The co-occurrence of SWCNTs versus MWCNTs for different applications is shown in the form of a Sankey graph (Figure ). The choices of which type of CNT to use in different circumstances depends on the necessary properties and their costs. SWCNT can be obtained as both semiconducting and metallic CNT. Conducting MWCNT have larger thermal and electrical conductivities than SWCNT 103 and may have a narrower range of conductivities. SWCNTs, however, have more predictable structures, making them more easily and predictably functionalized. In addition, SWCNTs have a Raman absorption at 532 nm and near-IR absorptions between 1000 and 1700 nm; 105 the near-IR absorption is in a region at which tissue is most permeable, making them amenable to use in biological sensing. The more predictable structures of SWCNT may allow them to be more predictably and easily functionalized by covalent and noncovalent methods. SWCNTs are roughly 1 nm in diameter, while MWCNT can have diameters between 2 and 100 nm. Purified SWCNT with reduced metal content can be obtained for 2000 USD/kg 106 though a 99% SWCNT content grade with <0.1% metal content is available, and MWCNT can be obtained for 100 USD/kg. As shown in Figure , for most of the applications of CNT, MWCNTs are predominantly used, likely because they are significantly less expensive than SWCNTs. Publications discussing solar cells, hydrogen storage, and actuators, however, use SWCNT and MWCNT at similar frequencies. SWCNTs are also used fairly extensively in biomedical applications, where their effective absorption and fluorescence properties at near-IR wavelengths make them amenable to tissue penetration and potential use in living things. SWCNT are likely to have more predictable functionalization behavior and toxicities, important for biocompatibility.