A new generation of high-performance and multi-functional anti-microbial nanocomposites and coatings is being developed as a result of technological advances in materials science and nanotechnology (Hasan et al. 2013; Cloutier et al. 2015). Their application is vast in areas such as in biomedical applications and hospital hygiene (implants, medical devices, surgical textiles and devices, hospital furniture), water purification and waste water treatment plants, industrial pipelines, dental medicine, food packaging, and many others. These exciting new materials with bioactive functional surfaces could become a powerful tool in the global effort to tackle antibiotic resistance and provide alternatives to antibiotic use. Antimicrobial resistance is undermining the treatment of an ever-increasing number of infectious diseases caused by bacteria, parasites, virus and fungi and is recognised as a serious global threat (WHO, 2016).
Bacteria and other pathogens’ adhesion to surfaces and biofilm formation are key mechanisms in their pathogenicity and a serious problem in both economic and human health terms. The successful adhesion to surfaces and biofilm formation depends on the characteristics and topography of the solid surface such as charge, hydrophobicity or composition. Bacteria in surface biofilms are significantly more resistant to antibiotics and external forces, and can more easily withstand the host’s immune response (Cloutier et al. 2015, Hajipour et al. 2012). A favourable environment will lead to bacterial adhesion and growth, but the increasing understating and knowledge of this environment has also opened up possibilities for a new generation of anti-bacterial and anti-biofilm agents that may become increasingly important in a post-antibiotic era (Paula et al. 2017). Evaluating the potential unintentional consequences of these materials for human health and potential impacts on ecosystems, as well as addressing regulatory requirements and product safety, is of paramount importance.
The main approaches adopted in antibacterial surfaces are:
The exact mechanism of nanoparticles as antibacterial agents against bacteria is not yet fully understood, but it is believed that electrostatic interactions may play a role in disrupting the cell membrane as the nanoparticle attaches itself to the bacterial cell wall. Nanoparticles may also cause the generation of reactive oxygen species, inducing oxidative stress by free radical formation (Swartjes et al. 2015).
Manufacturing methods for release-based anti-microbial coatings have evolved from the simple saturation of porous matrices with nanoparticles to form a composite, to more sophisticated deposition methods including plasma-based tools, sputtering/etching, imprinting, deposition and functionalisation, electrospinning, surface attachment and immobilisation of molecules and macromolecules. Paula et al. 2017 describes a number of functional nanoparticles that are currently being employed which include metallic elements (e.g. silver), inorganic compounds (e.g. zinc oxides), polymeric compounds (e.g. chitosan) and carbon-based compounds (e.g. graphene). Highly customised surfaces can be designed and directed by specific surface chemistry (chemical bonds and interactions), morphology, structure and function.
The development of these nanomaterials offers great promise in terms of their effectiveness, low cost and final material performance. However some unknowns remain in terms of their safety and sustainability and these issues are highlighted in more detail below. Key challenges also remain in terms of: (i) adequately controlling the nanoparticle release so that precise and steady doses are delivered within a certain time frame; (ii) the long-term durability and stability of the coatings, particularly if the functional surfaces are in contact with biological media or mechanical forces and weathering; (iii) the long-term efficiency and how they can be replenished as their active agents are released and wear off.
New areas of development include stimuli-responsive materials in the applications of self-healing coatings, nano-sized sensors and drug-release systems. These employ bacteria-triggered responses so that nanoparticles are only released when in contact with the bacteria. This is possible for example, using pH-sensitive coatings that react to a pH drop in the bacteria’s immediate environment. The pH drop occurs as a result of bacterial metabolic activity with the production of acidic substances, such as lactic acid (Cloutier et al. 2015). Smart release can also be associated with low oxygen levels caused by bacterial proliferation (Paula et al. 2017). These stimuli-responsive coatings could adequately control the antibacterial agent release, be active and effective for longer periods of time and offer possibilities in terms of designing coatings with specificity to certain bacteria strains.
The imparting of multi-functionality to coatings is also a new area of development to achieve enhanced performance. These “smart” coatings include:
In terms of safety and sustainability, it is important to evaluate the potential unintentional consequences for human health and potential impacts on ecosystems, as well as addressing regulatory requirements and product safety.
Employing a safety-by-design approach during product development and at early stages in the research activity represents a major step in integrating aspects of safety, stability, tolerability or biocompatibility characteristics (particularly important for biomedical applications) and will reduce uncertainty surrounding the potential risks of nanomaterials. It is vitally important to undertake toxicology and safety studies to gather sufficient information to understand the potential hazards and undertake comprehensive risk assessments (Paula et al. 2017, Swartjes et al. 2015).
Life Cycle Assessment (LCA) of novel nanomaterials remains a major tool in the assessment of environmental impacts, as well as understanding exposures throughout the product life cycle, including the potential for release into the environment during end-of-life scenarios (Lemire et al, 2013).
SAFENANO is currently involved in research to enable the safe and responsible development of advanced materials in a number of Horizon 2020 projects, where we are responsible for risk management through our expertise in nanoparticle hazard and exposure assessment.
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Hajipour, M.J., Fromm, K.M., Ashkarran, A.A., Aberasturi, D.J., Larramendi, I.R., Rojo, T., Serpooshan, V., Parak, W.J., Mahmoudi, M., 2012, “Antibacterial properties of nanoparticles”, Trends Biotechnol Vol. 30 (10), pp. 499-511.
Swartjes, J.J.T.M., Sharma, P.K., Van Kooten, T.G., Van der Mei, H.C., Mahmoudi, M, Busscher, H.J., Rochford, E.T.J., 2015, “Current developments in antimicrobial surface coatings for biomedical applications”, Current Medicinal Chemistry, Vol. 22, no.1, pp. 1-14.