Assembly of collagen matrices as a phase transition revealed by structural and rheologic studies

We have studied the structural and viscoelastic properties of assembling networks of the extracellular matrix protein type-I collagen by means of phase contrast microscopy and rotating disk rheometry. The initial stage of the assembly is a nucleation process of collagen monomers associating to randomly distributed branched clusters with extensions of several microns. Eventually a sol-gel transition takes place, which is due to the interconnection of these clusters. We analyzed this transition in terms of percolation theory. The viscoelastic parameters (storage modulus G′ and loss modulus G″) were measured as a function of time for five different frequencies ranging from ω = 0.2 rad/s to 6.9 rad/s. We found that at the gel point both G′ and G″ obey a scaling law G′(ω) ∝ G″(ω) ∝ ω^Δ, with the critical exponent Δ = 0.7 and a critical loss angle δ(tan δ = G″/G′) being independent of frequency as predicted by percolation theory. Gelation of collagen thus represents a second order phase transition.