Eye drops have a short window to work. Once a formulation reaches the eye surface, blinking, tear turnover, and drainage begin removing it almost immediately. For drugs with poor solubility or limited penetration into ocular tissues, that short residence time becomes a real formulation problem.
A study in International Journal of Pharmaceutics: X used acetazolamide (a carbonic anhydrase inhibitor used in glaucoma) as the test case. Rather than formulating it as a simple suspension, the team built hyaluronic acid nanocomposite eye drops loaded with acetazolamide nanocrystals.
The idea was fairly direct: use nanocrystals to improve apparent drug availability, then use a hydrogel matrix to help stabilize the formulation and increase contact with the ocular surface.
One part of the study was especially relevant from a mechanical testing perspective. The researchers used a CellScale UniVert 1kN to perform mucoadhesion testing by measuring the detachment force between the eye drop formulations and a mucin-coated agarose surface. That test gave a mechanical readout of how strongly the hydrogel formulations interacted with a mucosal model surface.
For researchers developing hydrogels, ocular formulations, drug delivery systems, or soft biomaterials, that is where the study becomes especially interesting. The question was not only whether the formulation could release drug. It was also whether the material could remain in closer contact with the surface long enough for that release profile to matter.
Why Mucoadhesion Testing Matters for Hydrogel Eye Drops
Topical ocular delivery is convenient, but the eye is not a passive surface. Tear flow, blinking, and drainage all act against residence time. A drug formulation may look promising in a vial and still struggle on the eye if it clears too quickly.
Acetazolamide is a good stress test for an eye-drop platform because it is poorly soluble and not especially permeable. Oral dosing is common but comes with systemic exposure. The appeal of a topical version is clear, but it only works if the formulation can maintain drug availability at the eye surface and support transport to the target tissues.
To approach that problem, the team combined two material strategies:
1. Nanocrystals, which can increase the apparent dissolution behaviour of a poorly soluble drug.
2. Hyaluronic acid hydrogel, which may improve formulation stability, ocular residence, and interaction with mucin on the eye surface.
That combination led to a set of eye drop formulations where mucoadhesion testing became part of the formulation screen. The authors were not treating adhesion as a separate material property. They were using it as one way to understand whether the hydrogel could stay in contact with a mucin-containing surface more effectively than a water-based nanocrystal dispersion.
Developing Hyaluronic Acid Nanocomposite Eye Drops
They started by making acetazolamide nanocrystals (wet bead milling followed by spray drying), then blended those nanocrystals into hyaluronic acid hydrogels. The comparison set included raw acetazolamide formulations, nanocrystal water dispersions, and hyaluronic acid hydrogels loaded with nanocrystals.
Their main formulation was HA_AZM-NC(5): a hyaluronic acid hydrogel with 5% w/v acetazolamide nanocrystals. The hydrogel concentration was set at 1.25% w/v hyaluronic acid after practical screening around pH, osmolarity, viscosity, and shear-thinning behaviour.
This early formulation work matters because the final eye drop had to sit in a narrow practical range. Too thin, and it may wash away quickly. Too viscous, and it may be uncomfortable or difficult to administer. In practice, the formulation needs enough structure to help with residence time, while still behaving like a usable eye drop.
Visual comparison of the eye drop formulations immediately after preparation and after 24 hours of storage. In panel A, the acetazolamide nanocrystal formulations appear more homogeneous than the raw acetazolamide formulations. In panel B, the hyaluronic acid nanocomposite formulation shows better apparent stability after storage than the nanocrystal water dispersion. Adapted from Falcone et al. International Journal of Pharmaceutics: X. 2026.
The figure above gives a useful visual summary of the formulation challenge. The raw acetazolamide systems did not appear homogeneous after stirring, while the nanocrystal formulations were more uniform. After 24 hours, the hyaluronic acid nanocomposite formulation appeared more stable than the nanocrystal water dispersion. That fits with the broader idea of using the hydrogel matrix as more than a passive carrier.
Mucoadhesion Testing with the UniVert 1kN
The mucoadhesion testing was done using both rheological and mechanical approaches. The UniVert was used for the mechanical detachment force portion of the study.
The setup was built around a mucin-coated agarose surface. The authors spread a 10% w/v mucin suspension onto an agarose mold and allowed it to adsorb. They then placed the formulation onto the mucin surface. A probe with a double-sided adhesive disk was lowered into contact, a constant compression force of 250 mN was applied, and then the probe was raised at 0.8 mm/s. The UniVert recorded the detachment force.
This is not tensile testing in the usual sense, and it is not peel testing. It is closer to a probe-tack or pull-off style adhesion measurement, adapted for mucoadhesion testing. The preload step brings the probe and sample into contact, then the pull-off portion measures how much force is needed to detach the interface.
For an eye drop formulation, that mechanical value is not meant to reproduce the entire blink cycle. It is a simplified test. Still, it gives a practical comparison between formulations under controlled contact and separation conditions.
Mucoadhesion testing results for the eye drop formulations. Panel A shows the rheological mucoadhesivity index, where hydrogel-based formulations had positive values while water dispersions showed negative values. Panel B shows mechanical detachment force testing, performed with the UniVert Texture Analyzer, where hydrogel-based formulations required higher detachment force than the water-based formulations. Adapted from Falcone et al. International Journal of Pharmaceutics: X. 2026.
The acetazolamide nanocrystal water dispersion showed a detachment force of about 0.55 N, while the hyaluronic acid nanocomposite hydrogel showed a higher value of about 0.66 N. The difference was not enormous, but it points in the expected direction. The hydrogel matrix appeared to increase the force needed to detach the formulation from the mucin-coated surface.
