Growing bone tissue-engineered niches with graded osteogenicity: an in vitro method for biomimetic construct assembly

3D Printed Piezoelectric BaTiO3/Polyhydroxybutyrate Nanocomposite Scaffolds for Bone Tissue Engineering

Bone defects are a significant health problem worldwide. Novel treatment approaches in the tissue engineering field rely on the use of biomaterial scaffolds to stimulate and guide the regeneration of damaged tissue that cannot repair or regrow spontaneously. This work aimed at developing and characterizing new piezoelectric scaffolds to provide electric bio-signals naturally present in bone and vascular tissues. Mixing and extrusion were used to obtain nanocomposites made of polyhydroxybutyrate (PHB) as a matrix and barium titanate (BaTiO3) nanoparticles as a filler, at BaTiO3/PHB compositions of 5/95, 10/90, 15/85 and 20/80 (w/w%). The morphological, thermal, mechanical and piezoelectric properties of the nanocomposites were studied. Scanning electron microscopy analysis showed good nanoparticle dispersion within the polymer matrix. Considerable increases in the Young's modulus, compressive strength and the piezoelectric coefficient d31 were observed with increasing BaTiO3 content, with d31 = 37 pm/V in 20/80 (w/w%) BaTiO3/PHB. 3D printing was used to produce porous cubic-shaped scaffolds using a 90° lay-down pattern, with pore size ranging in 0.60-0.77 mm and good mechanical stability. Biodegradation tests conducted for 8 weeks in saline solution at 37 °C showed low mass loss (∼4%) for 3D printed scaffolds. The results obtained in terms of piezoelectric, mechanical and chemical properties of the nanocomposite provide a new promising strategy for vascularized bone tissue engineering.

Translation of biophysical environment in bone into dynamic cell culture under flow for bone tissue engineering

Bone is a dynamic environment where osteocytes, osteoblasts, and mesenchymal stem/progenitor cells perceive mechanical cues and regulate bone metabolism accordingly. In particular, interstitial fluid flow in bone and bone marrow serves as a primary biophysical stimulus, which regulates the growth and fate of the cellular components of bone. The processes of mechano-sensory and -transduction towards bone formation have been well studied mainly in vivo as well as in two-dimensional (2D) dynamic cell culture platforms, which elucidated mechanically induced osteogenesis starting with anabolic responses, such as production of nitrogen oxide and prostaglandins followed by the activation of canonical Wnt signaling, upon mechanosensation. The knowledge has been now translated into regenerative medicine, particularly into the field of bone tissue engineering, where multipotent stem cells are combined with three-dimensional (3D) scaffolding biomaterials to produce transplantable constructs for bone regeneration. In the presence of 3D scaffolds, the importance of suitable dynamic cell culture platforms increases further not only to improve mass transfer inside the scaffolds but to provide appropriate biophysical cues to guide cell fate. In principle, the concept of dynamic cell culture platforms is rooted to bone mechanobiology. Therefore, this review primarily focuses on biophysical environment in bone and its translation into dynamic cell culture platforms commonly used for 2D and 3D cell expansion, including their advancement, challenges, and future perspectives. Additionally, it provides the literature review of recent empirical studies using 2D and 3D flow-based dynamic cell culture systems for bone tissue engineering.

Current strategies with implementation of three-dimensional cell culture: the challenge of quantification

From growing cells in spheroids to arranging them on complex engineered scaffolds, three-dimensional cell culture protocols are rapidly expanding and diversifying. While these systems may often improve the physiological relevance of cell culture models, they come with technical challenges, as many of the analytical methods used to characterize traditional two-dimensional (2D) cells must be modified or replaced to be effective. Here we review the advantages and limitations of quantification methods based either on biochemical measurements or microscopy imaging. We focus on the most basic of parameters that one may want to measure, the number of cells. Precise determination of this number is essential for many analytical techniques where measured quantities are only meaningful when normalized to the number of cells (e.g. cytochrome p450 enzyme activity). Thus, accurate measurement of cell number is often a prerequisite to allowing comparisons across different conditions (culturing conditions or drug and treatment screening) or between cells in different spatial states. We note that this issue is often neglected in the literature with little or no information given regarding how normalization was performed, we highlight the pitfalls and complications of quantification and call for more accurate reporting to improve reproducibility.

