BE 490 / 590: IMPLANTABLE DEVICE TECHNOLOGY

SAMPLE PROBLEM SET # 3

 

  1. A potentiometric sensor is being used to measure the concentration of potassium, [K+]. Electrodes are separated using a membrane permeable only to potassium. Reference solution has a known potassium concentration of [K+] = C, which is constant. Derive the equation describing the concentration of potassium in the sample solution. Is this a linear relationship?

    2. ANSWER:

     

     

  2. A bio-chemical sensor is built using optical means as shown in the diagram below. Explain how it works.

    3. ANSWER:

    This sensory system consist of a light source (producing the incident hn ), a light detector (sensing the returned/reflected hn ), an optical fiber catherter connecting the light source to the actual sensor, which is at the distal end of the catheter. Sensor consists of a dialysis bag which contains the antibodies (ABY) against the drug P. These antibodies are tagged with a fluorescent molecule which are symbolized with "o". Furthermore, the antigens (AGN - drug P) tagged with another fluorescent molecule symbolized with * are also in this dialysis chamber. On the absence of natural drug P, AGN* is docked on the ABYo, and their combined fluorescent characteristics are detected by the optical system. When the natural form of drug P diffuses in from the outside, it competitively bind to the ABYo, displacing the tagged AGN*. This reduces the intensity of the returned light. In other words, decreases in the fluorescent light is due to increases in the concentration drug P.

     

  3. A blood vessel has a diameter of 1 cm. An electromagnetic flow sensor with magnetic field intensity of 0.10 Tesla was used to measure the flow, and the voltage reading from the two electrodes on opposite sides of the vessel was 100 m -Volts. Calculate the flow velocity.

    4. ANSWER:

     

     

  4. A flow sensor using the Doppler-ultrasound principle was used to measure the blood flow in aorta. Speed of sound in tissue was determined to be 1,500 mt/sec. Excitation frequency was chosen as 5 MHz. Received signal was at 4,999,666.7 Hz. What is the velocity of the blood in the aorta? Is it moving towards or away from the transducer?

    5. ANSWER:

     

  5. A flow sensor using the Doppler-ultrasound principle was used to measure the blood flow in aorta. Speed of sound in tissue was determined to be 1,500 mt/sec. Operator noticed that the reflected signal was arriving 200  m -seconds after the transmission of incident wave. How far is the moving target? If the attenuation constant a is given as 0.25 cm-1, then what is the attenuation of the return signal, i.e. IRX/ITX?

    6. ANSWER:

     

  6. A flow sensor working on thermo-dilution principle has a heater element which produces 1 calorie/sec. Before the heater was turned on, thermistor signal indicated that the blood temperature was at 37° C, after the heater was turned on, temperature was increased to 38° C. If the cross sectional area of the blood vessel is 1 cm2, then what is the speed of the blood in the vessel?

    7. ANSWER:

    T1 = 37° C, T2 = 38° C, D T=1° C,

    Blood volume must be: D V = 1 cm3 since the heat capacity of blood can be assumed to be 1° C/cm3-calorie

    Flow velocity can be found as v = D V / A = 1 cm3 / 1cm2 = 1 cm/sec.

     

  7. We are trying to monitor the ventricular volume of a patient with Dilated CardioMyopathy (DCM) using impedance plethysmography technique. We have a catheter with five electrodes placed in the ventricular cavity of the patient. We are using the first and the fifth electrodes for excitation using a 10 m -Amp current source. We make voltage measurements in reference to the fifth electrode, i.e. we assume that the voltage on the fifth electrode is zero, and voltages of the remaining electrodes are measured with respect to the fifth electrode. Electrodes are equally spaced 2 cm apart from each other. Following measurements were made from the electrodes 1 through 4:

    8. V1=115 mV, V2=75 mV, V3=60 mV, V4=40 mV,

    Assuming that the resistance of blood is 150 W , calculate the ventricular volume.

    ANSWER:

     

  8. Show the photolithographic steps to produce a metal cantilever beam on a silicon substrate.

    9. ANSWER:

     

  9. Describe the differences between isotrophic and anisotrophic etch.

    10. ANSWER:

    Isotrophic etch removes silicon substrate at equal speed on each direction (omnidirectional). As a result of this, it creates geometries with smooth curves, not sharp edges. Agitation / no-agitation can be used to change the curvature to a certain extend, but sharp edges are not possible.

    Anisotrophic etch removes silicon depending on the crystal orientation. By cutting the wafer on the direction desired, one can form desired geometries on selected orientation. Sharp corners with well defined edges are possible.

     

  10. Define aspect ratio of a MEMS structure.

    11. ANSWER:

    Ratio of the vertical to horizontal dimension of a structure.

