I. Abstract of the Paper’s Core Research

This study has developed a dual‑atomization aerosol‑jet printing system, coupled with a water‑soluble PVA sacrificial‑substrate process, to sequentially fabricate a PI support layer and a silver–PI composite conductive layer. By dissolving the sacrificial layer, fully freestanding (substrate‑free) stretchable conductive wires (FSCW) are obtained. The system enables comprehensive validation of material fabrication, mechanical stretching, and multi‑sensing performance. The key findings are as follows:

• Dual-mist flow in-situ blending molding system

Ultrasonic atomization of silver ink and pneumatic atomization of PI ink enable real-time mixing of two aerosol streams within the nozzle, with precise control over the silver-to-PI ratio. The underlying pure PI layer provides mechanical support, while the upper silver‑composite layer ensures electrical conductivity, eliminating the need for pre‑mixing the slurry and thereby preventing nano‑silver agglomeration.

• Fabrication Process for Freestanding Structures with a Sacrificial Layer

A 400-nm-thick water-soluble PVA sacrificial layer was spin-coated onto a glass substrate. PI substrates and composite conductive traces were then printed in layers. After low-temperature curing, the PVA was dissolved by soaking in pure water, enabling the complete release of the support-free flexible conductive wires without damaging the conductive network.

• Excellent tensile properties and cyclic electrical stability

The self‑supporting serpentine structure exhibits a maximum tensile strain of 180% without fracture; within a 50% strain range, its electrical resistance varies linearly, with a strain sensitivity of 0.75. After 800 tensile cycles, its performance stabilizes, and even after 10,000 large‑strain cycles, the resistance increases only slightly—far outperforming substrate‑constrained printed circuits.

• Single-Structure Multifunctional Sensor Integration

A self-supporting structure with a double-layer sandwich configuration—conductive–PI dielectric–conductive—forms a capacitive humidity sensor, whose output signal in the 0–40% relative humidity range remains virtually unaffected by mechanical deformation. Meanwhile, a single-layer serpentine conductive trace can serve directly as an epidermal tactile and strain sensor, conforming to finger joints to stably acquire deformation signals.

• Integrated single-wire crossover circuit

Multi-layer aerosol stacking isolates conductive pathways, while a single self-supporting wire incorporates two independent conductive circuits, addressing the issue of cross‑shorts in conventional flexible circuits and simplifying soft‑circuit design.

II. Complete Preparation Procedure for Self-Assembled Stretchable Wire Aerosols

• Substrate Pre-treatment: A 5 wt% PVA aqueous solution was spin-coated onto a glass slide and dried at room temperature to form a 400 nm-thick water-soluble sacrificial layer.
• Ink Formulation: Silver nanoparticle ultrasonic atomization ink and polyimide (PI) pneumatic ink shall be stored separately.
• Aerosol Layer-by-Layer Printing: First layer: a pure PI support layer (3 print passes, 30 μm thick); Y‑shaped connector: two streams of atomized fluid are co‑extruded to form a silver–PI composite conductive layer; a PI insulating protective layer can be added as needed.
• Low-temperature curing: Maintain at 130°C for 12 hours to prevent high-temperature carbonization of PVA, which would render it insoluble.
• Peeling and release: Soak in pure water for 2–6 hours to completely dissolve the underlying PVA, then remove the self-supporting functional wire.
• Performance Characterization: Linear motor tensile fatigue testing, four-probe resistivity testing, humidity‑dependent capacitance measurement, and real‑world tactile validation.

III. Five Core Process Advantages of Aerosol-Jet-Printed Self-Supporting Flexible Structures

• Dual-ink in-situ mist-phase mixing with precise and controllable ratio control.

Two independent atomization channels enable real-time adjustment of silver and PI flow rates, eliminating the need for pre‑mixing and preventing nano‑silver sedimentation and agglomeration, while balancing electrical conductivity with tensile toughness.

• Maskless, multi-layer integrated direct writing; single-layer equipment enables multi-material stacking.

It enables the alternating deposition of a support layer, a conductive layer, and an insulating dielectric layer, allowing for the one-step fabrication of a sandwich‑type capacitive sensing structure without the need for multiple transfer or lithography steps.

• The self-supporting structure is free from substrate constraints, significantly enhancing its deformation capability.

Without elastic membrane constraints after delamination, the material can be stretched up to 180%, whereas substrate‑adhered circuits are limited to only 50% stretch, making them suitable for robotic joints undergoing large deformations.

• Gentle low-temperature processing, compatible with sacrificial layer removal techniques.

Cures at 130°C, preserving the PVA water-soluble substrate and ensuring clean delamination with no circuit damage.

• Micron-level high-precision molding, with a minimum feature size of 10 μm.

Serpentine micro‑wires and interdigitated sensing electrodes can be freely patterned to match miniature epidermal sensor chips.

IV. Five Core Technological Innovations of This Study

Innovation 1: A Novel Self-Standing Flexible Manufacturing Paradigm Combining Aerosol and Sacrificial Layers

By employing aerosol‑based layer‑by‑layer printing combined with water‑soluble sacrificial lift‑off, we have fabricated fully substrate‑free, stretchable conductive functional structures.

