Friction Measurements on Contact Lenses in Their Operating Environment
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Tribol Lett (2011) 44:387–397
DOI 10.1007/s11249-011-9856-9
METHODS PAPER
Friction Measurements on Contact Lenses in Their Operating
Environment
M. Roba • E. G. Duncan • G. A. Hill •
N. D. Spencer • S. G. P. Tosatti
Received: 14 April 2011 / Accepted: 5 September 2011 / Published online: 17 September 2011
Springer Science+Business Media, LLC 2011
Abstract An important issue concerning the use of soft Keywords Contact lens Microtribometer Mucin
contact lenses is comfort, which, among other factors, has Friction Hydrogel
been related to the level of friction between the anterior
side of the lens and the inner eyelid. Although several
studies have been carried out to investigate the frictional 1 Introduction
properties of contact lenses, these have not taken the
physiological environment of the eye into account. In use, The use of soft contact lenses is extremely widespread [1].
lenses are in contact with proteins present in tears, with However, issues concerning end-of-day comfort still exist
corneal cells and with the palpebral conjunctiva (clear [2]. Discomfort caused by contact lenses has been related
membrane on inner eyelid). The focus of this study was to to several factors such as dryness, protein adsorption,
establish a biologically relevant measurement protocol for physiological factors and friction occurring during the
the investigation of friction of contact lenses that would blinking process, especially between the anterior side of the
mimic the eye’s physiological environment. By optimizing lens and the inner eyelid [3–8]. Clinical tests have been
parameters such as the composition of the friction counter performed to understand and improve comfort, and have
surface, the lubricant solution, the normal load and the centered on the trial of different lens materials and oph-
velocity, an ideal protocol and setup for microtribological thalmic solutions [9–13]. The dryness issue has been
testing could be established and used to perform a com- investigated by studies of the incorporation of wetting
parative study of various commercially available soft agents into contact lenses [8] and on the effect of water
contact lenses. content of the lens [14]. A few studies have been carried
out to investigate friction of the lens [4–7]. Blinking is the
primary physiological contributor to the forces exerted on
contact lenses and their consequent motion. Typical values
M. Roba E. G. Duncan G. A. Hill S. G. P. Tosatti (&) of contact pressure are in the range of 3–5 kPa, sliding
SuSoS AG, Lagerstrasse 14, CH-8006 Duebendorf, Switzerland speed being around 12 cm/s [15, 16]. The friction studies
e-mail: samuele.tosatti@susos.com
carried out to date have been based on these reported
G. A. Hill N. D. Spencer values, despite the practical difficulties in achieving them
Department of Materials, Laboratory for Surface Science and because of resolution limits of the microtribometers used.
Technology, ETH Zurich, Wolfgang-Pauli-Strasse 10, However, the physiological environment has never been
CH-8093 Zurich, Switzerland
taken into account. Rennie et al. [4] investigated the fric-
G. A. Hill tion between glass and a contact lens by positioning the
VISTAKON, A Division of Johnson & Johnson Vision Care Inc, lens taken from borate buffer on a special holder, without
Jacksonville, FL, USA additional lubricant being added. Nairn and Jiang [5]
looked at friction, for both the anterior and posterior side of
G. A. Hill
GHBiomaterials, LLC 1918 Hickory Lane, Atlantic Beach, contact lenses, against polymethylmethacrylate (pMMA)
FL 32233, USA and polyhydroxyethylmethacrylate (pHEMA) counter
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388 Tribol Lett (2011) 44:387–397
surfaces, using ophthalmic solutions as lubricants. Dunn
et al. [7] carried out studies on friction between the anterior
side of a contact lens and corneal epithelial cells, while
Dong and Haugstad [17] looked at friction and adhesion of
polyvinylalcohol (PVA) contact lenses by scanning probe
microscopy (SPM). Lydon et al. [18] studied the coefficient
of friction (CoF) for contact lens versus glass and poly-
ethylene (PE) and, lastly, Ngai et al. [6] investigated fric-
tion for a contact lens versus glass system in the presence
of saline solution. Interestingly, they also considered the
possible effect of protein adsorption from the tear film by
incubating some of their lenses in a lysozyme and albumin
solution. Apart from the work performed by Ngai et al., Fig. 1 Silicone-rubber cover (left hand side) and PMMA ring (right
these studies generally did not take physiological aspects, hand side). Silicone cover was cast to perfectly match the geometry of
such as the eyelid counter surface and the presence of a tear the Teflon chamber, rounded holder and contact lens system. A hole
was made on the top part of the silicone cover to allow the contact
film, into account. Under the physiological conditions on lens anterior surface to be exhibited
the eye surface, the contact lens is in contact with proteins
present in tears and the anterior side of the palpebral
conjunctiva (inner eyelid) [7].
