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A Parylene MEMS Electrothermal Valve

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A Parylene MEMS Electrothermal Valve
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  1184 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 6, DECEMBER 2009 A Parylene MEMS Electrothermal Valve Po-Ying Li,  Member, IEEE  ,  Member, ASME  , Tina K. Givrad,  Member, ASME  , Daniel P. Holschneider,Jean-Michel I. Maarek, and Ellis Meng,  Senior Member, IEEE   Abstract —The first microelectromechanical-system normallyclosed electrothermal valve constructed using Parylene C is de-scribed, which enables both low power (in milliwatts) and rapidoperation (in milliseconds). This low-power valve is well suitedfor applications in wirelessly controlled implantable drug-deliverysystems. The simple design was analyzed using both theory andmodeling and then characterized in benchtop experiments. Oper-ation in air (constant current) and water (current ramping) wasdemonstrated. Valve-opening powers of 22 mW in air and 33 mWin water were obtained. Following integration of the valve withcatheters, our valve was applied in a wirelessly operated microbo-lus infusion pump, and the  in vivo  functionality for the appropri-ateness of use of this pump for future brain mapping applicationsin small animals was demonstrated. [2008-0322]  Index Terms —Drug delivery, electrothermal valve, Parylene C,wireless operation. I. I NTRODUCTION I NTHEPASTdecade,theuseofParylene[poly(  p -xylylene)]as a structural material in microelectromechanical systems(MEMS) devices has attracted significant attention, particu-larly for implantable drug-delivery systems that integrate sens-ing, pumping, and valving [1]–[3]. Parylene C, known forits biocompatibility, is widely used in implantable medicaldevices. Parylene C is also compatible with MEMS microfab-rication processes. Advanced drug-delivery systems can ben-efit from the excellent mechanical and electrical properties of Parylene C.Electrothermal valves are critical flow-regulating elements inmany microfabricated drug-delivery devices (Table I) [4]–[10].Valve operation is analogous to that of electrical fuses, and theprimary component is a resistive metallic element (typicallyplatinum or gold). The electrothermal valve is normally closedand single use. The valve-membrane blocks the flow pathuntil it is activated, at which point it is thermally removed. Inearlier work, metal, silicon, or silicon nitride composites were Manuscript received December 30, 2008; revised May 21, 2009. Firstpublished October 20, 2009; current version published December 1, 2009. Thiswork was supported in part by the National Institute of Neurologic Disordersand Stroke under Grant 1 R01 NS050171. Subject Editor R. Ghodssi.P.-Y. Li is with the Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089 USA (e-mail: poyingli@usc.edu).T. K. Givrad and J.-M. I. Maarek are with the Department of BiomedicalEngineering, University of Southern California, Los Angeles, CA 90089 USA(e-mail: givrad@usc.edu; jmaarek@bmsr.usc.edu).D. P. Holschneider is with the Departments of Psychiatry and BehavioralSciences, Neurology, Cell and Neurobiology, and Biomedical Engineering,University of Southern California, Los Angeles, CA 90033 USA (e-mail:holschne@usc.edu).E. Meng is with the Departments of Biomedical and Electrical Engineering,University of Southern California, Los Angeles, CA 90089 USA (e-mail:ellis.meng@usc.edu).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JMEMS.2009.2031689 investigated as electrothermal membrane materials [4]–[6].Whenelectricalpowerwasapplied,theresistiveelementmeltedthe thin-film membrane of the valve to open the flow path.However, significant power was required to generate the melt-ing temperature for these materials. By substituting with alow-melting-temperature polymer, power consumption is sig-nificantly decreased to improve valve performance. A fewelectrothermal valves employing polymer membranes, such asuncross-linked SU-8, polyethylene/polyethylene terephthalate,polydimethylsiloxane, and polymethylmethacrylate, were re-ported [7]–[10]. These polymer valves tend to take longer toopen compared to nonpolymer valves [4]–[6]. By decreasingthe thickness, the valve activation time for polymer valvescan be reduced. For implantable drug-delivery applications,valves must be biocompatible [11]. Only one existing valvehad biocompatible construction. However, due to the metallicmembraneconstruction(PtorTi),thevalvealsohadhighpowerconsumption (2.25 W) [4].Here, Parylene C (a biocompatible polymer [2], [3], [12]) isselected as the heater-supporting membrane