Quantifying the 3D macrostructure of tissue scaffolds

Design considerations when creating a high throughput screen-compatible in vitro model of osteogenesis

Inducing osteogenic differentiation in vitro is useful for the identification and development of bone regeneration therapies as well as modelling bone disorders. To couple in vitro models with high throughput screening techniques retains the assay's relevance in research while increasing its therapeutic impact. Miniaturizing, automating and/or digitalizing in vitro assays will reduce the required quantity of cells, biologic stimulants, culture/output assay reagents, time and cost. This review highlights the design and workflow considerations for creating a high throughput screen-compatible model of osteogenesis, comparing and contrasting osteogenic cell type, assay fabrication and culture methodology, osteogenic induction approach and repurposing existing output techniques.

Controlled pore anisotropy in chitosan-gelatin cryogels for use in bone tissue engineering

In tissue engineering, the development of an appropriate scaffold is crucial to provide a framework for new tissue growth. The use of cryogels as scaffolds shows promise due to their macroporous structure, but the pore size, distribution, and interconnectivity is highly variable depending on the fabrication process. The objective of the current research is to provide a technique for controlled anisotropy in chitosan-gelatin cryogels to develop scaffolds for bone tissue engineering application. A mold was developed using additive manufacturing to be used during the freezing process in order to fabricate cryogels with a more interconnected pore structure. The scaffolds were tested to evaluate their porosity, mechanical strength, and to observe cell infiltration through the cryogel. It was found that the use of the mold allowed for the creation of designated pores within the cryogel structure which facilitated cell infiltration to the center of the scaffold without sacrificing mechanical integrity of the structure.

Early Osteogenic-Induced Adipose-Derived Stem Cells and Canine Bone Regeneration Potential Analyzed Using Biodegradable Scaffolds

The complex process of bone regeneration is influenced by factors such as inflammatory responses, tissue interactions, and progenitor cells. Currently, multiple traumas can interfere with fracture healing, causing the prolonging or failure of healing. In these cases, bone grafting is the most effective treatment. However, there are several drawbacks, such as morbidity at the donor site and availability of suitable materials. Advantages have been provided in this field by a variety of stem cell types. Adipose-derived stem cells (ASCs) show promise. In the radiological examination of this study, it was confirmed that the C/S group showed faster regeneration than the other groups, and Micro-CT also showed that the degree of bone formation in the defect area was highest in the C/S group. Compared to the control group, the change in cortical bone area in the defect area decreased in the sham group (0.874), while it slightly increased in the C/S group (1.027). An increase in relative vascularity indicates a decrease in overall bone density, but a weak depression filled with fibrous tissue was observed outside the compact bone. It was confirmed that newly formed cortical bone showed a slight difference in bone density compared to surrounding normal bone tissue due to increased distribution of cortical bone. In this study, we investigated the effect of bone regeneration by ADMSCs measured by radiation and pathological effects. These data can ultimately be applied to humans with important clinical applications in various bone diseases, regenerative, and early stages of formative differentiation.

Identification and analysis of 3D pores in packed particulate materials

We took the classic 'guess the number of beans in a jar game' and amplified the research question. Rather than estimate the quantity of particles in the jar, we sought to characterize the spaces between them. Here we present an approach for delineating the pockets of empty space (three-dimensional pores) between packed particles, which are hotspots for activity in applications and natural phenomena that deal with particulate materials. We utilize techniques from graph theory to exploit information about particle configuration that allows us to locate important spatial landmarks within the void space. These landmarks are the basis for our pore segmentation, where we consider both interior pores as well as entrance and exit pores into and out of the structure. Our method is robust for particles of varying size, form, stiffness and configuration, which allows us to study and compare three-dimensional pores across a range of packed particle types. We report striking relationships between particles and pores that are described mathematically, and we offer a visual library of pore types. With a meaningful discretization of void space, we demonstrate that packed particles can be understood not by their solid space, but by their empty space.

3D printed hybrid scaffolds for bone regeneration using calcium methoxyethoxide as a calcium source

Introduction: Hybrids consist of inorganic and organic co-networks that are indistinguishable above the nanoscale, which can lead to unprecedented combinations of properties, such as high toughness and controlled degradation. Methods: We present 3D printed bioactive hybrid scaffolds for bone regeneration, produced by incorporating calcium into our "Bouncy Bioglass", using calcium methoxyethoxide (CME) as the calcium precursor. SiO2-CaOCME/PTHF/PCL-diCOOH hybrid "inks" for additive manufacturing (Direct Ink Writing) were optimised for synergy of mechanical properties and open interconnected pore channels. Results and Discussion: Adding calcium improved printability. Changing calcium content (5, 10, 20, 30, and 40 mol.%) of the SiO2-CaOCME/PTHF/PCL-diCOOH hybrids affected printability and mechanical properties of the lattice-like scaffolds. Hybrids containing 30 mol.% calcium in the inorganic network (70S30CCME-CL) printed with 500 µm channels and 100 µm strut size achieved the highest strength (0.90 ± 0.23 MPa) and modulus of toughness (0.22 ± 0.04 MPa). These values were higher than Ca-free SiO2/PTHF/PCL-diCOOH hybrids (0.36 ± 0.14 MPa strength and 0.06 ± 0.01 MPa toughness modulus). Over a period of 90 days of immersion in simulated body fluid (SBF), the 70S30CCME-CL hybrids also kept a stable strain to failure (~30 %) and formed hydroxycarbonate apatite within three days. The extracts released by the 70S30CCME-CL hybrids in growth medium did not cause cytotoxic effects on human bone marrow stromal cells over 24 h of culture.

