As any experienced clinician will tell you, administering injectables requires more than a good eye for aesthetics. The range of filler products (and tools to inject them) require a deeper understanding of the mechanics and physics to prevent accidents, maintain control, and achieve optimal results.
It’s essential to learn about the physical properties of injectables, including their viscosity level or how thick they are. But in addition to the viscosity, practitioners must also be mindful of the elasticity of a product. Elasticity, typically measured in pascals (Pa), is the ability of materials to return to their original shape and size after an applied deforming/distorting force is removed. Pascals are a measurement of pressure or stress, and are defined as the force per unit area. The combination of elasticity and viscosity is the science of G prime rheology.
G prime quantifies a product’s firmness, density, and ability to hold up under pressure. As the G prime goes up, sturdiness increases. A product with a lower G prime has more fluidity and movement to blend into softer areas. However, a stiffer product with a higher G prime number may be better suited for contouring. In addition, higher G prime fillers tend to last longer.
To have a better understanding of viscosity, we have included the table below showing the viscosity of common fluids.
|Type of Liquid||Viscosity (cP)|
*This data was collected from Viscosity Chart | Liquid Fill Viscous Data for Bottle Filling
Generally, elasticity and viscosity go hand-in-hand. As you can see in the table below, in most instances the higher the elasticity, the higher the viscosity. Radiesse® + 3% Lidocaine would be an example of an exception as it has a lower elasticity rating than Restylane-L®, but a higher viscosity rating.
Here’s a list of common fillers and their physical properties.
|Filler (Gel at 0.7Hz)||Elasticity (Pa)||Viscosity (cP)|
|Juvederm® Ultra Plus||75||17,699|
|Juvederm® Ultra Plus XC||136||32,152|
|Juvederm® Ultra XC||111||27,034|
|Radiesse® + 3% Lidocaine||554||143,100|
*This data was collected from Comparison of the rheological properties of viscosity and elasticity in two categories of soft tissue fillers: calcium hydroxylapatite and hyaluronic acid and Blunt-tipped microcannulas for the injection of soft tissue fillers: a consensus panel assessment and recommendations
Understanding a product’s characteristics helps you make informed treatment decisions. Fillers typically come with a standard needle. However, specialized needles allow for customized procedures that can boost client satisfaction and prevent issues like occlusion and pop-off.
There are several types of needles, including regular wall, thin wall, and ultra-thin wall varieties. Each has its benefits and drawbacks. Ultra-thin wall needles bring the advantage of a smaller needle for patient comfort while maintaining a large enough diameter to accommodate thicker (more viscous) materials. However, there are situations when a thicker needle wall provides much-needed rigidity. If you’re using a long needle, a regular wall is less likely to bend or break than a needle with a thin or ultra-thin wall.
Considering product viscosity is essential when selecting the appropriate needle gauge to reduce the risk of pop-off. Injections require an extrusion force to move the product through the needle. And fillers with higher G prime must be met with a greater extrusion force than more “runny” materials. If the injector has to push too hard, it becomes challenging to maintain control. As a result, the needle may detach from the syringe due to excessive force.
Another important and sometimes overlooked factor when choosing a needle is the clinician’s thumb strength. On average, males generate a maximum injector force of 95.4 N and females 64.1 N. After performing multiple repeated injections, clinician fatigue diminishes injector force ability. Choosing needles with a lower syringe volume, a smaller gauge, and a shorter length can help reduce flow resistance and the required injection force. As a result, clinicians may maintain better injection control over a long day.
Ultimately, needle selection must be based on multiple factors, including patient comfort, the viscosity and elasticity of the product, and the clinician’s physical strength and personal preferences. That’s why Air-Tite offers a wide variety of products to keep patients and providers happy for a multitude of treatments.
In addition to insulin-style fixed needle syringes, Air-Tite features low dead space needles that save you money by reducing product waste. Instead of needles, we understand that many providers prefer cannulas for filler administration. Our premium second-generation cannulas have the closest to tip port placement and dome-shaped tips prepared with a proprietary surface coating for precise product delivery and smooth tissue entry.
Frank K, Schenck TL, Gavril D, et al. Influence of rheological properties and needle size on extrusion forces of hyaluronic acid based soft tissue fillers. J Drugs Dermatol. 2021;20(5):498-502. doi:10.36849/JDD.5237
Harley Academy. Dermal filler: Know your G prime. October 5, 2021.
Sundaram H, Voigts B, Beer K, Meland M. Comparison of the rheological properties of viscosity and elasticity in two categories of soft tissue fillers: calcium hydroxylapatite and hyaluronic acid. Dermatol Surg. 2010 Nov;36 Suppl 3:1859-65. doi: 10.1111/j.1524-4725.2010.01743.x.
Sundaram H, Weinkle S, Pozner J, Dewandre L. Blunt-tipped microcannulas for the injection of soft tissue fillers: a consensus panel assessment and recommendations. J Drugs Dermatol. 2012 Aug;11(8):s33-9. PMID: 22859227.
Vo A, Doumit M, Rockwell G. The biomechanics and optimization of the needle-syringe system for injecting triamcinolone acetonide into keloids. J Med Eng. 2016;2016:5162394. doi:10.1155/2016/5162394