Computer-aided design and finite-element modelling of biomaterial scaffolds for bone tissue engineering

Modeling of the PHEMA-gelatin scaffold enriched with graphene oxide utilizing finite element method for bone tissue engineering

The development of computer-aided facilities has contributed to the optimization of tissue engineering techniques due to the reduction in necessary practical assessments and the removal of animal or human-related ethical issues. Herein, a bone scaffold based on poly (2-hydroxyethyl methacrylate) (PHEMA), gelatin and graphene oxide (GO), was simulated by SOLIDWORKS and ABAQUS under a normal compression force using finite element method (FEM). Concerning the mechanotransduction impact, GO could support the stability of the structure and reduce the possibility of the failure resulting in the integrity and durability of the scaffold efficiency which would be beneficial for osteogenic differentiation.

Lumbar Intervertebral Disc Herniation: Annular Closure Devices and Key Design Requirements

Lumbar disc herniation is one of the most common degenerative spinal conditions resulting in lower back pain and sciatica. Surgical treatment options include microdiscectomy, lumbar fusion, total disc replacement, and other minimally invasive approaches. At present, microdiscectomy procedures are the most used technique; however, the annulus fibrosus is left with a defect that without treatment may contribute to high reherniation rates and changes in the biomechanics of the lumbar spine. This paper aims to review current commercially available products that mechanically close the annulus including the AnchorKnot® suture-passing device and the Barricaid® annular closure device. Previous studies and reviews have focused mainly on a biomimetic biomaterials approach and have described some mechanical and biological requirements for an active annular repair/regeneration strategy but are still far away from clinical implementation. Therefore, in this paper we aim to create a design specification for a mechanical annular closure strategy by identifying the most important mechanical and biological design parameters, including consideration of material selection, preclinical testing requirements, and requirements for clinical implementation.

Experimental Characterization and Mathematical Modeling of the Adsorption of Proteins and Cells on Biomimetic Hydroxyapatite

Biomaterial development is a long process consisting of multiple stages of design and evaluation within the context of both in vitro and in vivo testing. To streamline this process, mathematical and computational modeling displays potential as a tool for rapid biomaterial characterization, enabling the prediction of optimal physicochemical parameters. In this work, a Langmuir isotherm-based model was used to describe protein and cell adhesion on a biomimetic hydroxyapatite surface, both independently and in a one-way coupled system. The results indicated that increased protein surface coverage leads to improved cell adhesion and spread, with maximal protein coverage occurring within 48 h. In addition, the Langmuir model displayed a good fit with the experimental data. Overall, computational modeling is an exciting avenue that may lead to savings in terms of time and cost during the biomaterial development process.

Digital Twins for Tissue Culture Techniques—Concepts, Expectations, and State of the Art

Techniques to provide in vitro tissue culture have undergone significant changes during the last decades, and current applications involve interactions of cells and organoids, three-dimensional cell co-cultures, and organ/body-on-chip tools. Efficient computer-aided and mathematical model-based methods are required for efficient and knowledge-driven characterization, optimization, and routine manufacturing of tissue culture systems. As an alternative to purely experimental-driven research, the usage of comprehensive mathematical models as a virtual in silico representation of the tissue culture, namely a digital twin, can be advantageous. Digital twins include the mechanistic of the biological system in the form of diverse mathematical models, which describe the interaction between tissue culture techniques and cell growth, metabolism, and the quality of the tissue. In this review, current concepts, expectations, and the state of the art of digital twins for tissue culture concepts will be highlighted. In general, DT’s can be applied along the full process chain and along the product life cycle. Due to the complexity, the focus of this review will be especially on the design, characterization, and operation of the tissue culture techniques.

