Circularly polarized implantable antenna characterization for retinal prosthesis systems

We describe a miniaturized antenna design for retinal prosthesis applications that enhances data transfer between the implant and external camera. The circularly polarized and conformal 2.4-2.48 GHz microstrip patch antenna was simulated inside the vitreous humor and is intended for biomedical applications. Modified Hilbert and serpentine geometries were used for the proposed implant antenna design. Capacitive radiator loading offered miniaturized dimensions of 5.8 x 6.5 x 2 mm(3) (width x height x thickness mm(3)). A truncated design enhanced the intrinsic circular polarization characteristics to >3 dB axial ratio. Polydimethylsiloxane substrate and superstrate materials were used to achieve an S rr value below -15 dB across the frequency range with biocompatible characteristics. The simulated peak gains for left-hand circular polarization and right-hand circular polarization at 2.45 GHz were -50 dBi and -60 dBi, respectively. These relatively small values were due to the high conductivity of the vitreous humor (sigma = 1.53 S/m), which imposed significant losses. Overall, the proposed antenna had an omnidirectional radiation pattern and +8.45 dBm input to meet the specific absorption rate regulation limit.