Biomedical Engineering Reference
In-Depth Information
to calcification, which is enhanced on the natural collagen surface as compared to the
smooth pyrolitic carbon. As should be now evident, the biomaterial scientist has a similar
job to a chef in that many different dishes can be created from the same basic ingredients;
it just depends on what components are added and how they are “cooked.” Many different
types of specialized equipment are used in this field.
5.5.3 Biomaterial Scaffold Fabrication Techniques (3-D)
The goal of a scaffold is to recreate important aspects of the cell microenvironment that
will allow cell proliferation, migration, differentiation, and synthesis of a new extracellular
matrix. When this combination of events occurs, new tissue can be regenerated. This is
important clinically for attaching/integrating host tissue into a synthetic body part replace-
ment, such as a total hip replacement implant, or for regenerating new tissue where none
remains. If a large enough wound has been created that cannot be filled with nature's scaf-
fold—a blood clot—then it is necessary to use a synthetic scaffold. Synthetically produced
biomaterial scaffolds are produced commercially and are made of ceramics and/or poly-
mers with or without a tissue-stimulating biological molecule and with or without cells.
Combining growth factors, cells, and biomaterial scaffolds is the premise behind tissue
engineering (see Chapter 6).
One of the most critical elements of the scaffold biomaterial is that it mimics the extracel-
lular matrix that normally serves to maintain space, support cells, and organize cells into
tissues. The section on surface chemistry in this chapter gave an example of how mimicking
components of the adhesive proteins of the ECM can enhance cell attachment and differen-
tiation. This section focuses on the three-dimentional structural and physical characteristics
of the extracellular matrix scaffold that appear to be critical to imitate in synthetic scaffolds
in order to stimulate cells and lead to the functional regeneration of tissues.
Pore size is a very important parameter of biomaterial scaffolds used for tissue regener-
ation. Through trial and error, optimal ranges of pore sizes have been determined for differ-
ent tissues and for different biomaterials. Some rules of thumb are used, such as the pores
must be at least 5-10
m for a cell to fit through. Successful bone scaffolds typically have
pores that traverse the full thickness of the scaffold and are 100-250
m
m in size. New blood
vessel formation, or neovascularization, has been shown to require pores within polymer
scaffolds that are between 0.8 and 8
m
m and to not be possible within polymers with pores
m
less than 0.02
m. Typically, the acceptable pore size in polymers is smaller than in ceramics
or metals, perhaps due to pore size expansion, which can occur in the body due to degra-
dation or swelling of the polymer.
The pore size determines many aspects of the scaffold, such as mechanical strength; deg-
radation rate; permeability to gases, fluids, and nutrients; and extent of cell ingrowth.
Interconnected porosity is essential for tissue engineering applications requiring nutrient
diffusion and tissue ingrowth. Early total joint replacements had smooth surfaces, but today
rough, porous coatings or grooved surfaces are used on hip implants to achieve bony
ingrowth. The hip and knee implants in Figures 5.2 and 5.3 have rough, porous coatings.
The porous coatings on orthopedic implants are achieved by partial fusion of small metallic
spheres to the implant surface. The interparticle spaces are the pores. The pores, as well as
grooves, seem to encourage bone cells to migrate into or along them. It has been observed
m
