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05/18/1999
Lithium and "Thin Hockey Pucks"
The Stanley Cup playoffs are in full swing as I write this.
Hockey pucks reminded me of that famous Swede, Arne H. W.
Larsson, the subject of an article last October 27 in the science
section of the New York Times. Mr. Larsson, 83 years old at
the time of the article, has the distinction of being the first
recipient of an implanted cardiac pacemaker, which in 1958
was roughly the size of a "thin" hockey puck. He suffered
from a disease which led to potentially fatal fainting spells,
sometimes 20-30 times a day! According to the Times article,
his wife had to plead with Dr. Ake Senning and Dr. Rune
Elmquist, an engineer, to implant the pacemaker they had
invented, even though they did not consider it ready for use in
humans. They finally agreed and, only 8 hours after the
implant operation, the device failed! In the middle of the night,
another operation and the only backup device was installed.
The batteries had to be recharged every few hours and that
pacemaker worked "on and off" for three years. Over the
years, the very healthy looking Larsson has had 26 different
pacemakers, one lasting more than 6 years.
The pacemaker idea dates back to at least 1889 when a Scot
named McWilliam suggested electrical pulses to insure a
steady heartbeat in patients with too low a pulse rate. In 1932,
an American, Albert Hyman, came up with a machine the size
of a sewing machine hooked to the heart by wires inserted
through the chest. Today, a typical pacemaker is more like a
thick quarter or half-dollar than a hockey puck and a simple
pacemaker can last 10 years or more. Some years ago there
were news reports questioning the large numbers of pacemaker
operations. But I had a friend, Lou, who not only had a
pacemaker but also a pig valve, both of which served him well
until his death in his 80''s last year. Lou joked that he would
live as long as there were enough spare pigs and pacemakers to
go around. Once, during an operation to replace his pacemaker
when the battery was running low, the surgeon took him off
any electrical stimulation and Lou''s pulse dropped to zero! In
his case, the pacemaker was literally keeping him alive.
My friend Lou could always tell when a nurse or doctor was
paying attention when taking his pulse. If their answer
deviated from 70 beats per minute, he could embarrass them by
pointing out that his pacemaker was programmed for that rate
and his pulse never deviated from that number. Today, more
advanced pacemakers are programmed for slower or faster
rates, depending, e.g., on whether the person is running or
resting. Some pacemakers can be queried as to the status of the
battery and even the history of the patient''s heart performance
since the last visit. Two developments are responsible for
these capabilities and for the fact that the hockey puck is no
longer a token for the size of the pacemaker. One is the silicon
chip (the first pacemaker contained two transistors, today''s
contain roughly 100,000); the other development is the lithium
battery.
Lithium is the lightest metal and is also extremely reactive,
especially when it melts. Being light and reactive make it very
attractive for a battery. A battery has two electrodes, a positive
and a negative. Lithium''s reactivity makes it the most negative
negative you can find (forgive the double negative). This
means lithium batteries have the highest voltages possible with
any given positive material. For at least two decades, the
battery of choice, and certainly the one I would want, for
cardiac pacemakers has been the lithium-iodine battery. When
I was a kid we would always paint a cut or scrape with
"iodine", actually iodine dissolved in alcohol. The positive
electrode is iodine in a polymer material with a chemical name
of poly 2-vinylpyridine. We''ll call it PVP.
We now have our two electrode materials, lithium metal and
iodine in PVP. But you might be saying, "Hey, we need an
electrolyte! My car battery has sulfuric acid in it." You''re
right. And here''s one of the beauties of this lithium-iodine
battery. When the lithium metal touches the iodine in PVP, the
lithium reacts with the iodine to form a film of a salt, lithium
iodide. Just as another highly reactive metal, sodium, reacts
with chlorine to form sodium chloride, common table salt.
Well, this lithium iodide film forms what we battery types call
a "separator", separating the metal from the iodine, stopping
the reaction and preventing a short circuit. I visited a
manufacturer of pacemaker batteries in Baltimore many years
ago and the way the battery was made was to form a U-shaped
cup of the soft lithium metal and simply pour the tar-like
mixture of iodine and PVP into the lithium cup. (I''ll never
forget that visit. On the way home via the Amtrak Metroliner I
felt a little bump. The speaker system happened to be on and
we passengers heard the engineer say, "I''ve just killed a man!"
Several years later, I mentioned this to my brother, whose
house in Delaware bordered the Metroliner tracks. It was
someone he knew who had committed suicide by walking in
front of our train!)
But I digress, and you''re saying, "Where''s the electrolyte?" It''s
the lithium iodide film, which is not just a separator but also a
"solid electrolyte". Solid electrolytes are nowhere near as
"good" an electrolyte as the sulfuric acid in your car battery,
which can deliver hundreds of amperes on demand. But a
pacemaker only requires roughly 20 microamperes of current, a
microampere being about a million times less current than your
100-watt light bulb uses. The lithium-iodine battery easily
handles the job at the 98.6*F temperature of a normal
functioning body. Now suppose you''re in an accident or run
into a wall and the pacemaker takes a blow that cracks the
lithium iodide film. The lithium metal simply forms more
lithium iodide, plugging the crack and is none the worse for
wear! Furthermore, it turns out that the product of the battery
reaction is, what else, lithium iodide, which builds up as the
cell discharges while running the pacemaker. Some of the
lithium iodide deposits on the film, making it thicker. This
increases the resistance so that after perhaps 10 years or so the
voltage begins to fall and the physician can detect that a new
battery or pacemaker is needed. This could be quite helpful,
especially to my old friend Lou, who certainly wouldn''t have
wanted his pacemaker to stop functioning without warning.
When I was at Bell Labs, we were considering using the
lithium pacemaker style battery to back up semiconductor
memories in telephones. When you program your frequently
called numbers into your pushbutton phone, chances are you''ll
find that you have to reprogram your phone (or reset your VCR
and digital clocks) when the power goes out. With battery
backup, this would not occur. We were concerned about the
safety aspects of using the lithium-iodine battery and opted for
a very sophisticated test. We put the battery on a hotplate
outside a small building, which happened to have an outdoor
power socket, turned the power on, retreated to a safe distance
to see what happened. At that time the pacemaker battery was
more like a fourth of a hockey puck in size and when we got
past the melting point of lithium (180*C) the battery began to
smoke and with a loud detonation the battery ended up on the
roof of the shed! A beautiful violet cloud of smoke, the violet
color typical of burning lithium, accompanied the detonation.
This demonstrates that safety is a relative thing. In the body at
98.6*F the battery is super-safe but, as the crematorium
workers found out when the first pacemaker-implanted bodies
showed up, you can lose your furnace if you''re not aware of the
power of lithium batteries out of control! We''ll have more on
other lithium batteries in later columns.
Allen F. Bortrum
Next article...Tuesday, May 25th
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