With a constantly growing percentage of the population having access to high-quality healthcare facilities, preventable pathogenic illnesses have been nearly eradicated in the developed parts of the world, which has led to a significant rise in the average human life expectancy over the last few decades. In such a highly developed world, age-related illnesses will lead to an immense burden on healthcare providers. Remote health monitoring enabled by wearable sensors will play a significant role in the growth and evolution of Health 3.0 by providing intimate and valuable information to healthcare providers regarding the progression of disease in patients with critical life-altering conditions. Especially, in the case of people suffering from neurodegenerative disorders, inexpensive and user-friendly wearable sensors can enable physiotherapists monitor real-time physiological parameters to design patient-specific treatment plans. This review provides a comprehensive overview of the recent advances and emerging trends at the convergence of biomimicry and nanomaterial sensors, with a specific focus on wearable skin-inspired mechanical sensors for applications in IoT-enabled human physiological parameters monitoring. Skin-inspired wearable mechanical sensors with relevance to the most common types of sensing mechanisms including piezoresistive, piezocapacitive, and triboelectric sensing are discussed along with their current challenges and possible future opportunities.@en

The development of lithium–sulfur (Li–S) batteries is hindered by the disadvantages of shuttling of polysulfides and the sluggish redox kinetics of the conversion of sulfur species during discharge and charge. Herein, the crystallinities of a titanium nitride (TiN) film on copper-embedded carbon nanofibers (Cu-CNFs) are regulated and the nanofibers are used as interlayers to resolve the aforementioned crucial issues. A low-crystalline TiN-coated Cu-CNF (L-TiN-Cu-CNF) interlayer is compared with its highly crystalline counterpart (H-TiN-Cu-CNFs). It is demonstrated that the L-TiN coating not only strengthens the chemical adsorption toward polysulfides but also greatly accelerates the electrochemical conversion of polysulfides. Due to robust carbon frameworks and enhanced kinetics, impressive high-rate performance at 2 C (913 mAh g −1 based on sulfur) as well as remarkable cyclic stability up to 300 cycles (626 mAh g −1) with capacity retention of 46.5% is realized for L-TiN-Cu-CNF interlayer-configured Li–S batteries. Even under high loading (3.8 mg cm −2) of sulfur and relatively lean electrolyte (10 μL electrolyte per milligram sulfur) conditions, the Li–S battery equipped with L-TiN-Cu-CNF interlayers delivers a high capacity of 1144 mAh g −1 with cathodic capacity of 4.25 mAh cm −2 at 0.1 C, providing a potential pathway toward the design of multifunctional interlayers for highly efficient Li–S batteries.@en

Self-powered wearable devices are gaining increasing attention to fulfil the sensing and energy harvesting needs for the next-generation wearable systems and body sensor networks. Here, we developed a wearable, flexible and lightweight nanofiber-based triboelectric nanogenearator (NF-TENG) for body motion energy harvesting. Polyvinylidene difluoride (PVDF) and nylon were electrospun into nanofiber films to form the two triboelectric materials. Under a load force of 10 N, the NF-TENG could generate an output voltage of 140 V, current of 0.5 μA and transferred charges of 50 nC. Preliminary body energy harvesting applications of the proposed NF-TENG were demonstrated, wherein the NF-TENG was able to generate an output voltage of 8 V and 10 V during typing and elbow bending, respectively.@en

During the past few decades, a significant amount of research effort has been dedicated toward developing skin-inspired sensors for real-time human motion monitoring and next-generation robotic devices. Although several flexible and wearable sensors have been developed in the past, the need of the hour is developing accurate, reliable, sophisticated, facile yet inexpensive flexible sensors coupled with neuromorphic systems or spiking neural networks to encode tactile information without the need for complex digital architectures, thus achieving true skin-like sensing with limited resources. In this work, we propose an approach entailing carbon nanofiber-polydimethylsiloxane composite-based piezoresistive sensors, coupled with spiking neural networks, to mimic skin-like sensing. The strain and pressure sensors have been combined with appropriately designed neural networks to encode analog voltages to spikes to recreate bioinspired tactile sensing and proprioception. To further validate the proprioceptive capability of the system, a gesture tracking smart glove, combined with a spiking neural network, was demonstrated. Wearable and flexible sensors with accompanying neural networks such as the ones proposed in this work will pave the way for a future generation of skin-mimetic sensors for advanced prosthetic devices, apparel integrable smart sensors for human motion monitoring, and human-machine interfaces.@en

