Particle size is a potentially important variable in pharmaceutical production and efficacy. In solid or suspension delivery systems, for example, dissolution and solubility characteristics, which impact the bioavailability of the API, are often controlled by particle size, though not necessarily in ways that are intuitively obvious. The Noyes-Whitney equation describes dissolution rate as being directly proportional to particle surface area, so one might expect small particles to dissolve more rapidly than large particles; however, actual dissolution studies show that this is not always the case. In the majority of these instances, this phenomenon can largely be explained by the relative surface roughness of the particles, which gives them a larger surface area.
Particle size can also affect the behaviour of a formulation during processing. In direct compression tabletting, particle size can influence segregation behaviour and the compressibility of a formulation that, in turn, can affect the consistency of tablet weight and composition, how the press operates, and the mechanical properties of the finished product.Particle size also has a critical effect on the content uniformity of solid dosage forms, with poor results often arising from a mismatch of drug and excipient particle size and density. Poor content uniformity may also result if drug particle size distribution is too large, but it is generally desirable to approach the upper edge of the permitted drug particle size range because reducing particle size too dramatically can exacerbate drug agglomeration and mixing problems.1
Similarly, particle size can affect the flow properties of powders and pastes. Increasing the polydispersity of particle size in powders can improve flow properties; for example, in the case of powders subject to flow in an industrial process, a bimodal distribution of particle sizes can also ensure better flow during processing.1 For pastes, where viscosity is often important, there is usually an optimum particle size distribution (PSD) that yields minimum viscosity whilst maintaining the particle volume fraction. The PSD will also have a direct influence on the texture of the finished product.
Overcoming the challenges
There are several methods, some of which are outlined below, that can be used to investigate particle characteristics. All give useful information on their own, but it is often more instructive, or simply necessary, to use the techniques to complement each other. Using laser diffraction methods to understand particle size distribution, and microscopy to understand shape, for example, helps to build a more detailed picture of why a powder behaves in a certain way.
It is also important to note that different techniques for measuring particle size can yield different answers. This is not a new observation. In 2004, the AAPS Journal published a report on particle size analysis and called for greater harmonisation of methods and a wider appreciation of the difficulty of reconciling results obtained by different methods of particle sizing.2 Since then, observers have noted that some progress has been made via the Laser Diffraction Measurement of Particle Size USP monograph (USP <429>),3,4 released in 2005, and the USP written standards regarding lipid emulsions (USP <729>). The European Pharmacopoeia also has a chapter on laser diffraction5 and, more recently, ISO13320:2009 was published, creating a new standard, and guiding the way for robust method development.