With plans to revisit the lunar surface and eventually send a crewed mission to Mars, future space exploration missions are likely to involve an extended surface stay [1]. For such missions, or perhaps even settlements, survival requires meeting basic human needs while Earth-independent. With an emphasis on in situ resource utilization [2], a sustainable extraterrestrial settlement must be a resource-efficient, closed ecological system [3].

To minimize energy cost, Martian manufacturing strategies capitalize on the abundant inorganic components readily available in the regolith of the planet’s surface. However, these manufacturing methods are based on technologies developed for the bountiful paradigm of Earth and are commonly characterized by processes involving elevated temperature and pressure [4, 5], polymers with complex and dedicated biosynthesis [6], limited reclamation [7], and niche uses [8]. Since any resource obtained on Mars comes at an opportunity cost, the sustainable production of these materials must be contextualized in a Martian ecosystem. Towards this objective, nature presents successful strategies of life adapting to harsh environments. In biological organisms, rigid structures are formed by integrating inorganic filler procured from the environment at a low energy cost (e.g., calcium carbonate) and incorporated into an organic matrix (e.g., chitin) produced at a relatively high metabolic cost [9, 10]. Chitin is a paradigmatic example of an organic matrix of mineralized composites; it is the second most abundant organic polymer on Earth (after cellulose) and biology’s recurrent solution to forming structural components. Chitin is produced and metabolized by organisms across most biological kingdoms, including most heterotrophs used as bio-converters of organic matter [11].

For food production and other life support systems on Mars, early explorers will rely on other biological life [12, 13] and, due to its ubiquity, chitin will likely be part of any artificial ecosystem (example in S1 Fig) [14]. Insects specifically have been identified as a key part of a potential Martian settlement as a high-yield protein complement to fulfil the caloric requirements of a human crew [15] and to process agricultural and biological waste [16]. Still, despite the almost unavoidable presence of large amounts of chitinous polymers in human-centered cycles and their potential to feed engineered bacteria populations [17], these biopolymers have limited nutritional value in vertebrates [18]. Therefore, in contrast to all biomolecules used to produce bioplastics, the extraction of chitin does not hamper or compete with the food supply; rather, it is a byproduct of it.

Chitin in its most acetylated form (e.g., taken from arthropods and mollusks) is a mostly inert molecule with the same manufacturability issues as cellulose. Highly deacetylated chitin chains, usually referred to as chitosan (e.g., those taken from fungi), can be easily protonated in a weak acid, inducing intermolecular repulsion forces and enabling dissolution in aqueous solvents. Working with simple chemistry suitable for early Martian settlement, we produced Martian biolith using chitosan derived from arthropod cuticle (shrimp, 75–85% Degree of Deacetylation) via treatment with sodium hydroxide, a component obtainable on Mars through electrolytic hydrolysis [19]. Chitosan was dissolved in a low concentration of acetic acid (i.e., 1% v/v), the simplest carboxylic acid and a common byproduct in both aerobic and anaerobic fermentations, which is a vital process in a biotechnological strategy suited for Mars [20, 21]. The pH-based, simplified chemistry used here for the extraction and manufacture of chitosan requiring only water (available in the form of subsurface ice), sodium hydroxide, and acetic acid, can be further simplified if the polymer is obtained from an ecosystem involving fungi and, therefore, not requiring deacetylation/NaOH, or can be avoided completely by the use of enzymatic fractionation [11].

Here, we approach the problem of staying on Mars from a bioinspired perspective by replicating chitinous bioinspired manufacturing [22] developed for the production of sustainable manufacturing on Earth [11]. The resulting Martian biolith and its associated chemistry involve Martian regolith simulant, ubiquitous biomolecules, and water-based solvents that are easily integrated into any ecological cycle(s) and avoids the need for complex polymer synthesis, shipping of specialized equipment, or dedicated feedstock. We demonstrate how this material, produced and used with minimal energy requirements, retains the versatility of its biological counterparts, enabling the rapid manufacturing of objects ranging from basic tools to perhaps even rigid shelters.

Chitosan forms transparent objects similar in appearance and mechanical characteristics to commodity plastics [23], a property lacking in the current materials deployable in early stage Mars settlements [7]. While chitosan on its own can be useful in specific applications, the composition of biolith (Fig 1A) has the minimum amount of metabolically expensive chitosan needed to produce a material with sound mechanical properties and general application. Three-percent chitosan in an acidic aqueous solution was used as external phase to form colloidal dispersions of Martian regolith simulant (Fig 1B) with weight ratios varying from 1:25 to 1:125, producing agglomerations of different bulk behaviors ranging from the inability to retain shape due to an excessive dispersive phase to the inability to form cohesive structures due to an excessive inorganic phase. Same trend was also observed and quantified in the mechanical testing of the material. Molecular and thermal analyses showed a lack of chemical interaction between the inorganic phase and the chitinous polymer (Fig 1C and 1D), suggesting that the composite is aggregated and internally compacted by strong intermolecular forces produced by the chitinous polymer during crystallization. After the aqueous solvent evaporated, the range of 1:75 to 1:100 led to the crystallization of the composite into solid structures with maximum flexural strength and stiffness (Fig 2A and 2B). These binder concentrations are between one-fifth and one-half of the amount of binder previously reported for “ultra-low-binder-content” composites (e.g., polystyrene (PS) or polylactic acid (PLA)) [24, 25].

