Living building material

A living building material is a material used in construction or industrial design that behaves in a way resembling a living organism. Examples include: self-mending biocement, self-replicating concrete replacement, and mycelium-based composites for construction and packaging. Artistic projects include building components and household items.

History

The development of living building materials began with research of methods for mineralizing concrete, that were inspired by coral mineralization. The use of microbiologically induced calcite precipitation in concrete was pioneered by Adolphe et al. in 1990, as a method of applying a protective coating to building façades.
In 2007, "Greensulate", a mycelium-based building insulation material was introduced by Ecovative Design, a spin off of research conducted at the Rensselaer Polytechnic Institute. Mycelium composites were later developed for packaging, sound absorption, and structural building materials such as bricks.
In the United Kingdom, the Materials for Life project was founded at Cardiff University in 2013 to "create a built environment and infrastructure which is a sustainable and resilient system comprising materials and structures that continually monitor, regulate, adapt and repair themselves without the need for external intervention". M4L led to the UK's first self-healing concrete trials. In 2017 the project expanded into a consortium led by the universities of Cardiff, Cambridge, Bath and Bradford, changing its name to Resilient Materials 4 Life and receiving funding from the Engineering and Physical Sciences Research Council. This consortium focuses on four aspects of material engineering: self-healing of cracks at multiple scales; self-healing of time-dependent and cycling loading damage; self-diagnosis and healing of chemical damage; and self-diagnosis and immunization against physical damage.
In 2016 the United States Department of Defense's Defense Advanced Research Projects Agency launched the Engineered Living Materials program. The goal of this program is to "develop design tools and methods that enable the engineering of structural features into cellular systems that function as living materials, thereby opening up a new design space for building technology... to validate these new methods through the production of living materials that can reproduce, self-organize, and self-heal." In 2017 the ELM program contracted Ecovative Design to produce "a living hybrid composite building material... genetically re-program that living material with responsive functionality wound repair... rapidly reuse and redeploy material into new shapes, forms, and applications." In 2020 a research group at the University of Colorado, funded by an ELM grant, published a paper after successfully creating exponentially regenerating concrete.

Self-replicating concrete

is produced using a mixture of sand and hydrogel, which are used as a growth medium for synechococcus bacteria to grow on.

Synthesis and fabrication

The sand-hydrogel mixture from which self-replicating concrete is made has a lower pH, lower ionic strength, and lower curing temperatures than a typical concrete mix, allowing it to serve as a growth medium for the bacteria. As the bacteria reproduce they spread through the medium, and biomineralize it with calcium carbonate, which is the main contributor to the overall strength and durability of the material. After mineralization the sand-hydrogel compound is strong enough to be used in construction, as concrete or mortar.
The bacteria in self-replicating concrete react to humidity changes: they are most active - and reproduce the fastest - in an environment with 100% humidity, though a drop to 50% does not have a large impact on the cellular activity. Lower humidity does result in a stronger material than high humidity.
As the bacteria reproduce, their biomineralization activity increases; this allows production capacity to scale exponentially.

Properties

The structural properties of this material are similar to those of Portland cement-based mortars: it has an elastic modulus of 293.9 MPa, and a tensile strength of 3.6 MPa ; however it has a fracture energy of 170 N, which is much less than most standard concrete formulations, which can reach up to several kN.

Uses

Self-replicating concrete can be used in a variety of applications and environments, but the effect of humidity on the properties of the end material means that the application of the material must be tailored to its environment. In humid environments the material can be used as to fill cracks in roads, walls and sidewalks, sipping into cavities and growing into a solid mass as it sets; while in drier environments it can be used structurally, due to its increased strength in low-humidity environments.
Unlike traditional concrete, the production of which releases massive amounts of carbon dioxide to the atmosphere, the bacteria used in self-replicating concrete absorb carbon dioxide, resulting in a lower carbon footprint.
This self-replicating concrete is not meant to replace standard concrete, but to create a new class of materials, with a mixture of strength, ecological benefits, and biological functionality.

Calcium carbonate biocement

Biocement is a sand aggregate material produced through the process of microbiologically induced calcite precipitation. It is an environmentally friendly material which can be produced using a variety of stocks, from agricultural waste to mine tailings.

