# Development of a Robotic Arm Based Hydrogel Additive Manufacturing System for In-Situ Printing
## 1 Introduction  
In-situ printing is a state-of-the-art technique which can directly repair the defect site according to its geometric features [1; 2]. Compared to common three-dimensional (3D) printing, also known as additive manufacturing (AM), in-situ printing is more applicable to injury repair due to the fact that the defects normally involve curved surfaces or even more complex geometry, while 3D printing is limited to a flat substrate. Thus far, studies on in-situ printing have focused on the repair of skin and cartilage [1].  
Researchers at Wake Forest University have repaired the skin defect (full thickness) on the back of a nude mouse using in-situ printing. A micro-valve is used to print fibrin and collagen mixed with fibroblasts and keratinocytes along the curved surface (one drop per millimeter) [3]. One issue with this method is that the curing time for thrombin and collagen is normally longer than 15 min and that possible material flow may occur during curved surface in-situ printing due to the fact that the printed liquid is not cured in time.  
A bio-printing system developed by the Scripps Research Institute is suitable for cartilage in-situ repairing. A modified HP 500 inkjet printer is used to print poly(ethylene glycol) dimethacrylate (PEGDA) with human chondrocytes. Results show the osteochondral defect is successfully repairedlayer by layer. The printed cartilage is firmly attached to the surrounding tissues and proteoglycan deposition is observed [4]. However, repairing the cartilage in this way does not involve curved surface forming, whereas in actual tissue repair, curved surface forming is unavoidable. Other studies on in-situ cartilage repair, using injection [5] or pouring [6] methods to fill the defects, demonstrate uncertain forming precision due to their manual operation.  
The development of in-situ printing technology makes it feasible to directly fill defects and repair tissue during surgical operations. A flexible and precise position control system is essential for in-situ printing. A versatile robotic arm (e.g., [PERSON] surgical robots) is a possible solution which occupies little operating-room space and can perform a variety of surgical procedures [7]. A robotically controlled additive manufacturing system has been developed by Massachusetts Institute of Technology using Kuka Agilus KR1100. The working space is \(1000\:\mathrm{mm}\times 500\:\mathrm{mm}\), and large-scaled biodegradable composite is produced using chitosan and sodium alginate [8]. However, the synchronous control of robotic movement and extrusion nozzle is not achieved.  
Hydrogel with a large amount of water and a cross-linked network structure is widely used in the study of tissue engineering [9; 10]. The curing time and forming control of hydrogel present difficulties in the manufacturing of biological scaffolds [11]. Light curing technology is one of the most widely used methods for 3D printing, and it is widely used in biological printing [12]. In our previous studies, a single-light-source curing method was applied [13]. However, due to the uneven distribution of light intensity, the formed hydrogel structures showed poor geometric accuracy. To solve this problem a symmetrical light source with more uniform light intensity is applied to replace the single light source.  
In this study, a brand new in-situ printing platform was set up and the feasibility study of in-situ printing was carried out. A small-scale robotic arm was used as the motion system. A special inkjet printing nozzle system was custom designed. PEGDA light curable biological hydrogel was used as the bio-material. Essential printing parameters were determined and the in-situ curved surface forming process was studied.