1. MEMS in the Medical World
Microelectromechanical devices are small, reliable, and stable and can be manufactured inexpensively
for use in disposable medical monitors.
Harold Joseph, Bob Swafford, and Stephen Terry EG&G IC Sensors
MEMS (microelectromechanical systems) have been used in the medical industry since the
1980s for a variety of silicon pressure, accelerometer, and custom microstructure applications.
These devices provide a number of advantages over other technologies. They are small, reliable,
and inexpensive. Much of the wafer processing equipment developed to make silicon electronic
components can be used to manufacture silicon sensors. Silicon, being nearly perfectly elastic,
will not give or creep over time. This quality well suits the material to medical applications,
where sensor output must be dependable. The sensing elements are made in a wafer fab and are
processed in batches from 2500 to 15,000 sensors, depending on structure size and complexity.
Pressure Sensors
Among the MEMS devices most commonly used in medical applications is the Wheatstone
bridge piezoresistive silicon pressure sensor. Typically measuring 2 mm by 2 mm to 3 mm by 4
mm, with diaphragm thickness in the range of 0.5 mil, these devices measure pressures from
<0.1 psi to >10,000 psi. The pressure sensing element combines resistors and an etched
diaphragm structure to provide an electrical signal that changes with pressure. As the diaphragm,
etched to a thickness dictated by the range to be measured, moves under pressure, stress is
concentrated in specific areas of the silicon element. Four ion-implanted resistors in these areas
change in value with compression or tension. Choosing the proper location for the resistors and
controlling the orientation of the resistors allows the MEMS designer to predict how the resistor
will change in value for a given deflection of the diaphragm.
Blood Pressure. To monitor the blood pressure of a patient in
intensive care, a sensor is attached to a bag of saline solution.
Fluid from the bag passes through tubing into the sensing
package and then on into the patient's arm. As the heart beats, a
pressure wave moves up the fluid path and is detected by the
sensor.
External blood pressure monitors originally designed for
sterilization and reuse incorporated a sensor mounted on a
metal base and connected to wire feedthroughs. The saline
solution passed through a plastic housing placed over the metal
base. A flexible diaphragm in the housing covered the pressure
sensor and separated it from the fluid. In today's version of the
monitor, a small sensing element (see Photo 1) is mounted on a
plastic or ceramic base with a cap designed to drop into the
manufacturer's housing. A gel separates the saline solution
from the sensing element. A similar package used to measure
intrauterine pressure during birth is housed in a catheter placed

2. between the baby's head and the uterine wall. During delivery, the baby's blood pressure is
monitored for problems during the mother's contractions.
Vital Signs. Vital signs monitors are instruments
used in hospitals and ambulances to measure blood
pressure and respiration. Sensors for these
applications are often placed in a TO-8, DIP, SIP, or
surface mount package (see Photo 2). The devices
can be scaled down to ~1/4 to 1/2 of blood pressure
sensing range and used in ventilators to monitor
breathing. In these applications, a tube attached to
the sensor brings the pressure measurement into the
monitor.
Eye Surgery. A system for removing debris
during retinal surgery incorporates several pressure sensors. During surgery, fluid is removed
from the eye, cleaned, replaced, and augmented if necessary. Two PCB mountable pressure
sensors measure and control the vacuum used to remove the fluid. The sensors are in parallel; if
either indicates a fault condition, the system is shut down. Another sensor in the vacuum control
loop measures barometric pressure and provides input to the pump's electronics.
Hospital Beds. Inflatable hospital bed mattresses designed for burn victims incorporate a series
of inflatable chambers regulated by pressure sensors in the control loop . Sections can be deflated
under burn areas to reduce pain and promote healing. For sleep apnea detection, breathing and
motion cause pressure changes in an inflated mattress. These changes can be monitored by a
low-range, very sensitive pressure transducer. If a time period passes without any movement, an
alarm is generated to awaken the sleeper.
Blood Analysis. PCB mountable pressure sensors (see
Photo 3) designed for in-office use provide barometric
pressure correction for systems that analyze
concentrations of O2, CO2, calcium, potassium, and
glucose in a patient's blood.
Inhalers. Individuals dependent on inhalers often
activate the devices during emergency situations when,
short of breath and agitated, they may trigger their
inhalers when their lungs are full or even when they are exhaling. The result is an insufficient
dose of medication. The optimum time for drug delivery is the beginning of inhalation, and
pressure sensors are accordingly being designed
into inhalers to detect where an individual is in the
breathing cycle and release the medication at the
proper time.
Kidney Dialysis Machines. When fluid pressure
is to be measured in a disposable plastic "set," a