This study has shown that measuring contact lens fric-
tional properties without taking the physiological envi-
ronment into account can lead to misleading results. It is,
therefore, important to perform such measurements under
biologically relevant conditions. Friction tests on a glass
versus contact lens system were performed by means of a
microtribometer. The measurement protocol was optimized
by varying the functionalization of the glass counter sur-
face, with the aim of mimicking the on-eye environment.
A comparative study was carried out by measuring the
coefficient of friction of several commercially available
contact lenses using the newly developed measurement
protocol.
2 Materials and Methods
2.1 Instrument and Setup
Friction tests were performed with a microtribometer
(Basalt Must, Tetra, Germany). Cantilevers (Tetra,
Germany) with different ranges of spring stiffness (N/m)
were used: kn = 23, kt = 23, ±10% and kn = 15, kt =
Fig. 2 Experimental setup for tribological tests: The contact lens is
15, ±10% (kn is the normal force spring constant and kt is placed on a rounded holder inside a Teflon chamber. The green
the tangential force spring constant). The contact lens was silicone-rubber cover and PMMA ring that hold the lens in position
placed inside a Teflon chamber on top of a sand-blasted are screwed into the Teflon chamber
rounded plastic holder (cyclo olefin polymer, Johnson &
Johnson Vision Care Inc., USA), matching the internal lens was facing upward. The counter surface consisted of a
radius of curvature of the lens, and was held in position by functionalized 5-mm diameter glass disk (cover glass,
a cast silicone-rubber cover (polyvinylsiloxane, Provil Thermo Scientific, Germany). Functionalization protocols
Novo, Germany) and plastic ring (poly(methyl methacry- are described in the following. A 6-mm long glass rod was
late), PMMA) (Figs. 1 and 2). Silicone cover and PMMA glued onto the tip of the tribometer cantilever. In turn, the
ring were screwed to the Teflon chamber by two screws functionalized glass disk was glued to the glass rod, being
placed at 180 to one another. The anterior surface of the the latter as centered as possible (Fig. 3). Gluing was
123
Tribol Lett (2011) 44:387–397 389
covered with lubricant solution during the friction test.
Figure 5 shows an example of contact lens and counter
surface mounted in the microtribometer. Contact area and
pressure between the flat glass counter surface and the soft
contact lens (Table 1) were estimated using the Hertzian
contact model as described by Chaudhri and Yoffe [19].
They related the radius of the contact area ah to the
mechanical properties of the materials by
a3h ¼ RP½ð1 m21 Þ=ðE1 Þ þ ð1 m22 Þ=ðE2 Þ ð1Þ
R being the radius of the contact lens, P the load, mi
Poisson’s ratio and Ei Youngs’s modulus.
French reported the Young’s modulus for a series of
Fig. 3 Example of cantilever modified for friction experiment: The contact lenses materials [20]. For the purposes of this
glass rod is glued to the cantilever tip; second, a glass disk is glued on
publication, values of contact area and pressure were
the other end of the glass rod
estimated for the contact lens materials with the highest
and lowest moduli. These materials are:
ACUVUE 0.3 MPa
NIGHT & DAY 1.5 MPa
ACUVUE brand contact lens is a cast-molded poly-
HEMA contact lens manufactured by VISTAKON, a
division of Johnson & Johnson Vision Care Inc., Jack-
sonville, FL, USA. The water content is around 58%. Focus
Night & Day is a cast-molded silicone hydrogel contact
lens manufactured by CIBA VISION. It has a water
content of about 24% and has a plasma-treated surface.
Poisson’s ratio has not been reported for these materials.
A value of 0.3 was used in the calculations. The values for
Fig. 4 Measurement setup: The functionalized glass disk, attached to
the glass counter surface are not important for this analysis.
the cantilever through a glass rod, is gently lowered until contact with
the lens occurs. Arrows in the drawing indicate normal force and The Young’s modulus is much greater than that of the
lateral force directions contact lenses; therefore, 1/E becomes negligible in Eq. 1.
performed using a cyanoacrylate-based glue (UHU GmbH
& Co. KG, Germany). The cantilever was then mounted in 2.1.1 Sample Preparation
the tribometer.
