Mandibular reconstruction plates are essential for restoring function after segmental defects, but current designs often lead to stress shielding and long-term complications. New porous implant architectures may reduce these problems, yet their performance is insufficiently validated in an experimental test environment under realistic loading conditions. Existing experimental setups either oversimplify the physiological fidelity or are too complex to reproduce reliable results.

In this thesis, a new experimental setup that combines anatomically relevant loading conditions with rigid and reproducible boundary conditions is designed, manufactured, and validated. It features moulded condyle fixation cups that ensure precise and repeatable alignment of the mandibular condyles, and a modular loading platform capable of accurately reproducing various bite configurations. The platform allows controlled repositioning of occlusal contact points and accommodates diverse mandibular geometries, enabling both unilateral and bilateral loading scenarios. This configuration creates a mechanically robust environment while preserving essential biomechanical characteristics of mandibular function.

Structural performance of the setup was investigated through finite element analysis to assess stress distribution, deformation behaviour, and potential failure locations. Experimental validation was carried out using quasi-static and progressive cyclic loading, confirming that the setup maintains stable boundary conditions, realistic load paths, and high reproducibility under repeated testing. Using this setup, three metamaterial implant designs were assessed and strain distributions were quantified using Digital Image Correlation, revealing distinct differences in strain concentration around screw regions.

Overall, this work provides a robust, reproducible, and physiologically informed platform for the mechanical evaluation of mandibular reconstruction implants. The setup enables systematic comparison of implant architectures and supports future extensions toward cadaveric studies and clinically validated testing protocols, contributing to the development of implants with improved long-term clinical outcomes. ... Read more

Slipped Capital Femoral Epiphysis (SCFE) is the most common hip disorder in adolescents characterised by the displacement of the femoral head relative to the femoral neck through the growth plate [3]. In-situ fixation, the current standard type of treatment, stabilises the epiphysis to prevent further slippage. Although it prevents further slippage of the femoral head, the altered morphology of the proximal femur remains [4], leading to long-term complications. Recent studies have, however, revealed that bone growth can continue after fixation, suggesting potential for guided bone growth through strategic screw placement. The aim of this study is to find the optimal screw position for in-situ fixation to stimulate longitudinal bone growth in desired areas, thereby potentially improving the femur morphology and reducing SCFE severity. The study is divided into two parts: the first part focuses on modeling healthy bone growth to validate the computational approach against results from a prior study and clinical data, while the second part concentrates on SCFE analysis to explore the optimal screw position for in-situ fixation to stimulate longitudinal bone growth in desired areas. A finite element (FE) model of a femur with a mild slip was developed to predict bone growth under various screw positions. The Osteogenic Index (OI), which quantifies the expected amount of bone growth, was used to predict growth patterns. The findings indicate that the conventional center-center screw position, commonly used in surgical practice, results in minimal bone growth. Conversely, anteriorly positioned screws, particularly in the anteromedial (AM) and anterolateral (AL) regions, were associated with the most significant and beneficial growth on the posterior side of the growth plate. The expected amount of growth in the AM and AL positions was approximately 211% and 138% greater, respectively, than in the conventional center-center position. These results suggest that anterior screw placement could enhance bone growth in desired areas, potentially mitigating the severity of SCFE and improving long-term functional outcomes. These results reinforce similar findings of a prior retrospective clinical study by [2]. This study contributes to a growing body of evidence supporting the reconsideration of traditional screw placement strategies in SCFE treatment. The insights gained could inform surgical practices aimed at optimising growth modulation and improving patient outcomes. Recommendations have been given to improve the FE analysis and validation. The largest uncertainty lies in the hip contact force in SCFE patients. Further research is needed to investigate this loading for different patients and slip severities and test the influence of the loading conditions on the growth pattern. In addition, clinical trials are necessary to fully validate these findings and assess their practical implications in the treatment of SCFE. ... Read more

A multi-material structure combines diverse materials, allowing for precise tailoring of properties in specific regions. The integration of dissimilar materials leads to superior performance compared to single-material components. Bimetallic structures, a subset of multi-material systems, offer site-specific performance owing to the contrasting properties of the materials involved.

This thesis focused on fabricating a thin, functionally graded bi-metallic wall using Wire Arc Additive Manufacturing (WAAM) with Inconel 625 and HSLA steel. Functional grading of metallic materials, like Inconel 625 and HSLA steel, enables precise customisation of component properties to fulfil distinct functions within a structure. Despite its promise, this combination presents challenges, notably the formation of intermetallic compounds that act as a catalyst for forming solidification cracks in combination with varying material composition and high heat input.