Systematic Review of Silk Scaffolds in Musculoskeletal Tissue Engineering Applications in the Recent Decade

During the past decade, various novel tissue engineering (TE) strategies have been developed to maintain, repair, and restore the biomechanical functions of the musculoskeletal system. Silk fibroins are natural polymers with numerous advantageous properties such as good biocompatibility, high mechanical strength, and low degradation rate and are increasingly being recognized as a scaffolding material of choice in musculoskeletal TE applications. This current systematic review examines and summarizes the latest research on silk scaffolds in musculoskeletal TE applications within the past decade. Scientific databases searched include PubMed, Web of Science, Medline, Cochrane library, and Embase. The following keywords and search terms were used: musculoskeletal, tendon, ligament, intervertebral disc, muscle, cartilage, bone, silk, and tissue engineering. Our Review was limited to articles on musculoskeletal TE, which were published in English from 2010 to September 2019. The eligibility of the articles was assessed by two reviewers according to prespecified inclusion and exclusion criteria, after which an independent reviewer performed data extraction and a second independent reviewer validated the data obtained. A total of 1120 articles were reviewed from the databases. According to inclusion and exclusion criteria, 480 articles were considered as relevant for the purpose of this systematic review. Tissue engineering is an effective modality for repairing or replacing injured or damaged tissues and organs with artificial materials. This Review is intended to reveal the research status of silk-based scaffolds in the musculoskeletal system within the recent decade. In addition, a comprehensive translational research route for silk biomaterial from bench to bedside is described in this Review.

Integrating finite element modelling and 3D printing to engineer biomimetic polymeric scaffolds for tissue engineering

The suitability of a scaffold for tissue engineering is determined by a number of interrelated factors. The biomaterial should be biocompatible and cell instructive, with a porosity and pore interconnectivity that facilitates cellular migration and the transport of nutrients and waste products into and out of the scaffolds. For the engineering of load bearing tissues, the scaffold may also be required to possess specific mechanical properties and/or ensure the transfer of mechanical stimuli to cells to direct their differentiation. Achieving these design goals is challenging, but could potentially be realised by integrating computational tools such as finite element (FE) modelling with three-dimensional (3D) printing techniques to assess how scaffold architecture and material properties influence the performance of the implant. In this study we first use Fused Deposition Modelling (FDM) to modulate the architecture of polycaprolactone (PCL) scaffolds, exploring the influence of varying fibre diameter, spacing and laydown pattern on the structural and mechanical properties of such scaffolds. We next demonstrate that a simple FE modelling strategy, which captures key aspects of the printed scaffold's actual geometry and material behaviour, can be used to accurately model the mechanical characteristics of such scaffolds. We then show the utility of this strategy by using FE modelling to help design 3D printed scaffolds with mechanical properties mimicking that of articular cartilage. In conclusion, this study demonstrates that a relatively simple FE modelling approach can be used to inform the design of 3D printed scaffolds to ensure their bulk mechanical properties mimic specific target tissues.

Utilization of Finite Element Analysis for Articular Cartilage Tissue Engineering

Scaffold design plays an essential role in tissue engineering of articular cartilage by providing the appropriate mechanical and biological environment for chondrocytes to proliferate and function. Optimization of scaffold design to generate tissue-engineered cartilage has traditionally been conducted using in-vitro and in-vivo models. Recent advances in computational analysis allow us to significantly decrease the time and cost of scaffold optimization using finite element analysis (FEA). FEA is an in-silico analysis technique that allows for scaffold design optimization by predicting mechanical responses of cells and scaffolds under applied loads. Finite element analyses can potentially mimic the morphology of cartilage using mesh elements (tetrahedral, hexahedral), material properties (elastic, hyperelastic, poroelastic, composite), physiological loads by applying loading conditions (static, dynamic), and constitutive stress-strain equations (linear, porous-elastic, biphasic). Furthermore, FEA can be applied to the study of the effects of dynamic loading, material properties cell differentiation, cell activity, scaffold structure optimization, and interstitial fluid flow, in isolated or combined multi-scale models. This review covers recent studies and trends in the use of FEA for cartilage tissue engineering and scaffold design.

Integrated Design Approaches for 3D Printed Tissue Scaffolds: Review and Outlook

Emerging 3D printing technologies are enabling the fabrication of complex scaffold structures for diverse medical applications. 3D printing allows controlled material placement for configuring porous tissue scaffolds with tailored properties for desired mechanical stiffness, nutrient transport, and biological growth. However, tuning tissue scaffold functionality requires navigation of a complex design space with numerous trade-offs that require multidisciplinary assessment. Integrated design approaches that encourage iteration and consideration of diverse processes including design configuration, material selection, and simulation models provide a basis for improving design performance. In this review, recent advances in design, fabrication, and assessment of 3D printed tissue scaffolds are investigated with a focus on bone tissue engineering. Bone healing and fusion are examples that demonstrate the needs of integrated design approaches in leveraging new materials and 3D printing processes for specified clinical applications. Current challenges for integrated design are outlined and emphasize directions where new research may lead to significant improvements in personalized medicine and emerging areas in healthcare.

