Microfluidic aspiration-assisted electrical impedance spectroscopy system is a reliable tool for the characterization of oocyte hardening

The fertilization rate in assisted reproduction depends significantly on the quality of the oocyte, especially on the hardening of the zona pellucida, which is a gelatinous layer of extracellular matrix surrounding the oocyte. Current reproduction techniques use elaborated and frequent manual manipulation steps, which are expensive, non-standardized, and observer dependent. However, a technically simple to perform, reliable and robust, indisputable and observer independent method is needed to characterize the biophysical properties of the oocyte, especially the hardening state of the zona pellucida. Therefore, a microfluidic-based cell trap technique is proposed, which offers a simple oocyte handling through hydrodynamic trapping. The zona pellucida aspiration at an aperture was measured observer-independently by electrical impedance spectroscopy for the first time. Impedance spectra were fitted by an appropriate equivalent electrical circuit model. A numerical in combination with an analytical model correlated the impedance changes with the aspiration length changes, from which Young’s modulus of the zona pellucida was calculated. As a proof-of-concept, the setup was applied to three different genetic mouse oocyte models owing a super soft, a normal and a stiff zona pellucida structure, respectively. The presented measurement technique can either be used for human and animal reproductive medicine to assist artificial fertilization, for the development of contraceptives using the zona pellucida as a barrier or, in general, for all kinds of characterization of rheological properties of single cells.