Paper of the Month - May

In the future he might be made of a sustainable biopolymer. 

In the future he might be made of a sustainable biopolymer. 

May’s ‘Paper of the Month’ is a collaboration between a number of different laboratories and Institutes: the Max Planck Institute of Colloids and Interfaces, Nanyang Technological University, Massachusetts Institute of Technology and the Wyss Institute for Biologically Inspired Engineering.

Their paper ‘Multi-scale thermal stability of a hard thermoplastic protein-based material’ determined the thermal properties of a potentially novel and sustainable biopolymer to replace the unsustainable petrochemical polymers we rely so heavily on today. 

Since the industrial revolution, petrochemicals have been used extensively: in aromatics, plastic production, and fuel. Not only are they environmentally damaging but reserves of petrochemicals are running out. They take millions of years to form, and substantial use of them over the last two centuries means future generations will have to go without. 

Yet their chemistry and structure are excellent for creating thermoplastics. When heated, weak forces of attraction between polymer chains – intermolecular forces – break, allowing the chains to move past each other and the plastic to be reshaped. When the plastic cools, these bonds reform and the new shape is held. 

In contrast, thermosetting plastics have a strong cross linked network of bonds, which allows them to retain their shape when heat is applied, although extreme temperatures will cause both thermoplastic and thermosetting plastics to permanently decompose. Thermosetting plastics have many useful applications but thermoplastics’ wide commercial use comes from their ability to be reheated and reshaped a number of times.

A simplistic representation of the effect of heat on bond interactions (red) between polymer chains (black) of thermoplastics and thermosetting plastics.

A simplistic representation of the effect of heat on bond interactions (red) between polymer chains (black) of thermoplastics and thermosetting plastics.

The search is on to find alternatives to petrochemical based polymers, but it is challenging because not many biopolymers have the chemical properties required to replace thermoplastics. Even when such properties are induced through chemical processing, the biopolymers often lose the integrity of their physical nature. However, one exception may be Sucker Ring Teeth (SRT), made of a protein called Suckerin, which is found in the tentacles of squid and cuttlefish. 

The research group first molecularly characterised the macromolecule to find it displayed many of the chemical properties true to thermoplastics. Hydrogen bonds – a type of intermolecular force – are found between SRT’s β-sheets and, just like thermoplastics, when heated these bonds break and SRT can be reshaped. 

Using X-ray scattering, spectroscopy and nano-mechanical techniques, the group determined its molecular mechanical and thermal properties. Using the Linkam THMS600 stage, which can control the temperature of the sample from -196°C to 600°C with an accuracy of 0.01°C, they precisely varied the temperature to test the thermal properties of SRT. 

Thermal extrusion experiments were conducted to test the thermoplastic nature of the polymers. With the addition of water and the plasticiser glycerol, it could be heated and cooled into various stable forms, reprocessed a number of times and proved successful in additive manufacturing. They determined SRT to be crystalline polymers of β-sheets with amorphous regions, which is structurally stable until 220°C. The properties of the treated SRT polymer are highly promising for synthetic commercial production. 

By determining the thermal and molecular properties of this biopolymer, the group has established a potential sustainable alternative to thermoplastics. With the results showing potential, the future range of applications for SRT may extend to 3D printing, synthetics and biomedical devices.

By Tabassum Mujtaba

Latza, V., Guerette, PA., Ding, D., Amini, S., Kumar, A., Schmidt, I., Keating, S., Oxman, N., Weaver, JC., Fratzl, P., Miserez, A., Masic, A. (2015). Multi-scale thermal stability of a hard thermoplastic protein-based material. Nature Communications. 6: 8313