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Diagnostic platforms are becoming increasingly sophisticated in their ability to monitor a variety of biomarkers across different biofluids. Current laboratory gold standard techniques including liquid chromatography/mass spectrometry (LC/MS), enzyme-linked immunosorbent assays (ELISA), and polymerase chain reaction (PCR) typically require costly instrumentation, complex protocols, and long time-to-results. Point-of-care (POC) devices seek to simplify sample processing and analysis using more compact and automated devices. Point-of-care approaches have already revolutionized the healthcare landscape by enabling more rapid and decentralized diagnosis while reducing costs and the requirement for trained personnel. Nonetheless, there is a significant push to take diagnostic devices beyond POC directly to the level of the individual. Wearable, or point-of-person (POP) devices, have made their way into the mainstream with products like the Apple Watch, Fitbit, and Oura Ring.While wearable devices have been met with commercial success and interest from the healthcare community and the public, they are mostly limited to monitoring physiological markers such as heart rate, temperature, and sleep activity. There is great interest in advancing wearable devices to monitor a variety biochemical species including metabolites, proteins, nucleic acids, neurotransmitters, and hormones. The ability to detect biochemical biomarkers in readily sampled biofluids like saliva, sweat, tears, and interstitial fluid has the potential to enable continuous and minimally invasive monitoring. Wearable devices can also facilitate personalized medicine, whereby there is an ever-increasing shift to provide feedback and therapies tailored to individuals based on their specific biometrics, as opposed to the long-standing one-size-fits-all approach to modern medicine. My dissertation details four projects I focused on during my doctoral research that seek to address four different primary challenges

The very recent concept of metaverse highlights the importance of virtual reality (VR) and augmented reality (AR), which associates with a wide variety of applications in entertainment, medical treatment, and human-machine interfaces. The current VR/AR technologies mainly rely on visual interaction, while immersive experience in VR and AR highly demands sensational feedback, such as haptic and temperature with noticeable quality in wearable or even skin-integrated formarts. In this article, we report a wearable and flexible haptic interface based on electromagnetic vibrotactile actuators with high wearability and stability. By adopting double layers of copper (Cu) coils at the top and bottom of the magnetic disc, an enhanced electromagnetic field can be generated. Additionally, the intensity of the haptic feedback can be modulated according to sensed pressure in the virtual world by adjusting the value of power input and frequency. The actuator exhibits high stability and tolerance upon environmental, cyclic, and impact resistance tests. Finally, the actuators are developed into the soft VR interfaces for mounting on forearms, fingers, and hands to verify their superiority over conventional haptic actuators in the aspects of performance and applications. © 2014 IEEE.

Wearable piezoresistive sensors are being developed as electronic skins (E-skin) for broad applications in human physiological monitoring and soft robotics. Tactile sensors with sufficient sensitivities, durability, and large dynamic ranges are required to replicate this critical component of the somatosensory system. Multiple micro/nanostructures, materials, and sensing modalities have been reported to address this need. However, a trade-off arises between device performance and device complexity. Inspired by the microstructure of the spinosum at the dermo epidermal junction in skin, a low-cost, scalable, and high-performance piezoresistive sensor is developed with high sensitivity (0.144 kPa-1 ), extensive sensing range ( 0.1-15 kPa), fast response time (less than 150 ms), and excellent long-term stability (over 1000 cycles). Furthermore, the piezoresistive functionality of the device is realized via a flexible transparent electrode (FTE) using a highly stable reduced graphene oxide self-wrapped copper nanowire network. The developed nanowire-based spinosum microstructured FTEs are amenable to wearable electronics applications.

Self-healing materials can prolong the lifetime of structures and products by enabling the repairing of damage. However, detecting the damage and the progress of the healing process remains an important issue. In this study, self-healing, piezoresistive strain sensor fibers (ShSFs) are used for detecting strain deformation and damage in a self-healing elastomeric matrix. The ShSFs were embedded in the self-healing matrix for the development of self-healing sensor fiber composites (ShSFC) with elongation at break values of up to 100%. A quadruple hydrogen-bonded supramolecular elastomer was used as a matrix material. The ShSFCs exhibited a reproducible and monotonic response. The ShSFCs were investigated for use as sensorized electronic skin on 3D-printed soft robotic modules, such as bending actuators. Depending on the bending actuator module, the electronic skin was loaded under either compression (pneumatic-based module) or tension (tendon-based module). In both configurations, the ShSFs could be successfully used as deformation sensors, and in addition, detect the presence of damage based on the sensor signal drift. The sensor under tension showed better recovery of the signal after healing, and smaller signal relaxation. Even with the complete severing of the fiber, the piezoresistive properties returned after the healing, but in that case, thermal heat treatment was required. With their resilient response and self-healing properties, the supramolecular fiber composites can be used for the next generation of soft robotic modules.

