The biocompatibility, mechanical properties and biodegradable nature of Mg alloys have made them attractive for biomedical applications, especially as bone implants. However, one of the main problems that limit the use of Mg alloy for several biomedical applications is their fast corrosion rates inside the body. Coating Mg-based implants is one of the most extensively studied approaches to address the fast corrosion rates of Mg alloys in the physiological environment. Micro arc oxidation (MAO) coating process has shown very promising results towards reducing the corrosion rates of Mg alloys due to the formation of a protective dense, well-adhered and wear-resistant oxide layer on the surface of the Mg alloy. In this study, the feasibility of coating an Mg-Zn-Ca-based alloy with a composite coating made using a micro-arc oxidation coating process and an immersion (dipping process) was investigated. The corrosion properties and surface characteristics of the coated alloy samples are assessed. The created protective composite coating is used to slow the corrosion rates of an Mg-Zn-Ca-based alloy. The developed composite coating resulted in a significant reduction in the corrosion rates. The results of this study show that it is possible to achieve more controlled corrosion rates of Mg-based implants towards patient-specific bone implant applications.

Fused deposition modeling (FDM), a material extrusion additive manufacturing process, has emerged as a method of choice for the fabrication of polymeric tissue engineering scaffolds. The FDM process is intrinsically complex, consisting of a multitude of parameters; in addition, there are material-machine-process interactions, which inevitably influence the mechanical properties, the surface morphology, and ultimately the functional integrity of fabricated bone structures. Consequently, physics-based process characterization and optimization in the FDM process is a burgeoning need. The overarching goal of this research work is to fabricate patient-specific, biocompatible, and biodegradable bone scaffolds for the treatment of osseous defects, fractures, and diseases. In pursuit of this goal, the objectives are to: (i) investigate the influence of consequential parameters of FDM on the functional properties of fabricated femur bone structures; and (ii) investigate the underlying physical phenomena behind the experimental observations using a computational finite-element model. In this study, biocompatible femur bone structures were FDM-deposited, based on a medical-grade polymer composite, composed of polyamide, polyolefin, and cellulose fibers. A new test specimen was designed, based on an X-ray micro-CT scan of a femur bone as well as the ASTM D638-14 (Type II) standard. In addition, the experimental characterization was on the basis of a cascade approach, composed of the following experimental deigns: (i) fractional-factorial design, utilized for factor screening and identification of consequential process parameters; (ii) Taguchi design, utilized for process optimization. Besides, a computational finite-element model was forwarded to investigate the underlying physical phenomena behind the experimental observations.

Pneumatic micro-extrusion (PME) is a high-resolution direct-write additive manufacturing process, which has emerged as the process of choice for tissue engineering and biofabrication of a broad spectrum of organs and tissues (e.g., bone, aortic valve, blood vessels, human ear, and nose). Despite the advantages and host of biomedical applications engendered by the PME process — including, for example, (i) accommodation of a wide range of material viscosity (enabled via thermopneumatic material deposition), (ii) large build volume and standoff distance for tissue engineering, (iii) in situ UV curing, and (iv) high-resolution multimaterial deposition — there are intrinsically complex design, material, and process factors as well as interactions, which influence the functional properties of PME-fabricated tissues and organs. Consequently, investigation of the impact and interaction of each factor aligned with establishment of a physics-based, optimal material deposition regime is inevitably a burgeoning need. In this study, using the Taguchi design, the influence of four significant factors, i.e., layer height, infill density, infill pattern, and print speed, is investigated on the compression properties as well as the dimensional accuracy of polycaprolactone (PCL) bone scaffolds, fabricated using the PME process. Furthermore, a 3D, transient two-phase flow CFD model is forwarded with the aim to observe the flow of material within the deposition head as well as the micro-capillary (nozzle). The results of this study pave the way for further investigation of the bio-functional properties of bone scaffolds, e.g., biodegradation, cell proliferation and growth rate.