Innovative Dual-Atomization Y-Shaped In-Situ Blending Ink Molding Technology

Ultrasonic and pneumatic dual atomization are independently controlled, with in-line mixing of the spray streams, addressing the challenges of incompatibility and agglomeration between silver and PI in the liquid phase.

Innovative 3‑Self‑Supporting Serpentine Structure with Ultra‑High Stretchability Mechanism

Without substrate constraints, the surface can undergo shape deformation, dissipating tensile stresses and surpassing the conventional limits of line stretch imposed by the substrate.

Innovation 4: Integration of multiple isolated conductive paths within a single wire

Aerosol‑based layered insulation and isolation, combined with a single self‑supporting conductor housing a dual‑channel circuit, prevents cross‑shorting and simplifies flexible wiring.

Innovative integrated design of a dual‑function sensor for both strain and humidity, featuring a five‑layer structure.

Silver—PI sandwich‑capacitor architecture that simultaneously enables strain monitoring and ambient humidity sensing, with multi‑sensor components fabricated in a single printing step.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Conventional flexible circuits rely on PDMS/PI substrates, and their stretchability is constrained by the substrate material; stress concentration can easily lead to fracture.

✅ Aerosol‑plus‑sacrificial‑layer fabrication yields a freestanding structure, free from substrate constraints; the surface morphology effectively disperses stress, resulting in a tensile strain that is more than three times higher.

Challenge 2: In silver and polymer inks, liquid-phase mixing readily leads to agglomeration, making it difficult to simultaneously achieve both conductivity and toughness.

✅ Two‑channel aerosol nozzles enable online mixing of the gas and liquid phases without prior contact, ensuring uniform dispersion of nano‑silver.

Challenge 3: Cross‑wiring in multilayer flexible circuits is prone to short circuits, and the manufacturing process is complex.

✅ Layered aerosol printing creates a PI insulating isolation layer, with each individual wire incorporating an independent conductive channel.

Challenge 4: The photolithographic transfer of freestanding thin films is a complex process, resulting in low yield and high costs.

✅ Digital aerosol direct writing, with a single-step fabrication followed by pure-water delamination—no lithography or transfer steps required.

Challenge 5: In conventional single-function printed devices, strain and humidity sensing must be fabricated separately.

✅ Integrated sandwich‑capacitor structure, enabling dual environmental signal detection with a single printing process.

VI. Future Diversified Industrial Application Scenarios

1 Wearable epidermal sensing patch for the human body

Finger and joint self‑standing strain/humidity sensor for noninvasive real-time monitoring of physiological parameters and skin‑environment conditions.

2 Soft-Robot Tactile Sensing Array

Large-deformation, substrate-free conductive circuits; pressure and deformation sensing units for robotic soft skins.

3 Lightweight Flexible LED Interconnect Circuitry

Self-supporting conductive wires remain intact under bending and stretching, enabling ultra-thin flexible display‑drive wiring.

4 Medical Flexible Implantable Sensing Elements

A lightweight, substrate-free, self-supporting structure that conforms to human soft tissues, enabling minimally invasive physiological monitoring.

5 Flexible Electronics Laboratory Rapid Prototyping Platform

Rapidly print multi‑composition self‑supporting conductive wires, perform batch testing of tensile and sensing properties, and shorten the device development cycle.

Image and Text Guide

Figure 1. AJP manufacturing process for freestanding stretchable conductive wires (FSCW): from dual‑ink printing to PVA dissolution and release.

• (a)   Process schematic: A PVA sacrificial layer approximately 400 nm thick is first spin-coated onto the glass substrate → a support line 200 μm wide and 30 μm thick is printed using PI ink (three printing passes, with a serpentine winding pattern and an inter-line offset of about 20 μm) → a conductive layer is then printed on top using an Ag/PI composite ink → finally, the structure is immersed in deionized water to dissolve the PVA, thereby releasing the freestanding FSCW.
• (b)   Silver nanoparticle ink (contained in a glass vial, atomized by ultrasonic nebulization); (c)   Polyimide (PI) ink (contained in a plastic canister, pneumatically atomized); (d)   Y‑shaped connector: It enables the aerosol streams of two inks to be mixed “in situ” within the channel, thereby preventing particle agglomeration in pre‑mixed inks while ensuring strong adhesion between the conductive layer and the underlying PI substrate.
• (e)   The FSCW serpentine pattern fully released from the substrate (scale bar: 1 cm); (f)   SEM of the wire cross-section: the bright layer is an Ag/PI composite conductive layer, while the dark layer is a PI support layer; the bilayer structure is clearly visible (scale bar: 100 μm).
• Data Under typical conditions, the mass ratio of Ag to PI is approximately 50:1 (volume ratio approximately 7:1); curing is carried out at 60 °C for 2 h followed by 130 °C for 12 h (with temperatures kept below 150 °C to prevent carbonization of PVA); the minimum reliable feature size in AJP is about 10 μm.

