Applications of Raman spectroscopy to the ophthalmological field : Raman spectra of soft contact lenses made of poly-2-hydroxyethylmethacrylate (PHEMA)

The Role of Thermal Accumulation on the Fabrication of Diffraction Gratings in Ophthalmic PHEMA by Ultrashort Laser Direct Writing

The fabrication of diffraction gratings by ultrashort direct laser writing in poly-hydroxyethyl-methacrylate (PHEMA) polymers used as soft contact lenses is reported. Diffraction gratings were inscribed by focusing laser radiation 100 µm underneath the surface of the samples. Low- and high-repetition rate Ti:sapphire lasers with 120 fs pulsewidth working at 1 kHz and 80 MHz respectively were used to assess the role of thermal accumulation on microstructural and optical characteristics. Periodic patterns were produced for different values of repetition rate, pulse energy, laser wavelength, distance between tracks, and scanning speed. Compositional and structural modifications of the processed areas were studied by micro-Raman spectroscopy showing that under certain parameters, thermal accumulation may result in local densification. Far-field diffraction patterns were recorded for the produced gratings to assess the refractive index change induced in the processed areas.

Experimental study of the mechanisms leading to the formation of glistenings in intraocular lenses by Raman spectroscopy

The phenomenon of glistenings, often appearing in intraocular lenses (IOLs) of patients after some time from the surgical operation, is potentially able to induce a poor quality of vision and, therefore, frustrate IOL implantation itself. In this paper, we combine optical microscopy with micro-Raman spectroscopy to get a deeper insight on the mechanism ruling, at microscopic scale, glistening formation. In particular, we have analyzed two types of IOLs, characterized by a different internal hydrophobicity but a similar polymer hydration coefficient. Raman imaging of single microvacuoles reveals that water creeps into the polymeric network, which traps water. Finally, applying the Principal Component Analysis (PCA) to Raman data, we provide information on the probable mechanism leading to water trapping in the two kinds of analyzed IOLs.

Silk Fibroin/Polyvinyl Pyrrolidone Interpenetrating Polymer Network Hydrogels

Silk fibroin hydrogel is an ideal model as biomaterial matrix due to its excellent biocompatibility and used in the field of medical polymer materials. Nevertheless, native fibroin hydrogels show poor transparency and resilience. To settle these drawbacks, an interpenetrating network (IPN) of hydrogels are synthesized with changing ratios of silk fibroin/N-Vinyl-2-pyrrolidonemixtures that crosslink by H₂O₂ and horseradish peroxidase. Interpenetrating polymer network structure can shorten the gel time and the pure fibroin solution gel time for more than a week. This is mainly due to conformation from the random coil to the β-sheet structure changes of fibroin. Moreover, the light transmittance of IPN hydrogel can be as high as more than 97% and maintain a level of 90% within a week. The hydrogel, which mainly consists of random coil, the apertures inside can be up to 200 μm. Elastic modulus increases during the process of gelation. The gel has nearly 95% resilience under the compression of 70% eventually, which is much higher than native fibroin gel. The results suggest that the present IPN hydrogels have excellent mechanical properties and excellent transparency.

Optical and vibrational properties of phosphorylcholine-based contact lenses-Experimental and theoretical investigations

The Raman, MIR and UV-vis spectroscopy have been used to characterize Omafilcon A material constructing the one of the Proclear family contact lenses. The Omafilcon A is hydrogel material composed of 2-hydroxyethyl methacrylate (HEMA) and 2-methacryloyloxyethyl phosphorylcholine (PC) polymers crosslinked with ethyleneglycol dimethacrylate (EGDMA). Vibrational and electronic properties of the Omafilcon A material were also investigated by quantum chemical calculations. Experimentally obtained Raman, MIR and optical spectra were compared to the theoretical ones calculated applying RHF and DFT methodology. The quantum chemical calculations were performed for isolated monomers of lenses compounds as well as for their dimers and trimers to elucidate the effect of Omafilcon A polymerization and the role of an individual components.

A poly(2-hydroxyethyl methacrylate)-based resin improves the dentin remineralizing ability of calcium silicates

