16-01-2014, 12:43 PM
SIMULATION OF MEMS PIEZOELECTRIC MICROPUMP FOR BIOMEDICAL APPLICATIONS
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Abstract
In this study, we demonstrate the usefulness of Finite Element Analysis (FEA) and simulation techniques in the design of MEMS micropumps. Such pumps provide for the handling of milliliter-scaled fluid volumes desired in many lab-on-a-chip chemical and biomedical applications. This work is focused on a micropump driven by the piezoelectric effect, which in turn invokes the dominant resonance behavior. Because the design of the device is the emphasis of this study, the model was originated in CAD and includes the fine-scale geometric details commonly encountered in a wide variety of micropumps. The model considered in this study is a rectangular micropump with a piezoelectrically actuated diaphragm on its top and two valves on its bottom. The mechanical efficiency of the pump hinges on using resonance to generate sufficient motion of the diaphragm. Mechanical Event Simulation (MES) commercial software from ALGOR was utilized to simulate this motion, and thus provide a method for optimizing the design. The results show that consideration needs to be given to the voltage-driving frequency because of its effect on the pump performance and the stress levels within it.
Introduction
The advent of micro fabrication methods has been used to manufacture a wide range of miniature pumps. These micropumps find their greatest application 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 many 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. This is a highly desirable feature, which could certainly have a significant impact on the multi-million, worldwide market for insulin delivery systems. Obviously, other medical markets exist for micropumps, with cancer treatments being the most prominent.
Theory
The dynamic nature of these pumps prompts us to consider a nonlinear FEA simulation capable of calculating stresses caused by both deformation as well as by inertial effects. One may argue that inertial effects should be insignificant at the length scales associated with micropumps, which tend to be on the order of a few micrometers. However, because of the high oscillation rates achieved by these devices, a proper design should account for inertial effects. A geometric nonlinear analysis is also required because of the relatively large geometric changes experienced by some components within these micropumps. In the micropump presented in this paper (see Figure 1), the diaphragm and valve flaps experience the largest deformation, and thus stresses.
The method of operation of this pump is to use the piezoelectric effect to excite the diaphragm at its first natural frequency. The resulting large-scale motion pumps the fluid through the pump chamber, with the inlet and outlet valves passively undergoing oscillatory movements. The resonant motion of the diaphragm, which is bonded to the piezoelectric component, makes its stresses critical to the design of the micropump.
Dynamic Analysis of the Micropump
The primary goal of this study was to develop a procedure to incorporate reliability considerations into the design of micropumps actuated using piezoelectric components. The first important step towards ascertaining the reliability of a pump design is to focus on the stresses experienced by the pump during its operation. Because the focus was to only consider the stresses experienced by the diaphragm, it was possible to avoid incorporating the valve flaps in the analysis. The remaining components of the micropump were included in the study.
Conclusions
In the current study, FEA is used to simulate the micropump operating conditions and investigate the design constraints for a displacement micropump actuated with a multi-layer piezoelectric material. In this pump model, the dominating multiphysics were simulated using electrostatics and nonlinear dynamics. A solution strategy coupling both of these analyses is applied using the commercial FEA software, ALGOR. In the time domain, a nonlinear geometric analysis is considered due to the large-scale deformation of the pump diaphragm. In addition, inertial effects are also considered because of their significant impact on the dynamic response of the micropump diaphragm during resonance. The maximum displacement and resulting stresses are calculated within a frequency range that contains the first five modes of the pump diaphragm. In terms of displacement, it is shown that the best performance is achieved when the pump is excited at its 1st natural frequency. This excitation will induce the maximum stress near the edge of the actuated diaphragm. To assure pump reliability for high cycle fatigue, it is, therefore, necessary to design this pump so that the maximum stress level is kept lower than the stress endurance limit of the diaphragm material. This requirement is vital for many types of micro devices considering the role micropumps play in sustaining the reliability of MEMS for biomedical applications, such as lab-on-a-chip devices.