A recently developed imaging mode, PeakForce, enabled concomitant measurement of topography and nano-mechanical properties of the sample, such as elastic modulus and deformation.
31 In this mode, the tip is oscillated at 1 kHz to acquire short range force-distance curves while the sample is scanned in the horizontal plane. The vertical distance, at which the force setpoint is reached, provides information about the topography of the sample, while the slope of the contact region provides estimations of mechanical properties, including deformation and elastic modulus, of lens cortical (
Fig. 2) and nuclear (
Fig. 3) cells. The cells' thickness (3–6
μm,
Figs. 2A and
3A) appeared uniform for both cell types (
Figs. 2B and
3B). The maximum deformation of the sample at the peak force is readily an indirect evidence for their difference in cell elasticity (
Figs. 2C and
3C). Cortical cells were found more deformable (221 ± 33.4 nm) than nuclear cells (45.0 ± 11.3 nm). This is supported further by the elastic modulus maps (
Figs. 2D and
3D), which show softer cortical cells compared to nuclear ones. While PeakForce imaging allows the direct structure-nano-mechanics assessment of a sample, due to the sharp pyramidal tip used and the shortness of the force-distance curves acquired, for real quantitative assessment of the mechanical properties of lens cells force-distance curves using spherical tips are preferred. Nevertheless, using PeakForce measurements, the elastic modulus for nuclear cells (
Fig. 3D) appeared more than an order of magnitude higher than the values for cortical cells (
Fig. 2D). Although qualitative, stiffness maps also allowed us to determine the degree of heterogeneity of large cell areas with a resolution of ∼ 100 nm. As can be observed, stiffness maps showed relatively low heterogeneity, suggesting that measurements obtained at few cell regions provided a reliable estimate of the overall cell elastic response. Representative force-distance curves of cortical and nuclear cells (
Fig. 4) varied in slope after the contact point clearly showing the difference in mechanical properties of two cell types (notice the different scales). The nuclear cell was stiffer and less deformable, which caused the cantilever to deflect more giving rise to a steeper slope.