Just before the test, the cantilever was slowly lowered so Contact lenses were removed from their packaging
that the glass disk approaches the center of the contact lens immediately before the test and installed on the sample
surface (determined by eye) (Fig. 4). The contact lens was holder, which had previously been cleaned with water and
Table 1 Contact area and contact pressure estimated values for ACUVUE and NIGHT & DAY pressing against a flat glass surface
Normal force (mN) Contact area (mm2) Contact pressure (kPa)
ACUVUE NIGHT & DAY ACUVUE NIGHT & DAY
2.50E - 01 0.093 0.032 2.7 7.8
5.00E - 01 0.15 0.057 3.4 9.5
1.00E ? 00 0.24 0.081 4.2 12.4
2.00E ? 00 0.37 0.13 5.4 15.6
3.00E ? 00 0.49 0.17 6.1 17.9
4.00E ? 00 0.59 0.20 6.7 19.7
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390 Tribol Lett (2011) 44:387–397
Table 2 Advancing contact angle measurements for treated glass
surfaces and ellipsometry thickness measurements for SiO2 model
surfaces
Treatment Contact angle () Adlayer thickness (nm)
Plasma \10 n/a (substrate base)
Hexamethyldisilazane [75 0.25 (±0.04)
Mucin \45 1.2 (±0.1)
Adlayer thickness measurements were performed on two separate
occasions with a total of six measurements
(from bovine submaxillary glands type I–S, Sigma-Aldrich,
Germany) in HEPES 1 (10-mM N-2-hydroxyethylpiperazine-
TM
N-2 ethanesulfonic acid, pH 7.4, BDH , UK) for 30 min and
rinsed with ultra-pure water to remove any non-adsorbed
species. Additionally, hexamethyldisilazane and mucin layer
thickness in the dry state were evaluated by ellipsometry (J.A.
Woollam Co., Inc., Ellipsometry Solutions, USA) on oxidized
silicon wafer model surfaces. The surface modification pro-
tocol applied to the silicon wafers was the same as for glass
model surfaces, with additional initial pre-cleaning with tol-
Fig. 5 Contact lens and cantilever mounted in the microtribometer.
uene and isopropanol. Adlayer thickness measurements were
The cantilever is slowly lowered until the glass disk is in contact with conducted under ambient conditions at an angle of incidence
the contact lens of 70 with respect to the surface normal, averaging 50 mea-
surements at each point. The spectral range was 370–995 nm.
detergent (1:1) (hydrochloric acid 300 mmol/L, detergent Data were fitted with the WVASE32 analysis software using a
1%, Cobas Integra, Roche, Switzerland) and wetted with three-layer model (Si/SiO2/Cauchy), where Si was assumed to
lubricant solution. Excess storage solution (from lens be constant for all wafers (1 mm). The SiO2 layer was fitted
packet) trapped between the lens and the holder was with the SiO2 model before hydrophobization with silanes and
avoided by gently tapping the edge of the lens on a clean mucin adsorption; the silane and mucin layers were fitted
surface. The contact lens was immediately covered with using the Cauchy (organic) layer model. The dry film thick-
lubricant solution to avoid drying. ness of the silane layer and of the mucin layer was assumed to
