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Optometry & Vision Science:
doi: 10.1097/OPX.0b013e318162261e
Original Article

Physical Properties of Soft Contact Lens Solutions

DALTON, KRISTINE OD; SUBBARAMAN, LAKSHMAN N. BSOptom, MSc, FAAO; ROGERS, RONAN MSc; JONES, LYNDON PhD, FCOptom, FAAO

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Centre for Contact Lens Research, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada

Received February 27, 2007; accepted October 12, 2007.

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Abstract

Purpose. To investigate the physical properties of commercially available soft contact lens solutions.

Methods. The pH, osmolality, surface tension (ST), and viscosity of various soft contact lens solutions were measured at room temperature. Viscosity measurements were also taken at 34°C. The solutions examined were Opti-Free Express (OFX), Opti-Free RepleniSH (OFR), Complete Moisture Plus (COM), UltraCare (UC), ReNu MultiPlus, Sensitive Eyes, AOSept (AO), Clear Care, SoloCare Aqua, and SoftWear saline. The peroxide solutions were measured before and after neutralization.

Results. The pH of most solutions was close to neutral (range 7.00–7.36), except for OFX and neutralized AO and Clear Care. The osmolality values of most solutions were in the 275 to 310 mOsm/kg range. OFX exhibited a significantly lower osmolality (225 mOsm/kg; p < 0.001), whereas UC was significantly higher (329 mOsm/kg; p < 0.001). Neutralized AO and SoftWear saline had ST values of approximately 67 mN/m. OFX, OFR, and SoloCare Aqua exhibited low ST values in the 30 to 35 mN/m range. The remaining solutions exhibited intermediate ST values of approximately 40 mN/m. These three groupings were significantly different (p < 0.001). The average viscosity of most solutions at room temperature was between 0.95 and 1.26 cP, except for COM (3.02 cP; p < 0.001). At 34°C, the average viscosity of most solutions was between 0.70 and 0.83 cP, except for COM, which had a viscosity of 1.92 cP (p < 0.001). The un-neutralized peroxide solutions had very different pH and osmolality values from all the solutions that would directly contact the eye (p < 0.001). Their viscosity and ST values were similar (p = NS).

Conclusions. This study detailed many physical properties of soft lens solutions that are not readily available and indicated that certain properties vary significantly among these products.

Soft contact lens solutions, which are used with both silicone hydrogel and traditional hydrogel lenses, account for the vast majority of care systems used by contact lens patients.1,2 It is possible that the physical properties of these solutions may influence both patient comfort and patient preference for one care system over another. Previous studies have shown that when the pH of a contact lens solution falls above the pH range of the tears (6.6–7.8),3 patients report that they experience both ocular discomfort and stinging.4–7 Osmolality of soft contact lens solutions may also play a role in patient comfort, as studies have demonstrated that tear film osmolality plays a significant role in the discomfort reported by dry eye patients,8–13 with higher osmolality values being associated with greater levels of discomfort.8–13 Surface tension (ST) and viscosity may also have the potential to influence patient comfort, either initially upon lens insertion or at the end of the day, through interactions between the solution, the lens, and the patient's tear film. The ST of human tears falls within the range of 40 to 46 mN/m,14,15 whereas the viscosity of human tears demonstrates marked shear-thinning when measured with a rheometer, which decreases from approximately 5.0 to 1.5 cP at 25°C for normal subjects.15,16

Soft contact lens solutions are composed of a wide range of components.17 All disinfecting solutions contain a preservative and a buffer and many contain other agents such as surfactants, chelating agents, and viscosity or lubrication-enhancing agents. The combination of these agents influences the physical properties of the final solution, which could potentially influence a patient's comfort, or discomfort, when using that particular solution.

Although information about the individual components of soft contact lens solutions is relatively easy to find, there is very little information about the physical properties of these solutions. The purpose of this study was to measure the pH, osmolality, ST, and viscosity of various soft contact lens solutions and to determine if these vary widely among commercially available products.

