Biomaterials

For the fabrication of implants, laser-based technologies are gaining rapidly growing importance. Additive Manufacturing techniques such as Selective Laser Melting (SLM) are regarded as THE key technologies for metal implants. They offer a great potential for the fabrication of patient-specific bone implants. In the focus of current research on bone implant materials, are novel (near-) beta Ti alloys, with biocompatible compositions. Owing to their low Young’s modulus values, they can lead to effectively reduced implant stiffness and thus, to improved adaption to bone properties. Likewise, implant surfaces with best suitable biofunctional properties are important. For novel Ti alloys, new concepts are being developed for attaining tailored implant surface functionality with regard to roughness, topography and chemistry enabling improved osseointegration. Those new developments are in particular relevant for patients with systemic bone diseases like osteoporosis.

Zhuravleva, K. et al., Production of Porous ß-type Ti–40Nb Alloy for Biomedical Applications: Comparison of Selective Laser Melting and Hot Pressing, Materials 6 (2013) 5700.

Project: SAB OsteoLas, DFG

Beta-type (β) Ti-(40-45)Nb alloys are promising materials for load-bearing bone implants in orthopaedics and trauma surgery. They exhibit very low Young’s moduli of ~60 GPa yielding a very low implant stiffness which contributes to the reduction of stress shielding effects.
For achieving optimum mechanical biofunctionality, different hardening strategies based on complex warm and cold rolling treatments in combination with annealing were developed. Most effective are routes comprise grain boundary hardening with preservation of the β-phase and further work hardening or precipitation hardening. Low temperature annealing (300°C) of recrystallized Ti-40Nb samples caused nanoscale ω-precipitations. Those stabilize the β-phase against stress-induced martensite formation and significantly increase the tensile strength to values >900 MPa which is comparable to values for Ti-6Al-4V. Further, the fatigue resistance is markedly increased.

Pilz, S. et al., Thermomechanical processing of In-containing ß-type Ti-Nb alloys, J. Mech. Behav. Biomed. Mater. 79 (2018) 283.
Helth, A. et al., Effect of thermomechanical processing on the mechanical biofunctionality of a low modulus Ti-40Nb alloy, J. Mech. Behav. Biomed. Mater. 65 (2017) 137.
Calin, M. et al., Elastic softening of Beta-type Ti-Nb alloys by indium (In) additions , J. Mech. Behav. Biomed. Mater. 39 (2014) 162.

Project:  DFG SFB Transregio 79

The application of powder metallurgy for β-Ti-Nb alloys enables new developments towards a variety of near-net shape techniques as well as tailored implants with an porosity for permanent bone implants. The introduction of an open porosity is advantageous for bone ingrowth and allows significant reduction of the implant stiffness.
Ti-(40-45m.%)Nb samples were successfully prepared by hot pressing or sintering of mechanically alloyed or gas-atomized β-TiNb powder without/with space holder phase. Due to their refined microstructure, fully dense samples show a significantly higher yield strength >900 MPa than conventionally casted β-TiNb-alloys. Scaffolds with 30-70% porosity were produced with NaCl as space holder, with very low Young’s modulus (10-60 GPa) but only limited yield strength. The empirical determination of the relationship between porosity and Young’s modulus was further assessed by finite element analysis.

Schmidt, R. et al., Powder metallurgical processing of low modulus beta-type Ti-45Nb to bulk and macro-porous compacts, Powder Technol. 322 (2017) 393.
Zhuravleva, K. et al., Phase transformations in ball-milled Ti-40Nb and Ti-45Nb powders upon quenching from the beta-phase region, Powder Technol. 253 (2014) 166.
Zhuravleva, K. et al., Determination of the Young’s modulus of porous beta-type Ti-40Nb by finite element analysis, Mater. Des. 64 (2014) 1.
Zhuravleva, K. et al., Mechanical alloying of beta-Type Ti-Nb for biomedical applications, Adv. Eng. Mater. 4 (2013) 262.

Project: DFG SFB Transregio 79

Alternative methods to the conventional powder production process were investigated to synthesis ultrafine-grained Ti and TiNb-powders for the production of alloys with a high strength to Young’s modulus ratio. Starting from pure Ti and Nb powders, β-type Ti-40Nb with a submicron grain size was produced by reactive milling in hydrogen atmosphere and subsequent thermal desorption treatment. Furthermore, a calciothermic synthesis route was developed to produce hydrogenated Ti and Ti–Nb powders directly from oxides. Fine grained powders with a particle size well below 45 µm and a rounded shape were achieved. These hydrogenated powders are suitable for the convectional press and sintering technique or MIM.

