Mineralized tissues facts for kids

Mineralized tissues are special parts of living things that mix minerals with soft body parts. Think of them like natural armor or strong support structures! You can find them in many places, like your bones and teeth, or in the shells of snails and clams. Even some deep-sea sponges and tiny ocean creatures called radiolarians have them.

These amazing tissues have become super strong and tough over millions of years of evolution. Scientists study them a lot to learn how nature builds such incredible materials. This field is called biomimetics, where we get ideas from nature to create new materials. Mineralized tissues are great examples because they are light, strong, and can do many jobs at once.

They get their strength from combining hard minerals (the non-living part) with soft protein networks (the living part). There are about 60 different minerals found in living things, but the most common ones are calcium carbonate (in shells) and hydroxyapatite (in bones and teeth). Even though minerals can seem fragile, mineralized tissues are actually 1,000 to 10,000 times tougher than the minerals alone! Their secret is a super organized, layered structure. This layering helps spread out forces and stop cracks from forming, making them very strong.

Two well-known examples of these tough natural materials are nacre (the shiny inner layer of some shells) and bone. Scientists use special tools like nanoindentation and atomic force microscopy to study them closely. While human-made materials are still catching up, new ways to create similar strong materials are being developed. Not all mineralized tissues are good for you, though. For example, kidney stones are mineralized tissues that form when your body isn't working quite right. Understanding how these tissues form, a process called biomineralization, helps us learn about diseases too.

How Mineralized Tissues Evolved

Scientists have wondered for a long time how mineralized tissues first appeared. One idea is that the first animal tissues to become mineralized were either in the mouth parts of ancient fish-like creatures called conodonts, or in the skin armor of early jawless fish called agnathans. This skin armor, called the dermal skeleton, was made of surface dentin and bone, sometimes covered by a hard layer like enamel.

It's thought that this dermal skeleton eventually turned into scales, which are similar to teeth. The first true teeth appeared in sharks and rays (chondrichthyans) and were made of all three parts of the dermal skeleton. Later, the way mammals form mineralized tissues developed further in bony fish (actinopterygians and sarcopterygians). Studying the genes of ancient jawless fish might help us understand even more about how these amazing tissues evolved.

Amazing Layered Structures

Mineralized tissues have special layered structures, called hierarchical structures, that give them their incredible strength. These layers exist at different sizes, from what you can see with your eyes down to tiny parts you need a powerful microscope to see. Let's look at how these layers work in nacre and bone.

Nacre: The Super Strong Shell Layer

Nacre, also known as mother-of-pearl, has several levels of amazing structure.

Big Picture: The Macroscale

Some mollusc shells have two layers to protect them from predators. The inner layer is nacre, and the outer layer is made of calcite. The calcite layer is hard and stops things from breaking through, but it can crack easily. Nacre, on the other hand, is softer and can bend a little without breaking, making it much tougher. Nacre is mostly made of a mineral called aragonite (a type of calcium carbonate), which makes up 95% of it. The other 5% is a soft, organic material (like proteins) that makes nacre 3,000 times tougher than aragonite alone! Nacre also has weaker lines called "growth lines" that can help stop cracks from spreading.

Tiny Details: The Microscale

Imagine a tiny brick wall. In nacre, the "bricks" are flat, microscopic tablets of aragonite, about 0.5 micrometers thick and 5-8 micrometers wide. The "mortar" holding these bricks together is a super thin layer (20-30 nanometers) of organic material. These tablets aren't perfectly flat; they're actually a bit wavy. This waviness is important because it helps the tablets lock together when pulled, making the material even stronger and harder to break.

Even Tinier: The Nanoscale

At the smallest level, we find the 30-nanometer-thick layer of organic material that "glues" the aragonite tablets together. We also see tiny aragonite grains that make up the tablets themselves. This organic "glue" is made of proteins and chitin.

So, to sum up nacre's amazing structure:

• On the big scale, you have the whole shell with its two layers (nacre and calcite) and the growth lines inside nacre.
• On the micro scale, you have the stacked aragonite tablets and their wavy surfaces.
• On the nano scale, you have the organic material connecting the tablets and the tiny grains within the tablets. All these layers work together to make nacre incredibly strong!

Bone: Your Body's Strong Framework

Like nacre, your bones also have a complex layered structure that helps them be strong and light. The mineral in bone is mainly hydroxyapatite, and the organic part is mostly collagen protein.

Big Picture: The Macroscale

At this level, you can see two main types of bone: compact bone (which is dense and solid) and spongy bone (which has a more open, porous structure). These are visible at the scale of several millimeters to centimeters.

Tiny Details: The Microscale

Inside compact bone, you'll find tiny cylindrical units called osteons, which are like small tubes. These are about 100 micrometers to 1 millimeter in size. Looking even closer, at 5 to 10 micrometers, you see the actual structure of these osteons and the small supporting struts in spongy bone.

