Exploiting contact lenses for ocular drug delivery is an emerging field in the area of biomedical engineering and advanced healthcare materials. Despite all the research conducted in this area, still, new technologies are in their early stages of the development, and more work must be done in terms of clinical trials to commercialize these technologies. A great challenge in using contact lenses for drug delivery is to achieve a prolonged drug release profile within the therapeutic range for various eye‐related problems and diseases. In general, desired release kinetics to avoid the initial burst release is the zero‐order kinetics within the therapeutic range. This review highlights the new technologies developed to achieve efficient and extended drug delivery. It also provides an overview of the materials and methods for fabrication of contact lenses and their mechanical and optical properties.

This paper aims to shed light on the microstructure of tough, “double-network” (DN) hydrogels synthesized by free-radical polymerization of a monomer within a highly cross-linked polyelectrolyte hydrogel and to discuss the most efficient topological microstructure for toughness enhancement. Fourier transform infrared (FTIR) characterization of a hydrogel synthesized from the potassium salt of 3-sulfopropyl acrylate (SAPS) and 2-hydroxyethyl acrylate (HEA) demonstrated that polymer chains synthesized during the second polymerization step of a conventional DN hydrogel are grafted to the skeleton of the polyelectrolyte network. Uniaxial tensile tests performed on hydrogels synthesized from SAPS and acrylamide (AAm) indicate that linear and nonlinear polymerization of a second monomer within a network without grafting to the first network, i.e., forming a semi-interpenetrating or interpenetrating network, does not produce a tough hydrogel. Toughness enhancement of a covalent hydrogel was optimized by grafting high molecular weight polymer chains with a free end to a first, highly cross-linked polyelectrolyte network with residual unsaturation. The concentration of the grafted chains is a crucial factor in determining the mechanical behavior of the hydrogel.

A detailed physical picture of necking phenomenon in semi-interpenetrating double-network (SIDN) hydrogels was developed using the tensile mechanics of DN hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm) with different molecular weights. Uniaxial tensile tests for DN hydrogels already damaged by a compression load demonstrated the necking required a first network with a brittle structure. Although necking originates from the brittle nature of the first network, the molecular weight of polymer chains of the second network is an important factor for neck propagation. The finite deformation of DN hydrogels is characterized by two regimes of deformation: one where the neck initiates and a second where it propagates.

A simple method was developed to fabricate tough hydrogels from a chemically cross-linked neutral hydrogel. Loosely cross-linked pseudoand true-interpenetrating polymer network (IPN) hydrogels with double-, triple-, and quadruple-network structures were synthesized from acrylamide (AAm), and their mechanical properties were studied. Increasing the number of polymeric networks significantly changed the mechanical properties of pseudo-IPN hydrogels even though the chemical composition and polymerization procedure of each individual network was the same. The SN and DN hydrogels showed behavior similar to extensible soft tissue, but the TN and QN hydrogels exhibited strain localization during tensile deformation. Loading−unloading−reloading tensile experiments indicated that tensile loading causes no damage to the SN and DN hydrogels. For pseudo-IPN hydrogels where strain localization occurred, unloading before strain localization resulted in no damage, but unloading after strain localization showed a large hysteresis due to the energy dissipation from damage to the internal structure of the sample. No damage occurred prior to failure in the SN, DN, and TN hydrogels during uniaxial compression, but QN hydrogel did suffer damage during compression.

A novel approach to zero-order constant-rate drug delivery from contact lenses is presented. Quasi-Case II non-Fickian transport is achieved by nonuniform drug and diffusivity distributions within three-layer bimodal amphiphilic conetworks (β-APCNs). The center layer is a highly oxygen permeable β-APCN matrix, which contains the drug and exhibits a high drug diffusivity. The outer β-APCN layers contain no-drug and are loaded with vitamin E, which slows diffusion. In contrast to single-layer neat-polymer and vitamin E-loaded films that display first-order "burst" kinetics, it is demonstrated experimentally and by modeling that the combined effect of nonuniform distribution of drug loading and diffusion constants within the three-layer lens maintains low local drug concentration at the lens-fluid interface and yields zero-order drug delivery. The release rates of topical antibiotics provide constant-rate therapeutic-level delivery with appropriate oxygen permeability for at least 30 h, at which time ≈25% of the drug was released.