3. Contact Lenses
Contact lenses are ocular prosthetic devices that have several functions, such as the correction of refractive errors in the cases of myopia, hypermetropia, and astigmatism
[60][58]. These devices are used to treat ocular dysfunctions, particularly corneal irregularities, and for post-surgical refractive rehabilitation. However, they can also be used as cosmetic lenses, such as colored and limbal ring lenses. Another interesting application of contact lenses concerns the prolonged administration of drugs
[61][59] first described by Sedlacek in 1965
[62][60]. Subsequently, significant attention has been paid to the ability of contact lenses to improve corneal penetration and drug bioavailability
[63,64][61][62]. The lens absorbs some of the drug from the tear film and then acts as a reservoir, slowly releasing the drug into the tears as the overall drug concentration in the tear film decreases. For this purpose, two methods are used. The lenses can be immersed in a solution of the drug for a period of time and then placed on the eye, resulting in a high initial release, followed by a slower and long-term release during the hours following the application, as in the administration of antibiotics or non-steroidal anti-inflammatory drugs. Alternatively, the drug can be applied to the contact lens after its application in the eye. This method is often adopted when the lens acts as a protective device (bandage lens), for example, following a corneal injury
[65][63]. However, both approaches prolong the contact time of the drug by improving its penetration through the cornea
[66][64].
The materials used for the manufacture of contact lenses include hydrogels (
Figure 5). Because hydrogels are composed of hydrophilic monomers containing electrochemical polarities, they can allow interaction with water. In addition, they are also oxygen permeable and flexible, and capable of retaining a large percentage of water within their polymer network. Due to these characteristics, hydrogels are an attractive material for the production of contact lenses.
[67,68][65][66]
Figure 5. Hydrogel-based contact lenses.
However, traditional hydrogels do not show excellent properties in encapsulating and controlling drug release due to the simple hydrophilic polymer chain, which does not undergo any additional interaction with the drug molecule. For this reason, Hu and collaborators synthesized functional hydrogels that are useful for the administration of ophthalmic drugs in the therapy of oculopathy
[69][67]. Specifically, the functional monoglucose methacrylate monomer (GMA)-β-cyclodextrin (mono-GMA-β-CD) and the functional methacrylate-β-cyclodextrin crosslinker (MA-β-C) were incorporated into the hydrogel by copolymerization in such a ratio to condition the swelling at equilibrium, the contact angle, the viscoelasticity, and the surface morphology of the hydrogel. Furthermore, the functional hydrogel containing the β-CD domain showed a better resistance to proteins and a significantly greater amount of encapsulated drug at equilibrium compared to the traditional hydrogel. Furthermore, in vivo studies have highlighted the better performance of functional hydrogels in reducing intraocular tension even compared to commercial eye drops.
In 2017, Horne et al. proposed an interesting non-aqueous approach for loading hydrophobic drugs into silicone hydrogel contact lenses and, in this regard, latanoprost was chosen as the model hydrophobic drug
[70][68]. Then, by immersing the lens in a n-propanol solution of the drug, the lens absorbed, in about 4 min, the organic solvent, causing the active ingredient to be dissolved into the contact lens by convection of the solvent, rather than by diffusion. Furthermore, it was shown that, through this loading system, the amount of hydrophobic drug inserted into the lenses was controllable, up to 450 μg per lens, and was proportional to the loading time and the concentration of the drug in the solution. The in vitro release of the drug from the lenses into the simulated tear fluid was proportional to the total amount of drug loaded, and diffusion was controlled for the first 3 days and completed within 4 days. The important result of this study was the demonstration of how loading a hydrophobic drug into silicone hydrogel contact lenses can occur within minutes and provide sustained release for many days.
In recent work, Silva and collaborators created a silicone-based hydrogel with a molecular imprint to be used in the production of contact lenses capable of releasing the antibiotic moxifloxacin hydrochloride (MXF) at an adequate rate for the treatment of eye infections (5 to 7 days), which is the recommended duration for topical treatment with conventional eye drops
[71][69]. Hydrogels were then prepared by the molecular imprinting method using acrylic acid (AA) as a functional monomer for the specific recognition of the antibiotic. In vitro experiments mimicking ocular surface fluid turnover showed that the imprinted hydrogel with the highest AA content released the MXF in the release medium at a concentration effective against
S. aureus and
S. epidermidis for approximately 2 weeks. Typical parameters of contact lenses were also analyzed, such as water absorption, wettability, transmittance, ionic permeability, and Young’s modulus. It was found that, even in the modified hydrogel, these parameters remained within the recommended ranges of values for contact lenses. Furthermore, the developed contact lenses did not show cytotoxicity and ocular irritation effects. Therefore, they may represent a promising strategy for the treatment of ocular infections.