Microbubble scavengers can remove carbon nanotubes from the body
(Nanowerk Spotlight) The toxicity concerns surrounding carbon nanotubes (CNTs) are highly relevant for two reasons: Firstly, as more and more products containing CNTs come to market, there is a chance that free CNTs get released during their life cycles, most likely during production or disposal, and find their way through the environment into the body.
Secondly, and much more pertinent with regard to potential health risks, is the use of CNTs in biological and medical settings. CNTs interesting structural, chemical, electrical, and optical properties are explored by numerous nanomedicine research groups around the world with the goal of drastically improving performance and efficacy of biological detection, imaging, and therapy applications. In many of these envisaged applications, CNTs would be deliberately injected or implanted in the body, for instance as intercellular molecular delivery vehicles (see for instance: "Nanotechnology based stem cell therapies for damaged heart muscles").
One of the issues researchers have been exploring is how – once the primary role of CNTs in a therapeutic application is fulfilled – they can promote the rapid removal of CNTs from the body, or the dispersal of aggregated clusters to sub-micron size in order to mitigate the harmful effects.
Researchers in India have now demonstrated a novel, optical tweezers based approach to scavenge CNTs from biological fluids such as blood. This method may potentially be of use in scavenging, transporting and dispersal of potentially toxic CNTs in biologically relevant environments.
(a) Schematic depiction of the experimental apparatus in which a fluid flow-cell is incorporated into an optical tweezers set-up. (b) Multiple microbubbles formed upon absorption of 1064 nm light by CNT bundles. (c)–(e) Formation of microbubbles and the attraction of a proximate CNT bundle towards the tweezer focal volume. Each of the frames is temporally separated from the preceding one by 40 ms. These snapshots were taken under static conditions wherein the fluid velocity in the flow-cell was zero. The bright patch at the junction of two bubbles is due to laser light scattered from the CNT bundle at the laser focus. (Reprinted with permission from IOP Publishing)
"We have succeeded in using a low-power infrared laser in an optical tweezers set-up to generate micro-bubbles in flowing, biologically-relevant fluids, including human whole blood," Deepak Mathur, a Senior Professor in Atomic & Molecular Sciences at the Tata Institute of Fundamental Research in Mumbai, tells Nanowerk. "These micro-bubbles are formed upon very localized heating of small bundles of carbon nanotubes that are suspended in the flowing fluid. The localized nature of the heating causes enormous temperature gradients to be set up in the fluid and these, in turn, set up surface tension gradients that give rise to complex flow patterns in the immediate vicinity of the microbubble. A consequence of this is that proximate CNTs are attracted towards the microbubble and appear to "adhere" to the bubble surface."
The physical manipulation of such CNT-encrusted bubbles enables the physical manipulation of CNTs by optical means – something that is generally not feasible under normal circumstances as it is not possible to optically trap CNTs using infrared laser light. As a matter of fact, CNTs are vigorously repelled from the focal volume of a tightly focused infrared laser beam; a dramatic illustration of this, along with a scientific rationalization, has recently been published by Mathur's group ("Bright visible emission from carbon nanotubes spatially constrained on a micro-bubble").
By extending this earlier work, the Tata Institute team went on to examine the formation of microbubbles in a variety of biologically relevant fluids, including human whole blood; the adhesion of proximate CNTs onto the bubbles; the optical micromanipulation of the cargo-laden bubbles through the fluid; and dispersal of CNT bundles by controlled explosion of CNT-encrusted microbubbles. They report their findings in the May 20, 2010 online edition of Nanotechnology ("Optical-tweezer-induced microbubbles as scavengers of carbon nanotubes").
"We managed to trap carbon nanotubes in fluid flow and demonstrated scavenging action by collecting the nanotubes onto an optically trapped microbubble and then moving the CNT-encrusted bubble both along and against the flow" explains Mathur.
The team also demonstrates shattering of a large cluster of CNTs by exploding a bubble. This leads to the possibility of using microbubbles as CNT scavengers in biomedical applications, that is, the use of microbubbles as a vehicle to aid CNT removal from the body once their intended task of drug-delivery or tissue-ablation has been completed.
Mathur points out that a key feature of his team's experiments is the use of very low power (5 mW) light in the infrared region (1064 nm wavelength) obtained from a continuous wave laser.
"Both the wavelength and power present obvious advantages from the point of view of potential applications in biomedical environments," he says. "We also found that CNT-encrusted microbubbles can be made to emit copious amounts of white (broadband) light. There may well be applications for such broadband light for photodynamic therapy type of work but conducted in spatially localized fashion, in physiological conditions, under full optical control."