A smart contact lens (CL) is an attractive wearable device that could enable noninvasive monitoring of physiological information and augmented reality display in addition to vision correction. Whereas, the CL is one of the causes of dry eye syndrome due to more rapid moisture evaporation through the CLs than that from the normal tear film covered by a lipid layer. Conventional rehydration by regular eye drops takes time and efforts. The only example so far of a smart CL for maintaining the moisture is that of Lee et al., who developed a CL coated with single layer of graphene which decreases the evaporation. Hence, there is still a high demand to develop a CL equipped with an anti-dehydration function. We here designed a charge-fixed soft CL generating electroosmotic flow (EOF) under an electric field for maintaining the moisture of the lens (Figure 1a) [1]. The CL supplies tears via EOF from the tear meniscus (temporary tear reservoir) behind the lower eyelid. EOF is the motion of water induced by an applied electric field across a fluid conduit like a capillary tube or a porous material such as hydrogels. When the fluid conduit contains fixed electrical charges, the dominant electromigration of the mobile counter ions produces net water flow.The soft CL made of negatively charged hydrogel were fabricated by copolymerizing methacrylic acid (MA, 0 – 15 wt%), 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA). At First, we evaluated the efficiency of EOF generation for each of the 0.2 mm thick hydrogel films. DC currents of 1.0, 2.0, and 3.0 mA were applied to the films sandwiched by the side-by-side Franz cell with a horizontal capillary, and the flow velocity of EOF was calculated from the movement of the water surface in the horizontal capillary. These results indicate that a hydrogel with a high MA content is suitable for effective EOF generation in hydrogels. Secondly, compression tests were performed to the films to evaluate the indentation fracture toughness. The larger MA content tends to make the hydrogel become brittle and the hydrogel with 15 wt% MA was easily broken by folding. By considering both the results of the EOF strength and the fracture toughness, we mainly used the 10 wt% MA hydrogel in subsequent experiments. The water content and the conductance of the films were separately monitored at 100 kHz during the slow drying under 75% humidity. This result supports that the change in water content in the hydrogel film can be traced by the conductance measurement (Figure 1b). Finally, the EOF-based moisturization for the battery-mounted CL was demonstrated using an enzymatic fructose–oxygen fuel cell [2], in which the CL was put on a hemispherical jig (ocular model) equipped with printed Ag/AgCl electrodes for conductance measurements (Figure 1c). The lower end (≈3 mm) of the CL was dipped in a buffer solution containing D-fructose to imitate a tear meniscus. Figure 1d shows the observed changes in conductance (moisture) of CLs without EOF (gray, natural drying) and with upward EOF generated by an enzymatic fuel cell (red). The results demonstrated that applying an upward EOF through the CL reduced the moisture loss of the lens compared to natural drying. Since the demonstration was conducted at a relatively dry condition (40% humidity) for the hanging CLs, it can be expected that the moisturization would be more effective in situations where CLs are put onto eye. From these results, we believe that the proposed hydrogel will promise development of a contact lens devices with an anti-drying function.Reference[1] S. Kusama, K. Sato, S. Yoshida, and M. Nishizawa, “Self-Moisturizing Smart Contact Lens Employing Electroosmosis”, Adv. Mater. Technol. 5, 1, 1–9 (2020)[2] Y. Ogawa, K. Kato, T. Miyake, K. Nagamine, T. Ofuji, S. Yoshino, and M. Nishizawa, “Organic Transdermal Iontophoresis Patch with Built-in Biofuel Cell,” Adv. Healthc. Mater. 4, 506–510 (2015). Figure 1
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Jul 16, 2024
Ivan Šoša 
Open Access