Polymer dynamics in layer-by-layer assemblies of chitosan and heparin

Area of Science:

• Materials Science
• Polymer Science
• Surface Chemistry

Background:

• Layer-by-layer (LbL) assembly is a technique for fabricating multilayer thin films.
• Polysaccharides like chitosan (weak polycation) and heparin (strong polyanion) are biocompatible materials.
• Understanding film structure and dynamics is crucial for applications.

Purpose of the Study:

• Investigate the structure and dynamics of chitosan/heparin (CH/HEP) multilayer thin films.
• Determine the effect of deposition conditions (pH, salt concentration) on polymer diffusion.
• Explain the mechanism behind the observed exponential film growth.

Main Methods:

• Fabrication of CH/HEP multilayer thin films on quartz using LbL deposition.
• Utilized fluorescence resonance energy transfer (FRET) and total internal reflection fluorescence (TIRF) for analysis.
• Employed fluorescence-labeled chitosan to track molecular diffusion.

Main Results:

• CH molecules exhibit out-of-plane diffusion within the CH/HEP film.
• Diffusion coefficient of CH was largely insensitive to deposition pH and ionic strength.
• A pH-sensitive label indicated sensitivity to the outermost layer's charge, even when buried.
• CH/HEP films showed a highly interpenetrated structure with no clear layer boundaries.

Conclusions:

• The interpenetrated structure of CH/HEP films is attributed to chitosan diffusion.
• Out-of-plane diffusion of chitosan explains the exponential growth observed in LbL assembly.
• LbL deposition of polysaccharides results in complex, dynamic film architectures.