have a refractive index of 1.45. Characterization results are
2.1.2 Counter Surface Preparation shown in Table 2.
To mimic the surface of the eyelid, glass disks with dif-
Before fixation onto the cantilevers, glass disks were oxygen ferent functionalizations were evaluated: oxygen plasma
plasma cleaned for 2 min (Diener Electronic, Germany) and cleaning (hydrophilic), silanization (hydrophobic) and pro-
hydrophobized with hexamethyldisilazane (Alfa Aesar, tein or polymer attachment. Proteins used were fibrinogen
Germany) from the gas phase for 30 min in a vacuum desic- (from bovine plasma, fraction I type I–S, Sigma-Aldrich,
cator. To assess successful hydrophobization, the dynamic Germany) and lysozyme (AppliChem, Germany), whereas
water contact angle ([75) was measured on hydrophob- poly(L-lysine)-graft-poly(ethylene glycol) (PLL(20)-g[3.5]-
ized glass model surfaces. Contact angle (advancing PEG(2), SuSoS AG, Switzerland) was used for polymer
(wCAadvancing) measurements were performed with a Krüss coating. Plasma cleaning and silanization were carried out
contact angle–measuring system (G2/G40 2.05-D, Krüss as described above. Fibrinogen and lysozyme coatings were
GmbH, Germany) with a drop speed of 15 lL/min. A movie achieved by immersing plasma-cleaned glass disks, already
with 100 images was recorded for the advancing contact angle glued onto the cantilevers, in 1.5 mg/mL fibrinogen in
and the analysis was carried out by means of the tangent phosphate-buffered saline (PBS) or 1 mg/mL lysozyme in
method 2 routine of the Krüss Drop-Shape Analysis HEPES 1 for 30 min. Finally, PLL-g-PEG functionalization
program (DSA version 1.80.0.2 for Windows 9x/NT4/2000, was performed by immersing plasma-cleaned cantilever-
1997-2002 KRUESS). Hydrophobized glass disks were mounted glass disks into 0.1 mg/mL PLL-g-PEG solution in
mounted on the cantilever as previously described. Immedi- HEPES 1 for 30 min. After incubation, the disks were rinsed
ately before the friction tests, the glass disk mounted on the with ultra-pure water and dried with nitrogen. Results are
cantilever was incubated in a 1 mg/mL mucin solution discussed in Sect. 3.
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Tribol Lett (2011) 44:387–397 391
2.1.3 Lubricant Solution 3 Results
A 0.9% sodium chloride solution with a borate buffer 3.1 Optimization Measurements
(pH * 7.4) was used as a control (packing solution [PS]
supplied by Johnson & Johnson Vision Care Inc. [JJVCI]). 3.1.1 Effect of Number of Sliding Cycles
As a lubricant solution, a tear-mimicking solution con-
taining PS plus serum (Precinorm U, Roche, Germany) Preliminary tests run on 1•DAY ACUVUE and 1•DAY
TM
(20:1) plus lysozyme (5 mg/mL) was used to perform ACUVUE MOIST brand contact lenses to investigate
comparative friction tests described in Sect. 3. the influence of ageing on friction revealed that CoF at 100
cycles at a normal load of 2 mN (maximum ageing) is
2.1.4 Experimental Conditions equal to or not significantly different than CoF at 0 cycles
(no ageing) (Fig. 6), indicating that there is no change in
All experiments for the optimization step were carried out the coefficient of friction for either lens type within the
TM
using 1•DAY ACUVUE and 1•DAY ACUVUE MOIST limits of this experimental protocol. 1•DAY ACUVUE
TM
brand contact lenses. In particular, the influence of sliding MOIST contact lenses exhibited significantly lower CoF
cycles and velocity was determined using tear-mimicking than 1•DAY ACUVUE contact lenses. Significant dif-
solution as a lubricant and mucin-coated silanized glass disks ferences in the results were evaluated by means of statis-
as counter surfaces. The effect of glass disk functionalization tical analysis (p value \0.05).
to mimic the eyelid was evaluated using packing solution as
lubricant. The comparative study was performed when a
3.1.2 Effect of Sliding Velocity
model to mimic the natural eye environment was established:
All contact lenses were tested in tear-mimicking solution
CoF was shown to significantly increase (p value \0.05)
against mucin-coated silanized glass disks.
with sliding velocity for both 1•DAY ACUVUE and
TM
1•DAY ACUVUE MOIST brand contact lenses
2.1.5 Measurement Program
(Fig. 7). Above 1 mm/s, distinguishing between the two
lenses investigated was difficult because of high and
The measurement program used for the optimization step
overlapping standard deviations.
and the comparative friction tests involved applying three
sets of seven normal forces varying from 0.25 to 5 mN.
