How exactly do surface structures cause functional properties? Which surface structures found in nature did you select to create water and oil repellence as well as antibacterial behaviour?

Thomas:
“Micro- and nanostructures present on surfaces generally increase the water repellency of hydrophobic surfaces. The contact angle of a drop of water to such surfaces becomes higher due to such structuring, compared to completely flat surfaces. Therefore, a drop of water rolls off very easily. Dirt particles or bacteria have only a low adhesion to such surfaces while the adhesion to water drops is higher. This means that such contaminations are picked up and rolled away by drops of water when those are rolling over these surfaces. Hence, hydrophobic surfaces cause also antibacterial activity to some degree. Such hydrophobic surfaces can often be observed in nature on the leaves of plants in our latitudes, this can especially be seen in the morning when the dew forms round drops on the leaves.

Furthermore, we also want to further enhance the antibacterial activity of the surfaces we design. In nature, we can find interesting structures on spider silk surfaces. The antibacterial properties of spider silk are based on several factors. First, spider silk contains components that possess antimicrobial properties. These can attack the cell walls of bacteria and inhibit their growth. In addition, the structure of spider silk, which is very strong and flexible, also influences its interaction with microbes. The physical properties of the silk can make it harder for bacteria to adhere to the surface, which also leads to reduced bacterial colonisation.
In addition to high water repellency and antibacterial behavior, we also strive for a certain level of oil repellency by applying special forms of microstructures. The living nature also shows us beautiful examples of this, such as certain surfaces on some fish species. However, such surfaces have not yet been scientifically researched and transferring them to industrial technology is very challenging.”

How does the transfer of our chosen special structures onto the coating of the textile work?

Thomas:
In our project we are working with lignin on a new type of coating on textiles. Lignin is a component of wood and is usually produced as a waste material during the extraction of cellulose for paper production. The properties of lignin as a coating on textiles are very suitable for many technical applications. Of course, the lignin still needs to be modified so that it meets the many demands of processing and the end products. We are currently working very intensively on this in the laboratories. You can read more about that in the previous blog article.

The goal is to give these coatings the microstructures on the surface that we have learned from living nature to repel water, oil and bacteria. Since we are working with thermoplastic lignin in the project, the surface can be structured using temperature and pressure. This is done using heated embossing rollers. These rollers have a negative structure on the metal surface that has been engraved with our chosen structure by laser. The lignin coating softens under the heat and the structure is pressed in with pressure by the embossing roller. The process is basically simple. However, the very special microstructures that we want to achieve require intensive development of all sub-components and process parameters in the embossing process.”

What are benefits of using the laser-based surface functionalisation, what are the reasons for using this method in BioFibreLoop?

Girolamo:
“A laser is a one-of-a-kind tool for manufacturing, capable of achieving both incredibly precise details at the micrometre scale and high-speed processing. Pulsed lasers, especially those with ultrashort pulses in the femtosecond range (1fs = 10⁻¹⁵ seconds!), have a unique ability to reshape surfaces on their own. 

In BioFibreLoop, we’re therefore able to mimic textures we found in nature with our laser-made patterns and “put them on textile”. Alongside surface chemistry, they help to recreate the same functional properties found in nature for the clothing produced.

To follow up Thomas’ answer to the surface structures: As a general rule, different surface patterns can lead to different functional properties. In recent years, a wide range of laser-created patterns has been developed, each designed to give surfaces special properties. Using a laser, we can create patterns at the nanometre, micrometre, or even multi-scale level (where nanometre structures form on micrometre patterns), resulting in what’s known as hierarchical structures. As consequence, hierarchical texturing allows us to create textiles with several functional properties! 

Additionally, unlike traditional industrial methods for creating patterns or surface textures, laser-based techniques don’t require any chemicals, making them a much safer and more environmentally friendly option.

To sum it up, the laser is a very efficient and safe tool to produce functionalised surfaces.”