The smart flexible devices that can implement both tactile and touchless sensing in real-time have attracted abundant interest due to the high demand for human-machine interaction (HMI), internet of things (IoT) and virtual reality (VR) system. However, obtaining the sensor that can precisely monitor signals via contact/non-contact modes without overlapping while in a facile and cost-effective methodology is still a challenge. Herein, we introduce a flexible dual-mode capacitive sensor for pressure (contact) and magnetic field (non-contact) detection based on magnetic tilted micropillar array (MTMPA) via a facile and tunable methodology. The MTMPA sandwiched in AgNW/PDMS electrodes can response to the external pressure and magnetic field with bidirectional deflection, allowing the sensor to precisely distinguish different stimuli in real-time without overlapping. With the optimal structure, the sensor exhibits high performance of magnetic response with sensitivity of −0.69 T-1, detection limit of 25 mT and excellent recoverability. The sensor also exhibits high pressure sensitivity of 0.301 kPa−1 (0–2 kPa) with the ultra-low detection limit of 1.2 Pa with excellent repeatability and mechanical robustness. As a proof of concept, practical wearable tactile applications (e.g. pulse-sensing, voice recognition, finger touching, pressure mapping and dynamic tracking) and touchless demonstrations such as magnetic field identification and trajectory tracking have been presented. The sensitive response of the sensor to external magnetic fields also enables us to customize the non-contact Braille recognition system through arranging specific magnet patterns, where the characters such as ‘a’, ‘b’, ‘c’, and ‘d’ have been well distinguished without direct contact. The results experimentally prove the profound significance of the methodology for potential uses towards health monitoring, body motion feedback, and human-machine interface, etc.

This paper reviews current progress on flexible and stretchable transistors and sensors for the next-generation multiplex electronics (commonly referred to as 'electronics skin' or 'e-skin') that is capable of simultaneous detection of multiple information from a variety of surfaces including human skin. Flexible chemical sensors for sweat analysis as well as physical sensors for detecting tactile force, bending, and temperature will be discussed, with emphasis on materials, detection mechanisms, and device demonstration to realize multiplex human-interactive devices. Next, system integration enabling the real-time monitoring of health conditions is also demonstrated as a proof of concept. Finally, perspectives on e-skin for moving toward realizing practical wearable electronics in the market are discussed. This paper targets the translation of the nano- and flexible technologies from academic innovations to industrial practical applications with high-impact breakthroughs.

Pressure distributions can be measured using many single force transducers, typically arranged in matrix configurations. Contrary to industrial applications, the mechanical interaction of the human body requires sensors that adapt to soft and pliable interfaces. Capacitive, piezoelectric, and resistive transducers are commonly used for force measurements. Modeling of the human body and its interaction with the environment often requires input of force and/or pressure distribution information. Pressure distribution patterns are closely related to the perception of comfort. Therefore, pressure distribution information is used in the design of footwear-, chair-, and sleeping mattress constructions. Athletic shoe companies use pressure measurements to improve footwear for preventing injuries and enhance performance. In the medical field, plantar pressures are measured for patients with insensitive feet to prevent ulceration (e.g., in diabetic feet). Similarly, pressure monitoring is required for patients who have to stay in bed or wheelchairs for extended periods of time. More recently, pressure distribution measurements are applied to the robotics field. The detection of pressure distribution of complex objects is necessary to mimic the handgrip of humans. For the development of electronic skin, the tactile sensing properties of natural skin have to be simulated by large arrays of miniaturized force sensors.

