Transitioning from a theoretical model to a functional underwater prototype involves a rigorous process of synthesis and troubleshooting. This chapter documents the technical execution of the project, detailing the mechanical assembly, electronic integration, and software architecture of the MARIS HABITATS system.

It serves as a technical log of the development lifecycle, highlighting how the theoretical foundations established in Chapter 2 were translated into physical components. From the challenges of ensuring watertight integrity for the sensor housing to the optimization of low-power data transmission, this section provides a comprehensive look at the engineering hurdles overcome during the fabrication and programming phases.

The goal of this project is to design artificial marine habitats that can help endangered fish species and corals thrive again. Climate change and the warming of the oceans are disturbing marine ecosystems and damaging the natural balance of life underwater. Because of this, many species are losing safe places to live, hide, feed, and grow. The design is therefore focused on supporting nature itself, especially fish, corals, and other marine organisms.

The structure had to meet several important requirements. It had to be modular, so that different units could be combined and adapted depending on the location and the needs of the ecosystem. It also had to be made from a material that would not harm the marine environment. Since corals need to grow on the structure, the material had to be suitable for marine life and preferably porous. At the same time, the habitat could not be excessively large or heavy, since this would complicate transport and installation. The design therefore had to balance practical deployment with sufficient weight and stability to remain in place and withstand sea currents.

To develop the concept, the team started with brainstorming sessions and research into similar existing projects. Different types of artificial reefs and marine restoration systems were researched, and also which materials could safely be used in the sea were studied. During this ideation phase, the creation of around six to seven different structural concepts was crucial. While the overall shapes of these concepts were quite similar, the main differences were in the materials and possible additional features such as sensors.

Several material options were explored, including basalt fabric-reinforced structures, polymer-clay, bacterial HSC, ECOncrete, recycled glass, and Biorock. Each material had its own advantages and disadvantages. Some were more expensive, while others were less suitable for coral growth or did not provide the level of porosity needed by marine organisms. Since fish and corals benefit from rough and porous surfaces, this became an important factor in the decision-making process.

After comparing the different options, basalt fabric-reinforced concrete was selected as the most suitable material for the project design. This material offered a strong balance between cost, weight, stability, and ecological suitability. It is not overly expensive, heavy enough to remain stable underwater, and easy to shape into modular forms. In addition, its porous surface makes it a good choice for encouraging coral growth and creating shelter for fish. The material can be formed by first making a mould in the desired shape and then placing the basalt fabric or mesh inside it. Depending on the required strength, the fabric can be arranged in one or several layers. Fine concrete or mortar is then poured, sprayed, or pressed around the reinforcement. After curing, the concrete becomes rigid while the basalt fabric remains embedded inside as reinforcement. For these reasons, basalt fabric-reinforced concrete was chosen as the best material for the final concept.

The final concept selected for MARIS HABITATS is a modular artificial marine habitat built from one repeated cone-shaped element, as shown in Figure 1. These modules can be connected side by side and stacked vertically, allowing the habitat to be expanded according to site conditions and ecological requirements. By using one repeated part, the system remains simple, scalable, and easy to reproduce, while still allowing a wide variety of structural arrangements.

The concept was selected because it provides a strong balance between ecological function and practical feasibility. Different configurations of the same module can create both smaller and larger shelter spaces, making the habitat suitable for a wide range of marine species. This adaptability is one of the main strengths of the concept, since different deployment sites may require different structural densities and sizes.

Another important aspect of the concept is the material choice. The habitat is intended to be made from basalt fabric-reinforced concrete, which combines structural weight and durability with a rough and porous surface suitable for marine colonisation. This surface can support the attachment and growth of algae, corals, and other marine organisms, while the weight of the material helps the structure remain stable under underwater currents.

Compared to other design directions explored during the project, this concept proved to be the most suitable. Earlier ideas such as spherical, hexagonal, and dome-based structures were less effective because they were either not modular enough or too difficult to manufacture and combine efficiently. For this reason, the cone-based modular concept was defined as the final structural direction of the project.

This section presents how the selected concept was developed into a feasible structural solution for underwater deployment. The design process focused on translating the general concept into a habitat that is modular, manufacturable, stable, and ecologically suitable for marine colonisation. The following subsections explain the main design decisions and the structural directions explored during the development process.

