Brillouin–Raman micro‐mapping of silicone‐hydrogel contact lenses: Revealing chemical and mechanical heterogeneities

Joint Brillouin-Raman micro-spectroscopy (BRMS) enables simultaneous, non-destructive characterisation of the mechanical and chemical properties of soft materials at the micrometre scale, combining GHz-range viscoelastic information from acoustic modes with the vibrational fingerprint of molecular species. We present an improved BRMS platform based on an inverted microscope with a single confocal pinhole shared by both spectrometers allowing a single spectrum spanning from 0.1 GHz to 100 THz to be acquired at each point of a raster scan. Applied to Delefilcon A silicone-hydrogel contact lenses, macroscopically homogeneous and optically transparent, the platform reveals micrometric bulk heterogeneities not previously reported. Their Raman fingerprint identifies these features as clusters of the copper phthalocyanine handling tint. Two-dimensional Brillouin mapping shows a reduction of the frequency shift by up to ∼0.5 GHz near the cluster centres, corresponding to a decrease of about 12% in the longitudinal elastic modulus. These results demonstrate the ability of BRMS to resolve chemical-mechanical heterogeneities that are inaccessible to either technique alone, with potential implications for the durability of biomedical soft materials.