Micromachines 2011, 2, 356-368; doi:10.3390/mi2030356
OPEN ACCESS

micromachines
ISSN 2072-666X
www.mdpi.com/journal/micromachines
ArticleLiquid Encapsulation in Parylene Microstructures Using
Integrated Annular-Plate Stiction Valves
Christian A. Gutierrez and Ellis Meng *Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089,
USA; E-Mail: cagutier@usc.edu* Author to whom correspondence should be addressed; E-Mail: ellis.meng@usc.edu;
Tel.: +1-213-740-6952; Fax: +1-213-821-3897.
Received: 1 August 2011; in revised form: 28 August 2011 / Accepted: 29 August 2011 /Published: 8 September 2011

Abstract: We report the design, fabrication and characterization of micromachined
Parylene structures for self-sealing liquid encapsulation applications. Automatic sealing isenabled through the use of an integrated annular-plate stiction valve which greatly reduces
device footprint over in-plane configurations. We achieve automatic wafer-level liquid
entrapment without using adhesives or processing at elevated pressures or temperatures.
The ability to track changes to the internal liquid volume through the use of electrochemical
impedance measurements is also presented.
Keywords: Parylene C; liquid encapsulation; stiction

1. Introduction
Many interesting MEMS applications such as variable-focus liquid lens optics [1], electrolysis
actuators, and electrowetting devices employ encapsulated liquids [2]. These applications requireappropriate fabrication techniques and structures for long term encapsulation of small liquid volumes.
The application of thermal or chemical bonding techniques is often challenging as the liquid must be
made compatible with the additional steps introduced into the process.
One such approach utilized thermo-compressive bonding of Parylene C to successfully encapsulate
water within microreservoirs for several days [3]. The technique required thermal excursion >200 °C,
and as such, the chemical stability of liquids other than water (or biologic material contained within:Micromachines 2011, 2357

proteins, cells, etc.) remains a concern. Various room temperature techniques have been proposed toaddress this challenge including chemical vapor deposition of Parylene C directly on droplets of low
vapor pressure liquids [1]. Others have exploited UV-curable resins to perform on-demandbonding-in-liquid with some success, however, alignment was found to be a critical factor [4].
Wafer-level room temperature sealing was demonstrated in gold microreservoirs patterned on siliconwafers and sealed through physical embossing of matching gold rings a glass sealing wafer. The seal
was shown to be hermetic but the embossing process required high pressure, epoxy reinforcement andvery smooth gold surfaces that were realized through additional lapping steps [5]. A sealingmechanism that introduces a minimum number of additional steps and simplifies the sealing process
would be an attractive alternative to current methods.
Stiction has been investigated as a possible sealing mechanism due to its simplicity and notoriously
strong surface interactions. Stiction is usually considered an unwanted failure mechanism in MEMS
and can render devices inoperable unless accounted for appropriately [6,7]. This phenomenon,
however, is beneficial when utilized as a sealing mechanism in valve structures for trapping fluid.
Parylene-based stiction valves have demonstrated excellent performance in wafer-level sealing of
liquids within microchambers [8,9], but are currently placed externally in relation to the liquidchamber and active structures of the device. This configuration increases the overall device footprint
and limits the ability to tailor the microchamber characteristics since the shared fabrication steps mustalso satisfy the stiction valve design constraints. Decoupling the valve construction by integrating it
within the active structure itself was attempted by [3] but liquid retention was limited to several
minutes due to suboptimal valve design. Here we combine the optimal annular-plate valve design
guidelines derived by [8] in an integrated device to reduce overall footprint by nearly 50% and achieve
automatic long-term liquid encapsulation [10].
2. Design and Fabrication
2.1. Device Design
The fluid encapsulation device consists of three main components: (1) an annular-plate stiction
valve; (2) peripheral fluidic access ports; and (3) a microchamber. The valve is connected via shortmicrochannels to the fluidic access ports located on the periphery and base of the microchamber
structure (Figure 1). Initially, the interior of the chamber and channels are filled with a sacrificial
material. After removing the sacrificial layer using a wet solvent, the chamber is filled by passive
soaking at the wafer level in the desired filling solution. Removal from the bath and exposure toambient conditions induces evaporation through the peripheral access ports. The evaporating liquid
fronts move along the connecting channels toward the stiction valve. The freed annular plate, if
mechanically compliant, can be pulled down to contact the substrate by capillary forces and if theadhesion energy is large enough, will remained pinned to the substrate forming a seamless
circumferential seal. The stiction-induced collapse and sealing of the integrated annular-plate traps theremaining liquid in the chamber above the plate (Figure 2).

