Many applications utilize flexible capacitive electrode implementations and analog front-end optimizations for improving noncontact electrocardiogram (ECG) measurements. However, the influence of the negative impedance in the circuit front end on noncontact measurements has not been investigated. The objective of this study was to develop negative impedance capacitive electrodes for improving the degraded noncontact ECG signals induced by the high-impedance interface between the skin and electrode. A capacitive electrode with a negative impedance of $-200\,\,\text{M}\Omega $ was designed to reduce the impedance mismatch between the body and measurement electrode. The proposed electrodes were applied on a subject’s chest to continuously measure the cardiac signals through a fabric layer and further evaluate the signal-to-noise (SNR) performance for noncontact ECG measurements. The negative impedance in the circuit front end reduced the coupling effect between the skin and the measurement electrodes, thus improved the SNR in theory and in practice. Importantly, our system produced a distinguishable heartbeat signal even with a maximum cloth thickness of 1.8 mm and skin–electrode capacitance exceeding 57 pF, which is suitable for wearable applications. This study is a pilot effort toward validating the efficacy of negative impedance capacitive electrodes for noncontact physiological measurements. This novel electrode design is expected to potentially address the limitations imposed by a fabric layer and its thickness in noncontact bioelectric sensing in the future.

中文翻译：

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用于通过织物层进行 ECG 传感的负阻抗电容电极

许多应用利用灵活的电容电极实施和模拟前端优化来改进非接触式心电图 (ECG) 测量。然而，电路前端的负阻抗对非接触式测量的影响尚未得到研究。本研究的目的是开发负阻抗电容电极，以改善由皮肤和电极之间的高阻抗界面引起的非接触式 ECG 信号退化。负阻抗为 $-200\,\,\text{M}\Omega $ 的电容电极旨在减少身体和测量电极之间的阻抗失配。建议的电极应用于受试者的胸部，以通过织物层连续测量心脏信号，并进一步评估非接触式心电图测量的信噪比 (SNR) 性能。电路前端的负阻抗降低了皮肤与测量电极之间的耦合效应，从而在理论上和实践中提高了信噪比。重要的是，即使最大布料厚度为 1.8 毫米，皮肤电极电容超过 57 pF，我们的系统也能产生可区分的心跳信号，适用于可穿戴应用。这项研究是验证负阻抗电容电极在非接触式生理测量中的功效的一项试点工作。