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METHODS

The reported compositions of the soft contact lens solutions investigated in this study are detailed in Table 1. The solutions included Opti-Free Express (OFX) and Opti-Free RepleniSH (OFR) (Alcon Laboratories, Fort Worth, TX), Complete Moisture Plus (COM) and UltraCare (UC) (Advanced Medical Optics, Santa Ana, CA), ReNu MultiPlus and Sensitive Eyes MPS (Bausch and Lomb, Rochester, NY), and AOSept (AO), Clear Care (CC) and SoloCare Aqua (CIBA Vision, Duluth, GA). A control saline solution [SoftWear Saline (SWS); CIBA Vision, Duluth, GA] was also included to determine how the solutions differed from a buffered saline. Two bottles of each solution were collected from different production lots, and the properties of each bottle of solution were measured in triplicate. As there were no significant differences between the different production lots for each solution, the data from both production lots were combined and analyzed (n = 6 measurements for each solution) and the mean value for these was reported.

Table 1
Table 1
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The pH, osmolality, ST, and viscosity of all solutions were measured. pH measurements were obtained using the VWR Model SB20 pH meter (Thermo Electron Corporation, Beverly, MA), osmolality measurements were acquired with the Advanced Model 3320 Osmometer (Advanced Instruments, Norwood, MA), and ST measurements were taken with the Cahn Dynamic Contact Angle Analyzer DCA-322 (CAHN Instruments, Madison, WI). All the aforementioned measurements were undertaken at room temperature (20.0°C). Viscosity measurements were performed with the ViscoLab 3000 Viscometer (Cambridge Viscosity, Medford, MA) at 20.0°C (room temperature) and 34.0°C,18 in an attempt to determine if viscosity of the products would be different at the temperature of the ocular surface.

The peroxide systems (AO, CC, and UC) were analyzed before and after neutralization. Neutralization was undertaken according to the manufacturer's instructions and the “neutralized” values were taken approximately 6-h after neutralization was initiated. As the non-neutralized peroxide systems will never (except inadvertently) be inserted into the eye, the values for these products are reported separately to all the other systems that are intended for contact with the eye.

All values quoted are means (±standard deviations). Statistical analysis was performed using SigmaStat 3.1 (Systat Software, San Jose, CA). Significant differences for each physical parameter were analyzed using a one-way analysis of variance across all solutions, with a post hoc Tukey test being conducted where differences were significant, to determine differences between individual solutions. In all cases p < 0.05 was considered to be statistically significant.

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RESULTS

pH

All the solutions, approved for contact with the eye, had pH values that fell within the reported tolerable pH range for the ocular surface,19 and many had pH values that were very close to neutral (Fig. 1). OFX (7.82) and OFR (7.88) demonstrated significantly higher (p < 0.001) pH values. Two of the neutralized peroxide systems (AO at 6.66 and CC at 6.76) were significantly lower (p < 0.001) than all other products. Interestingly, the third neutralized peroxide system (UC at 7.18) had a pH very similar to that of the preserved multipurpose solutions, but its pH was significantly different from the other two neutralized peroxide systems (p < 0.001).

Figure 1
Figure 1
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Osmolality

The osmolality values of the soft contact lens solutions are graphically shown in Fig. 2. Most osmolality values fell within a range of 275 to 310 mOsm/kg. OFX was significantly lower (p < 0.001) than other products at 225 mOsm/kg and neutralized UC was significantly higher (p < 0.001) at 329 mOsm/kg.

Figure 2
Figure 2
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The remaining products, with osmolalities in the range of 275 to 310 mOsm/kg, fell into two semidistinct groups. One group of solutions (SoloCare Aqua, COM and SWS) had significantly higher osmolalities (p < 0.001). The second group (OFR, ReNu MultiPlus, Sensitive Eyes MPS, and neutralized AO) had significantly lower osmolalities (p < 0.001). Neutralized CC (294 mOsm/kg) had similarities to solutions in both groups, but exhibited fewer statistical differences (p < 0.001) from solutions in the group of lower osmolality.

