Patent ID: 12251755

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Aspects and embodiments in accordance with the present invention are directed to providing biocompatible Ti-based alloys capable of exhibiting antimicrobial and bone-like mechanical properties. As previously noted, the embodiments may be applied to fabrication of any products which may benefit from such properties; for the purpose of this Detailed Description, however, the embodiments will be discussed mainly in relation to medical implants.

More particularly, the aspects and embodiments are directed to fabricating an article of alloy which may be characterized as bulk metallic glass (BMG). Unless indicated otherwise, “bulk metallic glass” or “BMG” is herein intended to mean an article formed of a metal alloy system, such article having an amorphous structure of various print dimensions. Though in specific metallic glass community, BMG is only limited to a group of glassy metals that could be cast at least 2 mm thick (‘casting thickness’) to achieve fully amorphous structure. Such minimum thickness may be increased in this publication as the parts could be 3D printed into larger dimension.

The antimicrobial effect comes predominantly from copper ions as they are released into certain bodily fluids. The release rates of copper ions depend on the four main parent phases: amorphous phase, Ti2Cu phase, elemental copper phase, and (Ti,M)2Cu phase, M being one or more alloying element selectable from the group of previously mentioned transition metals: Zr, Nb, Ta, Pd and Co. Phase segregation of copper is observed in amorphous phase both during heat treatment, and later inside bodily environment. Such phase segregation will help control the release of copper from amorphous phase, while chemical reactions and chemical gradient may drive release of copper ions from metallic copper islands (clusters) and/or intermetallic compounds. Other elements, such as Palladium (Pd), also exhibit anti-microbial effects with relatively different degrees of cellular responses. The bone-like mechanical property comes predominantly from amorphous phase that exhibits relatively low modulus of elasticity (more similar to natural bone property). The beta titanium phase (3-Ti) may also be preferably included in the microstructure to improve ductility and change in elastic Young's modulus Therefore, various metal elements were selected for alloy systems per the concept of present invention.

Titanium (Ti) with good biocompatibility and mechanical properties was chosen as base metal. Copper (Cu) was chosen predominantly for antimicrobial effect. Transition metals were chosen to enhance glass forming ability for amorphous formation. Among numerous transition metals that may be applicable to the concept of the present invention, a group comprising Zirconium (Zr), Niobium (Nb), Tantalum (Ta), Palladium (Pd), Cobalt (Co) is preferred. The chemical compositions (hereinafter, in term of atomic percentage) of such alloy systems comprises copper within a range of 5-30 atomic percent and transition metal within a range of 0-50 atomic percent. The transition metal may be one or a plurality of the abovementioned Zr, Nb, Ta, Pd and Co.

Aspects of the present invention are particularly compatible with rapid solidification (which is a consequence of fast cooling rate) typically found in laser-based additive manufacturing techniques or surface coating. These aspects, alone or in combination, provide means for fine tuning the following microstructuresFully amorphous microstructureAmorphous and beta titanium phaseAmorphous and elemental copper phaseAmorphous and Ti2Cu and/or (Ti,M)2Cu phase, M being one or more alloying element selectable from the group of previously mentioned transition metals: Zr, Nb, Ta, Pd and Co. Specific examples of (Ti,M)2Cu phase include (Ti0.4Zr0.4Nb0.2)2Cu. Moreover, (Ti,M)2Cu covers a tolerance of (Ti,M)2±0.2Cu phase as well. The atomic ratio of Ti (or M) to Cu is approximately within a range of 1.8:1 to 2.2:1.

Although the additive manufacturing process per the embodiments alone may produce the microstructure needed in an as-printed state, some heat treatment after the step of additive manufacturing is preferred. The heat treatment could be done both in-situ during additive manufacturing and post additive manufacturing. With customization of microstructure, the release of copper in bodily fluid could be tailor-made.

Alloy Compositions

Preferred alloy compositions in accordance with the concept of present invention are shown by way of example in Table 1. Additionally,FIG.1shows a pseudo equilibrium ternary phase diagram with different compositions of the alloys that could generate any one or combination of the following structures: (i) fully amorphous microstructure, (ii) amorphous and beta titanium phase, (iii) amorphous and elemental copper phase, and (iv) amorphous and (Ti,M)2Cu phase.

