Mechanical properties of collagen-based scaffolds for tissue regeneration

Abstract

Collagen-glycosaminoglycan (CG) scaffolds for the regeneration of skin and nerve have previously been fabricated by freeze-drying a slurry containing a co-precipitate of collagen and glycosaminoglycan. Recently, mineralized collagen-glycosaminoglycan (MCG) scaffolds for bone regeneration have been developed by freeze-drying a slurry containing a co-precipitate of calcium phosphate, collagen and glycosaminoglycan. Bi-layer scaffolds with CG and MCG layers have been developed for cartilage-bone joint regeneration. The mechanical properties (Young's modulus and strength) of scaffolds are critical for handling during surgery as well as for cell differentiation. The mechanical properties of the MCG scaffolds are low in the dry state (e.g. they can be crushed under hard thumb pressure) as well as in the hydrated state (e.g. they do not have the optimal modulus for mesenchymal stem cells (MSC) to differentiate into bone cells). In addition, there is interest in extending the application of CG scaffolds to tendon and ligament, which carry significant mechanical loads. This thesis aims to improve the mechanical properties of the both CG and MCG scaffolds and to characterize their microstructure and mechanical properties. Models for cellular solids suggest that the overall mechanical properties of the scaffold can be increased by either increasing the mechanical properties of the solid from which the scaffold is made or by increasing the relative density of the scaffold. In an attempt to increase the solid properties, the MCG scaffolds with increasing mineral content were fabricated.

(cont.) The mechanical properties were lower for the more highly mineralized scaffolds as a result of an increase in the number of defects such as cracked and disconnected walls. Next, we attempted to increase the mechanical properties by increasing the relative density of the MCG scaffolds. The volume fraction of solids in the slurry was increased by a vacuum-filtration technique. The slurry was then freeze-dried in the conventional manner to produce scaffolds with increased relative densities. Increasing the relative density by a factor of 3 increased the dry Young's modulus and crushing strength roughly by 9 and 7 times, respectively, allowing the dry scaffolds to withstand hard thumb pressure. The Young's modulus for the densest scaffold in the hydrated state was in the optimum range for MSC to differentiate into bone cells. Further, we attempted to improve the mechanical properties of the CG scaffold using the same technique. We were able to achieve an increase in its tensile Young's modulus in the dry state by a factor of aboutl0 times. Finally, the fraction of MC3T3 cells attaching to the CG scaffolds was found to increase linearly with the specific surface area of the scaffold, or with the number of binding sites available for cell attachment.