Model-Based Design to Enhance Neotissue Formation in Additively Manufactured Calcium-Phosphate-Based Scaffolds

In biomaterial-based bone tissue engineering, optimizing scaffold structure and composition remains an active field of research. Additive manufacturing has enabled the production of custom designs in a variety of materials. This study aims to improve the design of calcium-phosphate-based additively manufactured scaffolds, the material of choice in oral bone regeneration, by using a combination of in silico and in vitro tools. Computer models are increasingly used to assist in design optimization by providing a rational way of merging different requirements into a single design. The starting point for this study was an in-house developed in silico model describing the in vitro formation of neotissue, i.e., cells and the extracellular matrix they produced. The level set method was applied to simulate the interface between the neotissue and the void space inside the scaffold pores. In order to calibrate the model, a custom disk-shaped scaffold was produced with prismatic canals of different geometries (circle, hexagon, square, triangle) and inner diameters (0.5 mm, 0.7 mm, 1 mm, 2 mm). The disks were produced with three biomaterials (hydroxyapatite, tricalcium phosphate, and a blend of both). After seeding with skeletal progenitor cells and a cell culture for up to 21 days, the extent of neotissue growth in the disks' canals was analyzed using fluorescence microscopy. The results clearly demonstrated that in the presence of calcium-phosphate-based materials, the curvature-based growth principle was maintained. Bayesian optimization was used to determine the model parameters for the different biomaterials used. Subsequently, the calibrated model was used to predict neotissue growth in a 3D gyroid structure. The predicted results were in line with the experimentally obtained ones, demonstrating the potential of the calibrated model to be used as a tool in the design and optimization of 3D-printed calcium-phosphate-based biomaterials for bone regeneration.

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.

Bone intrinsic material and compositional properties in postmenopausal women diagnosed with long-term Type-1 diabetes

The incidence of diabetes mellitus and the associated complications are growing worldwide, affecting the patients' quality of life and exerting a considerable burden on health systems. Yet, the increase in fracture risk in type 1 diabetes (T1D) patients is not fully captured by bone mineral density (BMD), leading to the hypothesis that alterations in bone quality are responsible for the increased risk. Material/compositional properties are important aspects of bone quality, yet information on human bone material/compositional properties in T1D is rather sparse. The purpose of the present study is to measure both the intrinsic material behaviour by nanoindentation, and material compositional properties by Raman spectroscopy as a function of tissue age and microanatomical location (cement lines) in bone tissue from iliac crest biopsies from postmenopausal women diagnosed with long-term T1D (N = 8), and appropriate sex-, age-, BMD- and clinically-matched controls (postmenopausal women; N = 5). The results suggest elevation of advanced glycation endproducts (AGE) content in the T1D and show significant differences in mineral maturity / crystallinity (MMC) and glycosaminoglycan (GAG) content between the T1D and control groups. Furthermore, both hardness and modulus by nanoindentation are greater in T1D. These data suggest a significant deterioration of material strength properties (toughness) and compositional properties in T1D compared with controls.

Characterization of three-dimensional bone-like tissue growth and organization under influence of directional fluid flow

The transition in the field of bone tissue engineering from bone regeneration to in vitro models has come with the challenge of recreating a dense and anisotropic bone-like extracellular matrix (ECM). Although the mechanism by which bone ECM gains its structure is not fully understood, mechanical loading and curvature have been identified as potential contributors. Here, guided by computational simulations, we evaluated cell and bone-like tissue growth and organization in a concave channel with and without directional fluid flow stimulation. Human mesenchymal stromal cells were seeded on donut-shaped silk fibroin scaffolds and osteogenically stimulated for 42 days statically or in a flow perfusion bioreactor. After 14, 28, and 42 days, constructs were investigated for cell and tissue growth and organization. As a result, directional fluid flow was able to improve organic tissue growth but not organization. Cells tended to orient in the tangential direction of the channel, possibly attributed to its curvature. Based on our results, we suggest that organic ECM production but not anisotropy can be stimulated through the application of fluid flow. With this study, an initial attempt in three-dimensions was made to improve the resemblance of in vitro produced bone-like ECM to the physiological bone ECM.