TABLE 1Examples of preferred alloy compositionsComposition (atomic percent)Alloy No.TiCuZrNbTaPdCo1353035————244610—710233441010—710194441410—7101557015555——666204.64.74.7——762.319.59.24.54.5——858.51031.5————

Per the above Table 1, Alloy 1 and Alloy 8 may be represented as (Ti0.5+xZr0.5+x)100-yCuy, wherein x is within a range of 0-0.15 atomic percent and y is within a range of 10-30 atomic percent On the other hand, Alloys 2, 3 and 4 may be represented as Ti44Zr10Pd10Cu6+xCo23−xTa7, wherein x is within a range of 0-8 atomic percent. In addition, alloys 5, 6, and 7 may be represented as TixCuyZrzQa, wherein Q is one or a plurality of the other transition metals (Nb, Ta, Pd and Co); x is within a range of 60-70 atomic percent; y is within a range of 15-20 atomic percent; z is within a range of 5-10 atomic percent and a is within a range of 0-10 atomic percent.

Preparation of Alloy Powder

According to the concept of present invention, the foregoing alloy compositions are preferably used in their powdered form. Such powder may be prepared by way of pre-alloyed powder or blended powder, among others.

The pre-alloyed powder involves a process, usually melting, to obtain an ingot that having target alloy composition and a subsequent process, preferably gas-atomization, to turn this ingot into powder alloy with chemical homogeneity. Particularly, the pre-alloyed powder consists of a plurality of particles, each one having the same composition.

Alternatively, blended (i.e. pre-mixed) powder could be prepared by a process to intermix elemental powders (e.g. Ti, Cu, Zr, Ta, Pd, Co). Preferably, such powder comprises particles having a size within a range of 10-100 μm and a generally spherical shape. The pre-mixing process in accordance with preferred embodiments is novel in a way that no ball/bead is required in the pre-mixing container. As a result, higher purity of powder and lower contaminants (e.g. iron or carbon elements/compounds originating from such ball or bead) may be achieved. In order to achieve a preferable homogeneity, a rotational pre-mixing equipment with simultaneous 3-axis rotation is preferred. Preferably, the pre-mixing is carried out in a range of 5-30 rpm rotational speed with a pre-mixing time of 2-6 hours. The pre-mixing is as well preferably operated under Argon protective atmosphere to minimize level of oxygen, nitrogen and hydrogen contamination. Particularly, the mean composition of the blended powder after the completion of pre-mixing is within the target alloy composition, but the adjacent particles may have variation in their compositions and sizes.

For example, a batch of titanium elemental powder was intermixed with a batch of copper and zirconium elemental powders. Other alloying elements at proper ratios may be added to fine-tune the microstructure that is relevant to the release of copper ions. The intermixed powder will be used as raw material in a powder bed technique. The powder parameters, including but not limited to, (1) size distribution, (2) shape, (3) oxygen content, and (4) elemental or alloyed compositions, are important considerations. InFIG.2, as an example, particle size distributions of two mixtures of elemental powder (indicated as “Alloy 5” and “Alloy 6”) are shown in comparison with the narrower distribution of commercially available Ti-6Al-4V alloyed powder. Ti-6Al-4V size distribution has been used to produce >99.95% density 3D printed parts in our test. For the intermixed alloys, the powders were mixed together so that the size distributions were purposely different. The measurement was done in laser diffractometer particle size analyzer according to ASTM B822-10. The values for D10, D50 and D90 of Ti-6Al-4V powder are 21.6 microns, 32.1 microns and 47.4 microns respectively.

To properly control the rapid solidification process, the size distributions and shape of each elemental powders were carefully selected to correlate with laser parameters and scan strategy. In one example of the present invention, the geometry of the elemental powders was not in the spherical shape, but the mixing process could still provide good particle distribution and chemical homogeneity (i.e. elemental mapping) of the three elemental powders as shown in a scanning electron microscope (SEM) image ofFIG.3.

Fabrication by Additive Manufacturing

Powder prepared in accordance with the above embodiments are then used as raw materials for additive manufacturing process, whereby an intended article is fabricated in a layer-wise manner. Here, the preferred varieties of additive manufacturing techniques are power-bed laser melting techniques, including so-called selective laser melting (SLM).

In SLM, each cross-section of the component is built by means of consecutive scan of the laser beam to fully melt and fuse metallic powder. The characteristics of SLM process are fully melting and rapid solidification of metal as a result of small interaction volume and short interaction time between high-energy laser beam and material. This rapid solidification (cooling rates ranging between 103-108K/s) makes it suitable for the formation of amorphous metal. The cooling rate, as well as the resulting microstructure, can be tailored at any point within the component by controlling the processing parameters such as laser power, scan speeds, hatch spacing, layer thickness and scanning strategy.

The laser power may be selected so that the powder melts completely and homogeneously throughout the layer thickness. To manufacture the alloys in accordance with the present invention, the laser power is preferably at least 50 Watt.

