In the growing efforts of promoting patients' life quality through health technology solutions, implantable wireless biomedical telemetry systems have been identified as one of the frontrunners. In these systems, the design of implantable antennas is of prime importance in establishing the wireless data link. Small multitasking implantable antennas are an evolving wireless health technology for monitoring medical conditions. These implantable antennas are intended for different functions including wireless data transmission at Medical Device Radiocommunication Service (MedRadio) band (401-406 MHz) and Wireless Medical Telemetry Service (WMTS) bands (1395-1400, and 1427-1432), wireless power transfer, and control signals between sleep/wake-up modes at Industrial, Scientific, and Medical (ISM) bands (902-928 MHz and 2400-2483.5 MHz), respectively. In this paper, we present a meandered quad-band planar inverted-F antenna (PIFA) for wireless brain care. Employing the meandering miniaturizing technique, shorting the radiator to the ground plane, and high-permittivity substrate/superstrate layers (Rogers RO3210; varepsilon {r}=10.2, tan delta=0.003, h=0.635 mm) lead to downsizing the antenna volume greatly to 11 times 20.5 times 1.8 mm{3} that includes the biocompatible silicone coating. We developed and characterized the proposed antenna numerically utilizing a 7-layer human head model in full-wave electromagnetic field simulation, where the antenna was implanted in the cerebrospinal fluid (CSF) layer at the depth of 13.25 mm in the cranial cavity. Overall, we achieve a compact quad-band implantable PIFA with-42.3 dBi of gain with a radiation efficiency of 0.003 % at 402 MHz,-22.7 dBi gain with a radiation efficiency of 0.1 % at 902 MHz,-23.7 dBi gain with a radiation efficiency of 0.1 % at 1430 MHz, and-29.7 dBi gain with a radiation efficiency of 0.02 % at 2450 MHz. To our knowledge, this is the first self-matched quad-band antenna proposed for wireless implant communications.Peer reviewe

Björninen, T.

Pournoori, N.

Rahmat-Samii, Y.

Sydänheimo, L.

Ukkonen, L.