Osteoblasts in a Perfusion Flow Bioreactor-Tissue Engineered Constructs of TiO2 Scaffolds and Cells for Improved Clinical Performance

Combining biomaterial scaffolds with cells serves as a promising strategy for engineering critical size defects; however, homogenous cellular growth within large scaffolds is challenging. Mechanical stimuli can enhance bone regeneration by modulating cellular growth and differentiation. Here, we compare dynamic seeding in a perfusion flow bioreactor with static seeding for a synthetic bone scaffold for up to 21 days using the cell line MC3T3-E1 and primary human osteoblast, confocal laser scanning microscopy, and real-time reverse transcriptase-polymerase chain reaction. The secretion of bone-related proteins was quantified using multiplex immunoassays. Dynamic culture improved cellular distribution through the TiO2 scaffold and induced a five-fold increase in cell number after 21 days. The relative mRNA expression of osteopontin of MC3T3-E1 was 40-fold enhanced after 7 and 21 days at a flow rate of 0.08 mL/min, and that of collagen type I alpha I expression was 18-fold after 21 days. A flow rate of 0.16 mL/min was 10-fold less effective. Dynamic culture increased the levels of dickkopf-related protein 1 (60-fold), osteoprotegrin (29-fold), interleukin-6 (23-fold), interleukin-8 (36-fold), monocyte chemoattractant protein 1 (28-fold) and vascular endothelial growth factor (6-fold) in the medium of primary human osteoblasts after 21 days compared to static seeding. The proposed method may have clinical potential for bone tissue engineering.

18F- based Quantification of the Osteogenic Potential of hMSCs

In bone tissue engineering, there is a constant need to design new methods for promoting in vitro osteogenic differentiation. Consequently, there is a strong demand for fast, effective and reliable methods to track and quantify osteogenesis in vitro. In this study, we used the radiopharmacon fluorine-18 (¹⁸F) to evaluate the amount of hydroxylapatite produced by mesenchymal stem cells (MSCs) in a monolayer cell culture in vitro. The hydroxylapatite bound tracer was evaluated using µ-positron emission tomography (µ-PET) scanning and activimeter analysis. It was therefore possible to determine the amount of synthesized mineral and thus to conclude the osteogenic potential of the cells. A Student's t-test revealed a highly significant difference regarding tracer uptake between the osteogenic group and the corresponding control group (µ-PET p = 0.043; activimeter analysis p = 0.012). This tracer uptake showed a highly significant correlation with the gold standard of quantitative Alizarin Red staining (ARS) (r² = 0.86) as well as with the absolute calcium content detected by inductively coupled plasma mass spectrometry (r² = 0.81). The results showed that ¹⁸F labeling is a novel method to prove and quantify hydroxyapatite content in MSC monolayer cultures. The mineral layer remains intact for further analysis. This non-destructive in vitro method can be used to rapidly investigate bone tissue engineering strategies in terms of hydroxylapatite production, and could therefore accelerate the process of implementing new strategies in clinical practice.

Granular hydrogels: emergent properties of jammed hydrogel microparticles and their applications in tissue repair and regeneration

Granular hydrogels are emerging as a versatile and effective platform for tissue engineered constructs in regenerative medicine. The hydrogel microparticles (HMPs) that compose these materials exhibit particle jamming above a minimum packing fraction, which results in a bulk, yet dynamic, granular hydrogel scaffold. These injectable, microporous scaffolds possess self-assembling, shear-thinning, and self-healing properties. Recently, they have been utilized as cell cultures platforms and extracellular matrix mimics with remarkable success in promoting cellular infiltration and subsequent tissue remodeling in vivo. Furthermore, the modular nature of granular hydrogels accommodates heterogeneous HMP assembly, where varying HMPs have been fabricated to target distinct biological processes or deliver unique cargo. Such multifunctional materials offer enormous potential for capturing the structural and biofunctional complexity observed in native human tissue.

99mTc-Hydroxydiphosphonate quantification of extracellular matrix mineralization in 3D human mesenchymal stem cell cultures

Objectives

Bone tissue engineering is one of the fastest growing branches in modern bioscience. New methods are being developed to achieve higher grades of mineral deposition by osteogenically inducted mesenchymal stem cells. In addition to well established monolayer cell culture models, 3D cell cultures for stem cell-based osteogenic differentiation have become increasingly attractive to promote in vivo bone formation. One of the main problems of scaffold-based osteogenic cell cultures is the difficulty in quantifying the amount of newly produced extracellular mineral deposition, as a marker for new bone formation, without destroying the scaffold. In recent studies, we were able to show that ^99mTc-methylene diphosphonate (^99mTc-MDP), a gamma radiation-emitting radionuclide, can successfully be applied as a reliable quantitative marker for mineral deposition as this tracer binds with high affinity to newly produced hydroxyapatite (HA).

Methods

Within the present study, we evaluated whether this promising new method, using ^99mTc-hydroxydiphosphonate (^99mTc-HDP), can be used to quantify the amount of newly formed extracellular HA in a 3D cell culture model. Highly porous collagen type II scaffolds were seeded with 1 × 106 human mesenchymal stem cells (hMSCs; n = 6) and cultured for 21 days in osteogenic media (group A – osteogenic (OSM) group) and in parallel in standard media (group B – negative control (CNTRL) group). After incubation with ^99mTc-HDP, the tracer uptake, reflected by the amount of emitted gamma counts, was measured.

Results

We saw a higher uptake (up to 15-fold) of the tracer in the OSM group A compared with the CNTRL group B. Statistical analysis of the results (Student`s t-test) revealed a significantly higher amount of emitted gamma counts in the OSM group (p = 0.048). Qualitative and semi-quantitative analysis by Alizarin Red staining confirmed the presence of extracellular HA deposition in the OSM group.

Conclusion

Our data indicate that ^99mTc-HDP labelling is a promising tool to track and quantify non-destructive local HA deposition in 3D stem cell cultures.

Cite this article: T. L. Grossner, U. Haberkorn, T. Gotterbarm. ^99mTc-Hydroxydiphosphonate quantification of extracellular matrix mineralization in 3D human mesenchymal stem cell cultures. Bone Joint Res 2019;8:333–341. doi: 10.1302/2046-3758.87.BJR-2017-0248.R1.

