publications

2024

1. Sci. Adv.

   Ambient Health Sensing on Passive Surfaces Using Metamaterials

   Nguyen, Dat T., Zeng, Qihang, Tian, Xi, Chia, Patrick, Wu, Changsheng, Liu, Yuxin, and Ho, John S.

   Science Advances, Jan 2024

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   Ambient sensors can continuously and unobtrusively monitor a person’s health and well-being in everyday settings. Among various sensing modalities, wireless radio-frequency sensors offer exceptional sensitivity, immunity to lighting conditions, and privacy advantages. However, existing wireless sensors are susceptible to environmental interference and unable to capture detailed information from multiple body sites. Here, we present a technique to transform passive surfaces in the environment into highly sensitive and localized health sensors using metamaterials. Leveraging textiles’ ubiquity, we engineer metamaterial textiles that mediate near-field interactions between wireless signals and the body for contactless and interference-free sensing. We demonstrate that passive surfaces functionalized by these metamaterials can provide hours-long cardiopulmonary monitoring with accuracy comparable to gold standards. We also show the potential of distributed sensors and machine learning for continuous blood pressure monitoring. Our approach enables passive environmental surfaces to be harnessed for ambient sensing and digital health applications.

2. npj Flex. Electron.

   Digitally-defined Ultrathin Transparent Wireless Sensor Network for Room-scale Imperceptible Ambient Intelligence

   Jin, Yunxia, Yu, Mengxia,  Nguyen, Dat T., Yang, Xin, Li, Zhipeng, Xiong, Ze, Li, Chenhui, Liu, Yuxin, Kong, Yong Lin, and Ho, John S.

   npj Flexible Electronics, Feb 2024

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   Wireless and battery-free radio-frequency (RF) sensors can be used to create physical spaces that ambiently sense and respond to human activities. Making such sensors ultra-flexible and transparent is important to preserve the aesthetics of living environments, accommodate daily activities, and functionally integrate with objects. However, existing RF sensors are unable to simultaneously achieve high transparency, flexibility, and the electrical conductivity required for remote room-scale operation. Here, we report 4.5 µm RF tag sensors achieving transparency exceeding 90% that provide capabilities in room-scale ambient wireless sensing. We develop a laser-assisted water-based adhesion-reversion process to digitally realize computer-aided RF design at scale. By individually tagging multiple objects and regions of the human body, we demonstrate multiplexed wireless tracking of human-environment interactions and physiological signals at a range of up to 8 m. These radio-frequency identification sensors open opportunities for non-intrusive wireless sensing of daily living spaces for applications in health monitoring and elderly care.

2023

1. Nat. Comm.

   Implant-to-implant wireless networking with metamaterial textiles

   Tian, Xi, Zeng, Qihang, Kurt, Selman A, Li, Renee R,  Nguyen, Dat T., Xiong, Ze, Li, Zhipeng, Yang, Xin, Xiao, Xiao, Wu, Changsheng, Tee, Benjamin C. K., Nikolayev, Denys, Charles, Christopher J., and Ho, John S.

   Nature Communications, Jul 2023

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   Implanted bioelectronic devices can form distributed networks capable of sensing health conditions and delivering therapy throughout the body. Current clinically-used approaches for wireless communication, however, do not support direct networking between implants because of signal losses from absorption and reflection by the body. As a result, existing examples of such networks rely on an external relay device that needs to be periodically recharged and constitutes a single point of failure. Here, we demonstrate direct implant-to-implant wireless networking at the scale of the human body using metamaterial textiles. The textiles facilitate non-radiative propagation of radio-frequency signals along the surface of the body, passively amplifying the received signal strength by more than three orders of magnitude (>30 dB) compared to without the textile. Using a porcine model, we demonstrate closed-loop control of the heart rate by wirelessly networking a loop recorder and a vagus nerve stimulator at more than 40 cm distance. Our work establishes a wireless technology to directly network body-integrated devices for precise and adaptive bioelectronic therapies.

2. IEEE IMS

   Seatbelt-Embroidered Metamaterials for In-Vehicle Vital Sign Monitoring

   Zeng, Qihang, Tian, Xi,  Nguyen, Dat T., Yang, Xin, Chia, Patrick, Wu, Changsheng, and Ho, John S.

   In 2023 IEEE/MTT-S International Microwave Symposium - IMS 2023, Jul 2023

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   Continuous in-vehicle physiological monitoring can improve safety through early detection of driver fatigue and drowsiness. However, developing a system capable of effective and robust physiological monitoring in practical driving scenarios is challenging due to the noisy vehicular environment in which the sensor is exposed to engine vibrations, unpredictable road conditions and random body motions. In this work, we introduce a seatbelt-integrated metamaterial sensor for unobtrusive and continuous in-vehicle respiration and heartbeat monitoring. The metamaterial sensor is fabricated by digital embroidery of conductive threads and can continuously monitor respiration and heartbeat in a contactless manner. Our sensor supports confined microwave surface waves that are modulated by subtle local physiological motions, which are sensitively transduced into changes in the phase of the transmitted signal. We developed an algorithm based on variational mode decomposition (VMD) for noise-robust extraction of respiration and heart rate. We experimentally demonstrate the embroidered metamaterial in real-life driving scenarios and show accurate heartbeat detection with a median inter-beat interval (IBI) detection error of 23.5 ms during driving. Our results highlight the potential of embroidered metamaterial sensors in driver physiological monitoring applications, especially for the unobtrusive detection of fatigue and drowsiness.