The success of bone repair using an internal fracture fixation technique is critically dependent on the stability and biological process between the fragmented bones. However, the currently used bone plates mainly focus on stability rather than biology of healing, which subsequently (a) results in significant stress-shielding effects and (b) prevents stress from transferring from the bone plate to the bone during the healing process. This study proposes a novel design of a bone plate for the fixation of long fractured bones, which can mitigate these disadvantages to strike a balance between stability and biology. The new multi-material design adopts stainless steel (SS316L) and magnesium alloy (AZ31B) of three thicknesses such as SS316L (1mm)-AZ31B (2mm), SS316L (1.5mm)-AZ31B (1.5mm), and SS316L (2mm)-AZ31B (1mm). The mechanical properties (bending stiffness and moment) of the bone plates were evaluated according to the ASTM: F382-17 standard. Static corrosion tests were conducted in Hank’s Balanced Salt Solution (HBSS) at 37.5 °C. Compared with those of the original (non-corroded) bone plates, the maximum load-carrying capacities of the corroded bone plates decreased from 670 N to 495 N, 891 N to 518 N, and 928 N to 709 N in the case of SS316L (1mm)-AZ31B (2mm), SS316L (1.5mm)-AZ31B (1.5mm), and SS316L (2mm)-AZ31B(1mm), respectively. Digital image correlation was utilized to evaluate the inter-fragmentary strain (IFS) in the physical model of fractured bone plates. The IFS increased from 0.526 to 0.815, 0.484 to 0.784, and 0.455 to 0.533 in the case of SS316L (1mm)-AZ31B (2mm), SS316L (1.5mm)-AZ31B (1.5mm), and SS316L (2mm)-AZ31B (1mm), respectively, when a load of 200 N was applied. An optimized design of the bone plate of SS316L and AZ31B for granulation tissue formation based on Perren’s theory and IFS was successfully proposed.

Bone fractures are common in human beings, and the surgical operation performed to repair the fractured bone includes the drilling process. Axial cutting force during drilling plays an important role for success in osteosynthesis. Many processes like water jet machining, laser drilling, ultrasonic drilling, two phase drilling and rotary ultrasonic bone drilling (RUBD), have been developed to minimize the cutting force during orthopedic drilling. In the present work, a novel operation theatre compatible ultrasonic assisted rotary bone drilling machine prototype have been designed and fabricated in an attempt to reduce the cutting forces produced during orthopedic drilling. The experimental test were performed on fresh porcine bone and human femur. A comparative study was carried out between RUBD and Conventional drilling (CD), to find out the influence of rotational speed, drilling tool feed rate and drill diameter, on the cutting force produced during the drilling. The study of chip produced in RUBD and CD was also carried out with the help of scanning electron microscopy images. The result obtained by the developed machine showed that the cutting force obtained in human femur was greater (20–25 %) than porcine femur due to its greater hardness and RUBD process produced lower cutting force (20–30 %) than CD for both types of bone. The experimental results showed the increase in cutting force with decreasing rotational speed and increasing feed rate and drill diameter.

Deep-hole bone drilling is performed in many surgical implantation procedures. Unlike bone drilling of shallow holes (commonly 5 to 7 mm in depth), deep-hole bone drilling requires a larger hole depth (i.e., up to a depth of approximately 35 mm in cochlear implantation) using a high ratio of the length to diameter of the drill bit. This paper presents an experimental investigation of forces in deep-hole bone drilling. The hypothesis of this study was that the forces signals show a sudden transition with a considerable amount of increases during deep-hole bone drilling process. The objective of this study was to understand bone drilling behaviors as drilling depth increases. Drilling tests were performed on bovine cortical bones using a 2.5 mm diameter drill bit with a spindle speed of 3000 rpm and feed rates of 0.05 and 0.1 mm/rev at a drilling hole depth of 36 mm. The force signals showed two distinct states, which were referred to as normal and abnormal states in this study. After showing a constant force signals in the normal state once the drill tip became fully engaged in bone cutting, from a certain drilling depth, the force signals considerably increased, which is referred to abnormal state. This study observed that the chip morphology varies with respect to drilling depth, showing fragmented chips in the normal state and powdery chips in the abnormal state. The results of this study indicate that the abrupt increase in the force signals in the abnormal state is mainly attributed to chip clogging inside the flutes as the drilling depth increases.

