## 3 Technologies Used for Artificial Reef Design and Manufacturing
### Traditional Manufacturing Technologies vs. Additive Manufacturing Technologies  
The first artificial reefs were built with natural materials, such as stones. In the XX century, different components were tested as an artificial reef, including some kind of concrete cubic material, tire waste, materials from wrecks, and others [4]. Different kinds of cheap materials have gained popularity. The construction was rather simple, without necessary complications; mainly, concrete blocks and metal frames were applied [21]. Currently, additive manufacturing and reverse engineering have changed the rules. Technological progress allows for the use of 3D scanning and 3D printing as the background technology for the design of the coral reefs, in the same way as they are used for other applications [21; 45; 46]. Nowadays, there is a possibility to use scanned geometries of harvested corals to fabricate artificial coral skeletons [21] or design proper shapes dedicated to particular species, including the proper surface topology for settlements [23]. Alternatively, photogrammetric technologies can also be used to obtain 3D models [1]. Also, other advanced techniques are joined with additive manufacturing for customization, including eDNA metabarcoding, and lifecycle-based emergent traits [23]. The combination of these technologies gives the possibility to create biomimetic artificial structures with a high efficiency of restoration in selected ecosystems [23]. Thanks to modern additive technologies, it is possible to produce complicated shapes, potentially even in underwater conditions [47].  
Various 3D printing technologies are used for manufacturing artificial coral reefs; the most popular seems to be Fused Deposition Modeling (FDM). However, other technologies are also used, including fused filament fabrication (FFF), binder jetting, and VAT polymerization [48; 49]. [PERSON] et al. [21], in reaction to coral bleaching, created artificial corals using two different technologies, FDM and stereolithography (SLA), as supplementary processes to build artificial reefs [21]. [PERSON] et al. [49] applied digital light processing (DLP) for coral restoration projects. This technology was selected mainly because of the speed of production and model scalability [49]. In turn, [PERSON] et al. [50] selected particle bed 3D printing with Selective Cement Activation (SCA) for artificial reef manufacturing [50]. This technology allows for the creation of greatly accurate surrogate surfaces with the desired high roughness [50]. The additional advantages were complex geometry with high porosity and large overhangs, as well as the independence of production time from the model complexity [50]. For the reef projects, various types of devices are used, starting from partly self-modified FDM 3D printers, through concrete laboratory machines, up to large semi-industrial devices. The usage of particular solutions is usually connected with the scale of the project and its budget.  
The important aspect of each project is the costs. In comparison to the traditional approach, 3D printing is connected with higher efficiency in restoration, but also with higher costs. [PERSON] et al. [21] prepared the cost simulation for coral reef restoration using 3D printing technology [21]. According to his estimation, the cost of a fully functional 3D printing facility was approximately EUR 300,000 in 2021 [21].