Effects of Elasticity on Cell Proliferation in a Tissue-Engineering Scaffold Pore

Scaffolds engineered for in vitro tissue engineering consist of multiple pores where cells can migrate along with nutrient-rich culture medium. The presence of the nutrient medium throughout the scaffold pores promotes cell proliferation, and this process depends on several factors such as scaffold geometry, nutrient medium flow rate, shear stress, cell-scaffold focal adhesions and elastic properties of the scaffold material. While numerous studies have addressed the first four factors, the mathematical approach described herein focuses on cell proliferation rate in elastic scaffolds, under constant flux of nutrients. As cells proliferate, the scaffold pores radius shrinks and thus, in order to sustain the nutrient flux, the inlet applied pressure on the upstream side of the scaffold pore must be increased. This results in expansion of the elastic scaffold pore, which in turn further increases the rate of cell proliferation. Considering the elasticity of the scaffold, the pore deformation allows further cellular growth beyond that of inelastic conditions. In this paper, our objectives are as follows: (i) Develop a mathematical model for describing fluid dynamics, scaffold elasticity and cell proliferation for scaffolds consist of identical nearly cylindrical pores; (ii) Solve the models and then simulate cellular proliferation within an elastic pore. The simulation can emulate real life tissue growth in a scaffold and offer a solution which reduces the numerical burdens. Lastly, our results demonstrated are in qualitative agreement with experimental observations reported in the literature.

Bone quality in an ovariectomized monkey animal model treated with two doses of teriparatide for either 18 months, or 12 months followed by withdrawal for 6 months

Previous studies of ovariectomized (OVX) monkeys, treated with recombinant human parathyroid hormone (PTH) (1-34) at 1 or 5 μg/kg/day for 18 months or for 12 months followed by 6 months withdrawal from treatment, displayed significant changes in geometry, histomorphometry, and bone quality, but without strict tissue age criteria, of the midshaft humerus. Since bone quality significantly depends on tissue age among other factors, the aim of the present study was to establish the bone-turnover independent effects of two doses of PTH, as well as the effects of treatment withdrawal on bone quality by measuring bone material composition at precisely known tissue ages ranging from osteoid, to mineralized tissue older than 373 days. Raman microspectroscopic analysis of bone tissue from the mid-shaft humerus of OVX monkeys demonstrated that the clinically relevant dose of PTH administered for 18 months reverses the effects of ovariectomy on bone quality when compared against SHAM. Both doses investigated in this study restore the mineralization regulation mechanisms to SHAM levels. The study also showed that the beneficial effects induced by 12 months of clinically relevant PTH therapy were sustained after six months of therapy withdrawal.

A quantitative analysis of cell bridging kinetics on a scaffold using computer vision algorithms

Tissue engineering involves the seeding of cells into a structural scaffolding to regenerate the architecture of damaged or diseased tissue. To effectively design a scaffold, an understanding of how cells collectively sense and react to the geometry of their local environment is needed. Advances in the development of melt electro-writing have allowed micron and submicron polymeric fibres to be accurately printed into porous, complex and three-dimensional structures. By using melt electrowriting, we created a geometrically relevant in vitro scaffold model to study cellular spatial-temporal kinetics. These scaffolds were paired with custom computer vision algorithms to investigate cell nuclei, cell membrane actin and scaffold fibres over different pore sizes (200-600 µm) and time points (28 days). We find that cells proliferated much faster in the smaller (200 µm) pores which halved the time until confluence versus larger (500 and 600 µm) pores. Our analysis of stained actin fibres revealed that cells were highly aligned to the fibres and the leading edge of the pore filling front, and we found that cells behind the leading edge were not aligned in any particular direction. This study provides a systematic understanding of cellular spatial temporal kinetics within a 3D in vitro model to inform the design of more effective synthetic tissue engineering scaffolds for tissue regeneration. STATEMENT OF SIGNIFICANCE: Advances in the development of melt electro-writing have allowed micron and submicron polymeric fibres to be accurately printed into porous, complex and three-dimensional structures. By using melt electrowriting, we created a geometrically relevant in vitro model to study cellular spatial-temporal kinetics to provide a systematic understanding of cellular spatial temporal kinetics within a 3D in vitro model. The insights presented in this work help to inform the design of more effective synthetic tissue engineering scaffolds by reducing cell culture time; which is valuable information for the implant or lab-grown-meat industries.