Four-Dimensional Imaging of Soft Tissue and Implanted Biomaterial Mechanics: A Barbed Suture Case Study for Tendon Repair

Timely, recent developments in X-ray microcomputed tomography (XμCT) imaging such as increased resolution and improved sample preparation enable nondestructive time-lapse imaging of polymeric biomaterials when implanted in soft tissue, which we demonstrate herein. Imaging the full three-dimensional (3D) structure of an implanted biomaterial provides new opportunities to assess the micromechanics of the interface between the implant and tissues and how this changes over time as force is applied in load-bearing musculoskeletal applications. In this paper, we present a case study demonstrating in situ XμCT and finite element analysis, using a dynamically loaded barbed suture repair for its novel use in tendon tissue. The aim of this study was to identify the distribution of stress in the suture and tendon as load is applied. The data gained demonstrate a clear 3D visualization of microscale features in both the tissue and implant in wet conditions. XμCT imaging has revealed, for the first time, pores around the suture, preventing full engagement of all the barbs with the tendon tissue. Subsequent finite element analysis reveals the localized stress and strain, which are not evenly distributed along the suture, or throughout the tissue. This case study demonstrates for the first time a powerful in situ mechanical imaging tool, which could be readily adapted by other laboratories to interrogate and optimize the interface between the implanted biomaterials and the soft tissue.

Numerical optimization of cell colonization modelling inside scaffold for perfusion bioreactor: A multiscale model

Part of clinically applicable bone graft substitutes are developed by using mechanical stimulation of flow-perfusion into cell-seeded scaffolds. The role of fluid flow is crucial in driving the nutrient to seeded cells and in stimulating cell colonization. A common numerical approach is to use a multiscale model to link some physical quantities (wall shear stress and inlet flow rate) that act at different scales. In this study, a multiscale model is developed in order to determine the optimal inlet flow rate to cultivate osteoblast-like cells seeded in a controlled macroporous biomaterial inside a perfusion bioreactor system. We focus particularly on the influence of Wall Shear Stress on cell colonization to predict cell colonization at the macroscale. Results obtained at the microscale are interpolated at the macroscale to determine the optimal flow rate. For a macroporous scaffold made of interconnected pores with pore diameters of above 350 μm and interconnection diameters of 150 μm, the model predicts a cell colonization of 325% after a 7-day-cell culture with a constant inlet flow rate of 0.69 mL·min^-1. Furthermore, the strength of this protocol is the possibility to adapt it to most porous biomaterials and dynamic cell culture systems.

Optimal Load for Bone Tissue Scaffolds with an Assigned Geometry

Thanks to the recent advances of three-dimensional printing technologies the design and the fabrication of a large variety of scaffold geometries was made possible. The surgeon has the availability of a wide number of scaffold micro-architectures thus needing adequate guidelines for the choice of the best one to be implanted in a patient-specific anatomic region. We propose a mechanobiology-based optimization algorithm capable of determining, for bone tissue scaffolds with an assigned geometry, the optimal value L_opt of the compression load to which they should be subjected, i.e. the load value for which the formation of the largest amounts of bone is favoured and hence the successful outcome of the scaffold implantation procedure is guaranteed. Scaffolds based on hexahedron unit cells were investigated including pores differently dimensioned and with different shapes such as elliptic or rectangular. The algorithm predicted decreasing values of the optimal load for scaffolds with pores with increasing dimensions. The optimal values predicted for the scaffolds with elliptic pores were found higher than those with rectangular ones. The proposed algorithm can be utilized to properly guide the surgeon in the choice of the best scaffold type/geometry that better satisfies the specific patient requirements.

