Biomedical Electrodes and Sensors

Electrodes

should be made of metals which resist corrosion and oxidation

Pt, Au, Ag

Type of electrodes

• Superficial (disc / cup) electrodes

• Needle electrodes

• Micro-electrodes

Superficial (disc / cup) electrodes (10)

• ECG (Electrocardiography):

  • Ag/AgCl disposable surface electrodes

  • record electrical activity of the heart

• EMG (Electromyography):

  • Ag or Pt discs

  • measure muscle electrical activity

• EEG (Electroencephalography):

  • Pt cup electrodes

  • glued to the scalp

  • an electrolyte gel is injected into the electrode cup for better conductivity

Needle electrodes (2)

Invasive electrodes used to record biovoltages from a small area of tissue
fx.: individual nerves and muscle fibers

Micro-electrodes (3)

• Used for biovoltages from individual cells

• It is a glass capillary with an open end filled with an electrolyte of standard concentration

  • tip diameter: <0.5 μm

Superficial ECG Electrodes (photo)

Superficial EEG electrodes (photo)

Contact Voltage Problems in Superficial Electrodes

• Electrodes can generate unwanted "contact voltages" when in contact with the body

• Due to electrochemical reactions with skin fluids, sweat, and humidity

Polarisable electrodes

• Produce variable contact voltage that changes over time

• Create an uncontrolled variable bias, making them unsuitable for accurate measurements

Non-polarisable electrodes

• Generate a constant contact voltage, providing a measurable and consistent bias

• Suitable for accurate measurements as the bias can be accounted for

• Ag-AgCl electrode is used

Sensors Research Goal

To create sensors that can measure every variable in the human body, enhancing diagnostic and monitoring capabilities

Sensors design (3)

• need to be protected from body fluids

• designed to avoid insertion biases

• Act of measurement should not alter the value of the quantity being measured, ensuring reliability and accuracy in data collection

Main Sensor Categories (7)

• Mechanical variables

  • force, pressure, velocity

• Temperature

• Electromagnetic radiation

  • all ranges of the electromagnetic spectrum

• Chemical and biochemical concentrations

• Others: biomagnetism, gas-flow

Force (and Pressure) Sensors most common type based on

Piezoelectric Crystals - voltage produced across certain crystals when force/pressure is applied

Force (and Pressure) Sensors used in (4)

• balances

• pressure measurement devices

• force-platforms

• ultrasound imaging devices

Force (and Pressure) Sensors Pressure monitoring (5)

• intravascular

• urodynamic

• intracranial

• intra-uterine

• intra-ocular

Micro force Sensors

implanted in knee joint and provide ongoing information on forces and torques in joint

Velocity Sensors general

• Based on the Doppler Effect

• change in the frequency of reflected sound waves from a moving object is proportional to the velocity of the object

• Used for vascular measurements

• fx.: EchoHeart

EchoHeart

• first transvaginal audio Doppler probe

• It is intended as an economical and convenient method to determine fetal viability in very early stages of pregnancy

Velocity Sensors device

• Red-orange colour: blood moving in one direction

• Blue colour: blood moving in opposite direction (towards or away from sensor)

Temperature sensors

devices used to measure temperature in various environments and applications

used for central (nasopharynx - oesophagus - rectal) and skin temperature measurement

Temperature Sensors types

• Semiconductors temperature sensors

• Resistance wire temperature sensors

• Lead zirconate titanate capacitors

• Thermocouples

Semiconductors temperature sensors

change in electrical conductivity of semiconductor with temperature

Resistance wire temperature sensors

change in resistance of wire with temperature

Lead zirconate titanate capacitors

change of capacitance of capacitor with temperature

Thermocouples

voltage produced at a junction of two dissimilar metals varies with temperature

IR / Visible / UV Sensors definition

detect and respond to light in different regions of the electromagnetic spectrum, including infrared (IR), visible light, and ultraviolet (UV) wavelengths

fx.: Tympanic (ear) Thermometer

Types of Light Sensors (6)

• Semiconductor based

  • Photoresistors (Photoconductors)

  • Photodiodes

  • Phototransistors

  • Image Sensors (CMOS, CCD)