Lindemann, I. et al., Calciothermic Synthesis of Very Fine, Hydrogenated Ti and Ti–Nb Powder for Biomedical Applications, Adv. Eng. Mater. 3 (2019) 1901210/1-7
Lindemann, I. et al., Ultrafine-grained Ti-40Nb prepared by reactive milling of the elements in hydrogen, J. Alloys Compd. 729 (2017) 1244
Lindemann, I. et al., Synthesis of spherical nanocrystalline titanium hydride powder via calciothermic low temperature reduction, Scr. Mater. 130 (2017) 256

Projects:  DFG

Ti-alloys represent the principal structural materials in both metallic biomaterials and aerospace design development. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations.
In particular the class of β-stabilized Ti-alloys represents highly promising multifunctional materials with desirable structural and functional properties for various biomedical and engineering applications. Due to the thermoelastic martensitic transformation of the body centred cubic (bcc) β-phase to orthorhombic martensite α″ this alloy family also demonstrates shape memory (SM) behavior and superelasticity (SE). Up to the present day, these features have been stirring ever increasing interest.
Since the discovery of SM in Ti–Nb this system serves as a prototype to study SM and SE in Ni-free Ti-based alloys. Its importance is underlined by the fact that most of the recently developed low-modulus as well as SM and SE β-stabilized Ti-based alloys are modifications of the Ti–Nb. Hence, a better understanding of the phase transformations occurred in this binary alloy system will help explain, at least to some extent, the alloys derived from it.
In a cooperation work with partners from BioTiNet EU project (www.biotinet.eu) the mechanical and functional properties of metastable Ti-Nb alloys are optimized by controlled adjustment of the microstructural parameters via complex thermomechanical processing paths.

M. Bönisch et al., Routes to control diffusive pathways and thermal expansion in Ti-alloys, Sci. Rep. 10 (2020) 3045/1-9
M. Bönisch et al., Giant thermal expansion and a-precipitation pathways in Ti-alloys, Nat. Commun. 8 (2017) 1429/1-9
M. Bönisch, et al., Factors influencing the elastic moduli, reversible strains and hysteresis loops in martensitic Ti-Nb alloys, Mater. Sci. Eng. C 48 (2015), 511
M. Bönisch, et al., Thermal stability and phase transformations of martensitic Ti-Nb alloys, Sci. Technol. Adv. Mater. 14 (2013) 55004/1-9

Project: EU ITN BioTiNet

Titanium and its alloys are widely used in medical device industry due to their excellent corrosion resistance, low density, superior mechanical properties and high biocompatibility. A major problem is the reduced strength and hardness of these alloys, which may cause implant fracture. Limited wear resistance can also create the incidence of allergy and inflammatory cascades because of the release of particles and/or toxic metallic ions. Improvement of their mechanical properties plays an important role in enhancing the biomechanical compatibility of metallic implants, leading to a long-term safety and durability in the human body.
The development of composite-type structures based on titanium, can significantly improve the wear resistance, hardness and mechanical strength of implant materials. Our studies are focused on two types of Ti-based composites obtained with different processing routes: a) Ti-matrix composites reinforced with TiB particles, processed by additive manufacturing (selective laser melting-SLM) and powder metallurgy, and b) Ti-based metallic glass-matrix composite, produced by controlled casting.
We highlighted the potential to develop additive manufacturing of novel, low-cost bulk and porous titanium-matrix composites (Ti-TiB) to meet the needs for biomedical orthopaedic implants. We provided a pathway to their development through the application of alloy composition design and reinforcement strategies, manufacturing optimisation and identification of process-structure-property relationships controlling performance in combination with computer-aided structural design tools.

H. Attar et al., Nanoindentation and wear properties of Ti and Ti-TiB composite materials produced by selective laser melting, Mater. Sci. Eng. A 688 (2017) 20
L.-C. Zhang, H. Attar, M. Calin, J. Eckert, Review on manufacture by selective laser melting and properties of titanium based materials for biomedical applications, Materials Technology 31 (2016) 66-76
H. Attar et al., Mechanical behavior of porous commercially pure Ti and Ti-TiB composite materials manufactured by selective laser melting, Mater. Sci. Eng. A 625 (2015) 350
H. Attar et al., Selective laser melting of in situ titanium–titanium boride composites: Processing, microstructure and mechanical properties, Acta Mater. 76 (2014) 13
H. Attar et al., Comparative study of microstructures and mechanical properties of in situ Ti–TiB composites produced by selective laser melting, powder metallurgy, and casting technologies, J. Mater. Res. 29 (2014) 1941

