DEVELOPMENT OF UV-CURABLE, HYDROPHILIC GEL NETWORKS FOR SOFT CONTACT LENS MANUFACTURE Shane McGrath, Austin B. Coffey, Phillip R. Walsh and Niall E. Murphy Department of Engineering Technology, Waterford Institute of Technology, Cork Road, Waterford, Ireland. Corresponding author details: [email protected]
Abstract The development of functionally enhanced novel hydrogels for contact lens applications necessitates evaluation with a view towards optimising processability and materials selection for high volume manufacture. Six hydrophilic monomer resins (F1-F6) were successfully formulated and photo-polymerised, using in-house capabilities, to produce hydrogel contact lenses. A suite of characterisation methods were developed to determine the effects of resin composition on material curing performance and functional lens characteristics. By monitoring degrees of conversion and polymerisation kinetics for each formulation, enhanced curing performance was demonstrated with photoinitiator concentrations from 0.02mmol to 0.04mmol producing a 4.3% increase in the degree of conversion reached. The addition of a crosslinking agent was found to considerably increase the rate reaction in the early stages while enhancing crosslink density. Mechanical lens properties were shown to improve, while equilibrium water content (EWC) decreased, for lenses of enhanced crosslink density. The reported findings are expected to contribute significantly to current scientific knowledge and industrial practices in the field.
1 Introduction Photopolymerisation of monomers, also referred to as “curing”, is a well-accepted polymerisation method and is extensively used to manufacture coatings, lacquers, adhesives etc. [1, 2]. The process involves the transformation of a liquid resin into a solid, insoluble polymer network through exposure to UV radiation. [2-4]. Other significant developments of the technology have occurred in the area of bio-material manufacture. Using such techniques, hydrogel contact lenses can be formed from cured liquid resins. This has been particularly aided by the introduction of materials such as Polymethylmethacrylate (PMMA), Polyhydroxyethylmethacrylate (pHEMA), silane monomers and macromers . Selective combinations of these materials under the correct curing conditions can produce lenses with excellent optical clarity, high water contents, oxygen permeability and durability [5, 6].
However, the industrial processes by which lenses are manufactured remains shrouded in ambiguity and much of the technology used has been founded on years of hands on experience . This expertise has neither been well documented, nor well translated, throughout the scientific community leading to a lack of documentation, parameterisation and metricisation. This paucity of information produces a knowledge disparity concerning industrial practices and scientific understanding. Presently, the majority of research undertaken in the field has concentrated on the mechanistic and kinetic characteristics of lightinduced polymerisation, with a view towards developing new types of photoinitiators, monomers and functionalised oligomers for novel applications; ocular drug delivery being one example [2-4]. However, research from a manufacturing and processing viewpoint has not progressed with the same momentum. Fewer studies have targeted processing issues such as resin composition proficiency and the competent control of molecular weight distribution during polymerisation . These factors dramatically affect the mechanical and functional properties of finished lenses as well as their EWC which can lead to manufacturing and handling issues. Therefore, resin processability and material selection for high volume manufacture are important considerations as they impact on the manufacturer and patient requirements from a lens. It is obvious that while the needs of the patient must be met, the ensuing patient requirements for comfort, durability and optical clarity often conflict with manufacturing issues related to productivity, reproducibility, handling and transportation. Given growing patient demands for multiple ocular products spanning a wide variety of wear modalities (daily disposable, extended wear, overnight wear) it is not surprising that material driven development has been abundant. However increased demand from the contact lens wearer for multiple modalities intensifies commercial pressures from a manufacturing and processing standpoint [8-9]. This study seeks to bridge the current knowledge gap and provide a platform for further research. This is vital to the end goal of achieving optimised manufacturing processes and technological innovations.
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Liquid resins were dispersed between NaCl plates while KBr disc methods were used to study cured polymer. A resolution of 4cm-1 and a scan range between 4000cm-1 to 600cm-1 was employed.
2 Experimental 2.1 Materials Two hydrophilic monomers were selected; 2hydroxyethyl-methacrylate (HEMA) and Nvinylpyrrolidone (NVP). Azo-bisiso-butyronitrile (AIBN) was chosen as a free radical photoinitiator, di-ethylamino-ethyl methacrylate (DEAMEA) as a hydrophobic monomer and ethylene-glycol-dimethacrylate (EGDMA) as a crosslinker. Resins were prepared in accordance with Table 1. Prepared resins were sonicated, purged with nitrogen and stored in covered phials, in refrigerated conditions, to negate reactive effects of heat and ambient lighting.
