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We’ve talked about different materials engineers use to build things in the world, but there’s a special category of materials they turn to when building things to go inside our bodies. In this episode we’ll explore the world biomaterials like titanium and their coatings, the special chemistry of polyurethane, and the cross-linked structure of hydrogels. We’ll also look at the importance of safety & research, as well as the enormous future potential of biomaterials.

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CC Kids:
The efforts of engineers are all around you.

Take a stroll down the street and you’ll spot one feat of engineering after another. But what you can’t see are all the feats of engineering people are carrying around inside their bodies.

Because engineering isn’t just about what’s around us; it’s also about what’s inside us. From medical implants to artificial limbs and the bandages that cover your cuts, so many things have been designed from a special set of materials that work well with the human body. These biomaterials are vital to medicine, healthcare, and the world as we know it. [Theme Music] It takes special effort to design things that work directly with the human body.

Technically, you could try and use any material as a bio-material, but to be successful, it needs to be compatible with the biological system it’s interacting with. All the materials we’ve talked about in this course – metals, ceramics, polymers, and composites – and even living cells and tissues can be candidates for biomaterials, as long as they have the right skills for the job. That’s where bio-compatibility comes in.

That’s a general term for how compatible a material is with living tissue. It’s not only an important feature of biomaterials, it’s what makes something a bio-material. There are four main ways a material can interact with your body: it can hurt you; it can dissolve and be replaced by cells; your body can surround it with a protective layer; or it can bond with your living tissue.

Biocompatible materials are the ones that result in healthy bodily responses. They don’t tend to cause blood clots or infections, and lead to normal, uncomplicated healing. Which can be really important, because for most applications, your biomaterials will at least partly come in contact with bodily tissues or cells, and often for long periods of time.

Months. Years. Maybe decades.

Over the years, the materials we consider biocompatible have changed a lot – from the animal tissue used by ancient Egyptians to stitch wounds, to the wood used in olden-day peg legs, to the carefully-engineered materials we use today. Some of the more prominent ones today are titanium and stainless steel, used everywhere from joint replacements to dental implants. There’s also polyurethane, a type of polymer used in catheters, artificial heart valves, and other flexible devices, and hydrogels, for things like contact lenses.

Each type has different properties that make it useful as a bio-material. We’ve already talked about one use for alloys, like titanium and stainless steel, for example: braces. Titanium and some other alloys can be classified as biologically inert materials, which means they cause little or no reaction with nearby tissues.

Your body may recognize them as foreign materials and try to surround them in fibrous tissue, but it doesn’t outright reject them. In the US alone, over four million people wear braces, many of whom have the metal kind. And that’s not counting all the people who have already spent a couple of years of their lives with alloys digging into their gums.

You’ll also find these alloys in plenty of other things that we put in people’s bodies for an extended period of time. Titanium is often used in place of stainless steel because it lacks things like nickel that can sometimes cause allergic reactions. It also has a lower density compared to other metallic biomaterials, so a titanium implant will be lighter than a similar one made from stainless steel.

And when it comes to artificial body parts, “lighter” usually translates to “much more comfortable.” But both titanium and stainless steel have excellent mechanical properties like strength, resistance to bending, and durability. That’s why they’re used for things that tend to get a lot of wear and tear, like teeth and joints. Titanium is also often used to surround things that wouldn’t be biocompatible otherwise – a pacemaker, for example.

On their own, having a battery and electrical circuits inside you wouldn’t be great. Surround them with a biocompatible material like titanium, though, and it becomes much safer. Sometimes, you can change the surface of a material to do even better than just not causing harm.

For example, sometimes implanted devices that come into direct contact with blood, like pacemakers, cause dangerous blood clots or narrowing of blood vessels as the person’s body tries to heal around them. But scientists have found that coating the titanium in a mixture of collagen proteins and a blood thinner called heparin could someday help prevent those complications. The research is still in the early stages, but the fibrous tissues that form around implants are often collagen-based, so coating the titanium in the same proteins might speed up healing.

Meanwhile, the blood thinner might prevent clots. Other types of titanium coatings are already used for surgeries where you need to get an implant to bond with the surrounding bone. Some knee replacements, for example, are coated with things like proteins and silver, which give the alloy the ability to bond with living tissue and discourage bacterial growth.

With things like knee replacements, that can be really important, because for the surgery to be successful, you typically need to get the implant to bond with the surrounding bone. So when it comes to bio-compatibility, alloys like titanium are super useful. And with some clever engineering, you can make them even better.

Sometimes, though, you need a material that’s a little more flexible, like for a heart valve. And for that, engineers often turn to polyurethane. Like other polymers, polyurethane is made up of long, repeating chains arranged into larger molecules.

But the smaller segments of the polymer alternate between hard and soft, which gives it a super useful set of properties. Polyurethane has a high level of elasticity, so it’s pretty flexible, but it’s also durable and resistant to tearing. Combine these properties and you end up with a bio-material that performs well under both static and dynamic loads.

