publications

2024

1. Nature Electronics

   Metamaterial biosensors for driver monitoring

   John S Ho, and Qihang Zeng

   Nature Electronics, 2024

   Abs DOI

   Monitoring driver alertness could improve road safety, but the use of biosensors in moving vehicles is challenging. Now, a metamaterial biosensor has been designed that can be directly embroidered onto seat belts, enabling the contactless measurement of drivers’ vital signs. This biosensor shows reliable and consistent performance, even in challenging kinetic environments.

2. Nature Electronics

   A digitally embroidered metamaterial biosensor for kinetic environments

   Qihang Zeng, Xi Tian, Dat T. Nguyen, and 5 more authors

   Nature Electronics, 2024

   Abs DOI

   Biosensors could be used in vehicles to monitor driver alertness, detect impairment and gauge stress levels. However, measuring biomarkers, such as heart rate and respiration, without physical contact remains a challenge in these environments due to issues related to movement and external interference. Here we report a metamaterial biosensor that can capture cardiopulmonary signals in kinetic environments without being in contact with the body. Fabricated using digital embroidery, the biosensor can be integrated with a safety harness and can detect physiological motion through near-field interactions between wireless signals and the body. We show that the approach is capable of continuous, hour-long recording of heartbeat and respiration in an airline cabin simulator. We also show that the biosensor can operate in a moving car, and exhibits no degradation in accuracy compared with a stationary environment. A textile metamaterial sensor can wirelessly measure biomarkers—such as heart rate and respiration—in vehicles and shows no accuracy degradation when operating in moving environments.

3. Science

   Single body-coupled fiber enables chipless textile electronics

   Weifeng Yang, Shaomei Lin, Wei Gong, and 13 more authors

   Science, 2024

   Abs DOI

   Intelligent textiles provide an ideal platform for merging technology into daily routines. However, current textile electronic systems often rely on rigid silicon components, which limits seamless integration, energy efficiency, and comfort. Chipless electronic systems still face digital logic challenges owing to the lack of dynamic energy-switching carriers. We propose a chipless body-coupled energy interaction mechanism for ambient electromagnetic energy harvesting and wireless signal transmission through a single fiber. The fiber itself enables wireless visual–digital interactions without the need for extra chips or batteries on textiles. Because all of the electronic assemblies are merged in a miniature fiber, this facilitates scalable fabrication and compatibility with modern weaving techniques, thereby enabling versatile and intelligent clothing. We propose a strategy that may address the problems of silicon-based textile systems.

4. Science Adv.

   Ambient health sensing on passive surfaces using metamaterials

   Dat T. Nguyen, Qihang Zeng, Xi Tian, and 4 more authors

   Science Advances, 2024

   Abs DOI

   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.

2023

1. Biophysics Rev.

   Bioelectronic devices for light-based diagnostics and therapies

   Han-Joon Kim, Weni Sritandi, Ze Xiong, and 1 more author

   Biophysics Reviews, 2023

   Abs DOI

   Light has broad applications in medicine as a tool for diagnosis and therapy. Recent advances in optical technology and bioelectronics have opened opportunities for wearable, ingestible, and implantable devices that use light to continuously monitor health and precisely treat diseases. In this review, we discuss recent progress in the development and application of light-based bioelectronic devices. We summarize the key features of the technologies underlying these devices, including light sources, light detectors, energy storage and harvesting, and wireless power and communications. We investigate the current state of bioelectronic devices for the continuous measurement of health and on-demand delivery of therapy. Finally, we highlight major challenges and opportunities associated with light-based bioelectronic devices and discuss their promise for enabling digital forms of health care.

2. ACS Nano

   Technology Roadmap for Flexible Sensors

   Yifei Luo, Mohammad Reza Abidian, Jong-Hyun Ahn, and 139 more authors

   ACS Nano, 2023

   Abs DOI

   Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

3. PRL

   Stochastic Exceptional Points for Noise-Assisted Sensing

   Zhipeng Li, Chenhui Li, Ze Xiong, and 11 more authors

   Physical Review Letters, 2023

   Abs DOI

   Noise is a fundamental challenge for sensors deployed in daily environments for ambient sensing, health monitoring, and wireless networking. Current strategies for noise mitigation rely primarily on reducing or removing noise. Here, we introduce stochastic exceptional points and show the utility to reverse the detrimental effect of noise. The stochastic process theory illustrates that the stochastic exceptional points manifest as fluctuating sensory thresholds that give rise to stochastic resonance, a counterintuitive phenomenon in which the added noise increases the system’s ability to detect weak signals. Demonstrations using a wearable wireless sensor show that the stochastic exceptional points lead to more accurate tracking of a person’s vital signs during exercise. Our results may lead to a distinct class of sensors that overcome and are enhanced by ambient noise for applications ranging from healthcare to the internet of things.

