Viscosity Measurement Methods for Sodium Hyaluronate Prefilled Syringes
Accurate viscosity measurement is essential for sodium hyaluronate prefilled syringe manufacturing—affecting everything from fill accuracy to injectability to regulatory acceptance.
Why Viscosity Matters in Sodium Hyaluronate Prefilled Syringes
Sodium hyaluronate—the sodium salt of hyaluronic acid (HA)—is widely used in prefilled syringe medical devices for ophthalmic, orthopedic and aesthetic applications. Its therapeutic value depends on its molecular weight and concentration, both of which directly influence viscosity.
But sodium hyaluronate presents a manufacturing challenge: it exhibits non-Newtonian, shear-thinning behavior. Its viscosity decreases under applied shear stress, which means a single viscosity value measured at one shear rate does not fully characterize the material. This behavior affects compounding, filling, sterilization and ultimately the patient's injection experience.
For manufacturers, viscosity measurement is not just a specification check. It is a quality-critical control point that influences batch release, regulatory submissions and device performance.
The Rheological Challenge: Non-Newtonian Behavior
Unlike simple Newtonian fluids (such as water or saline), sodium hyaluronate solutions exhibit complex flow behavior. At low shear rates, the long-chain polysaccharide molecules remain entangled, producing high apparent viscosity. As shear rate increases—during filling or injection—molecular chains align and disentangle, reducing viscosity.
This shear-thinning property is clinically useful: it allows high-viscosity formulations to flow more easily through narrow-gauge needles during injection. But it also means that viscosity measurements must be performed under controlled, reproducible shear conditions to generate meaningful quality data.
Additionally, sodium hyaluronate is sensitive to thermal degradation. Sterilization processes—particularly moist-heat terminal sterilization—can reduce molecular weight and viscosity. Studies have shown that the degree of viscosity loss depends on both temperature and exposure time, with shorter high-temperature cycles often causing less degradation than longer low-temperature cycles.
Rotational Rheometry: The Standard Method
The European Pharmacopoeia (Section 2.2.10) and other regulatory standards specify rotational viscometry as the method for measuring viscosity of pharmaceutical formulations. For sodium hyaluronate, rotational rheometers with cone-and-plate or plate-plate geometry are the preferred instruments.
Cone-and-Plate Geometry
In cone-and-plate rheometry, the sample is placed between a flat plate and a shallow cone (typically 1° angle). The cone rotates at controlled speeds while the instrument measures the torque required to maintain rotation. This geometry provides uniform shear rate across the sample, which is essential for characterizing non-Newtonian fluids.
Key measurement parameters include:
Shear rate range: Typically 0.01 to 10 s⁻¹ for sodium hyaluronate, capturing both the zero-shear plateau and the shear-thinning region
Temperature control: Measurements are typically performed at 25°C or 37°C, depending on the application
Sample volume: Cone-and-plate geometry requires small sample volumes (often less than 1 mL), which is advantageous for expensive formulations
Interpreting Viscosity Curves
A complete rheological characterization of sodium hyaluronate includes:
Zero-shear viscosity (η₀): The plateau viscosity at very low shear rates, reflecting the fully entangled molecular network
Flow curve: Viscosity as a function of shear rate, showing the shear-thinning transition
Thixotropy testing: Time-dependent viscosity recovery after shear, relevant for formulations that must regain structure after injection
These measurements provide a more complete picture than a single-point viscosity value and support robust specification setting for batch release.
Connecting Viscosity to Device Performance
Viscosity specifications for prefilled syringes must account for downstream device performance. Key functional tests include:
Glide force: The force required to move the plunger during injection, directly influenced by formulation viscosity
Break loose force: The initial force needed to start plunger movement after storage
Injectability: The overall ease of injection, which affects patient experience and compliance
A formulation with viscosity outside specification may pass chemical identity tests but fail functional device testing—or worse, create an unacceptable injection experience for patients. This is why viscosity measurement must be integrated with device testing in a comprehensive quality control program.
Process Considerations: Protecting Viscosity Through Manufacturing
Maintaining target viscosity through the manufacturing process requires attention to several factors:
Compounding: Hydration time, temperature and mixing speed all affect the final viscosity of sodium hyaluronate solutions
Filling: High-viscosity gels require specialized filling equipment with appropriate pump designs and fill speeds to avoid shear degradation
Sterilization: Terminal sterilization cycles must balance sterility assurance with viscosity preservation; process development should include viscosity testing before and after sterilization
Storage: Long-term stability studies should include viscosity as a critical quality attribute
Manufacturing Expertise for High-Viscosity Prefilled Syringes
Successfully manufacturing sodium hyaluronate prefilled syringes requires more than standard fill-finish capabilities. It requires specialized experience with hydrogels, on-site analytical testing and integrated quality systems that connect formulation properties to device performance.
AbbVie Contract Manufacturing's Pringy-Annecy facility in France serves as the company's global Center of Excellence for prefilled syringe filling for medical devices. The site has decades of experience with hydrogels and highly viscous products, with manufacturing capabilities specifically designed for these challenging formulations.
The facility's on-site analytical testing capabilities include rheology, supporting viscosity characterization throughout development and commercial manufacturing. Full QC release testing—including HPLC, UPLC, spectroscopy (IR, UV), subvisible and visible particle analysis, and comprehensive device testing (break loose force, glide force, tip cap pull off force, leak test)—is performed at the site.
With three separate filling suites, including two high-speed commercial lines and one semi-automated clinical/launch line, the Pringy site offers flexibility for both development-scale and commercial-scale programs. The facility's aseptic fill-finish capabilities include experience with fill volumes from 1 mL to 10 mL in both glass and plastic syringes.
The integration of R&D with operations at a single site enables efficient tech transfer and process optimization—critical for formulations where viscosity control requires close coordination between development and manufacturing teams.
Conclusion: Viscosity as a Quality-Critical Attribute
For sodium hyaluronate prefilled syringes, viscosity measurement is not a routine specification check. It is a quality-critical control point that connects molecular properties to manufacturing process control to device performance to patient experience.
Effective viscosity testing requires appropriate rheological methods, controlled measurement conditions and integration with device functional testing. And successful manufacturing requires a partner with proven experience handling high-viscosity hydrogels and the analytical capabilities to support robust quality systems.
To discuss prefilled syringe manufacturing for sodium hyaluronate or other high-viscosity formulations, contact us.