How to Choose the Right Filler Type for Each Aesthetic Treatment

We know aesthetic practitioners can struggle with knowing how to choose the right filler type for any particular treatment.

Rheology is a complex topic but there are certain aspects that must be understood to ensure you’re always using the optimal filler product.

Here we break down the basic rheological properties of hyalyuronic gels, to inform your filler product selection. Plus, learn how GEM© can assist you in this.

Dr Tristan Mehta, Harley Academy Founder & CEO, shares this knowledge in an accessible way. Scroll down to watch a video with one of his top tips for how to really understand these hyaluronic acid gels...

Rheological parameters of soft tissue filler

Dr Tristan explains, “Manufacturers create these hyaluronic acid gels thinking about the tissue that they’re being injected within. These act as implants, lasting many months, sometimes years, so the filler that mimics the tissues the best will be the most natural gel.”

During the manufacturing process, he notes that laboratories can, “cross link the gels, alter the hyaluronic acid concentration, and the hyaluronic acid molecular weight. All of these factors determine the relative rheology and parameters of the gels that we use.

“This is a concept called ‘biomimicry’ where these gels are designed to be as close as possible to the tissue into which they’re being injected. And a lot of manufacturers have specific patented processes for doing this, to differentiate their gels from others’.”

If you’re interested in understanding these terms a bit more, we’ve summarised the topline information for you, below.

What is cross linking for hyaluronic acid?

Cross linking is one of those concepts that becomes clearer the more time you spend getting hands on with products. Next time you’re at a brand event or conference such as CCR, where manufacturers have products available for you to play with, give it a go!

At its core, cross linking is simply a way of stabilising hyaluronic acid (HA) so that it behaves consistently once placed in tissue. Without it, HA would dissipate far too quickly to be clinically useful.

Understanding this chemistry helps explain why certain filler products behave intuitively in some planes and less so in others. Cross linking is, essentially, the underlying architecture that informs filler product selection, safety and predictable outcomes.

Key points of cross linking for dermal fillers include:

• Hyaluronic acid chains are chemically bonded to each other using agents such as BDDE. This creates a three dimensional network that slows natural enzymatic breakdown
• The degree of cross linking determines how firm, elastic or cohesive the gel becomes, which directly affects clinical behaviour
• Higher cross linking typically increases longevity and structural support. It can also reduce softness or spread within the tissue plane
• Lower cross linking produces a more flexible, malleable filler that integrates easily. This may not hold projection under load
• Cohesivity and viscosity are influenced not only by the amount of cross linking but also by how those links are distributed throughout the gel
• Manufacturers use different technologies and patterns of cross linking.

How does hyaluronic acid concentration impact filler products?

Hyaluronic acid concentration is one of those factors that sounds simple on paper but becomes far more meaningful once you start comparing products side by side. It influences how a filler feels under the needle, how it settles in tissue and how long it stays present. It is never the only variable, but it’s a significant one.

For aesthetic practitioners, understanding hyaluronic acid concentration isn’t about memorising numbers. It’s about recognising how that chemistry translates into handling, tissue integration and long term clinical behaviour.

Here is how HA concentration impacts filler behaviour:

• Higher concentrations usually create a denser gel that holds shape more firmly and provides greater lift. However, they can feel less forgiving during injection
• Lower concentrations tend to be softer and spread more easily. This supports natural integration but won’t offer as much projection or structure
• Concentration interacts with cross linking, cohesivity and particle size. As such, two fillers with the same HA percentage can behave very differently in practice
• Products with higher HA loads may draw in more water post injection. This can be beneficial for hydration but requires thoughtful placement in areas prone to puffiness
• Lower concentration gels are often better suited to superficial planes where excessive swelling or stiffness would undermine aesthetic outcomes
• Manufacturers balance concentration with their specific cross linking technology to achieve predictable elasticity, flow and longevity.

Why does molecular weight matter for hyaluronic acid fillers?

Hyaluronic acid molecular weight quietly shapes how a filler behaves. It influences everything from how the gel interacts with enzymes to how it settles within different tissue planes. Once you understand it, product behaviour starts to make far more sense!

Here’s why molecular weight matters for hyaluronic acid-based filler gels:

• Higher molecular weight HA forms longer chains that can create a more robust, structured gel. This often contributes to lift and slower degradation
• Lower molecular weight HA consists of shorter chains that move more freely. This can improve spread and tissue integration but typically reduces filler longevity
• Most fillers use a blend of molecular weights, allowing manufacturers to fine tune elasticity, cohesivity and flow characteristics
• Molecular weight affects how the gel responds to mechanical stress. This is why some fillers feel more resilient while others feel more pliable
• Enzymatic breakdown is partly influenced by chain length, so fillers with more high molecular weight content may resist degradation for longer
• The interaction between molecular weight, cross linking density and HA concentration is what ultimately defines the filler’s clinical personality.

