Biomaterial challenges and approaches to stem cell use in bone reconstructive surgery

Treatment of a Large Tibial Non-Union Bone Defect in a Cat Using Xenograft with Canine-Derived Cancellous Bone, Demineralized Bone Matrix, and Autograft

A 17-month-old domestic short-hair cat was referred due to a non-union in the left tibia. The initial repair, conducted 3 months prior at another animal hospital, involved an intramedullary (IM) pin and wire to address a comminuted fracture. Unfortunately, the wire knot caused a skin tract, resulting in osteomyelitis. Although the wire knot was removed at that hospital, the draining tract persisted, continuously discharging exudate. Upon evaluation, the first surgery was reassessed and revised, involving the removal of the IM pin and the application of external skeletal fixation alongside an antibiotic susceptibility test. After 118 days post-revision surgery, while some cortical continuity was observed, a significant bone defect persisted, posing a substantial risk of refracture should the implant be removed. A second revision surgery was performed, utilizing a bone plate combined with cancellous bone autograft, recombinant human bone morphogenetic protein-2, and xenograft featuring a canine-derived cancellous chip mixed with demineralized bone matrix. Remarkably, the bone completed its healing within 105 days following the subsequent surgery. Radiography demonstrated successful management of the large bone defect up to the 2-year postoperative check-up. During telephone follow-ups for 3.5 years after surgery, no complications were identified, and the subject maintained a favorable gait.

Modelling cell adaptation using internal variables: Accounting for cell plasticity in continuum mathematical biology

Cellular adaptation is the ability of cells to change in response to different stimuli and environmental conditions. It occurs via phenotypic plasticity, that is, changes in gene expression derived from changes in the physiological environment. This phenomenon is important in many biological processes, in particular in cancer evolution and its treatment. Therefore, it is crucial to understand the mechanisms behind it. Specifically, the emergence of the cancer stem cell phenotype, showing enhanced proliferation and invasion rates, is an essential process in tumour progression. We present a mathematical framework to simulate phenotypic heterogeneity in different cell populations as a result of their interaction with chemical species in their microenvironment, through a continuum model using the well-known concept of internal variables to model cell phenotype. The resulting model, derived from conservation laws, incorporates the relationship between the phenotype and the history of the stimuli to which cells have been subjected, together with the inheritance of that phenotype. To illustrate the model capabilities, it is particularised for glioblastoma adaptation to hypoxia. A parametric analysis is carried out to investigate the impact of each model parameter regulating cellular adaptation, showing that it permits reproducing different trends reported in the scientific literature. The framework can be easily adapted to any particular problem of cell plasticity, with the main limitation of having enough cells to allow working with continuum variables. With appropriate calibration and validation, it could be useful for exploring the underlying processes of cellular adaptation, as well as for proposing favourable/unfavourable conditions or treatments.

A comprehensive review of properties of the biocompatible thin films on biodegradable Mg alloys

Magnesium (Mg) and its alloys have attracted attention as biodegradable materials for biomedical applications owing to their mechanical properties being comparable to that of bone. Mg is a vital trace element in many enzymes and thus forms one of the essential factors for human metabolism. However, before being used in biomedical applications, the early stage or fast degradation of Mg and its alloys in the physiological environment should be controlled. The degradation of Mg alloys is a critical criterion that can be controlled by a surface modification which is an effective process for conserving their desired properties. Different coating methods have been employed to modify Mg surfaces to provide good corrosion resistance and biocompatibility. This review aims to provide information on different coatings and discuss their physical and biological properties. Finally, the current withstanding challenges have been highlighted and discussed, followed by shedding some light on future perspectives.

SDF-1α/OPF/BP Composites Enhance the Migrating and Osteogenic Abilities of Mesenchymal Stem Cells

