Ex-vivo parametric study of laser ablation-based drilling of cortical bone.

Frequently orthopedic surgeries require mechanical drilling processes especially for inserted biodegradable screws or removing small bone lesions. However mechanical drilling techniques induce large number of forces as well as have substantially lower material removal rates resulting in prolong healing times. This study focuses on analyzing the impact of quasi-continuous laser drilling on the bone's surface as well as optimizing the drilling conditions to achieve high material removal rates. An ex-vivo study was conducted on the cortical region of desiccated bovine bone. The laser-based drilling on the bovine bine specimens was conducted in an argon atmosphere using a number of laser pulses ranging from 100 to 15,000. The morphology of the resulting laser drilled cavities was characterized using Energy dispersive Spectroscopy (EDS) and the width and depth of the drills were measured using a laser based Profilometer. Data from the profilometer was then used to calculate material removal rates. At last, the material removal rates and laser processing parameters were used to develop a statistical model based on Design of Experiments (DOE) approach to predict the optimal laser drilling parameters. The main outcome of the study based on the laser drilled cavities was that as the number of laser pulses increases, the depth and diameter of the cavities progressively increase. However, the material removal rates revealed a decrease in value at a point between 4000 and 6000 laser pulses. Therefore, based on the sequential sum of square method, a polynomial curve to the 6th power was fit to the experimental data. The predicted equation of the curve had a p-value of 0.0010 indicating statistical significance and predicted the maximum material removal rate to be 32.10 mm3/s with 95%CI [28.3,35.9] which was associated with the optimum number of laser pulses of 4820. Whereas the experimental verification of bone drilling with 4820 laser pulses yielded a material removal rate of 33.37 mm3/s. Therefore, this study found that the carbonized layer formed due to laser processing had a decreased carbon content and helped in increasing the material removal rate. Then using the experimental data, a polymetric equation to the sixth power was developed which predicted the optimized material removal rate to occur at 4820 pulses.

Surface Nanostructures Enhanced Biocompatibility and Osteoinductivity of Laser-Additively Manufactured CoCrMo Alloys.

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in orthopedic implants due to their excellent corrosion and wear resistance and superior mechanical properties. However, their limited capability to promote cell adhesion and new bone tissue formation, poor blood compatibility, and risk of microbial infection can lead to implant failure or reduced implant lifespan. Surface structure modification has been used to improve the cytocompatibility and blood compatibility of implant materials and reduce the risk of infection. In this study, we prepared CoCrMo alloys with surface nanostructures of various aspect ratios (AR) using laser-directed energy deposition (L-DED) and biocorrosion. Our results showed that medium and high AR nanostructures reduced platelet adhesion, while all of the alloys demonstrated good blood compatibility and antibacterial properties. Moreover, the medium and high AR nanostructures promoted cell adhesion and spreading of both preosteoblast MC3T3 cells and human bone marrow mesenchymal stem cells (hMSCs). Furthermore, the nanostructure promoted the osteogenic differentiation of both cell types compared with the flat control surface, with a substantial enhancing effect for the medium and high ARs. Our study proposes a promising approach for developing implant materials with improved clinical outcomes.

Underlying factors determining grain morphologies in high-strength titanium alloys processed by additive manufacturing.

In recent research, additions of solute to Ti and some Ti-based alloys have been employed to produce equiaxed microstructures when processing these materials using additive manufacturing. The present study develops a computational scheme for guiding the selection of such alloying additions, and the minimum amounts required, to effect the columnar to equiaxed microstructural transition. We put forward two physical mechanisms that may produce this transition; the first and more commonly discussed is based on growth restriction factors, and the second on the increased freezing range effected by the alloying addition coupled with the imposed rapid cooling rates associated with AM techniques. We show in the research described here, involving a number of model binary as well as complex multi-component Ti alloys, and the use of two different AM approaches, that the latter mechanism is more reliable regarding prediction of the grain morphology resulting from given solute additions.

Microstructure enhanced biocompatibility in laser additively manufactured CoCrMo biomedical alloy.

