Anti-microbial treatment for hardened metallic surfaces

Abstract

A simple and practical antibacterial treatment with nisin in cracked or uncracked metal tools is provided and easily monitored for its bacteriocin effect.

Claims

1. An antibacterial metal surface comprising an oxide layer with cracks created by a pulse laser irradiation (PLI) and an effective amount of an antimicrobial compound, wherein said cracks penetrated through the oxide layer and into the metal surface are filled with an effective amount of said antimicrobial compound, wherein said antimicrobial compound is nisin.

2. The antibacterial metal surface according to claim 1, wherein the oxide layer varies in thickness and displays different colors due to interaction with a visible light.

3. The antibacterial metal surface according to claim 1, wherein the metal is titanium or an alloy thereof.

4. The antibacterial metal surface according to claim 1, wherein the metal is a stainless steel.

5. The antibacterial metal surface according to claim 1, wherein the metal surface is a food processing knife, cutting board, packaging utensil, or surgical scalpel.

6. A method of providing antibacterial functionalities to a metal surface comprising: modifying the metal surface with pulse laser irradiation to create an oxide layer, with various depths of cracks generated by said puke laser irradiation, wherein said cracks penetrate through the oxide layer and into the metal surface; saturating said cracks in the oxide layer with effective amount of an antimicrobial compound; and optionally washing or scrubbing the oxide layer to remove extra said antimicrobial compound, wherein said antimicrobial compound is nisin.

7. The method according to claim 6, wherein said metal is titanium or an alloy thereof.

8. The method according to claim 6, wherein the metal is a stainless steel.

9. The method according to claim 6, wherein the metal surface is a food processing knife, cutting hoard, packaging utensil, or surgical scalpel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-1B depict backscattered electron images of the oxide film formed after PLI wherein FIG. 1A shows at 110 mm/s and FIG. 1B shows at 300 mm/s. Lines in the background, parallel to the arrows in the figures, are the result of the scanning direction of the laser path. There is no clear relationship between the laser direction and the formation of cracks.

(2) FIGS. 2A-2B depict FIB images of cross sections, wherein FIG. 2A shows the crack grows perpendicular to the interface but with some wobbling, probably due to the alternate expansion/contraction cycles between each laser pulse and the next and FIG. 2B shows the presence of a branch coming from the main crack.

(3) FIG. 3 shows decay of available driving force with increasing crack depth.

(4) FIG. 4 shows variation in stress intensity factor vs relative crack depth (penetrating into the substrate). Shaded area represents the relative crack depth for the films used in this study.

(5) FIG. 5 shows optical variation of oxide layer.

(6) FIG. 6 shows the benefit from the cracks, using cracks to be possible reservoir for storing other materials.

(7) FIG. 7 depicts that FTIR identifies peptides before and after mechanical impacts.

(8) FIG. 8 shows the peptides resistance to washing (water+detergent)+temperature.

(9) FIG. 9 demonstrates that antibacterial test shows effect of nisin treatment: the right side plate is nisin treated while the left side plate is not.

(10) FIG. 10 shows diagram of oxidation and bonding method to store nisin in oxide microcracks on a metallic surface, with the purpose to reduce pathogenic bacteria which can transfer to food products (meats, vegetables, diary, etc.)

(11) FIG. 11 shows diagram of showing Aspect ratio of cracks. Even a small crack has a high volume to store the peptide in the crack.

(12) FIGS. 12A-12B show comparison of nisin-coated specimens immersion into water. The process involves generally with nisin treatment to the specimen first, then the specimen is immersed in the water at room temperature, and nisin concentration in the solution is measured by typical BCA method. Cracked titanium is shown in FIG. 12A: loaded specimens were immersed in deionized water at room temperature and pH 6 for up to 21 days, but there was not release of nisin. Later, conditions were changed by increasing the temperature of water and even changing to acidic (pH 2) and basic (pH 10) conditions, but it was not possible to confirm any nisin release. GDS or XPS could likely be good techniques to confirm if nisin remains anchored to titanium oxide surface and into the cracks. An uncracked stainless steel is shown in FIG. 12B: loaded specimen was immersed in deionized water at room temperature and pH 6. The change in color in the colorimetric test was evident from the first days and deep purple after 21 days of immersion, confirming that nisin is effectively being released from the oxidized steel surface.

