Investigation of particle movement in irregularly shaped channels in inertial fluidics for scale up applications in bioprocessing
Date of Issue2019-07-03
School of Mechanical and Aerospace Engineering
Singapore Institute of Manufacturing Technology
Particle separation is a vital step in many analytical chemistry, biomedical diagnosis, and environmental applications. Inertial microfluidics has emerged in recent years as a promising tool for a wide range of flow cytometric tasks including cell separation, cell counting and mechanical phenotyping. In inertial fluidics, a transverse inertia-induced lift force across streamlines is inherently accompanied with higher order of magnitude convection mass transfer (channel Reynolds number >10), in contrast to microdevices working mainly based on diffusion mass transport phenomenon (Stokes flow) or low-Re flows, increasing throughput significantly. Emerging inertial focusing technique as an alternative method to microfiltration has brought remarkable benefits such as a continuous and clog-free system with lower maintenance costs. These features along with its relative ease of scalability to reach a relevant industrial scale will facilitate its potential adoption in various industries such as waste water treatment and bioprocessing. Of particular interest, dealing with a broader range of particle sizes up to one order of magnitude larger than cell sizes (a > 50 µm) in bioprocessing requires scaled-up channels to avoid clogging. However, Dean-coupled inertial focusing has not been studied in detail when the channel hydraulic diameter is greater than DH ≈ 0.3 mm. Moreover, with the advancement of cell therapy industry in recent years, cell purification at downstream processing introduces some new challenges. While removing particulates from manufactured cell products, using routine membrane technologies similar to protein manufacturing industry do not work as well. This work focuses on the design and development of a membrane-less filtration and separation device using inertial focusing for a large range of particle sizes. To this end, inertial focusing is investigated in straight and mainly curved channels due to their scalability, throughput and efficiency. Inertial focusing is profoundly reliant on the cross-sectional shape of channel and it affects not only the shear field but also the wall-effect lift force near the wall region. The wall-effect lift force is known as a determining factor for cross-lateral migration that leads to a reduced number of equilibrium positions. In order to investigate this, a rectilinear channel with trapezoidal cross-section is designed to break down the symmetrical condition in conventional rectangular microchannels for a broad range of channel Re number (20<Re<800). The altered axial velocity profile and consequently new shear force arrangement leads to a cross-laterally movement of equilibration toward the longer side wall; however, the lateral focusing starts to move backward toward the middle and the shorter side wall, depending on particle clogging ratio, channel aspect ratio, and slope of slanted wall, as the channel Reynolds number further increased (Re>50). Finally, a trapezoidal straight channel along with a bifurcation was designed and used for continuous filtration of a broad range of particle size (0.3<K<1) exiting through the longer wall outlet with ~99% efficiency (Re<100). Nonetheless, though the linear structure of channel can be scaled out relatively easy to reach higher volumetric throughput, it inherently suffers from low particle concentration. Thus, further investigations focused on a curvature-induced secondary flow in conjunction with inertial lift. The mechanism of Dean-coupled inertial focusing inside scaled-up rectangular and trapezoidal spiral channels (i.e., 5-10x bigger than conventional microchannels) is studied with an aim to develop a continuous and clog-free microfiltration system for bioprocessing. Scaling up channel hydraulic diameter one order of magnitude from ~0.1 mm (micron scale) to ~1 mm (millimeter scale) quenches the inertia of flow for a given channel Re number (Re≤500), resulting in deterioration of Dean-coupled inertial focusing (DH > ~1 mm). Accordingly, different scaled-up trapezoidal spirals are developed to (i) filter cell-microcarrier complex (retention device) and (ii) separate microcarriers from cell suspensions. Further biological experimentation validates the applicability of the developed devices. Since particulate contamination in GMP-grade biological products only exists at low concentrations, membrane-based filtration can eliminate large particulates without clogging. However, due to limitation on minimum screen size, particulates smaller than ~70 µm pass through the filter and end up in the manufactured cell products. To alleviate the particulates contamination, a scalable hybrid method using a combination of sieve and inertial-based separator is proposed as a generic method to remove particulates contamination larger than cell sizes ranging from visible to subvisible. Hence, a large-aspect-ratio trapezoidal spiral channel is established and then implemented to remove subvisible particles down to 25 µm from a mesenchymal stem cell suspension possessing a diverse cell size (10 µm ≤a<30 µm). Though the inertial-based separator can reduce the particulate burden in manufactured cell products, it is not a hundred percent reliable when the size of the particulate overlaps with that of cells. Thus, a combination of methods using inertial focusing together with active methods such as dielectrophoresis is needed to manipulate multi biophysical markers such as size and electrical properties.