The present work investigated biocompatibility of the unique nanostructural surface morphology inherently evolved in laser-based additively manufactured CoCrMo after biocorrosion in simulated body fluid at physiological temperature (37 °C). The extremely rapid thermokinetics intrinsically associated with the laser-based additive manufacturing technique resulted in heterogeneous cellular dendritic solidification morphologies with selective elemental segregation along the cell boundaries within CoCrMo samples. Consequently, a selective and spatially varying electrochemical response resulted in generation of a nanoscale surface morphology (crests and troughs) due to differential localized electrochemical etching. Also, depth of the trough regions was a function of the applied potential difference during potentiodynamic polarization which resulted in samples with varying morphological ratio (depth of trough/width of cell wall). CoCrMo with such nanoscale surface undulations were proposed for enhanced biocompatibility in terms of viability, spreading, and integration of MT3C3 pre-osteoblasts cells elucidated via MTT assay, immunofluorescence, and microscopy techniques. Furthermore, the influence of the morphological ratio, characteristic to the additively deposited CoCrMo after electrochemical etching (biocorrosion) on biocompatibility of MT3C3 pre-osteoblasts cells was qualitatively and quantitatively compared to a mirror-polished flat CoCrMo surface.

Machine learning based de-noising of electron back scatter patterns of various crystallographic metallic materials fabricated using laser directed energy deposition.

A novel machine learning (ML) method of refining noisy Electron Back Scatter Patterns (EBSP) is proposed. For this, conditional generative adversarial networks (c-GAN) have been employed. The problem of de-noising the EBSPs was formulated as an image translation task conditioned on the input images to get refined/denoised output of EBSPs which can be indexed using conventional Hough transform based indexing algorithms. The ML model was trained using 10,000 EBSPs acquired under different settings for additively manufactured FCC, BCC and HCP alloy samples ensuring enough diversity and complexity in training data set. Pairs of noisy and corresponding optimal EBSPs were acquired by suitable tweaking of the EBSP acquisition parameters such as beam defocus, pattern binning and EBSD camera exposure duration. The trained model has brought out significant improvement in EBSD indexing success rate on test data, accompanied by betterment of indexing accuracy, quantified through 'pattern fit'. Complete automation of the EBSP refinement was demonstrated where in entire EBSD scan data can be fed to the model to get the refined EBSPs from which high quality EBSD data can be obtained.

A multi modal approach to microstructure evolution and mechanical response of additive friction stir deposited AZ31B Mg alloy.

Current work explored solid-state additive manufacturing of AZ31B-Mg alloy using additive friction stir deposition. Samples with relative densities ≥ 99.4% were additively produced. Spatial and temporal evolution of temperature during additive friction stir deposition was predicted using multi-layer computational process model. Microstructural evolution in the additively fabricated samples was examined using electron back scatter diffraction and high-resolution transmission electron microscopy. Mechanical properties of the additive samples were evaluated by non-destructive effective bulk modulus elastography and destructive uni-axial tensile testing. Additively produced samples experienced evolution of predominantly basal texture on the top surface and a marginal increase in the grain size compared to feed stock. Transmission electron microscopy shed light on fine scale precipitation of Mg[Formula: see text]Al[Formula: see text] within feed stock and additive samples. The fraction of Mg[Formula: see text]Al[Formula: see text] reduced in the additively produced samples compared to feed stock. The bulk dynamic modulus of the additive samples was slightly lower than the feed stock. There was a [Formula: see text] 30 MPa reduction in 0.2% proof stress and a 10-30 MPa reduction in ultimate tensile strength for the additively produced samples compared to feed stock. The elongation of the additive samples was 4-10% lower than feed stock. Such a property response for additive friction stir deposited AZ31B-Mg alloy was realized through distinct thermokinetics driven multi-scale microstructure evolution.

A Review of Diagnostics Methodologies for Metal Additive Manufacturing Processes and Products.

