Given plans to revisit the lunar surface by the late 2020s and to take a crewed mission to Mars by the late 2030s, critical technologies must mature. In missions of extended duration, in situ resource utilization is necessary to both maximize scientific returns and minimize costs. While this presents a significantly more complex challenge in the resource-starved environment of Mars, it is similar to the increasing need to develop resource-efficient and zero-waste ecosystems on Earth. In a recent paper published on PLOS One, scientists at Singapore University of Technology and Design used recent advances in the field of bioinspired chitinous manufacturing to develop a Martian biolith to be used with additive manufacturing technology within the context of a minimal, artificial ecosystem that supports humans in a Martian environment.
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 polymers with complex and dedicated biosynthesis, limited reclamation, and niche uses. 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. 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.
For food production and other life support systems on Mars, early explorers will rely on other biological life and, due to its ubiquity, chitin will likely be part of any artificial ecosystem. Insects specifically have been identified as a key part of a potential Martian settlement as a high-yield protein complement to fulfill the caloric requirements of a human crew and to process agricultural and biological waste. Still, despite the almost unavoidable presence of large amounts of chitinous polymers in human-centered cycles and their potential to feed engineered bacteria populations, these biopolymers have limited nutritional value in vertebrates. 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, the researchers 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. 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 fermentation, which is a vital process in a biotechnological strategy suited for Mars. 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.
The researchers approached the problem of staying on Mars from a bioinspired perspective by replicating chitinous bioinspired manufacturing developed for the production of sustainable manufacturing on Earth. 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 avoid the need for complex polymer synthesis, shipping of specialized equipment, or dedicated feedstock. The study demonstrates 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, a property lacking in the current materials deployable in early-stage Mars settlements. While chitosan on its own can be useful in specific applications, the composition of biolith 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 an external phase to form colloidal dispersions of Martian regolith simulant 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.
The 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 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. These binder concentrations are between one-fifth and one-half of the amount of binder previously reported for “ultra-low-binder-content” composites such polystyrene (PS) or polylactic acid (PLA).
The versatility of biolith in applications without elevated temperature or pressure is demonstrated in an unprecedented range of manufacturing methods, which include additive manufacturing. 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 which was tested by tightening a hexagonal bolt. 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. 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. The researchers 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. 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).
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. The resulting structure was printed in 1.84 hours (Fig 4E). One of the advantages of the printing setup reported here 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, the researchers 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.
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, humanity is 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. The researchers 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 seem to indicate 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.