The importance of studying the biodurability of carbon nanotubes

Figure 1: The Fibre Paradigm (adapted from Donaldson et. al. 2006)

Key research studies on CNT bio-durability

One of the first major publications specifically addressing the durability of CNT was published by Allen et al. (2008). In their study, Allen et al. used single-walled carbon nanotube (SWCNT) which had been cleaned of metal contaminants via oxidative acidic treatment, which also induces the formation of carboxylic acid groups. Allen et al. incubated these SWCNT in presence of horseradish peroxidase (HRP), which is a protein which favours the insertion of oxygen onto molecules (using hydrogen peroxide) and can be found in cellular compartments specialised in ‘digestion’ of compounds, in animal and plant organisms. Over an incubation of 12 weeks, in presence of HRP and hydrogen peroxide, gradual degradation of the SWCNT was observed. The authors therefore suggested that in vivo, naturally occurring peroxidase enzymes (such as myeloperoxidase produced by the inflammatory neutrophils) could be agents involved in the bio-degradability of SWCNT, influencing their bio-persistence in the environment and their pathogenic potential.

Following this work, Liu et al. (2010) investigated the influence of surface chemistry of CNT on their bio-durability. For this purpose, the authors used SWCNT in their pristine (unmodified) form, SWCNT that had been treated with oxidising acids which induced the formation of carboxylic groups on their surface, and a range of other types of functionalised SWCNT. The various CNT were incubated in phagolysosomal simulant fluid which is an acidified physiological buffer with additional hydrogen peroxide to provide an oxidising environment, in order to mimic the phagolysosome, a cellular compartment dedicated to ‘digestion’ of compounds. SWCNT presenting carboxylic groups on their surface showed signs of degradation over time, supporting the previous findings by Allen et al. (2008). Moreover, the authors observed that this degradation was associated with breaking up and debundling of the SWCNT as well as oxidative attacks of the side walls resulting in release of carbon fragments. In contrast, pristine and other functionalised SWCNT were stable over time. The authors proposed that these results point toward a surface functionalisation-specific degradation mechanism and that understanding relationships between surface functionalisation and durability could be important for the safe development of SWCNT in medical applications. 

Kagan et al. (2010) used short-cut carboxylated SWCNT and took a dual approach to studying the bio-degradation of CNT, using both acellular and cellular systems. In the acellular studies, when CNT were incubated with hydrogen peroxide or myeloperoxidase (two natural components of the cellular defence system against pathogens), no signs of degradation were reported. In contrast, when the SWCNT were incubated in presence of both myeloperoxidase and hydrogen peroxide, the CNT were observed to degrade over time. Through further investigations, the authors showed that hypochlorite and radical intermediates of the myeloperoxidase were involved in this degradation process and that the chemical nature of the material surface was of importance in the positioning and further activity of the myeloperoxidase. In the cellular studies, these SWCNT were firstly incubated in the presence of activated neutrophils, which are cells from the immune system dedicated to defend the organism against foreign bodies using an arsenal of oxidative mechanisms. It was shown that these SWCNT could be biodegraded by neutrophils, provided that uptake occurred. Secondly, the same SWCNT were incubated in presence of macrophages, which are cells from the immune system specialised in scavenging of pathogens. However, macrophages were less efficient than neutrophils at degrading the SWCNT and it was suggested that this differential activity was related to their lower content in myeloperoxidase. Altogether these results suggested that if uptake occurs, carboxylated SWCNT could be degraded in vivo by myeloperoxidase expressed in cells such as neutrophils, and to a lesser extent macrophages. 

The most recent study concerning the bio-durability of carbon nanotubes is that of Osmond-McLeod et al. (2011), in which durability of a panel of both single and multi-walled CNT (MWCNT) was investigated. One SWCNT and three different MWCNT as well as asbestos and glass fibre controls were immersed in an acidified physiological solution for up to 24 weeks. The results showed that among this panel, only one MWCNT showed signs of degradation over time as assessed by mass loss and length reduction. The authors emphasised that not all CNT behave the same in terms of durability and that physico-chemical characteristics are important parameters influencing the potential degradability of a material. 

Both, Kagan et al. (2010) and Osmond et al. (2011) also performed in vivo studies and showed a reduced inflammatory response in mice exposed to the partially degraded or shortened CNT respectively, as compared to their non-degraded counterparts. This reinforces the importance of bio-durability and its consequences in terms of number of long fibres and therefore dose. 



In conclusion...

The latest studies show that CNT, and more generally HARN, can display a large range of shape such as ‘fibre-like’ or bundles and can also display very distinct and variable bio-durability. Moreover, structural defects, structure (single or multi-walled) and surface functionalisation are important physico-chemical parameters driving the (bio)durability of CNT in vitro and bio-persistence in vivo.

Research into this area is currently of importance, as a better understanding of the (bio)degradation of materials especially for fibrous ones is important for hazard assessment and could allow their safety by design including, for example, the design and production of new medical and pharmaceutical applications.

In conclusion, durability studies are an important aide in rapid screening of potential bio-persistence and in hazard consideration especially of fibrous nanomaterials such as some CNT.