The Prognosis for Medical Nanocomposites

Nanocomposites are undoubtedly making an impact in many fields of technology, where profound advantages over conventional composites can be readily identified. Whether they have a role in medical technology is yet to be determined. The answer may lie in the degree of subtly that is brought to the underlying scientific issues.

By: David Williams

The era of composite materials
When I was a young man, what we now know as materials science departments at universities were called metallurgy departments and metals ruled the world. Polymers, which gave rise to those inferior things we called plastics, were just being introduced and high performance ceramics barely existed. Within the then imperious world of metals, profound technologies were established that allowed major strengthening and toughening mechanisms to be developed that gave us ultrahigh performance alloys. Their strengthening mechanisms often depended on phase changes and precipitates, which were deliberately introduced to control plastic deformation and crack propagation. However, those magnificent alloys were expensive and heavy, and applications in areas such as aerospace and consumer electronic goods became limited. Before long, the same types of structural features were introduced into the polymer and ceramic fields and often gave good increases in strength to the intrinsically weak polymers, and acceptable increases in toughness to the inherently brittle ceramics. Foremost amongst the mechanisms were those associated with the development of multi-phase materials and the introduction of fillers into plastics and ceramics; the era of composite materials was born. In general, the principles of resistance to plastic deformation that were applied to the alloys also applied to these composites; the distributed particles or fibres acted as barriers to moving cracks and to plastic flow. Of equal importance was the fact that the fillers such as silica were often extremely cheap, which provided even greater cost savings.

In medical technology, there was a surge of interest in the possibilities of utilising composites, but it lacked urgency, because in many situations the twin advantages of cheapness and lightness did not outweigh the obligatory requirements for performance and biological safety. Certainly in those areas where lightness was an important factor and biological safety was less crucial such as in orthoses and artificial limbs the advantages were plain to see.

However, with the vast majority of implantable devices this was not the case. There were two major reasons for this. First, for
those materials where inertness and long-term performance were essential, the presence of a microscopic second phase could be detrimental; the release of small particles in this phase, often of micron dimensions, could stimulate a foreign body response far in excess of that associated with the principle matrix phase. It really did not matter what this second phase was made of, because small particles stimulate inflammation by virtue of their size and shape not their chemistry. The massive tissue reactions and appalling clinical consequences associated with the use of the polytetrafluoroethylene-alumina or polytetrafluoroethylene-carbon composites as components of temporomandibular joint prostheses in the 1980s showed just how bad an idea this could be. It should be noted that one area of healthcare has benefitted enormously from particulate resin based composites and that is dentistry. We would not have light-cured white fillings without these materials; the biological safety issues of swallowing the occasional microparticle released from a filling are nowhere near the same order of magnitude. The second reason that militated against implantable composites was the fact that the frequently claimed improvement in bioactivity was difficult to demonstrate or justify. If it is claimed that a dispersed second phase made of a putative bioactive material such as hydroxyapatite or a glass-ceramic improves the bioactivity (for example, bone-bonding activity) of a material, then we have to question how much of that second material actually occupies sites at the surface of the composite where they could exert their biological effect. It is unlikely that it will be a high percentage.

