I. Core Research Summary

• Aerosol Dual-Material Synchronous Manufacturing System

Ultrasonic atomization is used to print microchannels using water-soluble NaCl ink as a sacrificial mold; simultaneously, silver nanoparticles and PI insulating ink are switched to pattern interdigitated electrodes and a full‑coverage insulating layer on a flexible PI substrate, eliminating the need for photolithography and evaporation‑deposition steps in the entire microfabrication process.

• Hydraulic‑driven capacitive linear force response, with adjustable sensitivity.

When an external force compresses the PDMS reservoir, the dielectric fluid flows along the channel to coat the interdigitated electrodes, and the capacitance varies linearly with the applied force. The standard device exhibits a sensitivity of 4.3 pF/N and a linear force‑measurement range of 0–9 N; by adjusting the channel width and the dimensions of the reservoir, the sensitivity and measurement range can be freely tuned.

• High-stability curved surface compatible with flexible devices

With an overall thickness of only 1 mm, it can be bent and fixed on curved surfaces in either direction; after 2,200 reciprocating pressure‑cycling cycles, no significant signal drift is observed, and within the 10–50°C range, only a minor baseline shift occurs, with sensitivity remaining unaffected by temperature variations.

• Interdigitated electrodes significantly enhance the sensor’s signal-to-noise ratio.

Compared with conventional parallel‑plate electrodes, this design delivers nearly sevenfold signal enhancement at the same electrode area, enables stable detection of weak external forces, and supports low‑power capacitive sensing suitable for portable wireless devices.

• Closed-loop robot real-time force feedback validation

The sensor conforms to the curved surface of the container to measure squeezing force, while the microcontroller processes the capacitance signal in real time, precisely controlling the opening and closing range of the robotic gripper to enable hierarchical force‑controlled intelligent interaction.

II. Core Working Principle of Aerosol Jet Printing and the Complete Preparation Process

(1) Complete Aerosol Preparation Process for Sensors

• Microfluidic sacrificial mold printing : Aluminum foil substrate, ultrasonic atomization of a saturated NaCl aqueous solution, multilayer cyclic printing of reservoir chambers and microchannel arrays, and plate‑based drying at 95°C to form a soluble mold;
• PDMS casting and demolding : The liquid PDMS is coated onto the mold and cured at 70°C; the aluminum foil is peeled off, and NaCl is dissolved in pure water to obtain a hollow elastic microfluidic chamber.
• Interdigitated electrode printing : Kapton PI film substrate, ultrasonic atomization of silver nano‑ink, and aerosol direct printing to fabricate conductive traces with a 200 μm pitch;
• Insulation layer coating : Large‑diameter nozzle pneumatic atomization of PI‑insulated ink; full‑coverage silver electrodes with only external solder pads retained, isolating the conductive layer from dielectric‑liquid corrosion.
• Bonding and Liquid Injection Packaging : The PDMS microfluidic chamber is sealed to the electrode‑bearing silicone substrate; a 2:1 glycerol/deionized water mixture is injected into the channel, and the injection port is sealed to complete the device.

(II) Five Core Process Advantages of Aerosol Technology (Compared with Lithography and Molding)

• Mold sacrifice and conductive electrode fabrication completed in a single machine.

Conventional microfluidics and electrodes are fabricated using two separate sets of equipment; AJP can switch among three types of inks—water-soluble salts, metals, and polymers—and enables the simultaneous digital printing of molds and sensor circuits, significantly shortening the R&D cycle.

• Maskless, free customization of microchannel dimensions

CAD allows direct modification of the reservoir volume and channel dimensions, while print‑cycle count enables precise control of mold thickness (adjustable from 50 to 200 μm), facilitating rapid switching between two device architectures: high sensitivity and wide measurement range.

• Non-contact gentle forming, compatible with flexible substrates.

The nozzle exerts no mechanical pressure on the substrate, ensuring that thin substrates such as Kapton and aluminum foil remain undeformed; low-temperature drying with NaCl does not damage the substrate, making it compatible with a wide range of flexible electronic carriers.

• The consumable costs are extremely low, and the water-soluble molds can be completely removed.

NaCl is inexpensive and non-toxic; demolding requires only a rinse with pure water, eliminating the need for chemical etching. Silver and PI inks can be locally deposited as needed, avoiding waste from full‑area coating.

• Multi-layer, precision‑zone deposition ensures complete insulation coverage with no leakage.

The insulating ink precisely encapsulates all the sensor interdigitated fingers, leaving only the solder pads exposed to prevent direct contact between the dielectric liquid and the silver layer, thereby avoiding corrosion and signal drift.

III. Five Key Technological Innovations

Innovation 1: A Novel Process for Aerosol-Printed Water-Soluble NaCl Sacrificial Molds

Innovative 2-finger interdigitated electrode combined with a microfluidic liquid-driven coupled sensing architecture

Innovation 3: A Fully Parameter-Configurable Aerosol Manufacturing Platform

Innovative Integrated Insulation and Protection Aerosol Process 4

Innovative Additive Manufacturing Solution for 5 Flexible Curved Surfaces

IV. Existing Process Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Traditional photolithography masks are expensive to produce, require new tooling for redesigns, and have long iteration cycles.

Challenge 2: Parallel-plate capacitive sensors have low sensitivity, making it difficult to detect very small forces.

Challenge 3: The conductive silver layer is prone to oxidation when in contact with water-based liquids, leading to signal drift over prolonged use.