Additive manufacturing technologies based on metal are evolving into an essential advanced manufacturing tool for constructing prototypes and parts that can lead to complex structures, dissimilar metal-based structures that cannot be constructed using conventional metallurgical techniques. Unlike traditional manufacturing processes, the metal AM processes are unreliable due to variable process parameters and a lack of conventionally acceptable evaluation methods. A thorough understanding of various diagnostic techniques is essential to improve the quality of additively manufactured products and provide reliable feedback on the manufacturing processes for improving the quality of the products. This review summarizes and discusses various ex-situ inspections and in-situ monitoring methods, including electron-based methods, thermal methods, acoustic methods, laser breakdown, and mechanical methods, for metal additive manufacturing.

Crystallographic texture dependent bulk anisotropic elastic response of additively manufactured Ti6Al4V.

Rapid thermokinetics associated with laser-based additive manufacturing produces strong bulk crystallographic texture in the printed component. The present study identifies such a bulk texture effect on elastic anisotropy in laser powder bed fused Ti6Al4V by employing an effective bulk modulus elastography technique coupled with ultrasound shear wave velocity measurement at a frequency of 20 MHz inside the material. The combined technique identified significant attenuation of shear velocity from 3322 ± 20.12 to 3240 ± 21.01 m/s at 45[Formula: see text] and 90[Formula: see text] orientations of shear wave plane with respect to the build plane of printed block of Ti6Al4V. Correspondingly, the reduction in shear modulus from 48.46 ± 0.82 to 46.40 ± 0.88 GPa was obtained at these orientations. Such attenuation is rationalized based on the orientations of [Formula: see text] crystallographic variants within prior columnar [Formula: see text] grains in additively manufactured Ti6Al4V.

Laser fabrication of structural bone: surface morphology and biomineralization assessment.

The current work explores the surface morphology of the laser-ablated bone using Yb-fiber coupled Nd:YAG laser (λ = 1064 nm) in continuous wave mode. As the laser-ablated region contains physiochemically modified carbonized and nonstructural region, it becomes unknown material for the body. Thus, biomineralization on such a laser-ablated region was assessed by in vitro immersion test in noncellular simulated body fluid. The presence of hydroxyapatite was detected in the precipitated mineral product using scanning electron microscopy equipped with energy dispersive spectroscopy, and X-ray diffraction analysis. The effect of varying laser parameters on distribution of surface morphology features was identified and its corresponding effect on biomineralization was studied.

In-vitro biomineralization and biocompatibility of friction stir additively manufactured AZ31B magnesium alloy-hydroxyapatite composites.

The present study aims to evaluate effect of hydroxyapatite (HA, Ca10(PO4)6OH2), a ceramic similar to natural bone, into AZ31B Mg alloy matrix on biomineralization and biocompatibility. The novel friction stir processing additive manufacturing route was employed to fabricate Mg-HA composites. Various HA contents (5, 10, 20 wt%) were incorporated into Mg matrix. Microstructural observation and chemical composition analysis revealed that refined Mg grains and dispersion of HA particles at micro/nanoscales were achieved in Mg-HA composites after the friction stir processing. The biomineralization evaluation were carried out using immersion experiments in simulated body fluid followed by mineral morphology observation and chemical composition analysis. The wettability measurements were conducted to correlate the biomineralization behavior. The results showed improvement in wettability and bone-like Ca/P ratio in apatite deposit on the composites compared to as-received Mg. In addition, the increase of blood compatibility, cell viability and spreading were found in the higher HA content composites, indicating the improved biocompatibility. Therefore, friction stir processed Mg-20 wt%HA composite exhibited the highest wettability and better cell adhesion among other composites due to the effect of increased HA content within Mg matrix.

Computational Assessment of Thermokinetics and Associated Microstructural Evolution in Laser Powder Bed Fusion Manufacturing of Ti6Al4V Alloy.

Although most of the near non-equilibrium microstructures of alloys produced by laser powder bed fusion (LPBF) additive manufacturing (AM) are being reported at a rapid rate, the accountable thermokinetics of the entire process have rarely been studied. In order to exploit the versatility of this AM process for the desired properties of built material, it is crucial to understand the thermokinetics associated with the process. In light of this, a three-dimensional thermokinetic model based on the finite element method was developed to correlate with the microstructure evolved in additively manufactured Ti6Al4V alloy. The computational model yielded the thermal patterns experienced at given location while building a single layer through multiple laser scans and a whole part through multiple layers above it. X-ray analysis of the resultant microstructure confirmed the presence of acicular martensitic (α') phase of (002) texture within the build-plane. Computationally predicted magnitude of the thermal gradients within the additively manufactured Ti6Al4V alloy in different directions (X, Y, and Z) facilitated the understanding about the evolution of grain morphology and orientation of acicular martensite in prior ß grains. The scanning electron microscopy observations of the alloy revealed the distinct morphology of phase precipitated within the martensitic phase, whose existence was, in turn, understood through predicted thermal history.

