Flexible and stretchable metal oxide nanofiber networks for multimodal and monolithically integrated wearable electronics

The study addresses the need for scalable, low-cost, mechanically flexible and stretchable electronic materials and devices for wearable e-textile/e-skin applications. Conventional inorganic electronics often require complex mechanical designs (e.g., serpentine interconnects) to accommodate strain and can suffer from limited multifunctionality and stability. The authors propose blow-spun, three-dimensional (3D) inorganic nanofiber networks (FNs) of metal oxides and metals (IGZO, CuO, ITO, and Cu) as intrinsically stretchable, conformable platforms. The central aim is to demonstrate that these FN films can be fabricated at scale and integrated monolithically into multifunctional, reliable wearable electronics capable of sensing and distinguishing multiple stimuli (gas, light, strain, pressure, temperature, humidity, motion, respiration) while maintaining robust electrical performance under bending and stretching.

Prior work in stretchable and wearable electronics has relied on structural engineering of rigid inorganic materials (wavy/serpentine designs) or intrinsically stretchable organic composites, often trading off performance, stability, or multifunctionality. Metal oxide (MO) semiconductors such as IGZO are established for flexible thin-film transistors, but gas sensors based on MOs frequently require high-temperature operation (>300 °C), surface functionalization, or UV activation, and exhibit slower response/recovery and higher detection limits than desirable for wearables. Nanowire/mesh and fiber-based platforms have been explored, but scalable, monolithically integrated, multimodal sensing with robust stretchability remains challenging. The present work builds on blow-spinning methods for nanofiber formation and integrates MO FNs to overcome these limitations by leveraging large surface-to-volume ratios, porous 3D networks, and orientation effects to maintain conductivity under strain.

- Materials and precursor preparation: Metal salts In(NO3)3, SnCl4·5H2O (ITO), In(NO3)3, Zn(NO3)2, Ga(NO3)3 (IGZO), Cu(NO3)2 (Cu/CuO) dissolved in ethanol; poly(vinyl butyral) (PVB) added and stirred (~1 h). Ionic liquid [EMIM][TFSI]; PVDF-HFP; PEDOT:PSS; SEBS elastomer; PI precursor; PDMS were sourced as specified. - Blow-spinning fiber fabrication: Precursor solution loaded into a spray gun (nozzle 0.3 mm), sprayed at 20 cm, 0.1 mL min⁻1, gas pressure 100 kPa onto SiO2/Si or Al foil. PVB/metal salt nanofiber mats annealed with material-specific profiles: • IGZO: ramp RT→450 °C (10 °C min⁻1) in air; hold 15–30 min; cool in furnace. • ITO: ramp RT→500 °C (10 °C min⁻1) in air; hold 1 h; then RT→300 °C (2 °C min⁻1) in 5% H2/N2; hold 1 h; cool. • CuO: ramp RT→450 °C (10 °C min⁻1) in air; hold 1 h; cool. • Cu: further reduction of CuO fibers at 300 °C in 5% H2/N2; hold 1 h; cool. Fiber areal densities (cFN) of 0.15, 0.5, 2.0 µm⁻1 controlled by blow-spinning time (30 s, 90 s, 5 min). - Characterization: Optical microscopy; XRD (Rigaku Smartlab, Cu Kα); TGA/DSC (SDT Q600, 10 °C min⁻1, air); TEM/SAED (JEOL ARM300F). - Device fabrication: • Rigid IGZO FN TFTs (cFN=0.15 µm⁻1): IGZO FN on 300 nm SiO2/n++-Si; Al S/D (50 nm) via shadow mask; L=100 µm, W=1000 µm. • Flexible IGZO FN TFTs: IGZO FN contact-transferred to ultrathin PI (~1.5 µm) partially cured, then fully cured at 250 °C (1 h). Top-gate/top-contact with ion-gel dielectric (PVDF-HFP/[EMIM][TFSI], Ci=10.7 µF cm⁻2 at 1 Hz), Ti/Au S/D (20/100 nm). Gate via tungsten probe into ion-gel. • Stretchable resistors on SEBS: SEBS substrates drop-cast and cured. For strain and gas sensors, stretchable PEDOT:PSS/[EMIM][TFSI] electrodes patterned via PI stencil; IGZO FNs (cFN=0.5 µm⁻1) transferred. For temperature/UV/breath (IGZO) and pressure (CuO) sensors, Cr/Au (3/50 nm) electrodes used. Channel dimensions: IGZO W=1000 µm, L=100 µm; CuO W≈100 µm, L=50 µm. For high-coverage integration and ITO strain sensors, self-standing FN films (cFN=2.0 µm⁻1, 1×2 cm²) laminated onto SEBS with Cu wire contacts embedded during curing. - Electrical/physical testing: Agilent 4155C/B1500A analyzers in ambient (RH 30–40%). Bending tests by laminating on cylinders of defined radii; cyclic stretching via custom stretcher. Gas sensing in a controlled chamber with humidified air (~50% RH) and NO2, NH3, H2, CO2 inputs via MFCs; total flow ~550 sccm. Photodetection at 365 nm, ~7.3 mW cm⁻2; dark in black box. Temperature via resistive heater; infrared thermometer monitoring; bias 5 V. Humidity tests at RH 1%, 54%, 100%. Pressure tests 50 Pa–20 kPa; response measured at 20 ms sampling; finger contact isolated thermally by tape. - Finite element analysis (ABAQUS): 3D solid elements (C3D10M) for fibers embedded in elastomer. Material properties: fiber E=110 GPa, ν=0.34; substrate E=4.8 MPa, ν=0.5. Crack strain 0.8%. Simulated relationship between fiber orientation angle α, applied substrate strain ε, and fiber stress; simplified model relating ΔR/R to ε assuming evenly embedded straight fibers without mutual interference.