Piezoelectric Signals in Vascularized Bone Regeneration

The demand for bone substitutes is increasing in Western countries. Bone graft substitutes aim to provide reconstructive surgeons with off-the-shelf alternatives to the natural bone taken from humans or animal species. Under the tissue engineering paradigm, biomaterial scaffolds can be designed by incorporating bone stem cells to decrease the disadvantages of traditional tissue grafts. However, the effective clinical application of tissue-engineered bone is limited by insufficient neovascularization. As bone is a highly vascularized tissue, new strategies to promote both osteogenesis and vasculogenesis within the scaffolds need to be considered for a successful regeneration. It has been demonstrated that bone and blood vases are piezoelectric, namely, electric signals are locally produced upon mechanical stimulation of these tissues. The specific effects of electric charge generation on different cells are not fully understood, but a substantial amount of evidence has suggested their functional and physiological roles. This review summarizes the special contribution of piezoelectricity as a stimulatory signal for bone and vascular tissue regeneration, including osteogenesis, angiogenesis, vascular repair, and tissue engineering, by considering different stem cell sources entailed with osteogenic and angiogenic potential, aimed at collecting the key findings that may enable the development of successful vascularized bone replacements useful in orthopedic and otologic surgery.

Bioactive and Topographically-Modified Electrospun Membranes for the Creation of New Bone Regeneration Models

Bone injuries that arise from trauma, cancer treatment, or infection are a major and growing global challenge. An increasingly ageing population plays a key role in this, since a growing number of fractures are due to diseases such as osteoporosis, which place a burden on healthcare systems. Current reparative strategies do not sufficiently consider cell-substrate interactions that are found in healthy tissues; therefore, the need for more complex models is clear. The creation of in vitro defined 3D microenvironments is an emerging topographically-orientated approach that provides opportunities to apply knowledge of cell migration and differentiation mechanisms to the creation of new cell substrates. Moreover, introducing biofunctional agents within in vitro models for bone regeneration has allowed, to a certain degree, the control of cell fate towards osteogenic pathways. In this research, we applied three methods for functionalizing spatially-confined electrospun artificial microenvironments that presented relevant components of the native bone stem cell niche. The biological and osteogenic behaviors of mesenchymal stromal cells (MSCs) were investigated on electrospun micro-fabricated scaffolds functionalized with extracellular matrix (ECM) proteins (collagen I), glycosaminoglycans (heparin), and ceramic-based materials (bioglass). Collagen, heparin, and bioglass (BG) were successfully included in the models without modifying the fibrous structures offered by the polycaprolactone (PCL) scaffolds. Mesenchymal stromal cells (MSCs) were successfully seeded in all the biofunctional scaffolds and they showed an increase in alkaline phosphatase production when exposed to PCL/BG composites. This research demonstrates the feasibility of manufacturing smart and hierarchical artificial microenvironments for studying stem cell behavior and ultimately the potential of incorporating these artificial microenvironments into multifunctional membranes for bone tissue regeneration

Bone Substitutes in Orthopaedic Surgery: Current Status and Future Perspectives

Bone replacement materials have been successfully supplied for a long time. But there are cases, especially in critical sized bone defects, in which the therapy is not sufficient. Nowadays, there are multiple bone substitutes available. Autologous bone grafts remain the "gold standard" in bone regeneration. Yet, donor-site morbidity and the available amount of sufficient material are limitations for autologous bone grafting. This study aimed to provide information about the current status in research regarding bone substitutes. We report on the advantages and drawbacks of several bone substitutes. At the end, we discuss the current developments of combining ceramic substitutes with osteoinductive substances.

Surgical vacuum filter-derived stromal cells are superior in proliferation to human bone marrow aspirate

Background

During joint replacement, surgical vacuum suction guarantees a sufficient overview on the situs. We assume high concentrations of mesenchymal stromal cells (MSCs) on surgical vacuum filters.

We compared the in vitro proliferative and differentiation potency of cells from the following: (i) bone marrow (BM), (ii) cancellous bone (CB), (iii) vacuum filter (VF), and (iv) cell saver filtrate reservoir (SF) in 32 patients undergoing elective total hip replacement.

Methods

Mononuclear cells (MNC) were isolated, and cell proliferation and colony-forming units (CFU) were measured. Adherent cells were characterized by flow cytometry for MSC surface markers. Cells were incubated with osteogenic, adipogenic, and chondrogenic stimuli. Cells were cytochemically stained and osteoblastic expression (RUNX-2, ALP, and BMP-2) investigated via qPCR.

