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.