Retina microrheology via oscillatory atomic force microscopy

Viscoelastic properties of tissues, including elasticity and viscosity, are crucial for understanding development and disease progression. However, traditional atomic force microscopy (AFM) indentation methods provide limited insight into these complex tissue properties. This study establishes microrheology via oscillatory AFM to assess both the elastic and viscous components of tissue mechanics. We first compared indentation AFM to oscillatory AFM on mouse retinal tissue and found that the Young's modulus of indentation AFM (956.8 Pa) was statistically similar to the elastic component (storage modulus, E') of oscillatory AFM (920.2 Pa), while also providing the viscous component (loss modulus, E″ = 218.3 Pa), and the loss factor (tan(δ) = 0.238) across a wide range of biologically relevant frequencies (1-100 Hz). We also found that optimization of input probe parameters, such as approach length, approach speed, applied force, and oscillation amplitude, is key for accurate measurements. To examine whether this approach can detect differences between healthy and diseased tissues, we applied it to murine retinas from healthy control mice and diabetic retinopathy mice, using the oxygen-induced retinopathy (OIR) mouse model. OIR retinas exhibited increased stiffness (E' = 3564.0 Pa) and a higher loss factor (tan(δ) = 0.478) compared to healthy retinas (E' = 920.7, tan(δ) = 0.263), suggesting changes in the extracellular matrix and highlighting how retinopathy may alter matrix properties. Finally, to assess the feasibility of using microrheology AFM on banked tissues biospecimens, we examined how tissue fixation affects the measurements. We found that formaldehyde fixation increased stiffness and elasticity, with OIR tissues consistently stiffer than WT tissues in both fixed and unfixed tissues, enabling valid cross-treatment comparisons. Our findings establish the benefits of microrheology in capturing tissue mechanical behavior, which is important for studying disease impact on tissue mechanics. This approach offers new insights into tissue viscoelasticity with implications for studying the dynamics of tissue mechanics in diseases and regeneration.