Indentation of the cornea: A Bi-layer contact problem.

Histological observations of the cornea have identified the presence of multiple layers with differing thickness and function. The composition of the cornea consists primarily of collagen fibrils held together with proteoglycans but with an aqueous interstitial component being dominant. Indentation provides a means to quantify the spatial variation of the mechanical properties of the cornea, however the role of the different layers on the indentation response has barely been addressed. In addition, the response of the fluid content and its displacement during indentation has not been adequately considered. In this study indentation of the cornea with a relatively large spherical tipped indenter (R = 500 µm) is considered. It was observed that the initial phase of loading did not fit a classic Hertz elastic response but showed an initial steeper slope that gradually declines with increasing force and displacement. A relatively simple approach is developed that initially considers the cornea as a poro-elastic bi-layer contact problem, that is the presence of an outer thin stiffer Bowman's layer overlaying the thicker less stiff stroma.

Zone-dependent mechanical properties of human articular cartilage obtained by indentation measurements.

Emerging 3D printing technology permits innovative approaches to manufacture cartilage scaffolds associated with layer-by-layer mechanical property adaptation. However, information about gradients of mechanical properties in human articular cartilage is limited. In this study, we quantified a zone-dependent change of local elastic modulus of human femoral condyle cartilage by using an instrumented indentation technique. From the cartilage superficial zone towards the calcified layer, a gradient of elastic modulus values between 0.020 ± 0.003 MPa and 6.44 ± 1.02 MPa was measured. To validate the tissue quality, the histological tissue composition was visualized by glycosaminoglycan and collagen staining. This work aims to introduce a new protocol to investigate the zone-dependent mechanical properties of graded structures, such as human articular cartilage. From this knowledge, better cartilage repair strategies could be tailored in the future.

A simple basis for determination of the modulus and hydraulic conductivity of human ocular surface using nano-indentation.

This paper presents a simple analysis based upon Darcy's Law and indentation contact mechanics to determine the effective hydraulic conductivity and elastic modulus of fluid filled tissues. The approach is illustrated with the mechanical response of the human ocular surface using a 500µm radius spherical tipped indenter. Indentations of various regions of the ocular surface including the corneal stroma, limbal region and sclera have been conducted. Force-control indentations were made to a maximum force, which was maintained before unloading. Measurements of the indentation response of cornea at three different loading rates were also made. Elastic like response was observed during loading, which was followed by extensive creep prior to unloading. STATEMENT OF SIGNIFICANCE: This manuscript attempts to provide a relatively simply model for the contact loading of fluid containing tissues and materials. It shows that the response of such materials provides a basis for determining the effective modulus and effective hydraulic conductivity (permeability) in much the same manner that hardness and modulus do for the indentation of elastic-plastic materials. Eye tissue with its anisotropic elastic and permeability properties is used to illustrate the approach.