Optimal process parameters, notably reduced heat input, are critical in mitigating these issues. The current study involved process parameters optimisation for Inconel 625 single bead-on-plate welds to control dilution levels at dissimilar interfaces. Extensive characterisation using Light Optical Microscopy (LOM) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) was conducted to study the microstructural evolution of the as-fabricated sandwich structure.

Results indicated that interface 1 (Inconel 625/HSLA steel) exhibited minimal dilution and zero defects, while interface 2 (HSLA Steel/Inconel 625) exhibited substantial dilution and was prone to solidification cracks. The EDS results affirmed that the variations of elemental composition and heavy dilution at interface 2 lead to inhomogeneous microstructural features, elemental segregations and the formation of brittle intermetallic phases, thereby leading to a solidification crack at this particular interface.

X-ray diffraction (XRD) measurements were conducted to identify phases at the dissimilar interfaces, corroborating the microstructural features observed using SEM and LOM. The identified phases confirm the presence of brittle intermetallics, such as the laves phase, extensively present at interface 2. Vickers hardness testing was performed to assess the mechanical behaviour at both interfaces, revealing a consistent trend of decreasing values with a sudden increase in hardness values at both interfaces due to microstructural transition.

Based on the EDS results, an optimal range of elemental compositions was speculated, focusing primarily on the significant elements Fe, Ni, and Cr. These compositions, with Ni content of approximately 20-25 wt%, Fe content of around 70-75 wt%, and Cr content of approximately 5-10 wt%, are crucial in reducing cracking susceptibility at interface 2. This study elucidates the initiation and propagation of solidification cracks at interface 2, establishing a definitive link between microstructural evolution, mechanical behaviour, and crack formation.

Ultimately, this thesis lays the foundation for future research to delve deeper into these insights, fine-tune process parameters, and fabricate compositionally graded multi-material structures with enhanced susceptibility to solidification cracking at dissimilar interfaces. ... Read more

Introduction: Amputees still experience problems with the use of their upper limb prostheses, mainly in the lack of comfort and functionality. This results in rejection of the prosthesis or overuse injuries due to compensatory movements. The comfort and functionality should be improved by reducing weight and increasing the degrees of freedom. The wrist is an important joint for increasing the mobility. And for reducing the weight, another actuation method than the most available and heavy electrically driven device should be used. Methods: This study designs a hydraulic curved cylinder for the application of a wrist joint. Because of the complex and curved shape the manufacturing method used is 3D printing. The functionality of three 3D printing methods (FDM, MJF and SLA) and three sealing techniques (O-ring, X-ring, and Conus) will be evaluated by testing the amount of friction and leakage to find which configuration performs best and whether it results in an efficient actuator. Results: The SLA-cylinders and the Conus allowed the least leakage. The combination of these two even resulted in zero leakage. For friction the SLA and FDM-cylinders performed equally, only the MJF-cylinder caused more friction. The SLAcylinder is the only cylinder that can hold a constant pressure without leakage for 5 out of 8 pistons, when a 4 kg weight is attached. Finally, only piston-cylinder combinations with piston O-25 or Cylinder X.MJF do not match the requirements for the maximum allowed leakage and adjustment torque. Conclusions: The piston-cylinder combination of SLA-parts and Conus perform the best. For the application of a wrist joint in prostheses more research needed for the repeatability, scalability and long-term performance. ... Read more

Background: Wrist immobilization orthosis is a commonly adopted treatment method for wrist regarding pathologies. In recent years, a group of 3D-printed immobilization orthosis for the upper extremity has been proposed. However, they were mainly developed as substitutes for traditional casts for the upper extremity for short-term usage. There has been no definite attempt to design 3D-printed orthosis to immobilize the upper extremity used as an assistive device for long-term usage. Objective: Develop a patient-specific orthosis for wrist immobilization in long-term usage, which has suitable functionality, the comfort of wearing, and an efficient fabrication process by exploiting the benefits of Additive Manufacturing technology. Methods: Structure concepts were generated by modes of donning and doffing and evaluated with criteria. The prototype was assessed together with an orthopedic technician. For the next, the prototype was validated with actual patients who were suffering from degenerative wrist pathologies. The subjects wore their wrist immobilization orthosis for two months and were asked about their experiences. Results: In total, eleven wrist immobilization orthosis prototypes were validated with five subjects (two males and three females) suffering from degenerative wrist pathologies (Rheumatoid Arthritis: one subject, Osteoarthritis: three subjects, and Ehlers-Danlos syndrome: one subject). 45 % of prototypes provided excessive pressure on bony prominences. Around 55 % of prototypes caused unacceptable pain at distal palmar creases. Specifically, the movement of putting it on and off brought complications for 91 % of all prototypes. For three subjects who could use their orthoses for two months, the orthoses functioned well, decreasing pains in the wrist while having daily activities. Conclusion: First, the structure of the wrist immobilization orthosis was well-designed, performing proper support during activities of daily living. Second, the comfort evaluation was conditionally positive; due to the rigid hardness of the material, two out of eleven orthoses were too painful to wear for the further test. Since the morphology of bony eminences and the amount of the cutaneous fats around those areas are dissimilar for all people, it was hard to design pain-zero orthosis for two subjects. Lastly, SLS printing with PA12 material offered apparent advantages over traditional fabrication methods. All works were conducted digitally from the scanning to fabrication, and there was no waste of materials. Despite six working days of delivery time from the outsourcing company, the orthosis did not require any labor-intensive procedures. ... Read more

