## 4 Conclusions  
1. The SA-GEL printing material was prepared. The properties of SA-GEL composite material under various ratios were analyzed from viscosity tests and mechanical performance. The optimum material ratio (sodium alginate:gelatin = 3:8) was determined by comprehensive evaluation.
2. Modeling and simulation analysis of gel material extrusion process. First of all, the rheological properties of gel materials were analyzed, and then the cone needle model was simplified. Considering the slip effects, the mathematical model of extrusion flow was established from two facets of shear flow and slip flow. Finally, the printing line width and porosity prediction model was obtained, and the pressure and needle movement were analyzed by fluent simulation software. The variation law of flow rate, line width and porosity of support is affected by velocity and other factors.
3. In study of pressure, the flow model and fluent expansion simulation were validated by the measurement of extrusion flow rate and the experiment of extrusion swell. Then the needle speed of 10.7 mm/s was selected from the influence of needle speed on stent size and line width. In the analysis of fiber spacing, it was found that the mechanical strength of scaffolds was not enough when the spacing was too large, and the cells could not grow well when the spacing was too small, and the interval of porosity and mechanical strength was 1-1.4 mm. In the analysis of influence factors about different internal scaffold structure, by adjusting the proper internal structures, mechanical properties of scaffolds can be improved, when the internal structure of the scaffold was 0/45-1.2, the Young's and dynamic modulus were \(86.6\pm 10\) KPa and \(671.4\pm 31\) KPa respectively, which has good mechanical property.
4. C5.18 osteoblasts cells were implanted on the scaffold surface and cultured; the results indicate that the cells would survive well.
5. Finally, the performance of the printed composite SA-GEL scaffolds was tested and analyzed. The mechanical properties of the bracket show that the mechanical performance of the bracket is mainly affected by the porosity, the contact area between the extruded fiber lines and the relative position of the extruded lines. These experimental results support the original simulation that, under the control of printing parameters such as a temperature of 37 C\({}^{\circ}\), pressure of 1.8 bar, moving speed of 10.7 mm/s and an interval of 1.2 mm, better scaffold printing results can be obtained for newly prepared sodium alginate-gelatin composite bio-printing materials.  
Conceptualization, Y.G. and M.S.H.A.-F.; Data curation, F.W., Z.B., J.H., W.L. and G.C.; Formal analysis, Y.G., F.W., X.B. and H.L.; Funding acquisition, Y.G., H.S. and G.C.; Investigation, M.S.H.A-F., L.S. and J.H.; Methodology, Y.G., M.S.H.A.-F., X.B. and W.L.; Project administration, Y.G.; Resources, F.W., L.S. and W.L.; Software, Y.G., F.W., L.S., Z.B. and J.H.; Supervision, Y.G. and M.S.H.A.-F.; Validation, X.B., H.L. and A.B.S.; Writing--Original draft, Y.G., F.W., X.B. and H.S.; Writing--Review & editing, M.S.H.A.-F. All authors have read and agreed to the published version of the manuscript.  
The work is financially supported by the National Natural Science Foundation of China (51805475, 51675148, 51275141, 51305113), Key Discipline of The Ocean Techntronic Equipment's Technology, Nature Science Foundation of Zhejiang Provinces Technology 101, 51475129, 3D assembly of complex biological structure (LY14E050026), Zhejiang Province Higher Visiting Scholar project Technology 101, 51475129, Malaysia's Ministry of Higher Education (DIP-2017-001), and Outstanding Young Teachers Fund of Hangzhou Dianzi University (GK160203201002/003).  
The authors declare no conflicts of interest.