Modelling the role of surface stress on the kinetics of tissue growth in confined geometries

A mechano-regulation model to design and optimize the surface microgeometry of titanium textured devices for biomedical applications

In a technological context where, thanks to the additive manufacturing techniques, even sophisticated geometries as well as surfaces with specific micrometric features can be realized, we propose a mechano-regulation algorithm to determine the optimal microgeometric parameters of the surface of textured titanium devices for biomedical applications. A poroelastic finite element model was developed including a portion of bone, a portion of a textured titanium device and a layer of granulation tissue separating the bone from the device and occupying the space between them. The algorithm, implemented in the Matlab environment, determines the optimal values of the root mean square and the correlation length that the device surface must possess to maximize bone formation in the gap between the bone and the device. For low levels of compression load acting on the bone, the algorithm predicts low values of root mean square and high values of correlation length. Conversely, high levels of load require high values of root mean square and low values of correlation length. The optimal microgeometrical parameters were determined for various thickness values of the granulation tissue layer. Interestingly, the predictions of the proposed computational model are consistent with the experimental results reported in the literature. The proposed algorithm shows promise as a valuable tool for addressing the demands of precision medicine. In this approach, the device or prosthesis is no longer designed solely based on statistical averages but is tailored to each patient's unique anthropometric characteristics, as well as considerations related to their metabolism, sex, age, and more.

3D Printed Porous Zirconia Biomaterials based on Triply Periodic Minimal Surfaces Promote Osseointegration In Vitro by Regulating Osteoimmunomodulation and Osteo/Angiogenesis

The triply periodic minimal surface (TPMS) is a highly useful structure for bone tissue engineering owing to its nearly nonexistent average surface curvature, high surface area-to-volume ratio, and exceptional mechanical energy absorption properties. However, limited literature is available regarding bionic zirconia implants using the TPMS structure for bone regeneration. Herein, we employed the digital light processing (DLP) technology to fabricate four types of zirconia-based TPMS structures: P-cell, S14, IWP, and Gyroid. For cell proliferation, the four porous TPMS structures outperformed the solid zirconia group (P-cell > S14 > Gyroid > IWP > ZrO2). In vitro assessments on the biological responses and osteogenic properties of the distinct porous surfaces identified the IWP and Gyroid structures as promising candidates for future clinical applications of porous zirconia implants because of their superior osteogenic capabilities (IWP > Gyroid > S14 > P-cell > ZrO2) and mechanical properties (ZrO2 > IWP > Gyroid > S14 > P-cell). Furthermore, the physical properties of the IWP/Gyroid surface had more substantial effects on bone immune regulation by reducing macrophage M1 phenotype polarization while increasing M2 phenotype polarization compared with the solid zirconia surface. Additionally, the IWP and Gyroid groups exhibited enhanced immune osteogenesis and angiogenesis abilities. Collectively, these findings highlight the substantial impact of topology on bone/angiogenesis and immune regulation in promoting bone integration.

Design of a Haversian system-like gradient porous scaffold based on triply periodic minimal surfaces for promoting bone regeneration

INTRODUCTION

The bone ingrowth depth in the porous scaffolds is greatly affected by the structural design, notably the pore size, pore geometry, and the pore distribution. To enhance the bone regeneration capability of scaffolds, the bionic design can be regarded as a potential solution.

OBJECTIVES

We proposed a Haversian system-like gradient structure based on the triply periodic minimal surface architectures with pore size varying from the edge to the center. And its effects in promoting bone regeneration were evaluated in the study.

METHODS

The gradient scaffold was designed using the triply periodic minimal surface architectures. The mechanical properties were analyzed by the finite element simulation and confirmed using the universal machine. The fluid characteristics were calculated by the computational fluid dynamics analysis. The bone regeneration process was simulated using a in silico computational model containing the main biological, physical, and chemical variation during the bone growth process. Finally, the in vitro and in vivo studies were carried out to verify the actual osteogenic effect.

RESULTS

Compared to the uniform scaffold, the biomimetic gradient scaffold demonstrated better performance in stress conduction and reduced stress shielding effects. The fluid features were appropriate for cell migration and flow diffusion, and the permeability was in the same order of magnitude with the natural bone. The bone ingrowth simulation exhibited improved angiogenesis and bone regeneration. Higher expression of the osteogenesis-related genes, higher alkaline phosphatase activity, and increased mineralization could be observed on the gradient scaffold in the in vitro study. The 12-week in vivo study proved that the gradient scaffold had deeper bone inserting depth and a more stable bone-scaffold interface.

CONCLUSION

The Haversian system-like gradient structure can effectively promote the bone regeneration. This structural design can be used as a new solution for the clinical application of prosthesis design.

Multicellular dynamics on structured surfaces: Stress concentration is a key to controlling complex microtissue morphology on engineered scaffolds

Tissue engineers have utilised a variety of three-dimensional (3D) scaffolds for controlling multicellular dynamics and the resulting tissue microstructures. In particular, cutting-edge microfabrication technologies, such as 3D bioprinting, provide increasingly complex structures. However, unpredictable microtissue detachment from scaffolds, which ruins desired tissue structures, is becoming an evident problem. To overcome this issue, we elucidated the mechanism underlying collective cellular detachment by combining a new computational simulation method with quantitative tissue-culture experiments. We first quantified the stochastic processes of cellular detachment shown by vascular smooth muscle cells on model curved scaffolds and found that microtissue morphologies vary drastically depending on cell contractility, substrate curvature, and cell-substrate adhesion strength. To explore this mechanism, we developed a new particle-based model that explicitly describes stochastic processes of multicellular dynamics, such as adhesion, rupture, and large deformation of microtissues on structured surfaces. Computational simulations using the developed model successfully reproduced characteristic detachment processes observed in experiments. Crucially, simulations revealed that cellular contractility-induced stress is locally concentrated at the cell-substrate interface, subsequently inducing a catastrophic process of collective cellular detachment, which can be suppressed by modulating cell contractility, substrate curvature, and cell-substrate adhesion. These results show that the developed computational method is useful for predicting engineered tissue dynamics as a platform for prediction-guided scaffold design. STATEMENT OF SIGNIFICANCE: Microfabrication technologies aiming to control multicellular dynamics by engineering 3D scaffolds are attracting increasing attention for modelling in cell biology and regenerative medicine. However, obtaining microtissues with the desired 3D structures is made considerably more difficult by microtissue detachments from scaffolds. This study reveals a key mechanism behind this detachment by developing a novel computational method for simulating multicellular dynamics on designed scaffolds. This method enabled us to predict microtissue dynamics on structured surfaces, based on cell mechanics, substrate geometry, and cell-substrate interaction. This study provides a platform for the physics-based design of micro-engineered scaffolds and thus contributes to prediction-guided biomaterials design in the future.

