Reduced hardware complexity for viscosity measurements by optical knife-edge detection on micropillar-based microfluidic chips

Abstract

This study uses the innovative application of optical knife edge detection in micropillar-based microfluidic systems and aims to develop a new method for monitoring and analyzing viscosity changes of biological fluids such as blood, amniotic fluid, etc. in the microscale dimension. The basics of research are based on the placement of micropillars functionalized as both mechanical sensors and optical waveguides in a microfluidic chip. Each pillar within the microfluidic chip is designed to respond sensitively to liquid flow while maintaining the ability to effectively direct light. These dual functions become important for this study as they allow micropillars not only to detect changes in fluid flow but also to convert these changes into measurable optical signals. This microfluidic viscometer chip is made from Polydimethylsiloxane (PDMS); which is optically clear, inert, non-toxic polymer, and fabricated by basic soft lithographic processes Details were carefully considered when designing the experimental setup. This began with the selection of light emitting laser diode sources that could be used in microfluidic applications. Utilizing a carefully constructed lens system, this light beam is focused into a single micropillar. The focal lengths and positions of the lenses used were optimized to allow the maximum amount of light to enter the micropillar without significant loss. Through the micropillar, light is directed to an optical detection system for data acquisition with photodetector and CMOS camera for detection. These devices are calibrated to capture light emerging from micropillars and measure intensity changes caused by the bending of the pillars due to flow. In the first phase of the project, experiments were carried out with air flow using an air compressor. Afterwards, liquid tests were carried out at different flow rates using a glycerol-water mixture. To ensure control of experimental measurements, ray tracing simulations are performed to visualize and analyze how light behaves under different conditions within the micropillar structure. These simulations help improve the optical path setup and validate theoretical models developed to predict light behavior in response to flow-induced micropillar bending. The simulation results were used to adjust the detection system to improve sensitivity and specificity in measuring light intensity changes and the applicability of the method. Besides, an optical model that can be used to determine the detector size that can detect all of the light power coming from the micropillars without losing it is proposed. Additionally, a comparison of the normalized output light intensity with respect to the pillar displacement of the detector data from the simulation results and the results obtained from the analytical modeling is presented. Data from experiments show that the knife-edge detection technique provides reliable and consistent measurements of light intensity changes that agree well with liquid flow rates and viscosity. Experimental data confirms the effectiveness of the optical setup and micropillar design in capturing and converting liquid flow into optical signals that can be analyzed quantitatively. For the proposed technique, photodiode and CMOS camera based pillar bend detection were compared, and a good match between the results was shown. At the same time, advantages of the proposed technique, such as reduced computational load and cost reduction, are presented. In addition, a comparison of pillar bending angles based on the circle detection method, which will contribute to the accuracy of the proposed method, is also shown. Our work included an important investigation of bending limitations in micropillars, significantly influenced by the varying refractive indices of different liquids used in the microfluidic system for viscosity calculation. This variability has been identified as an important parameter because different fluids can change the conditions under which total internal reflection (TIR) occurs, for which micropillars can be used as waveguides. A simple mathematical model was proposed to determine these limits, and comprehensive ray tracing simulations were performed using different fluid indices available in the literature to predict how changes in the fluid refractive index affect the light-directing abilities of micropillars. This mathematical model was based on determining the limits of micropillar waveguides by measuring the critical angles and minimum radius of curvature for TIR in various liquids such as blood, cerebrospinal fluid, etc. The theoretical model was compared with data obtained from ray-tracing simulations. By gaining an understanding of the optical behavior within micropillars, its usability has been demonstrated in clinical diagnostic applications where the properties of fluids can vary significantly. The incorporation of a knife-edge detection system with micropillars into a microfluidic device represents a substantial advancement in optical fluid dynamics measurement. The research findings may facilitate the creation of novel microfluidic devices that are smaller in size, more cost-effective, and capable of delivering quicker and more precise outcomes. Subsequent investigations could prioritize the continued reduction in size of the device, broadening the scope of observable fluid characteristics, and exploring the practical implementation of this technology.