Micro ring, also known as micro-bead or micro loop in hair extensions use small metal rings or beads (usually aluminum) and can sometimes be lined with silicone to attach the extension hair. They are fixed to small sections of natural hair and tightened using a special tool that clamps the bead around the natural hair. The micro-beads are designed to be small enough so that they are not visible in normal use. http://www.angelfantastic.com/human-hair-extension/loop-in-hair-extension/