Engineering Graded Tissue Interfaces - Structural Interfaces and Attachments in Biology - page 300

Liu et al . [ 91 ] used a multi-nozzle low-temperature deposition and manufacturing
(M-LDM) system to make mechanically graded scaffolds using heterogeneous
materials, hierarchical porous structures, and different hydrophilicity. This system
could be effectively used for designing interfaces. Using CAD, the structure, pore
sizes, and porosity of two phases and interface could be designed initially. Further
graded structures could be fabricated using multi-nozzle and appropriate polymer
combinations. A Computer Aided System for Tissue Scaffolds (CASTS) aims to
bring automated production of graded scaffolds by providing a scaffold library data-
base that correlates scaffold porosity values and the corresponding compressive
stiffness and integrates this into the design process [ 92 ].
The soft tissue-bone interface has varying mechanical properties. Cell spreading,
motility, and phenotypic expression are found to be influenced by substrate stiff-
ness. A gradient in mechanical properties may be considered a key element for
engineering two tissues for smooth transfer of stress at the interface as well as for
regulating cell behavior. Scaffolds with gradients in mechanical properties can be
fabricated by blending polymers of different stress/strain behavior or by nucleation
of osteoconductive inorganic materials to mimic the tissue properties at the inter-
face. A gradient in porosity is also found to affect the mechanical properties of the
scaffolds. Advanced medical imaging systems like MRI and microCT help
to capture the micro-architecture of the interface, which together with the
bio-plotters/CAM can be used to recreate the complex tissue-interface structures
with specific mechanical requirements.
14.3.7 Graded Structures Using Nanofibers
Attempts have been made to fabricate graded scaffolds using nanofibers that mimic
the native ECM structure. Nanofibers have the advantage that they can be conve-
niently functionalized by encapsulation or attachment of bioactive species to control
the differentiation and proliferation of seeded cells. Additionally, the nanofibers can
be readily assembled into a range of arrays or hierarchically structured by
manipulating their alignment, stacking, or folding [ 93 ]. Nie and Wang [ 94 ] reported
the use of PLGA/hydroxyapatite composite nanofibers to deliver BMP-2 plasmid
DNA. Coaxial electrospinning was designed to produce a core-shell structure of the
nanofiber, which can encapsulate and release drugs more efficiently [ 95 ]. Nanofibers
were fabricated combining biodegradable polymers with inorganic bioactive
materials for osteogenic differentiation and calcification of bone matrix. This design
was to mimic the collagen fibers with hydroxyapatite nanocrystallites in the native
bone [ 96 ]. Organic-inorganic composite nanofibers made of gelatin-HA, collagen-
HA, and chitosan-HA have been designed to mimic the ECM of bone [ 97 - 99 ].
Furthermore, instead of adding particulate inorganic materials, degradable and bioac-
tive hybrid nanofibers were produced through the hybridization of inorganic and
organic phases in solution, by the sol-gel process. This process increased the chemical
stability by forming a hybridized network [ 100 ]. The approach of using bioactive
inorganic phases in concert with degradable polymers is continuing to attract attention