Iodine-Enhanced Micro-CT Imaging of Soft Tissue on the Example of Peripheral Nerve Regeneration

Microcomputed tomography (μCT) is widely used for the study of mineralized tissues, but a similar use for soft tissues is hindered by their low X-ray attenuation. This limitation can be overcome by the recent development of different staining techniques. Staining with Lugol's solution, a mixture of one part iodine and two parts potassium iodide in water, stands out among these techniques for its low complexity and cost. Currently, Lugol staining is mostly used for anatomical examination of tissues. In the present study, we seek to optimize the quality and reproducibility of the staining for ex vivo visualization of soft tissues in the context of a peripheral nerve regeneration model in the rat. We show that the staining result not only depends on the concentration of the staining solution but also on the amount of stain in relation to the tissue volume and composition, necessitating careful adaptation of the staining protocol to the respective specimen tissue. This optimization can be simplified by a stepwise staining which we show to yield a similar result compared to staining in a single step. Lugol staining solution results in concentration-dependent tissue shrinkage which can be minimized but not eliminated. We compared the shrinkage of tendon, nerve, skeletal muscle, heart, brain, and kidney with six iterations of Lugol staining. 60 ml of 0.3% Lugol's solution per cm³ of tissue for 24 h yielded good results on the example of a peripheral nerve regeneration model, and we were able to show that the regenerating nerve inside a silk fibroin tube can be visualized in 3D using this staining technique. This information helps in deciding the region of interest for histological imaging and provides a 3D context to histological findings. Correlating both imaging modalities has the potential to improve the understanding of the regenerative process.

Intensity of artefacts in cone beam CT examinations caused by titanium and glass fibre-reinforced composite implants

OBJECTIVES:

The aim was to compare titanium and glass fibre-reinforced composite (FRC) orbital floor implants using cone beam CT (CBCT). FRC implants are nonmetallic and these implants have not been analysed in CBCT images before. The purpose of this study is to compare the artefact formation of the titanium and the FRC orbital floor implants in CBCT images.

METHODS:

One commercially pure titanium and one S-glass FRC with bioactive glass particles implant were imaged with CBCT using the same imaging values (80 kV, 1 mA, FOV 60 × 60 mm). CBCT images were analysed in axial slices from three areas to determine the magnitude of the artefacts in the vicinity of the implants. Quantified results based on the gray values of images were analysed using analysis-of-variance.

RESULTS:

Compared to the reference the gray values of the titanium implant are more negative in every region of interest in all slices (p < 0.05) whereas the gray values of the FRC implant differ statistically significantly in less than half of the examined areas.

CONCLUSIONS:

The titanium implant caused artefacts in all of the analysed CBCT slices. Compared to the reference the gray values of the FRC implant changed only slightly and this feature enables to use wider imaging options postoperatively.

Osteogenesis for postoperative temporal bone defects using human ear adipose-derived stromal cells and tissue engineering: An animal model study

Mastoidectomy, the removal of infected mastoid bones, is a common surgical procedure for the treatment of chronic otitis media. Persistent and recurrent otorrhea and accumulation of keratin debris following open cavity mastoidectomy are still bothersome issues for both patients and otologists. In this study, we used human ear adipose-derived stromal cells (hEASCs) in combination with polycaprolactone (PCL) scaffolds and osteogenic differentiation medium (ODM) to regenerate temporal bone defects. The hEASCs showed stem cell phenotypes, and these characteristics were maintained up to passage 5. Mastoid bulla and cranial bone defects were induced in Sprague-Dawley rats using AgNO₃ and burr hole drilling, respectively, and the rats were then divided into five groups: (1) control, (2) hEASCs, (3) hEASCs + ODM, (4) hEASCs + PCL scaffolds, and (5) hEASCs + PCL scaffolds + ODM. Osteogenesis was evaluated by micro-computed tomography and histology. Compared with the control group, the groups transplanted with hEASCs and PCL scaffolds had significantly higher bone formation along the periphery of the mastoid bulla area. Moreover, ODM synergistically enhanced bone formation in mastoid bulla defects. Our results suggest that combining hEASCs with PCL scaffolds represents a promising method for anatomical and functional reconstruction of postoperative temporal bone defects following mastoidectomy. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 3493-3501, 2017.

Analysis of variability in additive manufactured open cell porous structures

In this article, a novel method of analysing build consistency of additively manufactured open cell porous structures is presented. Conventionally, methods such as micro computed tomography or scanning electron microscopy imaging have been applied to the measurement of geometric properties of porous material; however, high costs and low speeds make them unsuitable for analysing high volumes of components. Recent advances in the image-based analysis of open cell structures have opened up the possibility of qualifying variation in manufacturing of porous material. Here, a photogrammetric method of measurement, employing image analysis to extract values for geometric properties, is used to investigate the variation between identically designed porous samples measuring changes in material thickness and pore size, both intra- and inter-build. Following the measurement of 125 samples, intra-build material thickness showed variation of ±12%, and pore size ±4% of the mean measured values across five builds. Inter-build material thickness and pore size showed mean ranges higher than those of intra-build, ±16% and ±6% of the mean material thickness and pore size, respectively. Acquired measurements created baseline variation values and demonstrated techniques suitable for tracking build deviation and inspecting additively manufactured porous structures to indicate unwanted process fluctuations.

In vivo investigation of tissue-engineered periosteum for the repair of allogeneic critical size bone defects in rabbits

Aim: The aim of the study was to evaluate the efficacy of tissue-engineered periosteum (TEP) in repairing allogenic bone defects in the long term. Materials & methods: TEP was biofabricated with osteoinduced rabbit bone marrow mesenchymal stem cells and porcine small intestinal submucosa (SIS). A total of 24 critical sized defects were created bilaterally in radii of 12 New Zealand White rabbits. TEP/SIS was implanted into the defect site. Bone defect repair was evaluated with radiographic and histological examination at 4, 8 and 12 weeks. Results: Bone defects were structurally reconstructed in the TEP group with mature cortical bone and medullary canals, however this was not observed in the SIS group at 12 weeks. Conclusion: The TEP approach can effectively restore allogenic critical sized defects, and achieve maturity of long-bone structure in 12 weeks in rabbit models.

