Poly(vinyl alcohol) Hydrogels Reinforced with Nanocellulose for

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Poly(vinyl alcohol) Hydrogels Reinforced with Nanocellulose for Ophthalmic Applications: General Characteristics and Optical Properties Gopi Krishna Tummala, Ramiro Rojas, and Albert Mihranyan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b10650 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Poly(vinyl alcohol) Hydrogels Reinforced with Nanocellulose for Ophthalmic Applications: General Characteristics and Optical Properties Gopi Krishna Tummala,1 Ramiro Rojas,2 Albert Mihranyan1* *Correspondence to Prof. Albert Mihranyan. E-mail: [email protected]. 1

Nanotechnology and Functional Materials, Department of Engineering Sciences, Box 534

Uppsala University, 75121 Uppsala, Sweden 2

Fibre and Polymer Technology and Wallenberg Wood Science Center, School of Chemical

Science and Engineering, KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden

ACS Paragon Plus Environment

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ABSTRACT: Globally, uncorrected refractive errors are one of the main causes of visual impairment and contact lenses form an important part of modern day eye care and culture. Several hydrogels with varying physicochemical properties are in use to manufacture soft contact lenses. Hydrogels are generally too soft and reinforcement with appropriate materials is desirable in order to achieve high water content without compromising mechanical properties. In this study we have developed a highly transparent macroporous hydrogel with water content > 90%, by combining poly(vinyl alcohol) with nanocellulose. Furthermore, the results show that the composite hydrogel has refractive index close to that of water and very good UV blocking properties.


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1. INTRODUCTION Data from the World Health Organization states that there are 246 million people worldwide that are visually impaired.1 Globally, the main cause (43% of cases) of moderate and severe visual impairment is caused by uncorrected refractive errors, such as myopia, hyperopia or astigmatism, yet 80% of visual impairment can either be prevented or cured. However, 90% of those visually impaired live in low income settings in developing countries. Thus, there is a need for affordable vision corrective solutions from a global perspective. Vision corrective lenses include prescription eyeglasses and contact lenses. Compared to eyeglasses, contact lenses are lightweight and virtually invisible. Contact lenses remain an important part of the modern eye care and culture, and more than 85 million people worldwide wear contact lenses for corrective, cosmetic, and therapeutic purposes.2 Contact lenses can be broadly classified into two types: hard and soft contact lenses. Hard contact lenses had low oxygen permeability which led to unwanted clinical events such as corneal hypoxia and various types of edema. The quest for oxygen permeability led to the development of soft contact lenses. Using a hydrogel with high water content is favorable for oxygen permeability and wear comfort. Typically, the water content values for commercially available soft contact lenses vary between 40% and 75% depending on the product type.3 Although contemporary commercially available thin soft contact lenses have good oxygen permeability and good comfort level, they face the issue of proteinaceous deposits onto the polymer matrix.4 Further increase in the water content in order to reduce protein deposition has so far not been feasible as hydrogels are in general mechanically weak and this would further compromise their mechanical properties during use and handling. Hence, the benefit of high water content needs to be balanced by sufficient mechanical strength and flexibility. In this context, transparent poly(vinyl alcohol),


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PVA, hydrogels are interesting candidates for affordable, biocompatible, and mechanically strong contact lens materials with high water content and will be the subject of the present article. PVA has a long record of use in ophthalmic pharmaceutical formulations as a lubricant and viscosity-enhancing agent.5 The idea to use PVA hydrogels for contact lens and keratoprosthetic applications was first explored by Japanese researchers from Kyoto University in late 1980s-early 1990s. Hyon and co-authors developed a method to make a transparent PVA hydrogel using solvent mixtures.6 Water was mixed with several organic solvents such as ethanol, glycerin, ethylene glycol, and dimethyl sulfoxide (DMSO) to dissolve PVA and hydrogels were subsequently produced by freeze-thawing. Optimal mechanical and optical properties were achieved with the DMSO/water solvent system and in particular for 80/20 v/v combination.6 Kita and co-authors evaluated the PVA hydrogel material for soft contact lens application and concluded that, in addition to good mechanical strength and oxygen permeability, it also had low protein absorption, high water content, and transparency.4 In 1997, Tsuk et al. studied the application of PVA in keratoprosthesis in vivo.7 Tamura and co-authors showed that PVA is biocompatible and hemocompatible and therefore has great potential in regenerative medicine.8 In 2001, Wu et al. developed a synthetic cornea with a transparent PVAbased hydrogel optic center and demonstrated that limbal epithelial cells can migrate onto the synthetic cornea in a rabbit model.9 In 2014, He et al. developed a PVA/Chitin-based composite hydrogel which showed very good in vitro biocompatibility, flexibility, and transparency.10 It should be mentioned that in 2000 Mueller from Novartis patented a technology to develop PVAbased hydrogels for soft contact lens application, involving chemical modification of PVA