4. Nature Commun.

   Implant-to-implant wireless networking with metamaterial textiles

   Xi Tian, Qihang Zeng, Selman A. Kurt, and 11 more authors

   Nature Communications, 2023

   Abs DOI

   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. Implanted bioelectronic devices have proven useful for health sensing and therapy, while the interconnection of distributed implants remains challenging. Here, the authors demonstrate direct implant-to-implant wireless networking at the scale of the human body using metamaterial textiles.

2022

1. Nature Commun.

   Digitally-embroidered liquid metal electronic textiles for wearable wireless systems

   Rongzhou Lin, Han-Joon Kim, Sippanat Achavananthadith, and 4 more authors

   Nature Communications, 2022

   Abs DOI

   Electronic textiles capable of sensing, powering, and communication can be used to non-intrusively monitor human health during daily life. However, achieving these functionalities with clothing is challenging because of limitations in the electronic performance, flexibility and robustness of the underlying materials, which must endure repeated mechanical, thermal and chemical stresses during daily use. Here, we demonstrate electronic textile systems with functionalities in near-field powering and communication created by digital embroidery of liquid metal fibers. Owing to the unique electrical and mechanical properties of the liquid metal fibers, these electronic textiles can conform to body surfaces and establish robust wireless connectivity with nearby wearable or implantable devices, even during strenuous exercise. By transferring optimized electromagnetic patterns onto clothing in this way, we demonstrate a washable electronic shirt that can be wirelessly powered by a smartphone and continuously monitor axillary temperature without interfering with daily activities. Electronic textiles should maintain performance against repeated mechanical, thermal and chemical stresses during daily activities. Here, authors demonstrate digital embroidery of robust liquid metal-based fibres with wireless functionalities.

2. Phil. Trans. R. Soc. A

   Wireless interfaces for brain neurotechnologies

   Han-Joon Kim, and John S. Ho

   Philosophical Transactions of the Royal Society A, 2022

   Abs DOI

   Wireless interfaces enable brain-implanted devices to remotely interact with the external world. They are critical components in modern research and clinical neurotechnologies and play a central role in determining their overall size, lifetime and functionality. Wireless interfaces use a wide range of modalitiesincluding radio-frequency fields, acoustic waves and lightto transfer energy and data to and from an implanted device. These forms of energy interact with living tissue through distinct mechanisms and therefore lead to systems with vastly different form factors, operating characteristics, and safety considerations. This paper reviews recent advances in the development of wireless interfaces for brain neurotechnologies. We summarize the requirements that state-of-the-art brain-implanted devices impose on the wireless interface, and discuss the working principles and applications of wireless interfaces based on each modality. We also investigate challenges associated with wireless brain neurotechnologies and discuss emerging solutions permitted by recent developments in electrical engineering and materials science. This article is part of the theme issue ’Advanced neurotechnologies: translating innovation for health and well-being’.

3. Nature Nanotechnol.

   Wireless nanomedicine for brain tumors

   John S. Ho, and Yong Zhang

   Nature Nanotechnology, 2022

   Abs DOI

   Delivering light therapy using a remotely controlled bioelectronic device implanted above the brain might complement current glioblastoma therapies, reducing cancer recurrence and improving survival.

4. IEEE TAP

   Near-Reflectionless Wireless Transmission into the Body with Cascaded Metasurfaces

   Fengyuan Yang, Dat T. Nguyen, Brian O. Raeker, and 2 more authors

   IEEE Transactions on Antennas and Propagation, 2022

   Abs DOI

   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.

5. IEEE TAP

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

   Xin Yang, Xi Tian, Qihang Zeng, and 3 more authors

   IEEE Transactions on Antennas and Propagation, 2022

   Abs DOI

   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.

