# Development of a Robotic Arm Based Hydrogel Additive Manufacturing System for In-Situ Printing
## 4 Discussion  
Compared to traditional 3D printing platforms, a robotic arm has a more open printing space. In previous studies [8], robotic movement and extrusion are not explicitly linked, thereby the robotic arm and extrusion nozzle can only be controlled independently. In this paper, the motion of the robotic arm and the control of the nozzle and light sources were integrated together; the entire printing process was under synchronous control using the same computer operation interface. Therefore, the automation degree of the equipment was improved and in-situ printing was more convenient.  
Material flow is a critical concern for in-situ printing on large-scaled curved surfaces and the forming accuracy is normally affected. Light-curing illumination can be employed either after deposition of material or during the printing process, and the latter is more conducive to achieve a rapid curing [14]. In order to ensure a rapid solidification of liquid material, a double-light-source nozzle system was applied. Because the droplet size was small and had a relatively high energy density, the actual curing time could be reduced down to 0.8 s. Moreover, the symmetrical arrangement of light sources ensured the uniformity of light density so that the base material was evenly cured on both sides instead of on one side when using the single light source. The geometric accuracy of formed structures was therefore improved.  
Polyethylene glycol (PEG) is one of the most widely used synthetic materials because of its hydrophobicity, biocompatibility, and because it is easy to modify. PEGDA is synthesized from PEG by diacrylate modification, which is a hydrogel precursor. After the addition of the photo initiator, the light curing reaction can occur by irradiation of UV light [15; 16]. In mechanical testing, a monotonic positive relationship was observed. It is obvious that the mechanical property of the hydrogel is governed by the PEGDA solute. A higher PEGDA concentration leads to a higher elastic modulus. This phenomenon means that the mechanical properties of PEGDA hydrogel can be tailored to match versatile application requirements. For instance, a low mechanical strength hydrogel can be used to repair skin while a relatively high mechanical strength hydrogel can repair cartilage [17].  
Figure 7: (**a**) The defect was filled by ring path in-situ printing; (**b**) the defect was filled by parallel path in-situ printing; (**c**) mouse model with unfilled defect; (**d**) mouse model with filled defect.  
In this paper, a 20% (\(w/v\)) PEGDA solution was chosen as the printing concentration, because this concentration has the appropriate formability and mechanical properties. Cornell University uses 20% (\(w/v\)) PEGDA as biomaterial to optimize the parameters of a bio-printer [18]. The Illinois Institute of Technology chooses 15%, 25%, and 40% (\(w/v\)) PEGDA as the material to analyze cell adhesion and proliferation. The results show that increases in hydrogel concentration cause decreases in cell adhesion and increases in proliferation [19]. Therefore, 20% is a suitable concentration and can be adjusted if necessary for this study.  
Different PEGDA concentrations or the addition of other ingredients (e.g., cells, growth factors) may affect the curing time and droplet diameter. In future work, the effects of different material concentrations on printing parameters will be considered. Cells will be added to the material for in-situ printing on the basis of this feasibility study.