Living Surfaces

Incorporating microorganisms into polymer layers provides bioinspired functional living materials

Living organisms’ complex responses to environmental stimuli have spurred the development of synthetic materials that mimic many of these natural processes. Inspired by microorganisms living on surfaces such as the skin or certain cheeses, we incorporated the fungus Penicillium roqueforti into layers of polymer to create a self-cleaning living material. The authors suspended the microorganisms in an artificial habitat sandwiched between a supporting base layer made of polyvinyl chloride, and a porous cover layer that allowed for the exchange of gases and nutrients yet did not allow the fungus to grow out of its confined space. When the researchers added glucose-rich solutions to the material’s top layer, thereby mimicking a food spill, the enclosed organisms started to grow and consume the food, effectively cleaning the surface within 14 days. The fungi then entered a dormant state that enabled them to survive prolonged starvation, and were reactivated when nutrients were added to the surface 7 days later. The fungi also survived harsh conditions, such as surface disinfection with ethanol or soap, but were killed by 10-day starvation with severe dryness. According to the authors, the findings may prove useful in biotechnology and for a variety of consumer goods, such as self-cleaning or antibacterial packaging and indoor surfaces.

Design of a living surface. The simplest form of a living surface is composed of three layers. The middle layer (white, with black CFU) is the living layer and hosts microorganisms.The porous top layer is responsible for the confinement of the fungi, for nutrient, gas, and for mechanical, chemical, and biological stability. When a nutrient (yellow) is on the surface, the organisms within the living layer start growing and consuming the food. When all food is consumed, the fungi switch to a dormant state and await new food.

Highly efficient nano-silver composites enable self-disinfecting polymers

Antibacterial properties of products and goods gain importance. Common usage of antibacterial silver releases lots of heavy metal into the environment. As silver may have unwished adverse health and environmental side effects, a reduction of silver spread is advisable. This can be achieved by the use of reactive nanoparticle composites, which can be applied as fillers extruded with polymer e.g. for fibers or applied as coatings on polymer films.We investigate on the ability of silver-tricalcium phosphate (Ag/TCP) nanoparticles to build a reactive antibacterial system in polymers.

Sketch of the Ag release from reactive nanoparticle composites.

TEM image of silver-tricalcium phosphate nanoparticles (Ag = brighter smaller points).

These nanoparticles consist of 20-50 nm sized carriers, based on calcium phosphate, a major nutrient of bacteria. This bait is spiked with 1-2 nm silver particles. In the presence of growing bacteria, the tricalcium phosphate (TCP) carrier particles are biodegraded and trigger the release of the antibacterial silver. Applying silver in this complex form allows significant increases in antibacterial efficiency due to a larger contact area and triggered release of the active silver exclusively when TCP is consumed by bacteria. This reduces the required amount of silver if compared to non-reactive systems. Ag/TCP nanoparticles are produced by flame spray synthesis and incorporated in polymer surfaces or fibers. Such polymers show excellent antibacterial effects.

Incorporation in flat polymer surfaces

Self sterilizing polymer surfaces were produced by applying 20wt% Ag/TCP nanoparticles in a food-grade silicone as a coating on common polymer foils. The antimicrobial performance of the transparent films was investigated by using clinically relevant pathogens. 99.99 % of Escherichia coli, 99.99 % of Candida albicans and 100 % of Pseudomonas aeruginosa were killed within 24 hours, respectively. The triggered release of silver lead to an up to three orders of magnitude higher bacteria killing efficiency compared to conventionally used silver systems.

Photograph of a transparent polymer film containing highly antimicorbial Ag/TCP.

Incorporation in polymer fibers

For the production of self-sterilizing polymer fibers Ag/TCP particles containing 1.3 wt% silver were incorporated into fibers for antibacterial testing on two clinically relevant bacterial strains. The reactive fibers demonstrated significantly reduced plate count within 24 hours if compared to pure polyamide reference fibers. The number of colony forming units was reduced by 99.999 % with Escherichia coli and 99.6 % with Streptococcus sanguinis.

Picture of a FML tooth brush made of antibacterial fibres.

Further reading:

• L.C. Gerber, F.M. Koehler, R.N. Grass, W.J. Stark, Incorporating microorganisms into polymer layers provides bio-inspired functional living materials, Proc. Natl. Acad. Sci. USA, 109(1), 90-94 (2012).
  DOI: 10.1073/pnas.1115381109
  
• S. Loher, O.D. Schneider, T. Maienfisch, S. Bokorny, and W.J. Stark, Micro-organism-Triggered Release of Silver Nanoparticles from Biodegradable Oxide Carriers Allows Preparation of Self-Sterilizing Polymer Surfaces Small, 4 (6), 824–832 (2008).
  DOI: 10.1002/SMLL.200800047
  
• L.C. Gerber, D. Mohn, G. Fortunato, M. Astasov-Frauenhoffer, T. Imfeld, T. Waltimo, M. Zehnder, W.J. Stark. Incorporation of reactive silver-tricalcium phosphate nanoparticles into polyamide 6 allows preparation of self-disinfecting fibers, Polym. Eng. Sci., 51(1), 71-77 (2011).
  DOI: 10.1002/pen.21779