HIGH PERFORMANCE MICROPUMPS UTILIZING MULTILAYER PIEZO ACTUATORS T. Lemke1*, G. Biancuzzi1, C. Farhat1, B. Vodermayer2, O. Ruthmann3, T. Schmid2, H.-J. Schrag3, P. Woias1, F. Goldschmidtboeing1 1 FluidicMEMS Group, Laboratory for Design of Microsystems, Department of Microsystems Engineering, University of Freiburg, IMTEK, Georges-Koehler-Allee 102, Freiburg 79110, Germany 2 German Aerospace Center, Institute of Robotics and Mechatronics, Robotic Systems, Münchner Straße 20, Oberpfaffenhofen-Wessling 82234, Germany 3 Artificial Sphincter Study Group, Department of General and Visceral Surgery University Hospital, Hugstetter Straße 55, Freiburg 79110, Germany
*Corresponding author: Thomas Lemke, Phone: +49 (0) 761 203-7498, FAX: + 49 (0) 761 203-7492,
[email protected]
Abstract: We report on our latest results using PZT multilayer piezo-actuators for an advanced micropump. The piezo actuators were glued onto silicon membrane structures. Therefore a specially developed conductive epoxy resin was used. We have measured pump rates up to 4.36 ml/min @ 35 Hz for V= -15...+45 V. Furthermore the driving voltage of the piezo-actuator could be decreased dramatically to 30 Vpp. FEM simulation results of the membrane deflection were derived and compared to measured results. Keywords: micropump, piezo, multilayer
INTRODUCTION The application of micro actuator systems in medical implants [1], [2] is frequently hindered by their failure to comply with the high reliability requirements of these systems. In particular, their operation voltage has to be reduced to 60 V DC or 40 V AC [3] in a medical implant to ensure patient safety. While this is already a problem for several actuation principles, the too high energy consumption is another severe drawback that hinders an acceptable battery lifetime of the implanted system. Piezoelectric actuation has not found its way into medical implants from both reasons mentioned above. As mono- or bilayers these actuators will typically require AC driving signals above 100 V that have to be generated, with bad conversion efficiency, from low battery voltages. On the other hand, the high mechanical performance, the good scaling behaviour and the long lifetime of these actuators make them promising candidates for an application in medical implants. We have therefore conducted a systematic study to improve piezoelectric membrane actuators for a peristaltic micropump with regards to driving voltage and power consumption. We have applied a specially designed monolithic multilayer actuator for this purpose. These multilayer piezoactuators were mounted onto a silicon diaphragm to achieve a lowvoltage membrane actuator as used in previous studies with high-voltage monolayer and bilayer piezoactuators [4], [5]. The stroke of our new system was similar to these devices with a drastically decreased actuation voltage. In a
medical implant, this voltage has to be generated by a DC-DC converter from a low battery voltage. A decrease of the actuation voltage will increase the conversion efficiency and, hence, the battery lifetime [6]. As a third prerequisite, the reliability of the multilayer actuators has to be high. We have used a proven gluing process to mount the multilayer piezoceramics onto the silicon diaphragms [6] which delivers a high reproducibility, maximum stiffness and long term stability for our high performance micropumps driven at dramatically decreased voltages.
ASSEMBLY PROCESS We have used customized PZT multilayer piezo actuators from Morgan Electro Ceramic. The actuators consists of 9 layers each with a layer thickness of 15 µm (see a cross section SEM picture in Figure 1). PZT layers from PZT-506 were tape casted, printed with unfritted metal and afterwards co-fired. External metallization had been done through evaporation of chrome, nickel and gold [7]. The overall size of the actuators was 7x7 mm2 for the valve membrane and 10x10 mm2 for the pumping membrane respectively. As discussed in [8] and [6] the adhesion layer between the membrane and the piezo-actuator is crucible for the performance of plate bending actuation principles and has to fulfil different requirements such as high storage modulus (see Figure 4) for a maximum coupling and therefore a high plate bending and a good electrical conductivity for use as a bottom electrode. Both requirements were achieved by the use of a two compound epoxy-resin with a viscosity
589
of 160 cPas in the pre-mixed state, filled with electrically conductive graphite particles. The measured storage modulus of the glue layer was 2.7 GPa. The epoxy resin was filled with graphite nanoparticles (CTAB surface of 136mm2/g) in a ratio of 0.2 weight percent to achieve a sufficient electrical conductivity. Epoxy resin and graphite nanoparticles were stirred under controlled conditions to get a homogenous mixture containing electrically conductive agglomerates. The silicon membrane was covered with glue with a micropipette. A 10 µm thin glue layer between the PZT and membrane was produced by accurately controlling the amount of epoxy fluid covering the membrane. Afterwards the piezoactuator was mounted and thermal cured at 100°C for 15 min in a furnace.
the silicon valve membrane (7.38x7.38 mm2) and the pumping membrane structure (11.38x11.38 mm2) was modelled using quarter symmetry. Between the silicon membrane and the 9 layer multilayer PZT actuator a glue layer interface with a thickness of 10 µm was inserted. Changing polarization directions of the 9 piezo layers were modelled with the help of a rotating element coordinate system. Piezo material data were supplied by Morgan Electro Ceramic [6]. No intermediate electrodes were modeled due to their small thickness and therefore high overall aspect ratios. Proper boundary and symmetry conditions were applied onto the whole model. First the influence of a changing storage modulus of the glue layer to the membrane deflection had been determined. Driving voltages ranging from Vpp= -20...+45 V were applied onto the piezo layer`s top areas to study the deflection of the membrane at the center position. We also simulated the pressure dependent deflection of the membrane (without any driving voltage) by applying pressure ranging from 100 to 600 mbar onto the silicon membranes underside. A value integral over the displaced backside membrane area yields the displaced volume. Once these values were determined for changing membrane geometries (e.g. KOH etch depth and ICP backetch depth, etc.), the actuation behaviour could be described in detail.
SIMULATION RESULTS Figure 1. Cross section SEM image of the advanced multilayer actuator.
Aluminum wedge-wedge wire bonding (Delvotec 2512 Bonding tool) of the actuator and bond board was done to connect ground and signal electrodes properly (cf. Figure 2).
As it can be seen from Figure 3 the influence of the storage modulus of the glue layer plays a decisive role. Three different regimes could be identified. The first region, shows a very low storage modulus (
