BOSTON — Researchers at Mass General Brigham in Boston developed 4D-printed hydrogel tissue expanders that successfully underwent animal testing for ear and breast reconstruction, according to a 2026 study published in Nature Biomedical Engineering. The study was led by Di Wang, PhD, and Y. Shrike Zhang, PhD, of the Division of Engineering in the Mass General Brigham Department of Medicine.
Tissue expansion is a surgical technique used to stretch nearby skin to generate extra tissue for reconstructing areas such as the ear, breast, or nose. The current standard relies on silicone balloons gradually filled with saline over weeks or months through repeated injections—a process that can be painful and requires frequent clinic visits. Complications with these devices include bleeding, device shifting, and injection port issues, and many patients need a second surgery to remove excess stretched skin.
The new 4D-printed expanders are made from a gel-like hydrogel material whose expansion speed and final size can be precisely controlled. Fabricated using a light-based 3D printing method, the devices were shaped like a human ear or breast based on real patient scans, enabling customization to individual anatomy. Once implanted, the expanders grow autonomously without requiring injections.
In rabbit tests, the devices expanded to 10 to 30 times their original volume while maintaining structural integrity. Expansion occurred slowly and steadily, allowing natural skin stretching. The skin exposed to the expanders showed increased surface area, healthy thinning, and growth of new blood vessels.
The 4D-printed expanders eliminated the need for repeated injections and a follow-up surgery to trim excess skin. They also reduced overall surgery time and incision size compared to standard silicone expanders and demonstrated better positional stability. The hydrogel devices absorbed small amounts of bleeding, or hematoma, while continuing to expand normally—a feature that may address a serious complication in current procedures, as hematoma can increase pressure on tissue and reduce blood flow.
Earlier self-inflating expanders suffered from rapid expansion, insufficient strength, and limited shape complexity. The new approach overcomes these issues and could extend beyond reconstructive applications to cosmetic surgery.