Engineering Graded Tissue Interfaces - Structural Interfaces and Attachments in Biology - page 298

40-150 m for bone regeneration [ 30 ],

depending on the porosity and the scaffold materials used [ 68 , 69 ]. Thus, the
dimensions of scaffold pores can depend on the cell type. Awell-engineered scaffold
should be tailored with the appropriate pore sizes and porosities to meet the needs of
the specific cells and tissues.
One method reported for generating a porosity gradient was to stack porogen
mixture layers containing different volume fractions and/or particle sizes. O'Brien
et al . [ 68 ] fabricated collagen-based, porous tubular scaffolds with a graded
structure to facilitate the study of myofibroblast migration during peripheral
nerve regeneration. Oh et al . [ 70 ] fabricated scaffolds with a porosity and pore
size gradient along the cylindrical axis by a centrifugation method. They reported
that the porosity and the pore size ranges of the scaffold could easily be varied by
adjusting the angular velocity. The in vitro and the in vivo studies on this gradient
scaffold indicated that 380-405

m for fibroblast binding [ 67 ], and 100-400

m m

m pore sizes showed better cell proliferation for
chondrocytes and osteoblasts, while the scaffold section with 186-200

m

m pore
sizes was better for fibroblast growth. Fu et al . [ 71 ] looked into the influence of
pore architecture on bone regeneration. Hydroxyapatite scaffolds with approxi-
mately the same porosity (65-70%) but two differently oriented microstructures,
described as “columnar” (pore diameter 90-110

m

m) and “lamellar” (pore width

m

20-30 m), were prepared by unidirectional freezing of suspensions. The colum-

nar scaffolds with the larger pore width provided the most favorable substrate
for cell proliferation and function. They concluded that HA scaffolds with the
columnar microstructure and unidirectional pores favored bone repair applications
in vivo .Hsuetal . [ 72 ] fabricated porous ceramic implants with graded pore
structures to mimic the bimodal structure of cortical and cancellous bone. Func-
tionally graded scaffolds were made by vacuum impregnating an HA and tri-
calcium phosphate (TCP) ceramic slurry into a polyurethane foam that was
stitched or press fitted to form gradient templates. No interfacial weakness was
observed by the three point bending testing in these scaffolds. Sun et al . [ 73 ]
reported that minute pores that determine the surface topography had an influence
on the osteoconductivity. Osteoblasts cultured on porous silicon with pores on the
order of 1 m m enhanced osteoblast viability and mineralization, and maintained the
expression of the biomarkers of bone formation when compared to nanoscale pores
(50 nm or less). The pores and the surface topography influenced cell spreading,
mechanical signal transduction, and osteoconductivity.
Previous studies have suggested that different pore sizes favor the growth of
chondrocytes, fibroblasts, and osteocytes. Porosity and pore size play a role in
directing cell migration and altering permeability of the scaffolds. Scaffolds with
gradients in porosity have been obtained by stacking porogens of different sizes, by
centrifugation, varying the freeze drying conditions, or by using 3D plotters.
Even though gradients in pore architecture are not a critical element in the design
of scaffold for interface engineering, they were found to have significant influence
on the cell infiltration and behavior, vasculature, and mechanical properties at
the interface.

m