Nanoparticle Formation for Use in UV Blocking Contact Lenses

Motivation: There is a growing concern that continuous exposure of eyes to UV radiation could cause serious medical problems including cataract and retina damage. Protective gear such as hats and sunglasses provide some protection, but the UV damage could be minimized by wearing contact lenses, which provide superior protection against peripheral radiation. Currently only a few types of commercial contact lenses are approved as Class 1 UV blockers, and thus there is a need to develop new approaches for imparting UV blocking to contact lenses. We have developed a novel approach of preparing UV blocking lenses by incorporating UV blocking nanoparticles into the lenses. The particles can also be functionalized to impart color or fluorescence to prepare colored or fluorescent lenses. It may also be feasible to prepare photochromatic contact lenses using this approach.

Nanoparticle formation: The nanoparticles are prepared by controlling the polymerization dynamics by using chain terminating and chain transfer agents. In this approach, a multi-vinyl monomer, propoxylated glyceryl triacrylate (PGT) was used in our case, was mixed with a UV blocking monomer, 2-(4-benzoyl-3-hydroxyphenoxy)ethyl acrylate (BHPEA), a chain transfer agent, isooctyl 3-mercaptopropionate (IMP), and a chain terminating agent, dibenzoylmethane (DBM). DBM had the additional benefit of acting as a UV blocker in the UVA region. Thermal polymerization of the bulk solution was used to produce the nanoparticles using

0.1 wt% benzoyl peroxide (BP) as the initiator.
Our formulation was successful in forming particles that were less than 10nm in diameter whose average size could be controlled by adjusting the fractions of each component. The formation of nanoparticles was accomplished through the following proposed mechanism. The thermal initiator created free radicals that initiate polymer chains reacting. Due to its three vinyl groups, PGT causes the growing chains to become highly crosslinked, forming solid particles. IMP caps the radical ends on growing chains and produces a leaving group that can initiate another chain. DBM can also cap a growing chain, but the leaving group it produces has a much lower activity compared to other active molecules in the mix, thus acting as a chain terminating agent. While less reactive, this DBM leaving group is still able to react and start a new chain, thus a small fraction of DBM is incorporated into the particles. The reaction of the growing particles with PGT or BHPEA results in growth of the particle, while the reaction with IMP or DBM results in termination of the growing chain. After a majority of the active groups in a nanoparticle are terminated, the growth of the particle stops. Thus the particle size can be controlled by concentration of chain terminating and chain transferring agents. The results show that our approach can produce nanoparticles of controlled size with close to 10% conversion with mean particle sizes varying from 3-10nm depending on the formulation used. These particles were excellent UV blockers due to the incorporation of both BHPEA and DBM.

Application: Once formed, the nanoparticles were dialyzed with acetone and ethanol to remove the unreacted components from the nanoparticle solution. The dialyzed nanoparticles were loaded in silicone hydrogel contact lenses by soaking the lenses in a solution of particles in ethanol and acetone. This solution swelled the pores of the lens, allowing the nanoparticles to diffuse into the lens. The particle size was adjusted so that, once removed from the solution, the pores returned to their normal size and trapped the particles in the lens, preventing them from diffusing out while worn.

The UV absorption spectra of the gels are measured and mean absorbances are determined in the UVA and UVB ranges of the spectrum. Lenses loaded with about 2% particles
w/w in the hydrated lens block sufficient UV light to be classified as Class 1 blockers while maintaining transparency of the lens in the visible light region. The nanoparticles were tested to ensure that they remained in the lenses during wear by soaking them in phosphate buffer solution (PBS). The test showed that the nanoparticles are retained in the lens during soaking in PBS. The stability of the particle loaded lenses were also tested under heating and intense UV exposure. Placing the lenses in boiling water for an hour did not reduce the UV blocking. Similarly, exposure to high intensity UV light also did not degrade the UV blocking capabilities of the lenses.

Conclusions: A method has been developed for creating 3-10nm nanoparticles with good UV blocking capabilities. Incorporation of these nanoparticles into contact lenses is a promising approach for imparting Class 1 UV protection to contact lenses.