Single-Chip Two Antennas for MM-Wave Self-Powering and Implantable Biomedical Devices

─ Implantable biomedical applications arise the need for multi-band sensors with a wideband frequency channel for RF energy harvesting operation. Using a separate antenna for energy harvesting can simplify device circuit complexity and reduces operation frequency bands interference. This paper demonstrates the design of single chip with two separate integrated antennas for implantable biomedical applications. The two antennas have different structures with orthogonal polarization to achieve low mutual coupling and negligible interaction between them. The first antenna is a multi-band meander line (MBML) designed for multiple channels data communication, with quad operating bands in the MM-wave range from 22-64 GHz with area 1150 × 200μm. The second antenna is a wideband dipole antenna (WBDA) for RF energy harvesting, operates in the frequency range extend from 28 GHz to 36 GHz with area 1300×250μm. The proposed antennas are designed by using high frequency structure simulator (HFSS) and fabricated by using UMC180nm CMOS technology with total area 0.55 mm2. The MBML frequency bands operating bandwidths can reach 2 GHz at impedance bandwidth ≤ -10 dB. While, the WBDA antenna has gain -2 dB over the operating band extend from 28 GHz up to 36 GHz. The antenna performance is simulated separately and using the human-body phantom model that describes layers of fats inside body, and shows their compatibility for in body operation. Die measurements is performed using on wafer-probing RF PICOBROBES and shows the matching between simulation and measurement values. Index Terms ─ Implantable antenna, Multi-Band Meander-Line (MBML), on-chip antenna, radio frequency energy harvesting and UMC 180 μm CMOS, Wideband Dipole Antenna (WBDA).


I. INTRODUCTION
Currently, promising implantable systems have been proposed for healthcare monitoring and remote sensing to improve the lifestyle of the patients. UWB frequency bands are applied for implantable devices as it enables high data rate and multiple frequency channels for multimode operation [1][2][3][4][5]. Recently, MMW range is described as a complementary imaging technique instead of exposure of patient to radiations as magnetic resonance, X-Ray or Ultrasound [6][7][8]. The antennas depend on 5G technology to meet the capacity, latency and the bandwidth requirements to support the request of the growing number of wireless communication users [9][10]. Besides, antenna and associated electronics could be designed to support multiple frequency channels for data communications of a group of implantable sensors simultaneously, with the need for a wideband frequency channel for RF energy harvesting to obtain a suitable power level for device operation [11][12][13][14]. Figure 1 shows the energy/power required for device operation can be obtained using energy harvester/wireless powering transfer from external source to be applied to the implantable devices in different locations in-body. Different types of on-chip implantable antennas are described in the sub GHz range and MMW range [15][16][17][18][19][20].
Simultaneous data communication mode and RF wireless powering mode using same antenna and matching network [21][22] where the antenna toggles between the two communication modes, the data mode and power mode, but this needs special control technique to fulfil continuous switching between both modes. To reduce circuit complexity, the modes of operation can be split up by using two separate antennas, one is used for data communication (for multiple sensors) while the other operates for RF energy harvesting. In this paper, two separate integrated antennas on the same chip are proposed to be used for self-powering implantable biomedical devices. The antennas are designed for data communication and energy harvesting simultaneously, with two different structures are designed, a multiband meander line antenna (MBML) and a wide-band dipole antenna (WBDA), with no radiation coupling occurs. The MBML resonates at the 22, 36, 48, 60 GHz, such that it is suitable for multiple sensing applications, while the WBDA operates from 28 GHz-36 GHz to perform the wideband required for energy harvesting. The 3D configuration of the proposed antennas shown in Fig. 2, is fabricated using UMC 0.18µm CMOS technology with 6-Metal layers. As implantable devices are affected by the permittivity of human tissues which causes signal losses, antenna design should consider the human-body effect during antenna design as wireless transfer between the on-body device and the external electronic circuits. For human activity monitoring, the implant is set to be below the human scalp or embedded in the muscle [23]. In this paper, the chip is assumed for implantation in the fat layer of the human body. Phantom model is used to investigate the antenna signal transmission in the human body layers even in the presence of perturbations such as embedded muscle layers and blood vessels. Different sensor antenna are implemented in a chip 180 nm CMOS technology for radio frequency (RF) energy harvesting to increase the lifetime of the internal batteries or feed another system [30].
The paper sections are organized as follows; Section 2 shows detailed explanation of the proposed antennas configuration. In Section 3 the simulation and measurement performance of both antennas. In Section 4, the proposed antenna simulation using human-phantom is performed to investigate antenna compatibility with implantable applications. Finally, conclusions of the paper are shown in Section 5.

