Silicone Chemistry
Silicone (polydimethylsiloxane, PDMS) is a synthetic polymer built on a silicon-oxygen backbone with methyl side groups. Unlike silicon (the element), silicone is a stable, biologically inert polymer that does not degrade in physiological conditions. Its molecular structure can be varied to create materials ranging from thin oil to viscous gel to solid rubber by adjusting the chain length and degree of cross-linking. Breast implants use silicone in multiple forms: as the outer elastomeric shell, as the gel fill, and (in some designs) as internal baffle structures.
Gel Cohesivity
Modern silicone gel implants are described as cohesive — the gel molecules are cross-linked sufficiently to hold shape rather than flowing freely. Cohesivity is measured and classified: low cohesivity gels are softer and flow more (closer to earlier-generation implants); high cohesivity gels maintain shape under load and are used in anatomical form-stable implants. The degree of cross-linking directly determines the implant's feel and its behaviour if the shell is breached.
Shell Construction
The outer shell is made of silicone elastomer typically in multiple layers, ranging from 0.5mm to 2mm thick. The shell is the implant's primary containment system. Modern shells are substantially stronger than first or second generation implants. Some shells incorporate barrier layers that reduce gel bleed — the diffusion of small silicone molecules through the intact shell into surrounding tissue.
Surface Texturing
Implant surfaces range from smooth (polished) to various degrees of texturing. Textured surfaces were developed to reduce rotation of anatomical implants and reduce capsular contracture through tissue adherence. However, macro-textured implants have been associated with BIA-ALCL. Smooth implants have resurged in use following textured implant safety concerns. Nano-textured surfaces (very fine texturing) are being studied as a potential middle ground.
