Microfluidic Plasma Separation for Medical Diagnostics
Human blood is made up of red and white blood cells, platelets, and plasma. Plasma comprises about 55% of blood volume. It carries all elements of blood in suspension throughout the body. Plasma also carries nutrients, hormones, and proteins to where they’re needed in the body, and carries cell waste products away.
Antibodies, proteins, and blood clotting factors travel through the circulatory system in blood plasma. These substances can be separated from donated blood plasma and concentrated to provide treatment for chronic conditions such as hemophilia or autoimmune disorders. The concentrates also help treat burns, shock, and trauma. Providing plasma to patients suffering from these conditions boosts blood volume, combatting shock, and helps with blood clotting.
Plasma separated from blood cells is a yellowish liquid composed mostly of water, but also containing glucose, electrolytes, clotting factors, proteins, oxygen, and carbon dioxide. Plasma helps maintain a balanced electrolyte concentration in the bloodstream. It’s vital to the process of oxygen exchange— where red blood cells deliver oxygen through the body—and cleaning up carbon dioxide. Plasma plays a vital role in protecting against infections and blood disorders.
In addition to providing treatment to patients who need it, plasma can provide important information to clinicians and researchers. To do this, plasma must be separated from the blood cells it carries. Blood cells can interfere with analysis and detection of important biomarkers that indicate disease conditions or prior exposure to pathogens (antibodies).
Traditional Separation Methods
Traditionally, plasma has been separated from other blood components using a centrifuge. Blood in a tube that contains an anticoagulant to prevent clotting spins in the centrifuge, causing sedimentation—the blood cells fall to the bottom of the tube, leaving the plasma to be poured or suctioned off.
Traditional methods of plasma separation require a laboratory environment, bulky equipment, and extensive training. In the second decade of the 21st century, researchers began looking for ways to miniaturize the separation process to enable the development of point of care medical diagnostic devices. The goal was to create methods that could provide rapid results in a point of care or near point of care environment.
Microfluidic diagnostic devices are lightweight, portable, and require less intensive training for proper use. They provide fast results, which has been critical throughout the COVID-19 pandemic.
Microfluidic Devices for Plasma Separation
The challenge for microfluidic plasma separation for medical diagnostics was to find a way to replicate the sedimentation effect that traditional centrifuging provided, but with much smaller blood samples.
Microfluidic devices have many advantages: they’re portable, provide fast sample processing, and are easy to fabricate compared to traditional equipment. A variety of techniques have been proposed or developed, falling into two major groups: active and passive separation.
Active separation occurs when the clinician or technician applies an external force or field to the device, such as centrifugation, or a magnetic, acoustic, or electric field.
Passive separation doesn’t require an external force. Instead, it relies on the natural, characteristic behaviors sample’s components when introduced to the microfluidic system. Passive separation methods include filtration, lateral displacement, sedimentation on a cellular scale, and the hydrodynamic effects created in the device.
Capillary flow in lateral displacement type devices combined with a paper filtration membrane has achieved separation by agglutination of red blood cells. Unfortunately, in some of these devices, filtration resulted in a significant loss of proteins from the separated plasma, potentially affecting the accuracy of analyses. The filtration paper in these devices had a high surface-to-volume ratio, and these seemed to capture proteins along with blood cells in the agglutination process.
One solution proposed in a paper published online in April, 2020 that the analytical chemistry section of the American Chemical Society was to use a “synthetic paper” made of interlocked micropillar scaffolds. The patented synthetic paper used in testing this method was a porous material with a low internal surface area, made for use in lateral flow diagnostic testing devices.
In the study, it was described as inexpensive to make. The patented synthetic paper could be deployed in layers, creating efficient capillary action while retaining a lower internal surface area to reduce loss of proteins during cell agglutination.
The authors concluded that the synthetic paper performed well, providing efficient, fast plasma separation with high protein recovery. They posited that synthetic paper could find its most immediate use in lateral flow devices.
Another device design described in a June 2020 paper published on PLOS ONE by Siddhartha Tripathi and Amit Agrawal combined a passive separation device with an enzyme-linked immunosorbent assay (ELISA) to detect antibodies to COVID-19. The simple, passive device design relies on the geometrical and biophysical effects of blood flow in a microchannel. Blood cells drop out while nearly pure plasma moves on through the device. The design has been extensively tested and patented.
Still another technique would use thermopneumatic suction. The design employs a battery-powered heating wire to create a low-pressure environment. The difference in internal and external air pressure drives the sample into the device’s microchannel. The simple design employs an inlet for the sample, a microchannel, a filter “trench,” test strips, and a suction chamber. The filter trench employs gravitational sedimentation to separate plasma from blood cells.
Factors Affecting Device Performance
Each proposed type of microfluidic device for plasma separation described above had varying factors. These included the materials used, methods of fabrication, cost effectiveness, and purity of the derived plasma. The percent of plasma derived versus the volume of the sample was also a factor, as was the purity of the separated plasma.
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