But most of those species were rare: 2, of them were present in the navels of fewer than six people. In fact, most were only seen on a single individual. Despite the staggering diversity, the vast majority of bacteria found in human navels come from but a handful of species. Together, those eight species accounted for almost half of all bacteria found. The researchers also found three species of archaea, a type typically found only in extreme environments.
Interestingly, two of the three came from a single individual who said he hadn't taken a shower or bath for several years. Even if you shower every day, lint still collects Credit: Getty Images. Why so much belly button biodiversity?
They draw an analogy to fish within an estuary. The permanent residents have adapted to the estuarine habitat, while other species that may briefly show up just aren't equipped to take up long-term residence. Likewise, a disproportionate number of trees in any given rainforest are uniquely adapted to the tropics. Others may be able to grow in rainforest soil, but they can't establish a strong community.
While the sheer diversity makes it impossible to predict which types of bacteria might be found inside any individual human's belly button, what the researchers can do is predict which species are most frequent and which are rarer. So if your belly button doesn't routinely snag lint to form a ball of fuzzy fluff, fret not: your navel is still an exciting place. Truly, it teems with life. Body Matters Bacteria. The curious truth about belly button fluff.
Share using Email. By Jason G Goldman 10th July Some people have belly buttons devoid of fluff — while others must clean lint out of theirs every day. What exactly is belly button lint? What if my belly button lint smells? Read this next. Dirty Bellybutton. What Causes Belly Button Odor? Itchy Belly Button. Medically reviewed by Alana Biggers, M. Belly Button Pain. Medically reviewed by Andrew Gonzalez, M. Is My Belly Button Normal?
Medically reviewed by Judith Marcin, M. Can You Microwave A Sponge? Medically reviewed by Deborah Weatherspoon, Ph. Do Aqueous Ozone Cleaners Work? Plus, 3 to Try. Medically reviewed by Debra Rose Wilson, Ph. At any rate, these articles did not attempt to perform any analysis of the physical mechanism underlying the phenomenon of BBL formation. Steinhauser 2 further noted that the scaly structure and the direction of growth of the hairs and normal body movement could play a major role in the dislodging and transport of lint fibers.
But finer details of the mechanism and how all these factors lead to a constant driving force for the one-way transport remain elusive. Building upon these ideas, we attempt to model the phenomenon of BBL development by considering the dynamics of transport of textile fibers through the region between the piece of clothing simply referred to hereafter as shirt, but note that it could be a piece of lower clothing as well and skin.
A periodic forcing on the fibers is proposed to be generated via the respiratory cycle. The cuticle scales on the abdominal hair act like small ratchets pointed towards the navel effects.
We model this feature as an asymmetric tribological property of the hair. By solving the one-dimensional transport equation based on a uniform transport rate of the fibers, the accretion rate of lint in the navel is evaluated.
We obtain a reasonable match for the mass of the BBL with the values reported experimentally by Steinhauser 2. Other rarer body movements that could result in high amplitude forcing function e. However, all these effects are expected to only enhance the transport rate of lint to the navel; hence our results are conservative. Also, designing more efficient lint removers are important in medical field, given that lint contamination during surgical procedures is found to cause post-operative complications 4.
Before moving on to the theoretical framework, it is necessary to address its conceptual underpinnings. First, let us discuss the pertinent morphological features of human hair. A human hair consists of a central core called the cortex covered by a protective sheath of thin cellular layers called the cuticle. As revealed by scanning electron microscope SEM and atomic force microscopy AFM images 5 , 6 of a human hair fiber, the cuticle has a multi-layered structure, which is reminiscent of overlapping roof tiles see Fig.
The cuticle sheets run and overlap longitudinally in a root-to-tip direction along the hair fiber axis. Each cuticle sheet is typically 0. Expectedly, the ratchet-like architecture of hair surface results in a strong directional dependence of the frictional properties of hair. For example, the dynamic coefficient of friction measured in the tip-to-root direction is more than twice that in the root-to-tip direction 5 , 6.
This is expected because the protruding edges of the cuticle scales offer additional resistance to the motion of another surface on the hair surface in the tip-to-root direction.
On the other hand, the frictional resistance will be less in the root-to-tip direction. As discussed below, this directional asymmetry plays a pivotal role in facilitating the unidirectional transport of lint fibers. The saw-tooth like topology of hair surface could also be the reason for extricating lint fibers from fabrics through abrasion, when the fabric moves relative to the hair in the tip-to-root direction.
The fabric could be the shirt worn by the individual or the drying towel used after shower. Such dislodged lint fibers present over the abdominal skin finally get accumulated in the navel to form the BBL. Note that the possibility of drying towel acting as a source of lint fibers is not considered by any of the previous studies, which consider only the shirt worn by the individual as the source. While this could possibly explain the rare cases of discrepancy between the color of BBL and the shirt worn, as we will see later, the contribution from lint fibers of drying towel is very small in the production of BBL.
