**Nguyen-Phuong-Dung Tran and Ming-Chien Yang \***

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; thaonguyeng89@gmail.com

**\*** Correspondence: myang@mail.ntust.edu.tw; Tel.: +886-2-2737-6528; Fax: +886-2-2737-6544

Received: 13 April 2020; Accepted: 11 May 2020; Published: 14 May 2020

**Abstract:** In this study, silicone nanoparticles (SiNPs) were prepared from polydimethylsiloxane (PDMS) and tetraethyl orthosilicate (TEOS) via the sol-gel process. The resultant SiNPs were characterized by dynamic light scattering (DLS), transmission electron microscope (TEM), and scanning electron microscope (SEM). These SiNPs were then blended with 2-hydroxyethylmethacrylate (HEMA) and 1-vinyl-2-pyrrolidinone (NVP) before polymerizing into hydrogel contact lenses. All hydrogels were subject to characterization, including equilibrium water content (EWC), contact angle, and oxygen permeability (Dk). The average diameter of SiNPs was 330 nm. The results indicated that, with the increase of SiNPs content, the oxygen permeability increased, while the EWC was affected insignificantly. The maximum oxygen permeability attained was 71 barrer for HEMA-NVP lens containing 1.2 wt% of SiNPs with an EWC of 73%. These results demonstrate that by loading a small amount of SiNPs, the Dk of conventional hydrogel lenses can be improved greatly. This approach would be a new method to produce oxygen-permeable contact lenses.

**Keywords:** silicone nanoparticles; PDMS; TEOS; hydrogels; soft contact lenses

### **1. Introduction**

Contact lenses are employed for correcting eye vision. The global market for contact lenses is about US\$8.1 billion in 2018. Two major classes of soft contact lenses are silicone hydrogel and conventional hydrogel lenses. The market shares for the former is 69% while the latter takes 19% in 2018 [1]. Conventional hydrogel contact lenses are synthesized from hydrophilic monomers such as 2-hydroxyethyl methacrylate (HEMA), offering the wearer comfort due to their hydrophilicity. However, this class of contact lenses exhibits low oxygen permeability that may cause red-eye syndrome for long-term wearing [2,3]. To cope with this problem, silicon-containing polymers such as 3-(methacryloyloxy) propyltris(trimethylsiloxy) silane (TRIS) or polydimethylsiloxane (PDMS) are incorporated into the hydrogel to increase the oxygen permeability, leading to the inception of silicone hydrogel lenses [4–11].

Polydimethylsiloxane exhibits a hydrophobic nature and is low cost, simple to fabricate, and shows good biocompatibility, flexibility, thermal and oxidative stability, high optical transparency, and especially high oxygen permeability [12–18]. The main drawback of PDMS is the restriction of water absorptivity, surface wettability, and higher lipid deposition because of its inherent hydrophobicity [19–21]. These limitations can be improved through the combination with hydrophilic materials. The incorporation of hydrophobic and hydrophilic monomers usually takes two oppositional tendencies. The first trend is to increase equilibrium water content. However, this will decrease the oxygen permeability because of hydrophilic monomers. On the other hand, a higher PDMS content will enhance oxygen permeability but reduce the water uptake ability [11,22,23]. In silicone hydrogel contact lenses, water is a main factor to restrict the oxygen permeability [24–26]. However, the second

trend may improve both wettability and oxygen transmissibility when hydrophobic and hydrophilic monomers are cooperated at a proper ratio. However, the second trend may improve both wettability and oxygen transmissibility when hydrophobic and hydrophilic monomers are cooperated at a proper ratio.

silicone hydrogel contact lenses, water is a main factor to restrict the oxygen permeability [24–26].

In this work, a novel approach is adopted to improve the oxygen permeability of HEMA-based hydrogels. Instead of incorporating PDMS into the polymer chain, poly(dimethylsiloxane) dialkanol (PDMS diol) was reacted with tetraethyl orthosilicate (TEOS) through hydrolysis and condensation to form silicone nanoparticles (SiNPs) via the sol-gel process [27–29]. Thereafter, SiNPs were loaded into hydrogel synthesized from HEMA and 1-vinyl-2-pyrrolidinone (NVP). All the resultant hydrogels were subject to characterization, including Fourier transform infrared spectroscopy (FTIR), Raman, scanning electron microscope (SEM), transmission electron microscope (TEM), dynamic light scattering (DLS), equilibrium water content (EWC), oxygen permeability (Dk), optical transparency, mechanical strength, and contact angle measurements. We think this novel approach of loading silicone nanoparticles would improve the oxygen permeability without reducing the hydrophilicity and wettability of HEMA hydrogels. In this work, a novel approach is adopted to improve the oxygen permeability of HEMA-based hydrogels. Instead of incorporating PDMS into the polymer chain, poly(dimethylsiloxane) dialkanol (PDMS diol) was reacted with tetraethyl orthosilicate (TEOS) through hydrolysis and condensation to form silicone nanoparticles (SiNPs) via the sol-gel process [27–29]. Thereafter, SiNPs were loaded into hydrogel synthesized from HEMA and 1-vinyl-2-pyrrolidinone (NVP). All the resultant hydrogels were subject to characterization, including Fourier transform infrared spectroscopy (FTIR), Raman, scanning electron microscope (SEM), transmission electron microscope (TEM), dynamic light scattering (DLS), equilibrium water content (EWC), oxygen permeability (Dk), optical transparency, mechanical strength, and contact angle measurements. We think this novel approach of loading silicone nanoparticles would improve the oxygen permeability without reducing the hydrophilicity and wettability of HEMA hydrogels.

