Rheological characterisation and numerical flow simulation of the human eye vitreous humour fluid

Student thesis: Doctoral Thesis

Abstract

In this work, the dynamics of vitreous humour (VH) and of a range of pharmacological fluids used to replace VH in eye surgery were investigated, both experimentally and numerically. Vitreous humour is a gel-like viscoelastic material that fills the majority of the ocular globe. Age-related changes occur in the VH, and as a consequence VH becomes progressively liquefied with age, which has an impact on the corresponding rheological properties, and leads to the appearance of vitreoretinal pathologies.The most effective treatment for such diseases is the injection of a VH substitute in the vitreous cavity. Most of the vitreous substitutes commercially available are silicone oils (SiO) and perfluorocarbon liquids (PFLC), but so far none of them proven to be used as a permanent substitute.The results of the rheological characterisation of the VH and pharmacological fluids under shear and extensional flow conditions are presented in the first part of this thesis. Vitreous humour samples were obtained from New Zealand specimen rabbit eyes, whereas the pharmacological fluids were purchased from two different pharmaceutical companies.Two distinct VH phases were analysed: a liquid and a gel phase. The average surface tension measured for the VH liquid phase was 47.8 mN/m, at T = 21 ± 2 °C. Steady shear experiments demonstrated that VH liquid phase has a pronounced shear-thinning behaviour. Small amplitude oscillatory shear (SAOS) experiments showed that both gel and liquid VH phases present a solid-like behaviour, which is more dominant in the gel phase, and these results were corroborated by creep experiments. Experiments with the VH liquid phase under extensional flow yielded an average relaxation time of λ = 9.7 ms, at T ≈ 37 °C.None of the pharmacological fluids tested showed a rheological behaviour close to the VH. The PFLCs and SiO-based fluids exhibit a Newtonian behaviour, with the exception of Siluron 2000 fluid, which under extensional flow presents a relaxation time of λ = 6.8 ms, at T ≈ 20 °C. The PFLCs have viscosities close to the viscosity of water, whereas the SiO-based fluids present viscosities between 0.73 and 4.57 Pa s, at T = 37 °C.Surface tension experiments showed that the PFLCs present lower values than the SiO-based fluids, and that the surface tension of the VH liquid phase is more than double of that measured for each of the pharmacological fluids tested. In contact with VH liquid phase from rabbit eyes, all the pharmacological fluids tested yielded interfacial tensions around 30 mN/m.The dynamic response of VH during saccadic movements was studied numerically using the opensource software OpenFOAM®. A comparative study between the VH and pharmacological fluids was performed for a saccadic movement with an amplitude of 40°. The numerical results showed that the VH liquid and gel phases present distinct behaviour. The shear stresses at the walls for the VH gel phase are higher than those produced by the VH liquid phase.This shows that the phase balance in VH significantly affects the flow dynamics of the biofluid. The numerical results also showed that velocity and wall shear stress (WSS) profiles are significantly different for VH and pharmacological fluids, revealing that the latter are unable to mimic accurately the VH flow behaviour in conditions such as those experienced during saccadic eye movements.Finally, different degrees of saccadic movements were studied and the numerical results showed that, for all the fluids investigated, the momentum diffusion across the vitreous cavity and the WSS increase with the increase of the saccadic movement amplitude.The new experimental results provided in this thesis, complemented by the dynamic behaviour computed numerically for saccadic eye movements, offer new insights for the improvement and development of new VH substitutes to be used in eye surgery, with particula
Date of Award1 Apr 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorMonica Oliveira (Supervisor)

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