Mechanobiological modelling of tendons: Review and future opportunities

Tendons are adapted to carry large, repeated loads and are clinically important for the maintenance of musculoskeletal health in an increasing, actively ageing population, as well as in elite athletes. Tendons are known to adapt to mechanical loading. Also, their healing and disease processes are highly sensitive to mechanical load. Computational modelling approaches developed to capture this mechanobiological adaptation in tendons and other tissues have successfully addressed many important scientific and clinical issues. The aim of this review is to identify techniques and approaches that could be further developed to address tendon-related problems. Biomechanical models are identified that capture the multi-level aspects of tendon mechanics. Continuum whole tendon models, both phenomenological and microstructurally motivated, are important to estimate forces during locomotion activities. Fibril-level microstructural models are documented that can use these estimated forces to detail local mechanical parameters relevant to cell mechanotransduction. Cell-level models able to predict the response to such parameters are also described. A selection of updatable mechanobiological models is presented. These use mechanical signals, often continuum tissue level, along with rules for tissue change and have been applied successfully in many tissues to predict in vivo and in vitro outcomes. Signals may include scalars derived from the stress or strain tensors, or in poroelasticity also fluid velocity, while adaptation may be represented by changes to elastic modulus, permeability, fibril density or orientation. So far, only simple analytical approaches have been applied to tendon mechanobiology. With the development of sophisticated computational mechanobiological models in parallel with reporting more quantitative data from in vivo or clinical mechanobiological studies, for example, appropriate imaging, biochemical and histological data, this field offers huge potential for future development towards clinical applications.

A Mechanobiology-based Algorithm to Optimize the Microstructure Geometry of Bone Tissue Scaffolds

Complexity of scaffold geometries and biological mechanisms involved in the bone generation process make the design of scaffolds a quite challenging task. The most common approaches utilized in bone tissue engineering require costly protocols and time-consuming experiments. In this study we present an algorithm that, combining parametric finite element models of scaffolds with numerical optimization methods and a computational mechano-regulation model, is able to predict the optimal scaffold microstructure. The scaffold geometrical parameters are perturbed until the best geometry that allows the largest amounts of bone to be generated, is reached. We study the effects of the following factors: (1) the shape of the pores; (2) their spatial distribution; (3) the number of pores per unit area. The optimal dimensions of the pores have been determined for different values of scaffold Young's modulus and compression loading acting on the scaffold upper surface. Pores with rectangular section were predicted to lead to the formation of larger amounts of bone compared to square section pores; similarly, elliptic pores were predicted to allow the generation of greater amounts of bone compared to circular pores. The number of pores per unit area appears to have rather negligible effects on the bone regeneration process. Finally, the algorithm predicts that for increasing loads, increasing values of the scaffold Young's modulus are preferable. The results shown in the article represent a proof-of-principle demonstration of the possibility to optimize the scaffold microstructure geometry based on mechanobiological criteria.

Analysis of shear wave propagation derived from MR elastography in 3D thigh skeletal muscle using subject specific finite element model

The purpose of this study was to develop a subject specific finite element model derived from MRI images to numerically analyze the MRE (magnetic resonance elastography) shear wave propagation within skeletal thigh muscles. A sagittal T2 CUBE MRI sequence was performed on the 20-cm thigh segment of a healthy male subject. Skin, adipose tissue, femoral bone and 11 muscles were manually segmented in order to have 3D smoothed solid and meshed models. These tissues were modeled with different constitutive laws. A transient modal dynamics analysis was applied to simulate the shear wave propagation within the thigh tissues. The effects of MRE experimental parameters (frequency, force) and the muscle material properties (shear modulus: C10) were analyzed through the simulated shear wave displacement within the vastus medialis muscle. The results showed a plausible range of frequencies (from 90Hz to 120 Hz), which could be used for MRE muscle protocol. The wave amplitude increased with the level of the force, revealing the importance of the boundary condition. Moreover, different shear displacement patterns were obtained as a function of the muscle mechanical properties. The present study is the first to analyze the shear wave propagation in skeletal muscles using a 3D subject specific finite element model. This study could be of great value to assist the experimenters in the set-up of MRE protocols.

Assessment of activated porous granules on implant fixation and early bone formation in sheep

Background/Objective

Despite recent progress in regeneration medicine, the repair of large bone defects due to trauma, inflammation and tumor surgery remains a major clinical challenge. This study was designed to produce large amounts of viable bone graft materials in a novel perfusion bioreactor to promote bone formation.

