Nanocomposites for bone repair
Ian Mackay, Serena Best and Ruth Cameron at the Cambridge Centre for Medical Materials, UK, discuss a new chemical method to simultaneously produce and disperse mineral nanoparticles in a polymeric matrix for bone repair.
Living bone is, essentially, a composite of calcium phosphate minerals in a matrix of
collagen. The mineral is chemically similar to hydroxyapatite (Ca10(PO4)6(OH)2) with calcium deficiencies, and minority substitutions (for example, carbonate ions), and is found as nanometre-scale acicular crystals.
One of the research aims at the Cambridge Centre for Medical Materials (CCMM), UK, is to develop a material that mimics the structure of natural bone by incorporating nanocrystalline hydroxyapatite into a polymer matrix. However, it is intended that the polymer matrix will be biodegradable so that it is resorbed by the body over a period of time. The resulting material could be used for a wide range of orthopaedic applications, such as bone fixation (following fractures), filling defects left after excision of diseased bone, reconstruction of damaged joints, or fusion of spinal segments.
Once implanted, the material will be exposed to the natural regenerative processes constantly occurring in living bone, an equilibrium between the bone-resorbing osteoclast cells, and osteoblast bone generation. Over time, the material will be fully absorbed, and replaced by natural bone. In addition to encouraging rapid and full healing, such a material has the advantage that the implant naturally disappears, meaning that the patient does not need to undergo further surgery to remove the implant once its function is complete.
There is a wide selection of possible materials available for this application. Many polymers are known that can be successfully resorbed by the body – these include polyhydroxybutyrate, polycaprolactone, polylactide and polyglycolide. For the mineral component, there are different routes to produce hydroxyapatite, but one of the more successful synthesis routes involves reacting a solution of orthophosphoric acid with a suspension of calcium hydroxide. This has the advantage that the only by-product is water and, if carried out with care, it can yield high purity hydroxyapatite.
However, composites that incorporate mineral filler are often limited by problems associated with homogeneous dispersion of mineral particles into the polymer. The as-synthesised mineral particles have a tendency to aggregate into micrometer-sized clumps. This creates problems with reproducibility and, in particular, their dissolution behaviour and cellular activity are changed. Additionally, the larger mineral aggregates can impact on the mechanical properties of the material, which undermine its usefulness in orthopaedic applications.
Methods exist based on extrusion and on milling which use mechanical energy to re-disperse previously synthesised nanometre-scale hydroxyapatite into a polymer. However, these methods do not guarantee control over the morphology of the hydroxyapatite particles and the process can lead to greater variation in particle size distribution. A further disadvantage is that the mechanical process can incorporate impurities. Alternatively, dispersing agents such as citric acid or Darvan 7 can be used but additional chemical components in the material can have adverse effects on mechanical and biological properties.
To overcome these limitations, the CCMM has developed a new chemical method to simultaneously produce and disperse mineral nanoparticles in a polymeric matrix, through the production of hydroxyapatite-polyester composites. This technique allows careful control of the chemistry, morphology and size of the nanoparticles, and is simpler than the complex mechanical and surface-modification routes already known.
Under the microscope
A number of characterisation techniques have been used to investigate the materials produced in this way. The purity of the mineral content can be readily assessed using wide-angle X-ray diffraction, by comparing the composite to the original polymer, and to pure hydroxyapatite.
Thermogravimetric analysis can be used to determine the composite composition (and hence the efficiency of the synthesis in incorporating filler into matrix). The traces also changes in the temperature of mass loss due to degradation with the incorporation of calcium phosphate particles. For the samples depicted, the pure polymer (polylactide-co-glycolide) degrades at around 314°C. Once incorporated into the composite, however, the polymeric fraction (71wt%) loses mass at higher temperature, around 366°C. This may be due to altered transport of degradation products in the composite but may suggest that the bond between the polymer and mineral is strong, which would be ideal for improved mechanical properties, and may also result from small, well distributed mineral particles.
The microstructure of the composite can only be effectively imaged using transmission electron microscopy, by mounting a sample, thinly slicing it, and breaking the sample to produce a thin fragment of composite. This allows the mineral particles to be shown within a polymer matrix of much higher transmittance. The image shown has a vertical field of view of 1.125µm, the mineral particles are thus around 45nm long, and 15nm in width.
Having successfully dispersed nanoparticles of the desired bioactive ceramic into the polymer, with a method for controlling their size and morphology, the focus is now on optimising the mineral content and properties for biological response. Initially, this will be carried out using cell culture techniques in vitro, and also acellular degradation studies, to assess the effect of nanoparticle fillers on the timescales over which the polymer degrades.
Early results from such studies show that, as expected, increasing mineral content slows down the polymer’s degradation (because the mineral increases the pH, and buffers the acidic and autocatalytic polymer degradation products). This is valuable, as it may allow the degradation timescale to be tuned, and the acidity to be reduced, leading to lessened tissue irritation at the implant site.
Alongside this research, further work extending the usefulness of the synthesis technique to other systems, not necessarily biomaterial based, is ongoing – there are possibilities in structural composites, and semiconductor applications, as well as drug delivery systems that are closer to the biomedical home of the technology.