IT COULDN鈥橳 be simpler. Just take a length of stiff but bendable tubing and
connect the ends with a short piece of more flexible pipe to form a loop. Before
you seal the loop, fill the tubes with water鈥攑erhaps laced with some drops
of coloured oil so you can see any flow. Now rhythmically squeeze and release
the flexible section and watch what happens to the fluid. Bizarrely, it will
begin to move around the circuit.
The phenomenon has been known for centuries. But nobody understands how such
a crude arrangement can possibly get fluid moving round the circuit without a
valve to stop it flowing backwards between compressions. It鈥檚 a puzzle that has
a bearing on a range of problems, from how to build microscopic 鈥渓abs on a chip鈥�
to improving first aid for heart-attack victims. Now mathematician Eunok Jung is
trying to solve it.
She began her quest while a graduate student at New York University. Working
with her supervisor, Charles Peskin, she developed a computer program that
simulates a basic 鈥渧alveless pump鈥�. For simplicity鈥檚 sake they made their
virtual fluid circuit two-dimensional, rather like an oval racetrack. The
program computed the intricate details of what happens to the fluid and the
walls of the tube as one end of the flexible segment is rhythmically squeezed
and released. Running the simulation took several hours on a high-powered
workstation, but in the end it came up with the hoped-for result: the fluid
coursed around the circuit.
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Jung varied the rate of squeezing in her model, and carefully examined the
resulting motion of the walls and the fluid, hoping to find a clue to the
mechanism behind the flow. As she increased the frequency of squeezing, not
surprisingly the fluid flow sped up too. But then something extraordinary
happened.
As the frequency increased, the fluid began to slow down. Eventually, at
around three squeezes per second, it stopped, and then started moving in the
other direction. At even higher frequencies, the flow speed increased, then
slowed, then reversed direction again at five squeezes per second. The flow kept
on reversing as the frequency increased. No one in four hundred years of
valveless pumping had ever reported this happening.
At first Jung didn鈥檛 believe the result: she thought there must be some
mistake in the way she and Peskin had constructed the simulation. But after
thoroughly checking the programming, she became certain that this was a real
phenomenon鈥攁t least when working in two dimensions. Only an experiment
would reveal if the same thing would happen in three dimensions.
Fortunately, the NYU Courant Institute of Mathematical Sciences where Jung
was working is one of the few maths departments anywhere that has its own
experimental laboratory. WetLab does fluid-dynamic experiments inspired
by鈥攁nd inspirational to鈥攖he mathematical work done at the institute.
Simulation in hand, Jung went to see Zun Zhang, who runs WetLab. Zhang quickly
cobbled together a valveless pump: a loop of rigid plastic hose with a short
rubber section. Jung and Zhang filled the tube with water containing tiny
coloured markers to help them observe the flow. Then they began to squeeze the
rubber section with their fingers.
鈥淚鈥檒l never forget that day,鈥� Jung says. As she pumped the tube faster and
faster she saw for real what she had previously glimpsed only on the computer
screen. 鈥淚 ran to Charlie鈥檚 office on the fifth floor and I almost shouted at
him that the fluid was moving in the other direction.鈥�
Jung, now based at Oak Ridge National Laboratory in Tennessee, is still
analysing her data, hoping to unravel the mystery. There are some clues, but not
many. The exact frequencies at which the reversals occur depend on the size and
flexibility of the tubing. Her simulations reveal that each squeeze of the
flexible section of tubing sends a wave of compressions along it. This, she
reasons, might drive the fluid round in the same way that rhythmic muscular
contractions drive food through our intestines.
But that doesn鈥檛 explain why the fluid slowed down and then changed direction
as the frequency increased. After the reversal, the vibration of the flexible
portion of tubing is locked into a standing wave鈥攖he compressions travel
neither to the left or to the right鈥攁nd yet the fluid still flows. 鈥淲e
have no explanation,鈥� Jung says.
But uncovering the mechanism will be worth all the hard work it鈥檚 going to
take, because it could help with some real-world problems. There are molluscs
called pteropods, for instance, that swim using a method which looks like
valveless pumping. Biologists would dearly love to understand how they do this.
There is also potential for tiny valveless pumps to move fluids around the
鈥渓ab-on-a-chip鈥� silicon wafers being developed to perform diagnostic tests on
blood and DNA solutions.
And the work might also help researchers understand the way our circulatory
system develops in the womb. During the third or fourth weeks of pregnancy, the
human fetus has a circulation even though its beating heart has no valves.
Valveless pumping must somehow be involved in this.
But the most dramatic potential application is to cardiopulmonary
resuscitation, the vigorous rhythmic massaging of the chest that is often the
only hope of saving people who have suffered a serious heart attack or electric
shock. Even though CPR has been used to save lives for 40 years, doctors still
don鈥檛 fully understood why it works.
There are two competing theories. One holds that when the heart is compressed
between the sternum and the spine, it pumps as normal, with its valves working
to ensure the blood flows in the right direction. The other theory asserts that
the pressure on the chest as a whole is what drives the blood flow, with the
heart acting only as a passive conduit. If this theory is correct, then CPR is
an example of valveless pumping.
That would make Jung鈥檚 discovery vitally important, as it suggests that the
efficiency of CPR might depend on the frequency of the compressions. Get it
wrong and the blood might not flow at all, or even try to flow in the wrong
direction around the body.
Jung is now studying this by modifying a heart simulation developed by Peskin
in the 1970s. If successful, Jung鈥檚 work may tell us the best way to carry out
CPR. Her strange discovery, confirmed with a length of plastic hose, may not be
one of science鈥檚 more glamorous results. But don鈥檛 knock it鈥攐ne day it
might just save your life.