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[INTRO♪]

If you've ever given someone a piggyback ride, you know that even tiny humans can feel

heavy.

But we don't notice the weight of our own bodies.

And there's a song that explains why:

The knee bone is connected to the thigh bone.

The thigh bone is connected to the hip bone… and so on.

Prosthetic limbs generally feel much heavier than biological ones, because they aren't

connected to your skeleton.

Nowadays, though, engineers are trying to use the body's natural bony support system

to make prosthetics even better.

Your bones are connected to each other with ligaments.

And these ligaments have mechanoreceptors that help you feel how your joints are moving.

But they aren't able to sense how heavy things are.

That job goes to the Golgi tendon organs, which are mechanoreceptors in the tendons

connecting all your muscles to bones.

Because your leg is connected to your skeleton, your muscles don't have to provide that

much force to hold things in place.

So you don't really notice how how heavy your leg is — even though just one of them

makes up about 16% of your total weight.

Most prosthetic devices are attached with a socket held in place with a harness or suction.

Harnesses limit the range of motion, and the suction of the socket activates the Ruffini

corpuscles, which are mechanoreceptors that are sensitive to stretching skin.

That's why prosthetic limbs can feel like they're hanging onto the body, instead of

connected to it.

And if you had a prosthetic leg that was as heavy as a biological leg, it might feel like

walking around with like a toddler hanging on to you.

So scientists have had to design lighter prostheses, sometimes at the cost of function.

But as a different solution, they've been working on a way to connect prosthetic limbs

to the skeleton, using a technique called osseointegration.

Osseointegration is where bones grow into foreign materials.

Humans have actually been doing this for a long time, like in 600 CE when the Mayans

used mother of pearl to make teeth that integrated into the jawbone.

But it wasn't until 1965 that Per-Ingvar Brånemark used osseointegration to put titanium

dental implants in the first human patient.

Titanium is super biocompatible.

When there's oxygen around, it makes an oxide film that prevents lots of other chemical

reactions — so it doesn't, like, break down inside your body.

At the same time, it activates the immune system some, so the cells that are normally

responsible for bone resorption slow down, and new bone grows around the titanium fixture.

These titanium implants are made with lots of nooks and crannies that bone cells can

grow into, so they stay secure for decades.

Since dental implants became a thing, lots of researchers have applied osseointegration

to prosthetic limbs too.

All patients have to do is clamp their prosthesis onto the titanium implant, and they're good

to go.

Because there's no harness, there's more range of motion.

And because there's no suction socket, the prosthesis doesn't feel like it's pulling

on your skin.

There is one drawback: a metal rod sticking out of your skin is an open wound that you

have to take care of, or it can get badly infected.

Even though this technique is still experimental, nearly 1000 patients have undergone this surgery

for their prosthetic legs, arms, and even thumbs.

Researchers are working on ways to address the risks, and the technology can only improve

from here.

And we'll keep learning along with you here on SciShow!

If you're curious about other ways engineers are working with human bodies, check out our

episode about human interface technologies!

And if you want to delve into all kinds of science with us,

you can go to youtube.com/scishow to subscribe. to subscribe.

And ring that bell, I guess?

For more infomation >> Why Do Prosthetic Limbs Feel Way Heavier Than Biological Ones? - Duration: 3:27.

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Why Are Republicans Telling Democrats To Disown The Progressive Left? - Duration: 4:04.

Former FBI director James Comey made waves a couple days ago when he tweeted out the

Democrats don't need to lose their heads, and they need to avoid all these people who

are saying that we have to push the part to the left.

These are statements that have been echoed by other moderate anti-Trump Republicans throughout

the country in recent weeks.

These have been spurred in part by the victory of Alexandria Ocasio-Cortez.

They're worried that there's this big populous progressive wave through the United States

where people who actually represent the will of the America people are winning primaries

and potentially winning seats in the House of Representatives or the Senate.

So, we have right now all of these so-called moderate Republicans, and of course Joe Lieberman,

out there telling Democrats, "Whoa.

Hold your horses, guys.

Stop moving too far to the left.

Come back to the center."

Now, I don't know about you, but as a Democrat, I'm still a Democrat.

I have no intention of leaving at the moment.

If a Republican tells me how to run my own party, I'm probably going to go ahead and

do the opposite of whatever it is they tell me.

Why?

Because they have no interest in the Democratic party being successful.

What Republicans are trying to do, people like James Comey, people like these other

anti-Trump Republicans, like Steve Schmidt is another great example, they want to tell

us that we can't move too far to the left.

Go back to the center be centrists.

We know it costs you over a thousand seats over the last eight years, but trust me.

No, no, no.

It'll totally work this time.

Republicans right now, these disaffected, anti-Trump Republicans are trying to create

a new party, a new movement, somewhere where they fit in, you know, a party that's not

100% racist like Donald Trump, but still does whatever corporations want them to do, and

they found it.

They have found it with centrist Democrats.

That's why all of these Republicans are warning the party to not move further to the left

because they want to keep it in the middle so it's something that feels familiar to them,

something they can hitch their wagon to, and something that they can be a part of and possibly

run for office as a part of.

They want to make the centrist Democrats, the corpus Democrats into the new Republican

Party, and we can't let them do that.

There is a progressive wave sweeping through this country right now.

Maybe it's one or two candidates, maybe it's four or five, maybe it's 100, but when these

people have momentum, regardless of the party, as long as they're out there living their

story, not lying to us, and telling us that they're going to do the things that the majority

of people in this country actually want, you know, living wages, medicare for all option

for us to have, reducing student loan debt, protecting the environment, those aren't radical

far left ideas.

That's what a majority of people in this country from both political parties want.

Just because these people call themselves Democrats or they're self-described socialists,

what they really are are politicians that are listening to the people, that believe

in the people, and they put their faith in the people, unlike the centrist Democrats

and these moderate Republicans who want to ignore the people and put all of their faith

in corporations.

That hasn't worked very well for the Democrats in recent history, and we have no reason whatsoever

to believe that it's going to work for them in the 2018 midterms.