Mechanical Sensors in Biomedicine Xingwei Wang

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

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

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:

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

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

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

Pyroelectric effect Pyroelectric coefficient: p Flux density: D Temperature: T

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

Applications Infrared detection High sensitivity: 1/1000 °C

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

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

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

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

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

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

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.

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

Fiber optic pressure sensors

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

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

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

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.

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.