Analyzing the feasibility of discriminating between collagen types I and II using polarization‐resolved second harmonic generation

According to previous studies, the nonlinear susceptibility tensor ratio χ33/χ31 obtained from polarization‐resolved second harmonic generation (P‐SHG) under the assumption of cylindrical symmetry can be used to distinguish between fibrillar collagen types. Discriminating between collagen fibrils of types I and II is important in tissue engineering of cartilage. However, cartilage has a random organization of collagen fibrils, and the assumption of cylindrical symmetry may be incorrect. In this study, we simulated the P‐SHG response from different collagen organizations and demonstrated a possible method to exclude areas where cylindrical symmetry is not fulfilled and where fibrils are located in the imaging plane. The χ33/χ31‐ratio for collagen type I in tendon and collagen type II in cartilage was estimated to be 1.33 and 1.36, respectively, using this method. These ratios are now much closer than what has been reported previously in the literature, and the larger reported differences between collagen types can be explained by variation in the structural organization. According to previous studies, the susceptibility tensor ratio χ33/χ31 obtained from polarization‐resolved second harmonic generation, assuming cylindrical symmetry, can be used to distinguish collagen types. In this study, we demonstrate a method of excluding regions that do not fulfill cylindrical symmetry. Thereby, the lowest χ33/χ31‐value is unaffected by orientation. However, the results indicate that the χ33/χ31‐values for collagen types I and II are very close and difficult to distinguish.