The chemistry of condoms

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Today, we take contraception for granted to a certain extent. It’s easy to forget that, less than 200 years ago, the only condom available was one made from a length of sheep gut that you had to wash and re-use. Today, they’re made from a handful of different materials. This post looks at their recent material history, and some of the other chemistry modern condoms utilise.

The invention of rubber vulcanisation was the first step towards the modern condom. There’s some debate over who invented the vulcanisation process. Many sources attribute it to the American inventor Charles Goodyear. However, the English inventor Thomas Hancock actually patented the process in the U.K. 8 weeks before Goodyear took out a U.S. patent. Others claim Hancock simply got his hands on Goodyear’s sample and managed to work out his method.
 
Whoever was responsible for its invention, vulcanisation is useful for improving the properties of rubber. It involves heating rubber with sulfur to make it stronger, more elastic and flexible. The sulfur forms crosslinks between rubber polymer chains, leading to the change in properties.
 
Rubber was a big improvement on the condoms available at the time. As well as being more durable, it was more easily reusable. However, their cost and reduced sensitivity still limited their popularity.
 
The next advance in condom materials was the discovery of rubber latex. Latex is the term used to describe a mix of polymers in water. Rubber latex is a milky liquid found in many plants, notably rubber trees. Latex condoms were thinner and stronger than their rubber counterparts, able to stretch to up to 800% their original size before breaking. They also have a longer shelf life.
 
Latex condoms aren’t without issue though – some people can’t use them due to latex allergies. Natural proteins present in the latex trigger these allergies. Latex condoms also can’t be used with oil-based lubricants, as these damage them and make them more prone to breaking.
 
There was a long wait for another alternative, but it eventually came in the 1990s in the form of polyurethane. Polyurethane plastics have a range of uses (we’ve previously looked at their use in footballs). Though polyurethane condoms were slightly more prone to breakage than latex ones, they also offered improved sensitivity. They also have the obvious advantage of being usable by those with latex allergies.
 
More recently, polyisoprene (synthetic rubber) condoms have appeared as another latex alternative. They have the same stretchiness as latex condoms. Like latex condoms they are damaged by use with oil-based lubricants.
 
It’s not just the materials that condoms are made from that rely on chemistry. The first condoms with lubricants went on sale in the 1950s. As well as the oil-based lubricants already mentioned, water-based and silicone-based lubricants are available. Silicone-based lubricants involve polymers called siloxanes. Polydimethylsiloxane is an example, the compound which also gives silly putty its unusual properties.
 
Additives in the lubricants can assist with sexual performance. Some brands of condom contain a small amount of the anaesthetic benzocaine inside the tip. This has a numbing effect that helps prevent premature ejaculation. Others contain gels based on nitroglycerin. These cause the blood vessels in the penis to dilate, helping with erectile dysfunction problems. Others still claim to use L-arginine for a similar effect, though evidence for the efficacy of this seems scarce.
 
Lubricants also used to contain spermicidal compounds, primarily nonoxynol-9. This compound attacks the membranes of sperm cells, rendering them immobile. While it is effective in some of its other applications, it seems to be less so in condom lubricants. According to the World Health Organisation, there is no evidence that condoms with nonoxynol-9-containing lubricants provide any more protection against pregnancy than those without.
 
The investigation of new materials to use for condoms continues. Female condoms, previously made from polyurethane, are now more commonly made from nitrile rubber. More recently, some companies are trying to make more comfortable condoms from hydrogels. Meanwhile, others are trying to make condoms from a combination of graphene and latex. If amusing novelty is more your thing, glow-in-the-dark condoms are even available thanks to a mysterious ‘non-toxic phosphorous pigment’!
 
For more on contraception, check out this previous post on the pill.