An Empirical Model Linking Physico-Chemical Biomaterial Characteristics to Intra-Oral Bone Formation

Facial trauma, bone resection due to cancer, periodontal diseases, and bone atrophy following tooth extraction often leads to alveolar bone defects that require bone regeneration in order to restore dental function. Guided bone regeneration using synthetic biomaterials has been suggested as an alternative approach to autologous bone grafts. The efficiency of bone substitute materials seems to be influenced by their physico-chemical characteristics; however, the debate is still ongoing on what constitutes optimal biomaterial characteristics. The purpose of this study was to develop an empirical model allowing the assessment of the bone regeneration potential of new biomaterials on the basis of their physico-chemical characteristics, potentially giving directions for the design of a new generation of dental biomaterials. A quantitative data set was built composed of physico-chemical characteristics of seven commercially available intra-oral bone biomaterials and their in vivo response. This empirical model allowed the identification of the construct parameters driving optimized bone formation. The presented model provides a better understanding of the influence of driving biomaterial properties in the bone healing process and can be used as a tool to design bone biomaterials with a more controlled and custom-made composition and structure, thereby facilitating and improving the clinical translation.

On shape forming by contractile filaments in the surface of growing tissues

Growing tissues are highly dynamic, and flow on sufficiently long timescales due to cell proliferation, migration, and tissue remodeling. As a consequence, growing tissues can often be approximated as viscous fluids. This means that the shape of microtissues growing in vitro is governed by their surface stress state, as in fluid droplets. Recent work showed that cells in the near-surface region of fibroblastic or osteoblastic microtissues contract with highly oriented actin filaments, thus making the surface properties highly anisotropic, in contrast to what is expected for an isotropic fluid. Here, we develop a model that includes mechanical anisotropy of the surface generated by contractile fibers and we show that mechanical equilibrium requires contractile filaments to follow geodesic lines on the surface. Constant pressure in the fluid forces these contractile filaments to be along geodesics with a constant normal curvature. We then take this into account to determine equilibrium shapes of rotationally symmetric bodies subjected to anisotropic surface stress states and derive a family of surfaces of revolution. A comparison with recently published shapes of microtissues shows that this theory accurately predicts both the surface shape and the direction of the actin filaments on the surface.

Effect of Pore Size of Porous-Structured Titanium Implants on Tendon Ingrowth

Purpose

The reconstruction of a tendon insertion on metal prostheses is a challenge in orthopedics. Of the available metal prostheses, porous metal prostheses have been shown to have better biocompatibility for tissue integration. Therefore, this study is aimed at identifying an appropriate porous structure for the reconstruction of a tendon insertion on metal prostheses.

Methods

Ti6Al4V specimens with a diamond-like porous structure with triply periodic minimal surface pore sizes of 300, 500, and 700 μm and a porosity of 58% (designated Ti300, Ti500, and Ti700, respectively) were manufactured by selective laser melting and were characterized with micro-CT and scanning electron microscopy for their porosity, pore size, and surface topography. The porous specimens were implanted into the patellar tendon of rabbits. Tendon integration was evaluated after implantation into the tendon at 4, 8, and 12 weeks by histology, and the fixation strength was evaluated with a pull-out test at week 12.

Results

The average pore sizes of the Ti300, Ti500, and Ti700 implants were 261, 480, and 668 μm, respectively. The Ti500 and Ti700 implants demonstrated better tissue growth than the Ti300 implant at weeks 4, 8, and 12. At week 12, the histological score of the Ti500 implant was 13.67 ± 0.58, and it had an area percentage of type I collagen of 63.90% ± 3.41%; both of these results were significantly higher than those for the Ti300 and Ti700 implants. The pull-out load at week 12 was also the highest in the Ti500 group.

Conclusion

Ti6Al4V implants with a diamond-like porous structure with triply periodic minimal surface pore size of 500 μm are suitable for tendon integration.

3D printing of hierarchical porous biomimetic hydroxyapatite scaffolds: Adding concavities to the convex filaments