Recently, 3D porous graphene-polymer composite-based piezoresistive sensors have gained significant traction in the field of flexible electronics owing to their ultralightweight nature, high compressability, robustness, and excellent electromechanical properties. In this work, we present an improved facile recipe for developing repeatable, reliable, and linear 3D graphene-polydimethylsiloxane (PDMS) spongy sensors for internet of things (IoT)-enabled wearable systems and smart consumer products. Fundamental morphological characterization and sensing performance assessment of the piezoresistive 3D graphene-polymer sensor were conducted to establish its suitability for the development of squeezable, flexible, and skin-mountable human motion sensors. The density and porosity of the sponges were determined to be 250 mg cm-3 and 74% respectively. Mechanical compressive loading tests conducted on the sensors revealed an average elastic modulus as low as ∼56.7 kPa. Dynamic compressive force-resistance change response tests conducted on four identical sensors revealed a linear piezoresistive response (in the compressive load range 0.42-2.18 N) with an average force sensitivity of 0.3470 ± 0.0794 N-1. In addition, an accelerated lifetime test comprising 1500 compressive loading cycles (at 3.90 N uniaxial compressive loading) was conducted to demonstrate the long-term reliability and stability of the sensor. To test the applicability of the sensors in smart wearables, four identical graphene-PDMS sponges were configured on the fingertip regions of a soft nitrile glove to develop a pressure sensing smart glove for real-time haptic pressure monitoring. Similarly, the sensors were also integrated into the Philips 9000 series electric shaver to realize smart shaving applications with the ability to monitor shaving motions. Furthermore, the readiness of our system for next-generation IoT-enabled applications was demonstrated by integrating the smart glove with an embedded system software utilizing the an open source microcontroller platform. The system was capable of identifying real-time qualitative pressure distribution across the fingertips while grasping daily life objects, thus establishing the suitability of such sensors for next-generation wearables for prosthetics, consumer devices, and personalized healthcare monitoring devices.@en

In this work, we report a class of fabric-like flexible and wearable capacitive sensors utilizing electrospun polyvinyl acetate (PV Ac)-graphene nanofibers for human motion monitoring applications. The sensor reported in this work features flexible PV Ac-graphene thin film sandwiched between two layers of conductive fabric and fully encapsulated in transparent adhesive films. The fabricated pressure sensor demonstrates two linear regimes with sensitivity figures of 0.0136 kPa-1 (pressure < 75 kPa) and 0.0063 kPa-1 (pressure > 75 kPa), respectively. The realized pressure sensor is flexible and apparel integrable, and the facile fabrication method will enable reliable and repeatable performance over roll-to-roll manufacturing suitable for mass production.@en

This work reports the fabrication of a titanium carbide nanoparticle-based inkjet printed flexible bidirectional flow sensor. The design of the flow sensor consists of an inkjet printed titanium carbide piezoresistive strain gauge on a polyester cantilever. The sensors demonstrated a normalized flow sensitivity of 1.043/(ms-1) in the velocity range 0.15 - 0.55 m/s (for water flow). The fabrication method reported in this work potentially opens a new direction for fabrication of a class of robust, repeatable, and inexpensive flexible flow sensors.@en

Flexible piezocapacitive sensors utilizing nanomaterial-polymer composite-based nanofibrous membranes offer an attractive alternative to more traditional piezoelectric and piezoresistive wearable sensors owing to their ultralow powered nature, fast response, low hysteresis, and insensitivity to temperature change. In this work, we propose a facile method of fabricating electrospun graphene-dispersed PVAc nanofibrous membrane-based piezocapacitive sensors for applications in IoT-enabled wearables and human physiological function monitoring. A series of electrical and material characterization experiments were conducted on both the pristine and graphene-dispersed PVAc nanofibers to understand the effect of graphene addition on nanofiber morphology, dielectric response, and pressure sensing performance. Dynamic uniaxial pressure sensing performance evaluation tests were conducted on the pristine and graphene-loaded PVAc nanofibrous membrane-based sensors for understanding the effect of two-dimensional (2D) nanofiller addition on pressure sensing performance. A marked increase in the dielectric constant and pressure sensing performance was observed for graphene-loaded spin coated membrane and nanofiber webs respectively, and subsequently the micro dipole formation model was invoked to explain the nanofiller-induced dielectric constant enhancement. The robustness and reliability of the sensor have been underscored by conducting accelerated lifetime assessment experiments entailing at least 3000 cycles of periodic tactile force loading. A series of tests involving human physiological parameter monitoring were conducted to underscore the applicability of the proposed sensor for IoT-enabled personalized health care, soft robotics, and next-generation prosthetic devices. Finally, the easy degradability of the sensing elements is demonstrated to emphasize their suitability for transient electronics applications.@en

