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Mechanism allowing a blade to cut through tissue?

  1. Sep 21, 2006 #1


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    Here is a question posed to me recently that I'd like to post for discussion. It has elements of biology and physics.

    A butter knife does a poor job of cutting through a tomato or your hand, whereas a razor sharp knife cuts through quite easily.
    Why is this so? What mechanism is at work at the microscopic or molecular level? As the thickness of a blade becomes narrower, does it reach some critical dimension allowing it to separate the bonds of molecules along the cutting plane?

    Some thoughts; as you move though a cross-section of any tissue, there are non-uniform cohesive and adhesive forces holding it together. I am thinking these forces are related to the bond-energies holding the molecules together. The case of the butter knife might be easier to think about. The tip edge is "relatively" wide or blunt. As you cut, the surface is pressing over a wider area and pushes against cells or groups of cells. If you push hard enough, it will cut though tissue, but not very evenly, across the cutting plane.. Perhaps the downward force of a butterknife tears the tissue, with the weakest molecular bonds pulling apart initially and then successively stronger bonds breaking, until the tissue separates completely.

    So what is the mechanism that allows a sharp blade to easily cut though biological tissue?
  2. jcsd
  3. Sep 21, 2006 #2
    From a physics perspective, the more pressure you apply to something, the more apt it is to deform (i.e., be cut in this instance). Since pressure is force per unit area, and the area of the blade in contact with the tissue/tomato is smaller on a sharp blade than on a dull blade,, the pressure is increased.
  4. Sep 21, 2006 #3


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    I imagine it is simply stress concentration. A butter knife could cut with sufficient speed or pressure, but it would make an ugly wound. The sharper the blade the more concentrated the stress (shear) on the tissue.

    Stress-strain => Strain energy density, which may be key.
  5. Sep 23, 2006 #4


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    Great comments so far.. Now consider this at the microscopic level, how is the thinner blade interacting with the macromolecules of the tissue (proteins, lipoproteins, carbon-chained cellulose matrices)? How are the forces being distributed to these macromolecules to allow a smooth tangential shear along the cutting plane?
  6. Sep 23, 2006 #5


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    You may wish to google on "plastic surgery" and "obsidian". I learned how to knap flint and obsidian years ago, and the guy who put the course together sold his obsidian flakes to be used as scalpel blades. The flakes are essentially 1 molecule or so thick at the edge, making them sharper than razors or conventional scalpels, and less likely to cause tearing and scarring during surgery.
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