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(Nelfilcon A; poly(vinyl alcohol) partially acetalized with N-((formyl methyl)acrylamide) to enable crosslinking by UV irradiation in molds.11, 12 In order to understand the effect of solvent on transparency of PVA hydrogels it is important to understand the gelation phenomenon. When a homogenous PVA solution is quenched from high temperature to -20 °C, gelling occurs by physical crosslinking of PVA chains.13 DMSO is a better solvent for PVA than water, and mixtures of DMSO and water, with DMSO content greater than 60 % v/v, do not freeze up to -70 °C.14 PVA chains are more coiled in an aqueous solution, whereas they are more extended in DMSO solution.15 Upon cooling of PVA in a mixed solvent, the solubility of PVA is reduced, resulting in gradual phase separation and eventually local crosslinking of formed PVA crystallites.16 Once an infinite network is formed, it is hard for the solvents to phase separate resulting in a three dimensional gel structure.16 Factors such as PVA concentration, degree of deacetylation, molecular weight, solution composition and quenching temperature are thus important for gelling.13, 14 The strength of the PVA hydrogel primarily depends on its molecular weight and concentration.17, 18 Strong hydrogels can be obtained from very high molecular weight PVA but they are hard to manufacture, expensive to purchase, and difficult to process due to very high viscosity. In order to solve the intrinsic water content vs. mechanical strength conflict, one alternative is to reinforce the hydrogel material with another compatible mechanically strong component. However although various materials could potentially be used for reinforcement, the greatest challenge is to ascertain that the optical properties of the final product meet the requirements for ophthalmic use. Previous studies show that native cellulose is an excellent reinforcing agent for PVA polymer matrix.19-22 Normally, incorporating native cellulose in PVA hydrogels would reduce the transparency of the composite and may result in a patchy looking


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material. On the other hand, cellulose may be rendered transparent in the form of nanocellulose and proper blending with PVA may result in a suitable product. In particular, nanocellulose produced from TEMPO-oxidized cellulose is transparent and has high water holding capacity. Furthermore, TEMPO-oxidized cellulose was shown to be cytocompatible in a recent study.23, 24 Recently, we presented a new high water content hydrogel material consisting of cellulose nanowhiskers and PVA with promising properties for ophthalmic use.25 In the present study, the effect of inclusion of TEMPO-oxidized nanocellulose of varying aspect ratio and the manufacturing conditions on the physical-chemical properties of PVA hydrogels is investigated with special focus on optical profile.

2. EXPERIMENTAL SECTION 2.1.

Materials. Microcrystalline cellulose -Avicel

PH 101

(MCC),

2,2,6,6-

tetramethylpiperidine-1oxyl radical (TEMPO), 4-Acetamido-TEMPO, NaBr, NaClO, NaClO2, CH3COONa·3H2O, NaOH, poly(vinyl alcohol) with MwAVG 146,000-186,000 and degree of saponification 99.9%, (PVA), dimethyl sulfoxide (DMSO), aqueous hydroxylamine (50%), ethanol, HCl, CH3COOH were purchased from Sigma Aldrich. All the chemicals used were of reagent grade or better. Deionized water was used for all experiments. 2.2. TEMPO Mediated Oxidation of Cellulose. To produce cellulose nanocrystals (CNC), MCC was oxidized according to a previously published method.26 In short, 5 g MCC was dispersed in 400 mL of deionized water and 90 mg of TEMPO and 1 g of NaBr solubilized in 100 mL deionized water were added to it. The mixture was kept under magnetic stirring at 500 RPM. To the mixture, 10 mL of 10% wt NaClO with pH of 11 adjusted with 1M HCl was added at intervals of 30 min for 3.5 hours (70 mL in total). The reaction system was maintained at pH


