Kotov, a Professor in the Department of Chemical Engineering
at the University of Michigan, worked with members of his group and colleagues
from UMichigan's Departments of Materials Science and Biomedical Engineering as
well as from Prof. Chuanlai Xu's group at Jiangnan University in Wuxi, China. They published their findings in the November 7, 2008 online edition of Nano
Letters ("Smart Electronic Yarns and Wearable Fabrics for Human
Biomonitoring made by Carbon Nanotube Coating with Polyelectrolytes").
In their very simple process, the researchers repeatedly
dipped a regular cotton thread in a CNT dispersion and then let it dry. After
several repetitive dips, the cotton thread became conductive, with a
resistivity as low as 20 Ω/cm (a level low enough that it would allow for
convenient sensing applications that may not require any additional electronics
or converters). Interestingly, once the adsorbed CNT-cotton threads were dried,
it was impossible to remove the adsorbed CNTs from the fibers by exposure to
solvents, heat, or a combination of both.
"We found that the incorporation of CNTs into the
cotton yarn was much more efficient than their adsorption into carbon fibers,
which was tried elsewhere" explains Bong Sup Shim, a PhD student in Kotov's
group and first author of the above paper. "This could be a result of the
efficient interaction of polyelectrolytes with cotton and other natural
polysaccharide and cellulose-based materials, such as paper, which is well
known in industry. Additionally, the flexibility of the CNTs allowed them to
conform to the surface of the cotton fibers."
The scientists point out that polyelectrolytes are essential
for the stability of the CNT coatings on fibers and they are also essential for
comfortable wearing because they are hydrophilic.
SEM images of CNT coated cotton yarns. (Images: Bong Sup Shim)
He believes that further development of this
CNT-cotton material could lead to several useful applications:
- Reversible sensing schemes for
relevant biological compounds/markers;
- Various sensors for body
functions including monitoring of degree of contusion/blast damage (of
great interest to the project's funder, the Air Force); and
- Multiplexed sensing of five to
six analytes with yarns modified in different ways.
"We also might add that energy harvesting
materials and fabrics with charge storage capabilities become a possibility for
the fabrics described here" says Kotov. "The latter goal could be the
most challenging but nevertheless suitable for the nanotube-cotton composite
because of the nature of CNTs, the fairly high conductivity obtained, and
supercapacitor properties of carbon nanotubes."
Future aspects of this research will deal with
incorporating reversibility in the sensing mechanism and developing virtually
permanent coatings of carbon nanotubes on cotton and other fabrics.
One
issue the research team is very well aware of are toxicological concerns
surrounding CNTs. Although their extensive cell-culture data indicates that the
solid CNT-polymer composites are largely benign, they nevertheless emphasize
the need to further investigate the long-term contact between skin and
nanotubes.