Calcium-phosphate ceramics and polysaccharide-based hydrogel scaffolds combined with mesenchymal stem cell differently support bone repair in rats

Research in bone tissue engineering is focused on the development of alternatives to autologous bone grafts for bone reconstruction. Although multiple stem cell-based products and biomaterials are currently being investigated, comparative studies are rarely achieved to evaluate the most appropriate approach in this context. Here, we aimed to compare different clinically relevant bone tissue engineering methods and evaluated the kinetic repair and the bone healing efficiency supported by mesenchymal stem cells and two different biomaterials, a new hydrogel scaffold and a commercial hydroxyapatite/tricalcium phosphate ceramic, alone or in combination.Syngeneic mesenchymal stem cells (5 × 10⁵) and macroporous biphasic calcium phosphate ceramic granules (Calciresorb C35^®, Ceraver) or porous pullulan/dextran-based hydrogel scaffold were implanted alone or combined in a drilled-hole bone defect in rats. Using quantitative microtomography measurements and qualitative histological examinations, their osteogenic properties were evaluated 7, 30, and 90 days after implantation. Three months after surgery, only minimal repair was evidenced in control rats while newly mineralized bone was massively observed in animals treated with either hydrogels (bone volume/tissue volume = 20%) or ceramics (bone volume/tissue volume = 26%). Repair mechanism and resorption kinetics were strikingly different: rapidly-resorbed hydrogels induced a dense bone mineralization from the edges of the defect while ceramics triggered newly woven bone formation in close contact with the ceramic surface that remained unresorbed. Delivery of mesenchymal stem cells in combination with these biomaterials enhanced both bone healing (>20%) and neovascularization after 1 month, mainly in hydrogel.Osteogenic and angiogenic properties combined with rapid resorption make hydrogels a promising alternative to ceramics for bone repair by cell therapy.

Generation and Assessment of Functional Biomaterial Scaffolds for Applications in Cardiovascular Tissue Engineering and Regenerative Medicine

Current clinically applicable tissue and organ replacement therapies are limited in the field of cardiovascular regenerative medicine. The available options do not regenerate damaged tissues and organs, and, in the majority of the cases, show insufficient restoration of tissue function. To date, anticoagulant drug-free heart valve replacements or growing valves for pediatric patients, hemocompatible and thrombus-free vascular substitutes that are smaller than 6 mm, and stem cell-recruiting delivery systems that induce myocardial regeneration are still only visions of researchers and medical professionals worldwide and far from being the standard of clinical treatment. The design of functional off-the-shelf biomaterials as well as automatable and up-scalable biomaterial processing methods are the focus of current research endeavors and of great interest for fields of tissue engineering and regenerative medicine. Here, various approaches that aim to overcome the current limitations are reviewed, focusing on biomaterials design and generation methods for myocardium, heart valves, and blood vessels. Furthermore, novel contact- and marker-free biomaterial and extracellular matrix assessment methods are highlighted.

A computer-designed scaffold for bone regeneration within cranial defect using human dental pulp stem cells

A computer-designed, solvent-free scaffold offer several potential advantages such as ease of customized manufacture and in vivo safety. In this work, we firstly used a computer-designed, solvent-free scaffold and human dental pulp stem cells (hDPSCs) to regenerate neo-bone within cranial bone defects. The hDPSCs expressed mesenchymal stem cell markers and served as an abundant source of stem cells with a high proliferation rate. In addition, hDPSCs showed a phenotype of differentiated osteoblasts in the presence of osteogenic factors (OF). We used solid freeform fabrication (SFF) with biodegradable polyesters (MPEG-(PLLA-co-PGA-co-PCL) (PLGC)) to fabricate a computer-designed scaffold. The SFF technology gave quick and reproducible results. To assess bone tissue engineering in vivo, the computer-designed, circular PLGC scaffold was implanted into a full-thickness cranial bone defect and monitored by micro-computed tomography (CT) and histology of the in vivo tissue-engineered bone. Neo-bone formation of more than 50% in both micro-CT and histology tests was observed at only PLGC scaffold with hDPSCs/OF. Furthermore, the PLGC scaffold gradually degraded, as evidenced by the fluorescent-labeled PLGC scaffold, which provides information to tract biodegradation of implanted PLGC scaffold. In conclusion, we confirmed neo-bone formation within a cranial bone defect using hDPSCs and a computer-designed PLGC scaffold.

Bioactive glasses—structure and properties

Bioactive glasses were the first synthetic materials to show bonding to bone, and they are successfully used for bone regeneration. They can degrade in the body at a rate matching that of bone formation, and through a combination of apatite crystallization on their surface and ion release they stimulate bone cell proliferation, which results in the formation of new bone. Despite their excellent properties and although they have been in clinical use for nearly thirty years, their current range of clinical applications is still small. Latest research focuses on developing new compositions to address clinical needs, including glasses for treating osteoporosis, with antibacterial properties, or for the sintering of scaffolds with improved mechanical stability. This Review discusses how the glass structure controls the properties, and shows how a structure-based design may pave the way towards new bioactive glass implants for bone regeneration.

Porous 3D bioscaffolds by adaptive foam reticulation

Highly interconnected and 3D porous hydroxyapatite scaffolds with controllable macroporosity and microporosity have been produced using the adaptive foam reticulation technique, combining foam reticulation and freeze casting methodologies. Process optimized macropores were generated using a polymeric template while micropores are introduced by the sublimation of a porogen, in this instance camphene. The macropore (100–600 µm) and strut (20–100 µm) sizes of the bioceramic produced compares favorably with the template pore (200–600 µm) and strut (30–80 µm) sizes. X-ray diffraction analysis indicated a phase change from hydroxyapatite to whitlockite, a calcium-deficient phosphate incorporating magnesium and iron ions. Increasing the number of coatings and/or sintering temperature resulted in the increase of scaffold strength while reducing the pore size. Variations to the amount of porogen did not affect the macrostructure of the scaffolds and led to the controllable introduction of microporosity to the struts. Osteoblast-like MG-63 cell culture and assay tests indicated that the scaffolds are not cytotoxic and suitable for use as bone grafts.