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10.5 by adding 1 M NaOH through the entire period of reaction. The reaction was quenched after 3.5 hours by adding 10 mL of ethanol. Oxidized MCC was washed 3 times with deionized water by centrifugation and then dialyzed for 3 days. Cellulose was then collected by centrifuging and was dispersed in deionized water by ultrasonication (Vibracell 700 W, 20 KHz, USA) for 5 min to obtain a transparent gel. Cellulose nanofibrils, CNF, were prepared from never-dried spruce sulfite pulp (provided by Nordic Pulp in Säffle, Sweden) using 4-Acetamido-TEMPO as catalyst based on the method described by Tanaka et. al.27 The pulp (10 g) was suspended in 0.1 M acetate buffer (1000 mL, pH 4.8) and stirrer using an overhead stirrer at 150 RPM until fully dispersed. Thereafter, 4Acetamido-TEMPO (1 mmol) and sodium chlorite (0.1 mol) were added until dissolution. A buffered (pH 4.5); 30 mL of a 2 M NaClO solution (10.0 mmol) was added to the reaction flask in three steps, 10 mL each/2 hours. The reaction flask was maintained under nitrogen atmosphere over a period of 48 hours at 60 °C with continuous stirring. The oxidation was followed by washing thoroughly with acetate buffer and distilled water under vacuum filtration conditions. Afterwards the pulp fibers were mechanically dispersed in deionized water at 1% wt and passed through a microfluidizer M-110EH (Microfluidics Ind., USA) for a total of four passes, the first two through a set of “large pore” chambers (400 µm followed by 200 µm) and the second set of passes through “small pore” chambers (200 µm followed by 100 µm) with dilution in between. The obtained nanofibril suspension had a solid content of 0.6% wt. 2.3. Carboxylate Content. The carboxylate content of CNC and CNF were determined through conductometric titration in triplicates. About 100 mg of sample was dispersed in 60 mL of 10 mM aqueous NaCl through ultrasonication and pH was adjusted to 2.7 by addition of concentrated HCl. The dispersion was purged with nitrogen for about 30 minutes prior to


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titration. The dispersion was titrated under nitrogen atmosphere with 0.05 M NaOH until pH 5. The carboxylate content was determined by making linear fits to the titration curves using Origin 2015 software. The amount of carboxylic groups was calculated from the intercepts of the fitted curves. 2.4. Aldehyde Content. Schiff’s base reaction was employed using aqueous hydroxylamine to convert any aldehyde in the oxidized cellulose samples to oxime.28 Briefly, 100 mg of sample was dispersed in 40 mL aqueous 0.01 M acetate buffer (pH 4.5) followed by addition of 1.65 mL aqueous hydroxylamine solution (50%). The mixture was stirred at room temperature for 24 hours. The products were washed several times with 0.01 M HCl and finally with ethanol by centrifugation and dried. CHN analysis was carried out by Analytische Laboratorien (Lindlar, Germany). 2.5. Surface Charge. Dispersions of 0.0001% wt CNC and CNF in 10 mM NaCl were prepared through ultrasonication and pH was adjusted with solutions of HCl or NaOH. The electrophoretic mobility of the samples was measured at 25 °C using a universal dip cell (Malvern Instruments, UK) and a ZetaSizer Nano instrument (Malvern Instruments, UK). The zeta potentials were determined from electrophoretic mobilities using the Smoluchowski model. 2.6. Atomic Force Microscopy. The atomic force microscopy images were acquired in air using a Dimension Icon (Bruker, Germany) instrument. The samples were mounted on mica slides by first pre-coating the surface with 0.1% wt poly(L-lysine) solution and then fixing a dilute (0.2% wt) dispersion of nanocellulose onto the mica surface by gentle drying. The images were acquired in the peak-force tapping mode, using the manufacturer’s ScanAsyst Air cantilever and ScanAsyst automatic optimization algorithm.


