Most dermal fillers are made of hyaluronic acid, a sugar molecule that occurs naturally in your skin, joints, and cartilage. But hyaluronic acid is just one of several materials used in injectable fillers. The FDA recognizes four categories of filler materials: hyaluronic acid, calcium hydroxylapatite, poly-L-lactic acid, and polymethylmethacrylate microspheres. Each has a different composition, a different mechanism, and a different lifespan in your body.
Hyaluronic Acid: The Most Common Filler
Hyaluronic acid (HA) is a type of polysaccharide, essentially a long chain of sugar molecules, that your body already produces. It binds to water and swells into a gel, which is what gives skin its plumpness and hydration. The HA in fillers can come from bacterial fermentation or, less commonly, rooster combs.
Raw hyaluronic acid breaks down quickly in the body, sometimes within days. To make it last, manufacturers chemically bond the HA molecules together in a process called cross-linking. The industry-standard cross-linking agent is a compound called BDDE, which connects individual HA chains into a stable mesh. This mesh resists your body’s natural enzymes much longer than free-floating HA would. After the cross-linking reaction is complete, the finished product contains less than 2 parts per million of unreacted BDDE, a trace level the FDA has assessed as safe.
The degree of cross-linking determines how the filler behaves. More cross-links create a stiffer, firmer gel with greater lifting power, useful for areas like the jawline or cheeks. Fewer cross-links produce a softer, more pliable gel better suited for lips or fine lines. Manufacturers also adjust the molecular weight of the starting HA, the temperature and pH during production, and post-production steps like sieving to fine-tune the final texture. Many HA syringes include a small amount of lidocaine, a local anesthetic, mixed directly into the gel to reduce pain during injection.
The FDA estimates HA fillers last roughly 6 to 12 months, but degradation studies show more variation than that. MRI imaging has detected HA filler persisting up to 27 months in the midface, and laboratory analysis of commercial products found complete decomposition times ranging from about 740 days to over 2,000 days depending on the brand. One practical advantage of HA fillers: they can be dissolved with an enzyme called hyaluronidase if the results are unsatisfactory or a complication arises.
Calcium Hydroxylapatite: A Mineral-Based Filler
Calcium hydroxylapatite (CaHA) is a synthetic version of the mineral that makes up the hard structure of your teeth and bones. In filler form, it consists of tiny CaHA microspheres, each 25 to 45 microns in diameter, suspended in a gel carrier made primarily of sterile water, glycerin, and a small amount of carboxymethylcellulose (a common thickening agent). The brand most associated with this material is Radiesse.
When injected, the gel carrier provides immediate volume. Over the following weeks, the carrier is absorbed and the microspheres act as a scaffold that stimulates your body to produce its own collagen around them. The microspheres eventually break down through hydrolysis and enzyme activity. The FDA lists CaHA’s effects at roughly 18 months, though histological samples have found microspheres still present up to six years after injection in certain facial areas. Unlike HA fillers, CaHA cannot be dissolved with an enzyme, so the decision to use it is less easily reversible.
Poly-L-Lactic Acid: A Collagen Stimulator
Poly-L-lactic acid (PLLA) is a biodegradable synthetic polymer from the same family of materials used in dissolvable surgical stitches and orthopedic bone screws. It has a long track record in medicine. The best-known PLLA filler is Sculptra.
PLLA works differently from fillers that add immediate volume. It’s injected as a suspension of microparticles that gradually stimulate your body’s own collagen production. Results develop over several weeks and typically require a series of treatment sessions spread across a few months. The effects can last up to two years, though some studies have measured the material persisting longer. Like CaHA, PLLA is not reversible with an enzyme.
PMMA Microspheres: The Permanent Option
Polymethylmethacrylate (PMMA) is the only non-absorbable filler material approved by the FDA. The product Bellafill is 20% PMMA microspheres (roughly 6 million per syringe) suspended in 80% bovine collagen gel, with 0.3% lidocaine for comfort. Each PMMA microsphere is round, smooth, and 30 to 50 microns in diameter.
After injection, the bovine collagen gel provides initial volume and then is gradually absorbed. The PMMA beads remain permanently, serving as a scaffold around which your body builds its own collagen. Because the microspheres are not biodegradable, this is a lasting commitment. A skin test is typically required beforehand to rule out an allergic reaction to the bovine collagen component.
Fat Transfer: Using Your Own Tissue
Autologous fat grafting uses your own body fat as a filler. The process has four stages: selecting a donor site (commonly the abdomen or thighs), harvesting fat through a small liposuction procedure, processing the fat through centrifugation to separate viable fat cells from oil and fluid, and injecting the purified fat into the target area. Because the material comes from your own body, there is no risk of an allergic reaction.
Fat transfer is a more involved procedure than syringe-based fillers and is typically performed in a surgical setting. Not all transferred fat cells survive in their new location, so practitioners often slightly overfill the area. The fat that does survive can provide results lasting years, making it a semi-permanent option, though some volume loss over time is normal.
What All Fillers Have in Common
Regardless of the active material, injectable fillers share a few supporting ingredients. Most contain sterile water or saline as a base. Many now include lidocaine pre-mixed into the syringe at low concentrations so a separate numbing injection isn’t always necessary. Gel carriers like glycerin or carboxymethylcellulose help keep microspheres evenly distributed in the syringe and ensure smooth injection. These inactive components are absorbed by the body quickly after injection and play no role in the long-term cosmetic effect.
The differences between fillers come down to what your body does with the active material. HA adds volume directly by binding water. CaHA and PLLA act primarily as scaffolds that trigger your own collagen production. PMMA provides a permanent structural framework. Each material suits different goals, different facial areas, and different timelines, which is why practitioners often use more than one type across different parts of the face.