by Kevin Bullis
Angela Belcher leans in to watch
as a machine presses down slowly on the plunger of a syringe,
injecting a billion harmless viruses into a clear liquid. Instead of
diffusing into the solution as they escape the needle, the viruses
cling together, forming a wispy white fiber that's several
centimeters long and about as strong as a strand of nylon. A
graduate student, Chung-Yi Chiang, fishes it out with a pair of
tweezers. Then he holds it up to an ultraviolet light, and the
fiber begins to glow bright red.
In producing this novel fiber, the researchers have demonstrated a
completely new way of making nanomaterials, one that uses viruses as
microscopic building blocks. Belcher, a professor of materials
science and biological engineering at MIT, says the approach has two
main advantages. First, in high concentrations the viruses tend to
organize themselves, lining up side by side to form an orderly
pattern. Second, the viruses can be genetically engineered to bind
to and organize inorganic materials such as those used in battery
electrodes, transistors, and solar cells.
The programmed viruses coat themselves
with the materials and then, by aligning with other viruses,
assemble into crystalline structures useful for making
Tiny building blocks:
A small vial contains a billion viruses, each with a slightly
different genetic modification.
These can be screened
to determine which of them will bind to specific inorganic
materials, such as those used in rechargeable batteries.
Credit: Porter Gifford
But the approach is not just an
alternative way to make familiar devices; it could also be the
impetus for developing entirely new ones. In past work, Belcher has
created virus-based thin films for rechargeable batteries. Now that
she can spin viruses into fibers, she envisions threadlike batteries
and other electronic devices that can be woven directly into
"It's not really analogous to
anything that's done now," she says. "It's about giving totally
new kinds of functionalities to fibers."
The virus-based fibers have caught the
attention of U.S. Army researchers. They hope to incorporate future
versions of the fibers into uniforms, weaving them into the fabric
along with other supporting materials. The resulting fabrics could
have an array of advanced capabilities. Clothing made with them
could sense agents of chemical and biological warfare; it might
also store energy from the sun and power portable electronic
devices, such as night-vision gear.
Charlene Mello, a macromolecular
scientist at the Natick Soldier Research, Development, and
Engineering Center in Natick, MA, says that while such uniforms will
probably take decades to develop, Belcher's work has paved the way
Belcher uses different procedures to make different kinds of virus
fibers. To make the glowing fibers, she first used conventional
genetic-engineering methods to modify the virus DNA so that one of
the proteins that make up the body of the virus has extra copies of
a specific amino acid at one end. At the same time, the researchers
synthesized quantum dots (semiconductor nanocrystals that emit
intense light at precisely tuned wavelengths) with surface amine
groups that bind to the overproduced amino acid. The result:
hundreds of quantum dots glommed onto each virus, which combined
with similar viral particles to form a fiber that emits light.
Often, however, it's not obvious how to make a virus bind to
specific inorganic materials, such as gold particles. In these
situations, Belcher uses a method sometimes called "directed
evolution," which allows her to quickly modify viruses to work with
a range of materials.
In this case, directed evolution begins with a small vial that
Chiang pulls from a refrigerator. Inside is a clear fluid that
contains a billion viruses; they are nearly identical, but each has
a subtle genetic variation introduced by the researchers. The
variations are, in part, fortuitous: the researchers add a randomly
generated sequence of DNA to each virus. But the added DNA, which
codes for a short strand of amino acids called a peptide, is
inserted into the gene for a select protein.
Since there are so many variations among
the viruses in the vial, some of them should randomly have peptides
that bind to a useful inorganic material. The researchers simply
pour the contents of the vial onto a target material, such as a
small square of gold, and give the viruses a chance to bind. Then
they wash the material. After a few repetitions, only the viruses
that happen to bind strongly remain. The process allows the
researchers to quickly engineer viruses to bind to a particular
material, even if they don't know ahead of time what sequence of
amino acids is likely to work.
Once the right viruses have been made, getting them to form a fiber
is relatively simple. First, the researchers concentrate the viruses
so that their shape and chemical properties induce them to pack
closely together in a crystalline pattern. Then they force the
viruses through a needle and into a solution--a conventional
process, called spinning, that helps determine the diameter of the
After leaving the needle, the closely
packed viruses tend to hold together. But to further strengthen the
fiber, the researchers add a chemical linking agent to the solution;
this agent binds neighboring viruses to each other. The desired
inorganic materials can be added either before or after the fiber is
Encouraged by their success with the quantum-dot-studded glowing
fibers, Belcher and her coworkers hope to show that similar fibers
can be made into, among other things, sensors, solar cells, and
batteries. For example, they envision engineering two types of virus
fibers, one that serves as a negative battery electrode and another
that serves as a positive electrode. These fibers could be twisted
together, with a polymer electrolyte between them, to make a
rechargeable battery that could be woven into clothes.
Hurdles remain to be cleared, of course, before the technique will
yield complex practical devices. For one thing, Belcher will need to
invent fibers that do more than just glow red. But her methods make
it relatively easy to try out different materials and new designs.
The simple virus, says Belcher, gives her a great deal of
"It's just a wonderful unit," she
says. "Nature gives you the perfect starting material."
Kevin Bullis is the nanotechnology and
materials science editor of Technology Review.