In-body path loss models for implants in heterogeneous human tissues using implantable slot dipole conformal flexible antennas
© Kurup et al; licensee Springer. 2011
Received: 18 October 2010
Accepted: 3 August 2011
Published: 3 August 2011
A wireless body area network (WBAN) consists of a wireless network with devices placed close to, attached on, or implanted into the human body. Wireless communication within a human body experiences loss in the form of attenuation and absorption. A path loss model is necessary to account for these losses. In this article, path loss is studied in the heterogeneous anatomical model of a 6-year male child from the Virtual Family using an implantable slot dipole conformal flexible antenna and an in-body path loss model is proposed at 2.45 GHz with application to implants in a human body. The model is based on 3D electromagnetic simulations and is compared to models in a homogeneous muscle tissue medium.
A wireless body area network (WBAN) is a network, consisting of nodes that communicate wirelessly and are located on or in the body of a person. These nodes form a network that extends over the body of the person. Depending on the implementation, the nodes consist of sensors and actuators, placed in a star or multihop topology .
Applications of WBANs include medicine, sports, military, and multimedia, which make use of the freedom of movement provided by the WBAN. As WBAN facilitates unconstrained movement amongst users, it has brought a revolutionary change in patient monitoring and health care facilities. Active implants placed within the human body lead to better and faster diagnosis, thus improving the patient's quality of life. Implantable devices are increasingly proving their importance for biomedical applications. The use of active implants allows vital medical data to be collected over a longer period in the natural environment of the patient, allowing for a more accurate and sometimes even faster diagnosis. Active implants such as pacemakers and implantable cardioverter defibrillators (ICDs) need to relay information to other devices for control or monitoring . Thus a proper and efficient modeling of the channel is required to transfer data between implants and other devices. Moreover, the human body is a lossy medium which attenuates the waves propagating from the transmitter (Tx) considerably before they reach the receiver (Rx). Thus, to design an optimal communication link between nodes placed within or on the human body a proper and efficient path loss (PL) model is required.
To our knowledge very limited literature exists on propagation loss within the human body [2–6]. In  initial results of an in-body propagation model in saline water is presented. Inaccuracies lead to maximum deviations of 9 dB between the measurements and simulations. Also only a homogeneous medium is studied and there are no models available for heterogeneous medium.  considers a non-insulated hertzian dipole, hence the PL model can only be applied to very small dipole antennas.  provides various scenarios for channel modeling but does not provide a model for path loss.  discusses a link budget for an implanted cavity slot antenna at 2.45 GHz. However, no model for a heterogeneous medium is suggested that can be used for path loss simulation.  suggests a PL model for in-body wireless implants. However, it does not make use of biocompatible implantable antennas.  suggests a PL model for in-body wireless implants by making use of insulated dipole antennas in a homogeneous medium.
The goal of this article is to develop an empirical PL model for a heterogeneous medium, using implantable antennas, that describes the relationships between the PL, the distance between the antennas, and the power attenuation. Since it is difficult to carry out measurements in the human body, implantable antennas are designed by taking the dielectric properties of human muscle tissue into consideration.
Simulations are performed at 2.45 GHz in the license free industrial, scientific, and medical (ISM) band. This frequency band is chosen since there are no licensing issues in this band and the higher frequency allows the use of a smaller antenna. Moreover, 2.45 GHz allows higher bitrates due to the larger bandwidth . After carrying out the simulations in human muscle tissue, simulations are carried out in a heterogeneous medium for various scenarios using an enhanced anatomical model of a 6-year-old male child from the Virtual Family (Christ, in preparation). We use a child model because in children with evidence of internal bleeding and abdominal pain, correct diagnosis is a challenge and capsule endoscopy can be used for the diagnosis of such ailments. Capsule endoscopy has been accepted in adults by many gastroenterologists, however its usage in children has lagged due to the belief by pediatricians that the pills are too large to be swallowed by children [8, 9]. However, reports do suggest that children as young as two and a half years old are successfully undergoing capsule endoscopy, and most of the studies suggest that majority of pediatric patients can swallow the pill [10, 11].
The PL model developed in this article focuses on deep tissue implants, such as endoscopy capsules. In such applications the implants are placed deep inside the body, which we have selected up to a distance of 8 cm. A PL model will help in understanding the influence of the dielectric properties of the surrounding tissues and the power attenuation of such implants. As it is difficult for the manufacturers to test their system on actual humans, the proposed model can be used by them to evaluate the performance of in-body WBAN systems using well specified setups and to carry out link budget calculations.
The outline of this article is as follows. The setup and configuration of the simulations in the homogeneous muscle tissue medium and the heterogeneous human model are discussed in Sects. II and III, respectively. Section IV discusses the results including the reflection coefficient and the path loss of the implanted antennas in human muscle tissue medium and the heterogeneous model. Section V presents the conclusions.
Homogeneous Tissue: Human Muscle Tissue
A. Setup and configuration
Size of the folded slot dipole antenna
Simulations are performed using a 3D electromagnetic solver SEMCAD-X (SPEAG, Switzerland), a finite-difference time-domain (FDTD) program. SEMCAD-X enables non-uniform gridding. The maximum grid step in the muscle tissue medium is 1 mm at 2.45 GHz. The simulations are carried out using the implantable antennas up to a distance of 8 cm. The muscle tissue is modeled by using a cube (dimensions 150 × 150 × 280 mm3) with the dielectric properties of human muscle tissue. The implantable antennas are aligned for maximum power transfer and the source used is a voltage source.