Methods

Cylindrical defects were created bilaterally in the distal femurs of sheep, and titanium implants were inserted. The concentric gap around the implants was randomly filled either with allograft, granules, granules with bone marrow aspirate (BMA) or bioreactor activated granule (BAG). The viable BAG consisted of autologous bone marrow stromal cells (BMSCs) seeded upon porous scaffold granules incubated in a 3D perfusion bioreactor for 2 weeks prior to surgery. 6 weeks after, the bone formation and early implant fixation were assessed by means of micro-CT, histomorphometry, and mechanical test.

Results

Microarchitectural analysis revealed that bone volume fraction and trabecular thickness in the allograft were not statistically different than those (combination of new bone and residue of granule) in the other 3 groups. The structure of the allograft group was typically plate-like, while the other 3 groups were combination of plate and rod. Histomorphometry showed that allograft induced significantly more bone and less fibrous tissue in the concentric gap than the other 3 granule groups, while the bone ingrowth to implant porous surface was not different. No significant differences among the groups were found regarding early implant mechanical fixation.

Conclusion

In conclusion, despite nice bone formation and implant fixation in all groups, bioreactor activated graft material did not convincingly induce early implant fixation similar to allograft, and neither bioreactor nor by adding BMA credited additional benefit for bone formation in this model.

Challenges in engineering osteochondral tissue grafts with hierarchical structures

INTRODUCTION

A major hurdle in treating osteochondral (OC) defects is the different healing abilities of two types of tissues involved - articular cartilage and subchondral bone. Biomimetic approaches to OC-construct engineering, based on recapitulation of biological principles of tissue development and regeneration, have potential for providing new treatments and advancing fundamental studies of OC tissue repair.

AREAS COVERED

This review on state of the art in hierarchical OC tissue graft engineering is focused on tissue engineering approaches designed to recapitulate the native milieu of cartilage and bone development. These biomimetic systems are discussed with relevance to bioreactor cultivation of clinically sized, anatomically shaped human cartilage/bone constructs with physiologic stratification and mechanical properties. The utility of engineered OC tissue constructs is evaluated for their use as grafts in regenerative medicine, and as high-fidelity models in biological research.

EXPERT OPINION

A major challenge in engineering OC tissues is to generate a functionally integrated stratified cartilage-bone structure starting from one single population of mesenchymal cells, while incorporating perfusable vasculature into the bone, and in bone-cartilage interface. To this end, new generations of advanced scaffolds and bioreactors, implementation of mechanical loading regimens and harnessing of inflammatory responses of the host will likely drive the further progress.

Design, materials, and mechanobiology of biodegradable scaffolds for bone tissue engineering

A review about design, manufacture, and mechanobiology of biodegradable scaffolds for bone tissue engineering is given. First, fundamental aspects about bone tissue engineering and considerations related to scaffold design are established. Second, issues related to scaffold biomaterials and manufacturing processes are discussed. Finally, mechanobiology of bone tissue and computational models developed for simulating how bone healing occurs inside a scaffold are described.

Current strategies in multiphasic scaffold design for osteochondral tissue engineering: A review

The repair of osteochondral defects requires a tissue engineering approach that aims at mimicking the physiological properties and structure of two different tissues (cartilage and bone) using specifically designed scaffold-cell constructs. Biphasic and triphasic approaches utilize two or three different architectures, materials, or composites to produce a multilayered construct. This article gives an overview of some of the current strategies in multiphasic/gradient-based scaffold architectures and compositions for tissue engineering of osteochondral defects. In addition, the application of finite element analysis (FEA) in scaffold design and simulation of in vitro and in vivo cell growth outcomes has been briefly covered. FEA-based approaches can potentially be coupled with computer-assisted fabrication systems for controlled deposition and additive manufacturing of the simulated patterns. Finally, a summary of the existing challenges associated with the repair of osteochondral defects as well as some recommendations for future directions have been brought up in the concluding section of this article.

