Compact Implantable Antennas for Cerebrospinal Fluid Monitoring

The changes in dielectric properties of cerebrospinal fluid (CSF) can be utilized in diagnosis of cognitive diseases. The aim of this paper is to investigate the employability of an implantable antenna to concurrently operate as a radiator and sensor of the dielectric properties of CSF. The antennas exploit narrow slits and gaps as capacitors for sensing the permittivity of CSF. Three antennas are designed based on capacitively loaded loop (CLL), complementary split ring loop (CSRL), and interdigital capacitor loop (IDCL). To investigate if the dielectric properties of CSF are a function of age, in vitro measurements of different CSF from pigs with different ages are measured in the sub-1-GHz band (0.1-1 GHz). The results reveal a strong pairwise correlation exceeding 0.77 for permittivity and 0.83 for conductivity among samples. Hence, the dielectric properties of the CSF do not reflect age-dependence and sensing sensitivity is implicitly not affected by age. Finally, the implantable antennas are fabricated and tested in a realistic environment viz inside a piglet's head and CSF simulants. The shift in the resonance frequency when the permittivity is increased by 14% at 400 MHz ranges between 31-40 MHz with a relative resonance sensitivity between 6% and 8% for the proposed implantable antennas.