Power Consumption Design Challenges
The COVID-19 pandemic has created an unprecedented need for remote patient monitoring. Most of the patients who test positive for COVID-19 are in self-isolation at home, increasing the need for alternative medical solutions, including remote patient monitoring. On March 20, 2020, The FDA issued guidance for expanded use of certain remote monitoring devices to facilitate patient management while limiting physician-patient contact during the COVID-19 pandemic.
The guidance allows manufacturers of certain FDA-cleared non-invasive, vital sign-measuring devices to expand their use so that health care providers can use them to monitor patients remotely. The specific devices mentioned in the guidance are clinical electronic thermometers, ECGs, cardiac monitors, electrocardiograph software for over-the-counter use, pulse oximeters, devices to measure respiratory rate and breathing frequency, and electronic stethoscopes.
Device manufactures, therefore, have a unique opportunity to design new or expand current devices.
One of the key design challenges when designing a monitoring system utilizing multi-parameter data acquisition is the management of energy draw from the power source and battery selection
Choosing between primary or rechargeable power sources is a key decision in the use of the device and will determine clinical use-cases the device will support. Remote wearable and portable patient monitors are typically battery-powered, and battery life is critical because most patient monitors measure and monitor continuously. Designers use features such as standby, sleep, power save, hibernate, and shutdown to help reduce power consumption and extend battery life. By intelligently utilizing the power saving modes while fulfilling the monitoring requirements, monitor designers can save power. So, battery-powered systems require a careful choice of battery, efficient use of available charge, and efficient use of space.
Additionally, to optimize power management, it is important to choose a power architecture that is as efficient as possible and has extended battery run times. Power consumption not only depends on the use of power-saving features bit also interaction with the wireless network. The main physical connections to a network are Blue Tooth Low Energy BLE to a local gateway or mobile device, WiFI to a local router, or possibly a mobile device, and Low Power Wide Area networks such as LoraWan, LTE-M, and NbIOT.
Application matters. For instance, acquiring all parameters and streaming data in real-time would drive energy requirements higher, requiring large coin cells, primary cell(s), or rechargeable lithium ion solutions which require the use of a charger or charging device while deployed on the patient or be able to replace the device with a charged device in the clinical environment. Ideally, a primary battery source can support the use case without having to recharge or exchange the device.
Lithium remains the preferred choice for remote wireless applications due to its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest non-gaseous metal and offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries. Lithium cells, all of which use a non-aqueous electrolyte, have a normal operating current voltage (OCV) ranging between 2.7 and 3.6 V. The absence of water allows lithium batteries to endure more extreme temperatures.
One of the considerations of a primary power source is the voltage drop under higher or pulsed current draw. While Lithium coin cells offer high energy density, their higher internal impedance increases power supply complexity to store and release energy during higher power pulses, such as communications. Silver Oxide batteries have a lower internal impedance and are good for pulsed power applications.
Another key consideration of the system design and power source is to verify that under pulsed conditions, the processor and memory accesses are not compromised by a voltage drop present on the processor bus. Many of today’s processors can work down to 1.8 Volts without experiencing an upset and include brownout detection and protection mechanisms that can protect the system against upsets.
Energy harvesting systems combined with low power high integration silicon, are allowing the measurement of parameters, and transmission using lower power blue tooth BLE to mobile devices or BLE gateways. By harvesting energy from vibration, thermal and chemical sources, long-life sensors with no external power sources or battery change requirements are now possible.