(13) FIG. 13 depicts diagram showing the change of pH and temperature on nisin treated uncracked or cracked stainless steel undergoes different nisin release result in solution.

(14) FIGS. 14A-14B shows that Nisin desorption from stainless steel shows linear behavior. FIG. 14A shows that at close of neutrality (pH 6) the release of nisin is low and nisin keeps releasing even after 21 days. FIG. 14B shows that under acidic conditions (pH 2) the nisin release rate accelerates substantially. When pH 2 is combined with a rise in temperature, nisin release is estimated to be 100 times faster compared to normal conditions (pH 6 and room temperature).

(15) FIG. 15 shows a stainless steel specimen release of nisin under different conditions. A stainless steel with crack is put at room temperature and pH 6 the release is not detected even after 18 days; whereas at pH 2 and 50° C. nisin release is observed within one day.

(16) FIG. 16 shows a flow diagram showing antibacterial test. A specimen of either Titanium or stainless steel treated with different concentrations of nisin is in contact with the testing bacteria Listeria monocytogenes, cleaned with cotton swab and transfer any bacteria from the swab to a tube with PBS solution before any resulted bacteria is split into various concentrations to seed on the petri dish.

(17) FIGS. 17A-17B show that Nisin is effective on oxide layers. Under different treatments following the flow chart of FIG. 16, the amount of bacterial colonies on the petri dishes is kept low. FIG. 17A shows a petri dish with normal bacteria growth. FIG. 17B, the top shows bacteria growth pattern under different concentrations of nisin treatment to Titanium; while the bottom shows bacteria growth pattern under different concentrations of nisin treatment to stainless steel. It is confirmed that nisin on oxide layers of these metal has bacteriocin effect, while nisin coated on Titanium does not get released even after scrubbing and raised temperature, nisin coated on stainless steel is able to release at room temperature even after 21 days.

(18) FIG. 18 shows antibacterial activity displayed by diverse treatment of either Cracked Titanium or uncracked stainless steel. The treatment include treating Cracked Titanium or uncracked stainless steel with no nisin, with pure nisin, scrubbing the nisin treated metal, water immersion nisin treated metal at room temperature or raised temperature. Results show that pure nisin or 2.5% nisin in the petri dish is sufficient to inhibit bacteria growth, whether it comes from Titanium or from stainless steel. Raised temperature helps nisin release from cracked Titanium

(19) Tables

(20) TABLE-US-00001 TABLE I Chemical composition of the Ti alloy, obtained by EDS. % Ti % Al % V % Others Ti-6A1-4V 89.4 7.4 3.1 <0.1

(21) TABLE-US-00002 TABLE II Applied laser scan rates with resulting film thickness and color variation of the coupons. Irradiated area (colored centered square) is 6 × 6 mm. Laser scan rate 90 110 170 200 300 (m/s) Film thickness, 148 139 124 119 110 h (nm)

(22) TABLE-US-00003 TABLE III Crack density and crack spacing obtained by laser scanning at different rates. Laser scan 90 110 170 200 300 rate (mm/s) Crack density, 74.5 78.8 126.0 126.6 150.0 C. (cracks/mm) Average Crack 13.4 12.7 7.9 7.9 6.6 spacing, S (μm)

(23) TABLE-US-00004 TABLE IV Mechanical properties for film and substrate Film Substrate Young's modulus, E (GPa) 215 [4] 110 [14] Poisson's ratio, ν 0.3 0.3 Shear modulus, μ (GPa) 90 44 Residual stress, σ.sub.0 (GPa) 6.7-10.3 [4] —

DETAILED DESCRIPTION

(24) While the concepts of the present disclosure are illustrated and described in detail in the figures and the description herein, results in the figures and their description are to be considered as exemplary and not restrictive in character; it being understood that only the illustrative embodiments are shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.

(25) Unless defined otherwise, the scientific and technology nomenclatures have the same meaning as commonly understood by a person in the ordinary skill in the art pertaining to this disclosure.

