Breast implants are made of a silicone rubber outer shell filled with either silicone gel or sterile saltwater (saline). Every breast implant on the market today uses this basic two-part design: a flexible silicone envelope on the outside and one of these two filling materials on the inside. The differences between implant types come down to the specific formulation of these materials and how they’re engineered.
The Outer Shell
Every modern breast implant, whether filled with saline or silicone gel, uses the same basic shell material: a flexible silicone rubber called polydimethylsiloxane, or PDMS. The shell is built by dipping a mold into liquid silicone and chemically hardening (cross-linking) the layers to form a durable, elastic sac. To make the rubber stronger and more tear-resistant, manufacturers mix in tiny particles of silica, the same mineral found in sand and glass. These nanoparticles reinforce the silicone the way rebar reinforces concrete, increasing how far the shell can stretch before it fails.
Some implant shells have a smooth surface, while others are textured. Textured surfaces were developed to help the implant stay in position and reduce the risk of the scar tissue capsule around the implant tightening uncomfortably. The texturing is created through several methods. One common approach uses a salt-leaching process, where salt crystals are pressed into the silicone surface and then dissolved away, leaving behind a pattern of tiny pits and ridges. Other manufacturers create textures by pressing the silicone against foam molds or using nanoscale imprinting techniques to produce finer, more controlled surface patterns.
Silicone Gel Filling
Silicone gel implants are filled with a thick, sticky gel made from the same base polymer as the shell, but formulated to stay soft and pliable rather than firm and elastic. The gel is created by cross-linking silicone polymers to varying degrees. Less cross-linking produces a softer, more fluid gel. More cross-linking produces a firmer, denser gel that holds its shape better. The cross-linked silicone is considered chemically inert, meaning it doesn’t react easily with body tissues, which is a key reason it’s been used in medical devices for decades.
Modern silicone gel implants come pre-filled and sealed. If the shell ruptures, the gel typically stays in place rather than leaking freely into the body. This is a significant change from earlier generations of silicone implants, which used a thinner, more liquid gel that could migrate through tissues if the shell broke. The consistency of today’s standard silicone gel is similar to a soft gummy candy.
Form-Stable “Gummy Bear” Implants
Form-stable implants, commonly called gummy bear implants, use an even more heavily cross-linked silicone gel that is noticeably thicker and denser than standard silicone gel. The higher degree of cross-linking gives the gel enough structure to hold a specific shape, typically a teardrop, even if the outer shell is cut or broken. Where a standard silicone gel implant feels like a soft, squeezable mass, a form-stable implant feels firmer and maintains its contour more predictably. This firmness is purely a result of how tightly the silicone molecules are linked together during manufacturing.
Saline Filling
Saline implants use a silicone rubber shell that is inserted empty and then filled with sterile saltwater during surgery. The saline is a medical-grade sodium chloride solution, the same type used in IV bags. The FDA requires that only sterile, pyrogen-free saline drawn from a fresh container be used for each implant and each surgery to prevent bacterial contamination.
Because they’re filled after placement, saline implants allow the surgeon to make a smaller incision and fine-tune the volume during the procedure. The trade-off is that saline feels less like natural breast tissue than silicone gel. Saline implants can also develop visible rippling under the skin, particularly in people with less natural breast tissue to cover the implant. If a saline implant ruptures, the saltwater is harmlessly absorbed by the body, making a rupture immediately obvious as the implant visibly deflates.
What’s No Longer Used
Silicone and saline aren’t the only materials that have been tried. In the 1990s, an implant called Trilucent used refined soybean oil as a filler. The idea was that soybean oil would be more “natural” and would interfere less with mammogram imaging than silicone. It didn’t work out. The UK’s medical device regulator pulled Trilucent implants from the market in 1999 after discovering that the soybean oil broke down inside the body over time, producing compounds that could bind to DNA in surrounding tissue. An expert panel later confirmed that these breakdown products were genotoxic, meaning they had the potential to damage genetic material. No evidence of actual disease was found in patients, but regulators recommended removal of all Trilucent implants as a precaution because the shell also deteriorated faster than other implant types, increasing the risk of rupture with age.
Other experimental fillers, including polyvinylpyrrolidone (a water-soluble polymer) and various hydrogels, were tested in different countries over the years but never gained lasting approval. Today, the FDA approves only two categories of breast implants: saline-filled and silicone gel-filled, both with silicone rubber shells.
How the Two Types Compare
- Feel: Silicone gel implants more closely mimic the feel of natural breast tissue. Saline implants feel firmer and can sometimes slosh noticeably.
- Rupture detection: A saline rupture is obvious within hours because the implant deflates. A silicone gel rupture can go undetected for months or years (called a “silent rupture”), which is why the FDA recommends periodic imaging for silicone implants.
- Incision size: Saline implants are inserted empty, allowing a smaller surgical incision. Silicone gel implants come pre-filled, requiring a slightly larger opening.
- Placement age: The FDA approves saline implants for augmentation starting at age 18 and silicone gel implants starting at age 22.
What Determines Implant Firmness and Shape
The feel and behavior of an implant comes down to how its silicone molecules are engineered. A softer gel has fewer chemical bonds linking its polymer chains together, so it flows and compresses more easily. A firmer, form-stable gel has many more of these bonds, which locks the material into a more rigid structure. The shell thickness and the amount of silica filler also affect how the implant feels from the outside and how durable it is over time.
Manufacturers adjust these variables to create a range of implant profiles, from very soft and natural-feeling to firm and highly shaped. The choice between them depends on the person’s body type, the amount of existing breast tissue, and the desired cosmetic outcome. But regardless of the brand, shape, or profile, every implant currently available is built from the same fundamental material: medical-grade silicone in different configurations.