Self-cleaning paint and fabric inspired by the Sacred Lotus

Challenge: self-cleaning surfaces

Natural solution: Sacred lotus, Nelumbo nucifera

 

Many techniques are employed to clean man-made surfaces and fabrics: scrubbing, applying detergents, rinsing with hot water, sandblasting etc.  These activities are often expensive in terms of money, time and environmental cost.  In contrast many plants seem able to stay clean with nothing more than occasional rinse by dew or rainwater.

 

In the case of the lotus (Nelumbo nucifera, Gaertn.), this effect is so noticeable that the plant, which often grows in muddySacred lotus, Nelumbo nucifera © Wolfgang Stuppy water, has become a symbol of purity and is important in many Eastern cultures; Buddha is said to have been born in the heart of the lotus flower and Hindus associate the flower with the sun and the mother goddesses as a symbol of fertility1, 2.  This warm temperate to tropical wetland plant with a wide distribution, across Asia to Japan and into Australia, is also defined as a living fossil, being one of only two species in its genera, surviving in the world3, 4.  It has been managed and cultivated by humans for many thousands of years, for food, medicine, horticulture and cultural benefits5.   Although abundant in cultivation, local, wild populations of sacred lotus are declining in central mainland China.  This decline, due to the growing aquaculture industry and genetic invasion by cultivars, has led to the species being included on the official list of endangered species for China6, 7

 

Water droplets ‘bead’ simply carry dirt particles away with them as they roll over a lotus leaf © Sto Ltd

 

 

The leaves of many plants are coated with a waxy substance which makes them waterproof or ‘hydrophobic’8, 9.  Scientists began to take particular interest in the ‘self-cleaning’ phenomenon in the 1980s, culminating with the publication of a scientific paper in 199710 and the granting of a patent for the ‘Lotus-Effect’ in 199811.  What is this ‘Lotus effect’?

 
 
 

A double structured surface optimized for self-cleaning. Through the combination of micro- (cells) and nanostructure (wax crystals) contact areas are minimized © Lotus Effect®Examination of the surface of lotus leaves under a microscope reveals that they are not smooth but rather have an outer layer of cells (epidermis) which display a bumpy surface.  This bumpy epidermis is covered by a thin layer of wax crystals which are water-repellent (hydrophobic).  These two properties make the leaf super-hydrophobic and self-cleaning12.  Water falls onto the surface of the leaf and forms nearly spherical beads that easily roll off; any dirt clings to the water droplet rather than to the waxy, bumpy surface of the leaf, so being carried A water droplet takes up the particles loosely covering the leaf while rolling off, thus cleaning the surface © Lotus Effect®away13.  It is this complex surface coating that enables the leaf to be self-cleaning.  The ability to self clean is extremely important for lotus plants as they live in muddy water and need to keep their leaves clean to prevent infection by pathogens, such as fungi and bacteria, and to enable efficient photosynthesis14

 
 
 

One species of beetle with a similar coating on its shell is able to survive in the harshest desert conditions.  Daytime temperatures reach 50oC and water is scarce; the only available water comes in the form of a fog.  This beetle, the Namib Desert Beetle (Stenocara gracilipes,Solier) has developed a way to harvest water from the fog: it squats, head down and tail up, with its back facing the incoming fog.  Water condenses on its back and trickles down into its mouth15.  The beetle’s ability to The bumpy surface of the Namib Desert Beetle (Stenocara gracilipes) © Hans Hillewaert/Wikimediacollect water in this way is due to most of its back being covered in a bumpy, waxy, super-hydrophobic surface – just like the lotus leaf.  However, in the case of the beetle the top of the bumps on its back are free of wax and are hydrophilic, or water-attracting, so capture the moisture from the fog.  As the water collects on the top of the bumps, the drops grow in size until gravity causes the droplets to move down the bump to the waxy surface and from here the water is channelled to the beetle’s mouth16.

 

Biodiversity

This super-hydrophobic quality is not unique to the Lotus or the Desert beetle. It is an important biological characteristic in other species such as winged insects, like butterflies and dragonflies, that are unable to clean their wings with their legs as many other insects can17.  Another example of the water repelling trait is exhibited by the water strider whose legs display super-hydrophobic properties by virtue of numerous elaborately grooved needle-shaped tiny hairs, or microsatea, which enable it to walk-on-water18

 

These unique water repelling and self-cleaning characteristics exhibited have evolved in different species of plants and animals over millions of years and enable them to survive in diverse, and sometimes harsh, habitats with minimal energy cost.   

