Effects of TiO2 Synthesis Method and Loading on the Performance of Hydrogel Contact Lenses
(Jin-Wook Kim)
1
(A-Young Sung)
1,*
-
대구가톨릭대학교 안경광학과
(Dept. of Optometry and Vision Science, Daegu Catholic University, Daegu 38430, Republic
of Korea)
Copyright © The Korean Institute of Metals and Materials
Keywords
Hydrogel contact lenses, TiO2 nanoparticles, Hydrolysis, Precipitation method, Sol-gel method, Mechanical properties
1. INTRODUCTION
Contact lenses are medical devices that provide vision correction and everyday convenience,
and they are exposed to tears and the external environment for extended periods of
time[1,2]. Materials used for contact lenses therefore must not compromise oxygen transmissibility
while mut simultaneously ensuring high transparency and UV-B blocking performance[1,3,4]. It is also essential to reduce protein and bacterial adhesion and to maintain surface
wettability[1,11].
Hydrogels are widely used polymer materials that meet these requirements. Among them,
2-hydroxyethyl methacrylate (HEMA)-based structures are widely adopted owing to properties
such as processability and ease of tuning[2]. Although hydrogels offer excellent wearing comfort due to their high water content,
their mechanical strength is limited[5]. In addition, it remains difficult to achieve a substantial increase in UV-B blocking
while maintaining transparency in the visible region[6,11].
Organic UV blockers can provide high initial blocking, but issues such as leaching,
discoloration, and photodegradation remain. A material design that can simultaneously
provide mechanical reinforcement, UV blocking, and surface wettability therefore is
required. This study addresses these limitations by evaluating the effect of adding
TiO2 nanoparticles. TiO2 is chemically stable, has a high refractive index, and provides strong blocking in
the UV-B region. Particle size control in a range of tens of nanometers is effective
for maintaining transparency by suppressing scattering in the visible region[7]. Interactions with the hydrogel network through surface hydroxyl groups are expected
to improve dispersion stability. Reports also suggest that TiO2 may inhibit initial bacterial adhesion under certain conditions[8]. However, these effects are strongly influenced by synthesis conditions, including
particle size and morphology, crystal phase, and surface defects. TiO2 synthesis methods can be classified into hydrolysis precipitation and sol-gel routes[9]. Hydrolysis precipitation is simple and rapid, but fast particle growth and easy
agglomeration impede precise control of the size distribution and surface chemistry[10]. The sol-gel method provides control over the reaction rate and is favorable for
obtaining small particles with narrow size distributions[12], but it is sensitive to reaction conditions and requires additional post-treatment.
When incorporated into the same HEMA hydrogel, nanoparticles from these two routes
may differently affect dispersion, compatibility, and optical and mechanical properties.
The objectives of this study were to determine how the nanoparticle synthesis route
influences dispersion and compatibility in hydrogel-nanoparticle composites and to
identify the amounts of nanoparticles that improve optical properties, surface wettability,
and mechanical strength within a low loading range. In addition, by interpreting correlations
among micro-surface morphology observed by SEM and AFM and the contact angle, transmittance,
and tensile strength, we sought to explain the observed property improvements.
For the experiments TiO2 prepared by both synthesis routes was added stepwise to an identical HEMA hydrogel
composition. We evaluated UV-B transmittance, refractive index and transparency, water
content, contact angle and surface roughness, tensile strength, indicators of the
stability of leachables (pH, absorbance, and substances that reduce potassium permanganate),
and the antibacterial tendency using Escherichia coli. The goal was to quantitatively
compare the differences arising from the synthesis route and loading and to identify
the ranges of optimal conditions.
2. MATERIALS AND METHODS
2.1 Materials
2-Hydroxyethyl methacrylate (HEMA), ethylene glycol dimethacrylate (EGDMA), titanium
(IV) isopropoxide (TTIP), 2-propanol, nitric acid, glacial acetic acid, and sodium
hydroxide were purchased from Sigma-Aldrich (USA). Azobisisobutyronitrile (AIBN) was
obtained from Junsei (Japan). All reagents were of high purity and used as received
without further purification.
2.2 Synthesis of TiO2 nanoparticles
2.2.1 Hydrolysis–precipitation method
Titanium(IV) isopropoxide was added to deionized water (DIW) while stirring to induce
hydrolysis. Glacial acetic acid was then added and the reaction was allowed to proceed
under low-temperature conditions, followed by the addition of a sodium hydroxide solution
to form a precipitate. The resulting precipitate was collected by centrifugation and
washed five times with DIW to remove impurities and unreacted precursors. The washed
solid was freeze-dried to obtain TiO2 powder, which was designated as H_TiO2.
