Medical Device Link.

MATERIALS

Nanocomposites have been known to the world for many years and investigated for some time. The fastest growing area of research is in nanocomposite organic–inorganic materials, for example, polymers such as polypropylene and nylon(s) in combination with minerals such as montmorillonites and hydrotalcites. In literature, the term nanocomposite is generally used for polymers with submicrometer dispersions. In nanocomposites, submicrometer-size particles of inorganic materials are homogeneously dispersed as separate particles in a polymer matrix (Figure 1). This is one way of characterising this type of material. There is, in fact, a wide range of nanomaterials and a way to differentiate them is to classify them by the number of dimensions they possess (Figure 2). Their shapes vary and include:

Figure 1. Illustration of a nanocomposite showing a random distribution of inorganic nanoparticles.

• needle-like structures such as one-dimensional particles, for example, sepiolites, attapulgites and carbon nanotubes
• two-dimensional platelet structures, for example, layered double hydroxides and smectites
• sphere-like three-dimensional structures, for example, silica, zinc oxide and bariumtitanate.

Intrinsically hydrophilic particle surfaces and hydrophobic polymer chains are not miscible. Therefore well thought out engineering of the polymer–particle interactions can produce nanocomposites with a broad range of properties.

Figure 2. (click to enlarge) Examples of different nanoparticles with different dimensions.

In principle, nanocomposites are an extreme case of composites in which interface interactions between two phases are maximised. Because the remarkable properties of conventional composites are mainly due to interface interactions, the modified materials can lead to interesting results. Many exciting new materials with novel properties can be obtained by combining the parent properties into one single material. If a suitable combination is made, it is even possible for new properties to be discovered that are unknown in the parent constituent materials. For example, the patented planocolor applications (www.planomers.com) where dyes are attached to a nanoparticle surface, which combines enhanced UV colour stability and colouring polymers with immiscible organic dyes. An important aspect of the properties of nanocomposites involves determining whether or not the nanomaterial is intercalated or exfoliated.

Intercalated nanocomposites. Here, the polymer chains alternate with the inorganic layers in a fixed compositional ratio and have a well-defined number of polymer layers in the intralamellar space. These nanocomposites are more compound-like than the exfoliated versions because of the fixed polymer to layer ratio; it is their electronic and current carrying properties that make them interesting.

Exfoliated nanocomposites. Here, the number of polymer chains between the layers is almost continuously variable and the layers are >100 Å apart. These nanocomposites are interesting because of their superior mechanical properties:

• improved tensile and impact strength
• improved fatigue behaviour
• reduced oxygen- and water-vapour permeability
• improved flame retardancy
• reduced linear thermal expansion
• improved temperature stability
• enhanced thermal stability of additives
• improved crystallinity
• improved resistance to organic solvents
• improved surface finish
• new nanopigment basis (planocolor
• enhanced UV-light stability.

Commercialisation

Figure 3. (click to enlarge) Processing nanocomposite materials via reactive 3D-printing to scaffolds (left); electrospinning to nanofibres (right).

Although new, seemingly interesting nanocomposites keep appearing, unfortunately these are not yet attractive from a commercial point of view. In the end, what matters is the added value that can be obtained through the use of one of these technologies (Figure 3). There is a vast collection of literature and patents based on nanocomposites and a variety of technologies and commercial applications. These include polymer nanocomposite rigid tubing for catheters (Foster Corporation www.fostercomp.com) and a polyhedral oligomeric silsesquioxane based dental bonding system (Hybrid Plastics Corp. www.hybridplastics.com)

Interesting nonmedical applications include thermoplastic olefin for step-assist in cars (General Motors www.arifleet.com) and fire-retardant insulation for cable applications (Kabelwerk Eupen). Market predictions for nanocomposites used in packaging are 1.4 million kg for beer, carbonated soft drinks, meats, foods and condiment packaging.¹

Even though the advantages that can be obtained from the use of these materials are tremendous, their pricing has meant that applications have only started to appear in the past few years. Moreover, in most cases the use of these materials can be related to the ever-increasing need for miniaturisation and increased responsiveness with respect to actuated motion.² In addition to these mechanical-related enhancements, more interesting properties of nanocomposites have been discovered in recent years as stated above, which are opening up a new world of features that may be added to existing materials.

Next-generation possibilities

Figure 4. (click to enlarge) Examples of applications and market areas for nanocomposites.

The ultimate goal is the imitation of biomimetic self-organisation of structures. This will lead to new functional possibilities in materials and the development of new structures that result in a completely new generation of materials and devices. The essential elements for controlled organisation are the use of anisotropic building blocks (nanocomposite) and the introduction of additional functionality properties; these will make it possible to build up the desired structures. The possibility of incorporating additional chemical functions on the surface of these particles will also provide ways to control their organisation during materials form-ation. This will give rise to a new generation of “smart” nanocomposite materials that will provide options for materials with biomimetic structures and functionalities. It is envisaged that these developments will also lead to completely new possibilities for fine-tuning and extending the material and processing properties of the existing generation of polymer–inorganic nanocomposites. Furthermore, they will provide a basis for the development of smart materials and products with a higher (smart) degree of functionalities and even “intelligence.” Research on so-called “next-generation nanocomposites” is in a preliminary stage, but some examples (Figure 4) are as follows:

• Controlled (triggered) release/encapsulation with bioactive additives. This can be used, for example, in wound treatment, medicine release and lubricant release.
• Actuators (artificial muscles). Polymer nanocomposites filled with piezoelectric ceramic nanoparticles create the opportunity to combine flexibility and processability with high-force actuations.
• Controlled surface structuring for biointeraction. Migration to the surface can be achieved by controlling the surface properties of the nanoparticles. This results in surface selective structuring with unique features. An example of this is optimised interaction between tissue and these structured surfaces, which reduces friction and is interesting for applications such as catheters and needles.

Expanding design combinations

Although there are some commercial applications that make use of nanocomposites, this technology has not yet been used to its full capacity. The use of nanocomposites in medical device technologies can lead to further, improved properties: as well as added value in mechanical properties, it is also provides controlled-release features. The possibilities offered by nanocomposite technology give the designer the opportunity to use a much broader set of combinations of materials and properties, which will result in enhanced integration of different functions in medical devices.