 

The area of science which studies water-repelling and self-cleaning structures is a rapidly growing field of research. Increasingly it shows that differences at the nano-scale in a variety of structures can offer a range of solutions to important technological and biomedical challenges. 

 

Discoveries in this fast developing field provide further examples of how researchers are learning from nature and using nature’s designs to inspire technology. We should be encouraged to see that these discoveries ‘exploit’ biodiversity without actually depleting the natural resources themselves.  As technology develops, especially with regard to enabling research at the nano-scale, we recognise that there is still so much we do not know, so much we have to learn, about the many plants and animals with whom we share this planet.  Many other species could inspire a huge array of different innovative solutions, further supporting arguments for the need to conserve biodiversity.  

 

Economics

StoLotusan color, self cleaning paint, just like the lotus dirt rolls off the surface attached to the water droplet © Sto Ltd

 

The discovery of the phenomenon in the lotus plant has led to numerous commercial applications, including self-cleaning roof tiles developed by Erlus19, self-cleaning paint for buildings and render by Sto Ltd, soil-repellent fabrics developed by Hohenstein Institute20 and Nano-Tex21 and treatments for glass by Ferro AG22 which are currently used on the automated toll readers on the German Autobahn. 

 

Buildings painted with Sto Lotusan Color, self-cleaning paint © Sto Ltd

 

Environmental benefits of these coatings include decreased use of detergents, solvents and water.  Such coatings also save large volumes of traditional paints generally used for renovations work23 and improve the lifespan of hard-to-clean materials, such as stucco, concrete and other exterior wall finishes24

 

 

The beetle’s alternating hydrophobic/hydrophilic shell has inspired ideas for water-collecting technology in arid regions.  One of the first fog-collecting projects was set up in the Chilean village of Chungungo in 1987.  The system supplied 700 people with 15,000 litres of water each day25 until 2002 when the project ceased.  The new system show an increase in water capture compared to the inefficient nets that are used in many different areas around the world26.

 

Scientists at Massachusetts Institute of Technology (MIT) have developed a new material that can capture and control tiny amounts of water, just like the beetle does.  Applications include self-decontaminating surfaces that could channel and collect harmful substances, such as germs, which could then be easily killed or deactivated27.  Easy-clean/self-cleaning cloth is becoming widely available; popular with buyers of marquees, awnings and sails this type of use is expected to constitute the biggest market for lotus-effect finishes28.

 

Another potentially important area of growth is as a coating for solar panels.   Researchers at Boston University have estimated that 4g/m2 of dust on solar panels will have an estimated 40% reduction in power conversion29.  Dust deposition in Arizona is about 17g/m2 per month and dust storms have already caused a decrease in power production by 40% percent at a large, 10-megawatt solar power plant in the United Arab Emirates30

 

This area of research has now broadened into an entire new science of ‘wettability’, self-cleaning and disinfection31.  Bizerba has developed a VS12 slicing machine for use in the food industry that is water and stain resistant.  The treated compounds applied to the slicer are reported to be 30 times harder and 100% smoother than conventional anodised aluminium as well as being extremely temperature resistant and have been approved by the American Food and Drug Administrative (FDA)32.

 

Worldwide annual sales of products using the lotus effect are now over $100 million33.  Self-cleaning surfaces are gaining more and more in economic importance.  The market for nano-scale anti-microbial, easy-clean and self-cleaning coatings is projected to grow across all sectors over the next few years.  Improvements in coatings offering anti-bacterial and dirt repellents are also expected to drive growth in the medical, household care and food processing sectors34.

 

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Image Sources

Hans Hillewaert/Wikimedia – Namib Desert Beetle 

Wolfgang Stuppy Kew Gardens

Sto Ltd

Lotus-effect®

 