3.1.3 Choice of Counter Surface
Two cycles, backward and forward, were recorded for each
target normal force value. Only data points obtained from
The CoF was measured for different counter surfaces in
the second cycle were recorded and subsequently analyzed.
packing solution. Results show that the lowest CoF was
Ageing was simulated by performing 50 additional cycles
obtained for PLL-g-PEG-coated glass disks, whereas
at 2-mN normal force between each set of seven normal
higher CoF values were obtained for hydrophilic and
forces. In detail, the measurement protocol consisted of the
lysozyme-coated glass disks (Fig. 8). In the case of
following steps: measurement of coefficient of friction
hydrophilic glass disks, CoF for 1•DAY ACUVUE
(CoF) with seven normal forces, 50 ageing cycles, mea- TM
MOIST was significantly higher than for 1•DAY
surement of CoF with seven normal forces, 50 ageing
ACUVUE. In all other cases where there is a significant
cycles to give a total of 100 ageing cycles and again a final TM
difference, 1•DAY ACUVUE MOIST contact lenses
CoF determination. The measured stroke length was 1 mm
exhibited lower CoF than 1•DAY ACUVUE. No post
and the sliding speed was 0.1 mm/s. During optimization
measurement testing was performed on the mucin-coated
of the measurement protocol, 1 and 10 mm/s sliding speeds
counter surface. As the coefficient of friction did not
were also investigated. For data processing, lateral and
change between the first ramp (0 cycles) and the final ramp
experimentally determined normal force values were cal-
(after 100 cycles), the lens surface and counter surface
culated by averaging 20 data points at around 0.5-mm
were deemed unchanged during the experiment.
distance, and then plotted in a graph. Trace and retrace
curves obtained in this way were averaged and a coefficient
of friction was determined from the slope. 3.2 Comparative Study
2.1.6 Contact Lenses Various commercially available daily disposable and
reusable contact lenses were tested using the optimized
The lenses used in this study are listed in Table 3. The protocol, tear-mimicking solution as lubricant and mucin-
lenses were supplied by JJVCI. coated silanized glass as counter surface. Friction
123392 Tribol Lett (2011) 44:387–397
Table 3 Lenses used
Daily disposable lenses
Name of contact lens Manufacture Contains PVP? Silicone hydrogel? Surface treatment?
TM
CLARITI 1 day Sauflon No* Yes No
1•DAY ACUVUE MOIST Johnson and Johnson Yes No No
Vision Care, Inc.
TM
1•DAY ACUVUE TruEye (narafilcon B) Johnson and Johnson Yes Yes No
Vision Care, Inc.
TM
1•DAY ACUVUE TruEye (narafilcon A) Johnson and Johnson Yes Yes No
Vision Care, Inc.
1•DAY ACUVUE Johnson and Johnson No No No
Vision Care, Inc.
Focus DAILIES CibaVision
TM
All Day Comfort No No No
Proclear 1 day CooperCooperVision No No No
TM
ClearSight 1 Day CooperVision No No No
DAILIES AquaComfort Plus CibaVision No No No
Reuseable lenses
AVAIRA CooperVision No* Yes No
ACUVUE OASYS Johnson and Johnson Yes Yes No
Vision Care, Inc.
TM
CLARITI Sauflon No* Yes No
ACUVUE ADVANCE PLUS Johnson and Johnson Yes Yes No
Vision Care, Inc.
ACUVUE ADVANCE Johnson and Johnson Yes Yes No
Vision Care, Inc.
Biofinity CooperVision No* Yes No
ACUVUE Johnson and Johnson No No No
Vision Care, Inc.
[AIR OPTIX] NIGHT & DAY AQUA CibaVision No Yes Yes (methane air plasma)
PremiO Menicon No No Yes
AIR OPTIX AQUA CibaVision No Yes Yes (methane air plasma)
TM
AIR OPTIX (formerly O2 OPTIX ) CibaVision No Yes Yes (methane air plasma)
NIGHT & DAY CibaVision No Yes Yes (methane air plasma)
PureVision BAUSCH?LOMB No Yes Yes (oxygen plasma)
TM
Optima 38 BAUSCH?LOMB No No No
SOFLENS BAUSCH?LOMB No No No
* Indicates no PVP is reported in formulation but maybe formed in situ
coefficient was evaluated before (0 cycles) and after ageing measure the CoF of contact lenses in the most biologically
(50 and 100 cycles) (Tables 4 and 5; Figs. 9 and 10). relevant conditions.
