High molecular weight FAF–Rapa has the purity, size, and concentration–temperature phase behavior necessary for stability at body temperature. (A) Identity, purity, and fluorescence of FAF, FAF–Rapa, and rhodamine-labeled FAF–Rapa (Rho–FAF–Rapa) were analyzed by Coomassie blue staining and fluorescence imaging of SDS–PAGE. (B) Dynamic light scattering shows that all three formulations (Table 1) remain monodisperse at 37 °C (10 μM, PBS). (C) Optical density of FAF was monitored as a function of temperature at 350 nm to confirm solubility in PBS at physiological temperatures (shaded area). The Tt of FAF at 25 μM was 57.0 °C. (D) Using optical density, the phase transition temperature was plotted vs. concentration as a phase diagram, below which FAF remains soluble (n = 3, Mean ± SD). The shaded area indicates physiological temperatures. Dotted lines show a 95% confidence interval (CI) of the mean.