We calibrated the paper-based force sensor using a
precision balance, a LCR meter (for measuring the change in
resistance), and a multimeter (for measuring the circuit
output voltage). Figure 4A shows the experimental data of
the change in resistance of the carbon resistor as a function of
the mechanical strain applied to the carbon resistor. Based on
this calibration curve, we calculated the gauge factor of the
piezoresistive sensor to be 4.1. Figure 4B illustrates a
calibration curve of the output voltage of the Wheatstone
bridge circuit as a function of the applied force. The range of
force measurement was ±16 mN, and the resolution of force
measurement was 120 μN (corresponding to a voltage
detection limit of 0.1 mV). The sensitivity of the sensor was
0.84 mV/mN.