Current trends in the design of scaffolds for computer-aided tissue engineering

Advances introduced by additive manufacturing have significantly improved the ability to tailor scaffold architecture, enhancing the control over microstructural features. This has led to a growing interest in the development of innovative scaffold designs, as testified by the increasing amount of research activities devoted to the understanding of the correlation between topological features of scaffolds and their resulting properties, in order to find architectures capable of optimal trade-off between often conflicting requirements (such as biological and mechanical ones). The main aim of this paper is to provide a review and propose a classification of existing methodologies for scaffold design and optimization in order to address key issues and help in deciphering the complex link between design criteria and resulting scaffold properties.

Tendon mechanobiology: experimental models require mathematical underpinning

Mathematical and computational modeling is in demand to help address current challenges in mechanobiology of musculoskeletal tissues. In particular for tendon, the high clinical importance of the tissue, the huge mechanical demands placed on it and its ability to adapt to these demands, require coupled, multiscale models incorporating complex geometrical and microstructural information as well as time-based descriptions of cellular activity and response.This review introduces the information sources required to develop such multiscale models. It covers tissue structure and biomechanics, cell biomechanics, the current understanding of tendon's ability in health and disease to update its properties and structure and the few already existing multiscale mechanobiological models of the tissue. Finally, a sketch is provided of what such models could achieve ideally, pointing out where experimental data and knowledge are still missing.

Reconstruction of comminuted long-bone fracture using CF/CPC scaffolds manufactured by rapid prototyping

Background

Stabilization and bone healing of fractures in weight-bearing long bones are challenging. This study was conducted to evaluate the effect of a scaffold composed of chitosan fiber and calcium phosphate ceramics (CF/CPC scaffold) on stability and fracture repair in weight-bearing long bones.

Material/Methods

Comminuted fractures of paired radiuses were created in 36 healthy, mature dogs. The left radius of each dog was classified in the experimental group and treated with CF/CPC scaffold, and the right one was not filled, and was used as a blank control. Of the 12 animals in each group that were killed at week 4, 8, and 12 after the operation, 6 were used for histological analysis, and the other 6 used were for biomechanical studies. Both radiuses from each animal were dissected free and stored for these analyses. All the animals underwent X-ray radiograph pre- and post-operatively. Computer-aided rapid-prototyping technologies were adopted for the fabrication of three-dimensional scaffolds with precise geometric control.

Results

X-ray showed that the bone fracture area in the experimental group was filled with callus at week 12 after surgery. Histological examination detected slow resorption of the cement and new bone formation since week 4. At week 12, the scaffold material partially degraded and was still present in all specimens. Mechanical testing revealed that the failure strength of the radiuses treated with CF/CPC scaffolds was about 3 times that of the radiuses without implanted scaffolds.

Conclusions

The effect of using CF/CPC scaffold in treating comminuted weight-bearing long bone fractures is satisfactory.

Current views on calcium phosphate osteogenicity and the translation into effective bone regeneration strategies

Calcium phosphate (CaP) has traditionally been used for the repair of bone defects because of its strong resemblance to the inorganic phase of bone matrix. Nowadays, a variety of natural or synthetic CaP-based biomaterials are produced and have been extensively used for dental and orthopaedic applications. This is justified by their biocompatibility, osteoconductivity and osteoinductivity (i.e. the intrinsic material property that initiates de novo bone formation), which are attributed to the chemical composition, surface topography, macro/microporosity and the dissolution kinetics. However, the exact molecular mechanism of action is unknown. This review paper first summarizes the most important aspects of bone biology in relation to CaP and the mechanisms of bone matrix mineralization. This is followed by the research findings on the effects of calcium (Ca²⁺) and phosphate (PO₄³⁻) ions on the migration, proliferation and differentiation of osteoblasts during in vivo bone formation and in vitro culture conditions. Further, the rationale of using CaP for bone regeneration is explained, focusing thereby specifically on the material's osteoinductive properties. Examples of different material forms and production techniques are given, with the emphasis on the state-of-the art in fine-tuning the physicochemical properties of CaP-based biomaterials for improved bone induction and the use of CaP as a delivery system for bone morphogenetic proteins. The use of computational models to simulate the CaP-driven osteogenesis is introduced as part of a bone tissue engineering strategy in order to facilitate the understanding of cell-material interactions and to gain further insight into the design and optimization of CaP-based bone reparative units. Finally, limitations and possible solutions related to current experimental and computational techniques are discussed.