Pore Interconnectivity Influences Growth Factor-Mediated Vascularization in Sphere-Templated Hydrogels

Rapid and controlled vascularization within biomaterials is essential for many applications in regenerative medicine. The extent of vascularization is influenced by a number of factors, including scaffold architecture. While properties such as pore size and total porosity have been studied extensively, the importance of controlling the interconnectivity of pores has received less attention. A sintering method was used to generate hydrogel scaffolds with controlled pore interconnectivity. Poly(methyl methacrylate) microspheres were used as a sacrificial agent to generate porous poly(ethylene glycol) diacrylate hydrogels with interconnectivity varying based on microsphere sintering conditions. Interconnectivity levels increased with sintering time and temperature with resultant hydrogel structure showing agreement with template structure. Porous hydrogels with a narrow pore size distribution (130-150 μm) and varying interconnectivity were investigated for their ability to influence vascularization in response to gradients of platelet-derived growth factor-BB (PDGF-BB). A rodent subcutaneous model was used to evaluate vascularized tissue formation in the hydrogels in vivo. Vascularized tissue invasion varied with interconnectivity. At week 3, higher interconnectivity hydrogels had completely vascularized with twice as much invasion. Interconnectivity also influenced PDGF-BB transport within the scaffolds. An agent-based model was used to explore the relative roles of steric and transport effects on the observed results. In conclusion, a technique for the preparation of hydrogels with controlled pore interconnectivity has been developed and evaluated. This method has been used to show that pore interconnectivity can independently influence vascularization of biomaterials.

Characterization of porous glass fiber-reinforced composite (FRC) implant structures: porosity and mechanical properties

The aim of this study was to characterize the microstructure and mechanical properties of porous fiber-reinforced composites (FRC). Implants made of the FRC structures are intended for cranial applications. The FRC specimens were prepared by impregnating E-glass fiber sheet with non-resorbable bifunctional bis-phenyl glycidyl dimethacrylate and triethylene glycol dimethacrylate resin matrix. Four groups of porous FRC specimens were prepared with a different amount of resin matrix. Control group contained specimens of fibers, which were bound together with sizing only. Microstructure of the specimens was analyzed using a micro computed tomography (micro-CT) based method. Mechanical properties of the specimens were measured with a tensile test. The amount of resin matrix in the specimens had an effect on the microstructure. Total porosity was 59.5 % (median) in the group with the lowest resin content and 11.2 % (median) in the group with the highest resin content. In control group, total porosity was 94.2 % (median). Correlations with resin content were obtained for all micro-CT based parameters except TbPf. The tensile strength of the composites was 21.3 MPa (median) in the group with the highest resin content and 43.4 MPa (median) in the group with the highest resin content. The tensile strength in control group was 18.9 MPa (median). There were strong correlations between the tensile strength of the specimens and most of the micro-CT based parameters. This experiment suggests that porous FRC structures may have the potential for use in implants for cranial bone reconstructions, provided further relevant in vitro and in vivo tests are performed.

Micro-CT and PET analysis of bone regeneration induced by biodegradable scaffolds as carriers for dental pulp stem cells in a rat model of calvarial "critical size" defect: Preliminary data

Bone regeneration strategies in dentistry utilize biodegradable scaffolds seeded with stem cells able to induce bone formation. However, data on regeneration capacity of these tissue engineering constructs are still deficient. In this study micro-Computed tomography (micro-CT) and positron emission tomography (PET) analyses were used to investigate bone regeneration induced by two scaffolds [Granular deproteinized bovine bone (GDPB) and Beta-tricalcium phosphate (β-TCP)] used alone or in combination with dental pulp stem cells (DPSC) in a tissue engineered construct implanted in a rat critical calvarial defect. Bone mineral density (BMD) and standard uptake value (SUV) of tracer incorporation were measured after 2, 4, 8, and 12 weeks post-implant. The results showed that: (1) GDPB implants were mostly well positioned, as compared to ß-TCP; (2) GDPB induced higher BMD and SUV values within the cranial defect as compared to ß-TCP, either alone or in combination with stem cells; (3) addition of DPSC to the grafts did not significantly induce an increase in BMD and SUV values as compared to the scaffolds grafted alone, although a small tendency to increase was observed. Thus our study demonstrates that GDPB, when used to fill critical calvarial defects, induces a greater percentage of bone formation as compared to ß-TCP. Moreover, this study shows that addition of DPSC to pre-wetted scaffolds has the potential to ameliorate bone regeneration process, although the set of optimal conditions requires further investigation.

Novel resorbable glass-ceramic scaffolds for hard tissue engineering: from the parent phosphate glass to its bone-like macroporous derivatives

One of the major challenges of hard tissue engineering research focuses on the development of scaffolds that can match the mechanical properties of the host bone and resorb at the same rate as the bone is repaired. The aim of this work was the synthesis and characterization of a resorbable phosphate glass, as well as its application for the fabrication of three dimensional (3-D) scaffolds for bone regeneration. The glass microstructure and behaviour upon heating were analysed by X-ray diffraction, differential scanning calorimetry and hot stage microscopy. The glass solubility was investigated according to relevant ISO standards using distilled water, simulated body fluid (SBF) and Tris-HCl as testing media. The glass underwent progressive dissolution over time in all three media but the formation of a hydroxyapatite-like layer was also observed on the samples soaked in SBF and Tris-HCl, which demonstrated the bioactivity of the material. The glass powder was used to fabricate 3-D macroporous bone-like glass-ceramic scaffolds by adopting polyethylene particles as pore formers: during thermal treatment, the polymer additive was removed and the sintering of glass particles was allowed. The obtained scaffolds exhibited high porosity (87 vol.%) and compressive strength around 1.5 MPa. After soaking for 4 months in SBF, the scaffolds mass loss was 76 wt.% and the pH of the solution did not exceed the 7.55 value, thereby remaining in a physiological range. The produced scaffolds, being resorbable, bioactive, architecturally similar to trabecular bone and exhibiting interesting mechanical properties, can be proposed as promising candidates for bone repair applications.

