This paper introduces a fully compliant spherical joint with an optimized stiffness profile specifically for balancing a pendulum. The design builds on previous work that has successfully created a fully compliant spherical joint using tetrahedron-shaped elements connected in series. To efficiently compute and optimize the balancing behaviour of these compliant joints, a novel algorithm is developed and presented. Using this algorithm, optimizations are conducted to obtain simulated pendulum balancers under five different conditions. The performances of these results are analysed to assess the potential and limitations of the algorithm and these spherical joints. Based on one of the optimized results, a prototype is fabricated and expermentally validated, achieving a moment reduction of 90.5%. The deformation calculated by the TetraFEM tool closely matches the prototype’s deformation with an accuracy of 89.6%, demonstrating its potential for application in the development of shoulder exoskeletons.

The physical, chemical, and biological versatility and high water content of hydrogels have been taken advantage of to fabricate hydrogels that mimic the extracellular matrix (ECM) structure of native tissues and applied in the field of cell and tissue engineering to provide the cells with a 3D scaffold structure. Especially in tissue engineering, the creation of a suitable hydrogel scaffold with hard and soft interfaces that is capable of both supporting cell growth and stimulation for guided differentiation for tissue interface studies is a primary concern for advancements in tissue engineering. The ideal scaffold with hard and soft interfaces can be achieved by finetuning its biological, biochemical, structural, and mechanical properties for optimal cell proliferation and differentiation.

In this thesis, a magneto-responsive hydrogel scaffold composed of gelatin (Gel, 2.5%), alginate (Alg, 5%), and iron oxide microparticles (10% w/v) was developed and mechanically and rheologically characterized before, during and after the application of a uniaxial static magnetic field. The magneto-responsive hydrogel scaffolds were created through multi-material 3D printing using magnetic and non-magnetic hydrogel inks. The magnetic inks contained magnetic particle (MP) inclusions within its polymer network while the non-magnetic hydrogel ink had no MPs. The 3D printing process allowed for a local control in the magnetic and non-magnetic hydrogel distribution to create hard and soft hydrogel interfaces. The printability and shape fidelity of various ink compositions were evaluated, so that the final composition of Gel:Alg ratio of 1:2 (2.5%:5.0%) with 10% MP was chosen for further mechanical and rheological characterization. The magnetic hydrogel scaffold exhibited magnetorheological properties as it mainly increased the effective Young’s modulus, storage modulus, and damping factor, and decreased viscosity under a uniaxial static magnetic field application.

In tissue engineering, the developed hydrogel scaffold, which is locally responsive to magnetic cues, shows great potential for creating scaffolds capable of continuously stimulating embedded cells in a non-contact manner. As a proof of concept, a bi-layered, multi-material hydrogel scaffold was created with increased surface area attachment points between each hydrogel material to mimic the osteochondral tissue interface. The increased surface area between both layers was achieved through a checkered pattern design with alternating magnetic and non-magnetic hydrogel sections printed alongside each other.

Prosthetic socket design and fit are key for a successful amputee rehabilitation and comfort, directly influencing patient satisfaction and quality of life. However, there is a lack of quantitative data on stump pressure distribution and how it changes over time, which could greatly contribute to the efforts of designers, prosthetists and doctors to improve patient comfort.

This study explores the use of Force Sensitive Resistors (FSRs) to measure and visualize stump pressure distribution, specifically for transtibial prosthetic sockets. The research involves testing an experimental prototype equipped with FSRs on a cyclic loading machine, followed by a comparison of the results with a simulation.

The findings indicate that the highest loads are registered by FSRs positioned at the bottom of the stump and below the knee. Some anomalies were observed, potentially due to specific geometric features of the prototype and the way the load was applied during testing.
Overall, the experimental data suggests that FSRs are effective for measuring stump pressure distribution. However, further testing with increasingly complex load cases is necessary to validate the sensors' reliability.

In conclusion, FSRs demonstrate significant potential for enabling knowledge-based designs focused on patient well-being. Through the course of this project, valuable design insights and requirements for integrating sensors into prosthetic sockets were identified. Moreover, this systematic sensor testing approach can be applied to explore and compare between other pressure sensors.