In the physiological environment, contact lenses are in
contact with proteins present in tears, with corneal cells (on
4 Discussion the posterior side) and with the palpebral conjunctiva (on
the anterior side). Most studies carried out so far on the
The goal in the development of a model is to be able to use tribology of contact lenses have not taken such physio-
the in vitro data to predict in vivo performance. To cor- logical conditions into account [4–6, 18]. The coefficient of
relate clinical contact lens performance with the model friction is a property of the sliding pair; therefore, the
results and to evaluate potential sources of comfort-driven determination of the coefficient of friction using different
performance, possible lens discontinuation and adverse counter surfaces would be expected to produce different
effects to the coefficient of friction, it is desirable to results. When comparing the results found in this study
123Tribol Lett (2011) 44:387–397 393
Fig. 8 Mimicking the eyelid: CoF at 100 cycles for various
Fig. 6 Effect of number of sliding cycles (ageing) on friction using functionalized glass counter surfaces measured in packing solution.
TM
1•DAY ACUVUE and 1•DAY ACUVUE MOIST brand contact Sliding speed was 0.1 mm/s and normal forces ranged from 0.25 to
lenses versus hydrophobized mucin-coated glass disks. Tear-mim- 5 mN. Error bars are standard deviation of three repeat experiments
icking solution was used as lubricant
0.55, respectively. These variations show the need for a
standardized method for the measurement of the coefficient
of friction for contact lenses. Protein adsorption on contact
lenses was investigated by Garrett and Milthorpe [21] and
by Ngai et al. [6], who also performed friction tests. The
major finding was that CoF decreased at an early stage of
protein deposition, but it was hypothesized to increase in
time because of protein denaturation. Protein deposition on
contact lenses was also found to be related to comfort,
vision and increased inflammatory responses, as reviewed
by Jones [22]. In this study, the physiology of the eye was
taken into account by establishing a measuring protocol
that mimics the natural environment. For this optimization
Fig. 7 Graph showing the effect of sliding velocity on friction for step, not only parameters such as ageing and sliding speed
TM
1•DAY ACUVUE and 1•DAY ACUVUE MOIST brand contact were considered but also the role of eyelid and tears. The
lenses versus hydrophobized mucin-coated glass disks in tear- eyelid surface was mimicked by the counter surface, which
mimicking solution. Error bars are standard deviation of three repeat
experiments was required to meet several conditions. It had to be hard
and flat so that any lens deformation would be accommo-
with those from previous studies, the following observa- dated on contact. This would yield a constant contact area
tions can be made. Rennie et al. [4] evaluated Etafilcon A and eliminate the modeling required when using a spherical
using an untreated spherical glass counter surface at normal counter surface [4]. For this study, a hard–soft contact pair
forces of 3–20 mN. They report that the coefficient of was chosen rather than the soft–soft contact pair found in
friction obeys a sliding speed–dependent power law. Using the ocular environment. The hard–soft pair was chosen
their data, a coefficient of friction of about 0.15 would be because the goal of this study was to provide a method to
predicted for Etafilcon A. Our results, on the other hand, conveniently compare materials rather than to mimic the
show a coefficient of friction of 0.02–0.09 that was eye’s mechanics exactly. It was felt that the hard glass
invariant with normal force. Nairn and Jiang [5] evaluated surface would produce more reproducible results and has
the coefficient of friction for SeeQuence (Polymacon) less experimental difficulty than a soft counter surface.
brand contact lenses using a pin-on-disk tribometer. They Additionally, it was required not to adhere components
report coefficients of friction of 0.05–0.3 in saline. Our from the contact lens and to provide a surface representa-
TM
results for Optima 38, a Polymacon lens produced by the tive on the ocular environment. The various glass func-
same manufacture, were 0.55. Finally, Ngai et al. [6] tionalization procedures that were investigated were
reported the coefficients of friction for Lotrafilcon A deliberately chosen with the aim of mimicking the eyelid.
