The comparison of eight different common in vitro and ex vivo environments with in vivo conditions applying model collagen samples: Correlation possibilities and their limits

New biomaterials are routinely evaluated for their degradation behaviour in the real body environment. Following the 3R strategy, in vitro simulated body conditions are often preferred. No studies that simultaneously compare such conditions with the real body environment have been conducted to date. Model porous collagen scaffolds were exposed for 21 days to eight different environments: simple salt-based and enzymatic media, human blood plasma, cell culture media with and without human fibroblasts and ex vivo model cortical bone, and subsequently compared with an in vivo environment represented by a pig peritoneum. The mechanical properties of the scaffolds were then determined via uniaxial compression testing, and the structural properties via the micro-CT, weight loss, infrared spectroscopy, X-ray diffraction and histological methods. Interestingly, the various analysed simulated body conditions caused differing alterations in the collagen scaffold characteristics when compared with the real body environment. The mechanical properties were similar during the first 7 days of incubation but diverged after 14 and 21 days. The structural properties varied significantly after just 7 days of incubation. The histological evaluation of the scaffolds exposed to the cellular, ex vivo and in vivo conditions revealed the poor ability of cells to completely populate the scaffolds, accompanied by the massive ingrowth of connective tissue into the in vivo exposed scaffolds, which resulted in their variable global behaviour. In conclusion, the value of in vitro simulated body environments lies in their screening capacity and feasibility; however, direct extrapolation to real body conditions needs to be verified going forward.