Characterization of path loss and absorption for a wireless radio frequency link between an in-body endoscopy capsule and a receiver outside the body
- Karen Lopez-Linares Roman†1,
- Günter Vermeeren†1,
- Arno Thielens1,
- Wout Joseph1Email author and
- Luc Martens1
© Lopez-Linares Roman et al.; licensee Springer. 2014
Received: 1 October 2013
Accepted: 6 January 2014
Published: 1 February 2014
Physical-layer characterization is important for design of in-to-out body communication for wireless body area networks (WBANs). This paper numerically investigates the path loss and absorption of an in-to-out body radio frequency (RF) wireless link between an endoscopy capsule and a receiver outside the body using a 3D electromagnetic solver. A spiral antenna in the endoscopy capsule is tuned to operate in the Medical Implant Communication Service (MICS) band at 402 MHz, accounting for the properties of the human body. The influence of misalignment, rotation of the capsule, and three different human models are investigated. Semi-empirical path loss models for various homogeneous tissues and 3D realistic human body models are provided for manufacturers to evaluate the performance of in-body to out-body WBAN systems. The specific absorption rate (SAR) in homogeneous and heterogeneous body models is characterized and compliance is investigated.
A wireless body area network (WBAN) connects nodes that are situated on or in the body of a person. Applications of WBANs include medicine and health care, sports, and multimedia. As it facilitates movement among users, it has brought about a revolutionary change in patient monitoring and healthcare facilities. Active implants placed within the human body lead to better and faster diagnosis, thus, improving the quality of life of the patient. The development of an endoscopy capsule system enables the examination of areas of the small intestine that cannot be seen by other types of endoscopy. The benefits of the capsule endoscopy in terms of a better diagnosis are obvious. Also, the patient’s comfort improves, as there are no wires or tubes involved in the procedure.
The characterization of the physical layer of the network is an important step in the development of a WBAN. A lot of studies investigated on-body propagation [1–11]. Less literature is available on modeling of propagation loss within or in-to-out the human body [12–17] and often the focus is on 2.4 GHz. A path loss (PL) model for in-body wireless implants (biocompatible implantable antennas) is proposed in . Scenarios for channel modeling for an endoscopy application are proposed in . In  and  similar investigations were presented, but both of them proposed the 2.4 GHz frequency band, which led to a smaller antenna. In addition, in-body to in-body communication is examined. The influence of different tissues at various frequencies is characterized for an in-to-out channel in , but the antenna design was not specified. A multi-implant scenario at 2.4 GHz is investigated in  using insulated dipole antennas for specific locations such as the liver, heart, spleen, and the kidneys. A spiral antenna at 450 MHz for ingestible capsule endoscope systems is designed in .
The objective of this paper is to investigate numerically the PL of an in-to-out body radio frequency (RF) wireless link between an endoscopy capsule and a half-wavelength dipole (reference receiver) outside the body using a 3D electromagnetic solver, applying the finite difference time domain (FDTD) method. This study focusses on the 402 to 405 MHz Medical Implant Communication Service (MICS) band. The spiral antenna for an endoscopy capsule operating at 450 MHz proposed by  is selected as the transmitter and tuned to operate at 402 MHz by changing its dimensions. This spiral antenna fits inside a capsule and is experimentally validated in a human phantom and a pig under general anesthesia. Therefore, we selected this antenna for our propagation analysis. Both antennas are dimensioned with the FDTD simulation platform SEMCAD-X. Path loss for the in-to-out channel and the specific absorption rate (SAR) are investigated for homogeneous and heterogeneous tissues of three human models, namely, an adult, an obese adult, and a child. The influence of the dielectric properties of the tissue on the PL is evaluated and compliance of the electromagnetic absorption with the international guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP)  is evaluated for the maximum allowed radiated power.
The analysis of this research can be used by manufacturers to evaluate the performance of in-body to out-body WBAN systems and to carry out link budget calculations, since it is difficult for them to test their systems on actual humans. This research also enables to better estimate the energy consumption of the endoscopy capsule and thus, the battery lifetime, which is limited due to the small size of the pills. Another application of the models of this paper is cross-layer design, where the proposed models are used to evaluate communication protocols for WBANs and the energy consumption in short-range wireless networks .
The wireless connection between the devices of a WBAN can occur at different frequencies. Most published models use the industrial, scientific, and medical (ISM) frequency bands or UWB (ultra-wideband). Among these, the most commonly used frequencies in medical communications are 402 MHz, 868 MHz, and 2.4 GHz. The choice of the optimum frequency is an important factor taking into account attenuation, tissue conductivity, and also antenna size and orientation, which can affect not only radiation inside the body but can also determine the optimum distance of which good performance can be achieved in the surrounding environment. The MICS band is a licensed band used for implant communication and has the same frequency range (402 to 405 MHz) in most countries .
