A ribbon figure-8 coil design for scalable and selective Trans-Spinal Magnetic Stimulation (TSMS)

Trans-Spinal Magnetic Stimulation (TSMS) is gaining attention in medical research as a promising, non-invasive alternative to conventional treatments for refractory chronic neuropathic back pain, such as spinal cord stimulation (SCS) [1–2]. Through a time-varying magnetic field, TSMS delivers electrical pulses to the spinal cord through a coil positioned on the patient’s back. The stimulation must be focal and penetrate deeply enough to reach the spinal cord, which may vary in depth depending on the individual [3]. The various commercial TSMS coils are built by winding metal wires with different numbers of turns, which affect the inductance of the coil. The winding wire diameter may be increased to allow for larger currents and increase the magnetic field, $\mathbf{B}$, more profoundly in the tissue. In that case, the coil diameter tends to increase (Fig. 1A), and there may be a loss of focality and, consequently, a lower fiber selectivity.