Even Tinier: The Nanoscale

At the nanoscale, we find even smaller structures. There are fibrils (tiny fibers) and spaces between them, a few hundred nanometers in size. Then, at the smallest level (tens of nanometers), are the basic building blocks: tiny hydroxyapatite mineral crystals, cylindrical collagen molecules, other organic molecules like fats and proteins, and water. This amazing layered structure, found in all mineralized tissues, is key to their strength and performance.

The Mineral Part: What Makes Them Hard

The mineral is the non-living part of mineralized tissues that makes them hard and stiff. Besides hydroxyapatite and calcium carbonate, other minerals like silica can also be found. In mollusc shells, these minerals are carried to the building site inside tiny sacs within special cells. While inside these sacs, the minerals are soft and shapeless. But once they leave the cell, they become stable and form crystals. In bone, studies show that calcium phosphate starts to form inside tiny holes in the collagen fibers, then grows to fill those spaces.

The Organic Part: What Makes Them Tough

The organic part of mineralized tissues is made of proteins. In bone, for example, this is mainly the protein collagen. The amount of mineral in these tissues varies; the harder the tissue, the less organic material it usually has. However, without this organic part, the material would be very brittle and break easily. So, the organic part is super important because it makes the tissues tough and flexible.

Many proteins also act like tiny managers in the mineralization process. They can help minerals start forming (nucleation) or stop them from forming too much. For instance, the organic part in nacre helps control how the aragonite crystals grow. Some important proteins in mineralized tissues include osteonectin and osteopontin. In nacre, the organic material is porous, allowing tiny mineral "bridges" to form, which helps the nacre tablets grow in an organized way.

How Minerals Form in Nature

Understanding how living things create these mineralized tissues helps us try to build them artificially. While we still have questions about some details, we have some good ideas about how mollusc shells, bones, and sea urchin skeletons form.

Mollusc Shell Formation

The main parts involved in building a mollusc shell are:

• A sticky silk-like gel.
• Proteins rich in aspartic acid.
• A chitin framework (a tough, sugar-based material).

Here's how it generally works:

• First, the silk gel fills the space where the mineral will form.
• The organized chitin framework helps guide the direction of the growing crystals.
• The different parts of the shell-building material are kept separate in specific places.
• The mineral first appears as a soft, shapeless calcium carbonate.
• Once the mineral starts to form crystals on the framework, the calcium carbonate hardens.
• As the crystals grow, some of the acidic proteins get trapped inside them.

Bone Formation

In bone, mineralization starts from a mix of calcium and phosphate ions. The mineral begins to form inside tiny holes in the collagen fibers as thin layers of calcium phosphate. These layers then grow to fill the available space. Scientists are still studying exactly how the minerals are placed within the organic part of the bone. Some ideas are that it happens because calcium phosphate solution precipitates, or because biological inhibitors are removed, or due to the interaction of calcium-binding proteins.

Sea Urchin Embryo Skeleton

The sea urchin embryo is often studied to understand development. Its larvae form a complex inner skeleton made of two tiny spicules. Each spicule is a single crystal of calcite (another form of calcium carbonate). This calcite forms when a softer, shapeless calcium carbonate transforms into a more stable crystal. So, two different mineral forms are involved in building the larval spicule.

The Connection Between Organic and Mineral Parts

The way the mineral and protein parts connect, and the forces between them, are very important for how tough mineralized tissues are. Understanding this "organic-inorganic interface" helps us understand their amazing strength.

For example, in nacre, it takes a very strong force to pull the protein molecules away from the aragonite mineral, even though they aren't chemically bonded. Scientists use computer models to study how this interface behaves. Models show that when the material is stretched, the way the soft material deforms helps make the tissue harder. Also, tiny bumps on the tablet surfaces help resist sliding, making the material stronger. The waviness of the tablets helps them lock together, spreading out large deformations and preventing cracks.

Diseased Mineralized Tissues

In animals with backbones (vertebrates), mineralized tissues can sometimes form in unhealthy ways, not just through normal body processes. Some diseases where you see mineralized tissues include hardened arteries (atherosclerotic plaques), certain types of calcium deposits (tumoral calcinosis), a muscle disease called juvenile dermatomyositis, and stones in the kidneys or salivary glands. Most of these unhealthy deposits contain hydroxyapatite or a similar mineral.

Scientists use tools like infrared spectroscopy to find out what kind of mineral is present and how the mineral and other parts of the tissue have changed due to the disease. Also, there are special cells called clastic cells that normally break down mineralized tissue (like old bone). If these cells are out of balance, it can lead to diseases. In dentistry, studies look at how the mineral in dentin (the part of the tooth under the enamel) changes with age, leading to "transparent" dentin. It seems a "dissolution and reprecipitation" process causes this. Further studies on mineralized tissues might help us find the causes and cures for these conditions.