When inserting a flexible endoscope into a human colon during colonoscopy, limitations in the endoscope design can cause medical implications such as excessive colon stretching and buckling of the endoscope shaft. This thesis proposes a novel propulsion mechanism design for flexible endoscopes which changes the method of insertion as a way to prevent these implications. As inspiration for the design an analogy is drawn between plant roots growing through tortuous cracks in soil and flexible endoscopes moving through a tortuous human colon. The analogy was found to be relevant, resulting in eight biological features of which seven were suitable as a potential solution in the development of a flexible endoscope. Of these seven suitable solutions, apical extension and variable stiffness were implemented as functions in the proposed propulsion mechanism. The focus of the design process on these two functions ultimately culminated in a proof of concept flexible, extending endoscope design dubbed the Flextendoscope. The Flextendoscope consists of an inverted tube mechanism which can propel the endoscope through apical extension combined with a fiber jamming mechanism that can vary the bending stiffness. The concept design is found very promising as it theoretically allows insertion of a highly compliant shaft which is also able to provide the high tip rigidity required at the surgical location after insertion. Although evaluation of the Flextendoscope prototype validated the feasibility of the proposed proof of concept design, the performance of the apical extension function was of a more limited success. Due to internal frictional resistances the prototype shaft only extended under the lowest evaluated pressure (0.5 bar) and required a high manual force (up to 119 N) to do so. The behavior of the variable stiffness function on the other hand was found to be very desirable. The bending stiffness of the prototype increased with internal pressure level as well as radial deflection, showing initial elastic deformation followed by hysteresis. It showed considerable stiffening at a convenient threshold pressure of 0.7 bar, slightly above the pressure at which the shaft is extended. Future iterations of the Flextendoscope design should focus on overcoming the internal frictional resistances that limit the apical extension of the current design. They can do so by multiplying the number of inverted tubes inside the shaft. This multiplication would enclose the sliding shaft material of each inverted tube within their own central lumen thus avoiding contact between the sliding shaft material and other stationary components in the design, preventing frictional interactions. If the frictional resistance is overcome the Flextendoscope design could prevent colon stretching during insertion of an flexible endoscope and inherently prevent shaft buckling. The prevention of these implications by the Flextendoscope design could ultimately lead to colonoscopy becoming an easier, safer and less painful procedure with a higher success rate. ... Read more

Background The current workflow for the design and production of patient-specific surgical guides at the Elisabeth-TweeSteden hospital is out-sourced to external companies, making it a time-consuming and costly task. In order to minimize the production time and expenses, the design and fabrication of these guides could be implemented into the hospital’s workflow using an in-house available 3D printer. Compared to the current gold standard for the production of patient-specific surgical guides, a different printing technique and 3D print material will be used. Therefore, it should be determined whether these are suitable for the production of the guides. Moreover, the 3D printed models must be compatible with the in-house sterilization method in order to ensure quality and reliability. The aim of this thesis was to investigate the effect of the in-house available sterilization method on the mechanical, dimensional, and sterility properties of a novel biocompatible 3D printing material. Methods and results BioMed Clear resin specimens were tested according to the ISO standards for mechanical testing of plastics. Overall, steam sterilization changed the mechanical properties of the material, making it stronger and stiffer but more brittle, as compared to unsterilized specimens. The dimensional tests indicated that sterilization resulted in a dimensional change, up to 0.12 mm, which mostly occurred in the small crevices of the model. The deviations of the dimensions of the resulting 3D printed model compared to the digital 3D model were roughly less than 0.3 mm, which is comparable to the accuracy of the gold standard material, polyamide 12. Sterility tests showed that sterilization of the 3D printed models is indeed mandatory for the guides to be used intraoperatively. Also, the Central Sterile Services Department of the hospital is able to deliver sterile medical devices that can be stored for a minimum of four weeks at the sterile storage room. Conclusion With the feasibility confirmed for the hospital to be able to design and produce patient-specific surgical guides in-house, it is necessary to stress that the hospital must meet the Medical Device Regulations by complying with an appropriate quality management system, documenting the manufacturing process, evaluating the performance, and reviewing the experiences from clinical use. ... Read more

