Wearables have garnered significant attention in recent years not only as consumer electronics for entertainment, communications, and commerce but also for real-time continuous health monitoring. This has been spurred by advances in flexible sensors, transistors, energy storage, and harvesting devices to replace the traditional, bulky, and rigid electronic devices. However, engineering smart wearables that can seamlessly integrate with the human body is a daunting task. Some of the key material attributes that are challenging to meet are skin conformability, breathability, and biocompatibility while providing tunability of its mechanical, electrical, and chemical properties. Electrospinning has emerged as a versatile platform that can potentially address these challenges by fabricating nanofibers with tunable properties from a polymer base. In this article, we review advances in wearable electronic devices and systems that are developed using electrospinning. We cover various applications in multiple fields including healthcare, biomedicine, and energy. We review the ability to tune the electrical, physiochemical, and mechanical properties of the nanofibers underlying these applications and illustrate strategies that enable integration of these nanofibers with human skin.

In this article, we present a magnetically-tunable quad-band filter with high tunability in the frequency range of 2.1-3.9 GHz. A multi-band filter with four stop-bands comprises of a microstrip line coupled to four frequency-selective split-ring resonators (SRRs). We achieve tuning of individual frequency bands using magnetic reed switches connected in between the capacitive gaps of each split-ring resonator. Application of magnetic field tunes this capacitance affecting its resonance frequency. The measured reflection spectrum of the proposed device matches well with the simulation results. The results show more than 25% tunability for each of the four bands with bandwidth values in the range of 30-70 MHz with over 100% overall tunability in the 2.1-3.9 GHz frequency spectrum. Metamaterials are artificially engineered structures that can be designed for achieving tailored electromagnetic properties that do not occur naturally 1. In this respect, there has been a growing interest on the design of different metamaterials over the past couple decades. Key applications of metamaterials include the design of an effective medium with a negative magnetic permeability, perfect lens, perfect absorbers or imagers 2-5. The ability to manipulate and control permeability and permittivity in metamaterials has been used to realize innovative microwave and millimeter-wave circuits and sensing platforms 6-16. Hence, metamaterials can also be utilized in microwaves and antenna systems 17-22. Yet some other applications where tunable metamaterials have been realized and applied are in the area of optical or plasmonic metamaterials with applications in biosensing 23 and optics 24,25. In that sense, plasmonic metamaterials can be implemented in order to get negative refraction and negative refractive index 26,27. Metamaterials are built using individual resonators of which split-ring resonators (SRRs) are one of the most commonly used. Adjusting the dimensions such as the length and the gap of the SRR enables one to realize metamaterials with resonances from microwave to optical frequencies. Moreover, one can actively tune the resonance frequency by incorporating a tunable circuit element such as a transistor, diode or a varactor 5,28,29. Emerging new standards and demand for wireless communications have necessitated transceivers that can work across multiple frequency bands. A key element in such transceivers is compact multi-band filters with an ability to support multiple standards 30. Circuit elements such as diodes, varactors, MEMS switches have been demonstrated for frequency tuning of filters; however, such filters support only a few bands with limited tuning range or exhibit high insertion loss 31,32. CMOS-based active switches, diodes and varactors suffer from intermodulation and harmonics due to the undesirable nonlinear properties of such active devices at high power and high frequencies. Moreover, active switches have off-state leakage, and are sensitive to the process-voltage-temperature (PVT) variations resulting in u...

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