Recent Progress On Microfluidic Electrophoresis Device Application In Mass Spectrometry
With its ability and ease in use of low flow rates, ESI is shown to be compatible with microfluidic devices. Hence, it is a widely utilized method of ionization for use in online microfluidic-MS analysis applications. The advancement in the development of microfabrication technology, the coupling process of main types of microfluidic systems such as analog, digital and droplet microfluidics to MS through ESI has become increasingly more mainstream as the technology exhibits increased competence in use. The three kinds of microfluidics (analog, conventional and channel-based) have gained popularity thanks to their flexibility in a wide array of analyses such as sample preparation, pre-concentration, micro-reactions as well as separation. Here we describe the most commonly used couple means, the analog microfluidic systems to ESI-MS followed by specific applications when using these systems.
Those who are new to microfluidics may be partial to conventional emitters over integrated emitters as fabrication can remain challenging. Subsequently, numerous applications and approaches have been discussed in the literatures that rather use external emitters placed internally within the microfluidic substrate.
Droplet microfluidics has come to light as an extremely capable technique wherein highly monodispersed droplets of picoliter to nanoliter volume range can be spawned and manipulated with great frequency. Due to the inherent independent nature of the droplets that are generated in a multi-phase environment, each individual droplets can be thought of as independent micro-reactors wherein no cross-contamination or interaction between droplets occur. Due to the potential for high throughput detection in a digitalized manner, droplet composition analysis via Mass spectrometry has garnered significant attention. DMF exhibit similar pros as droplet microfluidics as individual droplets allow minimized crosstalk per analyses while imbuing an added edge of not requiring any particular pre-designed microfluidic channel layout before experiments as the electrode array provides the liberty for customized number of user-designed analyses that can be performed using a single device. In order for DMF devices to be coupled with ESI-MS, a couple of main issues must first be addressed; first, the transference of droplets from the DMF to the ESI emitter, and second, and dissociation of the voltages utilized in droplet manipulations and ESI spray voltage. Another preferred technique to couple microfluidics to mass spectrometry is MALDI, with the use of both microfluidics with MALDI primarily centering around the assumption of sample preparation steps such as fraction collection from separations or protein digestion processes. Furthermore, variability resulting from different users can be minimized MALDI spotting is automated, a welcome feature as the matrix mixing and spot application could impact sample analysis.
Microfluidics technologies are distinguished by their ability to manipulate and mix minute volumes of solutions and reagents through the use of network of channels and reaction chambers. As a result, the features of microfluidic devices also complement their use in analyses as well as culturing of cells and tissues. As cells represent the fundamental building blocks of all living things, the familiarity and fluency in the workings of cellular functions is critical cross several disciplines of cellular biology, human physiology, and tissue engineering with basic cellular studies involving the three utmost crucial steps – steps-isolation, culture, and analysis. Stem cells are able to undergo sustained self-renewal through replication and become precursor cells to specific tissue types, which offers a stable source of pathogen-free and physiologically relevant cells that can differentiate into mature somatic cells both invivo and invitro. However, novel LoC platforms allow better precise control and mimicry of various parameters inherent to complex invivo tissues, which can benefit our understanding of the biology and raise the clinical potential of stem cell-based therapies. A great deal of research that has been performed wherein stem cells have been utilized to examine various life processes. Klein et al. has developed a high-throughput droplet-MF method for barcoding of RNA sourced from thousands of individual cells for future analysis by next generation sequencing (NGS) and analyzed mouse embryonic stem cells.
Another field of interest for MF application is Neurology. MF has proven useful in both invivo and invitro drug delivery applications via on-chip reservoirs located on neural implants or in neuronal cells through high-precision delivery of growth and inhibitory factors using gradient-generating devices respectively. Complex interactions that exist among neural cells can be difficult to probe when using conventional means of analysis. Under these assumptions, MF has proven to be a most well-suited technique in neurology experiments. For example, when using MF devices with charged membrane-permeable, potential-sensitive dyes, it is possible to obtain a quick and highly sensitive determination of transmembrane potential with the use of minimal reagents. NMR (nuclear magnetic resonance) micro coils where NMR probes that are micro-fabricated on the glass substrate MF platform and sucrose solution have been used effectively in the study and testing of single non-perfused neurons. MF principles and techniques has also proven itself useful when applied to isolate brain tissue culture. Under invitro, separation of brain tissue remains a complicated task requiring impeccable control over experimental conditions as well as access to neural networks and synapses. The drug development process can be broken down to two phases, first, drug discovery, followed by drug testing. Phase one is comprised of selection of target, lead identification, and preclinical studies. The development stage includes clinical trials, manufacture, and the product lifecycle management. Compact MF devices small enough are coming to as novel tools in the study of target selection, lead identification, optimization, preclinical test and dosage advancement.
In biology, cell sorting is considered as a preliminary step in cellular level studies. As a result, Microfluidic chip-based sorters that leverages on system miniaturization have garnered a lot of interest over recent years. Microfluidic cell sorters employ a varied number of passive and active cell sorting mechanisms such as electric, magnetic, hydrodynamic and optical mechanisms. The most popular method is the electric sorting mechanism due to its flexibility, integration, and viability for automation. Sorting cells using the electro-kinetic switching is also a common approach, however, electro-kinetic interactions have shown to be relatively too weak when used with large cells or particles. Recently, Yao et al. combined both gravity and electro-kinetic forces for use in flow cytometry and fluorescence-activated cell sorting, with the system applied to determine the necrotic and apoptotic effects of ultraviolet light on HeLa cells. Discrimination of cells via magnetic nanoparticle tagging in microfluidic devices have also been reported by Ingris et al. The most common and standard method employed by microfluidic cell sorters is the hydrodynamic method. Here, cell sorting is done through determination of cells exhibiting fluorescent signals and sorting via active switching of flow directions. Recent advances in high-throughput hydrodynamic cell sorting has been reported by Chabert and Viovy et al. , wherein high throughput encapsulation of isolated cells is spontaneously self-sorted picoliter droplets. Optical sorting mechanism and their encompassing types, such as optical tweezers and optical traps, have proven attractive for microfluidic sorters due to the mechanism’s inherent non-physical contact and prevention of contamination. Separation of fluorescent cells using microfluidic chips via optical force switching has been demonstrated by Wang et al. In order for cells to be effectively analyzed on microfluidic chips, it is necessary for cells to be immobilized. Cell patterning in microfluidic channels by soft lithographic techniques such as micro-contract printing, patterning via microfluidic channels, and laminar flow patterning are common methods.
An important focal point of application for MF technologies have been in biomedical diagnostics, as MF technology possess unique features that make it appealing for the fabrication of Point of Care (PoC) testing devices. To date, some previous DNA separation techniques and diagnostics have been successfully miniaturized. Various molecules such as DNA, RNA and chemicals have often been analyzed with numerous MF systems for both general purpose and disease diagnostics.