A Human Clinical, Histological, Histomorphometrical, and Radiographical Study on Biphasic HA-Beta-TCP 30/70 in Maxillary Sinus Augmentation

BACKGROUND

By mixing hydroxyapatite (HA) and tricalcium phosphate (TCP), biphasic calcium phosphate ceramics can be obtained, and by varying their ratio it is possible to tailor the characteristics of the biomaterial.

PURPOSE

The aim of the present human study was to evaluate the histological and radiographical aspects of bone formation in maxillary sinus augmentation using a 30/70 HA-beta-TCP with a reticular structure.

MATERIALS AND METHODS

A total of 12 patients, undergoing two-stage sinus augmentation procedure using HA-beta-TCP at a ratio of 30/70, were included in the present study. After a 6-month healing period, during implant insertion, radiographical analysis was performed, and then the bone core biopsies were harvested and processed for histology.

RESULTS

At radiographic evaluation, the bone gain was on average 6.85 ± 0.60 mm. HA-beta-TCP 30/70 appeared to be lined by newly formed bone, with no gaps at the interface. The histomorphometric analysis revealed 26 ± 2% of residual grafted biomaterial, 29 ± 3% of newly formed bone, and 45 ± 2% of marrow spaces.

CONCLUSIONS

The present results indicate histologically the high biocompatibility and osteoconductivity of HA-beta-TCP 30/70, and clinically its successful use for sinus augmentation procedures.

Imaging challenges in biomaterials and tissue engineering

Biomaterials are employed in the fields of tissue engineering and regenerative medicine (TERM) in order to enhance the regeneration or replacement of tissue function and/or structure. The unique environments resulting from the presence of biomaterials, cells, and tissues result in distinct challenges in regards to monitoring and assessing the results of these interventions. Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are invasive, and do not provide information on functional status. Imaging techniques are needed that enable non-destructive, longitudinal, quantitative, and three-dimensional analysis of TERM strategies. This review focuses on evaluating the application of available imaging modalities for assessment of biomaterials and tissue in TERM applications. Included is a discussion of limitations of these techniques and identification of areas for further development.

Taking a deep look: modern microscopy technologies to optimize the design and functionality of biocompatible scaffolds for tissue engineering in regenerative medicine

This review focuses on modern nonlinear optical microscopy (NLOM) methods that are increasingly being used in the field of tissue engineering (TE) to image tissue non-invasively and without labelling in depths unreached by conventional microscopy techniques. With NLOM techniques, biomaterial matrices, cultured cells and their produced extracellular matrix may be visualized with high resolution. After introducing classical imaging methodologies such as µCT, MRI, optical coherence tomography, electron microscopy and conventional microscopy two-photon fluorescence (2-PF) and second harmonic generation (SHG) imaging are described in detail (principle, power, limitations) together with their most widely used TE applications. Besides our own cell encapsulation, cell printing and collagen scaffolding systems and their NLOM imaging the most current research articles will be reviewed. These cover imaging of autofluorescence and fluorescence-labelled tissue and biomaterial structures, SHG-based quantitative morphometry of collagen I and other proteins, imaging of vascularization and online monitoring techniques in TE. Finally, some insight is given into state-of-the-art three-photon-based imaging methods (e.g. coherent anti-Stokes Raman scattering, third harmonic generation). This review provides an overview of the powerful and constantly evolving field of multiphoton microscopy, which is a powerful and indispensable tool for the development of artificial tissues in regenerative medicine and which is likely to gain importance also as a means for general diagnostic medical imaging.

Near-infrared optical imaging for monitoring the regeneration of osteogenic tissue-engineered constructs

Millions of cases of bone injury or loss due to trauma, osteoporosis, and cancer occur in the United States each year. Because bone is limited in its ability to regenerate, alternative therapy approaches are needed. Bone tissue engineering has the potential to correct musculoskeletal disorders through the development of cell-based substitutes for osteogenic tissue replacement. Multiple medical imaging techniques such as magnetic resonance microscopy (MRM) were investigated recently; these techniques are able to provide useful information on the anatomical and structural changes of developing bone. However, there is a need for noninvasive approaches to evaluate biochemical constituents and consequent compositional development associated with growing osteogenic constructs. In this study, near-infrared (NIR) optical imaging with a bone-specific NIR-targeted probe, IRDye^® 800CW BoneTag™ (800CW BT), was applied in this study to longitudinally visualize regions of mineralization of tissue-engineered bone constructs in vivo. A fluorescent cell-based assay was performed to confirm the preferential binding of 800CW BT to the mineralized matrix of differentiated osteogenically driven human mesenchymal stem cells (hMSCs) in vitro. The hMSCs were seeded onto a biocompatible gelatin scaffold, allowed to develop, and implanted into a mouse model. Engineered constructs were examined in vivo using NIR imaging for bone mineralization, paired with MRM for verification of developing tissue. Results showed that NIR imaging with 800CW BT labeling can effectively assess the calcification of the developing osteogenic constructs, which is consistent with the analysis of excised tissue using NIR microscopy and histology. In conclusion, this study evaluated bone-like function of regenerating bone through tracking calcium deposition via NIR optical imaging with a fluorophore-labeled probe in a noninvasive manner.