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Figure 1. (Top): Top-view of device footprint with key radii indicated.
(Middle): Cross-section taken through connecting channel and fluidic access ports.
(Bottom): Cross-section through electrodes.

Figure 2. Principle of operation. Evaporation through the fluidic access ports moves theliquid fronts along the connecting channels toward the stiction valve. Capillary forces seal
the annular plate of the stiction valve against the substrate trapping liquid inside thechamber ([10]© IEEE 2010).

2.2. Stiction Valve Design
Parylene-based stiction valves were designed according to the guidelines developed by
Wang et al. [8,11]. Stiction occurs when the critical number, Nc, exceeds 1:
σr 4
N c = o 2 p(φ ,υ )(1)
Dh

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where σ = 2γLAcosθC is the surface energy of the Parylene-liquid interface, D = (Et3/12(1− υ2)) is theflexural rigidity of the plate, γLA is the liquid-air surface tension, θC is the Parylene contact angle, E isYoung’s modulus, t is the membrane thickness, h is the membrane height, υ is Poisson’s ratio, φ is theratio ri/ro, and p is a numerical function of φ and υ and has been evaluated through the use of numericalmethods. A combination of parameters that satisfy the design constraints were selected for thedevice (Figure 3, Table 1). Nc for these valve dimensions is well above 1, ensuring thatstiction occurs.
Figure 3. Partial cross-section of a fluid encapsulation device indicating key design parameters.

Table 1. Stiction valve radii and calculated critical number Nc. Valves were fabricated withfilm thickness tv = 2 µm and height hv = 2 µm.
Dimension (µm) Design Parameter Critical Number
ro/ri
φ
Nc
63/20
0.32
3.14
75/25
0.33
6.26
88/30
0.34
11.14
100/350.35
18.23
2.3. Fabrication
The entire fabrication process is realized at relatively low temperatures (90 °C) (Figure 4) whichenables its use on a variety of polymer-based substrates and facilitates adoption in temperaturesensitive processes. Standard soda-lime glass wafers were utilized as the carrier substrate. Fabrication
began with optional platinum electrodes (2,000 Å) patterned on a 10 µm Parylene film followed by a
1 µm Parylene insulation layer. The 10 µm Parylene film allows for release/handing of structures anddevices from the soda-lime carrier substrate for applications where flexibility is required. The optional
electrodes are exposed to the liquid within the chamber and serve as an electrical interface forelectrochemical measurement or actuation applications. In this work, electrodes were used forelectrochemical measurements to track the sealing capability of the stiction valves and to monitor
water vapor transmission through the Parylene film.
Photoresist was then spun-on and patterned (2 µm, AZ 4400, AZ Electronic Materials, Branchburg,
NJ, USA) forming the sacrificial mold for the valve structure. Parylene was then deposited (2 µm)
forming the valve membrane (tv) and sidewalls. Parylene over the valve center and access ports wasremoved in oxygen plasma (100 W, 100 mT) using a photoresist mask, thereby completing the annularvalve structure (ri) and access ports. The exposed state of the sacrificial photoresist forming the valve

