Strain stiffening in collagen I networks.

Biopolymer gels exhibit strain stiffening that is generally not seen in synthetic gels. Here, we investigate the strain-stiffening behavior in collagen I gels that demonstrate elasticity derived from a variety of sources including crosslinking through telopeptides, bundling through low-temperature gelation, and exogenous crosslinking with genipin. In all cases, it is found that these gels exhibit strain stiffening; in general, onset of strain stiffening occurs earlier, yield strain is lower, and degree of strain stiffening is smaller in higher concentration gels and in those displaying thick fibril bundles. Recovery after exposure to high strains is substantial and similar in all gels, suggesting that much of the stiffening comes from reversible network deformations. A key finding of this study is that collagen I gels of identical storage and loss moduli may display different nonlinear responses and different capacities to recover from high strain.

Pore size variable type I collagen gels and their interaction with glioma cells.

Gelation temperatures from 22 degrees C to 37 degrees C were used to control the pore size of collagen matrices independent of collagen concentration. To limit cell exposure to temperatures lower than physiological temperature, the putative nucleation and growth mechanism of collagen was investigated to determine the time at which gel fibril and network structure becomes independent of temperature. It was found that the temperature dependent portion of collagen gelation ends close to the time at which fibrils first form a network spanning structure. These findings were then exploited to prepare cell-embedded gels nucleated at 22, 27, or 32 degrees C and then incubated at 37 degrees C. This achieves fibrillar and network structure characteristic of gels formed solely at the nucleation temperature. Proof of principle studies of glioma invasion in these gels suggested pore size is a key determinant of glioma invasive speed in collagen gels.