Design and Fabrication of Bio-Enhancing Surfaces for Coral Settlement

Abstract

Coral reefs are vital ecosystems facing rapid degradation. This research explores architectural design solutions for bio-enhancing modular prototypes to support coral attachment and growth. Inspired by coral polyps, nine biomimetic designs were created using Maya and Rhinoceros 3D to optimise surfaces for coral settlement. A total of 75 prototypes (15 × 15 cm) were fabricated, incorporating four materials—PETG, concrete, oyster concrete, and clay—and seven colour variations—sand, translucent green, translucent brown, red, pink, grey, and reddish. The findings indicate that 3D printing with PETG was the most efficient fabrication method but required structural support and long-term underwater testing, while oyster concrete demonstrated potential for self-sustaining structures. This study highlights the role of architectural design in marine restoration, promoting biodiversity and resource-efficient solutions. By integrating corals into the design, these structures can self-grow and adapt, reducing material consumption and long-term maintenance.

1. Introduction

Coral reefs are multifunctional living structures that self-grow, self-adapt, and self-maintain. They are formed by living organisms, principally hard corals and algae. Their significance has been highlighted by many researchers [1,2,3]. Specifically, they increase local biodiversity, produce sediments, grow islands (cays), act as wave breakers, and are an essential element in people’s livelihoods as they provide food and resources [2,4]. According to the IPCC, reefs protect over 100 million people in over 100 countries and can reduce wave-driven flooding by almost 72% [2]. They efficiently dissipate wave energy and constitute some of the best-submerged wave breakers [4]. By mitigating the waves, reefs provide essential protection to the coastlines, reducing coastal erosion and preventing wave-induced flooding. This makes them the most critical ecosystems responding to sea-level rise and wave-induced floods [5]. Researchers analysed various concepts for integrating coral reefs as fundamental elements in the design of underwater structures [6]. Based on the analysis, coral reefs can provide various ecosystem services that should be considered when designing within aquatic coral reef environments. These are categorised as sediment production, surface accretion, biodiversity increase, and coastal protection, placing coral reefs as one of the best nature-based adaptation strategies for coastal areas.

Currently, coral reefs are threatened by human activities and climate change impacts [2,7]. Specifically, ocean warming causes coral bleaching, which is known to have the most severe impact on coral reefs. After mass bleaching events, coral reefs need years (5–25 years) to recover [8,9,10]. Ocean acidification increases the levels of CO₂, which can reduce coral settlement and growth [3,8,11]. Extreme weather events such as tropical storms physically damage the corals [12,13,14]. These high-intensity tropical storms are expected to increase [2]. Also, the ability of corals to adapt to sea-level rise is questioned by many researchers due to the unpredictable and accelerated rise rate [2,15]. Pressures from anthropogenic activities due to unthoughtful expansion in the water and overexploitation of natural resources further enhance these effects [16]. Activities such as sand mining, harbour construction, dredging, and land reclamation have severely impacted the coral reef ecosystem and altered the coastal dynamics [3,15]. Constructions have been built on top of the reef, or the reef has been dredged to create passages for the boats. Also, excess sedimentation during the construction process smoothers the corals and further damages the reef [17]. Coral heads have been used as a construction material, leading to the decline of the coral reef ecosystem [18]. The alteration of islands due to anthropogenic interventions also degrades the response of corals to adjust naturally to the impact of climate change.

The loss of coral reefs would lead to ecosystem degradation and, eventually, impact the survival of coral islands and human livelihood in atoll environments [19,20]. On 17 October 2009, the Maldivian government signed a document calling on all countries to reduce carbon dioxide emissions. The cabinet meeting was held underwater to emphasise the threat of rising sea levels on low-lying coral islands. Maldivian islands are less than 3 m above water, with 80% of the land area less than 1 m above mean sea level [15]. Current predictions by IPCC on the AR6 Summary for Policymakers identify approximately 1 m sea-level rise by 2120 under the scenario of Intermediate SSP3–7.0, which will cover a large portion of the country’s above-water land [21]. The degradation of coral reefs further exacerbates the threat to coral islands, as they lose their natural protective barriers. The same challenges apply to most of the low-lying coral islands around the world, such as Kiribati and Tuvalu [22,23]. IPCC’s sixth assessment report reported high confidence that human influence is the primary driver of climate change due to unsustainable energy use, land use, lifestyle consumption patterns, and production, placing low-lying coral islands and coral reefs at the forefront of climate change impact [3,7,24,25,26].

