I. Core Research Summary

• Low-Volatility Dual-Solvent Magnetic Specialty Ink System

A blended solvent of 90% toluene and 10% terpineol was employed. SPION nanoparticles at a concentration of 20 mg/mL remained stably dispersed for 24 hours without settling. The low-boiling-point toluene ensures efficient atomization, while the high-boiling-point terpineol suppresses evaporation‑induced drying, significantly mitigating coffee rings and material overflow. The ink’s viscosity is optimized to match the ultrasonic atomization aerosol system.

• Standardized airflow process window, enabling sub‑20 μm high‑precision magnetic lines.

The sheath gas/carrier gas focusing ratio FR is defined as the core control parameter, with an optimal range of FR = 1–4 and a carrier gas flow rate of 10–20 SCCM. Using a 100-μm nozzle, single-channel printing achieves a minimum line width of 17 μm, with controllable line thickness ranging from 125 to 256 nm, resulting in sharp edges and a uniform surface.

• Universal non-contact direct-write process for multiple substrates

Compatible with rigid glass, flexible PDMS silicone, and sub‑100‑nm‑thick PLLA biodegradable medical nanofilms; PDMS can achieve stable adhesion with only brief plasma activation, while the ultrathin PLLA film can be dissolved in water after printing, yielding a self‑supporting magnetic thin film.

• Retain complete superparamagnetic properties after printing.

SQUID hysteresis measurements confirm that the Fe₃O₄ thin film deposited from aerosols exhibits no remanence and no coercivity, with magnetic properties highly consistent with those of the pristine nanoparticles, indicating that the printing process does not compromise the superparamagnetic core’s functionality.

• Asymmetric magnetic patterns enable multidimensional, controllable motion of magnetic fields.

On PLLA films, asymmetric helical and cross‑shaped magnetic patterns can be printed; under an applied magnetic field gradient of 300 mT/m, the material exhibits bidirectional rotation, planar translation, and two‑dimensional-to‑three‑dimensional bending deformation, making it suitable for tetherless microrobots.

II. Complete Preparation Procedure for SPION Magnetic-Patterned Aerosols

(1) Complete Preparation Procedure

• Ink preparation: 10 nm superparamagnetic Fe₃O₄ particles are dispersed in a toluene/terpineol mixture (9:1) and subjected to ultrasonic treatment at 35°C for 30 minutes to form a stable magnetic colloid.
• Substrate pre‑treatment: Glass requires no activation; PDMS is subjected to air plasma treatment for 60 seconds to enhance wettability; PLLA is spin‑coated using a PVA sacrificial layer to fabricate an ultrathin nanomembrane.
• Optomec aerosol equipment employs ultrasonic atomization, with nitrogen serving as the carrier and sheath gas; it features a fixed nozzle-to-substrate distance of 2.5 mm and a printing speed of 2 mm/s.
• It regulates the airflow focusing ratio to enable batch printing of custom magnetic micro-patterns, such as straight lines, spirals, and crosses.
• 80°C oven, 10 min: low-temperature drying removes the solvent, eliminating the need for high-temperature sintering.
• AFM topography characterization, SQUID magnetic property measurements, and magnetically controlled deformation experiments conducted on a dual permanent-magnet platform.

(II) Five Core Process Advantages of Aerosol Jet Printing (Comparison with Lithography, Inkjet, and Soft Lithography)

• Ultra-high resolution far surpasses traditional inkjet.

The minimum line width achievable with inkjet printing is 50–100 μm; this process reliably produces magnetic micro‑lines as narrow as 17–25 μm, meeting the requirements of miniature magnetic sensors and precision structures for microrobots.

• Maskless digital rapid iteration, no templates required.

CAD enables direct modification of helical and custom‑shaped magnetic patterns, eliminating the need for tooling or molding; a single prototype can be produced in just tens of seconds, significantly shortening the validation cycle for new materials.

• Fully compatible with multiple materials, designed to fit ultra-thin bio-based substrates.

Rigid, flexible silicone, and biodegradable PLLA nanofilms can all be formed, while the non-contact mist jet causes no damage to ultra-thin medical films on the order of hundreds of nanometers.

• Precision ink deposition, magnetic nanomaterial consumables savings

SPION is sprayed only onto the target pattern area, whereas spin-coating or sputtering the entire substrate results in substantial waste of expensive magnetic nanopowder.

• Mild low-temperature molding with no degradation in magnetic properties.

The entire process employs only low‑temperature drying at 80°C, with no high‑temperature heat treatment, thereby perfectly preserving the superparamagnetic properties and preventing particle agglomeration or loss of magnetization.

III. Five Core Technological Innovations of This Study

Innovation 1: Compatible with ultrasonic atomization SPION dual-solvent specialty ink

Innovation 2: Magnetic Materials—Focusing on a Standardized Process System

Innovative In-Situ Magnetic Patterning of Ultra-Thin Biodegradable PLLA Films

Innovation 4: Fully preserves the superparamagnetic intrinsic properties after printing.

Innovative 5-Dimensional Multi-Drive Paradigm Based on Asymmetric Magnetic Patterns and Magnetic Fields

IV. Current Industry Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Inkjet-printed magnetic lines are thick, with severe edge bleeding.

Challenge 2: Lithography and soft lithography rely on templates, making it impossible to rapidly customize magnetic structures.

Challenge 3: Ultra-thin, biodegradable medical films struggle to withstand contact printing.