2.2.2 Sol–Gel method
Titanium (IV) isopropoxide was added dropwise to 2-propanol while stirring, after
which nitric acid and deionized water (DIW) were added to induce hydrolysis. The reaction
mixture was stirred at a controlled temperature to form a sol and then was allowed
to gel. The resulting gel was collected by centrifugation, washed five times with
DIW to remove impurities and unreacted precursors, and freeze-dried to obtain TiO2 powder, which was designated as S_TiO2.
2.3 Fabrication of contact lenses containing TiO2 nanoparticles
Hydrogel contact lenses were prepared using 2-hydroxyethyl methacrylate (HEMA) as
the base monomer, with azobisisobutyronitrile (AIBN) as the thermal initiator and
ethylene glycol dimethacrylate (EGDMA) as the crosslinker. The base formulation was
designated as Ref. Samples containing TiO2 synthesized by the hydrolysis precipitation method were assigned to the H group while
those containing TiO2 from the sol-gel method to the S group. Within each group, samples were denoted as
-3, -5, -7, and -10 according to the TiO2 loading level. All formulations were mixed using a vortex mixer for 2 h and then
subjected to ultrasonication for 30 min to ensure dispersion. The mixtures were dispensed
into contact lens molds and thermally polymerized at 100 °C for 1 h to produce the
lenses. Detailed compositions of each sample are provided in Table 1.
Table 1. Percent compositions of hydrogel samples (Unit : wt%)
|
Sample
|
HEMA
|
EGDMA
|
AIBN
|
H_TiO2 |
S_TiO2 |
Total
|
|
|
Ref
|
99.30
|
0.50
|
0.20
|
-
|
-
|
100.00
|
|
S-group
|
S-3
|
99.27
|
0.50
|
0.20
|
-
|
0.03
|
100.00
|
|
S-5
|
99.25
|
0.50
|
0.20
|
-
|
0.05
|
100.00
|
|
S-7
|
99.23
|
0.50
|
0.20
|
-
|
0.07
|
100.00
|
|
S-10
|
99.20
|
0.50
|
0.20
|
-
|
0.10
|
100.00
|
|
H-group
|
H-3
|
99.27
|
0.50
|
0.20
|
0.03
|
-
|
100.00
|
|
H-5
|
99.25
|
0.50
|
0.20
|
0.05
|
-
|
100.00
|
|
H-7
|
99.23
|
0.50
|
0.20
|
0.07
|
-
|
100.00
|
|
H-10
|
99.20
|
0.50
|
0.20
|
0.10
|
-
|
100.00
|
S_TiO2, TiO2 synthesized by sol-gel method
H_TiO2, TiO2 synthesized by hydrolysis-precipitation method
2.4 Analysis
Hydrogel contact lens specimens hydrated in 0.9% (w/v) NaCl physiological saline for
24 h were characterized. Optical properties were obtained with a UV-vis spectrophotometer
(Cary 60, Agilent, USA), and the average transmittance in the UV-B region was calculated
as a percentage. The refractive index of hydrated lenses was measured with an ABBE
refractometer (ATAGO NAR-IT, Japan). Water content was determined gravimetrically
by microwave-drying the hydrated lenses to constant mass and measuring wet and dry
masses with an analytical balance (PAG 214C, Ohaus, USA). Surface wettability was
evaluated by the sessile drop method to obtain static contact angles (DSA30, Krüss,
Germany). Surface topography and roughness were assessed by analyzing AFM images (NX10,
Park Systems, Korea). The morphology and size of the synthesized TiO2 nanoparticles were examined by SEM (Gemini 500, Zeiss, Germany), and particle sizes
were measured from micrographs to obtain representative values. Mechanical strength
was evaluated using a tensile tester (AGS-X, Shimadzu, Japan) at a constant loading
rate, and the maximum load at break was recorded.
Polymerization/leachable stability was assessed by immersing lenses in 10 mL of triple-distilled
water, and extracts collected at the five-day time point were analyzed by the KMnO4 consumption test, pH measurement, and UV-vis absorbance (Cary 60). Antibacterial
tendency was evaluated against Escherichia coli by co-incubating lenses with bacteria
in 0.9% saline for 24 h, removing surface moisture, shaking the lens with the saline
to dislodge adhered cells, and plating 1 mL of the resulting suspension onto 3 M PetrifilmTM for incubation at 36 ± 1 °C for 24 h (DS-210SL shaking incubator, Daewon Science,
Korea).
All measurements were conducted at least five times (n ≥ 5) and results are reported
as representative mean values.
