Micromachines 2022, 13, 1654. https://doi.org/10.3390/mi13101654 www.mdpi.com/journal/micromachines Article Comparative Study and Simulation of Capacitive Sensors in Microfluidic Channels for Sensitive Red Blood Cell Detection Wei Hu 1 , Bingxing Wu 1 , Soumya K. Srivastava 2 and Suat Utku Ay 3, * 1 Thermo Fisher Scientific, Jinke Road 2537, Pudong District, Shanghai 201206, China 2 Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506-6102, USA 3 Department of Electrical and Computer Engineering, University of Idaho, Moscow, ID 83844-1023, USA * Correspondence: suatay@uidaho.edu Abstract: Microfluidics provides an indispensable platform for combining analytical operations such as sample preparation, mixing, separation/enrichment, and detection onto a single compact platform, defined as a lab-on-a-chip (LOC) device with applicability in biomedical and life science applications. Due to its ease of integration, 1D interdigital capacitive (IDC) sensors have been used in microfluidic platforms to detect particles of interest. This paper presents a comparative study on the use of capacitive sensors for microfluidic devices to detect bioparticles, more specifically red blood cells (RBCs). The detection sensitivities of 1D, 2D, and 3D capacitive sensors were determined by simulation using COMSOL Multiphysics ® v5.5. A water-filled 25 μm × 25 μm PDMS microfluidic channel was used with different sizes (5–10 μm) of red blood cells passing across the capacitive sensor regions. The conformal mapping was used for translating the 1D IDC sensor dimensions into equivalent 2D/3D parallel plate capacitance (PPC) sensor dimensions, creating similar absolute sen- sor capacitance. The detection sensitivity of each capacitive sensor is determined, and a new 3D PPC sensor structure was proposed to improve the sensitivity for high-resolution RBC detection in mi- crofluidic channels. Proposed 2D and 3D sensors provide a 3× to 20× improvement in sensitivity compared to the standard 1D IDC structures, achieving a 100 aF capacitance difference when a healthy RBC passes in the structure. Keywords: microfluidics; lab-on-chip; capacitive sensor; interdigital capacitor sensor (IDC) 1. Introduction The use of capacitive structures as sensors has been widely investigated and pub- lished since 1960s, with them finding applications in many fields [1–5], recently including biomedical and life science areas [6–9]. Among them, coplanar interdigital capacitor (IDC) can be considered as a one-dimensional (1D) sensing structure that utilizes fringing elec- tric fields on interdigitated electrodes providing a one-sided investigation of the materials and any particulates passing above them, as shown in Figure 1a. These coplanar IDCs are the 1D mapping of the 2D parallel plate capacitors (PPCs) through conformal mapping [10–12]. Two- and three-dimensional parallel plate capacitors, as shown in Figure 1b,c, date back to the early days of electricity and electromagnetics [13,14]. They are used as sensors either by detecting changes in the suspending medium properties (i.e., permittiv- ity, εx) in where the electric field (E) is formed or by detecting changes that occurred in the physical properties of the capacitor structure (i.e., separation of the plates, s). For ex- ample, micro-machined accelerometers use suspended capacitor plates that can be dis- placed proportionally to the acceleration rate modulating plate separation (s) and hence the capacitance of the sensor [15,16]. In the case of microfluidic devices, the permittivity of the suspending medium in the microfluidic channel where the capacitive sensor is Citation: Hu, W.; Wu, B.; Srivastava, S.K.; Ay, S.U. Comparative Study and Simulation of Capacitive Sensors in Microfluidic Channels for Sensitive Red Blood Cell Detection. Micromachines 2022, 13, 1654. https://doi.org/10.3390/ mi13101654 Academic Editor: Xiujun Li Received: 26 August 2022 Accepted: 28 September 2022 Published: 30 September 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).