MECHANICAL AND THERMAL MODELING OF A
PARYLENE ELECTROTHERMAL VALVE FOR
MAPPING BRAIN FUNCTION IN FREELY MOVINGSUBJECTS
12Po-Ying Li1, Tina K. Givrad2,Daniel P. Holschneider3, Jean-Michel I. Maarek2, and Ellis Meng2

Department of Electrical Engineering, University of Southern California, USA
Department of Biomedical Engineering, University of Southern California, USA
3Department of Neurology, University of Southern California, USAABSTRACT
Mechanical and thermal modelling of the first normally-closed Parylene valve
operating on electrothermal principles for rapid and wireless drug delivery was investigated. The modelling and experimental results confirm the mechanical robustness of the valve membrane. Valve opening behavior obtained by transient thermal
finite element analysis are in accordance with the experimental microscopic images.
KEYWORDS: Parylene, Electrothermal valve, Neuroimaging, MIP
INTRODUCTION
A low power, fast response normally-closed Parylene valve operating on electrothermal principles for rapid and wireless drug delivery was developed to be used in
an implantable microbolus infusion pump (MIP) for neuroimaging in freely-moving
untethered animals [1]. Functional brain activation is achieved by rapid MIP injection of radiotracer into the circulation following the desired behavioral event, euthanasia administration, brain retrieval, autoradiographic imaging of brain slices, and3D image reconstruction. Previously, normally-closed electrothermal valves havebeen developed but only one is suited for implantable drug delivery [2]. This valve
consists of a resistive metallic membrane (Pt/Ti/Pt) suspended over a drug reservoir
that opens when an applied current melts the membrane (Fig. 1). Here, a Parylene
C/Pt composite membrane is used as our valve material to significantly reduce
power consumption since Parylene C can be thermally degraded at much lower temperature (125-200°C) and can form large membranes for larger effective valve opening area (330-500 μm).
THEORYOur valve consists of a platinum resistive element embedded in a flexible Parylene C membrane (10 μm) (Figs. 1a and b). The valve is situated in the lumen of acatheter and two contact pad flaps are wrapped along the catheter for connection tocontrol circuitry (Fig. 1c and d). RF power remotely triggering the valve is applied
to initiate Joule heating to thermally degrade or melt the Parylene membrane. Whenthe electrical and mechanical connections are broken, the pressurized reservoir
drives rapid release of fluids through the catheter and into the circulation.

Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
978-0-9798064-1-4/µTAS2008/$20©2008CBMS1105

Figure 1. Parylene MEMS electrothermal valve: (a) close-up view, (b) device
view, (c) assembly process, (d) operation principle
EXPERIMENTAL
The Pt element and membrane design must account for instantaneous peak pressure from the reservoir (1 atm); their mechanical performance was verified using
both large deflection approximation (Parylene only) and nonlinear finite element
modeling (FEM) (Parylene/Pt/Parylene) (Fig. 2). Large deflection theory provides asimplified model to investigate the mechanical performance of the Parylene vavle(Fig. 2a). In nonlinear FEM, a circular clamped Parylene/Pt/Parylene composite
membrane was subjected to uniform applied pressure (2.5 to 15 psi) (Fig. 2b) andthe membrane deflection and stress distribution were obtained. These results werethen verified in benchtop experiments. Transient thermal FEM of valve operation
was simulated with a 6 mA applied current and thermal contour plots were captured
every 133 msec. An experimental comparison was also obtained for the same conditions.
RESULTS AND DISCUSSION
The nonlinear mechanical model indicates the maximum stress of the Pt element
under 1 atm pressure (peak under normal operation) was 1.53GPa which is less thanits tensile strength (1.83GPa); the electrical connections are expected to survivepressurized conditions during radiotracer loading. Critical stress points in the membrane were identified in the stress distribution diagram. The nonlinear FEM modelshows good agreement with load deflection experiments (composite membrane)
demonstrating the mechanical robustness of the valve to the operating conditions forthe neuroimaging application (Fig. 2b).
Valve opening depends on thermal events which were simulated and captured
using time sequence microscopy(Fig. 2c). At 133 msec, a circular thermally oxdized
Parylene area was visually observed. The transient FEM results indicated that themajority of the valve area reached over 125°C, thermal oxidation initiation temperature of Parylene. At 266 msec, Parylene began melting and small openings between
Pt thermal elements formed and the Parylene melting temperature of 290°C wasreached in the central region in FEM. By 400 msec, the valve opening was nearlycomplete and most of the valve area exceeded the Parylene melting temperature in
FEM simulation. Both the expeimental and the transient FEM modeling images
demonstrate the valve opening mechanism. However, to achieve complete valve
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA1106

opening, the entire valve area must exceed the Parylene melting temperature. A
more efficient design is underway in which the Pt trace geometry is modified to obtain lower power consumption, higher opening efficiency, and higher heat transfer
efficiency into Parylene membrane instead of the surrounding packaging and fluid.

Figure 2. Modeling and experimental results: (a) equations for large deflectionapproach, (b) FEM model and results (deflection and stress distribution), (c) comparison of mechanical modelling and experimental results, and (d) experimentallyobtained microscope images compared to transient thermal modeling.
CONCLUSIONS
We successfully developed a disposable, low-power Parylene C MEMS valve toenable new neuroimaging approaches. The modeling and benchtop characterization
were performed and compared. Mechanical modeling demonstrated valve robustness to operation under 1 atm peak pressure. The themal modeling results suggest
an improvement to lower power consumption by redesigning the Pt trace. Also, additional experiments will evaluate the performance of the valve in the presence ofthe radiotracer agent and in vivo in preparation for neuroimaging experiments.
ACKNOWLEDGEMENTS
This work was funded in part by the NIBIB (1 R01 NS050171). The authors
would like to thank Dr. Donghai Zhu, Dr. Tuan Hoang, Mr. Neil Sardesai, and members of the Biomedical Microsystems Lab at University of Southern California.
REFERENCES[1] D.P. Holschneider, J.-M.I. Maarek, J. Harimoto, J. Yang, O.U. Sremin, An Implantable Bolus Infusion Pump for Use in Freely Moving, Nontethered Rats,
American Journal of Physiology 283 H1713 (2002).
[2] P.-Y. Li, T.K. Givrad, D.P. Holschneider, J.-M.I. Maarek, E. Meng, A Wirelessly-Activated Parylene Electrothermal Valve for Mapping Brain Function inFreely Moving Subjects, Solid-State Sensors, Actuators, and Microsystems
Workshop, Hilton Head Island, SC, pp. 32-35, (2008).

Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA1107