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G. Clare, A. Hopkinson, R. A. H. Scott, H. S. Dua; Biomechanical Properties of Freeze-Dried Amniotic Membrane. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2374.
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© ARVO (1962-2015); The Authors (2016-present)
Amniotic membrane (AM) may be a useful biomaterial on the battlefield in the management of ocular surface burns, and as an adjunct in other ocular injuries, as a barrier and as a scaffold for cell growth. Dried preparations of AM have a logistic advantage over cryopreserved (CP) tissue in eliminating the cold chain. AM is a heterogeneous viscoelastic tissue with regional structural variations. Freeze-dried (FD) AM has altered properties with respect to fresh (Fr) and CP AM, which may affect ease of surgical handling and durability. The purpose of this study is to characterise the biomechanical changes caused by cryopreserving and lyophilising AM.
AM from 5 healthy elective caesarean sections was isolated, washed and cut into sections. Sections were divided into 3 groups: FrAM, n = 35, CPAM, n = 32 and FDAM, n = 36. CPAM was thawed, and FDAM rehydrated. Samples were cut into strips of 30mm x 10mm. Digital micrometry and uniaxial tensile property testing were performed to determine the modulus of elasticity (E) and the following 4 measurements at the point of failure: load, engineering stress (σf, force applied per unit of initial AM cross-sectional contact area), strain (εf, the ratio of change in tissue length) and work done (the area under the load-elongation curve).
Load at rupture, work done to failure and εf, which are independent of thickness, were not significantly different between all three AM preparations. FDAM was more heterogeneous than FrAM or CPAM, showing more variability in biomechanical testing. FDAM was significantly thinner than CPAM, which was significantly thinner than FrAM (p<0.001). σf and E, both of which are dependent on thickness measurements, were significantly lower for FrAM than for CPAM (both p<0.05) and FDAM (p<0.001 and p<0.01 respectively).
Thickness-independent measurements are a more meaningful way of documenting gross changes in the tensile properties of biological tissues. Digital micrometry has limitations of practicality and use in the measurement of AM thickness. The variability of AM necessitates large numbers of tests to generate significant data. FDAM may be viable as an ocular surface dressing for military use.
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