(26) Pulsed laser irradiation (PLI) at different scan rates was used to modify the surface of Ti6Al4V, creating an artificial oxide layer under atmospheric conditions. Analysis of cross sections of samples by focused ion beam revealed that the oxide layers vary in thickness, displaying different colors due to interaction with visible light.

(27) Previous studies have shown the immobilization of antimicrobial peptides (AMP) as a way to confer antibacterial properties to other materials which are prone to biofilm formation; however, subsequent release of AMP from the surface may also contribute to inhibit bacteria in the surrounding medium.

(28) In this disclosure, antibacterial tests demonstrate that Ti6Al4V modified surfaces by PLI and treated with AMP inhibit the growth of Listeria monocytogenes. The response of the surface to mechanical impacts and different environmental conditions (dry, humid, wet, etc.) was evaluated, as well as measurements of the peptide released from the surface.

(29) Briefly, prevention of biofilm formation in a metal surface has practical applications, including in food processing and medicinal apparatus sterilization. In this disclosure, pulsed laser irradiation is employed to facilitate Ti oxide growth on a metal surface, forming various cracks to accommodate storage of different materials. A non-limiting list of materials that can be stored in the cracks are lubricants, anti-corrosive compounds, adhesives and antimicrobials, as seen in FIG. 6. In addition, depending on the type of laser used, frequency of the laser, pulse duration (time, ns), number of pulses, scan rate and working atmosphere, the microstructure, thickness and color of the film will be affected. For example, FIG. 5 shows optical variation of oxide layer in relation to the laser scan rate and thickness of the film. Such unique feature of metal surface cracks and its ability to be responsive to different light provides a convenient reservoir to harbor useful material and being monitored/replenished as needed.

(30) In one un-limiting example, nisin, an antibacterial peptide produced by Lactococcus lactis and active against a wide range of Gram-positive bacteria, is widely used as a food preservative approved by FDA and can be stored in the reservoirs for different uses in metal surface cracks. Nisin can be infused into metal surface cracks through an oxide film and vacuumed the solution. Through various methods such as FTIR a person skilled in the art is able to identify the peptides stored in the cracks even after some mechanical impact, such as scrubbing the surface or washing by water and detergent. See FIGS. 7 and 8. It is desirable to find out if those amine/amide groups correspond to nisin and if this organic material is coming out from the cracks.

(31) In one of antibacterial tests, the film on metal surface with or without nisin treatment is immersed into a Brain-heart infusion (BHI) medium with bacteria Listeria monocytogenes. After cleaning the surface with cotton swab, the solution is diluted and inoculated to petri dishes to observe bacteria growth. For non nisin-treated samples, at least 9 colony forming units were observed, whereas the nisin treated solution does not form colony forming unit, indicating nisin stored in the metal surface cracks still exerts its antibacterial effects.

(32) We have found that changing pH has an impact on nisin release in the solution. Nisin has very low solubility in alkaline conditions. Therefore, no nisin would release at pH10. It is noteworthy as shown in FIG. 15, with the stainless steel with cracks, the sample was loaded with nisin and immersed in deionized water (pH 6 and RT). After 18 days, no nisin release was detected. However, changing the same loaded specimen from normal to harsh conditions (pH 2 and 50° C.), the oxide layer underwent corrosion (spalling) in less than 24 hours. This corrosion destroyed the cracks and then nisin could be released. This is the confirmation that nisin was effectively into the cracks.

(33) It is also noted that stainless steel releases more nisin than Titanium. As shown in FIG. 12, Titanium with cracks and stainless steel without cracks are treated with nisin first, then immersed in water at room temperature. By BCA measurement, Titanium with cracks did not release any nisin after 21 days, whereas stainless steel with no cracks has shown colorimetric reading starting from day three and peaks on day 21.

(34) One advantage of instant disclosure is that the antibacterial material can be stored in various metal surface cracks, and such storage is easily monitored by optical observation based on the color change of the film.

