For thousands of groups all over the world, effective drug delivery is a subject of research. Using liposomes is one way in which drugs can be delivered. Liposomes are spherical vesicles that possess one or more lipid bilayers, the spherical form enables active molecules to be trapped inside. In this article different inks are studied using a visual microfluidic rheometer called FluidicamRHEO.
These molecules are then delivered into the target cells when the lipid bilayer of the liposome fuses with the bilayer of the cell membrane.
Drug delivery is the main use of liposomes, but there are other applications where they are employed such as: pesticides, dietary supplements, dyes for textiles and cosmetics.
Reminder of the Technique
To measure viscosity, FluidicamRHEO employs a co-flow microfluidic method. The sample and a reference solution are introduced into the microfluidic channel simultaneously (typically 2.2 mm X 150 μm) at controlled flow rates. This results in a laminar flow where the interface position between sample and reference relates the viscosity ratio and flow rates.
Figure 1: FluidicamRHEO measuring principle
Images collected during the measurement enable the software to calculate the position of the interface and directly plot an interactive flow curve.
Test one shows four different liposome samples which were all measured at 25 °C. Around 1 ml was taken to plot a flow curve. A 50 μm glass chip which enables a shear rate range between 1000-10,000s-1 and a low sample consumption was used.
Results could be correlated to the conventional rheometer at the lower shear rates and the higher shears correspond to shear happening in a needle. The second test shows sample B and D at both 25 °C and 37 °C.
FluidicamRHEO built-in camera enables observation the flow before, during and after the measurements. Example of Sample A is shown below:
Figure 2. Images of the inside of the microfluidic chip.
The flow rates are calculated automatically using the software, through a needle these flow rates can be correlated to the flow rate.
Results – Characterization of Injectable Liposome at 25 °C
Even when measuring low viscosity samples, FluidicamRHEO gives high precision because the laminar flow in the microfluidic chip is preserved.
Figure 3. Flow curves samples A-D at 25 °C.
Figure 4. Barchart showing the viscosity of samples A-D. Comparison of viscosity at 25 °C and 37 °C.
Samples B and D were chosen to be tested at 37 °C to mirror the body temperature. Figure 5 shows the results.
Figure 5. Flow curves samples B and D at 25 °C and 37 °C
Both of the samples are still exhibiting shear thinning behavior at 37 °C. The table below indicates how the temperature affects the sample’s viscosity, η.
Table 1: Viscosity of samples B and D at 25 °C and 37 °C
||η (mPa.s) Low shear 25 °C
||η (mPa.s) High shear 25 °C
||η (mPa.s) Low shear 37 °C
||η (mPa.s) High Shear 37 °C
A decrease in viscosity of 37% is observed at low shear when the temperature is increased to 37 °C for Sample B, but Sample D only exhibits a 32% decrease. At a higher shear, both samples show a 40% decrease in viscosity. Sample B is slightly more impacted by the temperature than sample D.
The high sensitivity when measuring low viscosity samples means very small differences can be observed and the impact of the temperature on the liposomes can also be studied in minutes. FluidicamRHEO is a powerful tool for the comparison of injectable liposomes with as little as 1 ml needed to plot a flow curve. Characterizing viscosity at real conditions provides deeper insight to injectability of analyzed formulations.
This information has been sourced, reviewed and adapted from materials provided by Formulaction.
For more information on this source, please visit Formulaction.