Synthesis and fabrication

Microscopic organisms are the key component in the formation of bioconcrete, as they provide the nucleation site for CaCO to precipitate on the surface. Microorganisms such as Sporosarcina pasteurii are useful in this process, as they create highly alkaline environments where dissolved inorganic carbon is present at high amounts. These factors are essential for microbiologically induced calcite precipitation, which is the main mechanism in which bioconcrete is formed. Other organisms that can be used to induce this process include photosynthesizing microorganisms such as microalgae, cyanobacteria, and sulphate reducing bacteria such as Desulfovibrio desulfuricans.
Calcium carbonate nucleation depends on four major factors:

Calcium concentration
DIC concentration
pH levels
Availability of nucleation sites

As long as calcium ion concentrations are high enough, microorganisms can create such an environment through processes such as ureolysis.
Advancements in optimizing methods to use microorganisms to facilitate carbonate precipitation are rapidly developing.

Properties

Biocement is able to "self-heal" due to bacteria, calcium lactate, nitrogen, and phosphorus components that are mixed into the material. These components have the ability to remain active in biocement for up to 200 years. Biocement like any other concrete can crack due to external forces and stresses. Unlike normal concrete however, the microorganisms in biocement can germinate when introduced to water. Rain can supply this water which is an environment that biocement would find itself in. Once introduced to water, the bacteria will activate and feed on the calcium lactate that was part of the mixture. This feeding process also consumes oxygen which converts the originally water-soluble calcium lactate into insoluble limestone. This limestone then solidifies on surface it is lying on, which in this case is the cracked area, thereby sealing the crack up.
Oxygen is one of the main elements that cause corrosion in materials such as metals. When biocement is used in steel reinforced concrete structures, the microorganisms consume the oxygen thereby increasing corrosion resistance. This property also allows for water resistance as it actually induces healing, and reducing overall corrosion. Water concrete aggregates are what are used to prevent corrosion and these also have the ability to be recycled. There are different methods to form these such as through crushing or grinding of the biocement.
The permeability of biocement is also higher compared to normal cement. This is due to the higher porosity of biocement. Higher porosity can lead to larger crack propagation when exposed to strong enough forces. Biocement is now roughly 20% composed of a self healing agent. This decreases its mechanical strength. The mechanical strength of bioconcrete is about 25% weaker than normal concrete, making its compressive strength lower. Organisms such as Pesudomonas aeruginosa are effective in creating biocement. These are unsafe to be near humans so these must be avoided.

Nucleation Mechanisms

Heterogeneous nucleation on microbial cell surfaces is common in MICP. Bacterial cell walls and extracellular polymers present negatively charged sites that selectively bind Ca²⁺ ions, effectively lowering the nucleation energy barrier. In essence, each bound cation–carbonate encounter forms a tiny crystalline embryo. Thus, microbes provide numerous nucleation templates, yielding calcite platelets or needles rather than uniform glassy films. For example, SEM studies show that calcite often precipitates as clustered platelets or needle-like aggregates on bacterial films. At high local supersaturation, unstable precursors like Amorphous calcium carbonate and Vaterite can initially form and later transform into calcite. In microbial consortia or in seawater, mixed metabolic pathways further modulate local pH and ion activities, affecting nucleation thresholds. These include the hydrolysis of urea or Photosynthesis.
Microscopy of microbially induced calcite often shows characteristic morphologies. Bacterial surfaces and exopolymeric sheaths concentrate Ca²⁺ and CO₃^2- ions and act as charged nucleation sites. The result is often aggregated "rafts" or needle-like clusters of calcite as shown in the image rather than smooth single crystals. Such textures are consistent with heterogeneous nucleation: crystals grow epitaxially on cell templates that locally elevate supersaturation. When supersaturation is relieved by rapid precipitation, calcium ions diffuse in from surrounding fluid, sustaining continued nucleation and growth around the microbe.
Extracellular polymeric substances secreted by bacteria also play a crucial role in CaCO₃ nucleation. EPS are complex biopolymers composed of polysaccharides, proteins, and nucleic acids that form a hydrated matrix around microbial colonies. These matrices can bind divalent cations such as Ca²⁺ and localize carbonate ions, thereby increasing ion activity at the cell-fluid interface. EPS mediates heterogeneous nucleation by concentrating reactants and lowering the interfacial energy barrier for crystal formation. Additionally, specific functional groups in the EPS such as carboxyl and hydroxyl moieties can template crystal orientation or polymorph selection. This microenvironmental control over supersaturation and binding energy is a fundamental example of biologically controlled mineralization.