Wearable and Implantable Devices for Healthcare
Authors: Wei Gao, Cunjiang Yu
Journal: Advanced Healthcare Materials (2021)

Over the past decades, wearable and implantable devices have demonstrated great potential for a wide range of personalized health monitoring and therapeutic applications. This special issue primarily focuses on functional and electronic materials, sensors technologies and capabilities, and the associated energy solutions for wearable and implantable devices toward healthcare applications. We have collected 17 reviews, four research articles, and one perspective, all of which are within the scope of this area and cover the topics in breadth and depth. Functional materials with reduced dimensions, such as nanomaterials, have unique properties to match with or to allow seamless integration with soft deformable tissues and organs, and/or to provide unique interfaces for sensing and therapeutic functions. Woon-Hong Yeo and co-workers summarize the recent progress and advances in various organic and inorganic nanomaterials and additive printing technologies for the development of soft, bioresorbable devices (article number 2100158). Nanomaterials, ranging from nanoparticles, to nanofibers, to nanomembranes, and to hydrogels, offer advantages in intimate interfaces, low impedance in sensing, biocompatibility, and degradability. Different printing technologies such as screen printing, inkjet printing, aerosol jet printing, and laser sintering for manufacturing implantable devices are also discussed. Owing to their low moduli, deformability, and processability, elastomers and polymers are an important class of materials for the construction of soft and bio-integrated electronics to form a conformal interface with soft and curvilinear biological tissues. Dae-Hyeong Kim and co-workers provide an overview of conductive elastomers, semiconducting elastomers, adhesive elastomers, and self-healing elastomers and their usages in building these soft and bio-integrated electronics (article number 2002105). Zhenan Bao and co-workers summarize the progress of various conjugated polymers, including semiconducting polymers and conducting polymers, and the design considerations of these polymers in various implantable neural interface devices (article number 2001916). Among many devices, soft electrodes are one of the most critical devices for both non-invasive and invasive electrophysiological signal recording. Yingying Zhang and co-workers discuss various soft, flexible electrodes in wires, meshes, and film formats as wearables and implants (article number 2100646). Wenlong Cheng and co-workers provide an overview of the soft materials and deformable microstructure design strategies of the future soft healthcare devices. Recent progress of the soft wearable healthcare devices for continuous monitoring physical, electrophysiological, chemical, and biological signals is also discussed (article number 2100577). The intrinsic stretchability and high conductivity of liquid metals make them a promising candidate for designing bio-interfaced electronics. Yang-Ung Park et al. review the featured properties and processing technologies of liquid metal-based soft wearable electronics. Example applications on biosensors, soft interconnects, and neural interfaces are discussed along with the challenges and prospects for future development (article number 2002280). Biological materials are another promising candidate in the development of wearable electronics as they are abundant, sustainable, biocompatible, and biodegradable. Tae-Woo Lee and co-workers summarize the biomaterials and structures used in wearable pressure sensors based on various sensing mechanisms such as piezoelectric, triboelectric, piezoresistive and capacitive effects (article number 2100460). Considering that transient materials are attracting growing interest for temporary biomedical implants, John A. Rogers and co-workers highlight recent advances in the development of implantable devices made of bioresorbable metals and metal alloys toward diagnostic and therapeutic applications (article number 2002236). This special issue also features multiple research articles reporting novel conductive materials and devices for reliable, robust and high fidelity soft, wearable sensors. George Malliaras and co-workers report the development of mm pitch-size surface electromyography arrays from composites of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) with the biocompatible ionic liquid (IL), cholinium lactate, to allow the high quality spatiotemporal recordings of forearms motions and activities (article number 2100374). Benjamin Chee Keong Tee and co-workers describe a new conductor of eHelix, a highly stretchable and reliable conductor composite made from helical copper wires and a soft elastomer, with robust and deformation-insensitive electrical conductivity (article number 2100221). The eHelix was successfully employed and validated its usages in several different devices, including wearable smart fabrics, a heart rate monitor, and a tactile sensing glove. While there are lots of recent advances in materials and device innovations toward wearable and implantable sensors and electronics, the ultimate sensing and continuous monitoring call for sustainable, bio-compatible, or battery-free medical devices. Seung Hwan Ko et al. summarize the progress of batteryless, wearable devices based on a variety of non-battery energy sources, such as electromagnetic energy, mechanical energy, biofuels, triboelectricity, thermoelectricity and wireless power transfer (article number 2002286). The power solution is even more critical and challenging in implantable devices compared with wearables. Wei Lan and co-workers outline the different energy sources, including 1) energy storage devices of batteries and supercapacitors; 2) energy-harvesting devices of biofuel cells, piezoelectric and triboelectric energy harvesters, thermoelectric and biopotential power generators; and 3) wireless power transfer devices based on inductive coupling, ultrasound, and photovoltaics (article number 2100199). In particular, wireless power transfer is considered to be favorable for many implantable devices. Jacob Robinson and co-workers review six different types of widely reported wireless power transfer technologies in implants, including inductive coupling, radiofrequency, mid-field, ultrasound, magnetoelectrics, and light (article number 2100664). Different technologies are summarized with respect to several critical tradeoffs or design considerations in power, miniaturization, depth, alignment tolerance, transmitter distance, and