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Surface Tension

ST values are shown in Fig. 3. Although many of the solutions have similar components, the ST of the solutions evaluated exhibited marked differences. The one-bottle systems had relatively low (<40 mN/m) ST values, whereas the neutralized peroxide solutions exhibited higher STs (>40 mN/m). The solutions with the highest values were neutralized AO at 70.3 mN/m and SWS at 67.9 mN/m, which were not different from each other (p = NS), but were different from all other products (p < 0.001). Within the group of neutralized peroxide systems, CC and UC were similar to each other (p = NS), but had significantly lower STs than AO (p < 0.001).

Figure 3
Figure 3
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The solutions with the lowest ST values were OFR at 29.7 mN/m and OFX at 31.2 mN/m, which were significantly lower than all other solutions (p < 0.05), but were not different from each other (p = NS). COM (40.5 mN/m) had the highest ST of any of the one-bottle products.

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Viscosity

The viscosity values at 20 and 34°C are shown in Figs. 4 and 5, respectively.

Figure 4
Figure 4
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Figure 5
Figure 5
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At 20°C, the most viscous solutions were COM (3.02 cP) and neutralized UC (1.26 cP), which were significantly different from each other (p < 0.001) and from the remaining products (p < 0.001). Neutralized AO (0.96 cP) and CC (0.96 cP) were the least viscous solutions. They were less viscous than all other products (p < 0.05), except for SWS (0.97 cP), to which they were not dissimilar (p = NS). The remaining solutions showed minor differences in viscosities.

At 34°C all the viscosities were lower (p < 0.001), but the order of solutions from most viscous to least viscous remained the same. COM at 1.92 cP remained the most viscous solution, followed by neutralized UC (0.86 cP). These were different from each other (p < 0.001) and were also higher than all other products (p < 0.05). The least viscous solutions were AO and CC, which were both lower in viscosity than all other products (p < 0.001), with the exception of SWS and OFR (p = NS).

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Un-Neutralized Peroxide Solutions

The un-neutralized peroxide solutions had very different physical properties than the rest of the solutions examined, as shown in Table 2. The pH values of un-neutralized UC (3.33), AO (6.40), and CC (6.53) were significantly more acidic than any of the other solutions tested (p < 0.001). AO and CC were not different (p = NS), but were significantly higher in pH than UC (p < 0.001).

Table 2
Table 2
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These solutions also had significantly higher (p < 0.001) osmolality values than any other products (Table 2). As with pH, there was no difference between the osmolality of un-neutralized AO and CC (p = NS), but un-neutralized UC was significantly lower than both these solutions (p < 0.001).

After neutralization, the osmolality of all three peroxide systems dramatically reduced. The osmolality for the two disc-based systems (AO and CC) reduced from approximately 1320 to 292 mOsm/kg, when compared with the tablet-based system (UC), which reduced from 945 to 329 mOsm/kg.

The ST of un-neutralized AO (67.7 mN/m) and un-neutralized UC (66.0 mN/m) were similar (p = NS) to the ST of SWS (67.9 mN/m). However, un-neutralized CC (42.9 mN/m) had a much lower ST than the other un-neutralized peroxides (p < 0.001), and exhibited a similar ST to that of the one-bottle multipurpose solutions (Fig. 3).

At 20°C un-neutralized CC (0.99 cP) had a statistically higher (p < 0.05) viscosity than either AO or UC, which were not different from each other (p = NS). When the un-neutralized peroxide solutions were re-tested at 34°C, all three solutions reduced in viscosity (p < 0.001). The three solutions were statistically different from each other (p < 0.001), with CC having the highest and UC the lowest viscosity.

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DISCUSSION

This study was designed to measure the physical properties of many soft contact lens solutions, which are not readily available from manufacturers.

