Microwave microfluidic sensors for analyzing magnetic bead-based bioliquids: from theory to experimental validation

Two novel microwave sensors integrated with microfluidic techniques are introduced and experimentally validated, intended for testing magnetic bead solutions. The first is a transmission line sensor that operates over a wide frequency range from 1 GHz to 110 GHz, utilizing an easily integrable coplanar waveguide (CPW) structure. The second sensor is based on resonant principles and operates at approximately 25 GHz, composed of a CPW structure and a spiral-shaped defect ground structure (DGS). By selecting this frequency band, the minimum size of the DGS sensing area is restricted to 296 μm, about 0.025λ0, which greatly reduces the volume of liquid samples needed. The transmission parameters of both sensors are analyzed using air, deionized (DI) water/ethanol solution, and magnetic bead solution as materials under test (MUTs). Micro-nano fabrication and on-chip measurements are conducted to validate the proposed sensors. Experimental results for the transmission line sensor demonstrate that the variation in |S11| can be used as a parameter to detect magnetic beads in solutions across a wide frequency range. For the resonance sensor, the measurements show that an increase in the real part of the complex relative permeability results in a redshift of the resonance frequency, whereas an increase in the imaginary part reduces the quality factor. These effects are consistent with those observed for complex permittivity. However, a notable difference is that an increase in the real part of permeability also causes a slight decrease in |S21|, which aids in signal detection. In addition to the proposed sensors, a terahertz (THz) spectroscopy transmission method is also experimentally employed to investigate the permittivity of different types and concentrations of magnetic bead solutions. Our study offers a new perspective for enhancing detection sensitivity in microwave microfluidic biosensors by co-utilizing permittivity and permeability. This approach shows great promise for applications in cell sorting and precise measurement of bioliquids.