|
Reproductive Health |
||
The Latex Condom: Recent Advances, Future DirectionsChapter 5 sidebar: New Tests for Product Development |
|
||
|
Coital Model. Recent laboratory studies evaluating the permeability of male and female condoms to viruses have used several types of apparatus that attempt to simulate physiologic conditions of intercourse. In two studies, the tests sent a microsphere suspension along the center of a penile form, covered with a condom. The suspension expanded the condom and was left there for 30 minutes. The test was designed to mimic the expected temperature, peak pressure and stretch of the condom during intercourse. (Lytle; Carey) The model did not incorporate motion, because, according to the Carey study, there are insufficient data to enable meaningful modeling of the dynamic aspects of coitus. Also, they assumed that the stretching of the pores of the condom came primarily from stretching the condom over the penis, not from the motion of coitus. Two earlier studies did use motion in their coital models. In one study, a suspension of HIV was dispensed in an untreated condom, which was then placed over a 20 milliliter (ml) disposable syringe and attached with a knot at the top of the plunger to achieve water tightness. The plunger and the condom were then put in the barrel of the syringe. The plunger was moved vigorously up to 50 times as a means of putting pressure on the condom to determine the effect on permeability. (Van de Perre) To test permeability of a female condom to STDs, 2 ml of virus suspension, which included HIV, was placed inside a female condom. That condom was then placed inside a second female condom, minus the insertion ring. A 35 ml plastic syringe case was inserted into the inner condom to serve as an artificial penis. The entire apparatus was then placed into a close-fitting foam model, designed to serve as an artificial vagina. The apparatus was plunged up and down 50 times. While the authors found the approach sufficient for testing impermeability, they also cautioned that the laboratory model has drawbacks for simulating intercourse. (Drew) Dynamic Mechanical Analysis (DMA). This is a technique to characterize polymers, such as latex rubber, and other materials whose mechanical behavior changes with temperature, under force/strain and due to other variables. The technique usually involves vibrating a sample at a certain frequency and measuring the response. Latex rubber is known as a "viscoelastic" material; i.e., it has a "viscous" or damping component and an "elastic" or springy component. DMA measures the combined viscoelastic response and separates the response into its components. The response of these components changes with the frequency of vibration and with temperature. For example, the children's toy called Silly Putty, which is a silicone rubber, becomes solid if it is very cold but flows under its own weight in a warm room. If pulled quickly, it fractures; if pulled slowly, it stretches. Analogous phenomena occur in latex rubber.
Since DMA measures how a material stores energy (the viscous component) and transmits energy (the elastic component), it may have some predictive value with respect to breakage and/or slippage. This testing technique is widely used in the polymer industry but has not been extensively considered for condom testing. Fourier Transform Infrared Spectroscopy (FTIR). This is a test of the chemical elements of a product, its chemical building blocks. For the condom, FTIR can give an indication of impurities, state of oxidation, the composition of plastic condoms, and other information. Some think of it as the chemical "fingerprint" of the condom. Current tests for pinholes and weak spots in the rubber look at mechanical properties of the condom, not the chemical properties. FTIR could be used, for example, to assess the extent of oxidation in condoms that have been stored for a long time. The advantage of the FTIR is that it is fast and inexpensive. The disadvantage is that it is not very sensitive. It requires at least 1 percent impurities to work well, and lower levels of impurities are hard to detect. The technique is not widely used in the condom industry even though it has been available for about 20 years. It is widely used in the chemical and pharmaceutical industries for quality control and structure determination. Short-Stem Air Burst Test. In the current air burst test, the condom is clamped on a mandrel-like device near the open end, about 150 mm from the tip. The standard condom is 160 to 180 mm long. Thus the test inflates virtually the entire condom with air, inflating the condom like a balloon. This obviously does not simulate human use and does not concentrate the air pressure where the latex experiences the most stress and strain. Due to thrusting, friction and ejaculation, the tip of the condom probably receives more pressure than do the sides. A "short-stem" test has been developed to concentrate the air pressure more towards the closed end of the condom, so as to simulate more closely the type of stress and strain the latex experiences in human use. This test would focus on the closed half of the condom, shortening the length tested from 150 mm to as short as 75 mm. FHI is conducting a study that compares the results of the standard air burst test and the short-stem test to condom breakage during human use, using stem lengths of 75 mm, 100 mm and 150 mm. Tear Test. This test measures the force necessary to cause a tear in the latex. In 1994, ASTM adopted a standard for a tear test for plastic film and thin sheeting, called the "tear-propagation resistance" test using a single-tear method. In this test, a thin plastic film is attached to a machine that separates the material. The force necessary to cause the tear is interpreted from a chart that measures the load of force and the time taken to cause a tear, called a "load-time" chart. The line graph in these charts clearly distinguishes a "low-extensible" from a "high-extensible" film. There is much more resistance to tearing in the high-extensible film. The standard cautions, however, that performance during actual use may not necessarily correlate with data from this test method. It also points out that data from specimens of dissimilar thickness are usually not comparable. Experts do not agree on whether the tear test can be useful for latex rubber. by Eli J. Carter and William R. Finger References
|
|||
 |
|||
