“Currently, the remote patient monitoring patch replaces the traditional bulky Holter device. The various sensors contained in the patch are capable of collecting heart rate, temperature and accelerometer data, which can transmit patient data to the cloud for real-time access by patients and doctors. While these patches help physicians improve care, they present challenges for power designers who must balance system performance with battery life requirements. As the second-generation SMD uses multi-modal sensing to improve accuracy and effectiveness, the challenges are further highlighted, and more stringent indicators are imposed on the power supply.
Fahad Masood, ADI Health and Medical Biosensing Applications Technician
The Internet of Things (IoT) revolution has brought about a paradigm shift in the way healthcare organizations care for patients in real-time. Among them, remote patient monitoring is an important area in which new medical equipment changes the way doctors and patients interact. With the miniaturization of integrated circuits and the evolution of wireless technology, traditional medical equipment has changed its appearance, its functions have been enhanced, and patient compliance and efficacy have gradually improved.
Challenges from Power
Currently, the remote patient monitoring patch replaces the traditional bulky Holter device. The various sensors contained in the patch are capable of collecting heart rate, temperature and accelerometer data, which can transmit patient data to the cloud for real-time access by patients and doctors. While these patches help physicians improve care, they present challenges for power designers who must balance system performance with battery life requirements. As the second-generation SMD uses multi-modal sensing to improve accuracy and effectiveness, the challenges are further highlighted, and more stringent indicators are imposed on the power supply.
Figure 1. Schematic diagram of the ECG patch power supply. Use a 235 mAh CR2032 coin lithium battery as the voltage regulator,
Powers the microcontroller, ECG front end, temperature sensor, and accelerometer.
Consider the ECG RPM patch (Figure 1), which continuously monitors the ECG and accelerometer while checking the temperature every 15 minutes.Data via Bluetooth Low Energy®(BLE) is transmitted every 2 hours for a total of 12 BLE transmissions per day. The patch supports three different load modes: standard monitor, temperature monitor and transfer mode. In standard monitoring mode, only the ECG and accelerometer are monitored. In temperature monitoring mode, another temperature sensor is also monitored. In transmit mode, the BLE radio monitors both ECG and accelerometer data and transmits data synchronously.
Designing RPMs, such as ECG patches, presents challenges for power supply designers on multiple levels. Designs are often space-constrained, and patches with multiple sensors may require multiple power rails. Because RPM patches are typically single-use products, designers typically opt for a cost-effective power source such as a coin cell battery. Designers must also consider the efficiency of the power subsystem if only using a coin cell battery to power the patch.
In addition, a challenge that power supply designers often overlook is how to extend the shelf life of their products. Because, turn-off current and battery self-discharge will shorten the life of any system. Therefore, the designer must determine whether the RPM patch can meet the runtime requirements beyond the shelf life, and if not, what can be done to preserve the battery life before the patch reaches the end user.
Gain insight into battery runtime
In order to accurately grasp whether the power solution meets the battery life requirements, the load curve must be determined. The load curve is a simple representation of the system load duty cycle. For the remote patient monitoring patch, three different working modes can be started from standard monitoring, temperature monitoring and transmission mode.
In standard monitoring mode, the current consumption of the patch shown in Figure 1 (including the 330 nA quiescent current of each buck converter and the current consumption of the MCU) is 1.88 mA. In temperature monitoring mode, the current consumption is 1.95 mA for 200 ms every 15 minutes. In transmit mode, when the patch transmits data via BLE, the current consumption is 7.90 mA for 30 seconds every 2 hours. These values can be found in the Active and Quiescent Current Specifications section of the appropriate device data sheet.
For load profile analysis, the time period of each operating mode of the day is used to determine the duty cycle calculation. Use Equation 1:
The duty cycle of the patch can be obtained, as shown in Table 1.
Table 1. The duty cycle of the patch in different operating modes
Figure 2. Load curve graph.
Using the load curve in Figure 2, the current consumption of the patch can be calculated. Taking the effective current consumption in each operating mode, an approximation of the average current consumption per day can be calculated by Equation 2.
Here is an example calculation:
Standard monitoring mode current per day = standard monitoring mode current × standard monitoring mode duty cycle × 24 hours
Standard Monitor Mode Current = 1.88 mA Standard Monitor Mode Duty Cycle = 0.9956
Current per day in standard monitoring mode = 1.88 mA × 0.9956 × 24 hours = 44.92 mAh/day
Once the daily current draw for each operating mode is determined, the battery life can be determined by Equation 3.
Here is another calculation example:
Battery capacity = 235 mAh
Current per day in standard monitoring mode = 44.92 mAh/day
Current per day in temperature monitoring mode = 0.01 mAh/day Current per day in transmission mode = 0.79 mAh/day
Battery life (days) = 235 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 5.14 days
These calculations show that the device will meet the 5-day operating time requirement with a battery life of over 5.1 days. However, this result is deceptive because the shelf life of the system is not taken into account. In the medical device industry, it is best to design for a 14-month shelf life (12 months for shelf life and two months for shipping).