The paper presents a methodology to optimize drill bits to realize safe drilling of bone materials for many surgeries like orthopedics and neurosurgery. First, a mechanistic model is introduced to relate drilling forces to main drill bit geometry parameters. Then a genetic algorithm is developed to optimize drill bit geometry parameters by minimization of drilling forces based on the mechanistic model. Finally prototypes of drill bits with optimized geometry parameters are produced and drilling experiments are conducted to verify the advantages of these new drill bits. The results show that by comparison with normal drill bit, the average drilling forces are reduced to more than 50% by drill bits with optimized geometry parameters under a wide range of drilling conditions.

Precise and firm fixation of the cranium is critical during craniotomy and delicate brain neurosurgery making head immobilization devices (HIDs) a staple instrument in brain neurosurgical operations today. However, despite their popularity, there is no standard procedure for their use and many complications arise from using HIDs in pediatric neurosurgery. In this paper, we identify biomechanical causes of complications and quantify risks in pin-type HIDs including clamping force selection, positioning and age effects. Based on our root cause analysis, we develop a framework to address the biomechanical factors that influence complications and understand the biomechanics of the clamping process. We develop an age-dependent finite element model (FEM) of a single pin on a cranial bone disc with the representative properties and skull thickness depending on age. This model can be utilized to reduce risk of complications by design as well as to provide recommendations for current practices.

Electro-hydrodynamic Jetting or E-Jetting is a process in which a polymer, dissolved in a solvent and extruded through a needle onto a substrate. A potential difference is applied between the needle and the substrate to facilitate the homogeneous extrusion of the fiber. This process is used to fabricate two dimensional scaffolds with porous mesh surfaces which act as a template for cell growth. As cells are very minute and are required to attach to the surface of the scaffold, it is essential to for the scaffold to have an adequate pore size that allows for nutrient transfer while preventing the penetration of cells through the scaffold. The fiber dimensions of the scaffold may be modified by varying the diameter of the needle through which the fiber is extruded. The change in fiber diameter subsequently results in the change in the bulk mechanical characteristics of the scaffold. It also causes a change in the net porosity of the scaffold. This paper aims to study the effect of the needle diameter on the bulk mechanical properties of the scaffold such as Young’s modulus, Tensile strength and Breaking Strength as well as morphological properties (porosity and pore size) of the Scaffolds are dependent on the cell type, as each type of cell has a different set of requirements depending on the functionality. Bone cells are smaller than soft tissue cells, hence a common scaffold design may not be suit either of the applications. Thus, a one size fits all approach is not suitable for the scaffold [9]. As seen in Figure 1, the Red Blood Cells are a fraction of the size of the fibroblasts and bone marrow stem cells [20–22]. Similarly, the stiffness of the cells is also different. Electro Hydrodynamic Jetting (E-jetting) is a process that is used to fabricate such 2D scaffolds by extruding a polymer solution through a needle and forming a fiber by applying a scaffold. For this study, twelve scaffolds belonging to three study groups were synthesized using e-jetting. By studying the effect of needle diameter on scaffold morphology and strength, we aim to develop a co-relation between the scaffold parameters, which will ultimately help in the creation of a knowledge database. The purpose of creating this database is to choose a select needle for a selected biomedical application.

Tensile and compression test specimens comprising lattice structures with simple cubic, crossing-rod and body-centered cubic (BCC) unit cells are produced via SLM additive manufacturing (AM) of AISI 316L stainless steel and CoCr powder. Equivalent stress-elongation curves are obtained, with equivalent strength, specific strength, stiffness modulus and specific stiffness calculated based on specimen density and sample cross-section. The obtained results highlight the fact that analogous structures can behave very differently depending on the chosen material. While large differences are obtained in strength and stiffness between the different unit cell types, specific strength and specific stiffness vary to a lesser extent. Two case studies are presented, including a porous structure suitable for bone implants in the field of biomedical engineering and an AISI 316L food packaging machine component. The results obtained in this study provide useful guidelines and equivalent properties for designers wishing to exploit the advantages of internal lattice structures in AM.