Bioactive polymeric composites have received great attention for their capability to remineralize the dentin tissue. This study was aimed at evaluating if a poly(HEMA-co-TEGDMA) resin (HEMA: 2-hydroxyethyl methacrylate; TEGDMA: triethyleneglycol dimethacrylate) may increase the in vitro apatite forming ability of a calcium silicate cement (CaSi), in view of developing a hydrophilic light-curable composite bio-remineralizing restorative material (R-CaSi). To this purpose, the following experiments were carried out: (1) In vitro apatite forming ability of R-CaSi and CaSi was comparatively assessed by micro-Raman spectroscopy after immersion of the cement disks in Dulbecco's Phosphate Buffered Saline (DPBS) at 37°C for 1-28days; (2) Previously demineralized human dentin slices were soaked for 7days in close contact with the CaSi and R-CaSi cements as well as poly(HEMA), poly(TEGDMA) and poly(HEMA-co-TEGDMA), and then were comparatively analyzed by IR spectroscopy. Micro-Raman spectroscopy showed that in calcium phosphate nucleation tests, the B-type carbonated apatite deposit formed on R-CaSi was thicker than that on CaSi; therefore, the poly(HEMA-co-TEGDMA) resin proved able to increase the in vitro apatite forming ability of the calcium silicate-based cement. Both cements were found to induce dentin remineralization, R-CaSi to a higher extent, in agreement with the calcium phosphate nucleation tests. This result may be ascribed to the positive role played by the polymeric component, which was found to interact with collagen and to chelate calcium ions. Upon remineralization, collagen underwent conformational rearrangements and the formed apatite phase, rather than a simple deposit, was intimately bound to the collagen matrix, thanks to the calcium ions chelated by it.

Chemical basis for alteration of an intraocular lens using a femtosecond laser

The chemical basis for the alteration of the refractive properties of an intraocular lens with a femtosecond laser was investigated. Three different microscope setups have been used for the study: Laser Induced Fluorescence (LIF) microscopy, Raman microscopy and coherent anti-Stokes Raman Scattering (CARS) microscopy. Photo-induced hydrolysis of polymeric material in aqueous media produces two hydrophilic functional groups: acid group and alcohol group. The spectral signatures identify two of the hydrophilic polar molecules as N-phenyl-4-(phenylazo)-benzenamine (C₁₈H₁₅N₃) and phenazine-1-carboxylic acid (C₁₃H₈N₂O₂). The change in hydrophilicity results in a negative refractive index change in the laser-treated areas.

Applications of Raman spectroscopy to ophthalmology: spectroscopic characterization of disposable soft contact lenses

Disposable soft contact lenses based on HEMA-MAA hydrogels are examined using Raman and ATR/FTIR vibrational spectroscopies and thermal analysis. The main factors dealing with physical, chemical, and biological biocompatibility are evaluated in relation to those of long-wear soft contact lenses with the aim of individuating the most biocompatible lens. The Raman spectra of HEMA-MAA lenses show that some biocompatibility factors are affected by environmental conditions and, in particular, by changes in pH and ionic strength values.

An FT-Raman spectroscopic investigation of dentin and collagen surfaces modified by 2-hydroxyethylmethacrylate

Although 2-hydroxyethylmethacrylate (HEMA) is commonly used for adhesive bonding to dentin, its role in promoting adhesion is not completely understood. Here, we use FT-Raman spectroscopy to elucidate further the nature of the interaction of HEMA with dentin. Ground dentin was exposed to 2.5% (w/w) nitric acid, washed, dried in air, and treated with HEMA. The samples were then sequentially washed with distilled water, with FT-Raman spectra being obtained after different wash times. Hydroxyapatite and bovine type I collagen were similarly treated with HEMA except for the acid exposure. The FT-Raman spectra of these samples were also recorded. The spectra of HEMA-treated water-washed dentin and collagen revealed the following changes: (1) The band intensities of HEMA absorbed on dentin and collagen decreased with increasing wash times (2) the nu(C=O) and nu(CCO) modes of HEMA at 1718 and 607 cm-1, respectively, either disappeared or decreased after extensive washing; (3) the nu (C=C) (1640 cm-1) and delta (=CH2), (1403 cm-1) bands exhibited minor variations in band position and relative intensity. These results demonstrate that HEMA interacts with dentin both physically and chemically. The chemical interaction can be interpreted by either hydrogen bonding or the formation of a new bond to the ester group of HEMA.

Raman spectroscopy in the study of biocompatibility

This review deals with the application of Raman spectroscopy to the study of the biocompatibility of orthopaedic and ophthalmological materials and includes an introduction, a brief theory on the fundamental concepts of the technique, a description of the method and of the traditional and non-traditional instrumentation and a brief treatment of the surface properties of the different classes of materials. In the orthopaedic field, Raman studies on the structures of glasses, ceramic materials, carbon fibres and polymers before and after implantation are reviewed, particularly as far as structural modifications at the biomaterial-tissues interface are concerned. In the ophthalmological field, the chemical biocompatibility of hydrophobic and hydrophilic polymers for intraocular and intrastromal implants and for soft contact lenses is considered with respect to the presence of monomeric reactive centres in hydrophobic materials and the water amount in hydrophilic ones. The progress of the multichannel Raman technique for 'in-vivo' measurements is also described.