# Mixing with Bubbles: A Practical Technology for use with Portable Microfluidic Devices.

Original entry: Nefeli Georgoulia, APPHY 226, Spring 2009

## Overview

Authors: Piotr Garstecki, Michael J. Fuerstman, Michael A. Fischbach,Samuel K. Siaa and George M. Whitesides

Source: Lab on a chip, Vol 6, p.207–212, 2006

Soft matter keywords: droplets, micromixing, Reynolds number, plasma oxidation

## Abstract

Fig.1 : Piotr Garstecki, Michael J. Fuerstman, Michael A. Fischbach, Samuel K. Siaa and George M. Whitesides, 2005, Lab on a chip

This publication presents a method to manufacture a portable, compact and low cost microfluidic device, intended for medical diagnostic use. The device is manufactured by a single step soft-lithography process (i.e. pouring PDMS in a micromold). To achieve successful mixing of reagents, all the while maintaining structural simplicity, air bubbles are introduced in the device to serve as mixers! Moreover, this microfluidic device is advantageous since its resistance to proteins has been proven and renders it compatible with all bodily fluids. The authors propose the device as a potentially low-cost solution, to be used in healthcare-developing countries. Still, one final issue has to be addressed for the device to evolve into a full-blown, marketable product. Plasma oxidation of the PDMS during the manufacturing process, renders the inner channels and surfaces hydrophilic. However hydrophilicity of PDMS gradually wears off (the process can take a few hours to days). This significantly limits the shelf life of the potential product. Authors suggest that coating surfaces might enhance long-term wetability of the device.

## Soft matter snippet

So, how does it work? Here are the components that the device features, also displayed in fig.2:

Fig.2 : Piotr Garstecki, Michael J. Fuerstman, Michael A. Fischbach, Samuel K. Siaa and George M. Whitesides, 2005, Lab on a chip
• Two entry ports for the analytes

Two wells of macroscopic dimensions serve as entry ports for the two analytes, that will subsequently be mixed in the device. The operational principle is that once the analytes mix, a reaction between the two will yield a noticeable result (such as a change of color) that will be medically significant (ex: presence/absence of infection).

• Filters
Fig.3 : Piotr Garstecki, Michael J. Fuerstman, Michael A. Fischbach, Samuel K. Siaa and George M. Whitesides, 2005, Lab on a chip

Multiple inlet channels serve as filters. These smaller channels connect the macroscopic ports of entry to the micro-mixer and minimize the chance of clogging in the device. The succession of filter-channels leading to the mixer is illustrated in fig.3.

• A micromixer

The T-juctions illustrated in fig.2, break down the air incoming in the device into separated bubbles, which serve as micro-mixers. The adjacent TWIST valves help regulate both air and liquid flow. Bubbles serve as mixres only when embedded in laminar liquid flow. The authors perform the following calculation, to ensure that flow in the device remains laminar for a span of biological analytes:

The rate of flow in the channels is defined through the Hagen Poiseuille equation:

$Q = (p_0-p) \frac{A^2}{\mu l}$

Where A is the cross-sectional area of the channel and $l$ its length. Also, $\mu$ is the viscosity of the liquid flowing through. By plugging in numbers, the authors find that Q ranges:

$Q = 0.15 \frac{\mu L}{s} - 1 \frac{\mu L}{s}$

The Reynolds number of the flow is given by the equation:

$Re = \frac{\rho Q}{\mu w}$

With $\rho$ being the density of the fluid and $w$ the width of the channel. Re ranges between:

Re = 0.1 - 10

Consequently, flow through the channels remains in the laminar regime, for analyte viscosities in the range of physiological fluids.

• A hand-operated air pump as a source of reduced pressure, to successfully pump liquid and gas

Something as cheap and simple as a syringe can be used as a vacuum pump (fig.1).

## Biocompatibility

Fig.4 : Piotr Garstecki, Michael J. Fuerstman, Michael A. Fischbach, Samuel K. Siaa and George M. Whitesides, 2005, Lab on a chip

The authors proceed in testing the device's performance in mixing protein-containing analytes. For this purpose, horseradish peroxidase (HRP) and Amplex red reagent are introduced in the two ports of entry. Upon mixing, HRP converts Amplex to a red fluorescent product with and emission maximum at $\lambda = 590nm$. The intensity of fluorescence is documented both before and after the liquids pass through the mixture. Moreover, these intensities are compared to the intensity of a pre-mixed solution on fig.4. Visibly, the microfluidic device successfully mixes the reagents to a fluorescence saturation point.