Uwe Schnakenberg

Ligand-gated ion channels are involved in numerous pathophysiological processes and are, therefore, important targets for drug discovery. We describe a new microfluidic lab-on-a-chip device that allows the recording of fast ligand-gated ion channel kinetics in Xenopus laevis oocytes by non-invasive electrical impedance spectroscopy (EIS). The device, designated as Aachen Fast Ion Channel Sensor (AFIS) system, consists of two chambers separated by an orifice for capturing an oocyte by applying suction to the lower chamber. For channel recordings, annular electrodes are placed above and below the orifice. As a proof of concept, we recorded the EIS responses of the X. laevis oocyte-expressed human P2X2 receptor (hP2X2R), which functions as a non-desensitizing adenosine triphosphate (ATP)-gated cation-selective channel. The impedance data were fitted with an appropriate equivalent electrical circuit model. After ATP application, both EIS and TEVC recordings showed similar fast responses, which reflected the rapid solution exchange. Concentration–response curves determined by EIS or TEVC were virtually similar, indicating that agonist potency can be reliably determined by EIS. The oocytes survived the EIS measurements intact and can be reused for further electrophysiological or other purposes. The AFIS system can be used with the noticeable electrical leakage between the oocyte and the orifice. This feature significantly reduces the amount of preparation time required to create a high-resistance seal. Therefore, the system offers the possibility to easy perform high-throughput drug screening, especially when multiple measurement sites are placed on one chip and a multiplexer is connected for simultaneous EIS measurements.

Digital microfluidic systems, based on the electrowetting-on-dielectric mechanism, allow the manipulation, dispensing, merging, splitting, and mixing of micro- to nanoliter droplets on hydrophobic surfaces by applying voltages to an array of planar electrodes. The manipulation of both a non-aqueous and an aqueous phase droplet in a single experiment has gained considerable interest. This study focuses on characterizing the dispensing and dosing of 1-octanol droplets, merging with a water droplet, and phase separation with minimal residue formation by shearing off the biphasic droplet at a tear-off edge of a hydrophilic well, using optimized actuation parameters. The volume of the 1-octanol droplet dispensed from an L-junction reservoir design increased with increasing dispensing speed. Dispensing can only occur within a certain reservoir volume range. Under identical conditions, 1-octanol droplets could be dispensed with volume variations of less than 0.55%, and manipulated at a maximum velocity of 5.6 mm/s when the frequency of the applied AC voltage was about 200 Hz. At the tear-off edge of the hydrophilic well, the 1-octanol residue on the water droplet was reduced to less than 0.15% of the original 1-octanol droplet volume. The results will be used for future applications, such as for the precise quantitative characterization of the reaction kinetics of complex parallel or sequential interfacial catalytic reactions, for the study of self-assembly processes or for liquid–liquid extractions at the 1-octanol–water interface.

The noninvasive and label‐free electrical characterization of single cells is increasingly gaining interest over the last years. Besides cancer cells, oocytes are in the focus of those investigations. The study of oocytes is relevant for two areas: on the one hand, the quality of mammalian oocytes is important in artificial reproduction technology for a high fertilization yield. On the other hand, amphibian oocytes are well established to be used as host cells for the characterization of ion‐channel kinetics for drug screening. For both areas, easy‐to‐perform and reliable characterization techniques are needed. This mini‐review summarizes important developments of characterizing oocytes with microfluidic systems in combination with observer‐independent noninvasive electrical sensing. Such miniaturized microfluidic systems can be improved to facilitate multiple oocyte characterizations simultaneously for higher throughput screening.

In in vitro fertilization, the status of the zona pellucida is of pivotal interest to determine the window of best fertilization. In this window, the zona pellucida becomes soft, so that a sperm can penetrate. Two lab-on-chip systems will be introduced which were developed to monitor the status of the zona pellucida of mouse oocytes with electrical impedance spectroscopy. The combination of microfluidic systems and impedance spectroscopy offer advantages of easy cell handling and observer-independent characterization.

