This document describes a microbubble pressure transducer (μBPT) with a microbubble nucleation core (μBNC) that allows for low power operation in wet environments. The μBPT uses electrochemical impedance measurement of a microbubble to detect pressure changes. It was improved by designing a circular Parylene chamber and flexible substrate that allows for localized bubble formation and short measurement paths. Testing showed the μBPT can repeatedly and efficiently generate stable microbubbles less than 1.5 nL in size via electrolysis in the μBNC and track pressure changes in real-time using low power.

1. A MICROBUBBLE PRESSURE TRANSDUCER
WITH BUBBLE NUCLEATION CORE
Author: Lawrence Yu, Ellis Meng
Reporter: 朱家君
Date: 2015/6/15

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

3. Introduction
• Purpose: μBPT + μBNC
→ achieve low power operation in wet environments.
• Operation: electrochemical impedance measurement
→ hydrostatic pressure change
→ μB size instantaneous response

4. MEMS pressure transducer
• Capacitive, piezoresistive, piezoelectric transduction:
• μBPT: no need of hermetic packaging
→ in vivo monitoring
→ reduce overall sensor footprint

5. Design
• Previous work: silicon microfluidic channels
→ poor control
(> 6% size variation)

6. 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 ↑

7. 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

8. Model
• Iac applied across the EI measurement electrodes:
→ monitor the volumetric conductive path (Rs)
• Rs:

9. 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.

10. 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.

11. Measurement
Current pulse v.s impedance Magnitude and phase
To eliminate capacitive effects, measurement
frequency was selected where phase ~0° (10
kHz)

12. Current injection v.s. impedance
Impedance-pressure correlation
(Type III)

13. Real-time pressure tracking (type III)

14. 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.

15. The End