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Explore the principles and applications of piezoelectric and piezoresistive sensors in biomedicine, from blood pressure measurements to flow sensors and more. Learn about the mechanisms, materials, and advantages of these sensors for accurate and dynamic measurements.
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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
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.