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layer (step c, d) made it difficult to remove the photoresist etch mask without erosion of the underlying
valve structure. For this reason, the chamber sacrificial layer was spun and patterned directly on top of
the etch mask (12 µm, AZ 4620, AZ Electronic Materials, Branchburg, NJ, USA), (rc = 150 µm). A
4.2 µm Parylene film was then deposited (tc) to enclose the chamber and a final Parylene etching step
was necessary to re-open the fluidic access ports (step h). Test dies were then diced and following
sacrificial photoresist removal in acetone, IPA, and DI water, the chambers were filled in a batchprocess by immersion in the desired fluid (DI water). Adhesion to the carrier substrate was realized
through treatment of the carrier wafer with A-174 silane prior to deposition of the base Parylene layer.
If desired, release from the soda-lime carrier substrate can be accomplished by skipping the A-174
treatment. This allows for a simple release from the substrate by cutting the Parylene film with a razor
blade and peeling; a process that is greatly facilitated if performed under water.
Figure 4. Low temperature (90 °C) multi-layer Parylene-C fabrication process.

3. Results and Discussion
3.1. Stiction Valve Sealing
Stiction valve sealing and liquid encapsulation capability were evaluated by optical observation of asample die containing several devices. The devices were filled, as described previously, with DI
water. The die was then removed from the bath and exposed to ambient laboratory conditions(typically <5 min) to initiate the fluid encapsulation process. Following sufficient fluid evaporation,
successful stiction valve sealing was observed and marked by the appearance of a dark central region
indicating conformal contact between the annular plate and substrate. Emergence of interference rings

Micromachines 2011, 2361

between the central region and the peripheral plate anchors indicated proximity of the valve plate to
the substrate as the valve plate transitioned from contact with the substrate to freestanding near theanchors (Figure 5). The mechanical deformation produced during stiction of the Parylene membrane ispredominantly elastic in nature and is consistent with small deflection models that assume maximum
deflections less than or equal to membrane thickness. Compared to more traditional diaphragm
materials like silicon nitride (E~300 GPa), the mechanical loading of the relatively soft Parylenematerial (E~4 GPa) produces only nominal stresses that fall well below the yield point of the material,
as verified through finite-element-modeling of membrane during deflection (data not shown here).
Figure 5. Optical micrograph of 300 µm diameter device filled with DI water. The
chamber is free standing while the valve is collapsed and pinned to substrate. The dark
central region indicates the seal between the annular-plate and substrate while interference
rings indicate proximity to substrate as plate transitions from contact at the center tofreestanding at the anchored edges ([10] © IEEE 2010).

3.2. Liquid Encapsulation
Successful encapsulation of liquid was evidenced by the appearance of a liquid front within thechamber after several hours of complete encapsulation. Initially, the chambers appeared uniform due tocomplete filling and entrapment of liquid within the chamber [Figure 6(a)]. This was followed by theappearance of a liquid front several hours later as a result of water vapor transmission [Figure 6(b)].
This is the first demonstration of uniform and complete device filling; in earlier attempts, air pockets
were often observed and thus, liquid entrapment was incomplete [3,11]. For comparison, devices
without a valve were also fabricated and tested [Figure 6(d–f)]. Within 1.5 h, valveless devices werecompletely dry while devices with a valve successfully encapsulated liquid up to 9 h at ambientconditions (20 °C, ~30% RH). The formation of a liquid front, even in valved devices, clearly
indicated that liquid loss still occurred. Water vapor transmission through the chamber membrane andnot the valve seal was hypothesized to be the root liquid loss mechanism; however a method toquantitatively track internal volume was needed. The following section describes the use ofelectrochemical impedance to track liquid volume within the chamber.

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Figure 6. Sequence of optical micrographs depicting the chamber state from filled toempty conditions. (a) Completely filled chamber with valve immediately after removalfrom immersion bath. (b) Valved chamber after 8 h of evaporative drying at ambientconditions. Complete valve seal evidenced by interference rings and darker central annularregion. (c) Empty chamber at 9 h. Valve remains pinned to the substrate. (d) Chamber
without valve immediately after removal from immersion bath. (e) Chamber drying occurs
rapidly; liquid evaporation front appears at 1 h. (f) At 1.5 h, the chamber is completely dry
indicating rapid evaporation without a stiction valve ([10] © IEEE 2010).

