Recent developments in microfluidic passive separation to enable purification of platelets for transfusion

Abstract

Platelet transfusion is a lifesaving therapy intended to prevent and treat bleeding. However, in addition to platelets, a typical unit also contains a large volume of supernatant that accumulates multiple pro-inflammatory contaminants, including residual leukocytes, microaggregates, microparticles, antibodies, and cytokines. Infusion of this supernatant is responsible for virtually all adverse reactions to platelet transfusions. Conventional methods for removing residual leukocytes (leukoreduction) and reducing the volume of transfused supernatant (volume reduction) struggle to mitigate these risks holistically. Leukoreduction filters can remove leukocytes and microaggregates but fail to reduce supernatant volume, whereas centrifugation can reduce volume, but it is ineffective against larger contaminants and damages platelets. Additionally, platelet purification based on these methods is often too logistically complex, time-consuming, and labor-intensive to implement routinely. Emerging microfluidic technologies offer promising alternatives through passive separation mechanisms that enable cell separation with minimal damage and drastically reduced instrumentation size and facility requirements. This review examines recent innovations in microfluidic cell separation that can be used for leukoreduction and volume reduction of platelets. It begins by defining the performance requirements that any separation method must meet to successfully replace conventional methods currently used to perform these tasks. Standard performance metrics are described, including leukocyte depletion efficiency, degree of volume reduction, processing throughput, and platelet recovery. Finally, the review outlines the primary challenges that must be overcome to enable simple-to-use, disposable microfluidic devices capable of both reducing the platelet unit volume and removing pro-inflammatory contaminants, while preserving most functional platelets for transfusion.

FIG. 1.

Purification of platelets for transfusion. (a) Current paradigm and limitations of conventional methods. (b) Potential benefits of applying microfluidic cell separation for purifying platelets.

FIG. 2.

(a) Microfluidic trapping: Microfabricated filter elements selectively capture larger cells while allowing smaller cells to flow through the structure. The design of the filter elements can be optimized to enable separation based on cell size, shape, and deformability. (b) Crossflow filtration: The sample flows parallel to the filter membrane while a pressure differential established across the membrane drives the flow of the filtrate. Cells smaller than the membrane pore size pass through with the filtrate, while larger cells are retained in the main flow stream. (c) Bifurcation law (plasma skimming or Zweifach–Fung effect): When flow splits at an unequal bifurcation, the daughter branch drawing less flow receives an even lower fraction of cells compared to the higher-flow branch. The size of cells carried by the fluid into the low-flow branch is determined by the width of the extracted flow lamina entering that branch. (d) Controlled incremental filtration: A small fraction of the flow in the main channel is extracted through a series of filtration gaps on either side. With each subsequent gap, the dimensions of the channels are incrementally adjusted to maintain the width of the extracted flow lamina, and thus the size of the cells carried into the side channels by the filtrate.

FIG. 3.

(a) Inertial focusing in straight microfluidic channels: Driven by the balance between the shear-induced and wall-induced lift forces, cells migrate laterally across the channel cross section into narrow equilibrium positions which depend on cell size, channel dimensions, flow rate, and fluid viscosity. (b) Hydrodynamic cell-wall interactions: The size-dependent effect of the wall-induced lift creates a separation between the smaller and the larger cells, the latter being carried off by the sheath flow. (c) Viscoelastic effects: Co-flow of a Newtonian (sample) and viscoelastic (sheath) fluid creates a stable interface, where the balance of the inertial and elastic lift forces causes a size-dependent cell separation.

FIG. 4.

(a) Deterministic lateral displacement (DLD): An array of posts where each subsequent row is shifted laterally to split off a relatively small flow lamina around each post. Cells that are small enough to be carried by the split flow laminae continue to follow the overall flow through the array. Larger cells are displaced laterally as they bump into each subsequent post. (b) Non-equilibrium inertial separation array (NISA) enhances the classical DLD design by taking advantage of the strong dependence of the inertial lift force on the size of cells flowing near rectangular posts.

FIG. 5.

(a) Dean drag (Dean's vortices or secondary flow): In curved channels, the difference in flow velocity between the center and the near-wall regions creates a pressure gradient, which causes secondary radial flow patterns to arise (so-called Dean vortices). The associated drag drives lateral migration of cells to the equilibrium positions across the channel cross section. These positions are determined by cell size, channel dimensions, radius of curvature, and flow rate. For rectangular channels with high aspect ratio, larger cells move closer to the inner wall, while smaller cells stay in the center of the channel. (b) Multi-dimensional double spiral (MDDS): The first rectangular spiral channel concentrates all cells toward the inner wall. The second spiral channel separates the smaller from larger cells by enhancing the Dean drag effect via its trapezoidal cross section. (c) Curved channels with U-shaped cross section: Larger cells are retained near the inner wall, while smaller cells migrate toward the outer wall. The use of U-shaped cross section minimizes the issues of mixing and recirculation that occur in channels with rectangular cross sections.