Locally Controlled Sensing Properties of Stretchable Pressure Sensors Enabled by Micro-Patterned Piezoresistive Device Architecture

For wearable health monitoring systems and soft robotics, stretchable/flexible pressure sensors have continuously drawn attention owing to a wide range of potential applications such as the detection of human physiological and activity signals, and electronic skin (e-skin). Here, we demonstrated a highly stretchable pressure sensor using silver nanowires (AgNWs) and photo-patternable polyurethane acrylate (PUA). In particular, the characteristics of the pressure sensors could be moderately controlled through a micro-patterned hole structure in the PUA spacer and size-designs of the patterned hole area. With the structural-tuning strategies, adequate control of the site-specific sensitivity in the range of 47~83 kPa−1 and in the sensing range from 0.1 to 20 kPa was achieved. Moreover, stacked AgNW/PUA/AgNW (APA) structural designed pressure sensors with mixed hole sizes of 10/200 µm and spacer thickness of 800 µm exhibited high sensitivity (~171.5 kPa−1) in the pressure sensing range of 0~20 kPa, fast response (100~110 ms), and high stretchability (40%). From the results, we envision that the effective structural-tuning strategy capable of controlling the sensing properties of the APA pressure sensor would be employed in a large-area stretchable pressure sensor system, which needs site-specific sensing properties, providing monolithic implementation by simply arranging appropriate micro-patterned hole architectures.


Electrical properties and linear fits of hole patterned APA pressure sensors
For more precise calculation of linearity of the pressure sensor, the linear fitting analysis was exhibited by Origin graph plot program. The linear equation has basic formula of y = a + b*x. As shown in figure S2, at first, D-10 sensor shows the best performance in linear characteristics that have the 31.24 of slope and 0.78 of standard error. Second, D-50 sensor shows the 40.35 of slope and 4.1 of standard error. At last, D-200 sensor shows 117.99 of slope and 17.32 of standard error. These results indicate the linearity to large pressure range (~20 kPa) is more correct in small size of diameter and the sensitivity is higher in large size of diameter pressure sensor.
With the same methods, the multi-mixed holes patterned pressure sensors linear fitting plots were shown in figure S3. D-10/50 sensor shows the 68.05 of slope and 3.9 of standard error. Also, D-10/200 sensor shows 177.57 of slope and 13.86 of standard error. These results indicate the D-10/200 sensor shows the highest-pressure sensing sensitivity with good linearity to low pressure range (~5 kPa) and D-10/50 sensor shows good linearity and sensitivity to middle pressure range (~8 kPa).

Electrical properties and linear fits with different thickness of APA pressure sensors
Above mentioned methods, the D-50 pressure sensors linear fitting plots with different PUA thickness were shown in figure S4. T-300 sensor shows the 125.34 of slope and 10.7 of standard error. Also, T-500 sensor shows 40.57 of slope and 1.54 of standard error. At last, T-800 sensor shows 43.125 of slope and 1.67 of standard error. These results indicate the T-300 sensor shows the highest-pressure sensing sensitivity with good linearity to low pressure range (~1 kPa) and T-500 sensor shows good linearity and sensitivity to middle pressure range (~5 kPa).   For more precise dynamic performance, the step up & down test was implemented to D-10, T-800 APA pressure sensor. The pressure was applied from 0, 2.5, 5,10,15 and 20 kPa until about 2 seconds in respect step, then applied from 20 kPa to 0 kPa as the same way with step-up level. As shown in figure S8, the result shows almost symmetric characteristics Figure S8. Change in current of the APA sensor fabricated with D-10 and T-800 under loading pressure of 2.5 kPa, 5.0 kPa, 10 kPa, 15 kPa, and 20 kPa.

Transmittance of the APA pressure sensors
The measured transmittance indicates APA sensor/PDMS, APA sensor/glass respectively. The average transparency shows the 56.89 and 45.94 %, respectively. Actually, the sensor shows not high transparency because the multiple coating of AgNW layer (twice as top, twice as bottom electrode) on PDMS. However, the area without AgNW layer, shows good transparency about 84.98%. Figure S9. The transparency of (a) the APA pressure sensor and (b) the PDMS substrate.
The APA pressure sensor was bent on thin spatula (~ Φ 2.5, 5mm) (figure S10 left) and with specific angle (30, 60 and 90°). As shown in figure S10 (right), the APA sensor operates like normal state even in bent without deterioration. The measurement equipment of sensor array consists of four main components such as Keithley 2636B (as a dual channel sourcemeter), and a homemade stretching endurance jig tester and a customized gauge force system. Typically, Keithley 2636B are measures an electrical resistance of each sensor. After measuring an electrical resistance, using the data acquisition software (DAQ; snM co,. Ltd) installed in PC, initial current and relative change in current for the pressure sensor were automatically saved and displayed. Figure S11 shows the photograph of the pressure sensor connected with measuring system in customized pressure gauge force. Figure S11. A measurement equipment and DAQ system for the pressure sensor.