The advent of nanocomposites

Thus, neither conventional particle nor fibre-filled polymers offer implantable medical devices many advantages. Now a new and maybe different composites paradigm has appeared and once again questions have to be asked about its suitability for medical technology. This paradigm concerns nanocomposites, where the dispersed phase will be in the dimension range of 100 nanometers or less. One thing seems certain to me: we will not gain many advantages if we treat nanocomposites as if they were the same old composites, just on a different scale. We have to learn at least two lessons. The first is that the nanoscale itself provides strengthening and toughening mechanisms. The second is that nature has known this for a long time and we are now ready to emulate nature at this level. Dealing with the latter point first, it has often been said that we can develop so-called biomimetic materials for medical applications by following the example of bone, which is of course a composite material that primarily consists of collagen reinforced by hydroxyapatite. There have been several attempts to prepare synthetic equivalents and use them for implantable medical devices. The problem here is that bone is not a simple two-phase composite. Instead, it has a complex hierarchical structure, with nanoparticulate apatite phases dispersed in a functionally predetermined manner, with appropriate orientation and anisotropy, in a collagen-dominated matrix that has nano-, micro- and macro-scale porosity. Moreover, bone, and indeed any naturally occurring composite, is developed by a bottom-up process, that is, the constituent parts are self-assembled during growth. By definition, this is radically different from a conventional synthetic composite that is manufactured by a conventional top-down method in which the ingredients are simultaneously placed together in a machine and forced into place by a combination of temperature and pressure. We read much about bio-inspired or biomimetic materials these days. We will not achieve true bio-inspired nanostructured materials unless we change this approach, as indeed several laboratories are now doing.¹

Returning to the first of the lessons mentioned above, we must recall that the nanotechnologies are based on the fact that as the dimensional scale falls below 100 nm, and especially when it reaches approximately 10 nm, two factors come into play that can dominate the performance of the material.² The first of these is that quantum effects can be observed, hence the profound influence of the nanoscale on medical imaging techniques such as those involving quantum dots.³ The second factor, which is of greater relevance here, is that the ratio of surface area to volume of nanoparticles increases significantly. Traditionally, the chemical composition of microscale composites is defined in terms of the volume fraction of the dispersed phase, which may vary from a few per cent up to more than 50 per cent depending on particle shape and size distribution. Imagine the effect on the number of particles in a composite if, for a given volume fraction, the size of individual particles is decreased to the nanoscale. Then stretch the imagination even further by considering the effect on the surface area of the nanoparticles, which translates into the interfacial surface area between nanoparticles and the continuous matrix. As described recently by Ruiz-Perez et al.,⁴ the effect of even small volume fractions of fillers, which may yield up to 10⁸particles/µm³ for less than 10% volume fraction and distances between particles of approximately 10 nm, is profound and gives what they describe as interface dominated materials. Here, we have major mechanisms of toughening to work with (bio-inspired by the toughness of bone, bamboo and ivory). But we will not achieve these effects if we still think of nanocomposites as working within the same paradigm as conventional composites, only on a smaller scale.

A great deal has to be learnt about the biological activity of nanoparticles and nanotopographies and, as usual, there is still time for us to get this all wrong again. But at least we have some hope now that the inspiration of nature can be put to really good effect.

References

1. R.M. Capito et al., “Self-Assembly of Large and Small Molecules into Hierarchically Ordered Sacs and Membranes,” Science, 319, 5871,
1812–1816 (2008).

2. D.F.Williams, “Defining Nanotechnology,” Medical Device Technology, 19, 3, 8–10 (2008)

3. D.F.Williams, “Quantum Dots in Medical Technology, Medical Device Technology, 17, 4 8–9 (2006)

4. L. Ruiz-Perez et al., “Toughening by Nanostructure,” Polymer 49, 4475–4488 (2008).

David Williams, Morgan & Masterson, Avenue de la Forêt 103,
Brussels 1000, Belgium, tel. +32 4 7597 0556
e-mail: peggy@morgan-masterson.com [5], www.morgan-masterson.com [6]

Professor David Williams DSc, FREng
Professor Williams retired from the University of Liverpool, after 40 years, at the end of 2007. He retains the position of Emeritus Professor there and now has a series of professorial appointments in the USA, Australia, South Africa and China. In the USA he is Director of International Affairs for the Wake Forest Institute of Regenerative Medicine. He offers consulting services from his company Morgan & Masterson, based in Brussels, Belgium. He is Editor-in-Chief of Biomaterials, the leading journal in the biomaterials field.

Published in European Medical Device Technology [7], February 2010, Volume 1, No. 2 [8]

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