A Microfluidic Device for the Hydrodynamic Immobilisation Of
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Sensors and Actuators B 192 (2014) 36–41 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb A microfluidic device for the hydrodynamic immobilisation of living ଝ fission yeast cells for super-resolution imaging a a,∗ b b c Laurence Bell , Ashwin Seshia , David Lando , Ernest Laue , Matthieu Palayret , c c Steven F. Lee , David Klenerman a Cambridge Nanoscience Centre, 11 J J Thomson Avenue, Cambridge CB3 0FF, United Kingdom b Department of Biochemistry, 80 Tennis Court Road, Cambridge CB2 1GA, United Kingdom c Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, United Kingdom a r t i c l e i n f o a b s t r a c t Article history: We describe a microfluidic device designed specifically for the reversible immobilisation of Schizosac- Received 24 April 2013 charomyces pombe (Fission Yeast) cells to facilitate live cell super-resolution microscopy. Photo-Activation Received in revised form 16 August 2013 Localisation Microscopy (PALM) is used to create detailed super-resolution images within living cells with Accepted 1 October 2013 a modal accuracy of >25 nm in the lateral dimensions. The novel flow design captures and holds cells in a Available online 28 October 2013 well-defined array with minimal effect on the normal growth kinetics. Cells are held over several hours and can continue to grow and divide within the device during fluorescence imaging. Keywords: © 2013 The Authors. Published by Elsevier B.V. All rights reserved. Microfluidics Single cell analysis Hydrodynamic trapping PALM imaging Single molecule localisation microscopy 1. Introduction normal growing environment (at least over the time span in which the cell is expected to be within the device) is critical [10–14]. This Super-resolution microscopy is an optical imaging technique means that a way to control the exchange of nutrients and dissolved which can allow three dimensional mapping of fluorescent par- gases, in suitable media and waste, to and from the cells, needs to ticles with accuracy far beyond the diffraction limit of visible light be found while holding the cell reliably in place long enough to (∼250 nm) [1]. However, its use in imaging tagged molecules within locate, manipulate (if necessary), and image. This must all be done living cells is currently restricted, by the need to hold the target cell without excessive movement or damage to the cell if the integrity very still over an extended period of time [2]. For immobilisation of the final data is to be reliable. to occur there must be some form of interaction with the cell and In order to increase the spatial precision in an imaging exper- various approaches to this have been described [3–5]. However, iment, PALM sacrifices temporal resolution in order to achieve for realistic investigations it is important that the trapping method optimal results. PALM, therefore, places strict requirements upon used permits normal growth and division of the cell to occur while the preparation of the samples, such as control over fluorescent protecting it from excessive shear forces. Hydrodynamic trapping contamination, avoidance of photo-conversion from ambient addresses many of these problems to some extent, and has also light sources and the need to keep the sample immobile in a been described before [6–9], but adapting these techniques to reli- well-defined space during the course of an imaging experiment. ably and automatically capturing and holding a large array of cells, Attempts to image living cells over long time periods in growing in a suitably high density is challenging; capabilities which are conditions have been limited because of this; many requirements ultimately necessary for a robust and user-friendly device suited for healthy growing conditions (such as allowing for the free to super-resolution microscopy. The maintenance of a healthy, exchange of nutrient and waste around the cells), are in stark contrast to those for super-resolution imaging. Microfluidics can provide a solution to this stalemate, but to be of any practical use, the device must be simple and robust enough to be adopted by ଝ This is an open-access article distributed under the terms of the Creative researchers who might have little experience with microfluidics, Commons Attribution License, which permits unrestricted use, distribution, and who in any case will want to focus on the microscopy and reduce reproduction in any medium, provided the original author and source are credited. ∗ the time and cost spent on sample preparation. In a recent review Corresponding author. Tel.: +44 1223 760333; fax: +44 1223 760309. E-mail address: [email protected] (A. Seshia). of Single Molecule Localisation Microscopy (SMLM), Patterson 