Introduction to Biomedical
Engineering
Biomedical sensors
Kung-Bin Sung
5/21/2007
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Outline
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Chapter 9: Biomedical sensors
– Biopotential measurements
– Physical measurements
– Chemical measurements
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Blood gases and pH sensors
– Biosensors
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Affinity biosensors: enzymatic and antibodybased
Direct and indirect detection
Fiber optic biosensors
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Introduction
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Transducer is a device that converts energy
from one form to another
In sensors, a transducer converts an observed
change into a measurable signal
Integrated with other parts to “read” out the
signal (electrically, optically, chemically)
Some are used in vivo to perform continuous,
invasive or non-invasive monitoring of critical
physiological variables
–
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pressure, flow, concentration of gas
Some are used in vitro to help clinicians in
various diagnostic procedures
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electrolytes, enzymes, metabolites in blood
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Introduction
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in vivo: inside a living body (human or animal)
ex vivo: outside the living body
in vitro: in a test tube
in situ: right in the place where reactions
happen (could be in the cells, tissue, test tube,
etc.)
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Physical measurements
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Displacement
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Inductive
Resistive
Capacitive
Ultrasonic
Air flow
Temperature
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Displacement transducers
Linear Variable Differential Transformer (LVDT)
The primary coil P is excited by an AC current
The induced potentials at the 2 secondary coils are canceled due to
the opposite polarities
When the core moves toward one coil, the induced potential in the coil
increases and the voltage in the other coil decreases
Induced voltage ∝ Displacement
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Displacement transducers
Potentiometer: resistance is
proportional to position
If the current through the resistor is
constant, the displacement (linear or
angular) will be proportional to
∆VO = I∆R
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Displacement transducers
Strain gauge: measures a small change in the length of an object as
a result of an applied force
l
R=ρ
A
Resistance of a conductive materiel with length l and
cross-sectional area A. ρ is a constant (resistivity)
The fractional change in length of an object is called strain
∆l
S=
l
Now consider a metal wire as a strain gauge
l
Original
Stretched
l+∆l
l + ∆l
l
∆l
∆R = ρ
−ρ ≈2 R
Al /(l + ∆l )
A
l
The gauge factor G
G=
∆R / R
=2
∆l / l
For silicon strain gauges, G > 100 (much more sensitive than metal)
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Displacement transducers
Strain gauge examples
Bonded to a semiflexible
backing material
Unbonded configuration for
measuring blood pressure
The change in resistance is quite small ⇒ amplifiers
Strain gauges are very sensitive to temperature ⇒ temperature
compensation
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Displacement transducers
Capacitive: change in distance between two parallel plates (an
insulating material sandwiched in the middle) results in a change in
capacitance
A: area
A
d: distance between two conductors
C = ε 0ε r
d
where ε0 is the permittivity of vacuum = 8.85×10-12 F/m
εr is dielectric constant of the insulating material
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Displacement transducers
Piezoelectric transducer: certain crystal (such as quartz) generates
a small electric potential when it is mechanically strained. The
charge Q induced at surface is proportional to the applied force F
Q = kF
(k is a constant and
specific to the material)
Then the voltage across the crystal is
V=
Q
C
Not used for measuring DC (static) displacement due to internal
leakage resistance that dissipates the surface charge
The inverse reaction can also happen ⇒ apply an voltage across
the crystal, and the crystal expands or contracts
In fact, piezoelectric transducers are often used to generate high
frequency vibrations such as ultrasonic pulse transducers and
resonant oscillators
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Air flow
Fleish pneumotachometer
Flow rate
∝ ∆P
Pressure is measured at
both sides of the resistive
screen
The screen obstruction provides some resistance to the air flow
and therefore generates pressure drop across the screen
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Blood flow
Apply a uniform magnetic field B
across blood vessel
r
r r
F = q (v × B )
If velocity of blood flow is v, F is force
experienced by charged particles in
blood
This force causes movement of
charges ⇒ distribution of charges
generates an electric field E
F
(blood flows into
the screen)
B
E
r V
E=
d
d: diameter of blood vessel
For charged particles, there is a second
force qE, at equilibrium:
V
qvB = qE = q
d
V
⇒v=
Bd
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Temperature transducer - thermocouple
Thermocouple: two different metal wires welded together
Seebeck (1821) effect:
- Thermal to electrical
- An electromotive force (emf) exists across the junction and is
