CHARACTERIZING MECHANICAL PROPERTIES OF CANCER CELLS BY NODE-PORE SENSING

Junghyun Kim1*, Andy Lei2, and Lydia L. Sohn1 1Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA, 94720, USA

and 2Department of Bioengineering, University of California at Berkeley, Berkeley, CA, 94720, USA ABSTRACT

We present a novel multiparametric microfluidic platform based on node-pore sensing to characterize the mechanical properties of cancer cells. Our platform quantifies cellular diameter, deformation under constant strain, and the transit velocity required to pass through a contraction region, simultaneously. Our results show that breast cancer cells (MCF-7 and MDA-MB-231) have greater transit velocities than, but similar deformation characteristics as, normal epithelial breast cells (MCF10A). We define a deformation index (DI), a mathematically derived dimensionless number based on the physical parameters our platform measures, to evaluate the deformability of cells and successfully use this parameter to differentiate cancer cell types. KEYWORDS: Mechanical properties, Cancer cells, Single cell analysis, Node pore sensing

INTRODUCTION

Mechanical properties of cancer cells are an important cue to grade the metastatic potential of cancers and consequently can be potentially used as a label-free biomarker [1]. A variety of quantitative systems have been employed and/or developed to evaluate the mechanical properties of cells, including atomic force microscopy [2], optical tweezers [3], microplate rheometery [4], hydrodynamic stretching cytometry [5], and microfluidic transit analyzer [6]. However, many of these platforms can only characterize one pa-rameter (e.g. deformation or surface friction) and few are applicable when a heterogeneous population of cells, such as that found in a tumor biopsy, is involved. Here, we have developed a novel microfluidic platform that integrates a node-pore sensor [7] with a contraction channel to measure multiple physical properties of cancer cells (cellular diameter, deformation, and transit velocity), simultaneously. We demonstrate that our platform is capable of distinguishing cells from different cancer cell lines based on the mechanical properties measured.

Figure 1: A) A photographic image of the microfluidic platform. B) Schematic of the platform show in A, with a close-up view (red dashed box) of the nodes, pore, and the contraction channel. An additional node-pore combination can be found on the other side of the contraction channel (not shown). C) Ex-pected current pulse produced by a cell (green circle) transiting the microfluidic channel. ΔInp and ΔIcont correspond to the magnitude of the current change when the cell passes through a node-pore or contrac-tion channel, respectively. The current pulse is symmetric in shape due to the symmetric arrangement of node-pores on either side of the contraction channel.

499978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA

MATERIALS AND METHODS Figure 1A shows an actual image of our overall platform, and Figure 1B shows a magnified

schematic view of the key microfluidics. As shown, the device consists of a polydimethylsiloxane (PDMS) mold that is bonded to a glass substrate with pre-defined platinum (Pt) electrodes and gold (Au) contact pads. Embedded within the PDMS mold is a primary microfluidic channel (30 µm x 25 µm x 8000 µm, H x W x L ) segmented by two nodes spaced 1155 µm apart. We refer to the segment between the nodes as the “pore” (Figure 1B). The primary channel subsequently connects to a “contraction” channel with dimensions 30 µm x 10 µm x 2055 µm (H x W x L). We use soft lithography to fabricate the PDMS mold and standard photolithography to pattern the electrodes and contact pads onto glass substrates. Using electron-gun evaporation, we deposit a 75/250 Å Ti/Pt thin film for the electrodes and 75/250/250 Å Ti/Pt/Au thin film for the contact pads. To complete the device, we first expose the PDMS mold and glass substrate to an oxygen plasma (470 mTorr, 80 W, 30 sec), then align and place the two together, and finally place the device on a hotplate at 80°C for 60 minutes.

Using a non-pulsatile pressure (3psi), we drove breast cancer cells (MCF-7 and MDA-MB-231, 1.5x105 cells/mL) and normal epithelial breast cells (MCF-10A, 1.5x105 cells/mL) through the completed device. We employed a four-point measurement with a DC voltage (1V) to measure the current across the entire microfluidic channel. As it transits the channel, a cell disrupts the electric field lines parallel to that channel, and a current pulse is subsequently produced. As shown in Figure 1C, the current pulse shape corresponds to the cell transiting the primary channel, the nodes, and the contraction channel. The different pulse magnitudes, ΔInp and ΔIcont, corresponds to the diameter of the cell without strain (Dcell) and the deformed diameter (Ddeformed) in the transverse direction when the cell is under constant strain [8], respectively. The transit time required for a cell to pass through the contraction channel is indicated by ΔT in the figure. Custom software written in Matlab was used to low-pass filter a pulse, identify its structure, and determine its magnitude (Figure 2A).

Figure 2: A) The current pulse produced by an MCF-7 cell (top), the same pulse low-pass filtered (mid-dle), and the derivative of the pulse with respect to time (bottom). The pulse derivative is employed to identify pulse widths. B) Transit velocity of MCF-7, MDA-MB-231, and MCF-10A cells through the con-traction channel (n=68 cells). Error bars represent standard deviation. C) Deformation (Ddeformed/Dcell) of the same MCF-7, MDA-MB-231, and MCF-10A cells (n=68). Error bars represent standard deviation. D) Distribution of the deformation index (DI) for the different cell types screened. RESULTS AND DISCUSSION

Through our measurements, we have found that the transit velocity (Ucell) of the cells in the contraction channel varies according to cell type: normal epithelial breast cells (MCF-10A) travel more slowly though the contraction channel than breast cancer cells (MCF-7 and MDA-MB-231) (Figure 2B). In contrast, all three cell types show similar cellular deformation, Ddeformed/Dcell, under constant strain in

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the contraction channel (Figure 2C). Using dimensional analysis [9], we define a new dimensionless parameter, the deformation index (DI),

cell

deformed

flow

cell

flow DD

UU

UkDI ×==µ

(1)

where k, Uflow, and µ are cell stiffness, the fluid velocity in the contraction channel, and viscosity of fluid, respectively. As shown in Figure 2D, cancer cells have larger DI values as compared to those of normal epithelial cells. Furthermore, MCF-7 cells have highest DI values among all three cell types screened. Despite their high degree of known invasive potential, MDA-MB-231 cells have relatively smaller DI values as compared those of MCF-7 cells. This surprising result can be explained by various external factors that are related to cancer metastasis such as interstitial flow patterns, the biophysical condition of extracellular matrix, and molecular interaction with the microenvironment [10]. CONCLUSION

Our integrated platform consisting of a node-pore sensor and a contraction channel enables one to measure multiple physical parameters of cancer cells simultaneously, thereby characterizing their mechanical properties. Furthermore, our newly defined deformation index (DI) can evaluate deformability and ultimately be employed to differentiate particular cancer-cell types within a heterogeneous cell population.

ACKNOWLEDGEMENTS

This research was funded by NIH 1R01CA190843-01 and NIH 1R21EB019181-01A1. J. K. is sup-ported by Jung-Song Fellowship.

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cancer cells in metastatic sites," Nature Reviews Cancer, 2, 563-572, 2002. CONTACT * Junghyun Kim; phone: +1-510-542-1631; jh.kim@berkeley.edu

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