Conformal in-ear bioelectronics for visual and auditory brain-computer interfaces

Brain-computer interfaces (BCIs) show promise in motor and language rehabilitation. Current methods, including cap-based, headband-based, and microneedle-based approaches, suffer from limitations such as inconvenience, limited applications, inflammation risks, and potential tissue damage. In-ear BCIs offer several advantages: gel-free EEG signal acquisition, wearability, and discretion, making them suitable for daily use. Existing in-ear EEG monitoring electronics face challenges due to the ear canal's complex geometry and sensitivity. Current designs often require supporting structures (earplugs or 3D printed attachments) which can cause discomfort, interface failure, and hinder communication. The goal of this research was to develop a novel in-ear bioelectronic device that overcomes these limitations, providing a comfortable and reliable platform for both visual and auditory BCIs.

Recent advancements in BCIs have led to innovative approaches for human-machine interaction. However, limitations of existing non-invasive and invasive techniques necessitate the exploration of alternative methods. The ear, with its unique anatomical features, has emerged as a promising site for EEG acquisition. Several ear-EEG monitoring electronics have been developed, demonstrating the feasibility of this approach. However, challenges remain in achieving comfortable and reliable contact with the ear canal's complex curvilinear shape and inter-subject variability. Many existing devices utilize rigid support structures, which can cause discomfort, impede communication, and compromise signal quality. The use of soft, deformable materials has been proposed to address these issues, but challenges remain in balancing conformability with sufficient contact pressure to ensure robust signal acquisition and mitigating irritation.

The researchers developed SpiralE, an in-ear bioelectronic device that uses electrothermal actuation to conform to the individual's ear canal. SpiralE is designed in a compressed spiral shape, which expands to a larger spiral upon application of Joule heating. This shape memory effect allows SpiralE to adapt to the unique geometry of each ear canal while maintaining a comfortable, stable, and reliable contact. The device consists of two layers of shape memory polymers (SMPs) with embedded electrothermal actuation layer (EAL) and an EEG detection layer (EEGDL). The EAL generates heat to activate the shape memory effect, while the EEGDL records EEG signals. Fabrication involved micro/nanotechnology for the creation of stretchable electrodes, followed by multilayer fast printing and reconfiguration into a spiral shape. The in vitro characterization of SpiralE included investigation of resistance modulation under strain, impedance spectroscopy, and assessment of sound transmission through the device. The device's ability to conform to complex curvatures was also demonstrated through experiments involving its placement in tubes and a right-angle elbow. In vivo experiments involved recording alpha rhythms and SSVEPs to evaluate the quality of EEG recordings. Visual BCI experiments included 9-target and 40-target online SSVEP speller experiments. Auditory BCI experiments used a cocktail party paradigm to assess auditory attention decoding (AAD). Signal processing involved bandpass filtering, downsampling, channel/epoch cleaning, and artifact rejection. Visual decoding employed TRCA and FBCCA algorithms, while auditory decoding used a linear regression model and Pearson's correlation. The performance of the BCI was evaluated using information transfer rate (ITR).

SpiralE demonstrated excellent conformability to the inner ear canal, ensuring comfortable and stable EEG recordings. The device achieved high accuracy in both visual and auditory BCI tasks. In the visual BCI experiments, offline accuracy for 2-target SSVEP classification reached 95%, and a calibration-free online 40-target SSVEP speller experiment showed successful typing of target phrases. Notably, in-ear SSVEPs showed a stronger second harmonic component compared to occipital SSVEPs, suggesting in-ear sensing could provide complementary information for understanding harmonic spatial distribution in SSVEP studies. In the auditory BCI experiments, using SpiralE alone, a cocktail party experiment achieved 84% accuracy in identifying the attended speaker. The ITR in the visual BCI reached 36.86 ± 15.53 bits/min, exceeding previous ear-EEG results. In vitro analysis showed that the impedance of SpiralE was less than 1 kilohm at 50 Hz, indicating suitability for EEG detection. The hollowness of the design allowed for minimal interference with sound transmission.

The findings demonstrate that SpiralE provides a viable platform for comfortable and effective in-ear BCIs. The high accuracy achieved in both visual and auditory tasks showcases the potential of this technology for real-world applications. The discovery of the stronger second harmonic in in-ear SSVEPs highlights a unique characteristic of in-ear EEG sensing. The successful implementation of a high-speed, calibration-free 40-target online SSVEP speller demonstrates the clinical potential of this technology. The successful auditory attention decoding in a complex auditory scene further supports the versatility and potential of SpiralE. This technology may pave the way for the development of user-friendly, wearable BCIs for a wide range of applications, such as assistive technology, communication aids, and neurorehabilitation.

This study successfully demonstrates the feasibility of in-ear BCIs using SpiralE, a novel conformal bioelectronic device. SpiralE's design addresses the challenges of existing in-ear EEG technologies, offering comfortable and reliable EEG acquisition. The high accuracy in visual and auditory BCI experiments and the novel findings regarding SSVEP harmonics highlight the potential of this technology for future advancements in BCIs and neuroscience research. Future work could focus on improving the signal-to-noise ratio, exploring different actuation methods, investigating long-term wearability, and expanding the range of applications for SpiralE.

While SpiralE demonstrated high performance, there are some limitations. The relatively small contact area between the device and the ear canal might lead to higher interface impedance, necessitating further research into optimizing contact force and exploring alternative actuation methods. The current study involved a relatively small number of participants, and further studies with larger sample sizes are needed to confirm the generalizability of the results. Long-term studies are also needed to evaluate the device’s durability and comfort during prolonged use.