I. Abstract of the Paper’s Core Research

This study employs the Optomec ultrasonic aerosol‑jet system to develop three 3D microstructural printing processes, tailored to three categories of functional inks—metallic conductive inks, conductive polymer inks, and bio‑collagen inks. The processes are systematically characterized in terms of micro‑pillar aspect ratio, print uniformity, and process repeatability, with complementary in vitro cytotoxicity assays used to delineate the biocompatibility limits of the materials. The key findings are as follows:

Differentiation Patterns of Forming Performance Among Three Types of Inks

① Diluted silver nanoink: The layer-by-layer LBL process yields the best results, enabling the fabrication of vertically aligned microcolumns with an aspect ratio of approximately 20 and the highest molding reproducibility; by contrast, the undiluted stock solution readily forms branched, disordered structures.

② PEDOT:PSS conductive polymer: A self‑developed modified formulation enables stable printing of microarrays with an aspect ratio of 4.5; standard inks are prone to bending and deformation, and the substrate temperature must be tightly controlled at 80°C.

③ Collagen–hydroxyapatite composite ink: The addition of glycerol enables the printing of pyramidal lattice structures with an aspect ratio of 2.5, exhibiting optimal cell compatibility; the entire printing process is conducted at ≤37°C to prevent protein denaturation.

Three types Classification of Application Scenarios for 3D Printing Strategies

CJD continuous deposition: single-point, continuous jetting; suitable for free‑form branches and isolated micro‑cones, but with poor repeatability.

LBL Layer-by-Layer Stacking: Multi-layer cyclic printing, the preferred choice for well-ordered microcolumn arrays, offering uniform dimensions and robust mass-production stability.

PW point-by-point lattice: localized intermittent deposition, enabling the fabrication of complex three-dimensional lattices such as pyramids and unit cells.

Quantitative Criteria for Critical Process Windows

Nozzle diameter ≤150 μm, focus ratio 1.67–4, printing speed ≤0.4 mm/s; metal substrate at 100 °C, conductive polymer at 80 °C, biological collagen ≤37 °C; stable operating ranges are 10–19 sccm for the carrier gas and 30–40 sccm for the sheath gas.

Biocompatibility Grading Verification

Silver ink releases silver ions, exhibiting high cytotoxicity toward neuronal cells and fibroblasts, thus requiring encapsulation for isolation; PEDOT:PSS is low‑toxicity and well‑suited for neural sensing; collagen–HAp elicits no cell death and demonstrates excellent osteoblast proliferation.

Summary of the Core Advantages and Disadvantages of 3D-AJP

Advantages: maskless, non-contact, micrometer-level precision, compatible with multiple materials, and adaptable to curved surfaces; Limitations: commercially available 3D inks are scarce, the CJD process suffers from poor reproducibility, and post‑processing involving sintering or crosslinking is required after fabrication.

II. Complete Preparation Procedure for 3D Aerosol Microstructures

Ink formulation: Silver-based Commercial silver paste diluted with deionized water at a 1:4 ratio, using a low-volatility binary glycol-based solvent blend.

Conductive polymers: PEDOT:PSS is blended with CMC and PEG to enhance the mechanical support during shaping.

Bio-ink I: Type I collagen combined with 200 nm hydroxyapatite in a biomimetic bone formulation; bioink II incorporates 1 M glycerol to enhance stereolithographic printability.

Substrate Pre-treatment: Glass slides are ultrasonically cleaned with isopropyl alcohol, and the surface charge is neutralized in a PBS buffer.

Equipment Commissioning: Ultrasonic atomization aerosol, with a fixed spray distance of 3 mm, and constant-temperature substrate control tailored to the ink type.

Three printing mode switches: CJD single-point continuous deposition / LBL multilayer cyclic array / PW site-specific layer-by-layer shrinkage printing;

Post-processing: Silver sintered at 200°C, PEDOT annealed at 140°C, and collagen chemically cross-linked with EDC;

Characterization: Optical and SEM imaging of three-dimensional morphology; live/dead staining of neurons and osteoblasts, and ATP-based cell viability assays.

III. Five Core Process Advantages of 3D Aerosol Jetting (Compared with Two-Photon, Extrusion, and Inkjet)

Fully compatible with all materials, offering integrated molding for conductive, bio‑compatible, and ceramic applications.

A single system simultaneously supports metal, conductive polymers, and natural bio‑protein inks, enabling the concurrent fabrication of electronic and biological microstructures.

Non-contact, adjustable nozzle-to-substrate distance; conformal 3D printing on curved and irregular substrates.

The nozzle offers an adjustable working distance of 2–35 mm, enabling three-dimensional deposition of flexible thin films and curved biological scaffolds without the need for support.

Micron-level high-precision 3D direct writing, with a minimum feature size of 50 μm.

