3.5.2. Tensile Test

The tensile test performed on the two directions of the film (MD and CD) is a simple method to analyse the anisotropy in the mechanical properties of the tissues and the effect of the coating on these properties [61]. Moreover, the comparison between treated and untreated samples gave information on the effect of the coating on the resistance of the fibre texture, regarding the complex mechanism that consists of different steps, such as the alignment of the fibre, the reduction in the intermesh voids, and sliding of the fibres [82].

In Figure 10a, the stress at break of treated and untreated tissues in both directions are shown and represent the maximum values of stress that the mesh can sustain before the first fibre (or bundle of fibres) started to break. A clear difference between TC and the other samples can be noticed. Similar values of stress at break were observed in TP, TW, and TA samples, with a higher resistance at break in MD with respect to CD. The high stress in MD can be explained considering the alignment of the fibres in that direction. The comparison between TP, TW, and TA samples showed that water treatment seems to slightly reinforce the fibres, while polyphenols reduce their ability to resist the tensile force. Regarding TC, stress values significantly higher with respect to the other samples were noticed and similar values were observed for both directions. Therefore, chitin was able to override the anisotropy of the material and increase its properties. This behaviour could be compatible with the filling of the intermesh space with nanofibrils and the formation of higher-density materials that can sustain a greater force, in agreement with puncture tests results.

**Figure 10.** Stress (**a**) and strain (**b**) at break of treated and untreated tissues for tensile test. Significance of standard deviation was investigated with a Tukey HSD post hoc test performed on 5 different specimens. On the top of each column, a letter was reported. Means that were identified as not significantly different were grouped under the same letter.

In Figure 10b, strain values related to maximum stress values are reported. Compared to stress at break values (Figure 10a), all samples showed similar results, although in TC, the difference between MD and CD was less evident than in the other samples. Strain at break is caused by a complex mechanism that comprehends the alignment and movement of the fibres in the tissue. Hence, the presence of chitin, filming on the treated surface and in between the different fibres, and thus enhancing inter-fibre linkages makes the materials more resistant but less deformable.

ANOVA of stress at break values (Figure 10a) showed the same grouping in both MD and CD, confirming a significant strengthening effect of chitin with respect to the pure and other coatings. Instead, the statistical analysis of strain at break (Figure 10b) showed some significant differences between MD and CD. The values measured in CD with respect to the same grouping observed in stress at break values showed a predominance of the chitin with respect to the other coating, while, in MD, all the means were not significantly different.
