MEMS Cardiopulmonary Management

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MEMS Cardiopulmonary Management

• Bruce Lau
• Stanley Wong
• Spring 07 MAE268

Advisors Prof. Prab Bandaru Prof. SungHo
Jin Prof. Frank Talke
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Overview

• 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?

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Introduction (Cardiopulmonary)

• 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 lungs 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 Ficks law.

Ficks Law
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Introduction (Cardiopulmonary)

• 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
• 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.

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Introduction (Design Components)

• 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)
• 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.

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Bradycardia

• Symptoms
• Characterized by an abnormally slow heart (lt60
  beats per minute)
• Chronic fatigue and exercise intolerance due to
  the heart not being able to provide enough
  nutrients to the body
• Diagnosis
• At the digression of doctors based upon several
  factors
• Age
• Level of activity
• Symptoms
• EEG Readings

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Bradycardia

• Causes
• Malfunction of the Sinus Node
• The Sinus Node is the hearts natural pacemaker
  and is responsible for generating electrical
  impulses to produce heart beats.
• Issues with the conduction pathway
• Intrinsic Factors
• Surgical Trauma
• Familial disease
• Infectious disease
• Extrinsic Factors
• Bodys response to drugs
• Situational Disturbances
• Imbalances in the body

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Current Treatment Options

• Drug Therapy
• 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
• External Pacemakers
• 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
• Intravenous Pacemakers
• 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
• 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

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Rate Adaptive Pacing

• Manufactures
• Rate adaptive pacing was first introduced by
  Medtronics in the mid 1980s in a device called
  Activitrax. Since then, different types of
  sensors have been developed to improve the
  pacemakers response to physiological needs
• Rate adaptive sensors
• 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

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Rate Adaptive Pacing

• Sensor type (continued)
• Temperature Sensor
• 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
• Minute Ventilation
• 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
• Direct measurement of respiration rate and tidal
  volume.

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Principles MEMS RF Induction Techonology

• 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 NASAs 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.

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Principles MEMS Piezoresistance

• the initial resistance R is dependent directly on
  ? (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 Poissons ratio

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Design Benefits

• 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 Darcys 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.

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Design Fabrication Process

• In regards to the fabrication process of the
  MEMS pressure sensing element, Pramaniks paper
  on design optimization of piezoresistive pressure
  sensor suggests the piezoresistor fabrication
  usually composed of p-type diffusion along the
  110 direction on n-type substrate with lt100gt
  orientation such that the longitudinal direction
  is 110 and the transverse direction is 1 -1 0
  for obtaining maximum response.

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Design Specifications/Optimizatoins

• 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.

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Design Challenges

• 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 (Darcys 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 cant be tweaked too much due to
  fabrication limitations.

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Questions?