II. ANTENNA CONFIGURATION AND DESIGN
Different shapes of on-chip integrated antennas are shown, as dipole [17], [24], [25], loop [18][19][20], zigzag [26], [27], and bowtie antennas [28], [29]. This is due to their circuits have differential input / output. The design of the two balanced antennas are utilized to obtain the benefits of the on-chip ground plane used to improve antenna radiation efficiency. Two different shapes of antennas are used to seeking different wireless applications, one for the data communication antenna and the second for wireless powering antenna integrated on the same chip with separate connecting PADS as shown in Fig. 3. The polarization of both antenna are linear polarized and there are orthogonal located to each other to reduced coupling between them. The chip is fabricated by using UMC 180nm CMOS process with silicon substrate and six metal layers. In this section, design and simulation for each antenna is demonstrated.

A. Data communication antenna
It is a multi-band meander line (MBML) implemented at 2 stacked layers to increase antenna total length-located at metal layers M6 and M4 as shown in Fig. 3. A layer of metal M1 is set as a ground plane to enlarge bandwidth with area of 1525µm × 250µm with four connections to the four ground PADS. Each meander layer -whether at M 4 and M 6 -has a finger line width 15µm, length 1150µm and line spacing 20µm between each two fingers. The meander structure at M 6 layer has five fingers while the meander structure at M 4 layer has only four fingers as shown in Fig. 2. The two layers have the same alignment but shifted 17.5µm, to increase number of resonance frequency bands. Five PADS are used with area 80µm×60µm for each and separated by 150µm. One of the PADs is connected tom the upper meander layer M6 to be used as signal PAD and the others are used as ground PADs. The MBML has a 50ohm line connection with signal PAD. The two meander layers are connected through via 1.   Figure 3 shows the structure of the wideband dipole antenna used for RF energy harvesting. It is composed of three layers, a dipole layer, a patch layer, and a ground plane. The dipole consisting of a half wavelength radiating dipole implemented at the upper metal layer M 6 with total area 1300µm×250µm. The patch layer is carried out in the intermediate metal layer M 4 with area 1100µm×50µm. This middle patch layer acts as a parasitic element to broaden the antenna bandwidth. A ground layer is set at the M 1 layer with area 1300µm×250µm. The patch layer is stacked to the dipole by VIA1. The signal and ground PADs (with area 80µm×60µm) are separated by 150µm. The ground M 1 layer is connected to the Ground PAD by VIA2. The proposed dipole antenna with patch layer is set to reduce the resonant frequency and increase the antenna bandwidth. The corresponding reflection coefficient response for both antennas is shown in Fig. 4 (a), where |S 22 | (as dashed blue line) represents the MBML response while |S 11 | (as black line) represents the WBDA. The green line explains the neglected coupling between both antennas. Current distribution at different tuning frequencies on both antennas' surfaces are shown in Fig. 5. The highest magnitude of current represents the corresponding element of radiation.