and Polymacon A to be 0.27 (calculated from normal and PLL-g-PEG, well known for its lubricating properties in an
tangential forces in their study). Our results were 0.38 and aqueous environment [23], gave rise to the lowest CoF,
123394 Tribol Lett (2011) 44:387–397
Table 4 Coefficients of friction measured on daily disposable contact lenses at 0.1 mm/s, arranged in ascending order of CoF at 100 cycles
Name of contact lens 0 Cycles Standard 50 Cycles Standard 100 Cycles Standard
deviation (±) deviation (±) deviation (±)
TM
CLARITI 1 day 0.017 0.001 0.014 0.002 0.017 0.004
TM
1•DAY ACUVUE MOIST 0.022 0.010 0.019 0.006 0.024 0.003
TM
1•DAY ACUVUE TruEye (narafilcon B) 0.032 0.013 0.030 0.006 0.034 0.009
TM
1•DAY ACUVUE TruEye (narafilcon A) 0.031 0.028 0.035 0.021 0.037 0.019
1•DAY ACUVUE 0.046 0.023 0.024 0.004 0.047 0.029
TM
Focus DAILIES All day comfort 0.122 0.021 0.113 0.016 0.091 0.009
Proclear 1 day 0.090 0.033 0.081 0.039 0.125 0.073
TM
ClearSight 1 Day 0.078 0.018 0.141 0.036 0.181 0.037
DAILIES AquaComfort Plus 0.344 0.000 0.474 0.012 0.424 0.051
Table 5 Coefficients of friction measured on reusable contact lenses at 0.1 mm/s, arranged in ascending order of CoF at 100 cycles
Name of contact lens 0 Cycles Standard 50 Cycles Standard 100 Cycles Standard
deviation (±) deviation (±) deviation (±)
AVAIRA 0.011 0.002 0.018 0.003 0.018 0.001
TM
ACUVUE OASYS 0.016 0.014 0.024 0.018 0.018 0.015
TM
CLARITI 0.034 0.014 0.023 0.005 0.022 0.009
TM
ACUVUE ADVANCE PLUS 0.022 0.005 0.021 0.001 0.024 0.004
TM
ACUVUE ADVANCE 0.029 0.002 0.028 0.002 0.042 0.010
Biofinity 0.033 0.009 0.057 0.004 0.050 0.002
ACUVUE 0.093 0.023 0.069 0.010 0.090 0.010
[AIR OPTIX] NIGHT & DAY AQUA 0.108 0.051 0.110 0.093 0.166 0.058
PremiO 0.177 0.010 0.207 0.023 0.176 0.012
AIR OPTIX AQUA 0.178 0.061 0.187 0.045 0.222 0.073
TM
AIR OPTIX (formerly O2 OPTIX ) 0.343 0.038 0.361 0.048 0.292 0.031
NIGHT & DAY 0.391 0.081 0.394 0.040 0.382 0.027
PureVision 0.415 0.037 0.443 0.041 0.423 0.032
TM
Optima 38 0.203 0.045 0.587 0.010 0.551 0.002
SOFLENS 0.562 0.063 0.542 0.033 0.513 0.017
whereas hydrophilic glass led to an unusual behavior be observed between 1•DAY ACUVUE and 1•DAY
TM
(Fig. 8). Against a hydrophilic counter surface, 1•DAY ACUVUE MOIST brand, most likely masked by the
TM
ACUVUE MOIST brand contact lenses exhibited a excellent lubricating properties of PEG. For these reasons,
higher friction coefficient than 1•DAY ACUVUE despite the first two functionalization protocols were discarded. All
the presence of polyvinylpyrrolidone (PVP). This is other protocols gave rise to the opposite trend, as expected.
inconsistent with the tactile sensation felt when rubbing Fibrinogen was chosen to have a more biologically ori-
TM
1•DAY ACUVUE MOIST lens surface between the ented counter surface and because it readily adsorbs on
fingers. 1•DAY ACUVUE brand does not feel as slip- glass. The use of lysozyme went one step further, as it is
pery. Belyakova et al. [24] reported a strong attraction the major component of tears. Both proteins lead to a
between PVP and silicon dioxide. The higher coefficient of clearly distinguishable behavior of 1•DAY ACUVUE
TM TM
friction observed for 1•DAY ACUVUE MOIST brand and 1•DAY ACUVUE MOIST brand, although friction
lens is attributed to an attractive interaction between PVP coefficients were relatively high. Mucin, a natural lubricant
TM
in the 1•DAY ACUVUE MOIST brand lens matrix and additive, became the surface coating of choice for per-
hydrophilic glass surface. Therefore, the high tangential forming friction tests on glass. It was chosen as it is a
force results from adsorption of the PVP on the counter glycoprotein found in the mucous membrane that consti-
surface rather than from an intrinsically high coefficient of tutes the palpebral conjunctiva, and is therefore in contact
TM
friction for 1•DAY ACUVUE MOIST brand. In the with lenses in their operating environment. In particular,
case of PLL-g-PEG, no difference in terms of friction could mucin from bovine submaxillary glands was used as it is a
123Tribol Lett (2011) 44:387–397 395
Fig. 9 Comparative study: CoF
for various commercially
available daily disposable
contact lenses tested using the
optimized protocol, which is
tear-mimicking solution as
lubricant and mucin-coated
silanized glass as counter
surface. Contact lenses are
represented in ascending (mean
of 0, 50 and 100 cycles) friction
coefficient order, according to
Table 4. Error bars are standard
deviation of three repeat
experiments
Fig. 10 Comparative study:
CoF for various commercially
available reusable contact lenses
tested using the optimized
protocol, which is tear-
mimicking solution as lubricant
and mucin-coated silanized
glass as counter surface. Contact
lenses are represented in
ascending friction coefficient
order, according to Table 5.
