Bioinspired Interfacially Engineered Flexible Island for Highly Stretchable Electronics

Significant research efforts toward integrating conventional rigid electronic components into stretchable substrates. Various strategies have been developed to create stretchable electronics, including serpentine, kirigami, and rigid island designs. The serpentine and kirigami approaches have used flexible substrates such as polyimide (PI) and polyethylene terephthalate (PET), where the substrate is carefully engineered to build stretchable electronics. In contrast, the rigid island approach is used to reduce the mismatch of the elastic modulus of the rigid island and stretchable substrate through several strategies, such as material-based and interfacial engineering-based methods. Material-based approaches employs stiffness gradients to minimize the discrepancy between high and low elastic moduli to improve the stretchability [1, 2]. A recent study using an interfacial engineering-based approach applied the mechanical interlocking method [3]. However, the proposed rigid island made from rigid polymer demonstrated low flexibility and considerable thickness, restricting its suitability for a wide range of applications.
In nature, plant roots anchor deeply into the soil, creating a robust interface (roots-soil interlocking) that minimizes the mismatch between the rigid plant body and the soft soil structure. This mechanism not only stabilizes the plant but also helps it withstand harsh conditions such as extreme weather and natural disasters. Roots-soil interlocking postulates high adhesion and secure interfacial behaviors and is also shown in mimicking applications such as stretchable electrodes and structural batteries.
In this work, we introduce bioinspired interfacial engineered flexible islands (BIEFI) that exhibits high stretchability and remarkable physical deformational performances for flexible-to-stretchable electronics. Drawing inspiration from roots-soil structure, the flexible-to-stretchable interface is designed using a flexible substrate with root structures embedded within a stretchable substrate. Root structure possesses primary and secondary roots, enhancing its effective and secure ground gripping through these diverse root types. BIEFI design achieves a stress distribution effect by including primary roots, thereby postponing interfacial failure. Secondary roots contribute to flexible mechanical interlocking effect that ceases crack propagation by establishing anchored Ecoflex zones. Through a parametric study of the BIEFI design, aimed at enhancing the interlocking mechanism, we achieved a remarkable increase in stretchability, reaching up to 700% strain. The flexible mechanical interlocking mechanism also significantly extends the fatigue life for various types of physical deformations (e.g., strain, poking, and twisting). The optimized BIEFI design showcases its potential in flexible-to-stretchable platforms, such as stretchable LED arrays and solar cell arrays, which remain functional under diverse deformations. A smart resistance band was developed to show highly stretchable applications by integrating the optimized BIEFI into a resistance band that monitors workouts with a strain sensor and accelerometer. This strategy offers a new perspective on the integration of rigid, flexible, and stretchable components, providing enhanced stretchability and durability for flexible-to-stretchable systems.

References

1. Matsuhisa, N., et al., Nature Communications, (2015). 6, 7461.
2. Libanori, R., et al., Nature Communications, 2012. 3(1): p. 1265.
3. Yang, J.C., et al., Science Advances, 2022. 8(22): p. eabn3863.

Acknowledgment
This work was supported by (1) the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1A2C3008742). (2) Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No.2022-0-00025, Development of soft-suit technology to support human motor ability).