Towards Improving the Quality of Electrophysiological Signal Recordings by Using Microneedle Electrode Arrays

Objective: Electrophysiological signals are recorded generally by metal electrodes placed on body surface. For long term use, the signal quality may decay with the change of interface impedance between electrodes and skin due to the conductive hydrogel dehydration. Besides, electrodes may shift during body movements, which causes unstable signal recordings. Methods: To improve the quality of electrophysiological signal recordings on human body surface, a type of microneedle electrode array (MEA) with microneedles around 550 μm in length was fabricated with a magnetization-induce self-assembly method. Results: Compared with the commonly used dry electrode array, the MEA has lower and more stable interface impedance, especially when the electrode-skin interface is under unstable pressures. For electrophysiological signal recording, the MEA can acquire electromyography (EMG) with significantly lower noise energy, higher signal-to-noise ratio, and higher motion-classification accuracy based on the EMG pattern-recognition method. Additionally, high quality electrocardiography (ECG) can be recorded by using the MEA, where more accurate R-peaks are extracted in different scenarios. Besides, there was no report about any discomfort like bleeding or inflammation by all the subjects. Conclusion: This research proved that the microneedles on the MEA can penetrate through the corneum and reach the epidermis of the subjects, which could avoid the influence of corneum and fix the electrode on the body surface for high-quality signal recording especially during body movements. Furthermore, the microneedles would not touch the dermis, enabling a painless signal acquisition. Significance: This work is beneficial to the applications of wearable human-machine interface technology.

I. Introduction

Electrophysiological signals are widely applied in health monitoring and disease prevention and diagnosis. Electrodes, also called electrophysiological sensors, are needed to record electrophysiological signals, and their performances are very critical to determine the acquired signal quality. Currently, there are several types of electrodes for electrophysiological signal recording, among which the wet electrodes are mostly applied in both scientific research and clinic applications though with some limitations. For instance, these electrodes require conductive hydrogels [1] to adhere to skin surface, and thus some preparation and cleaning processes are unavoidable before and after each use. For some people, the hydrogels may cause skin allergy [2], which would be lower wearing safety and comfort. During applications, the hydrogels become dry with time and the electrode-skin interface impedance (EII) will increase [3]. Besides, the single-use wet electrodes may increase the usage cost. Except for the wet electrodes, the dry electrodes can also be used, which mainly includes the metal and fabric types. The dry type is usually limited by the high and unstable EII [4], as well as the disturbance from the relative displacement between electrode and skin surface during body movements. Although the fabric type is characterized by the high flexibility that makes it conform well to the skin, it is more affected by humidity, temperature, structure, material, and pressure applied for fixation [5], [6]. In recent years, with the development of flexible electronic technology, several advanced electrodes have been proposed [7], [8]. E.g., there exists a kind of flexible electrode based on nano-gold materials [9], which is small, lightweight, easy to assemble and less disturbed by environments. However, this type of electrode can easily get damaged and its creep property often causes inconsistent EII.