The design phase focused on transforming the selected concept into a structure that could function in an underwater environment while remaining feasible to produce and deploy. From the beginning, the most important design requirement was modularity. The habitat had to be based on one repeatable element that could be combined in multiple ways without becoming too complex to manufacture or assemble. This requirement guided the entire design process and strongly influenced the selection of the final form.

Several structural directions were explored during this phase, including spherical, hexagonal, and dome-like concepts. Although these ideas offered interesting spatial qualities, they were gradually rejected because they did not satisfy the design goals strongly enough. Some concepts were too difficult to produce in a simple and repeatable way, while others did not provide the level of modularity needed to expand the habitat efficiently. In contrast, the cone-based element provided a clearer and more practical solution. Because the same unit can be repeated throughout the structure, the habitat can grow both horizontally and vertically while maintaining a simple and consistent construction logic.

The chosen design also supports ecological performance. By connecting and stacking the modules in different arrangements, the habitat can generate openings and sheltered spaces of different sizes. This is important because smaller and larger marine organisms require different types of refuge. The repeated units also create a more complex three-dimensional environment, which improves habitat quality and increases the suitability of the structure for fish, algae, corals, and other marine species.

Material selection was another important part of the design phase. The final design is based on basalt fabric-reinforced concrete, chosen for its combination of strength, weight, and ecological suitability. The material is heavy enough to improve stability under underwater currents, while its rough and porous surface can encourage biological growth over time. In this way, the design responds not only to structural and manufacturing requirements, but also to the biological purpose of the habitat.

Overall, the design phase transformed the initial concept into a clear and buildable solution. Instead of developing a complex habitat composed of many different parts, the design process focused on a single repeated module capable of generating a wide range of spatial configurations. This decision improved the scalability, manufacturability, and ecological potential of the habitat, and formed the basis for the structural development presented in the following subsection.

The structural development of the habitat began with a series of exploratory concepts. These early ideas were useful for identifying the design characteristics that were most important for the project, such as modularity, ease of production, structural repetition, and the creation of different shelter sizes. The figures below illustrate the main structural directions considered during this process. Figure 2 shows an initial idea.

One of the first directions explored was a more enclosed structure, shown in the early sketch above. This concept helped define the importance of shelter, internal space, and protection for marine species. However, although it offered enclosed refuge areas, it was not considered the most suitable direction because it did not provide the same level of modular flexibility as later concepts. As the design process continued, greater importance was given to repeatability and scalability.

Figure 3 illustrates the proposed habitat modular design.

A second concept was based on a hexagonal module supported by pillars. This idea introduced a stronger modular logic and allowed several units to be connected into a larger structure. The concept was relevant because it explored how repeated modules could create a more adaptable habitat system. It also supported the study of elevated structures and the possibility of generating openings of different sizes. However, this direction was not selected as the final solution because the cone-based concept provided a simpler repeated form and clearer scalability (see Figure 4).

Further structural exploration focused on varying the dimensions and arrangement of the elements in order to create openings suitable for different marine species. This stage was important because it highlighted the ecological value of structural diversity. By studying how repeated parts could generate different internal spaces, the development process produced a clearer understanding of how geometry could influence habitat quality. These studies confirmed that the final design should allow variation in shelter size while still remaining based on one simple repeated part (See Figure 1 and Figure 5).

After comparing the different structural directions, a concept based on one repeated cone-shaped module was selected. This solution was considered the most appropriate because it combines modularity, manufacturability, and ecological functionality. The same unit can be repeated many times, allowing the structure to expand horizontally and vertically while maintaining a simple construction logic. As illustrated in Figure 6, the arrangement of these modules creates a more complex habitat geometry with multiple shelter opportunities for marine organisms.

When several modules are combined, the habitat can cover a larger area of the seabed and create a more complex three-dimensional structure. This makes the system adaptable to different sites and allows the scale of the habitat to be adjusted according to the intended application. For this reason, the final structure is not defined by a single fixed form, but by a repeatable modular logic that can be expanded according to ecological and practical needs.

After defining the final modular structure, a suitable method still had to be developed for inserting the smart box. The development process of this solution is explained below.

Design Process of Sensor Structure

This was the first version of the smart block, in which the smart box was attached to the bottom of the modular structure. However, this solution was not aesthetically pleasing, so a new approach was explored in which the smart box could be integrated into the modular structure itself. It was also important to avoid changing the structure too much, since that would require separate moulds for these specific blocks, even though they would be produced in much smaller quantities than the standard blocks.

The first design iteration is shown in Figure 7, with a detailed view provided in Figure 8.