Thermo-responsive hydrogels are a class of smart material that show increasing promise and applications in the biomedical engineering industry. This thesis investigated a thermo-responsive hydrogel for 4D printing applications. A poly(N-isopropylacrylamide) (>97%) (pNIPAM) based hydrogel was developed and the concentration of NIPAM, as well as the molar ratio of crosslinker, N,N-methylenebis (acrylamide) (>99%) (Mbis), to monomer was investigated to determine an optimal resin composition, which was determined from the thermo-response of the hydrogel at the microscale.
The resins were used to print several bilayer beams with varying laser power, scanning speed and hatching angles. Once the optimal composition and printing parameters were determined, mechanical characterization of the hydrogel was performed to measure the Young’s modulus at various loading rates via nanoindentation.
The thermal expansion coefficient (TEC) of the hydrogel was measured in three orthogonal directions at different temperatures. Finally, based on the shape morphing behaviour of the bilayer beams, we developed shape morphing applications at the microscale including a thermo-responsive micro-gripper and thermo-responsive drug delivery valve. The purpose of this work was, therefore, to explore possible applications of such a hydrogel and the ability to print with it in the microscale via two photon polymerization.

Over two million bone grafts are performed worldwide, each year. The preferred method is using autografts, but there are two important downsides. There is often insufficient tissue to harvest and the scar at the harvesting side is painfull for the patient. Therefore there exists a great need to improve synthetic grafts.

Traditionally, synthetic bone scaffolds are made from only one material, this can either be a (bioactive) ceramic or metal. The former has the benefit of promoting bone growth, but has insufficient mechanical properties. Metals on the other hand have no issue competing with bone in terms of mechanical properties, but they may not be biocompatible nor aid osteo-induction.

In this study direct ink writing was used to produce multimaterial Ti6Al4V and akermanite scaffolds. The goal was to combine the favourable mechanical properties of Ti6Al4V alloy with the osteo-inductive properties of akermanite. Composites of Ti6Al4V and akermanite were evaluated as well, but similar to akermanite ceramic on itself, their mechanical performance was deemed insufficient. Akermanite and Ti6Al4V was found to react and form titanium silicide and a calicum compound, presumed to be calcium oxide. A core shell scaffold was designed which uses a Ti6Al4V shell and an akermanite composite core in order to achieve both adequate mechanical and improved bioactive properties. This scaffold performed comparable to cortical bone in stiffness, and boasted superior strength.

A hard-soft interface refers to the boundary between two materials or regions with significantly different mechanical properties, where one is rigid or hard and the other is flexible or soft. These interfaces are common in both engineered and natural systems and are characterized by the contrast in how each material deforms, transfers loads, or responds to environmental stress. Engineered hard-soft interfaces often experience failure due to high interfacial stresses, poor adhesion, and localized stress concentrations caused by mismatched mechanical properties. An example of engineered hard-soft interfaces can be found in tissue engineering, where they are used to replicate natural transitions between tissues, such as those between bone and cartilage. In contrast, hard-soft interfaces in nature, such as the root-soil system, demonstrate remarkable strength and adaptability, efficiently distributing loads and reducing stress concentrations despite differences in material properties.
The pull-out force was chosen to assess the strength of the root-soil interface, capturing the mechanical interactions of roots with their environment. This study focused on barley and mung bean seeds, chosen for their distinct root structures, barley with a fibrous system and mung bean featuring a taproot system. Over a 15-day growth period, various root characteristics such as length, diameter, tortuosity, and branching patterns were analyzed across soil and hydrogel substrates, each with distinct material properties and stiffness. The methodology included measuring growth in terms of days and stem height, along with 2D root trait extraction to analyze characteristics such as length, diameter and number of branches. Additionally, 3D computed tomography (CT) scanning was used to visualize root architecture, while pull-out tests provided key data on resistance and force-displacement curves, and finite element method (FEM) simulations enabled sensitivity analyses of various root structure configurations in a non-destructive manner. Lastly, experiments with hydrogel tested its viability for root growth, involving detailed protocols for hydrogel composition and seed preparation.
Plant growth measurements revealed a consistent increase in stem height over time, effectively captured by the logistic growth model. Laboratory pullout tests and root extraction demonstrated that increases in root characteristics such as length, diameter, and branching significantly improve pullout force in both barley and mung bean seeds. Moreover, pullout test results showed that barley roots have greater mechanical resistance and higher maximum forces than mung bean roots, although with greater variability in the data. FEM simulations indicated that a 45° vertical branching angle yielded the highest pullout force for barley in soil (5.09 N), while an 80° angle was most effective in hydrogel (4.98 N). In contrast, radial branching angles had negligible effects in both substrates. Tortuous root configurations significantly increased pullout force in soil, nearly doubling it from 5.09 N for straight roots to 9.80 N, but only slightly improved it in hydrogel, from 3.20 N to 3.61 N. The addition of branches in mung beans significantly increased pullout forces in both substrates due to the greater surface area, which enhanced root-substrate interaction. The FEM simulations showed that pullout forces were generally higher in soil due to its rigidity, which leads to a rapid increase in pullout force until root failure. Hydrogel, with its elastic properties, allowed roots to stretch more under load, providing uniform and gradual resistance. The FEM model was also validated through energy history output results and mesh convergence analysis. Lastly, initial experiments growing roots in hydrogel show promise for this substrate as a soil alternative, however further research is required to optimize its properties for plant growth.
Overall, this study provides a better understanding of the factors optimizing root anchorage and interface strength, offering design strategies for bioinspired engineered hard-soft interfaces. It also emphasizes the need to tailor natural design principles to the specific material properties of substrates in engineered contexts.

