Close
today i learned (til)

Science of Spider Silk

November 20, 2019

Every year I move into a new room on campus at Georgia Tech, I buy a package of sticky putty, hoping that it will be strong enough to hold my large posters of the periodic table and Chance the Rapper, because command strips are just way too expensive. They usually fall down in the middle of the night, waking me up in terror. Recently in my polymers class, I learned about the beauty of spider silks and how they secrete different silks to accomplish different tasks. When I googled “spider silk,” I found articles and youtube videos claiming that spider silk is stronger than steel. I wondered how spiderwebs are able to stick to walls better than my command strips, but when I accidentally run into one, it breaks easily.

After doing some research, I learned that strands of spider silk are actually made of thousands of close-packed, parallel nanostrands, and depending on the gland they are secreted from, their tensile strength is 100-400 kpsi, compared to steel, which is 40 kpsi [1]. The secret is in how the tensile strength is measured. Tensile tests consist of putting a material in tension, essentially stretching it until it breaks. Spider silk is made of semi-crystalline polymer nano strands that can stretch a lot before breaking, and having many strands stacked together makes it even stronger, while steel thread of the same diameter or density has a crystalline structure that doesn’t stretch. This scenario is analogous to breaking a single stick of strong wood versus breaking a bundle of many weaker but stretchier sticks of wood: the bundle wins.

Spiders have different glands that secrete silks with different properties as protein structures to accomplish different tasks [2]. The tubuliform gland creates the silk to make the hard outer shell for an egg sac, and the aciniform gland makes the inner lining of the egg sac. The aggregate and flagelliform glands produce the sticky droplets and capture spiral threads, respectively, to catch prey.  The major ampullate gland makes the frame of the web as well as the dragline silk that allows the spider to move around. Lastly, the pyriform silk gland is responsible for the silk that attaches the web to the walls, trees, your bedroom window, etc. Pyriform silk is described as a “protein glue” or a “glue-like cement” that holds small amino acid fibroins, which then connect larger dragline silks, and are sometimes reinforced by aciniform silk [3]. Together, all of these components create a very strong network of silks that makes the overall product very adhesive. 

In this picture from a study that took Scanning Electron Microscope (SEM) images of a single attachment point of a web, it looks like the pyriform silk is a splat of glue with the dragline silk (white arrow) stuck in it [4]. Looking closer, the “glue” looks like a network with many, smaller attachments to the surface of the wall. 

Meanwhile, the sticky putty that I use to try to hang up my posters isn’t as robust as the spider silk glue. The formula for sticky putty is a trade secret, but it is known that it works as a pressure-sensitive adhesive, so the extent of adhesion depends on the amount of pressure you apply to it, as well as the shape and geometry. So even though I add a lot of pressure to the poster when I first apply it, depending on the weight of the poster and the amount of putty I used, the poster will slowly pull the putty off the wall, depending on how much I used and the temperature of my room [5]. 

If spider silks are so strong and perform so many tasks well, why don’t we make adhesives or other high performance materials out of it? There have been many ideas for potential uses of spider silk in body armor due to its mechanical properties. It is also antimicrobial, so there are many ideas for biomedical applications, such as scaffolding for tissue generation [2]. However, spider silk is extremely difficult to harvest. This is because spiders are inherently cannibalistic and produce a small amount of silk in comparison to silk worms, which are typically used to harvest silk for textiles. Scientists have attempted to put spider genes in silkworms, but the silk they produce still isn’t of the same quality [6]. So unfortunately, I probably can’t get a spider as a pet and harvest the silk for my dorm posters. But doing all this research made me curious: How is Spiderman’s silk compared to actual spider’s silk? And where are the silk glands?

 

References:

[1]Hayashi, C. Y., Shipley, N. H., & Lewis, R. V. (1999). Hypotheses that correlate the sequence, structure, and mechanical properties of spider silk proteins. International journal of biological macromolecules, 24(2-3), 271-275.

[2] TED. (Dec 7, 2011). Hayashi, Cheryl: The Magnificence of Spider Silk [Video file]. Retrieved from https://www.youtube.com/watch?v=xossR6eHv3I&t=248s

[3]Hardy, John & Römer, Lin & Scheibel, Thomas. (2008). Polymeric materials based on silk proteins. Polymer. 49. 4309-4327. 10.1016/j.polymer.2008.08.006.

[4]Blasingame, Eric et al. “Pyriform spidroin 1, a novel member of the silk gene family that anchors dragline silk fibers in attachment discs of the black widow spider, Latrodectus hesperus.” The Journal of biological chemistry vol. 284,42 (2009): 29097-108. doi:10.1074/jbc.M109.021378

[5] “Blu Tack FAQs.” Blu Tack, Wayback Machine Internet Archive, 2010, web.archive.org/web/20100418062649/http://www.blutack.com/faq.htm.

[6] SciShow (Feb 4, 2019). Green, Hank: Why Can’t We Make Spider Silk? [Video file]. Retrieved from https://www.youtube.com/watch?v=UX2LHcLxjio

0Likes
About Author

You May Also Like

today i learned (til)
Neuroethics: expanding horizons
today i learned (til)
Psych Comic Interlude
today i learned (til)
Nicotine is Dope: a poem on vaping and addiction
today i learned (til)
‘IN Synch

Your journey starts here! Let’s talk!

ThemeRex © 2026. All rights reserved.
  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.