Assembling a molecular-sized robot using DNA was recently demonstrated by researchers aiming for nano-robots to detect disease markers on a cell, diagnose it and deliver a cargo of cancer-killing drugs as appropriate.
Deoxyribonucleic
acid (DNA) holds the instructions
that every living cell uses to grow, develop and fight disease. The material
itself, however, can also be used as a building block for tiny machines with
the ability to propel themselves—slashing aside tethers like a machete clears a
path, then attaching a bond to move forward the way a winch pulls a truck.
Several
years ago, Professor Milan Stojanovic at Columbia University demonstrated a DNA robot that resembled a
spider in its ability to use its tentacles for self-propulsion. The spider was
shown to be capable of "walking" through a "field" of DNA molecules, and cutting
and bonding with them to propel itself. Unfortunately, the direction that the
spider traveled was random, setting Stojanovic on a quest to find a way to
direct its movement to a goal.
Stojanovic’s
team reasoned that such a small robot could not be directed in the manner of a
normal robot, which has a microprocessor programmed with navigational
abilities. For tiny DNA robots, there is no
internal mechanism to program. Instead, the researchers decided to program the
environment with "bread crumbs," which could form a trail that the DNA robot could follow.
At such
tiny scales, however, proteins like the DNA itself remained the
best bet for creating such a trail. The team found the material it needed in
the work of CalTech Professor Paul Rothemund, who had invented what he named DNA origami. Using what is
called sequence-recognition in base pairs, DNA origami are
"folded" from a long single strand of DNA, with several shorter
helper strands that "staple" the long strand into the desired shape.
Artist’s rendering of
a 4-nanometer-diameter robot (blue) cutting its feet (green) from protruding
staples behind it as it walks forward by binding to staples ahead of it,
leading down a path to its goal (red).
Stojanovic's
team made its own DNA origami bread crumbs,
each programmed with a specific instruction to any walker that happens by—like
sign posts, except that they only measured 2 nanometers thick and 100 nanometers
long. Together, the DNA origami bread crumbs
were formed into a path that allowed Stojanovic's team to lead their DNA robots to a goal.
The 4-nanometer-diameter
spiders were also constructed from protein, using four symmetrically placed
"legs" that could selectively cut and bind to the DNA bread crumbs. By
cutting off the protruding origami staples that tethered it, then binding to
the next ones, the legs could propel the robot down the DNA path.
And by
programming the shape of each DNA origami with specific
intentions on which way the robot should go next, the researchers were able to
direct the spider to follow prescribed routes. These autonomous molecular DNA robots were
demonstrated to start, move, turn and stop while following a prescribed path.
Applications include therapeutic medical devices that navigate by following
natural DNA markers that identify
cancer cells and enable the robot to deliver drugs only to those cells.