Mesenchymal Stem Cells, Bioactive Factors, and Scaffolds in Bone Repair: From Research Perspectives to Clinical Practice

Mesenchymal stem cell-based therapies are promising tools for bone tissue regeneration. However, tracking cells and maintaining them in the site of injury is difficult. A potential solution is to seed the cells onto a biocompatible scaffold. Construct development in bone tissue engineering is a complex step-by-step process with many variables to be optimized, such as stem cell source, osteogenic molecular factors, scaffold design, and an appropriate in vivo animal model. In this review, an MSC-based tissue engineering approach for bone repair is reported. Firstly, MSC role in bone formation and regeneration is detailed. Secondly, MSC-based bone tissue biomaterial design is analyzed from a research perspective. Finally, examples of animal preclinical and human clinical trials involving MSCs and scaffolds in bone repair are presented.

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.

Recent Developments in Polyurethane-Based Materials for Bone Tissue Engineering

To meet the needs of clinical medicine, bone tissue engineering is developing dynamically. Scaffolds for bone healing might be used as solid, preformed scaffolding materials, or through the injection of a solidifiable precursor into the defective tissue. There are miscellaneous biomaterials used to stimulate bone repair including ceramics, metals, naturally derived polymers, synthetic polymers, and other biocompatible substances. Combining ceramics and metals or polymers holds promise for future cures as the materials complement each other. Further research must explain the limitations of the size of the defects of each scaffold, and additionally, check the possibility of regeneration after implantation and resistance to disease. Before tissue engineering, a lot of bone defects were treated with autogenous bone grafts. Biodegradable polymers are widely applied as porous scaffolds in bone tissue engineering. The most valuable features of biodegradable polyurethanes are good biocompatibility, bioactivity, bioconductivity, and injectability. They may also be used as temporary extracellular matrix (ECM) in bone tissue healing and regeneration. Herein, the current state concerning polyurethanes in bone tissue engineering are discussed and introduced, as well as future trends.

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.

Degradable poly(ester-ether) urethanes of improved surface calcium deposition developed as novel biomaterials

Bones, which are considered as hard tissues, work as scaffold for human body. They provide physical support for muscles and protect intestinal organs. Percentage of hard tissues in human body depends on age, weight, and gender. Human skeleton consists of 206 connected bones. Therefore, it is natural that the hard-tissue damage such as fractures, osteoporosis, and congenital lack of bone may appear. The innovative way of bone healing is an application of so-called tissue scaffolds. There are many synthetic polymers used in this field, but polyurethanes play a great role in this field. It is due to the possibility to control their degradation rate and to tune their surface to improve the calcification process, required for proper bone regeneration. In this article, we described the fabrication of degradable poly(ester-ether)urethane materials, having different hard-segment content (28% or 47%). PEEURs-28HS and PEEURs-47HS materials were obtained by two-step polymerization method and characterized by mechanical properties, ability to undergo oxidative degradation and surface calcification. Performed studies indicated that the PEEURs-28HS material possessed suitable properties to be proposed as a material for possible application in the bone tissue engineering.

Pre-osteoblast cell colonization of porous silicon substituted hydroxyapatite bioceramics: Influence of microporosity and macropore design

Silicate-substituted hydroxyapatite scaffolds containing multiscale porosity are manufactured. Model parts containing macropores of five cross-sectional geometries (circle, square, rhombus, star and triangle) and two sizes are shaped by microstereolithography. Three open microporosity contents (0.5, 23 or 37 vol%) are introduced in the ceramic. MC3T3-E1 pre-osteoblasts are seeded onto these scaffolds. Analysis of cell colonization inside the macropores after 7 and 14 days of cultivation shows that the cellular filling is proportional to the macropore size and strongly influenced by macropore shape. Straight edges and convex surfaces are detrimental. High aspect ratios, the absence of reentrant angles and the presence of acute angles, by creating concavities and minimizing flat surfaces, facilitate cell colonization. Rhombus and triangle cross-sections are thus particularly favorable, while square and star geometries are the least favored. An increase in the microporosity content strongly impairs cell growth in the macropores. The data are statistically analyzed using a principal components analysis that shows that macro- and microtopographical parameters of scaffolds must be collectively considered with correlated interactions to understand cell behavior. The results indicate the important cell sensing of topography during the initial step of cell adhesion and proliferation and evidence the need for an optimized scaffold design.

