Viscoelastic Characterization of the Biomechanics of Rabbit Iris using Micro-Indentation

Purpose : Iris biomechanics is essential in the pathophysiology of primary angle-closure glaucoma (PACG). Although some iris biomechanical properties have been reported [1-3], a physiologically accurate description of iris biomechanics, including its viscoelasticity, is not adequately addressed. Here, we quantified the iris viscoelastic properties using micro-indentation.

Methods : We used fresh 6-month-old NZW rabbit irides (2 males and 2 females), and conducted spherical indentation on their anterior limbal and pupillary regions, while the samples were in a 37○C PBS bath (Mach-1, Biomomentum; Fig. 1a). Using a ramp-relaxation protocol (indentation rate = 50 μm/s and relaxation = 20 s), we quantified the effective instantaneous (E*inst) and equilibrium moduli (E*eq) using a Hertzian model as E*inst=3/4Finst/eqR-0.5δ-1.5, where Finst/eq=peak/equilibrium force, R=indenter radius (250 µm), and δ=indentation depth (90 μm). We calculated Feq using the average of the last 5 s of relaxation. We used an exponential model (F=Feq+(Finst-Feq)exp(-t/τ)) to calculate the relaxation time (τ) via a logarithmic transformation and linear regression: ln(F-Feq)=ln(Finst-Feq)-t/τ. We compared regions using a t-test (α=0.05).

Results : We observed a clear viscoelastic response for all samples (Fig. 1b). Across all samples, we quantified E*inst=8.06±1.99 kPa (mean ± st.d.), E*eq=2.45±0.68 kPa, and τ=4.97±0.43 s with no significant difference between the regions (p>0.3, Fig. 2).

Conclusions : Our findings are comparable to the previous studies. For example, mean bovine iris short- and long-term moduli were 4.86±0.38 kPa (± SEM) and 0.24±0.16 kPa, respectively [2], comparable to our E*inst and E*eq, in order. However, rabbit iris elastic modulus measured using AFM was c. 0.7 kPa at the pupillary region and less stiff (c. 0.5 kPa) at the limbal region [3]. These differences may be due to methodological differences (e.g., indentation depth). Our findings provide an improved understanding of iris biomechanics and its role in PACG.