material. Parylenecan be thermally oxidized, degraded (between 125  ◦ C and200  ◦ C) [13], or melted (290  ◦ C) [14] at lower temperaturesthan previously used polymers and enables a low-power valvethat is suitable for wireless implants. Our electrothermal valvemembrane consists of a thin-film Pt element sandwiched inParylene (with thickness  <  10  µ m to improve both powerconsumption and valve-opening speed). The valve is placed inthe fluid flow path of a catheter and is opened by thermal degra-dation or melting following the application of electrical current(Fig. 1). The theory, finite-element modeling (FEM), design,fabrication, and testing of our valve are presented. The applica-tion of our Parylene electrothermal valve in a wireless poweredmicro infusion pump [1], [15]–[17] is also discussed. Thisimplantable pump stores radioactive solution for on-demandinjection into animals for neuroimaging of cerebral blood flow.A biocompatible and low-power valve allowing rapid and wire-lessly activated release of the radioactive drug is required forthis application. Thus, our valve enables, for the first time, anew functional neuroimaging paradigm for understanding be-havior in freely moving untethered mice.II. D ESIGN The electrothermal drug-delivery valve consists of anelectron-beam-evaporated thin-film Pt resistive element embed-ded in a flexible Parylene membrane (10  µ m) (Fig. 2). Thecenter portion of the Pt resistive element corresponds to theactive area of the valve. Parylene C(Specialty Coating Systems,Inc., Indianapolis, IN) was selected as the membrane materialforitsmechanicalstrength(Young’smodulusof2.76GPa[14]), 1057-7157/$26.00 © 2009 IEEE  LI  et al. : PARYLENE MEMS ELECTROTHERMAL VALVE 1185 TABLE IC OMPARISON OF THE  P ARYLENE  E LECTROTHERMAL  V ALVE  W ITH THE  V ALVES  R EPORTED IN  R ESEARCH  L ITERATURE Fig. 1. Parylene electrothermal valve. (a) Schematic diagram (cross-sectionalview) illustrating the components of the valve. (b) Illustration of the operationprinciple of the valve in which wireless power transfer is used. biocompatibility, and ease of integration. It is recognized bythe United States Pharmacopeia as a Class VI material that issuitable for the construction of implants, and furthermore, itis well established as a MEMS material [12]. Parylene furthersimplifies the valve design by obviating the need for an etchedsilicon membrane support.Three different serpentine geometries for the resistive el-ements were selected, each having a different occupied cir-cular area and element linewidth (Table II). The serpentineshape maximizes the total resistance of the element within thedesignated valve footprint. Two contact pads extending fromthe electrothermal element provide a convenient location forexternal electrical connections.The valve is situated in the fluid flow path, which, in theimplementation described here, is the lumen of a catheter. Fig. 1 Fig. 2. (a) Illustrations of the electrothermal valve layout in both top andcross-sectional views. (b) Photograph of a single valve. (c) Close-up of a valveelement. Both (b) and (c) are reprints of Fig. 1(a) and (b) in [16], reprinted withthe permission of the Chemical and Biological Microsystems Society. shows a wireless implementation of the electrothermal valve, inwhich the valve is triggered when the connected secondary coilis activated by an external primary coil. The power transmittedto the secondary coil allows current to pass through the resis-tive element and initiates Joule heating to thermally degradeor melt the Parylene membrane surrounding the element.When the electrical (Pt element) and mechanical connections(Parylene membrane) of the valve are broken, the valve opensand allows pressurized fluids to pass. The catheter implemen-tation facilitates incorporation of the valve with commercially  1186 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 6, DECEMBER 2009 TABLE IIV ALVE -D ESIGN  P ARAMETERS available tubing having circular cross sections. This simplesingle-use low-power valve is easily modified to accommodateother geometries commonly encountered in microfabricatedmicrofluidic systems.III. T HEORY AND  M ODELING  A. Mechanical Modeling The Parylene electrothermal valve prevents flow from apressurized source prior to activation. To ensure that the Ptelement can survive the typical peak pressure from a fluidreservoir [1 atm (101.3 kPa)], the mechanical robustness of themembrane was verified using both large-deflection approxima-tion and nonlinear FEM. First, a simplified nonlinear isotropichomogeneous circular clamped thin film (Parylene only) wasmodeled under uniformed applied pressure. Due to the com-plex composite structure of the electrothermal valve membrane(Parylene/Pt), this idealized analytical model may deviate fromthe actual mechanical response. Thus, a more realistic valvemodel using a nonlinear FEM model was also examined. 