The fast corrosion rate of magnesium (Mg) alloys is the main problem associated with the use of such biocompatible alloys for bone fixation applications. The corrosion resistance of Mg alloys can be improved by different post-fabrication processes such as heat treatment and coating. We have heat-treated a biocompatible Mg-1.2Zn-0.5Ca (wt.%) alloy at optimized heat treatment parameters to achieve the highest mechanical strength and corrosion resistance. Afterwards, the heat-treated alloy was coated with a ceramic layer using micro arc oxidation (MAO) process to further enhance the corrosion resistance. The microstructure of the prepared samples was investigated using optical microscopy and scanning electron microscopy (SEM). The corrosion characteristics were determined by conducting in vitro electrochemical and immersion corrosion tests. The results showed that the heat treatment process successfully improved the mechanical and corrosion properties of the Mg-1.2Zn-0.5Mn (wt.%) alloy. Both the in vitro electrochemical and immersion corrosion tests showed that the MAO-coated samples have a significantly higher corrosion resistance which results in a significantly lower corrosion rate. This study indicated that the biocompatible coating produced by MAO process may be suitable for providing heat-treated Mg-Zn-Ca-based alloys with protection from corrosion towards synthesizing bone fixation materials in clinical application.

Mg alloys are promising materials for bone implant applications mainly due to their low specific density, desirable stiffness and bioresorbability in the human body. Mg-Zn-Ca alloys are among the most promising materials for resorbable orthopedic fixation devices due to their superior biocompatibility. However, the mechanical and corrosion properties of the as-cast Mg-Zn-Ca alloys are insufficient. Heat treatment is a practical approach for strengthening Mg alloys especially after the fabrication of porous structures and 3D-printed components. We have investigated heat treatment of these devices and have studied the resulting microstructure of Mg-1.6Zn-0.5Ca (wt. %) alloys by hardness, compression, scanning electron microscopy (SEM), and electrochemical and immersion corrosion tests. Mg-1.6Zn-0.5Ca alloy was prepared with high purity Mg, Zn and Ca by casting. The cast ingots were solution-treated at 510 °C for 3 h then quenched in water. The quenched ingots were age hardened in an oil bath at 200 °C for 2 h. Pure Mg, as-cast and heat-treated Mg-1.6Zn-0.5Ca alloy ingots were cut into coupons to characterize their mechanical and corrosion properties. In vitro corrosion tests were conducted in modified simulated body fluid (m-SBF) at pH 7.4 and 37 °C. The hardness of the Mg-Zn-Ca alloy was significantly increased from 52.6 to be 66.8 HV after heat treatment. Also, the compression test results revealed that the heat-treated alloy has the highest compressive yield and ultimate strengths without significant change in stiffness and maximum strain. The mass loss of the Mg-Zn-Ca alloy by week 4 of the in vitro immersion test reduced from 174.6 mg/cm 2 for the as-cast alloy to 101.7 mg/cm 2 after the heat-treatment process. Heat-treatment was found to be a powerful post-shaping process not only to enhance the mechanical properties of the Mg-1.6Zn-0.5Ca (wt. %) alloy, but also to significantly improve its biocorrosion properties. Such heat-treated alloys can also be coated with biocompatible ceramics that provide additional protection from corrosion during the bone healing period (3–4 months).