2.3.2 Mechanical Properties A Stable Microsystems TA-XT plus Texture Analyser and a P/25, 25mm, cylindrical test probe (TA instruments) were used to investigate mechanical properties of swollen lenses (i.e. their final state when in contact with the cornea). Compression testing was employed and the compressive forces required to cause lens deformation were measured.
2.3.3 Swelling 2.2 Lens Manufacture Resins were placed in Polypropylene moulds using an automatic pipette and polymerised in a custom built, airtight, stainless steel polymerisation chamber; with a UV-Transmissible lid. The chamber was purged with nitrogen for 1 hour prior to polymerisation. Phillips Actinic BL PL-S 9W/10/2P 1CT mercury lamps (with a peak wavelength of 370nm) were used to polymerise the samples for 1 hour at room temperature (ca. 25oC); which was the optimal time and temperature determined from preliminary experimental data. Post curing, molds were dried in a vacuum oven at 60oC for 24 hours and lenses were subsequently swollen out of the molds with deionised water (DIW). Table 1: Resin compositions used Formulation
HEMA/AIBN (Lab Grade) HEMA/AIBN (Commercial) HEMA/AIBN/ EGDMA/DEAMEA HEMA/AIBN/ EGDMA/NVP HEMA/AIBN HEMA/AIBN
F1 F2 F3 F4 F5 F6
Molar Ratio (mole %) 99.95 : 0.05 99.95 : 0.05 69 : 0.05 : 29 : 2 67.5 : 0.05 : 27.5 : 5: 99.925 : 0.075 99.90 : 0.1
2.3 Characterisation Methods 2.3.1 Degree of Conversion Resin vibrational spectrums were studied (before, and after, photopolymerisation) using Fourier Transform Infra-Red Spectroscopy (FTIR) to monitor the characteristic vibrations of HEMA indicative of curing i.e. the symmetric stretching of C=C double bonds at 1636cm-1 and the symmetric stretching of CO-C single bonds at 1174cm-1.
Dried lenses (60oC for 24 hours under vacuum) were weighed and subsequently swollen in DIW for 24 hours at room temperature (ca. 25o). Swollen lenses were patted dry with lint-free cloth and their EWC calculated as per the following equation; EWC (%) =
- Ws = hydrated weight & Wd= anhydrous weight
2.3.4 Polymerisation Kinetics UV-DSC was performed using a TA instruments DSC Q2000 in conjunction with a custom fabricated attachment to enable the placement of UV lamps over the sample pans, in place of the standard lid. Liquid resin samples of 8mg by volume were introduced to open-topped DSC pans under a continuous nitrogen purge. The lamps were placed overhead once UV intensity had stabilised. The device was configured to collect data isothermally for 8 minutes. Exposure times in excess of 8 minutes were not found to produce any affect. In order to negate localised heat build-up, the lamps were allowed to fully cool down and restabilise between resin batches. Temperature was measured with a digital thermometer probe to confirm the lamps had cooled to room temperature. An ISO-Tech ILM 1332A lux-metre was used to determine lamp stability. Borchardt and Daniels Kinetic methods were applied to the thermograms generated in order to investigate the percentage conversion with time for all formulations. Analysis was applied in the isothermal region between 10% and 50% of the exothermic peak height observable on the thermograms.
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3 Results and Discussion Manufacturing conditions were developed by performing a number of characterisation tests on lenses produced with preliminary experimental architecture. From this, the polymerisation chamber and accompanying parameterisation was developed. Consistent geometrical uniformity and optical clarity (comparable to those commercially available) was found in the lenses produced. This demonstrates proof of concept of the manufacturing system developed and the parameters applied. Materials were not arbitrarily chosen but were selected from the literature due to reports of their ability to satisfy optimal lens criteria. Molar ratios for each resin formulation were determined from preliminary curing attempts having previously demonstrated a good ability to withstand handling, with minimal adhesion to glassware/lens moulds, while closely matching commercial lenses in terms of texture and feel. Photoinitiator quantities were varied to determine the effect on material curing performance and the resultant effect on functional lens properties. The addition of a crosslinker was included primarily to determine the effect of enhanced crosslink densities on lens properties.