3. flush diaphragm sensor is often used. These sensors incorporate either a plastic or a metal
diaphragm (see Photo 4) that mates up against a pressure diaphragm. Pressure is transmitted
across the diaphragms to the sensing element by means of a silica gel or silicone oil contained in
a disposable plastic set or tubing kit.
For kidney dialysis, catheters are attached to the patient's artery and vein. Blood flows from the
artery into the dialysis machine where it is cleaned, and then flows back into the vein. Cleaning
is accomplished by passing the blood across a thin membrane, on one side of which is a special
solution that matches the blood's mineral makeup. Through osmosis, waste products are removed
from the blood and move across the membrane into the fluid. Inlet and outlet pressures of both
blood and fluid are measured with a flush diaphragm pressure sensor, and the information is used
to regulate the operation of the dialysis system.
Infusion Pumps/Drug Delivery Systems. Infusion pumps and drug delivery systems meter
medication through a disposable plastic set or tube inserted into the patient. When a blockage
prevents fluid from moving down the tube, the sensing element detects a pressure spike and
indicates an alarm condition. The pressure range for these applications is in the 10 psi range and
the sensor can be a pressure device, a strain gauge, or a custom microstructure.
Several techniques are in current use. In some cases, the plastic set contains a diaphragm that
interfaces with a pressure sensor designed to have a matching diaphragm. Pressure spikes in the
plastic tubing increase the pressure across the diaphragm and the sensing element registers the
change. An alternative is a strain gauge connected to a flexible beam. The plastic tube rests on
the beam and moves it as pressure increases.
For infusion applications either in the home or for ambulatory use, a silicon microstructure flow
restrictor (see Photo 5) can provide the precise low flow rate control that is required.
The restrictor chip measures 1.8 mm by 4.5 mm and provides a flow rate of 0.5 ml/hr. The very
repeatable manufacturing process of the flow structure makes it an attractive alternative to
conventional glass capillary tubing.

4. Medical Drilling Equipment. In medical bores used for drilling bone and other hard tissue,
transducers measure the pressure of blood and other internal fluids during the drilling process.
The sensor interfaces with a disposable membrane. Fluid used in drilling exerts backpressure
through the membrane to the sensor and is monitored.
Accelerometers
The piezoresistive principle can also be used to make a silicon accelerometer. The sensing
element is typically 3 mm by 3 mm, and a typical production wafer lot contains 25, 4-in.-dia.
wafers, so a large number of sensors can be made from a single run. Accelerometers resemble
pressure sensors in combining ion-implanted resistors with a moving structure. The difference is
that accelerometers incorporate a sensing mass supported by beams in which resistors are
implanted. Caps above and below the sensing mass prevent damage to the sensor from an
overrange condition. The accelerometer mass moves when vibration occurs or as the structure is
rotated.
The sensor gives a constant output that is directly related to the position of the mass. This means
that the accelerometer can be used in applications to measure vibration frequency and amplitude,
or in DC applications to determine angle or register the amplitude of a single shock or pulse. The
operating range extends from below 0.1 g to over 500 g, and from DC to 3000 Hz. The devices
provide very good phase response for position and modal analysis.
Pacemakers. Pacemakers are designed to regulate heartbeat. When a person outfitted with a
pacemaker engages in physical activity, the heart rate needs to speed up. The accelerometer
detects increased movement and outputs this information to a microprocessor in the pacemaker,
which adjusts its signal to the heart.
Patient Activity Monitoring. Sleep and motion studies often entail one to three silicon
accelerometers, each designed to detect even slight motion in a single axis. Three of these
devices can be mounted orthogonally to provide an accurate description of movement in three
directions.
New Directions
Temperature. A remote temperature sensor can be made by depositing a thin layer of nitride-
oxide on an etched silicon diaphragm to form a thermocouple. The diaphragm isolates the sensor
from the immediate environment. IR radiation focused on the thermocouple creates a voltage.
This is known as the Sebeck effect. The sensor, or thermopile, is being evaluated for use in
remotely measuring temperature in the inner ear.
PCR Testing. Custom silicon microstructures are being considered for a number of new
specialized medical applications. Polymerase chain reaction (PCR), used in the analysis of DNA,
requires a carrier with a series of small wells containing a reagent. The carrier is rapidly cycled
between two temperatures. A silicon element works well as a carrier because it can be made very
small-an 11 mm by 11 mm array can provide up to 48 wells-with tight dimensional tolerances.
Depositing circuitry in each well to provide temperature control while testing requires less
reagent and permits shorter testing times.

5. Chemical Sensing with Silicon
An innovative silicon chemical sensing platform is
designed to replace more conventional devices that
dissipate power (heat) through a catalyst deposited on a
ceramic base. The sensor's small size requires less reagent,
resulting
in faster, more accurate, less expensive
operation. Suspending the dielectric
membrane on a silicon frame (see Photo 6)
achieves a high degree of thermal isolation
that translates into less power dissipation.
Platinum thin film elements built into the
membrane both heat the membrane and
measure its temperature. By changing the
geometry and coating of the suspended
mass, the sensor can be optimized to detect
combustible gases, CO, CO2, NOx, N2, and
H2O (moisture) for the consumer,
industrial, automotive, laboratory, and portable diagnostic
instrument markets.
Photo 6. The
chemical sensor in
this micro
photograph
measures 2 mm by
2 mm. Power
dissipation is 5 - 50
mW, and response
time is <1 s.
The current trend toward reducing the length of hospital stays is putting a greater emphasis on
outpatient and home care. Many of the monitoring products originally developed for hospitals
are being made less expensive and less complicated for use in home care environments. The
market for lower cost MEMS devices is accordingly expanding.
The authors wish to thank Steve Repetto, Gary Rendla, and Karin Gilles for their contributions to
this article.
Harold Joseph is Director of Sales and Marketing, Bob Swofford is Product Marketing
Manager, and Stephen Terry is Vice President of R&D, EG&G IC Sensors, 1701 McCarthy
Blvd., Milpitas, CA 95035