A structural model for the in vivo human cornea including collagen-swelling interaction

A structural model of the in vivo cornea, which accounts for tissue swelling behaviour, for the three-dimensional organization of stromal fibres and for collagen-swelling interaction, is proposed. Modelled as a binary electrolyte gel in thermodynamic equilibrium, the stromal electrostatic free energy is based on the mean-field approximation. To account for active endothelial ionic transport in the in vivo cornea, which modulates osmotic pressure and hydration, stromal mobile ions are shown to satisfy a modified Boltzmann distribution. The elasticity of the stromal collagen network is modelled based on three-dimensional collagen orientation probability distributions for every point in the stroma obtained by synthesizing X-ray diffraction data for azimuthal angle distributions and second harmonic-generated image processing for inclination angle distributions. The model is implemented in a finite-element framework and employed to predict free and confined swelling of stroma in an ionic bath. For the in vivo cornea, the model is used to predict corneal swelling due to increasing intraocular pressure (IOP) and is adapted to model swelling in Fuchs'' corneal dystrophy. The biomechanical response of the in vivo cornea to a typical LASIK surgery for myopia is analysed, including tissue fluid pressure and swelling responses. The model provides a new interpretation of the corneal active hydration control (pump-leak) mechanism based on osmotic pressure modulation. The results also illustrate the structural necessity of fibre inclination in stabilizing the corneal refractive surface with respect to changes in tissue hydration and IOP.     

Synthesis, characterization, and in vitro evaluation of a hydrogel-based corneal onlay

Blindness due to opacity of the cornea is treated by corneal transplantation with donor tissue. Due to the limited supply of suitable donor corneas, the need for synthetic corneal equivalents is clear. Herein we report the design and in vitro characterization of a hydrogel-based implant; this implant will serve as a permanent, transparent, space-filling onlay with a two-layer design that mimics the native corneal stratification to support surface epithelialization and foster integration with the surrounding tissue. The top layer of the implant was composed of a 2-hydroxyethylmethacrylate hydrogel containing methacrylic acid as the co-monomer (HEMA-co-MAA) with tunable dimensions and compressive modulus ranging from 700-1000 kPa. The bottom layer, which constitutes the bulk of the implant and is designed to provide integration with the corneal stroma, is a dendrimer hydrogel with high water content and compressive modulus ranging from 500-1200 kPa. Both hydrogels were found to possess optical and diffusion properties similar to those of the human cornea. In addition, composite implants with uniform and structurally sound interfaces were formed when the gels were sequentially injected and cross-linked in the same mold. HEMA-co-MAA hydrogels were covalently modified with type I collagen to enable corneal epithelial cell adhesion and spreading that was dependent upon the collagen coating density but independent of hydrogel stiffness. Similarly, dendrimer hydrogels supported the adhesion and spreading of corneal fibroblasts upon modification with the adhesion ligand arginine-glycine-aspartic acid (RGD). Fibroblast adhesion was not dependent upon dendrimer hydrogel stiffness for the formulations studied and, after in vitro culture for 4 weeks, fibroblasts remained able to adhere to and conformally coat the hydrogel surface. In conclusion, the tunable physical properties and structural integrity of the laminated interface suggests that this design is suitable for further study. The judicious tuning of material properties and inclusion of bioactive moieties is a promising strategy for promotion of implant epithelialization and tissue integration.     

Teleost fish scales: a unique biological model for the fabrication of materials for corneal stroma regeneration

The corneal stroma is composed of multiple lamellae, each containing closely packed collagen fibrils. The orientation of fibrils in a lamella is parallel, but those in different lamellae are orthogonal. As a result, the corneal stroma has a characteristic orthogonal plywood-like structure. Such a highly-regulated three-dimensional arrangement of collagen fibrils gives strength and transparency to the corneal stroma, but it also presents a challenge in the fabrication of materials to replace it. A bioinspired technology is required to process such materials, but the regulatory mechanism of collagen-fibril orientation is still unknown. The low regenerating activity of the corneal stroma seems to be a major factor preventing progress in this field of research. A similarly highly-ordered arrangement of collagen fibrils can be seen in the basal plates of teleost fish scales. Moreover, the scales have high regenerating ability. When a scale is mechanically lost, a new scale is rapidly regenerated. The cells that produce the basal plates are extremely activated; thus, production of the highly-ordered collagen fibrils is very rapid. Therefore, the regenerating scales should be a uniquely helpful biological model for studying the regulatory mechanism of collagen-fibril orientation. Fish-scale collagen has another advantage for use as a biomaterial: the low probability of zoonotic infection. Therefore, scale collagen is a most promising biomaterial for fabricating three-dimensionally arranged collagen fibers to substitute for the corneal stroma. Three tasks that must be clarified for the bioinspired production of a corneal substitute from fish scale collagen are proposed.     

