Biomedical Engineering Reference
In-Depth Information
single-stranded DNA. Subsections of the twisted DNA strand (146 base pairs each) are then
subcoiled around protein (histone) cores to form chromatosomes that cause the strand to
resemble a beaded necklace. The linked chromatosomes are then coiled to generate a
shorter, thicker fiber. The thicker fiber is then coiled upon itself to further shorten its length
to form the chromatid, two of which are linked together to form a chromosome.
Hierarchical design leads to a very efficient structure with maximum capability per
ounce of material. In bone, the multiple layers each have an interface between them that
leads to fracture resistance. In muscle, the many single fibers aligned and pulling together
demonstrate inordinate strength. The amount of information contained in our DNA is
staggering. Hierarchical design is thus another important concept that should be mimicked
in biomaterials design.
5.3.3 Biomineralization
Bones, antlers, teeth, coral, eggshells, and seashells are all examples of biomineralized
tissues. Although they serve different functions and have considerably different external
shapes, from the biomaterials perspective, they are all composed of numerous small, isolated
calcium phosphate or calcium carbonate crystals that are held together by a protein matrix.
The nucleation and growth of the mineral crystals are highly regulated by the organic protein
component that is secreted by cells and self-assembles to provide a template for the mineral
growth and nucleation. Three general processing principles for biomineralized tissues have
been identified that have significant implications for material scientists and engineers:
1.
Biomineralization occurs only within specific subunit compartments of microenvironments,
which implies stimulation of crystal production at certain functional sites and inhibition
or prevention of the process at other sites.
2.
A specific mineral phase is produced with a defined crystal size (frequently in the
nanometer range), shape, and orientation.
3.
Macroscopic shape forming is accomplished by packaging many incremental units
together, which results in unique net-shaped composites with hierarchical microstructures
that impart exceptional material properties. Point number 3 is another example of how
nature uses a bottom-up approach to assemble a complex tissue. Another powerful feature
of biomineralization is that, in most systems, remodeling of the original mineral structure
occurs as needed to optimize strength, accommodate organism growth, maintain mineral
ion equilibrium, and effect repairs. The initial biomineralized structure that is formed
after injury typically has a looser structure that restores some level of function, but is
not optimal. This is followed by a remodeling phase, so the biomaterials scientist may
not need to recreate the final tissue and could instead allow the cells to form the final,
most optimal, tissue over time.
In bone, controlled mineral nucleation and growth are accomplished within the micro-
compartments formed by the collagen matrix. The Type I collagen molecules secreted
by osteoblasts self-assemble into microfibrils with a specific tertiary structure having a
67-nm periodicity and 40-nm gaps or holes between the ends of the molecules (Figure 5.8).
The holes localize a microenvironment containing free mineral ions and bound side chain
