Biomaterials: An Introduction to Materials Science – Classes of Materials

This is the ceremonious first post to The Bioengineer.  I’m going to get straight to the heart/purpose of this blog, which is trying to bestow upon you, all that I know as a Bioengineer.

As is clear by the title of this post, the first lesson is going to be about Biomaterials.  In these first few weeks we will be covering biomaterials, and various biological systems and how engineers are interacting with them.

What are biomaterials?

Essentially, biomaterials are materials that are constructed in order to interact with biology, and natural cells and tissues.  It should be clear why biomaterials are important.  The body has a very limited ability to self-generate.  We humans aren’t like eartworms.  Our cells are highly, highly differentiated and specialized.  Simple example: if we lose an arm, that baby is gone.

We can think of biomaterials as a special class and subset of the more inclusive category of “materials”.  Thus, before we start to understand the nuances of biomaterials in particular, we need to develop a broad-based understanding of materials science.

Introduction to Materials Science

There are 4 general classes of materials: metals, ceramics, polymers and composites.

1. Metals – large numbers of non-localized electrons; good conductors, non-transparent, strong but deformable.
2. Ceramics – oxides, nitrides, carbides; clay, cement, glass; insulating; often hard and brittle
3. Polymers – long-chain molecules; synthetic and natural; range of properties
4. Composites – combinations of materials; filler in matrix

The distinctly different properties of each class can largely be attributed to the differences in bonding.  Ceramics are composed of atoms connected through ionic and covalent bonding; Metals through metallic bonding; and Polymers through covalent & secondary bonding.

To understand the effects of bonding we have to get through some terminology first.  Ionic bonding is described as one atom donating a valence electron to another atom.  Through this process 2 neutral atoms become 2 oppositely charged ions.  These ions are then attracted to each other due to their opposite charge, and this attraction creates an ionic bond.

Covalent bonding is when two atoms share electrons. 

Metallic bonding is a special type of bonding.  There is no such thing as a metallic bond (the singular is reserved for ionic, covalent, and hydrogen).  Metals are unique in that they have so many electron shells that their valence electrons are very loosely held down by their nuclei.  The electrons are free to move from one atom to another.  Thus, when we have slab of metal, we can visualize it on a microscopic level as who knows how many metal ions floating in a sea of electrons.

There are also a couple of “properties of bonding” that we need to familiarize ourselves with to quantitatively describe the different types of bonds:

• Bond length: describes the length of the bond, usually in angstrums
• Bond energy: the energy required to homolytically cleave the bond so that each atom gets half the elextrons; or the energy of the intermolecular force (for secondary bonding)
• Melting temperature: The temperature at which a compound will transition from a solid to a liquid ( or from a crystalline to an amorphous state)
  • Essentially, the larger the bond energy the higher the melting temperature.  This is because temperature is related to thermal energy.
• Coefficient of Thermal Expansion: the fractional change in length per degree of temperature change
  • The logic: as a material increases in temperature, this energy is stored in the intermolecular bonds, causing the length of the bonds to increase.  So materials will generally expand in response to an increase in thermal energy.  Hence the name.
  • Alpha = coefficient of thermal expansion.  L=length of material.  L0 = orignal length of the material.  T = temperature.

Now that we understand various properties of bonding, let’s related them to the four classes of materials. (What are they again?) Don’t worry, I already forgot also.  But after having looked them up, the four classes of materials are: ceramics, metals, polymers and composites.  In this exercise we won’t consider composites, because the properties of composites are highly variable.  They can be composed of any mixture of the other three materials.

Ceramics

• High melting temperature
• large bonding energy
• small coefficient of thermal expansion
• moderate density

Ceramics are generally characterized by ionic and covalent bonding.  These types of bonds have high bond energies and melting points.  Thus, we see a high bond energy and melting temperature in ceramics.  Ceramics have a moderate density as their ions often have special requirements for bonding (fourfold coordination for covalent bonds and charge neutrality for ionic solids)

Metals

• moderate melting temperature
• moderate bonding energy
• moderate coefficient of thermal expansion
• high density

Metals are often the densest packed of the three due to the metallic bonding and the minimal restrictions.  Also, metals are generally composed of atoms later on in the periodic table, with large atomic masses. 

Polymers

• low melting temperature
• small bonding energy
• large coefficient of thermal expansion
• low density

Polymers have the lowest density of the three classes.  Although single polymer chains are composed of several covalent bonds, these chains are held together to form the polymer solid through secondary bonds.  Secondary bonds are the weakest interactions, and thus have the lowest bond energy and melting temperature.

A general rule of thumb is that the weaker the intermolecular interaction, the longer the length of that interaction.  For example, a triple-bond is much shorter than a single-bond.  And accordingly, the polymer chains are kept at a further distance from each other by these secondary bonds than atoms are in metals and ceramics.