Hydroxyapatite is a high-value, biocompatible substance for use in dental and bone implants, restoration of archaeological artefacts, and other applications.
In a paper in Nature Communications, the researchers offer a techno-economic analysis of the technique to transform urine – which can seriously damage watersheds – into HAp, a calcium phosphate mineral projected to hold a market value of more than $3.5 billion by 2030.
“This process achieves two goals at the same time,” says coauthor David Kisailus, a University of California, Irvine Professor of materials science and engineering.
“On the one
hand, it helps remove human urine from wastewater streams, mitigating environmental pollution and the buildup of unwanted nutrients; and on the other hand, it produces a material that can be commercially marketed for use in a variety of settings.”
In the Nature Communications paper, he and his collaborators discuss how they developed and evaluated a biologically inspired system based on synthetic yeast cells they dubbed “osteoyeast” that drive the urine-to-HAp conversion.
In mammals, specialised cells called osteoblasts draw calcium phosphate from body fluids that is then processed and secreted as HAp.
Osteoblasts are not suitable for large-scale industrial HAp production, according to the researchers, so they turned to osteoyeast, which uses enzymes to break down urea and increase the pH of the surrounding environment. This triggers tiny cavities in the yeast to accumulate calcium and phosphate that are then secreted and crystallised into HAp.
The scientists found that their method can produce as much as 1g of HAp per litre of urine.
“This process to yield hydroxyapatite, or bone mineral, takes less than one day,” Kisailus says.
“The fact that it uses yeast
as a chassis, which is inexpensive and can be placed in large vats at relatively low temperatures – think about beer that’s made via fermentation processes and is well scaled – shows that this can be done easily without major infrastructural needs, and that has the added benefit of making it accessible to developing economies.”
Kisailus, who has a deep background in researching crystal growth mechanisms as well as making inorganic crystalline materials using biological and bio-inspired methods, says his main role in this project was to evaluate crystallisation pathways in
the osteoyeast platform.
According to the researchers, HAp composites are lightweight and offer significant mechanical strength, toughness and durability.
If synthetic hydroxyapatite that had its origins in urine can be produced economically and at scale, it can serve as a renewable and biodegradable alternative to commodities like plastics and other building materials.
“I am continuing to work with Professor Yasuo Yoshikuni from Lawrence Berkeley Laboratory, a corresponding author of this paper, to make other materials using this process, including materials for energy-based applications,” Kisailus says.
“We are currently developing strategies to leverage his yeast platform with our 3D printing and structural knowledge to make multifunctional architected materials.”