This image shows hardened arteries. The orange parts are dense calcium phosphate particles, and the green parts are the softer surrounding tissue.

Bioinspired Materials: Learning from Nature

Natural materials that combine hard and soft parts in clever, layered ways often have amazing properties. For example, materials like Bone, Nacre, Teeth, Silk, and Bamboo are light, strong, flexible, tough, resist breaking, and can even repair themselves! The main idea behind these materials is that the stiff parts give them strength, while the soft parts "glue" everything together and help transfer stress. Plus, the soft parts can deform a little without breaking, which helps the material resist cracks. Nature has perfected this strategy over millions of years, giving us great ideas for building the next generation of strong materials. Here are some ways scientists try to copy these natural designs:

Large Scale Models

One way to copy nacre's toughness is to make large models. Nacre is tough because cracks get stopped by the weak layers between its hard "bricks." Scientists try to copy this by making layered ceramic "bricks" held together by a weak "glue." These models can make brittle ceramics much tougher. Since nacre's wavy structure also helps, other models copy that waviness to spread out damage.

Biomimetic Mineralization

All hard materials in animals are made through a process called biomineralization, where special cells deposit minerals onto a soft protein framework. So, copying this process, called biomimetic mineralization, is a good way to build strong synthetic materials. The idea is to start with organic frameworks that have spots where ions can attach, encouraging minerals to grow there. This creates a composite material where the mineral acts as a super strong, wear-resistant surface, and the soft organic framework provides a tough base that can handle stress.

Ice Templating / Freeze Casting

This is a cool new method that uses how ice forms to create layered materials. Imagine freezing a ceramic mixture: as the ice crystals grow, they push the ceramic particles into layers. After the water is removed (sublimation), you're left with a layered ceramic structure that looks like a negative of the ice. Then, you can fill this structure with a soft material to create a hard-soft layered composite. This method is used to make super strong hydrogels, metal/ceramic, and polymer/ceramic materials that look like bricks and mortar. The "brick" layer is strong but brittle, and the soft "mortar" layer between them allows for a little bending, which helps relieve stress and makes the material more flexible without losing too much strength.

Additive Manufacturing (3D Printing)

Additive manufacturing, also known as 3D printing, uses computer designs to build structures layer by layer. Recently, many bioinspired materials with complex layered patterns have been made using this method, with features ranging from tiny (tens of micrometers) to super tiny (submicrometers). This means that cracks can only happen and spread on a very small scale, which prevents the whole structure from breaking. However, making these complex layered materials, especially at the nano and micro scales, can take a lot of time, which limits how much they can be used in large-scale manufacturing.

Layer-by-Layer Deposition

As the name suggests, this technique builds materials by adding layers one by one, like making a layered cake. Scientists have tried this to create materials similar to nacre, for example, by alternating layers of hard and soft materials like TiN/Pt. However, materials made this way don't always have the segmented (separated) layered structure seen in nacre. To fix this, a method called sequential adsorption has been suggested, where electrolytes are repeatedly adsorbed and rinsed, creating multiple layers.

Thin Film Deposition: Microfabricated Structures

This method focuses on copying the cross-layered structure of a conch shell, which is known for its highly organized structure, rather than nacre's layered structure. It uses microelectromechanical systems (MEMS) technology. In this process, the natural mineral aragonite and organic matrix are replaced by polysilicon and a material called photoresist. MEMS technology repeatedly deposits a thin silicon film. The interfaces are then etched away and filled with photoresist. Although MEMS technology is expensive and takes more time, it allows for very precise control over the shape of the material, and many samples can be made.

Self-Assembly

The self-assembly method tries to copy not just the properties of natural materials, but also how they are made. It uses raw materials found in nature to carefully control how minerals start to form and grow. This process happens at low temperatures and in water. Self-assembling films can act as templates that help ceramic phases form. However, a downside of this technique is that it can't create the segmented layered structure that is important in nacre for stopping cracks. So, this method needs more research to fully mimic nacre's complex structure.

The Future of Mineralized Tissues

Scientists have learned a lot about mineralized tissues, but there's still more to discover. We don't fully understand which tiny features are most important for their amazing strength. Also, we need to better understand how these materials behave under different forces. For nacre, the exact role of some tiny grains and mineral bridges still needs more study.

To successfully copy mollusc shells and other mineralized tissues artificially, we need to learn even more about all these factors, especially choosing the right materials that will give the best performance. And, any new technique for making these materials must be affordable and easy to produce on a large scale.

See also

• Shell growth in estuaries
• Biomineralization