temperature dependent
Nickel
If we use two such junctions,
one is at a known temperature
and the other is at the sample
I ∝ (T1 − T2 )
when the temperature difference is small
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Temperature transducer - thermocouple
The reference is typically in ice bath ⇒ 0°C
Thermocouples:
- very rapid response
- stable over wide range of temperatures (up to hundreds or even 1000)
- can be made quite small ⇒ in 30 gauge needle (outside diameter =
0.3mm)
- disadvantage: need a reference (ice bath)
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Temperature transducer – thermistor
Thermistor: semiconductor; the resistance is a function of
temperature
β (1 / T −1 / T0 )
RT = R0 e
where R0 is the resistance at a reference temperature, T0, and RT is
the resistance at temperature T. β is a material-specific constant. Both
temperatures are expressed in degrees K
Common shapes of thermistors
The size and mass of a thermistor probe must be small to produce a
rapid response to temperature variations
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Temperature transducer – thermistor
Thermistors:
- have high sensitivity (<<1°C)
- range is not as great as thermocouples (-50°C – 100°C), but
suitable for biological/physiological measurements
- need calibration (R vs. temperature curve)
- can also be made very small
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Cardiac output measured by thermodilution
Catheter with multiple ports
- Inject cold saline into the right
atrium (intravenous catheter)
- Measure temperature at the
pulmonary artery over time
- Conservation of energy: The
total heat content of the injected
saline will be
T
Vi ∆Ti ρ i ci = ρ b cb ∫ flow ⋅ ∆Tb (t )dt
0
Average blood flow (m3/s)
ρ: density (kg/m3)
c: specific heat capacity (J/kgK)
Vi: injection volume
T: temperature
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Chemical measurements
Important analytes and their normal ranges in blood, which indicate the
physiological status of the body: gas pressure and related parameters,
electrolytes, and metabolites
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Electrochemical cells as sensors
- Typically used for measurements of ion or gas-molecule
concentrations
- The sensor consists of two electrodes and an ionic conductive
material called an electrolyte
- Operation of electrochemical sensors is based on reactions and
their equilibrium at the electrode surfaces (interface between
electronic and ionic conductors)
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Blood oxygen measurement
Measuring arterial blood gases pO2: in operating room and intensive
care unit to monitor respiratory and circulatory condition of a patient
Clark electrode: measures partial pressure of O2
bias voltage ~0.7 V
O 2 + 2H 2 O + 4e − ↔ 4OH −
Measured current ∝
concentration of O2
Ag ↔ Ag + + e −
Ag + + Cl − ↔ AgCl ↓
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Transcutaneous pO2 measurement
The skin is heated to 43°C to increase local blood flow and enhance
diffusion of O2 through the skin
Mostly used on newborn babies in the ICU because their skin is
thinner
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Blood oxygenation
Oxygen saturation (% of oxygenated hemoglobin) can be measured
and used to represent blood oxygenation
Relationship between arterial blood oxygen saturation and partial pressure
of O2
mm Hg
Note different curves at different pH values
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Oxygen saturation
Saturation of O2
SO2
[
HbO 2 ]
=
× 100%
[Hb] + [HbO 2 ]
Oximetry: (color) measures light absorbance at one wavelength
where there is a large difference between Hb and HbO2 and at
another wavelength (or more wavelengths)
SO 2 = x + y
Absorbance
A(λ1 )
A(λ2 )
A: absorbance
x and y are constants
depending on optical
properties of blood
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Oxygen saturation
Pulse oximetry: use the pulsatile (AC) component to extract oxygen
saturation information and the non-pulsatile (DC) signal as a reference
for normalization
Change in arterial blood
volume associated with
periodic contraction of the
heart
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pH measurements
Membrane permeable to H+ is located at the tip of the active electrode
The potential across the membrane (Nernst equation)
V = 0.059 ⋅ log10 [H + ] + K
K is a constant
pH = − log10 [H + ]
V
ion-selective
membrane
The glass membrane is
highly selective to H+
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pCO2 measurements
For pCO2 measurement
H 2 O + CO 2 ↔ H 2 CO 3 ↔ H + + HCO 3
[CO 2 ] = a(PCO 2 )
−
a is a constant
It can be shown that
−
pH = log[HCO3 ] − logk − log a − log PCO 2
Linear relationship over range of 10-90 mmHg
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Biosensor – introduction
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Biosensor: is a sensor using a living
component or a product of a living thing for
measurement or indication. Generally a
biosensor consists of two parts
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A biological recognition element (enzyme, antibody,
receptor) to provide selectivity to sense the target of
interest (referred to as the analyte)
A supporting element which also acts as a
transducer to convert the biochemical reaction into
“signal” that can be read out
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Biosensor – introduction
“Receptor” provides selective molecular recognition
For example: enzymes, antibodies, receptors, nucleic acids,
polypeptides, etc.
