How much pressure does it take to pierce human skin?

Researcher X

I was wondering if any values for this were ever recorded. How strong is human skin when subjected to pressure? Since there are figures for steel which constitutes classical armor and values for bullet proof vests, I wanted to work out what magnitude of protection those things give you against weapons as opposed to bare skin.

Obviously, things like swords and bullets do more than just split skin - they go right through layers of muscle and bone, but I'm thinking of just the initial layer we call our skin first, which is the first indication of any kind of wound, small or not, when it is pierced/split and blood is drawn.

Shalashaska

It completely depends on the surface area of the piercing, how sharp it is, and to some extent where on the body.

There are figures of gross bullet penetration through skin+fat+muscle, but just skin? Too variable I think. The best research is by the FBI on the issue of over-penetration of human and inert target by their 10mm auto sidearms. That might help?

Researcher X

Well, the surface area is what creates pressure. The concept of sharpness is based on something with a very small contact area imparting a force.

I had another look about and found someone citing 100psi as the pressure yield of human skin, but I can't find any sources for it.

Shalashaska

True... consider these scenarios:

A chisel
An icepick
A hollowpoint round
A copper round with steel-tungsten core (FMJ, small tip)
A 1" ball bearing


Factors:

Surface area of the implement
Are we talking about skin on the side of your neck, or your hip, or wrist, or belly? The skin is a bit like kevlar, in that it will disperse some energy if it can, but that's limited by what's backing it.

Velocity of the round: piercing in a single stroke is easier, and may exert less pressure (as read by a gauge) than a steady push.

The material of the implement, and how it yields in relation to flesh. Mass, surface area, material, velocity, region of the body, variations in density of the dermis... it's too still too variable.

Remember, kevlar is great against a civilian round, but not so hot with round from an AR, or knives. Hence the need for impact plates of steel or ceramic, to allow the fabric to disperse the energy in the first place.

Shalashaska

Hey, on second thought, maybe that is in realtion to an air-injector?! That wouldn't answer your question, but it would explain the strange figure.

Researcher X

I don't see how that's possible. If you increase the force upon the same contact area, you will be increasing the pressure. A faster needle, knife, bullet will exert more pressure at its contact surface, than a slower version of the same mass and contact area.


But these variables all come back to varying the pressure, when the question is really about how much pressure the skin can take. There are lots of methods which can vary the amount of pressure you happen to apply, but 100psi of pressure is still 100psi of pressure no matter how you created it. A copper pin with the same contact area as a steel pin may deform more, but then it's producing less pressure, because it's producing less force on the same contact are (energy is being wasted through deformation).


That's true, though the small variation might not be a concern if you're just looking for a single example. At any rate, I'm thinking of the torso/upper body area.


No, someone just gave that figure to somebody else asking the question, without any other explanation.

Shalashaska

You're right, but skin is elastic and vascular. Slow pressure first causes blood to leave the area (through brute force) and that can make piercing a region difficult. In practice, I'm sure that every region has a specific psi for a given time of day, and person, but if you're interested in ballistic armor here is the difference: A 130 grain bullet traveling at 1400 fps is going to be exerting orders of pressure above what is needed to penetrate skin. Once the round has entered flesh, temporary cavitation from a fast and especially super-sonic round further reduce drag and again, less rebound. If you never have a chance to experience that return force from the skin, it requires less (in practice) as read by a gauge on the receiving end, to penetrate.

Sharp, dull, is a matter of surface area, but in the case of Kevlar (for example) the issue of the weave also becomes a factor. You can sever the threads which provide protection with a slice, which weakens the fabric as the slice continues. In the end, you require less force exerted on the same material, because you're not engaging more of the fabric. The pressure required to pierce the full thickness of the vest is high, but a blade works from the outside, in, destroying that protection without challenging it. Skin has some of the same issues: pierce to what depth? If you slice or needle the skin, you destroy the ability of the dermis to spread the force of that blow and dissipate it. In other words, there's piercing an epidermal cell, which is different from piercing an array of cells and collagen, actin, fat, and more.

In PRACTICE, the 1" ball bearing is going to drag much of the skin around it thus presenting more of it's surface area than a 1" rod, which cannot be fully enveloped as a sphere might. This also goes back to material, which isn't an issue if we're talking about shrapnel and bullets. In that case, issues you should be concerned with are best thought of in terms of how the kinetic energy is going to be received: shrapnel tends to have a lot of mass compared to a bullet, and is often very hot. If you're wearing Kevlar, it could melt outer layers of fabric, or rocket through you and out the back.

As for the 100psi figure, if we're not talking about specific material, location, and more, it's a meaningless figure. I'm sure you could model this system given enough human volunteers and really impressive computers, but as of not it's still a real challenge.

Deoxyribose

I bet a type I diabetic who has to prick his skin every day has a good idea of how much pressure it takes to puncture skin. But the skin's resistance to puncture depends on the proteins and molecules that cause your epithelial tissue to adhere together. Different levels of collagen and elastin will affect how much pressure is required to puncture skin. If you want to know how much force or energy is required to pull skin tissue apart, it might help to study the forces that hold skin tissue together.