An attempt to construct the stroma of cornea using primary cultured corneal cells

The number of patients currently awaiting corneal transplantation has resulted in the need to develop an artificial corneal replacement. In this study, we aimed to construct the corneal stroma using non-transformed corneal cells and a perfusion cell culture method. Corneal cells isolated from chicken embryos or rabbit and were embedded in the alkaline solubilized collagen gels crosslinked by TSG (Pentaerythritol polyethyleneglycol ether tetrasuccinimidyl glutarate). During culture, the majority of cells migrated from inside of the gel. The chicken and rabbit cells changed their morphology and stratified structures were constructed within the gels. These microstructures were similar to the natural corneal tissue. TEM analysis was performed to confirm the nano-microstructure of the constructs. Contrary to expectation, the cornea-like nanostructure of collagen fibrils was not observed within the gels. Further study including for example, such as the addition of dynamic stress or co-culture with endothelial cells, are therefore required in order to produce artificial constructs with the same superstructure as natural corneal tissue.     

Ultrasound characterization of normal ocular tissue in thefrequency range from 50 MHz to 100 MHz

The ultrasonic properties of ocular tissues including sclera, cornea, ciliary body and iris have been quantitatively evaluated over the frequency range from 50 MHz to 100 MHz at 37°C. Measurements were made with a wideband 60 MHz PVDF copolymer transducer in conjunction with a C-scan microscopy system developed in the authors' laboratory. Using this system, high resolution overview images were produced to identify homogeneous tissue regions for detailed quantitative analysis. The speed of sound for the four eye tissues ranged from 1542 m/s for iris to 1622 m/s for sclera. At 50 MHz the attenuation coefficient ranged from 1.3 dB/mm for cornea to 4.3 dB/mm for sclera. Scleral tissue also had the highest backscatter coefficient (0.0157 Sr^-1 mm^-1), while iris had the lowest (0.00184 Sr^-1 mm^-1). The measured ultrasonic properties are qualitatively related to their histological structure and imaging characteristics     

Linear dichroism of the cornea

The dichroic properties of in vitro sheep corneas were studied with a spectrophotometer in transmission mode for several angles of incidence of light beams. Several models of corneal anisotropy have been presented in the literature. The results presented here allow us to believe that the cornea behaves as a dichroic biaxial crystal. Furthermore, this dichroism is weak when the angle of incidence on the corneal surface stays small. The mathematical model that describes these optical properties of the cornea uses Mueller matrices.     

Anterior corneal and internal contributions to peripheral aberrations of human eyes

Anterior corneal and internal component contributions to overall peripheral aberrations of five human eyes were determined, based on corneal topography and overall aberration measurements. Anterior corneal position and orientation (tilt) were referenced to the line of sight. Ray tracing was performed through the anterior cornea for 6-mm-diameter pupils at angles out to 40 degrees in both the temporal and the nasal visual fields. In general, both component and overall Zernike aberrations were greater for the nasal than for the temporal visual field. In general, the anterior corneal aberration components were considerably higher than the overall aberrations across the visual field and were balanced to a considerable degree by the internal ocular aberration components. The component and overall levels of Zernike third-order aberrations showed linear trends away from the fixation axis, and the component levels of Zernike fourth-order aberrations showed quadratic trends away from the fixation axis. The second-order, but not higher-order, aberration components were susceptible to the choice of image radius of curvature, while disregarding corneal position and orientation affected second- and higher-order aberration components.     

硫氰酸钠浓水溶液中胶原蛋白的接枝改性及共混纺丝

在硫氰酸钠的浓水溶液中对胶原蛋白进行丙烯腈接枝聚合改性,并通过红外光谱、X射线衍射对接枝产物进行了表征。研究了几种不同比例胶原蛋白与聚丙烯腈共混纺丝液的纺丝性能,通过Instron5565型强力仪、扫描电镜等手段对纤维的力学性能与形貌进行了测试和分析。测试结果表明,胶原蛋白主要以无定形态存在于纤维中,随加入的胶原蛋白含量增加,纤维断裂强度逐渐下降。胶原蛋白含量15%以下时,纤维具有良好的力学性能,纤维具有圆形截面和表面的纵向沟槽。     

Laser ablation threshold dependence on pulse duration for fused silica and corneal tissues: experiments and modeling

The surface ablation threshold fluence of fused silica and two porcine cornea layers, the epithelium and the stroma, is characterized as a function of the laser pulse duration in the range of 100 fs-5 ps for a wavelength of 800 nm (Ti:sapphire laser system). The plateaulike region observed between 100 fs and 1 ps for the corneal layers indicates that for use in laser surgery, laser pulse durations chosen within this range should be practically equivalent. Our model predicts that the ablation threshold will decrease rapidly for pulse durations in the low end of the femtosecond regime.     

