A salt-based method to adapt stiffness and biodegradability of porous collagen scaffolds

Type I collagen scaffolds for tissue reconstruction often have impaired mechanical characteristics such as limited stiffness and lack of strength. In this study, a new technique is presented to fine-tune stiffness and biodegradability of collagen scaffolds by treatment with concentrated salt solutions. Collagen scaffolds were prepared by a casting, freezing and lyophilization process. Scaffolds were treated with 90% saturated salt solutions, the salts taken from the Hofmeister series, followed by chemical crosslinking. Treatment with salts consisting of a divalent cation in combination with a monovalent anion, e.g. CaCl₂, resulted in fast shrinkage of the scaffolds up to approximately 10% of the original surface area. Effective salts were mostly at the chaotropic end of the Hofmeister series. Shrunken scaffolds were more than 10 times stiffer than non-shrunken control scaffolds, and displayed reduced pore sizes and swollen, less organized collagen fibrils. The effect could be pinpointed to the level of individual collagen molecules and indicates the shrinking effect to be driven by disruption of stabilizing hydrogen bonds within the triple helix. No calcium deposits remained in CaCl₂ treated scaffolds. Subcutaneous implantation in rats showed similar biocompatibility compared to H₂O and NaCl treated scaffolds, but reduced cellular influx and increased structural integrity without signs of major degradation after 3 months. In conclusion, high concentrations of chaotropic salts can be used to adjust the mechanical characteristics of collagen scaffolds without affecting biocompatibility. This technique may be used in regenerative medicine to stiffen collagen scaffolds to better comply with the surrounding tissues, but may also be applied for e.g. slow release drug delivery systems.