• Photomultiplier Tubes (PMTs) - detect low light

Photodiodes (photo)

Photomultiplier tubes (photo)

Image Sensor (photo + example)

in digital cameras 20 Megapixel camera means 20 million light sensors

Semiconductor Tympanic (ear) Thermometer

based on the measurement of infra-red radiation emitted from the ear drum

Photodiode arrays

components of spectrophotometers

• Measure concentrations of metabolically important substances in body fluids

  • blood, cerebrospinal fluid, urine and amniotic fluid

Substances that can be quantitatively analyzed of Photodiode Arrays (lot so choose some to know)

hemoglobin, erythrocytes, hematocrit, amylase, bilirubin, cholesterol, glucose, urea, creatinine, lipase, triglyceride, albumin, alcohol, ammonia, copper, magnesium, lactate, calcium, iron, magnesium, aluminium, sodium carbonate, carbon monoxide and even certain enzymes

CMOS Image Sensor Projection Lens (photo)

Digital Microscopes (photo)

Pulse Oximeter (2)

part of – in vivo spectrophotometry

Measures the oxygen saturation of a patient's blood

Key Components and Functionality of Pulse Oximeter (5)

• Light Emitting Diodes (LEDs)

  • Red LED: Emits light at a wavelength of 660 nm.

  • Infrared LED: Emits light at wavelengths of 905 nm, 910 nm, or 940 nm.

• Photodiode

  • A sensor that detects the amount of light that passes through the patient's tissue

Pulse Oximeter Light Absorption and process (5)

• Oxyhemoglobin (oxygenated hemoglobin)

  • absorbs more infrared light

• Deoxyhemoglobin (deoxygenated hemoglobin)

  • absorbs more red light

The device calculates the ratio of oxyhemoglobin to deoxyhemoglobin

Infra Red Sensors (IR camera) (2)

• Temperature measurement at a distance

• Colour-scale coded temperature map

X / gamma Sensors types (8)

• Semiconductor-Based Sensors

  • Silicon Sensors

  • Germanium Sensors

  • Cadmium Zinc Telluride (CdZnTe) and Amorphous Selenium (aSe)

• Gas-Filled Sensors

  • Geiger-Muller Tube

  • Ionization Chamber

  • Proportional Counter

• Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) Crystals

Silicon Sensors

Measure individual X-ray photon energy

Germanium Sensors

Measure individual gamma photon energy

Cadmium Zinc Telluride (CdZnTe) and Amorphous Selenium (aSe)

Measure X/gamma energy fluence rate (energy per unit area per unit time)

Geiger-Muller Tube

Functions primarily as a detector rather than a sensor, providing simple count rates of radiation events

Ionization Chamber

Measures the total energy fluence (energy per unit area) from ionizing radiation

Proportional Counter

Measures the energy of individual alpha, beta, and gamma particles, providing detailed information about the radiation type and energy

Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) Crystals (2)

LiF : Mg : Ti

Measure total energy absorbed over a specific period

TL personal dosemeters (photo)

Radioactivity contamination monitors (photo)

Patient dose measurement meters (photo)

RF Sensors (antennas) (2)

• MRI system coil (generate/recieve signals)

• MRI antennas measure RF coming from the human body

Chemical and Biochemical Sensors

specialized devices designed to detect and measure specific chemicals or biological substances in a sample

Chemical and Biochemical Sensors types

• Ion-Selective Sensors

• Gas Sensors

• Biomolecular Sensors

Ion-Selective Sensors (2)

• Detect specific ions

  • fx.: pH sensors for hydrogen ions

• Measure ion concentration through selective membrane interaction

Gas Sensors (2)

• Measure gas levels like O₂, CO₂, and NH₃

• Important in medical diagnostics and environmental monitoring

Biomolecular Sensors (2)

• Detect biomolecules such as proteins and cancer biomarkers

• Significant in early disease detection and personalized treatment

Biomagnetic sensors (3)

• based on the Hall effect

• A semiconductor with current in a magnetic field generates a voltage perpendicular to the current direction

  • proportional to the magnetic field's strength