Biomedical Engineering Reference
In-Depth Information
the success of regenerative strategies. Synthesis of ceramic/polymer composites
represents a fascinating approach for designing “tissue-inspired composite materi-
als” with advantages over either pure ceramic or pure polymer to obtain superior
materials for specific applications. Traditionally, calcium phosphate-based ceram-
ics have proved to be attractive materials for biological applications. Among these
bio-ceramics, hydroxyapatite (HA)—Ca 10 (PO 4 ) 6 (OH) 2 —with an atomic ratio of
calcium to phosphorus (Ca/P) of 1.67 has been synthesized by different ways,
including aqueous colloidal precipitation and sol-gel methods. They allow synthe-
sizing HA micro/nanoparticles at room temperature, and directly control the par-
ticle and grain sizes [ 42 ]. In comparison with traditional strategies involving
physical mixing of particles, they ensure a more controlled and finer distribution of
crystallites in the polymer matrix. They also can enhance the mechanical response
in terms of strength, stiffness, toughness, and fatigue resistance to achieve com-
plete mechanical compatibility.
Besides, scaffold design has been also addressed to successfully reproduce the
microenvironment required to support and improve the molecular interactions which
occur among cells within the mECM [ 51 ]. In traditional composite scaffolds—ce-
ramic filler dispersed in a polyester matrix—differences in the chemistry of hydro-
philic particles and hydrophobic polymer may lead to inadequate homogenization
of the bioactive phase into the polymer matrix, favoring the cluster formation on
micrometric scale [ 42 ]. Wet chemical precipitation involving the precipitation of
HA in tetrahydrofuran, the same solvent utilized to dissolve the polycaprolactone,
may be an interesting strategy to overcome these limitations. The use of amphiphilic
surfactant (Span85, 75 % oleic acid) aided to improve the dispersion of HA in the
PCL matrix. Indeed, oleic acid is a kind of fatty acid with a long hydrocarbon tail
and a hydrophilic head [ 50 ]. Therefore, oleic acid is believed to mediate the homog-
enization of hydrophilic HA powders in the pool of a hydrophobic organic solvent,
such as tetrahydrofuran [ 30 ] .
Alternatively, HA/PCL porous scaffolds for bone tissue engineering have been
developed by combining chemical synthesis and phase inversion/salt leaching. This
strategy allows to reach sub-micrometric HA particles uniformly dispersed within
the polymer matrix [ 46 ]. Moreover, the use of low temperature approaches to syn-
thesize HA in a solvent in which the polymer is soluble provides a novel pathway to
generate homogeneous composite materials preserving the microstructural features
of the HA crystals. In this case, the pore architecture was assured by the removal of
solvents and used porogens (i.e., NaCl crystals), of predefined particle size. Both
chemical and morphological cues are necessary to ensure an adequate biological
response following implantation into bone tissue. Indeed, osteoconductivity is
essential for the recruitment of cells capable of forming bone matrices and ulti-
mately for successful bone ingrowth into scaffold. Bone invasion in scaffolds can
potentially be improved and accelerated by chemical surface modifications, or by
the addition of an osteoconductive component in the polymer [ 56 ] . To ef fi ciently
modify the inner pore wall surfaces, without altering the bulk structure and proper-
ties of the scaffolds, a biomimetic approach has been developed based on the sur-
face modification in order to promote the growth of apatite-like crystals onto
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