The MICS band does not support high data rate applications. The ISM 2.4 GHz band supports higher data rate applications and is available worldwide. However, there are high chances of interference as many wireless devices operate at ISM band and a higher attenuation in the body occurs due to the higher frequency. This results in a need to increase the transmitting power, which is a disadvantage in cases where the antenna is embedded within the human body, leading to higher power consumption and possible non-compliance with international exposure guidelines . Thus, MICS is a lower power frequency band, with less risk to encounter interferences. It allows longer communication links because the attenuation is lower. A disadvantage is that the size of the antennas increases. According to the authors’ knowledge, there is only a small amount of studies about in-to-out body propagation for this band.
Heterogeneous phantom models
Three heterogeneous models are investigated: a 6-year-old boy (denoted Thelonius), an adult man (denoted Duke), and an obese adult man (denoted Fats) . The Virtual Population models are based on magnetic resonance images (MRI) of healthy volunteers. The tissue properties for the frequency of 402 MHz are assigned from Gabriel’s database  and a maximum grid step in the body of 2 mm is used.
In this paper, it is not the goal to design suitable in-body antennas but to characterize the propagation from in-to-out the body. As a receiving antenna outside the body, a half-wavelength dipole is selected. The free-space dipole is modeled and tuned to resonate at 402 MHz. The thickness of the perfect electric conductor (PEC) structures is set at 2 mm and the gap is 1 mm. With respect to the transmitting antenna selection inside the body, PIFA (planar inverted-F antenna), dual-band microstrip patch antennas, and spiral antennas are often used for medical applications. In general, PIFA antennas are employed in artificial cardiac pacemakers and difficult to fit inside a pill. The size of the antenna is a factor of importance at 402 MHz. The spiral antenna  is the smallest among the different options and therefore it is selected for the investigation.
Spiral antenna in capsule
The length L is an important parameter in the design of the spiral because it determines the resonance frequency. In addition, the spiral has to fit in the capsule. In this case, an antenna with 4.98-mm radius is modeled.
The thickness of the shell is 0.25 mm. The ground plane has a radius of 5 mm and a height of 0.5 mm. The capsule is closed at the bottom by a circular Ultem sheet with a thickness of 0.5 mm.
Spiral antenna characteristics
Gap size (g)
S11 at 402 MHz
−5 dB Bandwidth
(muscle, skin, esophagus)
Path loss in homogeneous tissues
This section discusses the in-to-out body propagation and path loss for homogeneous tissues. PL models are presented and the influence of misalignment, rotation in the body, and the absorption are determined. We analyze here first homogeneous tissues because the influence of different parameters (dielectric tissue properties, tissue separation, alignment, etc.) can be investigated separately. This allows manufacturers to select a tissue with the highest absorption and thus highest path losses. Moreover, in this paper we aim at comparing the propagation in homogeneous tissues with the heterogeneous models. In contrary to layered tissues, homogeneous tissues can be replaced by liquids in practical setups to evaluate the performance of endoscopy capsules and systems of manufacturers.
Analysis of in-to-out body propagation
where the total antenna separation d is expressed in millimeter in this paper, PL0,dB is the path loss in decibel at a reference distance d0 (250 mm in this paper), and n [-] is the path loss exponent, which equals to 2 in the free space.
Influence of antenna separation for different tissues
Figure 6b shows the PL when moving the dipole away from the spiral. When the distance between the antennas is 160.5 mm, the dipole is only 10.5 mm from the tissue surface (Figure 1). At this distance, the coupling between dipole and tissue-air interface decreases the PL. For further separations the PL increases again.
Parameters for path loss models for homogeneous and heterogeneous tissues and phantoms ( d 0 =250 mm)
Muscle, aligned antennas
Skin, aligned antennas
Esophagus, aligned antennas
Esophagus, misaligned antennas
Influence of antenna misalignment
Influence of antenna rotation
The effect of the rotation around this axis is larger because the spiral changes from pointing to the dipole to pointing directly to the opposite direction. Also, the y- and z-components of the electric field E is higher than the x-component of E (Figure 1), so the effect of the rotation around y is larger.
Absorption: specific absorption rate
Path loss for heterogeneous phantom models
Path loss models
Figure 10 shows that the path loss values for Duke (adult) are higher than the path loss values for Thelonius (child). This is due to the position of the dipole antenna (see Section ‘Configurations’), since for the child Thelonius it is placed in front of the body and for the adult Duke it is positioned at one side with the arm between the two antennas. The tissues in the arm add additional losses. Also the distance between body and dipole is smaller for Thelonius (13.5 cm) than for Duke (18.5 cm). The path loss for misalignment in the homogeneous esophagus tissue can be considered as the worst case, as the PL values are higher than those for the adult man configuration.
Table 2 summarizes for n and PL0 values obtained using linear regression fitting (Equation 2). The PL exponents for Thelonius and Duke are similar and about 7 to 8 in Table 2. In the heterogeneous tissues, the path loss exponent is higher than the one in homogeneous tissues (about equal to 3) because of the influence of the surrounding tissues and the mixture of alignment and misalignment situations. The path loss is considerably higher when the antennas are misaligned (doubling of the path loss exponent n and shift of PL0,dB of 7 dB when misaligned for the esophagus). The regression models for the heterogeneous phantoms have R2 values (a coefficient of determination) of about 0.7, which is good. An F-test concluded that the regression is significant, i.e., n≠0, at the 5% significance level (F=78−81, p<10−9). Standard deviations of 2.0 and 3.0 dB for the child and adult model are obtained, respectively. Finally, Table 2 shows that larger standard deviations σ correspond with lower R2 values. These regression models can be used for link budget calculations for in-to-out body communications.
