Stiffening and Toughening Protein Hydrogels by Tuning Electrostatic Interactions

Area of Science:

• Biomaterials Science
• Polymer Chemistry
• Protein Engineering

Background:

• Electrostatic interactions significantly influence protein hydrogel swelling and mechanics.
• Understanding these interactions near the isoelectric point (pI) is crucial for material design.
• Chemically cross-linked protein hydrogels offer tunable properties but require precise control.

Purpose of the Study:

• To investigate the pH-dependent swelling and mechanical properties of a novel protein hydrogel.
• To explore the role of electrostatic interactions at the isoelectric point (pI).
• To demonstrate methods for tuning hydrogel mechanics via pH and buffer composition.

Main Methods:

• Fabrication of a chemically cross-linked hydrogel from engineered protein GRG5RG4R.
• Measurement of hydrogel swelling ratios across a range of pH values and ionic strengths.
• Assessment of mechanical properties, including Young's modulus and toughness, under varying conditions.

Main Results:

• Hydrogel swelling and stiffness were minimized at the isoelectric point (pI ≈ 4.6) in NaCl solution.
• Maximum Young's modulus and toughness were achieved at the pI in NaCl solution.
• In citrate and sulfate buffers, stiffness peaked below the pI, demonstrating a decoupling of swelling and stiffness.

Conclusions:

• Tuning electrostatic interactions via pH is a viable strategy for engineering stiff and tough protein hydrogels.
• Bivalent anions in buffers can act as secondary cross-linkers, enhancing hydrogel stiffness.
• pH and buffer choice offer effective mechanisms to modulate protein hydrogel mechanics without altering network chemistry.