# Polyimide Encapsulation of Spider-Inspired Crack-Based Sensors for Durability Improvement
## 3 Results and Discussion  
In general, metal layers on the crack lips can be delaminated by repetitive stretching, heating, and in water or chemical environments [23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. From repeated exposure to these environments, a sensor can have stress concentration on the edge due to different Poisson's ratio or thermal expansion mismatch [23; 25; 33]. A water or chemical environment causes degradation of the adhesion force between the metal film and the polymer substrate [32]. To measure the durability of our crack sensors, we have performed three kinds of experimental tests: thermal resistance test, water and saline solution resistance test, and marathon test. In order to protect the crack sensor from the harsh environment and improve the durability, we encapsulated the crack-based sensor with PI. The schematic illustration of PI encapsulated crack-based sensor is presented in Figure 1a. The crack on the thin films was generated by stretching them at two-percent strain before encapsulation (Figure 1b left). Additionally, the polyimide precursor solution was spin-coated on the thin cracked metal film. The cross-sectional SEM image (Figure 1b right) shows that the solution does not penetrate into the gap of a crack because the gap was too narrow (10-15 nm). Figure 1c illustrates the cracks generated in the transverse direction to the extension force, and the crack gap increases with applied strain as shown in Figure 1d. The gauge factor of the encapsulated sensor reached above 10,000 at a strain of two percent with high reproducibility and durability. The results in Figure 2 show that the thermal, water and saline solution durability of the crack sensor have been greatly improved with encapsulation by cured PI. In the thermal resistance test (Figure 2a), the PET substrate of the crack sensor maintained its gauge factor for about 90% at 200 \({}^{\circ}\)C because the deformation of the polymer surface reduced the stress concentration at the metal-polymer interface by going above glass-transition temperature (T\({}_{\text{g}}\) = 85 \({}^{\circ}\)C) [34]. However, it starts to break down when it is heated up to 300 \({}^{\circ}\)C, exceeding the melting temperature of the PET (T\({}_{\text{m}}\) = 290 \({}^{\circ}\)C) [35]. In the case of the PI substrate crack sensor without encapsulation, the relative GF rapidly decreased by heating, although the PI substrate was supposed to endure at the temperature above 300 \({}^{\circ}\)C. This was due to the delamination caused by the different thermal expansion coefficient and weak adhesive force between the metal layer and the polymer substrate [36]. However, for the encapsulated cracksensor, the gauge factor remained at 77% even after heating at 300 \({}^{\circ}\)C. The top encapsulating layer was responsible for enduring the heat treatment, which prevents the delamination of the metal layer from the substrate. The graphs in Figure 2b clearly show that sensors without encapsulation are very vulnerable in liquid environment. The metal layer of the PET substrate crack sensor delaminated as soon as it is immersed in water, and the PI substrate crack sensor also cannot resist liquid. However, the PI encapsulated crack sensor could endure 30 h in water and 24 h in 0.9% saline water with the gauge factor of two-percent strain even though the relative gauge factor decreased to 50%. However, we could see that straining two percent in a state where the adhesion force was degraded due to wet conditions accelerated degradation of its gauge factor. Figure S3 shows that when the strain range was from zero to one percent, the sensor maintains its GF of 90% or more for 130 h, and Figure S4 shows that there was no base resistance change for 190 h without straining. Because the actual operating range of the sensor would not be higher than one percent, it could endure longer in a wet environment. Figure 2c, PI is also verified as a very good passivation layer compared with PDMS (PDMS: polydimethylsiloxane) by its water permeability.