Modelling the biomechanics of the human iris with an active strain approach.

The iris is a deformable circular diaphragm that regulates pupil size in response to changes in illumination through the antagonistic actions of sphincter and dilator muscles. While the phenomenological relationship between pupil size and light intensity is well studied, the mechanical interplay between active muscle contraction and passive iris tissue remains poorly understood. In this study, we develop a finite-element model of the human iris using an active strain formulation to investigate the mechanics underlying pupil regulation under physiological conditions. The iris is represented as a fibre-reinforced soft tissue, with passive matrix behaviour modelled as isotropic, nonlinear elastic and active muscle contraction introduced via contractive strains along fibre directions. Numerical simulations are performed using a dedicated finite-element code. By progressively including active and passive tissue components, we analyse how tissue architecture affects pupil kinematics, stress distribution and interaction with supporting boundaries at the iris root. Results reveal a counterintuitive yet significant role of passive tissues in shaping three-dimensional iris deformation and moderating boundary reactions. This computational framework provides a mechanically consistent basis for understanding iris biomechanics and can support future studies extending to more complex physiological or pathological conditions.