Vial surface treatment and early creeping predict fogging in freeze-dried drugs

Vial fogging, the thin film of dried product that deposits on the inner glass wall of a freeze-dried vial above the cake, is one of those deceptively cosmetic defects that can escalate into a regulatory problem. When residue climbs high enough to reach the neck of the vial, it can interfere with the seal between the glass and the elastomeric stopper, threatening container closure integrity and sterility assurance, and driving up rejection rates during visual inspection.

The team evaluated six types of 10R (10 mL) borosilicate glass vials that differed in geometry, manufacturing origin and, crucially, inner surface treatment. The treated vials carried a proprietary modification called the Vialex process, applied during vial manufacture rather than as a post-production step: localized reheating during forming, particularly in the heel region of the vial, reduces alkali sublimates and produces a less hydrophilic glass surface. Standard vials of otherwise similar design did not receive this refinement.

The model formulation was deliberately chosen for its tendency to fog. It contained 4 percent mannitol, 2 percent sucrose, histidine buffer, the surfactant polysorbate 80 at 0.1 mg/mL, and 5 mg/mL of pyranine, a fluorescent dye used as a tracer. Surfactants like polysorbate 80 are common in biologic formulations because they stabilize proteins, monoclonal antibodies and antibody–drug conjugates, but they also lower surface tension and promote spreading of the liquid along the vial wall. The pyranine allowed the team to visualize liquid migration and residue deposits under ultraviolet illumination without, as far as the literature shows, substantially altering the flow behavior at the dilute concentrations used. Control cycles run without the dye produced similar fogging severity classifications, although minor differences in deposit morphology could not be ruled out.

The physical mechanism the authors invoke is Marangoni flow: fluid motion driven by gradients in surface tension at the liquid–glass–gas interface. Such gradients can arise from nonuniform surfactant distribution, from thermal gradients along the interface, or both. In thin wetting films these instabilities produce the characteristic branched or streaked residue patterns known as fingering instabilities. The paper lays out the lubrication-theory framework in which the evolution of local film thickness is governed by pressure-driven flow—capillary, hydrostatic and disjoining pressure contributions—balanced against surface-tension-gradient-driven Marangoni stresses, with mobility functions that depend on the assumed near-wall boundary condition. This mechanistic picture frames why surface energy, formulation composition and processing temperature all feed into the same interfacial instability.

Quantitatively, the surface measurements told a consistent story. Static contact angles measured with ultrapure water averaged 28.90 ± 10.48 degrees for Standard B vials versus 47.63 ± 15.78 degrees for Vialex B vials, and 43.78 ± 12.41 degrees for Standard L-V vials from Site A versus 66.55 ± 13.29 degrees for the corresponding Vialex L-V vials, with both differences statistically significant. Diiodomethane contact angles differed less between types. Surface energy calculated by the Owens–Wendt method was lower for treated vials: 56.73 ± 8.72 versus 65.90 ± 6.03 mJ/m² for the B pair, and 44.02 ± 6.99 versus 57.85 ± 7.74 mJ/m² for the L-V pair, both significant. The authors caution, however, that with substantial contact angle variability on heterogeneous glass, these values should be read as comparative indicators of relative wettability rather than absolute material constants—and notably, surface energy alone did not fully predict observed fogging severity.

The most striking result is how well the liquid’s behavior before freezing predicts the final defect. In small-batch MicroFD experiments, every one of ten Standard B vials showed visible creeping immediately after filling: in four vials the liquid rose more than 1 cm above the fill line but stayed below the shoulder, and in the remaining six it reached or approached the shoulder region. After freeze-drying, eight of those ten vials showed critical fogging with residue extending to the shoulder or higher, and the other two showed moderate fogging confined below the shoulder. The nine Vialex B vials, by contrast, showed no visible creeping at all before freezing, with stable menisci and completely clear walls, and after drying they exhibited only minimal, light fogging—residues below 1 cm above the cake, some likely attributable to mechanical splashing during handling.

