Purpose: Microfluidic approaches have numerous applications in cell biology. Microfluidics can be used to apply controlled hydrodynamic loads to cells, selectively treat cells with particular chemistries or drugs, and pattern proteins on surfaces. The traditional channel-based microfluidics have broad applications and have been widely used, but in some experiments having to enclose/confine a cell in closed channel can be limiting. The Turner group fabricates and uses devices that allow for the creation of local microfluidic flows in open liquid environments, such as Petri dishes and well plates. These devices, referred to Hydrodynamic Confined Microfluidic (HCM) Probes were first demonstrated by (Juncker, 2005) and other groups, including the Turner group at Penn, have further developed technology and pushed the application space (Christ, 2011). Applications of HCM probes include selective chemical treatment of single cells, biomolecular surface patterning, and cell mechanics studies.
Method of Fabrication/Use: The basic device concept is illustrated, a probe chip containing at least two ports is positioned a short distance above the surface of dish or plate in a liquid environment and fluid is pushed into the gap between the device and plate through one (inlet) port and then extracted through a second (outlet) port. If the gap, flow rates, and devices dimensions are appropriately chosen, a confined flow will be created. The devices are fabricated though a combination of silicon deep reactive ion etching (DRIE), PDMS casting, and bonding. The primary probe chip is structured by using photolithography and deep reactive ion etching to create the ports, mesa geometry, and backside channels. A PDMS backing layer with cast channels and holes for tubing connections is then bonded to the backsurface to facilitate connections to external syringe pumps. For use, the device is mounted on 5-axis positioning stage mounted on an inverted microscope that allows the device to positioned within a liquid-filled well (Christ, 2011).
Results: HCM devices have seen increasing use since their first demonstration in 2005. In the original HCM work, (Juncker, 2005) demonstrated patterning of proteins on surfaces and selective chemical treatment of cells in a dish. More recently, the Turner group has used HCM devices for applying controlled hydrodynamic loads to adherent cells. HCM probes are used to apply hydrodynamic shear stresses to cells in order to probe cell adhesion and cell stiffness. These tests are performed by applying an increasing flow rate to a cell and monitoring the deformation and detachment of the cells via optical microscopy. The HCM probes have been used to study the effect of cell shape (controlled via surface patterning) on cell adhesion (Christ, 2011). A recent review article (Qasaimeh, 2013) summarizes a number of applications of HCMs in the life sciences and medicine. One interesting application shown in this article is the use of HCM probes for the local immunohistochemistry on sections of ductal carcinoma breast tissues (Qasaimeh, 2013). This allows for staining on spots of ~100mm in size, thus permitting multiple staining on the same slice, resulting in a reduction of the amount of tissue needed.
Juncker D, Schmid H, Delamarche E. Multipurpose microfluidic probe. Nat Mater Aug 2005; 4 (8): 622-628.
Christ K. Hydrodynamically-confined microfluidics for cell adhesion strength measurement. Dept. of Mechanical Engineering 2011 Ph.D.
Christ KV, Turner KT. Design of hydrodynamically confined microfluidics: controlling flow envelope and pressure. Lab Chip Apr 21 2011; 11 (8): 1491-1501.
Qasaimeh MA, Ricoult SG, Juncker D. Microfluidic probes for use in life sciences and medicine. Lab Chip Jan 7 2013; 13 (1): 40-50.