Figure 1.
Block diagram of the MUlti-SEnsor array (MUSE) system composed of the following functional modules: A—pH sensor; B—impedance sensor; C—temperature sensor; D—stress sensor; E—cyclic voltammetry module; F—transient voltage analysis module; G—electrical stimulator; H—Cactus Semiconductor CSI021 (Cactus Semiconductor, Chandler, AZ, USA) stimulator (secondary stimulator); I—power monitoring to monitor the system; J—intelligent power routing to reduce power consumption; K—data storage (SD card); L—microcontroller and M—application software to control the system.
Figure 1.
Block diagram of the MUlti-SEnsor array (MUSE) system composed of the following functional modules: A—pH sensor; B—impedance sensor; C—temperature sensor; D—stress sensor; E—cyclic voltammetry module; F—transient voltage analysis module; G—electrical stimulator; H—Cactus Semiconductor CSI021 (Cactus Semiconductor, Chandler, AZ, USA) stimulator (secondary stimulator); I—power monitoring to monitor the system; J—intelligent power routing to reduce power consumption; K—data storage (SD card); L—microcontroller and M—application software to control the system.
Figure 2.
Conceptual sketch of the dual multi-sensor array configuration. A cross-sectional view of the array is shown (
top). It depicts a PtIr stimulation electrode (
blue) positioned above a stress strain gauge (
purple) as well as surrounded by an IrOx sputtered on gold pH working electrode (WE) (
red), an Au (
pink) and Pt (
turquoise) reference pH electrode (RE) and four Pt impedance electrodes (
black). The temperature strain gauge (
orange) is positioned around the electrodes and is prototyped with the strain gauge shown in (
a) [
17,
19]. The PtIr stimulation electrode is positioned directly above the stress strain gauge depicted in prototype (
b,
c) [
20]. Prototype (
d) shows a picture of the prototype multi-sensor array with the four Pt impedance electrodes surrounding the pH and stimulation electrodes. The pH electrodes allow a differential measurement to be made between the working electrode (IrOx) and reference electrodes (Au electrode, semilunar shaped Pt electrode and more distant IrOx for pH measurement of the surrounding medium, intended to be insensitive to stimulation) while stimulating via the PtIr electrode is performed. The pH sensor used to verify the pH electronics is shown in (
e).
Figure 2.
Conceptual sketch of the dual multi-sensor array configuration. A cross-sectional view of the array is shown (
top). It depicts a PtIr stimulation electrode (
blue) positioned above a stress strain gauge (
purple) as well as surrounded by an IrOx sputtered on gold pH working electrode (WE) (
red), an Au (
pink) and Pt (
turquoise) reference pH electrode (RE) and four Pt impedance electrodes (
black). The temperature strain gauge (
orange) is positioned around the electrodes and is prototyped with the strain gauge shown in (
a) [
17,
19]. The PtIr stimulation electrode is positioned directly above the stress strain gauge depicted in prototype (
b,
c) [
20]. Prototype (
d) shows a picture of the prototype multi-sensor array with the four Pt impedance electrodes surrounding the pH and stimulation electrodes. The pH electrodes allow a differential measurement to be made between the working electrode (IrOx) and reference electrodes (Au electrode, semilunar shaped Pt electrode and more distant IrOx for pH measurement of the surrounding medium, intended to be insensitive to stimulation) while stimulating via the PtIr electrode is performed. The pH sensor used to verify the pH electronics is shown in (
e).
Figure 3.
Schematic of the basic functional circuit of the developed pH readout system. WE: working electrode, RE: reference electrode.
Figure 3.
Schematic of the basic functional circuit of the developed pH readout system. WE: working electrode, RE: reference electrode.
Figure 4.
Circuit schematic of the AD5933 impedance analysis chip and signal scaling stage.
Figure 4.
Circuit schematic of the AD5933 impedance analysis chip and signal scaling stage.
Figure 5.