Trepo - Institutional Repository of Tampere University

1   Compact Quad-Band Meandered Implantable PIFA for Wireless Brain Care N. Pournoori1, L. Sydänheimo1, Y. Rahmat-Samii2, L. Ukkonen1, and T. Björninen1 1Faculty of Medicine and Health Technology, Tampere University, Finland 2Department of Electrical and Computer Engineering, University of California, Los Angeles (UCLA), Los Angeles, USA   Abstract—In the growing efforts of promoting patients’ life quality through health technology solutions, implantable wireless biomedical telemetry systems have been identified as one of the frontrunners. In these systems, the design of implantable antennas is of prime importance in establishing the wireless data link. Small multitasking implantable antennas are an evolving wireless health technology for monitoring medical conditions. These implantable antennas are intended for different functions including wireless data transmission at Medical Device Radiocommunication Service (MedRadio) band (401–406 MHz) and Wireless Medical Telemetry Service (WMTS) bands (1395 – 1400, and 1427 – 1432), wireless power transfer, and control signals between sleep/wake-up modes at Industrial, Scientific, and Medical (ISM) bands (902–928 MHz and 2400–2483.5 MHz), respectively. In this paper, we present a meandered quad-band planar-inverted-F antenna (PIFA) for wireless brain care. Employing the meandering miniaturizing technique, shorting the radiator to the ground plane, and high-permittivity substrate/superstrate layers (Rogers RO3210; εr = 10.2, tanδ = 0.003, h = 0.635 mm) lead to downsizing the antenna volume greatly to 11×20.5×1.8 mm3 that includes the biocompatible silicone coating. We developed and characterized the proposed antenna numerically utilizing a 7-layer human head model in full-wave electromagnetic field simulation, where the antenna was implanted in the cerebrospinal fluid (CSF) layer at the depth of 13.25 mm in the cranial cavity. Overall, we achieve a compact quad-band implantable PIFA with −42.3 dBi of gain with a radiation efficiency of 0.003% at 402 MHz, −22.7 dBi gain with a radiation efficiency of 0.1% at 902 MHz, −23.7 dBi gain with a radiation efficiency of 0.1% at 1430 MHz, and −29.7 dBi gain with a radiation efficiency of 0.02% at 2450 MHz. To our knowledge, this is the first self-matched quad-band antenna proposed for wireless implant communications.   1. INTRODUCTION Significant advances in wireless implantable medical devices (WIMDs) have brought revolutionary achievements in the modern medical industry. These devices enable continuous human health monitoring and provide their physiological parameters for early medical diagnosis and treatment in bio-telemetry systems. To set up a real-time bio-telemetry link in these systems with exterior monitoring/control equipment, the WIMDs need miniature implantable antennas. However, designing a miniature antenna that attains adequate radiation performance for wireless communications in the presence of the lossy human biological tissues is demanding. Dealing with the size restriction mainly depends on the resonant frequency of the required applications [1]-[3]. Authorized frequency bands for biomedical telemetry systems contain MedRadio band resonating at the frequency ranges of 401–406 MHz, WMTS bands (1395–1400 MHz, and 1427–1432 MHz) and the ISM bands of 433.1–434.8 MHz, 868–868.6 MHz, 902.8–928.0 MHz, and 2.4–2.48 GHz [4].  The MedRadio band is used for uninterrupted access to a patient’s health condition while the ISM bands are defined for WPT and switching control for sleep/wake-up signal modes [5]-[9].  Due to the difficulty of achieving proper radiation performance with a compact size at lower frequencies, considering a higher operating frequency is a promising approach to design an implantable antenna. On the other hand, a single band antenna may not fulfill all requirements of a biotelemetry system in either MedRadio/WMTS or at ISM bands; hence, it requires a multi-band implantable antenna for more versatile functions supporting wireless power, data transmission, and signal controlling simultaneously [6]-[11]. Correspondingly, we have recently witnessed extensive research on dual/multi-band implantable antennas [6]-[13]. Nevertheless, designing a multi-resonant antenna implanted in dissipative human tissues with a downsized volume and optimum radiation performance is still a topic of ongoing study.  In our previous work [14], we presented a prototyped small triple-band implantable PIFA with the dimension of 11×20.5×1.8 mm3 for wireless brain implant communication, where we utilized the meandered radiator for PIFA size reduction and obtain multi-resonant at both MedRadio (402 MHz) and ISM bands (902 MHzand 2400 MHz) with the satisfactory typical gain value and radiation efficiency.  