3.3. Impedance-Based Volume Measurement
When metal electrodes are immersed in liquid, the spontaneous creation of the so-called double-layerinterface is responsible for capacitive behavior at each electrode interface. In parallel with thiscapacitance is a charge transfer resistance which is typically very high and accounts for perturbationsof the double-layer interface when a potential is applied. The bulk solution, however, behaves much
like a liquid resistor that varies with path length and cross-sectional area, and lies in series with theinterfacial components. Utilizing a simplified model of this interface based on Randle’s equivalentcircuit, the measured impedance can be expressed as:

Z ( w) =2Rct+ Rs
1 + jwRctCdl(2)

where Rct is the charge transfer resistance of the interface, Cdl is the double-layer capacitance and Rs is
the solution resistance term.
The impedance of the liquid between the electrodes depends complexly on the volumetricconduction path of current-carrying ions in the solution as well as the total volume contained within
the chamber. As volume is lost through water vapor transmission, the chamber’s top surface deforms
to accommodate the lost volume resulting in changes to the volumetric cross-section over time. During
this liquid-loss phase, the impedance magnitude and phase vary due to changes in Rs which are directly
related to the bulk liquid volume. The impedance magnitude and phase are expressed as follows:Micromachines 2011, 2

363
12

2
2
⎛⎛
⎞ ⎛ 2wCdl Rct2 ⎞ ⎞⎟2Rct⎜⎟
+ Rs ⎟⎟ + ⎜⎜Z = ⎜⎜⎜ ⎝ 1 + ( wCdl Rct )2
1 + ( wCdl Rct )2 ⎟⎠ ⎟
⎠
⎝
⎝
⎠

⎛
⎞
2 wC dl Rct2⎟⎟
∠Z = arctan ⎜⎜2⎝ Rs (1 + ( wC dl Rct ) ) + 2 Rct ⎠

(3)

(4)

In order to relate impedance with liquid volume, Rs needs to be determined. To make the analysis
more tractable, a simplification of this system assumes that bulk resistance is inversely proportional tothe cross-sectional area through the central axis of the chamber through the simple relation Rs ≈ ρl/A,
where l is the distance between electrodes and A, the cross-sectional area, is calculated through thecenter of the device. In reality, the solution resistance calculation requires a more complex expressionthat integrates the differential cross sectional area dA over a path dl that accounts for the changing
membrane deflection profile with radius. The cross-sectional area through this mid section of thechamber can be easily calculated from the standard deflection equation for a circularly clamped
membrane of uniform thickness given by:y(r ) =

3(1 − μ 2 ) P 2( rc − r 2 ) 2
16 Et 3(5)where y is deflection, P is pressure, t is membrane thickness, a is diaphragm radius, E is modulus ofelasticity, r is radial distance, and µ is Poisson’s ratio. This simplification overestimates thecontribution of cross-sectional area to solution resistance as it assumes that the maximum deflection
profile is constant along the entire length of l, as would be case in a rectangular channel, however thisapproach still provides an adequate description of the observed phenomenon. Integrating a sweep ofEquation (5) using polar coordinates yields an expression for volume contained within the chamber:V =3π (1 − μ 2 ) Prc6
48 Et 3

(6)

Substituting y0 = y(r = 0) (maximum membrane deflection) into Equation 6 yields:V =π rc2 y 0
3

(7)

Through a similar substitution into the simple area integral of Equation (5), cross-sectional area canbe expressed in terms of maximum deflection as A = y0rc/10. Substituting in Equation (7) yields
an approximation that is useful for relating chamber volume with solution resistance andultimately impedance:
10πrc l(8)
Rs ≈ ρ
3V

