Micro rings are extremely common for attaching the hair extensions. They are used in past because of their minimal sizes. However nano rings are now available which are to beat the micro rings in this context.
They have the internal diameter of 2.5mm which makes them 90% smaller as compared to the micro rings (and have 3mm outer diameter). They are used to harness the hair extensions with the nano tip hair extensions. They are not detectable if fitted properly and it makes it possible to use the hair extensions again and again. Along with this they are very easy to apply and are very strong and durable.