You who are looking at your mobile phone, have you ever thought:
Why aren't our fingers holding the phone firmly?
Why can the phone screen and smart lock be unlocked with the touch of a finger?
These are inseparable from the same thing that has evolved in humans for a long time and is familiar to the point of being overlooked - fingerprints.
Today I will tell you about the mystery behind fingerprints.
Fingerprints are part of our skin and are protrusions of dermal tissue towards the epidermis [1]. Inside the fingerprint, there are also many sweat glands hidden.
We know that rough surfaces can increase friction; but if there is liquid on the contact surface, the friction will decrease and become very slippery.
So, when we take something, how exactly does the fingerprint work?
On November 30, 2020, a study published in the Proceedings of the National Academy of Sciences (PNAS)[2] revealed its secrets.
(Source: pnas.org)
Scientists from the United Kingdom and South Korea used advanced optical imaging, infrared domain optical coherence tomography (IR-OCT) and other techniques to observe the process of "finger grasping glass".
They found that when the drier fingertips touched the glass surface, the sweat glands in the fingerprint secreted sweat, allowing the skin surface to fully hydrate.
The meaning of "hydration" is actually "the combination of keratinocytes on the surface of the skin and water." Simply put, it's like the process by which a sponge absorbs water.
(Source: see watermark)
The surface cells become fuller after sweat hydration, increasing the contact area and friction between the skin and the glass, helping us to hold it better.
(A to C: image of the fingerprint pressed on the smooth glass; D: image of the spiral sweat gland catheter inside the skin of the fingerprint.) Source: Literature[2])
When the contact surface between the fingertip and the glass is large enough, the glass surface will completely cover the sweat gland outlet, blocking the sweat flow.
Then, the secretion of sweat glands is reduced, which prevents excessive sweat from slipping the fingertips and avoiding the glass from being "buried" at the scene.
What about people who are born to sweat? We have seen some students who were too nervous during the exam, and the whole paper was wet and dry, and it was wrinkled.
Don't worry, scientists have also analyzed fingertips in wet conditions.
When there is too much sweat between the fingerprints, the "evaporation system" begins to work.
With the help of the gap left by the groove between the fingerprints, the sweat and air contact surface is enlarged, which can help the moisture evaporate quickly, so that the friction between the fingertip and the glass surface is in the best condition.
(In a wet state, the sweat in the fingerprint evaporates and dries out within 180 seconds.) A: Optical image of fingerprint contact surface; D: cross-section of moisture change in fingerprint groove, Source: Literature [2])
Therefore, when our fingers come into contact with something as hard and impermeable as glass and metal, whether the fingertip is dry or wet, the secretion system and the evaporation system will work together perfectly to ensure that we can grasp objects and operate flexibly.
(A simulated pattern of the distribution of sweat in the fingerprint groove when the finger touches the glass.) Source: Literature[2], Translated by Tencent Medical Codex)
Speaking of which, you might think of another phenomenon —
When our fingers and toes are soaked in water, they will wrinkle after a long time.
That's right! Some studies believe that this is also to improve the ability of hands and feet to grasp the ground in the water [15].
After reading the conclusion, we finally know why our fingers can firmly hold the phone, without worrying about it falling on the ground and breaking the screen
(Friendly reminder: the hand skater may be too sweaty hands, or it may be that the fingers are not very obedient... It is recommended to bring your own non-slip phone case, lazy stand and hand towel. Source: soogif)
This research has also brought more new inspiration to humans in terms of bionic applications. Like what:
Design a more efficient haptic sensor (let's brush the touch screen more smoothly);
Use the grip principle of fingerprints to create more realistic prosthetics and robots;
Developing more responsive textile inspection equipment (Mom no longer has to worry about me picking fabrics).
Curious you might be wondering:
Since sweat evaporates, why do fingerprints on the surface of an item still leave traces and become evidence?
(Source: soogif)
In fact, in addition to water, the sweat secreted by the fingers also has a small amount of protein, inorganic salts, oils and so on. These substances not only make sweat sticky, but also make dust and small particles easy to adhere to.
Therefore, for hard, impermeable items such as plastic, glass, metal, and tiles, the criminal investigators will use some special powder (such as powder with obvious color difference or iron powder) to sprinkle on the surface, so that fingerprints can be extracted.
What about items that are soft or easily absorbent?
This requires the fingerprint to appear in its original form according to the physicochemical properties of the object itself.
For example, on the paper suspected of having fingerprints, fumigation with iodine vapor can see the fingerprints that turn brown, which is the "iodine steaming method"; similar to this, there is also the "silver nitrate method".
(Fingerprint extraction on the surface of the fabric.) Source: Shanghai Documentary Channel "True 25th Hour: Broken Fingerprints")
With the development of technology, some chemical means are not easy to detect the old fingerprint, the application of advanced optical instruments and auxiliary computer recognition means, can also make the fingerprint "visible" [3].
"One bucket is poor, two buckets are rich, three buckets are four buckets sell tofu, five buckets and six buckets open pawnshops, seven buckets and eight buckets are made by officials, and nine buckets and ten buckets enjoy Qingfu ..."
When you were a child, did you ever hear of such a smooth mouth, and still look at the fingerprints one by one with your fingers, looking for a bucket to find a dustpan? Please raise your hand~
The buckets and dustpans here really have scientific classification standards!
