Transcript MEMS Cardiopulmonary Management

MEMS
Cardiopulmonary
Management
Bruce Lau
Stanley Wong
Spring 07 MAE268
Advisors:
Prof. Prab Bandaru
Prof. SungHo Jin
Prof. Frank Talke
Overview
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CardioPulmonary system is the basic life sustaining system.
Patients with Bradycardia often require pacemaker intervention to
supply appropriate blood flow.
We would like to integrate MEMS wireless piezoresistive pressure
sensors to optimize the CardioPulmonary system in patients utilizing
current pacemaker.
We will propose ways of improving MEMS piezoresistive sensors for
our interests.
We will discuss benefits and future challenges of this design.
Questions?
Introduction: (Cardiopulmonary)
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The cardiopulmonary system is the major source of fuel to the entire
human anatomy.
The heart is responsible for providing pressure to pump oxygenated
and deoxygenated blood in and out of tissues.
The lung’s jurisdiction to delivery oxygen and excreting carbon
dioxide by diffusion from the blood pumped by the heart.
Both the lung and the heart are considered to be within the same
system due to their dependency on each other.
The primary mechanism for perfusion of oxygen and carbon dioxide
is by diffusion. Diffusion is governed by the simple Fick’s law.
dc
J  DA
dx
Fick’s Law
Introduction: (Cardiopulmonary)
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The glottis is a valve that opens and closes to allow
air movement.
Air then passes through the major airway, trachea,
down into bronchioles (smaller bifurcations of
airways).
When air reaches the alveolar (small air sacs),
oxygen and carbon dioxide diffusion takes places by
concentration gradient.
Thus, when oxygen rich air reaches the alveolar,
the heart must have already pumped deoxygenated
blood to the alveolar for diffusion.
With our MEMS management system, we are
hoping to optimize this.
There are two mismatches:
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Lean (more air and not enough blood)
Rich (more blood and not enough air)
Pacemakers patient have trouble with exercise
because the pacemaker cannot keep up for
ventilation causing “lean” situations. Our design
focus is to eliminate this.
Introduction: (Design Components)
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The lungs work similar to a vacuum to intake air. Analogous to an
internal combustion motor.
Heart is a pulsitile pump creating a pressure difference to drive
blood flow.
We will place 3 MEMS wireless pressure sensors (MEMS WPSs):
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1 MEMS WPS for measuring the pressure inside the lungs/respiratory
system. Implant under glottis.
2 MEMS WPS for measuring systolic and diastolic blood pressure to
derive pressure difference. Implants in vena cava and aorta.
External Management Charging system will mainly serve as a
diagnostic tool for the physician or even a charger for the patient.
Additional circuit including a RF receiver to retrieve signals from
MEMS WPSs for the pacemaker to optimize heart flow by altering
heart rate.
Bradycardia
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Symptoms
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Characterized by an abnormally slow heart (<60 beats per minute)
 Chronic fatigue and exercise intolerance due to the heart not being able to
provide enough nutrients to the body
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Diagnosis
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At the digression of doctors based upon several factors
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Age
Level of activity
Symptoms
EEG Readings
Figure 1: ECG reading of a healthy Individual with normal
PQRS waves.
Figure 2: Individual diagnosed with Bradycardia having P wave
durations of longer than 3 seconds
Bradycardia
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Causes
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Malfunction of the Sinus Node
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The Sinus Node is the heart’s natural pacemaker and is responsible for generating
electrical impulses to produce heart beats.
Issues with the conduction pathway
Intrinsic Factors
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Surgical Trauma
 Familial disease
 Infectious disease
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Extrinsic Factors
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Body’s response to drugs
Situational Disturbances
Imbalances in the body
Table 1: Intrinsic and Extrinsic causes of Sinus Node disorders
Current Treatment Options
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Drug Therapy
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Atropine Sulfate is used at doses of 0.05 mg every 3 to 5 mins in the emergency room
(First-line therapy) . Dopamine, Epinepherine, Isoproterenol are also alternatives
Temporary Pacemakers
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External Pacemakers
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Intravenous Pacemakers
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Placing electrodes on the surface of the skin near the heart and sending voltage pulses to control
heart rate
Typically a function in portable defibrillators
Require large current pulses (~50-100 mA) which may be uncomfortable to patient
Incision is made on a main artery to the heart and a catheter like device is routed to send localized
pulse to regulate heart rate
Permanent Pacemakers
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Implanted in an individual and uses a program to regulate heart rate based upon
information collected from a sensor. Measure natural pulse of a heart and uses algorithm to
determine whether interventional pacing is required
Figure 3: External
(www.orsupply.com)
Figure 4: Intravenous Pacemaker
(http://www.osypkamed.com)
Figure 5: Implantable Pacemaker
(http://www.medtronic.com)
Rate Adaptive Pacing
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Manufactures
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Rate adaptive pacing was first introduced by Medtronics in the mid 1980’s in a device called
Activitrax. Since then, different types of sensors have been developed to improve the
pacemakers response to physiological needs
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Rate adaptive sensors
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Vibration Sensing
- Commonly used. An accelerometer placed inside the pacemakers housing and do not require
additional electrodes
- Disadvantage: Respond to external vibration such as the tapping of the foot, driving on a
bumpy road, and external environmental noises
- Responsive to exercise may not be proportional to the intensity. Example of walking up and
down on stairs
Rate Adaptive Pacing
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Sensor type (continued)
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Temperature Sensor
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Minute Ventilation
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Blood temperature good indicator of physiological demand
Not widespread because it requires additional leads
Disadvantage:
- Slow response time. Physiological need may increase before sensor can indicate a change.
