Biomedical Sensors

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Biomedical Sensors
A key in knowing what is wrong with you.
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What is Measured and How?

• Measurand - a biomedical property that can be
  measured.

• Transducible property - that property of a
  measurand that allows it to be measured.

• Biosensor - converts a measurand into a
  measurable signal, usually electrical.

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Classification of Measurands

• Biopotentials, for example, ECG, EEG, etc. That
  is, those that arise from an electrical signal
  generated in the body.

• Nonbiopotentials, for example, temperature, blood
  pressure, rate of blood and air flow.

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Classification of Sensors

• Diagnostic - temperature, blood pressure, ECG

• Therapeutic- defibrillators, pacemakers

• Assistive - cochlear implants, functional
  neuromuscular stimulation

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Location of Sensor

• Noninvasive/noncontact - blood gases,
  electrolytes, etc.

• Noninvasive/contact - ECG electrodes,
  temperature, oximetry, etc.

Invasive/short-term - EMG electrodes, catheter,
etc.
Invasive/long-term - Pacemaker, FES electrodes,
etc.
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Measuring a Physiological Potential
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Measurable Physiological Potentials
Voltage (V)
0.1 1 10 100
1,000 10,000
Frequency (Hz)
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Electrodes

• Metal conductor in an electrolyte solution

Transduces an ion to an electron current
Characterized by electrode potentials
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Surface Electrodes
Foam
Self Adhesive
Tan Weave
Multi Use
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Electrode Potential
Metal
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Electrolyte Gel

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Skin
Half-cell potential
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Ag/AgCl Electrodes

• Very important clinically and in research.

Cl is the primary charge carrier and is present
in tissue.
Low noise electrodes.
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PHYSICAL MEASUREMENTS

• Force, acceleration, displacement

Temperature systemic, thermography
Pressure blood, pulmonary
Volume blood, pulmonary
Flow blood, air
Blood oxygen content (oximetry)
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Linear Variable Differential Transformer
Ferrite Core
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Differential Capacitive Transducer
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Electromagnetic Flowmeter
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Electromagnetic Flowmeters
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(a)
(b)
(a) Signals without noise are uncorrupted. (b)
Interference superimposed on signals causes
error. Frequency filters can be used to reduce
noise and interference.
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(a)
Figure 1.15 (a) Original waveform. (b) An
interfering input may shift the baseline. (c) A
modifying input may change the gain.
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(a)
(b)
Figure 1.16 Data points with (a) low precision
and (b) high precision.
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(a)
(b)
Figure 1.17 Data points with (a) low accuracy and
(b) high accuracy.