Impedance excitation circuit allowing voltage (two or three electrode setup) or current (two or four electrode setup) excitation of the system under test. (The current excitation signal in galvanostatic mode is defined using the current set resistance RSet = Vexcite_max/Iexcite_max).
Figure 5.
Impedance excitation circuit allowing voltage (two or three electrode setup) or current (two or four electrode setup) excitation of the system under test. (The current excitation signal in galvanostatic mode is defined using the current set resistance RSet = Vexcite_max/Iexcite_max).
Figure 6.
Circuit schematic showing the source multiplexing and the calibration resistance selection. Analog switch inputs A0 to A2 allow the different electrode configurations for the impedance measurement. Designation 1 and 2 correspond to the working and counter electrode, respectively.
Figure 6.
Circuit schematic showing the source multiplexing and the calibration resistance selection. Analog switch inputs A0 to A2 allow the different electrode configurations for the impedance measurement. Designation 1 and 2 correspond to the working and counter electrode, respectively.
Figure 7.
Possible impedance measurements for two multi-sensor arrays. Shown on the left is the working multi-sensor array with four impedance electrodes (A–D) located around the stimulation electrode (W). On the right is the counter multi-sensor array likewise with four impedance electrodes (A–D) located around the stimulation counter electrode (C). Each connecting line represents a possible impedance measurement option.
Figure 7.
Possible impedance measurements for two multi-sensor arrays. Shown on the left is the working multi-sensor array with four impedance electrodes (A–D) located around the stimulation electrode (W). On the right is the counter multi-sensor array likewise with four impedance electrodes (A–D) located around the stimulation counter electrode (C). Each connecting line represents a possible impedance measurement option.
Figure 8.
Five physical equivalent models (A—E) for characterization of the measurement setup and its calibration.
Figure 8.
Five physical equivalent models (A—E) for characterization of the measurement setup and its calibration.
Figure 9.
Circuit schematic of the Wheatstone bridge arrangements for (a) the temperature sensor and (b) the stress sensor. Digital potentiometers 1 and 2 are used to balance the resistive component of the bridge, while a dual parallel RCL arrangement is used to balance the imaginary component of the bridge.
Figure 9.
Circuit schematic of the Wheatstone bridge arrangements for (a) the temperature sensor and (b) the stress sensor. Digital potentiometers 1 and 2 are used to balance the resistive component of the bridge, while a dual parallel RCL arrangement is used to balance the imaginary component of the bridge.
Figure 10.
Circuit schematic of the square wave to sine wave converter based on a second-order Sallen-Key low-pass-filter (left amplifier) and active low-pass filter (right amplifier) design.
Figure 10.
Circuit schematic of the square wave to sine wave converter based on a second-order Sallen-Key low-pass-filter (left amplifier) and active low-pass filter (right amplifier) design.
Figure 11.
Circuit schematic showing a digitally controlled low-pass filter at each input stage for both sensor multiplexers.
Figure 11.
Circuit schematic showing a digitally controlled low-pass filter at each input stage for both sensor multiplexers.
Figure 12.
Circuit schematic of the AC coupled AD630 synchronous demodulator in a lock-in amplifier configuration.
Figure 12.
Circuit schematic of the AC coupled AD630 synchronous demodulator in a lock-in amplifier configuration.
Figure 13.
Circuit schematic of the preamplifier gain multiplexer.
Figure 13.
Circuit schematic of the preamplifier gain multiplexer.
Figure 14.
Circuit schematic showing the ADC buffering and low-pass filter stage of the AD630 output.
Figure 14.
Circuit schematic showing the ADC buffering and low-pass filter stage of the AD630 output.
Figure 15.
Circuit schematic showing a basic potentiostat setup for cyclic voltammetry interfacing to the WE, CE and RE.
Figure 15.
Circuit schematic showing a basic potentiostat setup for cyclic voltammetry interfacing to the WE, CE and RE.
Figure 16.
Circuit schematic for measuring the WE-RE and CE-RE transient voltages.
Figure 16.
Circuit schematic for measuring the WE-RE and CE-RE transient voltages.
