Atomic Force Microscopy

What is atomic force microscopy? What are its advantages and disadvantages?

Atomic force microscopy (AFM) relies on passing a sharp probe, mounted on a cantilever, over a surface to analyse its topography. A laser is reflected off the back of the cantilever, and the variations are measured by a photodetector and interpreted by software. The characteristics of the probe and cantilever can be varied according to the test sample. The mode of operation can also be changed, so that the probe can contact the surface continuously (contact mode), tap the surface intermittently (tapping mode), or hover above the surface (contact mode). The technique can also be used to analyse the adhesion, cohesion, stickiness and hardness of surfaces. The main advantage of using this method is that no prior preparation of the sample is required, so the dynamics of any changes can be monitored. Under the right conditions, the z-axis resolution can be close to atomic level. The main disadvantage is that the area of measurement cannot exceed 100µm by 100µm.

Walter B. McKenzie/ Photodisc/Getty Images

A comprehensive review of the applications of AFM can be found in the book "Atomic Force Microscopy in Process Engineering", edited by Professors Richard Bowen and Nidal Hilal. This also contains a chapter on the latest studies of AFM in a pharmaceutical context.

Mark Leaper

How is AFM being used for the qualitative and quantitative analysis of pharmaceutical powders?

By attaching a powder particle to a blank cantilever to create a "colloid probe", the interactions of the particle with other particles, equipment surfaces, and membranes can be measured. This can help with operations such as tablet compression, interactions of carrier particle with APIs in pulmonary drug delivery and interaction of excipients with membranes.

The micromechanical properties of particles can also be assessed by pressing the probe into the surface of the particle to reveal the hardness and stickiness of the surface. This can be done under controlled temperature and relative humidity. Because the sample requires no preparation, it's possible to obtain the dynamic responses to temperature and relative humidity changes for the same area.

In what ways does the force measurement aid the study of particle interactions between drugs and excipient particles?

As explained above, the technique can be used to examine the surface adhesion, cohesion, stickiness, and hardness of particles at varying humidities and temperatures. This can be done both in liquid and in air. It's also useful for assessing the interactions between the particles in solid dosage forms. The main applications, however, are in designing systems in which APIs are attached to inert carrier particles for pulmonary drug delivery. It's also useful for examining the effectiveness of dry granulation systems, to which interparticle forces are important.

How does AFM compare with other particle characterisation techniques, such as laser diffraction or Raman spectroscopy?

AFM should be used as complementary to such techniques, rather than as an alternative. AFM is primarily focusing on the surface properties of particles and topography, whereas the other techniques are used to identify the chemical composition of the entire particle, or the structure of the crystals/molecules present. AFM gives no indication of the degree of crystallisation or water content, but may indicate areas of different physical properties, which can be investigated further, using other methods. This can be useful in systems that are changing from an amorphous to a crystalline nature. AFM can detect the zones on the particle where this is taking place and other techniques can be used to examine this further.

How can characterisation of the morphology and roughness of pharmaceutical granules using atomic force microscopy result in improved granulation and pharmaceutical processing?

The flow properties of a powder can be influenced by the shape and roughness of the particles. A thorough knowledge of the surface properties of excipient powders and the resulting granules will enable process parameters to be optimised. It will also be helpful when modifying process methods, for example, a switch from wet granulation to dry granulation or direct compression. Rough particles also have a greater surface area, allowing the absorption of more moisture. This may be desirable or undesirable, depending on the application of the powder.

How might AFM be applied to the measurement of dynamic rheological properties, to benefit characterisation of a pharmaceutical formulation?

AFM can be used in liquids, and there have been several studies of suspensions and colloids in which AFM has been used to measure interactions between particles, as well as between the particles and fluid. There have also been studies that have examined the formation of bubbles in liquids and their interaction with particles in the suspension. AFM could be used to complement other techniques in optimising the design and manufacture of pharmaceutical suspensions, creams and lotions. In addition to the properties of the drug delivery systems, AFM can be used to characterise the areas to which the formulations are applied, such as skin, hair or dressing.

Are there any knowledge or technological gaps in the pharma industry that prevent AFM from being fully exploited?

The techniques and research strategies for using AFM in the pharma industry are now well established: examining the properties of powdered excipients, colloids, pulmonary drug delivery, microbiological systems and implants. The volume of data is still limited, however, and has not kept pace with the new formulations. As manufacturing companies modify their practices to accommodate the new quality by design approach, AFM will have a part to play. Along with bulk powder techniques, knowledge of the variation of surface properties will enable optimisation of solid-dosage manufacturing processes.

Mark Leaper is Senior Lecturer, School of Pharmacy,De MontfortUniversity (UK).