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2.7. Preparation of Hydrogel. To produce the hydrogels, 3 g of PVA was added to a mixed solvent system of DMSO and deionized water so as to obtain a concentration of 10% (w/w). The composition of solvent was varied by changing the mass ratios of DMSO and water to obtain DMSO:water 80:20, 70:30 and 60:40 mixtures. PVA solutions were obtained by heating the mixture at 100 °C under stirring in an oil bath for about 2 hours. For the preparation of a composite hydrogel, nanocellulose gel equivalent to 30 mg dry weight was added to the PVA solution and further mixed for an hour at 100 °C. The homogenous and transparent solutions obtained by this method were cast in to respective polypropylene molds and allowed to gel at -20 °C for 24 hours. The formed gels were collected from the molds, and the DMSO was exchanged with deionized water by dialyzing the gels in excess water for at least 48 hours. The obtained products after washing were stored in deionized water for further testing. 2.8. X-ray Diffraction. An X-ray diffractometer (D8 Twin-Twin, Bruker) with Bragg−Brentano geometry (CuKα radiation; λ = 1.54 Å) was used. The hydrogel samples were stored wet until measurement and the total time for measurement in air did not exceed 5 min. 2.9. Scanning Electron Microscopy. Carl Zeiss Merlin FEG-SEM instrument was used for electron microscopy. The samples were frozen in liquid nitrogen and then carefully freezedried to avoid shrinkage prior to analysis. The samples were mounted on aluminum stubs using adhesive carbon tape and sputtered with a thin layer of Au/Pd to minimize charging during imaging. 2.10. X-ray Micro Tomography. XµCT is a non-destructive method that is used to obtain the 3D structure of a sample. Bruker Skyscan 1172 was first used to acquire 2D X-ray images. The X-ray projections were used to computationally reconstruct the three-dimensional microstructure of the sample, or more precisely, the spatial distribution of local X-ray attenuation


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coefficient using NRecon 1.6.9 (Bruker, Belgium). The methods used for sample preparation and 3D image reconstruction were adopted from literature.29, 30 2.11. Equilibrium Water Content. Water content of all samples was determined by drying the samples to constant mass in a laboratory oven at 150 °C. Three measurements were made for each sample and in the case of lens samples 2 lenses were used in each measurement. 2.12. Light Transmittance. Light transmittance through 1 mm thick hydrogel strips immersed in quartz cuvette with deionized water at 25 °C was measured using a UV-Vis spectrophotometer (UV-1650PC, Shimadzu). Transmittance was measured in the range of 200 and 900 nm with deionized water as the reference for 100% transmittance. 2.13. Refractive Index. Refractive index of lenses was measured using a Mettler Toledo Refracto 30P, hand-held refractometer. The device determines the refractive index by measuring the critical angle of total reflection of a light beam of 589.3 nm falling on the sample. Calibration was performed before each measuring sequence with water at room temperature. The range of the instrument is between 1.32 and 1.50 with a resolution of 0.0001 and an accuracy of ± 0.0005.

3. RESULTS AND DISCUSSION In this study, CNC and CNF were used to reinforce PVA hydrogels to improve the mechanical properties of a model contact lens without compromising the transparency of the composite. The physical chemical properties of CNC and CNF gels are summarized in Table 1. It is evident from Table 1 that both CNC and CNF gels featured similar solids content. The carboxylate content value of CNC is higher than for CNF due to the difference in oxidation treatments. Both samples featured low aldehyde content, albeit it was slightly higher for CNF. The presence of carboxyl


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groups was further verified by ζ-potential values, which were approximately - 31-33 mV for pH 6-12 and - 6-12 mV for pH 2. Although the solids content of CNC and CNF were similar, the rheology was different owing to the differences in aspect ratio, further appreciated in AFM images below. Figure 1 illustrates the physical appearance of CNC and CNF samples. It is seen in Figure 1 that CNF gel is self-supporting, while CNC gel resembles a viscous hard-flowing dispersion. Both gels appear slightly translucent if not fully transparent. AFM imaging was used to investigate the morphology of the CNC and CNF samples. It is seen in Figure 2 that CNC consisted of individual whiskers of a few hundred nm long in contrast to entangled CNF, which are several µm long. The high aspect ratio of CNF is likely responsible for the high viscosity leading to formation of self-supporting gel as shown in Figure 1a. Accordingly, the free flowing viscous CNC dispersion is due to the lower aspect ratio of cellulose whiskers. Contact lenses obtained after solvent exchange with deionized water appeared very transparent as shown in Figure 3. The lenses were self-standing, sufficiently stiff, and flexible enough to conform to a convex surface. The stiffness of the lenses increased with reinforcement of CNC / CNF. Furthermore, it can be seen from Figures 4, the composite hydrogels were very elastic and could be pulled to a great extent without rupture. Detailed mechanical characterization will be covered in a separate article. In order to visualize the internal structure of the contact lenses, they were gently freeze dried to avoid shrinkage and then investigated using SEM imaging. It can be seen from Figure 5 that the composite formed an open, interconnected, continuous, and macroporous honeycomb structure. The areas that appear non-porous are due to artifacts that arose from collapsed pores of


