Flow-Induced Transport of Tumor Cells in a Microfluidic Capillary Network: Role of Friction and Repeated Deformation

Abstract

Introduction: Circulating tumor cells (CTCs) in microcirculation undergo significant deformation and frictional interactions within microcapillaries. To understand the physical parameters governing their flow-induced transport, we studied the pressure-driven flow of cancer cells in a microfluidic model of a capillary network.

Methods: Our microfluidic device contains an array of parallel constrictions separated by regions where cells can repetitively deform and relax. To characterize the transport behavior, we measured the entry time, transit time, and shape deformation of tumor cells as they squeeze through the network.

Results: We found that entry and transit times of cells are much lower after repetitive deformation as their elongated shape enables easy transport in subsequent constrictions. Furthermore, upon repetitive deformation, the cells were able to relieve only 25% of their 40% imposed compressional strain, suggesting that tumor cells might have undergone plastic deformation or fatigue. To investigate the influence of surface friction, we characterized the transport behavior in the absence and presence of bovine serum albumin (BSA) coating on the constriction walls. We observed that BSA coating reduces the entry and transit time significantly. Finally, using two breast tumor cell lines, we investigated the effect of metastatic potential on transport properties. We found that the cell lines could be distinguished only upon surface treatment with BSA, thus surface-induced friction is an indicator of metastatic potential.

Conclusions: Our results suggest that pre-deformation can enhance the transport of CTCs in microcirculation and that frictional interactions with capillary walls can play an important role in influencing the transport of metastatic CTCs.

Keywords: Capillary; Friction; Microfluidics; Repeated deformation; Tumor cells.

Figure 1

A microfluidic capillary network for probing flow induced squeezing and friction of tumor cells. (a) A schematic of the cell passing through a single constricted capillary (left) vs. in a network of capillaries (right). The surrounding fluid can bypass at capillary junctions; (b) a schematic showing repeated tumor cell deformation; (c) a bright field image of the microfluidic device (top) containing 4 columns (C1–C4) and 27 rows of constricted channels with W × L × H = 11.8 μm × 91.8 μm × 15 μm. Three cells (MCF-7) have been identified in red, blue and green boxes, and their corresponding trajectories are shown as lines in the same color. The x-axis shows marked gradations on a linear scale highlighting areas where cells undergo compressional strain and relaxation. Scale bar is 100 μm. The bottom row shows the three cells enlarged.

Figure 2

Metrics used to characterize tumor cell transport in the microfluidic capillary network. (a) The entry time is the time that a cell takes to fully enter the constriction channel upon its first encounter with the constriction walls; (b) the transit time is the time elapsed between when a cell has fully entered the constriction and prior to its exit; (c) the deformation index (DI) is the ratio of the major axis of the cell at each location in the device to the major axis prior to deformation in the constriction.

Figure 3

Influence of cell size and occlusion events on transport metrics. (a) Schematic illustrating the absence and presence of cell-to-cell contact before entering the constriction channels; (b) entry time as a function of cell size, in the presence and absence of cell-to-cell contacts; (c) transit time as a function of cell size. The legend is the same as (b). The data here is shown for MCF-7 cells.

Figure 4

Transport dynamics of cancer cells passing through the microfluidic capillary network. (a) A representative trajectory of a tumor cell in the microfluidic capillary network. Highlighted (in red) are the constriction zones C1–C4; (b) variation of the deformation index (DI) for the same cell shown in (a); (c) the average DI of five cells with very similar size while relaxing in the relaxation chambers after each constriction array. The DI values of C2–C4 are significantly different from C1, p value <0.05; (d) average entry and transit time ± SEM for the same five cells, shown in (c). The data here is shown for MCF-7 cells. *p-value <0.05. All the data shown was obtained in BSA-coated channels.

Figure 5

Influence of BSA coating on tumor cell transport. (a) A schematic showing that surface-dependent friction of tumor cells can be influenced by factors such as wall roughness, coating and electrostatic interactions; (b) the size distribution of MCF-7 cells used in the experiments with uncoated and BSA-coated constriction arrays. Fraction of cells is the ratio of cells in each size range to the total number of cells; (c) entry time ± SEM of MCF-7 cells in the presence (n = 61) and absence (n = 50) of BSA coating, shown for cells passing through the constriction arrays; (d) transit time ± SEM of MCF-7 cells in the presence and absence of BSA coating, shown for cells passing through the construction arrays. *p-value <0.05, N.S. not significant.

Figure 6

Deformability dependent friction and its relation to tumor cell metastatic potential. (a) A stiff cell (left) exerts strong normal force on the channel walls whereas a soft cell (right) exerts a weak force on channel walls; (b) entry and transit times ± SEM of breast tumor cells from their first deformation in BSA-coated and uncoated devices. The MDA-MB-231 and MCF-7 cells only differ in BSA coated devices. For BSA-coated devices, n = 61 and 49 for MCF-7 and MDA-MB-231 cells, respectively. For uncoated channels, n = 50 and 24 for MCF-7 and MDA-MB-231 cells, respectively; (c) the size distribution of MDA-MB-231 and MCF-7 cells used in obtaining the data for BSA coated channels; (d) the relaxation behavior (DI ± SEM) of MDA-MB-231 and MCF-7 cells in uncoated devices. The data shown is the mean obtained from n = 5 cells of nearly the same size; (e) the same as (d), except the data is obtained from BSA-coated devices. In both (d) and (e) there is no statistical difference between the two cell types. *p-value <0.05, N.S. not significant.

Figure 7

The relation between repeated deformation and metastatic potential of tumor cells. The entry time ± SEM of highly (MDA-MB-231) and weakly (MCF-7) metastatic breast cells in (a) uncoated (b) BSA-coated constriction arrays. The transit time ± SEM of highly (MDA-MB-231) and weakly (MCF-7) metastatic breast cells in (c) uncoated (d) BSA-coated constriction arrays. In untreated constrictions, the cells did not show difference in entry and transit times, whereas in BSA-coated constrictions, both entry and transit time showed a significant difference in all arrays, except for transit times of array C2. For BSA-coated devices, n = 61 and 49 for MCF-7 and MDA-MB-231 cells, respectively. For uncoated channels, n = 50 and 24 for MCF-7 and MDA-MB-231 cells, respectively. *p-value <0.05, N.S. not significant.