Simulation of MEMS Piezoelectric Micropump for Biomedical Applications

ALGOR, Inc., Pittsburgh, Pennsylvania

Since the advent of Micro Electro Mechanical Systems (MEMS) technology, microfabrication methods have been used to manufacture a wide range of miniature pumps. These micropumps find their greatest use in chemical and biomedical applications requiring the transport of small, accurately measured liquid quantities. When utilized in chemical applications, micropumps are often a component of a lab-on-a-chip device. Such devices are envisioned as providing for reasonably inexpensive, possibly even disposable, means to conduct laboratory experiments.

The same technology is utilized in biomedical applications, where micropumps can be used to administer small amounts of medication at regular time intervals. One recent key application of micropumps is to provide a means to deliver insulin to diabetic patients, thus providing an alternative to injections. Such types of micropumps can be programmed to administer insulin at a constant rate throughout the day, thus eliminating any surges or deficits of the drug in the patient's bloodstream.

Recently, a collaborative project between the University of Alberta and ALGOR, Inc. used finite element analysis (FEA) to determine the stress levels experienced by a micropump design based on a device manufactured by the Insititut fur Festkoerpertechnik (IFT) in Munich, Germany. The micropump measured 6000 x 6000 x 1000 μm for the length, width, and height, respectively, and its housing was composed of silicon nitride (Si3N4). The pump was driven by a multi-layer lead zirconate titanate (PZT) piezoelectric component bonded to a moving diaphragm that was 10 μm thick, which in turn forced fluid through a small chamber. The study examined how the dynamic stresses caused by the deforming diaphragm affected the pump effectiveness, durability, and reliability.

The researchers modeled the pump geometry in Pro/ENGINEER mechanical CAD software and used ALGOR's InCAD technology to seamlessly capture the model for analysis in ALGOR. Automatic mesh generation capabilities were used to create a finite element mesh of three-dimensional, 8-node brick elements.

First, an electrostatic analysis was performed to obtain the voltage distribution that was used to excite the piezoelectric component. A 200V load was applied on each of the ten layers of the PZT component. The bottom of the micropump was grounded so that the resulting voltage distribution was approximately zero everywhere except on the PZT component.

Next, a natural frequency (modal) analysis was conducted to determine the natural frequencies for the micropump. By examining the corresponding mode shapes, it was determined that exciting the micropump at its first natural frequency (118.47 Hz) would result in the greatest volume change. A linear static stress analysis was then performed in which the voltage distribution from the electrostatic analysis was used to determine the displacement magnitude from the piezoelectric effect.

Finally, a Mechanical Event Simulation (MES), which combines motion and stress analysis, was performed to obtain a history of the motion of the diaphragm and the resulting nonlinear transient stresses. During the event, the pump's bottom surface was fully constrained and the diaphragm was loaded with the displacement magnitude calculated from the linear static stress analysis, and a loading history determined from the exciting frequency. This value simulated the load induced by the voltage applied to the PZT component when oscillated at a frequency that maximized the motion of the diaphragm as well as the flow rate through the pump.

ALGOR's Monitor utility charted the time history of the vertical displacement of a point on the top of the PZT component. This displacement illustrated the expected effect of resonance at the first mode of vibration. The maximum stresses occurred near the edge of the diaphragm, where the largest bending moments exist. These maximum stresses were obtained at time 0.096 second, which was slightly shifted from the time point at which the maximum displacement was obtained: 0.092 second. This time shift between the calculated peaks of the stresses and the displacements was due to the influence of the inertial effects of the diaphragm. MES automatically accounted for these dynamic effects.

The results of the study indicated that driving the micropump near its first natural frequency would induce the maximum vertical motion, but also the maximum stress near the edge of the actuated diaphragm. The researchers concluded that, to ensure pump reliability for high-cycle fatigue, it was necessary to design the pump so that the maximum stress level was kept lower than the stress endurance limit of the diaphragm material.

This work was done by faculty at the University of Alberta in collaboration with ALGOR, Inc. For further information, visit UoAtechnology.ALGOR.com. For more information on using ALGOR's finite element analysis products, contact ALGOR, Inc., 150 Beta Drive, Pittsburgh, PA 15238; Tel. 1.800.48.ALGOR; or visit the Web site at www.ALGOR.com 

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