Geometrically engineered rigid island array for stretchable electronics

Stretchable electronics can be developed by integrating rigid components in a soft polymer matrix. However, it is challenging to eliminate cracks at the interface between soft and rigid materials. In a new report now published ...

Ferris wheel–shaped islands (FWI) Stretchable electronics allow a variety of hitherto unknown functions by offering various form factors that are usually incompatible with rigid electronics . For instance, stretchable displays , battery packs and logic circuits are vulnerable to lateral strain and can be protected against mechanical deformation via a variety of engineering strategies. However, mechanical mismatch between rigid islands and polymer materials can lead to interfacial cracks and device dysfunction. To solve this, researchers have devised several strategies . In this work, Yang et al presented geometrically engineered rigid islands with excellent mechanical stability at the interface.

Mechanistically, the interlocking structure of the proposed FWI effectively suppressed crack propagation at the interface. The optimized geometrical shapes relied on the mechanical properties of the constituent polymer materials. By design, the repetitive interlocking structure prolonged the fatigue life against various deformation and stretching modes. The team showed a variety of applications of the array, which they developed using Ecoflex polymer material to show stretchable electronics under various deformations and the formation of electronic skin to detect tactile stimuli with durability for daily use with commercial viability.

Online cover—a rigid, 3D printed “island” sits atop a stretchable polymer. Currently, repetitive and excessive strain cause stretchable electronics to fail, limiting their practical use. To remedy this, Yang et al. developed Ferris wheel-shaped islands (FWIs) that withstand the strain of continuous stretching, twisting, poking, and crumpling. Unlike conventional circle- and square-shaped islands in soft polymer matrix, the geometrically engineered FWIs are highly durable. Yang et al.’s approach applies to a wide variety of stretchable electronics and could hasten commercialization in the near future. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abn3863 Read more

FWIs embedded in Ecoflex substrate for highly durable stretchable electronics. (A) Schematic illustration of stretchable electronics with the FWI array in Ecoflex. (B) Left: schematic illustration of stretchable electronics operating under various deformations; right: schematic illustration of electronic skin (e-skin) detecting tactile stimuli. (C) Left: photographs of PLA islands embedded in Ecoflex; right: photographs comparing the maximum stretchability of circle-shaped island (CI) and FWI in Ecoflex substrate. The CI and FWI in Ecoflex are stretched to 75 and 175%, respectively. (D) Digital image correlation (DIC) images showing the progress of crack propagation for the CI and FWI in Ecoflex under stretching. (E) Stress versus strain for the CI (red trace) and FWI (blue trace) in Ecoflex under stretching. (F) The strain at failure according to the angle. The islands are rotated at specific angles, embedded in Ecoflex matrix, and stretched vertically. Scale bars, 1 cm (C) and 5 mm (D). Photo credit: J. C. Yang, Korea Advanced Institute of Science and Technology (KAIST). Credit: Science Advances (2022). DOI: 10.1126/sciadv.abn3863 Read more

Investigation of interfacial failure caused by crack propagation. (A) Schematic illustration of the process of deriving FWI. (B) Percent contribution of the design factors (the number of teeth, p/i ratio, a/b ratio, and c/i ratio) to the strain at failure. (C) Experimentally obtained strain at failure for CI, WMIs (n = 6 and 12), and FWIs (n = 6 and 12). (D) Schematic illustration of FE-simulated crack propagation between islands and Ecoflex substrate. (E) Overall strain versus crack opening displacement (COD) for CI, WMI (n = 6 and 12), and FWI (n = 6 and 12). (F) FE simulation images showing the process of crack propagation when the COD reaches 2.5 mm. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abn3863 Read more

Compatibility of FWIs with various polymer materials. (A) Schematic illustration of two types of complete interfacial failure in FWI in polymer substrate. (B to D) Strain at failure according to the p/i ratio change of (B) Dragon Skin, (C) Ecoflex, and (D) Ecoflex Gel. (E) Schematic illustration of FWIs with different p/i ratios. Each box represents the optimized FWI of each polymer. (F) Experimentally obtained strain at failure for CI (white bars) and FWI (blue bars) in three different polymer matrices (Dragon Skin, Ecoflex, and Ecoflex Gel). Scale bars, 5 mm (B to D). Photo credit: J. C. Yang, KAIST. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abn3863 Read more

Stretchable electronics consisting of rigid components capable of withstanding various deformation modes. (A) Schematic illustration, photograph, and SEM image of stretchable Ag flake/Ecoflex electrodes printed on the FWI in Ecoflex substrate. (B) Resistance versus lateral strain for the Ag flake/Ecoflex electrodes printed on CI and FWI in Ecoflex. The left inset shows that the electrode printed on CI in Ecoflex was disconnected at 50%. The middle and right insets show that the electrode printed on FWI in Ecoflex was connected at 125 and 220%, respectively. (C and E) Schematic illustration and photograph of (C) a stretchable LED array and (E) stretchable battery pack with FWIs. The encapsulation layer is Ecoflex. (D and F) Photographs of (D) the stretchable LED array and (F) the stretchable battery pack capable of withstanding various deformation modes (twisting, bending, stretching, and crumpling). Scale bars, 1 cm [(A), left image], 10 μm [(A), right image], 3 cm (C), and 4 cm (E). Photo credit: J. C. Yang, KAIST. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abn3863 Read more