3. IEEE EMBC

   Ambient Cardiovascular Monitoring with Metamaterial Textile Sensors

   Nguyen, Dat T., Zeng, Qihang, Tian, Xi, and Ho, John S.

   In 2023 45th Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC), Jul 2023

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   Contactless sensors embedded in the ambient environment have broad applications in unobtrusive, long-term health monitoring for preventative and personalized healthcare. Microwave radar sensors are an attractive candidate for ambient sensing due to their high sensitivity to physiological motions, ability to penetrate through obstacles and privacy-preserving properties, but practical applications in complex real-world environments have been limited because of challenges associated with background clutter and interference. In this work, we propose a thin and soft textile sensor based on microwave metamaterials that can be easily integrated into ordinary furniture for contactless ambient monitoring of multiple cardiovascular signals in a localized manner. Evaluations of our sensor’s performance in human subjects show high accuracy of heartbeat and arterial pulse detection, with ≥ 96.5% sensitivity and < 5% mean absolute relative error (MARE) across all subjects. We demonstrate our sensor’s utility for cuffless blood pressure monitoring on a human subject over a continuous 10-minute period. Our results highlight the potential of metamaterial textile sensors in ambient health and wellness monitoring applications. Clinical relevance — The contactless metamaterial textile sensors demonstrated in this paper provide unobtrusive, convenient and long-term monitoring of multiple cardiovascular health metrics, including heart rate, pulse rate and cuffless blood pressure, which can facilitate preventative and personalized healthcare.

4. IEEE TAP

   Textile-Integrated Phased Surfaces for Wireless Networking of Bioelectronic Devices

   Tian, Xi, Zeng, Qihang,  Nguyen, Dat T., and Ho, John S.

   IEEE Transactions on Antennas and Propagation, Oct 2023

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   Wireless networking of bioelectronic devices holds immense potential for physiological sensing and modulation. However, the human body presents formidable challenges to achieving efficient wireless communication due to signal obstruction, absorption, and reflection. In this paper, we propose a textile-integrated side-fed phased surface to overcome these challenges and enable efficient networking between wearable and implantable bioelectronic devices. The planar phased surface design with side-fed configuration allows for seamless integration with textile spoof surface plasmon (SSP) networks on clothing to effectively direct wireless signals from wearables toward the implants, thereby overcoming the limitations associated with conventional methods. We describe the procedure for designing the textile-integrated phased surface for operation in the 2.4–2.5 GHz industrial, scientific, and medical (ISM) band, and validate its functionality by full-wave simulations and experimental measurements. Using an adult pig, we experimentally demonstrate a 35 dB enhancement of wearable-to-implant wireless transmission at a depth of 3 cm and distance of 25 cm. These results highlight the potential of clothing to exploit textile-integrated microwave devices to enhance the integration of technology into daily life.

2022

1. IEEE TAP

   Near-Reflectionless Wireless Transmission into the Body with Cascaded Metasurfaces

   Yang, Fengyuan,  Nguyen, Dat T., Raeker, Brian O., Grbic, Anthony, and Ho, John S.

   IEEE Transactions on Antennas and Propagation, Sep 2022

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   The ability to transmit wireless signals into the body is important for medical communication, sensing, and powering technologies. However, the efficiency of signal transmission is limited by the large reflection encountered at the interface between air and biological tissue. Here, we demonstrate an approach to overcome this reflection using cascaded metasurfaces with deeply subwavelength thickness to transform the wave impedance at the air-tissue interface. We develop a procedure to systematically synthesize these metasurfaces using multilayer tissue model that can account for different body compositions. Using this procedure, we design a three-layer metasurface operating at 2.45 GHz with λ/24.5 thickness. Full-wave simulations in a computational human body model show that the metasurface provides transmission equivalent to replacing the background with a tissue-matched medium, while experimental measurements demonstrate 12.1 dB enhancement of transmission into a multilayer tissue phantom.

2. IEEE TAP

   Localized Surface Plasmons on Textiles for Non-contact Vital Sign Sensing

   Yang, Xin, Tian, Xi, Zeng, Qihang, Li, Zhipeng,  Nguyen, Dat T., and Ho, John S.

