By Tom Henderson
Small Times Senior Writer
Aug. 1, 2001 -- Robert Brown may not have discovered the secret of life nearly 175 years ago, but the English botanist might have stumbled upon the secret to powering the nanomachines of tomorrow.
Brownian motion, the random movement of tiny objects caused by thermal energy, could be harnessed and directed, said Ronald Fox, chairman of the physics department at the Georgia Institute of Technology.
If Fox is correct, scientists might have a power source for future nanoscale machines that cruise through the body to a cancer cell, then deliver tumor-killing enzymes; or for nearly invisible switches that make faster all-optical telecommunications a reality.
Nanoscale devices measure between 1 and 100 nanometers in scale, a nanometer being one-billionth of a meter. Unlike miniaturization to date, which has been top down -- that is, scaled down from larger to smaller -- nanoscale devices will be built up from the bottom, assembled one molecule at a time.
"There are lessons here for nanotechnology," Fox said. "Thermal motion can actually be harnessed to do many kinds of useful work."
In an article in the May issue of Physical Review E, Fox argues that while Brownian motion is generally random, the body has figured out a way to direct it in a controlled fashion to move enzymes and other chemicals inside cells.
What the body can do at the nanoscale, so, too, will nanotechnologists of the future, he said.
"When you get into this subcellular level on the nanometer scale, the dynamics and vitality of protein molecules are really due to thermal motion."
In 1827, Brown noticed the irregular but constant movement of tiny particles of pollen suspended in water. He was sure that what he was seeing through his primitive microscope was, in his words, "the secret of life."
But when he later saw the same motion by tiny inorganic particles, he disappointedly discarded his premise. The motion wasn't driven by any innate life force in the pollen, itself, but as a result of the thermal energy of the surrounding fluid, with molecules of the fluid randomly and constantly bombarding whatever small particle is suspended in it.
In 1999, Fox won a three-year National Science Foundation grant for $135,000, which funds the Brownian motion work of Fox and his postdoctoral colleague, Mee Hyang Choi.
The grant was for pure research into the study of random fluctuations in physical processes. "I didn't have to promise to build any devices," Fox joked.
But the work may nonetheless provide a vital link to the nanomachines and devices of the future.
Inside the body's cells, kinesin proteins acting like cellular tow trucks pull sacks of chemicals along tiny pathways called microtubules. Until now, it was thought that the kinesins' "walk" along the microtubules was fueled by an energy molecule called adenosine triphosphate (ATP).
In the article in Physical Review, Fox wrote that his research shows that there is a more efficient process that actually uses channeled Brownian motion to propel the proteins.
The kinesin proteins have a leg-like head at each end. One head attaches to the side of the microtubule, then the other head moves ahead and attaches, and the first lets go. Bit by bit, eight nanometers at a time, the protein advances. Fox's theory is that ATP merely acts as a switch to first bind the head of the protein to the microtubule and then to release it.
The protein's advance up the tube is fueled by Brownian motion, instead.
"Normally, Brownian motion cannot do anything concerted or with directionality, because it is random," Fox said. But asymmetric conditions created by the geometry of the microtubule and the switching of the ATP funnel the Brownian motion in a nonrandom direction.
An article in the May 24 issue of the journal Nature by researchers at the University of Tokyo and the University of California supported Fox's model of ATP switching, though Fox admitted further experiments are needed to win over skeptics.
Such experiments currently are impossible, Fox said, because of the tiny time and distance scales involved in monitoring such nanomotion.
Nonetheless, the Nature article prompted one of Fox's supporters to e-mail him with this enthusiastic message: "Did you see this? It's proof you are right!"
The supporter was Dr. Richard Fishel, professor of microbiology and immunology at the Kimmel Cancer Institute in Philadelphia. The focus of his work is on how DNA repair genes locate and bind to damaged DNA, and he believes Fox offers a reliable explanation.
"Ron Fox has now set the clear physical dimensions for how that has to work and how it has to work is [by] Brownian motion."
The repair of damaged DNA, Fishel said, involves a switching operation that controls a protein's affinity to bind to other proteins it encounters as Brownian motion moves it through cells.
"What Ron has provided here is the physical reasoning behind how these collisions can work," he said.
"It's clear Ron is right. Totally right. As a friend of mine said last night, 'This is a paradigm shift.' It's a completely different way of thinking."
EVENTUAL REAL-WORLD APPLICATIONS
MEMS devices operate like macroscale devices, and even look like them, with motors and gears and levers made of glass, ceramic or metal.
But NEMS -- nanoelectromechanical systems -- will act entirely differently, Fox said. At the tiniest of scales, the laws of physics change the ways things act and interact. For one thing, very tiny things have much more surface volume relative to weight.
Fox said that harnessing Brownian motion, in man-made ways that mimic the human cell, might be the most efficient way for nanotechnologists to design nanodevices.
At the macroscale, Brownian motion or diffusion -- it is the same process that causes gases over time to spread evenly through a confining area such as a room -- is relatively slow. But at the nanoscale, it is quite powerful, providing movement of up to 1,000 nanometers a second.
"I'm arguing that at that scale, moving parts could get their movement not by concerted and directed motion, but by random motion. It will work robustly. I think most nanotechnologists are unaware that these kinds of mechanisms exist. They need to know about them."
CONTACT THE AUTHOR:
Tom Henderson at email@example.com or call 734-528-6292.