One of the adaptation strategies for the resilience of corals is assisting coral growth, which promotes and supports their development. Three methods are the most used for coral growth. First, coral larvae settle on plates in control environments; another uses broken pieces of living corals called “fragments of opportunities” that are transplanted directly to the reef or a nursery, and the third is the micro fragmentation technique, where they detach a small portion of living coral and attach it to a surface [27,28,29,30]. The corals are planted directly on the reef (natural or artificial), or they first grow in a nursery and then transplanted. The design of the structure’s colour, texture, and material that supports assisted growth has a crucial role in the attachment and growth of corals and therefore in the success of the artificial reef [31,32]. For example, on Athuruga Resort Island in the Maldives, researchers monitored 78 Pocillopora verrucosa, and after a year, the out-planting success was 78% for 60 colonies. As determined, the main challenge was detachment, which affected 25% of the corals within the study [31].

The design of the artificial reef is affected by parameters such as environmental conditions, topography, benthic analysis, project scope, funding, material, technological, and human resources availability [27,29,33]. Currently, the most common artificial reef creation method focuses on placing steel rebar structures or concrete blocks (breezeblocks) in the water [27,33,34]. However, due to technological innovations, more research is being conducted on the structure’s form and material, with current structures focusing more on species-specific designs. New fabrication techniques involving 3D-printed (additive manufacturing) structures and parametric design allow for more haptic surfaces and detailed designs [35]. Moreover, additive manufacturing reduces waste material, energy efficient, can use recycled materials, and is readily available, cheap, and fast [36].

Artificial reef designs with morphological complexity and diversity, including various textures and roughness, are fundamental to supporting marine communities, biodiversity, and coral reef restoration [37,38]. Complex designs that look healthy and assemble the complexity of a natural reef can increase the settlement of coral larvae [32,37,38,39]. Researchers at the University of Hong Kong used a clay 3D printer to print artificial coral reefs. The design was conducted in Rhino and Grasshopper, and the form was based on Platygyra coral because the irregular form of the pattern offered more opportunities for coral attachment [35]. The structure was a natural clay reddish colour. In a different project, researchers used a clay 3D printer to print artificial reefs that assemble the complexity of the natural reef [38].

Another design firm that uses additive manufacturing is the Reef Design Lab in Melbourne. Their design comprises modules that can be assembled to create various grid structures. Each module is made of bio-enhancing surfaces that are 3D-printed and moulded in ceramic using the slip-casting process [40]. The surface has a rough, irregular texture to promote coral attachment, and the colour is light grey to white. This artificial coral reef was placed on Summer Island at North Male Atoll in the Maldives in 2018–2019. Footage from 2022 on the Lab’s website shows the structure’s growth of corals and other species. However, there is no official data on coral growth [41]. Three-dimensional printing using calcium carbonate (CaCO₃) in a binder jet 3D printer has also been used to construct artificial reefs [42]. The design focused on creating as much surface area as possible for the growth of various living organisms. The team (Objects and Ideograms) used a computational design to maximise the surface area and create houses for fish while providing surface variations that increased the roughness to attract and promote the growth of algae and microorganisms. Although this method was not intended for coral growth, 3D printing of calcium carbonate can be utilised for corals.

Researchers tested the effectiveness of Acropora kenti coral larvae attachment on three materials: alumina-based ceramic, calcium carbonate, and concrete [43]. Their research indicates that the concrete substrate had the highest larvae settlement and calcium carbonate had the highest survival rate. In another experiment, researchers showed that clay supports the growth of coralline algae, which increases the growth of corals [32,38,39]. Researchers have also shown that the colour variation is also significant for the settlement of coral larvae. For example, research on the settlement behaviour of Porites asteroids and Acropora palmata on seven different colours (red, green, blue, purple, white, pink, and orange) showed that both species preferred to settle on red surfaces, probably because it mimics the colour of the red algae. Their research suggested that coral larvae might use spectra cues for habitat selection [44].