#### **2. Materials and Methods 2. Materials and Methods**

#### *2.1. Materials 2.1. Materials*

Poly(dimethylsiloxane) dialkanol (PDMS-diol, KF-6001) was purchased from Shin-Etsu Chemical Co. Ltd., Tokyo, Japan. 1-Vinyl-2-pyrrolidinone (NVP), tetraethyl orthosilicate (TEOS), and 2-hydroxy-2 methylbenzene acetone (D-1173) were purchased from Sigma-Aldrich (St. Louis, Mo USA). Further, 2-hydroxyethylmethacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) were obtained from Acros Organics (NJ, USA). Phosphate buffered saline solution (PBS, 0.1 M, pH 7.4) was prepared in our laboratory. Poly(dimethylsiloxane) dialkanol (PDMS-diol, KF-6001) was purchased from Shin-Etsu Chemical Co. Ltd., Tokyo, Japan. 1-Vinyl-2-pyrrolidinone (NVP), tetraethyl orthosilicate (TEOS), and 2-hydroxy-2-methylbenzene acetone (D-1173) were purchased from Sigma-Aldrich (St. Louis, Mo USA). Further, 2-hydroxyethylmethacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) were obtained from Acros Organics (NJ, USA). Phosphate buffered saline solution (PBS, 0.1 M, pH 7.4) was prepared in our laboratory.

#### *2.2. Preparation of Silicone Nanoparticles 2.2. Preparation of Silicone Nanoparticles*

Silicone nanoparticles (SiNPs) were synthesized by cross-linking PDMS-diol with TEOS through the sol-gel process shown in Figure 1. The reaction was conducted in a solution containing 0.1 mL of HCl (37%), 4 mL of water, and 2 mL of ethanol (95%). Subsequently, 1 mL of TEOS was added into the solution and then stirred for 1 h at room temperature, followed by adding 4 mL of PDMS-diol dropwise into the reacting solution. The reaction was stirred for 24 h at room temperature in the dark after completion of the addition. After condensing in a vacuum oven at 80 ◦C for 24 h, SiNPs were harvested and purified with ethanol before sonication and centrifugation. Finally, SiNPs were stored after drying in an oven at 80 ◦C for 12 h. Silicone nanoparticles (SiNPs) were synthesized by cross-linking PDMS-diol with TEOS through the sol-gel process shown in Figure 1. The reaction was conducted in a solution containing 0.1 mL of HCl (37%), 4 mL of water, and 2 mL of ethanol (95%). Subsequently, 1 mL of TEOS was added into the solution and then stirred for 1 h at room temperature, followed by adding 4 mL of PDMS-diol dropwise into the reacting solution. The reaction was stirred for 24 h at room temperature in the dark after completion of the addition. After condensing in a vacuum oven at 80 °C for 24 h, SiNPs were harvested and purified with ethanol before sonication and centrifugation. Finally, SiNPs were stored after drying in an oven at 80 °C for 12 h.

**Figure 1. Figure 1.** Preparation of silicone nanopart Preparation of silicone nanoparticles from TEOS and PDMS-diol. icles from TEOS and PDMS-diol.

Figure 1 shows the synthetic reactions for silicone nanoparticles (SiNPs) from PDMS and TEOS. Firstly, the ethoxy groups of TEOS were hydrolyzed into the hydroxyl groups. In the subsequent condensation, silica and silicone were formed by removing hydroxyl groups of Si-OH [27,29,30]. Figure 1 shows the synthetic reactions for silicone nanoparticles (SiNPs) from PDMS and TEOS. Firstly, the ethoxy groups of TEOS were hydrolyzed into the hydroxyl groups. In the subsequent condensation, silica and silicone were formed by removing hydroxyl groups of Si-OH [27,29,30].

