The Resilient Monitor is a patch worn on the abdomen that monitors a user’s vital signs and alerts family, friends, and/or first responders on detecting an opioid overdose. The sensor is non-invasive and leverages a combination of photoplethysmography (PPG) and accelerometry to measure heart rate, respiratory rate, and oxygen saturation (SpO2). For the purpose of research studies, the raw sensor data is logged on a microSD card for later analysis.

The plan is for the device to last for an entire week on a single lithium AAA battery, like the Energizer L92. To put it in perspective, the iPhone XR Li-Ion battery has 6x the energy as this battery and often lasts for less than a day (in my case).

In order to use PPG, the sensor flashes red, green, and infrared LEDs at a rate of 25 times per second. Raw sensor data is logged to the microSD card periodically. These are the two main power consumers in our current implementation.

Before optimizing any of the design, the device lasted for 14 hours on a single AAA battery. Getting to a week of battery life has involved the following firmware modifications:

• Putting the microcontroller into a deeper sleep while not actively reading from the sensors.
  • It’s critical to ensure that components are only ON when they are being used.
• Decreasing the PPG sample rate, LED brightness, and measurement count.
  • Features of the physical design have helped us ensure optimal readings while minimizing power consumption: (1) The patch helps to block ambient light. (2) Multiple photodiodes allow for enhanced light capture.
• Switching to a microSD card with decreased power consumption in a standby state.
• Decreasing the brightness of the status indicator LED.

These changes only scratch the surface of what’s possible when it comes to optimizing a wearable sensor for a low-power use case like ours. We’re excited to explore further improvements.

With these changes in place, our sensor lasts for 8.5 days on a single AAA. Keep in mind that the final device will have a cellular connection for emergency alerts, will run vital sign algorithms onboard, but it will not need to store data on a microSD card. These features are in progress - and they will all affect the battery life. We’re confident in our ability to deliver a product that lasts for an entire week.