Injectable collagen as a pH-sensitive hydrogel.

Injectable collagen is a concentrated dispersion of phase-separated collagen fibres in aqueous solution. The structure and properties of collagen fibres are defined by the magnitudes of electrostatic and hydrophobic attractive forces between neighbouring collagen molecules within collagen fibres. The structure and mechanical properties of collagen fibre dispersions were studied by dynamic rheological measurements and by polarized microscopy. Rheological measurements were performed over pHs ranging from 6 to 9 and over temperatures ranging from 283 to 298 K. At higher pHs the fibre dispersions were found to possess more rigid fibres and stronger inter-fibre attractive forces. This response is argued to result from changes in the ionization of amino acid side chains, which result in larger net-electrostatic attractive forces. Raising the temperature caused fibres to rigidify through enhanced hydrophobic attractive forces. Gels formed by lower pH-higher temperature fibre dispersions possess different properties than gels formed at higher pHs and lower temperatures.