Biomedical Sensors

Dr. James A. Smith

What’s Important?

• Accuracy

• Operational Range

• Response Time

• Sensitivity

• Resolution

• Reproducibility

Classification

• Breakdown into general categories– Physical

• Goniometer

– Electrical• EMG (electromyographic)

– Chemical• Blood oxygen

Sensor Packaging

• “In vivo”– Inside the organism!– Risky!– Biocompatibility

• Host can affect the sensor & vice versa• Protein absorption / cell. Deposits

– Permeability affected

• Inflamation of tissue (latex!)• Sterilization

– Steam, ethylene oxide, gamma radiation

Electrolyte / Metal Electrodes

• Charge distribution at interface

• Different metals have different potentials

Physical Measurements: Displacement

• Linear Variable Diff. Transformer• Goniometer

– Knee & Elbow

• Strain– Wire length & width– Chest contraction– Force Plates

• Ultrasonics

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Temperature

• Tightly controlled body variable

• 37.0 +/- 0.5 C at core– Too high: disease or infection– Too low: trauma / shock

• Thermistor– Change resistance with temperature– Cavity or closed area

Oxygen Measurement

• Plasma O2: 2% of total oxygen– Clarke Electrode

• Chem. Reaction with Platinum• Electrode Current is prop. To oxygen content• Transcutaneous• Heat releases oxygen through skin

• Hemoglobin– Optical oximeter– Light absorption by blood

pH Electrodes• Acid: excess hydrogen ions• Base: excess hydroxl ions• Normal blood pH: 7.4 (basic)

– CO2 removal by lungs– Kidneys acid-base regulation

• Organic dye strips• Electrodes in solution

– a battery consisting of two electrodes, • each immersed in its respective solution• joined by a salt bridge

– Glass electrodes– Glass membrane can wear out

CO2

• CO2 in solution:– Weak acid

– Carbonic Acid (H2CO3)

• Use electrodes like in pH system

Enzyme Biosensors

• Biological specific mediators– Reagents for reaction or catalyzing

• Enzymes– 2000 proteins w/ biocatalytic properties– Accelerate reactions in cells

• Embed in neutral matrix• Glucose sensor

– Enzyme glucose oxidase– Useful for diabetics– Measure gluconic acid or hydrogen peroxide

Microbial Sensors

• Assimilation of organic compounds by microorganisms

• Look for secretions by micros– H2, CO2, etc.

• Need immobilized microbes

• Examples– Ammonia & Nitrogen Dioxide

EKG / ECG

• Electrocardiogram (ECG)

• Electrokardiogram (EKG)

• Electrode– Polymer & carbon / metal filler– Silver Chloride contact – Electrolytic foam– Motion artifacts!

EMG / EEG

• Electromyogram (EMG)• Nerve & Muscle• Surface are like EKG• Direct

– Needle– Bipolar electrode

• EEG– Brain– Cups (gel)– Subdermal (10mm, 0.5mm)

EMG & Muscles

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Source: http://www.eorthopod.com/images/ContentImages/elbow/elbow_anatomy/elbow_anatomy07c.jpg

EMG: locate muscles

Figure 1

Source: Vernier Inc.

Electrode Placement

Source: Vernier Inc.

Electrical Patterns

Source: http://www.unmc.edu/physiology/Mann/pix_14/emg.gif

EMG Video

• http://www.youtube.com/watch?v=k0uSpYd_Ics

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