While there are some other types of nanotubes made from other elements like boron nitride, the vast majority of nanotubes are made from carbon. These carbon nanotubes come in two primary forms:

* Single-walled nanotubes (SWNTs) - these are essentially rolled-up sheets of graphene, a single layer of carbon atoms arranged in a honeycomb lattice.

* Multi-walled nanotubes (MWNTs) - these consist of multiple concentric cylindrical shells of graphene.

Because of their unique structure and properties, carbon nanotubes have a wide range of potential applications, including:

* Electronics: transistors, sensors, displays

* Materials: composites, coatings, adhesives

* Energy: batteries, fuel cells, solar cells

* Medicine: drug delivery, tissue engineering