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Mohammad Vaezi and Shoufeng Yang

• Coating PEEK implants with biomaterials such incorporate both porosity and bioactivity into PEEK
as titanium and/or calcium phosphates, with fewer limitations in terms of cost, pore
• Incorporating porosity into PEEK implants to interconnectivity, and level of bioactive filler loading
enhance osteointegration and bone fixation. in comparison to other techniques.
Calcium phosphates including hydroxyapatite (HA) Current methods of bioactive PEEK processing, as
and β-tricalcium phosphate (β-TCP), or Bioglass can discussed earlier, do not permit control on distribution
be utilised as composite filler to produce PEEK com- of bioactive phase within the PEEK matrix. These
pounds with potential for osteointegration. Whilst ad- techniques rely on simple mixing of PEEK with bio-
dition of bioactive materials to PEEK offers an effi- active material powders/granules, and thus less control
cient method to engineer implants with tailored bio- on distribution. In addition, the wide range of physical
mechanical properties, it may result in reduced properties of different particles (size, shape, density)
[1]
strength and toughness of the implant . Different negates efficient and consistent mixing. The purpose
processing methods such as compounding and of this study is to develop a novel technique that
injection moulding [2–4] , compression moulding [5–8] , would permit greater control on incorporation of bio-
cold press sintering [9–11] and selective laser sintering active materials into PEEK than the existing tech-
(SLS) [12–14] have been used to produce bioactive niques.
PEEK/HA and β-TCP composites.
Compounding and injection moulding is a low-cost 2. Materials and Methods
process suitable for high-volume commercial near net Figure 1 depicts workflow of the technique applied to
shape manufacturing of PEEK compounds. However, make bioactive PEEK/HA composite. It comprises of
the quantity at which bioactive fillers may be loaded is fabrication of porous bioactive HA scaffold using ex-
strictly limited, as high loading increases melt viscos- trusion-based AM technology, followed by PEEK melt
[1]
ity resulting in inconsistent and unreliable mixing . infiltration into HA scaffolds through compression
Furthermore, material removal may be required to moulding process. To produce a fully interconnected
reveal bioactive particles on the surface of injection porous PEEK, the produced PEEK/HA composite was
moulded PEEK compound. In cold press sintering, further soaked into hydrochloric acid (HCl) solution
there is no limitation on the loading of bioactive fillers so that the HA network could be removed by HCl
but the process suffers from residual porosity in etching. Solvent-based extrusion freeforming process,
composite due to pressure reduction during sintering. first developed by Evans and Yang’s research
In contrast, compression moulding is a manufacturing group [15–25] , was used to print highly uniform HA 3D
platform offering greater flexibility, and shown to be lattice structures with controlled filament/pore size. In
well-suited to the synthesis of PEEK/HA composites. solvent-based extrusion freeforming, solidification is
Compression moulding is low-cost, suitable for based on solvent evaporation which has advantages
high-volume production of high-density PEEK com- over similar techniques such as robocasting, where the
pounds, and more critically tailored porosity. In this
technique, porous PEEK compounds can be realized state changes based on a dilatant transition.
by the addition of a fugitive particle (e.g., sodium The process of solvent-based extrusion freeforming
chloride) into the compound that is further leached out of the HA scaffolds involved the following steps: (i)
[1]
by soaking into a solvent post-moulding . SLS, a preparation of HA paste, (ii) 3D printing, and (iii)
powder-based additive manufacturing (AM) process, drying, debinding and sintering of the 3D printed
is capable of fabricating bioactive porous structures scaffold. An extrusion-based 3D printer was designed
with very complex architecture, thus permitting great- and built in-house for 3D printing of porous HA scaf-
er design freedom. This process has been applied to folds from HA paste.
form both porous natural grade PEEK and porous The following materials were used to form the HA
bioactive PEEK components. Use of SLS technique paste: (i) hydroxyapatite powder (HA, Ca 10(PO 4) 6
has been hampered by difficulty in loading the (OH) 2, Grade P221 S, Plasma Biotal Ltd. UK) with
–3
quantity of bioactive filler beyond 22% by volume density of 3156 kg·m and particle size within the
(v/v), and exceeding porosity beyond 70%–74% range of 1–5 µm (Figure 2(A)); (ii) polyvinyl butyral
(v/v) [13] . Compression moulding is perhaps the most (PVB, Grade BN18, Whacker Chemicals, UK) with
–3
appropriate form of material processing to produce density of 1100 kg·m ; (iii) polyethylene glycol (PEG,
bioactive PEEK compounds due to its ability to MW = 600, Whacker Chemicals, UK) with density of

International Journal of Bioprinting (2015)–Volume 1, Issue 1 67