In this report a design is proposed for a laparoscopic gripper that can be manufactured with metal 3D printing and polished with mass finishing. The design is a continuation of the development of a laparoscopic gripper that can be 3D printed in plastic. Laparoscopic grippers 3D printed solely out of metal have not yet been presented. Laparoscopic instruments are limited in width to 5 mm, which is bordering the manufacturing limits of selective laser melting 3D printing. The use of 3D printing for medical instruments has the potential to customise instruments specific to patient, procedure, and surgeon. Metal 3D printing can produce complex parts, albeit with a high surface roughness. Post-processing is required to reduce the surface roughness. Mass finishing techniques are a group of mechanical polishing techniques, of which centrifugal disc finishing was selected due to its capability to process parts in bulk without requiring workpiece fixation. To synthesise a suitable design, the processes of printing and polishing were analysed to formulate design guidelines. The analyses were part literature study, part experimental study. The experimental study had the aim to quantify and supplement the guidelines found in handbooks and articles. Using a novel visualisation technique, the polishing of different geometries could be distilled into quantitative design considerations. Here, a marking lacquer was applied to the surface of workpieces, which remained on unpolished surfaces. In this experiment a number of features were used, which corresponded to aspects that had potential to be used in the design. The use of channels was deemed unviable for polishing at the scale of laparoscopic instruments, which required the removal of these from the design. Mass finishing polishing removed the coarse surface structure present on metal produced with 3D printing, and brought surfaces of the test pieces to 0.05 mm below their desired width. Application of the design guidelines to the laparoscopic instrument was focused on making printing and polishing compatible joints. The laparoscopic gripper has two degrees of freedom for increased manoeuvrability. The features that comprise the joint are protrusions and cut-outs, sinusoidal gear arches, and actuation cable guides. Each of these features were dimensioned with values from the guidelines. The joint design required a number of components to be split so polishing access could be guaranteed, specifically for the cable guides. This had the added benefit of having each part be orientated during printing individually. The final design is based on application of the relevant design guidelines, and has been validated using scale models for mechanical stability. ... Read more

Many manufacturing industries have been impacted by the innovation of additive manufacturing (AM), and biomaterials manufacturing is no exception. One group of biomaterials impacted by the innovation of 3D printing are degradable biomaterials. Degradable biomaterials could eliminate the need for surgery to remove the implant. 3D printed porous degradable biomaterials provide both mechanical support and space for bone ingrowth. As of today, there is an absence of materials for this application that are biodegradable, biocompatible and can be 3D printed. Magnesium can be used as a degradable biomaterial but its corrosion resistance is not yet adequate for application in the human body. Applying zinc as an alloying element to magnesium increases its corrosion resistance compared to pure magnesium. As the addition of alloying elements changes the microstructure, which in turn changes its corrosion behaviour.

This thesis analysed the effect of microstructure on the corrosion behaviour of extrusion-based 3D printed porous Mg-4Zn (wt.%) scaffolds, using localised electrochemical techniques i.e. scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM). The microstructure of the Mg-4Zn scaffolds includes grains with secondary phases precipitated along the grain boundaries with the presence of micropores. The secondary phase particles showed increased Volta-potential compared to the magnesium-based matrix. Therefore, the addition of zinc caused micro-galvanic coupling between secondary phase particles and the matrix, but their contribution to corrosion is minimal due to postponed contact with the electrolyte and the protection by the corrosion products. Micropores in the Mg-4Zn scaffold increased the surface exposed to fluids and were pitting corrosion initiation sites. During corrosion however, the surface was covered with a more stable corrosion product compared to pure magnesium. As a result of this, the corrosion resistance of Mg-4Zn scaffolds is better than pure magnesium scaffolds. ... Read more

Literature reports a great deal of contradictory results concerning the effect of microstructure on the corrosion and passivity behaviour of advanced high strength steels. The difficulty in identifying the controlling cause of corrosion results from the inability of disentangling the coupled effects of individual microstructural features in a scientifically rigorous manner, thereby attributing the core behaviour to the wrong sources.
The aim of this thesis is to isolate and identify the effect of phases on the electrochemical response of high strength steels. To this end, a combined computational and experimental approach is taken. This work starts by analysing the connection between heat treatment, microstructure, and the resulting corrosion properties. After clarification of this interdependence, a finite element electrochemical model illuminates the corrosion behaviour of idealised two phase ferrite-martensite and ferrite-pearlite systems for different phase volume fraction combinations. The results from the simulations guide the microstructure creation for electrochemical experiments, where employed heat treatments result in ferrite-martensite and ferrite-pearlite microstructures with similar ferrite volume fractions. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) experiments in 0.1M and 0.01M H₂SO₄ solutions; potentiostatic polarisation, EIS and Mott-Schottky analysis in 0.1M NaOH solutions reveal the corrosion response and passive film barrier properties of the microstructures. Results demonstrate a clear phase dependency for both active and passive conditions, and are further discussed in light of microstructural features of secondary martensite and pearlite phases. ... Read more