Curvature- and fluid-stress-driven tissue growth in a tissue-engineering scaffold pore

Cell proliferation within a fluid-filled porous tissue-engineering scaffold depends on a sensitive choice of pore geometry and flow rates: regions of high curvature encourage cell proliferation, while a critical flow rate is required to promote growth for certain cell types. When the flow rate is too slow, the nutrient supply is limited; when it is too fast, cells may be damaged by the high fluid shear stress. As a result, determining appropriate tissue-engineering-construct geometries and operating regimes poses a significant challenge that cannot be addressed by experimentation alone. In this paper, we present a mathematical theory for the fluid flow within a pore of a tissue-engineering scaffold, which is coupled to the growth of cells on the pore walls. We exploit the slenderness of a pore that is typical in such a scenario, to derive a reduced model that enables a comprehensive analysis of the system to be performed. We derive analytical solutions in a particular case of a nearly piecewise constant growth law and compare these with numerical solutions of the reduced model. Qualitative comparisons of tissue morphologies predicted by our model, with those observed experimentally, are also made. We demonstrate how the simplified system may be used to make predictions on the design of a tissue-engineering scaffold and the appropriate operating regime that ensures a desired level of tissue growth.

Osteoconductive Microarchitecture of Bone Substitutes for Bone Regeneration Revisited

In the last three decades, all efforts in bone tissue engineering were driven by the dogma that the ideal pore size in bone substitutes lies between 0.3 and 0.5 mm in diameter. Newly developed additive manufacturing methodologies for ceramics facilitate the total control over pore size, pore distribution, bottleneck size, and bottleneck distribution. Therefore, this appears to be the method of choice with which to test the aforementioned characteristics of an ideal bone substitute. To this end, we produced a library of 15 scaffolds with diverse defined pore/bottleneck dimensions and distributions, tested them in vivo in a calvarial bone defect model in rabbits, and assessed the clinically most relevant parameters: defect bridging and bony regenerated area. Our in vivo data revealed that the ideal pore/bottleneck dimension for bone substitutes is in the range of 0.7-1.2 mm, and appears therefore to be twofold to fourfold more extended than previously thought. Pore/bottleneck dimensions of 1.5 and 1.7 mm perform significantly worse and appear unsuitable in bone substitutes. Thus, our results set the ideal range of pore/bottleneck dimensions and are likely to have a significant impact on the microarchitectural design of future bone substitutes for use in orthopedic, trauma, cranio-maxillofacial and oral surgery.

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.

Development of polyurethanes for bone repair

The purpose of this paper is to review recent developments on polyurethanes aimed at the design, synthesis, modifications, and biological properties in the field of bone tissue engineering. Different polyurethane systems are presented and discussed in terms of biodegradation, biocompatibility and bioactivity. A comprehensive discussion is provided of the influence of hard to soft segments ratio, catalysts, stiffness and hydrophilicity of polyurethanes. Interaction with various cells, behavior in vivo and current strategies in enhancing bioactivity of polyurethanes are described. The discussion on the incorporation of biomolecules and growth factors, surface modifications, and obtaining polyurethane-ceramics composites strategies is held. The main emphasis is placed on the progress of polyurethane applications in bone regeneration, including bone void fillers, shape memory scaffolds, and drug carrier.

Additive Manufacturing of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/poly(ε-caprolactone) Blend Scaffolds for Tissue Engineering

Additive manufacturing of scaffolds made of a polyhydroxyalkanoate blended with another biocompatible polymer represents a cost-effective strategy for combining the advantages of the two blend components in order to develop tailored tissue engineering approaches. The aim of this study was the development of novel poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/ poly(ε-caprolactone) (PHBHHx/PCL) blend scaffolds for tissue engineering by means of computer-aided wet-spinning, a hybrid additive manufacturing technique suitable for processing polyhydroxyalkanoates dissolved in organic solvents. The experimental conditions for processing tetrahydrofuran solutions containing the two polymers at different concentrations (PHBHHx/PCL weight ratio of 3:1, 2:1 or 1:1) were optimized in order to manufacture scaffolds with predefined geometry and internal porous architecture. PHBHHx/PCL scaffolds with a 3D interconnected network of macropores and a local microporosity of the polymeric matrix, as a consequence of the phase inversion process governing material solidification, were successfully fabricated. As shown by scanning electron microscopy, thermogravimetric, differential scanning calorimetric and uniaxial compressive analyses, blend composition significantly influenced the scaffold morphological, thermal and mechanical properties. In vitro biological characterization showed that the developed scaffolds were able to sustain the adhesion and proliferation of MC3T3-E1 murine preosteoblast cells. The additive manufacturing approach developed in this study, based on a polymeric solution processing method avoiding possible material degradation related to thermal treatments, could represent a powerful tool for the development of customized PHBHHx-based blend scaffolds for tissue engineering.

