Microscale Magnetic Field Modulation for Enhanced Capture and Distribution of Rare Circulating Tumor Cells

OPEN SUBJECT AREAS: ISOLATION, SEPARATION AND PURIFICATION LAB-ON-A-CHIP Received 9 July 2014 Accepted 2 February 2015 Published 4 March 2015 Correspondence and requests for materials should be addressed to X.J.Z. (john.zhang@ dartmouth.edu) Microscale Magnetic Field Modulation for Enhanced Capture and Distribution of Rare Circulating Tumor Cells Peng Chen1, Yu-Yen Huang3, Kazunori Hoshino2 & John X. J. Zhang3 1 Department of Biomedical Engineering, University of Texas at Austin, Austin, TX 78712, USA, 2Department of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA, 3Thayer School of Engineering, Dartmouth College, NH 03755, USA. Immunomagnetic assay combines the powers of the magnetic separation and biomarker recognition and has been an effective tool to perform rare Circulating Tumor Cells detection. Key factors associated with immunomagnetic assay include the capture rate, which indicates the sensitivity of the system, and distributions of target cells after capture, which impact the cell integrity and other biological properties that are critical to downstream analyses. Here we present a theoretical framework and technical approach to implement a microscale magnetic immunoassay through modulating local magnetic field towards enhanced capture and distribution of rare cancer cells. Through the design of a two-dimensional micromagnet array, we characterize the magnetic field generation and quantify the impact of the micromagnets on rare cell separation. Good agreement is achieved between the theory and experiments using a human colon cancer cell line (COLO205) as the capture targets. R are cell separation has been an important emerging process towards early diagnosis of diseases such as cancer1. In particular, Circulating Tumor Cells (CTCs), referring to the cells that have shed into the vasculature from a primary tumor site, and circulate in the bloodstream, have been demonstrated to be clinically significant due to its values in cancer diagnosis, prognosis and treatment monitoring2–4. The detection process usually involves the enrichment of the CTCs from interfering background hematocyte cells, before carrying on subsequent analyses5. To overcome the challenges of the natural rareness, a variety of approaches have been investigated towards efficient separation based on mechanisms such as adhesion6, filtration7, dielectrophoretic separation8, hydrodynamic manipulation9,10, and magnetic attraction11,12. Among these popular methods, the magnetic activated system in combination with immunoassay (also known as ‘immunomagnetic assay’) shows great potential, especially in its low detection limit, high sensitivity, specificity and throughput, which are all necessary for effective clinical applications12. Immunomagnetic assay usually works by selectively labeling the target cells with magnetic tags through specific biomarkers, and using magnetic force generated by permanent magnets to drive the cells for separation. It has been widely used for cell detecting, sorting and manipulating13–16, as summarized in previous review17. However, in traditional immunomagnetic assays, the efficacy of the magnetic field generated by permanent magnets (usually in the scales of centimeter or millimeter) is limited by the low value of magnetic field gradient and the low density of traps. Consequently, the target cells and magnetic tags tend to be captured and aggregated in a confined area. The aggregation may directly impact the structural integrity or quench the fluorescent signals from the target cells, all of which may interfere with cell imaging, identifying and weaken the strength of this approach. We propose a potential solution to the aggregation issue by modulating the in-channel magnetic field through implementing microscale magnetic structures – ‘micromagnets’, which are designed to generate localized strong magnetic field gradient upon magnetization and create multiple distributed capture sites. Modulating magnetic field is critical in a variety of applications, such as cell proliferation regulating18, magnetic particle trapping and manipulating19–21, and chemical kinetic modulation22,23. It usually associates with precise confinement of the magnitude and distribution of the magnetic field and gradient. As for separation purposes, several early studies have been reported on the integration of micromagnets with microfluidic systems. For example, nickel micro-strips have been fabricated to separate leukocytes from whole human blood as magnetic tracks24. Arrays of nickel posts are used in a microfiltration device to separate magnetic beads from non-magnetic beads25. Shrink-induced magnetic traps are used to extract DNA samples for qPCR studies26. Thermomagnetically SCIENTIFIC REPORTS | 5 : 8745 | DOI: 10.1038/srep08745 1 www.nature.com/scientificreports patterned micromagnets are used to separate magnetic and nonmagnetic micro-particles from a mixed solution27,28. However, for rare cancer cell studies, the aforementioned micromagnet structures might not serve the purpose. Since the cancer cells are rather fragile29, the relatively large thickness (.5 mm) of the previous structures might cause physical damages to the cells due to collisions. Therefore, we pursue an ultra-thin structure with submicrometer thickness to minimize possible damages to the cells. Additionally, in the demonstrated applications using aforementioned micromagnets to sort targets with large sub-populations, such as white/red blood cells24, magnetic/non-magnetic microbeads27,28, separation efficiency is the major key parameter that matters. However, when it comes to rare cell studies, each captured target cell needs to be individually addressable, structurally distinguishable, fluorescently visible, and potentially retrievable to facilitate downstream analyses. It posts extra requirements on avoiding cell aggregation. Therefore, we adopt an array design, anticipating the array captures cells discretely and provide a promising tool to generate better distribution of the captured CTCs. In the proposed device, we take a multi-dimensional approach – using permanent magnets for a long-range attraction, and using thin-film micromagnets for short-range retaining. Since magnetic field gradient increases as the size of the magnetic source downscaled, the interactions between target cells and magnetic field can be significantly enhanced on the channel substrate due to the ferromagnetic micromagnets. The patterned thin-film micromagnet approach is also appealing in that the magnetic field enhancement can be realized at ultimately single cell resolution, and can be well controlled by adjusting the geometries, materials, and distributions of the micromagnets during the design and fabrication stages. More importantly, considering the small size of the micromagnets, they can be easily implemented into most of the current immunomagnetic assays seamlessly without affecting other functional compone (...truncated)