Biomedical Porous Shape Memory Alloys for Hard-Tissue Replacement Materials

Abstract

Porous shape memory alloys (SMAs), including NiTi and Ni-free Ti-based alloys, are unusual materials for hard-tissue replacements because of their unique superelasticity (SE), good biocompatibility, and low elastic modulus. However, the Ni ion releasing for porous NiTi SMAs in physiological conditions and relatively low SE for porous Ni-free SMAs have delayed their clinic applications as implantable materials. The present article reviews recent research progresses on porous NiTi and Ni-free SMAs for hard-tissue replacements, focusing on two specific topics: (i) synthesis of porous SMAs with optimal porous structure, microstructure, mechanical, and biological properties; and, (ii) surface modifications that are designed to create bio-inert or bio-active surfaces with low Ni releasing and high biocompatibility for porous NiTi SMAs. With the advances of preparation technique, the porous SMAs can be tailored to satisfied porous structure with porosity ranging from 30% to 85% and different pore sizes. In addition, they can exhibit an elastic modulus of 0.4⁻15 GPa and SE of more than 2.5%, as well as good cell and tissue biocompatibility. As a result, porous SMAs had already been used in maxillofacial repairing, teeth root replacement, and cervical and lumbar vertebral implantation. Based on current research progresses, possible future directions are discussed for "property-pore structure" relationship and surface modification investigations, which could lead to optimized porous biomedical SMAs. We believe that porous SMAs with optimal porous structure and a bioactive surface layer are the most competitive candidate for short-term and long-term hard-tissue replacement materials.

Keywords: NiTi; biocompatibility; porous material; shape memory alloy; surface modification; β type Ni-free Ti alloy.

Figure 1

The stress-strain curves for dense NiTi shape memory alloys (SMAs), stainless steel, bone, and tendon [6].

Figure 2

Schematic determination of MT temperature in a NiTi SMA via a differential scanning calorimetry (DSC) plot [19].

Figure 3

Two-dimensional (2D) crystallographic mechanism of SME (V₁ represents one martensite variant, V₂ represents another variant).

Figure 4

Stress-strain curve of superelasticity (SE) in an SMA (ab—elastic deformation of austenite, bc—forming martensite by stress induced martensitic transformation (SIMT), cd—elastic deformation of martensite, de-recovery of elastic deformation of martensite, ef—austenite forming by reverse MT, fg—recovery of elastic deformation of austenite).

Figure 5

Microstructure illustration of human hard-tissue: (a) human bone [23] and (b) human tooth [34] (magnification image is SEM of actual tissue in a specific region).

Figure 6

Homogeneous pore structure in porous NiTi SMAs by various methods: (a) CS [12]; (b) SHS [57]; (c) thermal explosion mode in high temperature synthesis (SHS) [12]; (d) capsule-free hot isostatic pressing (CF-HIP) [12]; (e) SLM (the inset image is the designed part corresponding to the fabricated one) [63]; and, (f) SLM (the inset image is a macrograph) [64].

Figure 7

Inhomogeneous pore structure in porous NiTi SMAs: “sandwich-like” structure (a), radial gradient structure (b) by CF-HIP [84], radial gradient structure (c) and axial gradient structure (d) by low pressing sintering.

Figure 8

Schematic illustration in the filling of the powder mixture for fabricating two gradient porosity NiTi SMAs (Ti-1 or Ti-2 represents one Ti powder with a certain particle size, the same meaning for the Ni-1 or Ni-2).

Figure 9

Microstructure of porous NiTi SMAs by: (a) conventional sintering (CS) with Ni/Ti powders at 1100 °C for 2 h; (b) CS with Ni/TiH₂ powders at 1100 °C for 2 h [86]; (c) CF-HIP, 1050 °C for 3 h [54]; and, (d) SHS and post-reaction heat treatment at 1150 °C for 1 h [58].

