Indeed, during the past few years, industry has placed a growing emphasis on particle characteristics other than size, such
as shape, surface charge, and roughness. "Particle sizing is just one element of particle characterization," says Rideal.
"Other disciplines in this sector include particle shape, flow properties, both in the dry state and as suspensions (rheology),
morphology—including crystal structure, and porosity. Particle sizing alone is therefore not always the sole answer to efficacy....
A combination of physical properties is invariably investigated in arriving at the final solution."
According to Kippax, particle shape, in particular, has become an important parameter for both processing and final product
quality. "In direct compression tableting, for example, shape is known to have an impact on blend uniformity, the consistency
of powder flow through the press, compressibility, and the mechanical strength of the finished tablet," he says. "On the imaging
front, many are also already looking beyond shape to composition."
Particle characterisation is used in many areas of the pharma industry, including excipient characterization and other quality
control applications. Additionally, particle charcterization has a large role to play in the development of inhaled pharmaceuticals
(see sidebar 1: Inhaled pharmaceuticals). The industry’s drive for improved manufacturing efficiencies has also increased the uptake of particle characterization
technologies during process development. Particle characterization data help manufacturers to better understand their product,
and provide the opportunity for process optimization according to the product’s characteristics. In isolation, however, particle
characterization data do not always provide sufficient information to achieve this. In this case, techniques that characterize
the powder, rather than the constituent particles, can play an important role. Over the past few years, there have been a
number of advances in the area of powder characterization have helped manufacturers better understand their processes, ultimately
leading to more efficient manufacturing (see sidebar 2: Powder characterisation).
Sidebar 1: Inhaled Pharmaceuticals
Understanding particle characteristics can benefit process development in several ways. The latest technologies, such as the
on-line laser-diffraction systems noted above, enable processes to be monitored in real time during development work, which
provides better control and facilitates scale up. "The sector's interest in on-line, real-time measurement has risen sharply
as it works toward greater manufacturing efficiency," says Kippax. "Real-time monitoring accelerates the reliable identification
of 'cause and effect' during process development and has a valuable role to play in the transition to smarter operation."
Sidebar 2: Powder Characterization
Real-time monitoring, however, is not yet available in all areas of particle characterization. Tim Freeman, director of operations
at Freeman Technology, which specializes in powder characterization, adds, "I would perhaps highlight particle shape analysis—automated
imaging—as a prime example of a technique with proven applicability, that would be a desirable addition to the real-time selection
for pharma, but has yet to make this transition."