     

  11. Compare wet etch versus plasma etch.

    12. ANSWER:

    Wet etch utilizes acids in a solution to remove the silicon from the crystal. One can change the reaction speed by changing the temperature. Etch direction can be changed by the orientation of the crystal.

    In plasma etch, only the energy of the etchant is increased by application of RF energy. Furthermore, this process uses electric fields to drive the reactive agents directly into the silicon to control the etch direction. It is an active rather than a passive process.

     

  12. Explain why LIGA process is favored over simple wet etch, or even plasma etch. What is major problem with the LIGA process.

    13. ANSWER:

    LIGA is similar to the plasma etch. Instead of RF energy, one utilizes the synchrotron to generate high energy particles to drive the reaction within the silicon. Since the energy of the particles produced by synchrotron is much higher than the one available from RF driven plasma etchers, LIGA can produce much higher aspect ratios than plasma etchers. Main problem with LIGA systems is the requirement for synchrotron particle accelerator.

     

  13. To provide rate response feature for an implantable pacemaker, you are given the responsibility of designing a piezoelectric accelerometer. To keep the pacemaker light, you are not allowed to use a proof mass of more than 1 milli-gram. The inexpensive piezoelectric film that your purchasing department bought has the following properties:

    14. Electrical displacement constant, d = 2.33

    Thickness, h = 1 micron

    Relative permittivity, e r = 4.0

    We also know that Permittivity constant, e 0 = 8.85 x 10-12 F/mt, and gravitational acceleration is g = 9.81 mt/sec2

    If we want the accelerometer to produce 10 Volts at 0.01 g, what should be the surface are of the piezoelectric film under the proof mass?

    ANSWER:

     

  14. Show sketch of a MEMS fluid valve and a MEMS pump.

    15. ANSWER:

     

  15. List three means for telemetry between an implantable device and the external device (hint: one is optical). What are the advantages and disadvantages of each?

    16. ANSWER:

    RF (electromagnetic), acoustic, optical.

    RF: most common, power needs to be limited to prevent tissue damage. Tissue and the metal case of the implantable device are all conductive, and forms a Faraday Cage, reducing the efficiency of communications. However, still used most widely because of enormous technical background available for RF communications from radio, TV, cell phone etc applications.

    Acoustic: Suffers least amount of signal degradation with distance. Very harmless. Very prone to external noise and motion artifact. Used when simplicity is important such as trans-telephonic monitoring.

    Optical: Signal strength degrades very rapidly (half of the power is lost in less than 1 mm). Used when the bandwidth is very important, i.e. when there is a lot of data to be transmitted.

     

  16. What is even parity? What is it used for? Give an example.

    17. ANSWER:

    Parity: a method to detect and possibly correct single bit errors in digital communications

    Even parity: Parity bit is set to "1" if the number of 1's in a byte is an even number.

    Example: Even parity bit for 00101101 will be set to 1, since there are four 1's.

     

  17. What is modulation? Why does the telemetry signal need to be modulated? Give examples of three different modulation schemes. Draw the carrier signal for transmission of the digital sequence "010".

    18. ANSWER:

    Modulation: Modification of the characteristics of a carrier signal in accordance to the data to be transmitted.

    If not modulated, data transmission will have a lot of noise, will mix up with other signals, or may not even be received at the receiver (as in the case of coupled RF coils).

     

     

  18. Why is Morse Code more efficient for data storage/transmission than ASCII (American Standard Code for Information Interchange)? What makes a code more efficient?

    19. ANSWER:

    Morse code is more efficient because shorter codes are assigned to data symbols which occur frequently. In ASCII, all codes are the same length, so it takes just as long to transmit/store the frequently used symbols as the rarely encountered ones. A code for storage or communications is efficient if the coded volume is less than the generic uniform code (as ASCII).

     

  19. We have the following data sequence representing the blood glucose levels of a patient during the day (one sample per hour, total of 24 samples). Compress and code this data set using the Hoffman Algorithm.

    20. 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 2, 3, 4, 2, 1, 1, 1, 2, 3, 4, 2, 1, 1

    ANSWER:

    Probability table will be as follows:

    X

    N(X)

    P(X)

    1

    14

    7/12

    2

    6

    3/12

    3

    2

    1/12

    4

    2

    1/12

     

     

    Code assignments will be as follows:

    1

    0

    2

    10

    3

    110

    4

    111

     

     

  20. Describe the algorithm of the template matched compression technique. When is it efficient? Why? Give an example.

ANSWER:

Template matched compression for storage / transmission algorithms are efficient when the data being stored / transmitted is a periodic signal which is repeating, such as ECG. Instead of storing each beat in great detail, we store couple of templates, and log only the template number for each cycle. If the new data does not match to any of the stored templates, then we save that cycle as a new template.

Here is an example:

 

1111111111111111122222222222233333333333333333