Effect of electrostatic forces on the dynamic rheological properties of injectable collagen biomaterials

Injectable collagen is a concentrated dispersion of phase-separated collagen fibres in aqueous solution used to correct dermal contour defects through intradermal injection. The effect of electrostatic forces on the rheology of injectable collagen was studied by observation of the birefringence of collagen fibres through a polarizing microscope as well as by oscillatory rheological measurements on dispersions of varying ionic strengths (0.06-0.30). The birefringence of fibres progressively increased as ionic strength was reduced from 0.30 to 0.06. The linear viscoelastic measurements displayed a logarithmic relationship between storage (and loss) moduli and frequency over oscillation frequencies of 0.1-100 rad/s. The associated relaxation time spectra, interpreted using the theory of Kamphuis et al. for concentrated dispersions, show that collagen fibres become more flexible as ionic strength increases. This result was analysed at the molecular level from the perspective that collagen fibres are a liquid-crystalline phase of rigid rod collagen molecules which have phase-separated from solution. Electrostatic forces affect the volume fraction of water present in the collagen fibres which in turn alters the rigidity of the fibres. Flexible collagen fibre dispersions displayed emulsion-like flow properties whereas more rigid collagen fibre dispersions displayed suspension-like flow properties. Changes in fibre rigidity significantly alter the injectability of collagen dispersions which is critical in clinical performance.