Third-generation advanced high strength steels (AHSS) are a new class of steels that offer superior functional properties and significant weight savings in the body-in-white (BIW) structure of a car. Weight savings directly translate to reduced CO₂ emissions from cars which aids automotive manufactures to meet the vehicle emission guidelines put forth by regulatory bodies around the world. Further increase in weight savings and productivity can be realised when BIW components are fabricated using laser beam welding (LBW). However, the phenomenon of solidification cracking of AHSS during LBW poses challenges to not only its application in the automotive industry, and also in the production lines of steel manufacturers. From the body of literature pertaining to solidification cracking two fundamental conditions can be identified that result in solidification cracking in alloys. First is the development of thermo-mechanical stresses/strains during liquid melt solidification and second, is the formation of a crack susceptible microstructure. In addition to this, the welding conditions can influence the susceptibility of alloys to solidification cracking. The objective of the present study is to understand the influence of variable processing conditions during LBW on solidification cracking tendency of AHSS and how to control these conditions to minimise it. In particular, an attempt is made to understand the effect of keyhole configuration, welding speed and laser beam spot size on solidification cracking. Furthermore, finite element analysis is used to predict the size and shape of the weld pool during LBW, and to determine the net process efficiency which in turn is compared with the calculated process efficiency from the existing analytical model. Bead-on-plate LBW following the testing procedure of the VDEh (German Steel Institute) standard hot cracking test was performed on three third-generation AHSS at two LBW facilities with different beam quality. Due to this, the keyhole during the tests at the two facilities was identified to exist in the closed keyhole configuration and more towards the open keyhole configuration. The susceptibility to solidification cracking was found to increase when the keyhole prevailed in the closed keyhole configuration during LBW. The keyhole configuration was varied by altering the process parameters (welding speed and spot size of the laser beam) in comparison to the parameters corresponding to the closed keyhole configuration. Reducing the welding speed, while keeping the laser power and spot size constant, resulted in the open keyhole configuration and subsequent reduction in the solidification cracking tendency but, until a limit. Similarly, reducing the spot size, while keeping the welding speed and laser power constant, also reduced the solidification cracking tendency as the open keyhole configuration was enforced. The macroscopic area of the fusion zone (weld size) was found to corroborate with the solidification cracking tendency of the alloys. Consequently, the spot size of the laser beam was varied to determine the critical spot size which resulted in a critical weld size at which solidification cracking did not occur. Corresponding to this, the critical process efficiency was determined which is representative of the critical heat input above which solidification cracking occurs. However, the magnitude of the critical spot size, weld size and process efficiency is dependent on the solidification cracking susceptibility of the alloy in question. ... Read more

The incidence of osteoporotic vertebral compression fractures (VCFs) is rapidly increasing, necessitating the identification of the most appropriate treatment method. The well-established first and second generation surgical procedures are not capable of giving optimal outcomes. Moreover, my literature study has shown little to no improvements for four of the most prevailing third generation procedures. As a consequence, the med-tech company Amber Implants B.V. developed a new 3D-printed porous titanium implant that has the potential to resolve difficulties and drawbacks that were identified for all former procedures. Nonetheless, the performance of this implant has never been tested before. Thus, the aim of this study was to pre-clinically evaluate the biomechanical properties regarding the spinal deformities for this implant. For the quantification, the prevention of spinal deformities was subdivided into the anterior height restoration and the kyphotic angle correction. The pre-clinical evaluation was conducted on isolated vertebrae originating from three human cadaver specimens. First, the most abundant type of all osteoporotic VCFs, a wedge fracture, was generated on the vertebrae with 40% anterior height decrease. Thereafter, vertebral restoration was performed with the implants. Besides, vertebral restoration with the second generation surgical procedure, the Balloon Kyphoplasty procedure (BKP), was performed likewise to serve as a comparative. After vertebral restoration, half of the implant group and the total BKP group were tested in a cyclic loading test that mimicked the activity during the first week after surgical VCF repair, i.e. 10.000 cycles with loads ranging from 100N to 600N. The other half of the implant group was tested under higher loading conditions, i.e. 10.000 cycles with loads ranging from 100N to 1000N. At each test stage the spinal deformities, which were subdivided into the anterior height and the kyphotic angle, were evaluated from micro-CT (μCT) data. Eventually, the μCT scans were compared in order to quantify the implant's performance. The anterior height restoration after vertebral restoration with the implant was outstanding compared to the BKP procedure. Thereby, a statistical significance difference (p<0.05) was not only observed for the anterior height restoration, but also for the central and posterior height restorations, between the two surgical procedures. Additionally, the kyphotic angle correction of the implant procedure outperformed the outcomes observed for the BKP. Nonetheless, after cyclic loading various drawbacks for the implants were observed, resulting in substantial decreases in outcomes for the height restoration and the kyphotic angle correction. Regarding the height decrease, most was found in the trabecular bone inferior to the implant. Presumably, the plastic deformation was induced by high local stresses as a result of the minimal contact area between the implant and the trabecular bone. Complementary, the rotation of the implant was observed to effect the implant's performance. Ultimately, it was concluded that the implant in its current design could not compete with the BKP procedure, neither with the third generation surgical repair procedures for an osteoporotic VCF such as Vertebral Body Stenting (VBS) and SpineJack. Nevertheless, the outcomes after vertebral restoration were promising. Therefore, it was assumed that the implant has a decent chance of succeeding after implementing some adjustments on the implant's geometry and the corresponding surgical tools. ... Read more

