Chitin is widely found in fungal cell walls and arthropod exoskeletons, and is used as a biomedical material. However, chitin is not water-soluble, restricting its use for controlled release materials. We found that water-soluble chitosan can be acetylated to produce a chitin equivalent, or chitin gel. Chitin gel, produced by mixing chitosan solution with acetic anhydride, can be degraded by lysozyme and fetal bovine serum, so could provide an ideal means for controlled release in biological systems. We tested a combination of chitin gel with a chitin binding domain (CBD) fusion protein as a controlled release system regulated by chitin degradation. A fusion protein consisting of fibroblast growth factor 2 (FGF-2) fused to CBD bound the chitin gel, and was released time-dependently rather than as an initial burst during lysozyme degradation, suggesting that this system could provide a means for controlled drug release in biological systems. Contrastingly, the trinitrophenyl residue (TNP-X) covalently bound to chitin gel, and was released by lysozyme degradation with an initial burst. If release of CBD-FGF-2 were simply dependent on lysozyme degradation of the chitin gel, the release behavior of CBD-FGF-2 would be similar to that of TNP-X, with an initial burst. Therefore, we propose that CBD-FGF-2 repeats the cycle of binding, release, and re-binding to the chitin gel during degradation. This system allows for a time-dependent, controlled release of CBD-FGF-2 without an initial burst. (C) 2019, The Society for Biotechnology, Japan. All rights reserved.