Transducer types in biosensors: calorimetric, electrochemical,
optical, etc.
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Enzymatic biosensors
Enzymes are catalysts for biochemical reactions; large macromolecules
consisting mostly of protein, and usually containing a prosthetic group
(metals)
Substrate: the target molecule of an enzymatic reaction
S+E
ES
E+P
S: substrate; E: enzyme;
ES: enzyme-substrate complex;
P: product
Reaction rate vs. substrate
concentration at constant
enzyme concentration
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Enzymatic biosensors
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Advantages
– Highly selective
– Enzymes are catalytic, thus improving the
sensitivity
– Fairly fast
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Disadvantages
– Expensive: cost of extraction, isolation and
purification
– Activity loss when enzymes are immobilized
– Enzymes tend to be deactivated after a
relatively short period of time
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Enzymatic biosensors
- In an enzymatic biosensor, the enzyme is immobilized as the
“receptor”
- Enzymes are specific to their substrates which can be the analyte
Example: glucose sensors
glucuse oxidase
Glucose + O 2 + H 2 O ←⎯
⎯ ⎯⎯→ gluconic acid + H 2 O 2
Measures the amount of
O2 consumption
permeable to O2 only
Glucose
O2
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Enzymatic biosensors
Alternatively, the reaction product hydrogen peroxide can also be
measured through reduction of H2O2
H 2 O 2 → 2H + + O 2 + 2e −
Use a Clark-type electrochemical cell similar to the one for
measuring O2, but at a positive bias voltage of 0.65V
Problems:
1. The reduction/oxidation potentials are fairly high ⇒ other
materials will interfere
2. The concentration of O2 needs to be kept constant, which is
difficult since O2 is involved in the reactions
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Electron mediators in enzymatic biosensors
In the membrane:
S + O 2 ⎯oxidase
⎯⎯→ P + H 2 O 2
At the anode:
H 2 O 2 → 2H + + O 2 + 2e −
Oxygen is consumed and later regenerated, therefore it only serves
to transfer electron ⇒ can be replaced by a mediator
Example: multi-cycle chemical electrochemical reaction chain for
detecting glucose
(ox)
(red)
(ox)
GO: glucose oxidase
M: mediator
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Immunosensors
Immunosensors: based on highly specific antigen-antibody reactions
Antibody: works against the foreign body
Immunoglobin: globular proteins that participate in the immune
response
Structure of antibody
The Fc end of an antibody can bind to other immune cells and the
binding activate these immune cells
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Immunosensors
Detection of the reaction (binding/association) can be either direct or
indirect (labeled)
Direct detection: the binding of the analyte (antigen) to the “receptor”
(antibody) is detected directly through the presence of the analyte
However, most are indirect due to lack of signal from the analyte itself
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Immunosensors
Indirect detection: signal is provided by an exogenous reporter
competitive
sandwich
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Immunosensors
The interaction between antigen and antibody can be expressed as
ka
A+ B
kb
AB
A: free antigen; B: free antibody
ka: association rate constant; kb: dissociation rate constant
At equilibrium, the affinity constant (or association constant)
KA =
ka
[AB]
=
k d [A][B ]
(units M-1)
And the dissociation constant
k d [A][B ]
=
KD =
[AB]
ka
(units M)
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Immunosensors
Antigen-antibody interactions are reversible and the interaction halflife is determined by the magnitude of the kinetic rate constant
Dissociation rate constant kd and half-life of an antigen-antibody
binding complex
Kinetic rate constants of antigen-antibody interactions can be
obtained with biosensors based on surface plasmon resonance
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Immunosensors
The surface density of antigen molecules bound to the sensor
vs. the bulk concentration of antigen
K: binding constant of the antibody-antigen interaction
Γmax: maximum surface density of antigen molecules
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Indirect detection
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Radioimmunoassay (RIA)
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ELISA (Enzyme-Linked Immunosorbent Assay)
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The labels are radioactive isotopes (for example 131I)
Highly sensitive and specific but inconvenient and
expensive
Less popular now due to safety issues
Enzymes are used for labeling and signal is provided by
the reaction product
Simple in terms of procedure and equipment
Fluorescent immunoassay
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Labeled with fluorescent dyes
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ELISA
detection
antibody
secondary
detection
antibody
substrate
and
product
analyte
Capture
antibody
Typically colorimetric
measurement (absorbance at
certain wavelength ∝ analyte
concentration)
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Direct detection – Surface plasmon resonance
A surface plasma wave (SPW) is an electromagnetic wave (due to
charge density oscillation) which propagates along the boundary
between a dielectric (e.g. glass) and a metal
Magnetic field parallel to
the interface
n1
The magnitude of the surface
plasmon wave decreases
exponentially into both dielectric
