nanofibers compared to nanotubes that don't have the small features.
"These astrocytes can't make scar tissue unless they can adhere to the probe," Webster said. "Fewer astrocytes adhering to the nanotubes means less scar tissue will be produced."
The Purdue researchers pressed numerous nanofibers together to form discs and placed them in petri plates. Then the petri plates were filled with a liquid suspension of astrocytes. After one hour the nanotube disks were washed and a microscope was used to count how many of the dyed astrocytes washed out of the suspension, which enabled the researchers to calculate how many astrocytes stuck to the nanotubes. About 400 astrocytes per square centimeter adhered to the nanotubes containing the small surface features, compared to about 800 for nanotubes not containing the small surface features. The researchers repeated the experiment while leaving the nanotubes in the cell suspension for two weeks, yielding similar results.
When the nanotubes were placed in a suspension with neurons, the brain cells sprouted about five neurites, compared with the usual three neurites formed in suspensions with nanotubes that didn't have the small surface features.
Researchers plan to make brain probes and implants out of a mixture of plastics and nanotubes. The findings demonstrated that progressively fewer astrocytes attached to this mixture as the concentration of nanotubes was increased and the concentration of plastics was decreased.
"That means if you increase the percentage of carbon nanofibers you can decrease the amount of scar tissue that might form around these electrodes," Webster said.
The nanometer-scale bumps mimic features found on the surface of a brain protein called laminin.
"Neurons recognize parts of that protein and latch onto it," Webster said.
The crucifix-shaped protein then helps neurons sprout neurites, while suppressing the formation of scar tissue.
The tube-shaped molecules of carbon have unusual properties that make them especially promising for these and other applications. Researchers theorize that electrons might flow more efficiently over extremely thin nanotubes than they do over conventional circuits, possibly enabling scientists to create better brain probes as well as non-silicon-based transistors and more powerful, compact computers.
"Nano" is a prefix meaning one-billionth, so a nanometer is one-billionth of a meter, or roughly the length of 10 hydrogen atoms strung together. The nanotubes were about 100 nanometers wide, or roughly 1,000 times as thin as a human hair.
The research is funded by the National Science Foundation.
Webster also plans to test the effectiveness of silicon that contains the same sort of nanometer-scale features as the nanotubes, which could increase the performance of silicon probes and implants. In work with Spire Biomedical Inc. (Nasdaq:SPIR) in Bedford, Mass., Purdue researchers will analyze silicon that contains numerous pores, unlike conventional silicon, which has no such porous features. That research is funded by the National Science Foundation and the federal Small Business Innovation Research Program.
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Nano-biotechnology: carbon nanofibers as improved neural and orthopedic implants
Thomas J. Webster, Michael C. Waid, Janice L. McKenzie, Rachel L. Price and Jeremiah U. Ejiofor
Department of Biomedical Engineering, Purdue University
For the continuous monitoring, diagnosis, and treatment of neural tissue, implantable probes are required. However, sometimes such neural probes (usually composed of silicon) become encapsulated with non-conductive, undesirable glial scar tissue. Similarly for orthopedic implants, biomaterials (usually titanium and/or titanium alloys) often become encapsulated with undesirable soft fibrous, not hard bony, tissue. Although possessing intriguing electrical and mechanical properties for neural and orthopedic applications, carbon nanofibers/nanotubes have not been widely considered for these applications to date. The present work developed a carbon nanofiber reinforced polycarbonate urethane (PU) composite in an attempt to determine the possibility of using carbon nanofibers (CNs) as either neural or orthopedic prosthetic devices. Electrical and mechanical characterization studies determined that such composites have properties suitable for neural and orthopedic applications. More importantly, cell adhesion experiments revealed for the first time the promise these materials have to increase neural (nerve cell) and osteoblast (bone-forming cell) functions. In contrast, functions of cells that contribute to glial scar-tissue formation for neural prosthesis (astrocytes) and fibrous-tissue encapsulation events for bone implants (fibroblasts) decreased on PU composites containing increasing amounts of CNs. In this manner, this study provided the first evidence of the future that CN formulations may have towards interacting with neural and bone cells, which is important for the design of successful neural probes and orthopedic implants, respectively.
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