Porosity plays a key role on the osteogenic performance of bone scaffolds. Direct Ink Writing (DIW) allows the design of customized synthetic bone grafts with patient-specific architecture and controlled macroporosity. Being an extrusion-based technique, the scaffolds obtained are formed by arrays of cylindrical filaments, and therefore have convex surfaces. This may represent a serious limitation, as the role of surface curvature and more specifically the stimulating role of concave surfaces in osteoinduction and bone growth has been recently highlighted. Hence the need to design strategies that allow the introduction of concave pores in DIW scaffolds. In the current study, we propose to add gelatin microspheres as a sacrificial material in a self-setting calcium phosphate ink. Neither the phase transformation responsible for the hardening of the scaffold nor the formation of characteristic network of needle-like hydroxyapatite crystals was affected by the addition of gelatin microspheres. The partial dissolution of the gelatin resulted in the creation of spherical pores throughout the filaments and exposed on the surface, increasing filament porosity from 0.2 % to 67.9 %. Moreover, the presence of retained gelatin proved to have a significant effect on the mechanical properties, reducing the strength but simultaneously giving the scaffolds an elastic behavior, despite the high content of ceramic as a continuous phase. Notwithstanding the inherent difficulty of in vitro cultures with this highly reactive material an enhancement of MG-63 cell proliferation, as well as better spreading of hMSCs was recorded on the developed scaffolds. STATEMENT OF SIGNIFICANCE: Recent studies have stressed the role that concave surfaces play in tissue regeneration and, more specifically, in osteoinduction and osteogenesis. Direct ink writing enables the production of patient-specific bone grafts with controlled architecture. However, besides many advantages, it has the serious limitation that the surfaces obtained are convex. In this article, for the first time we develop a strategy to introduce concave pores in the printed filaments of biomimetic hydroxyapatite by incorporation and partial dissolution of gelatin microspheres. The retention of part of the gelatin results in a more elastic behavior compared to the brittleness of hydroxyapatite scaffolds, while the needle-shaped nanostructure of biomimetic hydroxyapatite is maintained and gelatin-coated concave pores on the surface of the filaments enhance cell spreading.

Model-based data analysis of tissue growth in thin 3D printed scaffolds

Tissue growth in three-dimensional (3D) printed scaffolds enables exploration and control of cell behaviour in more biologically realistic geometries than that allowed by traditional 2D cell culture. Cell proliferation and migration in these experiments have yet to be explicitly characterised, limiting the ability of experimentalists to determine the effects of various experimental conditions, such as scaffold geometry, on cell behaviour. We consider tissue growth by osteoblastic cells in melt electro-written scaffolds that comprise thin square pores with sizes that were deliberately increased between experiments. We collect highly detailed temporal measurements of the average cell density, tissue coverage, and tissue geometry. To quantify tissue growth in terms of the underlying cell proliferation and migration processes, we introduce and calibrate a mechanistic mathematical model based on the Porous-Fisher reaction-diffusion equation. Parameter estimates and uncertainty quantification through profile likelihood analysis reveal consistency in the rate of cell proliferation and steady-state cell density between pore sizes. This analysis also serves as an important model verification tool: while the use of reaction-diffusion models in biology is widespread, the appropriateness of these models to describe tissue growth in 3D scaffolds has yet to be explored. We find that the Porous-Fisher model is able to capture features relating to the cell density and tissue coverage, but is not able to capture geometric features relating to the circularity of the tissue interface. Our analysis identifies two distinct stages of tissue growth, suggests several areas for model refinement, and provides guidance for future experimental work that explores tissue growth in 3D printed scaffolds.

A multiscale optimisation method for bone growth scaffolds based on triply periodic minimal surfaces

Tissue engineered bone scaffolds are potential alternatives to bone allografts and autografts. Porous scaffolds based on triply periodic minimal surfaces (TPMS) are good candidates for tissue growth because they offer high surface-to-volume ratio, have tailorable stiffness, and can be easily fabricated by additive manufacturing. However, the range of TPMS scaffold types is extensive, and it is not yet clear which type provides the fastest cell or tissue growth while being sufficiently stiff to act as a bone graft. Nor is there currently an established methodology for TPMS bone scaffold design which can be quickly adopted by medical designers or biologists designing implants. In this study, we examine six TPMS scaffold types for use as tissue growth scaffolds and propose a general methodology to optimise their geometry. At the macro-scale, the optimisation routine ensures a scaffold stiffness within suitable limits for bone, while at the micro-scale it maximises the cell growth rate. The optimisation procedure also ensures the scaffold pores are of sufficient diameter to allow oxygen and nutrient delivery via capillaries. Of the examined TPMS structures, the Lidinoid and Split P cell types provide the greatest cell growth rates and are therefore the best candidates for bone scaffolds.

Additively manufactured patient-specific prosthesis for tumor reconstruction: Design, process, and properties

Design and processing capabilities of additive manufacturing (AM) to fabricate complex geometries continues to drive the adoption of AM for biomedical applications. In this study, a validated design methodology is presented to evaluate AM as an effective fabrication technique for reconstruction of large bone defects after tumor resection in pediatric oncology patients. Implanting off-the-shelf components in pediatric patients is especially challenging because most standard components are sized and shaped for more common adult cases. While currently reported efforts on AM implants are focused on maxillofacial, hip and knee reconstructions, there have been no reported studies on reconstruction of proximal humerus tumors. A case study of a 9-year-old diagnosed with proximal humerus osteosarcoma was used to develop a patient-specific AM prosthesis for the humerus following tumor resection. Commonly used body-centered cubic (BCC) structures were incorporated at the surgical neck and distal interface in order to increase the effective surface area, promote osseointegration, and reduce the implant weight. A patient-specific prosthesis was fabricated using electron beam melting method from biocompatible Ti-6Al-4V. Both computational and biomechanical tests were performed on the prosthesis to evaluate its biomechanical behavior under varying loading conditions. Morphological analysis of the construct using micro-computed tomography was used to compare the as-designed and as-built prosthesis. It was found that the patient-specific prosthesis could withstand physiologically-relevant loading conditions with minimal permanent deformation (82 μm after 105 cycles) at the medial aspect of the porous surgical neck. These outcomes support potential translation of the patient-specific AM prostheses to reconstruct large bone defects following tumor resection.

