Ординатура / Офтальмология / Английские материалы / Biomaterials and regenerative medicine in ophthalmology_Chirila_2010

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proteins/biological factors and the corresponding receptors on the cell membrane. The type and degree of contact between the cell and underlying matrix is the primary factor that determines cell behavior such as growth, migration, and health.3

7.1.1Surface energy

Many years of biomaterial research have focused on determining the material surface characteristics – such as charge, energy, and roughness

– that influence specific protein deposition, in the hope of developing the ability to tailor the Vroman effect to the specific type of cellular adhesion desired.4 Studies involving both fibroblasts and endothelial cells in culture have shown that positively charged surfaces enhance cellular proliferation and adhesion significantly better than negatively charged or non-ionic hydrogel surfaces.5, 6 The interfacial surface energy of the material also plays an important role in the thermodynamic free energy of adhesion for solutes in the surrounding media in the absence of any specific biochemical interactions (i.e. ligand–receptor interactions).7–9 High surface energy or wettability has been shown to interfere with both human fibroblast10, 11 and endothelial cell12, 13 attachment, and low surface energy or wettability has been shown to interfere with plasma protein adsorption.9, 14,15 In general, non-wettable materials have been shown to inhibit the attachment and growth of anchorage-dependent cells.16

7.1.2Topography

Studies have also shown that microscopically roughened surfaces of a given material demonstrate enhanced vascular cell adhesion and migration rates, compared with smooth surfaces of the same material.17, 18 Indeed, an atomic force microscopy study has quantified the fine structure of the basement membrane underlying the corneal epithelium as a complex topographical structure composed of pores and networks of fibers with diameters around

70 nm.19 Steele and colleagues20, 21 have shown that corneal epithelial cell migration was enhanced across surfaces with 0.1 μm track-etched pores over non-porous surfaces and that the effects were additive with hydrophilicity; however, pores greater than 0.9 μm inhibited migration even on hydrophilic surfaces. Other investigations have focused on the nanoscale roughness patterning of the substrate surface: smooth muscle cells have been shown to align preferentially to nanopatterned gratings (350 nm line width, 700 nm pitch, and 350 nm depth) on both polymethyl methacrylate and polydimethyl siloxane surfaces.22

7.1.3Adsorbed protein conformation

Although these investigations have helped to define some of the characteristics necessary for an optimal surface, they have also revealed that it is not only the type of protein at the surface that is important for good cellular response, but also the conformation of the protein and the ability of the cell to interact with it. Generally, proteins are intrinsically surface active and tend to concentrate at interfaces, in part because of their polymeric structure and in part because of their amphoteric nature.23 The opportunity for multiple modes of binding with many different types of surfaces is provided by the polar, charged, and non-polar amino acid side chains of the proteins. It has been observed that the general tendency for non-polar residues to be internalized in the native protein often requires structural alterations of the protein upon adsorption, in order to maximize the number of contacts with the surface.24 For example, protein adsorption on a hydrophobic surface could involve entropically driven, conformational changes to optimize the various bonding interactions between the hydrophobic and hydrophilic sites of the protein and the surface and water phases of the interface, respectively. Real-time Fourier transform infrared spectroscopy (FTIR) analysis of proteins binding

to surfaces has demonstrated conformational changes in the proteins as they first adhere and then adsorb to the material surface.25–27

Studies have shown that ECM proteins such as fibronectin (FN) and vitronectin must be adsorbed from the plasma or serum on to biomaterial surfaces as a prerequisite for successful cell adhesion and spreading.28–30

However, adsorption of FN on to hydrophobic materials is followed by

a decrease in cell adhesion and adhesion strength, and diminished cell spreading,31–33 a fact that has been attributed to the possible conformation

changes in the protein induced upon adsorption.31, 33 Extensive research evaluating the cellular response to different proteins pre-adsorbed on to surfaces varying in hydrophilicity has determined that the reason for the hydrophobic effect may be the mode of adsorption of attachment proteins

(e.g. FN), resulting in an impaired interaction with the corresponding integrin receptor, as well as the lack of a possible rearrangement of FN into ECM- like structures.16, 34–43 The results of these studies indicate that synthetic

materials must adsorb FN or other proteins in a relatively native conformation to improve their interaction with cells.44–47 If this is the case, the material

surface may interact with adhering cells in a manner similar to that of the native ECM.34 However, the overwhelming evidence indicating that protein biological functions are often mediated by specific amino acid sequences

(exposed surface epitopes or peptides released by proteolysis) has focused the majority of biomaterial development on direct chemical attachment of proteins and/or peptides to the polymer surface.48, 49

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7.2Engineering cellular adhesion

Over the last two decades, material science research has focused on specifically engineering the polymer surface to have the ability to interact actively with receptors on target cell membranes through the use of ligand-specific chemical sequences and tethered biological molecules. Short peptide sequences (such as YIGSR and RGD) responsible for cell-surface adhesion binding activity in extracellular adhesion proteins such as FN and laminin (LM) have proven to be sufficient for in vitro cell adhesion and spreading when chemically incorporated on to the surfaces in adequate numbers.48–55 Although these minimal binding sequences have only a fraction of the activity of the entire protein, their small size allows them to be incorporated at much higher concentrations than would be possible with entire protein structures. The short peptide sequences have the advantage of being relatively stable, and their synthetic nature renders them more amenable to chemical derivatization and covalent attachment.

