## 1 Introduction  
Recent years have witnessed a technological revolution in the field of humanoid robotics. This has led to advances in all disciplines, particularly in actuation technologies. The choice of the actuating technology depends upon the application. Electric actuators are typically used for light tasks, notably for humanoid robots designed for social interaction. On the contrary, hydraulic actuators are used when high force is required. For example, the humanoid robot ATLAS from Boston Dynamics [1], SARCOS from Carnegie Mellon University [2], and HYDROiD, a full-size humanoid under development at Paris Saclay [3], are hydraulically actuated as they are destined to perform heavy-duty tasks. HYDROID has 36 hydraulically actuated Degrees of Freedom consisting of multiple mechanisms designed to fit in a restrained volume to respond to an anthropomorphic design [4]. occurred in this domain where hydraulic cylinders became more electrically integrated and compact [5]. However, this technology requires the use of hydraulic tubes. The multitude of hydraulic tubes in humanoids has several disadvantages; they decrease their social acceptance, and the tubes face the danger of tearing off, which puts the global function of the system at risk. A compromise must, therefore, be made between the advantages and disadvantages of hydraulic actuation to maximize the effectiveness of this technology. One possible solution for eliminating hydraulic tubes is using decentralized hydraulic systems. This technology consists of integrating multiple electro-hydraulic actuators at joints to locally generate the required energy [6]. Another method consists of using a single hydraulic pump and integrating hydraulic pipes into the robotic parts to create internal oil passages. Three methods were defined for this purpose:  
1. Conventional manufacturing: Typically, these processes start with the rough machining of metallic blocks to create external shapes and then oil passages are drilled to form internal oil tubes. This process requires considerable labor, resulting in high machining times and costs. The first generation of HYDROiD--V1 with full hydraulic integration was manufactured using conventional technologies, mainly from steel, titanium, and aluminum. This work became more challenging when hybrid (serial + parallel) mechanisms were involved, which added to the complexity of the integration [7]. As shown in Figure 1, a high-pressure tube fed the robot arm at the shoulder, and the drilled tubes fed the arm and forearm joints internally [8]. The process supplied oil to all 36 hydraulically actuated active joints. Once the manufacturing process was complete, post-processing for corrosion protection or hardening was required, which added cost and time to the initial process. Thus, this manufacturing process is lengthy and costly and adds to the complexity of the design;
2. Metal additive manufacturing: With advances in manufacturing processes and materials, additive manufacturing has emerged as a new approach to designing portable devices. This technology allows the development of intricate shapes in which hydraulic oil passages can be pre-designed and printed within the structure. In 2016, Boston Dynamics announced that the next generation of ATLAS would incorporate structures manufactured with additive manufacturing [9]. The robot leg housed a hydraulic cylinder barrel and valve emplacements. Following the same logic, a smart  
Figure 1: HYDROiD’s arm and forearm-integrated hydraulic tubes manufactured with conventional manufacturing processes. integrated actuator was developed at IIT, Italy. It was printed with titanium alloys to permit the complex shapes to be made. The all-in-one actuator included integrated hydraulic paths, wire channels, valve emplacements, and position, force, and temperature sensors [10]. However, this technology requires post-processing of the material for stress relief and porosity elimination, which renders it ineffective in terms of cost and time;
3.