Editorial Feature

What are Self-Sterilizing Polymers?

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The ability to inactivate pathogens on contact by using inherently biocidal surfaces is at the core of some of the latest strategies for controlling the spread of harmful microorganisms in the healthcare industry, public transportation, and other sectors with a high risk of pathogen transmission via contaminated surfaces.

Today, infectious diseases are the second leading cause of death globally, surpassed only by heart diseases.

Traditionally, the most common strategies to mitigate disease-causing agents' spread are based on chemical disinfectants (such as bleach, hydrogen peroxide, and detergents) or repeated exposure to radiation (UV light). However, both approaches can damage the treated surface, adversely affect the environment, or introduce additional health concerns.

In particular, the uncontrolled use of antibiotics and disinfectants has caused environmental pollution and led to the emergence of drug-resistant pathogens.

Inherently Antimicrobial Surfaces for Effective Infection Prevention

The development of novel functionalized materials and coatings with biocidal properties offers an alternative approach that can prevent bacterial, viral, and fungal reproduction and proliferation.

Most of the engineered biocidal materials employ metal (silver or copper) or metal oxide (ZnO or TiO2) nanoparticles attached to a polymer substrate or embedded inside a polymer matrix.

The metal ions released from the nanoparticles can either interact with sulfur-containing proteins in the bacterial cell wall, causing cell malfunctioning or interfering with the ion transport across the cell respiratory chain. Under UV irradiation and humidity, some metal nanoparticles (such as TiO2) produce highly reactive free radical species. All these mechanisms could cause the microorganisms' death and are finding an ever-growing use in antimicrobial applications.

However, the metal nanoparticles are expensive to manufacture and there is a likelihood that nanoscale metal can leach into the environment and contaminate food chains and habitats.

Expanding the Arsenal of Antimicrobial Materials

Material scientists from North Carolina State University (NCSU) in the US, together with colleagues from Boston University and Kraton Corporation, have developed a novel self-sterilizing polymer that efficiently inactivates microorganisms, including SARS-CoV-2 - the virus that causes COVID-19 disease.

The polymer’s antimicrobial properties originate from its unique molecular architecture. It belongs to a class of materials called anionic multiblock-copolymers and consists of long macromolecular chains with dissimilar chemical properties linked together.

Self-Sterilizing Functionalized Block-Copolymers

Such macromolecular arrangements can spontaneously self-assemble into versatile nanostructured materials due to thermodynamic incompatibility between the polymer chain's constituent block.

The research group of Professor Richard Spontak at the Department of Chemical & Biomolecular Engineering, NCSU, in collaboration with Kraton Corporation, a leading global producer of specialty polymers based in Houston, Texas, US, initially investigated the potential application of a range of sulfonated block-copolymers as nanostructured membranes for water desalination and fuel cell technology.

One of the copolymers under investigation was a pentablock-copolymer (consisting of five separate blocks) known as TESET. The middle block was chemically functionalized with sulfonic acid (organic acid that contains sulfur).

Due to their unique structure, midblock-sulfonated block-copolymers can maintain a robust molecular network while permitting the transport of ions or polar liquids, such as water, throughout the polymer layer.

When TESET absorbs water from the environment, the hydrated sulfonic acid groups release protons that can travel through the nanostructured polymer network and drastically reduce the pH on the material's surface.

Although many microorganisms can exist in acidic environments with low pH, they need to maintain a neutral interior pH regardless of the external conditions. A sufficiently drastic and sudden change in pH can inactivate or kill any microbe placed in contact with the polymer.

Efficient Inactivation of Drug-Resistant Pathogens

The researchers tested the self-sterilizing polymer against several types of pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium, carbapenem-resistant Acinetobacter baumannii, and a strain of influenza virus.

Professor Spontak's team discovered that the biocidal activity of TESET could be controlled by adjusting the number of sulfonic acid functional groups along the polymer chain. When more than 40% of the polymer's middle block subunits were functionalized with sulfonic acid, the polymer killed 99.9999% of each of the tested bacterial strains (no colony formation observed) within five minutes.

Additional tests performed at Boston University's National Emerging Infectious Diseases Laboratories (NEIDL) Biosafety Level 4 laboratory demonstrated that self-sterilizing anionic polymers can rapidly inactivate SARS-CoV-2, reducing the viral activity by 99.9% in just five minutes. The polymers can also fully inactivate a common cold virus HCoV-229E, with a 99.998% activity reduction in 20 minutes.

Easily Restorable Antimicrobial Properties

The polymer’s antimicrobial properties could progressively degrade upon repeated exposure to microbial suspensions, as sulfonic acid groups get neutralized when interacting with positively charged ions (cations) in the aqueous environment. However, the scientists found that the polymer could be fully regenerated by being exposed to an acid solution that removes the cations from sulfonic acid groups.

A New Class of Antimicrobial Polymers for High-Contact Surfaces

The novel self-sterilizing polymer can be used as a coating on various high-contact surfaces in healthcare settings, public transport, and consumer products such as medical devices and cell phones.

TESET can also be used as a replaceable peel-and-stick film for personal protective equipment, textiles, and packaging.

A range of TESET polymers with different degrees of sulfonation is available from Kraton Corporation under the brand name BIAXAM™. Kraton is seeking regulatory approval from the US Environmental Protection Agency (EPA) to offer BIAXAM™ as a durable and long-lasting antimicrobial material.

References and Further Reading

North Carolina State University (2021) Researchers Demonstrate Self-Sterilizing Polymers Work Against SARS-CoV-2. [Online] www.news.ncsu.edu Available at: https://news.ncsu.edu/2021/02/polymers-inactivate-sars-cov-2 (Accessed on 16 March 2021).

Coatings World (2020) Kraton Seeking Approval for BIAXAM as Self-Sterilizing Sulfonated Polymer. [Online] www.coatingsworld.com Available at: https://www.coatingsworld.com/content-microsite/cw_covid-19/2020-09-11/kraton-seeking-approval-for-biaxam-as-self-sterilizing-sulfonated-polymer (Accessed on 16 March 2021).

B. Balasubramaniam, et al., (2021) Antibacterial and Antiviral Functional Materials: Chemistry and Biological Activity toward Tackling COVID-19-like Pandemics. ACS Pharmacol. Transl. Sci., 4, 1, 8–54. Available at: https://doi.org/10.1021/acsptsci.0c00174  

B. S. T. Peddinti, et al., (2021) Rapid and Repetitive Inactivation of SARS‐CoV‐2 and Human Coronavirus on Self‐Disinfecting Anionic Polymers. Adv. Sci., 2003503. Available at: https://doi.org/10.1002/advs.202003503

B. S. T. Peddinti, et al., (2019) Inherently self-sterilizing charged multiblock polymers that kill drug-resistant microbes in minutes. Mater. Horiz., 6, 2056-2062. Available at: https://doi.org/10.1039/C9MH00726A

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Cvetelin Vasilev

Written by

Cvetelin Vasilev

Cvetelin Vasilev has a degree and a doctorate in Physics and is pursuing a career as a biophysicist at the University of Sheffield. With more than 20 years of experience as a research scientist, he is an expert in the application of advanced microscopy and spectroscopy techniques to better understand the organization of “soft” complex systems. Cvetelin has more than 40 publications in peer-reviewed journals (h-index of 17) in the field of polymer science, biophysics, nanofabrication and nanobiophotonics.

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