Video documentation of a Standard B vial at room temperature captured the dynamics in real time: fingering patterns developed within seconds of filling and raced toward the vial shoulder, then faded over the next minute as the liquid redistributed, before gravity-driven drainage and evaporation ruptured the thin film and left isolated droplets on the wall. The state of that film at the moment of freezing determines the morphology of the final residue—branched, continuous-film or droplet-like—which explains why pre-freeze patterns do not always match the final fogging pattern exactly. Temperature during filling also mattered: as solution temperature rose, spreading intensified, and at 35 °C—a deliberately extreme condition, not a realistic manufacturing temperature—nearly complete wetting occurred in both vial types, showing that sufficiently destabilizing conditions can overwhelm even a treated surface, though Vialex B still crept and fogged less than Standard B at the same temperature.

Scale-up trials confirmed the pattern across all six vial types. Two identical lyophilization cycles with conventional freezing in a LyoStar3 dryer, each with roughly 150 vials arranged in a repeating sequence to neutralize positional heat-transfer bias near the dryer door, showed that the untreated lines—Standard A, Standard B and Standard L-V B—performed worst, dominated by critical fogging. Standard L-V B, which has a modified Lyo-Vial geometry but no surface treatment, was slightly better but inconsistent, shifting toward critical fogging in the second trial. The treated lines—Vialex A, Vialex B and Vialex L-V B—mostly fell into the no-, light- or moderate-fogging categories, with only isolated critical cases, primarily in Trial 2, underscoring the robustness of the surface treatment.

A complementary cycle using controlled ice nucleation by rapid depressurization (RD-CIN)—a 180-minute hold at −8 °C followed by depressurization from a 20 psig set point, with all other parameters unchanged—revealed that the benefit of controlled nucleation depends strongly on the vial surface. Under RD-CIN, Vialex A and Vialex L-V B vials were all classified as no fogging, a complete suppression of residue deposition. Vialex B showed intermediate behavior with most vials in the moderate category and a single critical case. Meanwhile, Standard A and Standard B exhibited critical fogging in 100 percent of vials, and Standard L-V B was mixed, predominantly moderate with nine critical vials. Controlled ice nucleation, in other words, is not a universal fix; it acts synergistically with reduced-wetting surfaces. The authors suggest the improvement may come from stabilization of the liquid interface during the pre-nucleation hold, or from partial regression of migrated liquid back into the bulk under elevated chamber pressure, though the exact mechanism requires further study.

The practical upshot is twofold. First, a simple visual assessment of creeping immediately after filling, before the vial ever enters the freeze-dryer, proved to be a strong indicator of post-drying fogging severity, offering manufacturers a rapid screening tool for formulation–container interactions—though the authors note such laboratory assessments cannot fully replicate commercial sterile manufacturing conditions. Second, inner surface treatment emerged as the most reliable lever: hydrophobic inner surfaces consistently suppress creeping by reducing liquid–solid adhesion, consistent with earlier reports that fogging falls sharply when water contact angles exceed roughly 40 degrees and largely disappears above 80 degrees. But the balance is delicate. Excessively hydrophobic vials have been reported to promote boiling and blow-up defects during vacuum-induced surface freezing, and very low surface energy can weaken cake adhesion to the vial wall. Fogging mitigation, the study concludes, requires a surface engineered to be just hydrophobic enough—low in wettability, yet not so low that it destabilizes freezing or the cake structure itself.

Subject of Research: How vial inner surface treatment and pre-freezing liquid creeping determine fogging severity in lyophilized injectable drug products

Article Title: Effects of surface treatment and pre-freezing behavior on vial fogging in lyophilization

Article References: Vorozhbit, E., Nail, S., Saca, V., McCormick, D., Fries, A., Crilley, B., & Alexeenko, A. (2026). Effects of surface treatment and pre-freezing behavior on vial fogging in lyophilization. AAPS Open, 12(1), Article 64. https://doi.org/10.1186/s41120-026-00194-y

Image Credits: AI Generated

DOI: 10.1186/s41120-026-00194-y

Keywords: lyophilization, vial fogging, freeze-drying, surface treatment, Marangoni flow, contact angle, surface energy, controlled ice nucleation, container closure integrity, pharmaceutical packaging, liquid creeping, borosilicate glass