characterization of porous ti-structures (.pdf)
TRANSCRIPT
Characterization of porous β-Ti structures
J. Luyten, J. Schrooten, J. Van Humbeeck,J.P. KruthKatholieke Universiteit Leuven
Belgium
1ste BioTiNet Workshop, 24-27 October 2011, Ljubljana, Slovenia“Advanced Methods for Material Characterization”
The outline• Introduction
• K.U.Leuven & BioTiNet• Bone Tissue Engineering• Material characterisation along the manufacturing process
• Principles of Selective Laser Melting (SLM)• SLM optimization of porous β-Ti scaffolds• Functionalisation of of β-Ti scaffolds• Biological in vitro & in vivo evaluation β-Ti scaffolds• Conclusions and plans
Introduction 1• The task of K.U.Leuven in
the BioTiNet project is• to produce β Ti – scaffolds by
SLM• To functionalised their surface
for orthopaedic use as part of a bone tissue engineering approach
Tissue Engineering for Bone generation
Bone defect
Operation Room
Patient own cells + growth factors
Scaffold
Healed bone defect
Bioreactor
Scaffold seeding and cell culturing
In vitro
Cells + medium
Introduction 2
• Using a β Ti as alloy for orthopaedic devices provides the following advantages– Improved biocompatibility by a combination of a
low E-modulus with a high strength (stress shielding)
– Reduced toxicity, materials free from elements as Al, V, Ni,…
– Improved ductility by the bcc crystallographic structure
– Possibility to improve wear and corrosion resistance by certain alloying elements
Introduction 3• Intensive characterization is used to control and to
optimise the envisioned requirements during the different synthesis steps
• The analyses are focused on– The starting powder properties– The SLM processing parameters– The heat treatments to get the microstructure needed– The functionalization of the scaffold surface– The cell-material surface interaction
Selective Laser Melting process (SLM)• SLM is an additive manufacturing technique in which functional,
complex parts can be created directly by selective, local melting of powder layers through interaction with a focused laser beam
• Applications: low production volume, complex parts made of expensive or non-machinable parts for the medical sector, aeronautics, electronics and tools making
• Main advantage:– Layer wise building = high geometrical freedom+ very flexible– Near net shape production
• Problems to solve- High temperature gradients = non-equilibrium microstructure + thermal stresses,
surface roughness- Line and layer wise building = porosity and anisotropy
Principles of SLM
Ti scaffolds by SLM
Controllable parameter Limitations
Design Minimal pore size
Morphology Minimal strut size
Mechanical properties Surface roughness
Powder requirementsRequirement Characterisation
techniqueFlowability (free flowing
powder)Hall, Carnet, Aptis
Particle size between 10 and 60 µm
Laser diffraction
Spherical particles (atomised)
SEM
Composition and impurities (O, N, H, C,)
IGA
Crystallographic phase XRD
SLM parameter optimisation• Development of the SLM manufacturing of porous β Ti scaffolds required 3 basic steps– Production of dens structures– Homogenisation of the structure by adapted heat
treatments– Manufacturing of porous structures
• Initial SLM optimisation– At first laser power, scan speed and line distance have to
be optimized including density control by Archimedes measurements
– Also O- content control is also very important for the ductility of the specimens
– Mechanical properties
Microstructural evolution during SLM
SLM optimisation and characterisation (with and without heat treatment)
Properties Characterization techniqueDensity Archimedes
O-content IGA
Microstructure SEM, EDAX, XRD, …
Mechanical properties Tensile ,compression, hardness tests
E-modulus, σ0.2, σUTS, δ (%) Pulse excitation, tensile tests
Fatigue Specific fatigue tests (compression)
Wear resistane Tribological tests
Mechanical properties • µCT combined with in-situ loading
• Global & local mechanical properties• 3D strain distribution