1) Large-Deflection Theory:  In large-deflection theory [18],[19], the nonlinear strain-displacement relations resulting frombending and stretching of a plate can be expressed as ε x  =  ∂u∂x  + 12  ∂w∂x  2 ε y  =  ∂v∂y  + 12  ∂w∂y  2 γ  z  =  ∂v∂x  +  ∂u∂y  +  ∂w∂x∂w∂y  (1)where  ε x ,  ε y , and  γ  z  are the strains and  u ,  v , and  w  arethe displacements in  x -,  y -, and  z -directions, respectively. Thecorresponding governing differential equations of the plate are ∂  4 φ∂x 4  + 2  ∂  4 φ∂x 2 ∂y 2  +  ∂  4 φ∂y 4  =  E    ∂  2 w∂x∂y  2 −  ∂  2 w∂x 2 ∂  2 w∂y 2  ∂  4 w∂x 4  + 2  ∂  4 w∂x 2 ∂y 2  +  ∂  4 w∂y 4 =  tD   pt  +  ∂  2 φ∂y 2 ∂  2 w∂x 2  +  ∂  2 φ∂x 2 ∂  2 w∂y 2  − 2  ∂  2 φ∂x∂y∂  2 w∂x∂y   (2) Fig. 3. Finite-element analysis using COSMOSWorks. (a) Nonlinear andtransient FEM model and its corresponding coordinate system. (b) Stressdistribution (1 atm). where  φ  is the stress function,  E   is Young’s modulus,  t  isthe thickness of the plate,  D  is the flexural rigidity, and  p is the applied pressure. By using the minimum-strain-energymethod, the solution for a clamped thin circular plate subject toa uniform load  (  p 0 )  is  p 0 a 4 64 Dt  = 0 . 488  w max t  3 .  (3) 2) Finite-Element Simulation:  Nonlinear static simulationof the electrothermal valve was also performed using FEM(COSMOSWorks 2007, SolidWorks Company, Concord, MA)(Fig. 3). The mechanical properties of the materials and applied  LI  et al. : PARYLENE MEMS ELECTROTHERMAL VALVE 1187 TABLE IIIM ATERIAL  M ECHANICAL  P ROPERTIES AND  P ARAMETERS  U SED IN  FEM loads used in this FEM study are listed in Table III. Severalcritical stress points were identified [Fig. 3(b)]. The resultsindicated that the maximum stress of the Pt element under1-atm applied pressure is 1.53 GPa, which is less than thetensile strength of the Pt thin film (4.8 GPa). Thus, the electricalconnections are expected to survive pressurized conditions thatarise during radiotracer loading.  B. Temperature of the Pt Element  The temperature of the thin-film Pt element can be predictedby a conventional calibration method commonly used in ther-mal anemometry [2]. This method includes determination of the temperature coefficient of resistivity (TCR) and overheattemperature (OHT) for the thin-film metal resistor. TCR is animportant parameter to predict Pt element resistance at differenttemperatures. The temperature dependence of Pt is approxi-mately linear over the range of interest and can be expressed as R ( T  ) =  R ( T  0 )[1 +  α ( T   − T  0 )]  (4)where  R ( T  )  is the resistance at temperature  T  ,  T  0  is an ap-propriate reference temperature, and  α  is the TCR. In addition,the OHT allows the temperature of the resistive element tobe calculated from its resistance. Pt element resistances fordifferent applied currents were obtained. Then, the temperatureof the valve Pt element was estimated by using the followingequation and the experimentally determined TCR: T   =  T  0  +  R ( T  ) − R ( T  0 ) αR ( T  0 )  .  (5) C. Thermal Modeling The valve-opening process is dependent on the heat transferfrom the resistive heating element to the valve-membrane ma-terial. Therefore, thermal modeling was performed to evaluateheat transfer in the electrothermal valve system. First, a 1-Dsteady-state heat-transfer analysis was used to model the tem-perature of the valve under constant applied electrical power.Due to the nonuniform geometry of the valve, this idealizedanalytical model is limited but provides some informationregarding the valve-opening mechanism. A transient finite-element simulation was also performed using a more realisticvalve model. 1) Analytical Modeling:  A 1-D steady-state heat-transferanalysis uses a uniform simplified thermal resistance model todiscuss the temperature of the valve (Fig. 4). Only the top half of the valve model is used in the analysis due to symmetry.Assuming steady-state conditions, the heat-transfer rate isexpressed as q  2 =  T  2 − T  ∞ R Parallel =  T  2 − T  ∞ 1 1 R air +  1 R glass =  T  2 − T  ∞ 1 h air A air + k glass A glass L glass (6)where  q   is the heat generated by the Pt thermal element,  T  2  isthe Parylene temperature at the Parylene–air boundary,  T  ∞  isthe ambient temperature,  R Parallel  denotes the combined paral-lel thermal resistances of the glass and air,  R air  is the thermalresistance of the air,  R glass  is the thermal resistance of the glasstubing,  h air  is the convective heat-transfer coefficient of theair,  k glass  is the thermal conductivity of the glass tubing,  A air is the contact area of the air and Parylene membrane,  A glass  isthe contact area of the glass catheter and Parylene membrane,and  L glass  is the thickness of the glass catheter. Therefore,the boundary temperature of the Parylene and air can be ex-pressed as T  2  =  T  ∞  +  q  21 h air A air  +  k glass A glass L glass .  (7)Similarly, if the membrane is in contact with water instead of air,  h air  and  A air  can simply be substituted with  h water  and A water . The thermal properties used in the calculations arelisted in Table IV. The calculated temperatures for 9 mA of applied current (corresponding to 40.5 mW) at the air–Paryleneand water–Parylene boundaries are 474.0  ◦ C and 126.6  ◦ C,respectively. In the case of air operation, the boundary temper-ature exceeds the Parylene melting temperature (290  ◦ C), andvalve opening is expected. However, to open the valve in water, T  2  must exceed 290  ◦ C corresponding to a minimum appliedpower of 105.6 mW. For a 500- Ω  Pt thermal element, thecorresponding minimum opening current is 14.5 mA.This simplified 1-D thermal resistance model can only pre-dict the temperature of a uniform, homogeneous, and layeredstructure. However, multiple heat propagation routes exist, andthePtthermalelementpossessescomplexgeometry.UsingFEM,the valve-opening mechanism can be examined in more detail. 2) Finite-Element Simulation:  The steady-state and tran-sienttemperatureprofilesoftheelectrothermalvalveweremod-eled using FEM (COSMOSWorks 2007, SolidWorks Company,Concord, MA). The same model [Fig. 3(a)] used in mechanicalFEM modeling was again applied in valve thermal FEM analy-sis. The heat-diffusion equation governs the thermal behaviorof the system and is expressed as ∇· ( k ∇ T  )+ q  = ρc  p ∂T ∂t , ∂T ∂t  =0 ,  at steady state (8)where  k  is the thermal conductivity,  T   is the temperature,  q  is the energy generated inside the system,  ρ  is the density,  1188 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 18, NO. 6, DECEMBER 2009 Fig. 4. (a) Schematic showing a 1-D heat-transfer model of the Parylene electrothermal valve. (b) and (c) Final model representing only the top half of the valvedue to symmetry.TABLE IVM ATERIAL  T HERMAL  P ROPERTIES AND  P ARAMETERS  U SEDIN  A NALYTICAL AND  FEM and  c  p  is the specific heat. A cylindrical coordinate system isused with two basic assumptions: 1)  k  is constant, and 2) ax-isymmetric geometry exists. Therefore, the governing equationbecomes k  ∂  2 T ∂r 2  + 1 r∂T ∂r  +  ∂  2 T ∂z 2  +  I  2 RV   =  ρc  p ∂T ∂t  (9)where  I   is the applied current,  R  is the resistance of the Ptelement, and  V   is the volume of the Pt element. The boundaryconditions at the interface between the Parylene and top andbottom air interfaces are − k Parylene ∂T ∂z  z =0 =  q  2  (10) − k Parylene ∂T ∂z  z = L Parylene2 = h air  T  ∞ − T  z = L Parylene2   (11)where  L Parylene  is the thickness of the Parylene membrane.Similarly, if the membrane is in contact with water instead of air,  h air  can simply be substituted with  h water .At the center of the Pt element, due to the axial symmetry of the temperature distribution along the  r –  z  plane, the boundarycondition is ∂T ∂r  r =0 = 0 .  (12)At the edge of the Parylene membrane, the boundary conditionis (assuming adiabatic conditions and no heat dissipation) ∂T ∂r  r = a = 0  (13)where  a  is the radius of the Parylene membrane.The initial condition at  t  = 0  is T  ( r,z,t  = 0) = 25  ◦ C .  (14)The governing equation (9), the boundary conditions (10)–(13),and the initial condition (14) are then used in the steady-state and transient FEM models to solve for the temperaturedistributions of the valve at different time increments.The thermal properties listed in Table IV were used in bothsteady-state and time-dependent thermal simulations. First,steady-state FEM was performed with air as the fluid medium.The maximum temperature of the valve calculated by steady-state FEM was 499.6  ◦ C and in close agreement with the 1-Danalytical value of 474.0  ◦ C. Transient FEM temperature pro-files are shown in Fig. 5. A 40.5-mW power was applied to thePt thermal resistive element. Convective cooling was appliedon the Parylene membrane and air boundary. By 133 ms, themajority of the valve area reached over 125  ◦ C, which isthe thermal oxidation initiation temperature of Parylene C. TheParylene melting temperature of 290  ◦ C was reached in thecentral region at 266 ms. By 400 ms, most of the valve area
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