Biomedical Engineering Reference
In-Depth Information
guidance cues to cells. Scaffold cores were fabricated from a suspension of type I
microfibrillar collagen and chondroitin sulfate [ 53 ].
The composite shell was created from a novel CG membrane fabricated via an
evaporative process from the same precursor suspension as the CG scaffold. Briefly,
degassed CG suspension was pipetted into Petri dishes and allowed to air dry in a
fume hood, resulting in a dense CG sheet with thickness on the order of tens to
hundreds of microns [ 1 ]. The amount of CG content within the suspension, mediated
by either total suspension volume or suspension density, correlated to the thickness
of the resultant membrane [ 1 ]. CG membranes displayed a consistent relative
density (75%) significantly larger than that of the scaffold core (0.6%); membrane
stiffness could be further modulated via post-fabrication crosslinking procedures.
Scaffold-membrane constructs were created via liquid-solid phase co-synthesis
[ 1 , 47 ] while degree of scaffold core anisotropy was separately modulated via a
directional solidification approach. Membrane pieces were cut to size, rolled, and
placed directly into a PTFE mold containing a copper bottom used to induce
directional solidification [ 53 ]. The CG suspension was then pipetted inside the
rolled membrane and allowed to hydrate the membrane [ 1 ]. The degree of mem-
brane incorporation can be tuned by adjusting the hydration time of the membrane
in the scaffold suspension prior to freeze-drying. The mold was then placed on a
precooled freeze-dryer shelf where the significant disparity in thermal
conductivities ( k Cu / k PTFE ~ 1,600) promoted unidirectional heat transfer through
the copper bottom during lyophilization. Conventional sublimation was then used
to remove the ice content from the frozen CG suspension, resulting in core-shell CG
composites containing an aligned pore microstructure [ 53 ]. The porous CG core
and dense CG shell are integrated into a single continuous biomaterial that
combines excellent porosity, permeability, and bioactivity with increased mechan-
ical competence necessary for tendon applications. SEM analysis of the scaffold
core shows elongated, aligned pores in the scaffold longitudinal plane as a result of
unidirectional heat transfer; in contrast, pores in the scaffold transverse plane are
circular and more isotropic (Fig. 16.6 ). The CG membrane displays a dense network
of fibrillar collagen content and shows excellent integration with the CG scaffold in
scaffold-membrane composites. Additionally, the membrane does not delaminate
from the scaffold core during the freeze-drying process or after hydration [ 1 ].
16.4.2 Characterization of CG Membrane Physical
and Mechanical Properties
CG membranes were fabricated via an evaporative process with thicknesses varying
over an order of magnitude (23-240
m) (Fig. 16.7 )[ 1 ]. The thickness could be
adjusted by changing the amount of solid collagen-GAG material used during
fabrication. Despite differences in thickness, the relative density was consistently
in the 0.75-0.80 range (CG scaffold relative density: 0.006). Swelling ratio tests
revealed that all membrane variants were at least 90% hydrated after 30 min in PBS
m
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