But here's a simple experiment:
Pull off a blister or some dead skin.
Get a needle.
Make a setup where the needle is resting directly above the skin (you're on your own for this one).
Measure how much weight must be added to the needle in order to puncture the skin.
Measure the area of the needle-point.
Divide the weight of the needle at breaking point by the area of the needle.
About that much.

Shalashaska

That is academically interesting, but it ignores the nature of ballistic impact, which is the point. Piercing dead skin of the outermost dermal layer really has little to do with the OP in the end. The dermis is much more complex, and given that he specified the abdominal/thoracic region, it's a different game in an area that is heavily muscles and laden with subcutaneous fat.

Your test would give you one information point: How much force for a given gauge of needle is requires to penetrate dead skin, separated from the fascia and the rest of the dermis. It would be unfortunate to use that as a model for anything else.

Andy Resnick

I was surprised- I couldn't find any measured data. Nothing from piercing guns (earlobes), nothing from glucose meters (finger sticks), and the only data I could locate from microinjections came out to 50,000 psi- which seems really high.

Shalashaska

I keep trying to explain why no numbers exist: it's too variable. Remember btw, that microinjectors need to pierce skin, fat, and deliver their payload into muscle tissue. That payload is usually a liquid, and you're displacing a small amount of tissue and fluid to do that. So, fast, and "hard" so to speak, for comfort's sake as well.

This is partly why designing armor is so very difficult, because shooting people in thousands of locations many MANY times isn't feasible or nice.

P.S. The rule for injections is: use more force and do it quickly: that minimizes discomfort by keeping the tissue form exerting more force on a larger surface area.

Andy Resnick

That may be true. Even so, I am surprised that (say) Bayer, or Maxima, or whomever manufactured this thing:

http://cgi.ebay.com/New-Steel-Ear-Bo...item20b08773bd

Doesn't see fit to post a spring constant.

Shalashaska

They don't?! Ok, that's a little odd. I know epi-pens have a clear load strength to pierce denim, leather, and more. I wonder if they consider that to be proprietary info? That seems like a very basic engineering issue. I have to research this.

Moonbear

According to this source, 0.1 to 3 N:
http://www.jbiomech.com/article/S002...473-1/abstract

And if I'm reading this one right, around .3 to .5 N for some commonly used needle guages:
http://www.emdt.co.uk/article/needles-comparison-study

Andy Resnick

wow... those numbers imply *huge* pressures are needed. I calculated the pressure to be around 5000 kPa from the second article, and that does not account for the fact that the needles are beveled.

I bet the actual pressures are closer to 50000 kPa... huh, that's comparable with the microinjection site.

cherieg

Re: What pressure is required to pierce human skin:

I am researching this subject also but my perameters are more specific. I have found the previous comments interesting and hope someone can help me find the answers I seek.

How much pressure is required to propel a jet of liquid at a velocity that will pierce human skin no deeper than the mid or sub dermis level?

The liquid would be of low density, forced under pressure through a very small orifice from the propelling apparatus producing a high velocity jet that would be smaller in diameter than a 33G needle. This method will result in a minute piercing in the skin with minimal surface tention / resistance.

But the estimated amount of pressure required for this "no needle injection" seems to vary considrably from one source to another. Can anyone help me to be more specific?

crasms

hi to all.

this has been a most interesting thread. it seems to me that we're not yet at the fundamentals of piercing (essentially point shearing) any material.

The amount of force required to break through any material is a function of the ultimate tensile strenght (UTS) and the area of the sheared surface. the surface area of the sheared materal is equivalent to the perimeter of the cutting tool and the thickness of the material to be sheared. so to use a real world example to punch a 1" square hole in steel that is 1/4 inch thick we take the perimeter of the punch which must be 4" times the depth (.25 inches) to get 1.0 square inches of material to shear. steel has a uts of 50,000 psi so it would take 50,000 pounds to punch this hole (25 tons)

What I haven't seen in the discussion so far is the equivalent parameters for human skin. What is the ultimate tensile strength of skin. and what is the thickness of the substrate that is resisting. (skin has multiple layers so It would be important to make sure that we don't assume the full thicknes of the dermus has the same strength.) furhter I sensed from others in the stream that skin thickness may vary from point to point on the body -- seems logical to me.

all of the talk about the area of the impact effecting the force is all true but it stems from the two fundamental properties I cite above.

So here's the project:

1. Can someone provide the UTS of skin -- and preferrable a range of UTS's observed or postulated. (here I wonder about the impact of things like tanned skin vs untanned, living skin vs dead skin etc.)

2. can someone provide the thickness of the top layer of skin that will resist the penetrating force. Again here the answer here is best expressed as a range of normal observed thicknesses for a given region of the body.

with the above it should be possible to predict the range of forces required to penetrate the skin assuming we know the shape and perimater of the cutting tool.

Facinating science project for someone! have fun!