The solute transport across the osteochondral (OC) interface is of crucial metabolic importance for a normal function of articular cartilage, and, therefore, for the OC interface as a whole. A better understanding how the mechanical- and physiological properties of the OC interface affect the solute transport across the OC interface could lead to new insights in repair strategies. The aim of this master’s thesis was to investigate how graded mechanical- and/or physiological properties of the OC interface affect the solute transport across the OC interface.

A combination of computational modelling and experimental diffusion tests on GelMA-based hydrogel plugs was used to approach the goal. Regarding the computational model, first, a multi-zone biphasic-solute finite element model that accurately replicates axial solute transport across the OC interface was designed and validated. Second, a power law function was used to apply several gradients on the initial values of the solid volume fraction (SVF), diffusion coefficient, and elastic modulus across the OC interface to study the effect of each parameter on the solute diffusion across the OC interface. On the experimental front, attempts were made to 3D-print GelMA-based hydrogel plugs but all failed. Alternatively, five groups (n = 3) of hydrogel plugs were created, each of which underwent different UV curing time, by casting GelMA into cylindrical plugs. Axial diffusion of an alizarin red solution through the hydrogel samples was recorded using a digital camera.

The results of the computational model show that only the SVF plays a small role in the height of the equilibrium concentration reached in the subchondral bone layer. However, the influence of both the SVF and diffusion coefficient on the time when the equilibrium concentration is reached in the subchondral bone is considerably large. It is shown that the elastic modulus has a negligible influence on the solute transport. Regarding the experimental diffusion tests, air bubbles and/or sincere light reflections made all but six hydrogel plugs unusable for further analysis. A relationship between sample thickness and diffusion is observed in the remaining hydrogel samples. The results of the SVF computational model and experimental diffusion tests were compared, but sufficient experimental data was lacking to draw any solid conclusions from this comparison.

This master’s thesis provides a new computational model of the OC interface which allows the implementation of graded parameters across the OC interface. It is concluded that the current experimental set-up is not suitable for obtaining consistent data on solute transport across hydrogel plugs. Suggestions to improve the experimental set-up are made.

Shape morphing is a prevalent feature in all living organisms. Incorporating shape-morphing capabilities into engineered scaffolds in the field of tissue engineering by virtue of 4D printing would allow for structures to closely mimic in-vivo conditions, and dynamically interact with changing cellular microenvironments. Engineered scaffolds subjected to external mechanical loads have recently drawn interest to study the impact of altering Poisson's ratio and pore size of cellular scaffolds on the properties of cells growing on it. However, this approach limits its applications to in-vitro environments. The ability to remotely actuate scaffolds would allow for precise, non-invasive, and controlled activation of these engineered structures. Towards this, the current research work aimed at the design and fabrication of a dynamic metamaterial-based unit cell microstructure capable of exhibiting a varying Poisson’s ratio in response to thermal stimulus. The microstructure was fabricated using a biocompatible Poly(N-isopropylacrylamide) (pNIPAM) based photoresist and a two-photon polymerization (2PP)-based direct laser writing technique. Systematic characterizations were first performed on a simplified model to evaluate the correlation between the printing parameters which include laser power, scanning speed, and hatching angles to the shape-morphing characteristics of the printed microstructures. The learnings were implemented to fabricate the proposed varying Poisson's ratio model. The fabricated microstructure exhibited a rapid and reversible change in Poisson’s ratio when subjected to a thermal stimulus by virtue of the inherent properties of hydrogels and heterogeneity introduced within the structures by varying the laser parameters during the printing process. In a broader context, this research serves as the first step towards the realization of a dynamic stimulus-responsive 3D scaffold for cell culture applications.

The mechanical failure of a temporomandibular joint (TMJ) replacement system can stem from various factors, with one notable cause being the unnatural kinematics and loading observed in prosthetic joint function. Given that,. the primary objective of the study was to investigate whether including functionally graded materials (FGMs) at the site of the prosthetic joint was feasible for achieving a greater resemblance to natural mandibular kinematics, while simultaneously reducing the reaction force exerted on the prosthetic joint. To this end, an FGM-based TMJ implant was designed with the intention of reducing joint reaction forces and enhancing joint flexibility to achieve a more natural kinematic behavior.