Although the design of the artificial reef material, texture, and colour has a significant impact on coral attachment and growth, research from a design perspective remains limited. Additionally, artificial reef designs that utilise additive manufacturing often conform to the constraints and capabilities of the fabrication tool. Current designs of artificial coral reefs lack a holistic approach. While individual research exists on material, colour, and texture, there is limited research that integrates all three factors. Bio-enhancing architectural design has the potential to address this gap by generating surfaces that respond holistically, aiming to promote coral attachment and growth. To achieve this, this research explores variations in texture, material, and colours to develop designs that could enhance coral colonisation. Considering the above precedents and direct observations of the coral reefs, it investigates the design and fabrication of biomimetic structures that mimic the function of the reef and the form of various coral species.

Biomimicry in architecture is defined as “a design inspired by the way functional challenges have been solved in biology” [45]. This research suggests learning from existing coral reef structures to develop bio-enhancing concept designs with specific shapes, voids, surfaces, and profiles to promote coral attachment. The hypothesis tests whether a coral-metric design can encourage coral attachment. The term coral-metric design is used here to describe the design approach informed by the coral measurements and characteristics. Key design parameters include texture (smooth or rough), colour, form, and material, all of which should attract and support coral growth. The findings of this design can serve as a foundation for designing underwater structures in atoll environments that encourage coral habitation. It is important to note that this research focuses only on hard corals that produce calcium carbonate and build the reef.

2. Materials and Methods

This research employed a mixed-methods approach, integrating qualitative and quantitative strategies to develop bio-enhancing underwater artificial coral reef designs. The study combined qualitative insights from literature reviews, field observations, expert interviews, and case study analyses with quantitative design simulations and fabrication of various materials to test the designs.

• First, a multidisciplinary literature review from ecology, coastal, civil engineering, and architecture was conducted to highlight existing knowledge on the design of artificial coral reefs and underwater structures. The review was carried out using the Griffith University library database for peer-reviewed publications with keywords such as “artificial coral reef design”. Additionally, industry insights published from grey literature such as reports, were examined to supplement information on the design of artificial coral reefs designs.
• Field observations between 2020 and 2024 were conducted at multiple locations, including the Great Barrier Reef (Heron Island, Lady Musgrave, and Lady Eliot) and the Maldives (North and South Male atolls). These observations provided insights into coral reef ecosystems, natural coral formations, and artificial reef constructions.
• Interviews with two marine ecologists specialising in coral reef restoration further informed the research (ethics approval GU Ref No: 2021/525). Their responses to the interviews are referenced in this research paper. Additionally, informal discussions with ecologists and coastal engineers were conducted in two organised workshops to evaluate and refine the design outcomes.
• A case study analysis was conducted on existing artificial coral reefs to identify design and fabrication strategies. Seven design approaches were analysed: breezeblocks, steel bars, ropes, mesh trays, 3D printed clay, calcium carbonate structures, and structures that grow with mineral accretion. The analysis focused on the materials, colours, and textures used in reef structures.
• Furthermore, using computer-aided design (CAD) software, including Rhino 7 and Maya 2021, design simulations were developed to test various biomimetic designs. These simulations analysed factors such as surface roughness, voids, and patterns to test the coral’s preferences on topographical formations. The design was guided by the actual form and metrics of corals as deducted from the Coral Finder 2021 [46]. Nine design variations were developed and are analysed in the section below.
• Additionally, the designs were fabricated as 15 × 15 cm squares using four different materials: thermoplastic PETG, clay, concrete, and oyster concrete. The selection parameters were developed based on the literature review, case study analysis and direct design and fabrication observations.
• The experiment’s aim was to test each material’s fabrication process, assess factors such as material breakage, the level of design detail achievable, colour experimentation, required tools and materials, fabrication time, and preliminary cost. The goal was determining which material offers the greatest design flexibility while requiring minimal technological, human and material resources.

