A MEMS thermal biosensor is described for metabolic monitoring applications. It consists of a thermal sensor chip integrated with a microfluidic system featuring two chambers - one for a sample solution and one for a reference buffer. Changes in temperature difference during biochemical reactions can be measured by the thermopile and indicate concentration changes. The device operates in either a flow-injection or flow-through mode. Fabrication involves depositing nickel and chromium layers for heaters and thermopiles, etching a backside cavity, and bonding a PDMS microfluidic chip. The sensor offers improved sensitivity for thermal monitoring of biochemical processes.

1. A MEMS Thermal Biosensor for
Metabolic
Monitoring Applications

2. What is a Biosensor?
 “Biosensor” is short for “Biological Sensor”.
 An Analytical device which converts a Biological response into an
Electrical Signal.
 A Biosensor typically consists of a Bio-recognition component,
Biotransducer component, and electronic system which include a
signal amplifier, processor, and display.

3. Components of a Biosensor

4. Thermal Biosensor
 THERMAL biosensors measure thermal energy released or absorbed in
biochemical reactions.
 Thermal activities exist ubiquitously in Biological Processes, and hence widely
applicable.
 Requiring no labeling of reactants, thermal bio sensing is a universally useful
method, allowing direct interrogations of elementary processes in biochemistry
without sophisticated cascades of reaction steps.

5. Thermal Biosensor
 MEMS thermal sensors are often based on temperature detection.
I. Thermistors
 Rely on changes in their electric resistance with temperature.
 Allow measurement of absolute temperatures.
 Limited in sensitivity
II. Thermopile
 Set of Thermocouple Junctions connected in series.
 Allows measurement of differences in temperature between two Junctions.
 Offers excellent common-mode noise cancellation and zero offset, and therefore can be highly
Sensitive.

6. Advantages of MEMS Thermal Biosensor
 Improved Thermal Isolation.
 Reduced Thermal Mass and Sample Volume.
 Offer improved Sensitivity and Linear Range.
 Reduced Power Consumption.
 Shortened Measurement Times.
 Allows batch fabrication and integration of miniaturized devices at
low cost
 Enable High-Throughput Operation

7. Design
 Device consists of a thermal sensor chip integrated with a microfluidic system featuring two
identical chambers.
 An analyte sample solution and a reference buffer solution are respectively loaded into the
chambers.
 The device can be used in Two modes: Flow-Injection mode and Flow-Through mode.
 An important feature of the microfluidic system is that the chambers are each based on a
freestanding polymer diaphragm.
 The temperature difference induces a voltage in the thermopile, which is the device’s direct
output

9. Operating Principle
 Changes during reaction:
o Enthalpy Change (ΔH)
o Change in Concentration(cs)
 Concentration change in Flow - injection mode
 cs = cs0e−λt.
 Concentration change in Flow - through mode
 cs = cs0e−(λA/Q)

10. Operating Principle
 Power Density
 ΔP =cs0λV ΔHe−λt (flow - injection mode)
 ΔP = cs0QΔH(1 − e−λV/Q) (flow - through mode).
 Temperature Difference
 ΔT = ReffΔP
 Thermopile Voltage
 ΔU = KΔP

11. Fabrication
 The fabrication process starts with a silicon
wafer with both sides covered with thermally
grown silicon dioxide.
 Chromium and nickel are then deposited via
sputtering and patterned to form the integrated
resistive heaters (nickel), thermopile
(nickel/chromium) and bonding pads
(chromium).
 Tetramethyl ammonium hydroxide is used to
etch the wafer from its backside with the oxide
there patterned as an etching mask.

12. Fabrication
 An SU-8 thin film is spin-coated and patterned to
form the diaphragms, which upon wafer dicing
are made freestanding by gas-phase XeF2 etching.
 The microfluidic chip was fabricated from
polydimethylsiloxane (PDMS) by replica
molding.
 PDMS is cast onto the SU-8 mold, cured at 70 ◦C
and peeled off to obtain a sheet bearing the
microfluidic features including the chambers,
channels and weirs.
 The fluidic interface is made by inserting Tygon
tubing into the inlet and outlet wells and then
sealed with epoxy.

14. References
1. Li Wang, David M. Sipe, Yong Xu, Qiao Lin, “A MEMS Thermal Biosensor for
Metabolic Monitoring Applications”, Journal of Microelectromechanical Systems,
Vol. 17, No. 2, April 2008.
2. B. Danielsson, “Calorimetric biosensors,” Biochem. Soc. Trans., Vol. 19, pp. 26–
28, 1991.
3. Martin F. Chaplin, C. Bucke, “Enzyme Technology”, 1991.