Figure 17.
pH series from pH 5.9 to pH 8.5 in steps of 0.5 for an iridium oxide electrode vs. an Ag/AgCl reference electrode. For each pH measurement one minute was taken to swap the pH electrode and give the sensor time to adapt to the new environment, followed by two minutes to measure the sensors response. Measurement using (a) a digital multimeter (Agilent model 34405A) and (b) the custom-made pH readout system.
Figure 17.
pH series from pH 5.9 to pH 8.5 in steps of 0.5 for an iridium oxide electrode vs. an Ag/AgCl reference electrode. For each pH measurement one minute was taken to swap the pH electrode and give the sensor time to adapt to the new environment, followed by two minutes to measure the sensors response. Measurement using (a) a digital multimeter (Agilent model 34405A) and (b) the custom-made pH readout system.
Figure 18.
pH value series from pH 5.9 to pH 8.5 in steps of 0.5 for an iridium oxide electrode vs. an Ag/AgCl reference electrode. One minute of open circuit potential recording followed by one minute to change the pH solution and allow the sensor some time to adapt to the new solution. Data recorded using the custom made electronics.
Figure 18.
pH value series from pH 5.9 to pH 8.5 in steps of 0.5 for an iridium oxide electrode vs. an Ag/AgCl reference electrode. One minute of open circuit potential recording followed by one minute to change the pH solution and allow the sensor some time to adapt to the new solution. Data recorded using the custom made electronics.
Figure 19.
Hysteresis plot of the pH vs. open circuit potential recorded during the stability test. Grey area indicates the standard deviation of the repeated pH series with trend lines. The average Nernstian slope of the pH curve is −57.301 mV/pH.
Figure 19.
Hysteresis plot of the pH vs. open circuit potential recorded during the stability test. Grey area indicates the standard deviation of the repeated pH series with trend lines. The average Nernstian slope of the pH curve is −57.301 mV/pH.
Figure 20.
Characteristic curves of the AD5933 impedance converter setup for a selection of resistor values for (a) the magnitude code and (b) the measured system phase versus frequency. The graph in (a) shows significant resonance behaviour at 300 Hz, however the effect becomes less severe at lower resistance values.
Figure 20.
Characteristic curves of the AD5933 impedance converter setup for a selection of resistor values for (a) the magnitude code and (b) the measured system phase versus frequency. The graph in (a) shows significant resonance behaviour at 300 Hz, however the effect becomes less severe at lower resistance values.
Figure 21.
Inverse of the magnitude curve versus resistance showing the linear behaviour of the AD5933. In the range of 1 kΩ to 10 kΩ the system showed best linear behaviour and represented the optimal measurement range of the device setup.
Figure 21.
Inverse of the magnitude curve versus resistance showing the linear behaviour of the AD5933. In the range of 1 kΩ to 10 kΩ the system showed best linear behaviour and represented the optimal measurement range of the device setup.
Figure 22.
The Bode plot for the arrangements A, B, C recorded with the AD5933 custom system are shown in figures (a,c), while the bode plots for the same arrangements recorded by an Autolab PGSTAT302N impedance module are shown in figures (b,d).
Figure 22.
The Bode plot for the arrangements A, B, C recorded with the AD5933 custom system are shown in figures (a,c), while the bode plots for the same arrangements recorded by an Autolab PGSTAT302N impedance module are shown in figures (b,d).
Figure 23.
The Bode plot for the arrangements D, E recorded with the AD5933 custom system are shown in figures (a,c), while the bode plots for the same arrangements recorded by an Autolab PGSTAT302N impedance module are shown in figures (b,d).
Figure 23.
The Bode plot for the arrangements D, E recorded with the AD5933 custom system are shown in figures (a,c), while the bode plots for the same arrangements recorded by an Autolab PGSTAT302N impedance module are shown in figures (b,d).
Figure 24.