- Scalable fabrication: Blow-spinning produced free-standing 3D FNs of IGZO, ITO, CuO, and Cu with controlled areal density (0.15–2.0 µm⁻1) and nanocrystalline structures confirmed by TEM/SAED. - IGZO FN TFTs (rigid, bottom-gate): Mobility µ ≈ 1 cm² V⁻1 s⁻1; Ion/Ioff ≈ 4×10⁴. As NO2 increases (20 ppb–20 ppm), linear ΔI/I0 vs concentration with sensitivity SG = 33.6% ppm⁻1 at 25 °C; response/recovery times 2–3 s/20–22 s at RT; LOD = 20 ppb. - IGZO FN TFTs (flexible, ion-gel gated on ~1.5 µm PI): Low-voltage operation (<10 V). Average µFE ≈ 0.52 cm² V⁻1 s⁻1; Ion/Ioff ~10⁴. Under 1 mm bending radius: µFE drops to 0.06 cm² V⁻1 s⁻1 with VT shift from +3.7 V to +1.3 V, both recover upon flattening. Mobility stable over 1000 bending cycles at r=5 mm. - Stretchability and mechanics: Single IGZO fibers crack at ~0.8% intrinsic strain; in FNs, randomly oriented fibers distribute stress. FEA shows substrate strain ε needed to reach 1.1 GPa in fiber rises polynomially with fiber angle α (e.g., ε≈0.8% at α=0°, ε≈22% at α=40°). IGZO FN resistors on SEBS show minimal ΔR/R0 up to ~25% strain; ΔR/R0≈5.1 at 50% strain; near-pristine recovery after release; stable over 5000 cycles (0–10% strain). - IGZO FN multifunctional sensing (stretchable resistor): • UV photodetector (365 nm, 7.3 mW cm⁻2, 5 V): Rph decreases from 36 to 16 mA W⁻1 when stretched to 10% strain; Iph/Ia increases from 123 to 403; D* increases from 4.4×10¹⁰ to 5.2×10¹⁰ Jones (at 10% strain). Table value: Rph ≈ 16 mA W⁻1, Iph/Id ≈ 403, D* ≈ 5.2×10¹⁰; operation up to 10% strain. • Gas sensing (NO2, 20 ppm, 5 V): Rapid response/recovery (~≤5 s cycles). Strain-dependent sensitivity: SG ≈ 38.0% ppm⁻1 (0% strain), 166% ppm⁻1 (10%), 46.5% ppm⁻1 (50%). High selectivity vs NH3 (20 ppm), H2 (10³ ppm), CO2 (10⁴ ppm). • Thermistor behavior (35–75 °C): Negative temperature coefficient with linear resistance vs temperature. Sensitivity Sr ≈ 2.1% °C⁻1 (0% strain) and 2.2% °C⁻1 (10% strain), exceeding IGZO films (~0.5% °C⁻1), graphene-based and polymer comparators. • Humidity: Between dry air (<1% RH) and 50% RH: ΔI/Ii ≈ 23%, response/recovery ~2 s/12 s. Between ambient and 100% RH: ΔI/Ii ≈ 260%, longer recovery (~120 s). • Breath analysis: Distinguishes bradypnea (~3–4 bpm), normal (~15 bpm), and tachypnea (~30 bpm post-jogging). After exercise, saturation current increases ~8×, reaching ~5×10⁴ nA due to elevated temperature/humidity. Alcohol intake (200 mL beer) decreases saturation current by ~50% at slow breathing; inhaling/exhaling current modulation ~10–20× lower than without alcohol. - CuO FN pressure sensing: High Young’s modulus fibers yield pressure-sensitive resistors (cFN~0.5 µm⁻1). Sensitivity Sp ≈ 0.04 kPa⁻1 for <4 kPa; detects ~50 Pa (dime) pressure; fast 40/60 ms response/recovery on finger touch/release. Attempted CuO FN TFTs could not be switched off (typical p-type oxide behavior). - ITO FN strain sensing: Stretchable resistors (cFN=2.0 µm⁻1) with resistance 3–7 MΩ and gauge factor ≈ 21; elongation ≥50%; stable ΔR/R0 behavior even after 1 year ambient storage. Effective for motion/gesture recognition with response/recovery ~2/7 s. - Monolithic integration (e-skin): IGZO, CuO, and ITO FN devices integrated on SEBS band patch on hand. Real-time differentiation of solar light, temperature, strain, breath, and pressure stimuli across daily activities. Cross-sensor signatures allow discrimination of confounded stimuli (e.g., temperature-involved events) that single sensors cannot uniquely identify.

