Open and closed microfluidics for biosensing

Biosensing is vital for many areas like disease diagnosis, infectious disease prevention, and point-of-care monitoring. Microfluidics has been evidenced to be a powerful tool for biosensing via integrating biological detection processes into a palm-size chip. Based on the chip structure, microfluidics has two subdivision types: open microfluidics and closed microfluidics, whose operation methods would be diverse. In this review, we summarize fundamentals, liquid control methods, and applications of open and closed microfluidics separately, point out the bottlenecks, and propose potential directions of microfluidics-based biosensing.

Paper-based open microfluidic platform for protein electrophoresis and immunoprobing

Protein electrophoresis and immunoblotting are indispensable analytical tools for the characterization of proteins and posttranslational modifications in complex sample matrices. Owing to the lack of automation, commonly employed slab-gel systems suffer from high time demand, significant sample/antibody consumption, and limited reproducibility. To overcome these limitations, we developed a paper-based open microfluidic platform for electrophoretic protein separation and subsequent transfer to protein-binding membranes for immunoprobing. Electrophoresis microstructures were digitally printed into cellulose acetate membranes that provide mechanical stability while maintaining full accessibility of the microstructures for consecutive immunological analysis. As a proof-of-concept, we demonstrate separation of fluorescently labeled marker proteins in a wide molecular weight range (15-120 kDa) within only 15 min, reducing the time demand for the entire workflow (from sample preparation to immunoassay) to approximately one hour. Sample consumption was reduced 10- to 150-fold compared to slab-gel systems, owing to system miniaturization. Moreover, we successfully applied the paper-based approach to complex samples such as crude bacterial cell extracts. We envisage that this platform will find its use in protein analysis workflows for scarce and precious samples, providing a unique opportunity to extract profound immunological information from limited sample amounts in a fast fashion with minimal hands-on time.

Arrayed isoelectric focusing using photopatterned multi-domain hydrogels

Isoelectric focusing (IEF) is a powerful separation method, useful for resolving subtle changes in the isoelectric point of unlabeled proteins. While microfluidic IEF has reduced the separation times from hours in traditional benchtop IEF to minutes, the enclosed devices hinder post-separation access to the sample for downstream analysis. The two-layer open IEF device presented here comprises a photopatterned hydrogel lid layer containing the chemistries required for IEF and a thin polyacrylamide bottom layer in which the analytes are separated. The open IEF device produces comparable minimum resolvable difference in isoelectric point and gradient stability to enclosed microfluidic devices while providing post-separation sample access by simple removal of the lid layer. Further, using simulations, we determine that the material properties and the length of the separation lanes are the primary factors that affect the electric field magnitude in the separation region. Finally, we demonstrate self-indexed photomasks for alignment-free fabrication of multi-domain hydrogels. We leverage this approach to generate arrayed pH gradients with a total of 80 concurrent separation lanes, which to our knowledge is the first demonstration of multiple IEF separations in series addressed by a single pair of electrodes.

On the halt of spontaneous capillary flows in diverging open channels

Due to their compactness and independence of exterior energy sources, capillary microsystems are increasingly used in many different scientific domains, from biotechnology to medicine and biology, chemistry, energy and space. Obtaining a capillary flow depends on channel geometry and contact angle. A general condition for the establishment of a spontaneous capillary flow in a uniform cross section channel has already been derived from Gibbs free energy. In this work, we consider spontaneous capillary flows (SCF) in diverging open rectangular channels and suspended channels, and we show that they do not flow indefinitely but stop at some location in the channel. In the case of linearly diverging open channels, we derive the expression that determines the location where the flow stops. The theoretical approach is verified by using the Surface Evolver numerical program and is checked by experiments. The approach is extended to sudden enlargements, and it is shown that the enlargements can act as stop and trigger valves.