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1. A Hydrogel-Based Ultrasonic Backscattering Wireless Biochemical Sensing

   https://doi.org/10.3389/fbioe.2020.596370  

   Nam, Juhong; Byun, Eunjeong; Shim, Hyunji; Kim, Esther; Islam, Sayemul; Park, Moonchul; Kim, Albert; Song, Seung Hyun (November 2020, Frontiers in Bioengineering and Biotechnology)

   null (Ed.)

   Wireless monitoring of the physio-biochemical information is becoming increasingly important for healthcare. In this work, we present a proof-of-concept hydrogel-based wireless biochemical sensing scheme utilizing ultrasound. The sensing system utilizes silica-nanoparticle embedded hydrogel deposited on a thin glass substrate, which presents two prominent interfaces for ultrasonic backscattering (tissue/glass and hydrogel/glass). To overcome the effect of the varying acoustic properties of the intervening biological tissues between the sensor and the external transducer, we implemented a differential mode of ultrasonic back-scattering. Here, we demonstrate a wireless pH measurement with a resolution of 0.2 pH level change and a wireless sensing range around 10 cm in a water tank. 

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2. Ferrogel-Based Wireless Acousto-Biochemical Sensing

   https://doi.org/10.1109/Transducers50396.2021.9495583  

   Lee, Jiseon; Jun, Chaerin; Oh, Eungyoul; Han, Jeonga; Kim, Albert; Song, Seunghyun (August 2021, 2021 21st International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers))

   Continuous monitoring of biochemical information is critical for health management. Hydrogel, a synthetic material that exhibits volumetric response to target stimuli, is an attractive material for such applications. However, wireless readout of the hydrogel's response over a longer distance, while maintaining the small sensor dimension has been challenging. In this work we present ferrogel-based wireless acousto-biochemical sensing system with small dimension (length: 7.5 mm, diameter: 2 mm) and long sensing distance (>10 cm). The sensor utilizes ferromagnetic hydrogel to convert pH to the change in resonance frequency; the wireless measurement is made through the RF signal emission under ultrasonic excitation. 

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3. Hydrogel-Fractal Piezoelectric Bilayer Transducer for Wireless Biochemical Sensing

   https://doi.org/10.1109/EMBC44109.2020.9175819  

   Islam, Sayemul; Park, Moonchul; Song, Seung Hyun; Kim, Albert (July 2020, 2020 42nd Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC))

   This paper reports on a novel transducer for wireless biochemical sensing. The bilayer transducer consists of a fractal piezoelectric membrane and pH-sensitive chemo-mechanical hydrogel, which overcomes many shortcomings in the chemical and biochemical sensing. The fractal design on the piezoelectric membrane enhances frequency response and linearity by employing periodically repeated pore architecture. As a basis of the pore, a Hilbert space-filling curve with modifications is used. On the surface of the fractal piezoelectric membrane, the hydrogel is laminated. When the bilayer transducer is introduced to a pH environment (e.g., pH = 4, 8, and 12), the hydrogel swells (or shrinks) and induces the curling of the bilayer transducer (10.47°/pH). The curvature then exhibits various ultrasound responses when the bilayer transducer was excited. The measured voltage outputs using an ultrasonic receiver were 0.393, 0.341, 0.250 mV/cm 2 when curvature angles were 30°, 60°, and 120°, respectively. Overall pH sensitivity was 0.017 mV/cm 2 /pH. Ultimately, the biochemical sensing principle using a novel bilayer ultrasound transducer suggests a simple, low-cost, battery-less, and long-range wireless readout system as compared to traditional biochemical sensing. 

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4. Defect localization in heterogeneous plate structures using the geometric phase change – index of Lamb waves

   https://doi.org/10.1016/j.ultras.2025.107654  

   Zhang, Guangdong; Kundu, Tribikram; Deymier, Pierre A; Runge, Keith (August 2025, Ultrasonics)

   Defect localization in homogeneous structures using ultrasonic waves is relatively easy to implement. However, locating defects in heterogeneous structures made of different materials can be challenging. This is because complicated reflections, refractions and scatterings occur when ultrasonic waves pass through the interfaces between two dissimilar materials of the heterogeneous structures. To address this issue, a localization methodology based on geometric phase change – index (GPC-I), derived from topological acoustic (TA) sensing, is proposed to adapt to the complicated scenarios when defects are present in heterogeneous plate structures. The GPC-I is adopted as the damage index (DI) to present the possibility of defects appearing on different acoustic sensing paths. A maximum peak value-dependent threshold in GPC-I plots (GPC-I vs. sensor sites) is defined to filter out unreliable sensing paths resulting from the heterogeneity. Different sensing modes (I and II) are combined to comprehensively provide a more reliable and accurate localization framework. Numerical modeling carried out by Abaqus/CAE software verifies the proposed GPC-I based localization technique. Comparison results among GPC-I and other two commonly used acoustic parameters—wave velocity differences (VD) and amplitude ratio (AR) (or wave attenuation) show that the GPC-I has superiority with higher sensitivity and stability for defect localization. This work can provide promising guidance for localizing defects in complex heterogeneous plate structures used in real-world engineering applications. 

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5. pH-Mediated nanoparticle dynamics in hydrogel nanocomposites

   https://doi.org/10.1039/d0sm02213f  

   Rose, Katie A.; Lee, Daeyeon; Composto, Russell J. (March 2021, Soft Matter)

   null (Ed.)

   The effect of static silica particles on the dynamics of quantum dot (QD) nanoparticles grafted with a poly(ethylene glycol) (PEG) brush in hydrogel nanocomposites is investigated using single particle tracking (SPT). At a low volume fraction of homogeneously dispersed silica ( Φ = 0.005), two distinct populations of PEG-QDs are observed, localized and mobile, whereas almost all PEG-QDs are mobile in neat hydrogel ( Φ = 0.0). Increasing the silica particle concentration ( Φ = 0.01, 0.1) results in an apparent change in the network structure, confounding the impact of silica on PEG-QD dynamics. The localized behavior of PEG-QDs is attributed to pH-mediated attraction between the PEG brush on the probe and surface silanol groups of silica. Using quartz crystal microbalance with dissipation (QCM-D), the extent of this interaction is investigated as a function of pH. At pH 5.8, the PEG brush on the probe can hydrogen bond with the silanol groups on silica, leading to adsorption of PEG-QDs. In contrast, at pH 9.2, silanol groups are deprotonated and PEG-QD is unable to hydrogen bond with silica leading to negligible adsorption. To test the effect of pH, PEG-QD dynamics are further investigated in hydrogel nanocomposites at Φ = 0.005. SPT agrees with the QCM-D results; at pH 5.8, PEG-QDs are localized whereas at pH 9.2 the PEG-QDs are mobile. This study provides insight into controlling probe transport through hydrogel nanocomposites using pH-mediated interactions, with implications for tuning transport of nanoparticles underlying drug delivery and nanofiltration. 

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