Figure 1. Confocal Brillouin microscope and stiffness calibration.

Optical setup diagram of the confocal Brillouin microscope (a) and longitudinal modulus of polyacrylamide hydrogels of different stiffness (b). The stiffness of the polyacrylamide gels was varied by adjusting the amount of crosslinker from 0.05% to 1.2% w/v bisacrylamide. The hydrogels (n = 3 for each crosslink concentration) were measured using the Brillouin microscope to determine their longitudinal modulus. The longitudinal modulus appears to increase linearly with the hydrogel crosslink concentration (R² > 0.99), indicating that Brillouin light scattering provides a meaningful measure of bulk stiffness.