An antenna is a device that beams EM waves into free space. Typically, the antenna is powered by a transmission line, such as a microstrip line or a coaxial cable, which transmits signals from the transmission source to the antenna . Over the past few years, several types of medical applications that use antennas have been thoroughly investigated and reported, including diagnosis and treatment of different chronic diseases . Among the existing types, the Yagi-Uda antenna, or simply Yagi antenna, features high directivity and, consequently, high gain in the direction of maximum energy radiation. This antenna consists of multiple director elements, which conduct the electromagnetic signal to the half-wavelength dipole, the element through which the signal is injected, called the radiator element, and a reflector element placed behind the half-wave dipole. Hence, the half-wave dipole is powered by the transmitter signal, which generates a current distribution along its length, resulting in the radiation of electromagnetic waves. Next, given their mutual coupling with the closest element, the director elements guide the electromagnetic radiation due to the current induction generated by the previous elements .
In this way, the pair of antennas used on Osseus corresponds to the model WA5VJB (Figure 2), marketed by Kent Electronics. This pair was designed to operate in a 2.4-2.48 GHz frequency range, and it has a radiation diagram, as indicated in Figure 3.
Additionally, it is typical for electronic circuits near the antennas to be sensitive to electromagnetic interference (EMI), and it is necessary to protect them from this effect. The technique used for such a process is called shielding, and it can be accomplished by several means. One of the simplest shielding methods for external interference is to surround the circuit with a metal box, usually made out of aluminum, responsible for reflecting these EM waves and thus preventing them from reaching the circuits.
Shielding can attenuate an electromagnetic wave. Such attenuation occurs through the reflection of the incident wave due to the use of conductive material in the shielding, like iron, steel, copper, or aluminum, the latter being more versatile and low cost. Then, its efficiency is calculated as a ratio of the power inside and that outside the shielding .
With the need to protect measurement equipment, applying EM shielding has become relevant, especially in electronic devices used in the medical field. Those are composed of sensitive analog amplifiers, microprocessors that can be affected by EM interference, among others.
Internally, there is a need to prevent the signal radiated by the transmitting antenna from reflecting off on the metallic walls of the box and returning to the electronic circuitry. Therefore, it makes it necessary to coat the metal box internally with an absorbing material. Electromagnetic absorbers are composed of materials that absorb the incident EM radiation in specific frequency ranges and release it as heat. These materials are obtained from the appropriate processing of polymeric matrices that act as absorbing centers for the incident radiation. This study used Eccosorb AN microwave absorbers designed to strongly attenuate a specific frequency range, confining the signal within the box and avoiding successive reflections on the metallic walls. Such absorbers are made of polyurethane foam treated with carbon and mounted on a laminated surface to generate a controlled conductivity gradient .
When it comes to Osseus, the processor is another significant device. Processors are devices of a few square centimeters in size and with high processing power. They can operate analog-to-digital conversions, with the ability to deliver results in a graphical environment over a computer network, perform data analysis, and compute them in a neural network by receiving programming instructions.
Osseus integrates a microcontroller with other peripheral devices to perform the tasks necessary to measure EM permittivity. Figure 4 depicts the flow of the device's operation. Thus, an operator fills in the patient's characteristics on an electronic form, performs the signal attenuation measurement, with no barrier between the antennas, selecting the point (frequency of the injected signal) of highest received power so that it can serve for reference or calibration. Then, the permittivity test is performed on the patient's finger. After that, the equipment (figure 5) displays the values obtained and sends the data for cloud storage and post-processing.
The device, through one of its antennas, emits an EM wave in a predefined frequency range. Then, on the opposite side, a similar antenna receives that wave, attenuated by the obstacle corresponding to the patient's finger—specifically, the middle phalanx of the middle finger. A circuit coupled to the antenna identifies the analog signal strength and converts it into a digital signal. Then, based on the patient's characteristics and the signal attenuation, the microcontroller can process the information to indicate whether or not the individual needs to get a DXA scan.
For generating the targeted frequency, we used a Voltage-controlled Oscillator (VCO). This component can generate a sinusoidal signal at a frequency that depends on the voltage applied to its input, and it is located at the transmitter side of the signal. On the opposite side, an RF Power Detector is connected to the receiving antenna. The sensor converts the received RF intensity into voltage so that it can be processed.
To obtain the desired directivity, we used a pair of Yagi Uda antennas printed on a fiberglass substrate, with dimensions of 6.6 cm × 7.0 cm, one being a transmitter and the other a receiver, operating in the frequency range of 2.45 GHz , fed by a microstrip line. After the prototype's three-dimensional modeling, we proceeded with the fabrication of the components. Part of it was manufactured with a 1.6 mm thick aluminum, ensuring a high standard of EM shielding, another in translucent acrylic, and the remaining components in a 3D printer. On the inside, the equipment has been coated with absorbing material designed for a 2.44 GHz frequency.
Proof of Concept
We have performed simulations to prove the operation of the device using samples of orthodontic plaster cylinders simulating human bone and encased in porcine skin. This kind of skin has been recommended for studies since it has physiological, histological, biochemical similarities and density akin to human skin. Each plaster cylinder was built with only water and plaster, 2 cm in diameter and 7.5 cm in height.
Next, we used the femur bones of Galliformes since, besides representing the first bird order associated with humans, such birds have relatively long femur bones. Moreover, we also used bones from the porcine proximal phalanx, given their remarkable similarity to human bone tissue and bones from cattle femurs since they satisfactorily reproduce aspects of human anatomy. First, the samples were classified and analyzed while still intact (original sample) and after undergoing mechanical perforations to increase porosity (altered samples - Figure 6) artificially. According to the perforations, the samples have undergone modifications in their attenuation measurements.