Introduction

This report describes a fictional project, and is intended to serve as an example project report for Data Acquisition Device Prototype projects.

In this report I describe a fictional device, the "Xineoph Ambulatory Blood Pressure Monitor Prototype", and a fictional project to design and build it.

Project Overview

The Xineoph Ambulatory Blood Pressure Monitor Prototype is a new kind of ambulatory blood pressure monitor that is an unobtrusive ABPM that records ambulatory blood pressure and heart rate readings for up to 7 days, then uploads those readings to a patient’s or doctor’s computer for analysis.  The device is useful for adults (age 13 and older) in preventive monitoring as well as post-diagnosis monitoring.

With appropriate analysis software (the Sphygmochron), the Xineoph monitor supports a significant new capacity to predict oncoming hypertension, stroke, heart disease, kidney disease, eye damage.  It gives doctors and their patients the ability to monitor the effectiveness of hypertension treatments and discover autonomic neuropathy affecting heart rate and blood pressure in diabetics without the currently very expensive test used for that purpose.

This increase in capacity to monitor and prevent disease is a direct result of the convenience and low cost (approximately $10) of the Xineoph monitor.

This project built and tested a prototype device.  The device contains battery power, sensors, a microcontroller, non-volatile memory, an interface for uploading the collected data, and minimal UI elements for controlling and testing the device.

The device is useful to the Phoenix Project to learn how to build open-source medical devices.  The Phoenix Project Coordination Team serves as the "customer" for this project, and is the major stakeholder.    The Phoenix Project provides funding for this project, specifically arranging for funds to purchase hardware components and software tools.


 

Requirements Analysis

R1. Device allows ambulation
R2. Device is electrically self-contained
R3. Device collects data from 2 sensors
R4. Device collects 40 samples/second from a piezo-electric film sensor
R5. Device collects user button pushes
R6. Device converts analog signals to digital
R7. Device turns user-perceivable light on and off
R8. Device identifies peaks and troughs from piezo-electric film sensor
R9. Device calculates HR, SBP, DBP from at least 5 cardiac cycles, every 30 minutes
R10. Device timestamps each collected measurement
R11. Device preserves and uploads at least 3 days worth of data
R12. Device preserves and uploads all acquired values
R13. Device preserves and uploads all calculated values
R14.

Product Description:

 
The device consists of a

�    dual piezoelectric pressure sensor and amplifier circuit,
�    microcontroller, firmware in ROM, power-on timer, dynamic memory, flash memory
�    radio frequency transmitter (Blacktooth),
�    battery, and
�    moisture-proof bandage.

The device is disposable – it is not expected to be reused.

The piezoelectric pressure sensors are mounted on a Monostyrofoam backing strip 3 inches apart.  The Monostyrofoam strip is � inch wide and 1/16 inch thick.  The exposed surface of the piezoelectric film is covered with a standard Flubber coating.  On the other side of the Monostyrofoam strip, at one inch from each end, are the microcontroller with built-in memory and Blacktooth radio transceiver modules.  This sensor assembly is adhered to a 4-by-1 inch bandage by the bandage’s adhesive, with the Flubber-coated side of the sensors exposed.  When applied to the inner upper arm, the sensor array is aligned with and placed over the brachial artery, Flubber-coated side against the skin, and held in place by the bandage.  See Fig 11-1 for schematic arrangement of the sensor assembly components.

The design and packaging of the device, a single unit that is held on the upper arm with a bandage, meets requirement R1 (ambulatorability) well.

Piezoelectric sensor and amplifier

The piezoelectric sensors are Measurement Specialties Inc. FLDT 028, coated on one side with a standard 1-molecule-thick Flubber coating.  The Flubber coating transmits pulse vibrations from the brachial artery to the sensor with no loss of signal, of course (because it is Flubber, after all).  The piezo electric film generates an electric pulse of up to 0.0000001 volt.  This signal is amplified by the amplifier circuit that is adhered to the reverse side of the piezoelectric film and converted to a digital 0 volt or 3 volt signal.  The amplifier circuit draws 0.22 microamps from the battery.

 The  FLDT 028 sensors were selected in a previous project by Akhila and it was a requirement (R3, R4) to use them in this prototype device.  Those requirements are met satisfactorily.  

Microcontroller

The microcontroller is a standard 8052Q microcontroller with 8K RAM, 8K Flash ROM, and 4K EEPROM built-in for memories.  The timer circuit (connected to the microcontroller’s first interrupt pin) causes the microcontroller, display, and sensor amplifier circuit to be powered up for up to one minute every 30 minutes.  The microcontroller firmware captures the timing of a pulse moving from the first sensor to the second sensor, and calculates the blood pressure from the time it takes for the pulse to move the 3 inches, using the method described in Chen et. al, US Patent No. 6,599,251, a soon-to-be-universal-standard technique in non-invasive blood pressure monitors to derive blood pressure from pulse delays.  The microcontroller firmware monitors the first sensor until 5 pulses arrive; this is used to calculate the heart rate.  The timestamp, blood pressure, and heart rate are burned into the Flash memory, and the microprocessor firmware powers itself down (until the timer powers it up again in 30 minutes, at which point the sequence repeats itself).  The microcontroller and built-in memory uses 0.55 microamps of power when powered up.

The microcontroller was selected from the Microchip catalog based on power allocation (XX milliamps), ability to read 3 sensors, and store the firmware in built-in-ROM.  It meets all these requirements adequately.  The 8502Q microcontroller has been around so long that it is now patent-free, and is available from multiple manufacturers, decreasing the risk that it will become unavailable.

