Design and optimization of arrays of neodymium iron boron-based magnets for magnetic tweezers applications

Abstract

We present the design methodology for arrays of neodymium iron boron (NdFeB)-based magnets for use in magnetic tweezers devices. Using finite element analysis (FEA), we optimized the geometry of the NdFeB magnet as well as the geometry of iron yokes designed to focus the magnetic fields toward the sample plane. Together, the magnets and yokes form a magnetic array which is the basis of the magnetic tweezers device. By systematically varying 15 distinct shape parameters, we determined those features that maximize the magnitude of the magnetic field gradient as well as the length scale over which the magnetic force operates. Additionally, we demonstrated that magnetic saturation of the yoke material leads to intrinsic limitations in any geometric design. Using this approach, we generated a compact and light-weight magnetic tweezers device that produces a high field gradient at the image plane in order to apply large forces to magnetic beads. We then fabricated the optimized yoke and validated the FEA by experimentally mapping the magnetic field of the device. The optimization data and iterative FEA approach outlined here will enable the streamlined design and construction of specialized instrumentation for force-sensitive microscopy.

A Horizontal Magnetic Tweezers and Its Use for Studying Single DNA Molecules

“…Magnetic tweezers are uniquely suited for studying the mechanical response of biopolymers under controlled forces—such experiments are said to be performed in the constant force ensemble in which the DNA’s thermally-averaged extension is measured as a function of externally-generated forces. Technical advances now allow applied forces in the 0.1–100 pN range and extensions down to angstroms to be reliably measured (although typical setups are limited to detecting extension changes in the order of 1–10 nm) [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. In addition to a pulling force, the DNA’s twist (or, more generally, linking number) [ 13 ] can also be modulated or, in a fixed-torque assay, direct, simultaneous and independent control of forces and torques on the DNA is also possible [ 14 , 15 , 16 ].…”

A Guide to Magnetic Tweezers and Their Applications

“…This is a key factor that defines performance of the tweezers. The generated force is proportional to the gradient of magnetic field and directed toward the area with the strongest field [1,[15][16][17]. As a result, magnetic tweezers display different properties depending on the configuration of the field.…”

Simple Horizontal Magnetic Tweezers for Micromanipulation of Single DNA Molecules and DNA—Protein Complexes

“…Tethers have superparamagnetic particles attached to one end that are subject to forces in the 0.1 piconewton (pN) to 100 nanonewton (nN) range when placed in non-uniform magnetic fields (sufficient to saturate the particles) (3,9,10). The magnetic fields are produced using permanent magnets or electromagnets (11,12). The twist of DNA can also be manipulated by rotating the magnets (2).…”