Programmable and flexible wood-based origami electronics

The study addresses the need for environmentally sustainable, flexible electronics capable of complex shape reconfiguration. Conventional flexible devices often rely on non-renewable, non-biodegradable plastics, exacerbating e-waste. Wood-based cellulose offers a renewable alternative, but prior wood-derived electronics have been limited in their ability to undergo complex, programmable shape changes. The research question is whether a top-down processed transparent wood film (TWF), combined with direct-ink-written cellulose-based conductive ink and origami-inspired design, can deliver programmable, shape-editable electronics with stable conductivity and high mechanical performance suitable for wearables, sensors, and microfluidics.

Prior work highlights the rise of flexible electronics for microfluidics, wearables, sensors, and soft robotics, typically built on polymeric substrates that raise sustainability concerns. Cellulose-based platforms have been explored via bottom-up (nanocellulose assembling to 2D/3D) and top-down (bulk wood/veneer) strategies, enabling strong, flexible, and optically functional materials. Origami and kirigami approaches have yielded reconfigurable structures in elastomers, stretch polymers, and isotropic paper, while pencil-on-paper sensors show high-fidelity biophysical detection but suffer from conductivity degradation upon folding due to weakly attached carbon. 3D printing (direct ink writing) with cellulose nanofiber-based gels enables precise, adherent conductive patterns with tunable rheology. However, reports on thin wood-film-based origami electronics are scarce, and programmable, editable wood-based devices remain challenging, motivating this study.

Substrate fabrication: Balsa wood veneers (Ochroma pyramidale; 80 × 80 × 1 mm³; 0.15 g cm⁻³) were delignified using 1 wt% NaClO₂ in acetate buffer (pH 4.6) at 80 °C for 6 h, followed by cell-wall softening and partial hemicellulose removal in 1 mol L⁻¹ NaOH at room temperature for 3–6 h. Samples were thoroughly rinsed and the process repeated three times. Wet treated veneers were placed between PES membranes and filter paper, densified under 3–5 kg at room temperature, and dried 24–48 h to produce TWF substrates of 50 ± 20 µm thickness and ~1.3 g cm⁻³ density. Conductive ink preparation: TOCNFs were prepared via TEMPO-mediated oxidation. For a 4 wt% CNT/TOCNFs ink, 1 g cellulose fibers in 150 mL water were treated with 0.016 g TEMPO and 0.1 g NaBr; NaClO (4 mmol g⁻¹) was added while maintaining pH ~10.5, then quenched and adjusted to pH 7 with HCl. The suspension was ultrasonicated (600 W, 30 min) to obtain TOCNFs, then mixed with CNTs at 1:1 mass ratio, stirred 30 min, ultrasonicated (300 W, 1 h), defoamed by high-speed centrifugal mixer, and concentrated to 4 wt%. 3D printing (DIW) and patterning: A pneumatic bioprinter (ROKIT) dispensed ink from a 1 mm needle stainless syringe; residual air was removed by centrifugation. Printing parameters were defined in Newcreator software, with circuit designs created in 3D MAX 2019 and converted to G-code. Printed conductive layers were ~1 µm thick (SEM), with overall printed features around 1 mm wide as patterned. Programmable origami design and simulation: Origami crease patterns (mountain, valley, border; adjustable flexible creases; faceted folds) were predesigned to exploit TWF anisotropy. Finite element method (FEM) analog simulations visualized strain and predicted stress–strain behavior during folding (0–70% folding states), examining telescopic and torsional folding sequences and anisotropic responses. Characterization: Morphology by SEM (JSM-7600F); composition by sulfuric acid hydrolysis (NY/T 1459-2007; GB/T 20805-2006); mechanical tensile tests (INSTRON 5966); AFM roughness; UV–Vis–NIR (Shimadzu UV3600 plus) for transmittance/haze; FTIR (VERTEX 80 V); confocal fluorescence (Leica SP5 II); WAXS at Australian Synchrotron (λ = 0.685 Å; detector distance 320 mm); nitrogen physisorption (ASAP 2020HD88). Electrical measurements: four-probe sheet resistance (ST2258C; 2 mm spacing; corrections applied), current via electrochemical workstation (CHI660E; 2 V, 0.1 s interval). Folding endurance testing (90°–180°) with Modular Flex Test System. Thermal tolerance by IR imaging (FOTRIC 600C; 25–115 °C; RH 65–95%). Adhesion via tape-peel and scraping. Recycling protocols for TWF membranes were also evaluated.