3. Results

3.1. Design

This research explores biomimetic surfaces that are derived from coral formations to develop topographies for coral attachment and growth. Three coral polyps forms were used as a reference for the design of the bio-enhancing surfaces—P. rugosa, Astrea, and Coelastrea [46]. The selection was intentionally based on their three distinct formations (Figure 1). The design was intended to be used for reproduction through micro fragmentation, facilitating various pocket sizes for coral fragment attachment and for coral larvae settlement.

Based on P. rugosa, the first two designs (a, b) comprise 1 mm thick extrusions. The soft curves extended vertically to create a rough, curved topography (Figure 1). The first (a) creates a more emphasised division through curve wall extrusions, whereas the second (b) develops a smooth topographic flow through surface extrusion. In both cases, the maximum height is 5 mm. Due to distinct formation, the first design creates more protective areas at the concave side of the extrusions than the second. Meanwhile, the dynamic design enhances water and sediment flow. These designs focus on corals that expand horizontally, such as encrusting or massive corals, by providing a continuous surface for expansion [6].

The following three designs (c, d, e) are inspired by Coelastrea (Figure 1). These designs have an irregular organic form with various pocket sizes ranging from a few mm to approximately 1 cm in width. The smaller pockets create protective environments for the first stages of coral larvae development. The larger ones can be used to attach various sizes of coral fragments. The first design has no variations in height, whereas the second and third (d, e) have a more pronounced topography, with the highest point of approximately 2.5 cm. Design (d) composes open holes that intend both to promote small fish recruitment and easier attachment of coral fragments.

Designs (f, g) are based on Astrea (Figure 1). These are composed of polyp-shaped modular 15 × 15 mm extruded vertically, 1 cm above the surface. These designs create a pattern form inspired by the natural formations of coral polyps. More protected areas for coral larvae attachment. The first creates a series of mountainous formations with deep and tight divisions, whereas the second creates crater shapes that intend to host a coral. Designs (h and i) are variations of the (d) with more irregular formations (Figure 1). The first (h) composes various heights of pointed shapes, giving the surface a less rigid appearance. The last one creates a more dynamic topography that intends to interplay with water flow and sediments.

3.2. Fabrication

The designs were fabricated using 3D-printed PETG, concrete, clay, and oyster concrete. First, they were 3D-printed using Bambu printers with PETG filament, a thermoplastic material known for its strong mechanical properties and resistance to seawater exposure [47,48]. The printing process was fast (between 1 and 6 h max for each tile based on the total volume) and the printed surfaces required no curing time after production. The printing diameter of the nozzle was 1.75 mm, enabling high-resolution textures that closely replicate the dimensions of the digital models. Also, PETG is available in fourteen solid colours and nine translucent variations, allowing for extensive colour experimentation. Five colour variations were selected based on coral reef observations: a solid, cream colour imitating sand, a solid red resembling red algae, a translucent pink reminiscent of certain algae, a translucent green evoking the colour of green algae, and a translucent brown mimicking the hues of most corals (Figure 2).

The other materials tested included grey concrete, reddish oyster cement, and pinkish clay. Three-dimensional printing was not available for concrete, clay, or calcium carbonate, nor was it recommended, as it cannot achieve millimetre-scale details. Instead, fast-set silicone moulds were created to cast these materials. In total, seventy-five models were produced (Figure 2): ten grey concrete models made from cement, sand and water; fourteen oyster cement models composed of crushed oyster shell powder, cement, sand, water, and red oxide for colour variation; ten red and pink PETG models; twenty sand-coloured PETG models; ten green and brown translucent PETG models; and eleven clay models, with red oxide added to create a natural pinkish hue.

This experimentation provided essential insights into the fabrication process and the level of design details achievable with each material (

Table 1. Fabrication summary of the four different materials.