Thickness of each powder layer may be determined from the particle size and applied laser power. According to the preferred embodiments, such layer thickness is within the range of 30-60 μm.

The scan speed and hatch spacing may be selected to facilitate the solidification conditions for the formation of amorphous metal and Ti2Cu phase. As discussed in alloy-design section, amorphous metal would result in a lower modulus of elasticity, and Ti2Cu phase would be beneficial for antimicrobial response According to the preferred embodiments, such scan speed is within a range of 65-2,000 mm/s, and such hatch spacing is within a range of 0.07-0.15 mm.

Even more preferably, the foregoing parameters may be adjusted so that the level of amorphousness and the amount of Ti2Cu phase are tailored for specific locations of the same article. For example, the additive manufacturing process may be configured such that two set of parameters are used for (i) the core and (ii) the surface of the same article. This is to promote antimicrobial response at the surface while maintaining appropriate modulus of elasticity at the core of same article.

The abovementioned processing parameters are directly related to energy density which determines not only the cooling rate but also the flow of liquid metal in the melt pool and heat affected zone (HAZ). High energy density leads not only to a high cooling rate, but also to a stronger chemical inhomogeneity (negatively affect amorphous formation) and larger HAZ. Thermodynamics modelling and finite element analysis of heat transfer may be used to identify potential processing parameters for experimental observation.

Scanning strategy may also be selected to accommodate homogeneous distribution of the constituent elements as well as heat transfer. A strategy having scanning vectors with 67-degree or 90-degree rotation in adjacent layers and scanning twice in each layer is preferred in order to promote a homogenous and amorphous microstructure.

Finally, heat treatments could be done either in-situ (i.e. during the additive manufacturing process) or post additive manufacturing to further promote the formation of copper-containing precipitates for antimicrobial property. The temperature of the heating plate may be in a range of 80-200° C. for in-situ heating. For the post additive manufacturing heat treatment, conditions with a temperature range of 790-950° C. and isothermal holding for 1-2 hours may be applied to the alloys.

EXAMPLE

Alloy No. 1 per Table 1 above, having a composition of Ti38Zr35Cu30(atomic percent), was prepared as blended powder and was fabricated using the following SLM processing parameters: The laser power of 95 W, scanning speed of 100 mm/s, hatch spacing of 105 μm and layer thickness of 40 μm. Scanning vectors with 90-degree rotation in adjacent layers and scanning twice in each layer were applied. The results from scanning electron microscopy (SEM), as shown inFIG.4, illustrates that a homogenous and amorphous as-built microstructure could be achieved. In addition,FIG.5demonstrates good homogeneous distribution of the constituent elements in the as-built microstructure. This observation confirms the efficiency of powder mixing and selective laser melting processes to fabricate an alloy per the preferred embodiments.

Verification of Amorphization

3D-printed articles could be fabricated with different processing parameters and scan strategies so as to control the formation of beta titanium phase (P—Ti), Ti2Cu and (Ti,M)2Cu.FIG.6Shows samples of different build volume that were printed on the platform in different configurations. Notice the different build topologies created by scan strategies. As part of our invention, the surface is fine-tuned so that there are local areas of metastable phase formation that could be tailored towards antimicrobial property.

According to an X-ray diffractometry (FIG.7A), scanning electron micrography (SEM) combined with energy dispersive x-ray spectrometry (EDX) and transmission electron micrography (TEM) were used to identify various phases such as: fully amorphous microstructure, amorphous and beta titanium phase, amorphous and elemental copper phase, and amorphous and (Ti,M)2Cu phase. SEM micrographs and EDX analysis revealed local areas where Ti2Cu is located as shown inFIG.7BandFIG.7C.

The samples were confirmed to be amorphous in selected area diffraction technique in TEM. X-ray diffraction was used to confirm the amorphous fraction in the as-printed and heat treated specimen. For good glass formers like Alloy 2, Alloy 3, and Alloy 4, the as-printed parts were fully amorphous.

The control of laser parameters and scan strategies could affect the melt pool which thereby limits copper diffusion, and combine such with proper heat treatment profile, some of the crystals could be formed towards our anti-microbial application as shown inFIG.7A. The middle graph shows inclusions of (Ti,M)2Cu crystals for the compositions centered around Alloy 5. The top graph shows later crystallization of beta-titanium phase which could be beneficial to the mechanical property to match better with human bone.

