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

Anatomy of the Heart Vena Cava Right Atrium Right Ventricle Pulmonary Arteries Lungs Pulmonary Veins Left Atrium Left Ventricle Aorta Body http://www.nhlbi.nih.gov/health/dci/Diseases/pda/pda_heartworks.html

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) http://www.ultrasoundspecialists.com/screenings.html

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 – 60-64 years old

Methods of Treatment Open Repair Endovascular Repair http://www.nhlbi.nih.gov/health/dci/Diseases/arm/arm_treatments.html http://www.vascularweb.org/_CONTRIBUTION_PAGES/Patient_Information/NorthPoint/Abdominal_Aortic_Aneurysm.html

EndoSure by CardioMEMS EndoSure Wireless AAA Pressure Measurement System Permanently implanted Radio frequency transmission Radio frequency powered Size of a paper clip Biocompatible http://www.cardiomems.com/content.asp?display=medical+mb&expand=ess http://www.physorg.com/news10533.html

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 http://www.physorg.com/news10533.html

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 “

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

1999-2002 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

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

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

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

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

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

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

Capacitance vs. Pressure

Power and Signal Transmission

Final Output

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

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

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

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. 337- 43 (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

References (continued) “New Medical Device Combines Wireless and MEMS Technology,” Physorg.com, February 03, 2006, December 08, 2007, <http://www.physorg.com/news10533.html> 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 40-1.