A PMMA valveless micropump using electromagnetic actuation
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- Yamahata, C., Lotto, C., Al-Assaf, E. et al. Microfluid Nanofluid (2005) 1: 197. doi:10.1007/s10404-004-0007-6
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.
KeywordsDiffuser micropumpLab-on-a-chipPowder blasting
Since the introduction of the concept of micro total analysis systems or µTAS in 1990 (Manz et al. 1990; Manz et al. 1991), multiple fluidic microchip technologies have been developed. A continuous challenge has been the transport and pumping of small quantities of biological fluids, of the order of a few microliters per minute. The first development of a micropump dates from the 1980s with the emergence of the field of microelectromechanical systems (MEMS). Van Lintel et al. (1988) and Van de Pol and Van Lintel (1990) presented a silicon micropump based on the piezoelectric or thermopneumatic actuation of a thin membrane. Since then, other integrated silicon-based micropumps, mostly based on piezoelectric actuation, have been proposed (Nguyen et al. 2002; Tay and Choong 2002). However, for many µTAS applications, microfluidic systems need to be disposable and low-cost rendering plastic materials more competitive. Plastic micropumps have already been shown to have great potential (Olsson et al. 1998; Böhm et al. 1999; Santra et al. 2002; Nguyen and Truong 2004). Innovative methods for fluid handling were simultaneously developed, such as the use of magnetic materials for the micropump actuation. For example, magnetic liquids or ferrofluids simultaneously provide the actuation and valving functions in a micropump. A ferrofluidic actuated pipette was presented by Greivell and Hannaford (1997). Hatch et al. (2001) and Yamahata et al. (2004) reported a micropump based on the pumping and valving action of a ferrofluidic plug in a microfluidic channel. Alternatively, magnetic actuation of a membrane with integrated magnetic parts has been shown to produce large forces (a few hundred μN) and a large membrane deflection (Böhm et al. 1999; Santra et al. 2002; Zhang and Ahn 1996; Liu 1998; Khoo and Liu 2000). Such an approach combines the benefits of the strong magnetic force with the potential of external electromagnetic actuation; moreover, direct contact of the actuating element with the fluidic circuit is avoided. The use of polymer-bonded magnets directly integrated with the membrane (Lagorce et al. 1999; Santra et al. 2002; Cho and Ahn 2003) offers additional miniaturization potential. Valves are also key elements of a micropump: ball valves (Accoto et al. 2000), normally closed check valves (Nguyen and Truong 2004; Yamahata et al. 2004), valves based on the modification of physical properties of liquids (Beebe et al. 2000) as well as Tesla elements (Forster et al. 1995; Morris and Forster 2003) have been proposed. A major advance in valveless pumping was demonstrated by Stemme and Stemme (1993) who substituted check valves by nozzle/diffuser elements with fluid-directing effects. Follow-up work by Olsson et al. (1995) and Olsson (1998) on the realization of in-plane nozzle/diffuser elements triggered great interest in the field of microfluidics (Gerlach et al. 1995; Gerlach 1998; Olsson et al. 1999; Nguyen and Huang 2001; Pan et al. 2003). Nozzle/diffuser micropumps are of particular interest for disposable µTAS applications because of the simple realization of the diffuser element that significantly lowers fabrication costs. Additionally, the use of the electromagnetic actuation principle, external to the pump, and the use of plastics appear to be appropriate solutions for the requirements of disposable devices.
In this paper, we describe the fabrication and characterization of an electromagnetically actuated polymethylmethacrylate (PMMA) valveless micropump. It consists of two diffuser elements and a polydimethylsiloxane (PDMS) membrane with an integrated composite magnet made of NdFeB magnetic powder. Diffuser theory is combined with numerical simulation tools to correctly dimension the micropump. Our micropump is a three-dimensional microfluidic structure consisting of four layers of PMMA, which are microstructured by powder blasting or by standard mechanical micromachining techniques. We also discuss the microfabrication of the composite magnetic membrane. A large-stroke membrane deflection (~200 μm) is obtained using external electromagnet actuation. We present a detailed analysis of the magnetic actuation force and the frequency-dependent flow rate of the micropump and analyze our experimental data using the theoretical models of Olsson et al. (1999) and Pan et al. (2003).
