8, 1998 Design News
SPECIAL MEDICAL ISSUE
Sensors invade the medical world
Advancements in sensing
technology provide clinicians with superior diagnostic
tools and smarter medical products
by Mark Allan Gottschalk, Western Technical
Take a tour of your local hospital and there's one
thing that probably won't cross your mind--sensors.
It's surprising, actually, since the place is probably
chock full of them. In fact, without sensors most modern
medical facilities would be nearly tossed back to an
era of leeches and blood letting.
"Disposable blood-pressure transducers are the
single largest application of sensors in the world today,"
says Roger Grace of Roger Grace Associates, a San Francisco
based sensor-industry analyst and consulting company.
"We're talking about sixteen to twenty million
devices per year."
Since Novasensor (now Lucas Nova-sensor, Fremont, CA)
introduced disposable blood pressure transducers (DPT)
to the world in 1982, this application has demonstrated
that a sizable market exists for accurate, inexpensive
sensing solutions. But pressure transducers aren't the
only application of medical-sensing technology. Other
areas of interest include:
Stress and strain. Piezoelectric
thin and thick film sensors from such folks as AMP
Sensors (Valley Forge, PA) are extremely versatile.
They can be turned into impact, vibration, sound,
and force sensors--to name a few--used in everything
from ultrasound machines to electronic stethoscopes.
Chemical. Non-invasive blood glucose
monitors, fluorescing sensors, and other near-term
technologies could be used to detect certain proteins
in blood or form the basis of an "electronic
nose." "Chemical sensing will be a major
growth area, especially for MEMS," says Grace.
Monitor on the move. Continuous monitoring
of blood pressure with DPTs might be common in hospitals,
but most of us are probably more familiar with the pressure
cuff and stethoscope method used during doctor's office
visits. Trouble is, some people are a little too
used to them, and they actually show short-term elevated
readings attributed to nervousness, a phenomenon called
"white coat hypertension."
"It's believed that up to 20% of people might
show this effect to some degree," says Paul Voith,
signal processing engineer in the Cardiology Div. of
Marquette Medical Systems. Voith and the company, though,
believe they have a solution. Why not monitor a patient
at discrete intervals all day and see what his real-world
blood pressure is?
Still in evaluation and not yet FDA approved, Marquette's
Ambulatory Blood Pressure system is intended to do just
that. It consists of a small, hip-worn case containing
battery-operated electronics and pneumatics used to
inflate a pressure cuff on the patient's upper arm.
As it deflates at a controlled rate, a piezo-film sensor
sandwiched between the patient's arm and the cuff tracks
the opening and closing sounds of the brachial artery--called
Korotkoff sounds--similar to the way a doctor listens
with a stethoscope.
From this information the usual systolic (high value)
and diastolic (low value) readings can be gathered and
saved to non-volatile EEPROM memory. The system is intended
to be worn continuously over a 24-hour period with readings
programmed to be taken every 3 to 30 minutes.
Other such systems exist, but Marquette's is patented
using a technique Voith developed for extracting the
signal from what can be substantial noise due to the
environment and patient movement. "These types
of devices are all called ambulatory, but there are
not many that you can actually ambulate with,"
Voith says. "Most require you to stop what you
are doing and hold still during the readings."
To maximize the signal and reject noise, Voith's method
involves the use of two pairs of piezoelectric polyvinylidene
fluoride (PVDF) film sensors from AMP Sensors laid out
in a linear array 6 cm long. Each sensor measures 2
3 1 cm. The first and third sensor form one pair while
the second and fourth form the other. Center-to-center
spacing between two sensors in a pair is 22 mm--roughly
1/4-wave length for the Korotkoff sounds that are centered
at 20 Hz.
Gathering pairs of readings serves two purposes. One,
it gives a greater total signal strength. Two, each
sensor will read the Korotkoff sound at a slightly different
time, and this can be used to differentiate the pulse
from ambient noise which all the sensors will see essentially
Competing systems use a similar method, measuring two
signals 1/2-wave apart and then subtracting the signals.
"This gives about a 3-db gain," says Voith.
"We do a little higher-priced math to obtain just
the part of the signal which is phase shifted versus
that which is not," he explains. This has the advantage
of rejecting common mode noise as well as all noise
that is in phase with the sensors, even if it is not
of the same magnitude--something simple subtraction
The higher-math method also makes it possible to use
a thin, flexible substrate, increasing patient comfort.
Competing arrays often have a stiff substrate in an
attempt to commonly couple the sensors for better noise
rejection. "We have no need to isolate the sensors
from the rest of the world," says Voith, "because
we do our noise rejection electronically."
Marquette's ABP also captures readings of pressure
oscillations from the inflatable air cuff. These are
highly affected by motion and muscular contractions,
but can provide additional data points for correlation.
Voith notes that possibly the most challenging aspect
of the project involved finding the best pattern for
the piezo sensors. Ultimately, he evaluated a dozen
different designs before hitting upon the layout featured
in the prototype. "It took awhile, but we finally
found one that works quite well."
Electronic stethoscope. Piezo film
also forms the basis for a versatile monitoring system
developed by FlowScan (San Mateo, CA). Called the LifeFlow
sensor, it can be used to track blood flow, respiratory,
digestive, or other internal body sounds even under
less than ideal conditions--say, inside an ambulance
or rescue helicopter.
It consists of two of AMP Sensors' piezoelectric elements
bonded to opposite sides of a flexible substrate. Overall,
the sensor measures 31.7 3 25.4 mm with a thickness
of less than 1 mm.
The package is secured to a patient's skin with hydrogel
at the desired monitoring site--over the heart for blood-flow
monitoring, for instance, or near the throat to track
respiration. Output signals can be passed to FlowScan's
portable LIfe Sound Amplifier (LISA)to drive an attached
stethoscope or, for in-office situations, to a computer
workstation for graphical display.
Though it might seem that the sensor is simply a microphone,
in actuality it responds to minute vibrations of the
skin. "It senses tiny amounts of flexure, not sound,"
says Jim Kassal, vice president of product development
at FlowScan. Bending the center of the sensor relative
to the edges by just 1 micron, he says, generates an
80-mV signal. Sounds outside the body tend to move the
entire sensor as a unit, but not bend it. "And
if it isn't bending, it doesn't care if it's moving,"
This makes LifeFlow ideal for use by emergency technicians
in chaotic situations where they need to be able to
clearly differentiate sounds the patient's body is making
from the surrounding din.
When the sensor package is bent, each element experiences
dynamic strain that is opposite in sign to the other.
Those signals are sent to a difference amplifier and
then the two are subtracted from each other to extract
the desired patient's sounds from the background.
One challenge was to find a connector that wouldn't
transmit vibration from the cord to the sensor. After
looking extensively--and unsuccessfully--for a stock
connector that was shielded, small, and had four spring
contacts, Kassal opted to design his own. By making
it much more compliant, he was able to eliminate the
need to tape the cord to the patient which had been
a minor annoyance.
Kassal conceived the LifeFlow sensor as a derivative
of previous work designing piezoelectric sonar transmitters
for the Navy. "The sensor is the crucial element,"
says Kassal. "Once you have a signal, you can do
all sorts of neat stuff with it. The trick is getting
Anesthesia analyzer. An old medical
anecdote ends with a patient saying to his anesthesiologist,
"so you're the guy who makes sure I go to sleep."
To which the doctor replies: "No, I'm the guy who
makes sure you wake up."
It's sobering, but factual. More than 20-million Americ