March 30, 2021
Design is motivated by function and the expected function of menstrual cups, put simply, is to hold the menstrual blood inside the vaginal cavity without leaking. Adding layers of complexity; it must be easy to place and remove, reusable and last long years, non-toxic, non-reactive with the fluids in the vaginal environment etc. However, menstrual cups also function to provide the unexpected delight of reduced cramps*, reduced duration of bleeding* and unparalleled comfort. The science behind its functioning can be diced into physical design of the cup, physics of fluid flow, material chemistry and material physics. The physical design of menstrual cups have been generically unaltered since 1937, the first modern instance of menstrual cup.
*Varies between menstruator’s in measure and nature
The major design features of a menstrual cup are:
- Collecting Chamber
Secondary features could include:
4. Air Holes
RIM: The rim of the menstrual cup acts as the seal on the vaginal wall, preventing flow of menstrual blood between the cup and the vaginal wall. The rim sits flush against the vaginal wall and changes its circumferential shape to follow the contour of the vaginal cavity. It does not retain the shape of a perfect circle when inside the vagina. If the rim remains in the shape of a circle, gaps will appear between the rim and the vaginal wall leading to leakage. Therefore the rim needs to be flexible. On the other hand, it is also essential that the rim does not collapse or fold from the forces of vaginal muscles, as this could also cause gaps followed by leakage. Therefore, the rim is made to be thicker than the body of the cup, painting a balance between flexibility and rigidity.
COLLECTING CHAMBER: The collection chamber has a volume of typically between 15–30 ml. It is shaped to taper away from the rim into a thin stem. Some designs contain markings on the side to indicate the volume collected.
AIR HOLES: Small holes are present in the upper portion of the collection chamber. These holes are present to allow air to pass through the cup while placing and removing it. It is necessary to release the seal between the cup and the vaginal wall before removing. If the cup is pulled down without releasing the seal, it causes suction in the vaginal cavity which in turn leads to painful forces on the vaginal wall and the cervix. To release the seal, the cup is squeezed. When the cup is squeezed the air between the cup and the cervix is compressed. This compressed air flows through the air holes making it easier to remove the seal. Similarly during placement of the cup, “popping open”of the folded cup creates a suction pressure between the cup and the cervix. The air holes allow air to pass through to the cup and negates the suction. This simple feature adds to the comfort of the using the cups.
Despite having holes in the body of the cup, the menstrual blood does not leak through. The surface tension of blood does not allow the blood to flow through the cavity. Surface tension is the property of a fluid to withstand penetration of its surface. Shown below is a picture of water strider walking on the surface of water. The surface tension of water renders the surface of water to act like an elastic film, not unlike a film of elastic rubber band. The strength or force of this elasticity is measured by surface tension. The higher the surface tension, the harder it is to penetrate the surface. By virtue of its low weight and distribution of its weight through the length of its legs (notice how the legs are the parallel to the surface), the water strider performs its theatre trick. The same way, the surface of menstrual blood can be compared to a stretched elastic film. When the hole is too small, the force required to break the surface is too high for it to naturally flow through the hole. Ultimately, the situation is metaphorical to pushing an elastic film through a pin hole.
This is not to say one cannot leak through the airholes at all. If the cup is filled to the brim and the vaginal vault is also full of blood, superseding the capacity of the cup to hold blood, it is likely that blood can exert the requisite force to push through the airholes.
RIBS: Ribs/Grooves are longitudinal/latitudinal lines of protrusions made by increasing the thickness of the material along that shape. Besides providing grip, they also ensure that the cup pops out of the fold while placement. A cup made out of softer and very flexible material will be easier to fold or squeeze. However, softer cups may succumb to the vaginal forces and may not pop out of the fold while placing it inside, depending on the strength of the pelvic floor. The ribs provide an extra structural support to the cup, to enable the cup to pop out. They are made out to be thicker than the body and can resist deformation against higher forces. The upshot of a soft, flexible body with a strong rib is a cup that is easy to fold with the hands but strong enough to pop out against the vaginal forces. It is this delicate balance between softness and strength that contends to make the menstrual cup a keeper to the user or not.
STEM: The stem is the conclusion to the taper of the collection chamber. The length of the stem varies for different cups and it can be cut according to user comfort. The stem acts as a gripping arrangement, to pull the cup out and to push it into place.
MYSTERY OF MENSTRUAL CRAMPS:
When the cup is properly positioned inside the vagina, the portion between the cup and the cervix is pneumatically sealed from the atmosphere. Simple movements of the body like walking, sitting etc. can potentially cause the cup to move slightly along the length of the vaginal cavity. These slight movements have the potential to cause a pressure differential which in turn can cause the menstrual blood from the uterus to exit the cervix faster than by natural means of uterine contractions. The quicker release of blood from the uterus could potentially reduce the contractions of the uterus thereby reducing the cramps. This is a hypothesis based on theory only. The actual mechanism could possibly also include physiological or other physical effects.
Menstrual research is currently very limited and most of the aforementioned concepts are based on theoretical understanding.
Abdul is a Chemical Scientist at CSIR-IMMT, Bhubaneswar. One can often find him scribbling on scraps of paper with confounding differential equations, never ending theories on feminism and Islam, sneakers on the go to compete with a flowing mane.
Abdul’s next contribution to Boondh will be on the material composition of Menstrual cups and an elaboration on the same. Period