Metallic glasses are known to combine high strength and hardness with comparatively low Young‘s modulus values. This has raised the interest to evaluate in particular Ti-based glasses as possible biomaterials. However, alloy design demands a compromise between the achievement of high glass-forming ability and the use of non-toxic constituents. Alloys of the Ti-Zr-Si(-Nb) system can be rapidly quenched to glassy or glass-matrix (nanocomposite) states. Mechanical testing predicts high wear resistance. Corrosion tests in Ringer’s solution revealed very low corrosion rates similar to those of beta-type Ti-Nb and stable anodic passivity. Nb addition improves the glass-forming ability and the mechanical properties and also supports a high pitting resistance even at extreme anodic polarization. Thermal treatments are possible in a defined temperature-time window to preserve alloy’s microstructure and yield oxide films with characteristic morphologies. The apatite-forming ability was proven. In collaboration with Univ. Pardubice the generation of self-organized double-wall oxide nanotube layers was demonstrated. These properties recommend these new alloys for application as coating materials in the biomedical field.

H. Sopha, D. Pohl, …, A. Gebert, J. Macak, Self-organized double-wall oxide nanotube layers on glass-forming Ti-Zr-Si(-Nb) alloys, Mater. Sci. Eng. C 70 (2017) 258.
S. Abdi, M. Bönisch et al., Thermal oxidation behavior of glass-forming Ti-Zr-(Nb)-Si alloys, J. Mater. Res. 31 (2016) 1264.
S. Abdi et al., Corrosion behavior and apatite-forming ability of glassy Ti75-xZr10NbxSi15 (x=0,15) alloys – potential materials for implant applications, J. Biomed. Mater. Res. Part B 104 (2016) 27.
S. Abdi et al., Effect of Nb adition on microstructure evolution and nanomechanical properties of a glass-forming Ti-Zr-Si alloy, Intermetallics 46 (2014) 156.
M. Calin at al., Designing biocompatible Ti-based metallic glasses for implant applications, Mater. Sci. Eng. C 33 (2013) 875.

Project:  EU-ITN BIOTINET

In an effort to develop novel biocompatible glassy metallic materials with an appropriate combination of biomedical properties as well as MRI compatibility, we have adopted a new composition design strategy based on the high-entropy alloys approach by exploring the central region of multi-component alloy phase space of the Ti-Zr-Nb-Hf-Si system. The newly developed multi-principal element alloys have biocompatible compositions, with magnetic susceptibility tailored to the medical MRI environment. They exhibit high thermal stability and excellent corrosion stability in simulated body fluid. All alloys show a weak paramagnetic nature advantageous for reducing the MRI artefacts. In addition, they exhibit higher X-ray linear attenuation coefficients relevant for interventional X-ray-based medical imaging. This two-fold advantage (lower magnetic susceptibility and higher radiopacity) allows the materials to be more precisely visualized via biomedical imaging methods, which is especially important for miniaturised implants such as coronary stents or aneurysm clips.These results provide a guideline on how to design equiatomic multicomponent metallic glasses with unique combination of biomedical properties by utilizing HEA characteristics, and thus providing a strategy for bridging MGs and HEAs fields for the discovery of new alloys of scientific significance and practical benefit.

M. Calin et al., Tailoring biocompatible Ti-Zr-Nb-Hf-Si metallic glasses based on high-entropy alloys design approach, Mater. Sci. Eng. C (2020).

Nano- and micro-patterning of biomaterials is a rapidly evolving technology used in the engineering sciences to control cell behavior.  The surface topography of the implant material plays an important role in enhancing the cell-material interaction.  However, the practical applicability of nano-/micro-patterning strategies with metals is restricted due to intrinsic limits in processability of high-strength metals. Bulk metallic glasses (BMGs) constitute a class of materials ideally suited to surface patterning in view of biomedical applications. They exhibit a unique temperature-dependent mechanical behavior, which enables polymer-like processability at temperatures slightly higher than the specific glass transition temperature of BMGs. We studied the thermoplastic forming ability of Ti- and Zr-based BMGs with potential biomedical applications.  The unique processability and intrinsic properties of this new class of amorphous alloys make them competitive with the conventional metallic biomaterials.

