Engineering Graded Tissue Interfaces - Structural Interfaces and Attachments in Biology - page 292

14.3.2 Engineering Graded Scaffolds: Why Are Graded
Structures Required?
Gradients in porosity, pore size, and mineral composition have functional
consequences with regard to stiffness, permeability, and biological activity in
tissues. These parameters are highly interrelated; a gradient in mineral content
and porosity results in a gradient in stiffness. Porosity and pore size have an effect
on permeability, while pore size and surface morphology are likely to influence
phenotypic expression. Studies have indicated that graded patterns of biologic
molecules were the driving force for migrating cells [ 26 ]. Gradients of secreted
signaling proteins were found to guide the growth of blood vessels during normal
and pathological angiogenesis [ 27 ]. Earlier attempts to engineer the osteo-
chondral interface used methods to fabricate independent layers and further
integrated the two components together by suturing or gluing [ 28 , 29 ]. Fabrication
of bi/tri/multilayered scaffolds with distinct phases resulted in an abrupt or
discrete interface created by the joining of two materials [ 30 ].Therewasasevere
lack of integration at the interface, limiting the biological performance of the
regenerated tissue. Consequently, investigators explored methods to construct
scaffolds with graded structures and better integration for a smooth transition of
properties at the interface. Malafaya and Reis [ 31 ] fabricated a chitosan-based
bilayered scaffold by a particle aggregation method where cross-linked chitosan
served as the chondrogenic layer and hydroxyapatite (HA) incorporated collagen
formed the osteogenic phase. HA was incorporated into collagen by random
assembly of particles. The cross-linking of collagen and the contact points of
adjacent HA particles formed the bonding sites, thereby creating a highly
interconnected network to overcome any risk of delamination. Ahn et al. [ 32 ]
designed a biphasic scaffold combining hyaluronic acid and atellocollagen for the
chondral (i.e., cartilage) phase and HA and beta-tricalcium phosphate for the
osseous (i.e., bone) phase. The two phases were freeze-dried together to create the
intermediate zone in which both the chondral phase and the osseous phase
coexisted. Thus scaffolds that can guide the chondrogenic and osteogenic differ-
entiation of cells in different regions of the same matrix were developed. Further-
more the challenge was in maintaining the appropriate chondrogenic and
osteogenic phenotypes under a single set of cell culture conditions [ 28 , 33 ].
Spalazzi et al . [ 34 ] designed a triphasic scaffold system mimicking the multi-
tissue organization of the native ACL-to-bone interface. Polyglactin knitted mesh
sheets formed the ligament phase, PLGA microspheres were used for the un-
mineralized fibrocartilage interface, and PLGA microspheres with bioactive glass
formed the osteogenic phase. The three phases were further sintered to form the
trilayer scaffold. Osteoblasts seeded on the osteogenic phase and fibroblasts
seeded on the ligament phase migrated to PLGA microspheres to form a
fibrocartilage interface. The study pointed out that tissue-specific gene expression
was observed in each of these phases and cocultured scaffolds maintained a higher
degree of structural integrity than the acellular scaffolds.