3D imaging of cell interactions with electrospun PLGA nanofiber membranes for bone regeneration

The interaction between resident cells and electrospun nanofibers is critical in determining resultant osteoblast proliferation and activity in orthopedic tissue scaffolds. The use of techniques to evaluate cell-nanofiber interactions is critical in understanding scaffold function, with visualization promising unparalleled access to spatial information on such interactions. 3D tomography exploiting focused ion beam (FIB)-scanning electron microscopy (SEM) was used to examine electrospun nanofiber scaffolds to understand the features responsible for (osteoblast-like MC3T3-E1 and UMR106) cell behavior and resultant scaffold function. 3D imaging of cell-nanofiber interactions within a range of electrospun poly(d,l-lactide-co-glycolide acid) (PLGA) nanofiber scaffold architectures indicated a coherent interface between osteoblasts and nanofiber surfaces, promoting osteoblast filopodia formation for successful cell growth. Coherent cell-nanofiber interfaces were demonstrated throughout a randomly organized and aligned nanofiber network. Gene expression of UMR106 cells grown on PLGA fibers did not deviate significantly from those grown on plastic, suggesting maintenance of phenotype. However, considerably lower expression of Ibsp and Alpl on PLGA fibers might indicate that these cells are still in the proliferative phase compared with a more differentiated cell on plastic. This work demonstrates the synergy between designing electrospun tissue scaffolds and providing comprehensive evaluation through high resolution imaging of resultant 3-dimensional cell growth within the scaffold.

STATEMENT OF SIGNIFICANCE

Membranes made from electrospun nanofibers are potentially excellent for promoting bone growth for next-generation tissue scaffolds. The effectiveness of an electrospun membrane is shown here using high resolution 3D imaging to visualize the interaction between cells and the nanofibers within the membrane. Nanofibers that are aligned in one direction control cell growth at the surface of the membrane whereas random nanofibers cause cell growth into the membrane. Such observations are important and indicate that lateral cell growth at the membrane surface using aligned nanofibers could be used for rapid tissue repair whereas slower but more extensive tissue production is promoted by membranes containing random nanofibers.

Stability of biodegradable waterborne polyurethane films in buffered saline solutions

The stability of polyurethane (PU) is of critical importance for applications such as in coating industry or as biomaterials. To eliminate the environmental concerns on the synthesis of PU which involves the use of organic solvents, the aqueous-based or waterborne PU (WBPU) has been developed. WBPU, however, may be unstable in an electrolyte-rich environment. In this study, the authors reported the stability of biodegradable WBPU in the buffered saline solutions evaluated by atomic force microscopy (AFM). Various biodegradable WBPU films were prepared by spin coating on coverslip glass, with a thickness of ∼300 nm. The surface AFM images of poly(ε-caprolactone) (PCL) diol-based WBPU revealed nanoglobular structure. The same feature was observed when 20% molar of the PCL diol soft segment was replaced by polyethylene butylenes adipate diol. After hydration in buffered saline solutions for 24 h, the surface domains generally increased in sizes and became irregular in shape. On the other hand, when the soft segment was replaced by 20% poly(l-lactide) diol, a meshlike surface structure was demonstrated by AFM. When the latter WBPU was hydrated, the surface domains appeared to be disconnected. Results from the attenuated total reflectance infrared spectroscopy and x-ray photoelectron spectroscopy indicated that the surface chemistry of WBPU films was altered after hydration. These changes were probably associated with the neutralization of carboxylate by ions in the saline solutions, resulting in the rearrangements of soft and hard segments and causing instability of the WBPU.