In situ cell recruitment is a promising regenerative medicine strategy with the purpose of tissue regeneration without stem cell transplantation. This chemotaxis-based strategy is aimed at ensuring a restorative environment through the release of chemokines that promote site-specific migration of healing cell populations. Stromal cell-derived factor-1α (SDF-1α) is a critical chemokine that can regulate the migration of mesenchymal stem cells (MSCs). Accordingly, here, SDF-1α-loaded microporous oligo[poly(ethylene glycol) fumarate]/bis[2-(methacryloyloxy)ethyl] phosphate composites (SDF-1α/OPF/BP) were engineered and probed. SDF-1α/OPF/BP composites were loaded with escalating SDF-1α concentrations, namely, 0 ng/ml, 50 ng/ml, 100 ng/ml, and 200 ng/ml, and were cocultured with MSC. Scratching assay, Transwell assay, and three-dimensional migration model were utilized to assess the migration response of MSCs. Immunofluorescence staining of Runx2 and osteopontin (OPN), ELISA assay of osteocalcin (OCN) and alkaline phosphatase (ALP), and Alizarin Red S staining were conducted to assess the osteogenesis of MSCs. All SDF-1α/OPF/BP composites engendered a release of SDF-1α (>80%) during the first four days. SDF-1α released from the composites significantly promoted migration and osteogenic differentiation of MSCs documented by upregulated expression of osteogenic-related proteins, ALP, Runx2, OCN, and OPN. SDF-1α at 100 ng/ml was optimal for enhanced migration and osteogenic proficiency. Thus, designed SDF-1α/OPF/BP composites were competent in promoting the homing and osteogenesis of MSCs and thus offer a promising bioactive scaffold candidate for on-demand bone tissue regeneration.

Bioengineering strategies for bone and cartilage tissue regeneration using growth factors and stem cells

Bone and cartilage tissue engineering is an integrative approach that is inspired by the phenomena associated with wound healing. In this respect, growth factors have emerged as important moieties for the control and regulation of this process. Growth factors act as mediators and control the important physiological functions of bone regeneration. Herein, we discuss the importance of growth factors in bone and cartilage tissue engineering, their loading and delivery strategies, release kinetics, and their integration with biomaterials and stem cells to heal bone fractures. We also highlighted the role of growth factors in the determination of the bone tissue microenvironment based on the reciprocal signaling with cells and biomaterial scaffolds on which future bone and cartilage tissue engineering technologies and medical devices will be based upon.

Effects of acid-alkali treatment on bioactivity and osteoinduction of porous titanium: An in vitro study

BACKGROUND

To elucidate the bioactivity and bone regeneration of porous titanium surfaces treated using acid-alkali combination, and to define the optimal alkali reaction time.

METHODS

Ten groups of porous Ti with at least 3 per group undergoing different acid-alkali treated time were prepared. The surface was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), bicinchoninic acid method (BCA), optical contact angle measurement and Raman spectrometry. Compression testing was performed with a universal testing machine. The bioactivity and osteoinduction were evaluated by a series of biological tests using a simulated body fluid (SBF) test, cell proliferation test, vinculin, ALP and OCN expression, and cell mineralization.

RESULTS

The acid-alkali treatment formed micro- and nano-scale structures on the sample surfaces. The alkali treatment for 12 h achieved the sharpest nano-scale surface relief and the most protein absorption. The treated porous surface was coated with a NaHTiO₃ layer. The acid-alkali etching did not compromise the elastic modulus and compressive strength of the porous Ti samples. In addition to hydroxyapatite, a perovskite phase was also formed on the treated porous samples in SBF. Non-treated dense Ti showed more cell adhesion and proliferation (P < 0.05), while osteoinduction and mineralization were more pronounced on the treated porous sample (P < 0.05).

CONCLUSION

Acid-alkali treatment is an effective means of generating nano-scale relief on porous Ti surface, and is beneficial for bioactivity and bone regeneration. The 15 min acid and 12 h alkali etching is the optimal combination. The osteoinductive efficacy may be attributable to the surface physical chemistry and the formation of hydroxyapatite and perovskite layers, rather than direct cell adhesion and proliferation.

Perfusion Enhances Solute Transfer into the Shell of Hollow Fiber Membrane Bioreactors for Bone Tissue Engineering