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the hydrogel during freeze drying. No distinct cellulose particles could be visualized in what appeared to be the pores, suggesting that they are distributed throughout the matrix. In Figure 5a, small projections are distinctly observed stretching outwards from the matrix cell walls. It is plausible that these projections are CNC whiskers. In Figure 5b, no such projections could be visualized; instead, occasionally long filaments could be seen extending over several honeycomb cells. Figure 6 is a 3D X-ray microtomogram of the CNC-PVA hydrogel in the wet state. The open interconnected network structure could be confirmed in this 3D image. Advanced imaging techniques need to be used in future for better resolution. The water content of the samples with different solvent composition used in their preparation was measured, and Table 2 summarizes the equilibrium water content and refractive index values of the studied samples. It can be seen from Table 2 that the samples featured very high water content, i.e. > 90%, which is by far the highest reported water content value for contact lens, surpassing any commercially available soft contact lens product. Thanks to the open macroporous structure of the hydrogel, it is believed that high water content is favorable for high optical transparency, good wear comfort, high oxygen permeability, low protein deposition, and high biocompatibility. The hygroscopic nature of nanocellulose present in the composite may additionally assist to retain water in the hydrogel. The refractive index values of the composite hydrogels from Table 2 are very close to that of pure water (1.3330 at 589.29 nm), meaning that when immersed in water the hydrogels are virtually invisible. The presented refractive index values are far better than those of commercially available contact lenses (typically around 1.38-1.44), which is expected given their lower water content.3


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Figures 7 summarize the transparency properties of the hydrogels as measured on 1 mm hydrogel strips in water. It is seen that all samples have high transparency, i.e. ≥ 90% in the visible light spectrum (390-700 nm). The produced high transparency is related to exceptionally high water content of the lenses and intricate mesh structure of the composite, which does not reflect light. It should be noted that, considering that the transparency of the samples was measured on 1 mm thick strips, even higher transparency is expected for real contact lenses as they are 2-3 times thinner. Compared with the commercially available chemically modified UV-crosslinked PVA based (Nelfilcon A) contact lenses, our contact lenses have better optical properties and water content. The refractive index of these lenses is 1.38 whereas our lenses have refractive index close to that of water, i.e. 1.335. The water content of these lenses is 69 % whereas our lenses have a water content of at least 90 %. Our model lenses are made with only 10% polymer whereas 30% polymer is used in these lenses and hence low water content. When compared to the Nelfilcon A lenses, mechanical strength is the only aspect our model lenses may be lacking. However, with respect to the application and handling, our model lenses are mechanically stable enough. Figure 7a shows the effect of solvent composition on the transparency of the hydrogel material with 10% PVA. With decreasing DMSO content the size of PVA crystallites tend to increase reducing the transparency in the visible region.4 Figures 7b and c show that the effect of cellulose reinforcement on transparency in the visible region is generally insignificant and is slightly more pronounced in CNF-reinforced samples compared to those reinforced with CNC. Figure 7b shows the effect of solvent composition on the transparency of the hydrogels reinforced with CNF. The trend is similar to that of pure PVA hydrogel in figure 7a but the


