Title: sensors
1 BIOMEMS Class I.
Introduction From MEMS to BIOMEMS/ Definitions
Winter 2009
Dr. Marc Madou
Aequorea victoria
2Content
- From MEMS to BIOMEMS
- BIOMEMS and analytical chemistry
- Definition of sensors
- Sensitivity
- Cross-sensitivity and crosstalk
- Signal-to-noise-ratio and drift
- Resolution
- Span or range and bandwidth
- Dynamic range, gain and dynamic error
- Selectivity
- Hysteresis
- Accuracy
- Calibration
3From MEMS to BIOMEMS
- Miniaturization engineering is a more
appropriate name than MEMS (NEMS), but the name
MEMS (NEMS) is more popular. It involves a good
understanding of scaling laws, different
manufacturing methods and materials. Initially it
involved mostly Si and mechanical sensors (e.g.,
pressure, acceleration, etc). Miniaturization
engineering or MEMS applied to biotechnology is
called BIOMEMS. In BIOMEMS the number of
materials involved is much larger, modularity is
often a must (not integration as in ICs !), costs
often need to be less than whats possible with
Si and batch processes are not always the answer
( continuous manufacturing need !).
4From MEMS to BIOMEMS
5BIOMEMS as part of analytical chemistry
- BIOMEMS may often be seen as a type of
analytical technique used in many research areas
- Chemistry
- Biochemistry
- Biology
- Geology
- Oceanography, etc.
- Analytical techniques which are also used in many
industrial areas - Forensic science (e.g. O.J.s DNA)
- Clinical diagnostics (e.g.glucose in blood)
- Product development (e.g. new drug)
- Quality control (e.g.pH of swimming pool)
- Both instruments and sensors (see next viewgraph
for definition) are used in BIOMEMS both will be
discussed in this course- the distinction between
the two is rather vague (e.g. size, complexity,
parts of an instrument might be called a sensor,
etc.)
6Definitions of sensors
- Chemical sensors are defined as measurement
devices which utilize chemical or biological
reactions to detect and quantify a specific
analyte or event. They are ususally a lot more
difficult to make than physical sensors which
measure physical parameters. - For the distinction between biosensors and
chemical sensors we define a biosensor as one
which contains a biomolecule (such as an enzyme,
antibody, or receptor), a cell or even tissue as
the active detection component. - A sensor, a transducer, transmitter and detector
or often used as synonyms. They are devices that
convert one form of energy into another and
provide the user with a usable energy output in
response to a specific measurable input. In the
chemical sensor area a transducer plus an active
surface is called a sensor.
Effector (magnetic, chemical, physical, etc.)
Active surface
Transducer
Sensor
Integrated sensor
Smart sensor
Amplification/Filtering/A/D, etc
Data storage and processing
Sensor system
Output
Control
7Sensitivity
- A sensor detects information input, Iin, and then
transduces or converts it to a more convenient
form, Iout i.e Iout F(Iin). So sensitivity is
the amount of change in a sensors output in
response to a change at a sensors input over the
sensors entire range. NOT THE SAME AS LOWER
LIMIT OF DETECTION! - Very often sensitivity approximates a constant
that is, the output is a linear function of the
input - Sensitivity may mathematically be expressed as
Germanium Resistance Thermometers
- Sensitivity 35,000 Ohms/K _at_ 4.2 K
- http//www.sci-inst.com/sensors/grt.htm
8Cross-sensitivity and crosstalk
- Cross-sensitivity The influence of one measurand
on the sensitivity of the sensor for another
measurand (e.g., OH- influences F- detection) - Crosstalk Electromagnetic noise transmitted
between leads or circuits in close proximity to
each other
9Signal-to-noise-ratio-S/N and drift
- S/N The ratio of the output signal with an input
signal to the output signal with no input signal - Drift Gradual departure of the instrument output
from the calibrated output. An undesirable change
of the output signal.
Noise is normally measured "peak-to-peak" i.e.,
the distance from the top of one such small peak
to the bottom of the next, is measured
vertically. Sometimes, noise is averaged over a
specified period of time. The practical
significance of noise is the factor which limits
detector sensitivity. A practical limit for this
is a 2 x signal-to-noise ratio.
10Resolution
- The smallest increment of change in the measured
value that can be determined from the
instruments readout scale.
11Span or range (also called bandwidth)
- Span or range The difference between the highest
and lowest scale values of an instrument - Bandwidth The range of scale values over which
the measurement system can operate within a
specified error range ( also used as another word
for span)
12Dynamic range, gain and dynamic error
- Dynamic range The ratio of the largest to the
smallest value of a range, often expressed in
decibels (dB), - GainThe ratio of the amplitude of an output to
input signal. - Dynamic error The error that occurs when the
output does not precisely follow the transient
response of the measured quantity.
13Selectivity
- Selectivity The ability of a sensor to measure
only one parameter, in the case of a chemical
sensor, to measure only one chemical species - Because of the lack of perfect selectivity arrays
are often implemented (e.g., electronic nose and
tongue)
The electronic nose The sensitivity of
certain gas sensors to different gases depends on
the choice of catalytic sensor material and the
operating temperature. By combining several
different gas sensors into a sensor array,
complex gas mixtures can be analysed. Although
the selectivity of the sensors is limited,
qualitative and quantitative gas analysis can be
performed using pattern-recognition techniques.
The combination of multiple gas sensors and
signal analysis using pattern-recognition
techniques is the concept behind the electronic
nose and tongue. These instruments have been
successfully used in a number of applications,
e.g., the quality estimation of ground meat, the
identification of different paper qualities, the
classification of grains with respect to
microbial quality, and the screening of
irradiated tomatoes.
14Hysteresis
- The difference in the output when a specific
input value is approached first with an
increaseing and then with a decreasing input.
Piezoelectric ceramics display hysteretic
behavior. Suppose we start at zero applied
voltage, gradually increase the voltage to some
finite value,and then decrease the voltage back
to zero. If we plot the extension of the ceramic
as a function of the applied voltage, the
descending curve does not retrace the ascending
curve - it follows a different path.
15Accuracy
- The degree of correctness with which a measuring
system yields the true value of a measured
quantity (e.g. bulls eye) --see calibration
http//ull.chemistry.uakron. edu/analytical/animat
ions/
16Precision
- The difference between the instruments reported
values during repeated measurements of the same
quantity. Typically determined by statistical
analysis of repeated measurements
http//ull.chemistry.uakron. edu/analytical/animat
ions/
17Accuracy, precision and standard deviation
- A measurement can be precise but may not not be
accurate - The standard deviation (s) is a statistical
measure of the precision in a series of
repetitive measurements (also often given as ??
with N the number of data, xi is each individual
measurement, and is the mean of all
measurements. The value xi - is called the
residual for each measurement
http//ull.chemistry.uakron.edu/ analytical/animat
ions/
18Calibration standard curve
- A process of adapting a sensor output to a know
physical or chemical quantity to improve sensor
output accuracy i.e. remove bias - A working or standard curve is obtained by
measuring the signal from a series of standards
of known concentration. The working curves are
then used to determine the concentration of an
unknown sample, or to calibrate the linearity of
an analytical instrument-for relatively simple
solutions
19What is Next?