Results

Dependent on the source, initial MNC amount as well as CFU number was significantly different whereas generation time did not vary significantly. CFU numbers from VF were superior to those from SR, BM, and CB. The resulting amount of MSC from the respective source was highest in the vacuum filter followed by reservoir, aspirate, and cancellous bone. Cells from all groups could be differentiated into the three mesenchymal lines demonstrating their stemness nature. However, gene expression of osteoblastic markers did not differ significantly between the groups.

Conclusion

We conclude that surgical vacuum filters are able to concentrate tissue with relevant amounts of MSCs. A new potent source of autologous regeneration material with clinical significance is identified. Further clinical studies have to elucidate the regenerative potential of this material in an autologous setting.

Fabrication of carbonate apatite honeycomb and its tissue response

Carbonate apatite (CO₃ Ap) block can be used as a bone substitute because it can be remodeled to new natural bone in a manner conforming with the bone remodeling process. Among the many porous structures available, honeycomb (HC) structure is advantageous for rapid replacement of CO₃ Ap to bone. In this study, the feasibility to fabricate a CO₃ Ap HC was studied, along with its initial tissue response in rabbit femur bone defect. First, a mixture of Ca(OH)₂ and a wax-based binder was extruded from a HC mold. Then the fabricated HC was heated for binder removal and carbonation at 450°C in a mixed O₂ -CO₂ atmosphere, forming a CaCO₃ HC. When the CaCO₃ HC was immersed in 1 mol/L Na₃ PO₄ solution at 80°C for 7 days, its composition changed from CaCO₃ to CO₃ Ap, maintaining the structure of the original CaCO₃ HC. Compressive strengths of the CaCO₃ and CO₃ Ap HCs were 65.2 ± 7.4 MPa and 88.7 ± 4.7 MPa, respectively. When the rabbit femur bone defect was reconstructed with the CO₃ Ap HC, new bone penetrated the CO₃ Ap HC completely. Osteoclasts and osteoblasts were found on the surface of the newly formed bone and osteocytes were also found in the newly formed bone, indicating ongoing bone remodeling. Furthermore, blood vessels were formed inside the pores of CO₃ Ap HC. Therefore, CO₃ Ap HC has good potential as an ideal bone substitute. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1014-1020, 2019.

Chemoradiotherapy screening in a novel biomimetic polymer based pancreatic cancer model

Pancreatic Ductal Adenocarcinoma (PDAC) is a deadly and aggressive disease with a very low survival rate. This is partly due to the resistance of the disease to currently available treatment options. Herein, we report for the first time the use of a novel polyurethane scaffold based PDAC model for screening the short and relatively long term (1 and 17 days post-treatment) responses of chemotherapy, radiotherapy and their combination. We show a dose dependent cell viability reduction and apoptosis induction for both chemotherapy and radiotherapy. Furthermore, we observe a change in the impact of the treatment depending on the time-frame, especially for radiation for which the PDAC scaffolds showed resistance after 1 day but responded more 17 days post-treatment. This is the first study to report a viable PDAC culture in a scaffold for more than 2 months and the first to perform long-term (17 days) post-treatment observations in vitro. This is particularly important as a longer time-frame is much closer to animal studies and to patient treatment regimes, highlighting that our scaffold system has great potential to be used as an animal free model for screening of PDAC.

Poly-urethane scaffold based 3D pancreatic cancer model enables realistic long term chemotherapy and radiotherapy screening. This model can be used for personalised treatment screening.

Bovine bone matrix/poly(l-lactic-co-ε-caprolactone)/gelatin hybrid scaffold (SmartBone®) for maxillary sinus augmentation: A histologic study on bone regeneration