Fabrication of customized Ti6AI4V heterogeneous scaffolds with selective laser melting: Optimization of the architecture for orthopedic implant applications

Orthopedic implants with heterogeneous porous structures were known as ideal bone osteointegration. This research introduced the selective laser melting (SLM), finite element analysis (FEA), and a hydrothermal process (HT) for manufacturing a three-level heterogeneous porous structure. The macroporous structure was designed via CAD and micropores were tuned via laser power regulation. A nano-size layer of hydroxyapatite crystals was coated by an HT process. The mechanical properties were reinforced via a core-shell structure with core reinforcement. The existence of micropores and nano-hydroxyapatite coating enhanced the in vitro proliferation of preosteoblasts and osteogenic cellular behaviors of rBMSCs. Thus, the three-level heterogeneous porous titanium implants could inspire researchers with potential clue of cyto-implant interaction mechanism, therefore building ideal orthopedic implants with accelerated osteointegration. STATEMENT OF SIGNIFICANCE: Porous structures of titanium implants play an important role in bone tissue regeneration; The geometrical environment influence cell behaviour and bone tissue ingrowth in all macro-/micro-/nanoscale. In this study, a novel method to fabricate heterogeneous scaffolds and its macro-/micro-/nanoscopic structures were studied. A CAD model was used to obtain the macroscopic structure and the insufficient laser power was introduced for porous microstructure. Therefore, a layer of nano hydroxyapatite was coated via hydrothermal process. Cytoproliferation and cytodifferentiation results indicated that a integrity of regular/irregular, macro-/micro-/nanoscale porous structure had advance in recruiting stem cells and promoting differentiation. This research is beneficial to the development of bone implants with better bone regeneration ability.

Modelling cell guidance and curvature control in evolving biological tissues

Tissue geometry is an important influence on the evolution of many biological tissues. The local curvature of an evolving tissue induces tissue crowding or spreading, which leads to differential tissue growth rates, and to changes in cellular tension, which can influence cell behaviour. Here, we investigate how directed cell motion interacts with curvature control in evolving biological tissues. Directed cell motion is involved in the generation of angled tissue growth and anisotropic tissue material properties, such as tissue fibre orientation. We develop a new cell-based mathematical model of tissue growth that includes both curvature control and cell guidance mechanisms to investigate their interplay. The model is based on conservation principles applied to the density of tissue synthesising cells at or near the tissue's moving boundary. The resulting mathematical model is a partial differential equation for cell density on a moving boundary, which is solved numerically using a hybrid front-tracking method called the cell-based particle method. The inclusion of directed cell motion allows us to model new types of biological growth, where tangential cell motion is important for the evolution of the interface, or for the generation of anisotropic tissue properties. We illustrate such situations by applying the model to simulate both the resorption and infilling components of the bone remodelling process, and to simulate root hair growth. We also provide user-friendly MATLAB code to implement the algorithms.

Customized additive manufacturing of porous Ti6Al4V scaffold with micro-topological structures to regulate cell behavior in bone tissue engineering

Scaffold micro-topological structure plays an important role in the regulation of cell behavior in bone tissue engineering. This paper investigated the effect of 3D printing parameters on the scaffold micro-topological structure and its subsequent cell behaviors. By setting of different 3D printing parameters, i.e., the 3D printing laser power, the scanning interval and the thickness of sliced layers, the highest resolution up to 20 μm can be precisely fabricated. Scaffolds' characterization results indicated that the laser power affected the forming quality of melt tracks, the scanning interval distance determined the size of regularly arranged pores, and the thickness of sliced layers affected the morphological and structural characteristics. By regulating of these printing parameters, customized porous Ti6Al4V scaffold with varied hierarchical micro-topological structure can be obtained. In vitro cell culturing results showed that the regular porous micro-topological structure of scaffolds with the aperture close to cell size was more suitable for cell proliferation and adhesion. The overall distribution of cells on regular porous scaffolds was similar to the orderly arrangement of cultivated crops in the field. The findings suggested that customization of the scaffold provided an effective way to regulate cellular behavior and biological properties.

Additive manufacturing pertaining to bone: Hopes, reality and future challenges for clinical applications

For the past 20 years, the democratization of additive manufacturing (AM) technologies has made many of us dream of: low cost, waste-free, and on-demand production of functional parts; fully customized tools; designs limited by imagination only, etc. As every patient is unique, the potential of AM for the medical field is thought to be considerable: AM would allow the division of dedicated patient-specific healthcare solutions entirely adapted to the patients' clinical needs. Pertinently, this review offers an extensive overview of bone-related clinical applications of AM and ongoing research trends, from 3D anatomical models for patient and student education to ephemeral structures supporting and promoting bone regeneration. Today, AM has undoubtably improved patient care and should facilitate many more improvements in the near future. However, despite extensive research, AM-based strategies for bone regeneration remain the only bone-related field without compelling clinical proof of concept to date. This may be due to a lack of understanding of the biological mechanisms guiding and promoting bone formation and due to the traditional top-down strategies devised to solve clinical issues. Indeed, the integrated holistic approach recommended for the design of regenerative systems (i.e., fixation systems and scaffolds) has remained at the conceptual state. Challenged by these issues, a slower but incremental research dynamic has occurred for the last few years, and recent progress suggests notable improvement in the years to come, with in view the development of safe, robust and standardized patient-specific clinical solutions for the regeneration of large bone defects.

3D printed titanium scaffolds with homogeneous diamond-like structures mimicking that of the osteocyte microenvironment and its bone regeneration study

Biofabrication of personalized titanium scaffold mimicking that of the osteocyte microenvironment is challenging due to its complex geometrical cues. The effect of scaffolds geometrical cues and implantation sites on osteogenesis is still not clear. In this study, personalized titanium scaffolds with homogeneous diamond-like structures mimicking that of the osteocyte microenvironment were precisely designed and fabricated by selected laser melting method. The effects of different geometric cues, including porosity, pore sizes and interconnection properties, on cellular behavior were investigated. Biomimetic mechanical properties of porous titanium alloy scaffold were predesigned and simulated by finite element analysis. In vitro experiment revealed that homogeneous diamond-like structures mimicking that of the osteocyte microenvironment triggered osteocyte adhesion and migration behavior. Typical implantation sites, including rabbit femur, beagle femur, and beagle skull, were used to study the implantation sites effects on bone regeneration. In vivo experimental results indicated that different implantation sites showed significant differences. This study helps to understand the scaffolds geometrical microenvironment and implantation sites effects on osteogenesis mechanism. And it is beneficial to the development of bone implants with better bone regeneration ability.

