Battery Life in Implantable Medical Devices

By Brad Jolly, Senior Applications Engineer, Keysight Technologies
Twitter: @Keysight

Heart disease continues to be a major health problem. The U.S. Centers for Disease Control and Prevention report it was the number one cause of death in 2019. In many cases, heart disease can be treated via lifestyle changes such as diet and exercise; in other cases, pharmaceutical approaches are useful.

In addition, the trend toward implanted medical devices applies to patients with cardiovascular diseases. Many people use pacemakers to maintain heart rhythms. Patients with atrial fibrillation and flutter may benefit from an implanted cardioverter defibrillator (ICD). Pacemakers and ICDs both run on battery power and a depleted battery can be dangerous to the patient. Fortunately, most modern pacemakers and ICDs have low battery warnings that allow the patient to plan for a replacement surgery ahead of the actual battery failure.

Costs of Failed Batteries
A depleted battery can mean the difference between life and death, so a timely device replacement is critical. In an outpatient procedure, there are many costs to consider, including the cost of the surgeon, the surgical suite and equipment, surgical nurses, the anesthesia and anesthesiologist, the recovery room or hospital room, and the follow-up care and supplies.

Beyond cost, there is risk to the patient, many of whom are elderly and may often have other health risk factors. Even in relatively healthy patients, there is always a risk of infection and other complications. Long battery life benefits patients and doctors, and it also reduces the risk of legal and regulatory consequences for the device manufacturer. Therefore, medical device manufacturers must take great care to ensure long battery life.

Current Measurement Instruments
To ensure long battery life for medical devices, design and validation can choose from many hardware tools that measure current. One of the most common tools is the digital multimeter (DMM), which is on almost every hardware engineer’s bench. The DMM is good for measuring low current levels very accurately, and it is also a good general purpose measurement tool for voltage, resistance, diodes, and more. One disadvantage of the DMM is that it lacks the bandwidth to capture fast transients accurately. It also may not have sufficient dynamic range to capture transitions between low-current sleep modes and relatively high current active modes.

The DC power analyzer is more expensive and less common than the DMM, but it has significantly better bandwidth. As a modular product, it is more flexible than the DMM and some of its modules include source-measure units (SMUs) with seamless ranging that can measure sleep and active modes without the glitching associated with range changes.

The oscilloscope is another commonly available instrument, and with purpose-built current probes, it can give very good current measurements. It lacks the source-measure and e-load capabilities of a DC power analyzer, but it has much higher bandwidth and more triggering capabilities.

The device current waveform analyzer is at the high end of current measurement devices. It has a use model like an oscilloscope, high current measurement bandwidth, and excellent accuracy for measuring low currents.

Turning Measurements into Insights
While all the instruments listed above can gather large amounts of data accurately, the data alone is not sufficient to provide the insights engineers need to optimize their designs. All that data must be quickly analyzed by sophisticated firmware and software that produces engineering statistics and clear graphs to reveal important information at a glance.

Data logging software is also an important tool. Implanted and body-worn medical devices often have behaviors that vary widely depending on the severity of the patient’s condition and the environment inside the patient’s body. For devices that communicate via RF signals, even electromagnetic conditions outside the patient can lead to excessive charge consumption, as devices facing difficult RF coexistence challenges may face multiple retries to ensure data is successfully transmitted.

Once you have gathered a large amount of data via data logging, you can use that information for more than battery drain analysis. For example, some software can use artificial intelligence techniques to analyze huge amounts of data and find anomalies in device current waveforms. For example, the segment between the red markers in the lower of the two current waveforms shown below is very different from normal device behavior. This could indicate an intermittent defect in the hardware – it could be a firmware bug, or it could be caused by malware, such as a Trojan lurking in the device.

Another type of software that helps engineers understand device behavior is event-based power analysis software. This software is part of an IoT device battery life optimization solution, and it correlates charge consumption with RF and DC events. It then analyzes that information to estimate battery life and identify the key events in a device that consume large amounts of time and battery charge. For example, the image below shows waveforms for device current (yellow), RF power (green), and LED supply voltage (blue). It also shows three sets of results along the bottom: complementary cumulative distribution functions (CCDF) on the left, battery life estimate statistics in the middle, and occupied time and charge consumption stacked bar charts on the right.

Specific Benefits for Medical Device Engineers
Of course, the benefits of the hardware and software tools described above would apply to any battery-powered IoT device. However, for medical device engineers, these tools can also provide data and graphics that can be included as documentation in a manufacturer’s quality management system (QMS). They can also provide critical information that engineers can use to reduce risk to the patient and to the medical professionals who rely on long battery life to mitigate the need for device replacement surgeries.

In summary, implanted devices can help people with heart issues, and longer battery life can benefit patients in numerous ways. Modern hardware and software solutions, such as IoT device battery life optimization solutions, device current waveform analyzers, and event-based power analysis software can help medical device engineers provide significant benefits for patients and medical professionals.