Bode plot of a small platinum electrode with a diameter of 0.85 mm (magnitude (a) and phase (b)). Measurement configured in a three electrode setup. For comparison both the measurement for the custom AD5933 system (black) and the Autolab system (orange) are shown.
Figure 24.
Bode plot of a small platinum electrode with a diameter of 0.85 mm (magnitude (a) and phase (b)). Measurement configured in a three electrode setup. For comparison both the measurement for the custom AD5933 system (black) and the Autolab system (orange) are shown.
Figure 25.
Simulation of an AC sweep across a balanced modified Wheatstone bridge showing (a) magnitude; (b) phase and (c) magnified phase at 10 kHz vs. applied AC frequency.
Figure 25.
Simulation of an AC sweep across a balanced modified Wheatstone bridge showing (a) magnitude; (b) phase and (c) magnified phase at 10 kHz vs. applied AC frequency.
Table 1.
External clock frequencies required to achieve a specific frequency range [
25].
Table 1.
External clock frequencies required to achieve a specific frequency range [25].
Clock Frequency | Frequency Range |
---|
16 MHz | 5–100 kHz |
4 MHz | 1–5 kHz |
2 MHz | 300 Hz–1 kHz |
1 MHz | 200–300 Hz |
250 kHz | 100–200 Hz |
100 kHz | 30–100 Hz |
50 kHz | 20–30 Hz |
Table 2.
Excitation voltage amplitudes and DC bias levels for a supply voltage of 3.3 V [
24].
Table 2.
Excitation voltage amplitudes and DC bias levels for a supply voltage of 3.3 V [24].
Voltage Mode | Output Excitation Voltage Amplitude | Output DC Bias Level |
---|
1 | 1.98 V p-p | 1.48 V |
2 | 0.97 V p-p | 0.76 V |
3 | 383 mV p-p | 0.31 V |
4 | 198 mV p-p | 0.173 V |
Table 3.
Truth table of the switching protocol for switches (1) to (5) of
Figure 5. Electrode configurations allow for two, three and four electrode impedance measurements using current or voltage excitation, respectively.
Table 3.
Truth table of the switching protocol for switches (1) to (5) of Figure 5. Electrode configurations allow for two, three and four electrode impedance measurements using current or voltage excitation, respectively.
Source | Voltage | Current |
---|
Electrode Configuration | 2 | 3 | 2 | 4 |
---|
Switch (1) | 0 | 0 | 0 | 1 |
Switch (2) | 0 | 0 | 1 | 1 |
Switch (3) | 0 | 1 | 0 | 0 |
Switch (4) | 0 | 0 | 1 | 1 |
Switch (5) | 0 | 1 | 1 | 1 |
Table 4.
Precision in relative standard deviations (RSD) and trueness error (averaged accuracy error) of the impedance sensor over an entire frequency sweep to determine the resistance of a series of test resistances. A two-point calibration method was used and the resistances were measured using a 4-point LCR meter (Escort ELC-3131D).
Table 4.
Precision in relative standard deviations (RSD) and trueness error (averaged accuracy error) of the impedance sensor over an entire frequency sweep to determine the resistance of a series of test resistances. A two-point calibration method was used and the resistances were measured using a 4-point LCR meter (Escort ELC-3131D).
Parameter | Resistance/Ω |
---|
995.1 | 1483 | 2698 | 3892 | 5602 | 6769 | 8251 | 9772 |
---|
Precision (RSD %) | 0.144 | 0.283 | 0.865 | 1.29 | 1.826 | 1.638 | 2.607 | 2.810 |
Trueness error (%) | −0.456 | −0.03 | −0.274 | 0.172 | −0.250 | 0.299 | −0.121 | −0.138 |
Table 5.
Estimated power consumption of the various sensor modules.
Table 5.
Estimated power consumption of the various sensor modules.
Module | Power Consumption/mW |
---|
pH (Single) | 58.0 |
Impedance | 48.9 |
Stress/Temperature | 119.0 |
Cyclic Voltammetry | 41.1 |
Transient Measurements | 72.4 |