3.4. Liquid Loss Mechanism
In order to verify that water vapor transmission through the chamber membrane was the primaryliquid loss mechanism, four devices were monitored optically under a microscope and electrochemicallyby solution impedance measurements via the integrated microelectrodes [Figure 7(a)]. Electrochemical
impedance of the encapsulated liquid was measured (5 kHz, 100 mVpp) over time using an Agilent
precision LCR meter (E4980A, Agilent Inc, Santa Clara, CA, USA). Impedance measurements wereMicromachines 2011, 2364

consistent across multiple devices and clearly tracked liquid volume changes over time [Figure 7(b)].
By correlating time stamped optical micrographs with measured impedance data, liquid loss wasquantitatively tracked and measured (Figure 8). Phase angle measurements clearly followed a linear
relationship with volume. As liquid was lost, the phase angle approached −90°, indicating thetransition from a mostly resistive phase angle contribution (i.e., solution present) to capacitive (i.e., nosolution present).
Figure 7. (a) Optical micrograph of device fabricated with optional electrodes (shown
while immersed in DI water). (b) Top: Impedance magnitude measured over time. Bottom:
Phase angle approached −90° indicating a switch from resistive solution impedance (wetchamber) to capacitive impedance (dry chamber).

(a)

(b)

Figure 8. Liquid volume measured over time (mean ± SD, n = 4). The measured WVTR
agreed closely with previously reported values for Parylene films at ambient conditions.
This indicated that the duration of liquid encapsulation was primarily limited by the water
vapor transmission of the Parylene film and not valve sealing. Volume was tracked
optically and measured using phase angle of impedance measurements ([10] © IEEE
2010).

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Four different valve designs were utilized in these experiments to assess the dependence of thestiction seal footprint on sealing performance. Generally for a given set of valve thickness and height,
the stiction footprint increased with valve diameter. Although the impedance magnitude data seemed
to indicate that that the larger valve diameter produced relatively smaller impedance changes and thus
maintained liquid volume somewhat more effectively, there was no significant benefit observed in thephase data. In addition, no significant difference was observed via optical methods. These observations
suggest that any potential performance gains of using larger valves versus smaller valves are minimal
and that the key requirement for valve sealing remains the initiation of stiction.
The change in liquid volume over time corresponds to a measured water vapor transmission rate
(WVTR) of 5.6 × 10−12 g·µm/µm2·h through top surface of the chamber. This value agrees closely withthe previously reported WVTR value of 5.78 × 10−12 g·µm/µm2·h (20 °C, 30% RH) for Parylene C
films [12]. This result confirms that the liquid loss mechanism is indeed water vapor transmission
directly through the Parylene membrane and not the stiction valve seal. Electrochemical impedance
measurements provide a simple and reliable method to quantitatively track internal liquid volumes
over time.
3.5. Applications
The performance-limiting factor for liquid encapsulation duration is the water vapor transmission
rate through as-deposited Parylene films. To further improve performance, additional techniques canbe implemented to enhance encapsulation lifetime (hencapsulation). The parameters affecting hencapsulation
are identified below:
hencapsulation =Mt
WVTR ⋅ SA(9)where M is mass of water (grams), t is thickness of the Parylene membrane (µm), and SA is exposed
surface area available for transmission in (µm2). Increasing membrane thickness improves
encapsulation lifetime proportionally while WVTR and exposed surface area are inversely proportional.
The WVTR of as-deposited Parylene-C films can be reduced by nearly 50% with additional hightemperature annealing steps [3,12]. Furthermore, deposition of thin films such as gold or aluminum
can also be used to reduce the SA available for transmission and provide a nearly impermeable
protective coating [11]. The use of a combination of these techniques can feasibly extend
encapsulation time for sub-nL volumes (in the current design) out to several days.
In addition to water, it is expected that this encapsulation technique is compatible with a variety ofother liquids provided that they are volatile (even if only weakly) and that the surface energy density
and contact angle for the desired solution is taken into consideration. The energy of Parylene-liquidinterface will vary according to the value of these parameters and appropriate design adjustments need
to be made to ensure conditions for stiction are preserved. For example, many common organic
solutions including acetone, methanol, ethanol, acetic acid and most oils have known surface energy
densities ranging 20–30 mJ/m2 at room temperature [13]. Typically these solutions also exhibit much
lower contact angles; use of organic solvents such as methanol and acetone are routinely used during
cleaning of Parylene and readily wet the surface indicating contact angles close to 0°.

