What do spiderwebs and your home’s air filter have in common? (no spider pictures)
Spiderwebs–and the creatures that make them–are not welcome in most homes. Not only are spiderwebs a “sign” of poor housekeeping (although they often appear overnight), spiders themselves are feared or despised (admittedly I’m in this group). Even many nature-lovers would rather relocate the spiders and tear down their webs rather than abide with them, but scientists have recently discovered that their webs contain a wonder material that filters air akin to the best air filters.
It’s obvious that spiders are probably not interested in reducing PM2.5 with their webs: their primary goal is getting dinner. Face it, though: we all know that spiderwebs do a great job of collecting dust! Scientists found that the electrostatic properties of the glue that coats spider webs causes them to reach out to grab all charged particles, from pollen and pollutants to flying insects. A quirk of physics causes webs to move towards all airborne objects, regardless of whether they are positively or negatively charged. Webs can catch particles as small as aerosols and pesticides, making them perfect environmental monitors if we choose to examine them. (How electricity helps spider webs snatch prey and pollutants)
It’s the statement that “all charged particles” are attracted to the web that caused us to investigate further. How does that work? Typically electrostatic filters work by charging all incoming particles either positively or negatively and then attracting them with an oppositely-charged filter. Spiderwebs are not exactly the same, because the web has no control of the charge of the particles (insects or dust) flying toward it, yet it actively “springs” out toward them if they are charged (click here to see it happen–no spiders included!)
The fact is that as things fly through the air, whether it’s dust, water droplets, insects or airplanes, they collect a static charge. This is why airplanes have little “antennae” or rods sticking out of the back of the wings: these static dischargers disperse the charge back into the air. Insects can easily acquire electrostatic charge by walking over charged surfaces or by flying in an airstream of charged particles. (Spiderweb deformation induced by electrostatically charged insects) Just as humans accumulate static by walking through dry air and carpets during winter, low humidity likely amplifies the static charge of insects, too. The deflection of the spider’s web depends on the mass of the particle or insect and their charge; small charged dust particles generate less deflection than larger insects. So, although insects have sensors on the tips of their antennae for detecting electric fields, and the glue spirals can distort Earth's electric field within a few millimeters of the web, sometime the total charge of the insect or their speed gets them in trouble, allowing the web to “reach out and grab” them!
But how does the spiderweb attract positive and negatively-charged objects? According to the scientists, this is due to the ion mobility within the miniscule water droplets that the web’s adhesive surface attracts. A combination of the spiders’ naturally compound-rich silk and the droplets (which serve as both glue carriers and electrostatic conductors) imbues the web with these amazing electrostatic properties. ('Electric' webs are spiders' secret to catching prey)
Orb weaver spiders are the common class of garden spiders. Their webs are formed roughly in a circular pattern, hence the “orb”. Their webs are also hygroscopic, meaning that they absorb moisture from the atmosphere, using salts to retain a specific amount of moisture.
Since there are more positive ions in the air than negative ones during calm weather, most insects gain a slight positive charge as they fly through the air. The web is usually “neutral” meaning it doesn’t have a charge, but as an insect nears it, moisture on the web allows electrons to migrate to the surface of the web near the insect and cause the silk to stretch out toward it.
Static induction is the principle that guides this phenomenon, which you've experienced if you've ever rubbed a balloon on your head and stuck it to a wall. Rubbing the balloon causes it to gain a static charge, and then it induces the opposite charge in the wall. Materials that are poor conductors, like rubber or silk, produce the best static induction.
