I. Core Research Summary

• Enables precise control over high aspect-ratio 3D microneedle structures.

By controlling the deposition duration of aerosols at specific locations, the exposed height of the microneedles can be tuned between 60 and 185 μm, with a diameter of approximately 19 μm and a maximum aspect ratio of 8.5. A single-chip partitioning design allows for the integration of microneedles of varying lengths, accommodating the signal‑acquisition requirements associated with different stratification depths in organoids, while enabling support‑free, site‑specific stacking and fabrication.

• Low-impedance, long-term electrochemically stable bioelectrode

PEDOT:PSS/MWCNT composite conductive microneedles exhibit an impedance as low as 7–8 kΩ at 1 kHz; after immersion in a 37°C culture medium for 42 days, no significant impedance drift is observed. The functional electrode yield is 90%, and the porous structure substantially enhances charge-storage capacity.

• Deep-stabilization neural signal acquisition in three-dimensional tissue structures

Microneedles can penetrate the internal network of neural spheroids and human brain organoids, enabling the recording of action potentials with a signal-to-noise ratio of 6.7–8.5 dB. Compared with commercial planar MEAs, they can capture synchronous firing signals from deep-layer neurons that are undetectable at the surface, and allow continuous monitoring for more than ten days within a single culture cycle.

• Dual-Printing Composite Integrated Mass-Production Process

Inkjet printing is used to fabricate silver/PEDOT‑based lead electrodes and insulating protective layers in a batch process, while aerosol‑based pinpoint stacking builds the micro‑needle tips. This maskless additive manufacturing system enables the construction of multilayer conductive–insulating structures, with design iterations completed in just a few hours.

• Animal-free in vitro neural research platform

Without the use of live animals, leveraging human brain organoids and a custom microarray of microneedles enables high-throughput electrophysiological screening of neural development and disease‑causing drugs, thereby reducing experimental ethical concerns and animal‑keeping costs.

II. Core Operating Principle of Aerosol Spraying and Its Unique Process Advantages

(1) Complete Composite Printing Preparation Process

• Pre-treatment of the flexible PEN substrate, followed by inkjet printing of the silver conductive bottom-layer traces and the PEDOT transparent interconnects;
• UV-curable inkjet-printed polyacrylic insulating layer, with printing windows reserved only at the electrode tips;
• Ultrasonic atomization–modified PEDOT:PSS/carbon nanotube bio‑conductive ink, with precise focusing achieved using a nitrogen sheath gas;
• Aerosol is deposited at a fixed location over an extended period, enabling the layer-by-layer stacking of self-supporting three-dimensional microneedles, with deposition duration precisely controlling the height.
• Following whole-chip sterilization, three-dimensional neuroelectrophysiological recordings are performed using a honeycomb scaffold or an organoid culture mold.

(II) Five Core Process Advantages (Compared with Silicon Lithography and Extrusion-Based 3D Printing)

• Fixed-point three-dimensional stacking, with freely customizable microneedle height.

Traditional etched microneedles have fixed dimensions; aerosols enable linear control of needle height by modulating the duration of localized printing. A single array can incorporate mixed-length probes to accommodate multi‑layer brain tissue depth profiling, eliminating the need for retooling.

• Dual‑print collaboration, balancing planar wiring with cutting‑edge 3D capabilities.

Inkjet technology excels at fabricating large-area planar interconnects, while aerosol jet printing specializes in micron-scale three-dimensional microstructures; the two approaches complement each other, enabling integrated multilayer insulation and conductive patterning with through‑via interconnections that are free of short circuits.

• Biocompatible, mild deposition—entire process free of high temperatures and strong acids.

Room-temperature atomization fabrication eliminates the need for etching, metal deposition, and other processes that can damage biomaterials. The PEDOT‑based composite ink exhibits no cytotoxicity and can be directly used in stem cell co‑culture microchips.

• Maskless Digital Rapid Iteration

With CAD, you can instantly adjust microneedle spacing, arrangement, and height; the new model can be printed and tested on the same day, dramatically shortening the prototyping cycle that typically takes several weeks with lithography.

• Low consumable costs, compatible with flexible substrates

High-valence conductive composite ink is deposited only at the electrode tip, resulting in material consumption that is far lower than that of full‑area coating; the flexible PEN substrate is bendable and compatible with microfluidic devices and implantable ex vivo biochips.

III. Five Key Technological Innovations

Innovation 1: A novel process for the site-specific stacking of aerosol‑based, self‑standing micro‑needles with high aspect ratios.

Innovation: 2 PEDOT/MWCNT High-Stability Bioconductive Specialty Ink

Innovative 3‑inkjet + aerosol composite multilayer integrated manufacturing system

Innovative Four-Layer Deep-Differentiated Detection Array Design

Innovative 5-Printed In Vitro Brain-Chip Standardization Platform

IV. Existing Process Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Conventional printing can only produce planar thin films and cannot fabricate microneedles with piercing capabilities.

Challenge 2: The single conductive PEDOT electrode exhibits high impedance and degraded long-term immersion performance.

Challenge 3: The lithographically fabricated microneedle array is fixed and cannot be readily adapted to accommodate the varying sizes of different organoids.