safety. In another review article, Dae-Hyeong Kim and co-workers summarize the recent advances in RF-based wireless power transfer and telemetry (for physiological data acquisition) for implantable bioelectronics to address the challenges resulted from the constraint and requirement for in vivo operation (article number 2100614). Michael A. Daniele and co-workers provide a review of the state-of-the-art ultrasound-powered implants as a promising wireless power transfer (article number 2100986). Different piezoelectric materials and harvesting devices including lead-based inorganic, lead-free inorganic, and organic polymers are summarized and their performance metrics and applications are presented. One key advantage of soft, flexible and wearable sensors is that they are capable of continuously tracking physiological parameters that are closely linked to an individual's health state. One key example application is cardiac activity monitoring to mitigate the incidence of and mortality caused by cardiovascular diseases. Chwee Teck Lim and co-workers provide an overview of recent progress in the development of flexible wearable sensors for personalized monitoring of cardiovascular biomarkers including electrocardiography, heart rate, blood pressure, blood oxygen saturation, and blood glucose (article number 2100116). Given the critical role of systolic and diastolic blood pressure monitoring for guiding clinical decision-making in the pediatric intensive care unit, the application of soft skin-interfaced wearable devices (optimized by careful selection of materials and mechanical designs) for wireless continuous tracking of systolic and diastolic blood pressure on pediatric patients is demonstrated by John A. Rogers et al. (article number 2100383). In addition to cardiac activities, wearable sensors can also be applied to monitor a number of biomarkers including humidity, temperature, as well as muscle and brain activities. An integrated wearable and flexible system consisting of a ZnIn2S4 nanosheet-based humidity sensor and a carbon nanotube/SnO2 temperature sensor is presented by Kuniharu Takei and co-workers and successfully evaluated on healthy volunteers to investigate the thermoregulatory responses under cold stimulation and exercise toward potential early prediction of thermoregulation disorders in the human body (article number 2100103). While commercially available wearable devices and most current research efforts are focused on monitoring health-related physical parameters, continuous tracking of molecules represents a major gap and opportunity to reveal a full picture of health. Joseph Wang and co-workers review the current progress, emerging trends, and unmet challenges of using microneedles as a minimally-invasive and effective means to access interstitial fluid for real-time monitoring of metabolites, electrolytes, and drugs (article number 2002255). Continuous monitoring of bodily fluid biomarkers could enable personalized medicine by tailoring therapeutics according to the real-time collected biomarker information. A great example here is closed-loop diabetes care in which one can tune the drug delivery based on the blood glucose levels. With a focus on materials and construction, Chi Hwan Lee summarize the recent innovations of wearable glucose monitoring and implantable insulin delivery approaches toward improved personalized diabetes management (article number 2100194). Successful translation of current proof-of-concept wearable and implantable technologies requires extensive human studies. In a perspective article, Wei Gao and co-workers present relevant ethical concerns on various aspects of early-stage human research (including reliability and validity, risk assessment, subject selection, data privacy and security, and informed consent) to evaluate the wearable prototypes from a researcher's perspective (article number 2100127). Machine learning is a promising approach to analyze and process the large sets of physiological data collected by the wearable biosensors from human studies. Xiaodong Chen and co-workers summarize different types of non-invasive biosensors and physiological signals collected from the human body, and review the machine learning algorithms used in data processing toward practical clinical practice and public health applications (article number 2100734). One could expect that, with further technological developments coupled with ethically conducted human validation, wearable and implantable devices could be readily applicable for both medical diagnosis and daily health monitoring in tackling a number of health conditions. This collection of reviews, research articles, and perspective in this special issue clearly show the fast advancement in materials and device technologies and also their usages and impacts in many clinical applications. Yet, as the past decade witnessed drastic advancement in this field, this collection only presents a small fraction of them. We expect that this special issue will spur a broader research community of science, engineering, biology, and medicine to join forces toward further, continuous development and innovations in the years to come. We would like to express our gratitude to all authors for their efforts and hard work to make this special issue possible. We also would like to thank those who we have not been able to include here due to the volume limitation of this special issue. Our deep appreciation finally goes to Dr. Irem Bayindir-Buchhalter and Dr. Conor Doss for their insights in recognizing the importance of this special issue topic, and all Wiley-VCH staff for their endless support in the editorial process. The authors declare no conflict of interest. Wei Gao is an assistant professor of medical engineering at the California Institute of Technology. He received his Ph.D. in chemical engineering at the University of California, San Diego in 2014. In 2014–2017, he was a postdoctoral fellow in the Electrical Engineering and Computer Sciences Department at the University of California, Berkeley. His research interests include wearable biosensors, digital medicine, micro/nanorobotics, bioelectronics, and nanomedicine. Cunjiang Yu is currently the Bill D. Cook Associate Professor in the Department of Mechanical Engineering at the University of Houston. He received his Ph.D. degree in mechanical engineering from Arizona State University in 2010. He was a postdoctoral fellow in the Department of Materials Science and Engineering at the University of Illinois at Urbana – Champaign from 2010 to 2013. His lab concerns the fundamental and application aspects of soft electronics and bioelectronics.