Physical properties of the HA-based powder gel
The prepared HA-based powder gel was uniformly rehydrated (Fig. 3a). A SEM image of cross-linked and freeze-dried HA-based hydrogel is presented in Fig 3b. The HA-based powder gel showed a swelling ratio of 671.4 % in PBS (Fig. 3c), which means that, in this study, the powder gel has sufficient capacity to load the BMP-2 solution. The HA-based powder gel was mixed with β-TCP spherical powder using the in situ mixing process described above.
Fig. 3  a SEM images of cross-linked and freeze-dried HA-based hydrogel; b Images of the hydrated HA-based powder-gel; c Swelling ratio of the HA-based powder-gel

rhBMP-2 release results
The collagen sponge was completely decomposed in 1 day and the impregnated rhBMP-2 was fully released within 1 day (Fig. 4). In contrast, the hyaluronic acid-based powder gel composite showed slow release compared to the collagen sponge, and the amount of rhBMP-2 released was less than 20 % of the total after 7 days.
Fig. 4 In vitro release of BMP-2 (A: Hyaluronic acid (HA)-based powder-gel; Control: collagen sponge)