I. Abstract of the Paper’s Core Research

In this study, we used Optomec’s aerosol‑jet printing system to fabricate 30–50 nm‑thick silver nanofilms and developed an atmospheric‑pressure dielectric‑barrier argon plasma jet sintering setup. We systematically characterized how plasma parameters, processing duration, and substrate type influence the microstructure, electrical conductivity, and thermal damage to the substrate of the silver films. The key findings are as follows:

Non-thermal plasma low-temperature sintering mechanism: Electron‑based high‑energy activation of nanoparticles, with an overall substrate temperature below 50°C—distinct from high‑temperature sintering that relies on thermal diffusion. A cyclic control strategy of 30 seconds on / 30 seconds off ensures precise temperature regulation, preventing overheating of the substrate.

Broad-spectrum substrate compatibility: Glass, porous printing paper, PET/PDMS flexible membranes, plant leaves, and the surfaces of fruits and vegetables can all be stably sintered to form conductive silver circuits, with no scorching or damage to the bio‑based substrates.

Excellent electrical conductivity: Paper substrate: 6 W plasma treatment, with a conductivity of 1.4 × 10⁶ S/m; glass substrate: under operating conditions of 11.7 W, the conductivity can reach 1.1 × 10⁶ S/m, and after 100 minutes of treatment, the resistance decreases by six orders of magnitude.

Microscopic densification mechanism: Plasma etching removes surface surfactants from the ink (reducing carbon content by nearly 50%), driving neck growth of silver particles and yielding a continuous, dense film; plasma penetration into the porous paper substrate is deeper, resulting in superior sintering compared to smooth glass.

Atmospheric open system, No vacuum chamber is required; it enables point‑by‑point localized sintering, allowing for differentiated conductive patterns on the same substrate and seamless integration with roll‑to‑roll continuous production lines.

II. Aerosol Printing + Complete Process Flow for Atmospheric-Pressure Plasma Sintering

1) Dilute the silver nanoparticle ink with deionized water at a ratio of 1:2.5, and disperse it uniformly by sonication.

2) The AJP device deposits a square-shaped conductive silver film via direct writing, compatible with various substrates such as glass, paper, and silicone.

3) An argon‑dominant plasma jet device, operating at a 50 kHz sinusoidal voltage with intermittent pulsed discharge for temperature control;

4) Plasma spot‑scanning treatment for 20–100 minutes, with real-time infrared monitoring ensuring the substrate temperature remains stable below 50°C.

5) SEM is used to observe the compactness of the particles, and the four-probe method and the Van der Pauw method are employed to measure the electrical conductivity of the thin film.

III. Core Advantages of the Five Major Processes (Compared with Hot Sintering, Laser Sintering, Vacuum Plasma Sintering, etc.)

Compatible with low-temperature, fully thermosensitive substrates.

Throughout the entire process, the substrate temperature remains below 50°C, ensuring that PET, paper, fresh produce, and biodegradable films do not undergo high‑temperature carbonization or deformation.

Atmospheric-pressure open‑type equipment eliminates the need for a vacuum chamber, significantly reducing production line costs.

No sealed low-pressure chamber is required; the equipment features a simple structure and can be integrated into a roll-to-roll continuous production line.

Localized, partitioned sintering enables gradient conductive pathways.

The plasma beam can be scanned in a displaced manner, enabling localized sintering and reinforcement on the same substrate without requiring uniform treatment across the entire surface.

Gentle, free of mechanical damage; suitable for ultra-thin, flexible substrates.

No intense light scorching, no high‑temperature radiation; thin paper and ultra‑thin PI film do not become brittle or crack.

Versatile for multiple materials; compatible with all metal-based aerosol inks.

The process is not limited to silver; nano-aerosol‑printed circuits on copper, nickel, and other materials can all be paired with low‑temperature plasma sintering.

IV. Five Core Technological Innovations of This Study

Innovation 1: Atmospheric-Pressure Non-Thermal Plasma Intermittent Temperature-Controlled Sintering System

A pulsed plasma start–stop process controls substrate temperature rise at the source, enabling non‑destructive densification below 50°C and overcoming the thermal limitations of all conventional high‑temperature post‑treatment processes.

Innovation 2: Elucidation of the Differentiated Sintering Mechanism for Porous and Smooth Substrates

It is confirmed that porous media facilitate plasma penetration and exhibit superior electrical conductivity at the same power level, providing a theoretical basis for optimizing paper-based electronic fabrication processes.

Innovative No-Vacuum Open-Structure Plasma Forming Equipment Solution

By abandoning the costly vacuum equipment of conventional low-pressure plasma systems, the atmospheric-pressure jet system is well-suited for industrial-scale, batch‑wise, continuous manufacturing.

Innovation 4: A New Process for Printing Conductive Circuits on Bio-Based Substrates

Stable conductive silver circuits are fabricated on the surfaces of living organisms such as fruits, vegetables, and leaves, paving the way for transient biosensing.

Innovation: Simultaneous realization of the dual effects of plasma surface activation at five levels.

While removing the organic dispersant from the ink, it simultaneously induces neck growth in the silver particles, thereby achieving both degreasing and densification of the conductive layer in a single step.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Conventional thermal sintering requires temperatures above 200°C, which can cause thermal degradation of flexible, paper‑based, or bio‑based substrates.