The values for elastic Young's modulus are approximately 8-24 GPa of cortical or solid bone. The scattered values are due to the completeness of the tests such as wet bone test, four-point bending test, three-point bending test of complex structure of combined cortical and trabecular bone, and ultrasonic test. For Ti-6Al-4V alloy, the elastic Young's modulus is of the order 113-125 GPa depending on test geometry and measurement techniques. For our cases, ultrasonic sound velocity measurement was employed on several Ti—Cu-based alloys and the elastic Young's moduli were in the range between 76-102 GPa. Examples include 81+/−3.2 GPa of combined amorphous and beta Ti/Ta phase for Ti40Zr15Cu30Nb8Ta7alloy after heat treatment, and 79+/−4.5 GPa for Ti60Zr17Cu18Pd10after heat treatment.

Cytotoxicity and Positive Biological Responses with Cells

Cytotoxicity evaluation was performed according ISO/EN 10993 part 5 as well as other methodologies related as extensions. Direct contact assays were done by using a cell line of human osteoblast like cells: SaOS-2. Osteoblastic cell line, SaOS2 or Sarcoma osteogenic, were used for in-vitro studies. After obtaining the cell line from cryopreserve vial, the cells were suspended in fresh medium then incubated in incubator supplied with 5% CO2, 95% O2at 37° C. At 5 days after thawing process, the photo was taking at confluency around 90% to show cell vitality. MTT assay is a colorimetric assay technique to assess the metabolically active cellular activities. The indicator involves the reducing of a yellow tetrazolium-based compound to a purple formazan product. As the number of living cells in the culture is directly related to the quantity of formazan product, the number of living cells may be quantified by the absorbance at 570 nm of wavelength. After cells were exposed to the surface for 3 days, 5 days and 7 days, no difference was detected when compared to the control as a screening method for cytotoxicity. The tests were repeated for many alloy recipes. Shown inFIG.8, six compositions were compared to show that in 7 days, there was no sign of cell cytotoxicity. The same test was repeated on MC3T3 and the results were comparable in both cellular responses. Cell viability assays (MTS test) also revealed that cells were viable after two weeks in culture. As shown inFIG.9, alkaline phosphatase (ALP) activity was higher in the supernatants collected from the pre-mineralized samples when compared to the control samples.

Antimicrobial Effects

Copper release test was carried out following ISO 10993-12 and ISO 10993-15. The printed specimens were soaked in NaCl solution at 0.9 weight percent concentration at 37° C. for one day. The surface area/volume ratio was kept constant at 15 cm2/cm3. The solution was then tested for the release of Cu, using Inductively Coupled Plasma Spectrometry (ICP-MS) to measure the copper release. The results are shown inFIG.10.

To test antimicrobial property of the alloy surface,Staphylococcus Aureusstrain was used as test subject to observe and count the colony forming units (CFU). First, all samples were autoclaved, then placed in 15 cc Falcon tubes containing MH broth. The tubes were inoculated with 20,000 CFU/mlS. aureus. The tubes were then incubated at 37° C. for 48 hours on a rocking table at 12 cycle/min. Samples were then removed from the original tubes, and rinsed with saline solution to remove planktonic cells. The specimens were then placed into new tubes with MH broth. Next, the tubes were sonicated to detach biofilm before being incubated again at 37° C. The samples were then transferred to MH agar plate and counted for CFU at specified time periods. The results are shown for the periods of 5 hours and 24 hours inFIG.11.

The CFU/ml ofStaphylococcus Aureusin control specimen were significantly higher than the counts in all new alloys (p value <0.02 in all cases), indicating an improved antimicrobial effect. It must be noted that there are several proposed methods for anti-microbial property. One evaluation is known as disk diffusion method using Mueller Hinton agar. The antibacterial activity of the material is controlled by diffusion in media and the zone of inhibition could be recorded. This method provides direct to surrounding diffusion measurement but requires rigorous dissolution of ions from the metal and diffusion through stationary agar. With clinical trials, it is known that blood supplies will circulate and affect the infection. Therefore a more dynamic test involving bodily fluid movement is more preferred.

One long term anti-microbial strategy is based on the ability to prolong the release copper ions on a daily basis for at least 1-2 weeks, such as shown in copper release test. This is a critical time in which patient has a high risk for infection. As it was demonstrated with a simple geometry, the release of copper could be prolonged and maintained at least for one week for persistent microbial effect. More complex geometry could be further created to match with required copper release amount via geometrical correlation with the surface area as long as the level of copper release could be matched with locally required dose. The zone of inhibition is shown inFIG.12to demonstrate long term anti-microbial effects of two example alloys per the embodiments (Alloy 5 and Alloy 6 per Table 1 above), in comparison with the benchmark specimen (Ti-6Al-4V).