Flexible Microsensor Array for the Monitoring and Control of Plant Growth System

1
Flexible Microsensor Array for the Monitoring and
Control of Plant Growth System

• Chang-Soo Kim1,2, D. Marshall Porterfield2,1,
• H. Troy Nagle3, Christopher S. Brown4
• (1) Dept. of Electrical Computer Engineering,
  University of Missouri-Rolla
• (2) Dept. of Biological Sciences, University of
  Missouri-Rolla
• (3) Dept. of Electrical Computer Engineering,
  North Carolina State University
• (4) Kenan Institute for Engineering, Technology,
  Science, North Carolina State University

2
Plant growth platform in microgravity

• Nutrient solution delivery using porous layer
  capillary interface between root and solution.
• Critical need of precise control of rhizosphere
  (root-environment interactions) wetness, oxygen,
  nutrients, temperature, etc.

3
Requirements for sensors in monitoring and
control of root zone

• Ideal configuration miniaturization, multiple,
  low power consumption, robustness and stability.
• Dependable "In situ diagnosis and/or calibration"
  method maintenance of accuracy and
  functionality.
• Microsensors for dissolved oxygen and wetness
  detection.

4
Electrochemical microelectrodes on polyimide
(Kapton) substrate(3-electrode polarographic
oxygen sensor)

• Substrate Kapton (polyimide)
• Metal (Pt) and lithography
• Polyimide lithography (thin)
• Polyimide lithography (thick)

5
Microsensor arrays on flexible substrates

• Oxygen microsensor strip
• (3-electrode electrochemical measurement)
• Conductivity sensor strip
• (4-electrode impedance measurement)

6
Porous tube - microsensor interface (animation)
Nutrient solution
Nutrient solution
Porous tube
Tube cross-section
Tube side-view

• Microsensor array on a flexible substrate
  enwrapping a porous tube.
• Oxygen microsensor dissolved oxygen detection.
• 4-point microprobe surface wetness detection.

7
Experimental setup with a porous tube

• A closer look of the oxygen sensor strip

• Flow system and Instrumentation

8
Surface dissolved oxygen measurement with a
commercial Clark-type mini-probe

• Reflecting O2 value of inner sol. at ()
  pressures.
• Convergence to 20 value (air-sat. value) at (-)
  pressures.

• Plotted with respect to O2 contents.

9
Surface dissolved oxygen measurement with a
microsensor array

• Reflecting O2 value of inner sol. at ()
  pressures.
• Scattering around 0 value at (-) pressures (due
  to surface dryness and absence of sensor
  permeable membrane).

• Plotted with respect to O2 contents.

10
Surface wetness measurement with a 4-point
microprobe array

• A steep decrease of surface impedance at the
  transition from (-) to () pressure.

11
On-chip intelligence of dissolved oxygen
microsensor with built-in microactuator in situ
self-diagnosis

• O2-rich or O2-depleted microenvironments
  established by water electrolysis.
• 2H2O ? 4H 4e- O2 (at AE as anode)
• 2H2O 2e- ? 2OH- H2 (at AE as cathode)

12
Sensor responses during O2-generating actuation
phases with various current densities (in bulk
solution)

• Approaching to O2-saturation value with
  increasing actuation current density.

• Response ratio (saturation _at_t90 / baseline
  _at_t0).
• Excessive responses (beyond 476, i.e.
  100/21).

13
Why excessive response (over 476)?

• Super-saturation
• excessive solubility of electrochemically
  generated dissolved gas near the electrode.
• pH shift accompanying to the gas generation
• dependence of the oxygen reduction coefficient
  (catalytic activity) on solution pH.
• In low pH (n4) O2 2H2O 4e- ? 4OH-
• In high pH (n2) O2 2H2O 2e- ? H2O2 2OH-,
  H2O2 2e- ? 2OH-

14
Why excessive response ? (continued)

• Pseudo-convection
• concentration-driven convection from the AE to
  enhance the mass transfer of oxygen to the WE.
• Electrochemical interference
• feedback of H2O2 to be converted at AE
• (H2O2 ? 2H O2 2e-, byproduct of oxygen
  reduction _at_WE).
• 5. Temperature
• local temperature elevation due to high current
  density in AE.

15
Sensor responses during O2-depleting actuation
phases with various current densities (in bulk
solution)

• Approaching N2-saturation (O2-depletion) value
  with increasing actuation current density.

• Response ratio (saturation _at_t90 / baseline _at_t0).

16
Summary

• Microsensor arrays for porous tube surface
  measurements.
• Dissolved oxygen.
• Wetness.
• Built-in, on-chip intelligence (in situ self
  diagnosis/calibration).
• Controlled oxygen microenvironments with an
  integrated actuator.
• Need of an external permeable membrane.
• Functional integration as well as structural
  integration.

17
Summary (continued)

• Prospects for microscale water electrolysis
• Microsensor application oxygen, pH.
• Microactuator application microfluidics for
  pumping/dosing, microreactor, in situ O2
  generation near root zone.
• Acknowledgement
• NASA grant 01-OBPR-01.