Home Application Note Characterization of roughness and mechanical properties of a contact lens surface using atomic force microscopy (AFM) with a specialized liquid cell

Characterization of roughness and mechanical properties of a contact lens surface using atomic force microscopy (AFM) with a specialized liquid cell

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Figure 1. (a) Schematic structure of the liquid cell design for contact lenses, (1) Cell adapter for AFM body (2) Cell Base (3) Cell bracket (4) O-ring between base and bracket (5) 2Ф bolts (6) 2.5Ф bolts; (b) Cell adapter installed on the AFM body; (c) Cell base and bracket components connected with soft contact lens inside.
Research Applications Technology Center, Park Systems Corp

Abstract

A soft contact lens is the most utilized type of contact lens, made of gel-type material. This slippery and transparent material, with a unique curvature, like a human eye, requires cutting, drying or freezing to fix for surface characterization. Considering that contact lenses are used in the moist environment inside of human eyes, these preparations result in inaccurate research preparation. The introduction of a liquid cell for contact lenses allows for the placement of a soft contact lens without deformation, and it provides a stable environment for AFM application. In this study, a soft contact lens is fully immersed in saline solution, and its surface, with color coating, is scanned using tapping mode and Park PinPoint™ mode of AFM. The roughness values, Young’s modulus, and adhesion energy of the lens surface are successfully acquired with nanoscale resolution using AFM with a liquid cell for the lenses.

Introduction

As a replacement for glasses, contact lenses are a commonly used for vision correction, providing comfort and convenience. Their use has expanded into the cosmetic market, and contact lenses are considered as a substitute for invasive surgery. Various studies have focused on the characteristics and material properties of contact lenses, which have led to versatility in properties and application fields [1, 2]. Technical reviews of contact lenses often name surface roughness and material analysis as key factors that define the comfort for users as well as the resistance against bacterial contamination [3, 4, 5]. High-tech manufacturing innovations of the lens mold facilitate lens surfaces with roughness in the nanometer-scale. Lens surfaces take on distinctive roughness and surface adhesion features depending on their manufacturing process and the after treatment [6]. These features establish guidelines for commercial use of contact lenses and lead to innovations in industry research. Among commonly used microscopy techniques, atomic force microscopy is best suited for studying surface properties on the nanometer scale. The capability of AFM at quantitative topography analysis surpasses any other microscopy system – features such as pores or surface defects can be mapped in three dimensions. Using force distance spectroscopy, AFM additionally obtains mechanical properties like surface adhesion and Young’s modulus. While this versatility is quite useful, the domed shape of a contact lens contrasts with the need for a flat surface for an AFM system to obtain useful surface data. Eventually, lenses are cut or deformed to flatten the surface enough to achieve stable measurement conditions. However, the deformation of the lenses for AFM measurements can entail a loss of information on actual mechanical properties. Here, we introduce a liquid cell for a contact lens measurement. This special cell design has a convex structure with a similar curvature to the human eye to hold a contact lens without deformation. A cover with a hole in the middle fixes the sample and enables access for the AFM tip. We demonstrate surface roughness measurements using tapping mode and Park PinPointTM Nanomechanical Mode to determine additional properties, such as the difference in Young’s modulus and adhesion energy between the center and the color coated layer. Figure 2. (a) Concept drawing of a soft contact lens with a color layer; (b) Optical camera image: Center of the lens surface on the convex top region; and, (c) color coated area. The red circles mark the AFM scan areas. The optical image scale for both (b) and (c) is 480 µm × 360 µm. Figure 3. All images are scaled to the same color. (a) Topography result of the center region on the top of the lens. Topography on the edge (b) and inside (c) of the color dot. (d) Peak to valley (Rpv), root mean square roughness (Rq), and average roughness (Ra) chart from (a), (b), and (c).

Materials and Methods

Liquid cell for contact lens

The cell design consists of three main parts: the cell adapter, the cell base and the cell bracket (bottom to top in Figure 1 (a)). The cell adapter is screwed into the AFM body first (Figure 1 (b)), and then the assembled cell is magnetically attached to the adapter. This configuration allows an independent assembly and manipulation of the liquid cell to facilitate an easy preparation for measurement. A dome shaped support structure on the cell base serves as the mount to affix the contact lens. The curvature radius of the dome is similar to that of a contact lens (~14 mm), so the lens rests on the base without deformation or transformation of its structure. As the collar structure of the cell cover (cell bracket) pushes down the lens, the sample is fixed with its top part open to allow surface access for imaging. In addition, an O-ring is added between bracket and cell base to enhance the stability of sample fixation. The exposed convex top point of the cell base is lower than the cell cover’s top surface; this way any contact lens sample can be immersed into saline solution for imaging to simulate real life application. Figure 4. Height (a), modulus (b), and adhesion energy images (c) at the center of the lens and at the color dot region (d), (e) and (f), respectively. (g) Modulus and adhesion energy mean values chart; modulus and adhesion energy values in center from (b) and (c), modulus of the edge and inside of color pigment were partially analyzed from (e), and adhesion energy for mean value of the whole image except some high-peaked distinctive points from (f). All images were performed using PinPointTM Nanomechanical Mode.