2021

1. PRApplied

   Wireless Magnetic Actuation with a Bistable Parity-Time-Symmetric Circuit

   Dong Zhenya, Kim Han-Joon, Cui Hongjian, and 3 more authors

   Physical Review Applied, Feb 2021

   Abs DOI

   doi:10.1103/PhysRevApplied.15.024023 url:https://doi.org/10.1103/PhysRevApplied.15.024023

2. Sci. Adv.

   A flexible multiplexed immunosensor for point-of-care in situ wound monitoring

   Yuji Gao, Dat T. Nguyen, Trifanny Yeo, and 17 more authors

   Science Advances, Feb 2021

   Abs DOI

   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.

3. Nature Electronics

   Metasurfaces for bioelectronics and healthcare

   Zhipeng Li, Xi Tian, Cheng-Wei Qiu, and 1 more author

   Nature Electronics, Feb 2021

   Abs DOI

   Bioelectronic systems typically rely on radiofrequency wireless components to interface with the human body, but such components are bulky and energy-demanding, which limits the performance of the systems. Metasurfaces—artificial two-dimensional materials with subwavelength structure—can be engineered to control electromagnetic fields around the human body and could be used to overcome the current limitations of bioelectronic interfaces. Here we review the development of metasurfaces for bioelectronics, and explore their potential for application in current and emerging healthcare technologies. We examine the use of metasurfaces to control electromagnetic fields in the vicinity of the human body, and discuss their application in microwave imaging, magnetic resonance imaging, biosensors, body networks and wireless power transfer. We also consider developments in materials science and artificial intelligence that can enhance the properties of metasurfaces for bioelectronics. This Review examines the development of metasurfaces for bioelectronic interfaces, exploring how they can be used to control electromagnetic fields in the vicinity of the human body and their potential application in current and emerging healthcare technologies.

4. Sutures for the wireless sensing of deep wounds

   Sameer Sonkusale

   Nature Biomedical Engineering, Feb 2021

   Abs DOI

   Deep surgical wounds can be monitored via conductive multifilament surgical sutures incorporating ‘sensing pledgets’ bearing capacitive sensors operated via harmonic radiofrequency identification.

5. Nature Biomed. Eng.

   Wirelessly operated bioelectronic sutures for the monitoring of deep surgical wounds

   Viveka Kalidasan, Xin Yang, Ze Xiong, and 19 more authors

   Nature Biomedical Engineering, Feb 2021

   Abs DOI

   Monitoring surgical wounds post-operatively is necessary to prevent infection, dehiscence and other complications. However, the monitoring of deep surgical sites is typically limited to indirect observations or to costly radiological investigations that often fail to detect complications before they become severe. Bioelectronic sensors could provide accurate and continuous monitoring from within the body, but the form factors of existing devices are not amenable to integration with sensitive wound tissues and to wireless data transmission. Here we show that multifilament surgical sutures functionalized with a conductive polymer and incorporating pledgets with capacitive sensors operated via radiofrequency identification can be used to monitor physicochemical states of deep surgical sites. We show in live pigs that the sutures can monitor wound integrity, gastric leakage and tissue micromotions, and in rodents that the healing outcomes are equivalent to those of medical-grade sutures. Battery-free wirelessly operated bioelectronic sutures may facilitate post-surgical monitoring in a wide range of interventions. Multifilament surgical sutures functionalized with a conductive polymer and incorporating pledgets with capacitive sensors operated via radiofrequency identification can be used to monitor physicochemical states of deep surgical sites.

6. Nature Biomed. Eng.

   A DIY approach to wearable sensor networks

   Rongzhou Lin, and John S. Ho

   Nature Electronics, Feb 2021

   Abs DOI

   Textile-integrated metamaterials can be used to propagate signals across the body, and between different people, allowing a network of sensors to be connected and powered.

7. Science

   A wireless optoelectronic skin patch for light delivery and thermal monitoring

   Han-Joon Kim, Yunxia Jin, Sippanat Achavananthadith, and 2 more authors

   iScience, Feb 2021

   Abs DOI

   Wearable optoelectronic devices can interface with the skin for applications in continuous health monitoring and light-based therapy. Measurement of the thermal effect of light on skin is often critical to track physiological parameters and control light delivery. However, accurate measurement of light-induced thermal effects is challenging because conventional sensors cannot be placed on the skin without obstructing light delivery. Here, we report a wearable optoelectronic patch integrated with a transparent nanowire sensor that provides light delivery and thermal monitoring at the same location. We achieve fabrication of a transparent silver nanowire network with >92% optical transmission that provides thermoresistive sensing of skin temperature. By integrating the sensor in a wireless optoelectronic patch, we demonstrate closed-loop regulation of light delivery as well as thermal characterization of blood flow. This light delivery and thermal monitoring approach may open opportunities for wearable devices in light-based diagnostics and therapies.

