Penetration of human skin I recently found an answer for the "toughness" of steel https://physics.stackexchange.com/a/16821 and was hoping to get a similar answer regarding human flesh. 
How much pressure is required to penetrate human skin?  to break bone? I have tried to google this and it seems surprisingly difficult to find. perhaps you know?
 A: I don't think that toughness is a particularly relevant materials property for describing human skin. The property of toughness arose out of the need of engineers to describe how rigid materials such as steel components may behave differently when subjected to sudden, sharp impacts. Some steel alloys may be able to absorb considerable impact energy before they break or shatter, and those alloys are said to have high toughness. Other steel alloys of the same yield strength may be more brittle and may fail when subjected to even small impact energies. Such alloys would be said to have low toughness. 
It might help if you described why you are interested in the "toughness" of skin, because there is probably a more appropriate engineering parameter to use than toughness. If what you are really interested is puncture resistance, there are probably engineering parameters to describe that.
A: A study that you can look at is by Shergold and Fleck, (2004), Mechanisms of deep penetration of soft solids, with application to the injection and wounding of skin, DOI: 10.1098/rspa.2004.1315.
From the abstract:

Penetration of the soft solid by a flat–bottomed punch is by the formation of a mode–II ring crack that propagates ahead of the penetrator tip. The sharp–tipped punch penetrates by the formation of a planar mode–I crack at the punch tip, followed by wedging open of the crack by the advancing punch. For both modes of punch advance the steady–state penetration load is calculated by equating the work done in advancing the punch to the sum of the fracture work and the strain energy stored in the solid.

and

For both geometries of punch tip, the predicted penetration pressure increases with diminishing punch radius, and with increasing toughness and strain–hardening capacity of solid. The penetration pressure for a flat–bottomed punch is two to three times greater than that for a sharp–tipped punch (assuming that the mode–I and mode–II toughnesses are equal)

A more recent experimental paper that does not use fracture mechanics is: Roesthuis et al. (2011) Mechanics of needle-tissue interaction, DOI: 10.1109/IROS.2011.6094969.
There is a vast amount of work on the subject in the scientific literature; mostly behind pay walls.