Experimental Set-up

To demonstrate the working principle of the cell, it was mounted to a Park NX12 AFM from Park Systems. We applied saline solution to the cell and fully immersed the contact lens under this solution to maintain the aqueous environment, in which actual usage of the lens mostly occurs. A Biolever Mini (BL-AC40TS) cantilever from Olympus was mounted on the liquid probe hand enabling in-liquid Tapping and PinPoint modes on the soft contact lenses with color coating. The resonant frequency of the cantilever is 110 kHz in air and 25 kHz in water. We chose the Biolever Mini cantilever with a small spring constant (k = 0.1 N/m) for its softness due to concerns that deformation by the cantilever could change the surface.

Result and Discussion

Using an optical camera with 10× magnification, we examined the sample surface prior to the AFM measurement. The center of this contact lens is transparent and the coating layer covers the rim of the lens as common in colored contact lenses (Figure 2 (a)). Optical images of these two different regions are shown in Figure 2; in the center (b) and on the color coated outer ring (c). Each of the optical images are marked with a red dotted circle where the AFM topography was scanned in the following. For the center region of the lens, we zoomed into the center of the optical position and scanned a 20 µm × 20 µm region (Figure 3 (a)). The same measurements were conducted on both the edge (Figure 3 (b)) and inside (Figure 3 (c)) of the color pigment for comparison. For analysis, three different roughness values were reported: peak to peak difference between the highest and lowest height value (Rpv); root mean square roughness (Rq); and, the average roughness value (Ra). All three roughness values (Rpv, Rq, and Ra) displayed the same trend, with the smallest roughness values for the center of the lens and the highest roughness in the color pigment, respectively. The color coating increased the surface roughness more than two times compared to the center region, without the coating layer. Furthermore, topography data obtained at the inside of the color pigment showed noticeable differences compared to outside of the color pigment. For a comparison of the mechanical properties, we scanned the center of the lens and the edge of the coating with the PinPoint technique. While the center region showed homogenous roughness and mechanical property characteristics (Figure 4 (a, b, c)), the edge of the color coating clearly indicated a structural change (Figure 4 (d, e, f)). The coating layer edge was scanned for immediate comparison between the edge (upper left side region) and the inside (bottom right side region) of the coating (Figure 4 (e)). As the roughness increased towards the color coating, the mechanical properties revealed that this coating consisted of two phases with hard coating pigments embedded in a softer matrix – with respect to the edge of coating, the color pigments were harder and the bottom matrix of the coating was softer. While the Young’s modulus difference is obvious, this feature did not appear in adhesion energy (Figure 4 (e, f)). There were points with high adhesion energy in Figure 4 (f), which were mainly distributed on soft matrix. In these distinctive points, the adhesion energy was 70.48 aJ which was over ten times higher than rest of the image region. Since the tip and sample reaction is acted under saline solution, adhesion energy was not following the regional contrast observed in the modulus image in color pigment images due to the cantilever damping in liquid.

Conclusions

We introduced a liquid cell with a curvature similar to the structure of the human eye. This cell provided a simple and stable measurement environment for soft contact lenses without deforming or destroying the lens and facilitated imaging in saline solution. Roughness measurements were obtained with a high resolution, comparing the color coated layer with the non-coated substrate. We discovered a clear change in topography as well as the Young’s modulus between the two regions of the lens. These results represent the first reference of an AFM study on contact lenses, which can then be used to analyze critical issues such as degradation and bio-fouling. The new, innovative cell design will allow AFM to become a reliable tool in the contact lens industry, and revolutionize the way contact lenses are studied on the nanoscale.

References

1. Carnt, N., Wu, Y., & Stapleton, F. (2017) Contact Lenses, Reference Module in Neuroscience and Biobehavioral Psychology. 2. Maulvi, Furqan A., Soni, Tejal G., and Shah, Dinesh O., A review on therapeutic contact lenses for ocular drug delivery. Drug Delivery, 23:8 (2016) 3017-3026. 3. Bruinsma , Gerda M., Rustema-Abbing, Minie, de Vries , Joop, Stegenga, Boudewijn, van der Mei, Henny C., van der Linden, Matthijs L., Hooymans, Johanna M. M., and Busscher, Henk J., Influence of Wear and Overwear on Surface Properties of Etafilcon A Contact Lenses and Adhesion of Pseudomonas Aeruginosa. Investigative Ophthalmology & Visual Science, Vol.43 (2002) 3646-3653. 4. Giraldez, Maria Jesus, and Yebra-Pimentel, Eva, (2012) ‘Hydrogel Contact Lenses Surface Roughness and Bacterial Adhesion.’ In Adedayo Adio (Ed.) Ocular Diseases. London: IntechOpen. 5. Ţălu, Ştefan, and Ţǎlu, M., Surface roughness of contact lenses investigated with atomic force microscopy. The Scientific Bulletin of VALAHIA University – MATERIALS and MECHANICS, Romania, no. 7 (year 10) (2012) 107-110. 6. Jones, Lyndon, Subbaraman, Lakshman, Rogers, R., and Dumbleton, Kathryn, Surface treatment, wetting and modulus of Silicone hydrogels. Optician. 232 (2006) 28-34.

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