A SUBNANOWATT MICROBUBBLE PRESSURE SENSOR BASED ON
ELECTROCHEMICAL IMPEDANCE TRANSDUCTION IN A FLEXIBLE ALLPARYLENE PACKAGE
Christian A. Gutierrez and Ellis Meng
University of Southern California, Los Angeles, California, USA
Table 1: Comparison to other sensing modalities
Modality
Packaging
PowerSize
E-chemical
OpenLow<300 μm
Piezoresistance
HermeticHigh>500 μm
Capacitance
Hermetic
Low>400 μm
OpticalHermetic
low>300 μm

ABSTRACT
We present the first all-Parylene microbubble pressuretransducer (μBPT) harnessing the ability of a microbubble(μB) to respond instantaneously to external pressurevariations. A μB is electrolytically generated and physicallytrapped within a Parylene microchamber such that pressureinduced bubble size variation is detected by electrochemical
impedance (EI) measurement. Real-time hydrostaticpressure measurement (-11.8 Ω/psi, ±0.1 psi) of negativeand positive pressures (-2 – 4psi) is demonstrated. The
open-package device design leverages the ambient liquid
environment obviating the need for hermetic packaging
techniques. μBPTs are biocompatible, flexible, ultraminiature (200 μm diameter, 10 μm thick), and can beoperated at very low power (≤ nW) making them especiallyattractive for wet, in vivo pressure measurement.

Figure 2: Completely released Parylene-based system witha 4×4 array of sensors (200 μm diameter). Sensor array isinterfaced via an integrated flat flexible Parylene cable.

INTRODUCTION
It is known that gas bubbles respond instantaneously to
external pressure variations [1] at frequencies below
resonance (55 kHz for a 50 μm bubble radius [2]), however
few efforts have successfully exploited this phenomena forsensing. Polymer-based MEMS fabrication technology isleveraged here to enable a new class of transducer that canprecisely produce, localize, and measure bubble response at
micron scales, realizing a novel pressure sensor with uniquecapabilities (Figure 1).
State-of-the-art ultraminiature pressure sensing
technologies generally occupy 400-500 μm diameter
footprints. Our unique electrochemical-based pressure
sensing technique does not utilize conventional diaphragmbased pressure transduction, enabling significant reductions
in overall footprint, packaging complexity, and power
consumption (Table 1). Parylene C is featured as both theflexible substrate and structural material enabling
electrolytic bubble generation, localization, and transduction
in a completely released and portable device compatible
junkkkwith various liquid environments (Figure 2). In particular,
these features address the unmet need for robust in vivo
pressure sensors. Our approach eliminates the need forhermetic packaging and the exclusive use of polymer
materials reduces cost, adds mechanical flexibility, andstreamlines integration for medical applications.

THEORY
Microbubble Dynamics
The fundamental dynamics of a suspended bubble fixedin an unbounded incompressible viscous liquid are governedby the Rayleigh-Plesset equation [3]:31⎛
2σ 4μ ⎞−rr + r 2 = ⎜ p g − p∞ −r ⎟(1)2
ρ⎝rr ⎠
where r is the bubble radius (dot denotes time-derivative),pg and p∞ are the pressure in the gas at the bubble wall andambient pressure far away from the bubble (infinitedistance), σ is surface tension, μ is fluid viscosity, and ρ isfluid density.
Under the condition that the pressure oscillation
frequency is much smaller than the bubble resonant
frequency the following pressure-radius relationship isobtained:b⎛r ⎞2σp ∞ = p i ⎜⎜ 0 ⎟⎟ −r⎝ r ⎠Figure 1: (Left) Model of pressure transducer. (Right)
Optical micrograph of device (200μm diameter) withentrapped microbubble (50μm diameter).

978-1-4244-9633-4/11/$26.00 ©2011 IEEE

(2)

where polytropic exponent b ≈ 1 . The pressure-radius
response can therefore be estimated by Eq. 2 which

549

MEMS 2011, Cancun, MEXICO, January 23-27, 2011

describes the balance of forces acting on the bubble. For agiven number of gas molecules produced by electrolysis anda non-varying surface tension, bubble size is a direct andinstantaneous measure of ambient pressure.
For bubble behavior over larger time scales, the effects
of mass transfer at the bubble liquid interface become
significant. The bubble will dissolve in the presence of anunder-saturated liquid as a consequence of diffusion acrossthe bubble-liquid interface. The bubble dissolution rate in anunder-saturated solution was derived by Epstein and Plesset[1]:⎞k (c s − c∞ ) ⎛⎜ 1
1dr⎟+=1/2⎜dt ρ ∞ + 2τ / 3r ⎝ r (πkt ) ⎟⎠(3)

where k is the coefficient of diffusivity of the gas in theliquid; cs and c∞ are the saturation concentrations of gas inthe liquid at the bubble surface and in the bulk, respectively;
ρ∞ the density of gas in the bubble with a gas-liquidinterface of zero curvature; and τ is the modified surface
tension. Henry's law establishes a connection between thepartial pressure of gas acting on a liquid surface, pg, and theequilibrium (or saturation) concentration of gas in theliquid:c s = ap g

Figure 3: (Left) Electrolytic μB generation and localization
within the Parylene microchamber. (Right) Bubble size
instantaneously responds to external pressure and istracked by electrochemical impedance measurement. Toprow = top views, bottom row = cross-sections.

METHODS
Materials and Fabrication
The transducer features all-Parylene construction withthin film platinum electrodes and traces. No further
materials are utilized nor any hermetic sealing or
encapsulation techniques required. The open-package
design enables a “wet” sensor that is open to, and filled by,
the surrounding aqueous liquid (via fluidic access ports)
which becomes the electrolyte in the sensor (Figure 1).
Isolation of the sensor from the environment is therefore notnecessary and the added costs and complexities of hermetic
packaging are avoided.
Fabrication techniques were previously reported [6].
Briefly, platinum electrodes (2000 Å) were deposited andpatterned on a Parylene coated (10 μm thick) soda lime
wafer followed by a 1 μm thick Parylene insulation layer. A
2 μm sacrificial photoresist layer then formed the fluidic
access ports followed by a second Parylene deposition step(2 μm thick). A 10-12 μm sacrificial photoresist layer wasthen spun-on and patterned to establish the chamber heightand diameter (200 μm). The chamber structure was enclosed
by a final 4.2 μm Parylene layer, followed by opening ofaccess port vias by oxygen plasma (Figure 4).
Following removal of sacrificial photoresist by
immersion in acetone and isopropyl alcohol, devices were
simultaneously filled at the wafer-level by passive soaking
in a bath of the desired electrolyte solution. The chamber
design allows the ionic conductive path to be constrained
within a known geometry and minimizes the potential forcross talk between adjacent sensors.

(4)

a is a constant characteristic of the particular gas–liquidcombination and is a function of temperature. If the ambientpressure is fixed and equal to p∞, then it is clear that unlessthe gas concentration c at the bubble surface satisfies Eq. 4,
the bubble will not be in equilibrium, and will either growor shrink when c > cs or c < cs, respectively. Mass transfer
driven bubble dissolution is a slow process; typical values
for bubbles of radius ~100 μm are estimated to be on theorder of ~0.1 μm/sec [3].
Electrochemically-based Transduction
The underlying principle of electrochemically-basedtransduction is the measurement of solution impedance. By
applying a small alternating current across a pair ofelectrodes (at a sufficiently high frequency, typically >1kHz), the solution resistance can be measured. The electricalresponse of this system has been well characterized inliterature [4]. Electrochemical measurements are possible atnanowatt power levels, making this approach attractive forwireless and implantable applications. The solution
resistance is a function of ionic concentration, distance
between the electrodes, and cross-sectional area ofelectrolyte between the electrodes. A trapped μB acts as an
insulating sphere in the conductive incompressible
electrolyte between the electrodes. Therefore, themonitoring of μB size variations by measuring electrolyte
impedance (resistance) correlates to external pressure
variations (Figure 3). Previously, pressure measurements
within a microfluidic channel were demonstrated using μBs
and electrochemical impedance transduction [5], however
this approach utilized rigid substrate materials and waslimited to measurement of intra-channel pressure.

Experimental Setup
The devices were filled with 1x phosphate bufferedsaline (PBS) as the electrolyte. A microbubble waselectrolytically generated within the chamber by applying a
DC current pulse (typically 1 μA, 5 s).

550

Figure 5: (Top) Profile of applied hydrostatic pressure fromcalibrated pressure source. (Bottom) Measured μBPT
impedance response (1x PBS) to applied pressure profile.

Figure 4: Fabrication process flow for a single transducer.
Device impedance was monitored using a LabVIEWinterfaced precision LCR meter (1 Vpp, 5 kHz) (AgilentE4980A). A calibrated pressure source metered externalhydrostatic pressure to the μBPTs housed in a test jig.

RESULTS AND DISCUSSION
Hydrostatic Pressure Measurement
Hydrostatic pressure oscillations were applied between-2 and 4 psi (-100-200 mmHg). Impedance measurements
tracked applied pressure closely (inversely) and exhibited
excellent resolution over this range (Figure 5).
Measurements were obtained within 100 seconds of bubbleformation; minimal change in bubble volume occurred overthis time. Measurement of discrete pressure steps was also
possible and demonstrated previously [7]. A calibrated
sensor response of -11.88 Ω/psi was obtained by a best fit
linearization and clearly demonstrated pressure tracking
capability (Figure 6). Pressure measurement resolution wasestimated to be ±0.1 psi (689.4 Pa) as determined by theminimum detectable pressure variation. It was also possible
to detect pressure variations at excitation powers less than 1nW (100 mVpp, 10 nA) with a signal-to-noise ratio >20 toachieve sub-nanowatt device operation (Figure 7). The
bubble generation phase does require somewhat higher
power levels but these are typically in the microwatt range
and only a few seconds in duration. Wireless μB generation
via inductive coupling (2 MHz, Class D) was achieved fordistances greater than 5 cm and <5 μW at the secondary(Figure 8). However, complete wireless operation stillrequires
implementationof
wireless
impedancemeasurement, which has been demonstrated by others [8].
Alternative device configurations are also possible; forexample, devices may utilize a trapped electrolyte thatdiffers in composition from the surrounding medium. Thiscan be realized through integration of liquid entrapment
structures, such as a Parylene-based stiction valve [9]. This
capability, combined with the small size, polymer
construction and low power operation makes this approach
attractive for wireless pressure monitoring.

Figure 6: Impedance data from figure 5 plotted againstpressure. Response is linear and yields excellent sensitivity.

Figure 7: Subnanowatt EI measurement possible with >20
SNR making technique attractive for wireless applications.
Measurement Application
Operation was demonstrated in the real-time tracking ofinternal pressure in an electrolysis pump utilized in a
MEMS drug delivery system [10]. The μBPT was placed incontact with fluid in the pump chamber. Pump on/off states
were clearly detected and calibrated pressure measurements
agree well with previously reported data measured with aconventional silicon pressure transducer (Figure 9).

551

Figure 8: μBs were successfully generated via inductive
coupling at distances greater than 5 cm and currents <1 μAusing DI water as the electrolyte. Bubble nucleation wasobserved at power levels typically <5 μW.

Figure 9: Impedance response to electrolysis pumpactivation/deactivation cycles (5 mA pump current).
Calibrated measurement of pressure (-10.7 Ω/psi) is shownat two points.

Static Bubble Approximation
Once the bubble is formed, the slow diffusion-limited
dissolution process maintains an approximately constantbubble size for a period of time (~20 min), and is dependent
on the kinetics and solubility of the gas/liquid interface.
Generally, smaller bubbles will have higher surface tension
and therefore higher internal pressures which serves tospeed up the dissolution according to Eq. 1. In our design,
the platinum electrodes are located at the periphery of thechamber, limiting contact between the recombining gas andthe electrodes, thereby minimizing any catalytic effect. In
addition, the bubble is compressed due to the fixed chamberheight and is forced to expand only in lateral directions, thus
the top and bottom surfaces of the bubble, which are incontact with the chambers top and bottom surfaces, do notcontribute significantly to the mass transport process.
These factors, taken together, give rise to longer bubble
lifetimes allowing their use as quasi-constant pressure
sensors over short to medium time scales (~15-20 min).
Other electrolyte-gas combinations can be utilized, such as
ethylene glycol or alcohols, which may provide insoluble
conditions for entrapped gases for applications requiring
long term constant-volume μB transduction capability.

like to thank Dr. Donghai Zhu, Mr. Connor McCarty, Ms.
Roya Sheybani and the members of the USC BiomedicalMicrosystems Laboratory for their assistance.

REFERENCES[1] P. S. Epstein and M. S. Plesset, "On the stability of gas
bubbles in liquid-gas solutions," J. Chem. Phys., vol.
18, pp. 1505-1509, 1950.
[2] B. Ran and J. Katz, "The response of microscopic
bubbles to sudden changes in ambient pressure," J.
Fluid Mech., vol. 224, pp. 91-115, 1991.
[3] M. S. Plesset and A. Prosperetti, "Bubble dynamics andcavitation," Ann. Rev. Fluid Mech., vol. 9, pp. 145-185,
1977.
[4] J. E. B. Randles, "Kinetics of rapid electrode reactions,"
presented at Discussions of the Faraday Society, 1947.
[5] D. A. Ateya, A. A. Shah, and S. Z. Hua, "Impedancebased response of an electrolytic gas bubble to pressurein microfluidic channels," Sensors and Actuators A,
vol. 122, pp. 235-241, 2005.
[6] C. A. Gutierrez, C. McCarty, B. Kim, M. Pahwa, and E.
Meng, "An Implantable All-Parylene Liquid-Impedance
based MEMS Force Sensor," in IEEE MEMS. Hong
Kong, China, 2010, pp. 600-603.
[7] C. A. Gutierrez and E. Meng, "Subnanowatt
Microbubble Pressure Transducer," in Hilton Head:
Solid-State Sensors, Actuators and Microsystems
Workshop. Hilton Head, SC, 2010, pp. 57-60.
[8] X. Zhang, J. Yan, B. Vermeire, F. Shadman, and J.
Chae, "Passive Wireless Monitoring of Wafer
Cleanliness During Rinsing of Semiconductor Wafers,"
IEEE Sensors, vol. 10, pp. 1048, 2010.
[9] C. A. Gutierrez and E. Meng, "Improved Self-Sealing
Liquid Encapsulation in Parylene Structures byIntegrated Stackable Annular-Plate Stiction Valve," inIEEE MEMS. Hong Kong, China, 2010, pp. 524-527.
[10] P.-Y. Li, R. Sheybani, C. A. Gutierrez, J. T. W. Kuo,and E. Meng, "A Parylene Bellows ElectrochemicalActuator," J. Microelectromech. Syst., vol. 19, pp. 215228, 2010.

CONCLUSION
We have designed, fabricated and tested a Parylenebased pressure transducer utilizing a unique liquidimpedance transduction technique. Microbubbles were
harnessed as transducers to electrochemically monitor
pressure. The sensor operates in communication with thesurrounding liquid environment (either DI water or PBS),
thereby eliminating the need for hermetic packaging andshows promise for low-power wireless applications.

ACKNOWLEDGEMENTS
This work was funded in part by the Engineering ResearchCenters Program of the NSF under Award Number EEC0310723, an NSF CAREER Award (ECS-0547544), and theBill and Melinda Gates Foundation (CG).The authors would552