III. ANTENNA MEASUREMENTS RESULTS
The measured antenna S-parameters are carried out by using cascade probe station on top with Anritsu Vector Network as shown in Fig. 6. The measured reflection coefficient is achieved by using on-wafer probing. The measurements setup shown in Fig. 6 The fabricated photo of the proposed antennas are shown in Fig. 6 with four ground PADS connected to the ground metal layer M 1 , the extra PADS have no effect on antenna performance but are added due to measurements concerns. To ensure the reliability and accuracy of measurements; the proper fixture is applied. The die is fixed on the probe-station holder using vacuum pump while the GSG probe and the positioner are held by a manipulator that has magnetic fixture to ensure accurate contact between the die and the GSG probe. The other measurement problems that could be cause uncertainty of measurements [29] which is avoided by using a calibration substrate at the beginning of the measurements to ensure accurate calibration for the complete setup (network analyzer, cabling and probe) whenever it produces a reading different than the standard. Besides, before each measurement experiment the VNA was calibrated using (the open short, through calibration kit). Figure 7 (a) shows the comparison between the simulated and measured reflection coefficients |S 11 | of the double meander antenna. However, the |S 11 | of the different dipole length is shown in Fig. 7 (b). The comparison between simulated and measured results showed that there are a good agreement between the lower and the upper of the operating frequencies' antenna bands. There are some ripples in the measured results and there are about −3 dB shift in the measured reflection coefficient results. These errors could be attributed to metal holder, which are not considered and many factors as the thickness of the layer, metal holder was not considered. In addition, uncertainty in the dielectric material properties are specified up to 15 GHz while the simulated dielectric material properties of the layer were identified up to 70 GHz. As well as the influence of larger wafer with the chuck and the uncertainty in the dielectric material properties. In the simulations, these values have been used for the characterization of the structure at 70 GHz. Table 1 shows the radiation pattern of the MBMA at different resonant frequencies. While table 2 shows the simulated polar radiation pattern for the WBDA antenna at three resonant frequencies 28 GHz, 32 GHz and 32 GHz, respectively.

IV. HUMAN-BODY PHANTOM MODELING
To validate the performance of the proposed antennas for implantable applications, phantom model is used to simulate the implanted antenna and evaluate its performance to be used in-body environment. The specific absorption rate (SAR) is important factor that show that if these antennas are safe or not on the human body. SAR factor depend of the electrical field radiation and conductivity of the human tissues. According to IEEE standard it should be the SAR over 1g and 10 g of tissue of less than 1.6W/Kg and 2 W/Kg, respectively. Figure 8 shows the homogeneous multilayered model with a dimension of 80 × 40 × 63 mm 3 used to model human tissue [30]. The values of electrical properties are tuned around 40 GHz, the implanted depth in skin layer is 2 mm. inside the human body. The resonant frequencies are reduced and the operating bandwidths are broaden. The results as shown in Fig. 9 for both proposed antennas showed that when the human body effect is included in the simulation, the antenna impedance matching is more significant for the chip implanted. Moreover, the phase of the reflection coefficient was slightly changed when the chip is placed outside a human body, and it was abruptly changed when the antenna was implanted in the human body. The changed results of the antenna performance are due to the conductivity of the human body, which adds extra load on the antenna surface and changes in the electrical properties of the substrate. However, when the chip is implanted in the human body, it became more dispersive and the operating bandwidths at −10dB (VSWR ≤ 2) were extended from 20 GHz to 45 GHz and from 57 GHz up to 65 GHz as shown in Fig. 9. However, in both simulation scenarios, the antenna bandwidth still operated in the selected bands of operations. The packaging proposed antenna are done by light-weight polymer-based materials that are biocompatible, RF-transparent, easy-to-process, and low-cost. On the other side the embedded antennas have a peak broadside power density of −60 dBW/m 2 at a distance of 1 m. For an isotropic radiator with a transmit power of 1 W, the power density at a distance of 1 m is equal to −11 dB W/m 2 , and for the embedded patch with a gain of −50 dB the power density would be equal to −61 dB W/m 2 .    The main drawback of such antennas design is the short communication range, which makes very hard reading process for the patient. As well as it is effect on the radiation efficiency due to the strong coupling of the near field components to the encapsulation of the antenna and electromagnetic absorption in the body tissues [30]. Which it could not to maintain a continuous communication link between the capsule and the external receiver. Table 3 tabulates different types of CMOS antennas operated at different frequencies; at high frequencies the gain is almost about -2 dB.

VI. CONCLUSION
This paper described two antennas integrated on the same-chip for implantable biomedical applications. Two separate antennas are designed with orthogonal polarization to ensure neglected mutual coupling between them. Institute since 1994 and her main research interests are wireless communication systems design, analog/mixedsignal/RF circuits, especially high speed PLL and frequency synthesizers. She also has many publications in low power digital circuit design, biomedical sensors read-out circuits, interconnect modeling and low power networks on chip (NoC). Now Heba is a member in nanotechnology lab and working in the field of nanomaterial electronics applications as graphene and CNT.