In this study, patient-specific finite element models of the intact and implanted mandible were simulated for two different biting tasks: incisal biting (INC) and left group biting (LGF). To incorporate FGMs into the implant design, an initial implant design was voxelized (with a voxel size of 0.25mm x 0.25mm x 0.25mm), allowing each voxel to be assigned a specific material. Subsequently, 10 voxel layers of the same size were added onto the implant head/prosthetic condyle. To investigate the impact of FGMs on mandibular biomechanics, a comprehensive analysis was performed on five distinct implant designs. These designs were differentiated solely by the functional gradient in material properties across the additional 10 voxel layers on top of the implant head. Each voxel (layer) was assigned a value of ρ, which represented the volume fraction of the hard phase.

The five distinct implant designs were as follows: (1) The first design was labelled as the "Polymeric Implant" consisting solely of the hard phase with a volume fraction of ρ=100%. It served as the reference for comparisons with other designs. (2) The second design featured an abrupt hard-soft connection without a gradient. This design was specifically examined to demonstrate the presence of stress concentrations at discrete interfaces between the hard and soft materials in layered composites. It was named "Implant with abrupt hard-soft connection without a gradient". (3) The third design was characterized by an FGM comprising ten discrete values of the volume fraction of the hard phase, ranging from ρ=0% at the surface layer to ρ=90% at the 10th layer with an increment of 10%. This design was designated as "FGM, ρ= [0, 10,…, 90]". (4) The fourth design encompassed an FGM featuring nine distinct values of the volume fraction of the hard phase, ranging from ρ=10% at the two surface layers to ρ=90% at the 10th layer with an increment of 10%. It was referred to as "FGM, ρ= [10, 20,…, 90]". (5) The fifth design comprised an FGM incorporating eight discrete values of the volume fraction of the hard phase, ranging from ρ=20% at the three surface layers to ρ=90% at the 10th layer with an increment of 10%. This design was named "FGM, ρ= [20, 30,…, 90]". The primary objective of investigating the fourth and fifth designs was to analyse the influence of a relatively less soft material at the surface (outer layer) of the implant head.

The biomechanical performance of an implant design was determined by utilizing normalized cross-correlation (similarity) and Euclidean distance (closeness) measurements between the displacement fields of the intact and implanted mandible. The performance of an implant design for a specific biting task was assessed by calculating the average of the percentage errors in each of these quantities. Additionally, the reaction forces on the prosthetic joint were compared between the implanted models.

The results showed that the second design, which featured an abrupt hard-soft connection without a gradient, was not viable. This was due to the presence of stress concentrations, stress increase up to more than ninefold was observed, at discrete interface between materials of contrasting hardness. This phenomenon could result in crack initiation and delamination, significantly compromising the overall performance and durability of the design. The study also revealed that material properties distribution on the surface of the implant head had a crucial impact on regulating mandibular displacements. Notably, greater displacements were observed when the outer layer of the implant head featured relatively softer material.

In comparison to the polymeric implant, “FGM,ρ=[0,10,…90]” implant exhibited an average performance increase of 19% for INC and a 3% performance decrease for LGF regarding mandibular displacement. Additionally, it reduced the joint reaction forces on the prosthetic joint by 8% and 12% during INC and LGF, respectively.

Moreover, the "FGM, ρ=[0,10,…90]" implant increased the mandibular range of motion by 20% during INC and 88% during LGF. Furthermore, it contributed to more symmetrical mandibular movement during INC, while the concept of symmetry was not applicable in the context of LGF.

These results have provided valuable insights for future research and development of TMJ implants, potentially leading to the creation of implants with improved long-term behavior.

