TWI Industrial Member Report Summary 1018/2012
By A Bahrami
CNTs are molecular-scale carbon fibres and can be thought of as one or many graphene (a single layer of graphite) sheets rolled into single or multi-concentric cylinders. CNTs are exceptionally black(Yang et al, 2007) and highly efficient light absorbers. The light absorption factor per graphene layer has been estimated to be 2.3% (Nair et al, 2008). This means that for multiwall tubes containing greater than 50 concentric tubes, most of the incident light is absorbed and mainly converted to heat. Indeed the consequences of exposing CNTs to radiant energy such as that from a domestic photographic flash have been known for a number of years, with the instantaneous combustion of CNTs when close to a flash gun first being reported by Ajayan et al (2002).
It is therefore intended to take advantage of the exceptional IR absorption properties of CNTs, and examine the possibility of removing CNTs from an aqueous environment by initiating a water-gas reaction and therefore provide the basis for methods of removing them from living tissue without causing detrimental temperature changes and at medically allowable laser exposure times and radiation wavelengths. Excessive heating of living tissue will cause transformations and tissue death starting at temperatures of 45°C. To avoid this, the radiation wavelength should be selected to minimise direct tissue absorption. The best approximation of a suitable wavelength is one at which water (70% of the body mass) is least absorbing, i.e. 600 to 1200nm, this is also the spectral range that biological systems are known to be highly transparent to (Poland et al, 2008 and Concise Science Dictionary, 1996). The wavelength chosen also determines the penetration depth; both optical scattering and absorption impede radiation. Scattering by tissue varies inversely with radiation wavelength, and major tissue chromophores have greater absorption at shorter wavelength. Therefore, in general, greater tissue depth penetration is possible with longer wavelengths, until the near-infrared absorption bands of water near 2000nm are reached. The wavelength of 940nm was chosen (within the available laser sources) to be as close as possible to that for the maximum penetration of soft tissue, with minimum amount of scattering and absorption by the tissue.
- Examine the hypothesis that CNTs can undergo water-gas reaction and what the activation energy and required temperature are for this process, using a modulated thermo-gravimetric analysis (TGA) methodology.
- Examine whether the water-gas reaction can also be driven by using laser radiation within acceptable safe exposure limits of human skin.
- Perform a proof of concept test on CNT-water suspensions sandwiched in between two layers of bacon which was considered an appropriate substitute for human skin in this study.