8. Sci. Adv.

   A wireless and battery-free wound infection sensor based on DNA hydrogel

   Ze Xiong, Sippanat Achavananthadith, Sophie Lian, and 23 more authors

   Science Advances, Feb 2021

   Abs DOI

   The confluence of wireless technology and biosensors offers the possibility to detect and manage medical conditions outside of clinical settings. Wound infections represent a major clinical challen...

2020

1. Nature Commun.

   Wireless battery-free body sensor networks using near-field-enabled clothing

   Rongzhou Lin, Han-Joon Kim, Sippanat Achavananthadith, and 6 more authors

   Nature Communications, Jan 2020

   Abs DOI

   Networks of sensors placed on the skin can provide continuous measurement of human physiological signals for applications in clinical diagnostics, athletics and human-machine interfaces. Wireless and battery-free sensors are particularly desirable for reliable long-term monitoring, but current approaches for achieving this mode of operation rely on near-field technologies that require close proximity (at most a few centimetres) between each sensor and a wireless readout device. Here, we report near-field-enabled clothing capable of establishing wireless power and data connectivity between multiple distant points around the body to create a network of battery-free sensors interconnected by proximity to functional textile patterns. Using computer-controlled embroidery of conductive threads, we integrate clothing with near-field-responsive patterns that are completely fabric-based and free of fragile silicon components. We demonstrate the utility of the networked system for real-time, multi-node measurement of spinal posture as well as continuous sensing of temperature and gait during exercise. Though wireless near-field communication (NFC) technologies that connect wearable sensors for health monitoring have been reported, the short range of NFC readers limits sensor functionality. Here, the authors report a wireless and battery-free body sensor network with near-field-enabled clothing.

2. IEEE TAP

   Conformal Propagation and Near-Omnidirectional Radiation With Surface Plasmonic Clothing

   Xi Tian, Qihang Zeng, Denys Nikolayev, and 1 more author

   IEEE Transactions on Antennas and Propagation, Oct 2020

   Abs DOI

   IEEE Transactions on Antennas and Propagation;2020;68;11;10.1109/TAP.2020.2998216

3. PRApplied

   Antireflection and Wavefront Manipulation with Cascaded Metasurfaces

   Fengyuan Yang, Raeker Brian O, Nguyen Dat T, and 3 more authors

   Physical Review Applied, Dec 2020

   Abs DOI

   doi:10.1103/PhysRevApplied.14.064044 url:https://doi.org/10.1103/PhysRevApplied.14.064044

2019

1. Nature Electronics

   Wireless body sensor networks based on metamaterial textiles

   Xi Tian, Pui Mun Lee, Yu Jun Tan, and 7 more authors

   Nature Electronics, Jun 2019

   Abs DOI

   Wireless networks of sensors, displays and smart devices that can be placed on a person’s body could have applications in health monitoring, medical interventions and human–machine interfaces. Such wireless body networks are, however, typically energy-inefficient and vulnerable to eavesdropping because they rely on radio-wave communications. Here, we report energy-efficient and secure wireless body sensor networks that are interconnected through radio surface plasmons propagating on metamaterial textiles. The approach uses clothing made from conductive fabrics that can support surface-plasmon-like modes at radio communication frequencies. Our body sensor networks enhance transmission efficiencies by three orders of magnitude compared to conventional radiative networks without the metamaterial textile, and confine wireless communication to within 10 cm of the body. We also show that the approach can offer wireless power transfer that is robust to motion and textile-based wireless touch sensing. Energy-efficient and secure wireless body sensor networks can be created by using conductive fabrics that support surface-plasmon-like modes at radio communication frequencies.