W3=0.8l2 = 6.4 W=11 W2= 1W2W1=2W1 W3W2l3 = 6.2 l4 = 7.4 l5 = 10 l6 = 6.7 d= 8.9 L = 20.5 Superstrate (εr=10.2), h=0.635 Subrstrate (εr=10.2), h=0.635 Ground planeFeeding-portl1 = 9.5 Skin (2)Fat (2)Muscle (2)Skull (5.5)Dura (0.5)CSF (4.9)BrainImplanted PIFAW(shorting-strip)= 1shorting-stripxy–zxyz(a) (b)Feeding portSilicone coatinghcoating =1.8 mmSuperstrate Radiating patchSubstrate Ground planeShorting-strip Feeding port W4 = 1.85 Figure 1: The proposed meandered quad-band implantable PIFA configuration. (a) The structure of proposed PIFA, (b) The 7-layer human head model with implanted PIFA. (Unit for all values is mm.). The developed antenna was embedded in CSF region of the seven-layer head model at the depth of 13.25 mm from the body surface. To provide a broad bandwidth PIFA and diminish its sensitivity to the properties’ variation in lossy human tissue, we employed 2-layer stacked structure and covered the radiator with a superstrate layer. This work advances upon our previous approach and proposes a self-matched quad-band meandered PIFA resonating at MedRadio (402 MHz), WMTS (1430 MHz), and ISM bands (902 and 2450 MHz) with the compact size of 11×20.5×1.8 mm3 implanted at 13.25 mm depth of the cranial cavity in the CSF layer. The numerical study of the developed antenna achieved proper radiation characteristics and appropriate performance for deep brain implants. 2. ANTENNA DEVELOPMENT AND DESIGN PRINCIPLES Figure 1 (a) illustrates the proposed quad-band meandered PIFA structure with the geometry of 11×20.5×1.8 mm3. The proposed model integrates a shorting strip to link the meandered radiator to the ground plane in the vertical y-z coordinate plane and an embedded 0.6 mm radius pin into the radiating patch to feed the PIFA via a 50 Ω coaxial cable. In the initial design procedure, we calculated the radiator geometry to operate at the ISM frequency band of 902 MHz by the given equation [15]-[18],                              ƒ=C0/4 (L + W + h −W(shorting-strip))               (1) where C0 is the light speed in free space, εr denotes the relative permittivity of the substrate, L and W are the length and width of the radiating patch, respectively, h denotes the substrate thickness and W(shorting-strip) is the width of shorting strip. Considering this equation, utilizing high-permittivity dielectric substrate /superstrate (Rogers RO3210; εr=10.2, tanδ=0.003, h= 0.635 mm) leads to antenna size reduction. The superstrate embedment over the radiating patch preserves the antenna from the lossy surrounding biological tissues [14]. Additionally, we covered the antenna with a thin layer of silicon-coating (εr=2.2, tanδ=0.007) for biocompatible isolation. In the next step, applying a few cutting slots (l2, l3, l4, l5, and W4) onto the radiating element shrank further the PIFA size and enhanced its bandwidth. Subsequently, lengthening the effective current path through meandering these cutting slots (l2, l3, l4, l5) led to exciting the antenna to resonate at the MedRadio band. Lastly, adding one more meandered cutting slot (l1) close to the PIFA feeding port created third and fourth resonances at 1430 MHz of WMTS bands and 2450 MHz of ISM band. Consequently, compact self-matched quad-band meandered PIFA brings a compelling approach for multitasking wireless implantable medical devices.  3. NUMERICAL SIMULATION MODEL, RESULTS, AND DISCUSSION 3.1. Numerical Design All the electromagnetic simulations and numerical modeling were conducted in the ANSYS High Frequency Structure Simulator (HFSS). In the simulation, we utilized a 7-layer human head model with the assigned thickness and the frequency-dependent electrical properties of the tissues according to the four-term Cole-Cole relaxation model [19] and we developed the PIFA implanted in the CSF layer of the head tissues at the depth of 13.25 mm, as shown in the cross-sectional view of layers in Figure 1(b). Table 1 presents the defined thickness, relative permittivity (εr) and conductivity (σ) of the employed head tissues model.  To investigate the electromagnetic characteristics of the antenna in this numerical head model and tune the desired resonant frequencies, we evaluated all crucial design parameters involving the effective lengths of the meandered slots and shorting strip, and the embedment place of the feeding port. As is observed in Figure 2, increasing the cutting slot length of l1 lowers the resonance frequency to 1430 MHz. Conversely, shortening the length of meandered slot (l3) gives rise to a higher resonance frequency at 2450 MHz. Simultaneously, creating the lower frequency bandwidths of around 402 MHz and 902 MHz Table 1: Electrical properties of the 7-layer head model. Tissue  Thickness (mm) 402 MHz 902 MHz 1400 MHz 2400 MHz εr σ (S/m) εr σ (S/m) εr σ (S/m) εr σ (S/m) Skin  2 46.7 0.7 41.4 0.9 41.4 0.9 38.1 1.4 Fat  2 5.6 0.04 5.5 0.05 5.5 0.05 5.3 0.1 Muscle  2 57.1 0.8 55 0.9 55 0.9 52.8 1.78 Skull  5.5 20.3 0.2 19.6 0.3 19.6 0.3 18 0.8 Dura  0.5 46.6 0.8 44.4 0.96 44.4 0.96 42.1 1.68 CSF  4.9 71 2.3 68.6 2.4 68.6 2.4 66.3 3.48 Brain (grey matter)  75 57.4 0.7 52.7 0.9 52.7 0.9 49 1.8    Figure 2: Effect of lengthening the cutting slot of (l1) and increasing the distance between shorting strip and feeding port (d) along with shortening the cutting slots length (l3) and slot width (W4). result from the longer shorting strip (l6) and the enhancement of the distance between the shorting strip and feeding port (d). To provide proper impedance matching at 402 MHz, we needed to employ one more slot at the end edge of the radiator (W4) to diminish slightly the effective length of the antenna. Considering these principles, we performed parametric sweeps adding these five key geometrical parameters (l1, l3, l6, W4, and d) in HFSS, and finally, we were able to improve and self-impedance-tune the proposed antenna for four desired frequencies resonating at MedRadio, WMTS, and ISM frequency bands. Fig 1 (b) indicates the deigned antenna dimension.  