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pH

pH, which is a measure of a solution's acidity or alkalinity, is also an indication of it's buffering capacity. It is an important property of soft contact lens solutions and has been linked directly to patient comfort.5–7 Reportedly, the human eye is capable of tolerating pH values in the range of 6.2 to 9.0 at 0.2 M strength.19 Although some patients may be more sensitive to changes in pH than others,3 solutions with pH values above this range have a tendency to cause discomfort.5,6 Buffering agents used in soft contact lens solutions directly affect their pH, and it is quite possible that the type of buffer used in a particular solution could also affect subsequent patient comfort.

Within the group of solutions tested, small but statistically significant differences existed, but the clinical significance of these differences on patient comfort is impossible to predict. However, one clear difference was the acidic pH seen in the un-neutralized peroxide solutions (particularly UC), which did not fall within the ocular pH tolerability range. These solutions must be neutralized before lens insertion or, understandably, discomfort will occur. Patients who may be especially sensitive to solution pH could potentially report some mild discomfort when using those products with slightly higher (OFX or OFR) or slightly lower (AO or CC) than average pH, but this requires further clinical investigation.

Neutralization had a significant effect on the pH of all peroxide systems (Fig. 1, Table 2). AO and CC, both of which use a platinum disc method of neutralization, showed only a minor alteration in their pH after neutralization. In comparison, UC, which uses a tablet method of neutralization, demonstrated a large change in pH after neutralization.

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Osmolality

Osmolality is a quantitative measure of the amount of dissolved solutes in a solution. These dissolved solutes, such as biocides, surfactants, chelating agents, and buffers, exert an osmotic pressure that is recorded in milliosmoles per kilogram (mOsm/Kg).

On average, the osmolality of tears is 305 mOsm/kg.8,11–13 Our results indicate that the majority of soft contact lens solutions are hypo-osmotic compared with human tears,1 with our osmolality values falling within the 275 to 310 mOsm/kg range. OFX and neutralized UC were the only solutions that had significantly different osmolalities from the group of solutions indicated for direct ocular contact. The remaining lens care solutions fell into two groups, higher and lower osmolalities. Examination of the results indicated that the two products with significantly different osmolalities (OFX at 225 mOsm/kg and neutralized UC at 329 mOsm/kg) could potentially cause patient discomfort, as they are quite different than human tears. However, both products are clearly commercially successful and thus the impact of osmolality on ocular comfort requires further investigation. All other solutions had relatively small differences in osmolality between them and differed only slightly from that of tears. Previous studies have shown that a relationship exists between a higher tear film osmolality and increased ocular discomfort in patients with dry eyes,8,11–13 so it seems logical that a dry eye patient has the potential to be more sensitive to the osmolality of their contact lens solution than a patient with a healthy, robust tear film.

The large difference in pH and osmolality of the peroxide systems before neutralization, along with the chemical properties of peroxide itself, likely contribute to the burning, stinging, and epithelial cell damage experienced by patients who are unfortunate enough to insert these solutions directly onto the ocular surface, as shown in a previous study.20

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Surface Tension

ST describes the residual inward attraction of molecules at the surface of a solution, and is measured in milli-Newtons per meter (mN/m). Pure water has a ST of approximately 72 mN/m, whereas human tears have a ST in the range of 40 to 46 mN/m.14,15 The dissolved proteins, mucins, and other molecules present in tears reduce their ST relative to pure water. The presence of surfactants in a solution, such as those often found in contact lens care systems, also reduces ST.

Examination of Fig. 3 and Table 2 shows that AO (un-neutralized and neutralized), UC (un-neutralized), and SWS all had STs of approximately 67 mN/m, which were significantly higher than any other solution. The ST of these solutions resembles that of water, because none of them contain a surfactant (Table 1). All the solutions that contain one or more surfactants had significantly lower STs, with values that were closer to that of human tears.14,15 OFX and OFR had significantly lower STs from that of human tears, as well as from the rest of the solutions tested.

