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.