In a cooperation work with partners from VitriMetTech EU project (http://www.vitrimettech.unito.it/) the unique thermoplastic behavior of Ni-free Ti- and Zr-based bulk metallic glasses has been investigated by utilizing the dramatic softening of the amorphous structure in the super cooled liquid region.

B. Sarac et al., Micropatterning kinetics of different glass-forming systems investigated by thermoplastic net-shaping, Scr. Mater. 137 (2017) 127
S. Bera et al., Micro-patterning by thermoplastic forming of Ni-free Ti-based bulk metallic glasses, Mater. Des. 120 (2017) 204

Project:  EU-ITN VitriMetTech

Besides Mg- and Zn-based alloys, Fe-based alloys belong to the new class of biodegradable (bioabsorbable or bioresorbable) metallic materials for biomedical applications. After providing a temporary metal-like functionality they should safely degrade in the human body without the generation of toxic residues.

● Fe-Mn layers were prepared by electrodeposition and by magnetron co-sputtering with envisioned application as component of biodegradable electronic devices and as alternative to polymers or pure metals used as packaging layers. Morphological, chemical, structural and magnetic characterization of the deposited layers was conducted and the biocompatibility characteristics were assessed. By electrodeposition, different ratios of Mn:Fe ions in the water-based sulfate electrolyte led to metallic and oxide based Fe-Mn layers. The electrodeposited layers showed different crystalline structure depending on their nature: a bcc-Fe crystal structure when they were metallic and fcc when they were oxidized. In addition, glycine was used as complexing agent in the electrolytic bath to reduce crack formation and to better control the Mn content of the layers (9-24 wt.%). In comparison, thin Fe-Mn films with Mn contents of 10-70 wt.% which were obtained by magnetron co-sputtering revealed a well-defined bcc structure that lowers its symmetry when the Mn content increases. The corrosion behavior of the deposited layers was evaluated in NaCl solutions by means of electrochemical studies and a trend of reactivity enhancement with increasing Mn content was detected. First in vitro cell tests revealed in particular for the magnetron co-sputtered films a high biocompatible characteristics with a cell viability of 85% even for the Mn-richest sputtered films.

M. Fernandez-Barcia, S. Kurdi et al., Comparative study of the sustainable preparation of FeMn thin films via electrodeposition and magnetron co-sputtering, Surf. Coat. Technol. 375 (2019) 182.
M. Fernandez-Barcia et al., Electrodeposition of mangenese layers from sustainble sulfate based electrolytes, Surf. Coat. Technol. 334 (2019) 261.

Project: EU ITN SELECTA

● Austenitic Fe-30Mn-1C- based alloys are fabricated at the IKM by means of casting and selective laser melting and they are developed with envisioned application as new materials, e.g. for biodegradable stents. For cast Fe-30Mn-1C samples fundamental electrochemical studies were conducted in different NaCl solutions as as well as in simulated body fluids in order to assess their corrosion behaviour. The alloy exhibited in all test solutions – from complex Tris-buffered SBF to non-buffered NaCl solutions with either very low concentration or strong acidification – an actively dissolving nature. This is different to the behaviour of 316L stainless steel used as reference material with a strong passivation tendency. At low NaCl concentrations, an effect of Mn in yielding much higher corrosion current densities of Fe-30Mn-1C compared to passivating pure Fe is detectable. Moreover, local Mn enrichment in austenite grain boundaries of Fe-30Mn-1C causes non-homogeneous surface dissolution in the initial corrosion stage and thus, formation of corrosion patterns. The role of Tris as organic buffer for simulated body fluids is critically assessed in particular with view to the sensitivity of the electrochemical response of Fe. This study supports the need for definition of suitable test conditions for the new class of biodegradable metals. Further studies employing electrochemical measurements and immersion testing with metal release analysis highlighted the positive impact of small S- or B-additions in enhancing the alloy degradations rate while not affecting the cytocompatibility.

A.Gebert, …, J. Hufenbach et al., Corrosion studies on Fe-30Mn-1C alloy in chloride-containing solutions with view to biomedical application, Materials and Corrosion 69 (2018) 167.
J. Hufenbach, …, A. Gebert et al., S and B microalloying of biodegradable Fe-30Mn-1C - Effects on microstructure, tensile properties, in vitro degradation and cytotoxicity, Mater. Design 142 (2018) 22.
J. Hufenbach, J. Sander, …, A, Gebert et al., Effect of selective laser melting on microstructure, mechanical, and corrosion  properties of biodegradable FeMnCS  for implant applications, Adv. Eng. Mater. (2020) 2000182