Screen-Printed Piezoelectric Sensors on Tattoo Paper Combined with All-Printed High Performance Organic Electrochemical Transistors for Electrophysiological Signal Monitoring

This work demonstrates sensitive and low-cost piezoelectric sensors on skin-friendly, ultrathin, and conformable substrates combined with organic electrochemical transistors (OECTs) for the detection and amplification of alternating low-voltage input signals. The fully screen-printed (SP) piezoelectric sensors were manufactured on commercially available tattoo paper substrates, while the all-printed OECTs, relying on an extended gate electrode architecture, were manufactured either by solely using SP or by combining SP and aerosol jet printing (AJP) on PET substrates. Applying a low-voltage signal (±25 mV) to the gate electrode of the SP+AJP OECT results in approximately five times higher current modulation as compared to the fully SP reference OECT. The tattoo paper-based substrate enables transfer of the SP piezoelectric sensor to the skin, which in turn allows for radial pulse monitoring when combined with the SP+AJP OECT; this is possible due to the ability of the conformable sensor to convert mechanical vibrations into voltage signals along with the highly sensitive current modulation ability of the transistor device to further amplify the output signal. The results reported herein pave the way toward all-printed fully conformable wearable devices with high sensitivity to be further utilized for the real-time monitoring of electrophysiological signals.


Measurement of the charge required in OECT transient measurements
The charges required for OECT transient measurements (when VG switches from 1.5 to 0 V, to minimize parasitic drain current contributions from the carbon-based source and drain electrodes) significantly differ for the two different OECT manufacturing approaches.
Approximately 47 nC was required to switch the fully SP OECT device from OFF to ON (Figure S1a), while only ~12 nC was required to switch the SP+AJP OECT (Figure S1b).The SP piezoelectric sensor on PET is also capable of monitoring the radial artery pulse, at least when pushed towards radial artery.In Figure S3, the peaks of the output voltage signal (after low pass filtering) are discernable, though with lower amplitudes and less repeatability as compared the tattoo paper-based SP piezoelectric sensor (Figure S2), which is explained by the rigidity and thickness (125 µm) of the PET substrate.

Polarization-electric (P-E) hysteresis measurement
The high electric field was applied across the screen-printed piezoelectric layer starting at 0 V and in (20 V) steps increasing to +500 V, to orient the dipoles and facilitate piezoelectric activity.After that, the voltage was decreased stepwise until -500 V and then increased till 0 V again.At about 300 V and -300 V, the polarization changes direction.The voltage required to change the direction depends on the thickness of the piezoelectric layer.

Tapping of the PET-based screen-printed piezoelectric sensor
As shown in Figure S5, the output voltage signal of the PET-based SP piezoelectric sensor is stable over time and repetitive in shape and amplitude.Average peak-to-peak voltage amplitudes of 2.6 ± 0.02 V were recorded, and in comparison with tattoo paper-based SP piezoelectric sensors (2.8 ± 0.06 V) the PET-based device generated comparable or slightly lower voltage output amplitudes, which may be explained by the more rigid and less conformable plastic substrate.

Figure S2 .
Figure S2.Comparison of filtered (5 Hz low pass filter; red graph) and non-filtered (blue graph) output voltage signals generated by the tattoo paper-based SP piezoelectric sensor upon pulsing radial artery.

Figure S3 .
Figure S3.Comparison of filtered (5 Hz low pass filter; red graph) and non-filtered (blue graph) output voltage signals generated by the PET-based SP piezoelectric sensor upon pulsing radial artery.

Figure S4 .
Figure S4.Photograph of the P-E hysteresis loop graph.The voltage sweeps are typically repeated a

Figure S5 .
Figure S5.Voltage output signals generated by a PET-based SP piezoelectric sensor.

Figure
Figure S6 depicts a circuit schematic that enables conversion from current measurement mode (of the OECT drain current) into voltage measurement mode.This circuit is designed and tested to compare the amplification obtained in SP+AJP and fully SP OECTs.The circuit consists of a digital oscilloscope with an integrated function generator that generates an alternating 50 mV peak-to-peak voltage signal (similar to the signal obtained from the TPbased piezoelectric sensor triggered by the radial pulse), an OECT (either SP+AJP or fully SP), and a resistor (R1; 20 and 2 kΩ when using the SP+AJP and the fully SP OECT, respectively).

Figure S6 .
Figure S6.Circuit used to record low voltage signals applied to the gate electrode of the OECT.

Figure
FigureS7shows the voltage output levels when using either fully SP or SP+AJP OECTs connected according to the circuit exemplified in FigureS6.In relation to the input signal supplied by the function generator (50 mV), the SP+AJP OECT provides approximately 2.5 times higher amplification as compared to the fully SP OECT (~128 mV vs. ~52 mV).This difference in voltage amplification highlights the benefit of using SP+AJP OECTs in combination with piezoelectric sensors.It may be noted that the current modulation differed by a factor of 5 in the graphs shown in Figure3.However, the lower voltage amplification factor (VOUT 128 mV vs. VIN 50 mV for the SP+AJP OECT) shown in FigureS7could possibly be enhanced by optimization of the resistor value and the supply voltage used in this measurement setup.

Figure S7 .
Figure S7.Voltage output signals when connecting either a fully SP (black) or a SP+AJP (green)