3D Bone Morphology Alters Gene Expression, Motility, and Drug Responses in Bone Metastatic Tumor Cells

Patients with advanced skeletal metastases arising from primary cancers including breast, lung, and prostate suffer from extreme pain, bone loss, and frequent fractures. While the importance of interactions between bone and tumors is well-established, our understanding of complex cell–cell and cell–microenvironment interactions remains limited in part due to a lack of appropriate 3D bone models. To improve our understanding of the influence of bone morphometric properties on the regulation of tumor-induced bone disease (TIBD), we utilized bone-like 3D scaffolds in vitro and in vivo. Scaffolds were seeded with tumor cells, and changes in cell motility, proliferation, and gene expression were measured. Genes associated with TIBD significantly increased with increasing scaffold rigidity. Drug response differed when tumors were cultured in 3D compared to 2D. Inhibitors for Integrin β3 and TGF-β Receptor II significantly reduced bone-metastatic gene expression in 2D but not 3D, while treatment with the Gli antagonist GANT58 significantly reduced gene expression in both 2D and 3D. When tumor-seeded 3D scaffolds were implanted into mice, infiltration of myeloid progenitors changed in response to pore size and rigidity. This study demonstrates a versatile 3D model of bone used to study the influence of mechanical and morphometric properties of bone on TIBD.

Cell proliferation and migration explain pore bridging dynamics in 3D printed scaffolds of different pore size

Tissue growth in bioscaffolds is influenced significantly by pore geometry, but how this geometric dependence emerges from dynamic cellular processes such as cell proliferation and cell migration remains poorly understood. Here we investigate the influence of pore size on the time required to bridge pores in thin 3D-printed scaffolds. Experimentally, new tissue infills the pores continually from their perimeter under strong curvature control, which leads the tissue front to round off with time. Despite the varied shapes assumed by the tissue during this evolution, we find that time to bridge a pore simply increases linearly with the overall pore size. To disentangle the biological influence of cell behaviour and the mechanistic influence of geometry in this experimental observation, we propose a simple reaction-diffusion model of tissue growth based on Porous-Fisher invasion of cells into the pores. First, this model provides a good qualitative representation of the evolution of the tissue; new tissue in the model grows at an effective rate that depends on the local curvature of the tissue substrate. Second, the model suggests that a linear dependence of bridging time with pore size arises due to geometric reasons alone, not to differences in cell behaviours across pores of different sizes. Our analysis suggests that tissue growth dynamics in these experimental constructs is dominated by mechanistic crowding effects that influence collective cell proliferation and migration processes, and that can be predicted by simple reaction-diffusion models of cells that have robust, consistent behaviours.

Relationship between the morphological, mechanical and permeability properties of porous bone scaffolds and the underlying microstructure

Bone scaffolds are widely used as one of the main bone substitute materials. However, many bone scaffold microstructure topologies exist and it is still unclear which topology to use when designing scaffold for a specific application. The aim of the present study was to reveal the mechanism of the microstructure-driven performance of bone scaffold and thus to provide guideline on scaffold design. Finite element (FE) models of five TPMS (Diamond, Gyroid, Schwarz P, Fischer-Koch S and F-RD) and three traditional (Cube, FD-Cube and Octa) scaffolds were generated. The effective compressive and shear moduli of scaffolds were calculated from the mechanical analysis using the FE unit cell models with the periodic boundary condition. The scaffold permeability was calculated from the computational fluid dynamics (CFD) analysis using the 4×4×4 FE models. It is revealed that the surface-to-volume ratio of the Fischer-Koch S-based scaffold is the highest among the scaffolds investigated. The mechanical analysis revealed that the bending deformation dominated structures (e.g., the Diamond, the Gyroid, the Schwarz P) have higher effective shear moduli. The stretching deformation dominated structures (e.g., the Schwarz P, the Cube) have higher effective compressive moduli. For all the scaffolds, when the same amount of change in scaffold porosity is made, the corresponding change in the scaffold relative shear modulus is larger than that in the relative compressive modulus. The CFD analysis revealed that the structures with the simple and straight pores (e.g., Cube) have higher permeability than the structures with the complex pores (e.g., Fischer-Koch S). The main contribution of the present study is that the relationship between scaffold properties and the underlying microstructure is systematically investigated and thus some guidelines on the design of bone scaffolds are provided, for example, in the scenario where a high surface-to-volume ratio is required, it is suggested to use the Fischer-Koch S based scaffold.

Novel Extrusion-Microdrilling Approach to Fabricate Calcium Phosphate-Based Bioceramic Scaffolds Enabling Fast Bone Regeneration

This study proposes a novel approach, termed extrusion-microdrilling, to fabricate three-dimensional (3D) interconnected bioceramic scaffolds with channel-like macropores for bone regeneration. The extrusion-microdrilling method is characterized by ease of use, high efficiency, structural flexibility, and precision. The 3D interconnected β-tricalcium phosphate bioceramic (EM-TCP) scaffolds prepared by this method showed channel-like square macropores (∼650 μm) by extrusion and channel-like round macropores (∼570 μm) by microdrilling as well as copious micropores. By incorporating a strontium-containing phosphate-based glass (SrPG), the obtained calcium phosphate-based bioceramic (EM-TCP/SrPG) scaffolds had noticeably higher compressive strength, lower porosity, and smaller macropore size, tremendously enhanced in vitro proliferation and osteogenic differentiation of mouse bone marrow stromal cells, and suppressed in vitro osteoclastic activities of RAW264.7 cells, as compared with the EM-TCP scaffolds. In vivo assessment results indicated that at postoperative week 6, new vessels and a large percentage of new bone tissues (24-25%) were formed throughout the interconnected macropores of EM-TCP and EM-TCP/SrPG, which were implanted in the femoral defects of rabbits; the bone formation of the EM-TCP group was comparable to that of the EM-TCP/SrPG group. At 12 weeks postimplantation, the bone formation percentage of EM-TCP was slightly reduced, while that of EM-TCP/SrPG with a slower degradation rate was pronouncedly increased. This work provides a new strategy to fabricate interconnected bioceramic scaffolds allowing for fast bone regeneration, and the EM-TCP/SrPG scaffolds are promising for efficiently repairing bone defects.

