# Rapid and Cost-Effective Fabrication and Performance Evaluation of Force-Sensing Resistor Sensors
## 2 Experimental Method  
### Fabrication of the FSR Sensor Sheet  
The fabrication of the FSR sensor sheet focuses on materials that exhibit both conductivity and piezoresistive properties, allowing for resistance changes in response to applied force. The materials chosen for this study were readily available and inexpensive, making the process accessible and cost-effective. The fabrication process is detailed in Table 1 and illustrated in Figure 1.  
Materials and Preparation  
1. Force-Sensitive Conductive Sheet (Velostat/Lingstat); Description: Velostat-1361 is a conductive polymer film made from polyolefin infused with carbon black, imparting electrical conductivity [33]. It is commonly used in packaging but has gained popularity among hobbyists for sensor applications due to its resistance-changing properties under bending or force.  
**Application:** Velostat is utilized in wearable devices such as shoes that light up when stepped on and systems analyzing the load on the foot's grounding surface. Preparation: The 280 mm \(\times\) 280 mm Velostat sheet was cut into 13.5 mm \(\times\) 13.5 mm pieces to serve as the sensing part of the FSR sensor (Figure 1a).
2. Hand-made 4B Graphite Sheet; Description: Cretacolor Monolith woodless graphite pencil, 4B, was used to create a conductive sheet on A4 copy paper. Method: The paper was uniformly shaded horizontally and vertically, alternating  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**FSR Sensor Material (Sheet)** & **Product Name** & **Comments** \\ \hline \hline Velostat-1361 & Adafruit-1361 (New York, NY, USA) & Purchased \\ \hline
4B Pencil + Paper (copy paper) & Cretacolor (Klagenfurt, Austria): Monolith 4B & Purchased \\ \hline
9B Pencil + Paper (copy paper) & Monolith 9B & Purchased \\ \hline TE Piezoelectricity Sheet & TE Conductivity (Schaffhausen, Switzerland) & Purchased \\ \hline
9B + Silicone Sealant+Paper (copy paper) & Silicone Sealant for bathroom: Ohgong & Purchased \\  & (Gwangju, Republic of Korea) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: FSR sensor materials.  
directions for 10 strokes each. This sheet was then cut into 13.5 mm \(\times\) 13.5 mm pieces (Figure 1b,c).
3. Hand-made 9B Graphite Sheet; Description: Similar to the 4B sheet but using a Cretacolor Monolith woodless graphite pencil, 9B. Method: The shading process was identical to the 4B sheet, with horizontal and vertical shading for 10 strokes each, then cutting into 13.5 mm \(\times\) 13.5 mm pieces (Figure 1d,e).
4. Piezoelectricity Sheet; Description: Manufactured by Tyco Electronics (TE), this sheet uses silver ink to exhibit piezoelectric properties under mechanical load. Preparation: Specific details about the preparation are indicated in Figure 1f.
5. Mixture of 9B Pencil Lead Powder and Silicone Sealant; Description: A mixture of 9B pencil lead powder and silicone sealant was used to create a piezoresistive sheet. Method: The 9B pencil lead was shaved into a fine powder using a cutter knife. The powder was then mixed with silicone sealant in a 4:1 ratio to achieve a gel-like consistency. This mixture was evenly applied to copy paper using fingers to ensure uniform thickness and allowed to dry naturally for at least 12 h (Figure 1g,h).  
As discovered [14; 18], the thickness of pencil graphite can significantly influence sensor performance, particularly in terms of conductivity and resistance. Therefore, to ensure uniform thickness, we applied the same number of strokes when fabricating the 4B and 9B graphite sheets, using 10 strokes as similarly employed in [14].