1. What is Rheology?

Rheology is the science that studies the deformation and flow of matter. It bridges the gap between elasticity (like a spring, which can recover its shape) and fluid mechanics (like water, which flows). It answers the question: How will this material respond when I push, pull, or stir it?

2. What is Rheological Properties of Hyaluronic Acid?

Hyaluronic acid solutions are not simple, water-like liquids. They are typical non-Newtonian fluids, and their rheological properties are mainly reflected in the following aspects:

–High Shear-Thinning

This is the most well-known and crucial rheological property of hyaluronic acid.

Phenomenon: At rest or under low shear rates, it is very viscous, even gel-like. However, when subjected to high shear rates (e.g., rapid stirring, application massage, or injection through a fine needle), its viscosity drops dramatically, becoming as fluid as water.

Principle: At rest, the long-chain HA molecules entangle with each other, forming a vast, disordered network structure that creates high resistance to flow. Under high shear, these long chains temporarily align in the direction of flow, disentangling from each other, which significantly reduces flow resistance.

Application Examples:

• Aesthetic Injections:High-concentration Hyaluronic acid fillers are very viscous before injection. The shear-thinning property is key: it allows the viscosity to decrease as it’s pushed through the syringe and needle, making injection feasible. Once inside the tissue, the shear force disappears, and it instantly recovers its high viscosity, providing support and volume.
• Skincare Application:When you rub a serum between your palms, it feels thinner. But after applying it to the face, it recovers its viscosity, allowing it to adhere well to the skin.

–Viscoelasticity

Hyaluronic acid exhibits properties of both a viscous liquid and an elastic solid.

When you rapidly compress or stretch it, it can temporarily resist deformation and partially recover its shape after the force is removed. This property allows it to cushion impacts and protect tissues and cells. For example, in synovial fluid, the elasticity of HA helps cushion the impact between bones during jumping or running.

Conversely, under slow, continuous force, it flows like a liquid and dissipates energy. Thus, in joints, the viscosity of hyaluronic acid provides lubrication for smooth movement.

–Pseudoplasticity and Thixotropy

These properties are related to shear-thinning but have subtle differences.

• Pseudoplasticity: Refers to instantaneous shear-thinning and recovery. It thins immediately when force is applied and thickens immediately when the force is removed.
• Thixotropy: Refers to the viscosity taking some time to recover to its initial value after the shear force is removed. This is like ketchup: you shake it (applying shear) to thin it, and after pouring, it doesn’t thicken back instantly but takes a few seconds to recover.

Why Are Rheological Properties So Important?

These properties directly determine the function and application efficacy of hyaluronic acid in various fields:

In Aesthetic Medicine and Healthcare:

Rheological properties determine a filler’s lifting capacity, molding capability, injection smoothness, and persistence in the body. A well-designed HA filler must have precisely controlled rheological performance. Furthermore, appropriate viscoelasticity can help reduce injection pain and post-procedural swelling. When used as a viscoelastic agent in ophthalmic surgery or injected into joint cavities for arthritis treatment, its rheological properties provide protection and lubrication.

In Skincare:

The shear-thinning property provides a smooth, easy-to-spread experience. Its high viscoelasticity forms a breathable, moisturizing film on the skin’s surface, locking in moisture and providing a tightening effect. Whether an Hyaluronic acid serum feels slippery and non-greasy or sticky and stringy depends entirely on the rheological properties determined by the molecular weight and concentration of the hyaluronic acid used.

In Biological Functions:

In the extracellular matrix and synovial fluid, the rheological properties of hyaluronic acid are crucial for maintaining tissue structural integrity, regulating cell migration, and transmitting mechanical signals.

Summary

The rheological properties of hyaluronic acid are the scientific code describing “how it flows and deforms under force.” It is not a single parameter but a collection of behaviors (like shear-thinning, viscoelasticity). Understanding these properties not only explains why HA has certain sensory characteristics and efficacy in skincare and aesthetic products but is also the core scientific basis for designing and optimizing related products. Stanford Chemicals Company offers sodium hyaluronate powders with varying molecular weights and viscoelastic properties.

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