Pore 0.8 Pore 1.0 Pore 1.2
Strain at max strength
[%]
Strength[MPa]
Stiffness[MPa]
As- produced
6.04 ± 0.32 13.00 ± 0.62
397.07 ± 29.95
Surface treated
7.02 ± 0.24 7.41 ± 0.88 226.15 ± 22.45
Bone scaffold requirements• Biocompatibility: bio-inert or bio-active• Bio-inert metals: Ti-6Al-4V, Ti, SS, Ta• Bioresorbable ceramics:
hydroxyapatite, α- or β-tri calcium phosphate
• Biodegradable polymers: PGA, PLA, PGLA
• Structural parameters:• High porosity• Open porosity
– Allowing osteoprogenitor cell seeding, cell attachment and cell migration
– Mass transport cell nutrition• Interconnectivity• Specific surface area• Adequate mechanical behavior
Requirements porous scaffoldsProperties Range order
Porosity >65%Interconnectivity of the pores >90%
Pore size (d) 50 µm <d < 500µmE-modulus < 3 GPa
Compression strength > 40 MPaDuctility (max strain) > 10 %
Characterization porous structuresProperty Characterization technique
Porosity IA, µCT, Hg-porosimetry
Interconnetivity IA, µCT, Hg-porosimetry
Pore size IA, µCT, Hg-porosimetry
E-modulus Pulse excitation, tension tests
Compression strength Compression tests
Ductility (max. strain) Compression testsPermeability Perfusion tests
y = 1.0994x - 0.1948R2 = 0.9976
y = 1.1699x - 0.2482R2 = 0.9636
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Designed pore size (mm)
Man
ufac
ture
d po
re s
ize
(mm
)
pore side planeLMpore top surfaceLM
0.2mm
0.2mm
Morphological 3D by µCTscaffold (n=4) po 1.00
Global porosity (%) 81.05 ± 0.51
Specific surface (1/mm) 4.41 ± 0.15
Average pore size (µm) 620.55 ± 2.91
Average strut size (µm) 239.72 ± 1.57
Interconnectivity (%) 100
Cell-scaffold interaction • Cell attachment and further, proliferation, differentiation and
migration are strongly influenced by the scaffold’s surface morphology and composition
• Modification of the surface morphology• Adjusting roughness, microporosity and specific surface• By changing the SLM parameters• By sand blasting• By (electro)chemical etching
• Modification of the surface composition by coatings• Bioactive glass & CaP• By electrophoresis• By electrodepostion• By sol gel• By biomimetic precipitation
Surface characterisationProperty Characterization technique
Roughness Roughness measurementMicroporosity Hg-porosimetry
N2 -adsorption/ desorption Specific surface BET
Surface composition SEM-+EDAX, XPS, FTIR, Raman
Biological activity Cell tests in vitro and in vivo
Surface treatment of Ti scaffolds• Combined chemical &
electrochemical polishing• Single strut roughness
quantification by SEM• 3D morphology by µCT
Etched Ti surface and CaP coating
Quantification cell-material interactions• 2D & 3D cell culture• Instrumented bioreactors• Cell imaging in 3D• Quantification protocols
• Viability, DNA, RNA, metabolic activity and cell distribution
• Surface functionalisation• Analysis, interpretation & feedback
Standard in vitro & in vivo SLM scaffold assessment
• Bioreactors• ZETOS: Biomechanical stimulation
cell-carrier constructs• 2D+ imaging perfusion bioreactor: cell
behaviour & bioresponse• Perfusion bioreactor: 3D cell seeding,
culturing & scaling-up
• Animal models• Ectopic nude mouse/rat model:
standard in vivo assay, in parallel with bioreactor experiments
• Orthotopic mouse/rat model: functional - Load-bearing – host integration
• Dynamic quantification
Conclusions and plans• The available characterisation techniques will be used during the
optimization of the SLM synthesis and the functionalisation of porous β Ti-scaffolds
• The obtained properties of the dense structures produced by SLM will be compared to– Conventional manufacturing techniques as hot forging and rolling– Other additive manufacturing techniques as E-Beam, 3D fiber deposition-, 3D
printing techniques whereby Ti and Ti6Al4V are used
• The final goal is to get not only a more biocompatible material but also a more bioactive scaffold by applying a specific surface functionalisation