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The vulcanization properties, physicochemical, dielectric, and electrical properties of natural rubber-based composites were investigated and compared. The composites contain 30–70 phr functional fillers having low (silicon dioxide (SiO2)) and high (titanium dioxide (TiO2)) dielectric constant values. The possibilities of using the composites as substrates in compact flexible antennas have been evaluated. Fillers are also characterized. It has been shown that they have a noticeable but different influence on all the properties of the composites studied. That is primarily due to the different structure and specific properties of SiO2 and TiO2, thereby determining the different "filler–filler" and "rubber–filler" interactions. The composite filled with TiO2 at 70 phr has a better capacity to act as a substrate in body area network antennas than the SiO2-filled composites. An antenna with such a substrate has features that meet all the requirements of the industrial, science, and medical spectrum. TiO2 composites do not change their resistance to pressure or deflection within certain limits, having a self-cleaning and antibacterial effect, which is also beneficial for their usage in antennas that are left on or near the human body. [ABSTRACT FROM AUTHOR]

We propose a new broadband flexible waterproof antenna suitable for Internet of Things applications. The antenna is comprised of three components: radiating elements fed by a 50-Ω coplanar waveguide transmission line and two layers from an elastomer composite (used as a substrate and top encapsulation) which completely covers the antenna's radiating elements to provide water and dust resistance. An antenna prototype is fabricated and tested. Experimental and numerical results show that the proposed antenna has broad bandwidth (|S11| ≤ −10 dB) over the frequency range of 0.83–2.52 GHz, high radiation efficiency of about 92% and good radiation patterns. The antenna was also immersed into water for 10 s, 10, 30 and 60 min, respectively and reflection coefficient magnitudes were measured, to verify its waterproof capability. The durability and flexibility tests were performed on the antenna prototype. Results show that there is no significant degradation in the antenna performance. [ABSTRACT FROM AUTHOR]

Glass ceramics from the CaO–P2O5–SiO2 system were synthesized. They were characterized by XRD, SEM and EDX methods. It was found that they contain two crystalline [Whitlockite, Ca3 (PO4)2 and Hydroxylapatite, Ca5 (PO4)3 (OH)] and one residual amorphous phase. The ceramics are composed of porous aggregates of fine particles with predominant macro- and mesopores in it. The ceramics were used as a functional filler in natural rubber based composites, as the filling is in the range of 5–50 phr. The influence of the chemical nature and structure of the ceramics and the degree of filling on the vulcanization characteristics, physicomechanical parameters, dielectric and dynamic properties, the coefficient of thermal conductivity and the volume resistivity, including external pressure or bending on the specimens were investigated. An influence of the filler and the degree of filling on all studied properties was found. All tested composites have dielectric parameters (ε′, real part of complex permittivity; ε′′, imaginary part of complex permittivity; σ, conductivity; tanδɛ, tangent from dielectric loss angle) suitable for use as substrates and insulating layers of flexible antennas. Particularly impressive are composites containing 30 and 50 phr fillers, which have the lowest tangent from dielectric loss angle values. [ABSTRACT FROM AUTHOR]

Wearable antennas exhibit numerous challenges in terms of design and optimization due to the specific environment in which they operate. Therefore, the design of such antennas is a non-trivial task, as multiple constraints have to be satisfied. We present an algorithm for design-and-optimization of flexible wearable antennas with high radiation efficiency and low specific absorption rate, that takes into account the dielectric loading of the human body. It provides a list of feasible antenna designs, not just a single solution, and identifies the optimal wearable antenna design. Numerical examples on the design and optimization of a wearable antenna (based on a dipole structure with a reflector) to demonstrate the validity and efficiency of the proposed algorithm are given. The optimal antenna design shows robust on-body performance and provides a suitable balance between small antenna size (antenna surface is 2214 mm²), high radiation efficiency (57.73%), and low value of the maximum 10 g average SAR (0.112 W/kg) on a homogeneous semisolid phantom. Finally, the optimal antenna design is fabricated. The antenna performance is studied under different conditions: on a homogeneous semisolid phantom, on a liquid phantom, on a three-layer semisolid phantom, on a human arm and in the free space. A good agreement between simulated and measured antenna performance is observed. [ABSTRACT FROM AUTHOR]