A microfilament is a small component of Federation technology shaped like a small, extremely thin, wire.
In 2374, Captain Kathryn Janeway and Seven of Nine were imprisoned aboard the USS Dauntless, an alien ship designed to look like a Starfleet vessel. In the brig, the captain removed a microfilament from her combadge. She then used the component to adjust Seven's cranial implant, which activated several of the former Borg's nanoprobes. Seven's bio-electric field was altered as a result, allowing her to easily walk through the force field, deactivate it, and escape with Janeway. (VOY: "Hope and Fear")