Selective laser melting (SLM) is an additive manufacturing technique, which is currently on the rise of being used for manufacturing bone implants. Spinal cage, dental and hip implants can for example be manufactured using SLM. Ti6Al4V lattice structures, categorised as metamaterials, can be printed by SLM with mechanical properties close to bone tissue. Due to the lattice structure the stiffness of the Ti6Al4V is decreased, by which stress shielding can be reduced. The lattice structures enhance bone ingrowth which in turn improves the implant’s integration into bone tissue. In light of this potential, this research is focused on biomechanical properties of additively manufactured Ti6Al4V metamaterials.

The current research is aimed at improving the fatigue resistance and wettability of diamond lattice structured Ti6Al4V by applying different microstructural designs and surface engineering through hot isostatic pressing (HIP), sand blasting (SB) and chemical etching (CE). Furthermore a comparison is made between the two SLM processes in terms of continuous and pulsed laser scanning. In order to verify the developed herein post treatment procedures, the tests were also upscaled to actual spinal cage implants. Furthermore, surface modifications affect its wettability which can be linked to cell adhesion and ultimately healing time of the implant. Hence Sessile drop tests were performed to assess the wettability and compare the effect of the various surface modifications.

For both SLM methods it was found that HIP reduces porosity of Ti6Al4V metamaterials, which reduces crack initiation sites and it also serves as a heat treatment increasing the b-phase fraction and thus increasing ductility
and fatigue resistance. SB and CE were found to reduce surface indiscrepancies, which decrease the effect of stress concentration and fatigue initiation sites. Finally SB induces compressive residual surface stresses which means the surface is work hardened, increasing the overall mechanical properties.

For continuous SLM samples an increase in yield strength from 89 MPa up to 115 MPa was found by applying HIP treatment. It should be noted, however, that static mechanical properties were not affected by SB and CE treatments. Fatigue resistance, both low cycle (LCF) and high cycle fatigue (HCF), was significantly improved by a combination of HIP, SB and CE. The observed trend was similar for both pulsed and continuous SLM samples. It is worth noting that SLM samples manufactured with pulsing laser were found in general to be inferior to the
continuous laser SLM, both in terms of static and dynamic properties. The difference is likely attributed to the nature of the laser scanning process, where for pulsing laser method each bead interconnection serves as stress concentration, while for continues laser it is rather the strut interconnections that act as weakest points. Furthermore, for the continuous SLM a preferred grain growth direction was observed which indicates anisotropy. This was not observed for pulsed SLM samples.

For the wettability results it was observed that SB decreases and CE increases the contact angle. A decrease in contact angle means the surface has become more hydrophilic, hence the in this study developed SB modification could be considered as more favourable for osseointergration.

The upscaled spinal cage implants post treatment procedure showed a decrease in yield strength and an increase in fatigue resistance for the HIP+SB+CE as compared to as-processed implants. The rather limited post treatment
improvement on implants was linked to the post process treatment method, which should be modified to account for the complex geometry of these structures. ... Read more

Atherosclerotic plaque rupture is the main cause of acute myocardial infarction and stroke, the two leading causes of death worldwide. Rupture of plaque tissue is a mechanical event, where plaque stress or strain locally exceeds its strength. Biomechanical studies agree with histopathological findings that a large lipid pool and thin fibrous cap overlying the lipid pool increase the likelihood of rupture, by showing increased plaque stresses for these geometries. Another plaque morphological feature that is frequently encountered is calcification. However, its role in the vulnerability of plaques to rupture is not fully understood, and biomechanical modeling studies do not agree on the effect of calcifications on plaque stress. These studies used isotropic material properties for the anisotropic and collagen rich plaque tissue, and generally focused on the effect of calcification on lumen stress or cap stress. However, histopathological findings revealed tissue damage at the interface between calcification and surrounding tissue.