2. Nature Electronics

   Wearable sensors get connected with plasmons

   Mehmet Rasit Yuce

   Nature Electronics, Jun 2019

   Abs DOI

   Nature Electronics, doi:10.1038/s41928-019-0265-7

3. Nature Electronics

   Sensitive readout of implantable microsensors using a wireless system locked to an exceptional point

   Zhenya Dong, Zhipeng Li, Fengyuan Yang, and 2 more authors

   Nature Electronics, Aug 2019

   Abs DOI

   Nature Electronics, doi:10.1038/s41928-019-0284-4

4. Nature Electronics

   Exceptional points enhance wireless readout

   Pai-Yen Chen, and Ramy El-Ganainy

   Nature Electronics, Aug 2019

   Abs DOI

   Nature Electronics, doi:10.1038/s41928-019-0293-3

5. PRApplied

   Enhancing Wireless Transmission from the Body with Wearable Diffractive Patterns

   Fengyuan Yang, Lee Pui Mun, Dong Zhenya, and 2 more authors

   Physical Review Applied, Nov 2019

   Abs DOI

   doi:10.1103/PhysRevApplied.12.054020 url:https://doi.org/10.1103/PhysRevApplied.12.054020

2018

1. PNAS

   In vivo wireless photonic photodynamic therapy

   A Bansal, F Yang, T Xi, and 2 more authors

   Proceeding of the National Academy of Sciences, Nov 2018

   Abs DOI

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2. Control of wireless power transfer to a bioelectronic device by harmonic feedback

   Xi Tian, Pui Mun Lee, and John S Ho

   AIP Advances, Nov 2018

   DOI
3. Methods for powering bioelectronic microdevices

   Pui Mun Lee, Ze Xiong, and John S Ho

   Bioelectronics in Medicine, Nov 2018

   DOI

2017

1. Nature Biomed. Eng.

   Conformal phased surfaces for wireless powering of bioelectronic microdevices

   Devansh R. Agrawal, Yuji Tanabe, Desen Weng, and 11 more authors

   Nature Biomedical Engineering, Mar 2017

   Abs DOI

   Wireless powering could enable the long-term operation of advanced bioelectronic devices within the human body. Although both enhanced powering depth and device miniaturization can be achieved by shaping the field pattern within the body, existing electromagnetic structures do not provide the spatial phase control required to synthesize such patterns. Here, we describe the design and operation of conformal electromagnetic structures, termed phased surfaces, that interface with non-planar body surfaces and optimally modulate the phase response to enhance the performance of wireless powering. We demonstrate that the phased surfaces can wirelessly transfer energy across anatomically heterogeneous tissues in large animal models, powering miniaturized semiconductor devices (<12 mm3) deep within the body (>4 cm). As an illustration of in vivo operation, we wirelessly regulated cardiac rhythm by powering miniaturized stimulators at multiple endocardial sites in a porcine animal model. A phased electromagnetic surface that conforms to the body surface can regulate cardiac rhythm in a porcine model through the wireless transmission of power to miniaturized semiconductor devices implanted at depths of over 4 cm.

2. PRApplied

   Enhanced Electromagnetic Energy Harvesting with Subwavelength Chiral Structures

   Zhenya Dong, Fengyuan Yang, and John S Ho

   Physical Review Applied, Oct 2017

   Abs DOI

   Efficient collection of energy from electromagnetic radiation is important to emerging technologies for wireless power transfer, but is challenging in dynamic environments, because performance generally depends on the light’s polarization direction. The authors show that engineering the chirality of a subwavelength structure can enable uniform collection of energy from all orientations of an incident polarization state. Such structures also enjoy enhanced performance when the chirality of the incident field matches that of the structure. This approach could enable miniaturization and more robust wireless powering of moving devices.

3. PLoS one

   High-performance wireless powering for peripheral nerve neuromodulation systems

   Yuji Tanabe, John S Ho, Jiayin Liu, and 10 more authors

   PloS one, Oct 2017

   Abs DOI

   Neuromodulation of peripheral nerves with bioelectronic devices is a promising approach for treating a wide range of disorders. Wireless powering could enable long-term operation of these devices, but achieving high performance for miniaturized and deeply placed devices remains a technological challenge. We report the miniaturized integration of a wireless powering system in soft neuromodulation device (15 mm length, 2.7 mm diameter) and demonstrate high performance (about 10%) during in vivo wireless stimulation of the vagus nerve in a porcine animal model. The increased performance is enabled by the generation of a focused and circularly polarized field that enhances efficiency and provides immunity to polarization misalignment. These performance characteristics establish the clinical potential of wireless powering for emerging therapies based on neuromodulation.