Challenge 4: The microchannel thickness is uniform, making it impossible to simultaneously meet the demands for high sensitivity and a wide measurement range.

Challenge 5: Rigid surgical devices cannot conform to the curved surfaces of surgical instruments and the human body.

V. Future Diversified Industrial Application Scenarios

1. Tactile Feedback in Minimally Invasive Surgical Robots

2. Wearable human physiological monitoring patch

3. Intelligent Prosthetics / Tactile Skin for Humanoid Robots

4. On-chip pressure sensing in laboratory microfluidic chips

5. Precision Clamping and Inspection of Industrial Curved Surfaces

Figure 1. Force sensor based on microfluidic technology: (A) Schematic diagram of the novel microfluidic force sensor, illustrating its constituent components: a PDMS microfluidic chip containing microchannels, reservoirs, and injection channels; these components are arranged and bonded onto an interdigitated silver electrode surface, with the electrodes themselves printed on a flexible Kapton substrate. The reservoirs are filled with a glycerol–water solution in a volume ratio of 2:1. When an external force is applied, the liquid moves along the channels and flows past the electrodes, resulting in a change in capacitance. (B and C) show the side view (B) and top view (C) of the force sensor, where FPC denotes a flexible printed circuit. (D and E) demonstrate that the sensor can be flexibly bent into either a convex (D) or concave (E) configuration.

Figure 2. Schematic illustration of the sensor fabrication process: (A) Sodium chloride molds are directly deposited onto an aluminum substrate using aerosol jet printing. (B) Following printing, PDMS is cast over the sodium chloride molds and allowed to cure. (C) After PDMS curing, the aluminum substrate is peeled off, and the sodium chloride is washed away with deionized water. (D) Silver interdigitated electrodes are fabricated directly on a polyimide (Kapton) substrate via aerosol jet printing; the inset shows an enlarged view of the electrode pattern. (E) An insulating polyimide layer is aerosol‑jet printed onto the electrode surface. (F) The printed Kapton covers only the interdigitated electrodes, leaving the connector regions uncoated. (G) The PDMS chip is aligned with the Kapton substrate bearing the electrodes and bonded using an adhesive (see Table S1 for details). (H) The sensing fluid—glycerol mixed with deionized water at a volume ratio of 2:1—is introduced into the microfluidic device through a short channel downstream of the reservoir.

Figure 3. Channel molds, electrodes, and measurement apparatus: (A) Photograph of a sodium chloride mold fabricated by printing. (B) SEM side-view image of the reservoir chamber within the mold. (C) SEM top-view image of the reservoir chamber, showing a 3×3×3 well array designed to prevent collapse of the PDMS replica‑chip reservoir under external forces. (D) Sodium chloride molds of varying thicknesses obtained by repeatedly printing the same design over multiple printing cycles. (E) SEM cross-sectional view of the sodium chloride mold embedded in PDMS. (F) SEM cross-sectional view of the PDMS microfluidic channel after removal of the sodium chloride and aluminum foil. (G) Photograph of electrodes printed on a PI substrate; the inset shows the finger‑like electrode structure as observed under an optical microscope. (H) Compression testing apparatus consisting of a linear motor, a pressing plunger (moving component), a force gauge (fixed component), and an impedance analyzer. The sensor is mounted on the force gauge, with its sensing area facing the pressing plunger, which is driven by the linear motor.

Figure 4. Experimental and simulation results on the performance and parameters of the force sensor: (A) Capacitance variation with applied force in a typical sensor; (B) Comparison of different electrode geometries shows that interdigitated electrodes exhibit higher sensitivity than parallel electrodes; (C) Comparison of capacitance responses for sensors with varying channel widths, including three cases: greater than 500 µm, equal to 500 µm, and less than 500 µm; (D) Simulation of displacement distribution in a 5 × 3 × 5 mm³ PDMS region containing a fluid reservoir, before and after applying a 7‑N force; (E) Schematic illustrating how, under a 1‑V voltage bias across the electrodes, the liquid column spreads along the electrode surface, leading to changes in interelectrode capacitance (the blue area indicates the liquid layer extending to the right); (F) Simulation results showing how the volume of the fluid reservoir varies with external force for PDMS layers of different thicknesses (0.5 to 2 mm); (G) Simulated relationship between the amount of liquid on the electrode surface and capacitance: capacitance increases linearly with the length of the liquid‑covered electrode; (H) Simulation results comparing the sensitivity of liquids with different relative dielectric constants (ε, ranging from 20 to 80); higher ε values enhance sensitivity. (I) Simulation results demonstrating the effect of varying the thickness of the PI insulating layer on the electrode surface (from 2 to 10 µm) on capacitance sensitivity; (J) Simulation results indicating that, for a given length of liquid coverage, the thickness of the liquid above that region (ranging from 50 to 300 µm) has only a minor impact on capacitance.

Figure 5. Stability testing of the force sensor and its interaction with the mechanical gripper: (A) Stability of a single sensor over more than 2,200 cycles: the force sensor undergoes repeated pressing and releasing at a frequency of 0.1 Hz while subjected to an applied force of up to 7 N. (B and C) The force sensor is mounted on the curved surface of a culture dish. (D–F) The sensor continuously monitors changes in the force exerted on the dish: (D) no compression; (E) moderate compression; (F) strong compression, and transmits the acquired data to the mechanical gripper’s control system, which then executes the corresponding gripping action.