Departments of Electrical Engineering and Bioengineering, School of Engineering, Santa Clara University, Santa Clara, CA 95053, USA.
Nanotechnology. 2010 Mar 26;21(12):125101. doi: 10.1088/0957-4484/21/12/125101. Epub 2010 Feb 25.
Identifying the neurophysiological basis underlying learning and memory in the mammalian central nervous system requires the development of biocompatible, high resolution, low electrode impedance electrophysiological probes; however, physically, electrode impedance will always be finite and, at times, large. Herein, we demonstrate through experiments performed on frog sartorius muscle that single multi-walled carbon nanotube electrode (sMWNT electrode) geometry and placement are two degrees of freedom that can improve biocompatibility of the probe and counteract the detrimental effects of MWNT/electrolyte interface impedance on the stimulation efficiency and signal-to-noise ratio (SNR). We show that high aspect ratio dependent electric field enhancement at the MWNT tip can boost stimulation efficiency. Derivation of the sMWNT electrode's electrical equivalent indicates that, at low stimulus voltage regimes below 1 V, current conduction is mediated by charge fluctuation in the double layer obviating electrolysis of water, which is potentially toxic to pH sensitive biological tissue. Despite the accompanying increase in electrode impedance, a pair of closely spaced sMWNT electrodes in a two probe (bipolar) configuration maintains biocompatibility and enhances stimulation efficiency and SNR compared to the single probe (unipolar) configuration. For stimulus voltages below 1 V, the electrical equivalent verifies that current conduction in the two probe configuration still proceeds via charge fluctuation in the double layer. As an extracellular stimulation electrode, the two sMWNT electrodes comprise a current dipole that concentrates the electric field and the current density in a smaller region of sartorius; consequently, the bipolar configuration can elicit muscle fiber twitching at low voltages that preclude electrolysis of water. When recording field potentials, the bipolar configuration subtracts the potential between two points allowing for the detection of higher signal amplitudes. As a result, SNR is improved. These results indicate that use of the high aspect ratio MWNT in a bipolar configuration can achieve a biocompatible electrode that offers enhanced stimulation efficiency and higher SNR.
确定哺乳动物中枢神经系统学习和记忆的神经生理学基础需要开发生物相容性、高分辨率、低电极阻抗的电生理探头；然而，电极阻抗在物理上总是有限的，有时还很大。本文通过对青蛙缝匠肌的实验证明，单根多壁碳纳米管电极（sMWNT 电极）的几何形状和位置是两个自由度，可以提高探头的生物相容性，并抵消 MWNT/电解质界面阻抗对刺激效率和信噪比（SNR）的不利影响。我们表明，MWNT 尖端的高纵横比相关电场增强可以提高刺激效率。sMWNT 电极的等效电路表明，在低于 1 V 的低刺激电压范围内，电流通过双层中的电荷波动来传导，从而避免了对 pH 敏感的生物组织具有潜在毒性的水的电解。尽管电极阻抗随之增加，但在双探头（双极）配置中，一对紧密间隔的 sMWNT 电极与单探头（单极）配置相比，仍能保持生物相容性，并增强刺激效率和 SNR。对于低于 1 V 的刺激电压，等效电路验证了双探头配置中的电流通过双层中的电荷波动来传导。作为一种细胞外刺激电极，两个 sMWNT 电极构成一个电流偶极子，将电场和电流密度集中在缝匠肌的较小区域；因此，双极配置可以在低电压下引起肌肉纤维抽搐，从而避免水的电解。在记录场电位时，双极配置可以减去两点之间的电位，从而可以检测到更高的信号幅度。因此，SNR 得到了提高。这些结果表明，在双极配置中使用高纵横比的 MWNT 可以实现一种生物相容性电极，从而提高刺激效率和 SNR。
