A dermal filler is subjected to a range of shearing, compression and stretching forces imparted upon it by muscle movement, external pressure and gravity. It also experiences incredibly high shear forces while being pushed through the tip of the needle, but needs to remain easy for the clinician to syringe accurately. Cohesion, cohesivity and crosslinking all play a part but can be difficult to measure. Targeting key rheological properties such as modulus, shear thinning and yield stress values allows you to optimise your dermal filler to resist shearing, maintain projection and spread out as intended.
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The rheology of a dermal filler is something a clinician considers every time they select a product, whether they know it or not. Any filler should be low viscosity and easy for the clinician to push through the needle, but high viscosity to stay in place once deposited. A fine line filler should spread and be easily molded within the tissue whereas a filler in the lower face, where there is more facial mobility, must still be easily molded but more resistant to spreading. While some of these properties may seem at odds with each other, by targeting specific rheological values and benchmarking to existing products it is possible to design a dermal filler to achieve specific properties without sacrificing others.
Viscoelasticity – Rotational Oscillation Testing
Dermal fillers in any position experience shear forces, especially those in the superficial dermal layers and in high movement areas of the lower face. Cheeks get rubbed, skin is pulled as the jaw moves and a dermal filler is expected to weather that stress and stay plump and in place for months at a time. Their ability to do that is based heavily on their viscoelastic properties.
Materials can react to force viscously like a liquid, elastically as a solid, or as a combination of the two. Dermal fillers fall into that third category displaying a combination of viscous and elastic deformation – viscoelastic deformation.
Viscoelasticity is measured by oscillatory testing. A rheometer gently probes the material, wobbling it back and forth, looking for delicate structure that is not detected by traditional viscometer testing. This kind of testing generates complex modulus (overall resistance to deformation), storage modulus (elastic response), loss modulus (viscous response) and phase angle (ratio between elastic and viscous response). When used together, these values tell us how spreadable and moldable the filler is once it is injected, as well as how hard or soft the filler will feel.
Compression Forces – Axial Oscillation Testing
As opposed to shear forces, compressive forces work in an axial motion, up and down. This kind of force is experienced frequently in the deep subdermal layers where muscle contractions are more common, or frequently during day to day wear when a patient does something as simple as resting their face on a pillow. Understanding how these axial forces will affect your dermal filler is vital to understanding how it will last, and how it will look, over time.
Measurement of compression forces is obtainable only on cutting edge rheometers with normal force transducers. Compression forces are measured in much the same way as shear forces, except instead of wobbling the sample back and forth, the sample is wobbled up and down. To produce a full profile for a dermal filler, both axial and rotational testing work in concert to provide a rounded set of information.
“Due to the constant compression forces that are applied on the implant throughout its in vivo life, the compression properties in both static and dynamic environments can provide valuable additional information for understanding the behaviour of a gel after implantation.”
Molliard, Samuel Gavard, et al. “Key Importance of Compression Properties in the Biophysical Characteristics of Hyaluronic Acid Soft-Tissue Fillers.” Journal of the Mechanical Behavior of Biomedical Materials, vol. 61, 2016, pp. 290–298., doi:10.1016/j.jmbbm.2016.04.001.
Dermal Filler Cohesion and Crosslinking
Many dermal filler formulations, including hyaluronic acid based formulations, rely on chemical crosslinking to impart many of the viscoelastic properties described above. The degree of crosslinking results in greater cohesion within the dermal filler, and can be a great indication of a number of properties such as spreadability and moldability. Cohesion can be measured in a few different ways: The level of cohesion is related to the storage modulus (measured oscillatory testing), where lower storage modulus means greater cohesion. For a more direct measurement specific to cohesion, drop weight analysis, where a drop of the filler is squeezed through a syringe and the weight measured, allows for a direct comparison of cohesion between samples.
Yield Stress and Shear Thinning Behaviour – The Tip of the Needle
A dermal filler that dispenses easily and smoothly from the syringe is vital to the clinicians accuracy whilst dispensing, and the overall experience when using the filler. There are two factors which play a huge part in creating this positive experience for the clinician: Yield stress and shear thinning.
The yield stress of a material describes the amount of force required to elicit significant flow. In this situation, it tells us how hard the clinician must press on the syringe in order to force the liquid through the needle. A low yield stress makes for a dermal filler which is easy for the clinician to smoothly push from the syringe. A yield stress too low however can lead to the dermal filler molding out of shape once deposited. Finding a middle ground, a yield stress high enough that the filler stays shaped whilst also being easy to syringe, is vital here.
Passing through the tip of the needle is a very high shear process considering the filler will spend the majority of its life at fairly low shear rates, at rest in a syringe or once deposited. As the clinician is syringing the filler, high viscosity will cause resistance and potentially reduce the applicators potential for fine control. Once deposited, high viscosity can be an advantage as it will help to keep the filler in place.
It is possible for a dermal filler to possess both of these at-odds properties by showing shear thinning behaviour. A material which shear thins can show high viscosities at low shear rates, but thin to show low viscosity at higher shear rates. This property can be vital for an easy to apply dermal filler. A dermal filler which shear thins too much however is liable to spread more than desired once applied. Thus finding a midground between desirable shear thinning for application and excess spreading once applied.
Dermal fillers are subjected to a number of forces throughout their working life. They are expected to be stable when stored and smooth to push through the tip of a needle. Once deposited they must resist a range of compression and shear forces. Depending on the type of filler, different spreading and moulding properties may be optimal. By targeting specific viscosity, yield stress and viscoelastic properties it is possible to design your dermal filler to perform to an exacting set of standards that clinicians and patients are looking for.