The aging process of hydrogel contact lenses studied by 1H NMR and DSC methods

Synthesis, characterization and biocompatibility of hybrid hydrogels based on alginate, κ-carrageenan, and chitosan filled with montmorillonite clay

New hybrid hydrogel composites based on a mixture of natural polysaccharides (sodium alginate, κ-carrageenan, and chitosan) filled with the clay mineral of natural origin, montmorillonite (MMT), were studied. The structure of intercalated/flocculated MMT distribution in the interpenetrating network of polysaccharide matrix was characterized using FTIR, X-ray diffraction, and SEM techniques. Swelling kinetics was investigated using the weight analysis, whereas the phase transition of water in the composition of hybrid hydrogels, by DSC method. Their biosafety was estimated using the Nelyubov method, germination test on cress (L. sativum) seeds, and metabolic fingerprinting of microbial communities and dehydrogenase assay. The obtained results indicated promising water-retaining properties of the synthesized materials. The hydrogels had a good sorption affinity for cadmium (Cd) ions confining bioavailability of the selected toxic heavy metal. They were safe for soil microorganisms and did not generate metabolic stress for them. Moreover, they did not reduce the viability of pea seeds. Thus, the development of biosafe hybrid hydrogel composites with a comprehensive, good effect on the environment could be considered as successful.

A facile method to synthesize strong salt-enhanced hydrogels based on reversible physical interaction

To overcome the adverse effects of salt on the mechanical properties of hydrogels, a facile double cross-linking method has been proposed to synthesize salt-enhanced tough hydrogels. Herein, a poly(hexafluorobutyl methacrylate-acrylamide) hydrogel [P(AAm-co-HFBMA) hydrogel] is prepared by the copolymerization of acrylamide (AAm) and hexafluorobutyl methacrylate (HFBMA) with N,N'-methylene bisacrylamide (NMBA) as a cross-linking agent in a dimethylformamide (DMF)/aqueous solution; DMF is then replaced by water. The results indicate that the tensile fracture stress of the P(AAm-co-HFBMA) hydrogel (20 mol% HFBMA) is as high as 0.43 MPa, which is far better than that of the PAAm hydrogel (ca. 30 kPa). Additionally, with a further increase in the hydrophobic structural units (25 mol% HFBMA), the tensile fracture stress of the P(AAm-co-HFBMA) hydrogel can be increased up to 2.34 MPa. The mechanical strength of the P(AAm-co-HFBMA) hydrogel is significantly enhanced to 3.50 MPa (2 M) from 2.34 MPa (0 M) after it is soaked in aqueous NaCl solutions with various salt concentrations. The mechanical properties and the results of the DSC analysis indicate that the main reason for its mechanical strength to exhibit a unique salt-enhancement trend can be explained as follows. After the P(AAm-co-HFBMA) hydrogel is soaked in the salt solution, the network gradually collapses with the penetration of the small molecules of salt. Thus, the hydrophobic C-F units easily form dynamic cross-linking junctions due to the switchable hydrophobic interaction between C-F groups, which can endow the P(AAm-co-HFBMA) hydrogel with a more effective dynamic energy dissipation mechanism in salt solution.

Carbon compositional analysis of hydrogel contact lenses by solid-state NMR spectroscopy

Contact lenses are worn by over 140 million people each year and tremendous research and development efforts contribute to the identification and selection of hydrogel components and production protocols to yield lenses optimized for chemical and physiological properties, eye health and comfort. The final molecular composition and extent of incorporation of different components in contact lenses is routinely estimated after lens production through the analysis of the soluble components that were not included in the lens, i.e. remaining starting materials. Examination of composition in the actual intact materials is always valued and can reveal details that are missed by only examining the non-incorporated components, for example identifying chemical changes to components in lenses during the production process. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for the direct compositional analysis of insoluble and heterogeneous materials and is also uniquely suited to determining parameters of architecture in contact lenses. We utilized ¹³C cross-polarization magic angle spinning (CPMAS) NMR to examine and compare the carbon composition of soft contact lenses. ¹³C NMR spectra of individual polymer components enabled the determination of the approximate molecular carbon contributions of major lens components. Comparisons of the conventional etafilcon A hydrogel (1 Day Acuvue MOIST) lenses and silicone hydrogel lenses (Acuvue Oasys, Dailies Total 1, Clariti 1 Day, Biofinity, and Pure Vision) revealed major spectral differences, with considerable variation even among different silicone hydrogel lenses. The solid-state NMR approach provides a direct spectral reporting of carbon types in the hydrogel lens itself. This approach represents a valuable complementary analysis to benefit contact lens research and development and could be extended to isotopically labeled hydrogel lenses to map proximities and architecture between hydrogel components.

Water Properties of Soft Contact Lenses: A Comparative Near-Infrared Study of Two Hydrogel Materials

The functionality of soft contact lenses depends strongly on the water content and their water-transport ability. This study was conducted in order to examine the state of water in two sets of soft contact lenses: VSO38, pHEMA Filcon I 1, and VSO50, copolymer of HEMA and VP Filcon II 1 (HEMA = 2-hydroxy-ethyl methacrylate; VP = vinyl pyrrolidone). Hydrogel lenses were studied using near-infrared spectroscopy and the novel Aquaphotomics approach in order to determine the state of water in materials based on their near-infrared spectra. Aquaphotomics approach investigates absorption at specific vibrational bands of water’s covalent and hydrogen bonds which can provide information on how the water structure changes with the structural change of the polymer network. Principal component analysis and specific star-chart “aquagram” were used to analyse water spectral pattern in hydrogel materials. The findings show that material VSO38 has water predominantly organized in bound state, while material with higher water content, VSO50, has more free and weakly hydrogen bonded water. Our findings define in detail exact water species existing and interacting with the polymer network. The results show qualitative and quantitative possibilities of Aquaphotomics for better modelling and understanding water behaviour in hydrogel materials.