The problem?
Faced with an issue related to the varying behaviour of its liquid product depending on different supplier formulations, one of our pharmaceutical clients wanted to measure the contact angle and surface tension of its samples deposited on three different substrates.
The client called on FILAB to verify that the formulation, in this case eye drops, exhibited optimal wetting behaviour on several types of surfaces.
Understanding the wetting behaviour of a drop on a substrate
Objective:
The aim of the study is to measure the contact angle and surface tension of several formulations on three different substrates.
- silicone support,
- neutral polyethylene support,
- polyethylene support with antimicrobial agent.
The approach used
Approach :
The technique used is goniometry, specifically the sessile drop method. A 4 µL drop of water (the model liquid) is placed on each sample. Measurements are taken a few seconds after placement, once the drop has reached a stable state. Each formulation is tested on the appropriate substrates.
For each formulation, surface tensions and contact angles are measured and compared between the different substrates.
Conclusion of the study
According to the analysis report, the nature of the substrate significantly influences the behaviour of the formulations. For the same formulation, the spreading and adhesion properties vary significantly depending on the contact surface.
Conclusion :
This study revealed that surface tensions are generally higher on polyethylene substrates than on silicone substrates.
Observation of the droplets shows that the silicone substrate is more hydrophobic than the polyethylene substrate, since the droplet forms a larger contact angle and lower surface tension on this substrate. This simple observation perfectly illustrates the interaction between this formulation and the tested surfaces.
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Frequently asked questions
Why compare a liquid formulation on several substrates?
Wetting behaviour depends on the interaction between the formulation and the surface it contacts. FILAB compared several eye-drop formulations on three substrates to examine these differences. A measurement performed on a single substrate does not necessarily describe how the same formulation will behave on another material.
Which substrates were compared in the wettability study presented by FILAB?
The study examined a silicone substrate, a neutral polyethylene, and a polyethylene containing a microbial agent. The comparison aimed to determine how the nature of these surfaces influenced the behavior of the formulations. These three substrates define the scope of the case studied and do not represent all materials that may be used with an eye drop.
What can a sessile drop contact angle measurement be used to compare?
The sessile drop method examines the shape of a drop deposited on a substrate and makes it possible to compare its contact angle under defined conditions. FILAB used it to characterize differences in wetting between the surfaces tested. Interpretation must take into account the liquid, the surface, and the measurement protocol.
Why control the volume and observation time of a droplet during measurements?
Comparable deposition and observation conditions make it possible to interpret differences between samples consistently. The protocol described by FILAB specifically calls for a measurement a few seconds after deposition, once the droplet has stabilized. The choice of reference liquid and test conditions must also be specified to understand the results.
What did the comparison of substrates show in this goniometry study?
The study highlighted the influence of substrate type on the behavior of the formulations. The observations notably reported a higher contact angle on silicone than on polyethylene under the conditions tested. These results document the liquid/substrate interaction; they do not constitute an assessment of the eye drops’ therapeutic efficacy.