1. MEMS Endovascular Pressure Sensors
Jonathan Brickey, Niels Black, Charles Wang
December 14, 2007

2. Anatomy of the Heart Vena Cava Right Atrium Right Ventricle
Pulmonary Arteries
Lungs
Pulmonary Veins
Left Atrium
Left Ventricle
Aorta
Body

3. Abdominal Aorta Aneurysm
2 cm
6 cm
Healthy Blood Pressure
Diastole: <80 mmHg (11 kPa)
Systole: <120 mmHg (16 kPa)
Hypertension Stage 2
Diastole: >100 mmHg (13 kPa)
Systole: >160 mmHg (21 kPa)

4. Prevalence of AAA 10th leading cause of death – 65-74 years old
5-7% men over 60 diagnosed with AAA
1-3% men over 65 experience aortic rupture
75-90% mortality rate from rupture
11:1 male:female ratio – years old

5. Methods of Treatment Open Repair Endovascular Repair

6. EndoSure by CardioMEMS
EndoSure Wireless AAA Pressure Measurement System
Permanently implanted
Radio frequency transmission
Radio frequency powered
Size of a paper clip
Biocompatible

7. Design Record Jay S. Yadav, M.D and Mark G. Allen
1995 – cofound CardioMEMS
2005 – EndoSure sensor invented
April, 2007 – granted FDA approval
Very stiff, very sensitive
Works 8in away
3.2cm, 4.1cm and 5cm

8. CNC wound coils, Kevlar membrane, Polyethelyene coating, Glass frame
Plastic deformation
Semipermiable
6,4,2 mm, max distance 3cm
1967 C. C. Collins “Miniature Passive Pressure Transensor for Implanting in the Eye “

9. 1992 Lars Rosengren 1995, William N.Carr, NJIT Hartley Oscillator
1992 – distance 2.2 cm, Low sensitivity, Stray Capacitance (oxide layer)
1995 – Very good sensitivity

10. Mark Allen, GA Tech
Wireless micromachined ceramic pressure sensors
GA Tech, under gov. grant for Intelligent Turbine Engines MURI Program
1 – glass conducts at high temp
2 – stiffer, less accurate, much higher temps, self packaged
High temperature self packaged wireless ceramic pressure sensor

11. 2006 – Mark Allen, GA Tech
Flexible Wireless Passive Pressure Sensors for Biomedical Applications
Standard flexible electronics packaging techniques
11mm diameter
1 – short term, permiable
2 – long term, ceramic reference pressure

12. Flexible Substrates: Types
Liquid Crystal Polymers (LCP)
Almost as ordered as fully crystalline solids
Chemically inert
Easy to fabricate
Polyamide Films
Kapton-E (DuPont)
thermal expansion coefficient same as Cu
13-50 micron thickness

13. Flexible Substrates: Advantages
For machining application:
Very high dimensional stability
High etchability – heavily isotropic
For biomedical applications:
Flexibility allows less invasive implantation
High levels of chemical inertness
Talk about rolling for catheter insertions

14. MEMS Screenprinting Additive process:
Mesh overlay – polyester or steel
Places where material does not go are “painted” over
Mesh screen placed on substrate, liquid poured over

15. MEMS Screenprinting Advantages/Disadvantages: Cheap!
Does not require pressurization or extremely expensive equipment, like lithography
Mesh can be reused
Not particularly precise
Features can be no smaller than mesh spacing (~50 µm)
50 micron =~ 4 layers of aluminum foil

16. Lithography Lithography mask for Inductor-Capacitor setup
Cross-section of Cu application
(Fonseca 2006)

17. Capacitance vs. Pressure

18. Power and Signal Transmission

19. Final Output

20. Problems in Simplification
Actual capacitor shape not circular:
“…tapered in the center to reduce deflection and avoid shorting out the capacitor…” (Fonseca 2006)
Circular model shorts out just before 13 kPa
Inductance
Very simplified:
Most MEMS inductors use complicated programs
13 kPa = standard blood pressure

21. Future Improvements
Major limitations: Size, Sensitivity, Transmission Distance
MEMS fabrication results in increased sensitivity
Size and Transmission Distance invariably linked

22. Other Possible Design Improvements
Finite element analysis of coil design inductance
Substrates with low dielectric constants
Hartley oscillators or other more complex CMOS for improving sensitivity or transmission distance

23. References
Wiemer, M., Frömel, J., Jia, C., Geßner, T., “Bonding and contacting of MEMS-structures on wafer level.” The Electrochemical Society - 203rd meeting, Paris (France), 2003 April 27- May 2
Fonseca, M.A.; English, J.M.; von Arx, M.; and Allen, M.G., "Wireless Micromachined Ceramic Pressure Sensor for High Temperature Applications," IEEE J. Microelectromechanical Systems, vol. 11, no.4, p (2002)
Fonseca, M.A., Kroh, J., White, J., and Allen, M.G., “Flexible Wireless Passive Pressure Sensors for Biomedical Applications,” Tech. Dig. Solid-State Sensor, Actuator, and Microsystems Workshop (Hilton Head 2006), June 2006

24. References (continued)
“New Medical Device Combines Wireless and MEMS Technology,” Physorg.com, February 03, 2006, December 08, 2007, <
Rosengren, L., Backlund, Y., Sjostrom, T., Hok, E., and Svedbergh, B., “A System for Wireless Intra-Ocular Pressure Measurements Using a Silicon Micromachined Sensor,” (1992)
Collins, C.C., “Miniature Passive Pressure Transensor for Implanting in the Eye,” IEEE Transactions on Biomedical Engineering, vol. BME-14, no. 2, April, 1967
Allen, M.G., “Implantable micromachined wireless pressure sensors: approach and clinical demonstration,” 2nd International Workshop on BSN 2005 Wearable and Implantable Body Sensor Networks, 2005, p