Testing hyaluronic acid gels through touch

Dr Tristan discloses his top tip for developing a solid understanding of filler products...

“What I like to do is test these gels with my fingers,” he explains. “One of the best tests you can do is to take some gel between your thumb and index finger. Tap them together, as I demonstrate in this video, and think about the texture and whereabouts in the face this would go. 

“In the video above, you’ll see how we can work through which treatment a specific gel is best suited to.”

What different dermal filler types feel like

Dr Tristan notes, “Rheology and biomimicry can be confusing, but often just feeling the gel can tell you a lot. 

“It’s actually a branch of physics where we’re looking at elasticity - or G prime - viscosity, and cohesivity, or tan delta. And, unless you’re a physicist who’s studied the different gels and experimented with the different gels in a lab, under the right conditions, a lot of this is meaningless. 

“As aesthetic practitioners, what we’re interested in is how does the gel feel? How long will it last? And how will it react in the tissues? These factors will help you to understand the different filler types across various product ranges.”

High G prime filler

“If you had something that was highly crosslinked, we’d call that a very ‘elastic’ gel and it would bounce. You’d feel it bounce between your fingers,” highlights Dr Tristan. “That type of ‘bouncy’ gel is something that would be injected deep, for example down onto the cheek bone, where you’d get projection.”

Medium G prime filler 

“With a medium G prime filler, this is a product that you’re going to want to be injecting in places such as the fat pads. The SOOF, for example. Or, if you want to get good projection, with lip filler, for instance. When you feel a medium G prime hyaluronic gel, you’ll notice it doesn’t have a lot of elasticity. It’s viscose and cohesive; the gel stays together as you manipulate it between your fingers, it’s not particulate. 

Low G prime filler

“Soft, low G prime filler products that offer low projection are good for blending and spreading. These feel very runny because of the difference in crosslinking, the lengths of the chains and concentration. You’ll find these gels are good to use for treatments under the eyes, in the skin and for the lips, for example.”

How do I know which filler product to use from the range I have in my clinic, for each treatment?

Many new aesthetic practitioners ask us “How do I know which filler products to use from a specific range, for each treatment?”

Dr Tristan confirms, “The good news is that, if you’re familiar with even just one brand range of dermal fillers - the one you used during your aesthetics training course, for example - it’s now very straightforward to translate this across other ranges.”

Using the Global Evidence Matrix to choose the right filler type

This is something Harley Academy has made really simple through the Global Evidence Matrix (GEM©). Whether you use GEM© via our online platform subscription, or through our physical matrices in each treatment room during trainee mentoring sessions, this information is made clear.

“What we’ve done with GEM© is, we’ve mapped across multiple product ranges for any given evidence-based technique.” Dr Tristan clarifies.

“The G prime and anatomical target information is included in GEM©. This is set out per treatment and correlated across different product ranges. Every product range will have a product that’s recommended for each particular target.”

Simplifying G prime in GEM©

Dr Tristan shares, “To make it really simple for our filler Global Evidence Matrix, we’ve categorised the G prime rating into ‘high’, ‘medium’ and ‘low’. We’re just focusing on the projection capacity. 

“Often, if something is - for example - highly projecting, it’s also highly viscose and highly cohesive. So, by focusing on a single parameter, when you’re looking through the different filler product ranges, that’s really your benchmark. You can use this to think about which is the right filler type that you want to use for each specific treatment. 

“Of course, there is additional nuance, but 9 times out of 10, you’re going to be thinking ‘Is this a stiff gel, or is this a really soft gel?’ And that is how we’ve broken down fillers for every injection you can do across the face, within GEM©.”

Information that applies across different filler brands

When Dr Tristan created the Global Evidence Matrix, he wanted to ensure the information it contained could be applied across different brands. He explained to us how this works:

“GEM© is based on anatomical targets and every filler range will have a product that’s designated as being appropriate for each of these targets. Once you know this information, you can translate it across. So, even if you’re opting for a range of hyaluronic acid-based soft tissue fillers that aren’t specifically listed in the Matrix, you’ll be able to apply the same logic.

“This is something I’ve always wanted to do as it creates a universal language for us as clinicians, that spans different product ranges.

With GEM© guidance, whatever the product range, you can be confident you’re choosing the right filler type for each treatment, every time.

Harley Academy’s prestigious medical aesthetics training in London, Manchester, and our international courses, all follows the GEM© framework. We include a free 12-month membership to the GEM© by Harley Academy platform with all our Level 7 Diploma courses, as well as our Aesthetics Accelerator and International Advanced Aesthetics Diploma programmes. For personalised advice on finding the best aesthetics training for you, as well as pricing information and details of our 0% interest payment plan opportunities, book a call with our team.

All information correct at time of publication