A medical system for electrical stimulation includes a first column of electrodes, a second column of electrodes, an expandable member disposed between first and second columns, and an expansion mechanism adapted to transmit an externally applied pressure to the expandable member. The pressure expands the expandable member in order to force the first column of electrodes apart from the second column of electrodes. The first and second columns, disposed side-by-side, may be inserted through a percutaneous needle and into a epidural space, alongside a spinal cord; after insertion, the first column may be forced apart from the second column by applying the pressure to the expandable member.