Figure 10

Schematic model of microstructural evolution during reactive sintering and furnace cooling: (a–d) Ni/Ti; (e–h) Ni/TiH₂. Region <942 °C is the solid-state reaction process. Dehydrogenation and separation of newly born Ti powders are shown in (f); Region >942 °C is the LPS; Region cooling is final structure below 620 °C after furnace cooling. White lines and black dots indicate the needle-like Ni₃Ti and spherical NiTi₂ eutectoid precipitates formed during cooling. [86].

Figure 11

DSC heating (a) and cooling (b) curves of porous NiTi SMAs fabricated by three methods (CS, SHS, and CF-HIP) [66].

Figure 12

Martensitic transformation temperature dependence of porosity for porous NiTi SMAs with different compositions: (a) Ni_50.8Ti_49.2 [54,66]; and, (b) Ni₄₉Ti₅₁ [76].

Figure 13

Compressive stress-strain curves: (a) homogeneous porous NiTi SMAs by CF-HIP with different porosities at RT; and, (b) gradient porosity of 20–61% [74].

Figure 14

Compressive load–unload recovery cycles under the compressive stress of porous NiTi SMAs by CS with 15% (a) and 28% (b) porosity [86].

Figure 15

Superelastic behavior of the porous SMAs: (a) elastic modulus for the samples with a porosity of 0.1, 0.2, and 0.4; (b) macroscopic critical SIM stress for the onset of transformation as a function of porosity; and, (c) average stress–strain responses for the samples with porosity. [90].

Figure 16

The relationship between elastic modulus (a), compressive stress at 3% strain (b), maximum superelastic strain (c) and porosity for porous NiTi (Ti_49.2Ni_50.8) SMAs prepared by various methods (data from refs. [160,161,162] are Ti_49.4Ni_50.6).

Figure 16

The relationship between elastic modulus (a), compressive stress at 3% strain (b), maximum superelastic strain (c) and porosity for porous NiTi (Ti_49.2Ni_50.8) SMAs prepared by various methods (data from refs. [160,161,162] are Ti_49.4Ni_50.6).

Figure 17

The S-N curves for porous NiTi SMAs with different porosities in terms of (a) the yield value and (b) the maximum applied stress [105].

Figure 17

The S-N curves for porous NiTi SMAs with different porosities in terms of (a) the yield value and (b) the maximum applied stress [105].

Figure 18

Loading-unloading curves for a sintered porous NiTi implant (Φ4 × 4 mm³ cylinders) deformed in compression mode before implantation (curve 2), after one month (curve 3) and after three months (curve 1) of implantation in rabbits [110].

Figure 19

Extracted electrochemical parameters from polarization curves in a 0.9% NaCl solution for 24 h with porosity in porous NiTi SMAs [110].

Figure 20

Ni ion releasing content of porous NiTi SMAs by CF-HIP with different pore sizes dependence of immersion duration (the safety line represents for the acceptable Ni ion content to the human body 0.5 μm/cm²/week [111]).

Figure 21

Ni leaching level of porous NiTi treated by different wet chemical treatments (a), in comparison with untreated NiTi and safety Ni level to human body; (b) is an enlarged area from (a). (PA: passivation in HNO₃; O-PIII: Oxygen PIII; NaOH-treated: treated in NaOH solution) [111].

Figure 22

Optical image (a) and SEM image (b) of porous NiTi SMAs fabricated by sintering and in-situ nitriding (inset image in (a) is a macrographic image) [165].

Figure 23

In vitro cell attachment (a) and in vivo Ni leaching (b) porous NiTi SMAs treated by in situ nitriding comparing with untreated porous NiTi [165].

Figure 24

Biomedical products made by porous NiTi SMAs: (a) Cervical spine implantation [169]; (b) lumbar spine implantation [169]; (c) intervertebral fusion device [169]; (d) acetabular cup [167]; (e) tooth [170]; and, (f) gum tissue replacement [167].

Figure 24

Biomedical products made by porous NiTi SMAs: (a) Cervical spine implantation [169]; (b) lumbar spine implantation [169]; (c) intervertebral fusion device [169]; (d) acetabular cup [167]; (e) tooth [170]; and, (f) gum tissue replacement [167].