EXPERIMENTAL

(35) Five Ti-6Al-4V (henceforth Ti alloy) coupons (12×12×3 mm) with a chemical composition shown in Table I, were irradiated using an Er-doped, glass-fiber laser (λ=1.54 μm) from SPI Lasers, Inc., at 7.6 W average power, pulse frequency of 225 kHz and pulse duration of 120 ns, in air, scanning a 6×6 mm area of the sample surface, line by line, with parallel laser traces at a controlled scan velocity [11].

(36) After laser exposure, the irradiated (colored) area of the coupons was characterized by scanning electron microscopy (SEM) combined with focused ion beam (FIB) machining, using a FEI Nova 200 NanoLab DualBeam™-SEM/FIB. SEM images were helpful to perform measurements of crack density and crack spacing, while FIB cuttings were used to observe the cross section of the Ti alloy, estimate the oxide thickness and crack depth into the film.

(37) Results

(38) Ti alloy coupons were superficially melted by the laser promoting oxygen diffusion through the molten material and, thus, to the oxidation of the Ti alloy [11]. These oxide layers (henceforth the oxide film) vary in color depending on the oxide thickness as shown in Table II.

(39) Oxide Film

(40) The oxides produced by laser irradiation are composed of TiO and Ti.sub.6O. The latter is mainly present in the interfacial layer, essentially intercalated in the HCP α-Ti substrate [4]. The film thickness, determined using images of the cross sections, is on the order of 100-150 nm, depending on the applied scan speed (Table II).

(41) A collection of interconnected cracks, resulting in the formation of oxide islands, covers the surface. This mudflat cracking (FIGS. 1A-1B) indicates the oxide film is stress relieved; residual stresses that developed during fabrication, likely due to difference in coefficients of thermal expansion between oxide and substrate, have been relieved through cracking. The average island diameter, or spacing between cracks, determined from line averages of SEM images, is on the order of 5-14 μm, and it decreases when the film thickness (h) decreases (Table III).

(42) Substrate

(43) FIB images of the cross section revealed that the cracks present in the film penetrate into the substrate (Ti alloy). The penetration is significant, mostly on the order of 1-6 μm beneath/below the interface.

(44) For all five Ti alloy coupons, cracks mostly grow perpendicular to the surface, with a few variations like the branch shown in FIG. 2B. No evidence of significant bifurcation within the substrate was observed, nor was there any evidence that the crack directions were altered significantly at the oxide-metal interface.

(45) Crack formation in materials is controlled by (a) the mechanical properties, including modulus (stiffness), toughness (energy required to break the material), and thermal expansion, (b) the geometrical properties (dimensions and flaw population), and (c) the loading (applied and residual strain, and temperature excursions) [12].

(46) Mechanics of the Film

(47) Every time that the native oxide layer is irradiated by a laser pulse, the local thermal excursion leads to the generation of local stresses and strains. The irradiation generates a change in temperature which drives to an expansion of the whole system (film+substrate), but film and substrate expand at different rates because the coefficient of thermal expansion of the film is different to that of the substrate. When a thin film is deposited on a substrate, a composite is formed in which the presence of the substrate affects the film and vice versa [13].

(48) After the few nanoseconds of irradiation, the whole system cools down. Film and substrate contract but, again, they do it at different rates. In the particular case of this study, the oxide film must have a higher coefficient of thermal expansion than the substrate; this way, the film contracts faster than the substrate but, because the substrate is substantially thicker than the film, even if the latter tends to shrink, the former restricts it from doing so, and thus the film experiences a tensile residual stress (σ.sub.0) that can be relieved by film cracking. However, at the same time, the interaction with laser promotes specific chemical reactions that change the chemistry in the oxide film; more oxygen (and probably other chemical species) diffuse into the film, creating a mixture of different oxides (TiO, Ti.sub.6O) and a thicker film. Because the chemical composition of the oxide layer is different depending on the thickness of the film, it is assumed that the fracture toughness of the film (Γ.sub.r) is not of the same magnitude for all the samples.