and metal media
Electric field is parallel to
the plane of incidence
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SPR biosensors
A surface plasmon wave can be excited optically: when the propagation
constant β of the incident light and the SPW are equal in magnitude and
direction for the wavelength
This corresponds to a certain angle of incident light at a fixed
wavelength ⇒ the reflected light reaches its minimum
reflectivity
Incident light with
various angles
Binding induced change in
propagation constant of SPW is
proportional to the refractive index
change in the sample ⇒ the angle
of incident light also changes
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SPR biosensors
Different parameters are measured in SPR biosensors to indicate the
refractive index change in the sample (due to binding)
Using monochromatic light
Angle of incident light
Using fixed incident and
reflection angles
Wavelength of incident light
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SPR biosensors
The kinetics of binding events can be monitored continuously sensorgram
BSA: bovine serum albumin, used
to non-specific binding
Analyte: SEB (staphylococcal
enterotoxin B)
a-SEB: antibody against SEB ⇒
sandwich binding
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SPR biosensors
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Direct detection ⇒ no need for labeling
High sensitivity
Fast ⇒ real-time monitoring
Used with sample in liquid ⇒ important
for biological samples
Relatively simple device, which makes
multichannel parallel detection easier ⇒
high throughput
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SPR commercialization - BIACORE
BIACORE optical system: light from different angle of reflection is imaged
onto different position of a photo-detector array
2D photo-detector
array
Multiple flow-cells ⇒ simultaneous detection of reflection intensity
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BIACORE products
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BIACORE features
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Specificity: different molecules interact with a
single partner immobilized on a sensor surface
Kinetics: rates of complex formation (ka) and
dissociation (kb)
Affinity: the level of binding at equilibrium (KD,
KA)
Concentration: can be determined for purified
molecules or for molecules in complex
mixtures such as serum (needs a calibration
curve)
Multiple interactions during complex formation
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Fiber optic based biosensors
Optical fibers are flexible and compact which are important features
for applications that need miniaturization such as in vivo
measurements
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Optical fiber
Typically made of glass SiO2, and consisting of a core and a cladding layer
Transmits light through the core (total internal reflection)
Diameter can be very small ~µm ⇒ very flexible
The angle of light rays that go into the fiber core and can be accepted
(transmitted) by the fiber is dependent on the core and cladding materials
Incident angle of light rays going into a fiber must be less than θ
θ
θt
NA of an optical fiber = sinθ =
n2
n1
n1 − n2
2
2
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Optical fiber sensor – examples
• Sensing area along side of an optical fiber
Total internal reflection
inside the fiber
• Sensing area located at distal end surface of an optical fiber
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Optical fiber sensor – examples
A fiber optic (imaging) bundle is used for detecting multiple
analytes simultaneously
- Each latex sphere can specifically binds to one analyte
- The analytes can be fluorescently labeled
- Use of reference channels could increase the reliability of
detection (e.g. spheres without recognition element still bind
to some analyte molecules non-specifically, so the measured
fluorescence from the reference spheres is considered
“background” and should be subtracted)
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Trends in biomedical sensors
Centralized laboratory
Point of care
Patients’ home
in vitro
Single test
in vivo
Multiple tests in a single run
Key technologies for miniaturization of biomedical sensors and
realization of biochips: microfluidics, semiconductor fabrication
processes, microelectromechanical systems (MEMS)
Equally important are: the development of more stable and
effective biomolecules used for recognition; search for new target
analytes that are of diagnostic and/or biological significance
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Biochips
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Miniaturized and highly integrated
Provide analytical or diagnostic function
Substrates can be glass, silicon, polymer,
plastic, etc
Desired features: high sensitivity and
specificity, high speed, small size, small
sample requirement, multi-channel and
parallel analyzing, reliable, cost-effective
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References
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Medical Instrumentation: Application and
Design, edited by John G. Webster
Chemical sensors and biosensors, by Brian R.
Eggins
Sensors in Biomedical Applications:
Fundamentals, Technology & Applications, by
Gábor Harsányi
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Ch7: Biosensors
Information about glucose meters
http://www.diabetesmonitor.com/meters.htm#fc
nim
Biomolecular sensors, edited by Electra Gizeli
& Christopher R. Lowe
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