Substrate curvature as a cue to guide spatiotemporal cell and tissue organization

Recent evidence clearly shows that cells respond to various physical cues in their environments, guiding many cellular processes and tissue morphogenesis, pathology, and repair. One aspect that is gaining significant traction is the role of local geometry as an extracellular cue. Elucidating how geometry affects cell and tissue behavior is, indeed, crucial to design artificial scaffolds and understand tissue growth and remodeling. Perhaps the most fundamental descriptor of local geometry is surface curvature, and a growing body of evidence confirms that surface curvature affects the spatiotemporal organization of cells and tissues. While well-defined in differential geometry, curvature remains somewhat ambiguously treated in biological studies. Here, we provide a more formal curvature framework, based on the notions of mean and Gaussian curvature, and summarize the available evidence on curvature guidance at the cell and tissue levels. We discuss the involved mechanisms, highlighting the interplay between tensile forces and substrate curvature that forms the foundation of curvature guidance. Moreover, we show that relatively simple computational models, based on some application of curvature flow, are able to capture experimental tissue growth remarkably well. Since curvature guidance principles could be leveraged for tissue regeneration, the implications for geometrical scaffold design are also discussed. Finally, perspectives on future research opportunities are provided.

Early osteointegration evaluation of porous Ti6Al4V scaffolds designed based on triply periodic minimal surface models

BACKGROUND

The graded porous structures were designed using triply periodic minimal surfaces models to mimic the biomechanical properties of bone. The mechanical properties and bone formation ability were evaluated to explore the feasibility of the design method in bone tissue engineering.

METHODS

The scaffolds were designed using a P-surface with different pore sizes. All materials were fabricated using 3D printing technology and the mechanical properties were tested by an electronic universal testing device. The biomechanical properties were then analyzed by finite element method, while the ontogenesis of the material in vivo was examined by implanting the scaffolds for five weeks in pigs.

RESULTS

According to the obtained results, the pore size ranged between 100 μm to about 700 μm and porosity were around 49.54%. The graded porous architectures can decrease the stiffness of implants and reduce the stress shielding effect. In addition, these porous structures can stimulate bone ingrowth and achieve a stable interface between implants and surrounding bone tissues after 5 weeks' implantation. The micro-CT results also demonstrated the obviously bone formation around all the porous structures.

CONCLUSION

To sum up, the triply periodic minimal surfaces based graded porous structure is effective in decreasing the stress shielding effect, promoting early osteogenesis and osteointegration. This is the first research to explore the effect of this kind of porous structures on bone formation in vivo where the obtained results supported the previous theoretical research on the application potential in bone tissue engineering.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Porous architecture designed using triply periodic minimal surface models can achieve gradually changed pore size and appropriate porosity for bone regeneration. This kind of structure can mimic the Young's modulus of natural bone tissue, improve the stress transmission capability and dismiss the stress shielding effect. It also can stimulate the early bone integration in vivo and enhance the binding force between bone and implants, which may bring a new design method for orthopaedic implants and their surface structure.

Engineering vasculature: Architectural effects on microcapillary-like structure self-assembly

One of the greatest obstacles to clinical translation of bone tissue engineering is the inability to effectively and efficiently vascularize scaffolds. The goal of this work was to explore systematically whether architecture, at a scale of hundreds of microns, can be used to direct the growth of microcapillary-like structures into the core of scaffolds. Biphasic bioceramic patterned architectures were produced using silicone molds of 3D printed parts. Grooves and ridges were designed to have widths of 330 μm and 660 μm, with periodicities respectively of 1240 μm and 630 μm. Groove depth was varied between 150 μm and 585 μm. Co-cultures of human dermal microvascular endothelial cells (HDMECs) and human osteoblasts (hOBs) were used to grow microcapillary-like structures on substrates. Bioceramic architecture was found to significantly affect microcapillary-like structure location and orientation. Microcapillary-like structures were found to form predominantly in grooves or between convexities. For all patterned samples, the CD31 (endothelial cell marker) signal was at least 2.5 times higher along grooves versus perpendicular to grooves. In addition, the average signal was at least two times higher within grooves than outside grooves for all samples. Grooves with a width of 330 μm and a depth of 300 μm resulted in the formation of individual, highly aligned microcapillary-like structures with lengths around 5 mm. Extensive literature has focused on the role of nano- and micro-topography (on the scale below tens of microns) on cellular response. However, the idea that architecture at a scale much larger than a cell could be used to modulate angiogenesis has not been systematically investigated. This work shows the crucial influence of architecture on microcapillary-like structure self-assembly at the scale of hundreds of microns. Elucidating the precise correspondence between architecture and microcapillary-like structure organization will ultimately allow the engineering of microvasculature by tuning local scaffold design to achieve desirable microvessel properties.

Additively manufactured porous metallic biomaterials

Additively manufactured (AM, =3D printed) porous metallic biomaterials with topologically ordered unit cells have created a lot of excitement and are currently receiving a lot of attention given their great potential for improving bone tissue regeneration and preventing implant-associated infections.

