Cardialen is closing in on a $4 million equity round, after raising $2.5 million from 18 unnamed investors.

Minneapolis and St. Louis-based Cardialen is developing a low-energy implant for atrial fibrillation designed to control the fibrillation without damaging heart tissue, according to its website.

The technology uses series of lower-energy pulses rather than a single high-energy shock to effect the therapy, according to the website.

The company has offered its 18 investors equity in Cardialen for the $2.5 million, according to the filing.