In the second variation of the smart block, the smart box was much more integrated into the structure. However, this version required many modifications to the standard block. Additional openings also had to be added to ensure that the sensors had sufficient exposure to the surrounding water. This made the solution inefficient and impractical. As a result, another approach was explored, with a stronger focus on modularity so that a completely new mould would not be required.

Consequently, an alternative solution was explored, with a stronger emphasis on modularity to avoid the need for a completely new mould. The second variation of the design is illustrated in Figure 9, while a front view highlighting the structural modifications is shown in Figure 10.

This led to the development of a modular sensor solution. The supporting structure is constructed from titanium alloy TC 4 and can be installed in any reef block as required. Its design allows for easy placement and removal, thereby simplifying maintenance and component replacement. The sensor housing is positioned on two supporting tubes and secured with a chain attached to the host block, ensuring stability during operation. (see Figure 11 and Figure 12).

To facilitate maintenance, a distinct block design is used for the sensor unit. Over time, biological growth such as algae is expected to accumulate on the reef structures, reducing visibility and making it difficult to distinguish individual components. By incorporating a visually and structurally identifiable block, the sensor unit can be reliably located and accessed, even after prolonged submersion.

Throughout this project, various approaches to data collection were explored. Initially, Version 1 was developed, while this version enabled continuous data communication, it proved to be highly complex and more prone to potential failures. Based on these findings, Version 2 was selected as the preferred because it prioritises robustness, reduced complexity, and ease of deployment, while accepting compromises regarding limited operational duration and the lack of continuous data communication.

Figure 13 and 14 presents the smart system black box diagram. The system corresponds to a living laboratory where the:

• Sun (top left corner) is the source of energy. A buoy equipped with solar panels on top will store power in a battery and provide power to the system.
• Sea water (bottom left corner) is the growth medium. Four sensors will monitor the water environmental conditions (diamond).
• Fish and sea life (bottom right corner) are under observation. Fish and algae will be monitored (presence and size) to determine biodiversity and measure photosynthetic effects and chlorophyll on surfaces.

All this data will be reunited and sent to the On Board Computer (OBC) while also getting a timestamp by a Real Time Clock (RTC). All this will be powered by the battery through a Power Management System (PMS) that received the power from the buoy.

Both the buoy and the structure will have a positioning module, that will count with an Inertial Measuring Unit (IMU), a Doppler velocity Log (DVL) and a Global Navigation Satellite System (GNSS) receiver, to have everything registered about the position of both elements and make sure nothing goes wrong due to external factors such as storms, currents or human factors.

From the structure to the buoy, there will be a chain and a cable, for both structural support and data and power connection between the 2 elements. Finally, all the data collected will be sent to a data center, this will be done via the standard Iridium Satellite Network.

The main output will be a report with all the obtained data.

Figure 15 illustrates a simplified, self-contained underwater monitoring system integrated into our modular artificial reef structure.

In this configuration, the system is based on a “Smart Block” that houses all electronic components, including the power supply, sensors, and data storage, eliminating the need for external infrastructure such as surface buoys, solar panels, or cable connections.

The entire system is powered by a lithium-ion battery. Environmental data is collected via sensors that measure pressure (depth), temperature, pH, and conductivity. All collected data is stored locally on a Secure Digital memory card (SD card); real-time transmission is not possible. Battery replacement and data retrieval are carried out through a scheduled maintenance procedure involving a diver. The estimated battery lifetime of the system is approximately 340 days, which limits the frequency of required maintenance operations to roughly once per 11 months. When battery replacement is necessary, a diver descends to the installation site and retrieves the Smartbox from the seabed. The enclosure must be brought to the surface in order to be opened safely. Battery replacement is performed aboard a boat, where the SD card is also replaced simultaneously to ensure secure and continuous data storage. After completion of the maintenance procedure, the Smartbox is redeployed and repositioned at its original location on the seabed. This integrated maintenance strategy allows both power supply and data storage components to be serviced during a single operation. After retrieval, the data is transferred to a research facility for analysis and evaluation, ultimately contributing to environmental monitoring and reporting.

An Arduino Uno R4 Minima was selected as the microcontroller for the system. This version is simpler than the normal Arduino Uno R4 and does not include built‑in Wi‑Fi or Bluetooth however, wireless communication is unnecessary for this application, as radio‑frequency signals are ineffective underwater.