More importantly, though, those hard and soft segments make polyurethane biocompatible. In the 1980s, researchers discovered that the molecules in the softer segments helped make the polymer more blood-friendly because the platelet cells that cause clots didn’t stick to them as much. Since then, engineers have worked to make polyurethanes with even better bio-compatibility by changing the chemical structure of the hard segments so they don’t accumulate platelets as much, either.

But polyurethane isn’t the only polymer that’s widely-used as a bio-material. There are also hydrogels, materials made up of things called hydrophilic structures, cross-linked polymers capable of holding large amounts of water. Think of the polymers we’ve talked about before, with those long chemical chains, but now they’re linked repeatedly between the chains, forming a network-like structure.

That network can then swell with water to fill in the gaps between the chains, leading to some interesting properties. For example, one of the most widely used hydrogels is something called PHEMA, a material that’s pretty unreactive and supportive of biological processes. It’s tough and doesn't degrade easily, isn’t absorbed by the body, and it can take on many different shapes and forms, making it the sort of Swiss army knife of biomaterials.

These characteristics are what make PHEMA hydrogels so well-suited for contact lenses. There’s even research on using them to deliver medicine to the eye! In fact, the unique chemical structure of hydrogels in general could make them ideal for delivering drugs across the body.

Say you swallow a hydrogel capsule filled with medication that you might normally take in a shot. When the pill enters your body, the water inside you will try to fill it up. The hydrogel will take in the fluid and swell up to maintain its equilibrium, effectively pushing out the drug into your body and releasing it where it needs to go.

And if you use a hydrogel that your body can gradually absorb, it will just dissolve after it’s done its job. The potential for future hydrogel applications is vast, including wound-healing bio-adhesives, artificial skin and cartilage, sexual organ reconstruction, and even vocal cord replacements. So far, we’ve mostly talked about building devices to put in the body.

But biomaterials can be used to help boost the body’s existing behaviors, too. For example, some researchers are working on a gel that can that can stick to your bodily tissues and help heal them faster. This gel can help prevent internal blisters and infections by filling the dead space that’s created by surgeries like tumor removals.

After your body has the time to heal, the gel degrades and is absorbed. On a much smaller scale – like, the size of individual cells – researchers are also looking into biomaterials that could deliver things like DNA directly into cells they otherwise couldn’t get into. By creating biomaterials that mimic viruses, you can trick cells into taking in these molecules, like some kind of cellular Trojan horse.

We might even be able to use them for cancer vaccines! But like anything else in science, and especially medicine, there are certain protocols bio-material engineers are expected to follow to make sure what they’re designing is more than just effective – it has to be safe. For example, early research might include testing the material on cells outside of the body, to get an idea of how biocompatible it is without risking anyone’s life.

Other times, it might involve animal testing, to see how a material affects the physiology of other animals before trying it on humans. But the proper precautions aren’t always taken, and there have been some cases where devices that were said to be safe caused reactions as mild as a rash or as serious as death. Like the Dalkon Shield, an early IUD contraceptive.

When it was introduced in the late 1960s, the Dalkon Corporation marketed the Shield as superior to other IUDs on the market. They said it had a lower failure rate, at about 1%, and that it was easier and less painful to insert and remove. And then people started getting infections.

Which was weird, because the uterus is normally sterile. By 1974, it had caused problems in tens of thousands of people, including at least 18 deaths. Later investigations revealed that the culprit was the tail string attached to the IUD to help remove it.

The string was made of a bundle of thin nylon fibers with a shell of nylon around them, and it acted like a wick, drawing bacteria up into the uterus. Turns out, the parent company that bought Dalkon had known about this problem the whole time. When one of their quality control employees insisted they address it, the company fired him instead – something that would be illegal in the US today.

On top of that, there were so many flaws in the human safety trial for the IUDs that it would take me another ten minutes just to list them all. We’ll discuss the ethics of engineering in more detail in a later episode, but what happened with the Dalkon Shield was a clear violation of even the most basic safety protocols. They released a product they knew could be dangerous, didn’t tell anyone about the risk, and hoped for the best.

That plan didn’t work out well for anyone. Biomaterials can be an incredibly powerful tool for improving people’s lives. But as engineers, it’s up to us to wield that power responsibly.

So today we learned all about biomaterials. We talked about alloys like titanium and their coatings, the special chemistry of polyurethane, and the cross-linked structure of hydrogels. We also looked at ways to use biomaterials to boost the body’s existing behaviors, as well as their enormous future potential.

Finally, we talked about the importance of safety research. I’ll see you next time, when we’ll shift directions a bit and learn about process control. Crash Course Engineering is produced in association with PBS Digital Studios.

If you want to keep learning new things, check out It’s Okay to be Smart, which is all about our curious universe and the science that makes it possible, hosted by Joe Hanson. Check out It's Okay to be Smart and subscribe at the link below. Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.