Congenital heart disease (CHD) affects almost 1% of newborns. Right ventricular outflow tract (RVOT) CHD affects 20% of newborns and includes anomalies such as tetralogy of Fallot (TOF) with or without pulmonary atresia, transposition of the great vessels, and truncus arteriosus. All these anomalies require RVOT reconstruction. Prosthetic heart valves are needed to improve the quality of life of patients suffering from CHD. One such prosthetic device is the pulmonary valved Conduit developed by XeltisTM. This study aimed to investigate the difference in the mechanical response of the XeltisTM pulmonary valved Conduit sizes 16, 18, and 20 (XPV16, XPV18, and XPV20) using mechanical experiments and a predictive finite element model.
Experiments of two load cases, Leaflet opening behavior (LC1) and parallel compression (LC2) have been done where measurements were taken for input parameters used for uncertainty quantification (UQ) in the FE model. Furthermore, the reaction force and displacement were measured to calculate the force values and stiffness values of the device during each experiment. The experiments were replicated with a developed FE model. From the results of the FE model, a metamodel (MM) was developed and a Monte Carlo simulation was performed to retrieve a distribution of the force values and stiffness values obtained in the simulation. Furthermore, UQ was performed and the sensitivity of the input parameters on the force values and stiffness values were quantified. Finally, with an area metric the accuracy of computational finite element (FE) models in simulating the mechanical response observed in the experiments of the XeltisTM pulmonary valved Conduit size 16, 18, and 20 mm was quantified.
For the Leaflet opening behavior, the reaction force increases when the device size increases while the stiffness doesn’t change with device size. As for the accuracy of the predictive FE model, the predictive FE model can simulate the mechanical response of the stiffness with an accuracy of at least 70.7% for the reaction force and at least 50.3% for the stiffness.
For the parallel plate compression, the reaction force and the stiffness increase when the XPV size decreases from size 18 to size 16. Furthermore, the difference between the XPV18 and the XPV20 is smaller for both the reaction force and the stiffness. As for the accuracy of the predictive FE model, the predictive FE model can simulate the mechanical response of the stiffness with an accuracy of at least 37.3% for the reaction force and at least 38.1% for the stiffness.
As for the important variables influencing the mechanical response, the reaction force and the stiffness are influenced mostly by the fiber stiffness of the component that is subjected to the load. Furthermore, the reaction force and stiffness are also influenced by the direction of the fibers. If the fibers are in the same direction as the load, the reaction force and stiffness increase. Although specifically for the Leaflet the amount of material has more influence on the reaction force and stiffness for larger XPV sizes with the Leaflet opening behavior load case. This indicates that for larger XPV sizes the material properties of the Leaflet have a smaller influence and the Leaflet geometry has a higher influence on the stiffness of the Leaflet opening.
Improvements are possible for a better agreement between the simulation and the experiment. These include performing more experiments with different samples from different production batches, and better estimation of input parameters with a high sensitivity to the output parameters.
This study provides a step in the direction of predictive computational device modeling that will help shorten the development time of new pulmonary heart valve devices. As the devices in this study are designed for pediatrics, this will help improve the quality of life of pediatrics suffering from congenital heart disease.

The widespread proliferation of cellular lattices in engineering design has entailed a revolution in the design of lightweight and multifunctional structures. Furthermore, the development of auxetic metamaterials has led to applications in fields ranging from biomedical to civil engineering. However, cellular lattices suffer from the development of localized concentrations of strains during deformation, which lead to sub-optimal mechanical performance. Furthermore, the mechanical properties of cellular lattices are dependent on their macro-scale geometries, limiting their design space. Thus, we seek to both enhance the mechanical properties of lattices by ameliorating strain concentrations and expand the design space of cellular lattices.

In attempting to do so, we can turn to nature. The bio-inspired design motif of the hierarchical arrangement of composite materials has lead to exceptional mechanical characteristics in natural materials. Moreover, morphogenetic processes such as bone remodelling play a vital role in the structural development of natural materials and their hierarchical material arrangement. Motivated by these concepts, we developed an algorithm that optimised the hierarchical material arrangement of hard and soft voxels in an auxetic and non-auxetic unit cell. Our algorithm does so based on a bone remodelling-like approach of strain energy optimisation, whereby elements are transitioned to a hard or soft material based on local strain energy.

Our finite element (FE) simulation results demonstrate the considerable expansion of the range of mechanical properties we can achieve simply by modifying material placement on a micron scale. We also observe clear mitigation of peak strain energies and more homogeneous distributions of strain energy density, which affirm the amelioration of strain concentrations. Furthermore, we were able to increase the stiffnesses of our unit cells without substantially affecting its Poisson’s ratio, disentangling previously interdependent mechanical properties. We further validated our FE simulations by additively manufacturing our optimised unit cells with state-of-the-art voxel-based printing. Both the mechanical properties predicted by our FE simulations and the strain distributions correlated well with digital image correlation recordings, validating our models. Our mechanical testing also revealed the considerable in- creases in ultimate tensile strength of our optimised designs, demonstrating the benefits of hierarchical material arrangement.

To conclude, we demonstrate the efficacy of strain energy-based material optimisation to improve the mechanical properties of unit cells and mitigate strain concentrations. The design paradigms of hierarchical arrangement of composite materials remarkably improved the material space of a unit cell without changing its macro-scale geometry. Findings from this study could enable the optimisation of existing cellular lattices and the development of novel unit cell designs with distinct intrinsic mechanical properties.

Bone defects are a common problem in healthcare today. When the damage is too extensive, the human body is not able to heal itself. Different solutions are available, but they all have limitations. Therefore, the search for a solution to overcome these limitations is important. In this theses a way to create a scaffold out of seperate crumpled elements is explored. Mechanical and morphological properties are examined.

