Inspired by the natural role of proteins in mediating charge transport in living biological systems, researchers from the Israel Institute of Technology in Haifa have created an environmentally friendly conductive polymer by using proteins as building blocks. The team's research was published in the journal Advanced Materials in July 2021.
A research team from the Israel Institute of Technology in Haifa has developed transparent polymer films derived from proteins. Image Credit: Design_Cells/Shutterstock.com
The innovative biopolymer is inherently biocompatible and biodegradable while demonstrating outstanding elasticity and electrical conductivity (among the highest measured in biological materials), which makes it highly suitable for biological and biomedical applications.
Modern society recognizes the ever-growing need for polymer materials derived from sustainable resources rather than fossil feedstock. Constantly depleting non-renewable resources and deteriorating environment drive industry and academia to reconsider their strategies when developing new materials.
Over the past decade, bio-based polymers emerge as feasible alternatives to traditional fossil fuel-based materials in various applications, such as additive manufacturing, nanotechnology, and tissue engineering.
Due to their availability and unique properties, proteins offer a great promise as a renewable source of sustainable biomaterials for various applications. Bio-based protein polymers can be derived from a variety of renewable feedstocks, including agricultural products (for example corn or soybeans), alternative sources such as algae or food waste, or from side-stream products (or by-products) from an existing industrial process.
Proteins often exhibit inherent structural and functional complexity unmatched by synthetic materials. Their remarkable functionality arises from molecular structures assembled from a relatively small number of simple building blocks. However, over time, that complex structure-function relationship evolved to sustain life in most complex environmental conditions.
Combining Environmental and Sustainable Chemistry
Inspired by real-life protein structures, Dr. Nadav Amdursky, an assistant professor at the Schulich Faculty of Chemistry in the Technion (Israel Institute of Technology in Haifa) and his colleagues developed a cost-effective polymerization process that relies on natural protein-protein interactions and results in a transparent proton-conducting elastomer.
Biological charge transfer processes, such as those involved in photosynthesis and aerobic respiration, form the basis for life on our planet. The ability of proteins to mediate charge transport (electrons, protons, and ions) relies on highly hierarchical protein structures spanning length scales ranging from nanometers up to millimeters.
Directional proton transport, which takes place across biological membranes, is one of the most fundamental processes in biology that drives the metabolism of all life on Earth. In biological systems, proton transport pathways involve structural water molecules and specific amino acids that form a hydrogen-bond network capable of long-range charge transport.
Nature-Inspired Bio-Polymerization Approach
Mimicking nature, Dr. Amdursky and his team created a proton-conducting polymer using affordable and readily available bovine serum albumin (BSA) protein as a feedstock.
The precursor BSA was subjected to a simple one-pot bio-polymerization process involving the dissolution of the protein in a trifluoroethanol (TFE)-water mixture (4:1 v/v ratio).
The solvent mixture helped to partially denature the protein and to protonate its nucleophilic side chains. After reducing the disulfide bonds in the precursor solution, it was drop-casted into a mold where intermolecular crosslinking was initiated by randomly re-forming disulfide bonds among the BSA molecules.
The resultant transparent polymer film formed after the solvent evaporation was highly elastic and stable in commonly used solvents (such as ethanol, hexane, water, and others).
Upon examination, the formed BSA film demonstrated attractive mechanical properties as an elastomer. It was possible to stretch the polymer film to an impressive five times its original length before failure, without any performance loss after a hundred stretching cycles at 80% strain. At the same time, the elastomer exhibited a tensile Young’s modulus of around 12 MPa, making it relatively strong yet highly elastic.
Stretchable Conductive Polymer for Biomedical Applications
However, the Technion researchers were most excited by the electrical properties of the novel polymer. Being a dense, water-containing, semicrystalline polymer bearing multiple amino acid side chains (that can participate in a hydrogen bond network) ensured excellent proton-conducting properties of the film.
The experimental data revealed high proton conductivity of approximately 5 mScm-1 (millisiemens per centimeter) at normal conditions (room temperature and humidity) – a value that is nearly two orders of magnitude higher than the conductivity of previously reported proton-conductive protein-based materials.
The BSA polymer remains highly conductive even upon bending and stretching due to its excellent mechanical properties. An additional advantage of the protein-based material is the extent of functional groups on its surface that can be targeted for post-synthetic modification to improve film's conductivity.
Affordable and Viable Alternative to Fossil-Based Polymers
The use of proton-conducting materials has rapidly grown in recent years, especially energy-related applications such as fuel cells, batteries, and capacitors.
Dr. Amdursky's team envisions that the biocompatibility and non-toxicity of the innovative polymer would make it suitable for biomedical applications as well. The researchers validated at the proof-of-concept level by using the BSA protein polymer as a standalone elastic electrode interface for electrocardiography and electroencephalography (replacing the inconvenient use of a conductive gel).
Unlike the majority of synthetic polymers, the novel proton-conductive polymer is fully biodegradable in less than 48 hours. The BSA feedstock proteins used by the Israeli scientists were sourced from waste by-products of the extensive bovine industry in the country, thus favoring the use of low-cost, renewable and sustainable raw materials.
References and Further Reading
Nandi, R., Agam, Y., Amdursky, N. (2021) A Protein-Based Free-Standing Proton-Conducting Transparent Elastomer for Large-Scale Sensing Applications. Adv. Mater. 2101208. Available at: https://doi.org/10.1002/adma.202101208
S. Mondal, et al. (2020) Exploring long-range proton conduction, the conduction mechanism and inner hydration state of protein biopolymers. Chem. Sci., 11, 3547-3556. Available at: https://doi.org/10.1039/C9SC04392F
Technion (2021) Researchers develop conductive biopolymers using proteins. [Online] Available at: https://latamisrael.com/researchers-develop-conductive-biopolymers-using-proteins/?lang=en (Accessed on 8 August 2021).
Jerusalem Post Staff (2021) Israeli researchers use food waste for breakthrough multiuse greentech. [Online] www.jpost.com Available at: https://www.jpost.com/jpost-tech/israeli-researchers-use-food-waste-for-breakthrough-multiuse-greentech-673503 (Accessed on 8 August 2021).
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