Diagnostic assays represent an integral step in the treatment and management of a broad spectrum of pathologies. Therefore, a principal aim of contemporary medical science is to refine and decentralize diagnosis by improving the rapidity, fluidity, and sensitivity of the sample-to-result process. Recent years have seen the emergence of micro total analysis systems, which offer portability, integration, and high-throughput processing, while concurrently lowering reagent use, total cost, and risks of mishandling.
Microfluidic techniques offer unique advantages over conventional bench-top methods by integrating sample preparation with analysis. By using microscale channels and tubes, microfluidic devices can minimize dead volume and sample waste. To further reduce processing volume, external tubing or pumps can be eliminated. Passive fluid actuation without the use of active pumps has been reported, including capillary-driven flow and vacuum-driven flow, which is ideal for resource-constrained environments at remote sites. Although multiple steps are typically required to prepare samples for biochemical assays, microfluidic devices can automate these steps in a single device, thereby minimizing manual sample handling and preserving sample integrity, as well as improving diagnostic accuracy and reproducibility.
Preparing plasma or serum is a requirement, and often the first step, in current medical diagnostics. Microfluidic platforms present many advantages for extracting blood plasma, owing to their microscale processing; these advantages include fast turnaround times, automated operation, reduced sample volumes, reduced cost, and portability. In addition, the microscale enables distinct physical phenomena to become apparent in streamlines and near boundaries, greatly facilitating the separation of smaller molecules, such as proteins and DNA, from larger species, such as cells.
Fig.1 The layout of an integrated centrifugal microfluidic system with three independent platforms for whole blood preparation and enzyme-linked immunosorbent assay (ELISA) operation. (Cui, 2015)
Often, as in the case of nucleic acid testing, target biomarkers must first be released from shielding membranes or viral protein capsids. Once large assay-inhibiting particulates have been removed and target analytes have been released into solution via cell or virion lysis, the chemical species of interest must be enriched and extracted from the lysate. This vital step serves to eliminate any inhibitors to downstream processes, to protect the analyte from enzymatic degradation and ensure stability, as well as to concentrate the analyte to improve the limit of detection and reduce background noise. Many methods of microfluidic cell lysis exist, and these utilize mechanical, thermal, or electrical energy to disrupt cell membranes, although the technique most commonly implemented in emerging micro total analysis systems devices is chemical lysis. This is often carried out using chaotropic agents, such as guanidinium thiocyanate, or, for bacterial lysis, enzymatic degradation by muramidase.
Fig.2 Device design for simultaneous purification of nucleic acids and proteins from serum using simultaneous cationic and anionic isotachophoresis (ITP) processes. (Cui, 2015)
The clinical analysis of urine is one of the most important noninvasive inspections used for medical diagnosis. In addition to its frequent use for tests for drug abuse and toxin exposure, urine can provide crucial diagnostic information about kidney diseases, metabolic disorders, or other pathologies. Unlike the preparation of blood samples, preparing urine samples for microfluidic assays is straightforward because it does not require pretreatments to prevent severe channel clogging. Nonetheless, since urine is also a complex biofluid that contains diverse entities, it is usually necessary to prepare samples for purification or enrichment, or both, prior to analyzing specific targets urine.
Saliva is a preferred biofluid sample for monitoring patients at home because collecting it is noninvasive. Nonetheless, processing saliva samples for analysis is extremely difficult, mainly because of the presence of oral particulate matter and adhesive mucins, which are responsible for the high viscosity and glycoprotein content. Untreated saliva that contains these interferents often fouls bioanalysis equipment and makes pipetting inaccurate. To successfully analyze saliva, it is necessary to remove these interferents while maintaining the target concentration. A simple microfluidic method to condition saliva samples before analysis has been proposed. Researchers used partially membrane-filtered saliva containing a reduced amount of mucins as the input for their microfluidic filtration channel to further eliminate mucins and glycoproteins. This microfluidic filter, called an H-filter, uses laminar flows in narrow microchannels for diffusive extraction of impurities. Using this conditioning protocol, 97% of mucins and 92% of total proteins are removed while retaining a significant amount of target analytes, which corresponds to a threefold enrichment of the analyte compared with the direct dilution of the filtrate sample.
The importance of microfluidics in sample processing is self-evident. Creative Biolabs has been focusing on microfluidics over years. We have established a comprehensive one-stop microfluidic solution platform and provide a variety of microfluidic-based services including but not limited to:
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