In-vitro bio-corrosion behavior of friction stir additively manufactured AZ31B magnesium alloy-hydroxyapatite composites.

Magnesium and its alloys have been considered for consumable bio-implant applications due to their similar mechanical properties to the natural bone and biodegradability. Nevertheless, uncontrollable corrosion rate and limited bioactivity of Mg based materials in biological environment restrain their application. In light of this, objective of the present study was to explore addition of hydroxyapatite (HA, Ca10(PO4)6OH2), a ceramic similar to bone mineral, into AZ31B Mg alloy and its effects on bio-corrosion behavior. Friction stir processing based additive manufacturing route was employed for producing AZ31B Mg-HA composites. Various HA contents (5, 10, and 20 wt%) were incorporated into Mg matrix. The microstructural observation revealed that the size of α-Mg grains reduced significantly after friction stir process. HA incorporation took place at micro/nanoscale in α-Mg matrix under the thermo-mechanical forces exerted by friction stir process. The corrosion behavior of friction stir processed Mg-HA composites was investigated using electrochemical methods in simulated body fluid. The results indicated an improvement in corrosion resistance for the composites compared to untreated AZ31B which was attributed to significant grain refinement upon friction stir process. On the other hand, incremental addition of HA had an opposing effect due to localized micro/nano-galvanic couples. As a result, friction stir process Mg-5 wt% HA composite demonstrated the highest corrosion resistance due to an optimum balance between beneficial effects of grain size refinement and limited number of local galvanic couples compared to the other friction stir process samples explored in the present work.

Thermal Assessment of Ex Vivo Laser Ablation of Cortical Bone.

As a potential osteotomy tool, laser ablation is expected to provide rapid machining of bone, while generating minimal thermal damage (carbonization) and physical attributes within the machined region conducive to healing. As these characteristics vary with laser parameters and modes of laser operation, the clinical trials and in vivo studies render it difficult to explore these aspects for optimization of the laser machining parameters. In light of this, the current work explores various thermal and microstructural aspects of laser-ablated cortical bone in ex vivo study to understand the fundamentals of laser-bone interaction using computational modeling. The study employs the Yb-fiber Nd:YAG laser (λ = 1064 nm) in the continuous wave mode to machine the femur section of bovine bone by a three-dimensional machining approach. The examination involved thermal analysis using differential scanning calorimetry and thermogravimetry, phase analysis using X-ray diffractometry, qualitative analysis using X-ray photoelectron spectroscopy, and microstructural and semiquantitative analysis using scanning electron microscopy equipped with energy-dispersive spectrometry. The mechanism of efficient bone ablation using the Nd:YAG laser was evaluated using the computational thermokinetics outcome. The use of high laser fluence (10.61 J/mm2) was observed to be efficient to reduce the residual amorphous carbon in the heat-affected zone while achieving removal of the desired volume of the bone material at a rapid rate. Minimal thermal effects were predicted through computational simulation and were validated with the experimental outcome. In addition, this work reveals the in situ formation of a scaffold-like structure in the laser-machined region which can be conducive during healing.

Manufacturing and Characterization of Hybrid Bulk Voxelated Biomaterials Printed by Digital Anatomy 3D Printing.

