determine the value of E of bulk gels, we also
used an atomic force microscope (MFP-3D; Asylum Research) equipped
with a tetrahedral cantilever (OMCL-AC240TS-C3; Olympus, Japan). The E value of the gels was determined by deforming the surface
and fitting the force–indentation curve to the Hertz (Sneddon’s
variation) model:33 (link) Here, F is the applied force
calculated using κ, and δ the indentation length according
to the difference between the piezo height and cantilever deflection.
The elastic modulus (E) and Poisson’s ratio
(ν) together define the material properties. Poisson’s
ratio ν was assumed to be 0.5 because the gelatin gel was considered
incompressible, as has been previously reported.34 (link) The cantilever properties were defined by the opening angle
α (=34°) and cantilever spring constant κ. A thermal
vibration based program installed in the AFM instrument was used to
calculate κ. The cantilever was calibrated before each experiment,
and the average κ for the experiments was 2 N/m. By analyzing
the approach curve, we obtained E.