Nonlinear Microwave Susceptibility Measurements Using Intermodulation Products on a Microfluidic Platform

Measuring the nonlinear responses of living cells has enabled an understanding of their behavior and functionality. Currently, this response has been studied at radio and optical frequencies, leaving an unexplored gap in the field of microwaves. This paper presents a system that combines microwave technology with a microfluidic platform to measure the nonlinear susceptibility of living organisms to electromagnetic fields. The applied technique involves feeding the system with two tones (2.1 GHz and 4 GHz) to generate third-order intermodulation products (PIMP) at 5.9 GHz. Nonlinear susceptibility was measured from the power levels of PIMP using a spectrum analyzer. Broadband electrodes based on the slot bowtie geometry were manufactured to operate at 5 GHz with a bandwidth of 4 GHz. Additionally, an engineering process is presented to optimize the power of the internal mixer of the spectrum analyzer to obtain the maximum dynamic range and improve the sensitivity of the system. Nonlinear susceptibility to microwaves was analyzed in four samples: pure ethanol, a mixture of ethanol and dimethyl sulfoxide (DMSO), live Escherichia coli (E. coli), and heat-killed E. coli. The results show that ethanol has zero nonlinear susceptibility, whereas when it is mixed with DMSO, a nonlinear response appears at a value of 4 dB with respect to the nonlinear susceptibility of the system in the absence of a sample. Finally, the nonlinear susceptibility of live E. coli to microwaves was detected, with a difference of 8 dB over the reference value and 6 dB with respect to the heat-killed E. coli sample.