3.1 Degree of Conversion An overlay of vibrational spectra for all polymerised resins is depicted in Figure 1. As polymerisation of HEMA occurs by the opening of C=C double bonds, this molecular group should not be present in wholly polymerised samples. However, F1 displays a noticeable C=C double bond peak at 1636cm-1 indicating incomplete polymerisation. No significant C=C double bond peaks are present for all other formulations suggesting near complete conversion was reached by these resins. This propounds their suitability for polymerisation using the manufacturing system developed and the parameters applied.
3.2 Mechanical Properties The principal mechanical features of interest were lens hardness (compressive force required), resilience (deformation recovery) and modulus of deformation (material elasticity); these being the key overlapping properties which relate to handling during manufacture and comfort on the cornea. Test results are provided in Table 2. Increased photoinitiator concentrations yield increased mechanical properties, with the exception of F6; which had the greatest photoinitiator quantity. This suggests residual components remain if photoinitiator quantities are too high causing a disruption in homogeneity which reduces mechanical performance. Mechanical properties are optimised with EGDMA (due to increased
crosslink densities) and the hydrophobic monomer DEAMEA. A combination of EGDMA and NVP produces intermediate mechanical properties as NVP’s hydrophilic attributes partially counteract the hardness and rigidity caused by additional crosslinking. F1 performed poorly, yet demonstrates high resilience. This indicates a more varied molecular weight distribution and a microstructural configuration of short chain components, with fewer crosslinks, greater polydispersity, and subsequently higher elastic tendencies. Table 2: Mechanical Properties of Finished Lenses Resin
SE = Standard error
3.3 Swelling The EWC of finished lenses is provided in Table 3. A trend is clearly discernible whereby EWC increases with increased photoinitiator quantities. This adds credence to earlier suggestions that residual contaminants remain as a result of excessive initiator concentrations, which enlarge the distance between adjacent chain configurations, increasing porosity. Therefore the water attracted into the structure by HEMA’s functional -OH groups has a larger number of sites to bind and distribute. F1 produced the largest EWC of all. This again suggests inferior curing of the lab-grade materials used. Swelling ability is dramatically reduced by the use of EGDMA and DEAMEA. Again the addition of NVP to crosslinked samples produces a lens with medium water absorption capabilities. All swelling study data is found to be in good correlation to mechanical test data. Resin
SE = Standard error
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3.4 Polymerisation Kinetics Using Isothermal UV-DSC methods a number of thermograms indicative of resin curing kinetics were obtained. An overlay of typical thermograms for each formulation is provided in Figure 2. UV polymerisation causes an exothermic reaction denoted by an increase in slope at time equal to zero (representing the introduction of the lamps). A rapid polymerisation reaction is signified by a steep exothermic slope, sharp peak and subsequent straightening of the curve; indicating the reaction has progressed rapidly and approached completion quickly. A slowly developing exotherm with a rounded peak, gradually rising from left to right before levelling out, indicates a slower reaction and longer time to reach full conversion. Application of Borchardt and Daniels Kinetic analysis yields a plot of percentage conversion with time (Figure 3). Figure 3 demonstrates that resin components and their quantities significantly impact on the speed and duration of the polymerisation reaction. The exothermic reaction of F1 developed the slowest and reached the lowest percentage conversion. This correlates to all other analysis methods which have shown that lenses cured from F1 were of the poorest quality. The inferiority of F1 has constantly been indicated throughout all test methods and is suspected to relate to the removal of inhibiting agents frequently added to HEMA pre-retail. Removal methods applied in house may not be as efficient as industrial practices at eliminating the inhibitor which deters photo-polymerisation. Resins containing EGDMA produce enhanced conversion rates, particularly in the early stages of polymerisation. It is proposed that this reaction speed relates to the increased number of C=C double bonds contained in the microstructure of EGDMA, compared to other resin components. This provides for supplementary reaction sites upon decomposition. As the number of these sites increases, it becomes easier for broken C=C double bonds to connect and so the reaction can progress at an enhanced rate. Variance in photoinitiator concentrations were found to impact on both the conversion speed and the final degree of conversion reached. Resin F6, which contained the greatest quantity of initiator, reached the highest final conversion. However, the reaction speed of this resin is slower in the early stages than resins containing EGDMA. F5 contains 50% less initiator than that of F6, but 50% more than all others, yet it displays a slow reaction speed until late in the process. While eventually reaching a high degree of conversion it is surprising that the rate reaction is slower. It was anticipated that increased initiator concentrations would allow polymerisation to progress quicker in
the early stages, as the number of free radicals generated upon exposure would be more plentiful. While the relationship between reaction speed and initiator concentration remains somewhat obscure from the results, the data suggests that a higher initiator concentration results in the polymer reaching a higher degree of conversion. However, this may occur at the expense of unreacted initiator remaining in the sample, as suggested by swelling studies and mechanical testing.