After neutralization, the ST of AO and CC were only marginally altered (although the change for AO was statistically significant at p = 0.004). However, the ST of UC decreased significantly. This is likely related to the components of the neutralizing tablet used in this system. AO and CC use a platinum-disc method of neutralization, resulting in only water and oxygen remaining after the chemical reaction and thus there are markedly lower pre- and post-neutralization ST measurements.

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Viscosity

Viscosity is a quantitative measure of the resistance of a fluid to flow when subjected to a shear stress of a particular shear rate, and is measured in centipoise (cP). The viscosity of the products tested was evaluated at two temperatures. As viscosity is influenced by temperature, we were curious to see what effect the temperature change at the ocular surface would have on the solutions tested. Figs. 4 and 5 show that all soft contact lens solutions had lower viscosities at the ocular surface temperature (34°C)18 than those at room temperature (20°C). The viscosity of water is 1.0 cP, whereas human tears demonstrate a non-Newtonian viscosity, such that the viscosity of the tear film demonstrates marked shear-thinning, with values of viscosity decreasing as shear-rate increases, from approximately 5.0 to 1.5 cP at 25°C for normal subjects.15,16

COM was by far the most viscous of any of the solutions tested, largely in part due to presence of propylene glycol and hydroxypropyl methylcellulose (HPMC) (Table 1), which are used as wetting agents in this product. At 25°C propylene glycol has a viscosity of 56 cP and HPMC has a range of viscosities from 4 to 78 000 cP. The impact of solution components on viscosity can also be shown by comparing neutralized UC to un-neutralized UC (1.26 vs. 0.96 cP at 20°C; p = 0.002). The neutralizing tablet for UC is coated with HPMC (Table 1) and, after its dissolution in the UC case, the HPMC is released and the solution viscosity clearly increases.

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CONCLUSIONS

This study demonstrates that soft contact lens solutions differ in certain physical properties. Clinically, these differences may have the potential to influence patient comfort initially and/or at the end of the day. However, whether or not the differences noted are clinically significant is unclear at this time and warrants further investigation.

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ACKNOWLEDGMENTS

The authors thank the Natural Science and Engineering Research Council of Canada (NSERC) for financial support through a Discovery Grant to LJ and Advanced Instruments for providing the Osmometer.

Lyndon Jones

Centre for Contact Lens Research

School of Optometry, University of Waterloo

200, University Avenue West

Waterloo, ON, N2L 3G1

Canada

e-mail: lwjones@uwaterloo.ca

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REFERENCES

1. Morgan PB, Efron N. A decade of contact lens prescribing trends in the United Kingdom (1996–2005). Cont Lens Anterior Eye 2006;29:59–68.

2. Morgan PB, Woods C, Jones D, Efron N, Tan KO, Gonzalez M, Pesinova A, Grein H-J, Runberg S, Tranoudis I, Chandrinos A, Fine P, Montani G, Marani E, Itoi M, Bendoriene J, van der Worp E, Helland M, Phillips G, Belousov V, Barr JT. International contact lens prescribing in 2006. Contact Lens Spectrum 2007;22:34–8.

3. Carney LG, Hill RM. Human tear pH. Diurnal variations. Arch Ophthalmol 1976;94:821–4.

4. Janoff L. The effective disinfection of soft contact lenses using hydrogen peroxide. Contacto 1979;23:37–40.

5. Tang I, Wong DM, Yee DJ, Harris MG. The pH of multi-purpose soft contact lens solutions. Optom Vis Sci 1996;73:746–9.

6. Harris MG, Torres J, Tracewell L. pH and H2O2 concentration of hydrogen peroxide disinfection systems. Am J Optom Physiol Opt 1988;65:527–35.

7. Harris MG, Higa CK, Lacey LL, Barnhart LA. The pH of aerosol saline solution. Optom Vis Sci 1990;67:84–8.

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15. Pandit JC, Nagyova B, Bron AJ, Tiffany JM. Physical properties of stimulated and unstimulated tears. Exp Eye Res 1999;68:247–53.

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Keywords:

contact lens solution; pH; osmolality; surface tension; viscosity

© 2008 American Academy of Optometry

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