This study investigated stresses and strains at the interface between calcification and fibrous tissue, how these stresses and strains are influenced by local anisotropy of the fibrous tissue and how geometric features of the calcification are related to these metrics. A morphometric study was conducted first, to investigate and categorize different patterns of fiber alignment around the calcifications, and to measure the calcification geometric features including its location in the plaque, its shape and its size. Biomechanical models including the local anisotropic material properties were constructed next, based on the observations and measurements made in the morphometric analysis. Stress and strain metrics were investigated at the calcification boundary, and subsequently related to fiber patterns and calcification geometric features.

Hundred forty five calcifications were segmented and measured in the morphometric analysis, and surrounding fiber alignments were studied. The analysis revealed that four main fiber patterns in the fibrous tissue surrounding calcifications exist: the Attached pattern, Pushed Aside pattern, Encircling pattern and Random pattern. Collagen fibers are attached to the calcification in the Attached pattern, are pushed aside by the calcification in the second pattern, encircle the calcification in the third pattern, and show a disorganized alignment in the Random pattern. The Attached pattern was the most prevalent fiber pattern, and its corresponding calcifications had larger aspect ratios and were on average larger than the other three fiber patterns. Large peak stresses and strains at the calcification boundary were identified in the biomechanical models for the Attached pattern and Pushed Aside pattern, while these metrics showed generally lower peak values for the Encircling pattern and Random pattern. Peak values for all stress and strain metrics were attained at the tip of the calcifications. Multivariate analysis showed that stresses and strains are related to the calcification geometric features; calcification interface stress and strain increased if the calcification was closer to the lumen, had a larger length/width ratio and was larger in size.

Histopathological examination of plaques evidenced damage at the interface between calcification and fibrous tissue, potentially caused by an adverse mechanical state at this boundary. Most studies focused on stresses at the lumen or in the cap however, and this study for the first time specifically investigated stresses and strains at the calcification boundary while simultaneously introducing local fibrous tissue anisotropy in the computational models. Results show that peak stresses and strains can develop at this boundary, which will remain undiscovered if isotropic materials are used. This study was also the first to extensively analyze calcification geometric features and surrounding fiber alignment in relation to these interface stresses and strains. Peak values were found to be dependent on these features and fiber alignment patterns, indicating a relation between these characteristics and mechanical stability of the plaque. The findings of this study further increase the clinical relevance of finite element modeling in rupture risk prediction by showing previously undiscovered peak stress and strain values for interfaces and geometric configurations which already were deemed to be destabilizing in clinical and histopathological studies. ... Read more

Glaucoma is the leading cause of irreversible blindness and in 2040, more than 111 million people are expected to have the disease. Vision loss in glaucoma is caused by the death of retinal ganglion cells (RGCs) primarily in the optic nerve head (ONH). Elevated intraocular pressure (IOP) is a well-known causative risk factor for glaucoma, highlighting the importance of biomechanics in the disease. All current therapies are focused on lowering IOP levels. However, these therapies are not effective for all patients. One limiting factor in the search for new therapies is the lack of knowledge about the cell processes that lead from biomechanical insult to RGC apoptosis. To better understand this relationship, animal models are used and amongst these models, rats are widely-used due to their pathophysiological similarities to human glaucoma. However, there are anatomical differences between the rat and human ONH, and these differences are likely to affect rat ONH biomechanics. Moreover, there are some anatomical alterations occur in the rat ONH due to remodeling and damage in the early stages of the disease. Computational modeling is a suitable approach for characterizing ONH biomechanics due to the complex anatomy of the posterior rat eye and there has been no previous attempt to characterize the glaucomatous rat eye ONH biomechanics. The aim of this thesis is to characterize glaucomatous rat ONH biomechanics by building two individual-specific glaucomatous rat ONH finite element models (FEMs). By comparing the 1st (mean and the 95th percentile) and 3rd (mean and the 5th percentile) principal strains on the anterior ON with previously built healthy rat eye individual-specific FEMs, the effect of geometrical differences occurred due to remodeling and damage in the early stages of the glaucoma were aimed to be better understood. The 1st principal strain (mean and 95th percentile) relative percentage differences between the glaucomatous and healthy rat eyes under the IOP of 30mmHg were found to be 37.74% and 69.66% for the MR04, and 35.66% and 66.02% for the MR05 rats. The 3rd principal strain (mean and 5th percentile) relative percentage differences were found to be 12.28% and 35.23% for the MR04, and 10.31% and 11.05% for the MR05 rats, in which the glaucomatous rat eyes had higher magnitude strains. When the experimentally measured mean IOP values were applied to the glaucomatous rat eye models, these calculated relative percentage differences were increased. Moreover, when the strain patterns in the anterior ON were observed, larger strains were located in the inferior ON in both of the glaucomatous rat eyes. Thus, the localization of high strain patterns observed in this study for the glaucomatous eyes were similar to the healthy rat eyes, which were dominantly located at the inferior ON. However, the magnitudes of the principal strains in the glaucomatous rat eye anterior ON were elevated due to the geometrical differences caused by remodeling and damage during the early stages of the disease. ... Read more

