Chapter 6. Condom testing

Condoms (the male variety) have long been the mainstay of last-second, desperate appeals for contraception, although with the advent of AIDS and hepatitis B virus they have taken on additional prominence in the prevention of sexually-transmitted disease (the only purpose for which they were legally sold in many states just a couple decades ago). In either case, a condom merely serves as a barrier against microbes (sperm or pathogens). And to make sure that condoms indeed function in this capacity, governments of Western countries test condoms to meet minimal standards, to ensure that the condoms don't break or leak when in use.

Any male who has used a condom realizes that disease prevention and contraception are not the only goals in condom design - if they were, the simplest solution would be to avoid sex. An additional goal is to minimize the loss of sensation realized during sex. The goal of sensation is in direct conflict with the goal of preventing disease and sperm transfer, because the way to manufacture a condom to be a faultless barrier is to make it thick, whereas the way to maximize sensation is to make it thin. Manufacturers thus make condoms as thin as possible while maintaining minimum standards for condom strength. And it is precisely this compromise that leads to occasional condom failure.

The material used in condoms is latex (rubber). Advantages of latex are that it can be produced in thin sheets, and it can stretch greatly without breaking. But latex is a biological material, and it is profoundly sensitive to various environmental conditions. Even temperatures as high as body temperature degrade it, albeit slowly, and oils (as in vegetable oil and baby oil) degrade it rapidly. So even if condoms meet reasonable standards in the factory, they may fail under a wide array of conditions experienced in "the field."

The simple models

Condoms need to be tested to ensure that they meet certain standards. It is obvious that the best and truest model of condom performance is sex itself, but government agencies such as our Food and Drug Administration aren't in the sex business, and no government has ever been willing to sanction sex labs as a way of testing condoms. So what do they do?

The usual ways of testing condoms involve subjecting entire condoms or parts of condoms to tests of strength and structural integrity. One current practice of the Food and Drug Administration involves filling a condom with 10 ounces of water and looking for leaks. Another method involves slicing a band from the shaft of a condom and testing its stretchability. And a different method used by many European countries, Canada, and now the U.S., is the airburst test: the condom is inflated with air until it bursts, and the maximum volume of air tolerated is used as the measure of strength. Regardless of which specific test is used, condom testing involves taking a sample of several condoms from a batch and calculating the fraction that pass the test. In the US, a batch of condoms cannot be sold if 5 or more condoms per 1000 fail the test.

So you can be relatively confident that any condom sold in the U.S. (and maintained under proper conditions) will survive the water test and stretch test or airburst test. Should we be comforted with that knowledge? Only to the extent that condom survival in these tests reflects condom survival during sex. That is, only to the extent that a water test or airburst test is a good model of the rigors of sex. Furthermore, the fact that different batches of condoms pass the FDA test does not mean that all of them are equivalent. Consumer Reports has evaluated several brands of condoms using the airburst test and has ranked them accordingly (to be presented in class). Perhaps surprisingly, a few brands had failure rates of 10% or more.

At this point, we may pause and consider how condom testing illustrates the theme of models. We will return to the template of the last chapter and show how one can apply this example to fill in the template.

There are several levels at which models apply, beginning with the lowly one in which a few condoms in a batch are treated as a model of the entire batch (otherwise, we would have to test every condom in the batch). At a higher level, we may regard one condom brand as a model of other brands (hence the advice from healthcare workers to "use a condom," which makes no statement about a brand). Then we have the government models of sex that are used to evaluate condom survival, such as the airburst test, the water test, and the stretch tests.

It takes little imagination to understand how these models are limited and may be seriously in error. However, although the airburst test is not anyone's idea of sex, when properly calibrated, it may give us a good idea of whether a condom will hold up during sex.

A better model

The Mariposa Foundation of Topanga California has constructed a more realistic model for testing condoms, which has been designated the Coital Simulating Instrument. It consists of a rubber "vagina" through which water (at body temperature) is circulated and into which a dildo (model of a penis) is thrust with a piston device. The condoms are inserted over the dildo and subjected to several "cycles" of piston thrusting. An ejaculation is also simulated. The various parameters have been established by a number of methods.

New models of STD prevention by condoms

Even if the models used to test condoms are reasonable indicators of whether a condom will break during sex, and thus whether they will function adequately in preventing sperm from reaching the female's reproductive system, but they may be rather poor indicators of whether a microscopic pathogen can pass from one partner to the other. For example, the FDA water test can detect holes only as small as 5 mm, but this sized hole is many times the size of sexually-transmitted viruses and even of the bacterium Chylamidia. Similarly, the airburst test is insensitive to small holes. So here we find new limitations of existing methods of testing condoms: these models don't give us a good understanding of the barrier to pathogens afforded by a condom. That is, these models have serious limitations when considering condoms as barriers to infectious disease.

Newer models are being tried. Several involve filling a condom with a pathogen (in water, for example) and determining whether the pathogen escapes to the outside - a passive transmission test. Here again, however, further levels of models are used. Some tests use the sexually-transmitted pathogen itself, which is the best model of a pathogen. But those tests are expensive because they require special facilities for working with pathogens. So other, simpler tests use the harmless bacterial virus fX174, which is somewhat smaller than the smallest sexually-transmitted pathogen (Hepatitis B Virus) and is easily assayed on bacterial plates. Perhaps the most realistic model of condom barriers against sexually-transmitted diseases is provided by the Mariposa Foundation. Condoms are "pre-stressed" on their coital simulator and then tested directly for HIV passage. They found that, in 10 trials per brand, brands ranked high on their airburst test did block HIV, whereas some of those ranked low on their airburst test did not block HIV.

To summarize this material in our templates for models: