A MICROBUBBLE PRESSURE TRANSDUCER WITH BUBBLE NUCLEATIONCORE
Lawrence Yu and Ellis Meng
University of Southern California, Los Angeles, USAABSTRACT
A microchannel-based microbubble (μB) transducer(μBPT) having a μB nucleation core (μBNC) was
developed to achieve low power operation in wetenvironments. In this work, we investigate μB dynamics
within the transducer structure and pressure transduction.
The transducer leverages electrochemical impedance (EI)based measurement to monitor the instantaneous response ofμB size changes induced by hydrostatic pressure changes.
We demonstrated on-demand μB nucleation by electrolysis
and real-time pressure tracking (-93 Ω/mmHg over 0-350mmHg). Repeatable, efficient electrolytic generation ofstable microbubbles (< 1.5 nL with < 2% size variation) was
achieved using a μBNC structure attached centrally to themicrochannel. Biocompatible construction (only Paryleneand Pt), small footprint, low power consumption (< 60 μW),
and liquid-based operation of μBPTs are ideal for in vivo
pressure monitoring applications.

INTRODUCTION
Typically MEMS pressure transducers consist of aflexible diaphragm affixed to a stiffer supporting structure.
Such a structure conveniently utilizes capacitive orpiezoelectric transduction mechanisms. Here, we employ analternative approach which exploits the ability of a
microbubble to respond instantaneously to external pressure
variations [1]. By combining localization of a microbubble
with electrochemical transduction, our pressure transduction
approach obviates the need for hermetic packaging requiredof conventional pressure sensor structures when used for invivo monitoring and thereby significantly reduces theoverall sensor footprint.

DESIGN
Transduction Principle
Previously, pressure response of μBs generated withinsilicon microfluidic channels was investigated [2] but withpoor control over the repeatability during electrolytic bubble
nucleation (> 6% size variation). We developed a μBPT
based on a circular Parylene chamber and flexible substrate
but achieved limited sensitivity due to the short EI
measurement path. Also, the electrolysis/measurement
electrodes were shared and thus switching was required forsensor operation [3]. Electrode spacing for optimal
electrolytic nucleation and EI measurement have opposing
requirements: increased spacing between electrodes results
in a larger measurement range while closely spaced
electrodes enables localized bubble formation. Therefore, toimprove sensitivity for in vivo applications, the use of
distinct pairs of smaller area electrodes for electrolysis

residing in a separate structure (a μBNC) and larger area
electrodes for high precision EI measurement wasinvestigated (Fig. 1).

Figure 1: Top-down illustration of pressure transduction
principle and layout of pressure transducer. (a)Microbubbles are electrolytically generated within anucleation core and coalesce into a single bubble. The
growing bubble extends outwards and fills measurement
region of the channel. (b) The μB then detaches from theμBNC and remains in the measurement channel where it
can respond to local pressure changes transmitted via theliquid interface ports.
In order to perform a pressure measurement, first a μB
is formed via electrolysis in the nucleation core and entersthe measurement channel as it grows (Fig. 1a), forming
capillaries of fluid in the channel corners (Fig. 2). Once a
sufficient size is reached, electrolysis current is terminatedand the μB detaches from the nucleation core. The
electrically insulating bubble now resides in themeasurement channel region and is available for pressuretransduction (Fig 1b). A low power, high frequency
alternating current applied across the EI measurement
electrodes permits monitoring of the volumetric conductive
path (solution resistance Rs in Fig. 3) [4]. External pressureof the liquid media is transferred through the interface ports(Fig. 1a), which directly influences bubble size. These
pressure variations induce μB size changes that are reflectedin the measured impedance, Z:| |ℓ(1)

where Rs is solution resistance, ρ is conductivity ofelectrolyte, ℓ is the length of the fluid capillary formed by aμB residing in the microchannel, and4

⁄

cos

⁄4

⁄4(2)

R is the inscribed inner radius of the channel, r the radius ofcurvature of the corner meniscus, and θ the contact angle ofthe μB with the channel wall (Fig. 3) [2].
This conductive path geometry improves sensitivity
over our previous design by leveraging the large changes in
impedance observed in submicron fluid capillaries [5].

Figure 2: Cross sectional view of bubble in microchannel inmeasurement region. Capillaries of fluid (shown in blue)
reside in corners between the bubble boundary and channel
walls. Total cross sectional area (STot) of conductive fluid
capillary path is dependent on contact angle of channel
sidewall material.

Figure 4: Bubble nucleation and formation process. (a)Nucleation via electrolysis in μB nucleation core. (b) μB
enters measurement channel. (c) Continued growth fills
microchannel. (d) Detachment of μB from μBNC andlocalization in the measurement channel.
Microchannel Measurement Area
A microchannel serves to confine the μB and provide
an area for EI measurements. Gating structures in themicrochannel keep μBs within the measurement area andprevent escape through the open-ended channels. Situating
the μBNC between and equidistant from the measurement
microelectrodes located at the ends of the microchannel
maximizes area for μB growth, thus maximizing
measurement range.

Figure 3: Impedance between measurement electrodes
modeled by the Randles circuit is proportional to length of
microbubble in measurement channel.
Nucleation Core
The μBNC consists of a pair of closely-spaced
microelectrodes (50 μm apart) enclosed within a tapered
cavity shaped so as to direct bubbles into the adjacentmicrochannel. Typically, nucleation via electrolysis creates
μBs within natural microcavities randomly distributed on
the electrode surface [6]; the cavity structure of the μBNC
forces μBs to coalesce before entering the microchannel.
When the μB spans the width of the microchannel, surface
energy of the hydrophobic Parylene sidewall causes thebubble to adhere, and termination of electrolysis current
results in μB detachment from the μBNC. Thus, a single
bubble is generated and localized in the measurement area.

