Resistors enable design of portable medical electronics

1st September 2017
Enaie Azambuja

Electronics equipment for medical applications has escaped the confines of medical facilities and entered the outside world. It’s not just the location of medical electronics that has changed, though – they are no longer the sole preserve of medical professionals. These changes have been enabled by modern design techniques. Today’s medical electronics devices are smaller, lighter and designed to be easy to operate.

By Phil Ebbert, VP of Engineering, Riedon Inc.

You can see examples in many public spaces – for example, defibrillators can often be found in schools, airports and sports arenas. Another example of this trend is targeted at people with diabetes.

Today’s blood glucose meters are small, portable machines that can easily fit in a pocket. Following the same design blueprint, other equipment for monitoring, diagnosis and the delivery of medication can also be found around the home, or worn on the body.

Because of their intended use, medical electronic devices are designed using the highest-quality components. These ensure that the equipment operates as safely and reliably as possible.

Of course, it’s not just about high-quality components: medical equipment, like the examples above, needs to be small enough to be portable, as well as rugged enough to cope with many different environments and efficient enough to be powered by batteries.

This article will look at two specific applications and the specifications that matter when choosing resistors for those portable medical applications. The article will then look at real-world resistor solutions from manufacturer and supplier Riedon.

Automatic external defibrillators (AEDs)

AEDs operate in much the same way as the defibrillators usually found in hospitals. The devices electrically shock the patient to return a heartbeat to a normal rhythm from potentially deadly cardiac arrhythmia (irregular heartbeat).

The defibrillators found in hospitals are usually only operated by trained personnel, such as doctors, who can calculate the correct level of shock to administer. The devices may also have (ECG) readers that assist the doctor with calculations. In many hospitals, the patient will already be hooked up to an extensive range of instruments, giving the doctor the information that is required.

In contrast, AEDs are devices that are intended for use by members of the public, though preferably the operator will be a first responder or other trained staff. AEDs may also be found in ambulances to complement manual defibrillators.

An AED intelligently and automatically analyses and diagnoses the patient’s heart rhythm, and then either provides the shock itself, or gives the operator the information on whether a shock is needed, and if so, at what level.

The removal of the need for a qualified operator means that the defibrillator will not be as powerful, and will be limited to the most common types of arrhythmia (VT and VF) and not suitable at all for a “flatline” condition.

AEDs can be found in many public areas, but more recently, body-worn and personal AEDs have been developed and are becoming more popular, even in some cases being promoted for home safety.

The charging circuit is simple, consisting only of a DC supply and a resistor (R1) in series. R1 is included to set the charge time and limit the current. The actual amount of energy discharged is dependent on the value of the storage capacitor and the calculated energy required, which can end up as high as 5000V.

If the AED is battery powered, the voltage will be supplied by a DC/DC converter. R2 and R3 form a voltage divider that monitors the voltage in the capacitor and provides a signal that disconnects the supply at the correct voltage level. The energy is discharged through the inductor (I) and R4 to provide a damped pulse, which usually delivers around 100 Joules in about 5ms. 

The circuit in figure 1 gives a fixed-polarity pulse, but most modern designs provide a biphasic polarity, alternating the polarity of pulses. The main advantages of biphasic design are that the circuits use less energy and have a better success rate.

As for the resistor requirements for such a circuit, they tend to be demanding. R1, R2 and R4 must be able to handle high voltages. Since R1 carries the charging current, value (usually around 100kΩ) and accuracy are not the most important factors, but it does need to be able to dissipate about 10W.

R4 is used to shape the pulse, so it does need to have a more accurate value (usually around 10Ω) and it also needs to be capable of pulse handling around 300 Joules. As for the potential divider, R2 and R3, they will normally require a ratio of around 500:1, which would reduce a 1000V voltage to around 2V to provide the comparator input to a microcontroller.

So, if the value of R2 were 10MΩ, R3 would be required to be close to 20kΩ. If the system is designed to be easily calibrated, the accuracy of the resistance values is not so critical. Other factors that are more important are stability under changing temperatures and voltage linearity.

These characteristics are defined as TCR (temperature coefficient of resistivity) and VCR (voltage coefficient of resistivity). In our example, typical values would be a TCR of ±100ppm/°C and from -1 to -5ppm/V for VCR – a negative coefficient.

As for practical examples for these resistors, Riedon’s HTE thick-film series would be ideal for R1, with a voltage rating of 48kV, up to 17W power rating and resistance values from 1kΩ to 100MΩ.

The HTE series would also be appropriate for R2 and R3, with a ±100ppm/°C TCR rating. For R4 a different type of resistor is required, one that can handle strong pulses. For this, Riedon recommends its UT wirewound resistor range that can withstand pulses over 1000 Joules. The resistor chosen should be non-inductive.

Blood glucose meters

The second medical electronics application is a blood glucose meter. These devices monitor diabetics and react to high and low blood sugar levels. Realising the reasons for blood sugar changes helps people with diabetes to regulate their diet and manage exercise and medication.

This information is crucial for people with Type 1 diabetes who inject insulin. The testing informs them of how effective the last injection was, and enables them to plan the next injection.

The blood glucose meter is a simple and effective way to measure levels of blood glucose. These devices are now small enough that they can be carried around and they tend to be simple to use, which allows diabetics to regularly test throughout the day.

The meter works by getting a sample of blood and applying it to a test strip that uses chemicals to react with glucose in the blood. The first blood glucose meters used a reaction that altered the strip’s color, allowing the user to compare the color with a chart, or measure using a colorimeter. Later versions take a different approach, using an electrochemical reaction.

In these systems, the test strip absorbs a known amount of blood. The blood’s glucose is oxidised with an enzyme electrode as the catalyst. It is then re-oxidised with a mediator agent, which reacts with the electrode and generates a current. The quantity of the charge is proportional to the amount of glucose.

As with most healthcare applications, accuracy is vital. In the example of a blood glucose meter, accuracy can be affected by a number of factors, which include size of the blood sample, quality of the sample, temperature, humidity, quality of the test strips and the quality of the measurement. The latter factor can be taken care of by using the correct current measurement technique with high-precision components.

Many manufactures use current-sensing resistors for this type of application. These resistors work on the same principle as the shunt resistors found in ammeters. They operate by measuring the drop in voltage over a low-value resistance placed in series with the load.

For glucometers that feature a digital display, the voltage is measured by an ADC that is usually a peripheral of the system microcontroller, which is also used to control and operate the meter.

The main requirements for a current-measuring resistor for this application are accuracy and stability, both over time and in any operating condition. Riedon has a number of different resistor types for current sensing. For this specific application, a resistor from the company’s CSR surface-mount range or through-hole MSR range would be the best choice.

The CSR range comprises metal strip resistors with resistances from 0.5 to 15 mOhm, ±1% tolerance and a stable TCR of ±50ppm/°C. Different packaging sizes provide power ratings of up to 3 Watts over a -55°C to +170°C temperature range. The TSR range resistances range from 5 to 100 mOhm with a ±1% tolerance, TCR of ±20ppm/°C and power ratings up to 5 Watts.

The same types of resistor are also often used in portable medical device applications for the purpose of monitoring the life of the battery. Resistors used for this application would be similar to the ones above – or, at times, with a similar specification but a higher power rating.

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