## 4 Discussion
### Manufacturing Process
#### 4.3.1 Extrusion-Based Additive Manufacturing  
The extrusion-based technique, ACEO's Drop-on-Demand (DoD) printing technology was used by [PERSON] et al. [10] and [PERSON] et al. [13,15] in three clinical studies. DoD technique involves the controlled ejection of individual droplets of silicone material from a print head onto a substrate, constructing the prosthesis layer by layer. Each droplet is deposited at specific coordinates onto a substrate, allowing for meticulous control over the prosthesis's geometry and material distribution. In one of the mentioned cases, [PERSON] et al. [13] successfully created an ear prosthesis using multi-material printing, which allowed for varying Shore hardnesses across different regions, enhancing both comfort and realism. This printing process utilized four nozzles to simultaneously deposit silicone materials with distinct Shore hardness levels tailored to different parts of the prosthesis (e.g., 60A for the bulk, 40A for the concha, and 20A for the lobe) [13].  
In all clinical cases utilizing DoD technology, effective printing procedures were outsourced to a central provider. The prosthesis designs were sent to the supplier for CAM using direct 3D silicone printing (ACEO Silicone General Purpose; Wacker Chemie AG) in all cases [10]. However, in 2021, their printing service was discontinued [11].  
Another extrusion-based technique, utilizing the Polylet Stratasys J750 3D printer, was employed in two clinical cases [12,16]. This method involves jetting photopolymer droplets onto a build tray layer by layer, with each layer immediately cured using ultraviolet (UV) light. The printer's capability to jet multiple materials simultaneously from different nozzles enables full-color, multi-material printing. In the challenging case of an adhesively retained facial orbital prosthesis presented by the [PERSON] et al. [12], the entire compound, including the characterized missing eye, was printed as part of the complete orbital prosthesis. This approach eliminated the need for a separate ocular prosthesis. The printer utilized multiple resin types (e.g., VeroFlexBlack, VeroFlexWhite, VeroFlexCyan) to achieve varied material properties and colors, with the capability of reproducing up to 500,000 colors in a single 3D-printed model. The voxel-based printing technology allowed for precise control over material distribution and color application, while the prosthesis was constructed with layers as thin as 14 microns, ensuring high resolution and fine detail [12].  
[PERSON] and [PERSON] [34] developed a cost-effective 3D printer designed to fabricate silicone structures using a micro-extrusion technique combined with intra-layer photo-cross-linking. Their goal was to create an affordable system (costing less than USD 1000) capable of printing silicone (PDMS) structures with high resolution and customizable mechanical properties. The results showcased the successful fabrication of intricate silicone constructs with excellent resolution (300 \(\upmu\)m channel width) and mechanical integrity, demonstrating significant potential for applications in biomedical engineering and maxillofacial prosthodontics [34].  
[PERSON] et al. [36] developed a 3D printable maxillofacial silicone using a customized extrusion-based printer designed for RTV silicone. The aim was to optimize the composition of a two-component RTV silicone elastomer by adjusting moderators, thixotropic agents, and filler content to achieve desirable mechanical properties, such as tear strength, tensile strength, and hardness, while ensuring printability without molds. Results demonstrated that the optimized formulations exhibited mechanical properties comparable to commercial silicones, making them suitable for fabricating facial prostheses with improved efficiency and consistency.