*Polymers* **2020**, *12*, x FOR PEER REVIEW 3 of 15

#### *2.3. Preparation of SiNPs-Loaded Hydrogel Composites 2.3. Preparation of SiNPs-Loaded Hydrogel Composites*

The hydrogels were polymerized from NVP and HEMA in the presence of SiNPs, cross-linking agent EGDMA and photo initiator D-1173 as shown in Table 1 and Figure 2. For all formulations, the mixture contained 0.5 wt% of EGDMA and D-1173. Then, the mixture was stirred in the dark at room temperature for 5 h. Afterward, the mixture was transferred to polypropylene molds and cured under UV light (365 nm) for 40 min. After demolding, lenses were soaked in 50% ethanol for 20 h at 50 ◦C to remove un-reacted monomers and photo initiator. Then, the lenses were immersed in distilled water for 4 h at 50 ◦C to wash out ethanol. Finally, the lenses were preserved in PBS (pH 7.4) at room temperature. The hydrogels were polymerized from NVP and HEMA in the presence of SiNPs, cross-linking agent EGDMA and photo initiator D-1173 as shown in Table 1 and Figure 2. For all formulations, the mixture contained 0.5 wt% of EGDMA and D-1173. Then, the mixture was stirred in the dark at room temperature for 5 h. Afterward, the mixture was transferred to polypropylene molds and cured under UV light (365 nm) for 40 min. After demolding, lenses were soaked in 50% ethanol for 20 h at 50 °C to remove un-reacted monomers and photo initiator. Then, the lenses were immersed in distilled water for 4 h at 50 °C to wash out ethanol. Finally, the lenses were preserved in PBS (pH 7.4) at room temperature.


**Table 1.** Formulation of soft lenses including HEMA, NVP, and silicone nanoparticles (SiNPs). **Table 1.** Formulation of soft lenses including HEMA, NVP, and silicone nanoparticles (SiNPs).

**Figure 2.** Preparation of SiNPs-loaded hydrogel lenses. **Figure 2.** Preparation of SiNPs-loaded hydrogel lenses.

#### *2.4. Elemental Analysis and Size of Particles 2.4. Elemental Analysis and Size of Particles*

The elemental composition of dry particles was determined using a field emission scanning electron microscope (FE-SEM/EDS, JSM-6500F, JEOL, Tokyo, Japan). The particle size was analyzed using dynamic light scattering (DLS-DKSH, Malvern Instruments Ltd., Malvern, UK), and transmission electron microscopy (TEM, JEM-2000FXII, JEOL, Japan). The elemental composition of dry particles was determined using a field emission scanning electron microscope (FE-SEM/EDS, JSM-6500F, JEOL, Tokyo, Japan). The particle size was analyzed using dynamic light scattering (DLS-DKSH, Malvern Instruments Ltd., Malvern, UK), and transmission electron microscopy (TEM, JEM-2000FXII, JEOL, Japan).

## *2.5. Equilibrium Water Content*

The equilibrium water content (EWC) of the hydrogel was calculated as follows:

$$\text{EWC } (\%) = \frac{\text{W}\_2 - \text{W}\_1}{\text{W}\_2} \times 100 \tag{1}$$

where W<sup>1</sup> and W<sup>2</sup> are the weights of the dry lens and the rehydrated lens in distilled water for one day at room temperature, respectively.

#### *2.6. Optical Transparency*

After swelling in PBS solution, the lens was adhered on the surface of cuvette containing 2 mL distilled water. The optical transparency was determined in a wavelength range of 400–700 nm using a UV-Vis spectrophotometer (Cary 300, Agilient Technologies, Santa Clara, CA, USA).

#### *2.7. Surface Characterization*

The contact angle of the contact lens was measured using a contact angle goniometer (DSA 100, Krüss GmbH, Hamburg, Germany) at room temperature. The contact angle was an average of three repetitions.

#### *2.8. Chemical structure*

The structure of SiNPs and contact lenses were examined using Raman Spectroscopy and FTIR. The FTIR of SiNPs (Nicolet 170 SX, Thermo Fisher Scientific, Madison, WI, USA) were performed in the wavenumber range of 600–4000 cm−<sup>1</sup> . SiNPs were pelletized with potassium bromide (KBr) before being scanned over 32 times by an infrared ray. The lenses were detected over 32 scans based on FTIR-ATR. The Raman spectroscopy (iHR550, Horiba Scientific, Kyoto, Japan) was determined in the wavenumber range of 400–4000 cm−<sup>1</sup> .

#### *2.9. Mechanical Properties*

The mechanical properties of hydrogel specimens were determined by modulus and tensile strength. Samples were cut as dog bone shape after hydrated in DI water. Modulus and tensile strength of specimens were measured based on a tensile tester (MTS 810, Material Test System, Eden Prairie, MN, USA) via ASTM D1708 standard at a crosshead speed of 50 mm/min.

### *2.10. Oxygen Permeability*

The oxygen permeability (Dk, barrer) of the lens was determined according to ISO18369-4:2006 which is based on polarographic method using an oxygen permeometer (201 T O2 permeometer, Createch, Chesterfield Twp, MI, USA). Polarography measures the oxygen permeation through a sample by measuring the current produced in a cell by reducing oxygen at a noble metal electrode. Before testing, guard ring polarographic cell (8.6 mm radius, CreaTech/Rehder Development Co., Chesterfield Twp, MI, USA), buffer solutions and the lenses were placed in a temperature and humidity-controlled box at 37 ◦C and 98% relative humidity till the temperature equilibrium. After fully hydrated, the lenses were stacked to measure the electronic current at a various number of lenses to correct the boundary effect. The linear plot of t/Dk versus thickness was drawn and determined Dk/t from the slope [31–33].