Mechanical metamaterials are materials that appear to have unusual elastic mechanical properties, with the most prominent being a negative Poisson’s ratio. The key to their unique properties lies in the microarchitecture of the material rather than the material itself. For advanced applications in the realm of prosthetics, such as form matching, aperiodic metamaterials have been recommended above repetitive ones. In a restricted lattice, metamaterials with disordered microarchitecture have displayed a wide variety of anisotropic conventional and auxetic behaviour. Because of the non-unique and non-intuitive link between the random network (RN) structures and their mechanical responses, their inverse design, which tries to extract the ideal structure according to specified requirements, remains a complex process. The current study looks at the forward prediction of mechanical response and the inverse design of 2D random network metamaterial unit cells for use in aperiodic combinatorial designs. The associated mechanical properties of stiffness and Poisson's ratio in two directions were numerically simulated for the unit cells. The configuration of random network designs that provide the least complicated unit cells with the greatest range of mechanical properties were selected. A dataset of such unit cells and their mechanical responses was then created for training machine learning models. A forward predictor of their mechanical response through a regression model was trained, to accelerate the production of the datasets and assist in the inverse design modelling. It was suggested to model the inverse design issue in a probabilistically generative manner, allowing for an interpretable investigation of the complicated structure–performance connection. A variational autoencoder (VAE) was trained to map complex microstructures into a low-dimensional, continuous, and organized latent space based on their mechanical response. A systematic approach to train machine learning models was investigated based on hyperparameter optimization methods, with cross-validation and a search space that expands over the model hyperparameters and data analysis alternatives. The trained forward predictor and generative model were evaluated, and the reported R^2 values were 0.99 and 0.96, respectively. Compared to numerical simulations, the estimation of mechanical properties through the regressor resulted in a hundredfold acceleration of the process. The trained deep generative model can be an effective tool for expediting the creation of RN unit cells. On-demand inverse creation of metamaterials for both observable and intentionally designed mechanical responses is achievable. To illustrate a practical use of such unit cells, a technique for aperiodic combinatorial design of unit cells for 2D form matching on two directions and all four sides was also provided. The mechanical properties of the unit cells, as well as the combinatorial design capabilities, were evaluated using experimental tests of 3D printed specimens. The experimental results of the unit cells were in good agreement with the numerical simulations. Similar forms to those programmed were achieved by tensioning the combinatorial patterns, albeit with a less degree of distortion. Overall, the findings show the RN unit cells' potential for combinatorial design.

Nature has always had randomness in many ways. Natural cellular structures, specifically, have used randomized microarchitectures to form mechanically efficient materials, such as wood, sponge, and trabecular bone. Mechanical metamaterials are a class of artificial microarchitected materials designed to carry tensions and compressions with lightweight. However, there had been some limitations regarding the range of mechanical properties and geometrical parameters of regular honeycomb mechanical metamaterials. Also, it is not possible for extreme auxetic honeycomb mechanical metamaterials, which are with extreme negative Poisson’s ratio, to have a high level of stiffness. The addition of randomness had been proven to extend the properties of mechanical metamaterials.

In this project, through the approach of the combination of bending- and stretching-dominated structures, based on the level of nodal connectivity of the structures, we aimed to explore the range of properties in both the in-plane and the out-of-plane contexts. In terms of in-plane properties, we proposed a simple design that can achieve extreme auxetic or double-side auxetic lattices, the corner-plus design. The design successfully created lattices with an over -0.8 negative Poisson’s ratio value and a 6 to 10 times level of stiffness as regular honeycomb metamaterials. Through recognition and rearrangement of the openings, the peninsula-like region that expands under tension, over one million different lattices are generated with extreme negative Poisson’s ratio values and a high level of stiffness. The design had expanded the stiffness versus Poisson’s ratio region compared to the ordinary materials and regular honeycomb mechanical metamaterials.

Furthermore, in total six meta-plates were designed from the selected metamaterials and additively manufactured for the investigation of out-of-plane behavior. Experimentally and computationally, their deformation and curvatures presented that the induced curvature along the transverse direction was adjustable, which can be manipulated with the level of connectivity and the value of Poisson’s ratio. Openings in these meta-plates also played a big role in the formulation of surface contours and induced curvatures both locally and globally.

In general, openings had evident contributions to the auxeticity and the unique surface contours, which can be a key factor in the future design of the random mechanical metamaterials. The extreme auxeticity and the modular surface topology can be further implemented into the design of wearable prostheses or medical implants for future applications.

Slipped capital femoral epiphysis (SCFE) is a condition in adolescents affecting the proximal femur. Increased mechanical forces result in fracture of the growth plate. Factors increasing the mechanical forces include for example obesity and endocrine disorders. As a result of the fracture a shift of the bone shaft towards the femur head occurs, causing mobility issues and pain. A treatment option for these patients is an intertrochanteric osteotomy with plate fixation. Restoring range of motion and pain reduction are the main aims of the surgery.