3D bone models to study the complex physical and cellular interactions between tumor and the bone microenvironment

As the complexity of interactions between tumor and its microenvironment has become more evident, a critical need to engineer in vitro models that veritably recapitulate the 3D microenvironment and relevant cell populations has arisen. This need has caused many groups to move away from the traditional 2D, tissue culture plastic paradigms in favor of 3D models with materials that more closely replicate the in vivo milieu. Creating these 3D models remains a difficult endeavor for hard and soft tissues alike as the selection of materials, fabrication processes, and optimal conditions for supporting multiple cell populations makes model development a nontrivial task. Bone tissue in particular is uniquely difficult to model in part because of the limited availability of materials that can accurately capture bone rigidity and architecture, and also due to the dependence of both bone and tumor cell behavior on mechanical signaling. Additionally, the bone is a complex cellular microenvironment with multiple cell types present, including relatively immature, pluripotent cells in the bone marrow. This prospect will focus on the current 3D models in development to more accurately replicate the bone microenvironment, which will help facilitate improved understanding of bone turnover, tumor-bone interactions, and drug response. These studies have demonstrated the importance of accurately modelling the bone microenvironment in order to fully understand signaling and drug response, and the significant effects that model properties such as architecture, rigidity, and dynamic mechanical factors have on tumor and bone cell response.

The influence of curvature on three-dimensional mineralized matrix formation under static and perfused conditions: an in vitro bioreactor model

Bone remodelling is the continuous turnover of bone by resorption and formation. It is controlled by interstitial fluid flow sensed by osteocytes. The refilling of bone resorption sites has been shown to be curvature driven. In vitro, curvature influences tissue growth and cytoskeletal arrangements under static and perfused conditions. Nevertheless, this has only been demonstrated for non-mineralized tissue in limited three-dimensional volumes. This study aims at investigating the influence of three different channel curvatures (S, -2.00 mm^-1; M, -1.33 mm^-1; L, -0.67 mm^-1) on mineralized tissue formation in three dimensions under static and perfused conditions. The ingrowth of mineralized tissue into the channels was dependent on curvature and was higher under perfusion (M and S channels). L channels were not closed in any group compared with partially (static) or fully (perfused) closed M and S channels. Mineralized tissue morphology was cortical-like in static samples and trabecular-like in perfused samples. Our results suggest that the three-dimensional in vitro model presented is not only able to reveal effects of curvature on mineralized tissue formation, but may be used as an in vitro model for critical size defects in trabecular or cortical bone.

Gradual conversion of cellular stress patterns into pre-stressed matrix architecture during in vitro tissue growth

The complex arrangement of the extracellular matrix (ECM) produced by cells during tissue growth, healing and remodelling is fundamental to tissue function. In connective tissues, it is still unclear how both cells and the ECM become and remain organized over length scales much larger than the distance between neighbouring cells. While cytoskeletal forces are essential for assembly and organization of the early ECM, how these processes lead to a highly organized ECM in tissues such as osteoid is not clear. To clarify the role of cellular tension for the development of these ordered fibril architectures, we used an in vitro model system, where pre-osteoblastic cells produced ECM-rich tissue inside channels with millimetre-sized triangular cross sections in ceramic scaffolds. Our results suggest a mechanical handshake between actively contracting cells and ECM fibrils: the build-up of a long-range organization of cells and the ECM enables a gradual conversion of cell-generated tension to pre-straining the ECM fibrils, which reduces the work cells have to generate to keep mature tissue under tension.

Engineering 3D Models of Tumors and Bone to Understand Tumor-Induced Bone Disease and Improve Treatments

PURPOSE OF REVIEW

Bone is a structurally unique microenvironment that presents many challenges for the development of 3D models for studying bone physiology and diseases, including cancer. As researchers continue to investigate the interactions within the bone microenvironment, the development of 3D models of bone has become critical.

RECENT FINDINGS

3D models have been developed that replicate some properties of bone, but have not fully reproduced the complex structural and cellular composition of the bone microenvironment. This review will discuss 3D models including polyurethane, silk, and collagen scaffolds that have been developed to study tumor-induced bone disease. In addition, we discuss 3D printing techniques used to better replicate the structure of bone. 3D models that better replicate the bone microenvironment will help researchers better understand the dynamic interactions between tumors and the bone microenvironment, ultimately leading to better models for testing therapeutics and predicting patient outcomes.

Modeling the Effect of Curvature on the Collective Behavior of Cells Growing New Tissue

The growth of several biological tissues is known to be controlled in part by local geometrical features, such as the curvature of the tissue interface. This control leads to changes in tissue shape that in turn can affect the tissue's evolution. Understanding the cellular basis of this control is highly significant for bioscaffold tissue engineering, the evolution of bone microarchitecture, wound healing, and tumor growth. Although previous models have proposed geometrical relationships between tissue growth and curvature, the role of cell density and cell vigor remains poorly understood. We propose a cell-based mathematical model of tissue growth to investigate the systematic influence of curvature on the collective crowding or spreading of tissue-synthesizing cells induced by changes in local tissue surface area during the motion of the interface. Depending on the strength of diffusive damping, the model exhibits complex growth patterns such as undulating motion, efficient smoothing of irregularities, and the generation of cusps. We compare this model with in vitro experiments of tissue deposition in bioscaffolds of different geometries. By including the depletion of active cells, the model is able to capture both smoothing of initial substrate geometry and tissue deposition slowdown as observed experimentally.