Three-dimensional modeling of angiogenesis in porous biomaterial scaffolds

Vascularization of biomaterial scaffolds is essential for the successful clinical application of engineered tissues. Experimental studies are often performed to investigate the role of scaffold architecture on vascularized tissue formation. However, experiments are expensive and time-consuming and synthesis protocols often do not allow for independent investigation of specific scaffold properties. Computational models allow for rapid screening of potential material designs with control over scaffold properties that is difficult in laboratory settings. We have developed and tested a three-dimensional agent-based framework for investigating the effect of scaffold pore architecture on angiogenesis. Software agents represent endothelial cells, interacting together and with their micro-environment, leading to the invasion of blood vessels into the scaffold. A rule base, driven by experimental findings, governs the behavior of individual agents. 3D scaffold models with well-defined homogeneous and heterogeneous pore architectures were simulated to investigate the impact of various design parameters. Simulation results indicate that pores of larger size with higher interconnectivity and porosity support rapid and extensive angiogenesis. The developed framework can be used to screen biomaterial scaffold designs for optimal vascularization and investigate complex interactions among invading blood vessels and their micro-environment.

Selective laser sintering in biomedical engineering

Selective laser sintering (SLS) is a solid freeform fabrication technique, developed by Carl Deckard for his master's thesis at the University of Texas, patented in 1989. SLS manufacturing is a technique that produces physical models through a selective solidification of a variety of fine powders. SLS technology is getting a great amount of attention in the clinical field. In this paper the characteristics features of SLS and the materials that have been developed for are reviewed together with a discussion on the principles of the above-mentioned manufacturing technique. The applications of SLS in tissue engineering, and at-large in the biomedical field, are reviewed and discussed.

Noninvasive, quantitative, spatiotemporal characterization of mineralization in three-dimensional collagen hydrogels using high-resolution spectral ultrasound imaging

As tissue engineering products move toward the clinic, nondestructive methods to monitor their development and ensure quality are needed. In this study, high-resolution spectral ultrasound imaging (SUSI) was used to noninvasively characterize mineral content in collagen hydrogels. SUSI was used to generate three-dimensional (3D) grayscale (GS) images of construct morphology with submillimeter resolution. Spectral analysis of the backscattered radio frequency (RF) ultrasound signals was used to determine the midband fit (MBF) and slope of the linearized RF spectrum. These parameters are operator and instrument independent, and were used to characterize the spatial distribution of mineral in constructs supplemented with hydroxyapatite particles. GS and MBF correlated closely with mineral content, while slope was not dependent on concentration. SUSI also was used to monitor mineralization of collagen constructs by immersion in simulated body fluid (SBF) over 21 days. The construct surface was mineralized before the interior, and there was a dose-dependent effect of SBF concentration on degree of mineralization and deposited particle size. MBF density was closely correlated with the amount of calcium deposited. These data demonstrate that SUSI has utility as a noninvasive imaging method for quantitative analysis of mineralization in 3D protein constructs. Such techniques may assist the development of engineered orthopedic tissues.

Mechanical stretching for tissue engineering: two-dimensional and three-dimensional constructs

Mechanical cell stretching may be an attractive strategy for the tissue engineering of mechanically functional tissues. It has been demonstrated that cell growth and differentiation can be guided by cell stretch with minimal help from soluble factors and engineered tissues that are mechanically stretched in bioreactors may have superior organization, functionality, and strength compared with unstretched counterparts. This review explores recent studies on cell stretching in both two-dimensional (2D) and three-dimensional (3D) setups focusing on the applications of stretch stimulation as a tool for controlling cell orientation, growth, gene expression, lineage commitment, and differentiation and for achieving successful tissue engineering of mechanically functional tissues, including cardiac, muscle, vasculature, ligament, tendon, bone, and so on. Custom stretching devices and lab-specific mechanical bioreactors are described with a discussion on capabilities and limitations. While stretch mechanotransduction pathways have been examined using 2D stretch, studying such pathways in physiologically relevant 3D environments may be required to understand how cells direct tissue development under stretch. Cell stretch study using 3D milieus may also help to develop tissue-specific stretch regimens optimized with biochemical feedback, which once developed will provide optimal tissue engineering protocols.

MR elastography monitoring of tissue-engineered constructs

The objective of tissue engineering (TE) is to create functional replacements for various tissues; the mechanical properties of these engineered constructs are critical to their function. Several techniques have been developed for the measurement of the mechanical properties of tissues and organs; however, current methods are destructive. The field of TE will benefit immensely if biomechanical models developed by these techniques could be combined with existing imaging modalities to enable noninvasive, dynamic assessment of mechanical properties during tissue growth. Specifically, MR elastography (MRE), which is based on the synchronization of a mechanical actuator with a phase contrast imaging pulse sequence, has the capacity to measure tissue strain generated by sonic cyclic displacement. The captured displacement is presented in shear wave images from which the complex shear moduli can be extracted or simplified by a direct measure, termed the shear stiffness. MRE has been extended to the microscopic scale, combining clinical MRE with high-field magnets, stronger magnetic field gradients and smaller, more sensitive, radiofrequency coils, enabling the interrogation of smaller samples, such as tissue-engineered constructs. The following topics are presented in this article: (i) current mechanical measurement techniques and their limitations in TE; (ii) a description of the MRE system, MRE theory and how it can be applied for the measurement of mechanical properties of tissue-engineered constructs; (iii) a summary of in vitro MRE work for the monitoring of osteogenic and adipogenic tissues originating from human adult mesenchymal stem cells (MSCs); (iv) preliminary in vivo studies of MRE of tissues originating from mouse MSCs implanted subcutaneously in immunodeficient mice with an emphasis on in vivo MRE challenges; (v) future directions to resolve current issues with in vivo MRE in the context of how to improve the future role of MRE in TE.

Attachment and Proliferation of Human-Adipose-Tissue-Derived Stem Cells on Bioactive Glass/PVA Hybrid Scaffolds

Bioactive glass/polymer hybrids are promising materials for biomedical applications because they combine the bioactivity of bioceramics with the flexibility of polymers. These advantages may be used in porous scaffolds for tissue-engineering applications. In previous works, hybrid foams bioactive glass/polyvinyl alcohol (PVA) were prepared by the sol-gel process. The hybrid foam 50% PVA/50% bioactive glass was chosen in the composition range studied as the scaffold with better compromise in terms of pore structure and mechanical behavior. The aim of the present study was to evaluate the adhesion, viability, and growth behavior of human-adipose-tissue-derived stem cells on bioactive glass/PVA foams in vitro and their potential as scaffold for application in bone-tissue engineering. The pore structure of the hybrid samples used in the study was analyzed by microcomputed tomography, showing a modal pore diameter of 284 μm and modal interconnect diameter of 138 μm. We found that cells are capable of adhesion, proliferation, growth, and ECM production on the scaffolds tested. The results show that the hybrid bioactive glass/PVA is a promising material for bone repair, providing a good environment for the adhesion and proliferation of human-adipose-tissue-derived stem cells in vitro.

