1

2  Scaffold architecture: High interconnected porosity, m size - for vascularization, cell /nutrient transport and, ultimately, bone regeneration  Surface: Osteoconductivity - friendly surface chemistry/ topography for cell attachment and function  Mechanical properties: Strength and stiffness to provide post-operative stability and adequate bio-mechanical environment for cell function Load-bearing bone scaffolds - REQUIREMENTS  Porous CaP ceramics and polymers do not posses the mechanical properties required for demanding load-bearing applications Metal foams can have high strength provided they possess regular pore architecture bone scaffold

3 Ti powder + I powder  TiI 4 (gas) Ni + TiI 4  Ni x Ti y  NiTi Ni foam Ti I TiI  m INCOFOAM ® Nickel foam Trabecular Nitinol - PIRAC processing PIRAC - Powder Immersion Reaction Assisted Coating NiTi foam

4 10  m inside strut 50  m PIRAC conversion is controlled by diffusion x ~ t PIRAC conversion into trabecular NiTi - different Ni foams 500  m 75% porosity 900  C, 8 h 500  m 93% porosity 900  C, 1 h Trabecular bone I. Gotman, Adv Eng Mater 12(7):B320 (2010)

5 Yield stress  y [MPa] Elastic modulus E [GPa] Porosity % As-processed % Compressed % Compressed % Compressed 1 , %, % Compression test , MPa mm 500  m Trabecular Nitinol - MECHANICAL BEHAVIOR compare with 12 MPa for TMT Light-weight: < 40% of TMT at the same porosity Easily formed into complex shapes before PIRAC processing (Ni foam) Easily machined (less ductile than TMT)

6 Trabecular Nitinol - preliminary compression FATIGUE TEST Under cyclic loading, Trabecular NiTi performs much better than either Trabecular Metal (Ta) or Actipore NiTi no failure no microcracks our trabecular NiTi 59% trabecular Ta ~69% Actipore NiTi ~65% 120 MPa, 1000 cycles P. Sevilla et al., J Alloys Compounds 439 (2007) 67

7 Coating trabecular NiTi with a Ti-rich layer  Ni ion release from NiTi is a concern 1st step: PIRAC nitriding at 900C, 2 h A uniform ~ 1 m thick TiN coating is formed on NiTi foam struts by nitriding 2  m TiN NiTi strut Etch Depth (nm) N KL1 Ni LM2 O KL1 Ti LM2 AES depth profile 2nd step: 2nd titanization at 900C, 2 h 20  m as-producedsurface treated 20  m  2 wt. % Ni ~ 37 wt.% Ni A near Ni-free strut surface with micro-faceted morphology is obtained after the two-step PIRAC nitriding/titanization treatment.

8 i corr at 0.25 V [A/cm 2 ] E b [V] E corr [V] as-produced NiTi surface- modified NiTi Trabecular Nitinol - corrosion behavior and ion release Low Ni ion release, especially after surface modification, should lead to excellent long-term biocompatibility of trabecular NiTi Trabecular NiTi surface-treated trabecular NiTi Ringer's solution, 37  C, 1 mV/s Ni release,  g/cm 2 Immersion time, days as-processed trabecular NiTi surface-modified trabecular NiTi Ringer's solution, 37  C

500  m 9 20  m 200  m cell spreading and adhesion after 1 day culture Stem cells (hMSC) on Trabecular Niti nol in osteogenic culture medium 500  m cell proliferation after 21 days culture hMSCs spread on trabecular NiTi, proliferated and differentiated down the osteogenic lineage (assessed by AlamarBlue, ALP activity and Alizarin Red S staining) I. Gotman et al., Acta Biomater, accepted

10

11

12 To improve the efficiency and thus save energy one of the trends is to increase the maximum service temperature at least up to ºC. One of the problems: oxidation resistance at higher temperatures higher temperatures Possible way to solve the problem - Oxidation resistant coatings Specimens placed into stainless steel containers with Cr powder mixed with 2% of Iodine and Ti as getter of nitrogen

13 Element Weight % Atomic %Formula Cr K Cr2N Fe K Fe N Totals100.00

14 Weight changes of the Cr PIRACcoated samples after oxidation at 750ºC in air PIRAC coated at 900ºC, 4h show the best results

15 SEM cross-section of as-received and Cr PIRAC coated sample oxidized at 750ºC, 4h Oxidation of as received specimen results in tens of microns thick oxide layer while at PIRAC Cr coated specimens the oxide layer is 2÷3 μm thick; To have better oxidation protective layer at very long exposures at 750ºC, thicker Cr based PIRAC coatings should be prepared

16

17 T24 steel (PIRAC in Cr-2I, 750C, 240 h) oxidized at 750C, 120 h h No iron (Fe) is detected on the surface of PIRAC-coated steel after 120 h oxidation at 750C. On the surface mainly Cr 2 O 3 The thickness of oxidized layer is about 5 µm

18

19 718Ni-based superalloy in (Cr-2% I) 900C, 4 h (Ti getter) Cr Ni Fe PointTiCrFeNi wt.% , PIRAC in Cr-2I, 900 ⁰ C,4h Cr Θ, deg. INTENSITY

20 718Ni-based superalloy in (Cr-2% I) 900C, 4 h (Ti getter) Coating thickness as function of exposure 850  C, 4.5 h 850  C, 9 h 800  C, 16 h 850  C, 3 h 6  m 7.5  m 10  m 12  m 850  C, 9 h before nitriding after nitriding 2Θ, deg INTENSITY 718, PIRAC in Cr-2I, 900 ⁰ C, 4h +900 ⁰ C, 2h in C 2 N Cr 2 N

in (Cr-2% I) 900C, 4 h + (Ti-2%I) 850C, 2 h + nitriding 900C, 2 h Surface composition (EDS), wt.% TiCr NTiCr before nitriding after nitriding 718 PIRAC in Cr-2I, 900 ⁰ C,2h Ti, 850 ⁰ C,2h; TiN, 900 ⁰ C, 2h TiN Ti 2 N INTENSITY 2Θ, deg INTENSITY 718, PIRAC in Cr-2I, 900 ⁰ C, 2h ;Ti,900 ⁰ C,2h, TiN, 900 ⁰ C,2h TiN Ti 2 N 2Θ, deg Microhardness, VHN.>2000 kg/mm 2

22

23 2nd stage - PIRAC nitriding

24