In the first kind, the organism does the manufacturing and is then halted. Mycelium binds agricultural waste into a foam over about a week and is heat-pressed. Bacteria secrete a sheet of cellulose at the surface of a sugar solution and the sheet is dried. Bacteria precipitate calcium carbonate between sand grains and the block is rinsed and cured. Growth happens at room temperature, on low-value feedstock, and the finished product can be composted. The advantage is in production rather than in performance, which is what makes these materials relevant to a circular economy.

In the second kind, cells stay viable inside a hydrogel, polymer, or mineral matrix and keep working after the material is installed. Synthetic biology supplies the behaviour: bacteria carry genetic circuits that make them secrete fibres, change colour, or release a compound when a signal arrives. A living brick reported in 2020 had cyanobacteria mineralise a sand and gelatin scaffold; one parent brick was split and regrown into eight over three generations (Heveran et al., 2020). Viability depended on humidity, and the conditions that kept the cells alive were the conditions that kept the brick weak. A living material can respond to its environment in ways an inert material cannot. It can also die, which makes viability a design requirement.

Chopped agricultural waste, hemp hurd or corn stalks, is inoculated, packed into a mould, and left for about a week while hyphae bind the particles; heat then kills the fungus and fixes the shape. The result has the density and strength of expanded polystyrene, which is why the products are packaging, insulation, and acoustic panels rather than structure (Ecovative; Mogu). Hy-Fi (2014) stacked roughly 10,000 such bricks into a 13-metre tower at MoMA PS1 and composted them afterward. Grown denser and pressed, the same material becomes a sheet used as leather; Reishi (MycoWorks) was made into a Hermès bag in 2021. Mycelium's weakness is water. Uncoated, it absorbs moisture and softens; coated, it stops being compostable.

Bacterial cellulose is grown by fermenting sweetened tea with a symbiotic culture of bacteria and yeast; a pellicle forms on the surface over two to three weeks, is lifted off, and is dried into a leather-like sheet. The bacterium Komagataeibacter xylinus produces cellulose fibres roughly 100 times thinner than plant cellulose, which gives the sheets their strength. Garments have been made this way since 2003 (BioCouture), and microbial textiles now guide the bacteria along a yarn scaffold so the cellulose grows into a designed structure rather than a flat mat (Modern Synthesis). The process has been used for wound dressings for decades. The newer development is treating the growth pattern itself as the designed element.

Cement production accounts for around 8 percent of global carbon dioxide emissions. Bio-cement avoids the kiln: Sporosarcina pasteurii bacteria are fed urea and calcium in a sand mould, hydrolyse the urea, raise the pH, and precipitate calcium carbonate between the grains, and the block is solid at room temperature in a few days (bioMASON). Self-healing concrete takes the opposite approach and puts the biology into ordinary concrete: dormant Bacillus spores and calcium lactate are mixed in, and when a crack admits water the spores revive, metabolise the lactate, and seal the gap with limestone (Jonkers, 2011). The method closes cracks under a millimetre wide and is sold commercially by Basilisk. Both respond to the same problem: concrete is cheap to make and difficult to reuse or unmake.