Sharpness vs. Hardness

A notable YouTuber, Kiwami Japan, has gained popularity by crafting knives from unconventional materials—yes, really unconventional ones, such as pasta and even gelatin. He shapes the gelatin, lets it dry until it becomes solid, and then sharpens it to a point where it can effortlessly slice through stiff paper:

However, it's crucial to differentiate between sharpness and hardness.

Hardness refers to the durability of a material, which ultimately determines whether it can cut through another material. Sharpness, on the other hand, influences how cleanly a material can make a cut. Unfortunately for those dreaming of achieving razor-sharp fingernails, while it's feasible to sharpen them to a fine edge, they won't be able to slice through any material that is denser than keratin, the substance that makes up our nails.

Kiwami Japan’s gelatin knife can produce an extremely sharp edge, enabling it to cut through softer items like paper, but it falls short against harder materials, such as wood.

What Can Our Nails Actually Cut?

Our fingernails are primarily composed of a tough protein known as keratin. This same protein is found in other structures like hair and rhinoceros horns (though it's worth noting that elephant tusks are specialized teeth made of a different material). Additionally, turtle shells and bird beaks contain a tougher form of keratin called beta-keratin, which is absent in mammals.

Delving deeper into the molecular structure of keratin, we find that it consists of long, coiled chains that form spiraling tubes. These chains are rich in sulfur, which allows for numerous stable connections between the tubes, enhancing their structural integrity. The amount of sulfur present in different keratins varies, affecting their flexibility—more sulfur leads to more connections, resulting in a stiffer material.

This variability in keratin's composition explains the unpleasant odor associated with burning hair, skin, or nails, as the sulfur breaks down and forms various pungent compounds.

The hardness of fingernails is measured on the Mohs scale, which ranks materials from 1 (very soft, like talc) to 10 (extremely hard, like diamonds). Fingernails score approximately 2.5 on this scale, meaning they can cut through softer substances like talc (1.0) or certain types of gypsum drywall (2.0) but are incapable of cutting through tougher materials like copper (3.5) or steel (6.5).

Even the addition of fingernail polish, composed of acrylic and rated around 3 on the Mohs scale, offers only marginal improvement; while it may enhance strength slightly, it does not elevate nails to the level of a functional cutting tool.

Theoretical Considerations: Monomolecular Edges

Science fiction often portrays futuristic cutting devices as having a "monomolecular edge," suggesting that the blade is sharpened down to a thickness of just one molecule. Such a feature would theoretically enable even softer materials to slice effortlessly through anything, from flesh to steel.

However, as we've established, sheer sharpness does not equate to the ability to cut through harder materials. A keratin blade, no matter how finely honed, would still fail against steel armor. Additionally, keratin's molecular structure is quite large compared to the smaller atoms of iron and carbon found in steel blades, meaning even a fingernail sharpened to a single-molecule thickness would not achieve the same cutting efficiency.

Regrettably, the dream of having naturally sharp blades at my fingertips remains just that—a dream. While we can hone our nails to a certain extent, their composition simply doesn't allow them to slice through metal or even wood. Nevertheless, it might still be wise to avoid crossing paths with anyone sporting particularly sharp nails!