I. Abstract of the Paper’s Core Research

This study employs the Optomec AJ300 ultrasonic aerosol system and utilizes three representative inks—silver nanoparticles (AgNP), PEDOT:PSS, and hydroxyapatite–collagen composites—to develop three 3D printing strategies: CJD continuous deposition, LBL layer-by-layer printing, and PW point-by-point fabrication. The study systematically investigates how ink formulations and process parameters influence microstructural morphology, high aspect ratios, and printing repeatability, and conducts comprehensive characterization of micro‑morphology, dimensional accuracy, and mechanical and electrical properties. The key findings are as follows:

• The three types of inks have clearly defined, differentiated 3D printing limits.

Layer-by-layer printing with diluted silver ink can fabricate hollow microcolumn arrays with a high aspect ratio of 20, enabling the creation of complex, bifurcating blade-like microstructures.

PEDOT:PSS, a conductive polymer, exhibits an optimal high aspect ratio of 7 and a dense, pore-free structure, making it well-suited for bioelectrodes.

The hydroxyapatite–collagen bio‑ink exhibits a maximum height-to-width-to-depth ratio of 3, enabling the fabrication of pyramidal lattice units that closely mimic natural bone tissue.

The applicable scenarios for the three major forming processes are clearly delineated.

CJD continuous定点 deposition: enables rapid fabrication of single, high‑aspect‑ratio microcolumns, ideal for single‑site functional microelectrodes.

L Layer-by-layer stacking: arrayed micro-pillar batch fabrication, involving multiple repetitions of the same steps, suitable for industrial-scale production.

PW point-by-point timed printing: enables the fabrication of gradient‑complex 3D lattice structures such as pyramids and lattices.

• The solvent system determines the success or failure of the molding process.

The combined use of high- and low-boiling-point co-solvents modulates the drying rate of droplets; excessively rapid drying during printing leads to rough dendritic structures, while excessive moisture causes structural collapse. The platform temperature and the sheath‑gas focusing ratio are the key control parameters.

• A complete system of material compatibility

Metal, conductive polymers, and natural biomolecular inks can all be stably aerosolized and 3D‑printed; a single system can switch between materials to fabricate both microelectronic devices and biological scaffolds, eliminating the need to retool the production line.

• All-digital maskless micro- and nano-fabrication

CAD‑customized 3D trajectories eliminate the need for photolithography molds, enabling rapid iteration of micron‑scale 3D structures and significantly reducing prototyping costs for small‑batch production.

II. Complete Preparation Process and Core Advantages of Aerosol 3D Microstructures

• Complete three-dimensional aerosol printing process:

1) The glass substrate is ultrasonically cleaned with isopropyl alcohol and deionized water to remove dust.

2) Three types of specialized inks—silver, PEDOT, and collagen—are prepared separately, with solid content and co-solvent ratios carefully adjusted.

3) Ultrasonic atomization generates an aerosol plume, with the nozzle and substrate maintained at a fixed safe distance of 3 mm.

4) Select one of three printing paths—CJD, LBL, or PW—based on the target microstructure.

5) Metals are sintered at 200°C, polymers are annealed at 140°C, and collagen is gently molded at the physiological temperature of 37°C.

6) Optical microscopy and SEM are used to characterize three-dimensional dimensions, porosity, and surface morphology.

• Five Core Advantages of 3D Aerosol Processing (Compared to Lithography / Micro‑3D Printing)

Multi-material universal 3D printing, covering metals, polymers, and biomaterials.

Most consumer-grade micro‑3D printers are compatible with only a single type of photosensitive resin; in contrast, AJP can handle conductive metals, organic conductive polymers, and bio‑collagen inks, enabling the integrated fabrication of electronic and biological devices.

Non-contact forming ensures that curved surfaces and heat-sensitive substrates remain undamaged.

At a spray distance of 3–5 mm, without mechanical extrusion, three-dimensional micro‑columns can be directly printed on flexible films, bio‑based substrates, and optical curved surfaces.

Micrometer-level high-precision three-dimensional control, with adjustable aspect ratio.

It can precisely fabricate microcolumn arrays with feature sizes on the order of tens of micrometers, achieving an aspect ratio (AR) as high as 20 for silver structures—far exceeding the capabilities of conventional inkjet-based 3D printing.

Three forming processes are matched on demand, balancing efficiency with complexity.

Continuous deposition is used to accelerate fabrication of simple microcolumns; layer-by-layer printing ensures reproducibility for standardized arrays; and complex lattice pyramids are fabricated via point-by-point sequential shaping.

Low-loss digital direct writing, requiring no masks or support structures.

Only functional materials are deposited in the target area, eliminating large‑area material waste; most microstructures do not require sacrificial support layers, thereby simplifying post‑processing.

III. Five Core Technological Innovations of This Study

Innovation 1: Three standardized AJP three-dimensional forming process systems

Provide comprehensive definitions of three 3D printing strategies—CJD continuous deposition, LBL layer-by-layer, and PW point-by-point—clarify the microstructural types each is suited to, and develop a reusable process‑selection guideline.

