## 4 Conclusions  
In this study, the piezoresistive characteristics of NCSSs were experimentally studied in terms of the sensor's geometric length to determine its sensitivity to the design. We fabricated bulk- and grid-type sensors with different filler contents (wt.%) and different sensor lengths.  
In the first experiment, we used silicone molds to prepare samples with three different lengths (30, 40, 50 mm) and two different filler compositions (0.35 and 0.5 wt.%). We measured the sensor resistance change with respect to the bending strain variation of a cantilever. For both compositions, we obtained higher sensitivity at longer sensor lengths, which suggested the possibility of using the geometric design to control the sensitivity.  
We deduced that sensor length may correlate with sensitivity, and we proposed a unit gauge factor model of NCSSs to explain the proportional relationship between their length and sensitivity. We supposed that the NCSS is a chain series of individual piezoresistive units and that the whole piezoresistive change can be a linear summation of these units. To verify our hypothesis, we performed a second experiment with more sophisticated samples. We designed grid-type sensors by using thin polyimide film masks. We printed the grid-type NCSS (0.5 wt.% filler) on a cantilever and repeated the same test. Results indicated that the improvement in NCSS sensitivity was exactly proportional to its incremental length ratio (because structural deformation brings piezoresistive change from whole units of the sensor). We were able to obtain higher sensitivity in the cases where the sensor had a longer length and denser pattern per unit area. We also achieved more linear and consistent data from the grid-type NCSS than from the bulk-type sensor.  
From the analysis, we concluded that the greater sensor length brings a greater change in resistance due to its piezoresistive unit summation. Eventually, the sensitivity of NCSSs directly relates to their length, and the patterned length can be used to easily control the sensor sensitivity. We also found that the fine sensor pattern can improve its performance, allowing better results to be achieved. Sensor geometric pattern design is one of the most important aspects of axial load and deformation measurements. For further study, we are studying advanced sensitivity design considering the three-dimensional pattern variables of the NCSS as well.  
Conceptualization: S.-Y.K., B.-G.C., and I.K.; methodology: S.-Y.K., B.-G.C., C.-J.K., Y.-S.J., J.-S.J., J.-H.S., and I.K.; formal analysis: S.-Y.K. and B.-G.C.; investigation: S.-Y.K. and B.-G.C.; data curation: S.-Y.K., B.-G.C., and G.-W.O.; writing--original draft preparation: S.-Y.K. and B.-G.C.; writing--review and editing: C.-J.K., Y.-S.J., J.-S.J., K.-Y.J., and J.-H.S.; visualization: S.-Y.K., B.-G.C., and G.-W.O.; project administration: I.K.; funding acquisition: I.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a Research Grant of Pukyong National University (2019).  
Not applicable.  
Data Availability StatementData available in a publicly accessible repository.  
The authors declare no conflict of interest.