5.3.2. Synthetic Hydrogel Based Nanocarriers

The synthetic hydrogels are easy to customize and indeed are a close model of biological matrices. Hydrogels exhibit high hydrophilicity and high hydration [142]. The most commonly used are poly(hydroxyethyl methacrylate) (PHEMA), poly(ethylene oxide) (PEO, also called poly(ethyleneglycol) (PEG)), polyvinylpyrrolidone (PVP), poly(acrylic acid)(PAA), polyacrylamide (PAM), and poly(vinyl alcohol) (PVA) [143]. These polymers are all used in several ophthalmic applications with good results [144–147].

Anti-VEGF antibodies (anibizumab or aflibercept) were loaded within poly(lactic-co-glycolic acid) microspheres, and successively suspended within poly(N-isopropylacrylamide)-based hydrogel and injected into a laser-induced rat model of choroidal neovascularization (CNV). Upon ocular administration, the rats showed smaller CNV lesions (less than 60%) with respect to non-treated animals [148].

Agrahari et al. [149] obtained novel pentablock (PB) copolymer (PB-1: PCL–PLA–PEG–PLA–PCL) based nanoformulations suspended in a thermosensitive gelling copolymer (PB-2: mPEG–PCL– PLA–PCL–PEGm) with a size of 150 nm. This composite nanoformulation was used to deliver macromolecules to slowly release drugs without toxic e ffects on the posterior segmen<sup>t</sup> of the eye. In vitro tests performed on ocular and mouse macrophage (RAW 264.7) demonstrated their biocompatibility and potential to be clinically translated.

Yang et al. [150] reported the production of non-toxic polyamidoamine (PAMAM) dendrimer hydrogel/poly(lactic-co-glycolic acid) (PLGA) NP platform (HDNP) to treat glaucoma. The dendrimers were loaded with brimonidine and timolol maleat, which underwent an enhanced uptake together with a slow release in vitro up to 28–35 days. The topical administration of the same drug-loaded denndrimers in normotensive adult Dutch-belted male rabbits strongly reduced the introcular pressure (IOP) of about 18% maintaining a glaucoma reduction for four days. Conversely, Individual Dendrimer Hydrogel Formulations (DH) and NP Formulations (NP) decreased the disease symptoms only for 48 h (Figure 6).

**Figure 6.** Fundus camera acquisitions of rabbits' eyes after hybrid polyamidoamine (PAMAM) dendrimer hydrogel/poly(lacticco-glycolic acid) (PLGA) nanoparticle platform (HDNP) and NP Formulations (NP) topical administration FluoSpheres were confined in nanosistems to follow the distribution and retention in the eye. The analysis was performed at the end of 1, 3, 5, and 7 days of formulations instillation. Left: regular fundus camera images of the eyes; right: fluorescent fundus camera images. Adapted with permission from [150], copyright (2012), American Chemical Society.

### *5.4. Active and Self-Propelling Nanoparticles: New Generation of Nanobots in Retinal Disease*

The customization of NPs formulation at a molecular level has interesting outcomes for reproduction of fundamental phenomena such as macro- and microorganisms' survival mechanisms [151]. For example, bacteria, uni- or multicellular organisms, and sperm cells are able to sense and actively respond to external stimuli by the generation of movement. In particular, they convert an external energy sources (temperature, magnetic fields, adhesion forces, chemical gradients) into mechanical work that induce migration. This is typical in processes such as thermotaxis, magnetotaxis, haptotaxis, chemotaxis [152]. By taking inspiration from nature, we have also now a panel of NPs able to self-propel towards a gradient of nutrients and to mimic biological taxis [153]. However, the development of such NPs still has many challenges because at nanoscale level, the physical principles of propulsion are different due to the presence of the Brownian effect that can interfere with nanomptors by collisions with solvent molecules. One of these challenges is the generation of particles having asymmetrical geometries at the nanoscale, as this provides guidance for the movement directionality. Bacteria like *Escherichia coli*, in fact, achieve propulsion by non-time-reversible motion of long flagella, and this asymmetric shape of the body is critical for motion generation in a specific direction [154].

It is clear that self-propelling NPs represent a new powerful tool in drug delivery, which is usually dominated by NPs passive diffusion. It should not come as a surprise that such strategies have been employed also for delivery in ocular medicine. For example, Wu et al. [155] designed micro-vehicles that can actively propel through the vitreous humor to enrich retina. The movement can be activated by helical magnetic micropropellers (~120 nm in diameter and ~400 nm in length) with liquid layer coating to reduce the adhesion of biopolymeric network. These nanovectors propelled in nanoporous hyaluronan solution, a fluid mimicking the vitreous. The inspiration for nanotool development is due to the liquid layer that characterizes carnivorous *Nepenthes* pitcher plant, which presents a slippery surface on the peristome to catch insects [156,157]. Porcine eyes were used as a model of human eyes: applying the external magnetic field, micropropellers exhibited controllable propulsion from eyeball to retina within 30 minutes. Instead, passive silica microparticles did not penetrate the vitreus (Figure 7). Nanorobots were also used to perform microsurgery on the retina and surrounding membranes thanks to the possibility of injecting them elsewhere in the body and bringing the drug to the target eye region [158]. Some intravitreal implants have recently developed to perform more clinical-inspired experiments. These nanotools, constituted by polymeric containers (silicon, poly(vinyl alcohol), or ethylene vinyl acetate), were placed in the posterior segmen<sup>t</sup> of the eye and they are permeable to lipophilic drugs. They did not display any toxicity and had a long retention time. These nanosystems have been extremely useful to treat proliferative diabetic retinopathy, retinal vascular occlusion, and uveitis [159].

**Figure 7.** (**a**) SEM and ESB-SEM images of the micropropellers. (**b**,**<sup>c</sup>**) incomplete rotation of an uncoated micropropeller for one period (p) in the vitreous was showed in schematic and time-lapse microscopy, whereas the coated propeller exhibited propulsion under magnetic stimuli in vitreous for 1.5 s (nine periods). Red arrows showed the propulsion direction of propeller. The original position of nanotools and after the rotating magnetic field application were showed with green and red dashed lines. Scale bars, 1 μm. (**d**) Schematic illustration of micropropeller movement in the vitreous. (**e**) Left: fluorescent acquisitions showed the micropropellers labelled red on retina. Right: passive fluorescent particles were accumulated near center of vitreous. Nuclei of cells were stained with DAPI. Scale bar, 20 μm. Adapted from reference [155] under the terms of the Creative Commons Attribution-Non Commercial license.
