3D designed battery-free wireless origami pressure sensor

The increasing demand for wearable sensors in precision medicine drives research into monitoring physiological signals for disease prevention, diagnosis, and treatment. Flexible pressure sensors, based on mechanisms like capacitance, piezoelectricity, piezoresistivity, and triboelectricity, are being developed for wearable applications. While self-powered triboelectric sensors address power supply issues, advancements are ongoing. Plantar pressure measurement is crucial for foot health assessment, early diagnosis of foot ulceration (e.g., in type 2 diabetes), and improvements in footwear design, gait analysis, and sports biomechanics. Current commercial plantar pressure detection systems are not wearable and limit continuous monitoring. Wearable plantar pressure devices are being developed, but wired connections limit user movement and comfort. Wireless communication technologies, like Bluetooth or LC-based sensors, offer greater freedom. LC sensors are attractive due to their durability, compact design, and lack of power requirements. However, the quality factor of capacitive-dominant LC sensors decreases with pressure, affecting measurement accuracy. Inductor-based pressure sensing offers a solution. Origami structures, particularly Miura-ori, provide advantages such as high surface area, stretchability, and predictable deformation under compression. This research presents a novel 3D-printed, flexible, origami-inspired insole for far-field wireless plantar pressure sensing. The multi-layer Miura-ori structure, combined with LC sensors, forms a pressure-sensing device converting mechanical deformation into electrical signals. 3D printing enables customization for user-specific weight, foot shape, and size, addressing limitations of existing systems.

The literature review extensively covers existing pressure sensing technologies, including capacitive, piezoelectric, piezoresistive, and triboelectric sensors, highlighting their advantages and disadvantages. It discusses the use of various materials like PVDF, PZT, AIN, and ZnO in high-performance pressure sensors. The limitations of wired foot pressure sensors and the advantages of wireless communication for enhanced user comfort and freedom of movement are emphasized. The paper also reviews existing wearable plantar pressure detection devices and commercial systems, comparing their capabilities and limitations. The advantages of using origami structures, specifically Miura-ori, in various engineering and sensing applications are discussed, focusing on their strength, stretchability, and energy absorption capabilities.

The study involved designing and fabricating a wireless origami pressure sensor using multiple 3D printing technologies. Three LC sensors with varying interdigitated capacitors were designed and 3D-printed using conductive silver ink on a polyimide (PI) substrate. S11 simulations were conducted to determine the resonant frequencies of these sensors, which were inversely related to capacitance. An L-shaped antenna was employed for wireless sensing. Experiments using the S21 method investigated the effect of LC sensor orientation relative to the antenna. Four LC sensors were embedded as pressure sensors within a 3D-printed insole featuring a Miura-ori origami structure. The insole design incorporated different origami orientations (parallel and perpendicular to the base plane) to optimize pressure sensing performance by varying the Young's modulus. The arrangement of the sensors under the heel and forefoot was based on average foot pressure distribution. Simulations were conducted to evaluate the performance of the complete insole with the four sensors under pressure.

The 3D-printed LC sensors exhibited resonant frequencies that decreased with increasing capacitor layers, consistent with theoretical expectations. Experimental S21 measurements showed that the resonant frequency of the LC sensors remained consistent regardless of their orientation relative to the L-shaped antenna. The 3D-printed insole with integrated LC sensors and Miura-ori origami design effectively measured plantar pressure. The differing compressibility of the origami structures (achieved through varying orientations) optimized pressure sensing across a range of pressures. The sensitivity of the sensor was tunable, demonstrating a range from 15.7 MHz/kPa (0-9 kPa) to 2.1 MHz/kPa (10-40 kPa). Simulation results showed the feasibility of simultaneously measuring pressure at four locations on the foot.

The results demonstrate the successful development of a battery-free, wireless plantar pressure sensing insole using 3D-printed LC sensors and a Miura-ori origami design. The tunable sensitivity of the sensor caters to varying pressure ranges. The use of an L-shaped antenna allows for efficient wireless signal transmission and reception, even with different sensor orientations. The origami structure not only provides structural support but also contributes to the sensor's sensitivity and adaptability to different foot shapes and sizes. This approach offers a significant improvement over existing wired and bulky wireless pressure sensing systems, promoting greater user comfort and convenience. The modular design of the insole facilitates customization for individual users based on their weight and foot morphology.

This research successfully demonstrated a novel battery-free, wireless pressure sensor integrated into a 3D-printed origami insole. The system’s tunable sensitivity and wireless operation offer significant advantages over existing technologies. Future work could explore the integration of more advanced signal processing techniques, expanding the number of sensing points for a more comprehensive pressure map, and testing the long-term reliability and durability of the sensor in real-world scenarios.

The current study focused on in vitro testing. Further investigation is needed to fully validate the performance of the sensor in real-world conditions. The long-term stability and durability of the 3D-printed components need to be assessed. A larger sample size of subjects is needed to confirm the robustness of the measurements. The range of pressures tested might be expanded to further evaluate the sensor's capabilities.