High-frequency electromagnetic waves are greatly scattered and attenuated by soil, rocks, water, and the human body, which impede their propagation across these media. However, low-frequency magnetic waves propagate with low attenuations through these dispersive media as long as these media are nonmagnetic. Unlike static electric fields that can be produced by “free” positive/negative charges, static magnetic fields are divergence-free and thus can be produced only by particles with noninteger spins (i.e., electrons), or permanent magnets, or current loops. Fields produced by these dipoles decay steeply as a third-order function of distance. Hence, magnetic current loops require excessively large currents to produce detectable magnetic field at large distances. However, rare-earth magnets with surface magnetic flux densities of 0.3–0.6 T, provide sufficiently large fields at large distances, thus the fields produced by these magnets that can be modulated by mechanically modulating the magnets for portable wireless magnetic field communication. Here, we present the development of an electromechanically modulated permanent magnet antenna operating at a frequency of 22 Hz, producing a magnetic flux density of $0.6~\mu \text{T}$ amplitude at 1 m. This antenna requires a starting energy of 0.48 J and it is capable of data rate of 8 b/s. This magnet antenna can reliably transmit binary data over a distance of up to 70 cm through obstacles that would severely hamper electromagnetic wave propagation.

This paper presents two novel techniques for monitoring the response of smart hydrogels composed of synthetic organic materials that can be engineered to respond (swell or shrink, change conductivity and optical properties) to specific chemicals, biomolecules or external stimuli. The first technique uses microwaves both in contact and remote monitoring of the hydrogel as it responds to chemicals. This method is of great interest because it can be used to non-invasively monitor the response of subcutaneously implanted hydrogels to blood chemicals such as oxygen and glucose. The second technique uses a metal-oxide-hydrogel field-effect transistor (MOHFET) and its associated current-voltage characteristics to monitor the hydrogel’s response to different chemicals. MOHFET can be easily integrated with on-board telemetry electronics for applications in implantable biosensors or it can be used as a transistor in an oscillator circuit where the oscillation frequency of the circuit depends on the analyte concentration.

We report for the first time the observation of low intensity (~10⁵ photons/cm².s) soft X-rays (1-30 KeV) from flowing gases (N₂, Ne, He, and Ar) in capillaries and piezoelectric lead zirconate titanate (PZT) crystals at room temperature. The no-slip boundary condition of gas flow at the capillary boundary may lead to friction between the gas molecules and surface dangling bonds of SiO₂ causing chemical bond formation and breakage that generates soft x-rays. The X-ray spectrum of the gas was Gaussian with no characteristic emission lines. The spectrum shape was dependent on the velocity of the flowing gas with the x-ray mean energy and full-width-at-half-maximum being inversely related to each other. The PZT x-ray also did not have any characteristic emission lines, and its mean energy depended on the amplitude and the type of excitation voltage (square, sinusoidal and pulse) waveforms applied to the PZT.