2015

1. PRApplied

   Self-Tracking Energy Transfer for Neural Stimulation in Untethered Mice

   John S Ho, Yuji Tanabe, Shrivats Mohan Iyer, and 5 more authors

   Physical Review Applied, Aug 2015

   DOI
2. PRB

   Planar immersion lens with metasurfaces

   John S. Ho, Brynan Qiu, Yuji Tanabe, and 3 more authors

   Physical Review B, Mar 2015

   Abs DOI

   The solid immersion lens is a powerful optical tool that allows light entering material from air or a vacuum to focus to a spot much smaller than the free-space wavelength. Conventionally, however, the lenses rely on semispherical topographies and are nonplanar and bulky, which limits their integration in many applications. Recently, there has been considerable interest in using planar structures, referred to as metasurfaces, to construct flat optical components for manipulating light in unusual ways. Here, we propose and demonstrate the concept of a planar immersion lens based on metasurfaces. The resulting planar device, when placed near an interface between air and dielectric material, can focus electromagnetic radiation incident from air to a spot in the material smaller than the free-space wavelength. As an experimental demonstration, we fabricate an ultrathin and flexible microwave lens and further show that it achieves wireless energy transfer in material mimicking biological tissue.

3. Nature Methods

   Wirelessly powered, fully internal optogenetics for brain, spinal and peripheral circuits in mice

   Kate L Montgomery, Alexander J Yeh, John S Ho, and 8 more authors

   Nature Methods, Aug 2015

   Abs DOI

   Nature Methods, (2015). doi:10.1038/nmeth.3536

2014

1. PNAS

   Wireless power transfer to deep-tissue microimplants

   J S Ho, A J Yeh, E Neofytou, and 5 more authors

   Proceeding of the National Academy of Sciences, Jun 2014

   DOI
2. Miniaturizing wireless implants

   Henry Mei, and Pedro P Irazoqui

   Nature Biotechnology, Jun 2014

   Abs DOI

   A breakthrough in wireless power transfer enables smaller implantable devices, opening the door to new applications.

2013

1. Proc. IEEE

   Midfield Wireless Powering for Implantable Systems

   John S Ho, Sanghoek Kim, and Ada S Y Poon

   Proceedings of the IEEE, Apr 2013

   DOI

2012

1. Wireless power transfer to a cardiac implant

   Sanghoek Kim, John S. Ho, Lisa Y. Chen, and 1 more author

   Applied Physics Letters, Aug 2012

   Abs DOI

   We analyze wireless power transfer between a source and a weakly coupled implant on the heart. Numerical studies show that mid-field wireless powering achieves much higher power transfer efficiency than traditional inductively coupled systems. With proper system design, power sufficient to operate typical cardiac implants can be received by millimeter-sized coils.

2. PRL

   Midfield Wireless Powering of Subwavelength Autonomous Devices

   Sanghoek Kim, John S. Ho, and Ada S. Y. Poon

   Physical Review Letters, May 2012

   Abs DOI

   We obtain an analytical bound on the efficiency of wireless power transfer to a weakly coupled device. The optimal source is solved for a multilayer geometry in terms of a representation based on the field equivalence principle. The theory reveals that optimal power transfer exploits the properties of the midfield to achieve efficiencies far greater than conventional coil-based designs. As a physical realization of the source, we present a slot array structure whose performance closely approaches the theoretical bound.

3. IEEE TAP

   Wireless Power Transfer to Miniature Implants: Transmitter Optimization

   Sanghoek Kim, John S. Ho, and Ada S. Y. Poon

   IEEE Transactions on Antennas and Propagation, Sep 2012

   Abs DOI

   This paper examines transmitter optimization for wirelessly powering a small implant embedded in tissue. The wireless link between the transmitter and receiver is first modeled as a two-port network and an expression for the power transfer efficiency derived. for a given small receiver in a multilayer tissue model, the transmitter is abstracted as a sheet of magnetic current density for which the optimal distribution is analytically found. The optimal transmitter is compared to the point and uniform source across a range of frequencies. At higher frequencies, the optimal current distribution is shown to induce fields that exhibit focusing. The effects of constructive and destructive interference substantially improves the power transfer efficiency and reinforces operation in the low GHz-range. The optimal transmitter establishes an upper bound on the power transfer efficiency for a given implant and provides insight on the design of the optimal transmit antenna.