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