Similarly, static induction occurs between the spider web and an insect. As an insect carrying a positive charge nears the threads of the web, that positive charge attracts electrons in the spider silk, creating a temporary negative charge. That negative charge can then be attracted to the positively charged insect, causing the spider threads to snap out and stick to the insect. (Note to Flies: Avoid Fuzzy Socks)
The drops of water on the web also allow glycoproteins in the web to move around it and coat any insects that become entangled in a sticky glue. Glycoproteins are proteins that have carbohydrates attached to them, which allows the glue to form a large number of hydrogen bonds. In these types of bonds, hydrogen forms a positive dipole in one molecule and fluorine, oxygen, or nitrogen form a negative dipole in another molecule. The positive dipole of hydrogen is attracted to the negative dipole on the electronegative atom, creating an attraction between the two molecules. (ChemistryTalk.org) Although the hydrogen bonds are relatively weak, they are collectively strong enough to keep insects and pollen from escaping the web. (Glue Stays Sticky When Wet)
Spiders depend on the invisibility of their webs to catch insects, so when the webs become “dirty”, many spiders clean and repair them on a daily basis (Spiders and Their Webs). To replicate the web and this cleaning action, other scientists took on the mission in 2020 of creating artificial webs that attract and release particles in a self-cleaning action (Ionic Spiderwebs)
Instead of repairing them, some spiders ingest the old web and its contaminants, including the water droplets on the web. Web material is hygroscopic, meaning that after it exits the spiders body, it attracts water from the atmosphere. (Water harvesting during orb web recycling) This actually helps the spider by giving it a source of water from the air. The pollen on the web is a bonus too: pollen makes up to a quarter of the diet of orb weavers. Unfortunately, a lot of urban spiders end up ingesting microplastics, chemicals and tire components (from road dust).
Spiders also build their webs with a minimum of material, to reduce waste and avoid having to clean or eat extraneous web. Because the web material is stretchy, sticky and because of static inductance, webs can be constructed with holes to let wind pass through, at the same time catching much more pollen and insects than any plain non-stretchy, sticky material. “Avoiding” capture is much harder for any insect or bit of pollen trying to fly “through” the web when its holes can close automatically by static attraction! Simply put, spider webs are amazing particle capture machines, also known as filters. It’s no wonder then that scientists are busy replicating them for different purposes.
Spider‐web‐inspired network generator (SWING) air filters, based on a unique electrospraying–netting technique, integrate properties of small pore size (200–300 nm) and innovative self‐charging capacity (3.7 kV surface potential), enabling the synergistic effect of physical sieving and electrostatic adhesion for PM removal. High efficiency (>99.995%), low pressure drop (<88.5 Pa), high transparency (>82%), robust bioprotective activity, energy‐saving, and long‐term stability for MPPS PM0.3/pathogen removal were achieved. The filters are made of electrospun nanofibers (PVDF material) and carbon nanotubes, which are uniquely formed by using a droplet spray–deformation–assembly process during electrospinning (Spider‐Web‐Inspired PM0.3 Filters Based on Self‐Sustained Electrostatic Nanostructured Networks)
The silk proteins in spider webs themselves were determined in the early 2000’s (Spider Silk Proteins – Mechanical Property and Gene Sequence). Spider silk is desirable not only for strength (it is superior to nylon, kevlar, silkworm silk and steel in elongation at break, tensile strength and breaking energy), but it’s also bio-compatible to humans and so can be used in medical applications. Artificial spider silk has not been easy to develop. Although the primary proteins were discovered earlier, It took a lot of gene-sequencing work to discover a formula for getting the optimal amount of nanocrystals in the silk. Once the protein sequence was determined, scientists needed to figure out who or what should be used to make the silk? Spiders themselves are too aggressive and territorial to be farmed. (Artificial Superstrong Silkworm Silk Is 70% Stronger Than Spider Silk) For this reason, bacteria, silkworms and goats have been bio-engineered with spider DNA to produce the silk. (Artificial Spider Silk Is Stronger Than the Real Thing, Spider Silk, BioSteel Goat)
Filter production methods: Traditional Needle Electrospinning (ES) requires extensive preparation, time, and post-treatment to produce filter material, as shown in this video. In this article, Centrifugal Electrospinning (CES) was found to be the most effective method in mimicking the fiber and composition of spider webs, albeit in a random non-woven way. The suitable spinning conditions for the recombinant spider silk protein eADF4(C16), including protein concentration, process ﬂowrate, electric ﬁeld strength,and rotational speed were analyzed. Experimentation with these variables enabled researchers to develop a roll-to-roll production process that is up to 1000 times faster than traditional electrospinning processes that also required no post-treatment.
So–knowing that spiderwebs are such efficient filters and their silk is now the object of much scientific research and investment, has this information changed your opinion of spiderwebs in your home? It’s ok, I know that fear of spiders is hard to dispel. So with you, I say, bring on the artificial spider silk, please!
Photo by Torbjørn Helgesen on Unsplash