It also appears that hyaluronic acid was doing most of the work in the mucoadhesion response. The hydrogel formulations showed stronger interaction with mucin than the water dispersions, while adding nanocrystals to the hydrogel did not dramatically change the mucoadhesive behaviour compared with the hydrogel controls. That makes sense. Hyaluronic acid can interact with mucin through physical entanglement and other non-covalent interactions, while the nanocrystals mainly contribute to drug loading and release behaviour.
This kind of detachment force measurement also connects with other adhesion-focused work, including a previous research highlight on switchable underwater adhesion pull-off testing.
Drug Release and Ocular Performance
After the formulation and mucoadhesion testing, the authors moved into release testing and in vivo ocular evaluation. In vitro release was measured using Franz diffusion cells in simulated tear fluid at 37°C.
The hyaluronic acid nanocomposite formulation showed an initial release phase followed by more prolonged release. The authors described the release behaviour as a combined effect of hydrogel relaxation and nanocrystal hydration. In practical terms, the hydrogel network appeared to slow and organize the release process, while the nanocrystals provided a drug reservoir.
That point is easy to miss if the study is viewed only as a glaucoma paper. It is also a biomaterials formulation paper. The mechanical and physical properties of the carrier affect how the drug is presented, retained, and released.
For another drug delivery-focused example, CellScale recently covered silk microneedles designed for antifreeze protein delivery, where material design also shaped how the therapeutic payload was delivered.
In Vivo Testing in Rabbit Eyes
The authors then tested ocular tolerance and intraocular pressure response in albino normotensive rabbits. The tolerance results were encouraging within the limits of this preclinical model. The treated eyes did not show apparent signs of corneal damage, opacity, conjunctival redness, irritation, or fluorescein-detected epithelial lesions after administration.
The intraocular pressure data were more striking. The hyaluronic acid nanocomposite eye drop produced a sustained reduction in intraocular pressure after a single administration. Compared with saline, the HA_AZM-NC(5) formulation showed significant IOP reduction across the measured time points. It also produced a more prolonged response than the commercial brinzolamide formulation used as a reference in the study.
Intraocular pressure response after topical administration of the tested formulations in normotensive rabbit eyes. Panel A shows the change in intraocular pressure over time, with the hyaluronic acid acetazolamide nanocrystal formulation showing a more sustained reduction. Panel B shows the response after 8 hours, where the nanocomposite hydrogel group had a larger pressure reduction than the comparison groups. Adapted from Falcone et al. International Journal of Pharmaceutics: X. 2026.
In the performance summary, the authors report that HA_AZM-NC(5) produced a larger overall IOP-lowering response than the commercial reference when they compared the area-under-the-effect curve. They also point out that the hydrogel formulation achieved IOP reduction using a lower acetazolamide dose than their earlier nanocrystal-only work.
It is still an early-stage result in a rabbit model, not a finished product. Questions like manufacturing, sterilization, longer follow-up, and clinical translation remain open. But within this study, the hydrogel plus nanocrystal approach changed multiple parts of the chain: stability in the vial, detachment force in mucoadhesion testing, release behaviour, and the in vivo pressure response.
This study also fits alongside work on scleral cross-linking for myopia, where mechanical changes in ocular tissue were part of the experimental story.
What Stands Out About the Study
One thing that stands out is how many measurements were needed to make the formulation story convincing.
The vial images showed whether the formulations remained visually uniform. Rheology helped narrow down a practical hyaluronic acid concentration. FTIR, DSC, DLS, and SEM were used to check whether the nanocrystals remained physically embedded and dispersed in the polymer matrix. Release testing showed how drug availability changed over time. The animal work then checked tolerance and pressure response.
Mucoadhesion testing sits in the middle of that workflow. It does not replace the release study or the in vivo work. Instead, it gives a mechanical way to compare how formulations interact with a mucin-containing surface before moving to biological performance.
That is an important role for a mechanical test. In soft biomaterials and drug delivery research, the useful question is often not “what is the material property?” in isolation. It is more often “how does this material behave in the context where it is supposed to work?”
For hydrogel eye drops, that context includes a wet, mucosal surface under repeated disturbance. A detachment force test cannot fully recreate that environment, but it can help compare candidate formulations under repeatable conditions.
Using the UniVert for Mucoadhesion Testing and Soft Biomaterials
The CellScale UniVert is a flexible mechanical testing platform used for applications such as tension, compression, bending, shear, torsion, pressure, and custom fixture-based workflows. In this study, the UniVert was used as a texture analyzer for mucoadhesion testing, where force was recorded during separation from a mucin-coated surface.
That kind of setup is a good example of why soft biomaterial testing often needs more than standard grips or platens. Hydrogels, films, coatings, mucosal delivery systems, and adhesive interfaces can each require a different fixture, contact geometry, test environment, or loading protocol.
For researchers developing mucoadhesive hydrogels, ocular drug delivery systems, wound dressings, tissue adhesives, or other hydrated soft materials, the UniVert can be configured around the test question rather than forcing every sample into the same conventional test format.
In this paper, the test question was straightforward: how much force is needed to detach the formulation from a mucin-coated model surface?
The answer helped connect formulation composition to mechanical interaction at the surface. For an eye drop, that interaction may influence how long the formulation remains available before it is cleared away.
That is what makes mucoadhesion testing useful here. It gives researchers a controlled mechanical measurement for a biological interface that is otherwise difficult to describe with formulation chemistry alone.
