Researchers seek alternatives to established methods for determining the porosity of poly lactic-co-glycolic acid (PLGA) microparticles. These methods, such as mercury intrusion porosimetry (MIP) and nitrogen adsorption based methods, often require large sample volume and produce toxic metal waste.
Seeking an alternative method, Sediq et al. (2017) evaluated the ability of the FlowCam® to determine density and porosity of PLGA microparticles using sedimentation veolocity and Stoke's Law. Sediq et al. used the FlowCam and VisualSpreadsheet to image, analyze, and calculate the settling velocities of micropaticles. The FlowCam only requires a few milliliters of sample.
METHOD I: PLGA Microparticle Porosity from Sedimentation Velocity in Fluids with Varying Densities (Density-Matching Method)
In the first method, cesium chloride-containing phosphate buffered saline solutions of various densities were impregnated with a one of three microparticles, ranging in porosity from 4, 21.6, to 51.9%. Porosities were pre-determined using MIP. Sediq et al. would load 1.5mL of sample into the FlowCam and start with a flow rate of 0.20mL/min and a camera rate of 10 frames/second. After the flow cell and associated tubing were filled with sample, the flowcell was manually clamped to cease the flow of liquid. The FlowCam camera continued to image the settling of the particles suspended in the fluid. The particle displacement over time was calculated for accurately sized particles that met criteria for normal settling behaviors, as determined by Sediq et al., and used in the displacement rate plot.
METHOD II: PLGA Microparticle Porosity from Sedimentation Velocity using Stokes' law
In the second method, Sediq et al. compared the velocity rates of particles suspended in fluids with densities close to the particle density. Low sedimentation velocities resulted in more accurate density determinations due to the high volume of displacement over time measurements from the many frames captured during the settlement process, as described in Method I.
Sediq et al. generated a plot of metric displacement per particle over time. The slope of the linear regression for this plot is equivalent to the particle velocity.
Density values were calculated using the particle velocities. From the density values and known composition of the particles, the porosity was calculated.
Sediq et al. tested the validity of the FlowCam method using non-porous polystyrene beads. The density calculated from the linear regression of the velocity rate of the polysytrene beads was statisically equal to the reference values of the manufactuer, therefore demonstrating the validity of this method.
Both methods tested by Sediq et al. yielded porosity values similar to those obtained by MIP and required 100-fold less sample volume than MIP, therefore validating the use of sedimentation velocity and the FlowCam for determining porosity of PLGA microparticles.
Sediq, A.S., Waasdorp, S.K.D., Nejadnik, M.R., van Beers, M.M.C., Meulenaar, J., Verrijk, R., Jiskoot, W. (2017), Determination of the Porosity of PLGA Microparticles by Tracking Their Sedimentation Velocity Using a Flow Imaging Microscope (FlowCAM). Pharmaceutical Research. doi: 10.1007/s11095-017-2120-8