Preparation of tissue engineered (TE) 3D constructs to repair large bone defects is limited by the difficult supply of nutrients and oxygen to cells in the innermost regions of constructs cultured in bioreactors. Poor oxygenation negatively affects cell viability and function. Bioreactor design optimization may help relieve these limitations. Bioreactors in which cells are cultured outside bundles of hollow fiber membranes (HFMBs) are structurally similar to natural bone. HFMB operation in pure diffusion has been reported to suffice for fibroblasts, but is deemed insufficient for bone cells. In this paper, the effect of perfusion flows in the cell compartment on solute transfer was investigated in HFMBs differing in design and operating conditions. HFMBs were designed and operated using values of non-dimensional groups that ensured solutes transfer towards the cell compartment mainly by diffusion; in the presence of low to high Starling flows; in the presence of pulsatile radial flows obtained by periodically stopping the solution flow leaving the bioreactor using a pinch valve. Distribution of matter in cell-free HFMBs was evaluated with tracer experiments in an optimized apparatus. Effectiveness of solute transfer to cell compartment was assessed based on the bioreactor response in terms of the shell volume actively involved in mass transfer (VMTA) according to transport models developed specifically for the purpose. VMTA increased with increasing Starling flows. In the pulsatile radial flow mode, tracer concentration in the shell increased 3 times faster than at high Starling flows. This suggests that controlled perfusion flows in HFMBs might enable the engineering of large TE bone constructs.

Biomimetic Tissue-Engineered Bone Substitutes for Maxillofacial and Craniofacial Repair: The Potential of Cell Sheet Technologies

Maxillofacial defects are complex lesions stemming from various etiologies: accidental, congenital, pathological, or surgical. A bone graft may be required when the normal regenerative capacity of the bone is exceeded or insufficient. Surgeons have many options available for bone grafting including the "gold standard" autologous bone graft. However, this approach is not without drawbacks such as the morbidity associated with harvesting bone from a donor site, pain, infection, or a poor quantity and quality of bone in some patient populations. This review discusses the various bone graft substitutes used for maxillofacial and craniofacial repair: allografts, xenografts, synthetic biomaterials, and tissue-engineered substitutes. A brief overview of bone tissue engineering evolution including the use of mesenchymal stem cells is exposed, highlighting the first clinical applications of adipose-derived stem/stromal cells in craniofacial reconstruction. The importance of prevascularization strategies for bone tissue engineering is also discussed, with an emphasis on recent work describing substitutes produced using cell sheet-based technologies, including the use of thermo-responsive plates and the self-assembly approach of tissue engineering. Indeed, considering their entirely cell-based design, these natural bone-like substitutes have the potential to closely mimic the osteogenicity, osteoconductivity, osteoinduction, and osseointegration properties of autogenous bone for maxillofacial and craniofacial reconstruction.

Mechanical and biological effects of infiltration with biopolymers on 3D printed tricalciumphosphate scaffolds

The aim of this study was to evaluate the influence of infiltrating 3D printed (TCP) scaffolds with different biodegradable polymers on their mechanical and biological properties. 3D printed TCP scaffolds with interconnecting channels measuring 450±50 µm were infiltrated with four different biodegradable copolymers. To determine the average compressive strength, a uniaxial testing system was used. Additionally, scaffolds were seeded with MC3T3 cells and cell viability was assessed by live/dead-assay. Uninfiltrated TCP had an average compression strength of 1.92±0.38 MPa. Mechanical stability was considerably increased in all infiltrated scaffolds up to a maximum of 7.36±0.57 MPa. All scaffolds demonstrated high cell survival rates with a maximum of 94±10 % living cells. In conclusion, infiltration of 3D printed tricalcium phosphate scaffolds with biodegradable polymers significantly improved mechanical properties and biological properties were comparable to those of uninfiltrated TCP scaffolds.

Biomimetic Rotated Lamellar Plywood Motifs by Additive Manufacturing of Metal Alloy Scaffolds for Bone Tissue Engineering

Additive manufacturing presents opportunities to treat bone defects using biomimetic tissue scaffolds. Past investigations have explored modulating scaffold mechanical properties through varying materials and geometric motifs. Herein, we applied the rotated plywood structure of bone tissue to a 3D printed scaffold with the goal of improving mechanical performance compared to an orthogonal mesh design commonly used in tissue scaffold applications. The scaffolds were subjected to uniaxial compression followed by scanning electron microscopy and microcomputer tomography. The uniaxial compression test was characterized through elastic modulus (mean 1.32 GPa biomimetic, 0.196 GPa orthogonal, p < 0.001), ultimate compressive strength (mean 16.546 MPa biomimetic, 6.309 MPa orthogonal design, p < 0.001), and ultimate compressive strain values (4.867% biomimetic, 9.000% orthogonal, p < 0.005). Correlation of microfracture imaging to bulk scaffold mode of failure suggest that utilizing the biomimetic plywood design not only improved mechanical performance, but also reduced asymmetrtic buckling, plastic deformation, and fracture propagation similar to bone tissue.