Additively manufactured metallic porous biomaterials based on minimal surfaces: A unique combination of topological, mechanical, and mass transport properties

Porous biomaterials that simultaneously mimic the topological, mechanical, and mass transport properties of bone are in great demand but are rarely found in the literature. In this study, we rationally designed and additively manufactured (AM) porous metallic biomaterials based on four different types of triply periodic minimal surfaces (TPMS) that mimic the properties of bone to an unprecedented level of multi-physics detail. Sixteen different types of porous biomaterials were rationally designed and fabricated using selective laser melting (SLM) from a titanium alloy (Ti-6Al-4V). The topology, quasi-static mechanical properties, fatigue resistance, and permeability of the developed biomaterials were then characterized. In terms of topology, the biomaterials resembled the morphological properties of trabecular bone including mean surface curvatures close to zero. The biomaterials showed a favorable but rare combination of relatively low elastic properties in the range of those observed for trabecular bone and high yield strengths exceeding those reported for cortical bone. This combination allows for simultaneously avoiding stress shielding, while providing ample mechanical support for bone tissue regeneration and osseointegration. Furthermore, as opposed to other AM porous biomaterials developed to date for which the fatigue endurance limit has been found to be ≈20% of their yield (or plateau) stress, some of the biomaterials developed in the current study show extremely high fatigue resistance with endurance limits up to 60% of their yield stress. It was also found that the permeability values measured for the developed biomaterials were in the range of values reported for trabecular bone. In summary, the developed porous metallic biomaterials based on TPMS mimic the topological, mechanical, and physical properties of trabecular bone to a great degree. These properties make them potential candidates to be applied as parts of orthopedic implants and/or as bone-substituting biomaterials.

STATEMENT OF SIGNIFICANCE

Bone-substituting biomaterials aim to mimic bone properties. Although mimicking some of bone properties is feasible, biomaterials that could simultaneously mimic all or most of the relevant bone properties are rare. We used rational design and additive manufacturing to develop porous metallic biomaterials that exhibit an interesting combination of topological, mechanical, and mass transport properties. The topology of the developed biomaterials resembles that of trabecular bone including a mean curvature close to zero. Moreover, the developed biomaterials show an unusual combination of low elastic modulus to avoid stress shielding and high strength to provide mechanical support. The fatigue resistance of the developed biomaterials is also exceptionally high, while their permeability is in the range of values reported for bone.

Additively Manufactured and Surface Biofunctionalized Porous Nitinol

Enhanced bone tissue regeneration and improved osseointegration are among the most important goals in design of multifunctional orthopedic biomaterials. In this study, we used additive manufacturing (selective laser melting) to develop multifunctional porous nitinol that combines superelasticity with a rationally designed microarchitecture and biofunctionalized surface. The rational design based on triply periodic minimal surfaces aimed to properly adjust the pore size, increase the surface area (thereby amplifying the effects of surface biofunctionalization), and resemble the curvature characteristics of trabecular bone. The surface of additively manufactured (AM) porous nitinol was biofunctionalized using polydopamine-immobilized rhBMP2 for better control of the release kinetics. The actual morphological properties of porous nitinol measured by microcomputed tomography (e.g., open/close porosity, and surface area) closely matched the design values. The superelasticity originated from the austenite phase formed in the nitinol porous structure at room temperature. Polydopamine and rhBMP2 signature peaks were confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy tests. The release of rhBMP2 continued until 28 days. The early time and long-term release profiles were found to be adjustable independent of each other. In vitro cell culture showed improved cell attachment, cell proliferation, cell morphology (spreading, spindle-like shape), and cell coverage as well as elevated levels of ALP activity and increased calcium content for biofunctionalized surfaces as compared to as-manufactured specimens. The demonstrated functionalities of porous nitinol could be used as a basis for deployable orthopedic implants with rationally designed microarchitectures that maximize bone tissue regeneration performance by release of biomolecules with adjustable and well-controlled release profiles.

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.

Effect of Ceramic Scaffold Architectural Parameters on Biological Response

Numerous studies have focused on the optimization of ceramic architectures to fulfill a variety of scaffold functional requirements and improve biological response. Conventional fabrication techniques, however, do not allow for the production of geometrically controlled, reproducible structures and often fail to allow the independent variation of individual geometric parameters. Current developments in additive manufacturing technologies suggest that 3D printing will allow a more controlled and systematic exploration of scaffold architectures. This more direct translation of design into structure requires a pipeline for design-driven optimization. A theoretical framework for systematic design and evaluation of architectural parameters on biological response is presented. Four levels of architecture are considered, namely (1) surface topography, (2) pore size and geometry, (3) porous networks, and (4) macroscopic pore arrangement, including the potential for spatially varied architectures. Studies exploring the effect of various parameters within these levels are reviewed. This framework will hopefully allow uncovering of new relationships between architecture and biological response in a more systematic way as well as inform future refinement of fabrication techniques to fulfill architectural necessities with a consideration of biological implications.

Tissue growth controlled by geometric boundary conditions: a simple model recapitulating aspects of callus formation and bone healing

The shape of tissues arises from a subtle interplay between biochemical driving forces, leading to cell growth, division and extracellular matrix formation, and the physical constraints of the surrounding environment, giving rise to mechanical signals for the cells. Despite the inherent complexity of such systems, much can still be learnt by treating tissues that constantly remodel as simple fluids. In this approach, remodelling relaxes all internal stresses except for the pressure which is counterbalanced by the surface stress. Our model is used to investigate how wettable substrates influence the stability of tissue nodules. It turns out for a growing tissue nodule in free space, the model predicts only two states: either the tissue shrinks and disappears, or it keeps growing indefinitely. However, as soon as the tissue wets a substrate, stable equilibrium configurations become possible. Furthermore, by investigating more complex substrate geometries, such as tissue growing at the end of a hollow cylinder, we see features reminiscent of healing processes in long bones, such as the existence of a critical gap size above which healing does not occur. Despite its simplicity, the model may be useful in describing various aspects related to tissue growth, including biofilm formation and cancer metastases.

Bone tissue regeneration: the role of scaffold geometry

A review of how the geometrical design of scaffolds influences the bone tissue regeneration process.

Morphological patterns of a growing biological tube in a confined environment with contacting boundary

Morphological transition from creasing to detachment of a growing biological tube in a confined environment with the contacting boundary.