Owing to the unique combination of material properties, CNTs are used in a wide variety of applications where they play critical functional roles. Figure shows the distribution of carbon nanotube applications referenced in journal and patent publications. The largest fraction of CNT publications focused on their use in composites, including polymermatrix composites (Figure ). The structure of CNT gives them strength along their longest dimension, while the length of CNT allows them to act on macroscopic length scales. In addition, the ability to functionalize the exterior of CNT either covalently or noncovalently can be used to make them compatible with a variety of materials. The electrical and thermal conductivity of CNT allows them to act as functional components and not just as structural components. Composites are used in many products on both large and small scales; their ubiquity and the useful characteristics of CNT in them can account for their large fraction in CNT applications. Some of the applications for CNT in composites include their use in batteries, in biocompatible hydrogels for heart cell growth, for use with a glycidyl polyketone to form nonconductive layers, to increase strength and wear resistance in composites with high-density polyethylene, to absorb high-frequency electromagnetic waves (CN117042427A 67 ), and as components in heatresistant aerogels (CN114275770 68 ). One caveat with the analysis of composites is that the term "composite" can refer to both a direct use of CNTs for their physical properties (i.e. structural composites where CNTs are incorporated into a matrix to provide mechanical strength, or electrical or thermal conductivity) and a more general structure type in which CNTs are combined with other components (such as their use as catalyst supports); classification as a composite thus may not exclude the use of CNTs in other applications.