Great question! Breast tissue expanders are most commonly filled with saline because saline is safe, easily adjustable, and well-tolerated by the body. Saline expanders allow gradual filling through a valve, which helps stretch the skin and muscle gently over time in preparation for the permanent implant. That said, there are air-filled tissue expanders available, but they are much less commonly used. Air expanders can be lighter and may provide some advantages in specific clinical scenarios, but they are not as widely adopted because air can be more difficult to control and may have risks related to air embolism or leakage. Overall, saline remains the standard choice for tissue expansion due to its safety profile, ease of monitoring, and surgeon familiarity.