The system is powered by a 12 V 20 Ah LiFePO₄ battery. To prevent excessive battery degradation and to extend its service life, the battery is not discharged fully. A minimum state of charge of 20 % is enforced, meaning that only 80 % of the nominal battery capacity is used. The battery’s nominal voltage is 12,8 V. The total power consumption of the system during active operation is 1,505 W.

To minimize energy usage, measurements are performed once per hour. The system is designed to remain active for only 1 minute per hour, which is sufficient for sensor stabilization and for writing the collected data to the SD card.

Battery capacity: 12,8 V × 20 Ah × 0,8 = 204,8 Wh

Daily energy consumption (1 min/hour operation): 1,505 W/60 × 24 h = 0,602 Wh/day

Number of days: 204,8 Wh / 0,602 Wh/day = 340,199 days

Based on these calculations the system can operate for approximately 340 days on a single battery charge.

Selecting sensors was quite challenging, as most sensors such as pH and conductivity probes are designed for temporary measurements and not for long term submersion. Additionally, the sensors must withstand the high pressure at the seabed, and many are not suitable for seawater. This resulted in expensive sensors, mainly sourced from suppliers in the United States.

The BarXT sensor [1] measures both pressure and temperature. From the pressure data, the depth can be calculated. An Inter-Integrated Circuit (I²C) level converter is required to convert the 3,3 V logic signal to 5 V so it can be read by the Arduino.

The pH sensor [2] is sourced from Atlas Scientific. It is used together with a pH module [3], which converts the signal into an analog signal that can be directly read by the Arduino’s analog inputs.

For the conductivity sensor, no similar ready-made solution was available. It outputs a current signal of 4 mA–20 mA, which must be converted into a voltage of 0 V–5 V to be read by the Arduino. This is done using a 250 Ω resistor, according to Ohm’s law (U=I×R).

0.004 A × 250 Ω = 1 V

0.020 A × 250 Ω = 5 V

The enclosure is one of the most critical and costly components of the system, as it must withstand high external pressure at the seabed. Suitable enclosures are therefore difficult to source.

Condensation is expected to form inside the enclosure as the air trapped inside is cooled by the surrounding seawater. This temperature difference can lead to moisture accumulation, increasing the risk of corrosion and electrical failures. To mitigate this risk, silica gel desiccant packets are placed inside the enclosure to absorb excess moisture.

The sensors from Atlas Scientific use ¾“ NPT threads, while the enclosure is designed with M10 threads. This requires sealing the existing holes and machining new threaded openings in the enclosure.

However, the sensor from Blue Robotics is equipped with M10 threads, allowing direct installation into the enclosure without modification.

Table 1 presents a comprehensive overview of all sensors and components included in the system. The electrical schematics are shown in Figure 16.

The use of an imaging system was initially considered to monitor fish growth. However, this approach has been deprioritized, as the primary focus of the project has shifted toward the analysis of quantitative sensor data.

Sensor-based measurements provide continuous, objective, and scalable insights into environmental conditions, which are more closely aligned with the project’s core objectives.

The inclusion of a camera system is therefore limited to supporting species identification, specifically to document the presence of fish within the reef environment. In addition, visual documentation of the reef structure will be conducted during annual maintenance operations, during which images of the installation will be captured once per year.

In Figure 17 you can see what the enclosure will look like with all the electronics inside. The picture on the left is with the sensors included. The picture on the right is without the sensors to show how much space the rest of the electronics will use.

Describe in detail the: (i) use cases or user stories for the smart device and app; (ii) selection of development platforms and software components (use tables to compare the different options); (iii) component diagram.

Present and explain the: (i) initial packaging drafts; (ii) detailed drawings; (iii) 3D model with load and stress analysis, if applicable.

The prototype is designed to measure similar parameters to a CTD system, but instead of using a conductivity sensor to estimate salinity, it uses a TDS sensor. This is a significantly cheaper alternative and is sufficient for early-stage testing, where the main goal is to validate the system concept rather than achieve final measurement accuracy. The pH sensor is also excluded from the prototype in order to reduce cost, since it is not essential for testing the basic functionality of the system. Apart from the sensor selection and reduced measurement precision, the prototype follows the same general system design as the final product. For the enclosure, a simple airtight plastic container (e.g. from IKEA) is used as a temporary solution. This significantly reduces costs compared to waterproof enclosures and is sufficient for controlled testing environments. To ensure watertight cable penetrations in the prototype, a silicone‑based sealant will be used. The same sealant may also be applied around the enclosure lid if leakage is detected during testing.

