CONTACT LENSES (CLs) are convenient and popular optical devices used for vision correction, worn by an estimated 175 million people worldwide (Ackerman, 2018). Although CLs are usually safe and well tolerated, poor hygiene while handling lenses, noncompliance with disinfection regimens, poor storage case hygiene practice, and extended or overnight wear could lead to CL contamination and ocular infection, which may result in visual impairment or even blindness (Fleiszig et al, 2020; Ting et al, 2021; Stapleton, 2020).

CL-related infections involve adherence to the CL and/or the CL case and their colonization with organisms. The development of CLs and CL cases that prevent those events has been an expanding field of research for decades (Campolo et al, 2022). The strategy is to use a chemical compound, or a combination of several, as a coating or as an infused material to add antimicrobial or antiadhesion properties.

Antimicrobial agents interfere with essential cell processes, leading to microorganism death, while antiadhesion molecules prevent microorganism adherence to the CL surfaces instead of killing them directly. In both strategies, the coating or infused material must be nontoxic to the ocular surface and have a minimal impact on the normal ocular microbiome (Xiao et al, 2018).

Silver is one of the most investigated biocidal compounds, and silver-impregnated CL cases are already commercially available. Although studies using CLs with silver ions and silver nanoparticles have shown efficient antimicrobial activity (Meretoudi et al, 2021; Lakkis et al, 2011) and no impact on the microbial environment around the cornea (Willcox et al, 2010), they are unfortunately not feasible, mainly due to the toxic effect of their positive charge, which inhibits corneal cell proliferation (Campolo et al, 2022).

However, it has been recently reported that silver toxicity can be decreased when encapsulated silver nanoparticles are combined with other polymers or biomolecules (Kharaghan et al, 2019; Khan and Lee, 2020). Besides its toxicity, the relatively high cost of silver also limits its application. Less expensive metals, such as zinc and copper, have been considered as potentially viable alternatives to silver. A significant reduction of bacterial adhesion to silicone hydrogel CLs with Zn-CuO nanocoating has been demonstrated in the literature (Nahum et al, 2019; Tuby et al, 2016).

Although they are efficient and less expensive than silver, to date there is a lack of in vivo studies evaluating zinc and copper toxicity on ocular cells. CLs coated with organoselenium have shown no toxic effects on corneal epithelial cells, while successfully reducing bacterial adhesion and growth (Tran et al, 2012; Mathews et al, 2006).

Another strategy is the use of antimicrobial cationic peptides. Several studies have shown that CLs coated with those peptides, such as melamine and its derivatives, can significantly reduce bacteria colonization on some silicone hydrogel CL surfaces (Cole et al, 2010; Dutta et al, 2018) as well as fungi and amoeba CL contamination (Dutta et al, 2013). In a human clinical trial, although the incidence of corneal infiltrative events was low, the use of CLs coated with Mel-4 reduced that incidence by at least 50% when compared to an uncoated CL control (Kalaiselvan et al, 2021).

Among the compounds with antiadhesion properties, phosphorylcholine has been explored in studies, showing significant reduction of bacterial attachment on hydrogel and silicone hydrogel CLs. Due to its hydrophilic nature, phosphorylcholine increases CL wettability, improving wearer comfort (Court et al, 2001; Yao et al, 2006; Huang et al, 2007). Studies have evaluated the use of distinct complex polymer compounds with antibacterial and/or antiadhesion coatings, although further exploration of these areas is needed (Yeh et al, 2014; Xu et al, 2014; Cheng et al, 2015).

Clearly, a wide variety of compounds have been extensively explored to provide antimicrobial and antiadhesion properties to CLs. Some drawbacks, such as ocular toxicity and elevated cost, may hinder the development of effective antimicrobial contact lenses. Further research is required to overcome those drawbacks and to demonstrate the biocompatibility of the CLs if they were to be commercialized. CLS

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