Page 81 - IJB-2-2

Kuppuswamy Hariharan and Ganesan Arumaikkannu

infrared range (λ = 1060 nm). Surface features like closer to a low and adequate temperature that they can
microstructure, topography, crystal structure, func- be handled and exposed to ambient temperature and
tional groups and mechanical properties like hardness, atmosphere. Finally, the parts are removed from the
Young’s modulus and adhesion strength were investi- powder bed and loose powder is cleaned off from the
gated along with cell response to the coated surface. parts [46,47] .
Generally large sized particles and hard coating will
be evident when the film was formed using Nd:YAG 2.2 Preparation of Hydroxyapatite Target for Coating
laser in infrared range, which in turn favours the bio- The chosen coating material, HA, was synthesised via
compatibility by bone-implant anchoring [44] . wet chemical precipitation process. It involves stirring
3
2. Materials and Methods 0.5 mol/dm of calcium hydroxide [Ca(OH) 2] for a
period of 30 min in 1000 mL distilled water. Ammo-
3
2.1 Fabrication of Substrate nium phosphate [(NH 4) 3PO 4, 0.3 mol/dm ] is stirred in
the same manner in another solution of 1000 mL dis-
A 3D CAD model with a dimension of 25 × 25 × 3 mm tilled water and this is added in drops to the Ca(OH) 2
was designed and the data was sliced into layers be- solution. After a couple of hours of preparation, main-
cause AM fabricates parts in layer by layer. The part tained at a pH level at 7 and above, a gelatinous pre-
has been built using SLS technique (EOS FORMIGA cipitate is obtained. In order to extract the HA powder,
P100). SLS fuses thin layers of polyamide powder [48]
(EOSINT P/PA2200) [45] which has been spread across the precipitate is calcined at 100°C for 5 h . The
the build area using a counter-rotating powder level- powder is then hydraulically pressed at 400 MPa to
ling roller or blade. The part building process takes form a 25 mm diameter deposition target that is fur-
place inside an enclosed chamber and to minimise ther furnace sintered at 800°C for a period of 5 h.
oxidation and degradation due to atmospheric gases, 2.3 Pulsed Laser Deposition
nitrogen was allowed to flow inside the chamber. The
powder in the build platform was maintained at a Polyamide was pulsed laser deposited with HA using
temperature just below the melting point or glass tran- Nd:YAG laser (Quanta Laser, USA) consisting a
sition temperature of the polyamide material and it wavelength (λ) of 1064 nm and energy of 135 mJ. Af-
was preheated using infrared heater which was placed ter evacuating the deposition chamber with a base
–5
above the build chamber as well as the powder feed pressure of 10 Torr, the laser beam was brought to
chamber. This heater was generally used to maintain focus on the rotating target at an incident angle of ~45°
the elevated temperature throughout the fabrication for deposition. The laser beam scans the continuously
process, which minimises the laser power require- rotating target to serve three purposes: (a) minimisa-
ments of the process, with pre-heating, less laser en- tion of craters formation, (b) reduction of undesirable
ergy is required for fusion and could prevent warping melting and (c) reduction in re-solidification. The ro-
of the part during the build due to non-uniform ther- tation of the target also helps enhance the erosion rate.
mal expansion and contraction (curling). Once appro- While maintaining the substrate at a distance of 4.5
priate preheating of the powder was done, a focused cm from the target and at a temperature of 150°C the
laser beam is directed onto the powder bed and moved HA layer deposition took place at a period of 30 min.
according to the CAD design to thermally fuse the
material to form the sliced cross-section. Surrounding 2.4 Film Characterisation
un-sintered powders act as a support structure to the The microstructure of the coated surface was exam-
part and eliminate the external secondary powder to ined using field-emission scanning electron micros-
use as a support material. After completing a layer, the copy (CARL-ZEISS Supra 40VP, FE-SEM) and en-
build platform is lowered according to the defined ergy dispersive X-ray spectroscopy (EDX) (Oxford
layer thickness and a new layer of powder is laid and Instruments). Analysis was carried out to find the
levelled using the counter-rotating roller. The laser stoichiometry of the coated layer for 60 s and electron
beam scans the subsequent slice cross-section. This beam energy of 15 keV. Atomic force microscope
process repeats until the complete part is built. Once (AFM) (NTMDT, Ireland) analysis was carried out at
the part has been completed, a cool-down period is an ambient pressure, room temperature and humidity.
usually required to allow the parts to uniformly draw AFM images were prepared with non-contact tapping

International Journal of Bioprinting (2016)–Volume 2, Issue 2 87