Design, Characterization, and Optimization of a Broadband Mini Exposure Chamber for Studying Catecholamine Release From Chromaffin Cells Exposed to Microwave Radiation: Finite-Difference Time-Domain Technique

A free-space in vitro exposure system for identifying specific microwave (MW) parameters in the frequency range of 1-6 GHz that can induce nonthermal effects on exocytosis, which is the process by which neurotransmitter release occurs, has been designed, constructed, characterized, and optimized. The exposure system is placed within an anechoic chamber and incorporates continuous online monitoring of basal and stimulated catecholamine release from cultured bovine adrenal-medullary chromaffin cells, which are well-established models of neural-type cells. The cells are immobilized inside a cell-perfusion apparatus (CPA) and are continuously superfused with temperature-controlled balanced salt solution, with the entire CPA placed within a mini exposure chamber (MEC) constructed out of a MW-absorbing material. All relevant equipment for carrying out the experiments is shielded from the MW field by being housed in an aluminum conductor box located behind the MEC. Detailed distributions of the electric field and the specific absorption rate (SAR) at the location of the cells within the MEC were computed using the finite-difference time-domain (FDTD) method. FDTD computations were also used for optimizing the exposure system, so that the highest intensity of electric field could be delivered under dynamic temperature control and with an acceptable degree of field homogeneity (to within 30%) over the entire frequency range of 1-6 GHz. A major finding is that maintaining an acceptable level of homogeneity of the electric field and SAR for exposing cells to 1-6 GHz MW fields requires a different distribution of the cells within the CPA for exposures carried out at the lower versus the higher end of the frequency range of interest