A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering

The combined potential of hydrogels and rapid prototyping technologies has been an exciting route in developing tissue engineering scaffolds for the past decade. Hydrogels represent to be an interesting starting material for soft, and lately also for hard tissue regeneration. Their application enables the encapsulation of cells and therefore an increase of the seeding efficiency of the fabricated structures. Rapid prototyping techniques on the other hand, have become an elegant tool for the production of scaffolds with the purpose of cell seeding and/or cell encapsulation. By means of rapid prototyping, one can design a fully interconnected 3-dimensional structure with pre-determined dimensions and porosity. Despite this benefit, some of the rapid prototyping techniques are not or less suitable for the generation of hydrogel scaffolds. In this review, we therefore give an overview on the different rapid prototyping techniques suitable for the processing of hydrogel materials. A primary distinction will be made between (i) laser-based, (ii) nozzle-based, and (iii) printer-based systems. Special attention will be addressed to current trends and limitations regarding the respective techniques. Each of these techniques will be further discussed in terms of the different hydrogel materials used so far. One major drawback when working with hydrogels is the lack of mechanical strength. Therefore, maintaining and improving the mechanical integrity of the processed scaffolds has become a key issue regarding 3-dimensional hydrogel structures. This limitation can either be overcome during or after processing the scaffolds, depending on the applied technology and materials.

Review paper: Critical Issues in Tissue Engineering: Biomaterials, Cell Sources, Angiogenesis, and Drug Delivery Systems

Tissue engineering is a newly emerging biomedical technology, which aids and increases the repair and regeneration of deficient and injured tissues. It employs the principles from the fields of materials science, cell biology, transplantation, and engineering in an effort to treat or replace damaged tissues. Tissue engineering and development of complex tissues or organs, such as heart, muscle, kidney, liver, and lung, are still a distant milestone in twenty-first century. Generally, there are four main challenges in tissue engineering which need optimization. These include biomaterials, cell sources, vascularization of engineered tissues, and design of drug delivery systems. Biomaterials and cell sources should be specific for the engineering of each tissue or organ. On the other hand, angiogenesis is required not only for the treatment of a variety of ischemic conditions, but it is also a critical component of virtually all tissue-engineering strategies. Therefore, controlling the dose, location, and duration of releasing angiogenic factors via polymeric delivery systems, in order to ultimately better mimic the stem cell niche through scaffolds, will dictate the utility of a variety of biomaterials in tissue regeneration. This review focuses on the use of polymeric vehicles that are made of synthetic and/or natural biomaterials as scaffolds for three-dimensional cell cultures and for locally delivering the inductive growth factors in various formats to provide a method of controlled, localized delivery for the desired time frame and for vascularized tissue-engineering therapies.

Biomechanics in bone tissue engineering

Biomechanics may be considered as central in the development of bone tissue engineering. The initial mechanical aspects are essential to the outcome of a functional tissue engineering approach; so are aspects of interface micromotion, bone ingrowths inside the scaffold and finally, the mechanical integrity of the scaffold during its degradation. A proposed view is presented herein on how biomechanical aspects can be synthesised and where future developments are needed. In particular, a distinction is made between the mechanical and the mechanotransductional aspects of bone tissue engineering: the former could be related to osteoconduction, while the latter may be correlated to the osteoinductive properties of the scaffold. This distinction allows biomechanicians to follow a strategy in the development of a scaffold having not only mechanical targets but also incorporating some mechanotransduction principles.

Mathematical modeling of degradation for bulk-erosive polymers: applications in tissue engineering scaffolds and drug delivery systems

The degradation of polymeric biomaterials, which are widely exploited in tissue engineering and drug delivery systems, has drawn significant attention in recent years. This paper aims to develop a mathematical model that combines stochastic hydrolysis and mass transport to simulate the polymeric degradation and erosion process. The hydrolysis reaction is modeled in a discrete fashion by a fundamental stochastic process and an additional autocatalytic effect induced by the local carboxylic acid concentration in terms of the continuous diffusion equation. Illustrative examples of microparticles and tissue scaffolds demonstrate the applicability of the model. It is found that diffusive transport plays a critical role in determining the degradation pathway, whilst autocatalysis makes the degradation size dependent. The modeling results show good agreement with experimental data in the literature, in which the hydrolysis rate, polymer architecture and matrix size actually work together to determine the characteristics of the degradation and erosion processes of bulk-erosive polymer devices. The proposed degradation model exhibits great potential for the design optimization of drug carriers and tissue scaffolds.