References

  1. Tan, R.  (2001) Lotus (Nelumbo nucifera) Article online  Accessed October 2010
  2. Stuppy, W.  Nelumbo nucifera (sacred lotus) Kew Online Accessed October 2010
  3. Jianhua, X, Lihuan, Z & Shiliang. Z  (2006) Genetic diversity and geographic pattern of wild lotus (Nelumbo nucifera) in Heilongjiang Province.  Chinese Science Bulletin  Vol: 51 No: 4 421-432.  Accessed October 2010
  4. Aona, YLS & Zappi, D  Neotropical Nelumbonaceae  Kew Online  Accessed October 2010
  5. La-ongsri, W, Trisonthi, C & Balslev, H (2008) Management and use of Nelumbo nucifera (Gaertn.) in Thai wetlands.  Wetland and Ecology Management. Volume 17, Number 4, 279-289 Springer Online article  Accessed October 2010
  6. Wolfgang Stuppy  Nelumbo nucifera (sacred lotus) Kew   
  7. Xue Jianhua, Zhuo Lihuan & Zhou Shiliang. (2006) Genetic diversity and geographic pattern of wild lotus (Nelumbo nucifera) in Heilongjiang Province.  Chinese Science Bulletin  Vol. 51 No: 4 421-432
  8. Post-Beittenmiller, D. (1996)  Biochemistry and molecular biology of wax production in plants.  Annual Review of Plant Physiology and Plant Molecular Biology 47: 405-430
  9. 2009 Introducing superhydrophobicity in plants  Nature’s Raincoats  Accessed October 2010
  10. Barthlott, W. et al (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces.  Planta 202: 1 – 8
  11. Forbes, P  (2005)  The Gecko’s Foot.  Harper Perennial, London, UK.  Chapter 2, pp 29 - 54
  12. Nees Institute Research University of Bonn  Accessed October 2010
  13. Barthlott, W. et al (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces.  Planta 202: 1 – 8
  14. Neinhuis, C. et al (1996) Characterization and distribution of water-repellent, self-cleaning plant surfaces.  Annals of Botany 79: 667 – 677
  15. Forbes, P  (2008)  Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology: Scientific American  Online article  Accessed October 2010
  16. Forbes, P  (2008)  Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology: Scientific American  Online article  Accessed October 2010
  17. Modi, A & Surana, P (1999)   Bio – Mimicry of Lotus Leaf   Indian Institute of Technology, Kanpur.  Accessed October 2010
  18. Feng, X, Gao, X,  Wu, Z, Jiang, L & Zheng, Q  (2007) Superior Water Repellency of Water Strider Legs with Hierarchical Structures: Experiments and Analysis.  Langmuir Vol: 23, No: 19, 4892-4896 American Chemical Society  Full article available online  Accessed October 2010
  19. Self cleaning clay tiles from Erlus Lotus  Erlus website Accessed October 2010
  20. Hohenstein Institute (2010) A new approach optimises "lotus effect" of soil-repellent textiles  Online Article   Accessed October 2010
  21. Nano-Tex Organisation website  Accessed October 2010
  22. Baumann, M, Fritsche, K-D, Sakoske, G &  Tünker, G.  (2005)  Easy printable nano coatings for glass.  Ferro AG article available on line Accessed October 2010
  23. Deutsche Bank Research (2008) World Chemicals Market Accessed October 2010
  24. Sto Lotusan Colour.  The self-cleaning facade paint with the Lotus-Effect®.  Accessed October 2010
  25. Whitefield, J  (2001)  Water wings aid desert survival Innovations Report for Forum for Science, Industry and Business  Accessed October 2010
  26. Patel, P  (2006) Super Plastic Both Attracts and Repels Water.  An odd new material could be a boon in dry regions with limited access to clean water Technology Review (MIT Publishing)  
  27. Anon  (2007)  Beetles Are Inspiration For New Antibacterial Coatings, Materials Scientists Copy Beetle Anatomy To Develop New Coatings.  Science Daily online Journal.  Accessed October 2010
  28. Forbes, P  (2008) Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology: Scientific American  Online article  Accessed October 2010
  29.   Self-cleaning solar panels could find use in the dusty environs of Arizona, the Middle East or Mars    Scientific American Online Accessed October 2010
  30. Bullis, K  (2010)  Self-Cleaning Solar Panels  Technology Review,  MIT Publications.  On line article  Accessed October 2010
  31. Forbes, P  (2008) Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology: Scientific American  Online article  Accessed October 2010
  32. Meat Trade News Daily  (2010) Self-cleaning surfaces with lotus effect  Online article Accessed October 2010
  33. McKeag, T  (2010)  Lotus Leaf Demonstrates Business Case for Bio-Inspired Products GreenBiz  Accessed October 2010
  34. Anon (2010) Market Applications of Nanoscale Anti-microbial, Easy-clean and Self-cleaning Coatings  Future Markets Inc  Online Article  Accessed October 2010
 

Further reading

Lotus Effect Research Group

S.C.S. Lai (2003) Mimicking nature: Physical basis and artificial synthesis of the Lotus-effect Universiteit Leiden