METHODSFabrication
The μBPT was fabricated (Fig. 5) on a flexible
Parylene C substrate with thin film Pt electrodes based on
previously reported techniques [7]. Platinum was deposited(2000 Å thick) and patterned on a Parylene coated (10 μmthick) silicon wafer. Following deposition of a 2 μm
Parylene insulation layer, sacrificial photoresist waspatterned to establish the microchannel and μBNC
structures. A final 2 μm thick layer of Parylene was used toenclose the structure, and access ports were patterned andopened with oxygen plasma.
Devices were released in an acetone bath, and anelectrical connection was established with a ZIF connector,
using previously reported methods [8]. The microchannel
was first soaked in isopropyl alcohol to facilitate filling with
1X PBS, the measurement solution.

Figure 5: Overview of transducer microfabrication process.
(a) Deposit 1st Parylene and perform Pt lift-off. (b) Pattern
sacrificial photoresist, deposit 2nd Parylene layer, and etchinterface ports. (c) Release device from silicon substrateand soak in electrolyte to fill channel.
Experimental Setup
Microbubbles were electrolytically generated (Fig. 4) in
1X PBS under LabVIEW control over 1-4 μA for 5-25 s atthe μBNC and consistently exited into the measurement
channel. During current injection, multiplexed impedance
measurements were taken (Fig. 6) with a precision LCR
meter (Agilent E4980A). Bubble size was quantified
optically and with EI measurement. Electrochemical
impedance spectroscopy (EIS) yielded 10 kHz as theoptimum frequency (minimum system phase) for
maximizing the solution resistance component of theimpedance response (Fig. 7). Hydrostatic pressure
measurement used a calibrated pressure source attached to acustom test fixture housing the μBPT. A μB was generated
at 0 mmHg and subsequent pressure oscillations weretracked in real-time (Fig. 9).

Figure 6: Impedance tracking of bubble nucleation in 1X
PBS superimposed on the injected current pulse.

RESULTS AND DISCUSSION
Localized and metered bubble nucleation wasrepeatable. Power draw was ~1 nW for EI measurement(1Vp-p) and < 60 μW for electrolytic bubble generation.
Impedance monitoring during μB nucleation enabled precise
control of bubble formation, and this rate is hypothesized tobe correlated to the hydrostatic pressure [9].

Figure 7: Magnitude and phase for measurement electrodes
in 1X PBS. To eliminate capacitive effects, measurement
frequency was selected where phase ~0° (10 kHz).
Three types of μBs were observed (Fig. 8): type I μBs
were confined in the μBNC (Fig. 4a, < 16 μC); type II (Fig.
4b) spanned both the μBNC and microchannel (16-40 μC);
type III (Fig. 4c) resided in the microchannel, detached (Fig.
4d) from the μBNC (> 40 μC). Bubble type can be
categorized based on the total injected charge during
electrolysis. Pressure transduction was performed only with
type III μBs. The gas evolved from electrolysis of PBS isnearly all hydrogen [10], resulting in the short lifetime (< 5s) of type I and II μBs due to H2 contact with Pt electrodes,
which catalyze recombination. Type III μBs maintained
constant size (> 15 min) due to diffusion-limited dissolution(Fig. 6; partial time course shown). High precision EI
measurement (SD < 2%) of bubble size was achieved.

Figure 8: Metered current injection for μB nucleation
correlates to a change in impedance. Dashed lines
distinguish bubble morphologies based on injected charge.
A linear trend between EI and applied pressure wasobserved (Fig. 9), suggesting bubble length is proportional
to applied pressure as described in Eq 1. For the samemeasurement range, sensitivity was improved three ordersof magnitude compared to the previous circular chamberdesign (0.22Ω/mmHg); this improvement is attributed to thehigh impedance response of the submicron fluid capillaries.

ECCS-1231994. The authors would like to thank Mr. J.
Nishida, Mr. A. Jibodu, and the members of the USC
Biomedical Microsystems Laboratory for their assistance.

REFERENCES

Figure 9: Impedance-pressure correlation for Type III μBs.
Obtained highly linear response with improved sensitivity
compared to previous chamber design.
On a type III μB, pressure oscillations were tracked inreal-time (Fig. 10). Dissolution of the bubble was relativelyslow and dependent on mass transport and interfacial
kinetics, enabling stable pressure tracking measurement foran adequate time period (> 15 min). Bubble detachmentfrom the μBNC also favors the slow diffusion-limited
process over the Pt-catalyzed recombination reaction.

Figure 10: Real-time pressure tracking with type III
microbubble.

CONCLUSIONS
Precise control of μB formation is essential in pressuretransduction based on μB response to pressure variations.
The use of a μBNC structure for high precision μB
nucleation and pressure tracking using the localized μB in
an adjacent microchannel was demonstrated. Generated μBs
were stable (> 15 min) and the new sensor geometry
allowed improved sensitivity over the previous circularchamber design. Further sensor characterization undervarying environments (electrolyte composition, temperature,and pH) is underway.

ACKNOWLEDGEMENTS
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CONTACT*E. Meng, tel: +1-213-8213949; ellis.meng@usc.edu