I. Abstract of the Paper’s Core Research

This study, based on the Optomec AJ200 aerosol‑jet printing system and utilizing four types of functional inks—silver, polyimide, PVDF‑TrFE, and PU—directly fabricated the rotor and stator micro‑grating arrays of a triboelectric sensor. Linear displacement sensors and three‑channel rotational angle sensors were then prepared, and comprehensive characterization—including morphological, electrical, and dynamic response testing—was performed. The key findings are as follows:

• Ultra-fine 25μm micro-grating fabrication, micron-level positioning Movement Perception

Aerosol‑based precision printing achieves 25‑μm line widths, with a stator interdigitated electrode pitch of 50 μm; linear displacement resolution reaches 50 μm, enabling the capture of low‑speed micro‑slip motions at 50–300 μm/s. The open‑circuit output voltage remains stable at 20–25 mV, and performance shows no degradation after thousands of cycles.

Multi-material gratings can be freely combined to enable transparent, flexible sensing.

It supports a variety of triboelectric pairing configurations, including silver–polyimide, PVDF–nylon, and all‑polymer systems. All‑polymer devices are optically transparent and can be directly laminated onto display screens, while flexible substrates are compatible with curved‑surface devices. The silver/Polyimide (PI) combination delivers the highest output signal amplitude in triboelectric charging.

• AJP Mask-Assisted Fabrication of Large-Area Wide Electrodes

PU ink printing enables the fabrication of peelable photolithographic masks; after gold evaporation, the mask is peeled off, allowing for the rapid preparation of large‑area interdigitated electrodes while balancing high‑precision microstructures with the mass‑production requirements of large‑area arrays.

• Three-channel rotary sensing, accurately detecting both angle and direction of rotation.

Three gratings are phase-shifted by one-third of a period, and signals are acquired synchronously. The direction of rotation—clockwise or counterclockwise—is determined by the sequence in which the pulses appear. The angular resolution of a single channel is 8°, while combining the three channels triples the overall resolution, with no misidentification of the direction of motion.

• Fully Digital Rapid Prototyping Development

CAD enables free‑form design of linear and radial gratings, with one‑click switching between metal and polymer inks, allowing multiple sensor‑structure prototypes to be fabricated in a single day—without the need for custom photolithography masks—thereby significantly shortening the R&D cycle for self‑powered sensors.

II. Complete Process Flow for Fabricating Aerosol‑Jet Friction Sensors and Their Core Advantages

• Complete sensor fabrication process:

1) Ultrasonic cleaning and plasma activation of PI, nylon, and glass substrates;

2) An aerosol‑direct‑write process is used to pattern the stator interdigitated electrodes with silver nanoparticle ink, followed by printing a PI insulating isolation layer; a second printing step forms the upper electrode, resulting in a sandwich‑type interdigitated structure.

3) Switch the PVDF/PI inkjet print carriage friction grating;

4) Stepwise curing at 130–200°C to fabricate the linear sensor;

5) The rotary sensor employs a radial three-channel staggered grating configuration.

6) A PU ink‑based printing stripping mask is used, and gold evaporation is employed to fabricate large‑area electrodes.

7) Calibration of triboelectric signals acquired by devices mounted on linear motors and rotary stages.

• Five Core Process Advantages (Comparison of Lithography, Inkjet, and Screen Printing)

Metal / Polymer inks are versatile, breaking through the material barriers of traditional processes.

Lithography is suitable only for processing metallic thin films, while our aerosol‑compatible inks cover the full range—conductive silver, piezoelectric PVDF, insulating PI, and peelable PU—enabling the fabrication of fully transparent, metal‑free friction devices.

25 μm Extremely fine lines, far surpassing conventional printing precision.

Inkjet printing achieves a minimum feature size of 30 μm, whereas screen-printed thick lines cannot realize micron-scale gratings. AJP, with its sheath‑gas‑assisted focusing, stably forms 25‑μm microstructures, significantly enhancing displacement‑detection resolution.

Flexible / Curved-Surface Conformal Non-Contact Printing

A safe spray distance of 3–5 mm ensures that nylon and ultra-thin, flexible PI films remain undamaged by compression, while curved‑surface touch interfaces and soft‑robotic skins can directly incorporate molded sensor arrays.

Dual‑path compatibility: direct‑write microstructures + mask‑assisted large‑area fabrication

It can directly print high‑precision microgratings, as well as peelable PU masks for fabricating large‑area metal electrodes, thereby meeting both laboratory prototyping and mass‑production line requirements.

