So I’m co-hosting this new show for CNBC called “Make Me A Millionaire Inventor” – obligatory link here – in which Deanne Bell and I look for the best/coolest/weirdest/craziest inventions never made and work with them to build or improve the prototype, market test, and pitch to investors. In today’s episode, I advise a team of four skaters and businessmen on a product they’ve created called the Skull Cap to help protect extreme sports athletes from head injury.
As you might guess, a big part of getting ready to pitch a product like this is testing it to make sure it actually does what you say it does. So we took the guys to an impact testing facility to do a linear drop test (basically, dropping an approximately-head-shaped steel weight in a straight line onto the different samples we were testing and measuring the impact in g’s).
I’m a chemist by training. By no means do I specialize in anything related to dropping fake heads on hard surfaces and seeing how much they crack (and I apologize to any physicists offended by unintentional misuse of terms like “impact” and “force” in the rest of this post). So I called in the big guns – in this case, Dr. Peter Matic at the Naval Research Laboratory in Washington, DC.
We would have loved to visit NRL and test the Skull Cap in Dr. Matic’s lab, but the guys are based in Southern California, so that was out. But Dr. Matic was incredibly helpful in helping us think about the results of the test, especially when it came to figuring out how to best summarize the data for a TV audience.
We spent an hour testing 27 different samples, and as you’ll see, the final scene on TV is just a few minutes. So Dr. Matic and I had a very long e-mail conversation about how to best and accurately convey how the Skull Cap performed, in 3 sentences or fewer.
I thought I’d share part of that conversation to show: (1) how much work goes into writing a few sentences on TV, (2) the delicate balance between keeping something accurate and also entertaining, and (3) how awesome it was for a scientist with no prior connection to the TV show to give so much of his time and energy for the sole purpose of making sure someone he doesn’t know that well says the right thing on TV.
So, without further ado:
The very first draft of the results from the impact testing facility data was something like:
“Any impact above 300 g’s will kill you. That’s the line. Below 300 g’s you’re alive, above 300 g’s you’re dead.”
If you have any experience with science or engineering, you’re (rightfully) raising your eyebrows – there are very few black-and-white statements in science. So while it’s generally true that the higher the g force your head experiences during an impact, the more likely that impact is to seriously harm or even kill you, there is no “kill line” above which you automatically die. Damage to the brain depends on so much more than just the peak impact force. So we redrafted the quote, and at this point I reached out to Dr. Matic to ask his opinion.
George Zaidan here, from the CNBC show “Make Me a Millionaire Inventor.” We chatted on the phone a while back, and I’m wondering if you could help me answer a quick question.
As you know, the product we discussed on the phone is a soft-foam hat/helmet liner, and as part of the show, we performed a linear drop test on the product. One test compared a commercially available skateboard helmet with a newer prototype. (The test was a 5 kg weight being dropped from ~4.5 ft.) The accelerometer registered a peak g-force of 1000 g’s for the commercial helmet vs. 285 g’s for the new prototype.
That’s a pretty substantial reduction, but I’d like to be able to put this result in terms that a lay audience would understand, especially as it relates to head injury. Ideally what I’d like to say is something like, “A 1000 g impact will almost certainly kill you – but at 285, you’ll live. You won’t walk away from it, but you’ll live.”
Do you think that would be accurate? Let me know what you think.
Thanks so much,
Dr. Matic wrote back:
If they have instrumented a head form and are measuring the acceleration of the head (and not the helmet or the impactor acceleration), then 1000 g’s is very high and probably fatal for a blunt type of impact, like from a skateboard height. An acceleration of 285 g’s on the head is still significant, and could cause a concussion and perhaps some other injuries. (The shorter the impact duration, the higher peak acceleration you can actually endure. The sports type of impact would typically occur over about 15 millseconds of contact.)
You would generally want to keep the average head acceleration (calculated from many tests) under perhaps 150 g’s (maybe even less than 100 g’s) with perhaps no single response exceeding 300 g’s.
So your statement (slightly modified)… “A 1000 g (head acceleration from) impact will almost certainly kill you — but at 285g, you’ll live. You (may or may not) walk away from it, but you’ll live.” would be true.
But like any good scientist, he didn’t stop there. He asked a bunch of questions to clarify:
As I recall from watching the company’s video, they were doing the linear drop test on soft foam material samples lying flat and measuring the impactor’s acceleration. Are they still doing the same tests? How are they testing the skateboard helmet, if you can’t lay that material flat?
The reason I ask is that the head acceleration is generally measured to indicate the severity of the impact to the head. The measured head accelerations are usually much less than the helmet accelerations (factor of ten less would not be uncommon). The head accelerations would be measured when an instrumented headform with a helmet is dropped onto a steel base, using the linear drop test.
But if they instrumented the helmet (or the drop weight), and the g-forces are on the helmet or the drop weight, the high g-forces are actually not surprising since the conventional skate boarding helmet is probably hard (at least the ones I see on the internet). Hard-hard contact will generate the higher g’s, while hard-soft contact will generate lower g’s.
The fact that they get a reduction in the acceleration still seems to be encouraging either way.
By “instrumented” I assume you mean the place where the accelerometer was located? They were dropping a 5kg steel head-shaped mass from 4.5 ft onto various flat samples – and the accelerometer was located in the head-shaped mass. The samples were flat versions of the same materials that would be used in helmets, and they were placed on a flat surface (they didn’t actually go out and buy the helmet from Wal-Mart).
When the test facility did a control (dropping the steel head-shaped mass directly onto the steel surface below with no padding at all) from about 1 ft, the measured peak was 1000 g’s.
Does this jibe with your expectations? Based on my reading of your e-mail, we shouldn’t be surprised at the high g-forces since the dropweight was instrumented and no human-analogue headform was used, and the samples tested lacked any curvature. (And yes, the skateboarding helmet they tested had a hard outer shell.)
After a few more back-and-forths, Dr. Matic clarified some other stuff further:
Hypothetically, keep in mind that if you had an instrumented human-analogue headform with an instrumented skull and an instrumented soft brain simulant, and it was wearing an instrumented helmet, if the helmet was measuring acceleration in the 1000’s of g’s, then the skull acceleration would be in the 100’s of g’s (similar to the solid headform) and the soft brain would only be in the 10’s of g’s. But when you talk about brain tissue itself, is it more sensitive.
Notice how you can have:
Hard-hard contact if you have a [metal headform-surface] impact test.
Hard-soft-hard contact if you have [metal headform-foam-surface] impact test.
Hard-hard-soft-hard contact if you have [metal headform-helmet foam-helmet shell-surface] impact test.
Soft-hard-soft-hard-soft contact if you have [headform brain-headform skull-helmet foam-helmet shell-surface] impact test
In the real human-helmet system, the hard protective components (skull and shell) try to protect the brain from penetrating contact injuries but don’t really decelerate things very well. The soft components (foam) are good at deceleration but not so good at preventing penetration.
Helmets try to make both work together do both jobs.
And based on all that info, and that I needed to keep the quote very short for our limited time on TV, I proposed the final draft of it:
So, all things considered, I feel comfortable using a slightly modified version of the quote you sent over: “A 1000 g head impact will almost certainly kill you — but at 285g, you’ll live. You might not walk away from it, but you’ll live.”
I know “1000 g head impact” isn’t really a thing, but I think omitting the word “acceleration” makes it more clear.
Let me know if you think this is OK.
And the final blessing from Dr. Matic:
I think your statement based on the performance of the test items as described are a quite reasonable conclusion.
It was my pleasure to help out. I enjoyed the discussions and learning about the concept.
Thanks Dr. Matic and NRL!