Because of the proximity of the human body, antennas cannot be separated from the wireless propagation loss. Therefore, using the method of , one can calculate the maximum allowable path loss for sensor modules operating in the MICS band. With the maximum permitted level of −16 dBm EIRP (equivalent isotropically radiated power) or 25 μ W and a sensitivity level of −90 dBm for 152 kbps and −99 dBm for 51 kbps , a nominal maximum path loss (PLmax) of 74 dB for 152 kbps and 83 dB for 51 kbps is obtained, respectively. When comparing these maximum allowable path losses with the path loss models obtained in Figure 10, one can conclude that up to 400-mm separation wireless communication for the heterogeneous phantoms (adult and child) is possible for both data rates for the considered configurations (for the adult configuration higher separations can be difficult). However, for misalignment in the esophagus tissue, separations up to about 293 mm are possible for 152 kbps. The provided models of Table 2 can be used for further link budget calculations accounting for misalignment and rotation of the endoscopy capsule. To specifically account for the rotation, one can add a margin of 4 dB (x-axis) or 10 dB (y-axis) in the link budget. Also the application of the model with misaligned antennas is a possibility. A rotation of the capsule can result in a variation of the path loss up to 14 dB (10 and 4 dB margins).
Comparison of path loss for adult and obese adult men
Figure 10 also compares the obese human model (Fats) with the other tissues. In general, larger path loss is observed in the obese model due to the larger separations between transmit and receive antenna and the larger distance the electromagnetic waves have to propagate in the lossy human body tissue. We remark that a comparison of the PL in two human body models purely on the basis of the separation distance is not straightforward: for the same separation in Duke and Fats, the endoscopy capsule in Fats can be horizontally aligned with the dipole outside the human body and, thus, no misalignment loss is present. But in the Duke, the capsule will be positioned somewhere else in the body; the endoscopy capsule and dipole outside the body will not be horizontally aligned and misalignment loss occurs, which can result in a larger PL than for the obese model. Moreover, in the particular cases of Duke and Fats, when the dipole and the capsule are horizontally aligned, the propagation distance of the electromagnetic waves through the arm of Duke is about 2 cm longer, because near the wrists the arm of Fats is 2 cm smaller than the arm of Duke.
The mean and the maximum path loss in Fats equals to 70 and 82 dB, respectively. With respect to the adult this is 64 and 76 dB. This analysis shows that one has to be careful when making link budget evaluations: communication is possible for Fats for the considered configuration of 51 kbps.
Figure 9b shows also the psaSAR for the adult and child human body models (heterogeneous tissues) along the samples of the digestive tract. There are of course more samples for the adult (Duke) than for the child (Thelonius) because of the larger dimensions of the adult and thus longer tract. The absorption values are averaged in 10-g mass for an input power of 25 μ W. PsaSAR values for both phantoms are similar and compliant with the ICNIRP guidelines is obtained (psaSAR 10 g values are below 2 W/kg). The highest values are about 2.2 mW/kg for the maximum allowed transmission power of 25 μ W. Figure 9a,b has the same scale, enabling visual comparison. The absorption values in heterogeneous tissues are comparable (slightly higher maximum) with the absorptions for the homogeneous muscle tissue. Peaks for the heterogeneous models occur due to the proximity of tissue boundaries.
Path loss and absorption of an in-to-out body radio frequency (RF) wireless link between an endoscopy capsule and a receiver outside the body are numerically investigated. Three relevant homogeneous tissues (muscle tissue, skin tissue, and esophagus) and three heterogeneous human body models of an adult, obese adult, and a child are considered. A spiral antenna in the endoscopy capsule is tuned to operate in the MICS band at 402 MHz accounting for the properties of the human body. The misalignment between antennas causes an increase of the path loss exponent from 3 to 7 and an additional shift of 7 dB in the esophagus tissue. An increase in the path loss due to the rotation of the antenna up to 10 dB is observed. Path loss models for different homogeneous tissues and two realistic human body models are provided, enabling manufacturers to evaluate the performance of in-body to out-body WBAN systems. Path loss exponents in heterogeneous models are about 7 to 8 and higher than for homogeneous tissues. The obtained absorption values are around 2.2 mW/kg for the maximum allowed transmission power in the MICS band, which is about 1,000 times lower than the ICNIRP guidelines for localized absorption both in homogeneous and heterogeneous tissues. In this paper three realistic heterogeneous phantom models are investigated. As there are many variations among people, future research should account for morphology.
WJ is a post-doctoral fellow of the FWO-V (Research Foundation - Flanders).
This research is partly funded by the Fund for Scientific Research - Flanders (FWO-V, Belgium) project G.0049.09N.
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