Maskless digital iteration has significantly reduced R&D costs.

No custom lithography masks are required; grating periods and channel layouts can be modified, with samples ready on the same day, and comparative experiments involving multiple friction‑material pairs can be completed rapidly.

III. Five Core Technological Innovations of This Study

Innovation 1: An Integrated Micro-Grating Fabrication System for Aerosols Across Multiple Materials

By leveraging AJP, conductive metals, piezoelectric polymers, and insulating layers can be printed simultaneously, enabling a single device to fabricate the complete set of functional layers for both the stator and rotor of a triboelectric sensor, thereby overcoming the limitation of lithography, which is restricted to metal patterning.

Innovation: A 25-μm ultra-fine grating achieves an ultra-high displacement resolution of 50 μm.

Micrometer-scale periodic friction structures amplify charge‑induction signals, enabling stable identification of low‑speed, minute sliding motions and filling the gap in high‑precision, self‑powered sensing.

Innovation 3: A Novel All-Polymer Transparent Triboelectric Sensing Scheme

Without a metallic conductive layer, the PVDF–nylon pairing enables transparent, flexible devices suitable for optical applications such as AR glasses and in‑vehicle transparent touchscreens.

Innovation 4: Three-Channel Phase-Shifted Rotational Sensing Architecture

Three‑channel grating timing is staggered, enabling precise steering detection based on the sequential order of signals, thereby eliminating motion‑recognition errors in single‑ and dual‑channel modes and tripling angular resolution.

Innovative 5 AJP Peelable Mask–Assisted Large-Area Electrode Process

PU-printed temporary mask, which can be peeled off directly after evaporation deposition, simplifies the fabrication of large-area interdigitated electrodes while balancing precision and production efficiency.

IV. Existing Process Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Conventional lithography can only fabricate metallic gratings and is unable to produce transparent polymer-based sensors.

✅ Compatible with functional polymers such as PVDF and PI, enabling direct-write fabrication of fully transparent friction layers, suitable for display and optically integrated devices.

Challenge 2: Inkjet and screen-printed lines are thick, with poor displacement resolution, making it impossible to detect minute motions.

✅ Sheath‑gas‑focused shaping with a 25‑μm ultra‑fine periodic grating elevates displacement‑measurement accuracy to the micrometer level.

Challenge 3: Single- and dual-channel rotary sensors are prone to steering misjudgments and have low resolution.

✅ Three‑channel phase‑shifted grating synchronous acquisition, with forward and reverse rotation distinguished via pulse timing, significantly enhances measurement accuracy.

Challenge 4: Achieving simultaneous high-precision microstructuring and large-area patterning remains difficult.

✅Dual-process route: direct-write micro‑scale high‑precision devices combined with a PU‑masking method for fabricating large‑area electrodes, meeting both production requirements.

Challenge 5: Traditional microfabrication suffers from long fabrication cycles and high tooling costs.

✅ Maskless digital direct writing; CAD enables one-click adjustment of grating period and channel arrangement, facilitating rapid iteration of multiple friction‑pairing schemes.

V. Future Diversified Industrial Application Scenarios

1 Flexible Transparent In-Vehicle Touch Panel

Windshield and center‑console curved transparent triboelectric sensors, requiring no external power supply, enable swipe‑ and rotation‑based gesture recognition.

2 Soft-Robotics Biomimetic Tactile Skin

A curved nylon‑based, solvent‑borne sol–gel grating array that is self‑powered to sense limb sliding displacement and rotational angle.

3 Wearable smart wristbands / AR interactive lenses

Ultra-thin, flexible, self-powered sensing that detects finger swipes and knob rotations, reducing device power consumption.

4 Industrial Automation Micro Displacement Encoder

A compact, battery-free position-sensing module for assembly lines, designed to replace active optical grating encoders and reduce wiring costs.

5 Medical Rehabilitation Joint Angle Monitoring Patch

Flexible rotational sensors attached to the limbs enable passive acquisition of joint angles for home-based rehabilitation monitoring.

6 University-Driven Sensing R&D Platform

A desktop aerosol‑based device enables the rapid fabrication of multi‑material triboelectric prototypes, eliminating the need for a cleanroom lithography line.