Electronic skins (E-skins) with sensory capabilities mimicking human-skin or beyond are essential for wearable healthcare monitoring devices, robotic technologies, human-machine interface (HMI) and artificial intelligence (AI). The ability to rapidly detect small pressure variation represents an important function of E-skins. Here we report a new design of pressure-sensing E-skins by integrating a unique conductive microstructured air-gap (CMAG) gate with two-dimensional (2D) semiconductor transistors to achieve unprecedented combination of ultrahigh sensitivity, rapid response, low power consumption and long-term stability. We first show that the CMAG can be used to create the capacitor-based E-skins with a highest sensitivity up to 770.4 kPa-1 (vs. ~ 4.5 kPa-1 reported previously). By employing the CMAGs as pressure sensitive gate for 2D MoS2 transistors, we show that the pressure sensitivity can be further amplified to achieve a highest value of ~ 2.61 � 107 kPa-1 (vs. the previous record of 192 kPa-1). Along with ultrafast response (< 0.1 ms), lower power consumption (~ 9 pW−270 nW), low minimum pressure detection (< 0.05 Pa) and excellent stability, our device delivers an overall performance well above the existing pressure-sensing E-skins, enabling highly sensitive static pressure mapping, real-time human pulse wave measurements, E-skin microphone for speech pattern recognition, and remote pressure monitoring.

Moravec's paradox shows that low-level sensor-imotor skills are more difficult than high-level reasoning in artificial intelligence and robotics. So simplifying every sensing unit on electronic skin is critical for endowing intelligent robots with tactile and temperature sense. The human nervous system is characterized by efficient single-electrode signal transmission, ensuring the efficiency and reliability of information transmission under big data conditions. In this work, we report a sensor based on a single-electrode piezoelectric nanogenerator (SPENG) by electrospun polyvinylidene fluoride (PVDF) nanofibers that can realize steady-state sensing of pressure integrating cold/heat sensing on a single unit. Piezoelectric signals appear as square wave signals, and the thermal-sensing signals appear as pulse signals. Therefore, the two signals can be acquired by a single unit simultaneously. The SPENG overcomes the shortcoming of electronic skins based on a single-electrode triboelectric nanogenerator (STENG), which can sense only dynamic movement and cannot sense temperature variations. The new sensor configuration uses a capacitor instead of the STENG's ground wire as a potential reference, allowing it to be used for truly autonomous robots. At the same time, the traditional advantages of polymer piezoelectric materials, such as flexibility, transparency, and self-powered advantages, have also been preserved.

This dissertation presents the research on tactile sensing with compliant structures towards human-robot interaction. It would be beneficial for robots working collaboratively with humans to be soft or padded and have compliant tactile sensing skins over the padding. To allow the robots to interact with humans via touch effectively and safely and to detect tactile stimuli in an unstructured environment, new tactile sensing concepts are needed that can detect a wide range of potential interactions and sense over an area. However, most highly sensitive tactile sensors are unable to cover the forces involved in human contacts, which ranges from 1 newton to thousand newtons; to implement area sensing capabilities, there have been challenges in creating traditional sensing arrays, where the associated supporting electronics become more complex with an increasing number of sensing elements. This dissertation develops a novel multi-layer cutaneous tactile sensing architecture for enhanced sensitivity and range, and employs an imaging technique based on boundary measurements called electrical impedance tomography (EIT) to achieve area tactile sensing capabilities. The multi-layer cutaneous tactile sensing architecture, which consists of stretchable piezoresistive strain-sensing layers over foam padding layers of different stiffness, allows for both sufficient sensitivity and an extended force range for human contacts. The role that the padding layer plays when placed under a stretchable sensing layer was investigated, and it was discovered that the padding layer magnifies the sensor signal under indentation compared to that obtained without padding layers. The roles of the multi-layer foams were investigated by changing stiffness and thickness, which allows tailoring the response of multi-layer architectures for different applications. To achieve both extended force range and distributed sensing, EIT technique was employed with the multi-layer sensing architecture. Machine

Printing technologies offer large-area, high-throughput production capabilities for electronics and sensors on mechanically flexible substrates that can conformally cover different surfaces. These capabilities enable a wide range of new applications such as low-cost disposable electronics for health monitoring and wearables, extremely large format electronic displays, interactive wallpapers, and sensing arrays. Solution-processed carbon nanotubes have been shown to be a promising candidate for such printing processes, offering stable devices with high performance. Here, recent progress made in printed carbon nanotube electronics is discussed in terms of materials, processing, devices, and applications. Research challenges and opportunities moving forward from processing and system-level integration points of view are also discussed for enabling practical applications.

