A MICROBUBBLE PRESSURE TRANSDUCER WITH BUBBLE NUCLEATION CORE

Author: Lawrence Yu, Ellis Meng

Reporter: 朱家君Date: 2015/6/15

Outline• Introduction• MEMS pressure transducer• Design and operation• Measurement results• Conclusions

Introduction• Purpose: μBPT + μBNC

→ achieve low power operation in wet environments.• Operation: electrochemical impedance measurement

→ hydrostatic pressure change

→ μB size instantaneous response

MEMS pressure transducer• Capacitive, piezoresistive, piezoelectric transduction:

• μBPT: no need of hermetic packaging

→ in vivo monitoring

→ reduce overall sensor footprint

Design• Previous work: silicon microfluidic channels

→ poor control

(> 6% size variation)

This work• Circular Parylene chamber and flexible substrate:

→ short EI measurement path

→ limited sensitivity

• Trade-off:

Optimal electrolytic nucleation EI measurement

Spacing between electrodes ↓ Spacing between electrodes ↑

localized bubble formation measurement range ↑

Improvement

• μB formation:

1) generated within a nucleation core

2) coalesce into a single bubble

3) growing bubble extends outwards

4) fill measurement region of the channel

5) electrolysis current is terminated

6) μB detaches from μBNC

7) remains in the measurement channel

(respond to local pressure changes

transmitted via the liquid interface ports)

↑External pressure

Model

• Iac applied across the EI measurement electrodes:

→ monitor the volumetric conductive path (Rs)

• Rs:

Operation • a) Nucleation via electrolysis

in μB nucleation core.• b) μB enters measurement

channel.• c) Continued growth fills

microchannel• d) Detachment of μB from

μBNC and localization in

the measurement channel.

Fabrication • a) Deposit 1st Parylene and perform Pt lift-off.• b) Pattern sacrificial photoresist, deposit 2nd Parylene

layer, and etch interface ports.• c) Release device from silicon substrate and soak in

electrolyte to fill channel.

Measurement

Current pulse v.s impedance Magnitude and phase

To eliminate capacitive effects, measurement frequency was selected where phase ~0° (10 kHz)

Current injection v.s. impedance

Impedance-pressure correlation (Type III)

Real-time pressure tracking (type III)

Conclusion • μB nucleation by electrolysis and real-time pressure

tracking (-93 Ω/mmHg over 0-350mmHg). • Repeatable, efficient electrolytic generation of stable

microbubbles (< 1.5 nL with < 2% size variation) was achieved using a μBNC structure attached centrally to the microchannel.

• Biocompatible construction (only Parylene and Pt)• Small footprint• Low power consumption (< 60 μW)• Liquid-based operation of μBPTs are ideal for in vivo

pressure monitoring applications.

The End