In the 1880s, Sir Francis Galton first proposed the fingerprint classification in his book,[4] which divided fingerprints into three main categories: bucket patterns, miliforms, and curved patterns, and corresponding subtypes. With the increase in fingerprint applications, the classification methods are also different.
Studies have found that about 60 to 65% of the population is michi-shaped; 30-35% are bucket-shaped; and only 5% are curved[10].
According to this speculation, people with "buckets" may be born with fewer than those with "dustpans".
Then, the more "fighting" with the "fight" in the slip of the tongue, the more "fate is good", perhaps it is just a psychological premise that things are scarce and expensive. People who believe this statement will also unconsciously attribute and move closer to it (similar to "blessed earlobes" and "good luck finding clover").
When we were fetuses, fingerprints were formed around 3 months into the mother's belly [9].
The basic pattern of the fingerprint is determined by the gene, so the fingerprints of different people are not the same. Even if it is the same person, the fingerprints of 10 fingers are not the same.
So, there is a special group of people - identical twins (two people are differentiated from the same fertilized egg), their genes are exactly the same, will the fingerprints be different?
The answer is: it's not the same.
The study found that although the starting point of fingerprint development of identical twins is the same, the growth process is different.
For example, the amniotic fluid around the fetus and the posture of the fetus in the womb will affect the formation of fingerprints. So the "bump" fingerprint situation will not happen!
In detective novels, there are often plots of "criminals use pineapple juice to make fingerprints disappear". Will this happen in reality?
Indeed, the protease in pineapple degrades protein, "bites" our tongue, and puts it in the meat to make the meat tender.
However, some people have found that frequent contact with pineapple juice will only make the fingerprint less obvious for the time being, and the key is that the fingers will peel off and burn pain (touching the sheets are painful, which undoubtedly increases the difficulty of committing the crime)! Even workers who cut pineapples always wear gloves.
(Fingers soak pineapple juice twice a day, changing after 8 days.) Source: Video blogger Tom Scott[13])
As early as 1935, experts studied the "fingerprint changes that apply acid to the fingers", and found that the fingerprints were not completely destroyed, and the surrounding circle could still be identified [14].
(Acid-treated fingerprints of early 20th-century American robber John Dillinger.) Source: Literature[14], Translated by Tencent Medical Code)
If you want to really eliminate fingerprints, at least to reach a depth of 1 mm, the range should be larger, otherwise it will grow back to its original state.
However, there is one type of person in the world that cannot be traced through fingerprints - that is, skin stripe disease patients [11].
This is a rare disease. They are born without fingerprints, and their fingers are bare (fingerprinting on various documents is a big problem). Fortunately, this disease does not have a particularly large impact on physical health.
Amazingly, they don't "slip" when they hold things. Because the palms and soles of the palms of the hands and feet become thicker and the ability to wick sweat is slightly reduced, it can also produce grip friction similar to that of ordinary people.
In addition, the anti-tumor chemotherapy drug "capecitabine" will also cause fingerprints to become shallow and disappear, and can be recovered after 2 to 4 weeks of discontinuation.
(A 57-year-old female with breast cancer uses a thumb behind capecitabine.) Source: Literature[12])
The little fingerprint learned a lot, and the strange knowledge increased. Today, have you learned to scrap it?
Contributing Author: Gaga | Ph.D. in Pharmacology, Macau University of Science and Technology
Review expert: Zhang Hongtao (pen name a section of ginger) | Associate Professor of Research, Department of Pathology and Experimental Medicine, University of Pennsylvania School of Medicine, USA
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[4] Yager N, Amin A. Fingerprint classification: a review[J]. Pattern Analysis and Applications, 2004, 7(1): 77-93.
[5] Yohannes S, Bekele E. Ethiopian population dermatoglyphic study reveals linguistic stratification of diversity[J]. PloS one, 2015, 10(6): e0126897.
[6] Jain A K, Prabhakar S, Pankanti S. On the similarity of identical twin fingerprints[J]. Pattern Recognition, 2002, 35(11): 2653-2663.
[7] Mehta A A, Mehta A A. Study of fingerprint patterns in type II diabetes mellitus[J]. Int J Anat Res, 2015, 3(2): 1046-1048.
[8] Desai S D, Hadimani G A. Dermatoglyphics and Health[J]. Anatomica Karnataka, 2013, 7(1): 1-9.
Liu Rundong. Research on Calculation Method of Fingerprint Ab Crest Line Number Based on Bresenham Algorithm[D]. Yunnan University, 2016.
[10] Wijerathne B T B, Rathnayake G K, Adikari S C, et al. Sexual dimorphism in digital dermatoglyphic traits among Sinhalese people in Sri Lanka[J]. Journal of physiological anthropology, 2013, 32(1): 27.
[11]https://www.nationalgeographic.com/news/2011/8/110809-fingerprints-skin-disease-health-science-weird/
[12] Cohen P R. Capecitabine-associated loss of fingerprints: report of Capecitabine-induced Adermatoglyphia in two women with breast cancer and review of acquired Dermatoglyphic absence in oncology patients treated with Capecitabine[J]. Cureus, 2017, 9(1).
[13]https://www.quora.com/Can-someone-really-erase-their-fingerprints-using-pineapple-juice
[14] Cummins H. Attempts to alter and obliterate finger-prints[J]. Am. Inst. Crim. L. & Criminology, 1934, 25: 982.
[15] Changizi M, Weber R, Kotecha R, et al. Are wet-induced wrinkled fingers primate rain treads? [J]. Brain, behavior and evolution, 2011, 77(4): 286-290.
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