I. Abstract of the Paper’s Core Research

• Micron‑scale miniature coils are stably mass‑produced, perfectly suited to the narrow cavities of cochlear implants.

The aerosol direct‑write coil has a diameter of 1800 μm, with a wire width and spacing of 112.5 μm each, a film thickness of 12 μm, and an inductance ranging from 11.9 to 17.7 nH; the mean value for standard samples is 15.5 nH. Its geometric dimensions are matched to the cochlear entrance canal and can be further scaled down to the target specification of 600 μm.

• Magnetic stimulation markedly suppresses the propagation of neural activation.

The magnetic field penetrates biological tissues uniformly, with a total depolarization–hyperpolarization range of only 1.5 mm for a single coil—far smaller than the conventional electrode current spread—enabling precise frequency‑specific stimulation and enhancing auditory frequency resolution.

• Dual-layer verification of the magnetic-field activation mechanism, combining simulation and physical testing.

First, electromagnetic radiation calibration is achieved using a 60-mm‑diameter inkjet coil, with an error of only 3.3%; subsequently, a miniaturized aerosol coil is developed to establish a quantitative model of the neural activation function (AF), enabling precise prediction of the strength of axonal stimulation and accurate identification of the cochlear nerve’s response region.

• Integrated multi-layer additively manufactured coil structure

First, an aerosol is used to print a helical conductive silver layer; then, SU‑8 insulating bridges are manually fabricated to interconnect the inner and outer terminals of the coil. A second printing step creates the lead-out pads. This entire maskless process eliminates the need for a cleanroom MEMS production line, with batch resistance tolerances maintained within ±12%.

• Flexible substrates have enormous adaptation potential.

The non-contact deposition characteristics of AJP enable the direct printing of microcoil arrays onto flexible cochlear silicone implant carriers, thereby mitigating the risk of insertion‑induced damage associated with rigid silicon‑based MEMS devices and facilitating the development of multi‑channel, high‑density auditory stimulation arrays.

II. Complete Process Flow for Aerosol Jet Fabrication of Microcoils and Its Core Advantages

(1) Complete Microcoil Fabrication Process

• The slides were washed with isopropyl alcohol, dried with nitrogen gas, and mounted on an 80°C heating stage.
• Ultrasonic atomization of silver nano‑ink, with a sheath gas flow of 24 ccm and an atomizing gas flow of 12 ccm, using a 150‑μm nozzle; the main body of the helical coil is printed at a low speed of 0.2 mm/s.
• Bake the silver film on a 200°C hot plate for 15 minutes to cure it and reduce its resistivity.
• Manually spin-coated SU‑8 photoresist insulating bridges, followed by high-temperature curing to prevent short circuits across isolation vias.
• The secondary aerosol is printed at the center, with lead pads extending to the periphery; the device is then annealed again to complete fabrication.
• Resistance and inductance are measured on a probe station, and electromagnetic radiation calibration is performed inside a Faraday cage.

(II) Five Key Process Advantages (Comparison of MEMS Lithography and Inkjet Printing)

• Unparalleled micro‑molding capabilities—beyond the reach of inkjet technology.

Inkjet printing achieves a minimum line width of several hundred micrometers, while aerosol-based techniques can stably fabricate spiral circuits at the hundred-micrometer scale. The overall coil size is reduced by 97%, making it compatible with millimeter‑scale cochlear cavities.

• Conformal compatibility with flexible and irregular substrates

Coils can be directly printed on the curved surface of the silicone‑based flexible cochlear implant, whereas rigid MEMS silicon wafers are prone to damaging inner ear tissues, making this process safer for implantation.

• Maskless Digital Rapid Iteration

CAD allows direct modification of coil diameter, number of turns, and wire width, eliminating the need for custom photomasks and enabling rapid iteration of a 600 μm ultra‑small coil design.

• Extremely low consumable waste, enabling low-cost, small-batch R&D.

Noble silver is deposited only in the coil region, whereas etching the entire MEMS device significantly reduces nano‑silver consumption; this has been validated at low cost in the laboratory.

• Multilayer conductive / insulating stepwise fabrication

The same device enables printing of conductive traces and, combined with a simple insulation‑layer process, achieves inter‑trace bridging; the integrated helical coil structure is formed in a single step.

