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MATERIAL MATTERS COLUMN

The meaning of the nanoscale

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Professor David Williams DSc, FREng is Professor of Tissue Engineering at the University of Liverpool and Director of the UK Centre for Tissue Engineering located in the Universities of Liverpool and Manchester. He is Editor-in-Chief of Biomaterials, the leading journal in the biomaterials field. He is Scientific Director of STEPS, the European Commission Framework VI Programme on a Systems Approach to Tissue Engineering Products and Processes. Professor Williams is also a Managing Partner of Morgan & Masterson LLC, a consulting partnership that focusses on global health-care issues.

I have written several times on the subject of nano-technology, especially as it relates to medical devices and the balance between benefits and risks that is associated with the introduction of new technologies into society in general, and into health care in particular. One interesting and important matter that often accompanies the rapid emergence of new areas of science and engineering is the potential for confusion over the terms that are introduced into our language. This often happens through the simultaneous use of the same terms by different organisations and individuals, but with slight or even substantial differences in meaning. This is perhaps inevitable, but it has the unfortunate consequence of introducing confusion into terminology where there may be scientific, technological, regulatory or legal ramifications. This is certainly the case with nanotechnology. The confusion largely arises from differences in emphasis between the technologists who are introducing new substances and designs at the nanoscale at an exceptional rate, and the toxicologists, biologists, pathologists and environmental scientists who are attempting to understand the implications to the environment and to human health from the use and dissemination of these entities.

One good example of the confusion is seen with the term “nanomaterial,” which is now widely used. One has to ask what can possibly be meant by this term. By conventional definitions, a material is a substance useful for making objects, and “nano” describes something extremely small. If we place the prefix “nano” in front of “material,” we appear to have an extremely small material, which is nonsense. The object that is produced may be extremely small, but what can be extremely small about a material? Do we mean that the material is composed of extremely small units and if so, are they atoms, or molecules, or crystals, or grains, or domains or phases, in which case everything is a nanomaterial. Should we confine the term to collections of nanoparticles? Each of which could be extremely small, but rarely do collections of particles lead to a useful object.

A defining framework

I was recently involved in writing a scientific opinion for the European Commission that attempted to deal with some of these uncertainties in definitions in nano-technology.¹ The work was aimed at providing a framework for the development of these definitions and a basis for the rational formulation of some of the words. In developing this framework we have to start with the concept of the nanoscale itself, because the many different facets of nanotechnology are concerned with the manipulation of, or engineering at, the nanoscale. The prefix “nano-” specifically means a measure of 10-9 units; the nature of this unit is determined by the word that follows. Thus a nanosecond is 10-9 seconds and a nanometre is 10-9 metres. It is widely accepted that in the context of nanoscience and nanotechnologies, the nanoscale should only refer to units of dimensions, rather than of any other unit of scientific measurement. However, it is unrealistic for practical purposes to consider the prefix “nano-” to solely and precisely refer to 10-9 metres, just as it is not considered that “micro-” specifically and solely concerns something with a dimension of precisely 10-6 metres. It is important to note that there are two characteristics that confer special properties to products of nanotechnologies: the quantum effects that are associated with extremely small dimensions, and the large surface area to volume ratio that exists at these dimensions. It is widely agreed that dimensions in the order of 100 nm and below are most likely to be associated with these properties, and therefore the starting point for a framework of definitions must be that the nanoscale is considered to be in the order of 100 nm or less. The concept of the nanoscale does not imply being smaller than the microscale; just because an event occurs at less than a micron does not confer nanoscale characteristics to it.

Clarification is needed on how precisely the term refers to the 10-9 unit of measure. The changes in characteristics that are seen with reducing dimensions do not occur uniquely at the 100 nm dimension. Thus, a range of dimensions at this level must be allowed and some latitude has to be given with respect to the meaning of “in the order of.” I do not consider that 999 nm comes within this range and it is more correctly termed submicron; but 101 nm does.

The field of nanotechnology has given rise to a large number of new terms prefixed by “nano-” and indeed by “nanobio-” (or “bionano-”). Without being too rigorous or prescriptive, it is important to keep some perspective here and I suggest that science at the nanoscale is best served by a combination of good discipline accompanied by pragmatism. Most of the concepts and behaviour patterns seen at extremely small dimensions are not new and can be described by existing terminology that is also used at larger scales. It is crucial that a new language is not adopted unnecessarily by the scientific community and that on those occasions when it is required, it is consistent with established terminology. The earlier example of nanomaterial demonstrates this point. If it was not for the fact that the word is in current, constant, use, its very existence should be questioned because etymologically the word nanomaterial is a nonsense. A “nanostructured material” is far more logical, but a pragmatic approach would suggest that a nanomaterial is “any form of a material that is composed of discrete functional parts, many of which have one or more dimensions in the order of 100 nm or less.” Other definitions logically follow this approach such as a nanocrystalline material is “a material that is comprised of many crystals, the majority of which have one or more dimensions in the order of 100 nm or less;” and a nanocomposite is a multi-phase material in which the majority of the dispersed phase components have one or more dimensions in the order of 100 nm or less.

Differentiation of terms based on shape

Terms that differentiate between discrete entities that have one, two or three dimensions in the nanoscale are important and the use of the term “nanoparticle” here has created some difficulties. For example, many traditional biological domains have entities that exist at a nanoscale such as vesicles, proteins and liposomes, and there has been much discussion about their relationship to nano-particles derived through the use of nanotechnology.

Also, in toxicology, a particle is usually taken to be one that has three dimensions of approximately comparable size. But in nanotechnology the term “nanoparticle” is often used as a collective term for any material consisting of discrete entities with one, two or three dimensions in the order of 100 nm or less. Because of the immense importance of nanoparticle toxicology, it is considered preferable to differentiate particles at the nanoscale according to their shape. Thus, a nanosheet should be considered to be a discrete entity that has one dimension in the order of 100 nm or less and two long dimensions. Other entities such as a nanofilm, nanoplate or nanolayer also comply with this definition, but they may differ from each other by other characteristics. A sheet is usually free and a layer is usually supported and there may be considerable differences in flexibility. A nanorod should be considered to be a discrete entity that has two dimensions that are in the order of 100 nm or less and one long dimension. Similarly, nanofibre, nanowire and nanowhisker comply with this definition and are characterised by their aspect ratio (the ratio between length and diameter of the structure). A nanotube is considered to be a discrete hollow entity that has two dimensions in the order of 100 nm or less and one long dimension.

Particles are considered to be individual discrete entities and a nanoparticle is considered to be a discrete entity that has three dimensions in the order of 100 nm or less. It is inappropriate to discuss heterogeneous collections of nanoparticles simply as nanoparticles. The preferred terminology in the case of deliberately manufactured products containing nanoparticles is “nanoparticulate matter.”

Some may argue that a common language will eventually emerge and there is no cause for concern about terminological confusion in the short term. I would argue that a little common sense and rigour at this stage will serve this new area of science even better.

David Williams, Morgan & Masterson, Avenue de la Forêt 103, Brussels 1000, Belgium, tel. +32 4 7597 0556, e-mail: peggy@morgan-masterson.com, www.morgan-masterson.com.