3D designed battery-free wireless origami pressure sensor

The study addresses the need for comfortable, wearable, and battery-free pressure monitoring for health and sports biomechanics. Prior wearable pressure sensors (capacitive, piezoelectric, piezoresistive, triboelectric) have advanced significantly, but wired connections complicate device structure and limit user movement, while active wireless systems add bulk and complexity. Plantar pressure monitoring is valuable for diagnosing and managing conditions (e.g., diabetes-related foot ulceration, rheumatoid arthritis, Parkinson’s disease) and for gait analysis and footwear design. Existing platforms like Zebris provide comprehensive pressure distribution but are not wearable for continuous monitoring. Passive LC wireless sensors offer durability, compactness, and no power source, yet capacitive-dominant LC sensors suffer from decreasing quality factor with pressure, affecting accuracy. To overcome these issues, the authors propose integrating pressure-sensitive inductors and a mechanically tunable origami (Miura-ori) structure. They develop a 3D-printed, flexible origami-inspired insole embedding multiple LC sensors and a monopole (L-shaped) antenna for far-field wireless plantar pressure sensing, enabling personalized sensitivity through structural design parameters (thickness, unit cell size, orientation) tailored to user weight, foot size, and shape.

The paper reviews wearable pressure sensing mechanisms: capacitive (fast response, high sensitivity), piezoelectric (direct charge generation under force; materials such as PVDF, PZT, AlN, ZnO), piezoresistive (resistance change via conductive path modulation; simple structure), and triboelectric (self-powered). β-phase PVDF is highlighted for reliability and chemical stability. Commercial plantar pressure systems (e.g., Zebris) are comprehensive but non-wearable, motivating ambulatory solutions. Wearable plantar devices using various sensing mechanisms have been reported. Wireless approaches using Bluetooth or passive LC sensors improve comfort and mobility. Passive LC sensors are durable, compact, and battery-free, though capacitive-dominant designs suffer reduced quality factor under pressure. Prior RF insole approaches using coupling between a ring antenna and LC sensors require repositioning the antenna over each sensor, which is inconvenient and error-prone. Origami (Miura-ori) structures provide high surface area, stretchability, rigid foldability, tunable mechanical properties, and strong, predictable deformation with high impact energy absorption; multi-layer mirror-stacked Miura-ori enhances strength compared to single-layer designs, making them attractive as architectured substrates for sensors.

- Sensor design and fabrication: Three LC sensors were designed with identical spiral inductors (3.75 turns; outer diameter 17.4 mm) and varied interdigitated capacitors to tune resonant frequency. Conductive silver ink was 3D printed on a polyimide (PI) substrate to form the inductors and capacitors. A bridge connecting the outer end of the spiral inductor to the central interdigital capacitor was printed horizontally over a dielectric layer to insulate it from the inductor. Multiple 3D printing technologies, including multi-directional 3D printing, were used to fabricate the sensors and embedded inductors. - Electromagnetic simulations: ANSYS HFSS simulations of S11 were performed to determine the resonant frequencies of the LC sensors. The LC resonant relation f = 1/(2π√(LC)) guided design. Simulated resonant frequency decreased with increased capacitor layers (e.g., 4 to 8 layers reduced f from ~2.76 to ~2.72 GHz). - Wireless measurement setups: Two approaches were used. 1) S11 reflection with a primary ring to excite the LC sensor and detect peaks at resonance. 2) S21 transmission with an L-shaped (monopole) antenna to enable far-field wireless sensing. Two commercial log-periodic antennas (800 MHz–6 GHz) were connected to ports 1 and 2 of a vector network analyzer (VNA) and placed at 90 degrees to minimize interference. LC sensors were positioned on the L-shaped antenna in three orientations to study coupling sensitivity: (a) bridge perpendicular to the horizontal segment; (b) bridge parallel and close; (c) bridge parallel and offset from the horizontal segment. S21 spectra were recorded for each orientation. - Insole architecture and integration: A flexible insole was 3D printed using a multi-layer Miura-ori origami structure. Four LC sensors were embedded at plantar-relevant locations: under the head of the 5th metatarsal (LC1), lateral border (LC2), under the head of the 1st metatarsal (LC3), and the heel (LC4). The L-shaped antenna was integrated at the bottom of the insole beneath the sensors for simultaneous far-field interrogation. The insole’s mechanical response was tuned by orienting the Miura-ori foldcores differently: the overall insole stack used layers perpendicular to the base plane (higher stiffness), while cylindrical origami blocks (six layers) at sensing sites were stacked parallel to the base plane (lower stiffness) to enhance compressibility over sensors. Finite element/3D geometric analyses visualized gap change in serpentine conductors within the origami under compression; stress–strain studies quantified effective modulus (cylindrical origami ~0.0134 MPa). - System-level simulations: S21 simulations were performed for the L-shaped antenna coupled with individual LC sensor–serpentine pairs and for simultaneous measurement of all four pairs to assess spectral separability and readout feasibility for multi-site sensing.

- The wireless pressure sensor exhibits tunable sensitivity: 15.7 MHz/kPa in 0–9 kPa and 2.1 MHz/kPa in 10–40 kPa ranges, enabling low- and mid-pressure plantar measurements. - Wireless monitoring of resonant frequency and signal intensity changes allowed discrimination of foot pressure under different postures using passive LC sensors interrogated via a monopole antenna. - Increasing interdigital capacitor layers decreased resonant frequency in agreement with LC theory (e.g., simulations: ~2.76 GHz to ~2.72 GHz for 4 to 8 layers). - Orientation studies with the L-shaped antenna showed consistent trends across placements: higher capacitance yielded lower resonant frequency; representative measured S21 peaks appeared around 1.543–1.546 GHz, 1.601–1.605 GHz, and 1.601–1.604 GHz for the three respective orientations. - The origami architecture enabled spatially tailored stiffness: cylindrical Miura-ori blocks over sensors provided higher compressibility (effective Young’s modulus ~0.0134 MPa) compared to the stiffer overall insole stack, enhancing pressure transfer to sensors while maintaining structural support. - The integrated L-shaped antenna and LC sensor array facilitated simultaneous, far-field interrogation without repositioning a ring antenna, improving convenience and reducing measurement error risk.

The work demonstrates a battery-free, passive wireless plantar pressure sensing platform that addresses limitations of wired sensors (complexity, restricted movement) and active wireless systems (bulk, power requirement). By leveraging LC resonance and integrating an inductor element sensitive to mechanical deformation, the design mitigates the accuracy degradation associated with purely capacitive-dominant LC sensors under load. The Miura-ori origami architecture provides tunable mechanical properties through geometric design and orientation, enabling personalized sensitivity and range adjustments for different users (weight, foot size/shape). The embedded L-shaped antenna supports far-field, multi-site readout, eliminating the need to manually align a ring antenna over each sensor and thereby enhancing usability and reducing potential errors. Collectively, the findings confirm that structural mechanics (origami) and passive RF sensing can be co-designed to achieve robust, customizable plantar pressure monitoring suitable for wearable applications.

The study presents a 3D-printed, origami-inspired insole integrating passive LC sensors and a monopole antenna for battery-free, far-field wireless plantar pressure sensing. It achieves tunable sensitivity across relevant pressure ranges and demonstrates simultaneous, orientation-robust RF readout at multiple plantar sites. The architectured Miura-ori design enables customization of mechanical response and sensing range for individual users. The platform shows promise for applications in orthotics, prosthetics, and sports gear.