III. Five Core Technological Innovations of This Study

Innovation 1: A novel coupling scheme integrating aerosol microwire coils with cochlear magnetic stimulation.

Innovation 2: Simulation–Prototype–Miniaturization—A Three-Tier Verification R&D Paradigm

Innovation 3: Neural Activation Function Adapted to the Alternating Magnetic Field Model

Innovative 4‑layer maskless spiral coil additive manufacturing process

Innovation 5: A New Path for Manufacturing Implantable Flexible Auditory Arrays

IV. Existing Process Challenges and Corresponding Solutions Plan

Challenge 1: Conventional electrode-based electrical stimulation is subject to interference from lymphatic fluid conductivity, resulting in a broad activation zone and low spatial resolution.

Challenge 2: MEMS lithography devices are highly rigid, and their implantation can easily damage the cochlear soft tissues, while also being prohibitively expensive.

Challenge 3: Insufficient inkjet printing precision, making it impossible to fabricate millimeter-scale microhelical coils.

Challenge 4: The inner and outer terminals of the planar spiral coil are prone to short circuits when bridged, making it difficult to form.

Challenge 5: Microcoil electromagnetic testing and simulation face high benchmarking difficulties.

V. Future Diversified Industrial Application Scenarios

• Next-generation high-resolution cochlear implant electrode array

• Minimally Invasive Magnetic Stimulation Medical Device for Peripheral Nerves

• Brain–computer interface flexible microelectromagnetic array

• Animal Auditory Nerve Research and Experimental Platform

• Micro RF‑inductive microcoil chip

Figure 1. (A–D) The tonotopic organization of the human cochlea. From a first-order approximation, acoustic frequencies can be mapped onto spatial locations (Encyclopaedia Britannica [5]). (E) MED‑EL cochlear implant electrode array (MED‑EL blog [6]). (F) A thin-film array fabricated on a polyimide substrate using commercial microelectromechanical systems (MEMS) technology [7,8].

Figure 2. (a) Top view of the COMSOL coil model; (b) Pattern for a large-scale (100× magnification) inkjet‑fabricated coil; (c) Inkjet‑manufactured coil, with a maximum diameter of 60 mm.

Figure 3. (a) Process flow diagram of the AJP microcoil, showing top and side views; (b) Corresponding AutoCAD file.

Figure 4. When the angle is 90 degrees, the surfaces of the coil pair are parallel to each other. (a) Three-dimensional schematic of the COMSOL model showing the far-field radiation pattern for a coil with a diameter of 60,000 µm—its geometry is identical to that of the 600‑µm coil; units: V/m. (b) Two-dimensional schematic of the COMSOL model depicting the far-field radiation pattern for the 60,000‑µm coil; units: V/m, with directivity of 3.709 dB. (c) Average far-field radiation pattern generated from data collected across four experimental runs; voltage values along the entire coil length are in mV, and the directivity is 4.0494 dB.

Figure 5. (a) Schematic diagram of the activation mechanism and the coordinate system; (b) The AF(x) curve illustrates the spatial distribution of neuronal responsiveness to stimulation as the current amplitude is set to its maximum value of 1 A and the coil separation z varies from 100 to 400 µm. Neurons located at x = 0 are virtually unresponsive to stimulation by the coil.

Figure 6. (a) A successfully printed AJP coil, with a thickness of 12 µm and a trace width of 112.8 µm, connected to a dual-probe system for performance testing. (b) If leakage or blockage occurs during printing, defective coils may form; this can be avoided by following the operating guidelines in the Aerosol Jet Printing Handbook. (c) An AJP coil featuring an SU‑8 insulating bridge prior to the interface pad‑printing stage. (d) Close-up of a printed coil with both an insulating bridge and an interface pad: air bubbles are present within the SU‑8 insulating bridge (an undesirable phenomenon), but they do not affect functionality after baking and curing. Furthermore, although the interface pad traces make contact with the coil portion before the central end, the coil will operate at a resonant frequency corresponding to the larger loop, with negligible impact on the magnetic field. (e) The complete device structure: coil, SU‑8 insulating bridge, and interface pad.

Figure 7. A Wyko NT2000 (Veeco) profilometer showing the output signal of the coil section with an SU‑8 bridge.

Table A1. Units of Electromagnetic Properties

Table A2. Characteristics of 1800 µm Diameter Microcoils