sensors

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BIOMEMS Class I.
Introduction From MEMS to BIOMEMS/ Definitions
Winter 2009
Dr. Marc Madou
Aequorea victoria
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Content

• 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

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From 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 !).

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From MEMS to BIOMEMS
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BIOMEMS 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.)

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Definitions 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
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Sensitivity

• 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

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Cross-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

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Signal-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.
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Resolution

• The smallest increment of change in the measured
  value that can be determined from the
  instruments readout scale.

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Span 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)

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Dynamic 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.

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Selectivity

• 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.
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Hysteresis

• 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.
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Accuracy

• 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/
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Precision

• 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/
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Accuracy, 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/
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Calibration 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

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What is Next?