The advent of 3D digital printers has led to the evolution of realistic anatomical organ shaped structures that are being currently used as experimental models for rehearsing and preparing complex surgical procedures by clinicians. However, the actual material properties are still far from being ideal, which necessitates the need to develop new materials and processing techniques for the next generation of 3D printers optimized for clinical applications. Recently, the voxelated soft matter technique has been introduced to provide a much broader range of materials and a profile much more like the actual organ that can be designed and fabricated voxel by voxel with high precision. For the practical applications of 3D voxelated materials, it is crucial to develop the novel high precision material manufacturing and characterization technique to control the mechanical properties that can be difficult using the conventional methods due to the complexity and the size of the combination of materials. Here we propose the non-destructive ultrasound effective density and bulk modulus imaging to evaluate 3D voxelated materials printed by J750 Digital Anatomy 3D Printer of Stratasys. Our method provides the design map of voxelated materials and substantially broadens the applications of 3D digital printing in the clinical research area.

Evolution of surface morphology of Er:YAG laser-machined human bone.

The extensive research on the laser machining of the bone has been, so far, restricted to drilling and cutting that is one- and two-dimensional machining, respectively. In addition, the surface morphology of the laser machined region has rarely been explored in detail. In view of this, the current work employed three-dimensional laser machining of human bone and reports the distinct surface morphology produced within a laser machined region of human bone. Three-dimensional laser machining was carried out using multiple partially overlapped pulses and laser tracks with a separation of 0.3 mm between the centers of consecutive laser tracks to remove a bulk volume of the bone. In this study, a diode-pumped pulse Er:YAG laser (λ = 2940 nm) was employed with continuously sprayed chilled water at the irradiation site. The resulting surface morphology evolved within the laser-machined region of the bone was evaluated using scanning electron microscopy, energy dispersive spectroscopy, and X-ray micro-computed tomography. The distinct surface morphology involved cellular/channeled scaffold structure characterized by interconnected pores surrounded by solid ridges, produced within a laser machined region of human structural bone. Underlying physical phenomena responsible for evolution of such morphology have been proposed and explained with the help of a thermokinetic model.

Optimization of biocompatibility in a laser surface treated Mg-AZ31B alloy.

Biodegradable bone implants can remove the need for subsequent bone-implant surgeries by controlled biomineralization and degradation. Although Mg-alloys generally possess biocompatible properties, they corrode rapidly, thereby preventing sufficient hydroxyapatite formation and biomineral growth. In an attempt to address these limitations, laser surface treatments were performed via the employment of a continuous wave Nd:YAG laser on the Mg-AZ31B alloy using laser fluences in the range of 1.06-4.24 J/mm2 (250-1000 W). The laser-treated samples were investigated for their wettability in simulated body fluid. In vitro analyses were performed in simulated body fluid to examine corrosion and biomineralization behavior on the laser-treated samples. Statistical optimization algorithms based on wettability data predicted an optimal laser fluence of 3.286J/mm2 (775 W) within the range of laser fluences used in the present study for achieving a balance between biodegradation and biomineralization. Confirmatory tests on optimized samples indicated an up to 84% grain size reduction in laser-treated surface regions, a several-fold increase in Mg17Al12 (ß) phase volume fraction, a reasonably abundant formation of hydroxyapatite, and increased rates of biomineralization that exceeded degradation. These findings indicate the potential of laser surface engineering to realize Mg-AZ31B alloy as a viable biodegradable bone implant material.

Integrated experimental and computational approach to laser machining of structural bone.

This study describes the fundamentals of laser-bone interaction during bone machining through an integrated experimental-computational approach. Two groups of laser machining parameters identified the effects of process thermodynamics and kinetics on machining attributes at micro to macro. A continuous wave Yb-fiber Nd:YAG laser (wavelength 1070 nm) with fluences in the range of 3.18 J/mm2-8.48 J/mm2 in combination of laser power (300 W-700 W) and machining speed (110 mm/s-250 mm/s) were considered for machining trials. The machining attributes were evaluated through scanning electron microscopy observations and compared with finite element based multiphysics-multicomponent computational model predicted values. For both groups of laser machining parameters, experimentally evaluated and computationally predicted depths and widths increased with increased laser energy input and computationally predicted widths remained higher than experimentally measured widths whereas computationally predicted depths were slightly higher than experimentally measured depths and reversed this trend for the laser fluence >6 J/mm2. While in both groups, the machining rate increased with increased laser fluence, experimentally derived machining rate remained lower than the computationally predicted values for the laser fluences lower than ∼4.75 J/mm2 for one group and ∼5.8 J/mm2 for other group and reversed in this trend thereafter. The integrated experimental-computational approach identified the physical processes affecting machining attributes.