• Transparent wood film (TWF) fabrication: Delignification removed ~92.1% lignin while retaining >90% hemicellulose in DW; subsequent alkali treatment removed 58.3% hemicellulose in PW, increasing specific surface area to 46.9 m² g⁻¹ (vs. OW 1.1; DW 17.8 m² g⁻¹).
• TWF structure/properties: Dense, anisotropic film of 50 ± 10 µm thickness with collapsed, layered cell walls exposing cellulose fibril bundles; optical transmittance 78% and haze 88% at 550 nm; Young’s modulus 43.68 GPa and tensile strength 393.8 MPa (longitudinal), far exceeding OW, DW, printer paper, and PET; orientation index 0.88 from WAXS; surface roughness ~±100 nm.
• Flexibility/endurance: TWF tolerated rolling/knotting and >8000 fold cycles without cracking; as-printed circuits maintained function under bending.
• Conductive ink and printing: CNT/TOCNFs inks (2–8 wt%) exhibited shear-thinning; 4 wt% selected for print fidelity. Printed layer ~1 µm thick formed a dense, entangled CNT/TOCNFs network strongly adhering to TWF via hydrogen bonding and van der Waals forces.
• Electrical performance: Average conductivity 0.24 S cm⁻¹; circuits powered an LED when bent. Relative sheet resistance remained nearly unchanged for 500 180° folding/unfolding cycles; bending-induced resistance change ~3%. After 500 cycles, cracks appeared in the ink layer, while TWF substrate showed only a crease with no damage.
• Environmental stability: Stable current across RH 65–95%, with higher currents at higher RH (water-assisted electron transport). Thermal tolerance: TWF stable up to 115 °C (PET melted at 95 °C, deformed at 115 °C). Current at 115 °C decreased due to moisture evaporation but was stable between 35–95 °C.
• Adhesion: Tape-peel (15 cycles) caused only minor ink loss on TWF with pattern integrity preserved; on printer paper, ink detached by cycle 8 and resistance increased. Scraping tests further confirmed superior adhesion on TWF vs printer paper and PET.
• Programmability and sensing: Origami patterns enabled reversible on/off switching by folding (e.g., connecting a circuit at 90° rotation) and two-dimensional reconfiguration. FEM predicted max principal strain <5% during folding, consistent with elastic, reversible deformation. Wearable demos on knee and elbow produced stable, stepwise current signals; anisotropy led to direction-dependent signal amplitudes; increased running speed (5–20 km h⁻¹) raised relative resistance.
• Recyclability: Proposed closed-loop recycling; recycled wood-based membranes retained favorable mechanical stability after 10 cycles, comparable to cellulose nanopaper.

The work demonstrates that a top-down processed, anisotropic transparent wood film, combined with strongly adhering CNT/TOCNFs conductive ink and origami crease design, overcomes key limitations of prior wood-based electronics by enabling complex, programmable shape transformations without sacrificing electrical stability. The high mechanical strength and orientation of cellulose fibers, coupled with extensive hydrogen bonding, provide a robust, flexible substrate that survives repeated folding, while the ink’s dense percolating network ensures reliable electron transport across deformations and varying humidity. FEM simulations corroborate that folding strains remain within elastic regimes (<5%), supporting reversible reconfiguration. The origami-enabled programmability allows reconfigurable circuit connectivity (on/off by folding) and adaptable conformability for wearables, as validated in motion sensing on joints. The approach advances sustainable electronics by replacing petroleum-based substrates with recyclable wood-derived films, suggesting broader impact for sensors, microfluidics, e-skins, and soft robotics where shape editability and environmental compatibility are crucial.

This study introduces programmable, flexible wood-based origami electronics by integrating a densified transparent wood film substrate with DIW-printed CNT/TOCNFs conductive patterns. The devices combine high mechanical performance (E ≈ 43.7 GPa; σ ≈ 393.8 MPa), optical transparency/haze, strong ink–substrate adhesion, and stable conductivity under bending and hundreds of folding cycles. Origami design endows reversible, editable 3D configurations and functional switching, enabling wearable motion sensing with anisotropy-informed signal responses. A closed-loop recycling concept underscores sustainability. Future research could optimize long-term cycling robustness of conductive traces, improve high-temperature electrical stability, integrate active components, and develop more complex multi-degree-of-freedom origami architectures for advanced sensing, microfluidics, and soft robotics.

Electrical resistance increases after extensive folding (≥500 cycles) due to cracking in the printed ink layer, indicating the conductive trace is the durability bottleneck despite the TWF substrate remaining intact. Electrical output diminishes at elevated temperature (115 °C) owing to moisture evaporation. Demonstrations are proof-of-concept (e.g., simple LEDs and joint motion sensing) without long-term on-body or harsh-environment testing. The study does not report biocompatibility, sweat/salt exposure stability, or comprehensive lifetime under repeated washing or abrasion.