This intertrochanteric osteotomy alters the femur anatomy, resulting in a mismatch between bone and conventional plate fixation. These plates do not fit to the reconstructed bone, resulting in high chances of plate failure. The best option for these patients is to use patient-specific designed bone plates for femur fixation. Optimal mechanical properties of the patient-specific plate are important in limiting plate failure. These properties are dependent on design parameters, which can be adapted dependent on patient criteria and anatomy. Investigations towards patient-specific bone plates and the optimal design parameters for efficient and stable fracture fixation is lacking.

The focus of this study is on a specific SCFE patient case treated with corrective osteotomy using a patient-specific Ti-6Al-4V plate and screws for bone fixation and stability. A valid numerical model (i.e., finite element analysis (FEA)) is created, with physiological loading corresponding to two leg stance and walking, for an initial designed (ID) patient-specific bone plate to analyse plate performance. Screw configuration analysis and topology optimization of the ID plate were performed to design an alternative topology optimized (TO) plate. Biomechanical experiments with digital image correlation (DIC) techniques were performed for mechanical strength analysis and validation of the finite element model (FEM).

Both ID and TO plate were able to withstand two leg stance loading conditions according to both FEA and experimental testing. For walking loading conditions the ID plate showed instability, which could result in higher probability of failure in the plating construct. Quasi-static compression experiments showed failure in the most lateral proximal screw in all tests. The failure in the screws was caused by high bending moments. Optimization of the ID plate resulted in an alternative plate design in SCFE treatment. The TO plate consisted of six screws, decreasing plate length by 18.1%. The TO plate had a variable thickness, where thickness was increased around the lateral side of the plate compared to the ID plate.

The steps followed in this study to design a patient-specific implant and evaluation plate mechanical behaviour with FEM and additional experimental testing have shown to be adequate in investigating plate performance and can be used in the evaluation of newly designed patient-specific bone plates in SCFE patients.

Integration of the sensory systems inside patient-specific medical devices (e.g., implants or prosthesis) can provide additional measurement data at different time points. This can result in improving the traditional designs of those medical devices, and consequently leading to increasing the patient’s comfort and quality of life. By doing measurements directly on or inside the patient, information can be gathered more specifically, which is important for improving detection and diagnosis. Next to that, it enables better decision-making for therapies, treatments and the allocation of prosthesis, causing improved rehabilitation. Problems with the comfort and fitting of the prosthesis can have a big impact on the life of the patient wearing the prosthetic. If there is lack of comfort, the patient is less likely to wear the device and that could limit the rehabilitation and satisfaction about their life in multiple areas. To get a better insight in the transtibial prosthesis in particular, and possibly solve the problems that occur, measurements can be done at the interface of the patient and the socket of this prosthesis. The data can be used to visualize the problem more accurately and, as a result, the response and treatment process can be chosen in a more precise manner. First, a case study about the lower limb prosthesis was conducted. Several questions will be reviewed, including defining elements of the satisfaction of patients about their prosthesis, what current existing problems with prosthesis are and the reasons of patients why they are not satisfied with their prosthesis. Then, two types of sensors will be used to visualise the problems occurring at the interface of the socket and the residual limb. One type of sensor is the piezoelectric pressure sensor. It is used to map the pressure on the limb inside the socket. Several experiments have been done with an existing 3D printed socket design and the data is compared to a finite element model that predicts stress distributions to validate this model. The model itself was changed on multiple areas to get closer to test conditions and improve model outputs. To be able to improve such models, the test-procedure should be further revised. After that, thermistors were used to measure the temperature within a 3D printed socket. A design for a new socket has been made to actively use the temperature data and cool the residual limb inside the socket when needed. This is done by integrating air-channels within the structure of the 3D printed socket and air is forced through those channels with a power-fan. The design has been validated with several tests and it is improved based on that. Those tests, calculations and estimations have shown that the current design may be able to cool the residual limb when further improved and optimized. In the end, several suggestions and recommendations for improvements were provided. Some of those improvements could be made in the size of the air-channels and the interface between socket and residual limb. Finally, present and potential (ongoing) developments on how to change the patient's condition, as well as how to apply promising developments in innovative ways, will be addressed.