	PETG	Concrete	Oyster Concrete	Clay
Design detail	3D-printing can achieve design details in 1.75 mm.	3D-printing can achieve design details in 4 mm. Pouring the concrete in moulds can achieve less.	Pouring the concrete in moulds can achieve design details. Oyster shell should be crushed into a powder.	Clay shrinks and hardens during the dry process makes it challenging to control details in mm.
Colour experimentation	Available in fourteen solid colours and nine translucent variations. Very vibrant colour variations.	Natural colour dyes can be added to the mix.	Natural colour dyes can be added to the mix.	Natural colour dyes can be added to the mix.
Material cracking	No cracking was observed.	Minimal cracking was observed.	Moderate cracking was observed.	Severe cracking was observed. Clay needs to be fired in very high temperatures.
Tools	3D printer	Hand mixing equipment such as buckets, shovel, and gloves. Also design moulds.	Hand mixing equipment such as buckets, shovel, and gloves. Also design moulds.	Hand mixing equipment such as buckets, shovel, and gloves, design moulds and kiln for firing
Material resources	PETG filament	Cement, sand, water	Cement, oyster shell powder, sand, water, red oxide	Clay
Fabrication time	1 to 6 h each. For nine tiles approximately 3 days	For mixing, pouring and cleaning approximately 3 h. However, requires 4 weeks to set fully.	For mixing, pouring and cleaning approximately 3 h. However, requires 4 weeks to set fully.	For mixing, pouring and cleaning approximately 2 h. However, also requires 4 weeks to set fully.
Material Cost (AUD/per tile)	3 AUD	1 AUD	7 AUD (cost increase due to the oyster shell powder)	4 AUD
Additional notes	Requires extra support	Requires moulds	Requires moulds	Requires firing

). Overall, 3D printing with PETG was the fastest process, requiring minimal material and producing the most refined details. However, PETG is a lightweight material and requires support to prevent floatation in seawater. Also, even though research exists on the resistance of PETG in seawater environments, and it has been widely used for underwater structures and reef restoration, further studies are needed to assess its long-term durability and its symbiosis with corals. Additionally, PETG is not structurally self-supporting and requires additional support for underwater structures. Concrete structures can serve as a foundation for PETG finishes.

The concrete models were relatively easy to manufacture. However, they required four weeks to set fully. Despite this, concrete remains one of the most durable materials and, as mentioned, provides a suitable substrate for coral attachment. Its natural colour is grey, but red oxide can be added for colour variation (Figure 2). However, concrete is very heavy, making underwater deployment more challenging. Additionally, cement production is one of the most significant contributors to climate change, making it crucial to minimise its use wherever possible. Three-dimensional printing with concrete would not achieve the fine details that are required for this particular design. Therefore, pouring the material into moulds was necessary to preserve the design details. Some cracks were observed in the final design models (Figure 2).

The next material tested was oyster concrete. Oyster shell powder was used to replace part of the aggregates in the mixture. The final mix consisted of one part cement, one part oyster powder, three parts sand, water, and red oxide for colouring. The mixture also required four weeks to set fully. The resulting models had a texture similar to concrete but appeared slightly more refined. However, they exhibited more cracks (Figure 2). Oyster concrete is considered a more sustainable alternative to conventional concrete. Various studies have explored the ratio of oyster shells in the mixture [49,50,51], though the ideal proportion depends on the function of the structure. For greater structural stability, a lower amount of oyster powder should be used [52]. Red oxide was added to mimic the colour of red algae and potentially attract more coral larvae.

Although clay is the most natural material from the selection, and extensive research supports the successful attachment of coral to clay structures, the clay trials for this design were not very successful. To capture the intricate details of the design, the clay needed to be highly liquid, which significantly extended the settling time. Even after settling, cracks were observed in the structure. Additionally, clay must be fired at very high temperatures, approximately 900 °C, to achieve stability, making the fabrication process challenging in areas without access to kilns.

In conclusion, 3D printing with PETG was the fastest and most efficient fabrication process. However, additional support is required for structural stability. Oyster concrete has the potential to produce self-supported structures, but further research is needed to assess its feasibility for underwater construction and its long-term structural integrity when exposed to seawater.