2 Design and theory
3 Numerical simulation
4 Microfabrication technology
4.1 Magnetic membrane
4.2 Microfluidic chip
5 Characterization of the micropump
5.1 Magnetic membrane and electromagnet
We measured the magnetic induction generated by the permanent magnet membrane using a Hall probe and obtained a value of 0.09 T at the surface of the magnet. From this value we derived a z component of the magnetization Mz≈1.44×105 A.m–1, taking into account the demagnetization factor of the permanent magnet (~0.5), the polymer magnet being characterized by a diameter of 5 mm, a height of 2.2 mm and a volume of 4.3×10−8 m3. We can now evaluate Eq. 7 considering the magnetic field generated by the electromagnet at the center of the polymer permanent magnet (see Fig. 8). For example, when the polymer magnet is in contact with the soft iron core of the electromagnet, its center position is at z=1 mm providing a force of about 100 mN. During normal pumping operation the middle of the polymer magnet is 3 mm away from the soft iron core and the magnetic membrane deflection is not larger than 0.25 mm, giving a typical magnetic force of 75 mN. For a PDMS membrane area in contact with the liquid of 3.8×10−5 m2 this corresponds to an effective actuation pressure of ~20 mbar. This value is of the same order as the measured backpressure of the micropump (see further).
5.3 Resonance frequency
By monitoring the membrane deflection as a function of pressure, the membrane stiffness was measured to be Kp=6 MPa. Kv is the ratio of the pumping chamber volume variation amplitude to the deflection of the membrane at the center, which we estimated to be 1.96×10−5 m2<Kv<3.85×10−5 m2. The resonance frequency calculated with Eq. 8 was estimated to be 14 Hz<f0<20 Hz for water, which is in good agreement with the measured resonance frequency of 12 Hz (see Fig. 12). Note that the efficiency η, while important for giving a high flow rate, has a minor influence on the resonance frequency.
Using Eqs. 10 and 12, one finds that Eqs. 8 and 13 are equivalent, demonstrating that Eq. 8 is a particular case of Eq. 9. Putting in the various parameters into Eqs. 9 and 10, with ρ=1.3 kg m−3 for air, we find that the membrane resonance frequency f1~200 Hz, and the experimental pumping resonance frequency for air f0~180 Hz is close to this frequency (see Fig. 13).
5.4 Frequency-dependent flow rate
For our micropump, losses take place in the diffusers, in the channels, and in the membrane. These multiple sources complicate the estimation of the parameter ξ. The following values were used for the fitting curve of Fig. 12: α=0.2, δstatic=170 μm, ξ=35, fo=12 Hz, f1=200 Hz; while for the curve of Fig. 13, we find that α=0.2, δstatic=55 μm, ξ=0.7, fo=180 Hz, f1=200 Hz, in good agreement with the geometrical parameters of the micropump. We note that the damping effect is more important for water than for air, which can be related to the higher dynamic viscosity of water (μwater=1.0×10−3 Ns/m2 at 20°C) compared to air (μair=1.8×10−5 Ns/m2 at 20°C). For comparison, if we fit the theoretical curves of Pan et al. (2003) with the damped forced oscillator model, we find for the dimensionless loss coefficient ξ=0.35 and 0.007 for water and air, respectively. Our experiments are the first ones that directly demonstrate the damped resonance behavior of a nozzle/diffuser micropump. Although resonance frequencies are correctly predicted by the models of Olsson et al. (1999) and Pan et al. (2003), the damping behavior is strongly underestimated in the theory of Pan. Indeed, to correctly fit our experimental data, the losses in the theory of Pan need to be overestimated by a factor 100. A possible explanation for the discrepancy between our experimental data and the model of Pan et al. (2003) might be the importance of losses in the complete microfluidic circuit (and not only in the nozzle/diffuser elements), in particular losses caused by squeezed film damping in our pumping chamber, which has a vertical dimension of the order of the membrane deflection amplitude.
6 Conclusion and outlook
We have designed and experimentally realized nozzle/diffuser structures for application in a PMMA micropump. The combination of the nozzle/diffuser elements with an electromagnetically actuated PDMS membrane, characterized by a large deflection amplitude, resulted in a self-priming micropump with which we successfully pumped both water and air. Its flow rate–backpressure performance has been characterized and the micropump resonance frequency showed good agreement with the theoretical models of Olsson et al. (1999) and Pan et al. (2003). The frequency-dependent flow rate can be well understood by a fluidic damped oscillator model. Our results indicate that electromagnetic actuation is a simple actuation method that presents a serious alternative to piezoelectric actuation. In particular, we anticipate that the technology presented above can be advantageously applied for lab-on-a-chip applications where low cost and/or disposable aspects are of primary importance. Future developments include an enhancement of the maximum operation pressure by increasing the volume of the magnet, by using powders with improved magnetic characteristics or by creating an electromagnetic actuation circuit with higher effective permeability.
The authors gratefully acknowledge the Swiss Commission for Technology and Innovation for financially supporting this project (Project CTI-Medtech 4960.1 MTS) and Dr D. Solignac and Dr A. Donzel for useful discussions.