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relative decrease in transparency is higher, especially for DMSO:water 60:40 composition. The decrease of transparency may be related to higher aspect ratio of CNF fibers and hence stronger light scattering. Contrary to CNF samples, it is seen in Figure 7c that CNC reinforced hydrogels do not show much variation in transparency in the visible region, probably due to smaller aspect ratio of CNC and less scattering. For convenience, the spectra for DMSO:water 80:20 samples are summarized separately in Figure 7d. Interestingly, while the hydrogels were highly transparent in the entire visible light spectrum, they exhibited substantial UV-light blocking properties, as manifested by a sharp drop for the transparency in the region between 200 and 400 nm. Considering the damaging role of UV light for eyes associated with the development of cataract and pterygium, the intrinsic UVblocking properties of PVA based hydrogels are highly beneficial for ophthalmic applications.31 The UV-blocking properties in nanomaterials are complex and may simultaneously involve UVlight absorption, reflection, and scattering. Pure absorption of UV light is due to excitation of electrons in double bonds or non-bonding molecular orbitals, i.e., lone pairs, to higher energy levels, which are normally unoccupied.32 UV-blocking behavior of PVA has previously been observed and utilized in the context of keratoprosthesis.7 Class I contact lenses offer the highest level of UV protection, blocking more than 90% UV-A and 99% UV-B, whereas Class II lenses must block at least 70% of UV-A and 95% of UV-B radiation. Non-UV blocking contact lenses may have some residual blocking properties, on average not exceeding 10% UV-A and 30% UVB.31 The CNF containing hydrogels showed stronger UV-blocking properties, most likely due to stronger scattering properties. For comparison, CNF reinforced DMSO:water 80:20 hydrogels showed 60% transparency for UV-A (320-400 nm) and 50% transparency for UV-B (290-320 nm) as compared to CNC which showed 80% transparency for UV-A and 60% transparency for


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UV-B, respectively. The CNC containing samples, exhibited less UV-scattering but stronger UV-absorption properties as manifested by peaks at 280 and 330 nm, which are ascribed to substantially higher content of UV-absorbing carbonyl (C=O) groups in CNC as compared to CNF. The UV-blocking properties of the hydrogels are substantially improved by reducing the amount of DMSO in the original mixture from 80:20 to 60:40, probably because of stronger scattering due to slightly larger crystallites. In all, the UV-A-blocking properties of nanocellulose-reinforced samples are comparable to those of Class II contact lenses albeit UV-B properties are somewhat inferior. The hydrogel samples were analyzed further with XRD (see supplementary info). Neither the characteristic peak of cellulose I (22.6°) nor that of PVA (20°) is present. Instead, a broad halo in the diffractogram was observed, which arises due to the scattering effect of large amount of water present in the PVA network.33

4. CONCLUSIONS In the present study we found that introducing CNC or CNF in PVA hydrogels allows producing a set of high water content self-standing hydrogels with exceptional optical properties, such as similar-to-water refractive index, high transparency for visible light and strong UV-blocking properties. Furthermore, the hydrogel has a macroporous structure filled with water which is important for wear comfort, high oxygen permeability, low protein adsorption and biocompatibility. Future studies will be directed towards thorough mechanical characterization and biocompatibility of the composite.


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ASSOCIATED CONTENT Supporting Information: XRD data

AUTHOR INFORMATION Corresponding Author *Prof. Albert Mihranyan, E-mail: [email protected] Author Contributions G.T., A.M. designed the material and conceived project. G.T., A.M. and R.R. carried out the experiments. G.T., A.M. and R.R., contributed to data analysis. G.T. and A.M. prepared the manuscript. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This works has been funded by the Wallenberg Wood Science Center. One of the authors (A.M.) is a Wallenberg Academy Fellow and thanks the Knut and Alice Wallenberg Foundation for their long-term financial support. The authors thank Dr. Thomas Joffre and Dr. Seung Hee Jeong for their valuable contributions to 3D imaging and data acquisition. ABBREVIATIONS PVA, poly(vinyl alcohol); DMSO, dimethyl sulfoxide; TEMPO, 2,2,6,6-Tetramethylpiperidine1oxyl radical; CNC, cellulose nanocrystals; CNF, cellulose nanofibrils; MCC, microcrystalline cellulose; XRD, X-ray diffraction; AFM, atomic force microscopy; SEM, scanning electron microscopy; UV, ultraviolet light.