The ideal scaffold for bone regeneration is required to be highly porous, non-immunogenic, biostable until the new tissue formation, bioresorbable and osteoconductive. This study aimed at investigating the process of new bone formation in patients treated with granular SmartBone^® for sinus augmentation, providing an extensive histologic analysis. Five biopsies were collected at 4-9 months post SmartBone^® implantation and processed for histochemistry and immunohistochemistry. Histomorphometric analysis was performed. Bone-particle conductivity index (BPCi) was used to assess SmartBone^® osteoconductivity. At 4 months, SmartBone^® (12%) and new bone (43.9%) were both present and surrounded by vascularized connective tissue (37.2%). New bone was grown on SmartBone^® (BPCi=0.22). At 6 months, SmartBone^® was almost completely resorbed (0.5%) and new bone was massively present (80.8%). At 7 and 9 months, new bone accounted for a large volume fraction (79.3% and 67.4%, respectively) and SmartBone^® was resorbed (0.5% and 0%, respectively). Well-oriented lamellae and bone scars, typical of mature bone, were observed. In all the biopsies, bone matrix biomolecules and active osteoblasts were visible. The absence of inflammatory cells confirmed SmartBone^® biocompatibility and non-immunogenicity. These data indicate that SmartBone^® is osteoconductive, promotes fast bone regeneration, leading to mature bone formation in about 7 months.

Processing large-diameter poly(L-lactic acid) microfiber mesh/mesenchymal stromal cell constructs via resin embedding: an efficient histologic method

In this study, we performed a complete histologic analysis of constructs based on large diameter ( >100 μm) poly-L-lactic acid (PLLA) microfibers obtained via dry-wet spinning and rat Mesenchymal Stromal Cells (rMSCs) differentiated towards the osteogenic lineage, using acrylic resin embedding. In many synthetic polymer-based microfiber meshes, ex post processability of fiber/cell constructs for histologic analysis may face deterring difficulties, leading to an incomplete investigation of the potential of these scaffolds. Indeed, while polymeric nanofiber (fiber diameter = tens of nanometers)/cell constructs can usually be embedded in common histologic media and easily sectioned, preserving the material structure and the antigenic reactivity, histologic analysis of large polymeric microfiber/cell constructs in the literature is really scant. This affects microfiber scaffolds based on FDA-approved and widely used polymers such as PLLA and its copolymers. Indeed, for such constructs, especially those with fiber diameter and fiber interspace much larger than cell size, standard histologic processing is usually inefficient due to inhomogeneous hardness and lack of cohesion between the synthetic and the biological phases under sectioning. In this study, the microfiber/MSC constructs were embedded in acrylic resin and the staining/reaction procedures were calibrated to demonstrate the possibility of successfully employing histologic methods in tissue engineering studies even in such difficult cases. We histologically investigated the main osteogenic markers and extracellular matrix molecules, such as alkaline phosphatase, osteopontin, osteocalcin, TGF-β1, Runx2, Collagen type I and the presence of amorphous, fibrillar and mineralized matrix. Biochemical tests were employed to confirm our findings. This protocol permitted efficient sectioning of the treated constructs and good penetration of the histologic reagents, thus allowing distribution and expression of almost all the tested molecules to be revealed. Our results demonstrated that it is possible to perform histologic analyses of large-diameter PLLA-based microfiber scaffold/MSC constructs that face the failure of standard histologic procedures.

Use of autologous human mesenchymal stromal cell/fibrin clot constructs in upper limb non-unions: long-term assessment

BACKGROUND

Tissue engineering appears to be an attractive alternative to the traditional approach in the treatment of fracture non-unions. Mesenchymal stromal cells (MSCs) are considered an appealing cell source for clinical intervention. However, ex vivo cell expansion and differentiation towards the osteogenic lineage, together with the design of a suitable scaffold have yet to be optimized. Major concerns exist about the safety of MSC-based therapies, including possible abnormal overgrowth and potential cancer evolution.

AIMS

We examined the long-term efficacy and safety of ex vivo expanded bone marrow MSCs, embedded in autologous fibrin clots, for the healing of atrophic pseudarthrosis of the upper limb. Our research work relied on three main issues: use of an entirely autologous context (cells, serum for ex vivo cell culture, scaffold components), reduced ex vivo cell expansion, and short-term MSC osteoinduction before implantation.

METHODS AND FINDINGS

Bone marrow MSCs isolated from 8 patients were expanded ex vivo until passage 1 and short-term osteo-differentiated in autologous-based culture conditions. Tissue-engineered constructs designed to embed MSCs in autologous fibrin clots were locally implanted with bone grafts, calibrating their number on the extension of bone damage. Radiographic healing was evaluated with short- and long-term follow-ups (range averages: 6.7 and 76.0 months, respectively). All patients recovered limb function, with no evidence of tissue overgrowth or tumor formation.

CONCLUSIONS

Our study indicates that highly autologous treatment can be effective and safe in the long-term healing of bone non-unions. This tissue engineering approach resulted in successful clinical and functional outcomes for all patients.