Bone tissue regeneration: the role of scaffold geometry

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

Spatial optimization in perfusion bioreactors improves bone tissue-engineered construct quality attributes

Perfusion bioreactors have shown great promise for tissue engineering applications providing a homogeneous and consistent distribution of nutrients and flow-induced shear stresses throughout tissue-engineered constructs. However, non-uniform fluid-flow profiles found in the perfusion chamber entrance region have been shown to affect tissue-engineered construct quality characteristics during culture. In this study a whole perfusion and construct, three dimensional (3D) computational fluid dynamics approach was used in order to optimize a critical design parameter such as the location of the regular pore scaffolds within the perfusion bioreactor chamber. Computational studies were coupled to bioreactor experiments for a case-study flow rate. Two cases were compared in the first instance seeded scaffolds were positioned immediately after the perfusion chamber inlet while a second group was positioned at the computationally determined optimum distance were a steady state flow profile had been reached. Experimental data showed that scaffold location affected significantly cell content and neo-tissue distribution, as determined and quantified by contrast enhanced nanoCT, within the constructs both at 14 and 21 days of culture. However, gene expression level of osteopontin and osteocalcin was not affected by the scaffold location. This study demonstrates that the bioreactor chamber environment, incorporating a scaffold and its location within it, affects the flow patterns within the pores throughout the scaffold requiring therefore dedicated optimization that can lead to bone tissue engineered constructs with improved quality attributes.

Additive manufacturing of star poly(ε-caprolactone) wet-spun scaffolds for bone tissue engineering applications

Three-dimensional fibrous scaffolds made of a three-arm star poly(ε-caprolactone) were developed by employing a novel computer-aided wet-spinning apparatus to precisely control the deposition pattern of an extruded polymeric solution as a filament into a coagulation bath. Star poly(ε-caprolactone)/hydroxyapatite composite scaffolds composed of fibres with a porous morphology both in the outer surface and in the cross section were successfully produced with a layer-by-layer approach achieving good reproducibility of the internal architecture and external shape. Changes in processing parameters were used to fabricate scaffolds with different architectural parameters in terms of average pore size in the xy-axes (from 190 to 297 µm) and in the z-axis (from 54 to 126 µm) and porosity (in the range of 20%–60%). Based on the mechanical characterization, processing variations and hydroxyapatite loading have an influence on scaffold compression properties. Cell cultures, using a murine pre-osteoblast cell line, had good cell responses in terms of proliferation and osteoblastic differentiation. Thus, this technique appears to be an effective method for producing customized polymeric scaffolds for bone tissue engineering applications.

Tissue growth into three-dimensional composite scaffolds with controlled micro-features and nanotopographical surfaces

Controlling topographic features at all length scales is of great importance for the interaction of cells with tissue regenerative materials. We utilized an indirect three-dimensional printing method to fabricate polymeric scaffolds with pre-defined and controlled external and internal architecture that had an interconnected structure with macro- (400-500 μm) and micro- (∼25 μm) porosity. Polycaprolactone (PCL) was used as model system to study the kinetics of tissue growth within porous scaffolds. The surface of the scaffolds was decorated with TiO2 and bioactive glass (BG) nanoparticles to the better match to nanoarchitecture of extracellular matrix (ECM). Micrometric BG particles were also used to reveal the effect of particle size on the cell behavior. Observation of tissue growth and enzyme activity on two-dimensional (2D) films and three-dimensional (3D) scaffolds showed effects of nanoparticle inclusion and of surface curvature on the cellular adhesion, proliferation, and kinetics of preosteoblastic cells (MC3T3-E1) tissue growth into the pore channels. It was found that the presence of nanoparticles in the substrate impaired cellular adhesion and proliferation in 3D structures. Evaluation of alkaline phosphate activity showed that the presence of the hard particles affects differentiation of the cells on 2D films. Notwithstanding, the effect of particles on cell differentiation was not as strong as that seen by the curvature of the substrate. We observed different effects of nanofeatures on 2D structures with those of 3D scaffolds, which influence the cell proliferation and differentiation for non-load-bearing applications in bone regenerative medicine.

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

In a previous paper we presented a theoretical framework to describe tissue growth in confined geometries based on the work of Ambrosi and Guillou [Ambrosi D, Guillou A. Growth and dissipation in biological tissues. Cont Mech Thermodyn 2007;19:245-51]. A thermodynamically consistent eigenstrain rate for growth was derived using the concept of configurational forces and used to investigate growth in holes of cylindrical geometries. Tissue growing from concave surfaces can be described by a model based on this theory. However, an apparently asymmetric behaviour between growth from convex and concave surfaces has been observed experimentally, but is not predicted by this model. This contradiction is likely to be due to the presence of contractile tensile stresses produced by cells near the tissue surface. In this contribution we extend the model in order to couple tissue growth to the presence of a surface stress. This refined growth model is solved for two geometries, within a cylindrical hole and on the outer surface of a cylinder, thus demonstrating how surface stress may indeed inhibit growth on convex substrates.

