## 2 System Design and Method
### Design and Measurements  
The fabricated sensor was sandwiched within wire electrodes and fixed with a polyimide tape film which was employed as a soft surface. The distribution of the piezoresistive tactile sensors was first characterized by demonstrating that it could record data from human fingers when making different hand gestures. Hence, to obtain stable signals, three sensors--made using the above-presented process--were fixed to the thumb, index, and middle fingers of five subjects. The piezoresistive tactile sensor obtained is shown in Figure 1a. The morphological characterization in Figure 1b shows the unique porous microstructure filled with conducive nanoparticles. We evaluated the reflection of deformation on the sensor performance by subjecting the central area of the sensor to different loadings on an electrochemical workstation connected to a Mark-10 force gauge and multi-meter converter. Pressure deformation loading was increased gradually from 10-100 kPa at intervals of 10 kPa, i.e., the range of values that fit both human physiological signals and robotics applications [33]. Figure 1: (**a**) Optical image of the piezoresistive sponge before and after coating with the conductive network; (**b**) scanning electron microscope (SEM) images of the tactile sensor. The piezoresistive sensor response values were recorded with real-time sensor feedback using computer software, namely, KBD-24. Additionally, the testing conditions were repeatedly applied to 10 different sensor samples to obtain data from different units and properly analyze the stability and accuracy of the sensor prototype. Figure 2a shows the gradual response obtained during each iteration, where it can be seen that the sensor produced data even up to tiny pressure values. Notably, the response curve was very close to a linear shape when loading values higher than 70 kPa were applied, as shown in Figure 2b. This demonstrates the relationship that exists between the applied deformation and the response outputs. Similarly, the piezoresistive effect showed a decrease in electrical resistance with an increase in pressure loading. We measured the performance using the sensitivity (S) metric as defined in Equation (1); \(\Delta R\) is the difference between the resistance \(R\) and the initial resistance (\(R_{0}\)), while \(\Delta P\) is the difference between the applied pressure \(P\) and the initial pressure \(P_{0}\). The statical results reveal that there were three linear levels of sensitivity produced by the force sensor: S1 = 0.0311 under a loading range of 10-30 kPa, S2 = 0.038 under a loading range of 30-70 kPa, and S3 = 0.075 under a loading range of 70-100 kPa. \[S=\frac{\Delta R}{(R_{0}\times\Delta P)}. \tag{1}\]  
Additionally, we investigated the sensor's response to normal and shear forces. Normal force was applied at 15.75 N for 300 s. As shown in Figure 2c, the piezoresistive  
Figure 2: Resistance variations in response to the tiny stimulus pressures (**a**) and large loading range from 10 to 100 kPa. (**b–d**) Resistance response of the piezoresistive tactile sensor to an applied normal force of 15.75 N and shear force. effect decreased the electrical resistance under the normal force due to the continuous deformation of the sensor. This decreased the diameter of the inner pores and increased the contact of the conductive nanoparticles located on the walls. Conversely, shear force was applied to the top surface parallel to the _XY_ plane without applying any normal force. The sensor showed an obvious resistance variation in response to any changes in the surface, as shown in Figure 1d, owing to the unique elastic porous structure which enabled a fast interaction between the conductive fillers located on walls. Lastly, to further characterize the electrical properties and explain the voltage distribution, the sensor was powered by a constant resistor and a loading force of 15.75 N was applied with different frequencies between 1 kHz and 10 kHz.