In this work, we report a class of wearable, stitchable, and sensitive carbon nanofiber (CNF)-polydimethylsiloxane (PDMS) composite-based piezoresistive sensors realized by carbonizing electrospun polyacrylonitrile (PAN) nanofibers and subsequently embedding in PDMS elastomeric thin films. Electro-mechanical tactile sensing characterization of the resulting piezoresistive strain sensors revealed a linear response with an average force sensitivity of ~1.82 kN−1 for normal forces up to 20 N. The real-time functionality of the CNF-PDMS composite sensors in wearable body sensor networks and advanced bionic skin applications was demonstrated through human motion and gesture monitoring experiments. A skin-inspired artificial soft sensor capable of demonstrating proprioceptive and tactile sensory perception utilizing CNF bundles has been shown. Furthermore, a 16-point pressure-sensitive flexible sensor array mimicking slow adapting low threshold mechanoreceptors of glabrous skin was demonstrated. Such devices in tandem with neuromorphic circuits can potentially recreate the sense of touch in robotic arms and restore somatosensory perception in amputees.@en

With growing life expectancy and a rise in population with sedentary lifestyles, the load on existing healthcare infrastructure is expected to increase manifolds in the next two decades. As per National Institute for Public Health and the Environment (RIVM) the number of centenarians will quadruple by 2040 [1]. The need of the hour is robust, reliable, sensitive, and inexpensive wearable sensors capable of providing intimate and detailed information regarding a person’s physiological parameters. However, it is also critical that such sensors are environmentally friendly and that production is sustainable. In this work, we present graphene-polyvinyl acetate (PVAc) electrospun nanofibrous membrane-based degradable piezocapacitive sensors for applications in wearables. The sensor featured in this work comprises of 0.25 wt.% graphene-loaded electrospun nanofiber membrane sandwiched between two layers of fabric-based flexible electrodes. The degradability of the sensors has been demonstrated previously [2]. It is expected that sensors similar to the one presented here will gain widespread acceptance for future applications in flexible electronics and wearable devices. @en

In this age of advanced smartphones and wearable devices, the need of unlimited power has become a basic necessity. Most of the gadgets rely on some sort of power source in the form of batteries or power adapters. For example, smart watches have become very common these days and have a huge potential for implementation of energy harvesters. In near future it will be really desirable to have self-powered smart wearable devices which meet their energy needs by scavenging mechanical energy produced by physical activities. In order to solve the problem of fast battery depletion in modern smart devices, a lot of research has been carried out in the field of energy harvesters especially using thin film technologies and polymer nanofibers. Most interesting among them are the nanogenerators using polymers with piezoelectric properties like PVDF due to their low production cost and high conversion efficiency. Polymer-based nanofiber energy harvesters are not only relevant for wearable devices and smartphones but also for biomedical energy scavenging applications primarily due to their biocompatibility. Chapter 2 particularly deals with current scenario of different types of nanofiber-based energy harvesters. A comprehensive review related to current research work going on in the field of nanofiber-based energy harvesters is presented here.@en

Chapter 3 presents a comprehensive review of the various biomimetic self-powered and low-powered MEMS pressure and flow sensors that take inspiration from the biological flow sensors found in the marine world. The sensing performance of the biological flow sensors in marine animals has inspired engineers and scientists to develop efficient state-of-the-art sensors for a variety of real-life applications. In an attempt to achieve high-performance artificial flow sensors, researchers have mimicked the morphology, sensing principle, materials, and functionality of the biological sensors. Inspiration was derived from the survival hydrodynamics featured by various marine animals to develop sensors for sensing tasks in underwater vehicles. The mechanoreceptors of crocodiles have inspired the development of slowly and rapidly adapting MEMS sensory domes for passive underwater sensing. Likewise, the lateral line sensing system in fishes which is capable of generating a three-dimensional map of the surroundings was mimicked to achieve artificial hydrodynamic vision on underwater vehicles. Harbor seals are known to achieve high sensitivity in sensing flows within the wake street of a swimming fish due to the undulatory geometry of the whiskers. Whisker inspired structures were embedded into MEMS sensing membranes to understand their vortex shedding behavior. At the outset, this work comprehensively reviews the sensing mechanisms observed in fishes, crocodiles, and harbor seals. In addition, this chapter presents an in-depth commentary on the recent developments in this area where different researchers have taken inspiration from these aforementioned underwater creatures and developed some of the most efficient artificial sensing systems.@en