# Additive Manufacturing in Underwater Applications
## 5 Areas of Application
### Biological Research  
The devices connected with biological research are probably the most diverse group according to the level of complexity. They include very simple devices for eco-monitoring, such as a 3D-printable instrument designed to measure the Secchi depth and water color [109], and also complicated systems of replicas of living organisms for hydrodynamic tests--for example, benthic organisms and shark skin membranes for the analysis of marine animals' behavior [110]. This group also includes different biomechanic robots that mimic efficient natural swimmers (e.g., boxfish, seahorse, snailfish) [16]. Another area of inspiration is the locomotion of cephalopods and jellyfish. This type of motion is the inspiration for some underwater vehicles [23; 111; 112; 113]. For the design of this type of vehicle, 3D printing technology is necessary, because it requires non-typical elements [23; 111]. The same applies for other bio-inspired soft robots [114; 115; 116].  
An important element of soft robots is also the source of energy. It should be waterproof and sustainable in the long term [117]. As a solution to this problem, AM can be useful. [PERSON] et al. [117] have researched this topic [117]. They developed a bionic, stretchable nanogenerator for underwater energy harvesting that was based on marine fauna, namely an electric eel [117]. They confirmed that the designed solution has the potential to be an effective power source for soft underwater robots as well as wearable electronics used underwater [117].  
In the area of soft robots, research was also provided by [PERSON] et al. [76]. As a source of inspiration, they used the morphology of Anura, such as Rana esculent (a semi-aquatic frog) [76]. As a result of this work, a prototype solution, using AM technology, of a bio-inspired frog robot was created. Its capability for underwater synchronous swimming was confirmed. This robot is dedicated to underwater environmental monitoring [76].  
This type of activity can be useful not only for the research of modern animal species but also in understanding evolution. [PERSON] and [PERSON] [118] used 3D printing technology for the simulation of the motion of ancient sea animals (externally shelled cephalopods--_Baculties compressus_) to investigate their functional morphology and ecology over geological timescales [118]. This research is particularly interesting, because this type of animal has no modern analogs that allow for a comparison of the possible means of moving [118]. The use of 3D-printed models allows us to verify possible means of movement of this organism and select the most probable theories about their life habits. The provided experiments give some first-order constraints in this area [118].  
In addition, plants can be a source of inspiration for the use of 3D printing for underwater purposes. [PERSON] et al. [119] designed a new hydrophobic material based on the structure of the leaf _Salvinia_[119]. The biomimetic artificial surface successfully imitated the super-repellent capability of the leaves and also fully retrieved air mattresses created by nature [119]. The results of this work allow us to better understand the mechanisms of the water-repellent property and, on this basis, design materials for underwater applications [119].  
Another solution that is potentially useful in biological research, supported by 3D printing technology, is untethered stimuli-responsive soft materials with programmed sequential self-folding [120]. This type of solution, composed of hydrogels and elastic polymers, displays reversible self-folding properties without changing the chemical composition or structural dimensions, which makes its application possible many times [120]. The designed solutions can capture, hold, and fasten objects in the underwater environment, which can be useful in many areas for the investigation of small objects [120].