The work demonstrates that blow-spun 3D inorganic nanofiber networks provide intrinsically stretchable electronic platforms without complex mechanical layouts, addressing key challenges in wearable electronics. Random fiber orientation in FNs enables strain accommodation while maintaining conductive pathways, as validated by FEA and experiments. IGZO FN TFTs show robust electrical characteristics under bending and exceptional room-temperature NO2 sensing with ppb-level detection and second-scale dynamics, overcoming common high-temperature requirements in MO gas sensors. As stretchable resistors, IGZO FNs deliver multimodal sensing—UV, gas, temperature, humidity, and respiration—exhibiting high sensitivity and fast response/recovery. CuO FNs contribute high-sensitivity pressure sensing with millisecond dynamics, and ITO FNs provide reliable, high-gauge-factor strain sensing suitable for motion/gesture recognition. The monolithically integrated e-skin combining IGZO, CuO, and ITO devices differentiates among complex, simultaneous stimuli, enabling practical human–machine interfaces. Collectively, these findings validate the hypothesis that scalable FN films can yield multifunctional, mechanically resilient wearable electronics with strong performance across sensing modalities.

This study establishes a scalable blow-spinning strategy to fabricate metal oxide and metal nanofiber network films (IGZO, CuO, ITO, Cu) and integrates them into flexible/stretchable transistors and multimodal sensors. The FN architecture imparts high mechanical tolerance while delivering strong device performance: flexible IGZO TFTs stable over 1000 bends, room-temperature NO2 sensing with ppb LOD and fast dynamics, stretchable resistive sensors for light, gas, temperature, humidity, pressure, and strain, and monolithically integrated e-skins capable of distinguishing real-world stimuli combinations. These advances provide a pathway to robust, sensitive, and versatile wearable electronics with potential applications in artificial skin, biomedical implants, and smart protective gear. Future work can build on this platform to expand analyte specificity (e.g., via surface functionalization), integrate low-power/readout circuits, and explore long-term on-body operation in varied environments.

- Material/device constraints: CuO FN-based TFTs could not be switched off, limiting their use as active transistor channels (typical of p-type oxides). IGZO single fibers crack at low intrinsic strain (~0.8%); at high macroscopic strain (e.g., 50%), increased crack density in aligned fiber segments raises resistance and can reduce gas sensing response due to fewer effective conductive pathways. - Cross-sensitivity and selectivity: IGZO FN resistors respond to multiple stimuli (temperature, humidity, light), so single-sensor readouts can be ambiguous (e.g., IGZO alone cannot distinguish among temperature-involved events), necessitating multi-material integration for unambiguous discrimination. - Strain-induced performance variations: Flexible IGZO TFT mobility and threshold voltage shift under tight bending (r=1 mm), recovering on flattening; strain also modulates photodetector metrics (Rph decreases, Iph/Ia and D* increase), implying calibration is needed for accurate sensing under deformation. - Environmental dynamics: Humidity sensing shows longer recovery at high RH (up to ~120 s), which may affect rapid-cycling applications in very humid conditions.