(49) The first cracks appear with uniform spacing and only when the stress in the film reaches a critical value. Once the first cracks are formed, the distribution of stress transferred across the interface that causes more fracture is different from that which originally existed when the film was continuous [13]. The cracking produces a stress relaxation in the film that lowers the stress to a value below that of the critical stress, especially in the regions very close to the crack. As the substrate is strained further, the stress in the cracked oxide segments (islands) will increase and, to relieve this added stress, more cracks will form, generally in the midpoints of the unbroken islands, those points being the farthest from the cracks and thus experiencing the least stress relaxation (maximum stress) [13]. In the case of this study, the process may be repeated several times with a laser pulse irradiating the surface every few nanoseconds. We begin with the assumption that the film would not have undergone multiple fracture (mudflat) if it were not bonded to a more pliable material as substrate beneath it [13].

(50) Once the laser irradiation process has been completed, crack density (C) in the oxide film may be estimated using SEM images. Then the average spacing between cracks can be calculated with the equation:

(51) $\begin{array}{cc}S=\frac{1}{C}& \left(\mathrm{Eq}.1\right)\end{array}$
where S is the crack spacing. Values of crack density and crack spacing for each one of the samples are shown in Table III.

(52) With the application of nanoindentation, Lawrence [4] determined that residual stresses in oxide films grown using similar laser scan rates are in the order of 6.7 to 10.3 GPa. Using this data and

(53) $\begin{array}{cc}{\sigma }_0=\frac{E_f{\mathrm{.Math.}}_0}{\left(1-v_f\right)}& \left(\mathrm{Eq}.2\right)\end{array}$
the applied strain in the film (ε.sub.0) may be calculated. With the parameters in Table IV, ε.sub.0 is estimated in the 0.022-0.034 range. E.sub.f and υ.sub.f are the Young's modulus and the Poisson's ratio of the film, respectively.

(54) According to Thouless [14], the crack spacing is proportional to the square root of the film thickness by

(55) $\begin{array}{cc}S\simeq 5.6h\sqrt{\frac{{\Gamma }_f\left(1-v_f\right)}{\left(1+v_f\right)E_f{\mathrm{.Math.}}_0^{2}h}}& \left(\mathrm{Eq}.3\right)\end{array}$
where Γ.sub.f is the fracture toughness of the film (in J/m.sup.2).

(56) Using Eq. 3, it is estimated that Γ.sub.f should be between 4600 J/m.sup.2 and 7500 J/m.sup.2.

(57) Mechanics of the Substrate

(58) Channel cracks in the film have developed at some stage during the cooling of the film/substrate system and their growth may be stopped either because the substrate is much tougher or because sufficiently large substrate surface defects are not readily available [15]. Hu et al. [16] demonstrated that Cr films subject to tensile intrinsic stress first split generating a mudflat cracking pattern, and then the cracks subsequently can extend into glass substrates, growing along the plane for which K.sub.II=0. Guo et al. [17] also found that cracks penetrate into the substrate when a brittle TiN film is placed on brass substrate.

(59) In this current study, the film is stiffer than the substrate (E.sub.f>E.sub.s). A compliant substrate provides less constraint for crack propagation [8]. Because the laser pulses are directly applied to the oxide film, the stress and strain reside in the film, not in the substrate. The available driving force (G.sub.a) decays significantly for deep cracks, implying stable propagation. The crack arrests at a certain depth due to this decline.

(60) Since cracks can propagate in a stable fashion below the film-substrate interface when they are driven by a residual tension in the film [18], the energy-release rate (driving force) at the tip of a single crack of depth a, where a>>h, tends to a limit of

(61) $\begin{array}{cc}\frac{G_a}{{\stackrel{\_}{E}}_{s}h{\mathrm{.Math.}}_0^2}=2.14\left(\frac{1+\alpha }{1-\alpha }\right){\left(\frac{a}{h}\right)}^{-1}& \left(\mathrm{Eq}.4\right)\end{array}$
where Ē.sub.s is the Young's modulus of the substrate and α is the Dundurs' parameter,

(62) $\begin{array}{cc}\alpha =\frac{{\stackrel{\_}{E}}_f-{\stackrel{\_}{E}}_s}{{\stackrel{\_}{E}}_f+{\stackrel{\_}{E}}_s}& \left(\mathrm{Eq}.5\right)\end{array}$
where Ē=E/(1−υ.sup.2) in plane strain. If α is positive, means that the film is stiffer than the substrate; if negative, then substrate is stiffer than the film.