No photolithographic mask is required; microcolumns, pyramids, and lattice arrays are designed in-house using CAD, achieving a resolution far surpassing that of extrusion-based bioprinting.

Fast fabrication speed: a single 6×4 microarray can be completed in 5 minutes.

Compared with two-photon lithography, it significantly reduces equipment costs and offers exceptional cost-effectiveness for small‑batch, rapid prototyping.

Mild low-temperature processes protect bioactive materials.

The collagen system forms entirely below 37°C, preventing high-temperature denaturation of the protein and making it well-suited for the fabrication of tissue-engineering scaffolds.

IV. Five Core Technological Innovations of This Study

Innovation 1: A Systematic Comparative Study of Three 3D Printing Strategies

Provide a comprehensive definition of the three aerosol-based additive manufacturing processes—CJD, LBL, and PW—and clearly delineate the structural applications and stability limits of each.

Innovation 2: Standardized Process Window for Three Types of Functional Ink Systems

Develop a comprehensive set of process parameters—covering temperature, gas flow rate, and nozzle size—for silver nanoparticles, conductive polymers, and collagen, and compile them into a standardized R&D manual.

Innovative Preparation Scheme for Controllable High-Aspect-Ratio Gradients

Diluted silver ink enables AR≈20 ultra-high‑aspect‑ratio vertical microcolumns, filling a critical gap in the fabrication of high‑aspect‑ratio aerosol‑based microstructures.

Innovation 4: Glycerol-Modified Lattice‑Shaping Technology for Bio‑inks

Glycerol regulates the evaporation rate of the collagen solvent, stabilizes the printed pyramidal three-dimensional biomimetic lattice, and expands the fabrication pathways for bone scaffolds.

Innovative Five-Stage Grading System for the Biocompatibility of Inks

Distinguish between the cytotoxicity of conductive electrons and that of biomaterials, and establish clear guidelines for the packaging and material selection of biomedical devices.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Commercial aerosol inks are compatible only with two-dimensional films, and dedicated 3D inks are extremely scarce.

✅ Self-formulated bonding and moisturizing additives; custom ink formulations for silver, PEDOT, and collagen can be directly replicated.

Challenge 2: The CJD continuous deposition process exhibits poor forming repeatability, and the branching structure is uncontrollable.

✅ For regular arrays, the LBL (Layer-by-Layer) process is preferred; for complex, standalone structures, parameter calibration is employed to enhance consistency.

Challenge 3: Collagen is prone to denaturation at high temperatures, making it impossible to achieve stable shaping through high-temperature drying.

✅ Limited to a 37°C low-temperature substrate, relying on glycerin for moisture retention and chemical crosslinking instead of thermal curing.

Challenge 4: Gas‑path parameters are highly sensitive; imbalances in focus ratio and flow rate can easily lead to material overflow and line distortion.

✅ Fixed within the standard focus ratio range of 1.67–4, with dedicated sheath gas and carrier gas flow rates for three types of inks.

Challenge 5: Silver nanoparticle ink exhibits high biotoxicity and cannot be directly contacted with cells.

✅ The outer PDMS dielectric encapsulation of the device prevents the release of silver ions, or it can be replaced with PEDOT or collagen-based bio‑inks.

VI. Future Diversified Industrial Application Scenarios

1 Biomedical 3D Micro-Scaffolds

Collagen–hydroxyapatite three-dimensional lattice, a scaffold for osteoblast proliferation, and an in vitro biomimetic culture scaffold for orthopedic repair.

2 Bioelectric Sensing Microelectrode Array

PEDOT:PSS three-dimensional micropillars, neural signal acquisition chips, and in vitro neuronal stimulation and detection devices.

3 MEMS Microsensor Arrays

High-aspect-ratio silver conductive microcolumns, pressure‑ and electrochemical‑based microscale detection chips, and microelectrodes.

4 Micro-scale Energy Storage 3D Current Collectors

Silver three-dimensional conductive framework, high‑specific‑surface‑area electrode for microsupercapacitors.

5 High-Throughput Materials Research and Development Platform for Laboratories

Rapid switching between metal and bio‑inks enables the batch fabrication of three‑dimensional microstructures with varying aspect ratios, thereby shortening the formulation‑screening cycle.

Image and Text Guide

Figure 1. Three physical sub-processes of ultrasonic atomization AJP (U-AJP) and three 3D printing strategies.

Table 1. Comparison of Composition and Physical Properties of Three 3D AJP Inks

Figure 2. 3D AJP results of AgNP ink (CJD and LBL, comparison before and after dilution)

Figure 4. 3D AJP results of PEDOT:PSS ink compared with those of inferior quality.

Figure 5. 3D AJP results of collagen-based inks (Col-HAp with +1 M glycerol)

Figure 6. In vitro biocompatibility of three types of ink (rATP cell viability)

Table 3. Summary of 3D Microstructure Results for Each Ink × Printing Strategy Combination