The versatility of biolith in applications without elevated temperature or pressure (Fig 2C and 2D) is demonstrated in an unprecedented range of manufacturing methods, such as casting, using it as mortar and in additive manufacturing (Fig 2E and 2F). The initial products of biolith could be consumable tools and equipment mass fabricated by hand or casting. A pragmatic test for the casting potential of biolith was performed by producing a functional wrench (Fig 3A), which was tested by tightening a hexagonal bolt (Fig 3B). While biolith could not replace metallic tools, the simple wrench was able to sustain a torque of 2.83±0.92 Nm before breaking, matching the torque specified for M5 bolts used in non-critical space applications (Fig 3C) [26]. This demonstrates the practical utility of a tool made with biolith and its usefulness on Mars, as requirements would likely be different due to the lower gravity and atmospheric pressure. We also tested the ability of biolith to reproduce geometries via molding, from a simple cylinder to a more angular companion cube and, ultimately, a geometry involving both rounded and angular shapes (Fig 3D). The fidelity of these replicas was measured via comparative surface analysis using a high-resolution 3D scanner. While the average volumetric shrinkage among the three geometries was 9.9±3.6% due to the evaporation of water, the average surface deviation for the individual artifacts was small (-0.61±0.69 mm for the cylinder; -0.34±0.81 mm for the cube; -0.32 ±1.04 mm for the astronaut-like geometry).

Biolith could also be used as mortar for general maintenance purposes. Fig 3E shows biolith being used to fix a 12.5 mm diameter circular hole in a pipe. The elastic behavior of biolith enables a buckled internal plug to form and anchor the dried patch, producing a sealing effect based on mechanical principles rather than chemical bonding (i.e., like a rivet). No leakage was observed even after several weeks. The strength of the material in sustaining hydraulic pressure was measured using a modified burst test for biomaterials (ASTM F2392) under both negative and positive pressures (Fig 3F). In the more demanding operating condition that off-seats biolith sealing on precut biolith discs, the sealing was able to sustain pressures in the range of 117.70 ±49.67 MPa before the biolith sealing dislodged. For comparison, the atmospheric pressures on Earth and Mars are about 101.325 kPa and 0.61 kPa, respectively, while the Martian lander cabin pressure is likely to be around 56.53 kPa [27]. The hydraulic pressure sustained by the biolithic repairs of an aluminum disc (1.09±0.52 MPa) was significantly lower than that on the repairs of a synthetic polymers disc (PLA, 163.32±29.01 MPa) and the biolith disc. The difference was initially attributed to the different surface interactions between biolith and aluminum or PLA; however, this hypothesis was discarded due to the lack of a significant difference in bonding strengths between those materials (Fig 3G). This suggests that the low hydraulic pressure borne by biolithic repair on aluminum might be related to mechanical effects, such as a stiffness mismatch between the repaired part (aluminum) and the repair material (biolith).

The self-adhesion of biolith is indicative of its suitability for additive manufacturing. For this purpose, a scaled ovoid replica inspired by the design of a NASA 3D printed habitat winner (MARSHA) was extruded in three segments and assembled using biolith (Fig 4A, 4C and 4D). The resulting structure was printed in 1.84 hours (Fig 4E). One of the advantages of the printing setup reported here (Fig 4B) is the ability to tune the printing process so as to balance speed and definition, as well as the ability to scale printed artifacts to several orders of magnitude using the same material and manufacturing technology. For example, using this technology, we printed a 5 m structure in 48h with a 5 mm definition, and its replica at 40 cm in 12 h with a 0.5 mm definition [11, 28].

While scarce resources in an extraterrestrial environment pose extreme challenges to the establishment of a closed ecological cycle that supports human activities, it is conceptually similar to the problem of sustainable development on Earth. With past development based on the false premise of unlimited resources, we are facing the effects of a development model that is disconnected from Earth’s ecological cycles, resulting in both a shortage of primary resources and an accumulation of waste. Here, we applied the principles of bioinspired chitinous materials and manufacturing, initially developed for production within circular regional economies on Earth, to develop a composite with low manufacturing requirements, ecological integration, and versatile utility in a Martian environment. The results presented here demonstrate that the development of a closed-loop, zero-waste solutions to tackle unsustainable development on Earth may also be the key to our development as an interplanetary species.