Corneal Nanoparticle Uptake: Size and Surface Chemistry

Topical eye drops remain the dominant format for ophthalmic treatment, but their performance is constrained by rapid clearance from the ocular surface and limited penetration through the tear film and cornea. The study by Marjan Azadi and Allan E. David addresses a central formulation question: how do nanoparticle size and surface chemistry influence uptake by human corneal epithelial cells (HCECs)? The answer is important because a nanoparticle can be designed to remain near the ocular surface, penetrate mucus, or interact efficiently with epithelial cells, but these objectives may not be achieved by the same surface properties.

Study Background and Research Question

The ocular surface presents several sequential barriers. The tear film contains a superficial lipid layer and a mucoaqueous layer with mucin gradients, while the cornea contains multiple tissue layers with different physicochemical properties. The epithelial layer represents approximately 10% of corneal thickness but contributes about 90% of the corneal barrier function, according to the reference study. Tight junctional organization and the continuous clearance of tear-film components make passive delivery of many molecules inefficient.

Conventional approaches, including viscosity enhancers, penetration enhancers, ointments, prodrugs, and in situ gels, can improve residence time or solubility but may also introduce irritation, inflammation, blurred vision, or systemic exposure. Polymeric nanoparticles offer a different strategy: they can protect unstable payloads, provide sustained release, alter surface retention, and potentially improve contact with ocular tissues. Poly(lactic-co-glycolic acid), or PLGA, is particularly useful because its formulation permits adjustment of particle size and surface characteristics.

Before this work, many studies had shown that nanoparticles with diverse properties can enter corneal cells, but the relationship between those properties and the underlying uptake route was less clear. Azadi and David therefore investigated whether size and polymeric surface modification could shift nanoparticle–cell interactions and whether uptake could be assigned to specific endocytic pathways.

Key Innovation from the Reference Study

The study’s main innovation is its integrated comparison of particle size, surface charge, and surface polymer identity within a human corneal epithelial model that includes simulated mucosal conditions. Rather than evaluating a single optimized formulation, the researchers created a physicochemically varied PLGA nanoparticle panel. Alginate and chitosan were used as mucoadhesive surface polymers, whereas polyethylene glycol (PEG) represented a mucopenetrative modification.

This design is valuable because ocular nanoparticle performance cannot be interpreted from size alone. A smaller particle may be more readily internalized, but a polymer coating can change hydration, electrostatic interactions, mucin association, and the accessibility of the particle surface to cellular membranes. By measuring uptake alongside cytotoxicity and by adding pathway inhibitors, the study connects formulation attributes with a plausible cellular mechanism rather than reporting uptake as an isolated endpoint.

The work also separates two questions that are often conflated: whether nanoparticles are tolerated by HCECs and how they enter those cells. A formulation can show high uptake but poor viability, or low toxicity but limited internalization. Assessing both properties provides a more useful basis for ocular delivery design.

Methods and Experimental Design Insights

PLGA nanoparticles were synthesized using the emulsion–solvent evaporation method. The resulting particles were surface modified with alginate, chitosan, or PEG to generate distinct interfacial properties. The reported formulations were spherical and monodisperse, with a polydispersity index below 0.2. Their sizes ranged from 100 to 250 nm, and zeta potentials ranged from −25 to +15 mV, as reported in the published article. These measurements are important controls: broad size distributions or unstable surface charge would make pathway comparisons difficult to interpret.

Short-term cytotoxicity was evaluated with the MTT assay in HCECs. The authors then used an in vitro model consisting of an HCEC monolayer integrated with a simulated mucosal solution. This configuration is more informative than a cell suspension because it introduces a simplified barrier and mucus-associated environment. Nanoparticle uptake was measured across the formulation panel, followed by experiments using inhibitors intended to distinguish energy-dependent endocytosis, macropinocytosis, caveolae-mediated uptake, clathrin-mediated uptake, and phagocytosis.

Protocol Parameters

• Nanoparticle platform: Use PLGA particles prepared by emulsion–solvent evaporation, with surface modification selected to produce mucoadhesive or mucopenetrative behavior.
• Particle quality: The reference formulations were spherical, 100–250 nm in size, and had a polydispersity index below 0.2; these values provide a literature-based benchmark for a relatively uniform particle panel.
• Surface potential: The reported zeta-potential window was −25 to +15 mV. Because coating chemistry affects both charge and hydration, zeta potential should be interpreted together with polymer identity rather than as an independent variable.
• Cell viability condition: HCECs were incubated with nanoparticles for 24 hours at concentrations up to 100 μg/mL in the MTT analysis, producing 70–100% viability across the tested conditions according to the reference paper.
• Uptake model: Evaluate formulations in an HCEC monolayer combined with simulated mucosal solution when the objective is to approximate epithelial contact under mucus-associated conditions.
• Mechanism testing: Use pathway-inhibitor experiments as comparative evidence for endocytic contributions, and pair them with viability and barrier measurements in a follow-up workflow.

