PSI - Issue 37

Shirsha Bose et al. / Procedia Structural Integrity 37 (2022) 131–138 Bose et al., 2021/ Structural Integrity Procedia 00 (2019) 000 – 000

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2.5%, 3.75% and 5%) were calculated (Fig. 4b). Basically, delamination in this context refers to the failure of the collagen-chromium bonded layers. The area of delamination was estimated with the Image-J software analysis and the normalization was done with respect to the total interfacial surface area. Initially (at 1.25% and 2.5% strain), there was negligible difference in the area of delamination between the two studied micro-islands; however, this increased at higher strain levels (especially at 5% strain) for the square micro-island. From this study, it might be concluded that using circular structures for flexible electronics with collagen substrates would reduce the effect of potential damage occurring from tension as compared to the structures with defined vertices such as square-edged micro-islands. In addition, using round configuration would reduce the excess metallic materials used in square ones, thus, minimizing

the cost/weight of the metal used. 3.3. Dry vs. wet collagen substrate

As discussed, previous studies showed the drastic change in the mechanical properties of collagen film when tested in wet environment (Bose et al., 2020a; 2020b). The stress-strain graphs of collagen film tested in dry and wet environment (Fig. 5a) demonstrate the weak mechanical behaviour of collagen in the latter case. Hence, due to the potential applications, it is of immense interest to consider the wet environment in numerical analysis.

Fig. 5: (a) Stress-strain graphs for collagen film in dry and wet environments. (b) Maximum stress normalised with yield stress generated for metallic layers for square micro-islands in both environments. (c) Stress distribution in collagen substrate for both environmental conditions (left – dry; right – wet). The square micro-island was modelled with wet collagen substrate. The metallic layers of gold and chromium layers revealed a heavy drop in the maximum stress level approximately by 95% (from 145 MPa to 6 MPa) and 72% (from 165 MPa to 45 MPa), respectively (Fig. 5b), when tested in the wet conditions. The stress generated in wet collagen substrate (0.93 MPa) dropped by two orders of magnitude as compared to the dry ones (0.006 MPa, see Fig.