(Sudden activities such as sprinting)
- May respond to ingesting hot or cold foods
Defined as (respiration rate x tidal volume)
Can be approximated by measuring the impedance of the thoracic cavity and pacemaker case
- Electrode placed inside chest wall followed by pulses (the Meta pacemaker uses 1 mA at 50
ms intervals)
Earlier models incorporating Minute Ventilation technology had issues with slow response times
Disadvantages:
- Increase power drain due to the need to provide constant pulses
- Impedance can be sensitive to motion artifacts
Ideal Sensor
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Direct measurement of respiration rate and tidal volume.
Good response time
Not sensitive to motion artifacts
Will provide an appropriate response based upon the physiological needs of an individual.
Principles: MEMS RF Induction
Techonology
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The top plate on the left represents the
external receiver unit sending energy
through induction or receiving RF
telemetry signals for the sensor.
The bottom plate placed on the
surface of the biological tissue
represents the sensor itself with a
diaphragm in the middle that offers
piezoresistive sensing.
Notice the edges of the diaphragm will
be a coil similar to the external unit.
Optimal frequency from NASA’s design
is 330 MHz with approximate 10 cm
charging and signal transduction
distance.
Coil serves as an antenna for signal
transport and as an inductor for
charging.
Principles: MEMS Piezoresistance
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the initial resistance R is dependent directly on ρ
(resistivity), l (length of piezoresistor) and inversely to t
(thickness of the piezoresistor) and w (width of the
contact)
When pressure is introduced, the new resistance is
represented by R’
With the diaphragm of the MEMS WPS on the bottom, the
diaphragm will face compressive stress and can be further
simplified.
We have sensitivity as the magnitude of dR/R. It is this
change in resistance that allows for the absolute pressure
to be measured while the sensitivity represents how well
this resistance change can correlate to the pressure range
of interest.
Sensitivity is dependent on the average longitudinal and
transverse stresses, both types of stresses are dependent
on the dimensions of the diaphragm or membrane
r is the radial distance from the center of the diaphragm, a
represents the radius of the circular diaphragm for
simplicity, and v represents the Poisson’s ratio
Design: Benefits
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Diagram on the left shows the MEMS WPS can
be controlled and charged wirelessly.
With our proposed MEMS WPS sensors, we
are able to measure the different absolute
pressure inside the respirator tract and giving
us the air density reading with the addition of a
temperature sensor built into the MEMS WPS.
The air density measurement is derived from
the ideal gas law where the flow rate is
proportional to the product of air density
measured and the speed (inspiration or
expiration rate).
With the direct absolute pressure reading, we
can achieve better response time, a direct
method of measuring air flow and less
susceptible to vibration effects allowing our
proposed design to be more accurate and
precise then all of the precious methods
mentioned above.
If the systemic vascular resistance of the
patient is know, we can calculate blood flow by
utilizing Darcy’s law as the difference in
pressure of the vascular system is measured
by the two other MEMS WPSs located in the
vena cava and the aorta.
Design: Fabrication Process
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In regards to the fabrication
process of the MEMS pressure
sensing element, Pramanik’s paper
on design optimization of
piezoresistive pressure sensor
suggests the piezoresistor
fabrication usually composed of “ptype diffusion along the [110]
direction on n-type substrate with
<100> orientation such that the
longitudinal direction is [110] and
the transverse direction is [1 -1 0]
for obtaining maximum response.”
Design: Specifications/Optimizatoins
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Pressure range: 0 to 1 bar
Sensitivity desired: 30 mV V-1 bar-1
Nonlinearity: less than 0.5% of full output
Resolution: 1 mbar
Operational temperature: 25 oC to 80 oC
The piezoresistive coefficient decreases as
doping concentration increases. However, the
etching rate is highly dependent on the doping
concentration and in order to achieve certain
geometry of the MEMS WPS sensor, certain
amount of doping concentration on Si is
needed.
In figure a, we observe the initial resistance is
dependent on the length of the piezoresistor
and thus affecting the sensitivity. Contrary to
this hypothesis, Pramanik set out to examine
the sensitivity comparing piezoresistors of
length 25um and 100um showing an
insignificant change for the length to increase
four folds.
Observing figure b, one can realize the
tremendous upward shift in the sensitivity curve
by decreasing the membrane thickness by 33
percent.
Design: Challenges
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MEMS WPS implanting technique is in question.
As we decrease the diaphragm, balloon effects might result and
nonlinearity is introduced.
Due to permanent implant nature, we have to deal with internal
heating by the MEMS WPS in addition with its internal battery.
Piezoresistive coefficient is a fucntion of temperature, with internal
heating, resistive measurements might be off.
No direct way of measuring systemic vascular resistance for blood
flow (Cardiac Output) measurements (Darcy’s law).
Current size of MEMS WPS is around 1m x 1m with internal
antenna. The smaller the size, the better in case of implants being
loose.
Doping can’t be tweaked too much due to fabrication limitations.
Questions?