Innovation 2: Quantitative Comparison of the Three-Dimensional Forming Capabilities of Three Typical Functional Inks

The system identifies that silver, conductive polymers, and bio‑collagen inks can achieve the highest aspect ratios, morphological characteristics, and process windows, providing a quantitative basis for material selection.

Innovative Three-Component Co-Solvent Drying–Driven Three-Dimensional Forming Mechanism

This study elucidates the impact of droplet evaporation rates on the collapse of three-dimensional structures and dendritic defects, and proposes a general formulation strategy for binary solvent systems with high and low boiling points.

Innovative 4-Metal–Biomimetic Integrated Manufacturing Paradigm

A single aerosol‑based device simultaneously supports microelectronic three‑dimensional electrodes and biomimetic bone scaffolds, thereby breaking down the barriers between electronic and biomanufacturing technologies.

Innovative 5‑axis supportless micro‑lattice direct‑write process

Point-by-point sequential control of printing height enables direct fabrication of pyramidal lattice units, eliminating the complex process of sacrificial layer removal in microfabrication.

IV. Current Industry Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Conventional additive manufacturing equipment can only process a single type of resin, making it impossible to print conductive metals or bio‑proteins.

✅ An aerosol-based, wide-viscosity atomization system enables seamless switching among silver, PEDOT, and collagen inks, allowing the fabrication of multi-material 3D microstructures on a single platform.

Challenge 2: 3D printing is prone to defects such as collapse, dendritic growth, and porosity, resulting in poor part‑to‑part consistency.

✅ By optimizing the sheath‑gas focusing ratio, platform temperature, and solvent evaporation rate, the layer‑by‑layer printing process significantly enhances array repeatability.

Challenge 3: Biological collagen is sensitive to high temperatures, and most microfabrication processes cannot be used to shape it.

✅ Collagen is processed at a gentle 37°C, preserving protein activity without high-temperature damage, making it compatible with tissue engineering scaffolds.

Challenge 4: High-aspect-ratio structures are more difficult to fabricate than three-dimensional microcolumns, and conventional lithography yields extremely low length-to-diameter ratios.

✅ Diluted silver ink combined with a layer-by-layer stacking strategy enables the stable fabrication of hollow conductive micropillars with an aspect ratio of 20.

Challenge 5: Complex lattices and gradient‑structured three‑dimensional architectures require multiple photolithography steps, resulting in long fabrication cycles.

✅PW enables pixel-by-pixel control of exposure and deposition duration, enabling one-step fabrication of a pyramidal dot array and eliminating the need for multi-layer alignment.

V. Future Diversified Industrial Application Scenarios

1 MEMS Microscale Three-Dimensional Electrodes and Energy-Harvesting Devices

Silver high aspect-ratio microcolumn arrays, used in miniature supercapacitors and three-dimensional piezoelectric sensing contacts, enhance the effective surface area of the devices.

2 Neurostimulation Bioelectronic Stent

PEDOT:PSS three-dimensional micropillar array, for in vitro neuronal cell culture and implantable biosensing electrodes, with excellent biocompatibility.

3 Biomimetic Bone Tissue Engineering Scaffolds

Collagen–hydroxyapatite composite three-dimensional lattice, mimicking the composition of human bone, used as an in vitro culture scaffold for bone repair.

4 Three-dimensional detection array for microfluidic chips

In-situ printing of micrometer-scale sensing microcolumns within the chip enables highly sensitive biochemical detection of trace liquid samples.

5. Multifunctional Micro- and Nanoscale R&D Platform for the Laboratory

A single aerosol-based system enables the concurrent fabrication of microelectronic and bio‑3D prototypes, eliminating the need for multiple expensive microfabrication tools.

Image and Text Guide

Figure 1. The three physical sub-processes of the AJP process under ultrasonic configuration (U-AJP):

• i) Atomization & Transport: Approximately 1.7 MHz ultrasonic excitation atomizes the selected ink into droplets, which are then conveyed by a nitrogen carrier gas (CGF) through a pick-up tube.
• ii) Collimation & In-flight Jet: The droplets are pneumatically focused by an N₂ sheath gas (SGF) and form a focused jet as they pass through a nozzle equipped with a shutter; the infrared thermographic image shown (12.9–100.4 °C) depicts the temperature field at a 100 °C benchtop temperature, along with the particle/aerosol flow.
• iii) Impaction & Impingement: The jet is deposited onto the substrate (with an offset distance of approximately 3 mm), and three printing strategies can be selected as needed: continuous jet deposition (CJD), layer-by-layer printing (LBL), and pointwise printing (PW).

