(a–c): Confocal microscopy images of glucagon fibrils with initial concentration 3 mg/mL in aqueous buffer (50 mM glycine HCl, pH 2.5) at three consecutive times: 64, 87 and 126 mins after the onset of fibrillation. Scale bar shows 5 μm. Each circle represents a data point and the red line represents the cumulated tracked positions of the growing fibril end. (d) Growth of 20 fibrils at the glucagon concentration of 3 mg/mL. Plateaus correspond to arrested states while fibrils elongate outside the plateaus. (e) The average end-to-end-distance squared () as a function of fibril length. The solid green line is obtained by fitting Eq. (1) to combined experimental data (red points) from all glucagon concentrations. The persistence length of fibrils is returned by the fit as 60 ± 2μm.