A. Setup and configuration
Scenario I-- Esophagus: For each scenario, the Tx and the Rx are placed at three different positions. The first position is at location 1 as shown in the Figure 2. Here the Tx antenna is placed in the esophagus (εr = 62.15 and σ = 2.2 S/m) of the VFB and the receiving antenna is placed at a separation of 1 cm from the Tx up to 5 cm in steps of 5 mm as shown in the Figure 2. The Rx antenna traverses through the lungs (εr = 34.42 and σ = 1.24 S/m) in this position. Position 2 in this scenario is such that the Tx and Rx antennas are placed 1 cm below location 1 and this is indicated as location 2 in the Figure 2. In position 3 the Tx and Rx antennas are placed 2 cm below location 1 and are indicated as location 3 in the Figure 2. At all these positions the Rx antenna again traverses through the lungs.
Scenario II--Stomach: Here, the implantable antenna is placed such that the Tx lies in the stomach and the Rx moves from 1 cm to 2 cm through the stomach lumen (εr = 52.72 and σ = 1.74 S/m), which is enclosed by stomach (εr = 62.16 and σ = 2.21 S/m), and then moves partially into the liver (εr = 54.81 and σ = 2.25 S/m) starting from 3 cm and and then entirely up to 8 cm as shown in location 4 in Figure 2. Position 2 and position 3 in this scenario are such that the Tx and Rx antennas are placed 1 cm and 2 cm below location 4 indicated as location 5 and location 6 in the Figure 2.
Scenario III--Small intestine: In the first position of this scenario the Tx antenna is placed in the small intestine (εr = 54.42 and σ = 3.17 S/m at 2.45 GHz) of the VFB as shown in Figure 2 at location 7. The Rx antenna is placed starting from 1 cm up to a separation of 8 cm from the Tx antenna. In this position the Rx antenna traverses through various tissues such as the kidney (εr = 52.74 and σ = 2.43 S/m), gall bladder (εr = 68.36 and σ = 2.8 S/m), liver (εr = 54.81 and σ = 2.25 S/m), and also the artery (εr = 58.26 and σ = 2.54 S/m). Position 2 and position 3 in this scenario are such that the Tx and Rx antennas are placed 1 cm and 2 cm below location 7 indicated as location 8 and location 9 in Figure 2.
Scenario IV-- Large intestine: The Tx antenna is placed in the large intestine (εr = 53.87 and σ = 2.03 S/m at 2.45 GHz) of the VFB as shown in Figure 2 at the location 10. Here the Rx antenna traverses through the large intestine and also through fat (εr = 5.28 and σ = 0.105 S/m). Position 2 and position 3 in this scenario are such that the Tx and Rx antennas are placed 1 cm and 2 cm below the location 10 indicated as location 11 and location 12 in Figure 2.
In total, 162 simulations are carried out in the heterogeneous VFB for the various scenarios.
A. Return loss for the implantable antenna: muscle tissue and heterogeneous medium
B. Path loss
1) PL in human muscle tissue and heterogeneous VFB
C. PL model
1) Homogeneous human muscle tissue
Parameter values and SD of the fitted models for PLdB in human muscle tissue and the heterogeneous
C i (dB)
Homogeneous muscle tissue
VFB Small Intestine
VFB Large Intestine
2) Heterogeneous model for esophagus--scenario I
3) Heterogeneous model for stomach--scenario II
4) Heterogeneous model for small intestine--scenario III
5) Heterogeneous model for large intestine--scenario IV
Table 2 lists all the attenuation constants α i , the constant C|dB and the SD from all the scenarios considered. It can be seen for the two scenarios of small intestine and large intestine the effective attenuation constants are higher than the attenuation constant of the homogeneous muscle tissue. The conductivity, σ = 3.17 S/m of the small intestine is much higher than the conductivity, σ = 2.01 S/m, of the muscle tissue causing more attentuation in the small intestine. In this scenario the Rx antenna traverses through various tissues such as the kidney (εr = 52.74 and σ = 2.43 S/m), gall bladder (εr = 68.36 and σ = 2.8 S/m), liver (εr = 54.81 and σ = 2.25 S/m), and also the artery (εr = 58.26 and σ = 2.54 S/m). The tissues through which the antenna passes through have the conductivity higher as compared to the muscle tissue. In the large intestine the huge variation in the dielectric properties of the tissues (large intestine and the fat layer) through which the Rx traverses gives rise to higher attenuations. In case of the esophagus and the stomach, Table 2 shows that the effective attenuation constant lower than the muscle tissue medium as in both the scenario the Rx antenna traverses through tissues having lower conductivities compared to the muscle tissue. In case of the esophagus the tissues are esophagus (εr = 62.15 and σ = 2.2 S/m) and lungs (εr = 34.42 and σ = 1.24 S/m). In case of stomach the antenna traverses through the stomach lumen ((εr = 52.72 and σ = 1.74 S/m).
The path loss in homogeneous human muscle tissue and various heterogeneous media using implantable slot dipole conformal flexible antennas is investigated at 2.45 GHz. An in-body path loss model for the homogeneous medium and a heterogeneous human model is derived. Simulations based on FDTD and the fitted models show excellent agreement. It is observed from the considered scenarios, that the 95th percentile of the PL in heterogeneous VFB lies above the PL in the homogeneous medium. Thus, PL model in the heterogeneous VFB will help in understanding the power requirements for implants working at 2.45 GHz. The PL model in the heterogeneous human model for the considered deep tissue implant scenarios can also be used to evaluate the performance of in-body WBAN systems using well specified setups and to carry out link budget calculations.
European Radiocommunications Committee
implantable cardioverter defibrillators
industrial: scientific: and medical
magnetic resonance images
short range device
wireless body area network.
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