Table 1. Three categories of AJP inks and their properties for 3D microstructure printing research:

• AgNPs-based ink (electronic): Novacentrix Metalon JS-A221AE, a nano-dispersion; viscosity 5.9 mPa·s, surface tension 35.2 mN/m; primary solvent: deionized water; co-solvents: diethylene glycol (3–10 v/v%) + isopropyl alcohol (2–10 v/v%); loaded with 35 nm AgNPs (50 wt%); sheet resistance 50–100 mΩ/sq;
• PEDOT:PSS-based ink (bioelectronics): Sigma Aldrich + in-house formulation; viscosity ≤10 mPa·s, surface tension 75 mN/m; in-house co-solvent system comprising polyethylene glycol and ethylene glycol; loaded with PEDOT:PSS (1.3 wt%); additive: carboxymethyl cellulose; conductive, sheet resistance 19–24 Ω/sq;
• Collagen-based ink (bio-interface): Enzyme-extracted bovine type I collagen (3 mg/mL) supplemented with hydroxyapatite HAp nanoparticles (200 nm, 3–6 mg/mL); viscosity 100–150 mPa·s, surface tension 76 mN/m; primary solvent: 0.01 M HCl; co-solvent/additive: 1 M glycerol; bioactive, osteoconductive ink.

Table 2. 3D AJP printing parameters for three types of inks (23 °C, 50% RH, Optomec AJ300s, offset distance 3 mm):

• AgNPs-based / water dilution (1:4): CJD was fabricated using a 300 μm nozzle (focus ratio Rf = 40:10, stage temperature 40 °C, speed 0); LBL was performed with a 100 μm nozzle (Rf = 30:18, stage temperature 100 °C, speed 0.4 mm/s, 50 layers, 6×4 array, Ø50 μm); post‑processing involved thermal sintering at 200 °C for 1 hour.
• PEDOT:PSS-based (standard and in-house formulations): 150 μm nozzle, Rf 35:18, stage temperature 80 °C, speed 0.4 mm/s, 50 layers; post-processing: thermal annealing at 140 °C for 1 hour.
• Collagen-based: 150 μm nozzle, Rf 40:19, chamber temperature 37 °C (simulating physiological conditions); CJD speed 0, LBL 0.4 mm/s, PW 0.01 mm/s, 50 layers (LBL); post‑processing with EDC–NHS chemical crosslinking.

Figure 2. 3D AJP microstructural printing results of AgNP-based inks (including printing strategies and characterization):

• a) Undiluted ink + CJD: Branch-like columns, with a rough surface and significant overspray, are virtually non-reproducible;
• b) Undiluted ink + LBL: Regular, repeatable structures (tubular bodies with a diameter of 2 mm and frost/tree-like columns with a base diameter of approximately 100 μm);
• c) Water dilution (1:4) + CJD: Cone‑like/branched structure—cone height approximately 374 μm, AR 3–12 (at Rf = 4, base diameter 141.47 ± 11.81 μm, tip diameter 31.65 ± 2.59 μm); leaf‑shaped multi‑branched structures arise from the division of single branches (printing time < 3 s), but branch orientation is uncontrollable and reproducibility is poor.
• d) Water dilution + LBL: 6×4 vertical microcolumn array, with an average height of 960.42 ± 37.69 μm (50 layers), a diameter of 47.74 ± 5.62 μm, and an aspect ratio of approximately  20 , hollow structures (inner diameter <20 μm) with a growth rate of approximately 19 μm per layer; it is recommended that the inter‑pillar spacing be ≥200 μm to prevent interconnection. Overall, LBL exhibits superior reproducibility and controllability compared to CJD.

Figure 3. 3D AJP microstructural printing results of PEDOT:PSS-based ink:

• a) CJD + standard formula: Microcolumn bending exceeds 130°, with poor repeatability; b) CJD + proprietary formulation: Similarly curved and low in repetitiveness;
• c) LBL + In-house formulation: Regularly arranged micropillar array, with an average height of 322.69 ± 7.86 μm (50 layers), a diameter of 45.52 ± 2.22 μm, and a maximum aspect ratio of  7 , with a dense microstructure and a growth rate of approximately 6.5 μm per layer;
• d) KU Leuven logo printed by LBL (With a feature size of approximately 70 μm), it demonstrates the capability to fabricate complex patterns. Only the combination of “in-house formulation + LBL” can successfully pattern 3D structures; a substrate temperature of 80 °C yields optimal results (too low for stacking, too high for deformation).

Figure 4. 3D AJP microstructural printing results of collagen-based ink (including printing strategy and ink formulation):

• a) Col-HAp + CJD: Dense, slender micro-cones (AR ≈ 4);
• b) Col-HAp + LBL: Hollow microcolumns (AR = 2.6, height 216.5 ± 8.2 μm, width 84.1 ± 4.6 μm);
• c) Col-HAp-1M glycerol + PW: Pyramidal structures (which can serve as lattice units) exhibit a significant improvement in formability upon the addition of 1 M glycerol.
• d) Col-HAp-1M glycerol + LBL: 3×3 microcolumn array; e) University logo printed by LBL Note: Glycerol thickens the walls of hollow columns and may occlude the central channel, thereby enhancing 3D printing capability while reducing resolution and accuracy; maintain a stage temperature of 37 °C to simulate physiological conditions.