A tunable, microfluidic filter for clog-free concentration and separation of complex algal cells

Abstract

An inherent problem with microfluidic filters is the tendency to clog, especially when applied to cells due to their geometrical complexity, deformability, and tendency to adhere to surfaces. In this work, we handle live algal cells of high complexity without signs of clogging, achieved by exploiting hydrodynamic interactions around trilobite-shaped filtration units. To characterize the influence of cell complexity on the separation and concentration mechanisms, we compare the hydrodynamic interactions to those of synthetic, rigid microparticles. We discover that simple rolling along the filter structures, which prevents clogging for particles, cannot be applied to cells. Instead, we find that inertial effects must be employed to minimize the filter interactions and that this modification leads to only a minor reduction in device performance.

Appendix

1.1 Swimming behavior of algae

The marine dinoflagellates (M1,M2) have two flagella and have a helical swimming pattern. C. rostratiformis (F1) has two flagella, the forward with thin hairs is dragging the cell forward while erected. Figure 8 shows trajectories of the fastest swimmer used in this study, namely algae of P. reticulatum (M2). Figure 8a was obtained using a 4 $\times$ objective and Fig. 8b was obtained using a 10 $\times$ objective, and the exposure time of both images was 5 s. Based on the length of the streaks, the swimming speed is estimated to $\sim 0.15\,\hbox {mm/s}$, which is negligible compared to the speed of cells in the flow field around the separation units ($\sim 2\,\hbox {m/s}$). Therefore, the swimming activity of cells used in this study does not influence the separation dynamics.

Fig. 8

Long-exposure images of swimming cells in drops deposited onto a microscope slide: a trajectories of Protoceratium reticulatum (M2) in a 2 $\upmu$l drop viewed through a 4$\times$ objective clearly shows the helical swimming pattern. The speed is negligible compared to the speed of a cell in the flow field around the separation units, and thus, the swimming does not influence the separation and concentration dynamics. b Long-exposure image of swimming Protoceratium reticulatum (M2) in a 110 $\upmu$l drop viewed through a 10$\times$-objective. Scale bar is $400\,\upmu \hbox {m}$. Exposure time is 5 s

To be able to be stationary in the liquid without sinking or rising and to ease the task of swimming, the densities of the swimming organisms are only about $5\%$ higher than that of water. The F2 cell is not a swimmer, but its disk-shaped body enables migration across streamlines, which allows it to be easily transported by whirls and currents over large distances. To fully exploit this transportation method, this cell is also nearly neutrally buoyant.

1.2 Algal cultures

The micro-algae used in this study were obtained from the Norwegian Culture Collection of Algae. The marine dinoflagellates P. minimum (strain UIO 089) and P. reticulatum (strain UIO 232) were isolated from the Oslofjorden, Skagerrak, Norway, in 1986 and 2001, respectively. The freshwater micro-algae M. truncata (strain NIVA-CHL 34) was isolated from River Storelva, Ringerike, in S. Norway in 1978 and C. rostratiformis (strain NIVA-3/81) from Lake Helgetjernet in 1981.

The marine cultures were grown in the algal medium IMR $\frac{1}{2}$ medium (Eppley et al. 1967), supplemented with 10 nM selenium (Edvardsen et al. 1990) with salinity of 25 PSU. The freshwater algal strains were grown in the medium Z8 (Kotai 1972) or a modified Z8 medium. The media were sterilized by pasteurization at $80\,^{\circ }\hbox {C}$ for 20 min. All four strains were grown at $16\,^{\circ }\hbox {C}$ under fluorescence white light with an irradiance of approximately $50\,\upmu \hbox {mol}$ photons/$\hbox {m}^{2}/\hbox {s}$, and a 14h:10h light:dark cycle. They were grown as batch cultures in 5 l Erlenmeyer flasks and harvested in the exponential or beginning of stationary growth phase.