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Knowledge on the availability of dissolved oxygen inside microfluidic cell culture systems is vital for recreating physiological-relevant microenvironments and for providing reliable and reproducible measurement conditions. It is important to highlight that in vivo cells experience a diverse range of oxygen tensions depending on the resident tissue type, which can also be recreated in vitro using specialized cell culture instruments that regulate external oxygen concentrations. While cell-culture conditions can be readily adjusted using state-of-the-art incubators, the control of physiological-relevant microenvironments within the microfluidic chip, however, requires the integration of oxygen sensors. Although several sensing approaches have been reported to monitor oxygen levels in the presence of cell monolayers, oxygen demands of microfluidic three-dimensional (3D)-cell cultures and spatio-temporal variations of oxygen concentrations inside two-dimensional (2D) and 3D cell culture systems are still largely unknown. To gain a better understanding on available oxygen levels inside organ-on-a-chip systems, we have therefore developed two different microfluidic devices containing embedded sensor arrays to monitor local oxygen levels to investigate (i) oxygen consumption rates of 2D and 3D hydrogel-based cell cultures, (ii) the establishment of oxygen gradients within cell culture chambers, and (iii) influence of microfluidic material (e.g., gas tight vs. gas permeable), surface coatings, cell densities, and medium flow rate on the respiratory activities of four different cell types. We demonstrate how dynamic control of cyclic normoxic-hypoxic cell microenvironments can be readily accomplished using programmable flow profiles employing both gas-impermeable and gas-permeable microfluidic biochips.

We present a novel micromachined fast diffusion based mixing unit for the study of rapid chemical reactions in solution with stopped-flow time resolved Fourier transform infrared spectroscopy (TR-FTIR). The presented approach is based on a chip for achieving lamination of two liquid sheets of 10 microm thickness and approximately 1 mm width on top of each other and operation in the stopped-flow mode. The microstructure is made on infrared transmitting calcium fluoride discs and built up with two epoxy negative photoresist layers and one silver layer in between. Due to the highly laminar flow conditions and the short residence time in the mixer there is hardly any mixing when the two liquid streamlines pass through the mixing unit, which allows one to record a mid-IR transmission spectrum of the analytes prior to reaction. When the flow is stopped, the reactant streams are arrested in the flow-cell and rapidly mixed by diffusion due to the reduced interstream distances and the reaction can be directly followed with hardly any dead time. On the basis of two model reactions-neutralisation of acetic acid with sodium hydroxide as well as saponification of methyl monochloroacetate-the performance of the mixing device was tested revealing proper functioning of the device with a time for complete mixing of less than 100 ms. The experimental results were supported by numerical simulations using computational fluid dynamics (CFD), which allowed a reliable, quantitative analysis of concentration, pressure and flow profiles in the course of the mixing process.

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A new concept for the study of chemical reactions in solution by time-resolved Fourier transform infrared spectroscopy (TR/FT-IR) is presented. The key element of this concept is a micromachined mixing unit for fast and highly reproducible diffusion-based mixing that is incorporated in a flow cell for transmission measurements and operated in the stopped-flow mode. The mixing unit achieves multilamination of two liquid streamlines inside the flow cell. When the flow in both feeding channels is maintained, there is almost no mixing of the liquids, because of the short residence time inside the mixer, hence allowing for the recording of a reference spectrum of the reactants prior to reaction. When the flow is stopped by rapid switching of a dedicated injection valve, highly reproducible diffusion-controlled mixing takes place inside the flow cell so that spectral changes induced by the reaction under investigation can be directly followed. The total volume required for one experiment is ∼ 5 μL, and mixing times achieved so far are in the millisecond range. Factors governing time resolution in this new concept are the time required to stop the flow, the spacing of the individual streamlines, the diffusion coefficients of the reactants involved, and the signal strength of the spectral changes induced by the reaction under study. In this paper, the possibilities and limitations of the new concept are studied with the use of three model reactions, which are an acid-base neutralization reaction, the addition of sulfite to formaldehyde, and the basic hydrolysis of methyl monochloroacetate. In addition, the complete mixing process in the system was studied by computational fluid dynamics (CFD) simulations, which provided valuable insights into details of the mixing process itself as well as confirming the experimental results obtained.

Physiological‐relevant in vitro tissue models with their promise of better predictability have the potential to improve drug screening outcomes in preclinical studies. Despite the advances of spheroid models in pharmaceutical screening applications, variations in spheroid size and consequential altered cell responses often lead to nonreproducible and unpredictable results. Here, a microfluidic multisize spheroid array is established and characterized using liver, lung, colon, and skin cells as well as a triple‐culture model of the blood‐brain barrier (BBB) to assess the effects of spheroid size on (a) anticancer drug toxicity and (b) compound penetration across an advanced BBB model. The reproducible on‐chip generation of 360 spheroids of five dimensions on a well‐plate format using an integrated microlens technology is demonstrated. While spheroid size‐related IC50 values vary up to 160% using the anticancer drugs cisplatin (CIS) or doxorubicin (DOX), reduced CIS:DOX drug dose combinations eliminate all lung microtumors independent of their sizes. A further application includes optimizing cell seeding ratios and size‐dependent compound uptake studies in a perfused BBB model. Generally, smaller BBB‐spheroids reveal an 80% higher compound penetration than larger spheroids while verifying the BBB opening effect of mannitol and a spheroid size‐related modulation on paracellular transport properties.

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