Session: SYMP 1-8: Wearables

Paper Number: 147950

147950 - Enhancing Sensitivity of Hydrogel-Based Resonant Sensors Through Additively Manufactured Porous Structures 

Wearable sensors have revolutionized personalized healthcare by enabling real-time monitoring of physiological parameters. Traditional rigid sensors encounter limitations such as discomfort and inaccurate readings stemming from their rigidity, fragility, and mechanical incompatibility with human skin. The advent of flexible wearable sensors marks significant advancement, addressing these issues by offering improved conformity to the human body, reducing discomfort, and minimizing the risk of false readings. In fact, hydrogels, a 3-D crosslinked polymer network with high water content, have been extensively applied in flexible wearable sensors due to their inherent flexibility, softness, stretchability, biocompatibility, permeability, and tunable mechanical properties. However, integrating these sensors with electronics, including rigid chips and bulky batteries, remains challenging. To address these challenges, this study presents a wireless passive sensor approach based on resistor-inductor-capacitor (RLC) resonators integrated with hydrogel materials. They are interrogated via inductive coupling and do not need internal batteries but instead are powered by interrogating radio frequency signals. These sensors were designed to detect changes in temperature and pressure through shifts in resonant frequencies facilitated by inductive coupling between the sensor and reader coil. Furthermore, this work explores the effect of additive-manufactured porous structures on the sensing performance of hydrogel-based RLC resonators. Poly(N-isopropylacrylamide) (PNIPAM)-based and poly(ethylene glycol) dimethacrylate (PEGDMA)-based hydrogels were additively manufactured using the direct ink writing (DIW) method, with varying infill densities to create temperature and pressure sensors. Biocompatible sodium alginate was utilized as a rheology modifier to construct 3D hydrogel architectures that are both chemically and physically cross-linked. With the rheology modifier, the viscosity of PNIPAM-based and PEGDMA-based hydrogel precursor inks was improved and showed shear-thinning behavior. This facilitates smooth ink flow through a syringe nozzle and ensures the structural integrity of the printing object. The temperature sensing results showed that porous PNIPAM-based sensors exhibited significantly greater temperature-induced frequency shifts compared to their solid counterparts, indicating higher temperature sensitivity. Furthermore, surface morphology analysis of PNIPAM-based sensors at 24 °C and 45 °C provided insights into the temperature sensing mechanism, linking changes in opacity and hydrogel dimensions to sensor responsiveness. Similarly, PEGDMA-based sensors with the lowest infill density (30%, higher macro-porosity) showed the highest sensitivity compared to those with 40% and 60% infill due to the increased compressibility and dynamic changes in relative permittivity. These findings suggest that the sensing performance of hydrogel-based sensors can be optimized by manipulating 3D macro-porous structures with versatile additive manufacturing techniques. This study contributes valuable insights into the design, fabrication, and optimization of hydrogel-based sensors, paving the way for their broader applications in healthcare and beyond.

Presenting Author: Bo Mi Lee Missouri University of Science and Technology

Presenting Author Biography: Bo Mi Lee is an assistant professor in the Department of Mechanical and Aerospace Engineering at Missouri University of Science and Technology. Prior to this, she was a postdoctoral associate in the Department of Mechanical and Aerospace Engineering at the University of Central Florida. She received her Ph.D. in Structural Engineering from the University of California, San Diego, in 2019. She obtained her M.S. in Civil and Environmental Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Korea, in 2011, and a B.S. in Applied Physics from Hanyang University, Korea, in 2009. She was the recipient of the UCF Pre-eminent Postdoctoral Program (P3) Award, the Rising Stars Women in Engineering from the 2019 Asian Dean’s Forum, the Best Paper Award from ASME Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS) in 2018, and a dissertation fellowship from UC San Diego in 2019, among others. In addition, her paper was selected as a 2017 highlight by IOP Publishing. Her current research interests include multifunctional materials, stimuli-responsive nanocomposites, and data-driven approaches for enhancing advanced sensor technologies, biomedical systems, and energy solutions.

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