Nonlinear analysis of static axisymmetric deformation of the human cornea

This paper presents a simple mechanics model for analyzing the stresses, strains and displacements of the human cornea under the action of the intraocular pressure. The analysis is performed under the assumptions that when the cornea undergoes small displacements the stress–strain constitutive relation includes an isotropic, linear material of ground substance and an orthotropic, nonlinear material of collagen fibrils. The present results highlight the influence of corneal geometry and its local microstructural organisation on the corneal overall mechanical properties.     

Hyaluronan hydration generates three-dimensional meso-scale structure in engineered collagen tissues

Here, we show that the local incorporation of osmotically active hyaluronan into previously compressed collagen constructs results in further rapid dehydration/compression of collagen layers, channel formation and generation of new interfaces; these novel structures, at the nano–micro (i.e. meso-scale) were formed within native collagen gels, in a highly predictable spatial manner and offer important new methods of fabricating scaffolds (e.g. tubes and open-spirals) with potential for use in tissue regeneration such as in peripheral nerves and small vessels. This paper tests the possibility that the local fluid content of a dense collagen network can be controlled by incorporation of an osmotically active (native) macromolecule—hyluronan. This is an exemplar physiological, osmotic swelling agent. Hyaluronan is commonly secreted by cells deep in connective tissues, so is a good candidate for this role in a cell-driven system balancing mechanical compaction of bulk tissue collagen. These constructs may have potential as functional in vitro models representing developmental and pathological processes.     

An automated approach for three-dimensional quantification of fibrillar structures in optically cleared soft biological tissues

We present a novel approach allowing for a simple, fast and automated morphological analysis of three-dimensional image stacks (z-stacks) featuring fibrillar structures from optically cleared soft biological tissues. Five non-atherosclerotic tissue samples from human abdominal aortas were used to outline the multi-purpose methodology, applicable to various tissue types. It yields a three-dimensional orientational distribution of relative amplitudes, representing the original collagen fibre morphology, identifies regions of isotropy where no preferred fibre orientations are observed and determines structural parameters throughout anisotropic regions for the analysis and numerical modelling of biomechanical quantities such as stress and strain. Our method combines optical tissue clearing with second-harmonic generation imaging, Fourier-based image analysis and maximum-likelihood estimation for distribution fitting. With a new sample preparation method for arteries, we present, for the first time to our knowledge, a continuous three-dimensional distribution of collagen fibres throughout the entire thickness of the aortic wall, revealing novel structural and organizational insights into the three arterial layers.     

J-integral and CMOD for external inclined cracks on autofrettaged cylinders

Autofrettage is a well-known method to increase the load carrying capacity of the pressure vessels. The autofrettage outcomes can be contrary when the imperfections or material discontinuities are on the outer surface. In this study, the influences of the external inclined cracks on the fracture parameters in autofrettaged cylinders were studied. The inclination angles of cracks are different but they have the same depth. Crack mouth opening displacements and J-Integral values along the cracks front were investigated. The effects of autofrettage ratio, inclination angle, crack depth and length, the ratio of inner to outer radius and applied pressure were studied. The J-Integral values were calculated by using a modified equation taken from the recent literature, which includes the residual stress effects.     

The effect of circular bridge piers with different inclination angles toward downstream on scour

A fundamental principle in the safe design of bridge piers is to estimate maximum scour depth. As such, many researchers devoted their efforts to study the phenomenon of scour around bridge piers and to present relationships to estimate maximum scour depth. In the current research, scour around circular bridge piers with two different diameters and different inclination angles toward downstream has been investigated experimentally under clear water and different discharges. The obtained results revealed that increase in the inclination angle leads to a significant decrease in the scour depth, dimensions of sediment ridge and scour hole. Moreover, by comparing the data obtained from vertical bridge pier and well-known relationships, a relationship having the most agreement with the results was selected and modified to estimate equilibrium scour depth around inclined piers.     

Corneal epithelialisation on surface-modified hydrogel implants: artificial cornea

The objective was to investigate corneal re-epithelialisation of surface-modified polymethacrylate hydrogel implants in order to evaluate them as potential materials for an artificial cornea. Polymethacrylate hydrogels were modified with amines and then coated with different extracellular matrix proteins (collagen I, IV, laminin and fibronectin). The modified hydrogels were surgically implanted into bovine corneas maintained in a 3-D culture system for 5 days. The epithelial growth across the implant surface was evaluated using fluorescent, light and electron microscopy. Full epithelialisation was achieved on 1,4-diaminobutane-modified hydrogels after coating with collagen IV. Hydrogels modified with 1,4-diaminobutane but without further coating only showed partial re-epithelialisation. Hydrogels modified with other amines (1,2-diaminoethane or 1,3-diaminopropane) showed only partial re-epithelialisation; further coating with extracellular matrix proteins improved epithelialisation of these surfaces but did not result in complete re-epithelialisation. Evaluation of the corneas implanted with the 1,4-diaminobutane-modified hydrogels coated with collagen IV showed that the artificial corneas remain clear, integrate well and become covered by a healthy stratified epithelium. In conclusion the 1,4-diaminobutane surface-modified hydrogel coated with collagen IV supported the growth of a stable stratified epithelium. With further refinement this hydrogel has the potential to be used clinically for an artificial cornea.     

