In-Ear Sensors Record Health Metrics From Brain Activity and Sweat

October 10, 2023 by Jake Hertz

The in-ear sensor array can track a person's physiological and biochemical data to help diagnose neurologic conditions, track physical activity, and even monitor mental health.

In the growing field of wearable technology, the quest for unobtrusive yet comprehensive health monitoring solutions is relentless. While wristbands and smartwatches have made strides in tracking physical metrics, they fall short when it comes to capturing intricate physiological and biochemical data.


The UCSD team’s in-ear sensors

The UCSD team’s in-ear sensors. Image used courtesy of UCSD

To answer this challenge, a group of researchers from the University of California San Diego (UCSD) have developed an in-ear sensor array that not only monitors brain activity but also tracks lactate levels in sweat. This article delves into the technical construction and operation of this sensor technology and explores the challenges and benefits of in-ear sensing.


Earbud Sensors Harvest Brain and Lactate Data

The UCSD researchers say their in-ear integrated sensor array can simultaneously monitor brain activity through electroencephalogram (EEG) and lactate levels in sweat, offering a comprehensive, non-intrusive solution for real-time health monitoring. The in-ear placement of the sensor is strategically advantageous, leveraging the ear's proximity to the central nervous system and major vasculature to capture high-quality, reliable data. Data is then wireless transmitted from the earbuds back to a centralized dashboard on a user’s phone or computer.

The importance of this work lies in its potential to revolutionize health monitoring systems. Traditional wearable devices like wristbands and smartwatches are limited in the range of physiological metrics they can capture. In contrast, this in-ear sensor provides a rich source of data that could be invaluable for medical diagnostics, performance optimization, and even mental health assessment. The dual-modality of the sensor—capturing both electrophysiological and electrochemical data—makes it a comprehensive tool for monitoring a broad spectrum of health indicators.


How Does the Sensor Work?

One of the major successes of this research is the sensor's intricate design and fabrication process, which addresses several challenges, such as limited space, anatomical variability, and signal integrity. 

The sensor array is fabricated using a layer-by-layer screen printing method, the foundation of which is a 150-μm-thick TPU (Thermoplastic Polyurethane) substrate. Sequential layers of stretchable Ag (silver) for interconnection, SEBS (Styrene-Ethylene-Butylene-Styrene) as an insulating layer, and stretchable PB (Prussian Blue) for electrochemical electrodes are printed and cured on this substrate. 


The array consists of both electrochemical and electrophysical sensors

The array consists of both electrochemical and electrophysical sensors. Image used courtesy of Nature

For electrophysiological sensing, the researchers constructed 3D stretchable Ag electrodes using a 3D-printed tough polylactic acid mold. These electrodes are then bonded to a flexible printed circuit board (PCB) using silver liquid solder, ensuring robust electrical connections.

This sensing array is based on potentiometric measurement techniques—techniques that measure the concentration of a chemical component in a liquid-based solution situated above the sensor's surface. The array includes three in-ear electrophysiological electrodes, one reference electrode at the cymba conchae and one driven right leg (DRL) electrode at the concha cavum. These electrodes are connected to a portable data acquisition (DAQ) system operating in single-ended mode. The DAQ system employs a fully differential programmable gain amplifier at each channel to suppress common-mode interference from power lines and other sources.

For lactate sensing, the electrochemical sensor employs a working electrode and a counter electrode made of stretchable PB material. The sensor geometry is specifically designed to match the sweat secretion distribution in the tragus area of the ear, optimizing the sensor's performance.


A Comfortable Diagnostic Tool

UCSD claims this research offers a more nuanced and comprehensive approach to monitoring health and well-being. The sensor's ability to capture high-quality EEG data could help diagnose neurologic conditions, making it easier to monitor conditions like epilepsy, Alzheimer's, or other cognitive impairments in real-time. The lactate sensing capability adds another layer of diagnostic potential, offering insights into metabolic health and potentially aiding in the early detection of conditions like diabetes or cardiovascular disease. The sensor's unobtrusive design also makes it a candidate for long-term mental health monitoring. 

In future prototypes of the device, the UCSD researchers plan to integrate edge processing capabilities and glucose monitoring.