0925-4005/$ – see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.10.002 L. Bell et al. / Sensors and Actuators B 192 (2014) 36–41 37 Fig. 1. Image of the fabricated device showing external fluidic interconnects. The trapping layer is 52 mm from tip to tip, 10 mm wide and 5 m thick, and is covered by a thicker control layer. et al. listed this limitation as one of the main challenges facing these techniques, along with improvement of the instrumentation and the choice of fluorescent probe [15] (Fig. 1). Fission yeast (Schizosaccharomyces pombe) is a genetically tractable eukaryotic organism that is used extensively in molec- ular and cell biology. They are a unicellular rod-shaped eukaryote, which typically measure ∼4 m in diameter. Importantly, the dimensions of the long-axis of the cell (7–14 m) is directly cor- Fig. 2. COMSOL simulation output showing the evolution of the flow velocity over related to the point in the cell division cycle [16], allowing the 11 rows. The pressure difference is 1 kPa between the inlet, the outlet and the walls interrogation of biological phenomena, such as DNA replication, are assumed to exhibit full slip. The colour scale has been limited to values between 0 (dark blue) and 25 mm/s (scarlet). (For interpretation of the references to colour repair, etc., within the context of the cell cycle. As the diameter in this figure legend, the reader is referred to the web version of the article.) of the cell does not change during replication S. pombe is espe- cially useful for immobilisation with a hydrodynamic flow for super-resolution imaging in defined orientations and locations, The device contains three fluidic inlets feeding into four trapping over extended periods. While a number of previous groups have regions all draining to a single, common drain (see Supplementary reported cell trapping and live cell studies in microfluidic devices, Information). The thicker control layer allows much higher flow the particular requirements of PALM imaging in fission yeast place volumes, providing temperature control by rapidly drawing heated significant requirements on device design. or cooled water through the device. We present herein a device that was developed to meet the The trapping region was designed with a zigzagging main chan- criteria necessary for PALM imaging of structures within living nel forming 69 rows, each with 28 basins along it. Each basin is S. pombe cells over multiple cell cycles. These included hydro- 22 m wide with a central plug channel at the bottom, which is 2 m dynamically trapping cells within 1 m of the imaging surface wide and 35 m long. This plug channel forms a shorter path to the with minimal movement over extended time frames ranging from next row. It is this reduction in path length, which increases flow approximately 5–10 min to several hours, with control over the through the plug and helps to draw a cell into the basin. Success- supply of fresh media as well as the temperature within the ful immobilisation of a cell removes this flow conduit, forcing later device. cells to continue past the basin and towards the next empty one. To improve the flow through each basin, the main channel gradually 2. Device design narrows along each row, decreasing in width by 1 m after every basin, and returning to 50 m at the beginning of each new row. The device utilises a ‘basin’-style design for hydrodynamic As the main channel needs to maintain sufficient flow to carry trapping [17], consisting of small trapping wells, each with a filter- cells and debris past the end of each row, the length of the rows ing ‘plug’ channel. This allows a slight pressure difference to build was limited to 28 basins, leaving the main channel 22 m wide at up when the plug is blocked by a cell. This holds the cell in place its thinnest point. The channels and trapping basins are designed whilst allowing further flow to move past the site and so avoid to be 5 m high to allow for S. pombe cells to be flushed through the blockage. The relative amount of flow through a plug compared to device and simultaneously enable effective flow reduction across the flow around the basin determines the likelihood that a pass- the trapping basin once a cell has been trapped. Fig. 2 shows a ing cell will be drawn into it – consequently the flow around the simplified 2D simulation of the fluid flow velocity through 11 rows basin must be minimised to make the flow through dominant. This of the trapping region. It was modelled using Comsol Multiphysics was done by forming rows of basins and gradually constricting the 4.0a, and shows that the fluid flow in the trapping region quickly thickness of the main channel passing them to further increase the forms a repeating pattern from row to row as it moves away from flow through the plugs.