Mechanical Sensors in Biomedicine

1. Mechanical Sensors in Biomedicine Xingwei Wang

2. Noninvasive blood pressure measurements • Latex bag inside a Velcro cuff • Pump to compress the vessels until bloodstream is stopped • During the slow cuff deflation, listen to the Krortkoff sound • Arterial pressure-wave propagation caused by the heartbeats

3. Piezoelectric effect • Pressure -> mechanical strain/stress variation -> separate the center of gravity of the positive charge from the center of the gravity of the negative charge -> dipole moment -> polarization change -> electricity • Materials do not have a center of symmetry. • Converse effect: electric field -> strain

4. Piezoelectric materials • Electric polarization: P • Mechanical stress: T • Piezoelectric coefficient: d • Subscript E: the field is constant • Mechanical strain: S • Applied field: E • Converse effect: d* • Subscript T: stress is constant • Laws of thermodynamics:

5. Why piezoelectric effect can only be used for dynamic process? • Charge dissipation -> voltage generated by stress decays

6. Single-crystal materials • Total: 32 • Symmetry and nonpolar: 11 • Piezoelectric effect: 20 • Noncentrosymmetry but no piezoelectric: 1: cubic system • Classical example for piezoelectric crystal: quartz

7. Pyroelectric effect: Heating->electricity • Spontaneous polarization: the centers of gravity of the positive and negative charges are separated, even without stress. • Atmosphere normally contains sufficient free positive and negative ions to neutralize the free surface charge • Heating->desorb the surface neutralizing ions -> change polarization->surface charge change

8. Pyroelectric effect • Pyroelectric coefficient: p • Flux density: D • Temperature: T

9. Capacitor • Pyroelectric voltage signal: ∆U • Permittivity: εrε0 • Thickness of pyroelectric film: d • Temperature change -> excess charge on the pollar faces -> current flow in the external circuit. • Similar to time-dependent behavior of piezoelectric materials

10. Applications • Infrared detection • High sensitivity: 1/1000 °C

11. Piezoresistive effect • Metal films, semiconductors • Resistance variation when mechanical stress and/or strain is applied • Due to • Piezoresistivity: resistivity change versus stress • Geometrical piezoresistivity: pure geometrical effect caused by deformation

12. Resistivity change • T : mechanical stress • Resistivity: ρ • П: piezoresistivity coefficient.

13. Deformation sensitivity of a resistor • Gauge factor: the ratio of the fractional change in resistance to the fractional change in geometrical sizes: • L: resistor length

14. Advantages over piezoelectric sensor • More accurate in static pressure and force measurement. • No interference from pyroelectric effect.

15. Piezoresistive silicon pressure sensors • Pressure difference -> membrane deformation -> resistance changes -> Wheatston-bridge

16. Resistance change • Resistance change: • G: gauge factor • E: Young’s modulus of the membrane • a: thickness • K: constant depending on geometrical sizes

17. Hemodynamic invasive blood pressure sensors • Package sensor chip in a sterilizable plastic housing (dome). • A pipe transmits the blood pressure to the dome and the sensor membrane. • Fill silicon oil between intermediate membrane and sensor chip.

18. Catheter (invasive) blood pressure sensor • Miniature silicon chip: 5 mm x 1 mm x 15 µm; • Pressure/temperature sensor + circuit • Ultraminiaturized piezoresistive pressure sensor chip: 0.5 x 0.5 x 2.3 mm

19. Fiber optic pressure sensors

20. Mechanical sensors in spirometry • Respiratory flow measurement • Fleisch tube: measure pressure difference across a grid as a function of the flow • Flow-resistance: Rf • Flow rate:v • Pressure difference: ∆p

21. Upper airwasy to prevent obstructive sleep apnea syndrome (OSAS) • 7 transducers • 1 at the tip of the catheter • placed into the esophagus • to measure pressure in the chest • 6 arrayed over 20 mm intervals • Measure pressure from the back of the nose to just above the larynx

22. Transducers • Each is a series of 3 optical fibers • 1 emitting; 2 receiving • Bend radius of 50 mm (typical) for insertion into the nasopharynx • Transduction element: silicone gel coated with reflective titanium dioxide • Pressure -> Meniscus deforms -> reflected intensity of light modulated • Diameter of a single transucer element: 0.94 mm. • Resolution: 10 Pa • Range: 5 kPa

23. Flow measurements in anaesthesia and respiratory function analysis • Turbine flow meter: measure the number of rotations of a turbine wheel placed inside the flowing medium.

24. Vortex shedding flow meter • Fluid flows around an obstacle -> creates vortices behind it • Above a certain velocity, uniform vortices are shed alternately from either side of the obstacle • Vortex shedding frequency is proportional to the flow velocity • Vortices create local pressure variation -> detected by piezoelectric capacitors.