Challenge 4: Multilayer conductive traces are prone to interlayer leakage and signal crosstalk.

Challenge 5: A single printing process struggles to simultaneously accommodate both large-area wiring and micron-scale three-dimensional tips.

V. Future Diversified Industrial Application Scenarios

1. High-Throughput In Vitro Drug Screening for Neurological Disorders

2. Stem Cell–Based Research Platform for Neural Development

3. Microfluidic Brain Organ Integrated Sensing Chip

4. Minimally Invasive Extracorporeal Tissue Electrophysiological Testing Equipment

5. Rapid Development of Next-Generation Flexible Brain–Machine Interface Prototypes

Figure 1. MnEA structures fabricated by aerosol jetting and inkjet printing. (A) Schematic layout of the MnEA, showing the inkjet‑printed silver (gray) and PEDOT:PSS (light blue) supply channels, as well as the position of the μ‑needle electrodes (yellow) produced via aerosol jetting. (B) Photograph of the MnEA with μ‑needles (average height 158 ± 13 μm) used for electrochemical measurements. (C) Bright-field microscopy image of the MnEA without a passivation layer; the μ‑needles are invisible because they are aligned perpendicular to the substrate. (D) SEM images of a 4×4 grid of μ‑needles printed on a gold‑plated wafer at varying printing durations (60 to 300 s). (E) Magnified SEM image of multiwall carbon nanotubes deposited on the surface of the printed structure. (F) Schematic illustration of the measured dimensions of the μ‑needle electrodes at different printing times (150, 240, and 300 s), along with the distribution of the passivation layer on the MnEA.

Figure 2. Electrochemical characterization of the μ‑needle electrode. (A) Example cyclic voltammograms of the μ‑needle electrode at different scan rates (10, 50, 100, and 200 mV·s⁻¹), showing the current I as a function of the working electrode potential. (B) Specific volumetric capacitance C values at various scan rates (n = 11, ± standard deviation). (C) Example plots of impedance |Z| and phase angle as functions of the applied frequency f, presented in logarithmic coordinates. (D) Impedance |Z| values at 1 kHz after incubation in PBS solution within a 37°C humidified incubator for up to 14 days (n = 19, ± standard deviation), 21 days (n = 18), and 42 days (n = 17).

Figure 3. MnEA recordings performed on interconnected neural spheroids derived from human induced pluripotent stem cells (hiPSCs) and supported by a micro‑cellular scaffold. (A) Schematic of the micro‑cellular scaffold bearing neural spheroids placed on the MnEA (left, not to scale) and a bright-field microscopy image (right), illustrating μ‑needle electrodes in different configurations; the μ‑needles are indicated by white arrows in the microscopy image. (B) Time traces of the recorded signals when penetrating the micro‑cellular chamber with μ‑needle electrodes of average height 185 ± 26 μm (i), curved μ‑needle electrodes of average height 185 ± 26 μm (ii), slightly bent μ‑needle electrodes of average height 60 ± 20 μm (iii), and μ‑needle electrodes of average height 60 ± 20 μm (iv) (left). The traces show the mean waveform of action potentials ± standard deviation (n = 119 for (i), 213 for (ii), 245 for (iii), and 378 for (iv); signal-to-noise ratios were 8.0 ± 0.9 for (i), 6.7 ± 0.6 for (ii), 7.3 ± 0.7 for (iii), and 7.7 ± 0.8 for (iv).

Figure 4. Results of MnEA recordings performed on hiPSC‑derived brain organoids. (A) Photograph of a brain organoid in culture prior to its mounting on the MnEA. (B) Schematic of the experimental setup: four organoids are secured in the four wells of the MnEA, which is inserted into the head of an imaging system; after medium is perfused, the glass annular chamber on the MnEA is sealed with a PDMS cover equipped with an Ag/AgCl reference electrode. (C) Bright-field microscopy image of an organoid positioned on a 12‑μ‑needle electrode array, with the well edges indicated by yellow arrows. (D) Schematic illustrating the architecture of an organoid housed in the MnEA’s 12‑μ‑needle electrode wells (the schematic does not reflect actual dimensions). (E) Confocal microscopy images of an organoid derived from hiPSCs expressing RFP‑tagged β‑actin, when mounted on the MnEA’s 12‑μ‑needle electrode array: the upper and right panels show cross‑sectional views along the blue and red lines in the x–y plane, respectively, with the base of the organoid marked by a yellow arrow. (F) Time courses of recorded signals from an organoid on day 77, measured using the MnEA equipped with μ‑needle electrodes (average height 158 ± 13 μm) (i), and from an organoid on day 63, measured with a commercial two‑dimensional MEA (ii) (left panel); the curves depict the mean waveform of action potentials, together with standard deviation (n = 63 for (i), n = 25 for (ii)). Signal‑to‑noise ratios (SNR) were 8.5 ± 1.9 for (i) and 7.7 ± 0.8 for (ii). (G) Fluorescence microscopy images of immunofluorescent signals in a 20‑μm‑thick section of the brain organoid, 78 days after differentiation: Sox2 appears magenta; Tau appears cyan; Hoechst stain appears blue.