Finite element method (FEM), mechanobiology and biomimetic scaffolds in bone tissue engineering

Techniques of bone reconstructive surgery are largely based on conventional, non-cell-based therapies that rely on the use of durable materials from outside the patient's body. In contrast to conventional materials, bone tissue engineering is an interdisciplinary field that applies the principles of engineering and life sciences towards the development of biological substitutes that restore, maintain, or improve bone tissue function. Bone tissue engineering has led to great expectations for clinical surgery or various diseases that cannot be solved with traditional devices. For example, critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of bone tissue engineering is to apply engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. The total market for bone tissue regeneration and repair was valued at $1.1 billion in 2007 and is projected to increase to nearly $1.6 billion by 2014.Usually, temporary biomimetic scaffolds are utilized for accommodating cell growth and bone tissue genesis. The scaffold has to promote biological processes such as the production of extra-cellular matrix and vascularisation, furthermore the scaffold has to withstand the mechanical loads acting on it and to transfer them to the natural tissues located in the vicinity. The design of a scaffold for the guided regeneration of a bony tissue requires a multidisciplinary approach. Finite element method and mechanobiology can be used in an integrated approach to find the optimal parameters governing bone scaffold performance.In this paper, a review of the studies that through a combined use of finite element method and mechano-regulation algorithms described the possible patterns of tissue differentiation in biomimetic scaffolds for bone tissue engineering is given. Firstly, the generalities of the finite element method of structural analysis are outlined; second, the issues related to the generation of a finite element model of a given anatomical site or of a bone scaffold are discussed; thirdly, the principles on which mechanobiology is based, the principal theories as well as the main applications of mechano-regulation models in bone tissue engineering are described; finally, the limitations of the mechanobiological models and the future perspectives are indicated.

A dynamical study of the mechanical stimuli and tissue differentiation within a CaP scaffold based on micro-CT finite element models

The control of the mechanical stimuli transmitted to the cells is critical for the design of functional scaffolds for tissue engineering. The objective of this study was to investigate the dynamics of the mechanical stimuli transmitted to the cells during tissue differentiation in an irregular morphology scaffold under compressive load and perfusion flow. A calcium phosphate-based glass porous scaffold was used. The solid phase and the fluid flow within the pores were modeled as linear elastic solid material and Newtonian fluid, respectively. In the fluid model, different levels of viscosity were used to simulate tissue differentiation. Compressive strain of 0.5% and fluid flow with constant inlet velocity of 10 μm/s or constant inlet pressure of 3 Pa were applied. Octahedral shear strain and fluid shear stress were used as mechano-regulatory stimuli. For constant inlet velocity, stimuli equivalent to bone were predicted in 80% of pore volume for the case of low tissue viscosity. For the cases of high viscosity, fluctuations between stimuli equivalent to tissue formation and cell death were predicted due to the increase in the fluid shear stress when tissue started to fill pores. When constant pressure was applied, stimuli equivalent to bone were predicted in 62% of pore volume when low tissue viscosity was used and 42% when high tissue viscosity was used. This study predicted critical variations of fluid shear stress when cells differentiated. If these variations are not controlled in vitro, they can impede the formation of new matured tissue.

Tissue differentiation in an in vivo bioreactor: in silico investigations of scaffold stiffness

Scaffold design remains a main challenge in tissue engineering due to the large number of requirements that need to be met in order to create functional tissues in vivo. Computer simulations of tissue differentiation within scaffolds could serve as a powerful tool in elucidating the design requirements for scaffolds in tissue engineering. In this study, a lattice-based model of a 3D porous scaffold construct derived from micro CT and a mechano-biological simulation of a bone chamber experiment were combined to investigate the effect of scaffold stiffness on tissue differentiation inside the chamber. The results indicate that higher scaffold stiffness, holding pore structure constant, enhances bone formation. This study demonstrates that a lattice approach is very suitable for modelling scaffolds in mechano-biological simulations, since it can accurately represent the micro-porous geometries of scaffolds in a 3D environment and reduce computational costs at the same time.