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Many common aqueous salt solutions such as KCl, NaCl and Na2SO4 exhibit slightly higher surface
energy densities than pure water (73.4, 73.6 and 74.7 mJ/m2 respectively at 1 M) and vary linearly withconcentration following the approximate realation γLA = γLAH2O + MΔγ, where M is molar concentration
and Δγ is typically between 1–2 mJ/m2 for most salt solutions. Contact angle values for water on
Parylene have been reported throughout the literature but values for other liquids such as acids andliquid metals, to our knowledge, have yet to be reported.
The valve-in-chamber configuration is useful for applications in electrochemical-based transducers
such as force and pressure sensors. The capability of small encapsulated liquid volumes to measurecontact forces and hydrostatic pressures has been demonstrated with excellent sensitivity [14,15].
Furthermore, electrolysis-based actuation can be fully exploited by this design as increasing chamberpressure further enforces the seal between the valve plate and substrate in contrast to previously
reported in-plane designs in which the valve opened at sufficiently high internal pressures [9,11].
Figure 9. Additional configurations utilizing the valve-in-chamber design. Complex
encapsulated microfluidic systems can be realized. White dots indicate dummy electrodesites for impedance-based measurements.

There are numerous applications and configurations for such devices beyond single valve-chamberconfigurations. Complex interconnected geometries are possible through the use of multiple valvesdistributed throughout a microfluidic network (Figure 9). This approach reduces to the clearing time ofthe sacrificial material while still maintaining sealing capability thus allowing for encapsulation withinlarger structures. The introduction of microelectrodes throughout such structures enables multi-point
impedance measurement which has been demonstrated and applied to biomimetic tactile sensing
applications at the macroscale [16]. The ability to realize a diverse array of sensor configuration
using this liquid encapsulation technology is also potentially useful across a wide array of
biomedical applications including tactile feedback for minimally-invasive surgery [17], tissue healthdiagnostics [18-20], and surgical tool instrumentation [21].
4. Conclusions
The technique utilized here improves encapsulation performance by integrating an optimized
stiction valve within a chamber structure. This reduces overall device footprint by nearly 50% overin-plane approaches, extends encapsulation lifetime, is performed in batch at the wafer-level, involves

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no high temperature or pressure steps and is achieved without adhesives. This is accomplished at thecost of only a few additional standard micromachining processing steps. This improved approach has
the potential to enhance many applications where liquid encapsulation is required.
Acknowledgments
This work was funded in part by the Engineering Research Centers Program of the NSF underAward Number EEC-0310723 and the Bill and Melinda Gates Foundation (CG). The authors would
like to thank Donghai Zhu and the members of the USC Biomedical Microsystems Laboratory for theirassistance.
References
1.

Khiem, N.B.; Matsumoto, K.; Shimoyama, I. Polymer thin film deposited on liquid for varifocal
encapsulated liquid lenses. Appl. Phys. Lett. 2008, 93, 124101.
2. Lee, J.; Kim, C.-J. Surface-tension-driven microactuation based on continuous electrowetting.
J. Microelectromech. Syst. 2000, 9, 171-180.
3. Matsumoto, S.; Ichikawa, N. New Methods for Liquid Encapsulation in Polymer MEMS
Structures. In Proceedings of the IEEE 21st International Conference on Micro Electro
Mechanical Systems, MEMS 2008, Tucson, AZ, USA, 13–17 January 2008; pp. 415-418.
4. Okayama, Y.; Nakahara, K.; Arouette, X.; Ninomiya, T.; Matsumoto, Y.; Orimo, Y.; Hotta, A.;
Omiya, M.; Miki, N. Characterization of a bonding-in-liquid technique for liquid encapsulation
into MEMS devices. J. Micromech. Microeng. 2010, 20, 095018.
5. Lapisa, M.A.; Niklaus, F.; Stemme, G. Room-Temperature Wafer-Level Hermetic Sealing for
Liquid Reservoirs by Gold Ring Embossing. In Proceedings of the International.Solid-State
Sensors, Actuators and Microsystems Conference, Transducers 2009, Denver, CO, USA, 21–25
June 2009; pp. 833-836.
6. Mastrangelo, C.H.; Hsu, C.H. Mechanical stability and adhesion of microstructures undercapillary forces. I. Basic theory. J. Microelectromech. Syst. 1993, 2, 33-43.
7. Mastrangelo, C.H.; Hsu, C.H. Mechanical stability and adhesion of microstructures undercapillary forces. II. Experiments. J. Microelectromech. Syst. 1993, 2, 44-55.
8. Wang, Z.; Xu, Y. Theoretical and experimental study of annular-plate self-sealing structures.
J. Microelectromech. Syst. 2008, 17, 185-192.
9. Gutierrez, C.A.; Meng. E. A Dual Function Parylene-based Biomimetic Tactile Sensor andActuator for Next Generation Mechanically Responsive Microelectrode Arrays. In Proceedings ofthe 2009 International SolidState Sensors Actuators and Microsystems Conference, Transducers
2009, Denver, CO, USA, 21–25 June 2009; pp. 2194-2197.
10. Gutierrez, C.A.; Meng, E. Improved Self-Sealing Liquid Encapsulation in Parylene Structures byIntegrated Stackable Annular-Plate Stiction Valve. In Proceedings of the IEEE 23rd International
Conference on Micro Electro Mechanical Systems, MEMS 2010, Hong Kong, China,
24–28 January 2010; pp. 524-527.

