  Force spectroscopy using magnetic tweezers (MT) is a powerful method to probe
the physical characteristics of single polymers. Typically, molecules are
functionalized for specific attachment to a glass surface at one end and a
micron-scale paramagnetic beads at the other. By applying an external magnetic
field, multiple molecules can be stretched and twisted simultaneously without
exposure to potentially damaging radiation. The majority of MT utilize moving
permanent magnets to produce the forces on the beads (and the molecule under
test). However, translating and rotating the permanent magnets may require
expensive precision actuators, limits the rate at which force is changed, and
may induce vibrations that disturb tether dynamics. Alternatively, the magnetic
field can be produced through an electromagnet which allows much faster force
modulation and eliminates motor-associated vibration. Here, we describe a
low-cost quadrapolar electromagnetic tweezer design capable of manipulating
DNA-tethered MyOne paramagnetic beads with forces of up to 20 pN. The
solid-state nature of the generated B-field modulated along two axes is
convenient for accessing the range of forces and torques relevant for studying
the activity of DNA motor enzymes like polymerases and helicases. Our design
specifically leverages technology available at an increasing number university
maker spaces and student-run machine shops. Thus, our design is not only
applicable to a wide range biophysical research questions, but also an
accessible tool for undergraduate education.