3. RESULTS AND DISCUSSION
3.1. UV-B Transmittance measurement
Relative to Ref, the UV-B transmittance of the fabricated lenses decreased monotonically
with increasing TiO2 loading. At identical loading, the S-group exhibited lower transmittance than the
H-group, indicating the former provided superior UV-B shielding. The transmittance
results for each sample are presented in Fig. 1.
Fig. 1. UV-B Spectral transmittance of samples.
3.2 Refractive index and water content
Relative to Ref, the refractive index increased progressively with higher TiO2 loading. The S-group rose from 1.438 to 1.446 and the H-group from 1.437 to 1.445.
At identical loading, the S-group consistently showed a higher refractive index than
the H-group. In contrast, water content remained nearly constant across all samples
(37.18–37.22%), indicating that adding TiO2 increased the refractive index without altering hydration characteristics. The results
are shown in Fig. 2.
Fig. 2. Comparison of refractive index and water content of samples.
3.3 Wettability
Relative to Ref, the static contact angle decreased consistently as TiO2 loading increased. In the S-group, the values of the contact angle decreased from
≈52° at S-3 to 35.5° at S-10; in the H-group, they fell from ≈52° to 36.4°. At identical
loading, the S-group generally exhibited lower contact angles than the H-group, indicating
superior wettability. At the highest loading, the reduction relative to Ref was ≈37.6%
for S-10 and ≈36.0% for H-10. Detailed values and percent changes for each sample
are presented in Fig. 3 and Fig. 4.
Fig. 3. Contact angle samples comparison.
Fig. 4. Contact angle images and samples comparison. ([A]: Ref, [B]: S-10, [C]: H-10).
3.4 Surface analysis
In the AFM results S-10 exhibited uniformly distributed micro-undulations with few
pronounced protrusions or abrupt height changes. Height variations were generally
limited to a few tens of nanometers, indicating a relatively smooth and homogeneous
surface. By contrast, H-10 showed location-dependent irregularity in the size and
spacing of surface features, with larger overall height deviations. These observations
indicate that S-10 has lower surface roughness and higher uniformity than H-10.
SEM showed small, bright dot-like nanoparticles in S-10 that were finely and evenly
dispersed, with few distinct aggregates. The particle-size distribution of H-10 was
broader and localized clusters were present. These morphological differences are consistent
with the observed decreases in static contact angle and suggest that the S-10 surface
is more favorable for wettability. AFM and SEM images employed for the surface analysis
are presented in Fig. 5 and Fig. 6.
Fig. 5. SEM images of samples. ([A]: Ref , [B]: S-10, [C]: H-10).
Fig. 6. AFM images of samples. ([A]: Ref, [B]: S-10, [C]: H-10).
3.5 Tensile strength
Relative to Ref, the tensile strength of the fabricated lenses increased monotonically
with increasing TiO2 loading. For the S-group, the strength values rose from 0.160 kgf/mm (S-3) to 0.210
kgf/mm (S-5), 0.240 kgf/mm (S-7), and 0.260 kgf/mm (S-10). For the H-group, the values
increased from 0.145 kgf/mm (H-3) to 0.190 kgf/mm (H-5), 0.210 kgf/mm (H-7), and 0.230
kgf/mm (H-10). At identical loading, the S-group was generally higher by approximately
0.010–0.030 kgf/mm, with the maximum observed for S-10 (0.260 kgf/mm). These results
confirm a clear positive correlation between TiO2 concentration and tensile strength. The data are presented in Fig. 7.
Fig. 7. Comparison of tensile strength of samples.
3.6 Polymerization stability and leachables
To assess the polymerization stability, pH, UV-vis absorbance, and a KMnO4 consumption test were performed. The pH values were essentially identical between
the control and test groups for all samples, spanning a range of ~7.2–7.5 with no
systematic differences as a function of additive presence or concentration. They also
indicated negligible variation attributable to leachables (Fig. 8A). The absorbance remained low across all samples (0.14–0.18). Although a moderate
upward trend was observed with increasing TiO2 loading, all values stayed at a low level. This suggests that no significant organic/inorganic
species were detected at the measurement wavelength(s) (Fig. 8B). In the KMnO4 test, the control deionized water (D-W) required ~21.7 mL, whereas the samples ranged
from ~19.2 to 20.9 mL. The titration volume gradually increased toward the control
value with higher TiO2 loading, and inter-sample differences were small. These results indicate a low level
of reducing residues in the extracts and suggest that increased loading diminishes
the influence of residual species (Fig. 8C). Overall, lenses containing TiO2 exhibited stable polymerization/leachables behavior for all metrics. Detailed results
are presented in Fig. 8.
Fig. 8. Elution test of samples. ([A]: pH test, [B]: Absorbance, [C]: KMnO4 reduction test).