Image and Text Guide

Figure 1. Triboelectric motion sensor with an ultra‑fine grating structure:

• a)   Schematic diagram of a free‑moving grating‑structured triboelectric sensor: the mover (containing Ag or nylon strips) slides over the stator, whose back side is equipped with interdigitated electrodes (Electrode 1/2). An insulating PI film separates the electrodes from the friction surface, preventing direct contact with the mover and thereby mitigating frictional wear.
• b)   Photograph of the Ag actuator printed on a PI film (scale bar: 5 mm);
• c)   Photograph of the stator printed on a polyimide (Kapton) film (scale bar: 1 cm); the printed layout is shown in…   d) : First, print the Ag interdigitated electrode → then print the PI insulating layer → followed by printing the second Ag electrode layer; the two electrode layers are offset by half a pitch to form an interdigitated pair (period 50 μm);
• e)   SEM of the stator thin-film cross-section: Two rows of interdigitated electrodes are well insulated by a thin PI layer in the middle (scale bar, 50 μm).
• f)   Physical image of the sensor mounted on a LinMot linear motor (scale: 5 cm);
• g)   Normalized open-circuit voltage (Voc) at different sliding speeds (50/100/200/300 μm/s): the amplitude is approximately 20–25 mV and is essentially independent of speed, while the signal period varies with speed, enabling continuous monitoring of both sliding distance and velocity.
• h)   After converting the horizontal axis from time to distance based on velocity, the maximum sensitivity of Voc as a function of the voice coil position is approximately   630 μV/μm , enabling displacement resolution at the micrometer乃至sub-micrometer level.

Figure 2. Comparison of material selection for grating structures and large-area electrode fabrication via AJP-assisted dry etching:

• a)   Printed grating‑structured movers composed of five material combinations (period: 600 μm, scale: 1 cm): Ag/Kapton, PVDF‑TrFE/Kapton, Kapton/Teflon, Kapton/Nylon, Kapton/Al;
• b)   Normalized Voc comparison at a sliding speed of 1 mm/s: Ag–Kapton output is at its highest. , higher than that of Kapton–Nylon (although the latter is farther apart in the triboelectric series), attributed to the rougher surface of the printed silver and the larger actual contact area, resulting in greater charge transfer;
• c)   Polyurethane (PU) mask printed on glass (serpentine path, scale bar 1 cm); d)   Cross-section of the mask line measured by a profilometer (Dektak): width 250 μm, height 50 μm;
• e)   A ~200 nm gold film was sputtered onto the mask glass. f)   Directly peel off the PU mask (“Peeling off PU”); g)   Interdigitated gold electrodes observed under backlight; h)   SEM image of the gold electrode edge (scale bar: 200 μm). This “AJP mask + metal deposition + dry lift‑off” process enables rapid fabrication of large‑area, wide‑electrode interdigitated structures for high‑speed motion monitoring (PU can also be applied to soft substrates such as nylon, Teflon, and aluminum foil).

Figure 3. Three-channel rotary triboelectric sensor:

• a)   Schematic diagram: The mover is a polyimide film with a radially printed grating, while the stator consists of three concentric interdigitated silver electrodes (Channels 1/2/3) printed on a Kapton film.
• b)   Physical specimen of a radial grating mover printed on a nylon film (scale bar: 1 cm); c)   Physical representation of a three-channel interdigital silver electrode stator printed on Kapton film (scale bar: 1 cm; each channel comprises 45 electrode pairs, with a single-channel angular resolution of 8°).
• d)   Detailed CAD view of the mover: the gratings in each channel are staggered relative to one another.   ±1/3 cycle (Channel 1/2/3); e)   Stator CAD detail: The radial alignment of all channels is perfect. With simultaneous monitoring and comparison across the three channels, the angular resolution can be improved to that of a single channel.   3 times (With an inter-peak spacing of 8/3°), reliable angle and direction measurements are achieved.

Figure 4. Three-channel signal processing and rotational direction discrimination:

• a)   Signal processing circuit: The three-channel triboelectric outputs are fed into the MCU’s analog inputs Ch1, Ch2, and Ch3 via the load resistor R₂ (20 MΩ).
• b)   The three-channel signals acquired simultaneously—red, green, and blue corresponding to Ch1 inner ring, Ch2 middle ring, and Ch3 outer ring—allow differentiation of waveforms between counter‑clockwise (anti‑clockwise) and clockwise rotations.
• c)   Clockwise rotation of the local magnification: the signal peaks appear in the sequence Ch1→Ch2→Ch3→… (repeating 1→2→3). d)   Rotate the local magnification counterclockwise: the peak sequence is reversed, from Ch3 to Ch2 to Ch1 (repeating 3→2→1). The direction of rotation can be determined by examining the order in which peaks appear in each channel, and the absolute angular position can be calculated from the inter‑peak spacing (8/3°).