✅Pulsed atmospheric-pressure plasma, with the substrate maintained at a constant temperature below 50°C, leaving all types of heat-sensitive and biodegradable substrates undamaged.

Challenge 2: Low-pressure plasma equipment is expensive, requires a vacuum chamber, and is difficult to scale up for mass production.

✅ Atmospheric-pressure open-type argon jet system, with no vacuum components, significantly reducing equipment capital and operational maintenance costs.

Challenge 3: Laser sintering causes localized high-temperature scorching of the thin film and substrate, limiting pattern design.

✅ Whole‑area low‑temperature plasma, with no localized hot spots, enabling uniform sintering of large‑area, complex‑shaped patterns.

Challenge 4: Conventional sintering of porous paper substrates readily leads to bubbling and delamination, resulting in poor electrical conductivity.

✅ Plasma can penetrate deep into the pores, enabling thorough particle fusion; paper-based devices exhibit higher electrical conductivity than glass substrates.

Challenge 5: Uniform high-temperature treatment cannot achieve regionally differentiated conductivity.

✅ The plasma beam enables mobile, localized scanning, allowing on-demand control of the sintering degree in specific regions to fabricate functionally graded circuits.

VI. Future Diversified Industrial Application Scenarios

1 Disposable paper-based sensing electronics

Paper-based electrocardiogram and humidity sensors, sintered without damage using low-temperature plasma, enabling low-cost, disposable, environmentally friendly electronics.

2 Wearable silicone flexible sensing patch

PDMS and silicone‑gel substrates with conductive traces ensure that wearable monitoring devices remain securely attached even after prolonged bending.

3 Transient Biomedical Electronics

Epidermal and implantable short-term sensing, biodegradable thin-film substrates, and low-temperature processing that preserves the biocompatibility of materials.

4 Agricultural Plant Physiology Monitoring Electrode

In situ printing of conductive sensing circuits on leaf surfaces and fruit/vegetable skins enables nondestructive monitoring of plant water status and hormonal signals.

5 Roll-to-Roll Mass-Production Flexible Display Wiring

Flexible PET roll-to-roll aerosol printing combined with continuous low-temperature plasma sintering, suitable for large-area printed electronics production lines.

Image and Text Guide

Figure 1. AJP-printed silver nanoparticle film and atmospheric-pressure, room-temperature plasma jet sintering apparatus

Figure 1(a): Schematic of the AJP deposition head: ultrasonic atomization of silver nanoparticles, with an N₂ sheath gas confining the aerosol jet to strike the substrate. Figure 1(b): Plasma sintering setup: Ar gas is passed through a quartz tube to generate a plasma jet via dielectric barrier discharge; the printed silver film is positioned directly facing the jet. R_x = 10 kΩ and C_x = 2.2 nF are used for electrical measurements, and an IR camera measures temperature from the backside. Figure 1(c): Photograph of the plasma jet impacting PEL paper. Figure 1(d): IR thermal image: the active plasma region reaches a maximum of 45 °C. Figure 1(e): AJP‑printed Notre Dame logo on a leaf. Figure 1(f): Plasma jet striking the surface of a tomato. Figures 1(g) and 1(h): SEM images of the silver film before and after sintering (scale bar: 50 nm).

Figure 2. Variation of sheet resistance and electrical conductivity with plasma sintering time.

Sheet resistance (Ω/□) on the left axis and electrical conductivity (S/m) on the right axis versus sintering time (min), for glass (red) and PEL paper (black). During the first 20 min, the sheet resistance drops from 10⁷ to ~3 Ω/□—a reduction of six orders of magnitude—and continues to decrease gradually thereafter; after 80 min, the conductivity levels off, reaching 1.1×10⁶ S/m on glass and 1.4×10⁶ S/m on PEL paper.

Figure 3. SEM densification of the silver film on glass and PEL substrate as a function of sintering time.

Figure 3(a–d): On glass, from unsintered to 20, 50, and 100 min, silver nanoparticles evolve from isolated spherical particles into neck‑to‑neck connections, ultimately forming a dense, continuous film; Figure 3(e–h): On PEL paper, particles fill the pores, and after 100 min a wavy, continuous structure emerges. Scale bar: 50 nm.

Figure 4. Surface temperature and current waveforms under a pulsed plasma switch

Figure 4(a) shows the surface temperatures under a 30‑s on/30‑s off cycling: the glass reaches a peak of approximately 49 °C, while the PEL paper, being thin and thermally conductive, peaks at about 45 °C; throughout the cycle, neither exceeds 50 °C. Figure 4(b) depicts the sinusoidal plasma current for a single cycle at 50 kHz and 7.5 kVpp: the red curve for glass exhibits a negative‑half‑cycle current amplitude significantly larger than that of the black curve for PEL, corresponding to deposition powers of 11.7 W for glass and 6.1 W for PEL.

Figure 5. Sintering of a silver film on the surface of a tomato and demonstration on flexible/bio‑based substrates.

The sheet resistance of the silver film printed on tomatoes decreases with increasing sintering time: 0 min—insulating (>10⁷ Ω/□, at the multimeter’s upper limit); 15 min—~300 Ω/□; 30 min—~20 Ω/□; 45 min—~10 Ω/□. Insets: (a) a 1×5 mm² silver film on a tomato before sintering; (b) the sintered silver film; (c) the sintered pattern on a finger wrapped in silicone; (d) an interlocking Notre Dame ND logo printed by AJP on a grape.