Figure 25

DSC curves of the sintered Ti-(8–18) at.% Nb alloys (0.8 wt.% oxygen in the alloys) [194].

Figure 26

The effect of Nb on Ms (a) for Ti-Nb binary alloys and alloy elements on Ms (b) for Ti-Nb-based ternary alloys [172].

Figure 27

The stress-strain curves obtained by tensile test for Ti-19Nb-9Zr SMAs: (a) cyclic loading-unloading; (b) after five cycles of loading-unloading tests [209].

Figure 28

Compressive stress-strain curves of Ti-11Nb and Ti-22Nb alloys (a), and constant strain (pre-strain = 5%) cycling curves (b) of Ti-11Nb after solution treatment at RT [212].

Figure 29

Extended phase stability index diagram based on $\overline{\mathrm{Bo}}$ and $\overline{\mathrm{Md}}$ parameters in which about 80 shape memory and low elastic Ti alloys. The shadow zone indicates the coexistence of both properties [216].

Figure 30

Mechanical properties of Ti-based SMAs, in comparison with other materials: (a) tension strength vs. elongation [217]; and, (b) yield strength v.s. young’s modulus.

Figure 30

Mechanical properties of Ti-based SMAs, in comparison with other materials: (a) tension strength vs. elongation [217]; and, (b) yield strength v.s. young’s modulus.

Figure 31

Fatigue properties of Ti-Nb-Ta-Zr alloys subjected to aging treatment at 573 K for 10.8 ks (AT10.8), solution treatment (ST), and severe cold rolling (CR) [223].

Figure 32

Attached number of living-MG63 cells (a) and cell proliferation index (b) as a function of the incubation time. The groups marked with the same symbol have no statistically significant differences at different times of culture [230].

Figure 33

Histotomy of bone contact of the Ti–Nb alloy (a,c) and Ti (b,d) at 4 weeks and 12 weeks illustrated by fluorescence-dyeing reagents, respectively. Green (#) revealed the new bone formation of two-week duration dyed by calcein, and the yellow (Δ) revealed that new formation of four-week duration by tetracycline [231].

Figure 34

Oxygen, carbon, nitrogen, and hydrogen contents in bulk, powder and porous Ti-Nb-Zr specimens [210] (Inset contains maximum concentrations according to ASTM F67-00).

Figure 35

Pore morphology of porous Ti-22Nb-6Zr SMA with various porosities by different methods: (a) 6.7% by CS [241]; (b) 12% by CF-HIP [234]; (c) 57.6% by CS with NH₄HCO₃ [241]; and, (d) 65% by CS with pore forming agent from alloy powder [217].

Figure 36

Porous Ti-25 wt.% Nb alloys with 70% porosity fabricated by PM using polyurethane as space holder [243]: (a) macrographic image, and (b) pore structure.

Figure 37

(a) The morphology of the electron beam melting (EBM)-produced porous Ti-24Nb-4Zr-8Sn (wt.%) SMAs; (b) the single unit of 3D rhombic dodecahedron modeling; and, (c) the surface morphology [239].

Figure 38

Compressive stress-strain curves of porous Ti-Nb-Zr alloys and cortical bone [210].

Figure 39

Stress-strain cycle curves for porous Ti-21Nb-5.5Zr alloys under various testing modes at RT: (a) compressive; (b) tensile; and, (c) bending [210].

Figure 40

The relationship between elastic modulus (a); compressive strength; (b) [210]; and, superelastic strain (c) [210,217,241] and porosity for Ni-free SMAs.

Figure 41

(a) The normalized S-N curves of the porous Ti2448 and Ti-6Al-4V alloys with different porosities, and (b) the relationship of Young’s modulus and the fatigue strength for the porous Ti2448 and Ti-6Al-4V specimens [239].

Figure 42

Confocal micrographs of cell growth: (a) inside pores and (b) on the surface of the porous TiNbZr alloys [14].

Figure 43

Biomedical applications made by porous Ni-free SMAs: (a) spine implantations [167]; and, (b) spine complete replacements [263].