3.7 Antibacterial activity
Ref showed numerous dense, dot-like colonies of Escherichia coli across the grid,
indicating clear bacterial growth. By contrast, S-10 had no visible colonies or levels
below the detection limit and the plate was essentially clean, thus demonstrating
strong inhibition. H-10 also had markedly fewer colonies than Ref, indicating an inhibitory
tendency, although the effect was less pronounced than for S-10. Antibacterial activity
of H-10 and S-10 against Escherichia coli was thus confirmed (Fig. 9).
Fig. 9. Antimicrobial images of the samples (A: Ref, B: S-10, C: H-10).
4. CONCLUSIONS
This study quantitatively compared the effects of adding TiO2 nanoparticles to HEMA-based hydrogel contact lenses along two axes: synthesis route
and loading. The synthesis routes were hydrolysis precipitation and sol-gel and the
nanoparticle loading was set at four levels in a range from 0.03 to 0.10 wt%.
All samples were prepared in an identical hydrogel matrix, and optical, surface, and
mechanical properties, as well as stability of leachables and antibacterial activity,
were evaluated. Comparisons with the control and between the two groups showed that
the synthesis route modulated material properties.
UV-B transmittance decreased as TiO2 loading increased. The sol-gel (S) group consistently showed lower transmittance
than the hydrolysis precipitation (H) group at the same loading, with the largest
blocking effect at 0.10 wt%. This is attributed to the more uniform dispersion of
sol-gel particles within the hydrogel, which enabled more effective absorption and
scattering. Thus, TiO2 addition improves UV shielding and the synthesis route determines the magnitude of
the improvements.
The contact angle analysis showed improved surface wettability: the value of the control
was ~56.9°, S-10 ~35.5°, and H-10 ~36.4°. The S-group values were typically 1-4° lower
than the H-group values at identical loading. Because improved wettability is directly
related to tear-film stabilization on the lens surface, these results suggest that
adding TiO2 adjusts the surface water distribution to render the hydrogel surface more hydrophilic.
The tensile strength increased consistently with TiO2 addition and the S group exceeded the H group by ~0.010–0.030 kgf/mm at the same
loading. This is attributed to more efficient load transfer arising from reduced agglomeration
and better interfacial adhesion of sol-gel particles.
Antibacterial activity was assessed using Escherichia coli. S-10 showed virtually
no colonies on the plate and H-10 also showed a marked reduction versus the control;
the inhibitory effect was stronger in the S group. This may reflect the combined effects
of improved wettability and nonspecific interactions at nanoparticle surfaces that
reduce initial adhesion and proliferation. Notably, despite strong antibacterial effects,
changes in water content and extractable metrics were small. This indicates that the
TiO2 formulations achieved both property balance and hygiene.
Overall, the nanoparticle synthesis route was identified as a key factor that improves
the performance of hydrogel-TiO2 composites. The S group outperformed the H group in UV-B shielding, surface wettability,
and tensile strength at equal loading. Both routes showed similar trends of an increased
refractive index while maintaining water content, and the stability of leachables
remained within the reference range irrespective of the route. The most pronounced
performance gains occurred at TiO2 content of 0.05–0.10 wt%, where S-10 showed the highest overall performance. These
results confirm the general relationship between the TiO2 synthesis route/loading and material properties and contribute to foundational research
on hydrogel contact lens materials.
ACKNOWLEDGEMENT
This research was funded by research grants from Daegu Catholic University 2025-1009.
REFERENCES
Musgrave C. S. A., Fang F., Mater., 12, 261 (2019)
Kopeček J., J. Polym. Sci. A Polym Chem., 47, 5929 (2009)
Dharma A. A., Kaidzu S., Ishiba Y., Okuno T., Tanito M., Mater., 18, 4784 (2025)
Kim D. H., Sung A. Y., J. Integr. Nat. Sci., 11, 1 (2018)
Ning X., Huang J. A. Y., Yuan N., Chen C., Lin D., Int. J. Mol. Sci., 23, 15757 (2022)
Gause S., Chauhan A., J. Mater. Chem. B., 4, 327 (2016)
Wang W., Polymers., 11, 458 (2019)
Pan Y., Int. J. Nanomed., 19, 11143 (2024)
Gupta S., Tripathi M., Open Chem., 10, 279 (2012)
Simakov S. A., Tsur Y., J. Nanopart Res., 9, 403 (2007)
Park H.-I., Sung A.-Y., Korean J. Met. Mater., 63, 217 (2025)
Heo S.G., Kim S., Seo S.-J., Sim J.J., Shin J., Mirzaei A., Choi M.S., Korean J. Met.
Mater., 62, 631 (2024)