Flexible Toolbox of High-Precision Microfluidic Modules for Versatile Droplet-Based Applications

Although the enormous potential of droplet-based microfluidics has been successfully demonstrated in the past two decades for medical, pharmaceutical, and academic applications, its inherent potential has not been fully exploited until now. Nevertheless, the cultivation of biological cells and 3D cell structures like spheroids and organoids, located in serially arranged droplets in micro-channels, has a range of benefits compared to established cultivation techniques based on, e.g., microplates and microchips. To exploit the enormous potential of the droplet-based cell cultivation technique, a number of basic functions have to be fulfilled. In this paper, we describe microfluidic modules to realize the following basic functions with high precision: (i) droplet generation, (ii) mixing of cell suspensions and cell culture media in the droplets, (iii) droplet content detection, and (iv) active fluid injection into serially arranged droplets. The robustness of the functionality of the Two-Fluid Probe is further investigated regarding its droplet generation using different flow rates. Advantages and disadvantages in comparison to chip-based solutions are discussed. New chip-based modules like the gradient, the piezo valve-based conditioning, the analysis, and the microscopy module are characterized in detail and their high-precision functionalities are demonstrated. These microfluidic modules are micro-machined, and as the surfaces of their micro-channels are plasma-treated, we are able to perform cell cultivation experiments using any kind of cell culture media, but without needing to use surfactants. This is even more considerable when droplets are used to investigate cell cultures like stem cells or cancer cells as cell suspensions, as 3D cell structures, or as tissue fragments over days or even weeks for versatile applications.

Parametric studies on droplet generation reproducibility for applications with biological relevant fluids

Although the great potential of droplet based microfluidic technologies for routine applications in industry and academia has been successfully demonstrated over the past years, its inherent potential is not fully exploited till now. Especially regarding to the droplet generation reproducibility and stability, two pivotally important parameters for successful applications, there is still a need for improvement. This is even more considerable when droplets are created to investigate tissue fragments or cell cultures (e.g. suspended cells or 3D cell cultures) over days or even weeks. In this study we present microfluidic chips composed of a plasma coated polymer, which allow surfactants-free, highly reproducible and stable droplet generation from fluids like cell culture media. We demonstrate how different microfluidic designs and different flow rates (and flow rate ratios) affect the reproducibility of the droplet generation process and display the applicability for a wide variety of bio(techno)logically relevant media.

Formulation of organic and inorganic hydrogel matrices for immobilization of β-glucosidase in microfluidic platform

The aim of this study was to formulate silica and alginate hydrogels for immobilization of β-glucosidase. For this purpose, enzyme kinetics in hydrogels were determined, activity of immobilized enzymes was compared with that of free enzyme, and structures of silica and alginate hydrogels were characterized in terms of surface area and pore size. The addition of polyethylene oxide improved the mechanical strength of the silica gels and 68% of the initial activity of the enzyme was preserved after immobilizing into tetraethyl orthosilicate-polyethylene oxide matrix where the relative activity in alginate beads was 87%. The immobilized β-glucosidase was loaded into glass-silicon-glass microreactors and catalysis of 4-nitrophenyl β-d-glucopyranoside was carried out at various retention times (5, 10, and 15 min) to compare the performance of silica and alginate hydrogels as immobilization matrices. The results indicated that alginate hydrogels exhibited slightly better properties than silica, which can be utilized for biocatalysis in microfluidic platforms.

Uncovering toxicological complexity by multi-dimensional screenings in microsegmented flow: modulation of antibiotic interference by nanoparticles

The technique of microsegmented flow was applied for the generation of two- and higher dimensional concentration spaces for the screening of toxic effects of selected substances on the bacterium Escherichia coli at the nanolitre scale. Up to about 5000 distinct experiments with different combinations of effector-concentrations could be realized in a single experimental run. This was done with the help of a computer program controlling the flow rates of effector-containing syringe pumps and resulted in the formation of multi-dimensional concentration spaces in segment sequences. Prior to the application of this technique for toxicological studies on E. coli the accuracy of this method was tested by simulation experiments with up to five dissolved dyes with different spectral properties. Photometric microflow-through measurement of dye distribution inside the concentration spaces allowed the monitoring of microfluid segment compositions. Finally, we used this technique for the investigation of interferences of the antibiotics ampicillin and chloramphenicol towards E. coli cultures and their modulation by silver nanoparticles by measuring bacterial autofluorescence. Each concentration point in this three-dimensional concentration space was represented by 4 or 5 single segments. Thus, a high reliability of the measured dose/response relations was achieved. As a result, a complex response pattern was discovered including synergistic and compensatory effects as well as the modulation of the range of stimulation of bacterial growth by a sublethal dose of chloramphenicol by silver nanoparticles.