Micromachines 2011, 2368

11. Zhang, H.; Wang, S.; Xu, Y. Study and Applications of a Parylene Self-Sealing Structure. In
Proceedings of the 19th IEEE International Conference on Micro Electro Mechanical Systems,
2006. MEMS 2006, Istanbul, Turkey, 2006; pp. 282-285.
12. Menon, P.R.; Li, W.; Tooker, A.; Tai, Y.C. Characterization of Water Vapor Permeation ThroughThin Film Parylene C. In Proceedings of the International Solid-State Sensors, Actuators andMicrosystems Conference, Denver, CO, USA, 21–25 June 2009; pp. 1892-1895.
13. Joseph, J.J. The surface tension of pure liquid compounds. J. Phys. Chem. Ref. Data 1972, 1,
841-1010.
14. Ateya, D.A.; Shah, A.A.; Hua, S.Z. Impedance-based response of an electrolytic gas bubble topressure in microfluidic channels. Sens. Actuat. A 2005, 122, 235-241.
15. Gutierrez, C.A.; Meng, E. A Subnanowatt Microbubble Pressure Sensor based on Electrochemical
Impedance Transduction in a Flexible All-Parylene Package. In Proceedings of the 2011 IEEE
24th International Conference on Micro Electro Mechanical Systems, MEMS 2011, Cancun,
Mexico, 23–27 January 2011; pp. 549-552.
16. Wettels, N.; Santos, V.J.; Johansson, R.S.; Loeb, G.E. Biomimetic tactile sensor array. Adv. Rob.
2008, 22, 829-849.
17. Eltaib, M.E.H.; Hewit, J.R. Tactile sensing technology for minimal access surgery—A review.
Mechatronics 2003, 13, 1163-1177.
18. Han, L.; Noble, J.A.; Burcher, M. A novel ultrasound indentation system for measuring
biomechanical properties of in vivo soft tissue. Ultrasound Med. Biol. 2003. 29, 813-823.
19. Plewes, D.B.; Bishop, J.; Samani, A.; Sciarretta, J. Visualization and quantification of breast
cancer biomechanical properties with magnetic resonance elastography. Phys. Med. Biol. 2000,
45, 1591-610.
20. Samani, A.; Bishop, J.; Luginbuhl, C.; Plewes, D.B. Measuring the elastic modulus of ex vivo
small tissue samples. Phys. Med. Biol. 2003, 48, 2183-2198.
21. Tokida, M. Integration of Cell Sheet Sucking and Tactile Sensing Functions to Retinal Pigment
Epithelium Transplantation Tool. In Proceedings of the IEEE 23rd International Conference on
Micro Electro Mechanical Systems, MEMS 2010, Hong Kong, China, 24–28 January 2010;
pp. 316-319.
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