Physical Characterization of Nano Particulates Used in the Pharmaceutical Industry

Introduction

The formal definition of nano particle is a discrete entity that has a dimension of 100 nm or less. A nano-scaled material, on the other hand, may be comprised of nano-scaled structures, though the physical dimensions of the material may exceed 100 nm. A spray-dried agglomerate of nano-sized particles would be described as nanoscaled. For pharmaceutical applications, one does not resort to the use of nano materials unless there is a therapeutic driving force to do so. If the compound is poorly soluble in-vivo or if the molecule is highly potent, and the desired presentation of the drug product is a tablet, capsule, or suspension, the decision may be implemented to produce the active pharmaceutical ingredient as a nano material.

The actual form of the nano materials covers a number of technologies, from actual nanocrystals, to emulsions, to liposomes. Regardless of the formulation or carrier, once this size range is implemented, the drug product performance is intimately connected to the physical properties of the active pharmaceutical ingredient. Physical characterization becomes a crucial part of the drug substance and drug product control strategy.

Physical Characterization Techniques

Sample Preparation

To obtain accurate and meaningful particle size data, the challenge for nano materials lies in the sample preparation. Because the material is so small and so surface active, generally accepted scientific practices can often end up producing artifacts, or altering the sample during measurement.

Toxicity concerns for highly potent nanocrystals suggest that the material should not exist as a dry powder, but can be more safely handled dispersed in a liquid. To prevent particle aggregation, either the pH can be adjusted, or surfactants can be used. Solubility of drug substance particles, however, is often pH dependent; therefore, if the pH is adjusted to provide adequate particle dispersion, the material could dissolve. The same phenomenon can occur with surfactants, where the particles can be solubilized by their addition.

If the particles are not solubilized by the use of surfactants, then for some characterization techniques, the adsorbed layer of surfactant will skew the particle size, making the particles appear larger. This, however, may be relevant to the performance of the drug, and such measurements can be valuable in predicting in vivo performance. Factors such as these must be considered when interpreting physical characterization data for nano materials in pharmaceutical applications.

Particle Size Measurement Via Dynamic Light Scattering

 Nano particle with adsorbed surfactant layer. Particle size data obtained by PCS will also measure the effect of the surfactant

Laser diffraction, the dominant platform for sizing most pharmaceutical powders, is an ensemble technique (i.e. measures many particles rather than a single particle measurement) that has a lower size limit of approximately 0.1 μm. To measure objects below that limit, dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS), is one of the few ensemble techniques that can be employed in this size range (0.005 – 1 μm). The technique is well established and several commercial platforms are available. 

Stable, well-dispersed particles are placed in the sample cell, where Brownian motion causes the particles to move randomly in the suspension. A laser beam passes through the sample cell and is scattered by the particles. The randomly changing diffraction pattern is converted into a histogram of intensity vs. size. Note that this data representation is not what one typically encounters, which is frequency vs. time. For this reason, PCS should be used to measure an average particle size rather than to produce a particle size distribution.

Particle Size Measurement by Electron Microscopy

Since the inception of nano technology, electron microscopy has served as the gold standard for measuring particle size and morphology. Initially, scanning electron microscopes were incapable of imaging nano-scaled objects; therefore transmission electron microscopy was the only microscopic technique available. With the advent of field emission electron guns for scanning electron microscopes, this scenario changed. Both instruments now offer clear images of nano materials. Figure 2 shows a scanning electron micrograph of drugcarrying biodegradable nanoparticles comprised of a polysebacic acid core and a shell of polyethylene glycol. 

Scanning electron microscopy offers a three-dimensional representation of the particles. The gray scale produced in SEM images, however, makes quantitative measurements on these images using appropriate image analysis software very difficult because of the lack of contrast provided.

For both transmission and scanning electron microscopy, the particles must be isolated, and are analyzed in a high vacuum environment.

Both techniques are insensitive to adsorbed surface layers such as surfactants that would be required to disperse the particles in a liquid (typically aqueous) environment.

To obtain these high resolution images, in both instances, the sample is bombarded with a very high energy electron beam. Prolonged exposure of the particles to this beam can cause degradation. This must be considered when opting for this type of analysis.

In summary, both PSC and electron microscopy provide information on particle size. If at all possible, both techniques should be employed for the size characterization of pharmaceutical nanoparticles. They are complimentary orthogonal techniques that provide valuable information.

Surface Area Measurements

Transmission electron micrograph of silica particles used for drug delivery

Particle diameter is perhaps the most relevant data to collect for scenarios where the physical size of the particle controls the in vivo performance of the drug product. When nano-scaled formulations are considered based upon the poor solubility of the drug substance, surface area measurement provides equally meaningful data. It has long been realized that surface area controls the dissolution of solid oral dosage forms. 

Because nanoparticles can easily pass through the filters and frits found in commercial surface area instruments, either the sample must be exposed to vacuum very slowly, or the material should be presented in an agglomerated form (i.e. nano-scaled). If the nanoparticles are contained in aggregates, a two-tiered strategy for characterization could be adopted, whereby the particle size measurement would measure aggregate size, and surface area measurements would provide an indication of the nano particle size. Often, high energy sonication is employed in an attempt to attain the primary particle size. This is not a preferred choice. The data obtained is often an indication of the degree of sonication, rather than the primary particle size.

by

Akshaya Srikanth,  Ronald Iacocca

Pharm.D Internee, Professor of Advanced Pharmaceutical Technology

Hyderabad, India