Magnetically actuated micropumps

"Lab-On-a-Chip" (LOC) systems are intended to transpose complete laboratory instrumentations on the few square centimetres of a single microfluidic chip. With such devices the objective is to minimize the time and cost associated with routine biological analysis while improving reproducibility. At the heart of these systems, a fluid delivery unit controls and transfers tiny quantities of liquids enabling the biological assays. This explains the need for robust integrated micropumps as a precondition for the development of many LOC devices. In this context, we have developed a rapid prototyping method for the fabrication of microfluidic chips in plastic and glass materials. The microfabrication principle, which is based on the powder blasting microstructuring process, was used to build devices in either polymethylmethacrylate (PMMA) or borosilicate glass. Various types of micropumps have been developed which were all based on external magnetic actuation. The use of ferrofluids (or magnetic liquids) has been the subject of the first part of the research. A piston pump using a ferrofluid plug moved by an external magnet has been studied. The integration of a rare-earth material (NdFeB) in a flexible polydimethylsiloxane (PDMS) membrane, in the form of a powder or as a classical permanent magnet, has then been proposed. An external electromagnet was used to actuate the magnet-containing diaphragm of a reciprocating micropump. Different types of valves, which constitute the critical element in reciprocating micropumps, have also been investigated. We have studied silicone membrane valves, nozzle-diffuser elements and ball valves. While nozzle-diffuser elements present the simplest valving solution from a manufacturing point of view, ball valves have been proposed as a very promising alternative due to their high efficiency. Together with the detailed characterization of the prototypes, we have proposed analytical models that predict the hydrodynamic behaviour of the micropumps. The performances of our micropumps indicate that magnetic actuation is well adapted for LOC microsystems. While we have demonstrated that our proposed microfabrication technique is an excellent rapid prototyping method for disposable plastic devices, our glass micropumps present a competitive low-cost alternative satisfying criteria of biocompatibility and high temperature (130 °C) resistance.

Rapid Prototyping of Microfluidic Devices

Meng Shen

With the growing importance of miniaturized energy applications and the development of micro Total Analysis Systems (μTAS), we have realized microfluidic devices, namely, magnetic micropumps, microfluidic fuel cells and membrane-based protein preconcentrators, all having high application potential in future. The choice of rapid prototyping microfabrication technologies and the selection of affordable materials are important aspects, when thinking of commercialization. Thus, we have employed powder blasting, polymer molding and assembly technologies during devices fabrication throughout the thesis. The first type of microfluidic device that we present is a poly(methyl methacrylate) (PMMA) ball-valve micropump with two different designs of the electromagnetic actuator, as optimized by the finite element method. The integration of a permanent magnet in a flexible polydimethylsiloxane (PDMS) membrane, which is clamped into PMMA structure, is proposed for providing a large stroke of the pumping membrane, making the micropump bubble-tolerant and self-priming Focusing on low power consumption for μTAS integration, another type of magnetic micropump with active valves is realized. It consists of a microfluidic chamber structure in glass that is assembled with a PDMS sheet, which comprises two valving membranes and a central actuation membrane, having each an integrated permanent magnet that is peristaltically actuated by a rotating arc-shaped permanent magnets assembly. A lumped circuit model is developed to predict and describe the frequency-dependent flow rate behavior for this type of pump. Powder blasting and PDMS molding rapid prototyping technologies are employed for realization of these two types of micropumps. Fuel cells with fluid delivery and removal options, having chemical reaction sites and electrode structures that can be realized in a microfluidic format, have high potential for applications. Therefore, microfluidic direct methanol fuel cells with embedded ion- permselective medium are studied and such type of fuel cell is realized by integrating a narrow Nafion strip in a molded elastomeric structure. A mechanical clamping assembly technology enables leakage-free operation and stable performance. The characterization reveals its output power density, using H2O2-based oxidant, is among the high-performance direct methanol fuel cells in microscale. Re-using the technology of the fuel cell chip, with its particular ion-permselective Nafion membrane and assembly method, we also have developed a protein preconcentrator with high purification performance. Our device can preconcentrate negatively charged biomolecules located at the anodic compartment side of the Nafion strip within only a few minutes with a high preconcentration factor. Moreover, a complex microfluidic finite element model is proposed to study and understand the physics of the preconcentration effect. Finally, we conclude the thesis with an outlook on future developments based on our work of the project and on the assembly technologies for microfluidic device integration.

EPFL2010

Développement et étude d'une technique de microsablage à haute résolution

Anne-Gabrielle Pawlowski

We have used powder blasting as a technique to structure brittle materials like glass using an erosion resistant mask. The study of the erosion rate shows that the erosion of glass is maximum at a normal incidence of the powder beam while the erosion rate of metals and elastomers is minimum. A metallic contact mask is very resistant, easy to use, but its resolution as defined by its manufacturing process (laser ablation) is limited. The resolution of the mask depends on the thickness of the metal piece. However due to the stress generated by the powder blasting technnique, the metal mask used must have an important thickness (about 0,5 mm), which limits the resolution (50 μm). The photolithographic masking method we developed allows to increase the resolution. We have demonstrated that the erosion resistance of Polydimethylsiloxane (PDMS), an elastomer very frequently used in microsystems realisation is excellent, and that the use of a photosensitve epoxy resist like SU8 allows to define high-resolution structures. The replication quality is good as the majority of structures defined in PDMS have the same size compared to the features defined by the SU8. In our process, the SU8 is patterned into a mould structure that is filled with PDMS. The SU8, has a similar erosion behavior as glass and is quickly removed by powder blasting. The resolution of the process determined by the particle size ; we have used particles with 10 μm diameter to explore the ultimate limits of powder blasting. The study of the etching by powder blasting shows that the etching rate changes with the width of the channel and the etching time. Furthermore, dimensions of channels or hole structures change during powder blasting, they are increased by the underetching effect. This underetching depends on the depth of the channels and can induce changes in resolution.The erosion mask resistance is function of the thickness of the mask, of the etching time and of the applied pressure. We typically have chosen a PDMS mask thickness of 50 μm. When the applied pressure of the powder jet increases, the PDMS erosion is more important. When an energetic particle impacts on the PDMS, the glass underneath the mask can be damaged and the resolution of the structures is decreased. Experimentally, this PDMS/SU8 mask is resistant and efficient for an applied pressure up to 200 kPa and for a glass erosion depth of 300 μm for large channels. The minimum feature size is 20 μm, but this depends also on the shape ; for example triangular holes are only well defined for a minimum dimension of 60 μm.

EPFL2006

A PMMA valveless micropump using electromagnetic actuation

Martinus Gijs, Christophe Yamahata

We have fabricated and characterized a polymethylmethacrylate (PMMA) valveless micropump. The pump consists of two diffuser elements and a polydimethylsiloxane (PDMS) membrane with an integrated composite magnet made of NdFeB magnetic powder. A large-stroke membrane deflection (~200 µm) is obtained using external actuation by an electromagnet. We present a detailed analysis of the magnetic actuation force and the flow rate of the micropump. Water is pumped at flow rates of up to 400 µl/min and backpressures of up to 12 mbar. We study the frequency-dependent flow rate and determine a resonance frequency of 12 and 200 Hz for pumping of water and air, respectively. Our experiments show that the models for valveless micropumps of A. Olsson et al. (J Micromech Microeng 9:34, 1999) and L.S. Pan et al. (J Micromech Microeng 13:390, 2003) correctly predict the resonance frequency, although additional modeling of losses is necessary.