Characterization of pneumatically activated microvalves by measuring electrical conductance

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Abstract

Accurate determination of the mechanical response of pneumatic activated microvalves is important for the optimization of a number of microfluidic elements. In the case of multi-layers PDMS devices, pneumatic activated valves are used in combination for pumping, mixing and other functionalities. In such devices, the mechanical response of a valve depends on the PDMS toughness, the membrane thickness as well as the detail of the channel configuration and the pressure applied in both flow and control channels. We describe characterization technique of a pneumatic activated valve based on conductance measurements of electrolytes in the microfluidic channel. We also show that the well calibrated microvalves can be used for the development of more complex functionalities such as high precision dose control of chemical solutions.

Introduction

Pneumatic microvalves can be easily produced on chip based on multi-layer soft lithography techniques [1], [2]. With elastic polymer such as polydimethylsiloxane (PDMS), two microchannels can be made in a cross-configuration separated with a thin film of PDMS. Applying a pressure in one of the channels, the PDMS film is deformed so that the cross-section of the adjacent channel is changed, resulting an effective actuation or valving. Several other functionalities including peristaltic pumping and solution mixing can also be obtained based on the same principle of PDMS thin film actuation [1]. This has lead to a successful integration of more than 1000 microfluidic elements on a chip in the same way of microelectronic integrated circuits [3]. Similarly, advanced microfluidic devices can then be fabricated for chemical and biological applications [4], [5], [6]. In all cases, accurate determination of the mechanical properties of pneumatic activated membranes is important for the optimization of microfluidic elements. In this work, we study the response of the PDMS microvalves by measuring the conductance of an electrolyte solution through the microvalve section controlled pneumatically [7], [8]. By changing systematically the pressure applied in the flow and control channels, we were able to determine the optimal condition for actuation which should be useful for the improvement of performances of several types of microfluidic components.

Section snippets

Experimental

Multi-layer soft lithography has been used for the rapid prototyping. Fig. 1 shows a schematic presentation and a microphotograph of a microfluidic device used in this work. It consists of a two layer PDMS microchannel structure bonded on a glass substrate. Both the flow and the control channels are designed to have an initial channel width of 100 μm and a height of 10 μm. To obtain these channels, molds were fabricated by optical lithography with SJR 5740 (from Shipley) and SU8 2010 (from

Results and discussion

Firstly, we study the dependence of the mechanical properties of microvalves on the pressure in both control and flow channel in a static mode. In Fig. 2 is shown the current measured between the two electrodes as a function of the control pressure for several values of the pressure presented in the flow channel. As can be seen, higher the pressure in the flow channel, higher the pressure is required in the control valve for a perfect closing. For zero pressure applied in the control channel,

Conclusion

We have performed a systematic investigation on the pressure actuation of PDMS thin film in microfluidic configurations. The conductance measurement appears to be suitable for the characterization of dynamic response of pneumatically actuated microfluidic elements. We found that the surface adhesion at low actuation frequencies influences the PDMS membrane opening time. Clearly, the response of the PDMS membrane is delayed respect to the switch off of the applied control pressure and once the

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