# Brain Implantable End-Fire Antenna with Enhanced Gain and Bandwidth
## 1 Introduction  
Brain signal monitoring has gained considerable attention not only from brain scientists but also from electronics engineers. Real-time neural data extracted by a brain-machine interface (BMI) or brain-computer interface (BCI) can be used for various applications, such as restoring sensory functions and controlling robotic prostheses [1]. A review of the basic concept of BCI, its applications, and challenges were extensively discussed in [2]. A conventional technique such as electroencephalography (EEG) requires a wired electrode to be attached on the scalp to monitor brain signals [3]. However, several studies have demonstrated the need for implanting a wireless BMI deep into the brain to monitor both the electroencephalogram (EEFG) and electrocorticography (ECoG) for cognitive and speech control [4; 5; 6; 7; 8; 9]. Neuralink [10], a neurotechnology company, has recently presented a pioneering deep BMI with wireless communication and power-charging functions. A wireless BMI consists of electrodes, an analog integrated circuit, a digital signal processing unit, a radio frequency (RF) front-end, and an antenna. They are packed into a biocompatible housing whose footprint typically ranges from 10 \(\times\) 10 mm\({}^{2}\) to 20 \(\times\) 20 mm\({}^{2}\)[5; 6; 7; 8; 9; 10; 11; 12; 13]. The slightly large size of a brain implant is primarily due to the antenna size. For instance, the size of a half-wavelength patch antenna at the 2.4 GHz industrial-scientific-medical (ISM) frequency band in free space is approximately 60 \(\times\) 60 mm\({}^{2}\). The antenna size can be reduced to fit into the housing by considering the high dielectric constants of brain tissues (e.g., cortical bone or dura) and by applying antenna miniaturization techniques(e.g., meandering, folding, and shorting). However, most of the reported works suffer from a low antenna gain (\(<\)\(-\)20 dBi) that is due to the high dielectric loss of brain tissues. The study showed that the antenna gain reduced by 10.7 dB after implanting it in the brain [5]. Typically, a brain-implanted antenna is located under the skull, immersed horizontally in the dura or cerebrospinal fluid (CSF), as shown in Figure 1a. This circumstance lowers the antenna gain because the skull is thick (\(\sim\)7 mm at the bregma) and lossy (loss tangent, tan\(\delta\), \(\sim\)0.3) [14; 15]. A full-wave electromagnetic simulation study indicated a reduction of at least 2 dB in antenna gain because of the thick skull. Deformation of the antenna radiation pattern is another issue. A broadside antenna horizontally placed under the skull is expected to radiate toward the zenith. However, unexpected large back lobes and side lobes are often seen after implanting an antenna in such a complex brain environment [16], i.e., the broadside radiation is not guaranteed because of the impact of the thick skull layer, high permittivity brain tissues, and small antenna ground plane. This paper presents an end-fire radiating antenna implanted vertically in the skull. Figure 1b shows the conceptual illustration of the vertical placement. The antenna was placed along the thickness of the skull and then connected upright to the integrated circuit (IC). Hence, the antenna had more design freedom as long as the height conformed to the thickness of the skull. More importantly, the antenna gain can be significantly improved throughout the bandwidth of interest. The proposed antenna was designed for 3-5 GHz impulse radio ultra-wideband (IR-UWB), which could transfer the data at a high rate with a low power consumption because of its wider operation bandwidth [17; 18]. A tapered slot antenna, the so-called Vivaldi antenna, was employed to cater to this wide bandwidth. The Vivaldi antenna is a well-known end-fire radiating antenna that provides broadband impedance matching and radiation performance owing to its gradual tapered structure [19]. However, we found that an abrupt tapering profile was required because the end-fire length was limited to the skull thickness.