Development of Extended-Lifetime Organic-Inorganic
Scaffolds for Orthopaedical Applications
A new generation of biomaterials for orthopaedic applications has been developed in this project. A novel
processing technique, called robocasting or direct-write assembly, has been successfully applied to fabricate
porous bioceramic (hydroxyapatite, β-tricalcium phosphate) scaffolds for bone tissue engineering. The
scaffolds consist of a 3D network of perfectly joined calcium phosphate rods fabricated in a pre-determined
geometry. This is achieved by the computer-controlled robotic deposition (see figure below) of dense water-
based suspensions of calcium phosphate powders capable of supporting their own weight. The calcium
phosphate scaffold encourages bone cells to proliferate into its designed pore structure, helping the body to
regenerate a damaged tissue region after its implantation. The processing technique used has significant
advantages over more conventional techniques to fabricate porous scaffolds that do not allow a precise control
of their three-dimensional external shape and internal morphology, but also over other technologies capable of
producing analogous controlled structures (stereolithography, 3D printing, etc), in terms of simplicity and cost,
and avoiding the use of potentially toxic binders.
calcium-phosphate-based (hydroxyapatite, HA, and β-tricalcium phosphate, β-TCP) inks suitable for
robocasting were developed and used to fabricate porous scaffolds with controlled distribution of porosity (see
SEM micrographs below):
Calcium phosphate scaffolds are inherently brittle and this study has allowed us to identify the damage modes
occurring in the scaffold structures under compressive stresses. In-situ observations during uniaxial
compression tests and post-mortem SEM micrographs (see figures below) enabled the identification of the
principal damage modes developed in the scaffold: contact induced-damage and longitudinal cracks. Although
contact-induced damage was observed sometimes as the first damage mode to develop, due to its localized
nature it does not jeopardize the integrity of the structure. Instead, the failure mode consists of longitudinal
cracks running through the structure parallel to the load axis. Depending on the testing direction this cracks cut
or separate the rods comprising the structure (see figures below). As the test progresses, they accumulate till
the structure is reduced to a bunch of disconnected pillars:
Tests performed orthogonal to the printing plane (direction 3): the longitudinal cracks divide
the unsupported rod segments.
Tests performed along rod directions (directions 1 or 2): the longitudinal cracks detach the rods.
Independently of the testing direction, even after several
longitudinal cracks have developed the structure retains certain
mechanical resistance. Such residual load-bearing capacity is
provided by the columnar structure that remains oriented along
the load axis. Of course, when the load is removed the columnar
structure collapses with no connection between the pillars to
provide some mechanical integrity. However, if the scaffolds
were infiltrated with some secondary, more ductile, phase to
provide a link between pillars, the resulting composite would
most likely exhibit a good level of mechanical integrity even
after the failure of the ceramic structure. This possibility
encourage us to continue pursuing our goal of creating damage tolerant scaffolds for load bearing applications
in bone tissue engineering through the infiltration of bioceramic scaffolds (HA, tricalcium phosphate, bioactive
glasses, etc.) with biodegradable polymers (PLA, PCL, PGA, etc.). Preliminary infiltration test have been
performed but the process need to be optimized yet to progress in this direction.
β-TCP/PLA composite scaffold
A Finite Element Model was developed to simulate the scaffolds and analyze the origin of the identified
damage modes. ABAQUS/Standard was selected as FEM software and the elastic properties of the materials
necessary for the simulations were determined through nanoindentation, yielding values of E = 96 ± 6 GPa
and E = 42 ± 9 GPa for HA and β-TCP, respectively.
The FEM simulations
performed allowed the
identification of the stresses
responsible for the longitudinal
cracks in both testing directions.
Equating those stresses to the
fracture strength of the
individual rods, as measured b
by 3-point bending tests (σF =
68 ± 12 MPa and σF = 27 ± 9
MPa for HA and β-TCP,
respectively) it was possible to
predict the compressive
strengths of the scaffolds. The
values obtained were in
excellent agreement with the experimental data. Some additional work needs to be done in order to optimize
the porous scaffold geometry but once this is achieved it would be relatively simple to extend the methodology
to the composite scaffolds we ultimately intend to fabricateThese computer models will allow us to optimize in
the near future the geometrical design of the scaffolds in order to improve their mechanical performance for
load-bearing orthopaedic applications.
The experimental data showed that hydroxyapatite scaffolds have a much greater (about 3 times) compressive
strength than the β-TCP structures. This is consistent with the flexure strength values reported previously and
the low strength values obtained for β-TCP are attributed to the development of microcracks in this latter
material during sintering. Additionally, the effect of immersing the scaffolds in simulated body fluid (SBF) for
20 days on their mechanical properties was analyzed. A significant improvement, about a two-fold increase, of
the fracture strength of HA was found after the immersion while β-TCP remained unaffected. The strength
enhancement observed in HA after immersion in SBF is attributed to the bone-like HA formation on the HA
scaffolds that was observed by SEM, while β-TCP scaffolds did not promote such HA deposition and growth
and therefore its strength remained unaltered.
The results obtained in this project suggest that the mechanical performance of the scaffold could be greatly
improved by infiltration of the porous structure with biodegradable polymers (PLA, PCL, PGA, etc.) to create
a composite. The porosity necessary for bone in-growth into the composite scaffolds will be created in-situ
after implantation during the degradation of the polymer material. This possibility opens a most promising and
unexplored way to create damage tolerant scaffolds for load bearing applications in bone tissue engineering.
The viability of this concept has been preliminarily explored in this project with very promising results, and
additional work is under way to bring this concept to reality.