(63) For the film/substrate system in this study, α=0.32, while G.sub.a depends on the crack depth according to FIG. 3. It can be observed that the decay is very significant in the first 500 nm, therefore there is enough driving force to cross the interface and penetrate into the substrate.

(64) Drory and Evans [19] determined that for a semi-infinite substrate, the opening mode stress intensity factor K.sub.I for a line force on a crack surface is given by

(65) $\begin{array}{cc}\frac{K_I}{{\sigma }_0\sqrt{h}}=\frac{2.6}{\sqrt{\pi \lambda }}& \left(\mathrm{Eq}.6\right)\end{array}$
where λ is the relative depth and is defined as λ=(a−h)/h. So, K.sub.I is inversely proportional to the square root of the relative depth as shown in FIG. 6 from [19] for λ up to 10.

(66) In this study, the average crack depth is around 30-35 times larger than the film thickness, and the behavior of K.sub.I vs λ is shown in FIG. 4. It is important to remark that the K.sub.I values of the Ti alloy are in the order of 1 MPa√{square root over (m)}. This value is considerably lower than the average fracture toughness for Ti-6Al-4V, which is on the order of 65 MPa√{square root over (m)} [20].

(67) This low toughness is likely due to embrittlement of the substrate from either oxygen or hydrogen. As the laser irradiates the film, a flaw is activated and the cracks grow towards the interface but, at the same time, several chemical reactions occur; the main one is the diffusion of oxygen that enriches the film, creating a mixture of oxides. As the growth is in ambient atmosphere both oxygen and water vapor are present, and the harsh environment of the pulsed laser could lead to dissociation of water, making possible either embrittling species could be present. Literature reports of O embrittling α-Ti [21] or H embrittling Ti [22, 23] originating the decay in K.sub.I. Unfortunately, given the rapid diffusion of H in Ti, the time between fabrication and analysis would be prohibitive to identify if H had been the embrittling species; similarly the fact that the oxide intermixes with the matrix makes identifying the actual concentration near the end of the crack tip challenging.

(68) Therefore, the application of PLI on Ti-6Al-4V alloy specimens creates an oxide film composed by a mixture of different Ti oxides with 100-150 nm thickness; strong presence of mudflat cracking is observed, with cracks that not only penetrate the film but also the substrate, attaining depths estimated at 1-6 μm.

(69) Given that the oxide film is stiffer than the substrate and α>0, the energy release rate in the film increases when the ratio a/h increases, which helps the cracks reach the interface.

(70) When cracks start to penetrate into the substrate, the energy release rate (driving force) considerably decays because the stress forms in the film. At some depth, the energy is not enough to continue the fracture of the substrate, and the cracks arrest.

(71) The toughness at which cracks arrest in this system (on the order of 1 MPa√{square root over (m)}) is significantly lower than the bulk toughness of Ti-6Al-4V; this may be due to embrittlement caused by the diffusion of chemical species (e.g. hydrogen or oxygen) during oxide growth due to the interaction between the laser pulse and the atmosphere.

(72) We have shown that a mudflat cracked oxide layer may be created on the surface of Ti-6Al-4V by irradiation with a pulsed laser. These cracks penetrate 2-6 μm into the substrate, and can be used as reservoir for second-phase compounds like antimicrobial agents.

(73) FTIR spectra show that, even after mechanical and environmental impacts, some amine/amide groups are still on the surface of the treated oxide.

(74) Antibacterial tests show that, even after mechanical impact (scrubbing), the oxide surface treated with nisin still shows antibacterial properties.

