High Force Magnetic Tweezer Device

The high force magnetic tweezer device is a scientific rheometer used to study the mechanical properties of biological materials in the micrometer range, especially the cytoskeleton of cells. Especially according to cells it is necessary to understand the dynamics of cellular force responses in order to understand cell growth and differentiation. The high force magnetic tweezer setup is based on the setup designed by Kollmannsberger et. al. 2007, which is used to estimate the viscoelastic cell properties both in the linear and non-linear deformation regime. This noninvasic technique consists of an electro magnet applying precise forces without any contact to small micron sized magnetic particles that are bound via focal adhesion complex to the cytoskeleton of cells. Using optical bead tracking during force application the displacement of the magnetic particles can be measured in real time with a precision of one thousandth of the particle diameter.

Figure 1| Magnetic tweezer priciple. (A) Photographic image of the magnetic tweezer device. (B) A high magnetic field gradient is generated by a needle-shaped high-permeability core of a solenoid attached to a micromanipulator. The gradient force generated by the magnetic tweezer acts on superparamagnetic beads coated with proteins from the extra cellular matrix. Beads are bound to the cell surface via integrin receptors that connect the extracellular space with the intracellular cytoskeleton.

High precisional electro magnet

The principal item of the magnetic tweezer device is the magnetic tweezer arm shown in figure 2. It consists of a special steel bar with a diameter of 4.5 mm where the end closest to the specimen is tapered to concentrate the flux so that a large field and field gradient is created. The magnetic field itself is generated through a cooper coil wound around a brass body connected to the steel core. The magnetic field is measured by a hall probe sitting at the other end of the steel core.

Figure 2| Magnetic tweezer arm. The main body of the magnetic tweezer device consists of a steel bar (2) which is tapered at one end (1) Die needle shaped tip has a tip radius of about 10µm. A copper coil (3) with about 250 windings generates the magnetic field. At the back end of the core a hall probe (5) is installed with which the induced magnetic field can permanently be measured. The whole construction can be positioned by the micromanipulator (4).

High force application

Left animation loop shows the strong deformation of a cell due to an applied force ramp up to 80nN with the high force magnetic tweezer device. This particular cell could withstand this force without bead disruption. The bead trajectory is tracked in real time. Forces of up to over 200nN can be used to investigate the adhesion strength of cells to the extra cellular matrix. Disruption forces for each cell are detected and used for further analysis.

Figure 3| Animation loop of a strong cell deformation with a force ramping up to 80 Nanonewtos. (A) Viscoelastic cell response (blue) due to an applied force step (red). (B) Cell response with rupture event (blue) due to an increasing force ramp (red)

Experimental setup

The complete tweezer setup is shown in figure 4. The setup was maximally user friendly designed. During the whole measurement, the user has all controls in striking distance and can thus concentrate exclusively on the measurement itself. From his place all necessary adjustments can be performed from adjusting the intensity, probe positioning as well as the tweezer positioning. A wireless numpad is adjusted to the micromanipulator containing all necessary software commands needed for the tweezer software. A wireless mouse pad and keyboard enables the user to control any other adjustments on the computer.

Figure 4| User-friendly magnetic tweezer setup. (1) Control desk for operating the magnetic tweezer, the microscope and the computer (2) The arm of the magnetic tweezer is built on the side of the microscope, reaching into the probe, which can be positioned via motorized x-y microscopic stage. (3) During measurement, all information are shown on the pc screen. (4) Magnetic tweezer power supply. References
P. Kollmannsberger, B. Fabry, High-force magnetic tweezers with force feedback for biological applications, Rev. Sci. Instrum. 78 (2007) 114301–114306.