Determining the helical pitch and relative polarity of collagen using polarization-resolved SHG

Abstract

Collagen fibrils are nanoscale biological ropes which are the primary load bearing structures within the human body. Due to the non-centrosymmetric structure and high nonlinearity of the triple helical collagen molecule, collagen fibrils are able to produce significant second harmonic generation (SHG) when placed at the focus of an ultrafast laser. There have been many publications which have utilized polarization resolved SHG (PSHG) for investigating collagen organization in tissues, mainly for applications in disease diagnosis. However so far there has been little work on investigating individual isolated collagen fibrils using PSHG. This presents an exciting opportunity for research since PSHG imaging of individual fibrils can help us to gain a better understanding of PSHG images of collections of fibrils in tissues, and therefore improve the utility of PSHG as a potential diagnostic tool. Additionally, the high sensitivity of PSHG to changes in molecular structure can provide us with a better understanding of the nanoscale origins of injuries to tissues such as skin, tendons, and ligaments. Here we will review our recent work which has applied PSHG to study individual collagen fibrils, including investigation of how out of plane orientation affects PSHG images of collagen, and how differences in hydration alter collagen ultrastructure. We also present new data on the effect of applied tension on individual fibrils.