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 shares that same basic silicone shell, but the filling, the surface texture, and the firmness of the gel inside can vary significantly. Understanding what goes into each component helps explain why different implants look, feel, and behave differently in the body.
The Outer Shell
Regardless of what’s inside, every breast implant is wrapped in a shell made of silicone elastomer, a flexible, medical-grade rubber. The FDA identifies the specific silicone polymers used as polydimethylsiloxane and polydiphenylsiloxane. During manufacturing, these polymer chains are chemically crosslinked, meaning they’re bonded together into a strong, elastic network rather than left as a loose liquid. Small amounts of silica (in its amorphous, non-crystalline form) are mixed into the shell to reinforce it, similar to how filler strengthens rubber in other industries.
Modern shells are built in multiple thin layers, dipped and cured repeatedly to reach the right thickness and durability. This layered construction also helps limit “gel bleed,” the slow migration of microscopic silicone molecules through the shell wall. Earlier generations of implants had thinner, single-layer shells that allowed more of this seepage, which is one reason implant design has gone through six generations of refinement since the 1960s.
Silicone Gel Filling
Silicone gel implants come pre-filled with a fixed amount of gel and are sealed at the factory. The gel itself is also made of silicone, but in a softer, more fluid form than the shell. What distinguishes one silicone implant from another is the gel’s cohesivity, which is essentially how firm or soft the filling is. A less cohesive gel flows and deforms more easily, producing a softer feel that many patients prefer. A more cohesive gel holds its shape better, maintaining projection and upper fullness in the breast.
The firmest option is often called a “gummy bear” implant because the gel inside behaves like the candy: if you cut it in half, it holds its shape rather than oozing. These highly cohesive implants are less likely to ripple or wrinkle under thin skin, and if the shell ever ruptures, the gel tends to stay in place rather than migrating into surrounding tissue. That said, firmer implants feel less natural to the touch and carry a slightly higher chance of flipping inside the breast pocket, especially in larger sizes with high projection.
Softer gels, by contrast, allow more natural movement and a feel closer to breast tissue. They tend to settle into the lower part of the breast over time, which some patients prefer for a more natural drape. The tradeoff is a greater tendency toward visible rippling, particularly in patients with less natural tissue covering the implant.
Saline Filling
Saline implants use the same silicone elastomer shell but are filled with sterile saline, a simple solution of sodium chloride (salt) and water at the same concentration found in your body. Unlike silicone gel implants, saline implants are inserted empty and then filled during surgery through a built-in valve. This means the surgeon can fine-tune the volume in the operating room and the incision can be smaller.
The valve itself is a small silicone component built into the shell. Standard models have a self-sealing valve on the front of the implant that closes permanently once filled. Adjustable models place the valve on the back, allowing saline to be added or removed after surgery to tweak the size during recovery. Deflation, the most common failure mode for saline implants, happens when saline leaks through a damaged valve or a break in the shell. When this occurs, the body safely absorbs the saltwater, but the implant visibly loses volume.
Surface Texture Options
The outside of the shell isn’t always smooth. Manufacturers offer different surface textures designed to interact with the body’s tissue in specific ways. Smooth shells are the simplest: the silicone surface is left as-is after curing. Textured surfaces are created through more involved manufacturing steps.
One common technique uses a “lost-salt” method. Uncured silicone is pressed into a bed of fine, granular salt, then heat-cured. When the salt is washed away, it leaves behind an irregular pattern of tiny pores, roughly 600 to 800 micrometers across and 150 to 200 micrometers deep. Another approach stamps the silicone surface with polyurethane foam to create a finer, more uniform texture. These surface variations affect how the implant interacts with the scar tissue capsule that naturally forms around it.
Polyurethane-Coated Implants
Some implants add a layer of polyurethane foam, about 1 millimeter thick, over the silicone shell. This sponge-like coating was first introduced in the 1970s and works through a different mechanism than standard texturing. The foam gradually breaks down in the body over time, and the irregular absorption pattern discourages the formation of tight scar tissue (capsular contracture), one of the most common complications with any breast implant.
Polyurethane-coated implants were briefly pulled from the U.S. market in 1991 after concerns that a breakdown product of the foam could be cancer-causing. Subsequent research put the theoretical lifetime cancer risk at roughly 1 in a million for a single pair of implants, and follow-up studies found that the concerning byproduct was undetectable in patients’ blood, appearing only in trace amounts in urine. These implants remain available in many countries outside the United States and are considered equivalent to standard textured implants for augmentation, with notably lower capsular contracture rates.
Lightweight Implants
A newer design reduces the weight of silicone gel implants by up to 30 percent. These lightweight implants use standard medical-grade silicone gel reinforced with hollow borosilicate glass microspheres, tiny glass bubbles that are chemically bonded to the gel network so they can’t shift or migrate. The microspheres are surface-treated to repel water and are locked in place during a specialized curing process.
Borosilicate glass was chosen for its crush resistance, chemical stability, and long track record of biocompatibility in medical devices. For patients receiving larger implants, the weight reduction can meaningfully decrease the long-term gravitational pull on breast tissue and skin.
How Implant Materials Are Regulated
The FDA classifies all breast implants, both saline and silicone, as Class III medical devices, the highest risk category. This means every implant design must go through premarket approval, the most rigorous pathway for medical devices. Manufacturers are required to submit detailed chemical analyses of the shell, valve, patch, and filler materials. Biocompatibility testing follows international standards (ISO 10993) that evaluate how the materials interact with living tissue, including tests for immune system reactions.
An international manufacturing standard specific to breast implants (ISO 14607) sets requirements for materials, design, sterilization, and packaging. The FDA partially recognizes this standard, diverging on certain testing requirements for biological evaluation where it applies its own guidelines instead.