Soft, curved electrode systems capable of integration on the auricle as a persistent brain-computer interface

Abstract

Recent advances in electrodes for noninvasive recording of electroencephalograms expand opportunities collecting such data for diagnosis of neurological disorders and brain-computer interfaces. Existing technologies, however, cannot be used effectively in continuous, uninterrupted modes for more than a few days due to irritation and irreversible degradation in the electrical and mechanical properties of the skin interface. Here we introduce a soft, foldable collection of electrodes in open, fractal mesh geometries that can mount directly and chronically on the complex surface topology of the auricle and the mastoid, to provide high-fidelity and long-term capture of electroencephalograms in ways that avoid any significant thermal, electrical, or mechanical loading of the skin. Experimental and computational studies establish the fundamental aspects of the bending and stretching mechanics that enable this type of intimate integration on the highly irregular and textured surfaces of the auricle. Cell level tests and thermal imaging studies establish the biocompatibility and wearability of such systems, with examples of high-quality measurements over periods of 2 wk with devices that remain mounted throughout daily activities including vigorous exercise, swimming, sleeping, and bathing. Demonstrations include a text speller with a steady-state visually evoked potential-based brain-computer interface and elicitation of an event-related potential (P300 wave).

Keywords: auricle integration; brain–computer interface; soft electronics; text speller.

Fig. 1.

Fractal device architectures and mechanical properties of EEG measurement systems. (A) Epidermal electronics with fractal layouts, composed of three electrodes (REC, GND, and REF) and interconnect (Left), with magnified view of the latter (Right). (B) Device laminated on the auricle and mastoid (Left) and the magnified interconnect (Right). (C) FEM results of fractal structures upon mechanical bending (180°) with the radius of curvature of 0.5 mm. (D) FEM results for simultaneous bending along two orthogonal axes (R, distance between tragus and outer edge of the ear; d, thickness of ear lobule). (E–H) Images of mounted devices on different regions of the ear, including the crura of antihelix, helix, and lobule. (I) Schematic illustration and a flowchart about the overall EEG recording process.

Fig. 2.

Assessments of biocompatibility by infrared thermography (IRT) and through cell-based studies. (A) Keratinocytes cultured on a control (Petri dish) with fluorescence microscopic image (Left) and quantitative measurement of cell viability (Right). (B) Keratinocytes cultured on a fractal device for comparison. (C) Image of a fractal device mounted on the forearm. (D) IRT images of the skin surface collected during 2 wk reveal no adverse effects. (E) Image of a gel electrode removed after 1 d (Left) and IRT image showing erythema, elevated temperature on the skin (Right).

Fig. 3.

Long-term recording of EEG. (A and B) Set of P300 data collected with an LTE electrode, immediately after mounting (A) and after 24 h (B). The SNR values are similar. (C–E) EEG alpha rhythms measured with a set of LTE electrodes for 14 d. (Left) Spectrograms showing the amplitudes as a function of the frequency for 1 min. Signals after 30 s are clearly detected when the eyes are closed. (Right) Plots presenting raw EEG signals. The data show no significant differences in signal amplitudes or patterns during 2 wk of continuous wear.

Fig. 4.

EEG electrodes with tripolar concentric ring (TCR) and capacitive designs. (A) Schematic illustration of TCR electrodes where a single set of three rings occupies the same area as a conventional metal electrode. (B–F) Images of TCR electrodes. (B) Integrated set of electrodes on the auricle and mastoid. (C) Enlarged view of the REF (upper antihelix). (D) Magnified view of fractal meshes. (E) GND on lobule. (F) REC on mastoid. (G) Spectrograms of EEG alpha rhythms recorded by the TCR electrodes including inner, middle, and outer rings. (H) Capacitive electrode mounted on the skin by a silky fabric. (I) Process of washing the electrode in soap water. (J) EEG alpha rhythms to compare signals before and after washing the electrode for ten cycles. The RMS values show the retained device functionality.

Fig. 5.

Recording of steady-state visually evoked potentials (SSVEP) and P300 and their use for brain–computer interfaces. (A) Experimental setup for SSVEP-based text speller, showing a visual stimulation, a volunteer wearing electrodes, and a signal amplifier and classifier. (B) Brain mapping with 40 electrodes to identify locations that yield the highest signals. (C) Plot of SSVEP signal quality according to the location. (D) Image of the text speller interface including visual stimulation, classification algorithm, and a volunteer watching the flickering windows. (E) Flowchart of the simplified classification process. (F) Summarized performance of three subjects in the spelling task. The averaged spelling rate for “computer” with word prediction is 2.37 characters/min, and the averaged accuracy is 93%. (G) Comparison of the signals for LTE and conventional gel electrodes. Both exhibit similar patterns and amplitudes. (H) Image of the P300-based text speller to record the event-related potentials to identify the desired letter.