#168 from R&D
Innovator Volume 4, Number 7
Sussman, a professor of chemical engineering at Tufts University,
has written two textbooks on thermodynamics and has twenty
invented a process to that improves the physical properties and
manufacturing productivity of textile and industrial fibers.
That a specialist in thermodynamics came up with this
process shows how seemingly unrelated experiences can serve as the
germ for innovation.
The story starts
over 20 years ago, when I had heard that an Israeli biophysicist,
Aharon Katchalsky, was interested in non-equilibrium
thermodynamics, my specialty, concerns systems that go from an
initial equilibrium or rest state to another such equilibrium
state, while non-equilibrium thermodynamics has to do with
continually changing systems or systems that are experiencing
dynamic inflows and outflows, even when at rest.
thermodynamics is applicable to such biological systems as muscle
movement. I contacted
Dr. Katchalsky and arranged to spend my sabbatical in his lab to
better understand his attempts to apply thermodynamics to biology
and to what are sometimes called “steady state” systems.
The Israeli lab
workers were fascinated by the properties of collagen fiber.
Collagen is a basic building material of tendons.
Under the name “catgut,” it serves as an absorbable
suture in surgery. Collagen
fiber contracts forcefully to half its length when placed in
certain salt solutions. In
fresh water, it expands again, and the expansion-contraction
process can be cycled over and over again.
Here was a
material that changed its linear dimension upon the "chemical
command" of a salt solution.
If you can get something to change dimension, it doesn't
take too much ingenuity to use it to drive an engine.
In fact, that's how humans have always built engines,
starting with the steam engines of Mr. Watt and his predecessors.
They didn't use a chemical command, but instead used heat
to change the volume of a gas (steam), and the volume change of
steam moved a piston. The
use of heat to produce motion is effective, but inherently
inefficient. The direct
conversion of chemical energy to mechanical energy would be more
efficient and is routine in muscle, but man-made machines are
generally not driven this way.
Katchalsky's lab assembled a “chemical” engine in which a loop
of collagen fibers ran around four pulleys.
Two pulleys were immersed in fresh water and two in salt
solution. By simple
gearing, the pulleys were interconnected and constrained to move
at rates matching the extent of contraction and expansion of the
collagen. As the
fiber contracted in the salt water and expanded in fresh water, it
drove the pulleys slowly.
"That's clever, but you guys are so absorbed in
non-equilibrium thermodynamics that you've ignored classical
efficient engine always moves against a resisting force only
infinitesimally smaller than the force it generates."
(For example, to raise a weight with a rope and pulley, an
efficient process would use the minimal force that would move the
weight.) And they
said, "Oh? Well,
how do we do this with a fiber?"
My suggestion was that they replace the pulleys with two
cones with collagen fiber wrapped in a helix about the cones.
The cones, unlike the pulleys, allowed the collagen to
contract gradually in many small decrements; and thus release its
contractile energy very efficiently as the fiber wrapped around
the pulleys moved from the large to the narrow ends.
We built such a device.
It was a novel mechanochemical turbine that rotated as
smoothly and quietly as an electric clock! The output power was taken from a pulley on the cone's shaft.
Our little engine
simulated, efficiently but crudely, the chemical roots of animal
muscle action. All
man-made engines use heat to expand something.
Instead, our "mechanochemical turbine" derived
its energy directly from a chemical reaction.
We described the work in Science
(167: 45, 1970) and our article attracted wide attention from the
engine wasn’t practical. The
amount of work it could perform was very low.
You'd need a truckful of salt water to power a bicycle.
Also the collagen lost its strength after two days of
engine operation. It
didn't seem worth patenting.
We hoped, however, that our success would stimulate ideas
for other types of mechanochemical engines.
Perhaps, the thermodynamics of our engine would provide
insights into biological systems like muscle—or more
promising—perhaps someone would synthesize a better contractile
the Engine Backwards
One day, while
watching the collagen engine run, I asked myself, "What if we
were to run it backwards? What
use could that have? Could
we do something by stretching
the fibers in this manner? In
the engine, the contracting fiber rotated the cones.
So, how about rotating the cones specifically to elongate a
brought back memories of fiber production at du Pont, where I used
to work. When nylon
is made, it is stretched on rotating cylinders to align the
molecules and strengthen the fiber.
Now that I was working with rotating cones (rotating due to
collagen shrinking), it seemed that cones may have a distinct
advantage over cylinders to produce fibers with more finely
controlled molecular alignment and structure.
I returned to
Tufts and started working on the idea, which I call the
"Incremental Draw Process."
Remember, the mechanochemical turbine worked because small
incremental contractions forced the collagen fiber to rotate the
metal cone. Working
backwards, I now incrementally stretch the fiber as it moves in an
expanding helix from the smaller to the larger circumference of a
pair of rotating cones.
It was also
possible to impose a temperature gradient on the cones, allowing
the polymer molecules to remain mobile as their orientation along
the fiber's length increased.
This allows increased stretching and gives the fiber
improved properties, including greater tensile strength.
We now could vary the deformation rate, temperature, and
temperature gradient to control molecular structure far more
precisely than is possible in conventional fiber processing,
making the incremental draw process applicable to many kinds of
industrial and textile fibers.
I published this work in Polymer
News (18: 70,
1993), and elsewhere. This
attracted quite a bit of attention from the fiber industry.
I worked with a
number of fiber companies to test the concept. It was difficult to obtain the commitment needed for our
realization of a commercial process.
However, I persisted, as did Allied Signal and finally a
viable manufacturing process was achieved.
As a consequence, other companies are now showing interest.
In my lab, I'm
collecting a "library" of fibers made by changing
parameters during the incremental draw process.
We analyze this library using x-ray diffraction, and other
tools, to try to quantify and prepare mathematical models of
structure formation in crystalizable polymers as a function of
the mechanochemical turbine and the incremental draw process were
rewarding experiences. When
I think about a new concept or device, I go out of my way to
consider what would happen if I ran it backwards.
It's a powerful way to trigger new ideas.