Anyone want to knit a microscopic sweater?
By Peter Weiss
Rod-shaped molecules can organize themselves by, say, lining up like sardines or grouping
into distinct layers when they form orderly fluids known as liquid crystals. If exposed to ultraviolet
(UV) light or heat, the molecules of some liquid crystals self-organize in a second way:
They link into long chains called polymers.
Now, scientists investigating such materials have accidentally created microscopic polymer
tubes that tangle themselves into clumps that resemble balls of yarn, report Pavel A. Kossyrev
and Gregory P. Crawford of Brown University in Providence, R.I.
“We were actually just trying to align the liquid crystals … [but] ended up with this weird,
yarn-ball thing,” Crawford says. “It was totally unexpected.”
These bundles, described in the Dec. 4 Applied Physics Letters, represent more than just a
new type of polymer structure, Kossyrev explains. The balls behave in an unusual, possibly
useful way: In strong electric fields, they flatten out and partially unravel to form a flat, overlapping
pattern of loops. It’s as if “you took a piece of [cooked] spaghetti and threw it on the
table,” Kossyrev says.
“There are some neat things that one can propose to do with these,” says Joe B. Whitehead
of the University of Southern Mississippi in Hattiesburg. For example, they could be used for
controlling drug delivery by being loaded with medicine and then unfurled electrically.
To make the balls, Kossyrev and Crawford deposited diacrylate molecules on the inner,
cylindrical walls of pores in a membrane. After adding the fatty molecule lecithin, which
adjusted the diacrylate molecules’ orientations, the scientists solidified the mixture with UV
light. Finally, they dissolved the membrane with caustic soda. Hollow polymeric threads that
had formed in the now-vanished pores immediately tangled up into balls.
“Without lecithin, you don’t see any winding,” Kossyrev notes. Lecithin molecules shrink
from the water in the caustic solution, crumpling the 60-micrometer-long tubes into balls a few
micrometers in diameter, about the size of a sperm cell. An electric field introduces into the
thread a separation of charge that makes the structure flatten out.
In the drive to create nanotechnology (SN: 3/1/97, p. S14: https://www.sciencenews.org/pages/sn_arc97/75th/cw_essay.htm), many researchers are exploring
molecules that assemble themselves into useful structures. By using a membrane’s pores for
this process, Whitehead notes, this new work may open additional routes to that goal.
