# 3D Printed Robotic Hand with Piezoresistive Touch Capability
## 2 Three-Dimensional Printing of the Prosthesis  
At the manufacturing level, the production method was critical to maximizing functionality and cost. Utilizing 3D printing allowed the choice of different types of material depending on the specific purpose of each part of the robotic hand. The chosen prosthetic model was taken from an open-source research platform, which allowed the needs of each part to be quickly identified and the model to be divided into sections, with the hand and forearm being separate parts. All 3D prints were made on a _Prusa I3 MK3S+_ printer with a building volume of 25 \(\times\) 21 \(\times\) 21 cm (XYZ) and a standard nozzle size of 0.4 mm. The printer itself is connected and programmed through its own software called _Prusa Slicer_, which allows the user to fully control every aspect of the production phase, allowing control in production speed, axis speed movement, layer weight, positioning on the feeding bed, percentage in support use, etc. For the robotic hand, the raw material chosen had to be able to withstand considerable mechanical stress (tensile and flexural strength), since the fingers, the middle of the hand and the palm would be subjected to mechanical forces generated by servomotors during the instrumentation of the prosthesis. There were several materials that could have been used, such as polycarbonate (PC) and polyamide (PA) (nylon), but PLA was chosen as the raw material for the hand parts due to its cost effectiveness and long-term production characteristics. It provided the required tensile and flexural strength for each part, while allowing production time to be reduced. In contrast, the other polymers mentioned would provide greater tensile strength, but would increase production time and would be difficult to handle in the 3D printer, causing problems such as clogged nozzles, among others [17; 18]. Each part was printed separately (Figure 1), allowing more accurate production customization and cost savings. Each part was monitored for size, geometric position on the print bed, infill percentage, layer weight, support percentage, ironing, surface quality, print speed and nozzle diameter. These parameters affect the production time, especially the geometric position, as it can change the amount of support material and can change the layer deposition during printing, affecting the strength of the part and the orientation of the layer. Figure 1: Representation of each of the parts of the prosthetic hand: (**A**) the core piece of the hand; (**B**) all the phalanges of each finger; (**C**) the attachments to the core of the hand: on the left, the attachment of the thumb, which allows it to move more freely; and on the right, the completion of the palm; (**D**) the attachments between the different phalanges, which allow the fingers to move. Depending on the size of the part, the infill percentage can be changed, as parts with larger dimensions tend to reduce the infill percentage, compared to smaller parts. This parameter also has a significant effect on production time. Another aspect taken into account was the aesthetics of the robotic hand, i.e., the surface quality of the printed parts, which plays a fundamental role not only for the prosthesis user but also for the efficiency of the sensory system, since the surface of the palm and associated irregularities influence the magnitude of the touch felt by the sensors [18; 19]. As mentioned above, each part was printed using specific polymeric 3D printing materials, which varied from part to part depending on the end use. Figure 1A-C shows the parts produced using PLA as the primary material. The only parts wherein PLA was not used are shown in Figure 1D. These specific parts were used for the attachments between the prosthetic fingers (Figure 1B), allowing them to move against each other, making the general movement of the prosthetic hand similar to the movement of a human hand.