Biomedical Engineering Reference
In-Depth Information
4.4.4
Porosity
Porosity is defined as a percentage of void spaces in solids and it is
a morphological property independent of the material. The surface
area of porous bodies is much higher, which guarantees a good
mechanical fixation in addition to providing sites on the surface that
allow chemical bonding between the bioceramics and bones [374].
Furthermore, a porous material may have both closed (isolated)
pores and open (interconnected) pores. The interconnected pores
look like tunnels and are accessible by gases, liquids and particulate
suspensions [375]. The open-cell nature of reticulated materials is a
unique characteristic essential in many applications. Furthermore,
dimensions of open pores are directly related to bone formation,
since such pores grant both the surface and space for cell adhesion
and bone ingrowth. On the other hand, pore interconnection
provides the ways for cell distribution and migration, as well as
it allows an efficient
blood vessel formation suitable for
sustaining bone tissue neo-formation and possibly remodeling [155,
376-385]. Namely, porous HA bioceramics can be colonized by bone
tissues [381, 386-396]. Therefore, interconnecting macroporosity
(pore size > 100 μm) [111, 374, 381, 397, 398], which is defined
by its capacity to be colonized by cells, is intentionally introduced
in solid bioceramics (Fig. 4.6). Macroporosity is usually formed
due to a release of various volatile materials and, for that reason,
incorporation of pore-creating additives (porogens) is the most
popular technique to create macroporosity. The porogens are
crystals or particles of either volatile (they evolve gases at elevated
temperatures) or soluble substances, such as paraffin, naphthalene,
sucrose, NaHCO
in vivo
, gelatin, polymethylmethacrylate or even hydrogen
peroxide [112, 295, 400-407]. Obviously, the ideal porogen should be
nontoxic and be removed at ambient temperature, thereby allowing
the ceramic/porogen mixture to be injected directly into a defect site
and allowing the scaffold to fit the defect [408]. Sintering particles,
preferably spheres of equal size, is a similar way to generate porous
3D bioceramics of calcium orthophosphates (Fig. 4.7). However,
pores resulting from this method are often irregular in size and
shape and not fully interconnected with one another.
3
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