非接触式眼压计检定用模拟人眼装置关键技术研究

为解决非接触式眼压计的量值溯源问题,深入分析了眼压检测中所涉及的Imber-Fick以及Glodmann眼压计的原理,设计了一套非接触式眼压计检定用模拟人眼装置,采用角膜与巩膜复合结构,控制模拟人眼在溯源和检测时相同的角膜变形量与内压预设值,并利用L型转矩平衡法,将模拟人眼的外部标准眼压值溯源至质量与长度。实验结果表明,装置具有良好的测量重复性和稳定性,分辨率高达1 Pa,且压力连续可调,整体扩展不确定度达到0.08 kPa（k=2）。     

Design and characterization of a real-time angular scatter ultrasound imaging system

A novel ultrasound imaging system has been implemented using two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays, in real time, a conventional backscatter image and an angular scatter image formed using side-scattered echoes from the same region. The design of the system is shaped by the influence of the scatter angle on the spatial resolution and receive signal processing requirements of the instrument. The subtended scatter angles are restricted to values >90 degrees to ensure that the angular scatter receiver effectively tracks the transmitted pulse and that the spatial resolution in the two images is comparable. The system is sufficiently tolerant of small variations in the average acoustic velocity of the medium to guarantee reliable pulse tracking in biomedical applications. The angular scatter signal magnitude is significantly weighted by the directivity of the receive array. The imaging system will most effectively demonstrate angular variations in scattering at scatter angles between 125 and 145 degrees , where the angular response of the receiver is near its peak.     

体外模拟胶原纤维矿化的研究进展

矿化胶原纤维是天然骨的主要成分,掌握胶原纤维的生物矿化过程、原理和调控机制对于骨修复材料的设计及骨组织的再生修复发展有重要意义。综述了体外模拟胶原纤维矿化的研究进展,主要包括胶原纤维内矿化机理和非胶原蛋白对胶原生物矿化的调控作用,以期为胶原生物矿化机理的探讨及骨组织再生修复的研究提供借鉴。     

Application of structural analysis to the mechanical behaviour of the cornea.

Structural engineering analysis tools have been used to improve the understanding of the biomechanical behaviour of the cornea. The research is a multi-disciplinary collaboration between structural engineers, mathematical and numerical analysts, ophthalmologists and clinicians. Mathematical shell analysis and nonlinear finite-element modelling have been used in conjunction with laboratory experiments to study the behaviour of the cornea under different loading states and to provide improved predictions of the mechanical response to disease and injury. The initial study involved laboratory tests and mathematical back analysis to determine the corneal material properties and topography. These data were then used to facilitate the construction of accurate finite-element models that are able to reliably trace the performance of cornea upon exposure to disease, injury or elevated intra-ocular pressure. The models are being adapted to study the response to keratoconus (a disease causing loss of corneal tissue) and to tonometry procedures, which are used to measure the intra-ocular pressure. This paper introduces these efforts as examples of the application of structural engineering analysis tools and shows their potential in the field of corneal biomechanics.     

Measurement of small-signal absorption coefficient and absorption cross section of collagen for 193-nm excimer laser light and the role of collagen in tissue ablation

A 193-nm ArF excimer laser transmission was measured at subablative fluence through varying strength solutions of dissolved collagen, yielding an absorption cross section of 1.14 x 10(-17) cm2 for the peptide bond, which accounts for 96% of the total collagen attenuation that is based on additional transmission measurements through solutions of isolated constituent amino acids. The measured absorption cross sections, in combination with typical corneal tissue composition, yield a predicted corneal tissue absorption coefficient of 16,000 cm(-1). In addition, dry collagen films were prepared and ablation-rate data were recorded as a function of laser fluence. Ablation rates were modeled by use of a Beer-Lambert blow-off model, incorporating a measured ablation threshold and an absorption coefficient that are based on the measured collagen absorption cross section and the film bond density. The measured ablation rates and those predicted by the model were in very good agreement. The experiments suggest that collagen-based absorption coefficients are consistent with predicted corneal tissue ablation rates and previously observed dynamic changes in tissue properties under ablative conditions.