Selective laser melting (SLM) is an additive manufacturing technique in which complex parts can be fabricated directly by melting layers of powder from a CAD model. SLM has a wide range of application in biomedicine and other engineering areas and it has a series of advantages over traditional processing techniques. A large number of variables including laser power, scanning speed, scanning line spacing, layer thickness, material based input parameters, etc. have a considerable effect on SLM process materials. The interaction between these parameters is not completely studied. Limited studies on balling effect in SLM, densifications under different processing conditions, and laser re-melting, have been conducted that involved microstructural investigation. Grain boundaries are amongst the most important microstructural properties in polycrystalline materials with a significant effect on the fracture and plastic deformation. In SLM samples, in addition to the grain boundaries, the microstructure has another set of connecting surfaces between the melt pools. In this study, a computational framework is developed to model the mechanical response of SLM processed materials by considering both the grain boundaries and melt pool boundaries in the material. To this end, a 3D finite element model is developed to investigate the effect of various microstructural properties including the grains size, melt pools size, and pool connectivity on the macroscopic mechanical response of the SLM manufactured materials. A conventional microstructural model for studying polycrystalline materials is modified to incorporate the effect of connecting melt pools beside the grain boundaries. In this model, individual melt pools are approximated as overlapped cylinders each containing several grains and grain boundaries, which are modeled to be attached together by the cohesive zone method. This method has been used in modeling adhesives, bonded interfaces, gaskets, and rock fracture. A traction-separation description of the interface is used as the constitutive response of this model. Anisotropic elasticity and crystal plasticity are used as constitutive laws for the material inside the grains. For the experimental verification, stainless steel 316L flat dog bone samples are fabricated by SLM and tested in tension. During fabrication, the power of laser is constant, and the scan speed is changed to study the effect of fabrication parameters on the mechanical properties of the parts and to compare the result with the finite element model.

The aim of this article is to investigate the effect of two different fixation hardware materials on bone remodeling after a mandibular reconstruction surgery and to restore the mandible’s function, healthy appearance, mastication, swallowing, breathing, and speech. The hypothesis is that using fixation hardware with stiffness close to that of the surrounding bone will result in a more successful healing process in the mandible bone. The finite element model includes the material properties and forces of the cancellous bone, cortical bone, ligaments, muscles, and teeth. The reconstruction surgery is modeled by including the fixation hardware and the grafted bone. In the sectioned mandible, to best mimic the geometry of the mandible, two single barrel grafts are placed at the top of each other to form a double barrel graft set. Two different materials were used as the mandibular fixation parts, stiff Ti-6Al-4V, and porous superelastic Nickel-Titanium (NiTi) alloys. A comparison of these two alloys demonstrates that using porous NiTi alloy as the fixation part results in a faster healing pace. Furthermore, the density distribution in the mandibular bone after the healing process is more similar to the normal mandible density distribution. The simulations results indicate that the porous superelastic NiTi fixation hardware transfers and distributes the existing forces on the mandible bone more favorably. The probability of stress shielding and/or stress concentration decrease. This type of fixation hardware, therefore, is more appropriate for mandible bone reconstruction surgery.

This study characterizes the forces in high-speed bone cutting and grinding for the use of haptic devices in surgical simulations. Unrealistic force feedback due to the lack of vibrational features is one of the most common drawbacks. Generally, the force profile can be decomposed to a mean force and a vibrational force magnitude. These forces are experimentally measured under various motions, including feed rate and tool orientation, to mimic manual operations and to understand the effects of these parameters. Change in feed rate was found to be insignificant in the overall force feedback, while the change in tool orientation showed statistically significant effects. The grinding burr and cutting burr also exhibited different forces under an identical condition. The explanation for the behavior of the forces based on the cutting and grinding conditions is discussed along with the results.

A set of monotonic tensile tests was performed on 3-D printed plastics following ASTM standards. The experiment tested a total of 13 “dog bone” test specimens where the material, infill percentage, infill geometry, load orientation, and strain rate were varied. Strength-to-weight ratios of the various infill geometries were compared. It was found through tensile testing that the specific ultimate tensile strength (MPa/g) decreases as the infill percentage decreases and that hexagonal pattern infill geometry was stronger and stiffer than rectilinear infill. However, in finite element analysis, rectilinear infill showed less deformation than hexagonal infill when the same load was applied. Some design guidelines and future work are presented.

