(249b) Chemical Engineering Approaches to the Design and Fabrication of Extrusion-Based Functionally Graded Scaffolds Targeting Tissue Engineering Applications

Repair of large segmental bone defects, i.e., critical-sized defects, which
occur due to blunt trauma, tumor resection surgeries, pathological degeneration
and congenital deformities, are challenges for orthopedic surgery [1-4].
Currently, about 5-10% of procedures applied to repair critical-sized defects
result in delayed unions or non-unions [1-2]. Autografting
is still the ?gold standard? in clinical bone repair procedures [3,5-7]. However, up to 30% of the autografting
procedures have complications associated with the donor-site morbidity, limited
tissue availability, varying quality and longer hospital stays [6-8]. Other
alternatives, i.e. allografts and xenografts, suffer
from limited sources of supply, contamination risks, immunogenic
incompatibility and inability to incorporate with the host bone [3,7,8]. Designing synthetic bone grafts offers an alternative
route in treatment of critical-sized defects [1-8]. Clinical trials have
demonstrated the possibility of using metallic, ceramic and polymeric bone
graft substitutes [5,9-14]. It is generally understood
that the bone grafts need to be (i) biocompatible,
(ii) preferably bioresorbable to prevent second surgery for removal after full
recovery or due to late immunologic response, wear or dislodgement, (iii)
mechanically adequate and stable until functional bone tissue forms within the
defected area, and (iv) most importantly osteoconductive and preferably
osteoinductive [1,6-8,12].

A second approach, i.e., bone tissue engineering, attempts to utilize porous
polymeric scaffolds which are typically seeded with patient's own stem
cells.  Tissue constructs are formed upon the proliferation and
differentiation of the cells on the scaffold within a bioreactor and implanted
to defect site. Typical polymeric materials for such tissue engineering
scaffolds are polyglycolide, polylactide,
polycaprolactone (PCL) and their copolymers or their biocomposites with
particles like hydroxyapatite and tricalcium phosphate as well as other bioagents like growth factors [6,7,9-14].

Problem statement: The designing and fabrication of bioresorbable
polymeric bone graft substitutes and porous scaffolds for bone graft
substitutes and tissue engineering are both challenged by the complex
structural and compositional gradations found in human tissues [15,16]. For example, it would be desirable for the bone graft
substitutes used in the repair of critical sized defects in long bones like
femur and tibia to accommodate their changing porosities and moduli along their
transverse and axial directions [17,18]. Therefore,
mimicking of such complex gradations found in native tissues can require
correspondingly complex gradations in bone graft substitutes and tissue
engineering scaffolds which exhibit tailored three-dimensional distributions in
composition, structure and properties.  Conventional methods of generating
graded scaffolds include layer-by-layer casting, freeze-drying, phase
separation, and rapid prototyping techniques including fused deposition modelling, 3D printing, selective laser sintering, and stereolithography [15,16]. These
methods have recently been supplemented via scaffold fabrication methods that
are within the arsenal of the chemical engineering profession and that rely on
the twin screw extrusion process as a base, i.e., twin screw extrusion and
spiral winding, twin screw extrusion and electrospinning and co-extrusion to
furnish additional flexibility in design and manufacturing of tissue
engineering scaffolds [19-27].

The tools of chemical engineering that have been applied to generate the new
methods based on extrusion, co-extrusion and fiber spinning/electrospinning
include the solution of the coupled transport equations to mathematically model
and simulate the thermo-mechanical history of the biopolymers and biosuspensions in the extruder and the die (to provide
mixing effectiveness, stress, velocity, temperature, and residence time
distributions) in conjunction with rheological characterization of the material
functions, the characterization of  microstructural
distributions and mechanical properties of the scaffolds and assessments of the
biocompatibility of the scaffolds graded in porosity and bioactive
concentrations to determine cell proliferation and differentiation principally
using histology and PCR analysis [19-27].

Examples of extrusion based methods developed and applied to generate
functionally-graded bone graft substitutes and scaffolds: In twin screw
extrusion and spiral winding the extrudates, the composition and porosity of
which can be altered as a function of time, are wound on a simultaneously
translating and rotating mandrel to generate radially
and axially-graded scaffolds [21,22]. The twin screw
extrusion process can also be applied concomitantly with electrospinning to
generate nanofibrous meshes which can be graded with
porosity, pore size and bioactive concentration distributions [19,20,23].

For example, concentration gradients of  two
bioactives, ß-gloycerophosphate and insulin could be
generated using TSE-electropinning. The regions that
are rich in insulin were determined to give rise to chondrocytic
differentiation of adipose-derived stem cells, whereas the regions that were
rich in ß-gloycerophosphate were determined to
furnish greater degree of mineralization [23].  Finally,
 axially and radially graded scaffolds of
PCL could be fabricated via the co-extrusion method to generate distributions
of porosity, pore size and concentration distributions of TCP and HA in both
the axial and radial directions (cage/core structures). The modulus of the
co-extruded scaffolds could be manipulated systematically in both the radial
and axial directions. When tissue constructs were generated by seeding bone
marrow derived stem cells onto the co-extruded graded scaffolds and samples
were harvested as a function of time during cell proliferation and
differentiation in media, the gene expression levels and µ-CT analysis revealed
the osteogenenic differentiation, bone ECM formation
and mineralization as affected by the gradations of the bioactive ingredient, i.e.,
HA and TCP concentrations [24- 27].

Conclusion: During the last three decades the chemical engineering
profession has gained specialized capabilities in rheology, mathematical
modeling and simulation on various types of extrusion and fiber spinning methodologies,
microstructural analysis and tailoring, and
development of mechanical properties to allow the development of extrusion
based technologies for the development of bone graft substitutes and tissue
engineering scaffolds. The role of chemical engineering is expected to increase
with greater utilization of these industrially-relevant methods upon commercial
scale implementation to advance patient care by providing better graft
substitutes and scaffolds which better mimic the complex elegance of native
tissues.

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