Shelf life factors that must be considered
Summing up the shutdown currents of the devices in the system, while using the typical self-discharge rate of 1% to 2% per year for the CR2032 battery, it can be seen that after 14 months, the battery capacity is not sufficient to support 5 days of working time, and it is necessary to carry out The battery is sealed.
Table 2. Battery capacity after 14 months
After 14 months on the shelf, the battery capacity will decrease significantly. When the CR2032 is idle on the shelf, nearly 40% of its energy will be dissipated through shutdown current and battery self-leakage. Substituting this battery capacity into Equation 3 gives a more accurate run time:
Battery life (days) = 146.66 mAh/(standard monitoring mode + temperature monitoring mode + transmission mode)
Battery life (days) = 146.66 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 3.21 days
On shelves for more than one year, battery capacity will be affected by battery self-discharge and system shutdown current. Battery self-discharge is related to the battery chemistry and environment. The chemistry of the CR2032 battery is lithium manganese, which has a self-discharge rate of 1% to 2% per year. After a year, a coin cell battery can lose 2% of its capacity in hibernation. In contrast, the BR2032 battery chemistry is a lithium-fluorocarbon polymer with a self-discharge rate of 0.3% per year. We often think that the best battery chemistry for an application is the one with the lowest discharge rate, but this is not the case. Although the BR2032 battery has a lower discharge rate, it also has a lower capacity than the 200 mAh CR2032 battery. Recalculation using the previous formula can determine whether such a low-capacity battery has sufficient power.
In this ECG patch, when the system is powered off, the IC shutdown current is the biggest factor reducing battery life. Shutdown current occurs when the IC is disabled and there is no active load. These currents are usually due to leakage in the IC and ESD protection devices within the IC, which consume small amounts of current even when there is no load. These currents are typically small (under 1µA) but have a large impact on battery life. In this RPM patch, the shutdown current can reduce battery capacity by as much as 40% within a year. Using a battery seal limits the system’s ability to draw excessive current from the battery during shutdown.
Battery seals are typically done in two ways: mechanical battery seals in the form of Mylar pull tabs, and electrical battery seals in the form of load switches. Mylar/plastic pull tab is a mechanical battery seal where the plastic pull tab is located between the battery and the system. When the device is ready to use, the user simply pulls out the plastic tab and the battery starts powering the system. This is a simple, low-cost, proven battery mechanical seal that has been in use for many years. For medical devices, however, this solution is not always possible. For ECG patches that need to be waterproof, the protruding grooves in the mylar make the patch vulnerable to water damage. In addition, the small plastic sheet may not work well for the less dexterous end user.
Simple load switches, such as the Vishay SiP32341, are a good choice for electrical battery sealing. The device is a field-effect transistor that, when turned on, isolates the battery from the rest of the system, making the SiP32341’s off current the only current drain on the battery. The load switch has a logic control line that can be turned on by a push button when the device is ready to use. The shutdown current of the SiP32341 is typically 14 pA, a significant improvement with battery sealing compared to the current consumption of the entire system without battery sealing. If SiP32341 is used as the cell seal, the CR2032 primary cell can maintain 99.97% capacity for 14 months. Without the cell seal to protect the cell from the ECG patch turn-off current, the CR2032 primary cell can only hold 62.39% of its original charge. Once this 37% difference in capacity is eliminated, the ECG patch can still meet the 5-day lifetime requirement after a 14-month shelf life.
Table 3. Battery capacity after 14 months with battery seal
The battery seal preserves battery capacity by preventing all devices in the system from drawing battery shutdown current. After 14 months of idle RPM patch, the remaining battery capacity remained above 99.9%.
Substituting this battery capacity into Equation 3 gives a more accurate run time:
Battery life (days) = 230.25 mAh/(standard monitoring mode + temperature monitoring mode + transmission mode)
Battery life (days) = 230.25 mAh/(44.92 mAh/day + 0.01 mAh/day + 0.79 mA/day) = 5.04 days
In ADI’s view, battery analysis while the system is active and in shutdown/low power modes is critical in order to design a power supply that meets all the requirements of a medical device. It is not difficult to find the ECG patch case that collects heart rate, temperature and acceleration data through BLE communication. Based on the analysis and principles of ADI, it can be applied to any number of medical equipment systems powered by primary batteries for more equipment manufacturers’ reference.
About the Author
Fahad Masood is a health and medical biosensing applications technician at Analog Devices. He has nearly a decade of experience in electronics product development for healthcare, computing and industrial applications. Fahad holds a BS in Biomedical Electronics from Rochester Institute of Technology. Contact: [email protected]