## 3 Results
### In Vivo Animal Study  
VECs-laden composite hydrogels with different delivery systems were implanted into necrosis site of femur head in Japanese rabbits by injection, all rabbits resumed normal ambulation and daily activity within one or two days after surgery. No signs of infection and wound split, and appetite loss was not observed for rabbits. Firstly, the host reaction around tissues and bone regeneration were evaluated by X-ray scanning and histological images, the results were presented in Figures 3 and 4. Figure 3a-d confirmed the successful construction of a femur head necrosis model by using the anhydrous alcohol method within 3 weeks. Soft tissue swelling around femur head was found before injection, and such swelling vanished after 4 weeks. The necrosis site couldn't be self-headed without intervention. However, new bone tissue was observed at the necrosis site with composite hydrogels treatment since week 4, and new bone tissue was filled up at necrosis site completely at week 8. The vessels density and mean vessel diameter were quantitatively obtained by ImageJ software according to H&E and Masson staining results (20 images for each sample, which is the same necrosis site for the X-ray image), and the results were shown in Figure 5. The vessels density and mean vessel diameter in composite hydrogels with different delivery systems increased over the weeks compared to the control group. At week 4, the rabbits treated by composite hydrogel with delivery system II showed the highest value of the mean vessel diameter. However, at week 8, the highest data of the vessel density and mean vessel diameter were observed in the delivery system III group. H&E and Masson staining results confirmed that the new blood vessels were formed at the necrosis site of the rabbit femur head and that the ingrowth of new bone tissue was consistent with cell vessels density and mean vessel diameter. Figure 2: The VECs proliferation while cultured with composite hydrogels with different delivery systems at day 3. Optical microscope images of VECs in composite hydrogels with (**a**) delivery system I, (**b**) delivery system II, and (**c**) delivery system III. Meanwhile, the fluorescence images of VECs were obtained with (**d**) delivery system I, (**e**) delivery system II, and (**f**) delivery system III. The cell nucleus was represented by blue color, and cytoskeleton was represented by pink color. Scale bar: 25 μm. Figure 3: The X-ray images of the rabbit femur head in the right-posterior position at 4 th and 8 th weeks post-operation. The femoral head necrosis models were constructed by using the bone injection of anhydrous alcohol method (**a–d**), the circles indicated the necrosis district. Non-intervention treatment as a control group, images (**e**,**i**) showed femur head without treatment (control group) at 4 th and 8 th weeks respectively; images (**f**,**j**) showed femur head treated by composite hydrogel with delivery system I at 4 th and 8 th weeks respectively; images (**g**,**k**) showed femur head treated by composite hydrogel with delivery system II at 4 th and 8 th weeks respectively; images (**h**,**l**) showed femur head treated by composite hydrogel with delivery system III at 4 th and 8 th weeks, respectively. Scale bar: 1 cm, the arrows indicated the regeneration district. Figure 4: The representative optical images of H&E and Masson staining slices of cross sections at the necrosis site where the composite hydrogels were implanted at the 4 th and 8 th weeks.