Design and calibration of a 3D printed magnetic tweezer combined with cellphone-based microscope for biomolecular force spectroscopy

Abstract

Motor proteins are a vital component of cell cytoskeleton which are responsible for performing many cell functions. These proteins walk on actin filaments or microtubules, which can be a plus or minus end walker dependingon the super and sub families of the protein. Our understanding of single protein force generation has improved due to recent studies using optical tweezers for force spectroscopy, however real cell functions involve multiple motors working together and generating large forces. These high force (>101 piconewtons) measurements cannot be done using optical instruments due to fundamental and practical limitations. These can be overcome by implementing magnetic tweezers. Calibrated magnetic fields can allow us to apply forces on a micrometer sized superparamagnetic bead ranging from 102 of femtonewtons to 101 piconewtons. Here I report the design and calibration of a low-cost 3D printed permanent magnet based single- and multi-molecular force spectroscope, in combination with a cellphone for image acquisition mounted on a low-cost compound microscope. 2D field simulations were performed to understand the magnetic field and energy profile. We optimized the 3D CAD drawings to result in large and uniform field gradients for accurate force measurements. Calibration of the device was performed on 2.8 µm diameter superparamagnetic beads. The viscous drag and speed of bead movement from single particle tracking of bright-field images was used to experimentally estimate the force. We measure an average force of 1.2±0.6 pN acting on the beads. The setup can then potentially be used to study aspects of force generation by different kinds of molecular motors in both in vivo and in vitro experiments.