1. Why Does Hair Feel Different When Stroked in Opposite Directions?
Imagine holding a single fallen hair between your fingers and sliding it from root to tip, then from tip to root. A research paper on cosmetics explains that moving from root to tip is easier than moving from tip to root [4]. This is because the cuticle on the hair’s surface has a directional bias [4]. The cuticle is the outermost layer of the hair, shaped like scales. It is made mostly of hard protein and overlaps like tiles to protect the inner part of the hair from outside stimuli [5]. But how big is one of these "tiles"?
2. How Big Are the Cuticle Cells and How Much Do They Overlap?
The cuticle cells in finished hair are flat, rectangular sheets. One cell is about 0.5 micrometers thick and about 50 micrometers long [2]. One micrometer is one-thousandth of a millimeter. While hair thickness varies by person, one observation example had a diameter of about 100 micrometers [2]. In that case, the cell length is about half the hair’s width, and its thickness is less than one-hundredth of the width [2]. These cells overlap like roof tiles, with up to five-sixths of their length covered by the next cell, leaving the edge tilted slightly toward the tip [2]. The number of overlapping layers varies. One study lists 5 to 10 layers [1], while a report using an electron microscope to observe cross-sections of Japanese hair found 5 to 6 layers [3]. How does this tilt affect touch?
3. How the Overlapping Cuticle Edges Catch Your Fingers Moving Tip to Root
Think of roof tiles on a house. Sliding your hand down from top to bottom causes no resistance, but stroking upwards catches your fingers on the tile edges [4]. Hair surfaces are similar. The edges of cuticle cells are said to be spaced about 5 micrometers apart along the hair [4]. Because they overlap pointing toward the tips, the surface shape has directionality [2]. This is why texture feels different depending on the direction. However, the difference in friction changes based on hair condition and measurement methods [4]. Note that the roof tile analogy is for clarity; real cuticles are much smaller structures, only about 0.5 micrometers thick [2]. What is inside these thin tiles?
4. What Layers Make Up a Single Cuticle Cell?
One cell is reported to consist of layers from the outside in: epicuticle, A-layer, exocuticle, and endocuticle [1]. The cell membrane complex (CMC) is the part between neighboring cells [1]. The outermost epicuticle is about 13 nanometers thick and made mostly of protein (about 80%) [1]. Thirteen nanometers is a few tens of times smaller than the cell’s total thickness of 0.5 micrometers (500 nanometers) [1]. The outer layers are said to be hard because they have many cross-links connecting proteins, while the inner endocuticle is soft because it has fewer links [6]. Inside each cell, hard layers are on the outside and soft layers are on the inside. These cells overlap pointing toward the tips on the hair surface.
5. How Can You Try This Test Yourself With a Fallen Hair?
You can try this today: pick up a long fallen hair, gently hold it between your fingers, and slide it from root to tip and then tip to root to compare the feel [4]. Some people feel the difference clearly, while others do not. Do not pull hard. You can also visit science museums or school labs to ask if they can show magnified images of hair surfaces to see if the tile-like overlap is visible. Official information is available on the Saitama City Science Navi website regarding hair and cuticles [5], and the Journal of the Microscopy Society of Japan contains reports on electron microscope observations of hair cross-sections [3]. Note that this article’s sources do not cover surfaces of non-human hair, such as wool or pet fur. Investigating those could expand your comparison.