behind it. So the spider is most likely to
step forward, not back. Following the
track laid out on the 65-by-90 nanometer origami field, the spider can walk
straight, turn a corner and make its way
along the path with no outside help. After
about 30 minutes, the spider reaches a
“stop” strand that its foot enzymes can’t
cut. Mission accomplished.
Stojanovic says his next goal with this
spider will be to increase the number
of steps it can take and program more
complex movements into the origami.
He also wants to design spiders that
can link together and cooperate on a
task. Spiders might also read each other’s trails, he says, the way ants or other
social insects do.
One day in the distant future, these
spiders may be able to crawl around on
cell membranes, recognizing diseased
cells and helping destroy them.
“That’s a dream, not something that is
around the corner,” Stojanovic says.
For that dream to become a reality,
the crawlers would need to graduate
from their artificial DNA tracks and be
able to traverse a more natural land-
scape, such as the surface of a cell. But
since cell membranes aren’t covered in
DNA, spiders would need to be designed
to interact with a different molecule,
perhaps an intermediate protein that
scientists would insert onto a cell.
Burned bridges
One problem with an autonomous spider
that cuts up the track behind it is that
the origami is used up after only one run.
“If your motors are forever destroy-
ing the tracks, you’ve got to rebuild
the tracks, which would cost you a
huge amount of energy,” says physicist
Andrew Turberfield of the University of
Oxford in England. “An automobile that
chewed up the road behind it would be a
bit unpopular.”
Turberfield is working on ways for
walkers to move by themselves without
destroying their tracks. His team has
come up with a two-footed walker that
walks along a reusable strand of DNA by
flipping over itself, like a gymnast doing
handsprings across a mat.
A “fuel” strand added to the surrounding solution raises the walker’s back foot.
The walker then flips over and moves a
step forward on the track. It can go backward simply by switching to a fuel strand
that reacts with the front foot.
This type of nanobot takes its inspiration from kinesin, a natural molecular
motor that carries cargo around the cell,
Turberfield says. Kinesin’s two feet coordinate so that the back foot is always the
one to pick up first and move forward.
“We’re looking at what cells do with
motors and are trying to emulate the
cell,” Turberfield says. “If you look at
biology as inspiration, then you won’t
go far wrong.”
Nano assembly lines
Another breed of DNA spider new to the
nanotech world does more than walk. It
also picks up cargo with three DNA arms.
A group led by DNA nanotechnology pioneer Ned Seeman of New York
University has designed a four-legged,
three-armed spider that picks up gold
nanoparticles from stations along an
origami track. The spider can’t walk by
itself, but rather requires the scientists
to add short, single strands of DNA into
the surrounding solution at each step to
coax the feet forward.
Researchers embed three stations
into the origami. Each station holds a
gold nanoparticle wrapped in a single
strand of DNA that is complementary
to the DNA in the spider’s arms. When a
spider stops at a station, one of its DNA
DNA in the fold dnA can be folded into almost any shape, even a smiley
face (computer illustration below). A more detailed simulation (right) shows
how a single strand of dnA (gray and black) is folded to create a tower shape.
Short “staple strands” (colored) bind across the folds, clipping the shape in
place. Once the pieces are mixed in solution, the shapes self-assemble.
