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Mohammad Vaezi and Shoufeng Yang

ing, etc. have several limitations such as shape restric- ers, and then the mixture is densified by pressure, and
tions, manual intervention, inconsistency, and inflex- finally, compression moulded. A similar compression
ibility. The use of solvent-based extrusion freeforming moulding process was used in this study for infiltra-
in this project guaranteed reproducibility and unifor- tion of 3D printed HA lattice structures, barring the
mity of the 3D printed HA scaffolds. Moreover, the fact that there is no requirement for mixing and densi-
technique can deliver much finer filaments (down to fication of PEEK and HA powders.
50 μm) and avoid the thermal management issues of The proposed technique provides new possibilities
other extrusion-based techniques. The versatility of for producing both bioactive PEEK compounds and
the method is not just in terms of structure and fila- porous PEEK structures with theoretically enhanced
ment dimensions, but also in terms of the capability biological performance. In addition, both bioactive
to utilize a range of ceramic powders (such as zirconia, phase and PEEK matrix are fully interconnected,
alumina, HA, TCP, Bioglass, etc.) according to the which is a superior advantage compared to existing
desired properties. The key feature of our in-house techniques. Whilst HA, a well-known osteoconductive
developed solvent-based extrusion freeforming me- material was used in this work, other bioactive mate-
thod is its unique nozzle design with minimum die rials such as Bioglass, β-TCP, etc. with higher biode-
land, and paste formulation, which allows us to make gradation rates can be used. The interconnected bioac-
very uniform pastes quickly with an excellent extru- tive network can be fully absorbed in vivo, leaving 3D
dability and less nozzle jamming during extrusion. interconnected channels for further in-growth and pro-
The additive nature of the process ensures minimal liferation. In this way, a 3D locked bone/PEEK struc-
waste of biomaterial and renders it suitable for mass ture could be achieved in vivo which can remarkably
production of porous bioactive structures [33] , and mi- improve graft fixation compared with existing tech-
cro-scale woodpile structures [34] for use as scaffolds. niques.
The printed HA scaffolds were highly uniform which In addition to PEEK/HA composite, the proposed
makes the overall process very consistent and repeata- technique could be utilized to produce PEEK with
ble for further use in compression moulding process. fully interconnected pores and controlled porosity.
A number of researchers have reported the use of Conventional techniques such as particulate leaching
polymer infiltration into bioceramic scaffolds to in- exert poor control on porosity, and suffer from limita-
crease mechanical properties [35–40] . In these studies, tions such as inconsistency and require manual inter-
bioceramic porous structures were immersed in mol- vention. Using the proposed technique, porous PEEK
ten polymer or solution to achieve infiltration of either could be easily produced with much greater control on
the filaments or the entire structure. Martínez-Vázquez porosity and enhanced reproducibility, a key require-
et al. [35] immersed extrusion freeformed β-TCP scaf- ment in production of medical devices. Furthermore,
folds into molten polylactic acid (PLA) and polyca- this technique is a low-cost process in comparison to
prolactone (PCL) for 2 hours to produce composite SLS, while affording greater control of pore size (the
bioceramic/polymer composite. These techniques minimum achievable in SLS is currently 400–500 μm).
were not suitable for use in this study as viscosity of Therefore, while SLS as an AM technology has great-
easy flow medical grade PEEK is still much greater er control on pore size and architecture of biocon-
than PCL or PLA. This renders the process of infiltra- structs [42,43] , its use in clinical applications is currently
tion of fine pores by immersion unreliable, resulting in limited to those in which small pore sizes are not re-
scaffold material being retained on the surface of quired. In contrast, with the use of the proposed tech-
viscous PEEK melt. In addition, medical grade PEEK nique, a wide range of pore sizes (200–1000 µm) is
tends to degrade when it is held in excess of 30 mi- achievable. This makes the technique a versatile ap-
nutes at temperatures above 400℃ in the absence of a proach, enabling formation of porous PEEK which
vacuum. Ma et al. [41] used compression moulding to can be tailored to specific biomechanical requirements
make a simple functionally graded PEEK/HA compo- of a variety of clinical applications.
sites. Roeder’s group at University of Notre Dame, To summarize, the main features of these new
USA, reported successful use of compression mould- PEEK/HA composite are:
ing and particulate leaching to make porous bioactive • Greater control on distribution of bioactive phase
PEEK/HA-whisker composite [5–8] . Bioactive particles within PEEK matrix (controlled by computer design),
and/or a fugitive particle are mixed with PEEK powd- • Ability to tailor mechanical and biological proper-

International Journal of Bioprinting (2015)–Volume 1, Issue 1 73