Potential applications of carbon nanotubes

Carbon nanotubes (CNTs) are cylinders of one or more layers of graphene (lattice). Diameters of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) are typically 0.8 to 2 nm and 5 to 20 nm, respectively, although MWNT diameters can exceed 100 nm. CNT lengths range from less than 100 nm to 0.5 m. Individual CNT walls can be metallic or semiconducting depending on the orientation of the lattice with respect to the tube axis, which is called chirality. MWNT's cross-sectional area offers an elastic modulus approaching 1 TPa and a tensile strength of 100 GPa, over 10-fold higher than any industrial fiber. MWNTs are typically metallic and can carry currents of up to 109 A cm−2. SWNTs can display thermal conductivity of 3500 W m−1 K−1, exceeding that of diamond. As of 2013, carbon nanotube production exceeded several thousand tons per year, used for applications in energy storage, device modelling, automotive parts, boat hulls, sporting goods, water filters, thin-film electronics, coatings, actuators and electromagnetic shields. CNT-related publications more than tripled in the prior decade, while rates of patent issuance also increased. Most output was of unorganized architecture. Organized CNT architectures such as "forests", yarns and regular sheets were produced in much smaller volumes. CNTs have even been proposed as the tether for a purported space elevator.

Recently, several studies have highlighted the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic (>1 mm in all three dimensions) all-carbon devices. Lalwani et al. have reported a novel radical initiated thermal crosslinking method to fabricated macroscopic, free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes as building blocks. These scaffolds possess macro-, micro-, and nano- structured pores and the porosity can be tailored for specific applications. These 3D all-carbon scaffolds/architectures may be used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaics, and biomedical devices and implants.

Biological and biomedical research Researchers from Rice University and State University of New York – Stony Brook have shown that the addition of low weight % of carbon nanotubes can lead to significant improvements in the mechanical properties of biodegradable polymeric nanocomposites for applications in tissue engineering including bone, cartilage, muscle and nerve tissue. Dispersion of low weight % of graphene (~0.02 wt.%) results in significant increases in compressive and flexural mechanical properties of polymeric nanocomposites. Researchers at Rice University, Stony Brook University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone tissue engineering and bone formation. Compared to the commonly used polyurethane in prosthetic feet, CNT natural rubber composites with just 1% carbon nanotubes show a 25% boost in tensile strength. Additionally, this small CNT addition improves dimensional stability by 20% and heat resistance by 15% over natural rubber. CNTs exhibit dimensional and chemical compatibility with biomolecules, such as DNA and proteins. CNTs enable fluorescent and photoacoustic imaging, as well as localized heating using near-infrared radiation. SWNT biosensors exhibit large changes in electrical impedance and optical properties, which is typically modulated by adsorption of a target on the CNT surface. Low detection limits and high selectivity require engineering the CNT surface and field effects, capacitance, Raman spectral shifts and photoluminescence for sensor design. Products under development include printed test strips for estrogen and progesterone detection, microarrays for DNA and protein detection and sensors for NO2 and cardiac troponin. Similar CNT sensors support food industry, military and environmental applications. CNTs can be internalized by cells, first by binding their tips to cell membrane receptors. This enables transfection of molecular cargo attached to the CNT walls or encapsulated by CNTs. For example, the cancer drug doxorubicin was loaded at up to 60 wt % on CNTs compared with a maximum of 8 to 10 wt % on liposomes. Cargo release can be triggered by near-infrared radiation. However, limiting the retention of CNTs within the body is critical to prevent undesirable accumulation. CNT toxicity remains a concern, although CNT biocompatibility may be engineerable. The degree of lung inflammation caused by injection of well-dispersed SWNTs was insignificant compared with asbestos and with particulate matter in air. Medical acceptance of CNTs requires understanding of immune response and appropriate exposure standards for inhalation, injection, ingestion and skin contact. CNT forests immobilized in a polymer did not show elevated inflammatory response in rats relative to controls. CNTs are under consideration as low-impedance neural interface electrodes and for coating of catheters to reduce thrombosis. CNT enabled x-ray sources for medical imaging are also in development. Relying on the unique properties of the CNTs, researchers have developed field emission cathodes that allow precise x-ray control and close placement of multiple sources. CNT enabled x-ray sources have been demonstrated for pre-clinical, small animal imaging applications, and are currently in clinical trials. In November 2012 researchers at the American National Institute of Standards and Technology (NIST) proved that single-wall carbon nanotubes may help protect DNA molecules from damage by oxidation. A highly effective method of delivering carbon nanotubes into cells is Cell squeezing, a high-throughput vector-free microfluidic platform for intracellular delivery developed at the Massachusetts Institute of Technology in the labs of Robert S. Langer. Carbon nanotubes have furthermore been grown inside microfluidic channels for chemical analysis, based on electrochromatography. Here, the high surface-area-to-volume ratio and high hydrophobicity of CNTs are used in order to greatly decrease the analysis time of small neutral molecules that typically require large bulky equipment for analysis.

Composite materials

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