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Disposable Medical Instrumentation

Tue, 05/08/2007 - 7:25am
In an effort to enhance safety, sterility, and convenience, device manufacturers are seeking solutions to enable them to create disposable medical instrumentation. New sensor technology and innovative modular design are helping to make these types of devices a reality. This article reviews these technological advances.

By Steve Kennelly
In medical electronics applications, sensors respond to stimuli such as temperature, protein, pressure or motion and relay the data to signal-processing circuitry. Depending upon the application, there may be a display (usually an LCD) that conveys useful information to

A Tympanic Thermometer measures temperature by detecting infrared energy radiated from the eardrum. Designers can guard users against possible erroneous operation by posting interactive messages on the meter’s display.

the user. Advances in semiconductor technology have resulted in microcontrollers (MCUs) that can directly interface with most of the sensors and process the received data.
Importance of Signal Conditioning
Medical instrumentation demands sensor probes to be of the disposable type, based on the nature of test, safety, or hygienic (sanitation) considerations. To accomplish this, considerable design ingenuity needs to go into these sensor probe designs, which are based on the “front-end detection and backend processing” principle. A sensor produces a change in an electrical property to denote a change in its environment. Designers need to condition this change to a digital format using signal-conditioning circuits. This signal conditioning is a challenging task, because most sensors provide outputs that have non-linear portions.

For example, temperature sensors—when used at room temperatures—are accurate to within 1°C. However, at hot or cold temperature extremes, their accuracy decreases non-linearly. Typically, this non-linearity has a parabolic shape that is described by an empirical equation (Figure 1).


Figure 1: Typical sensor accuracy before and after compensation
From the graph, it can be observed how compensation provides an accurate and linear temperature reading over the sensor’s operating temperature range. From the vendor’s datasheet, it is possible to derive an equation that shows the relationship between the sensor’s voltage and its changes to varying temperature. This equation provides a linear voltage change of a few hundred microvolts per degree centigrade. Next, this voltage can be either amplified for analog output sensors or interfaced to an analog-to-digital converter (ADC) for analysis and display. Designers can use an MCU or a digital signal controller (DSC) to solve this equation in firmware and obtain a higher-accuracy temperature reading. A similar technique can be used to compensate other types of sensors involving pressure, protein, or light.
Designing Disposables
Modular design of medical instrumentation can offer many benefits to the user and also help the manufacturer. Instrument designers need to correctly partition their designs into signal detection, data acquisition, and analysis blocks. Where possible, they must design the signal-detection blocks to be disposable. This can be understood by identifying two varieties of disposable medical instruments. First, there are the disposable “sample-element” types of medical instruments. Second, there are medical instruments that are “wholly disposable” in nature. In the case of the disposable sample-element type of instrument, the sensor probes used to sample medical data can be dispensed after each usage. As the name implies, a wholly disposable medical instrument can be discarded entirely after one or more uses.

An example of the sample element type is the sleep-apnea-syndrome diagnostic instrument. Disposable probes, comprising thermistors and signal-conditioning circuitry, are placed on the patient’s nose and mouth to report the respiration airflow temperature back to the instrument’s controller. Based on the sensors’ output, the system controller calculates the parameters of the respiration pattern. The instrument’s non-volatile memory houses a list of normal breathing-pattern values and their plus and minus deviations. The system software uses the measured sensor probe values and makes a match against the normal breathing pattern—where a deviation exists, the apnea-risk value is sent to the display. Based on this information, doctors can diagnose the patient for further tests.

Blood glucose meters measure the glucose value in blood and they are available in many sizes with varying capabilities. The meter can be made more user-friendly by adding a few extra function buttons.
 
In another disposable sample-type instrument example, using specially coated test strips, blood glucose meters measure the amount of glucose in the blood. The latest blood glucose meters use a coulometric technique where a capillary action on the test strip takes in the blood sample that is fed to an electrode containing glucose oxidase enzyme. After reoxidizing the enzyme with ferrocyanide ions, the total charge passing through the electrode yields the glucose concentration in the blood. An MCU evaluates the current passing through the blood and returns the glucose value for display on an LCD.

Calibration in medical instrumentation is a crucial issue. For instance, glucose meters are accurate to within 䔮% to 15% during normal operation, based on a variety of factors such as meter calibration, ambient temperature, handling of test strips, size of blood sample, humidity, and shelf life of test strips. Advanced MCUs enable instrument makers to account for these variants and prepare the users by displaying warnings of inaccurate results.

Blood glucose meters today collect extra data from which patterns and suggestions for patients could be developed (some meters perform data analysis, but do not record historical data). With the available embedded semiconductor technology and with some redesign of meters based on modular principles, designers can present blood-sugar data in innovative ways.