Error bars are standard
deviation of three repeat
experiments
readily available mucin that functions at pH 7, as do eye drops for severely diseased patients. In their studies,
mucins in the eye. Hydrophobization of glass disks before they diluted the serum 1:20 with saline, and did not add any
incubation in mucin solution was performed to enhance of the tear-specific proteins. Hill et al. [27] described a
mucin adsorption. The major finding with these experi- synthetic tear fluid of 5% blood plasma supplemented with
ments is that the CoF can significantly vary according to lysozyme at 4.5 g/L and lactoferrin at 1.7 g/L. The tear-
the measuring protocol. mimicking fluid used in this study was modified increasing
Another important aspect of the physiology of the eye is the lysozyme to 5 g/L and eliminating the lactoferrin.
the presence of tears, which play a fundamental role in Lipids are an important part of the dacruon [28] and
lubrication. For the evaluation of different contact lens have been shown to adsorb onto contact lenses [29]. Cher
materials, the use of a lubrication solution that is closely [28] has proposed dacruon as a new description of the tear
related to human tears is preferred. Human tears contain a film. The historical model describes the tear film as a lipid
protein array that is similar to serum but is lacking lacto- layer, an aqueous layer and a mucus layer. The dacruon
ferrin, lysozyme and tear-specific prealbumin [25]. Yoon model describes it as a continuous concentration gradient
et al. [26] reported the use of diluted antilogous serum as of mucin from the ocular surface to the lipid layer of the
123396 Tribol Lett (2011) 44:387–397
tear film. Lipids are reported to be floating on the surface of to produce a PVP analog in situ. This frictional behavior
the dacruon [28], and therefore would not be involved in was expected because of the lubricating properties of PVP
the lubrication of the lid on the contact lens unless a break or its analogs and confirmed the validity of our measure-
in the tear film would occur, allowing lipids to adsorb on ment protocol, which proved to be successful in distin-
the surface of the lens. This eventuality can take place as a guishing between contact lens frictional properties.
consequence of eye dryness, irritation and on insertion of
the contact lens. Our model does not contemplate such
situations, but rather focuses on mimicking the eyelid– 5 Conclusion
contact lens system in standard conditions, where the tear
film is intact and lipids are not adsorbing on the contact Friction tests were run on commercially available contact
lens surface. Moreover, the inclusion of lipids into the tear- lenses with a microtribometer. The measurement protocol
mimicking fluid results in a heterogeneous lubricating was optimized according to sliding speed, counter surface
fluid, which could affect the reproducibility of measure- and lubricant–solution composition, to achieve a set-up that
ments. For these reasons, lipids were not included in the mimics the physiological environment. The best combi-
described model. nation was found to consist of 0.1 mm/s sliding speed,
For the determination of our eye-mimicking system, mucin-coated glass as a counter surface and a lubricant
contact pressure and sliding speed were also taken into based on packing solution containing lysozyme and serum.
account. Typical in vivo pressure and speed values are Measuring contact lens frictional properties without taking
reported to be around 3–5 kPa and 1.2 mm/s, respectively. physiological conditions into account was shown to lead to
We chose normal loads that allow our setup to be in the different and potentially misleading results, demonstrating
contact pressure range described in literature. Contact the importance of establishing a broadly valid biologically
pressures for 1•DAY ACUVUE were estimated by using relevant measurement protocol.
the Hertzian contact model and were found to be in the
range of 2.6–6.5 kPa for the normal loads used in this study. Acknowledgments This study was supported by VISTAKON, a
division of Johnson & Johnson Vision Care Inc., Jacksonville, FL, USA.
The purpose of this model is to evaluate the coefficient
of friction of contact lenses against a realistic counter
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