Challenge 4: High-temperature processing can lead to magnetic particle agglomeration and the loss of superparamagnetism.

Challenge 5: Conventional continuous magnetic thin films can only be dragged in a single direction, resulting in a limited range of motion.

V. Future Diversified Industrial Application Scenarios

1 Minimally Invasive Medical Magnetic Microrobots

2 Cable-free soft micro-robots

3 Flexible Magnetic Sensor Array

4 Magnetic Manipulation and Cultivation of Biological Cells

5 High-Throughput Research and Development of Magnetic Materials in the Laboratory

Figure 1. (a) Schematic of the AJP process using an ultrasonic atomizer. (1) The ink‑formulated material is subjected to ultrasonic atomization. (2) An inert gas (N₂) is used to increase the pressure within the atomization chamber. (3) The aerosol is conveyed to the deposition head by a carrier gas. (4) The aerosol is focused and accelerated by an additional annular sheath of inert gas. (5) The resulting high‑velocity jet is deposited onto the substrate surface through the nozzle. (6) An automated stage moves to form the desired pattern. (7) Switching‑type patterning is achieved by interrupting the jet with a mechanical shutter. (b) A program containing operational instructions is generated based on digital design data. (c) Magnetic material with the desired pattern is directly deposited onto the substrate surface.

Figure 2. Effect of specific alcohol on printed lines: (a) without specific alcohol; (b) 5% (v/v) specific alcohol; (c) 10% (v/v) specific alcohol. Printing parameters: nozzle diameter 100 µm, sheath gas flow rate 20 SCCM, carrier gas flow rate 10 SCCM, scan speed = 2 mm/sec, working distance = 2.5 mm. Scale bar = 50 µm.

Figure 3. Optical microscopy images of SPION‑printed lines on glass slides, showing the trend in line width as the focusing ratio (FR = sheath‑gas flow rate / carrier‑gas flow rate) increases at different carrier‑gas flow rates. Lines printed at the same focusing ratio are grouped together. Scale bar: 20 µm.

Figure 4. Atomic force microscopy (AFM) analysis of SPION‑printed lines on a glass slide. (a) An example AFM scan showing a line printed with SPIONs (carrier gas flow rate: 10 SCCM; sheath gas flow rate: 40 SCCM; focus ratio FR = 4), along with the cross‑sectional profile indicated by the horizontal red line. The average line thickness was determined by calculating the mean height difference between a selected region on the line surface (SPION‑containing area) and a corresponding region on the glass substrate (glass area); the base width (black) and the half‑height width (red) are also shown. (b) Variation of line thickness with focus ratio at different carrier gas flow rates: (black triangles) 10 SCCM, (black diamonds) 15 SCCM, (black squares) 20 SCCM; error bars represent the standard deviation of line height (root‑mean‑square roughness) obtained from AFM scans. (c) Dependence of base width (black) and half‑height width (red) on focus ratio at various carrier gas flow rates: (black triangles) 10 SCCM, (black diamonds) 15 SCCM, (black squares) 20 SCCM. (d) Relationship between half‑height width and base width. (e) Surface scan image of a 10 μm × 10 μm area (carrier gas flow rate: 10 SCCM; sheath gas flow rate: 40 SCCM; focus ratio FR = 4).

Table 1. Roughness of SPION trajectories. Roughness was determined from atomic force microscopy (AFM) scans with a 10 μm × 10 μm field of view, using the formula Ra = (1/N) |Zi − z|.

Figure 5. Hysteresis loops of pristine SPIONs (black triangles) and SPION patterns fabricated by aerosol jet printing on a silicon wafer (black diamonds) and on PTFE tape (white squares).

Figure 6. (a) Optical microscopy images comparing SPION‑based helical structures printed on a glass slide, a PDMS thin film, and a PLLA nanofilm (carrier gas flow rate: 10 SCCM; sheath gas flow rate: 40 SCCM; FR = 4). Scale bar: 20 µm. (b) A patterned PDMS thin film. (c) A patterned PLLA nanofilm floating on the water surface after being released from the fabrication substrate (the nanofilm’s edges are indicated by dashed lines). (d) and (e) show optical microscopy images of helical structures with different pitches and outer radii, printed on PDMS and PLLA surfaces, respectively. (f) and (g) depict a patterned nanofilm (15 mm × 15 mm) released from the substrate, floating on the water surface, along with its optical microscopy magnified views. (h) Schematic illustrations of the injection (top) and jetting (bottom) processes for patterning the nanofilm. (i) The patterned nanofilm collected on a glass slide and dried, together with an optical microscopy magnified image after ten injection–jetting cycles.

Figure 7. (a) A dual external permanent magnet (dEPM) platform used for magnetic manipulation experiments. The petri dish containing a magnetically patterned nanofilm and floating on the water surface is indicated by a dashed line. The different patterns tested include: (b) a cross‑patterned nanofilm; (c) a double‑corner patterned nanofilm; (d) a double‑sided patterned nanofilm; (e) a homogeneous nanofilm used as a control; (f) the double‑corner patterned nanofilm performing a prescribed sequence of rotational and translational motions along a square trajectory; (g) a double‑sided patterned nanofilm that undergoes a two‑dimensional-to-three‑dimensional shape transformation via out-of‑plane bending: starting from a flat state (left), the nanofilm transforms, in response to an applied magnetic field gradient, into a mirror‑image N‑shaped structure (middle) and then into an N‑shaped structure (right).