Copyright (c) 2010 IEEE. Personal use is permitted. For any other purposes, Permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. SENSORS-3716-2009 1 Abstract— Dissolved oxygen (DO) is an important parameter with significant effect on cellular development and function. Micron-scale laminar flow and hydrodynamic focusing provide ideal tools for the generation of controlled chemical micro- environments and their application as stimuli to cells. In this paper we demonstrate the generation and characterization of multi-stream laminar flow and hydrodynamically focused sample streams with defined dissolved oxygen concentrations on chip. A solid-state oxygen sensor layer was integrated into PDMS-based microchannels and calibrated. Several combinations of sample and buffer streams with concentrations ranging from 0 to 34 mg/l DO were generated and measured for up to three independent parallel flow streams. In addition, diffusion-based stream broadening measured with the sensor was used to determine the coefficient of diffusion of O 2 in the flow medium. The devices have the potential to provide novel insights into cell biology and improve the relevance of in-vitro cell assays. Index Terms—Microfluidics, hydrodynamic focusing, optical oxygen sensor, PtOEPK/PS, spatial measurement. I. INTRODUCTION HE characteristics of laminar flow, as observed in microfluidic devices, allow one to generate parallel multi- stream flows with stable inter-stream interfaces in a single microchannel. Material transport across these interfaces is by diffusion only and can be controlled using the flow speed of the individual streams. In cell biology, this phenomenon can be applied to produce controlled chemical microenvironments down to sub-cellular dimensions [1, 2], enabling one to study the biochemical and biophysical processes of cells. To this day the use of multiple parallel flow streams has been explored mostly for the partial treatment of individual and patterned cells with biochemical reagents [1-3]. In an extension of this concept, it has further been shown that the shape of the interface between the parallel flow streams can be Manuscript received. This work was supported in part by the University of Canterbury through a Targeted Doctoral Scholarship and the MacDiarmid Institute for Advanced Materials and Nanotechnology through a Post Doctoral Fellowship. Part of this paper was presented in an earlier version at the 2009 IEEE SENSORS Conference and was published in its proceedings. V. Nock is with the MacDiarmid Institute for Advanced Materials and Nanotechnology, Department for Electrical and Computer Engineering, University of Canterbury, Christchurch, Private Bag 4800, New Zealand (phone: +643364 2987 ext 7123; e-mail: volker.nock@elec.canterbury.ac.nz). R. J. Blaikie is with the MacDiarmid Institute for Advanced Materials and Nanotechnology, Department for Electrical and Computer Engineering, University of Canterbury, Christchurch, Private Bag 4800, New Zealand (e- mail: richard.blaikie@canterbury.ac.nz). selectively modified by modulating the driving pressures to produce arbitrary shaped chemical signal streams [4]. Beyond the use for the delivery of reagents or nanoparticles, multi-stream laminar flows also have the potential to be used to generate microenvironments with controlled oxygen concentrations inside a single channel. In cell-based applications in particular, the oxygen concentration of a sample stream itself represents a parameter with significant effect on cellular development and function. For example, the dissolved oxygen (DO) concentration has been found to be intimately linked to cell survival, metabolism and function [5, 6]. The capability to expose regions of a cell- culture or individual cells and regions on the cell surface to controlled DO levels therefore has the potential to yield novel insights into cell biology. Furthermore, measuring and controlling the DO concentrations of sample streams will increase the relevance of existing small-molecule delivery applications, which previously have been performed mostly with media equilibrated under atmospheric oxygen conditions [1-4, 7]. Achieving this requires a means of measuring spatially- distributed DO concentrations inside the particular microdevice. Thus, we have recently developed a robust deposition and patterning method for optical oxygen sensors based on Platinum(II) octaethylporphyrin ketone (PtOEPK) in polystyrene (PS) as microporous oxygen-permeable matrix [8]. This material system has attracted considerable interest due to the long wavelength shift and long-term photo stability exhibited by the PtOEPK molecule [9]. In addition, the homogeneous nature of spin-coated sensor films as obtained with our fabrication process allows one to visualize spatially-varying DO concentrations in-situ, such as generated by two independent flow streams [8]. In this paper we extend the concept by demonstrating the advanced capabilities of the integrated sensor system for spatially-resolved measurement of DO in two microfluidic devices related to the generation of localized microenvironments. The in-situ measurement of local oxygen concentration is demonstrated both in flow streams hydrodynamically focused to cellular dimensions, as well as multiple parallel streams with variable concentration levels. The former represents an important example of how the spatial resolution of applied stimuli, such as oxygen, can be improved towards cellular and sub-cellular dimensions. While this has previously been demonstrated as a means to improve the sample control for small molecular reagents [7], this paper Spatially-Resolved Measurement of Dissolved Oxygen in Multi-Stream Microfluidic Devices Volker Nock, Member, IEEE, and Richard J. Blaikie, Member, IEEE T Authorized licensed use limited to: University of Canterbury. Downloaded on August 09,2010 at 03:03:42 UTC from IEEE Xplore. Restrictions apply.