The parameters above distinguish reported study conditions from general workflow recommendations. In particular, the inhibitor experiments are most useful for ranking pathway contributions under the tested conditions; they should not be treated as definitive molecular proof without complementary imaging, genetic perturbation, or trafficking assays.

Core Findings and Why They Matter

The MTT results indicated only mild short-term toxicity across the tested nanoparticle conditions. At concentrations up to 100 μg/mL and after 24 hours, cell viability remained within the reported 70–100% range. This finding supports the feasibility of the formulation platform for uptake studies, but it does not establish long-term ocular tolerability or therapeutic safety.

Uptake was primarily energy-dependent, demonstrating that passive diffusion was not the dominant explanation for nanoparticle entry into HCECs. Among the tested formulations, 100 nm PLGA nanoparticles and PEG-PLGA particles with a nominal size of 150 nm produced the highest uptake levels. This comparison is mechanistically interesting because it does not yield a simple rule that the smallest particle is always superior. The result suggests that size and surface chemistry operate together: a moderately sized PEG-coated particle may engage the epithelial interface differently from an unmodified particle of similar dimensions.

Inhibitor studies identified macropinocytosis and caveolae-mediated endocytosis as the dominant uptake pathways under the experimental conditions. Clathrin-mediated endocytosis also contributed partially, whereas phagocytosis did not appear to play a role within the studied size and surface-chemistry range. These observations help explain why formulation changes can alter intracellular delivery even when all particles share the same PLGA core.

For ocular drug delivery, the practical implication is that a successful carrier must be evaluated at the level of mechanism. Increasing mucoadhesion may prolong contact with the ocular surface but could also alter mucus interactions and cellular access. PEGylation may support movement through mucosal material, yet its effect on epithelial uptake depends on particle size and the biological interface. The study therefore supports a design-of-experiments approach in which size, coating, charge, uptake, and cell response are analyzed together.

Comparison with Existing Internal Articles

A related internal overview, Nanoparticle Uptake Mechanisms in Corneal Epithelial Cells, emphasizes the same study’s conclusion that energy-dependent endocytosis dominates and that macropinocytosis and caveolae-mediated uptake are particularly important. The present analysis adds a stronger focus on why the comparison is informative: the work links a controlled PLGA formulation series to a mucosal epithelial model, identifies the best-performing size and surface combinations, and clarifies that clathrin-mediated uptake is contributory rather than dominant.

The two discussions are complementary rather than contradictory. The internal article is useful as a concise mechanism-centered summary, while the reference paper provides the experimental basis for interpreting uptake in relation to particle uniformity, surface modification, cytotoxicity, and model design. Neither source supports the assumption that one nanoparticle coating will perform identically across all ocular tissues or disease states.

Limitations and Transferability

The model is a meaningful improvement over simple suspension cultures, but it remains an in vitro approximation. A monolayer of HCECs with simulated mucosal solution does not reproduce the complete tear film, blinking, mucus turnover, multilayered corneal architecture, stromal diffusion, resident immune cells, or vascular and inflammatory responses. Consequently, high cellular uptake should not be equated with improved delivery across the intact cornea or with increased drug concentration in a specific ocular compartment.

The study also evaluates a defined range of spherical PLGA particles and surface chemistries. Its conclusions may not transfer directly to particles with substantially different shapes, compositions, payloads, degradation rates, or aggregation behavior. Because surface modification changes several variables at once—including charge, hydration, steric accessibility, and mucin interaction—the experiments do not fully isolate the effect of any single physicochemical property.

Mechanistic interpretation is another boundary. Chemical inhibitors can indicate that a pathway is involved, but they may have incomplete selectivity, affect cell physiology independently of endocytosis, or alter nanoparticle association with the membrane. Follow-up studies could strengthen the conclusions with live-cell imaging, genetic suppression of pathway components, intracellular trafficking analysis, transepithelial transport measurements, and assays of epithelial barrier integrity. Testing drug-loaded particles would also be necessary because payload release and biological activity can change both uptake and cell response.

Why this cross-domain matters, maturity, and limitations

The study’s carrier-design principles may be conceptually useful when a small-molecule signaling modulator is being considered for ocular delivery, but that bridge remains early-stage. The reference work did not test a PKC inhibitor, measure PKC signaling, or establish efficacy in an ocular disease model. Its evidence supports a workflow for examining nanoparticle uptake and tolerability; it does not demonstrate that a particular therapeutic payload will benefit from the same size or surface chemistry. Payload solubility, release kinetics, intracellular localization, and target-cell biology must therefore be assessed independently.

Research Support Resources

For adjacent signaling experiments, researchers can use Rottlerin (SKU B6803), a PKC inhibitor, in workflows examining PKC-dependent responses such as cell proliferation inhibition and apoptosis induction. In suitable cellular models, related readouts may include caspase-3 activation and PARP cleavage. These applications are separate from the ocular nanoparticle study and should be validated with appropriate controls, including vehicle controls and orthogonal measures of pathway engagement.