Kavli Affiliate: Gijsje H. Koenderink

| First 5 Authors: Melle T. J. J. M. Punter, Bart E. Vos, Bela M. Mulder, Gijsje H. Koenderink,

| Summary:

Soft living tissues like cartilage can be considered as biphasic materials

comprised of a fibrous complex biopolymer network and a viscous background

liquid. Here, we show by a combination of experiment and theoretical analysis

that both the hydraulic permeability and the elastic properties of (bio)polymer

networks can be determined with simple ramp compression experiments in a

commercial rheometer. In our approximate closed-form solution of the

poroelastic equations of motion, we find the normal force response during

compression as a combination of network stress and fluid pressure. Choosing

fibrin as a biopolymer model system with controllable pore size, measurements

of the full time-dependent normal force during compression are found to be in

excellent agreement with the theoretical calculations. The inferred elastic

response of large-pore ($mathrm{mu m}$) fibrin networks depends on the strain

rate, suggesting a strong interplay between network elasticity and fluid flow.

Phenomenologically extending the calculated normal force into the regime of

nonlinear elasticity, we find strain-stiffening of small-pore (sub-$mathrm{mu

m}$) fibrin networks to occur at an onset average tangential stress at the

gel-plate interface that depends on the polymer concentration in a power-law

fashion. The inferred permeability of small-pore fibrin networks scales

approximately inverse squared with the fibrin concentration, implying with a

microscopic cubic lattice model that the thickness of the fibrin fibers

decreases with protein concentration. Our theoretical model provides a new

method to obtain the hydraulic permeability and the elastic properties of

biopolymer networks and hydrogels with simple compression experiments, and

paves the way to study the relation between fluid flow and elasticity in

biopolymer networks during dynamical compression.

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