The most common method for mandibular reconstructive surgery is the use of a Ti-6Al-4V fixation device and a fibular double barrel graft. This highly stiff fixation hardware (E = 112 GPa) often shields the bone graft (E = 20 GPa) from carrying the load, which may result in bone resorption. Highly stiff Ti-6Al-4V fixation hardware is also likely to concentrate stress in the fixation plate or at screw threads, possibly leading to hardware cracking or screw pull-out. As a solution for that, we have proposed and studied the effect of using a low stiffness, porous NiTi fixation device [1–4]. Although the stress in the fixation device is increased, using such low stiffness fixation hardware, is preferable to have an even higher stress on the graft in order to minimize the risk of resorption or hardware failure. We assume that preloading screws allows them to better engage the fixation hardware with the plate and the surrounding bone and causes an increased von Mises stress. The fixation device can be patient-specific and additively manufactured, such that the shape would match the outer surface of the cortical bone. In this study, we modeled a healthy cadaver mandible via CT-derived 3D surface data. The mandible was virtually resected in the molar region (M 1−3 ). The model simulated the result of reconstructive surgery under the highest chewing loading regime (i.e., 526 N on first right molar tooth [5, 6]) where reconstruction was done with either Ti-6Al-4V fixation hardware or patient specific, stiffness-matched, porous NiTi fixation hardware. The calibration of the material properties for this simulation was done using experimentally obtained data (DSC and compression tests) of Ni-rich NiTi bulk samples. The analyzed term in the finite element analysis was stress distribution in the cortical and cancellous bone. Porous NiTi fixation devices were also produced using Selective Laser Melting (SLM) using the geometry of the aforementioned cadaver mandible. In this paper we have studied the effect of additional torque or preload on the performance of the fixation plates. The finite element analysis demonstrated that applying a preload to the screws increased the stress on the bone. Under similar levels of applied preload, the porous NiTi fixation device showed an increased level of von Mises stress in the bone, particularly in the graft. Additionally, the analysis indicated the higher level of stress on the bone surrounding the screws for the case of using NiTi, which could contribute to increasing screw stability. The fabricated patient-specific fixation hardware not only matched the shape of cortical bone but also contained the level of porosity that defines the appropriate modulus of elasticity.

Functionally graded bioactive glass coatings on bioinert metallic substrates were produced by using continuous-wave (CW) laser irradiation. The aim is to achieve strong adhesion on the substrates and high bioactivity on the top surface of a coating material for load-bearing implants in biomedical applications. The morphology and microstructure of the bioactive glass from the laser coating process were investigated as functions of processing parameters. Laser sintering mechanisms were discussed with respect to the resulting morphology and microstructure. It has been shown that double layer laser coating results in a dense bond coat layer and a porous top coat layer with lower degree of crystallinity than an enameling coating sample. The dense bond coat strongly attached to the titanium substrate with a ten microns wide mixed interfacial layer. A highly bioactive porous structure of the top coat layer is beneficial for early formation of a bone-bonding HCA layer. The numerical model developed in this work also allows for prediction of porosity and crystallinity in top coat layers of bioactive glass developed through laser induced sintering and crystallization.

Hydroxyapatite, a bioactive ceramic, has been combined with biodegradable polymers to create composite three-dimensional interconnected porous scaffolds for bone graft substitutes. The materials and fabrication methods of these composite scaffolds are reviewed. The resulting mechanical and biological properties of scaffolds produced from the combination of certain materials and fabrication methods are analyzed. Requirements for a bone graft substitute and third generation scaffolds with the addition of osteoinductive and osteogenic features to composite scaffolds including biomolecule delivery and cell seeding are also introduced. Finally, the benefits of using additive manufacturing technologies to enable high level of control over the design of interconnected pore structure are discussed.

This paper presents a new analytical modeling of ultrasonic vibration assisted drilling of bones for medical surgery operations. Due to the heterogeneous bone structure and the uneven bone surface, bone surgery cutting suffers from inaccuracy and difficulty in high precision bone surgery cutting. In this paper, a new method of ultrasonic vibration assisted drilling is proposed for bone surgery cutting. An analytical force modeling is presented for ultrasonic vibration assisted bone drilling. Experimental result validates the analytical modeling presented in the paper. Preliminary testing result also shows a significant improvement of drilling accuracy based on the proposed ultrasonic vibration assisted bone drilling. The proposed cutting techniques can be used in bone cutting surgery to increase the accuracy of born drilling position and reduce trauma damage of bone and surrounding soft tissues.