The development of wearable sensor devices brings significant benefits to patients by offering real-time healthcare via wireless body area networks (WBANs). These wearable devices have gained significant traction due to advantageous features, including their lightweight nature, comfortable feel, stretchability, flexibility, low power consumption, and cost-effectiveness. Wearable devices play a pivotal role in healthcare, defence, sports, health monitoring, disease detection, and subject tracking. However, the irregular nature of the human body poses a significant challenge in the design of such wearable systems. This manuscript provides a comprehensive review of recent advancements in wearable and flexible smart sensor devices that can support the next generation of such sensor devices. Further, the development of direct ink writing (DIW) and direct writing (DW) methods has revolutionised new high-resolution integrated smart structures, enabling the design of next-generation soft, flexible, and stretchable wearable sensor devices. Recognising the importance of keeping academia and industry informed about cutting-edge technology and time-efficient fabrication tools, this manuscript also provides a thorough overview of the latest progress in various fabrication methods for wearable sensor devices utilised in WBAN and their evaluation using body phantoms. An overview of emerging challenges and future research directions is also discussed in the conclusion.

A compact, conformal, all-textile wearable antenna is proposed in this paper for the 2.45 GHz ISM (Industrial, Scientific and Medical) band. The integrated design consists of a monopole radiator backed by a 2 × 1 Electromagnetic Band Gap (EBG) array, resulting in a small form factor suitable for wristband applications. An EBG unit cell is optimized to work in the desired operating band, the results of which are further explored to achieve bandwidth maximization via floating EBG ground. A monopole radiator is made to work in association with the EBG layer to produce the resonance in the ISM band with plausible radiation characteristics. The fabricated design is tested for free space performance analysis and subjected to human body loading. The proposed antenna design achieves bandwidth of 2.39 GHz to 2.54 GHz with a compact footprint of 35.4 × 82.4 mm². The experimental investigations reveal that the reported design adequately retains its performance while operating in close proximity to human beings. The presented Specific Absorption Rate (SAR) analysis reveals 0.297 W/kg calculated at 0.5 W input power, which certifies that the proposed antenna is safe for use in wearable devices.

.This paper presents a microstrip patch antenna array fed with a low loss and highly efficient feeding structure, called Empty Substrate Integrated Waveguide (ESIW). The proposed antenna array targets 5G wireless communication systems operating at 28 GHz. The antenna array consists of 4 × 4 radiating patches where every four radiating elements are grouped together forming a 2 × 2 subarray and each of the subarray is fed by ESIW feeding network by aperture coupling. Experimental results demonstrate good agreement with the simulated results, offering a wide |S11| < -10 dB bandwidth of 12.4%, i.e., 26.5 — 30 GHz. A peak gain of 18.2 dBi and a high efficiency of about 91% is achieved using the proposed structure. Furthermore, the low-profile substrate of the ESIW feeding structure makes seamless integration of proposed antenna array with the millimetre-wave front-end. The advantages, such as low fabrication cost and high efficiency, make the proposed antenna array a suitable solution for millimetre-wave 5G wireless communication systems.

The suspended triple-layer graphene modulator is firstly investigated theoretically. We find there appear two modulation depths for electro-absorption modulation. The light-graphene interaction is enhanced to its maximum by our designed waveguide structure. The highest modulation depth for electro-absorption modulation can be 0.834 dB/µm, causing a 3-dB footprint of only 0.94 µm². For electro-refractive modulation, there appear several 100% modulations with a much smaller π-phase shift length of only 11.3 µm. This modulator also shows great potential for high-speed modulation with a prediction value of 759.85 GHz, while the switch energy can be as low as 0.61 fJ/bit with low applied voltage. Moreover, the verification simulation by COMSOL is also presented, which shows very good agreement with our calculation results, and the figure of merit (defined as the ratio of modulation depth to insertion loss) of this modulator can be 2105. We believe these results can pave the way to design practical high-speed, compact-footprint, and high-efficiency devices.

A Rectangular Dielectric Resonator Antenna (RDRA) design for mmWave-frequency-band MIMO metrics is proposed, with a compact, low-complexity, high-gain structure that is easy to fabricate and offers reduced inter–port isolation. The RDRA operates in the mmWave spectrum, featuring a compact size of 1.307${\lambda }_0$ × 1.307${\lambda }_0$, an impedance bandwidth of 6%, and a resonant frequency of 28 GHz, with a peak gain of 7 dBi. A four element MIMO system iteration was developed while maintaining the performance of the single element antenna. Additionally, a simple, low-complexity slot-etching technique was applied to achieve an average inter-port element isolation of 14 dB. The design also achieved a novel four-beam petal-splitting radiation pattern. The MIMO metrics, with an envelope correlation coefficient (ECC) of <0.5 and a diversity gain (DG) < 10, were successfully met. The simulated and measured results are in good agreement.

Theory of five kinds of layered structure THz waveguides is presented. In these waveguides, the modified and hybrid THz surface plasmon-polaritons (SPPs) are researched in detail. On these modes, the effects of material in each layer are discussed. The anti-resonant reflecting mechanism is also discussed in these waveguides. The mode characteristics of both TM mode and TE mode are analyzed for guiding TM mode with low loss and TE modes with huge loss in one waveguide: the TE modes filter application is put forward. The mode characteristics for one waveguide have useful sensor applications: for TE₁ mode, we find that the low cut-off frequency has a sensitivity (S) to the refractive index of the dielectric slab. The highest S can be 666.7 GHz/RIU when n₂ = 1.5, w = 0 and t = 0.1 mm. We believe these results are very useful for designing practical THz devices for SPPs, filter and sensor applications.