Surface characterisation and biomechanical analysis of the sclera by atomic force microscopy

The sclera is an important collagenous based connective tissue that gives the eye its shape and protects the sensitive layers within the globe. The elasticity and resilience of the sclera mainly come from the stroma, which contains a dense network of collagen fibrils comprising 90% of the thickness of the tissue. However, the outermost layer of the scleral tissue (∼10 μm) is known as the episclera, which is mostly uncharacterised and seldom investigated. Here, we use AFM scanning of porcine eyes to show that the surfaces of these two distinctive layers are structurally different. Furthermore, we use AFM nanoindentation to show that the episclera has significantly different mechanical properties than the stroma. The mechanical properties of the stroma are shown to be distributed between its two component parts (proteoglycan matrix and collagen fibrils).

Research highlights
► First AFM imaging on fresh, unfixed scleral tissue in physiological conditions.
► High resolution imaging, highlighting periodic banding of collagen fibrils within the tissue.
► Nano-mechanical testing on the two discrete outer layers of the sclera (episclera and stroma).
► Modulus results on the episclera are correlated to the matrix phase of the tissue and are in agreement with bulk compression testing.
► Modulus results on the stroma are distributed between the two separate tissue components (matrix & collagen fibrils) and are in agreement with bulk tensile testing