Original source: Materials Today
Spider silk has long been celebrated for its extraordinary mechanical behaviour. The combination of high strength and high toughness makes it unique amongst natural fibres. It’s also known to be biocompatible and biodegradable, which has prompted many studies into its relevance to biomedical devices. But spider silk also has another superpower – despite being entirely composed of proteins, typically a nutrition source for microorganisms, it repels them, and can do so for years. For the health care sector – where surface contamination by pathogenic microbe films can lead to life-threatening infections – properties like these are in high demand. But without a detailed understanding of the specific mechanism that confers this microbe-repellence, spider silk can’t easily be adapted for use in medical applications.
But a new paper may change that. Writing in Materials Today [DOI: 10.1016/j.mattod.2020.06.009], a group of German researchers have developed a series of 2D and 3D materials from specially-engineered recombinant spider silk proteins. Based on the dragline silk of the European garden spider Araneus diadematus, they engineered two protein sequences, eADF3 and eADF4, plus several variants thereof. These were processed to produce smooth and patterned films, as well as hydrogels. For comparison, the same structures were made from three other materials – a silkworm protein (B. mori), a biopolymer (PCL), and gelatin, a common material in tissue engineering applications.
In order to systematically analyse the antimicrobial properties of each of these biomaterials, they were tested against a diverse selection of biofilm-forming microbes, including pathogenic bacteria (S. mutans, S. aureus, E. coli) and fungi (C. albicans, P. pastoris). The researchers first investigated the effect of protein structure, charge and molecular weight on microbial adhesion, using two different techniques – one based on fluorescence, which measured microbial viability, and the other on atomic force microscopy, which directly measured adhesive forces. For tests involving E. coli, S. aureus, and P. pastoris, negligible adhesion was found on three of the smooth, 2D spider silk films (eADF4(C16), eADF4(C32NR4), and eADF3((AQ)12)) – they outperformed the control films in all cases.
Next, the University of Bayreuth team compared smooth and micropatterned versions of the films, to explore the effect of topography. Two bacteria and two fungi were seeded on top of these films, and after drying, each sample was imaged with a scanning electron microscope. Their findings were clear – regardless of the topography, the spider silk films again displayed low attachment compared to the control films. To examine the behaviour of 3D spider silk materials, hydrogels were produced for each protein variant and the controls. Bacteria and fungi were easily detected on the gelatin and B. mori hydrogels. In contrast, even after being incubated with the microbes for 10 days, no growth was seen on the spider silk hydrogels.
One drawback to biomaterials that are inherently non-fouling is that they’re often not bioselective. In other words, they’re so effective that they repel all cells, even human ones. This limits their usefulness in applications such as tissue engineering. To elucidate whether these spider silk proteins might be able to display a bioselective behaviour, the team had one final test. They added a cell-binding motif (RGD), known to enhance cell attachment, to the silk proteins. In both film and hydrogen form, this material had pronounced bacterial and fungal-repellent properties. Importantly, when the hydrogel was incubated within a cell culture for 10 days, they found good viability for the cells accompanied by no microbial contamination.
The authors say, “To the best of our knowledge, this is a completely new finding, which opens the door for novel applications of spider silk materials, e.g., as bioselective coatings in various biomedical applications, and for…regeneration medicine.”
Sushma Kumari, Gregor Lang, Elise DeSimone, Christian Spengler, Vanessa T. Trossmann, Susanne Lücker, Martina Hudel, Karin Jacobs, Norbert Krämer, Thomas Scheibel. “Engineered spider silk-based 2D and 3D materials prevent microbial infestation”, Materials Today (2020), article in press. DOI: 10.1016/j.mattod.2020.06.009