Two-Terminal Longitudinal Hotwire Sensor for In-Line Monitoring of Sub-Nanoliter Volume in Microfluidic Channels Kee Suk Ryu, Kashan Shaikh, Edgar Goluch, Patrick Mathias and Chang Liu Micro and Nanotechnology Laboratory University of Illinois at Urbana-Champaign Urbana, IL USA Email: changliu@uiuc.edu Abstract— We report a simple and practical sensor for monitoring both the absolute position and advancing speed of liquid front in a microfluidic channel. The sensor consists of a longitudinal hot wire element – a two-terminal electrical device, with its length spanning the entire channel. The design, materials, fabrication method, and use of this sensor are extremely simple. Characterization results are presented. I. INTRODUCTION Today, microfluidic laboratory-on-a-chip (LOC) devices are being investigated for realizing biochemical detection protocols on chip 1,2 . Microfluidic chips offer many potential advantages, including automation 3 , reduction of reagent volume 4,5 , acceleration of reaction processes 5 and miniaturization/portability 3-6 . To confidently operate a microfluidic system and carry out complex biochemical protocols, it is imperative to incorporate sensors for feedback control and process monitoring/verification. In this work, we focus on the task of identifying the position and speed of advancing liquid fronts in a plug flow situation, which is often encountered in lab chip applications. In macroscopic and manual bench-top assay protocols, the volume of liquid reagents is often large enough for direct visual observation. However, as the size of microchannels and volumes of reagents shrink, it becomes increasingly more difficult to measure the position of advancing liquid fronts and the speed of its movement. Such measurements are vital information to verify the functioning of a system. Simple tasks such as introducing liquid into an empty channel (priming) or knowing if the channel is emptied are not straightforward for microfluidic channels. For example, channels may be blocked due to bubbles, particles, or hydrophobic wall conditions. Flow measurement can be achieved by using optical detectors such as microscopes, optical fibers, and CCD chips. However, these solutions are not amenable to miniaturization, system simplification, and cost reduction. Additionally, the field-of-view of optical measurement systems is often too small to observe the process in a long microfluidic channel (e.g., often more than 1 mm long). In order to continuously and accurately monitor the position of fluid fronts in a micro channel, conventional practice requires embedding a series of sensors with electrical output along the entire length of the channel with given intervals. For example, an array of micromachined pressure sensors has been integrated along a micro channel to monitor pressure distribution 7 . (Note that the purpose of this sensor is not for measurement of velocity and position of liquid front.) However, the accuracy for position measurement using an array of sensors is limited and the complexity is high. For example, the use of many sensors increases the number of electrical terminals and wiring complexity. II. DESIGN AND PRINCIPLE OF THE SENSOR We have designed a sensor that measures the absolute position and advancing speed of liquid front movement. The operation principle, fabrication and packaging method, as well as testing results are discussed in the following. The schematic diagram of the sensor is illustrated in Figure 1. A resistive element made of a thin metal film traverses the length of the channel section of interest. We select a metal that offers finite temperature resistive sensitivity, i.e., its resistance is a function of the temperature.