Figure 2. Electrical and mechanical properties of FSCW: tunable resistance, aqueous stability, linear strain response, and 950 cycles.

• (a)   At a fixed Ag flow rate of 30 SCCM and with the PI flow rate gradually increased (from N/A to 160 SCCM), the sheet resistance of the Ag/PI composite line rises as the PI content increases—allowing precise tuning of the conductor’s resistance via ink flow rate.
• (b)   The electrical resistance of four samples remained virtually unchanged after 6 hours of immersion in water: the PVA dissolution–molding process does not affect the electrical properties of FSCW.
• (c)   LinMot linear motor tensile testing rig + four-probe resistivity measurement setup (scale: 2 cm); (d)   A physical photograph of an FSCW stretched from its original length to 0%→50%→100%→150%.
• (e)   The resistance change ratio R/R₀ varies with strain: it remains approximately linear up to about 50%, increases by roughly 140% at around 170% strain, and fractures at 180% (by contrast, a comparable structure embedded in a polymer film fails at strains below 50%).
• (f)   0%–50% strain stability testing: response remains stable for approximately 950 cycles, with no significant change observed after about 800 cycles (indicating that the wire has reached a stable, repeatable conductive configuration); the initial resistance is approximately 80 Ω (equivalent electrical conductivity of 1–2×10⁵ S/m), and the sensitivity (dR/R₀)/(dL/L₀) is approximately 0.75.

Figure 3. Multilayer structural design (dFSCW): coaxial stacking of conductive layers, dielectric isolation, and end‑face electrode interconnection.

• (a1–4)   Schematic cross-sections of four stacking designs: double-layer (PI + Ag/PI), three-layer, four-layer, and five-layer—where the conductive layers (Ag/PI) are separated by PI dielectric layers to prevent short circuits. (b1–4)   Corresponding SEM cross-section (scale bar: 50 μm): the bright layer is an Ag/PI composite, while the dark layer is PI, demonstrating that AJP can sequentially stack different materials layer by layer.
• (c1–4)   Cross-sectional thickness profile of the wire, measured with a profilometer (Dektak 6M), with a height of approximately 30 μm and a width of approximately 250 μm. (d1–4)   Top-view SEM of the wire end (scale bar: 1 mm): The conductive layers are interconnected at the end to form connection electrodes, with a PI layer above protecting the conductive layers (e.g., d2/d3).
• Key Data : The design with a top-layer PI protection (b2) is extremely insensitive to tensile strain yet remarkably robust—after 75,000 cycles of repeated stretching at 100% of the original length, its resistance increases by less than 20%; the double-layer dFSCW is essentially a “Ag–PI–Ag” capacitor structure.

Figure 4. Application demonstration: dFSCW circuit with a standalone tactile sensor and a single-wire-powered LED.

• (a)   Physical prototype of the AJP‑printed standalone stretchable tactile sensor (with two stretchable conductive wires bridging interdigitated electrodes); (b)   Interdigital electrode SEM (scale bar: 1 mm), which constitutes the functional core of the sensor.
• (c,d)   The sensor is attached to the finger and maintains reliable adhesion and functionality even as the finger extends and flexes. (e)   Real-time conductance signal: Whether the finger is extended or bent, touching the forked electrode structure with the fingertip causes a sudden change in conductance (1/R)— Tactile sensing is unaffected by bending. 。
• (f)   Dual‑path FSCW (dFSCW) supplies power from the battery via a single wire to surface‑mount LEDs at both ends (5 mm scale); (g)   During the stretching of dFSCW to 100% of its original length, the LED’s current and voltage remain nearly constant. (h–k)   The LED remains lit under 0%, 50%, and 100% tensile strain.
• Meaning : A single printing step suffices to fabricate self-supporting circuits, while the multilayer architecture enables “a single wire to traverse an entire circuit,” significantly simplifying the wiring complexity of stretchable circuits.

Figure 5. Stretchable humidity sensor: The dFSCW capacitive structure is sensitive to humidity but insensitive to stretching.

• (a)   dFSCW is mounted side by side with the commercial humidity module on a rod (scale: 1 cm); (b)   The entire assembly is placed inside a test chamber modified from a syringe (scale: 2 cm); (c)   Humidity is regulated by introducing a controlled airflow into the chamber via a water bubbler.
• (d)   Capacitance versus frequency (80 Hz–50 kHz) at different relative humidities (43%–91%): Higher humidity corresponds to higher capacitance—moisture absorption alters the dielectric constant of the PI dielectric layer.
• (e)   Capacitance varies with humidity: at low frequencies (100 Hz), the response is more sensitive than at high frequencies (10 kHz), but the error bars are smaller at high frequencies, indicating greater stability.
• (f,g)   Capacitance measured at 56%/66%/76% relative humidity and over a range of 0%–40%: The impact of stretching within a range of 10%–40% on capacitance is negligible. — Moreover, epidermal electronics typically exhibit a tensile strain of about 30%; accordingly, this humidity sensor can maintain accurate and reliable performance in wearable applications. Strain can also be calibrated by measuring resistance changes as the conductive layer is subjected to mechanical deformation.