Nanotube sheets at 0 kV (A), 5 kV (B), and 3 kV at 1500 K (C) .
From printer drivers, to pacemakers, to robotic limbs on space crafts, actuating materials are the key components in controlling movement in a machine. Actuators convert various energy types such as electrical, chemical, hydraulic, or thermal energy into mechanical energy, and can be made of a variety of materials. Even human muscles can be considered actuators, as they convert energy from calories into motion. Recently, Multi-Walled Carbon Nanotubes (MWCNTs) in aerogels have been explored as actuators, acting similarly to human muscles. These artificial muscles use electrical stimulation captured by the conductive nanotubes to expand and contract the electroactive polymer, which in turn generates mechanical energy .
MWCNT aerogels are created by dispersing nanotubes into the polymer poly(3-hexylthiophene)-b-PTMSPMA (P3HT-b-PTMSPMA), creating strong chemical bonds between the nanotubes as the polymer condenses on their surfaces. By maximizing the interactions between the nanotubes through the polymer a lower critical concentration of MWCNTs are required to form the aerogel, making it one of the lightest materials in the world with a density of ~1.5 mg/cm3 . Unlike 2-dimensional graphene sheets that are semimetals, carbon nanotubes are conductive along their tubular axis, making them a more powerful semiconductor. This allows them to efficiently harness electrical energy from a stimulus which then expands the gel by disrupting the ions in the electroactive polymer. The use of carbon nanotubes in artificial muscles is advantageous not only for its efficiency as a conductor but also for its durability due to the sp2 bonds between carbon atoms (stronger than the sp3 bonds of carbon atoms in diamonds) . Additionally, MWCNT aerogels are able to withstand temperatures ranging from -196-1600oC, as carbon uniquely becomes denser as it is heated rather than softer, and due to the lack of liquid in the aerogels, which would freeze otherwise . Even under extreme temperature conditions, the nanosheets are capable of expanding up to 220% in milliseconds.
The durability, efficiency, and low density of these artificial muscles make them prime candidates for use in actuator systems on space crafts and satellites. MWCNT aerogels can also be permanently expanded and frozen across a substrate to create Transparent Conducting Films (TCFs) used in LED screens and solar cells. Due to the wide band gap of carbon nanotubes, more light will be able to pass through the TCFs, creating a wider transparency range which ultimately allows for the full use of the solar spectrum in solar cells . While the electrical energy required to actuate the MWCNT aerogels is too high for safe use in prosthetics, piezoelectric nanogenerators which function in a similar fashion are being explored as a biomedical device .
- Aliev, A. E., J. Oh, M. E. Kozlov, A. A. Kuznetsov, S. Fang, A. F. Fonseca, R. Ovalle, M. D. Lima, M. H. Haque, Y. N. Gartstein, M. Zhang, A. A. Zakhidov, and R. H. Baughman. "Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles." Science 323.5921 (2009): 1575-578. Web.
- Zou, Jianhua, Jianhua Liu, Ajay Singh Karakoti, Amit Kumar, Daeha Joung, Qiang Li, Saiful I. Khondaker, Sudipta Seal, and Lei Zhai. "Ultralight Multiwalled Carbon Nanotube Aerogel." ACS Nano 4.12 (2010): 7293-302. Web.
- Lu, X.; Chen, Z. (2005). "Curved Pi-Conjugation, Aromaticity, and the Related Chemistry of Small Fullerenes (C60) and Single-Walled Carbon Nanotubes".Chemical Reviews. 105 (10): 3643–3696.doi:10.1021/cr030093d. PMID 16218563.
- Bland, Eric. "World's Lightest Material Made into Muscle." NBC News, 19 Mar. 2009. Web. 09 Aug. 2016.
- Howell, Abigail. "Zinc Oxide Nanowires in Piezoelectric Nanogenerators for Biomedical Devices." Nanotechnology in City Environments (NICE). Center for Nanotechnology in Society, Arizona State University, Aug. 2015. Web.
These carbon nanotube artificial muscles can elongate further and at faster rates than natural muscles, and they also have the ability to actuate at extreme low and high temperatures.
MWCNT aerogels require a high voltage for actuation, which makes them unsafe for use in prosthetics and creates the same risks as any highly conductive material not properly handled, including electrocution.