## 4 Discussion and Conclusions  
Implantable devices have been playing a critical role in the field of biomedical research and health monitoring. Implantable antennas can support the collection of vital data from a body and reprogramming in-body sets through wireless communications, which can be used in a variety of medical applications, such as heart signal/beat analysis, reprogramming of pacemakers, daily activity recording, skeletal issues and diseases diagnosis, implantable medicine, nerve stimulation, and drug delivery without the need for multiple surgeries.  
The resonance frequency of antennas implanted in various people may change and alter the antenna's impedance matching because the characteristics of human tissues differ from individual to individual. In order to solve this problem, implantable antennas should have a broader bandwidth (BW). Additionally, miniaturization methods are needed to design compact implantable antennas that are compatible for biomedical purposes. Therefore, here, three triple-band implantable antennas with wide bandwidth characteristics and appropriate sizes were designed, analyzed, and compared. These antennas include fractal, meandered, and comb antennas. The antennas operate at three bands: 400 MHz, 2.4 GHz, and 5.7 GHz. The sizes of all antennas are optimized to be implanted, and they are smaller or comparable with similar reported samples. It is also notable that other recent works include only single- or dual-band antennas, and triple-band implantable antennas have rarely been reported in the literature. The BWs of all three presented designs are more than the other reported works. The comb antenna has the best BW among others. Since wider BW is more desirable in case of tissue characteristics variations, the comb antenna is more preferable in our designs. Rogers 3210 was chosen as the substrate and we obtained 120, 320, and 1200 MHz bandwidths at the specified resonant frequencies. The antenna dimensions are 1.4 mm\(\times\)12 mm \(\times\) 12 mm. The fractal type antenna is slightly smaller in size among the designs.  
To study the performance of the antenna implanted in different body tissues and body types, antennas were placed in different body parts of the human voxels and based on our analysis in all placements operational bands were covered. In human voxel, meander antennas had a better performance stability at 400 MHZ band and comb antenna showed a better performance stability at 2.4 GHz and 5.7 GHz. The meander antenna was selected because of the ease of fabrication for validation and a liquid phantom was provided to measure the S\({}_{11}\) of the antenna. The antenna satisfied the SAR limit and the data connectivity requirement. The measurement results were compared to simulations and good agreement was observed.  
Conceptualization, K.S. and S.H.; methodology, K.S., S.H. and H.C.; software simulation, K.S. and S.H.; validation, K.S, S.H. and S.S.; formal analysis, K.S. and S.H.; investigation, K.S., S.S. and H.C.; resources, K.S., S.H. and H.C.; writing--original draft preparation, K.S., S.H. and S.S.; writing--review and editing, K.S. and H.C; supervision, H.C.; project administration, H.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** H.C. receives support from the National Science Foundation (NSF) CAREER award #1917105.  
**Institutional Review Board Statement:** Not applicable.