7.2.1Direct attachment

Direct attachment of these cell-surface receptor recognition sequences has been associated with an increase in cell culture adhesion of a variety of cell types, including human foreskin fibroblasts, bovine pulmonary endothelial cells, human umbilical vein endothelial cells, and porcine pulmonary aortic endothelial cells.48–55 However, the increase in cell adhesion in these systems, while significant, was not as high as was anticipated and did not reach the levels seen in natural systems. Generally, the increase seemed to be of a non-specific nature and was not found for all cell types tested.56 Use of these sequences on surfaces cultured with rabbit corneal epithelial cells has not demonstrated the same marked increase in cell adhesion as has been reported for other cellular systems.38, 57

The reason for the limited results may be threefold: (a) the attached short peptide sequence is held very close to the surface and may be sterically hindered from orienting itself into optimal attachment positions; (b) the short peptide sequence is not of sufficient length to extend out and away from proteins non-specifically adsorbing to the surface and, therefore, is obscured to some extent from the cell receptor (Fig. 7.1); and (c) the single short peptide sequence does not adequately mimic the complex adhesionpromoting abilities found in a protein with quaternary structure. Several studies support these hypotheses.

7.2.2Spacers

versus

7.1 Directly attached RGD peptide sequences (triangles) can be obscured by non-specifically adsorbed proteins, whereas the tethered biological molecules (squares) are held away from the adsorbed proteins.

3400) results in a 50% increase in specific cell attachment, compared with the response to a surface with directly attached peptides. PEG is widely

recognized for its lack of interactions with macromolecules found in body fluids.59–61 Use of PEG as a spacer arm or tether has been shown to

increase the activity of bound enzymes. D’Urso et al.62 found that the Km (concentration of substrate that gives ‘half-maximal’ activity) of enzymes bound to a surface by a PEG tether is increased 10–20 times over that of the native enzyme. Tethering of the active molecule decreases unproductive diffusion and increases the probability of successful interactions with target molecules or cells. Further investigations59, 63 have reported the tethering of biologically active protein molecules to polymer scaffolds for tissue regeneration. By covalently linking epidermal growth factor (EGF) on to a star-poly(ethylene oxide) tether and then anchoring the tether on to the surface of a biodegradable scaffold, Griffith-Cima64 showed a 40% increase in rat hepatocyte cell adhesion and migration; this investigator also reported that DNA synthesis within the cells was comparable with the levels found when the medium contained free EGF.

Although the use of the PEG spacer arm diminishes the steric constraints and the effects of non-specifically adsorbed proteins, it does not address the problem of the need for adhesion of a specifically targeted cell type and exclusion all other cell types. For example, five peptide sequences on the

LM α1 chain carboxyl-terminal globular domain have been found to exhibit cell-type-specific attachment activities, including SIYITRF, IAFQRN, and LQVQLSIR.65 PHSRN (from the ninth type II repeating unit), a synergistic peptide that enhances the activity of RGD, has been found in the central cell-adhesive domain of FN.66 Additionally, Cameron et al.67 have shown

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that rabbit corneal epithelial cells interact with FN via multiple adhesionpromoting sequences within the intact FN molecule. Another study using osteoblasts showed that, by linking PHSRN and RGD sequences to recapitulate the native spacing of FN prior to surface tethering cell adhesion, spreading and focal contact formation was increased compared with RGD alone.68 It is apparent that for most cells the use of just one peptide sequence is insufficient to mimic these complex matrix binding proteins.

7.3Engineering corneal epithelium attachment and growth

7.3.1Synthetic replacement/augmentation of the cornea

Synthetic augmentation of the cornea to produce refractive change has been investigated by a number of researchers during the past 30 years,69–72 while complete synthetic replacement of diseased and opacified corneas has been

investigated for more than 200 years.73, 74 Initial replacement studies included the use of glass, metal, and bone as substitute materials;75–77 studies over the past 50 years have focused on the use of polymers.78–82 As the field of

polymer chemistry and its medical applications grew in the mid 1980s, there was a shift from the use of hard polymers such as poly(methyl methacrylate) (PMMA) to soft hydrogels such as collagen, polyurethanes, and poly(2- hydroxyethyl methacrylate) and co-polymers thereof.83–90 Hydrogels are used not only because their compliance (softness) allows them to respond to the pressure fluctuations caused during blink without inducing high shear forces at the material–tissue interface and damaging surrounding tissue, but also because of their ability to maintain a hydrated environment similar to that of the natural tissues.

In the past decade there has been some clinical success in both corneal augmentation and replacement. For replacement devices, the use of a soft, porous polymer attachment skirt has improved anchoring of the central optic and decreased extrusion of the devices.88, 91–96 The AlphaCor keratoprosthesis (KPro) (Addition Technology Inc., Des Plaines, IL, USA) has been successful in a number of uncomplicated corneal replacement cases; however, the list of exclusion criteria for its implantation is extensive and long-term viability is uncertain.93 For augmentation devices, the use of hydrogels that allow nutrients to permeate the material to supply the tissue anterior to the implant has significantly increased the functional life of these devices.97–99 Both synthetic replacement and augmentation devices share the requirement that corneal epithelial cells must be able to form a viable confluent layer across the anterior surface of the optical material.96, 99 However, corneal epithelial cell attachment to, and growth over, these hydrogel surfaces is generally minimal.94