Ligand-gated ion channels (LGICs) are involved in many pathophysiological processes and represent a relevant target for drug discovery. In this work, we developed a novel setup based on a microfluidic chip in combination with non-invasive electrical impedance spectroscopy to study ligand-induced changes in Xenopus laevis membrane impedance. As a proof-of-concept, the responses of the human P2X2 receptor hP2X2R as a cation-selective, ligand-gated ion channel, which is gated by adenosine triphosphate, is reported. The impedance measurements show short capacity transients compared with two electrode voltage clamp measurements. In future, the measurement sites can be multiplied and facilitate multiplexed drug screening of ligand-gated ion channels.

This work presents research on thin magnetron-sputtered platinum (Pt) films deposited over commercial gas diffusion electrodes and applied to convert and pressurize hydrogen in an electrochemical hydrogen pump. The electrodes were integrated into a membrane electrode assembly with a proton conductive membrane. Their electrocatalytic efficiency toward hydrogen oxidation and hydrogen evolution reactions was studied in a self-made laboratory test cell by means of steady-state polarization curves and cell voltage measurements (U/j and U/p_diff characteristics). The achieved current density at a cell voltage of 0.5 V, the atmospheric pressure of the input hydrogen, and a temperature of 60 °C was more than 1.3 A cm⁻². The registered increase in the cell voltage with the increasing pressure was only 0.05 mV bar⁻¹. Comparative data with commercial E-TEK electrodes reveal the superior catalyst performance and essential cost reduction of the electrochemical hydrogen conversion on the sputtered Pt films.

Computational fluid dynamics simulations are used to obtain a radially symmetrical flow profile at an aperture of a microfluidic channel by manually arranging flow‐directing obstacles around the aperture. For the first time, the flow profile is improved by calculating the coefficient of variation (CoV), which is applied to the flow velocity magnitude along coaxially arranged circles below the aperture in the microfluidic channel. A minimum CoV of 0.03 is obtained at the circle with the same diameter as the aperture. Channels with the optimized design are fabricated by lamination of two layers of dry film resist and patterned with UV‐lithography. The aperture is formed by using an inductively coupled plasma reactive ion etching process. Video recording of the flow behavior confirmed the simulation results experimentally. The proposed channel design and the optimization method can favorably be used for a balanced hydrodynamic trapping of particles or cells at apertures.

The success rate in vitro fertilization is significantly linked to the quality of the oocytes. The oocyte’s membrane is encapsulated by a shell of gelatinous extracellular matrix, called zona pellucida, which undergoes dynamic changes throughout the reproduction cycle. During the window of highest fertility, the zona pellucida exhibits a softening phase, while it remains rigid during oocyte maturation and again after fertilization. These variations in mechanical properties facilitate or inhibit sperm penetration. Since successful fertilization considerably depends on the state of the zona pellucida, monitoring of the hardening process of the zona pellucida is vital. In this study, we scrutinized two distinct genetic mouse models, namely, fetuin-B wild-type and fetuin-B/ovastacin double deficient with normal and super-soft zona pellucida, respectively. We evaluated the hardening with the help of a microfluidic aspiration-assisted electrical impedance spectroscopy system. An oocyte was trapped by a microhole connected to a microfluidic channel by applying suction pressure. Transient electrical impedance spectra were taken by microelectrodes surrounding the microhole. The time-depending recovery of zona pellucida deflections to equilibrium was used to calculate the Young’s modulus and, for the first time, absolute viscosity values. The values were obtained by fitting the curves with an equivalent mechanical circuit consisting of a network of dashpots and springs. The observer-independent electrical readout in combination with a fitting algorithm for the calculation of the viscoelastic properties demonstrates a step toward a more user-friendly and easy-to-use tool for the characterizing and better understanding of the rheological properties of oocytes.