Preparation of polymer-based microfluidic systems

Initial situation

The quantitative detection of toxic substances (e.g. botulinum neurotoxins) with microfluidic systems and integrated local impedance spectroscopy is showing promise. As they require smaller quantities of substances, microfluidics would make it safer to work with toxic substances (smaller quantities required). To achieve this goal, it is necessary to know the properties of the materials used and gain proficiency in handling them.

Goals

The goal of the project is to develop procedures for the microstructuring of polymers for use in microfluidic systems. Specifically, this project works with polydimethylsiloxane (PDMS) and poly-3,4-ethylenedioxythiophene (PEDOT:PSS). PEDOT:PSS is an electrically conductive polymer.

Microfluidics

Microfluidic systems make it possible to manipulate and analyse small quantities of liquid substances (e.g. mixing processes). Smaller quantities of substances make it possible, for example, to carry out processes more cheaply (expensive reagents) or more safely (for toxic substances). The long-term goal is to design and manufacture a microfluidic system for the detection of toxic substances (e.g. botulinum neurotoxins). The analysis can be performed by impedance spectroscopy, for example.

Impedance spectroscopy

In impedance spectroscopy, the frequency-dependent resistance (impedance) of a sample is measured using a four-point measurement. If microelectrodes are used for this purpose, local changes, such as in an emulsion, can also be investigated. This is known as local impedance spectroscopy. ALPS has used local impedance spectroscopy in collaboration with BFH HAFL, for instance, to explore the processes involved in the whipping of cream or the properties of milk foams. One goal of the project is to integrate microelectrodes for local impedance spectroscopy into microfluidic systems.

Microelectrodes

Measurements using local impedance spectroscopy require electrodes with lateral dimensions of a few hundred microns. ALPS can manufacture such electrodes from metals using thin film and lithography processes in a clean room laboratory. As an alternative, the project will investigate to what extent metallic electrodes can be replaced by electrically conductive polymer structures (PEDOT:PSS).

Procedure

Clean room laboratory

One cubic metre of normal ambient air contains several billion particles, which would make the manufacture and correct functioning of microsystems or microelectrodes impossible. Processes in this field therefore have to be carried out under isolated, virtually particle-free conditions, such as are guaranteed by clean rooms. The ALPS Institute has a class ISO 6 clean-room laboratory for microstructuring processes.

Structuring processes

For the production of PDMS-based microfluidic systems, the first step is to make a mould of SU-8 photoresist using a lithography process. These SU-8 moulds can then be used to cast PDMS structures. In many of the process steps, the polymer surfaces are additionally pretreated in a plasma.

Solution

With the microstructuring processes thus developed, fully functional microfluidic systems, such as micromixers with integrated Au electrodes for monitoring the mixing process using local impedance spectroscopy, can be produced from PDMS. It is likewise possible to produce lateral microstructures from layers of the electrically conductive polymer PEDOT:PSS.
Working with the HAFL department at BFH, local impedance spectroscopy was used to observe chronological sequences in the process of whipping cream.

Publication

Local Impedance Spectroscopy: A Potential Tool to Characterize the Evolution of Emulsions and Foams.
Michael Held, Patrick Schwaller, Markus Vaihinger and Christoph Denkel
Journal of Food Science and Engineering 7 (2017) 249-261
doi: 10.17265/2159-5828/2017.05.003