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(10) He, M.; Wang, Z.; Cao, Y.; Zhao, Y.; Duan, B.; Chen, Y.; Xu, M.; Zhang, L. Construction of Chitin/PVA Composite Hydrogels with Jellyfish Gel-Like Structure and Their Biocompatibility. Biomacromolecules 2014, 15, 3358-3365. (11) Muller, B., Especially contact lenses. Patent No: US6106746: 2000. (12) Muller, B., Photocrosslinked polymers. Patent no: US5789464, 1998. (13) Takeshita, H.; Kanaya, T.; Nishida, K.; Kaji, K. Small-angle neutron scattering studies on network structure of transparent and opaque PVA gels. Phys. B 2002, 311, 78-83. (14) Ohkura, M.; Kanaya, T.; Keisuke, K. Gels of poly (vinyl alcohol) from dimethyl sulphoxide/water solutions. Polymer 1992, 33, 3686-3690. (15) Hoshino, H.; Okada, S.; Urakawa, H.; Kajiwara, K. Gelation of poly (vinyl alcohol) in dimethyl sulfoxide/water solvent. Polym. Bull. 1996, 37, 237-244. (16) Takeshita, H.; Kanaya, T.; Nishida, K.; Kaji, K. Gelation process and phase separation of PVA solutions as studied by a light scattering technique. Macromolecules 1999, 32, 78157819. (17) Gupta, S.; Sinha, S.; Sinha, A. Composition dependent mechanical response of transparent poly (vinyl alcohol) hydrogels. Colloids Surf. B. Biointerfaces 2010, 78, 115-119. (18) Kobayashi, M.; Toguchida, J.; Oka, M. Development of an artificial meniscus using polyvinyl alcohol-hydrogel for early return to, and continuance of, athletic life in sportspersons with severe meniscus injury. I: Mechanical evaluation. The Knee 2003, 10, 47-51. (19) Cho, M.-J.; Park, B.-D. Tensile and thermal properties of nanocellulose-reinforced poly (vinyl alcohol) nanocomposites. J. Ind. Eng. Chem. 2011, 17, 36-40. (20) Frone, A. N.; Panaitescu, D. M.; Donescu, D.; Spataru, C. I.; Radovici, C.; Trusca, R.; Somoghi, R. Preparation and characterization of PVA composites with cellulose nanofibers obtained by ultrasonication. BioResources 2011, 6, 487-512.


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Figure 1. Physical appearance of (a) CNF and (b) CNC samples.


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Figure 2. AFM images of TEMPO oxidized (a) CNC and (b) CNF.

Figure 3. Typical appearance of self-supporting and transparent contact lens made from 10% PVA and 1% nanocellulose composite hydrogel.


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Figure 4. Demonstration of the elastic behavior of hydrogel in its pristine state, extended state, and relaxed state after stretching.


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Figure 5. SEM image of freeze dried contact lens with (a) 10% PVA and 1% CNC (b) 10% PVA and 1% CNF made in a solvent system with DMSO to water ratio 80:20.


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Figure 6. X-ray microtomogram of 10% PVA and 1% CNC hydrogel made in a solvent system with DMSO to water ratio 80:20. Darker areas show water and brighter areas show matrix.


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Figure 7. Comparative UV-Vis transmission spectrum of 1 mm hydrogels with (a) 10% PVA, (b) 10% PVA and 1% CNF, (c) 10% PVA and 1% CNC, (d) 10% PVA and 1% CNF/CNC. Table 1. Physical Chemical Characteristics of CNC and CNF Samples Parameter/sample CNC CNF Solids content (%) 0.58 ± 0.198 0.53 ± 0.023 Carboxylate content (mmol/g)

2.05 ± 0.088

0.70 ± 0.059

0.001

0.007

Zeta potential at pH 12

-30.70 ± 0.885

-31.70 ± 1.080


-33.00 ± 2.780

-33.00 ± 1.670


-6.67 ± 1.270

-11.90 ± 1.480

Aldehyde content (mmol/g)


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Table 2. Equilibrium Water Content and Refractive Indices of Composite Hydrogel Contact Lenses Sample Equilibrium Refractive Index, 20 °C Water Content % DMSO:Water 80:20_PVA 10% 90.7 ± 0.49 1.3340 ± 0.0007 DMSO:Water 70:30_PVA 10%

93.3 ± 0.07

1.3347 ± 0.0016

DMSO:Water 60:40_PVA 10%

94.2 ± 0.01

1.3330 ± 0.0005

DMSO:Water 80:20_PVA 10%_CNF 1%

92.8 ± 0.26

1.3344 ± 0.0019


93.2 ± 0.28

1.3359 ± 0.0008


93.1 ± 0.53

1.3334 ± 0.0014

DMSO:Water 80:20_PVA 10%_CNC 1%

93.1 ± 1.76

1.3345 ± 0.0018


93.3 ± 0.36

1.3341 ± 0.0012


93.5 ± 0.41

1.3349 ± 0.0011

TOC GRAPHIC


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Poly(vinyl alcohol) Hydrogels Reinforced with Nanocellulose for

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