Fully Printed Minimum Port Flexible Interdigital Electrode Sensor Arrays

Screen-printed interdigital electrode-based flexible pressure sensor arrays play a crucial role in human-computer interaction and health monitoring due to their simplicity of fabrication. However, the long-standing challenge of how to reduce the number of electrical output ports of interdigital electrodes to facilitate integration with back-end circuits is still commonly ignored. Here, we propose a screen-printing strategy to avoid wire cross-planes for rapid fabrication of flexible pressure sensor arrays. By innovatively introducing an insulating ink to realize electrical insulation and three-dimensional interconnection of wire crossings, the improved sensor array (4 × 4) successfully reduces the number of output ports from 17 to 8. In addition, we further constructed microstructures on the laser-etched electrode surfaces and the sensitive layer, which enabled the sensor to achieve a sensitivity as high as 17 567.5 kPa-1 in the range of 0-50 kPa. Moreover, we integrated the sensors with back-end circuits for the precise detection of tactile and physiological information. This provides a reliable method for preparing high-performance flexible sensor arrays and large-scale integration of microsensors.

We conducted an electrical insulation cycle experiment.It is verified by experiments that the electrical connection between the upper and lower layers of the electrode is reliable under the three-dimensional layer-by-layer insulation stacking mode.Figure S1 shows that there is no electrical connection between the two electrodes when the insulating ink is screen printed between the upper and lower electrodes and is pressed for 20,000 times under a force of 50 kPa.
The thickness of the sensitive layer is affected by many factors, such as ink characteristics, screen printing parameters, ambient temperature, humidity, ink knife angle and so on.Therefore, the consistency of the thickness of the sensitive layer cannot be guaranteed, and the depth of the microstructure cannot be known.Nonetheless, the quantity of laser engravings can take the place of the depth value.Different laser engraving times cause different microstructure depths.
We took measurements of the sensor performance curve when the number of laser engraving is 0, 5, 10, 15 and 20 times.The experiment demonstrates that the number of laser engravings is different, resulting in different microstructure depths, but it has little effect on the performance of the pressure sensor, the sensor performs poorly if the sensitive layer is not laser etched.The sensor performance is reasonably near and falls within the permitted error range when the sensitive layer is laser engraved 5, 10, 15, and 20 times.It is impossible to guarantee the constancy of the sensitive layer's thickness because it depends on a variety of conditions.As a result, the sensitive layer's microstructure depth is chosen for this investigation when it has been totally penetrated by 20 rounds of laser engraving.The sensing unit is fixed to a beaker with a specific radian in the strain experiment (Fig S6a).At this point, mechanical deformation generates the associated strain.The data diagram showing how strain affects sensor response is found in Figure S6b.The figure shows that the strain alters the sensor response by around 0.02 μA.This value is significantly less than the pressure sensor's response value upon detecting a change in pressure，indicating that the strain's impact on the response can be disregarded, multi-signal decoupling is not required for the sensor.In the temperature experiment, we contrasted the sensor's response and sensitivity data at 10, 30, 50, 70, and 90 degrees Celsius. Figure S8 shows that when the temperature rises from 10 °C to 50 °C, the sensor's response increases by roughly 0.003μA.At 70 °C and 90 °C, the response curve of the sensor first rises and then falls to a stable level.The temperature rising is to blame for this, as it causes the sensor to respond more strongly.But as the humidity drops and the moisture in the sensor unit evaporates, the sensor's response is diminished.The sensor response experiences an increase of around 0.005 μA in its final stable condition.It can be concluded that the influence of temperature on the sensor response is weak and negligible.On the other hand, the temperature has little effect on the performance of the sensor.As shown in the sensitivity diagram of the sensor corresponding to each temperature (Fig. S8f), the sensitivity data of various humidity fall within the allowable error range.(The error curve is shown in Figure 4a.)Therefore, multi-signal decoupling is not required for the sensor.

Fig. S1
Fig. S1 Performance test.(a)Electrical insulation performance test image.(b) 20,000 cycles of experimental data graph.(c) Effect of microstructure depth of sensitive layer on pressure performance of sensor.

Fig. S4
Fig. S4 Bottom electrode EDS layered image.(a) The energy dispersive X-ray spectrometer was used to analyze the images of "C", "Ag", "O" and "N" elements in the electrode layer.(b) The electrode layer analyzes the image of the "C" element.(c) The electrode layer analyzes the image of "Ag" element.(d) The electrode layer analyzes the image of "O" element.(e) The electrode layer analyzes the image of "N" element.

Fig. S5
Fig. S5 EDS layered image of sensitive layer.(a) The "C", "O" and "N" elements in the sensitive layer were analyzed by EDS.(b) The sensitive layer analyzes the image of "C" element.(c) The sensitive layer analyzes the image of "N" element.(d) The sensitive layer analyzes the image of "O" element.

Fig. S6
Fig. S6 The influence of strain on the performance of the sensor is tested.(a) Diagram of strain test.(b) Data diagram of the influence of strain on sensor response.

Fig. S7
Fig. S7 The influence of humidity on sensor performance.(a) -(e) Data diagram of the influence of different humidity on sensor response.(f) An experimental data chart comparing the performance of sensors at varying humidity levels.

Fig. S8
Fig. S8 The influence of temperature on sensor performance.(a) -(e) Data diagram of the influence of different temperature on sensor response (f) Comparison of experimental data charts for sensor performance at various temperatures.

Fig. S9
Fig. S9The mechanical arm was tested with 20 kinds of pulse data, including firm pulse, feeble pulse, tense pulse, long pulse, wiry pulse, weak pulse, floating pulse, small pulse, intermittent pulse, short pulse, rapid pulses, full pulse, swift pulse, tremulous pulse, tympanic pulse, infrequent pulse, weak-floating pulse, hollow pulse, deep pulse and hidden pulse.

Fig. S10
Fig. S10 Test of sensor pressure distribution.(a) The hand-held beaker's pressure distribution test and (b) heat map of pressure distribution.(c) Plantar pressure distribution test and (d) heat map of pressure distribution.