Now taking into consideration the periodic abdominal motion induced by the breathing cycle and the fact that all body hairs in the area around the navel are stooped toward the navel see Figs 1a and 2 , we arrive at a simple physical picture explaining the transport process of lint fibers to the navel.
During the respiratory cycle, the size of the abdomen periodically changes, setting up a relative motion between the skin and the shirt whether loose or tight. Let us focus on the consequences of this motion along the line connecting the navel and the middle of the chest.
Similar arguments can be applied to other directions too, but are not addressed here to keep the discussion short. During the inhale phase of the respiratory cycle, in the upright position of the body, the motion of shirt is upwards relative to the skin see Fig.
A cartoon illustrating the sliding motion of a lint fiber in an area superior to the navel. Two consecutive phases of a breathing cycle: a inhale, b exhale are shown. The spikes on the hair fibers represent the cuticle scale edges.
The net effect of the relative oscillatory motion of the fabric, as shown by the red arrow, is to transport the lint fiber toward the navel in a cycle because the hairs point toward the navel.
The final initial, respectively location of the lint in each phase is shown by the solid dashed curve. Since the frictional force will be more in the tip-to-root direction the sliding velocity of the lint fiber in the exhale phase shown by the blue arrow is less in the inhale phase than it is in the exhale phase.
Consider a lint fiber dislodged from the shirt that is sandwiched between a hair fiber and the shirt worn by the individual. As the shirt moves relative to the hairs, the lint fiber will tend to move along with shirt. In the inhale phase, when the shirt moves against the stooped direction the hair fiber i. Note that since the thickness of the cuticle scales is about two orders of magnitude lesser than that of the lint fiber, the chances of the lint fiber getting snagged to the cuticle scale are very small.
Instead, the lint fiber will slide over the microscopic protuberances, facing higher friction. Subsequently, in the exhale phase of the respiratory cycle the shirt moves along the direction of the orientation of the hair see Fig. As the next respiratory cycle begins, the direction of relative motion of the shirt reverses and the cycle repeats.
Since the net sliding velocity in a cycle will be directed in the root-to-tip direction, the lint fiber makes an incremental advance in the root-to-tip direction of the hairs in every breathing cycle. Note that for smooth transition of lint fiber between two cosecutive hair fibers, the hair density must be high enough to ensure that the adjacent hair fibers are overlapping as shown in Fig.
This explains why bellies with sparsely populated hair as in Fig. Thus over time, the lint fiber will be transported toward the navel, where it becomes trapped.
The reason for this trapping becomes apparent in a close inspection of Fig. The hairs in the immediate vicinity of the navel stoop towards the depression of the navel, as depicted in Fig. This causes the lint fiber to proceed deep enough into the depression, when its contact with the fabric will be lost and thereby the traction force on the lint fiber vanishes. This also explains why navels that stick out colloquially called outties rarely collect lint 1 as in such cases the contact between the shirt and lint fibers always exist and there is no deep repository for the buildup of BBL.
The continuous influx of lint fibers from all the directions around a lint-collecting navel leads to an accretion of lint fibers in the navel. Over time, the accumulated lint fibers mix with sweat and other foreign matter to form a compact mass of navel fluff.
Now let us turn to the theoretical analysis of the phenomenon. We wish to mathematically model the transport process of the lint fiber population on the body and thus provide a reasonable estimate of the accretion rate of the fibers in the navel. This will help us to predict the mass of the BBL as a function of time and thus verify our model by comparing our results with the experimentally obtained data on the mass of BBL 2.
We begin by examining the different forces in play in the transport of a lint fiber to understand its transport rate. As depicted in Fig. These normal forces lead to frictional forces in the sliding direction at the contact points. These forces are given by the following expressions:. Thus the net traction force acting on the fiber is.
This finite skin pressure induced by the clothing suggests that the fabric is under tension. To obtain a first order estimate of this tension, one may use the Laplace equation 9. This relation can be obtained by a simple force balance and is similar to that of the pressure jump across a curved cylindrical interface between a fluid and a gas or the circumferential hoop stress in a thin-shelled cylinder.
The analogy to the fluid system stems from the fact that due to surface tension a fluid-gas interface will also behave like an elastic membrane under tension. The tension in the fluid interface is balanced by force due to excess internal fluid pressure, while the tension in the garment is balanced by force due to skin pressure.
Cross-sectional view represented by the circle of the lint showing the different forces acting on it. Note that since the fabric is moving rightward, the frictional force exerted by the lint on the fabric will be leftward. Assuming that the lint is also under the same amount of pressure, the normal force transmitted to the lint fiber from the shirt is given by.
If we idealize the lint-fabric system as a cylinder in contact with a flat surface as in Fig. However, in practice, the roughness on the surfaces and localized solid deformation at the contact points will result in a finite real area of contact. Accurate determination of the real contact area remains a major challenge in tribological calculations As explained earlier, this is attributed to the fact that the cuticle layers are aligned in the root-to-tip direction; hence the friction is weaker in that direction.
Consequently, as per Eq. Assuming that the inhale and exhale phase are of the same duration, the average velocity of a lint fiber in a breathing cycle is then given by.
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