Highly stretchable hydrogels from complex coacervation of natural polyelectrolytes

Area of Science:

• Biomaterials Science
• Polymer Chemistry
• Materials Engineering

Background:

• Hyaluronic acid and chitosan are biocompatible polysaccharides with opposing charges, suitable for polyelectrolyte complexation.
• Complex coacervation is a method for forming hydrogels with tunable mechanical properties.
• Controlling solution conditions like pH and ionic strength is crucial for directing hydrogel formation and properties.

Purpose of the Study:

• To investigate the impact of pH on the complex coacervation of hyaluronic acid and chitosan.
• To understand how pH influences the mechanical properties (elasticity, strength, stretchability) of the resulting hydrogels.
• To explore the potential for creating advanced hydrogel materials through controlled polyelectrolyte assembly.

Main Methods:

• Preparation of high ionic strength solutions of hyaluronic acid (HA) and chitosan (CS).
• Desalting via dialysis to induce complex coacervation.
• Systematic variation of pH during polyelectrolyte assembly and characterization of hydrogel properties.

Main Results:

• Slight coacervation occurred at pH 5.5-7.5 due to insufficient chitosan protonation.
• Free-standing, handleable hydrogels formed at pH 3.0-5.5 via coacervation and syneresis.
• Unusual hydrogel stretchability was observed at pH 2.0-3.0, near hyaluronic acid's pKa, attributed to HA chain pre-folding and HA/CS complexation.

Conclusions:

• pH is a critical parameter for controlling the complex coacervation process and hydrogel characteristics.
• Tailoring pH allows for the fabrication of hydrogels with specific mechanical properties, from robust to highly stretchable.
• This pH-tunable complex coacervation offers a versatile strategy for designing functional hyaluronic acid/chitosan hydrogels.