Compared to the final designed solution, several modifications are made for the prototype. The structural block is downscaled to 1:6, resulting in a model size of about 10 cm. This allows the structural concept to be tested in a smaller and more practical format. In addition, alternative materials are considered for the prototype structure. The block may either be produced using 3D printing or cast in standard concrete, rather than using the final material and full-scale production method.

The Smart Box is also simplified compared to the final design. Instead of using a dedicated underwater enclosure, a waterproof plastic box is used as the prototype housing. Holes are drilled in the enclosure for the water-measuring sensors, which are installed through the openings and sealed with adhesive to prevent leakage.

The enclosure will initially be tested without oil to determine whether it can withstand underwater pressure while protecting the internal electronic components. If the enclosure is not sufficiently pressure-resistant, it will be filled with oil as a pressure-compensation solution. This reduces the pressure difference between the inside and outside of the box while protecting the battery, sensors, and electronic components from direct contact with water.

These changes are made to simplify prototype construction and enable early testing of the system concept before developing the final full-scale solution.

The prototype is designed to measure parameters similar to those measured by a CTD system, Conductivity, Temperature, and Depth. However, instead of using a conductivity sensor to calculate salinity, the prototype uses a TDS, Total Dissolved Solids, sensor.

Conductivity measures water’s ability to carry an electrical current. This ability is directly related to the concentration of dissolved ions, such as salts, minerals, and other inorganic materials. TDS, on the other hand, represents the total amount of dissolved substances in the water, including inorganic salts such as calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates, as well as small amounts of organic matter. TDS is typically measured in mg/L or ppm and is commonly used as an indicator of water quality.

Using a TDS sensor provides a significantly cheaper alternative to a conductivity sensor. For early-stage testing, this is sufficient because the main goal is to validate the overall system concept rather than achieve final measurement accuracy.

The selected sensors used in the prototype are presented in Table 2.

The pH sensor is also excluded from the prototype in order to reduce cost, since it is not essential for testing the basic functionality of the system. Apart from the sensor selection and reduced measurement precision, the prototype follows the same general system design as the final product.

The other electronic components used in the prototype are listed in Table 3.

For prototype testing, a low-cost solution is used both for the enclosure and structural elements. A simple plastic lunchbox can serve as a temporary enclosure, where holes can be drilled for sensor placement, making it suitable for controlled testing before investing in the final underwater housing. In addition, standard cement is used for structural testing, as it provides sufficient strength at a very low cost. These materials are summarized in Table 4.

PLA filament can be used either as an alternative material for the blocks. It can be used to create moulds for casting concrete blocks, or as the structure for the prototype instead of concrete. This allows for greater flexibility and repeatability during the design and testing phase. However, PLA is not suitable for long-term structural use in harsh environments, and is therefore primarily intended for prototyping and tooling purposes.

For the enclosure, a simple airtight plastic container (e.g. from IKEA) is used as a temporary solution. This significantly reduces costs compared to waterproof enclosures and is sufficient for controlled testing environments. To ensure watertight cable penetrations in the prototype, a silicone‑based sealant will be used. The same sealant may also be applied around the enclosure lid if leakage is detected during testing.

If the external hydrostatic pressure exceeds the enclosure’s mechanical limits at greater test depths, an oil‑filled enclosure may be used as a pressure‑compensation solution. Transformer oil would be the preferred choice due to its superior electrical insulation properties, however, it is difficult to obtain for small‑scale prototyping. Therefore, cooking oil is considered as a low‑cost and readily available alternative. Although it doesnt provide the same electrical insulation, it is expected to be sufficient for this low‑voltage prototype and suitable for short‑term experimental testing.

The electrical schematics for the prototype is presented in 18.

Detail and explain any changes made in relation to the designed solution, including different software components, tools, platforms, etc.

The code developed for the prototype (smart device and apps) is described here using code flowcharts.

Perform the hardware tests specified in Tests. These results are usually presented in the form of tables with two columns: Functionality and Test Result (Pass/Fail).

Software tests comprise: (i) functional tests regarding the identified use cases / user stories; (ii) performance tests regarding exchanged data volume, load and runtime (these tests are usually repeated 10 times to determine the average and standard deviation results); (iii) usability tests according to the System Usability Scale.

Provide here the conclusions of this chapter and make the bridge to the next chapter.