4 Conclusion This study has focused on addressing the outlined knowledge gap by (i) formulating monomer resins suitable for in-house photopolymerisation to produce hydrogel contact lenses and (ii) to developing a suite of characterisation techniques capable of investigating the effects of resin composition on material curing kinetics and functional lens characteristics. Both (i) and (ii) were successful. Manufactured lenses demonstrated characteristic lens criteria comparable to commercial lenses in relation to geometrical uniformity, optical clarity and handling ability. Investigation of resin curing ability by FTIR has shown that formulations F2-F6 reached complete (or near complete) conversion under the manufacturing parameters selected. Correlations between mechanical test data and swelling studies have successfully illustrated (i) the advantages of using a crosslinker in terms of functional performance and (ii) while increased photoinitiator concentrations provide for greater processing ability by ultraviolet radiation and allow for greater swelling capabilities, the presence of unreacted free radicals disrupts microstructural homogeneity at the detriment of mechanical properties. Analysis of polymerisation kinetics has established the relationship between resin composition and curing performance in terms of the rate reaction and overall degree of conversion achievable. Elevated initiator quantities were found helpful in reaching full conversion but impact the polymerisation rate to a lesser extent. Polymerisation rates were found to be significantly assisted by the addition of a crosslinker (EGDMA) owing to the number of reactive sites provided by its microstructure upon decomposition. Further study is necessary in augmenting processing constraints and resin formulations to develop optimised processes capable of repeatedly producing lenses with optimised functional characteristics. However, the findings presented serve to demystify photopolymerisation processes by reducing the disparity in scientific understanding. This is necessary in providing a platform for future research from which optimisations and innovations can be realised.
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5 Acknowledgements Commercial monomer and photoinitiator (Hydroxyethyl-methacrylate and Azo-bisisobutyronitrile respectively), along with polypropylene moulds, were kindly supplied by Bausch and Lombe, Waterford, Ireland.
6 References 1. McDermott, S., Investigation of UV-LED Initiated Photopolymerisation of Bio-compatible HEMA. Thesis, 2008. Dublin IT. 2. Decker, C., Photoinitiated crosslinking polymerisation. Progress in Polymer Science, 1996. 21(4): p. 593-650 3. Decker, C., Light-induced crosslinking polymerization. Polymer International, 2002. 51(11): p. 1141-1150. 4. Decker, C., The use of UV irradiation in polymerization. Polymer International, 1998. 45(2): p. 133-141. 5. Efron, N. and C. Maldonado-Codina, 6.633 Development of Contact Lenses from a Biomaterial Point of View – Materials, Manufacture, and Clinical Application, in Comprehensive Biomaterials, D. Editor-in-Chief: Paul, Editor 2011, Elsevier: Oxford. p. 517-541.
6. Nicolson, P.C. and J. Vogt, Soft contact lens polymers: an evolution. Biomaterials, 2001. 22(24): p. 3273-3283. 7. Lyons, S., Naturelle Curing. Educational Presentation, 2011. Baush + Lombe. 8. Turnbull, C., Introduction To Ocular Delivery. Stable Microsystems, TAXT Plus Application Notes, 2006. 9. TA-Instruments, TAXT Plus Application Studies - Test Methods for Pharmaceutical & Medical Devices. Measuring Contact Lens Mechanical Strength, 2006.
KEYWORDS Hydrogels, Soft Photopolymerisation
Figure 1 - Overlay of Vibrational Spectra Obtained by FTIR for All Formulations
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Figure 2 - Overlay of Typical Thermograms Obtained by UV-DSC for All Formulations
Figure 3 - Conversion with Time for All Formulations Determined by Borchardt & Daniels Analysis
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