Effect of polyacrylamide gel elasticity on collagen type II fibril assembly

Collagen type II fibrils provide structural integrity to the articular cartilage extracellular matrix. However, the conditions that control the fibril radial size scale, distribution, and formation inside of dense networks are not well understood. We have investigated how surrounding elastic networks affect fibril formation by observing the structure and dynamics of collagen type II in model polyacrylamide gels of varying moduli. Cryogenic transmission electron microscopy (cryo-TEM) is used to image the fibril structure and is verified qualitatively with optical microscopy of fluorescently-tagged collagen within the gels. Using fluorescence correlation spectroscopy super-resolution optical fluctuation imaging (fcsSOFI), the diffusion dynamics of the collagen in low pH and neutral pH conditions are determined. Overall, the fibril bundle diameter and concentration were found to decrease as a function of gel modulus. The single fibril diameter remains constant at 30 nm within the gels; however, the diameter was found to be smaller when compared to in solution. Additionally, the mode of diffusion of the collagen triple helices changes within gel environments, decreasing the diffusion coefficient. Understanding the intricate relationship between network topology and collagen type II fibril formation is crucial in gaining deeper insights into the transport phenomena within complex acellular tissues that are necessary for the development of future therapeutic materials.