Figure 3 shows the impedance matching improvement of the quad-band PIFA compared to the triple-band PIFA from our previous work [14]. It is obvious, the added WMTS operating frequency band in our proposed antenna at 1430 MHz could obtain the reflection coefficient value of −11.5 dB, the other resonant frequencies at MedRadio and ISM bands involve 402, 902 and 2450 MHz with the corresponding reflection coefficient values of −5.5 dB, −5 dB, and −4.3 dB at 402, 902, and 2450 MHz, respectively. As can be seen from Fig. 3, despite the promising quad-band S11 response, presently the antenna meets the classical goal of S11 < −10 dB only near the 402 MHz MedRadio, and 1430 MHz WMTS bands. In this regard, a topic of our ongoing work focuses on using multiparameter numerical optimization schemes for fine-tuning the geometry of the antenna to satisfy this goal at all four operating frequencies.  Figure 3: Reflection coefficient of the proposed quad-band PIFA compared to our previous triple-band antenna [14]. (b) XZ ZY (a) 402 MHz902 MHz1430 MHz2450 MHzPhi= 0 Phi= 90  Figure 4: 2D simulated antenna directivity. (a) at XZ plane (phi=0), and (b) at YZ plane (phi=90). 3.2. Antenna Radiation Performance and Far-Field Pattern We evaluated the antenna radiation characteristics including antenna gain, directivity, realized gain, and radiation efficiency to ensure the far-field data telemetry communications and wireless transfer power in WIMDs. Figures 4 (a) and (b) illustrate the 2D simulated directivity of the antenna at both the XZ (phi=0) and YZ planes (phi=90) at our desired resonant frequencies. The results demonstrate that the proposed quad-band PIFA radiates properly at the direction outwards head tissues, where antenna with the linear polarization with E field in YZ-plane attained the peak directivity of 2.6 dBi, 6 dBi, 5.8 dBi, and 6.7 dBi on −z-axis at the desired frequencies of 402, 902, 1430 and 2450 MHz, respectively. Figure 5 depicts the 3D simulated far-field gain radiation pattern with a maximum gain of −42.3 dBi at 402 MHz with the 0.003% efficiency, −22.7 dBi at 902 MHz with the 0.1% efficiency, −23.7 dBi at 1430 MHz with the 0.1% efficiency, and −29.7 dBi at 2450 MHz with the 0.02% efficiency.  Finally, we calculated the realized gain (GR) of the PIFA applying the antenna gain (G) in the given formula of GR = G(1−|S11|2). Figure 6 compares the realized gain of the proposed quad-band PIFA and our previous triple-band implantable antenna [14]. It is evident from the simulation results that the realized gain achieved similar values to the maximum gain, which verifies that impedance-tune has been obtained without compromising the PIFA gain. The attained realized gain involves −43.8 dBi, −24.5 dBi, −24.3 dBi, and −31.7 dBi at 402, 902, 1430, and 2450 MHz. Table 2 compares our antenna with only two other reported implantable quad-band antennas [10], [11] in terms of the antenna size, structure, operating frequency, gain, and implant depth. From this comparison, we were able to achieve a self-tuned antenna with state-of-the-art radiation performance for deep implants.  (a)  (b)  (c)  (d) ZYX Figure 5: 3D simulated antenna gain (dBi), (a) 402 MHz, (b) 902 MHz (c) 1430 MHz, and (d) 2450 MHz.  Figure 6: Realized gain of the proposed quad-band antenna versus frequency compared to the realized gain of our previous triple-band implantable antenna [14]. Table 2: Comparison of quad-band implantable antennas. Ref Type Frequency (MHz) Volume (mm3) Implant Depth Gain (dBi) Shorting Pin Layers Matching component [10] PIFA 401 433 1427 2400 245 4 mm in skin –22 –23 –17 –16 Yes 3 No [11] Meandered-line patch 403 915 1470 2400 8.43 50 mm in tissue –34 –29.6 –28.2 –22.4  Yes 2 Yes This work PIFA 402 902 1430 2450 286.4 13.25 mm in CSF –42.3 –22.7 –23.7 –29.7  Yes 2 No  4. CONCLUSION Miniature multitasking implantable antennas are an evolving technology for wireless brain care. We developed a brain-implantable quad-resonant PIFA using a meandering miniaturization technique with diminished volume to 11×20.5×1.27 mm3 for the data telemetry, WPT, and signal controlling applications simultaneously at MedRadio, WMTS, and ISM bands. The numerical simulation results of the developed PIFA verified the attainment of the satisfactory gain values with the adequate broad bandwidths and suitable radiation efficiencies for desired resonances at the 13.25 mm brain implant depth. In contrast to the stated multi-band implantable antennas, here, we could attain a self-matched quad-band implantable antenna for the first time with an appropriate performance providing multi-channel communications for deep brain implants. In future work, we aim to improve the impedance matching, implement, and test the antenna to validate the numerical results we have presented in this work. ACKNOWLEDGMENT This research was funded by the Doctoral Programme in Biomedical Sciences and Engineering under the Faculty of Medicine and Health Technology of Tampere University, Academy of Finland under funding decision 294616 and supported by The Finnish Foundation for Technology Promotion (TES) and Nokia Foundation. REFERENCES  1. Rahmat-Samii, Y. and E. 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Compact Quad-band Meandered Implantable PIFA for Wireless Brain Care

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Compact Quad-band Meandered Implantable PIFA for Wireless Brain Care

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