The osteochondral interface is known to smoothly mitigate the stress concentration over the surface area of the subchondral bone, with a structural transition from the cartilage to bone. These sites in the body are exposed to high forces, which makes them prone to failure. To improve the repair strategies, a better understanding of the mechanical property-structure relationship of osteochondral interface is crucial. The aim of this study was to observe the structure and map the stiffness of the equine osteochondral interface during maturation and detect a possible mechanical property-structure relationship.
Osteochondral samples from the metacarpo- and metatarsophalangeal joint of 9 days, 12 days, 7 weeks, 13 months old and three mature horses were investigated in this study. Histology was performed to observe the structure. The local elastic modulus was determined using nanoindentation and the strain distribution with compression tests combined with digital image correlation (DIC).
The cartilage thickness decreased with age and an emerging tidemark was detected in the 13 months old joint, indicating calcification of the cartilage starts at this age. The local elastic modulus showed a clear transitional region between low and high values for the mature interface, indicating the articular cartilage and subchondral bone region was indented, respectively. For the immature samples, a less clear transition was found and in the 9 days and 12 days old interface the entire matrix consisted of low elastic modulus values. The strain distribution from the compression tests over the interface confirmed the decreasing cartilage thickness with age. The transition location from high to low strain decreased with age.
The mechanical transition from cartilage to bone showed significant changes within the interface and during maturation for the local elastic modulus and the strain distribution. These changes can be compared to the structural changes demonstrated in the histology images. The data obtained in this research provides new insights into the maturing osteochondral interface and the mechanical and structural changes within this complex biological tissue.

Restoration of the normal mandibular form and function is attempted with reconstructive surgery. The current standard procedure to restore continuity defects of the mandible involves free tissue transfer with an autologous bone flap. Even though the success rates are high, several critical drawbacks have been associated with this procedure, including severe donor site morbidity, long hospital stay and re­covery process, need for high surgical expertise, insufficient bone graft height, and mechanical failure of the plating system. A systematic approach for the designing and testing of patient­-specific implants used as an alternative to the free flap procedure appears to be still lacking. The goal of this thesis project was to develop a semi­automatic workflow for designing patient­ specific mandibular reconstruction implants and assess the effect of topology optimization on the biome­chanical performance of the implant using the finite element analysis (FEA) and experimental validation. Using the proposed workflow, a fully porous implant (lattice implant) and a topology optimized implant (TO implant) were designed according to the anatomy of a synthetic mandible analog subjected to lateral resection, and were additively manufactured using selective laser melting. Both implants were designed in the shape of a cage to allow for the insertion of a bone graft and the integration of dental implants. The mechanical performance of the reconstructed mandibles was predicted with computa­tional FEA and evaluated through quasi­-static and cyclic experimental testing. Digital image correlation was used to validate the finite element model through principal strain field comparison. An excellent fit between the implants and mandibles was established, indicating the capability of the workflow to develop customized implants with accurate dimensions. The results obtained with FEA were in agreement with the DIC measurements and quasi-­static testing results. No significant differences (P < 0.01) in mean stiffness, mean ultimate load, and mean ultimate displacement were found between the non-­implanted control mandibles, lattice­-implanted mandibles, and TO-­implanted mandibles during quasi-­static testing. No implant failure was observed during static nor cyclic testing at loads substantially higher than the average maximum biting force of healthy individuals, indicating the high resistance of the implant designs to mechanical failure. Yet, the lattice implant would likely be preferred over the TO implant for clinical application due to its lower weight (16.5%), higher porosity (17.4%), and shorter workflow time (633.3%). The workflow proposed in this study may offer surgeons and medical engineers the tools to sys­tematically design and evaluate patient­-specific reconstruction implants. This would result in more cost­-effective and time­-effective pre-­surgical planning and result in implant designs that can minimize morbidity and maximize aesthetic and functional outcomes.

The mechanical properties of biological hard-soft interfaces change gradually over a small surface area to prevent the formation of stress concentrations through variations in the interface’s chemical composition, microstructure and geometry. The bone-tendon/ligament and bone-cartilage interface are the most prominent hard-soft interfaces within the human body and need restoration once injuries occur. It is difficult to repair these interfaces and simultaneously maintain the initial quality of the native tissues. This study used hybrid 3D-printing techniques, i.e., fused deposition modeling (FDM) and an extrusion-based technique, to develop a proof of concept for the fabrication of hard-soft interface structures. The first part of the study concentrates on the design of geometrical interlocking structures. A parametric study with multiple simulations was performed for two specific geometries, an anti-trapezoidal and a double hook design. The double hook design demonstrated the highest stiffness under tensile stress conditions. The second part concentrates on manufacturing hard-soft interface structures, for which 2D and 3D models were fabricated. The 2D models were used to explore different geometrical interlocking designs and validate the computational models. The 3D models were used to create a proof-of-concept for a hard-soft interface made from PLA (FDM technique) and alginate (extrusion-based technique). The 3D-printing techniques were combined by extruding hydrogel into the interlocking system of the PLA part and printing a soft alginate scaffold on top of the interlocking structure. In conclusion, this study suggests several practical solutions to improve interfacial designs and to manufacture hard-soft interface structures. Combining 3D-printing techniques opens up new possibilities for the fabrication of state-of-the-art hard-soft interfaces structures.