ABSTRACT.Purpose: Scleral biomechanical weakness and thinning is known to be one of the main factors in the pathogenesis of progressive myopia. We tried to strengthen rabbit sclera by cross-linking scleral collagen using ultraviolet A (UVA) and the photosensitizer riboflavin. Methods: Circumscribed 10 · 10 mm sectors of the posterior -equatorial sclera of six chinchilla rabbit eyes were treated in vivo using a UVA double diode with 4.2 mW/cm 2 UVA at 370 nm and applying 0.1% riboflavin-5-phosphate drops as photosensitizer for 30 min. 1 day postoperatively biomechanical stressÀstrain measurements of three treated scleral strips were performed using a microcomputercontrolled biomaterial testing device and compared to non-treated contralateral control sclera. In addition, three treated eyes were examined histologically by light microscopy, TUNEL staining and electron microscopy to evaluate side-effects. Results: Following the cross-linking treatment, the ultimate stress was 11.87 -1.8 MPa versus 3.63 -0.40 in the controls (increase of 227.9%, p = 0.014), Young's modulus 27.67 -4.16 MPa versus 4.9 -2.15 MPa in the controls (increase of 464.7%, p = 0.021) and ultimate strain 92.2 -7.43% versus 165.63 -19.09% in the controls (decrease of 54.52%, p = 0.012). Histologically, serious side-effects were found in the entire posterior globe with almost complete loss of the photoreceptors, the outer nuclear layer and the retinal pigment epithelium (RPE). Conclusions: Our new method of scleral collagen cross-linking proved very effective in increasing the scleral mechanical strength; the new treatment may represent an option for strengthening scleral tissue in progressive myopia. However, serious sideeffects were observed in the outer retina. In future studies these side-effects could be avoided by reducing the irradiation dose below the cytotoxic level of the retina. Before its clinical application, the new method should be tested in a myopia animal model.

The plastic deformation mechanism operating in polymer glasses is analyzed. The whole process consists of two main stages: nucleation of special shear defects, called PSTs (plastic shear transformations), and their disappearance. The important feature of plastic deformation of glasses is the storage of a large amount of internal energy ΔUdef upon straining. Such energy storage is the critical issue for mechanical performance of polymeric material: if the amount of stored energy is high, the appearance of macroscopic failure is very probable while glassy materials collecting a small amount of stored deformation energy are quite ductile. It is proposed that the rate of disappearance of PSTs is a key factor in dissipation of stored deformation energy. A parameter describing the dissipation ability of material upon deformation is introduced.

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Experimental results on work W(ε), heat Q(ε) and stored energy U(ε) of deformation for glassy polymers such as linear PS, PC, PMMA, Polyimid, amorphous PET, thermotropic aromatic polyesters, Vectra™ for example, crosslinked epoxy are presented. All the data was obtained by a deformation calorimetry technique. Loading and unloading of samples were performed at room temperature with strain rate έ = 10-2 - 10-4 sec-1 under uniaxial compression up to engineering strains of εdef = 40-50%. During straining all polymers accumulate an excess of the latent energy U(ε). Elastic fraction of the energy is released completely at sample unloading and only residual Ures(ε) energy is conserved in samples. The latent energy Ures(ε) grows up to εdef =20-25% and levels off then. Shapes of the Ures(ε) curves are the same (S-shape) for all polymers. However, the saturation level is different for each polymer. The ratio U(ε)/W(ε) was also measured. It was found that at strains εdef < εy (εy - strain at the yield point) U(ε)/W(ε) ≈100%. I.e. all W is stored by sample in a form of U. The ratio decreases up to 60-30% for different polymers at higher strains. Release of the residual energy Ures (DSC measurements) and strain εres (thermally stimulated strain recovery technique) was measured for deformed and unloaded samples at heating. It was found that about 85-90% of Ures stored by samples is released in glassy state of polymers (below Tg). The Ures is related to a small fraction of εres, only to 7-10%. The rest of Ures and εres are recovered at the softening (devitrification) interval, around Tg. Computer modeling (molecular dynamics) of an isothermal shear deformation was performed for 2-dimentional two component atomic glass containing 500 Lennard-Jones particles of two different diameters. It was found that localized deformation events are of anelastic nature. The εan appears at early deformation stage in a form of localized shear events (transformations). Such events are nucleated in a sample and merged and united at later deformation stages, when concentration of the events becomes high enough. Finally, merged transformations form kind of shear band crossing entire sample. On the basis of experimental data and computer modeling the deformation mechanism for glassy polymers is proposed. The first stage of the process is the nucleation of “the carriers of non-elastic strain”, anelastic shear transformations (ASTs). All these ASTs are energetically excited. The concentration of the ASTs is responsible for the amount of Ures(ε) stored by a sample. It is suggested that such nucleation is the rate-controlling step in non-elastic deformation of any non-covalent glass. Saturation of the stored energy is defined by the reaching the steady state regime in carrier’s concentration. In this regime the rates of nucleation and termination (decrease of the stored local energy by AST) of carriers becomes equal. The termination proceeds spontaneously and easy (fast). The decrease of local energy of ASTs follows by local uncoiling of chains and by an appearance of new, extended chain conformers. However, such uncoiling is not the rate-controlling step for entire deformation process. Suggested mechanism very well describes all existing experimental facts. Deformation mechanisms for glasses seriously differ from that operating in rubbers and crystals.