Here’s a bit of a riddle for you: What has the consistency of both toothpaste and dough, can absorb twice its weight in water and still remain sticky, was used by medieval artists as a binder in paint and has been hypothesized by physicians to repel insects?
It’s earwax.
David Ancalle and David Hu
The medical term for earwax is cerumen, based on keros, the Greek word for beeswax. People have been pondering earwax since ancient times. Ancient hygiene kits including picks and spoons to remove earwax have been found in archaeological sites worldwide.
Even today, it remains a perpetual question: to remove or not to remove your earwax? The answer: Don’t do it. The general medical belief is that ears are self-cleaning, and earwax brings benefits such as lubricating the ear canal, fighting infection and acting like a sticky net to trap debris before it reaches the eardrum. By poking in there you can accidentally make matters worse, pushing wax deeper or scratching your delicate ear canal.
Cerumen impaction – or blocked earwax – affects about a fifth of the American population, leading to 12 million patient visits per year. Blockages are worsened by daily activities such as wearing earbuds or hearing aids.
Another common problem with ears is wedged debris. For children, much of this is self-inflicted, in the form of items such as Legos or erasers. But for many adults who have something stuck in their ear, the debris is insects, such as ants, beetles, flies or cockroaches.
While rare for the average person, insects in the ear occurred with such frequency over the years that physicians began to wonder whether earwax evolved as a natural barrier for insects. But how would such a barrier work?
To answer this question, my Ph.D. student David Ancalle led experiments with earwax and insects. We teamed up with Yuhang Hu and her student Donjing He, experts in the behavior of soft solids such as hydrogels. As mechanical engineers, we wanted to focus on the physics rather than the chemistry of earwax.
Our research, published in the journal Soft Matter, shows that earwax can, in fact, stop insects without the use of dangerous chemicals.
Watching ants encounter earwax
To do experiments, we needed voluminous amounts of earwax.
Initially we obtained earwax from deceased mammals such as pigs, sheep and rabbits. Eventually we turned to our friends and family. A person has only a grain of rice’s worth of earwax in each ear. Due to a mutation in a single gene, East Asians have dry, flaky earwax, while people of African or European descent have wet, squishy earwax.
To simulate an ant climbing into an ear canal, we used a glass beaker with its rim coated in earwax. Ants, like many insects, are climbing specialists, capable of traveling across many kinds of surfaces. To climb rough surfaces, ants have spikes on their feet, like mountain climbers do, that helps them grasp onto ridges and cracks. For smooth surfaces, ants unfurl wet suction cups, which can support 60 times their body weight on smooth glass.
Watching ants trying to climb earwax showed us how strange a substance it really is. Rough earwax slowed them down, but with their claws they made progress.
But as soon as ants tried to climb smooth earwax, they fell immediately.
From these experiments, we concluded that earwax had anti-stick properties when in contact with the insects’ feet.
We next conducted experiments with ants crawling through an earwax-coated tunnel, more similar in size to a human ear canal. As the ants attempted to travel through the tunnel, earwax stuck to them. Once coated, they groomed furiously, unwilling to walk until they had removed as much as possible. Even after several minutes of effort, the ants still had earwax sticking to hard-to-reach places.
David Ancalle and David Hu
So, earwax is a paradoxical material: It is simultaneously slippery, thanks to the lipids it contains, but also sticky. Similar to many materials – including ketchup, paint and shampoo – it thins out when it is physically deformed, what physicists call shear-thinning.
Figuring out earwax’s physical properties
The dual qualities of earwax have to do with its unique composition. Earwax is made up of flakes of skin cells held together with a binder. This blend forms a kind of sponge with microscopic holes. When a body overproduces earwax, it heaps up in boulders that ants can easily climb over.
When the substance is smooth, however, ants rely on their wet suction cups. The holes in the earwax absorb the drops produced by the ants, preventing them from attaching. Absorbency makes earwax fundamentally different from commonly used forms of pest control.
In your local store, you will find sticky surfaces such as fly paper or chemicals that kill the insects. Some researchers look to nature to find bio-friendly alternatives to using chemicals; for example, plants have properties that prevent insects from climbing.
From this study, we learned that the ear is also a model for insect repellency. Earwax is made of natural materials but has a structure that prevents ants from sticking to it.
The porosity of earwax also gives it unique properties. We showed it can absorb twice its weight in water yet still cling to the walls of the ear. This water-absorbing ability may help prevent water from accumulating on the delicate eardrum and fend off infections such as swimmer’s ear.
Scientists don’t yet understand the complex relationship between earwax, water and bacteria. Some studies show that earwax kills bacteria, but other studies find that earwax harbors bacteria.
Our experiments showed that earwax is most challenging to insects when similar to the thin layer of earwax maintained by a healthy human body. Wax would accumulate more in human ears if not for two self-cleaning mechanisms. The skin in the ear canal grows outward like a conveyor belt. Over 8-10 weeks, this process – called epithelial migration – pushes the oldest earwax out of the ear.
Simultaneously, earwax dries over time, transforming from a gooey state to a brittle one that readily cracks. Every time you talk or chew, your ear canal deforms, and physicians found that this movement helps to dislodge the drier, older earwax.
As a test of our concept, a member of our team, chemical engineer Rafeen Rahaman, designed an artificial earwax based on the physical structure rather than the chemistry of the earwax. Ants struggled when they touched it, just like when they interacted with natural earwax.
The cost of harvesting natural earwax and our recipe for artificial earwax are not cheap. We don’t foresee our methods replacing insect repellents produced at large scale. But perhaps the concept might inspire new ways to prevent earwax from coating our earbuds or lead to the manufacture of insect control methods using safe formulations.
