Synthesis of Layered, Graded Bioscaffolds - Structural Interfaces and Attachments in Biology - page 342

( 200 mTorr) produces a highly porous scaffold where the scaffold pore size and

shape is defined by the size and shape of the ice crystals formed during solidifica-
tion (Fig. 16.1 )[ 38 ]. Modifying suspension freezing rate, final freezing tempera-
ture, and inclusion of annealing steps have all been shown to significantly alter
final scaffold pore structure [ 19 , 37 , 39 ]. These scaffolds resemble low-density,
open-cell foams, with an interconnected network of struts and are typically
fabricated with relative densities ( r */ r s ) significantly less than 5% (porosities
greater than 95%).
Recently, CG fabrication schemes have been modified to enable integration of
calcium phosphate (CaP) mineral content into the prototypical CG scaffold [ 40 - 42 ],
including control over the mineral to organic ratio (CaPmass fraction) of the collagen-
GAG-CaP (CGCaP) triple co-precipitates (0-80 wt%) in order to cover a range that
includes the mineral content of developing osteoid and natural (cortical) bone (75 wt%
CaP) [ 40 , 43 ]. CGCaP scaffolds are of particular interest for orthopedic applications
due to their potential to mimic the native biochemistry of bone, notably the creation of
an interpenetrating collagenous matrix (organic) and CaP (mineral) network, as well
as the significant improvement in overall mechanical properties of the CGCaP
scaffolds relative to non-mineralized CG scaffolds [ 44 - 46 ]. The addition of a mineral
phase to the classic CG scaffold archetype enabled the development of composite
materials with the requisite biochemical and biomechanical properties for bone and
interfacial tissue engineering. CGCaP scaffold technology has been used as the basis
for creating multiphase collagen scaffolds for the repair of interfacial tissues, notably
osteochondral defects [ 40 , 41 , 47 ].

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16.3 Multi-compartment Scaffolds for Interfacial
Tissue Engineering

Tissue interfaces are found throughout the body; they link distinct tissue
compartments with a stable interface most often containing gradients in ECM
content (i.e., collagens, GAGs, CaP) and soluble biomolecules (i.e., growth factors,
proteins, cytokines) across the interface. Notably, articular joint surfaces contain
two distinct tissue types, bone and articular cartilage, meeting at a smooth, stable
interface (the “tidemark”). A major contributing factor to the stability of the
osteochondral interface is the smooth compositional transition between mineralized
bone and unmineralized cartilage [ 48 ] that occurs as CaP content gradually
decreases from the levels found in the subchondral bone plate (~75 wt%) across a
zone of calcified cartilage to zero in articular cartilage. Over this same transition,
type II collagen content dramatically increases from zero in the subchondral bone to
the levels found in native cartilage. Most significantly, collagen fibrils extend across
this tidemark from each side, ensuring a continuous organic phase [ 49 - 51 ]. Here we
describe integration of CG and CGCaP scaffold technologies to develop a multi-
compartment collagen scaffold for the repair of orthopedic interfaces.