The firmware to capture a reading and store it in flash memory is described by the following pseudo-code:

If State is not “None”
   // Upload or reading is in progress, don’t interrupt it
   Return from interrupt
Set State to “Recorder”
If the Memory Address Register is 0
   // Hardware error has occurred, e.g., lost power
   Execute Halt Mode
If the Memory Address Register is at Max Address
   // Memory is full, no room for more readings
   Execute Halt Mode
Load reading from systolic sensor, store in next memory location
Load reading from diastolic sensor, store in next memory location
Load reading from heart rate sensor, store in next memory location
Load reading from clock (for timestamp), store in next memory location
Advance Memory Address Register
Set State to “None”
Execute Halt Mode

Upload Button

A small "upload" button is connected to the microprocessor’s second interrupt pin.  When that button is pushed, the microprocessor is powered up, and it executes a second firmware program which initializes the Blacktooth radio transceiver, transmits all stored blood pressure and heart rate data, and erases the Flash memory in preparation for capturing more readings.  

The uploader firmware is described by the following pseudo-code:

If State is not “None”
   // Upload or reading is in progress, don’t interrupt it
   Return from interrupt
Set State to “Uploader”
If the Memory Address Register is 0
   Set Memory Address Register to Minimum Address
   Fill Memory with “Hardware Error” indicator reading (-1,-1,-1,-1)
   Advance Memory Address Register
If the Memory Address Register is at Minimum Address
   // There are no readings to upload
   Execute Halt Mode
Calculate standard 16-bit checksum on data in Memory
Store checksum in next memory location
Advance Memory Address Register
Invoke Bluetooth on Chip module, passing Memory address as argument
// Prepare to record more readings
Reset Memory Address Register to Minimum Address
Set State to “None”
Execute Halt Mode

Event Button

The Event button is a simple push button connected to the microcontroller's second interrupt pin.  When pushed once, the microcontroller flashes the light toggles between collecting data every 30 minutes and not collecting data every 30 minutes.  When pushed twice, the microcontroller stores the timestamp and an "event happened" indicator into the flash memory.

This button meets requirements R5 and R7 adequately, but the project noted that it was easy for two button pushes to be interpreted as two single button pushes instead of a double push, and recommends a different design for the final device.

Radio transceiver (Blacktooth)

The Blacktooth transceiver is Company XXX, model YYY (might get some idea of what to use from here: http://www.pdacortex.com/fda_approves_bluetooth_med_system.htm ).  It draws 0.22 microamps from the battery when powered up and transmitting/receiving.

 

Battery

The battery is a � inch diameter lithium medical-grade battery, 3 volts, type DL2032 from DurableCell, used in many home-quality non-invasive medical devices.  It is capable of supplying 1.00 microamp of power for up to one minute every 30 minutes, more than enough power for the 0.99 microamp load placed on it by the amplifier, microprocessor, and radio transceiver.

 

Moisture-proof bandage

The bandage is a standard water-proof 4x1 bandage from Yohnson and Yohnson, with the absorbent pad removed and replaced with the sensor array.  This bandage is rated to stick to human skin for up to 7 days.

Patient-contacting materials

The materials that contact the patient’s skin while applying, wearing, and removing the blood pressure monitor are the Flubber coating on the piezoelectric sensors, the plastic bandage, and the adhesive on the bandage.

 

User Instructions

Intended Use: The Xineoph Ambulatory Blood Pressure Monitor is used for monitoring a persons blood pressure and pulse rate in thirty minute intervals over a period of a week. The Xineoph (BPM) is disposable after the seven day period. DO NOT REUSE.

Directions For Use: Place the Xineoph Ambulatory Blood Pressure Monitor sensor array on the arm with the piezoelectric film strips three inches apart over the brachial artery. Then place bandage over the sensor array to hold in place. When close to computer that is being used to monitor patient, push button on sensor array to transmit the blood pressure readings to the computer via Blacktooth RF transmitter. When the seven days are up dispose of sensor array. DISPOSE OF BATTERY PROPERLY.

Indications For Use: The Xineoph Ambulatory Blood Pressure Monitor is designed for the sole purpose of monitoring blood pressure and heart rate. It is not reusable and is designed to take blood pressure at thirty minute intervals for a period of up to a week.

Contraindications: Do not use if patient is allergic bandage adhesive.

Warnings: Patients allergic to the bandage adhesive could develop rash on skin where contact occurred.

Precautions: For most effective results place the piezoelectric film strips three inches apart over the brachial artery.

Special Patient Population: Patients 13 years or older.

Adverse Reactions: If irritation of skin occurs, stop use immediately. If lasts more than 24 hours contact physician.

 

Project Testing Report

Jill installed the prototype device on Jack on Monday.  Jack pushed the button, and the light flashed "on" indicating it had started taking data.  On Tuesday at about 11:45, worried that his blood pressure might be unnecessarily high, Jack pushed the button twice to log the event that the cafeteria lady made him eat broccoli again.  On Friday, Jack pushed the button again, and the light flashed "on" to indicate that it had stopped taking data.  He took the device off, confirmed no issues occurred with operation or skin irritation and returned it to Jill.  Jill uploaded the data and verified that the device took the correct number of readings by looking at the timestamps.  Jill declared success.  Jack thought that one data point doesn't make a trend.

Conclusion

About This Page

This page is maintained by Larry A. Beaty.  It was last updated on 13 August 2008.

The author(s) provide this information as a public service, and agree to place any novel and useful inventions disclosed herein into the public domain. They are not aware that this material infringes on the patent, copyright, trademark or trade secret rights of others. However, there is a possibility that such infringement may exist without their knowledge. The user assumes all responsibility for determining if this information infringes on the intellectual property rights of others before applying it to products or services.

(C) 2008 Larry A. Beaty. Copying and distribution of this page is permitted in any medium, provided this notice is preserved.

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