# Polyimide Encapsulation of Spider-Inspired Crack-Based Sensors for Durability Improvement
## 3 Results and Discussion
Compared to a sensor encapsulated with a PDMS of 30 \(\upmu\)m thickness, it showed superior performance in saline solution. The major cause of GF decreases in liquid environment is delamination. In Figure S5, the PI or PDMS encapsulated crack sensor immersed in saline solution shows that the metal layer is separated from the substrate, which indicates an increase of the base line. However, the delamination issue in these environments can be enhanced by a thicker encapsulation layer. Figure 3c shows that if the thickness of the encapsulation layer is increased to 10 \(\upmu\)m, it can last more than 100 h in saline solution, but it must be appropriately adjusted because it is in a trade-off relationship with the sensitivity of the sensor (Figure 3a,b). These results show a large expandability of the crack sensor in a humid environment, such as a wearable device. Figure 3: Comparison of performance changes according to PI encapsulation thickness. (**a**) Resistance response in two-percent strain for various encapsulation thicknesses. (**b**) Gauge factor comparison by encapsulation thickness. (**c**) Performance comparison by encapsulation thickness in saline solution (0.9% NaCl). Figure 2: The performance comparison of the crack sensors in different environments. (**a**) Performances of the sensors after thermal treatment; (**b**) Performances of the sensors after dipping in water; (**c**) Performances of the sensors dipping in saline solution (0.9% NaCl). PET: polyethylene terephthalate; PDMS: polydimethylsiloxane. Figure 4 shows durability (marathon test) performance of the sensors up to 15,000 cycles. First, the crack sensor on PET substrate without the encapsulating layer lost its gauge factor rapidly by 20% from the first 1000 to 15,000 cycles of repetitive stretching (Figure 4a). Second, the GF of the crack sensor on PI substrate without the encapsulating layer increased until 3000 cycles and started to decrease gradually until 7000 cycles, and it became 30% of maximum GF at 15,000 cycles (Figure 4b). The reason why the sensitivity increased first and then decreased was that deepening of crack depth by penetration and incomplete recovery of crack gap by residual stress occurred simultaneously. When a crack penetrates into the substrate, sensitivity increases by deepening the crack depth [16]. However, repetitive straining accumulates residual stress on the crack vertex, disturbing closing of the crack gap, causing the gauge factor to decrease [37]. Third, the crack sensor on PI substrate with the encapsulating layer had lower sensitivity than that of the crack sensor on PI substrate without encapsulation, but sensitivity drop with repeated tensile load improved slightly. It maintains its GF above 65% until 11,000 cycles (Figure 4c). These results show that the encapsulating layer contributes to delay of the progress of the crack. The resistance of the metal films on the polymer substrate dramatically increases due to the crack gap opening by stretching it up to two percent. To use it as a sensor, the reverse process should return the initial crack gap and its initial resistance as well. Figure 5a-c show that a 10-cycle set of strains from zero to two percent at a sweeping speed of 20 mm/min with one second of the interval was nearly identical. The linearly fitted gauge factor of crack sensor on PET substrate with the strain range from zero to two percent by regression analysis was \(\sim\)2800, that of the crack sensor on PI substrate was 28,000, and that of the PI encapsulated crack sensor was 12,500. Interestingly, it shows that by only changing substrates from PET to PI, sensitivity increased up to 10 times. We assume that this phenomenon was caused by crack penetration into the PI substrate because the sensitivity of the sensor increases as the depth of crack increases [16]. Figure S6+ shows the change in resistance of three samples when applied strain from zero to two percent.