A group of researchers at Zhejiang University have published an article entitled “A flexible and degradable hybrid mineral as a plastic substitute” in the journal Advanced Materials. The research is led by Prof. Liu Zhaoming and Prof. Tang Ruikang from the Department of Chemistry.
According to the article, the mineral contains over 80% calcium phosphate (CaP) and less than 20% polymers. Its flexibility and mechanical properties appear to be similar to those of polymer plastics. Additionally, it has improved hardness and thermostability, and more importantly, it can be sourced from nature and recycled after use. Since minerals are inexpensive, this technology could be suitable for extensive use.
Nevertheless, minerals are hard, brittle, and difficult to be shaped. It is therefore essential to overcome their intrinsic brittleness.
“These natural biomineralized materials [shells, teeth, and bones] are composed of inorganic minerals, organic macromolecules and polymers. It is this small quantity of macromolecules and polymers that control the size and morphology of inorganic minerals,” said Prof. Liu. “This inspired us to explore similar means to achieve the regulation of inorganic ionic polymerization.”
Prof. Liu and his team selected polyvinyl alcohol (PVA) and sodium alginate (SA) as ideal biomimetic polymers and added them to CaP ionic oligomers to produce a hybrid material. In terms of chemical functional groups, these two polymers reportedly resemble collagen in bones.
“More than 80% of the hybrid mineral is inorganic CaP, and the remaining components include PVA and SA,” said Prof. Liu. “These inorganic nanofibers are bonded by organic materials on a mesoscopic scale to form bulk materials on a macroscopic scale.”
As reported in the article, by using transmission electron microscopy, the researchers noted that CaP nanofibers possessed periodic structural defects, exhibiting a lower ionic cross-linking degree than hydroxyapatite (HAP, a crystalline phase of CaP). As a result, the stable and tough inorganic ionic network could be assembled into a structure with three or four parallel ionic chains, thereby rendering the inorganic structure flexible.
In the experiments, the hybrid material showed tensile strength amounted to about 20 MPa and its elastic modulus reached about 600 MPa; meaning it could be stretched and bent. In addition, due to its high inorganic content, it exhibited greater hardness at roughly 0.8 GPa.
The researchers found that this hybrid mineral remained mainly unchanged while burning and turned into a crystalline mineral and losing its toughness.
Due to pollution, Liu and his team conducted experiments simulating weathering, precipitation and ingestion by animals. They concluded that if soaked in water for more than 3 months, the material’s PVC and SA components would be dissolved and they would not cause any pollution because they are environmentally friendly. Furthermore, if it were accidentally ingested by an animal, it would hypothetically be gradually degraded and digested.
Prof. Liu and his team will conduct follow-up research concerning how to reduce the cost in manufacturing.
“It is a significant breakthrough to produce a hybrid mineral whose inorganic content reaches 80%, but the remaining 20% of polymers makes it hard to turn into a real mineral,” said Prof. Liu. “We hope that we can discover the mechanism for the formation of CaP with periodic structural defects and completely remove its organic polymers, thus elevating the understanding, synthesis and structural control of inorganic materials to a higher level.”