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 fiber. 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 formed.
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 flexibility. “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.