Printed electronics and sensors enable new applications ranging from low-cost disposable analytical devices to large-area sensor networks. Recent progress in printed carbon nanotube electronics in terms of materials, processing, devices, and applications is discussed on page 4397 by A. Javey and co-workers. The research challenges and opportunities regarding the processing and system-level integration are also discussed for enabling of practical applications.

Inorganic nanomembranes are shapeable (flexible, printable and even stretchable) and transferrable to virtually any substrate. These properties build the core concept for new technologies, which transform otherwise rigid high-speed devices into their shapeable counterparts. This research is motivated by the eagerness of consumer electronics towards being thin, lightweight, flexible, and even wearable. The realization of this concept requires all building blocks as we know them from rigid electronics (e.g. active elements, optoelectronics, magnetoelectronics, energy storage) to be replicated in the form of (multi)functional nanomembranes, which can be reshaped on demand after fabrication. There are already a variety of shapeable devices commercially available, i.e. electronic displays, energy storage elements, and integrated circuitry to name a few. From the beginning, the main focus was on the fabrication of shapeable high-speed electronics and optoelectronics. Only very recently, a new member featuring magnetic functionalities was added to the family of shapeable electronics. With their unique mechanical properties, the shapeable magnetic field sensor elements readily conform to ubiquitous objects of arbitrary shapes including the human skin. This feature leads electronic skin systems beyond imitating the characteristics of its natural archetype and extends their cognition to static and dynamic magnetic fields that by no means can be perceived by human beings naturally. Various application fields of shapeable magnetoelectronics are proposed. The developed sensor platform can equip soft electronic systems with navigation, orientation, motion tracking and touchless control capabilities. A variety of novel technologies, like smart textiles, soft robotics and actuators, active medical implants and soft consumer electronics will benefit from these new magnetic functionalities. This review reflects the establishment of shapeable magnetic sensorics, describing the entire

This paper reports the radio frequency (RF) performance of InAs nanomembrane transistors on both mechanically rigid and flexible substrates. We have employed a self-aligned device architecture by using a T-shaped gate structure to fabricate high performance InAs metal-oxide-semiconductor field effect transistors (MOSFETs) with channel lengths down to 75 nm. RF measurements reveal that the InAs devices made on a silicon substrate exhibit a cutoff frequency (f t) of approx. 165 GHz, which is one of the best results achieved in III-V MOSFETs on silicon. Similarly, the devices fabricated on a bendable polyimide substrate provide a f(t) of approx. 105 GHz, representing the best performance achieved for transistors fabricated directly on mechanically flexible substrates. The results demonstrate the potential of III-V -on-insulator platform for extremely high-frequency (EHF) electronics on both conventional silicon and flexible substrates., Published in the Journal of Nano Letters, v12 n8 p4140-4145, 2012. Supported in part by DARPA, DOE and AFOSR Grant no. FA9550-10-1-0113.

Touch (or tactile) sensors are gaining renewed interest as the level of sophistication in the application of minimally invasive surgery and humanoid robots increases. The spatial resolution of current large-area tactile sensors (greater than 1 cm2) lag human fingers by over an order of magnitude. Using metal and semiconducting nanoparticles, a ~100 nm thick, large area thin-film device working on the principles of electron tunneling is self-assembled, such that the change in current density through the film and the electroluminescence light intensity are linearly proportional to the local stress. By pressing a United States 1 cent coin (and also a copper grid) on the device a well resolved stress image by focusing the electroluminescence light directly on CCD is obtained. Both the lateral and height resolution of texture are comparable to human finger at similar stress levels of ~10 KPa. The fabrication of the film is based on self-assembly of polyelectrolytes, and metal and semiconducting nanoparticles in a layered architecture. The polyelectrolyte layer functions as the dielectric tunneling barrier and the nanoparticles function as the base for tunneling electrons. The assembly of the device can be simplified by incorporating the functionality of the polyelectrolyte and the nanoparticles in a single composite medium. A non-micellar mineralization process for the synthesis of multifunctional nanocomposite materials is also reported as a possible building block for the assembly of tactile sensor. The non-micellar method results in the synthesis of monodisperse semi-conducting nanoparticles templated on polymer chains dissolved in solution at high yield. The monodispersity is achieved due to the beaded necklace morphology of the polyelectrolyte chains in solution where the beads are nanometer-scale nodules in the polymer chain and the nanoparticles are confined to the beads. The resultant structure is a nanoparticle studded necklace where the particles are imbedded i