Viscoelastic characterization of rat cerebral cortex and type I collagen scaffolds for central nervous system tissue engineering

Abstract

In the field of tissue engineering and regenerative medicine for the central nervous system, therapeutic strategies may involve implantation of biomaterial scaffolds into the brain. An understanding of the relationship between the brain and the scaffold mechanical properties can help in the selection of a safe and effective biomaterial. This research demonstrates the use of indentation testing along with viscoelastic modeling to characterize and compare mechanical properties of in situ rat cerebral cortex and collagen scaffolds of varying collagen concentration. The stress-relaxation solution for indentation of a viscoelastic material was derived based on a five-element Maxwell model and use of the correspondence principle. Applying the model to experimental stress-relaxation data, the brain was characterized by three shear moduli G(1)=1.6±0.10 kPa, G(2)=2.0±0.15 kPa, G(3)=1.8±0.20 kPa, and two viscosities η(2)=11.0 ± 0.44 kPa⋅s, η(3)=148.7 ± 6.70 kPa⋅s, with corresponding relaxation time constants τ(1)=5.7±0.3 s and τ(2)=88.4 ± 7.6 s. The brain showed average relaxation of 74% from its peak force during loading to an approximately asymptotic force over a 5 minute hold at constant displacement. Collagen scaffolds generally showed increasing trends in the shear moduli, viscosities, and percentage relaxation with increasing collagen concentration. While the brain had similar stiffness to the 1.0% collagen scaffold during the loading phase, the brain's relaxation behavior was distinct from all of the scaffolds. Similarities and differences between the mechanical behavior of the brain and collagen scaffolds of varying collagen concentration are discussed in relation to application of biomaterials for regenerative medicine.