REFERENCES

(75) [1] Nam, K., I. Park & S. Ko. Patterning by controlled cracking. Nature, 485, 221-224 (2012). [2] Mani, S. & T. Saif. Mechanism of controlled crack formation in thin-film dielectrics. Applied Physics Letters, 86, 201903(1-3) (2005). [3] Seghir, R. & S. Arscott. Controlled mud-crack patterning and self-organized cracking of polydimethylsiloxane elastomer surfaces. Scientific Reports, 5:14787, DOI 10.1038/srep14787 (2015). [4] Lawrence, S., D. Adams, D. Bahr & N. Moody. Deformation and fracture of a mudflat-cracked laser-fabricated oxide on Ti. Journal of Materials Science, 48, 4050-4058 (2013). [5] Lim, Y., Y. Sohida, C. Andres & M. Barco. Surface characterizations of variously treated titanium materials. The International Journal of Oral & Maxillofacial Implants, 16, 333-342 (2001). [6] Tarasenko, V., S. Alekseev, A. Fedenev, I. Goncharenko, N. Koval, K. Oskomov, V. Orlovskii, N. Sochugov & M. Shulepov. Study on IR and UV—lasers interaction with metal surfaces. Gas and Chemical Lasers and Intense Beam Applications III, 4631, 234-243 (2002). [7] Akman, E. & E. Cerkezoglu. Compositional and micro-scratch analyses of laser induced colored surface of titanium. Optics and Lasers in Engineering, 84, 37-43 (2016). [8] Hutchinson, J. & Z. Suo. Mixed mode cracking in layered materials. Advances in Applied Mechanics, 29, 63-146 (1992). [9] Xia, Z. & J. Hutchinson. Crack patterns in thin films. Journal of the Mechanics and Physics of Solids, 48, 1107-1131 (2000). [10] Guo, T., L. Qiao, X. Pang & A. Volinsky. Brittle film-induced cracking of ductile substrates. Acta Materialia, 99, 273-280 (2015). [11] Pérez del Pino, A., P. Serra & J. Morenza. Oxidation of titanium through Nd:YAG laser irradiation. Applied Surface Science, 197-198, 887-890 (2002). [12] Choul-Kim, B., C. Moraes, J. Juang, M. Thouless & S. Takayama. Fracture-based micro- and nanofabrication for biological applications. Biomaterials Science, 2, 288-296 (2014). [13] Wojciechowski, P. & M. Mendolia. Fracture and cracking phenomena in thin films adhering to high-elongation substrates. Thin Films for Emerging Applications (Edited by M. Francombe & J. Vossen), Academic Press Inc., 16, 271-282 (1992). [14] Thouless, M. Modeling the development and relaxation of stresses in films. Annual Review of Materials Science, 25, 69-96 (1995). [15] Zhou, J., S. Huang, L. Zuo, X. Meng, J. Sheng, Q. Tian, Y. Han & W. Zhu. Effects of laser peening on residual stresses and fatigue crack growth properties of Ti-6Al-4V titanium alloy. Optics and Lasers in Engineering, 52, 189-194 (2014). [16] Hu, M., M. Thouless & A. Evans. The decohesion of thin films from brittle substrates. Acta Metallurgica, 36, 1301-1307 (1988). [17] Guo, T., L. Qiao, X. Pang & A. Volinsky. Brittle film-induced cracking of ductile substrates. Acta Materialia, 99, 273-280 (2015). [18] Thouless, M., Z. Li, N. Douville & S. Takayama. Periodic cracking of films supported on compliant substrates. Journal of Mechanics and Physics of Solids, 59, 1927-1937 (2011). [19] Drory, M. & A. Evans. Experimental observations of substrate fracture caused by residually stressed films. Journal of American Ceramic Society, 73, 634-638 (1990). [20] Niinomi, M. Mechanical properties of biomedical titanium alloys. Materials Science and Engineering A, A243, 231-236 (1998). [21] Liu, Z. & G. Welsch. Effects of oxygen and heat treatment on the mechanical properties of alpha and beta titanium alloys. Metallurgical Transactions A., 19A, 527-542 (1988). [22] Meyn, D. Effect of hydrogen on fracture and inert-environment sustained load cracking resistance of α-βtitanium alloys. Metallurgical Transactions, 5, 2405-2414 (1974). [23] Tal-Gutelmacher, E. & D. Eliezer. Hydrogen-assisted degradation of titanium based alloys. Materials Transactions, 45, 1594-1600 (2004).