Engineers often use high temperatures, pressures and polluting chemicals to make synthetic materials. By contrast, biology produces remarkable materials like wood and bone using widely available chemical resources in water and at ambient temperature. The ability of organisms to create materials under such mild processing conditions relies on the intricate biological machinery of living cells. Notably, natural selection processes have evolved such machinery for hundreds of millions of years to fulfill the demands of biological environments. In this context, we asked the question: can we harness the machinery and evolutionary processes of biology to create materials more sustainably while still meeting engineering needs? To explore this question, we have utilized a microfluidic platform to evolve material-forming microorganisms towards cell mutants that meet the functionalities and high productivity needed in industrial processes. This directed evolution approach is illustrated using cellulose-producing bacteria commonly found in fermented beverages. With the help of the high-throughput microfluidic tool, it was possible to isolate a bacterial mutant that produces up to 70% more cellulose than its native counterpart. Such overproducing bacterial strain offers an attractive alternative to wood to meet the growing demand for cellulose in the textile, medical and packaging industries. Beyond cellulose, we expect the proposed technology to offer a compelling new approach to isolate bacteria for the bio-fabrication of other sustainable materials, such as silk, polyesters and clay-based bricks.