Electrochemical impedance spectroscopy with interdigitated electrodes at the end of hypodermic needle for depth profiling of biotissues

By Yun, Joho; Kang, Giseok; Park, Yangkyu; Kim, Hyeon Woo; Cha, Jung-Joon; Lee, Jong-Hyun
Published in Sensors and Actuators B: Chemical NULL 2016

Abstract

Abstract An EoN (electrochemical impedance spectroscopy-on-a-needle) with bipolar interdigitated electrodes fabricated at the end of a curved surface of a hypodermic needle has been introduced to electrically discriminate biotissues. A flexible photomask and photoresist spray coating were employed in the photolithography process to fabricate fine electrode patterns on the round surface of the needle, which was insulated with parylene C. The electrodes were coated again with parylene C to enhance biocompatibility and durability. A repeatability test using a {PBS} (phosphate buffered saline) solution with known properties was carried out ten times; the maximum standard deviation was approximately 7% in electrical impedance. To analyze the EoN as an electrical circuit, equivalent modeling was introduced for the design of {EIS} (electrochemical impedance spectroscopy) devices with a target performance. In order to evaluate the discrimination capability of the EoN, the electrical depth profiling was conducted for three-layered (fat-muscle-fat) porcine tissues over a frequency range of 100 Hz-1 {MHz} as the needle sequentially penetrated the biotissues. The discrimination index (DI), defined as the ratio of the mean difference and the square root of sum of two variances, was introduced to find the optimal frequency at which the largest {DI} occurred for two boundaries of three-layered tissues. To improve the DI, the measured electrical impedance of three-layered porcine tissue was compensated by excluding the unwanted measurement error induced by the connection lines inserted in the biotissues. As a result, the discrimination index was considerably increased by 9.3 times in real (Z?) part of impedance and 3 times in imaginary (Z??) part of impedance. We believe that the proposed EoN can be utilized to find the exact location of specific tissues and to discriminate between normal and abnormal lesion tissues prior to biopsy.

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