Microstructure and corrosion behavior of laser surface-treated AZ31B Mg bio-implant material.

Although magnesium and magnesium alloys are considered biocompatible and biodegradable, they suffer from poor corrosion performance in the human body environment. In light of this, surface modification via rapid surface melting of AZ31B Mg alloy using a continuous-wave Nd:YAG laser was conducted. Laser processing was performed with laser energy ranging from 1.06 to 3.18 J/mm2. The corrosion behavior in simulated body fluid of laser surface-treated and untreated AZ31B Mg alloy samples was evaluated using electrochemical technique. The effect of laser surface treatment on phase and microstructure evolution was evaluated using X-ray diffraction and scanning electron microscopy. Microstructure examination revealed grain refinement as well as formation and uniform distribution of Mg17Al12 phase along the grain boundary for laser surface-treated samples. Evolution of such unique microstructure during laser surface treatment indicated enhancement in the corrosion resistance of laser surface-treated samples compared to untreated alloy.

Laser surface modification of AZ31B Mg alloy for bio-wettability.

Magnesium alloys are the potential degradable materials for load-bearing implant application due to their comparable mechanical properties to human bone, excellent bioactivity, and in vivo non-toxicity. However, for a successful load-bearing implant, the surface of bio-implant must allow protein absorption and layer formation under physiological environment that can assist the cell/osteoblast growth. In this regard, surface wettability of bio-implant plays a key role to dictate the quantity of protein absorption. In light of this, the main objective of the present study was to produce favorable bio-wettability condition of AZ31B Mg alloy bio-implant surface via laser surface modification technique under various laser processing conditions. In the present efforts, the influence of laser surface modification on AZ31B Mg alloy surface on resultant bio-wettability was investigated via contact-angle measurements and the co-relationships among microstructure (grain size), surface roughness, surface energy, and surface chemical composition were established. In addition, the laser surface modification technique was simulated by computational (thermal) model to facilitate the prediction of temperature and its resultant cooling/solidification rates under various laser processing conditions for correlating with their corresponding composition and phase evolution. These predicted thermal properties were later used to correlate with the corresponding microstructure, chemical composition, and phase evolution via experimental analyses (X-ray diffractometer, scanning electron microscope, energy-dispersive spectroscopy).

Integrated experimental and theoretical approach for corrosion and wear evaluation of laser surface nitrided, Ti-6Al-4V biomaterial in physiological solution.

A laser based surface nitriding process was adopted to further enhance the osseo-integration, corrosion resistance, and tribological properties of the commonly used bioimplant alloy, Ti-6Al-4V. Earlier preliminary osteoblast, electrochemical, and corrosive wear studies of laser nitrided titanium in simulated body fluid clearly revealed improvement of cell adhesion as well as enhancement in corrosion and wear resistance but mostly lacked the in-depth fundamental understanding behind these improvements. Therefore, a novel integrated experimental and theoretical approach were implemented to understand the physical phenomena behind the improvements and establish the property-structure-processing correlation of nitrided surface. The first principle and thermodynamic calculations were employed to understand the thermodynamic, electronic, and elastic properties of TiN for enthalpy of formation, Gibbs free energy, density of states, and elastic properties of TiN were investigated. Additionally, open circuit potential and cyclic potentio-dynamic polarization tests were carried out in simulated body fluid to evaluate the corrosion resistance that in turn linked with the experimentally measured and computationally predicted surface energies of TiN. From these results, it is concluded that the enhancement in the corrosion resistance after laser nitriding is mainly attributed to the presence of covalent bonding via hybridization among Ti (p) and N (d) orbitals. Furthermore, mechanical properties, such as, Poisson׳s ratio, stiffness, Pugh׳s ductility criteria, and Vicker׳s hardness, predicted from first principle calculations were also correlated to the increase in wear resistance of TiN. All the above factors together seem to have contributed to significant improvement in both wear and corrosion performance of nitride surface compared to the bare Ti-6Al-4V in physiological environment indicating its suitability for bioimplant applications.