Implantable CPW fed M Shaped Antenna for Biomedical Applications

Implantable medical device has become very popular in medical sector for proper diagnosis and treatment. If, this devices are wireless, then it will very effective for remote communication. For this wireless communication, antenna is a suitable thing. So, this paper presents the design Implantable CPW fed M-shaped antenna for biomedical applications. Coplanar waveguide fed technique is used with ground plane in this antenna. The antenna is designed by Alumina ceramic (Al2O3) substrate and Copper ground plane with dimensions (8mm × 7mm × 0.85mm) and Silicon material of 0.1mm thickness is used as a coating for human body safety. This antenna works under 2.42 GHz resonate frequency of ISM band (2.40-2.48 GHz). The bandwidth percent of the antenna at -10dB is 7.66%. The return loss characteristic of the antenna is -56.96 dB. Based on the conductivity and permittivity of different tissue (such as: Muscle, Fat. Skin) a simple 3D human body phantom structure is designed for implantation of CPW fed antenna. The antenna is designed and simulated with the help of CST software. The antenna parameters such as: Return loss, Smith Chart, VSWR, Gain, SAR have been discussed in the paper.


Introduction
At presents, Different diseases such as: Diabetic, High per tension, Cancer, Heart attack etc have become a part of some human life. Old aged people suffer from these diseases very much. Regularly checkup is the best treatment of these diseases. But, Regularly checkup is very painful for old aged people at hospital or to the doctor. The implantable device has become an effective solution for these problems. Because, implantable device is that device which collects information from patient's body and sends it to external device by wirelessly [1,2]. This wireless communication is done by antenna. In this case, Small implantable device is implanted inside human body very easily. For this small device, small antenna is much suitable. Antenna have been used in many medical applications such as: Biomedical telemetry, Wireless capsule endoscopy, Hyperthermia treatments, Microwave imaging, Microwave coagulation therapy [3][4][5]. The Industrial scientific and medical (ISM) band (2.40-2.48 GHz) is allocated by FCC for medical applications. This band gives higher bit rates and high frequency [6,7].
Recently, Implantable antenna have been reported by some researchers. In paper [8], an implantable medical antenna was proposed, which operating at 2.45GHz of ISM band. The bandwidth of this antenna was 40. 3 MHz. An implantable dual spiral antenna has been proposed in the frequency 2.41 GHz of ISM band. The total size of this antenna was (30mm × 30mm × 1.6mm). The return loss of this antenna was -26 dB at 2.41 GHz [7]. A rectangular shaped implantable antenna were presented at 2.46 GHz of ISM band. The dimension of this antenna was (17.635mm × 17.325mm × 1.3314mm). The return loss of this antenna was -46.42 dB at 2.46 GHz [6]. Most of the implantable antennas proposed in literature have large size and complex shape. It is very difficult to implant into the human body for large size. So, Main purpose of this research is to reduce the size of antenna as much as possible at proper operating frequency band. For reducing the dimension of antenna, high permittivity dielectric substrate Alumina ceramic (Al 2 O 3 ) is used for simulation [9,10]. Since, this antenna is operated inside human body (such as: muscle, fat, skin). Implantable device is must biocompatible to prevent unwanted short circuits and patient safety for long-term implantation [11][12][13]. This paper has introduced Implantable CPW fed M-shaped antenna with CPW fed technique and coated with silicon as biocompatible material. This silicon coated antenna has been designed and simulated inside human body phantom using CST microwave studio [14]. This CPW fed M-shaped antenna is resonating at 2.42 GHz of ISM band. This paper is organized as follows: section 0, describes the antenna design and structure. Section 0, shows simulation results of proposed antenna. Section 0, denotes conclusion for this research.

Antenna Design and Structure
The design specifications for the proposed antenna are: 1. Substrate material for the design is Alumina ceramic (ε r =9.9). 2. Patch and Ground material for the antenna is Copper.

Biocompatible material for coating of the antenna is
Silicon (ε r =11.9). CPW fed technique has been used for this antenna. CPW fed has the advantages such as to simplify the fabrication and reduces the return loss. CPW fed ground plane and good impedance matching have their own merits and their physical structure makes them suitable for biomedical applications [15]. The details dimensions of the proposed antenna are shown in Figure 1.
Human tissues are electrically conductive. There has possibility to short circuit condition with patch and ground material of antenna. So, A shield can protect from this conditions. Silicon material has been utilized as a shield of this antenna. Because, Silicon is biocompatible material for the purpose of developing implantable biomedical devices [9,16]. After designing this antenna, the antenna is simulated inside human body phantom. Human body phantom structure consists of muscle, fat and skin tissues of different dielectric properties. The proposed antenna inside human body phantom is shown in Figure 2 and the values of different dielectric properties of human tissues are specified in Table 2.

Results and Discussion
The software is used to model and simulate the CPW fed antenna in CST Microwave Studio. It analyzes 3D and multilayer structures of human body phantom. It has calculate Return loss plot, Smith chart, VSWR plot, Farfield radiation pattern. The simulation results of the proposed antenna are presented in the below figures:   The voltage standing wave ratio (VSWR) is the measure of how well the antenna terminal impedance is matched to the characteristic impedance of transmission line. VSWR of the proposed antenna is shown in Figure 5. The VSWR of the proposed antenna is 1.0028 at resonant frequency 2.42 GHz. Which is suitable for this antenna.    Specific absorption rate is a measure of how much transmitted RF energy is absorbed by human tissues. IEEE C95.3-2002 standard says that for an input power of 2mW, the 1-g averaged SAR should not exceed 1.6 W/kg as given by FCC and ICNIRP guidelines for public exposure [6].
In this research, 1-g averaged SAR (in Figure 7 ) is 0.456 W/kg and 10-g averaged SAR (in Figure 8 ) is 0.0932 W/kg for an input power of 2mW.

Conclusion
The Implantable CPW fed M-Shaped antenna has been designed for biomedical application with dimensions of (8mm × 7mm × 0.85 mm) in the ISM band (2.40-2.48 GHz). This antenna have been simulated in (60 mm × 60 mm) bio-tissue. The designed antenna resonates at 2.42 GHz and provides a bandwidth of 185.6 MHz. Due to its wideband property, this antenna can be used for remote distance health monitoring system. Size reduction and biocompatibility are main focus of this antenna design. For size reduction, high dielectric material is used and for biocompatibility the antenna is encased in a biocompatible material. Considering all these factors the antenna is designed using Alumina Ceramic (Al 2 O 3 ) substrate inside human body phantom model and silicon coating of 0.1 mm is used as a biocompatible material for human body safety. Due to superior permittivity of the Alumina ceramic substrates, implantable antenna exhibits lower return loss (-56.961 dB), good VSWR, better impedance matching with CPW structure. Therefore, the proposed antenna is proper structure for ISM band frequency of biomedical engineering.