To evaluate cell adhesion on SF hydrogels, hMSCs were cultured on SF for 24 h. The hMSCs well attached and spread on SF hydrogels from relatively higher Mn SF hydrogels (i.e., L0, L10, and L30) while the cells maintained spherical shape and less cells were observed on the relatively lower Mn SF hydrogels (i.e., L90 and L180) (Fig. 8A). The metabolic activity of hMSCs cultured on SF hydrogels showed a corresponding decrease as the Mn of SF decreased (Fig. 8B). Fig. 8C depicts F-actin structure of hMSCs on SF 24-h post-seeding. It was shown that F-actin was well developed on the whole cytoplasm of hMSCs on the intact SF hydrogel (L0, Fig. 8Ca) while observed at round boundary near cell membrane (L180, Fig. 8Cb). It is generally known that cell attachment and spreading are governed by the matrix stiffness [31] and [32]. As expected, a fewer hMSCs attached on L180 gel surface with less spreading while much more hMSCs remained and spread on the surface of stiffer SF hydrogels (i.e., L0) with well-developed actin cytoskeletons. Hence, it is proved that SF hydrogel properties could be altered in very wide ranges like other synthetic polymer-based hydrogels by molecular weight control of SF. Especially, the improved transparency of depolymerized SF hydrogel will be very useful for cell morphology observation and various biological assays. Also, this technique can expand the optical device application of silk-based materials.