Evaluation of 3-D bioactive glass scaffolds dissolution in a perfusion flow system with X-ray microtomography

Bioactive glass has high potential for bone regeneration due to its ability to bond to bone and stimulate osteogenesis whilst dissolving in the body. Although three-dimensional (3-D) bioactive glass scaffolds with favorable pore networks can be made from the sol-gel process, compositional and structural evolutions in their porous structures during degradation in vivo, or in vitro, have not been quantified. In this study, bioactive glass scaffolds were put in a simulated body fluid flow environment through a perfusion bioreactor. X-ray microtomography (μCT) was used to non-destructively image the scaffolds at different degradation stages. A new 3-D image processing methodology was developed to quantify the scaffold's pore size, interconnect size and connectivity from μCT images. The accurate measurement of individual interconnect size was made possible by a principal component analysis-based algorithm. During 28 days of dissolution, the modal interconnect size in the scaffold was reduced from 254 to 206 μm due to the deposition of mineral phases. However, the pore size remained unchanged, with a mode of 682 μm. The data presented are important for making bioactive glass scaffolds into clinical products. The technique described for imaging and quantifying scaffold pore structures as a function of degradation time is applicable to most scaffold systems.

In vitro 3D culture of human chondrocytes using modified ε-caprolactone scaffolds with varying hydrophilicity and porosity

Two series of 3D scaffolds based on ε-caprolactone were synthesized. The pore size and architecture (spherical interconnected pores) was the same in all the scaffolds. In one of the series of scaffolds, made of pure ε-polycaprolactone, the volume fraction of pores varied between 60% and 85% with the main consequence of varying the interconnectivity between pores since the pore size was kept constant. The other scaffolds were prepared with copolymers made of a ε-caprolactone-based hydrophobous monomer and hydroxyethyl acrylate, as the hydrophilic component. Thus, the hydrophilicity and, presumably, the adhesion properties varied monotonously in the copolymer series while porosity was kept constant. A suspension of human chondrocytes in culture medium was injected in the 3D scaffolds and cultured in static conditions up to 28 days. SEM and immunofluorescence assays allowed characterizing cells and extracellular matrix inside the scaffolds after different culture times. To do that, cross sections of the scaffolds were observed by SEM and confocal microscopy. The quantity of cells inside the scaffolds decreases with a decrease of the volume fraction of pores, due to the lack of interconnectivity between the cavities. The scaffolds up to a 30% of hydrophilicity behave in a similar way than the hydrophobous; a further increase of the hydrophilicity rapidly decreases cell viability. In all the experiments production of collagen type I, type II, and aggrecan was found, and some cells were Ki-67 positive, showing that some cells are adhered to the pore walls and maintain their dedifferentiated phenotype even when cultured in three-dimensional conditions.

Spiral-structured, nanofibrous, 3D scaffolds for bone tissue engineering

Polymeric nanofiber matrices have already been widely used in tissue engineering. However, the fabrication of nanofibers into complex three-dimensional (3D) structures is restricted due to current manufacturing techniques. To overcome this limitation, we have incorporated nanofibers onto spiral-structured 3D scaffolds made of poly (epsilon-caprolactone) (PCL). The spiral structure with open geometries, large surface areas, and porosity will be helpful for improving nutrient transport and cell penetration into the scaffolds, which are otherwise limited in conventional tissue-engineered scaffolds for large bone defects repair. To investigate the effect of structure and fiber coating on the performance of the scaffolds, three groups of scaffolds including cylindrical PCL scaffolds, spiral PCL scaffolds (without fiber coating), and spiral-structured fibrous PCL scaffolds (with fiber coating) have been prepared. The morphology, porosity, and mechanical properties of the scaffolds have been characterized. Furthermore, human osteoblast cells are seeded on these scaffolds, and the cell attachment, proliferation, differentiation, and mineralized matrix deposition on the scaffolds are evaluated. The results indicated that the spiral scaffolds possess porosities within the range of human trabecular bone and an appropriate pore structure for cell growth, and significantly lower compressive modulus and strength than cylindrical scaffolds. When compared with the cylindrical scaffolds, the spiral-structured scaffolds demonstrated enhanced cell proliferation, differentiation, and mineralization and allowed better cellular growth and penetration. The incorporation of nanofibers onto spiral scaffolds further enhanced cell attachment, proliferation, and differentiation. These studies suggest that spiral-structured nanofibrous scaffolds may serve as promising alternatives for bone tissue engineering applications.

Visualizing biointerfaces in three dimensions: electron tomography of the bone-hydroxyapatite interface

A positive interaction between human bone tissue and synthetics is crucial for the success of bone-regenerative materials. A greater understanding of the mechanisms governing bone-bonding is often gained via visualization of the bone-implant interface. Interfaces to bone have long been imaged with light, X-rays and electrons. Most of these techniques, however, only provide low-resolution or two-dimensional information. With the advances in modern day transmission electron microscopy, including new hardware and increased software computational speeds, the high-resolution visualization and analysis of three-dimensional structures is possible via electron tomography. We report, for the first time, a three-dimensional reconstruction of the interface between human bone and a hydroxyapatite implant using Z-contrast electron tomography. Viewing this structure in three dimensions enabled us to observe the nanometre differences in the orientation of hydroxyapatite crystals precipitated on the implant surface in vivo versus those in the collagen matrix of bone. Insight into the morphology of biointerfaces is considerably enhanced with three-dimensional techniques. In this regard, electron tomography may revolutionize the approach to high-resolution biointerface characterization.