Root-inspired, template-confined additive printing for fabricating high-robust conformal electronics

Abstract

Conformal electronic devices on freeform surface play a critical role in the emerging smart robotics, smart skins, and integrated sensing systems. However, their functional structures such as circuits tend to tear-off, break, or crack under mechanical or thermal influence when in service, thus limiting the application reliability of conformal electronics. Herein, inspired by the tree root system, template-confined additive (TCA) printing technology was presented for reliable fabrication of robust circuits. TCA printing technology involves the penetration of adhesive into the functional material, thereby enhancing the mechanical robustness of the circuits, allowing them to maintain their electrical performance despite the presence of external damaging factors such as scratching, abrasion, folding, and high temperatures. For example, herein, the circuits could withstand mechanical abrasion at temperatures as high as 350 °C without compromising electrical properties. Benefiting from the confines of template, the printed circuits achieved resolutions of up to 300 nm, suitable for various materials such as P(VDF-TrFE), MWCNTs, and AgNPs, which enabled the multi-material self-aligned fabrication. Furthermore, the versatility of TCA printing was presented by fabricating circuits on arbitrary substrates, and realizing various devices, such as conformal temperature/humidity sensing system and epidermal ultra-thin energy storage system. These applications present the significant potential of TCA printing in fabricating intelligent devices.

Fig. 1. TCA printing.

a Schematic illustration of the analogy between a tree against the natural load and the circuits against the mechanical loads. b Fabrication process of the TCA printing. c SEM images of the section and top view of the printed MWCNTs and Ag circuits. d Demonstration of strong robustness of printed circuits. When the LED is lighted using the printed serpentine circuit, it still lights up when the circuit is subjected to tweezers scraping, sandpaper grinding, and repeated hammered, respectively

Fig. 2. Characterization of the adhesion stability of printed circuits.

a Schematic of cross-section of traditionally printed circuit (top) and TCA-printed circuit (bottom). b Cross-section SEM image of a traditionally printed circuit. c Cross-section SEM image of a TCA printed circuit. d Testing of traditionally printed circuits and TCA-printed circuits for taping, solderability, scratching, and folding, respectively, schematic (top) and resistance comparison of circuits after testing (bottom), and (e) SEM images presenting comparison of the morphology of the circuits after testing

Fig. 3. Demonstration of printing results.

a SEM image of top view (top) and cross-section (bottom) of printed AgNPs grids with a line width of 300 nm. b SEM images of printed AgNPs, MWCNTs, TiO₂, and P(VDF-TrFE) materials with a line width of 1 μm. c SEM images of different thicknesses of printed circuits. d SEM images of printed circuits with different layers. e SEM images of printed circuits with different morphologies

Fig. 4. Demonstration of the ability to print circuits on multiple substrates.

a Optical image of circuits printed on the surface of a glass Pasteur pipette with significant curvature. The partially enlarged SEM image of the printed circuit at three different curvature positions. b Demonstration of circuits printed on smooth substrates (acrylic, glass, chili, and nitrile gloves). c Demonstration of circuits printed on rough surfaces (egg, pigskin, sandpaper, and magnesium alloy). Insets: Roughness characterization of these substrate surfaces

Fig. 5. Conformal multifunctional sensing system.

a Schematic diagram of the integrated temperature and humidity sensing system on the surface of the device. b Optical image of the temperature and humidity sensor system fabricated on the surface of a ceramic vase. c The printed circuits, and d soldered electronic components. e Peel strength of TCA printed AgNP circuits on different substrates. f Block diagram of the sensing principle of the conformal temperature and humidity sensing system. g The test results of the parts in a dry and humid environment. h The test results of the parts under cold and warm air currents

Fig. 6. Conformal energy storage system.

a Schematic and equivalent circuits of the integrated energy storage system on the surface of the device. b Optical image of three series-connected interdigitated MSCs systems fabricated on the surface of a ceramic vase. Insert Optical diagram of MSCs lighting up the LED. c Fabrication process of the integrated MSCs. Insert: Optimized structure of the interdigital electrode. d SEM image of the top view of structured interdigital electrode mentioned above and SEM image showing cross-sectional view below. e CV curves of the integrated MSCs at different scan rates from 0.2 to 2 V·s⁻¹. f GCD curves of the integrated MSCs at different current densities from 0.1 to 2 mA·cm⁻³. g Nyquist impedance plots of the integrated MSCs. h Capacitance of the integrated MSCs at different scan rates from 0.2 to 2 V·s⁻¹. i Capacitance over 1,000 charge/discharge cycles. j Electrode volume–specific capacitance comparison of different CNT-based MSCs