Illustration of the magnetic micropost array. (A) Cells are plated onto the micropost arrays that contain embedded cobalt nanowires. Microposts have diameters, heights, and center-to-center spacing. Nanowires have diameters and are in length. (B) Traction forces from the cell impart deflections to the microposts. These deflections are measured to calculate the local traction force. (C) Application of a uniform magnetic field induces a magnetic torque on the nanowire that causes an external force on the cell. Force stimulation causes a change in traction forces that can be readily detected.
Fabrication of magnetic micropost arrays. (A) SU-8 photoresist is spin coated onto a silicon wafer and exposed with light through a photomask to pattern the SU-8. (B) Developing the resist results in freestanding SU-8 microposts. (C) Micropost arrays are cast in PDMS to create negative molds. (D) Nanowires suspended in ethanol are aliquoted into the negative molds while under a magnetic field to draw the nanowires into the holes. (E) PDMS is poured into the template to encapsulate the nanowires. (F) The array is peeled from the template and contains both magnetic microposts with nanowires and nonmagnetic microposts.
(A) Preparation of the magnetic microposts starts with microcontact printing of fibronectin onto the microposts. A hydrophobic, fluorescent dye (DiI) impregnates the PDMS for fluorescent microscopy. Pluronics F127 NF (F127) is adsorbed to the PDMS to block cellular adhesion from the sidewalls and base. (B) Cells are plated on the microposts and allowed to spread on the fibronectin surface. (C) After culturing overnight , the cells are ready for testing.
Schematic of the setup for live-cell measurements. Cells on the micropost array are placed in a custom-built microscope chamber that has a sliding rail system with NdFeB magnets to apply a uniform horizontal magnetic field across the array. Temperature and levels are controlled to ensure viability of the cells. The arrays are placed inside a glass-bottom cubic culture dish with medium and video recorded on an inverted fluorescence microscope (not drawn to scale).
Measurement at room temperature of magnetic moment per cobalt nanowire vs magnetic field shows different magnetizations for applied field angle . Inset: Schematic of oriented at angle to the long axis of the nanowire and magnetic moment components and in the parallel and perpendicular directions.
Microscopy imaging of embedded nanowires in the magnetic microposts. (A) Phase contrast image of a cross-sectioned array showing a nanowire embedded in the microposts. (B) Scanning electron micrograph of the array with the contrast of the cobalt nanowire enhanced with backscattering.
Characterization of magnetic micropost deflections. [(A) and (D)] Phase contrast image of magnetic microposts under zero field. [(B) and (E)] Applying a field causes deflection in the magnetic microposts. [(C) and (F)] Displacement vs applied field for magnetic and nonmagnetic microposts labeled in panels A and D.
(A) Phase contrast image of the micropost array with a NIH 3T3 cell attached. (B) Fluorescent image of the same micropost array. The cell outline is traced from the corresponding phase contrast image. The arrow indicates a nonmagnetic micropost nearby the cell. The arrowhead indicates a nonmagnetic micropost to which the cell is attached. The circle indicates the location of the magnetic micropost.
Image analysis for measuring the deflections of the microposts. (A) Image of a micropost [arrowhead in Fig. 8(b)] with an applied traction force by the cell. The center of the micropost is identified with an -center line (red) and -center line (green). (B) Two-dimensional Gaussian curve fit for the image data in panel B. (C) Gaussian fit data (red line) compared with image data (black dots) along the -center line. (D) Gaussian fit data (green line) compared with image data (black dots) along the -center line. [(E) and (F)] Plots of calculated - and -deflections vs time for the post in panel A (red, subscript “C”) and a free post (blue, subscript “F”) identified in Fig. 8(b) with an arrow. The field is turned on at and a force is applied in the positive -direction at the magnetic post. Error bars indicate uncertainty in image analysis.
Live-cell force microscopy results. [(A) and (B)] Red arrows show traction forces before and after application of external force calculated from fluorescent image of the array. Force stimulation at leads to nonlocal changes in the contractility of the cell. (C) Displacement and traction forces vs time for the cell posts (pink), a subset of cell posts labeled in panel B (red), and the magnetic micropost (black).
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