Experimental and Theoretical Studies of Serpentine Interconnects on Ultrathin Elastomers for Stretchable Electronics

Abstract

Integrating deformable interconnects with inorganic functional materials establishes a path to high-performance stretchable electronics. A number of applications demand that these systems sustain large deformations under repetitive loading. In this manuscript, the influence of the elastomeric substrate on the stretchability of serpentine interconnects is investigated theoretically and experimentally. Finite element analyses (FEA) reveal a substantial increase in the elastic stretchability with reductions in substrate thickness. Low-cycle fatigue tests confirm this trend by examining the stretch required to form fatigue cracks associated with plastic deformation. To elucidate the mechanics governing this phenomenon, the buckling behaviors of deformed serpentine interconnects on substrates of various thicknesses are examined. The analytical model and FEA simulations suggest a change in the buckling mode from local wrinkling to global buckling below a critical thickness of the substrate. Scanning electron microscope and 3D optical profiler studies verify this transition in buckling behavior. The global buckling found in thin substrates accommodates large stretching prior to plastic deformation of the serpentines, thereby drastically enhancing the stretchability of these systems.

A Metal‐Like Conductive Elastomer with a Hierarchical Wrinkled Structure

“…Such elastomeric conductors must have metal‐like electrical conductivity that is stable under repeated mechanical deformations, such as stretching, bending, and compression . Several approaches have been attempted to achieve the desired electrical and mechanical performance, including serpentine or fractal‐based layout design, chemical reduction of a metal precursor, and incorporation of conductive fillers with a high aspect ratio (i.e., metal nanowires (metal NWs) or carbon nanotubes (CNTs)) . However, to our knowledge, the elastomeric conductors based on nanomaterials reported to date have not exceeded an electrical conductivity of ≈10 5 S cm −1 (e.g., electrical conductivity of bulk gold and bulk iron ≈ 4.1 × 10 5 and 1.0 × 10 5 S cm −1 , respectively), despite the incorporation of a high loading amount of conductive nanomaterials into the elastomers.…”

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including serpentine or fractal-based layout design, [9,10] chemical reduction of a metal precursor, [11][12][13] and incorporation of conductive fillers with a high aspect ratio (i.e., metal nanowires (metal NWs) or carbon nanotubes (CNTs)). [14][15][16][17] However, to our knowledge, the elastomeric conductors based on nanomaterials reported to date have not exceeded an electrical conductivity of ≈10 5 S cm −1 (e.g., electrical conductivity of bulk gold and bulk iron ≈ 4.1 × 10 5 and 1.0 × 10 5 S cm −1 , respectively), despite the incorporation of a high loading amount of conductive nanomaterials into the elastomers.The inadequately low conductivity of existing elastomeric conductor is due to the contact resistance between neighboring conductive components or the intrinsically low conductivity of the carbon-based materials used.

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Biaxially Stretchable Ultrathin Si Enabled by Serpentine Structures on Prestrained Elastomers

“…For example, the maximum stretchability in sinusoidal‐shaped serpentine is about 57%, regardless of other limiting factors, such as serpentine width. Analytical studies on geometrical factors and stretchability have been reported elsewhere . In addition, serpentine networks naturally result in low areal fill factor configuration of the materials or devices, i.e., the ratio between the actual area of the material and the area of the structure occupied.…”

A High Aspect Ratio Serpentine Structure for Use As a Strain‐Insensitive, Stretchable Transparent Conductor

“…This result could be reduction of the effective stretching stiffness of the combined a high aspect ratio serpentine structure and PDMS substrate due to the decrease in elastic modulus of substrate. As a result, the entire serpentine structure and PDMS substrate can stretch in perpendicular direction with reduced strain at the peak of serpentine wave . Considering the elastic limits of typical noble metals (≈2.0%), a strategy based on decreasing the Young's modulus below 150 kPa (≈30:1 PDMS) is expected to provide high aspect ratio serpentine TC with an elastic stretching ability in excess of 100% applied strain …”

“…As a result, the entire serpentine structure and PDMS substrate can stretch in perpendicular direction with reduced strain at the peak of serpentine wave. [38] Considering the elastic limits of typical noble metals (≈2.0%), a strategy based on decreasing the Young's modulus below 150 kPa (≈30:1 PDMS) is expected to provide high aspect ratio serpentine TC with an elastic stretching ability in excess of 100% applied strain. [10] Figure 3b illustrates the effect of the height (aspect ratio) of the Au high aspect ratio serpentine structures, 40 nm in width and with a PDMS substrate 150 kPa Young's modulus, on the 100% applied strain.…”