4. Discussion

This study explored the design of bio-enhancing surfaces for coral attachment by developing nine designs inspired by coral formations. These designs were fabricated using four different materials derived from a literature review and incorporating seven colour variations that resemble the natural reef, aiming to enhance coral larvae attachment through biomimetic design. Fabrication results suggested that oyster concrete could provide self-supported bio-enhancing structures. However, further research is necessary to assess long-term coral growth.

The developed designs can be applied to coral fragment attachment and coral larvae settlement. Moulds based on the above design patterns can be used to cast oyster concrete, or the designs can be 3D-printed and attached to other structures. Material selection should be guided not only by resource availability but also by functional requirements. For example, moulds can be developed for underwater columns, into which oyster concrete can be poured to form structural components.

The selection criteria for each design should consider coral species and environmental conditions such as sedimentation and wave intensity. For example, corals that expand horizontally, such as encrusting or massive coral types, require extensive horizontal surfaces, making designs (a) and (b) (Figure 1) more suitable. In contrast, branching corals which attach at a single point, need pockets to support their vertical growth, as seen in designs (c), (d), and (g) (Figure 1). Beyond coral preferences, several other factors influence the design of these structures, including material sustainability, carbon emissions, and the potential use of recycled materials. Although these variables are beyond the scope of this article, they are important considerations in the design process. Additionally, environmental variables like sunlight availability, temperature, salinity, and seabed substrate conditions also affect coral growth and survival and must be considered when designing artificial reef structures.

5. Conclusions

The architecture perspective in this research helps develop holistic design approaches that do not focus solely on material, colour, or form, as most studies do, but instead integrate these factors to create optimal design opportunities for coral attachment. Typically, artificial coral reef design is dictated by material availability and fabrication skills. To the best of our knowledge, no similar study has tested the architectural design of these structures in a systematic way. This research builds upon previous studies, combining various approaches to establish a more comprehensive design framework for coral attachment. Since corals are the “clients” in this context, this study investigates the best possible habitats to support their growth and survival.

The developed designs should be applied to the surfaces of underwater structures to encourage coral attachment. In underwater projects, considering texture, material, and colour is crucial for enhancing biodiversity and promoting the development of marine life. Architectural design is known for its holistic approach to the built environment. Architects are not solely responsible for creating structures for human habitation but also for supporting ecosystems, promoting biodiversity, and adapting to climate change. Various projects worldwide are being developed with similar goals, such as the Living Breakwaters project in Manhattan by the SCAPE studio, which focuses on oyster reef restoration.

Building on the Living Architecture approach, this research suggests that incorporating corals as fundamental design elements could lead to structures that self-grow, self-adapt, and self-maintain, reducing the need for additional funding, materials, and technological resources over time. The on-site growth of these structures would lessen the demand for construction materials and sourcing. Moreover, living structures have the potential to grow stronger with age, and their ability to self-repair and self-maintain is particularly valuable in hard-to-access areas, such as underwater environments. However, the corals’ slow growth rate is a limitation that could, to some extent, be mitigated by using bio-enhancing surfaces and materials. Furthermore, in the case of coral fragment transplantation, the survival success rate for coral survival depends on numerous factors, including environmental changes, continuous monitoring and maintenance, and funding, making it very hard to predict the long-term success or failure of the designs.

In conclusion, the impact of architectural design on the construction of artificial reefs to promote coral attachment and growth has largely been overlooked. However, this study has demonstrated that the strategic design of a structure can play a significant role in adapting to environmental conditions and the needs of corals. To evaluate coral larvae attachment and growth, the tiles were placed in the water in November 2024 and March 2025 and are currently being tested in a coral facility in Bundaberg. The ongoing study investigates how material and colour influence the habitation of coral larvae. Since corals require time to grow, this experiment necessitates a long-term study. Further research will explore the synergy of architecture with ecology for the construction of large-scale structures in the water that could support both human and coral habitation.