The effect of pore geometry on the in vitro biological behavior of human periosteum-derived cells seeded on selective laser-melted Ti6Al4V bone scaffolds

The specific aim of this study was to gain insight into the influence of scaffold pore size, pore shape and permeability on the in vitro proliferation and differentiation of three-dimensional (3-D) human periosteum-derived cell (hPDC) cultures. Selective laser melting (SLM) was used to produce six distinct designed geometries of Ti6Al4V scaffolds in three different pore shapes (triangular, hexagonal and rectangular) and two different pore sizes (500 μm and 1000 μm). All scaffolds were characterized by means of two-dimensional optical microscopy, 3-D microfocus X-ray computed tomography (micro-CT) image analysis, mechanical compression testing and computational fluid dynamical analysis. The results showed that SLM was capable of producing Ti6Al4V scaffolds with a broad range of morphological and mechanical properties. The in vitro study showed that scaffolds with a lower permeability gave rise to a significantly higher number of cells attached to the scaffolds after seeding. Qualitative analysis by means of live/dead staining and scanning electron micrography showed a circular cell growth pattern which was independent of the pore size and shape. This resulted in pore occlusion which was found to be the highest on scaffolds with 500 μm hexagonal pores. Interestingly, pore size but not pore shape was found to significantly influence the growth of hPDC on the scaffolds, whereas the differentiation of hPDC was dependent on both pore shape and pore size. The results showed that, for SLM-produced Ti6Al4V scaffolds with specific morphological and mechanical properties, a functional graded scaffold will contribute to enhanced cell seeding and at the same time can maintain nutrient transport throughout the whole scaffold during in vitro culturing by avoiding pore occlusion.

How linear tension converts to curvature: geometric control of bone tissue growth

This study investigated how substrate geometry influences in-vitro tissue formation at length scales much larger than a single cell. Two-millimetre thick hydroxyapatite plates containing circular pores and semi-circular channels of 0.5 mm radius, mimicking osteons and hemi-osteons respectively, were incubated with MC3T3-E1 cells for 4 weeks. The amount and shape of the tissue formed in the pores, as measured using phase contrast microscopy, depended on the substrate geometry. It was further demonstrated, using a simple geometric model, that the observed curvature-controlled growth can be derived from the assembly of tensile elements on a curved substrate. These tensile elements are cells anchored on distant points of the curved surface, thus creating an actin “chord” by generating tension between the adhesion sites. Such a chord model was used to link the shape of the substrate to cell organisation and tissue patterning. In a pore with a circular cross-section, tissue growth increases the average curvature of the surface, whereas a semi-circular channel tends to be flattened out. Thereby, a single mechanism could describe new tissue growth in both cortical and trabecular bone after resorption due to remodelling. These similarities between in-vitro and in-vivo patterns suggest geometry as an important signal for bone remodelling.

Trabecular bone remodelling simulated by a stochastic exchange of discrete bone packets from the surface

Human bone is constantly renewed through life via the process of bone remodelling, in which individual packets of bone are removed by osteoclasts and replaced by osteoblasts. Remodelling is mechanically controlled, where osteocytes embedded within the bone matrix are thought to act as mechanical sensors. In this computational work, a stochastic model for bone remodelling is used in which the renewal of bone material occurs by exchange of discrete bone packets. We tested different hypotheses of how the mechanical stimulus for bone remodelling is integrated by osteocytes and sent to actor cells on the bone's surface. A collective (summed) signal from multiple osteocytes as opposed to an individual (maximal) signal from a single osteocyte was found to lead to lower inner porosity and surface roughness of the simulated bone structure. This observation can be interpreted in that collective osteocyte signalling provides an effective surface tension to the remodelling process. Furthermore, the material heterogeneity due to remodelling was studied on a network of trabeculae. As the model is discrete, the age of individual bone packets can be monitored with time. The simulation results were compared with experimental data coming from quantitative back scattered electron imaging by transforming the information about the age of the bone packet into a mineral content. Discrepancies with experiments indicate that osteoclasts preferentially resorb low mineralized, i.e. young, bone at the bone's surface.