# Concept, Design, Initial Tests and Prototype of Customized Upper Limb Prosthesis
## 3 Results
### Prototyping the Mechatronic System Parts  
One of the modern, and relatively cost affordable, techniques for prototyping is 3D printing. Mechanical components of the mechatronic system were prototyped using several rapid prototyping techniques due to various offers on the market. The scope was to obtain accurate prototypes for the parts made of fit materials (polymers, resins) and, not least, with moderate costs. The first technique used was FDM (fused deposition modeling), the material was biodegradable thermoplastic polymer, PLA, and the equipment was Creality Ender 3-Pro [27]. Only the exterior surface of the index finger was printed for preliminary tests (see Figure 16). The prototype proved to be accurate enough for manually testing finger mobility and for preliminary tests of the pressure sensors. Still, the technique had limitations because of shape deviation from the nominal values (outside the acceptable upper/lower limits) and the material's characteristics (relatively high density and, consequently, not lightweight enough). One more step in prototyping the mechanical components was that of considering only the motion of thumb and index fingers--due to their importance in handling everyday objects. If the mechatronic structure with only these two fingers \"worked\" correctly, then it could be assumed that all the rest of the structure would do so. Figure 15: Experiments on the sensors. (**a**) FSR pressure sensors—for force, (**b**) tilt sensor—for finger inclination angle, (**c**) EMG sensor test. The 3D model to be prototyped, conventionally named, 2-FPRINT, is presented in Figure 17.  
The first prototyping technique considered was the one from EnvisionTEC that is based on \"3 SP (Scan, Spin and Selectively Photocure) technology to quickly 3D print highly accurate parts from STL files, regardless of geometric complexity\" [28]. The printing equipment is Envisiontec ULTRA 3 SP, and the resin is a special one, EnvisionTEC's premium E-Model Light [29], used mainly for orthodontic purposes. Some of the complex components were printed, so as to test this 3 SP technique's results. Images taken while prototyping are shown in Figure 18.  
Figure 16: FDM prototype of the index finger surface. (**a**) 3D model, (**b**) FDM prototype. Figure 17: 3D model of the two fingers prototype, 2-FPRINT. After having the printed parts photocured, one important task was that to check their geometric precision (see Figure 19). Dimensions of the holes for mounting the rolling bearings and the micro-motors, respectively, as well as their tolerances, were determined. Additionally, the thread dimensions (pitch of 0.5 mm) were checked. Furthermore, the worm-worm gear transmission system was tested. Figure 18: 3 SP printing of mechanical parts. (**a**) Model construction and position, (**b**) models at the final printing process, (**c**) printed parts. The conclusion was that this printing technique did not prove to be adequate for the accurate and small parts of the mechatronic system designed. There were tolerances larger than the designed ones (0.02 mm), and most hole dimensions were smaller than nominal, i.e., smaller than the lower limits. Moreover, all exterior dimensions resulted in higher than upper limit values, so that additional processing would be required. The thread pitch was not correct, and almost proved to be threadless. It could not be assigned a set of \"rules\" for overcoming these geometric problems, as the deviations from nominal dimensions were not similar for all the printed parts. Another printing technique used for the rapid prototyping of the mechatronic system parts was DLP (digital light processing). DLP technology uses a digital projector to project an image of the layers, processed with specialized software, across the entire build platform, curing the whole surface of the layer through a single flash. The printed components of the mechatronic system were built using the printer \"Figure 4 Standalone\" from 3 DSystems.