It is important to note here that we have only posed a few generic questions; we could
have asked additional questions had we considered the specifics of the application. For example,
when selecting materials for implants in bioengineering applications, one must consider many
additional factors, including: Is the material toxic to the body? Can the material be sterilized?
When the material comes into contact with body fluid, will it corrode or deteriorate? Because
the human body is a dynamic system, we should also ask: How would the material react to
mechanical shock and fatigue? Are the mechanical properties of the implant material compat-
ible with those of bone to ensure appropriate stress distributions at contact surfaces? These are
examples of additional specific questions that one could ask to find suitable material for a specific
application.
By now it should be clear that material properties and material cost are important
design factors. However, in order to better understand material properties, we must first
understand the phases of a substance. We discussed the phases of matter in Chapter 9; as a
review and for the sake of continuity and convenience, we will briefly present the phases of
matter again here.The Phases of Matter: Solids, Liquids, Gases, and Plasma
As we discussed in Chapter 9, when you look around, you will find that matter exists in various
forms and shapes. You will also notice that matter can change shape when its condition or its sur-
roundings are changed. We also explained that all solid objects, liquids, gases, and living things
are made of matter, and matter itself is made up of atoms or chemical elements. There are 106
known chemical elements to date. Atoms of similar characteristics are grouped together and
shown in a table — the periodic table of chemical elements. Atoms are made up of even smaller
particles we callelectrons,protons,andneutrons. In your first chemistry class, you will study these
ideas in more detail, if you have not yet done so. Some of you may decide to study chemical
engineering, in which case you will spend a great deal of time studying chemistry. But for now,
remember that atoms are the basic building blocks of all matter. Atoms are combined naturally,
or in a laboratory setting, to create molecules. For example, as you already know, water molecules
are made of two atoms of hydrogen and one atom of oxygen. A glass of water is made of billions
and billions of homogeneous water molecules. A molecule is the smallest portion of a given mat-
ter that still possesses its microscopic characteristic properties.
Matter can exist in four states, depending on its own and the surrounding conditions:
solid, liquid, gaseous, or plasma. Let us consider the water that we drink every day. As you
already know, under certain conditions, water exists in a solid form that we callice.At a stan-
dard atmospheric pressure, water exists in a solid form as long its temperature is kept under 0C.
Under standard atmospheric pressure, if you were to heat the ice and consequently change its
temperature, the ice would melt and change into a liquid form. Under standard pressure at sea
level, the water remains liquid up to a temperature of 100C as you continue heating the water.
If you were to carry out this experiment further by adding more heat to the liquid water,
eventually the liquid water changes its phase from liquid into a gas. This phase of water we
commonly refer to assteam. If you had the means to heat the water to even higher temperatures,
temperatures exceeding 2000C, you would find that you can break up the water molecules
into their atoms, and eventually the atoms break up into free electrons and nuclei that we call
plasma.554 Chapter 17 Engineering Materials
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