Yet another type of disposable sample-type medical device can be found in lactate measurement instruments. The lactate measurement instrument enables sports medicine practitioners to precisely calculate an athlete’s heart-rate training zones. Lactate meters work the same way as blood-sugar meters—testing for the lactate enzyme through reflectance photometry on a small blood sample, using a specific wavelength of light.

The leading blood lactate test meters come in credit-card sizes, where a test strip works on a small blood sample and returns results in under a minute.

Blood-pressure meters display both blood-pressure readings and pulse-rate values. Deploying microcontrollers that directly drive LCDs on the instrument facilitate ease-of-use operation.
 
These instruments feature automatic temperature compensation, using a built-in sensor. Lactate testing determines an individual response to heart rates. Since the relationship between exercise intensity and heart rate differs for different exercises, training programs can be based on individual responses.

Possible advanced techniques based on modular design principles could include remote connection to a doctor’s office and a keyboard for data entry into the meter or to a PDA to upload the results to a computer. Using the instrument’s memory, designers could modify the firmware to keep track of testing patterns, and even present them as a simple graph with low-cost LCDs.

In some instruments, the electrodes are of the disposable type. A defibrillator is such an instrument that delivers a carefully controlled electric shock, based on the feedback received through the sensory pads attached to a patient’s chest. Because each second is vital, the defibrillator instrument needs to correctly calculate the precise first-time dose of energy, every time. The electric shock is administered either through electrodes on the exterior of the chest wall or directly to the exposed heart muscle, to reset the heart's electrical impulse and normalize heart rhythms. Voltage, current, waveform timing, and delivery all impact the amount of energy delivered to the patient’s heart.

Some defibrillators use biphasic waveform designs and clever circuitry to deliver less current than the traditional monophasic waveform systems. These biphasic instruments have been designed to be effective at comparatively lower peak currents. To be power efficient, defibrillators need a low-capacitance design to efficiently generate a waveform personalized to the patient’s impedance. To accomplish this, the instrument needs to perform complex mathematical calculations to accurately compute waveforms that deliver the correct amount of charge. Designers can use DSCs that support DSP instructions and have the necessary signal-conditioning peripherals to design a modular defibrillator instrument.

An electrocardiogram (ECG) is a non-invasive test instrument that measures electrical activity in the heart. Electrical sensors attached to predetermined positions on the arms, legs, and chest record electrical activity and help assess heart function. Based on the sensors’ outputs, the instrument displays the heart’s rhythm pattern. An abnormal pattern reveals a disturbance of the heart's rhythm—such as a lower blood supply to part of the heart wall—or a heart attack. Based on modular design principles, leads on an ECG sensor can be made to connect to a disposable electrode.
MCUs in Disposable Instruments
Designers can consider deploying Flash-based, power-managed MCUs that are capable of driving LCDs in sleep mode while maintaining desired functional features. By using these MCUs, it is possible to design instruments that are wholly disposable in nature. For example, sensors in pregnancy testers detect the pregnancy hormone called Human Chorionic Gonadotrophin in the urine, and display the result on an LCD, which typically reads either “pregnant” or “not pregnant.” Another example of a disposable medical instrument is compact blood glucose meters, which feature a built-in display mounted onto a test-strip vial. Testing with these meters is easy—a test strip is removed from the vial, inserted into the meter on top, and tested. When the test strips run out, the unit is discarded.

Figure 2: Touch-screen displays driven by the latest Flash-based MCUs enable more user-friendly medical instrumentation.
Some of the MCUs offer a 10-bit ADC that provides an interface for resistive touch-screen displays. These touch-screen displays enhance the ease-of-use of the instrument and eliminate specialized keyboards and other pointing devices. For example, blood-glucose meters or lactate meters can even display interactive messages from a touch screen (Figure 2) as to what percent of readings are within target range, based on the frequency of highs and lows.

These examples demonstrate the power of the latest MCUs, which are now enabling a variety of home-based medical testing functions in disposable instruments. Expect to see a variety of disposable medical-testing instruments becoming available to users at a lower cost than ever before.
Conclusion
Highly capable, low-cost MCUs that can interface with a variety of sensors have made disposable instrumentation a reality and a cost-effective proposition. In addition, economical DSCs facilitate complex mathematical analysis of sensors’ outputs to not only provide health diagnostics but also certify the correct functioning of the instruments themselves. The instruments based on these controllers tend to be more compact and require simple or no calibration from the user. In summary, these controllers enable low-cost medical instruments—where the value of the medical testing function has gone up while the cost of the instruments has come down.
ONLINE
For additional information on the technologies and products discussed in this article, visit Microchip Technology Inc. at www.microchip.com/medical.

Steve Kennelly is the senior manager of the Medical Products Group at Microchip Technology Inc. He is responsible for leading this newest vertical market group from Microchip, which addresses the specific needs of the medical device industry. Kennelly can be reached at steve.kennelly@microchip.com.
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