Our ageing, and increasingly overweight population demands better implant designs and testing methods. More realistic finite element method (FEM) models are a great approach to reduce costs of the implant design process. FEM models could be used to evaluate implant performance against lack of bone ingrowth and medial tibial collapse. Failed bone ingrowth of the tibial component can result in early loosening, suggested to occur due to high micromotions at the implant interface. Additionally, post-operative misalignment, high patient BMI, and stumbling accidents cause medial tibial collapse. These phenomena have been assessed via FEM, where a realistic simulation, adequate constitutive model, loads, and boundary conditions must be selected. Also, the implantation process must be included to account for bone deformations. The purpose of this study was to investigate which constitutive model better predicts implantation and subsequent micromotions, bone ingrowth, and medial collapse.

A FEM model was created from a CT-scanned tibia implanted with a Monoblock and a Persona (Zimmer, Inc., Warsaw, IN) trabecular metal tibial components. Four activities were considered. Four different heterogeneous material models were used: A linear elastic (LE), a softening Von Mises (sVM) model, an ideal isotropic crushable foam (iICF) model, and a hardening isotropic crushable foam Model (hICF) model. The hardening function of the latter model was mathematically demonstrated and validated against existing data. Implantation was performed prior to every analysis. The resulting micromotions were compared to an ingrowth threshold of 40 um to estimate extent of ingrowth. The same material models were used to evaluate medial collapse under stumbling conditions. The ingrowth results of the Monoblock were compared to retrieval. Implant performance was evaluated between the Persona and NexGen.

The hardening function was able to predict yield when compared to experimental data. For the implantation simulations, the sVM model presented the most volume of plastic elements around the implant. The hICF and the sVM models were the best to predict ingrowth, where the latter under-predicted ingrowth and the former over-predicted it. The LE model results were incapable of predicting ingrowth, especially in the regions where press-fit conditions should be present. For medial tibial collapse, the sVM model results presented structural instabilities at relatively low loading thresholds. The iICF model results were incapable to predict medial collapse due to material instabilities. The hICF model was able to predict collapse without instabilities.

Implantation results demonstrate the importance of using ICF plastic models in cementless implant analysis, as it provides the necessary contact stresses around the implant interface. Bone ingrowth results of every material model were not equivalent to the retrieval data, suggesting extra modelling considerations are required. Using a sVM or a hICF model is suggested for micromotions and ingrowth research. Ingrowth results show that the complexity of the plastic models was enough to predict that the Persona implant was not going to outperform the NexGen. Differences in collapse behaviour between material models showed important instabilities that must be considered in future medial collapse studies. Further research is required to realistically simulate the studied phenomena. ... Read more

Treatment of liver diseases is often done by means of interventional radiology and thereby needles are widely used. These needles are under constant development and therefore needle-tissue interaction data are necessary. The goal of this study is to provide data on needle forces during puncturing of liver blood vessels. These data can be used to mimic blood vessels in a polyvinyl alcohol (PVA) liver phantom, intended for needle development and training purposes. Needle insertion experiments were done on human livers, by puncturing portal veins, hepatic veins, hepatic arteries and liver tissue. The resulting force data show that peak forces are higher during puncturing of liver veins (median=2.20 N, interquartile range=1.46 N to 3.67 N) than during puncturing of liver tissue (median=0.38 N, interquartile range=0.31 N to 0.51 N). The force data were used to find a blood vessel mimicking material. Silicone, with the addition of a mesh fabric, was found to mimic the peak forces of liver veins during needle insertion. A silicone blood vessel was created with use of a 3D-printed blood vessel structure made of water-soluble PVA. The addition of the mesh fabric, furthermore ensures proper bonding between the silicone blood vessel and the PVA liver tissue. The methods described in this study can be used to implant artificial veins in a PVA liver phantom. ... Read more