   IEEE Transactions on Antennas and Propagation, Sep 2022

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   Radio-frequency technologies are capable of remotely sensing human vital signs for broad applications in healthcare. However, their use during daily life is hindered by challenges such as environmental interference, low sensitivity, and non-ideal form factors. Here, we report a radio-frequency sensor made from a conductive textile that provides sensitive non-contact monitoring of vital signs. The sensor (13 cm2 in size) comprises a radial pattern of grooves that support spoof localized surface plasmon (LSP) modes and a ground plane with a complementary window that controls the sensor’s interaction with the body. These spoof LSP modes provide a sharp resonant dip and a broadly tunable evanescent field that enables tracking of heartbeat and respiration via the |S11| spectrum measured in a connector-free manner. We establish the optimal operating mode of the sensor and show that it provides a resonant frequency shift of 250 MHz for respiration and 100 MHz for heartbeat motions. Experiments with a healthy human subject show that the measured heart rate (HR) and respiratory rate (RR) are in close agreement with gold standard sensors and are robust to daily activities performed while sitting.

3. IMBioC

   Non-Contact Vital Sign Monitoring With a Metamaterial Surface

   Nguyen, Dat T., Zeng, Qihang, Tian, Xi, and Ho, John S.

   In 2022 IEEE MTT-S International Microwave Biomedical Conference (IMBioC), May 2022

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   Vital sign monitoring is important for determining the health status and well-being of an individual. Despite their high level of accuracy, clinical sensing methods for vital signs, such as heart or respiration signals, often require direct skin contact and the use of wires, making them restrictive and inconvenient. In contrast, alternative sensing approaches using wireless means can be comfortable for users and are suitable for long-term, continuous health monitoring scenarios. Recent advances in physiological sensing using Doppler radars present great potential for non-contact vital sign monitoring, but face many challenges due to background clutter and large body motions. In this work, we develop an integrated system for non-contact vital sign monitoring based on microwave metamaterials and software-defined Doppler radar. Our sensor is thin, flexible, and able to monitor health through clothing. We demonstrate our system’s capability in respiration and cardiac sensing through experiments on a healthy volunteer and validate its cardiac sensing accuracy against electrocardiography as the gold standard. Validation results show a Pearson’s correlation coefficient r ≈ 0.9 and Bland-Altman agreement limits of ±37.1 ms between our sensor’s and the gold standard’s estimation of heart beat-to-beat intervals.

2021

1. Sci. Adv.

   A Flexible Multiplexed Immunosensor for Point-of-Care in Situ Wound Monitoring

   Gao, Yuji,  Nguyen, Dat T., Yeo, Trifanny, Lim, Su Bin, Tan, Wei Xian, Madden, Leigh Edward, Jin, Lin, Long, Ji Yong Kenan, Aloweni, Fazila Abu Bakar, Liew, Yi Jia Angela, Tan, Mandy Li Ling, Ang, Shin Yuh, Maniya, Sivagame D/O, Abdelwahab, Ibrahim, Loh, Kian Ping, Chen, Chia-Hung, Becker, David Laurence, Leavesley, David, Ho, John S., and Lim, Chwee Teck

   Science Advances, May 2021

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   Chronic wounds arise from interruption of normal healing due to many potential pathophysiological factors. Monitoring these multivariate factors can provide personalized diagnostic information for wound management, but current sensing technologies use complex laboratory tests or track a limited number of wound parameters. We report a flexible biosensing platform for multiplexed profiling of the wound microenvironment, inflammation, and infection state at the point of care. This platform integrates a sensor array for measuring inflammatory mediators [tumor necrosis factor–α, interleukin-6 (IL-6), IL-8, and transforming growth factor–β1], microbial burden (Staphylococcus aureus), and physicochemical parameters (temperature and pH) with a microfluidic wound exudate collector and flexible electronics for wireless, smartphone-based data readout. We demonstrate in situ multiplexed monitoring in a mouse wound model and also profile wound exudates from patients with venous leg ulcers. This technology may facilitate more timely and personalized wound management to improve chronic wound healing outcomes. An integrated smart microfluidic dressing with a multibiomarker immunosensor can provide in situ wound status profiling on site. An integrated smart microfluidic dressing with a multibiomarker immunosensor can provide in situ wound status profiling on site.

2020

1. Phys. Rev. Appl

   Antireflection and Wavefront Manipulation with Cascaded Metasurfaces

   Yang, Fengyuan, Raeker, Brian O.,  Nguyen, Dat T., Miller, Joseph D., Xiong, Ze, Grbic, Anthony, and Ho, John S.

   Physical Review Applied, Dec 2020

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   Layered structures are widely used in optics to control wave reflection and propagation at interfaces but their thickness relative to the wavelength hinders their application in the radio-frequency regime. Here, we design and experimentally demonstrate cascaded metasurfaces with deeply subwavelength thickness that provide both antireflection and wavefront manipulation. We describe the systematic synthesis of the metasurface layers from a prescribed scattering response and demonstrate designs capable of providing near-perfect microwave transmission through glass as well as antireflection focusing of wireless signals from air into water.