Effect of hypoxia on the proliferation of porcine bone marrow-derived mesenchymal stem cells and adipose-derived mesenchymal stem cells in 2- and 3-dimensional culture

OBJECTIVE

Bone marrow-derived mesenchymal stem cells (MSCs) and adipose-derived mesenchymal stem cells (ASCs) currently represent a promising tool for the regeneration of large bony defects. Therefore, it is pivotal to find the best cell source within the body and the best conditions for in vitro cellular expansion. This study compared cellular response of MSCs and ASCs from a porcine animal in normoxic (21% O2) and hypoxic (2% O2) cell culture conditions via 2D and 3D experimental settings.

MATERIALS AND METHODS

The effect of constant exposure to hypoxia on primary pig stem cells was evaluated by two methods. First, a cumulative population doublings (cumPD) over a period of 40 days, a metabolic activity assay in both 2D and 3D beta-TCP-PHB scaffolds, followed by analysis of osteogenic differentiation potential in cell monolayers.

RESULTS

Our results displayed enhanced cell culture proliferation in 2% O2 for both MSCs and ASCs, with impaired osteogenic differentiation of MSCs. The impact of constant hypoxia on porcine MSCs and ASCs exhibited a statistically significant decrease in osteogenic differentiation under hypoxic conditions with the MSCs.

CONCLUSIONS

Our data suggest that MSCs and ASCs expanded in hypoxic culture conditions, might be more suitable for use in the clinical setting where large cell numbers are required. When differentiated in normoxic conditions, MSCs showed the highest osteogenic differentiation potential and might be the best choice of cells with consideration to bone repair.

Application of platelet-rich plasma with stem cells in bone and periodontal tissue engineering

Presently, there is a high paucity of bone grafts in the United States and worldwide. Regenerating bone is of prime concern due to the current demand of bone grafts and the increasing number of diseases causing bone loss. Autogenous bone is the present gold standard of bone regeneration. However, disadvantages like donor site morbidity and its decreased availability limit its use. Even allografts and synthetic grafting materials have their own limitations. As certain specific stem cells can be directed to differentiate into an osteoblastic lineage in the presence of growth factors (GFs), it makes stem cells the ideal agents for bone regeneration. Furthermore, platelet-rich plasma (PRP), which can be easily isolated from whole blood, is often used for bone regeneration, wound healing and bone defect repair. When stem cells are combined with PRP in the presence of GFs, they are able to promote osteogenesis. This review provides in-depth knowledge regarding the use of stem cells and PRP in vitro, in vivo and their application in clinical studies in the future.

In vitro evaluation of three different biomaterials as scaffolds for canine mesenchymal stem cells

PURPOSE

To evaluate in vitro ability the of three different biomaterials - purified hydroxyapatite, demineralized bone matrix and castor oil-based polyurethane - as biocompatible 3D scaffolds for canine bone marrow mesenchymal stem cell (MSC) intending bone tissue engineering.

METHODS

MSCs were isolated from canine bone marrow, characterized and cultivated for seven days with the biomaterials. Cell proliferation and adhesion to the biomaterial surface were evaluated by scanning electron microscopy while differentiation into osteogenic lineage was evaluated by Alizarin Red staining and Sp7/Osterix surface antibody marker.

RESULTS

The biomaterials allowed cellular growth, attachment and proliferation. Osteogenic differentiation occurred in the presence of hydroxyapatite, and matrix deposition commenced in the presence of the castor oil-based polyurethane.

CONCLUSION

All the tested biomaterials may be used as mesenchymal stem cell scaffolds in cell-based orthopedic reconstructive therapy.

Fabrication of porous titanium implants by three-dimensional printing and sintering at different temperatures

This study evaluated the feasibility of using three-dimensional printing (3DP) to fabricate porous titanium implants. Titanium powder was blended with a water-soluble binder material. Green, porous, titanium implants fabricated by 3DP were sintered under protective argon atmosphere at 1,200, 1,300, or 1,400°C. Sintered implant prototypes had uniform shrinkage and no obvious shape distortion after sintering. Evaluation of their mechanical properties revealed that titanium prototypes sintered at different temperatures had elastic modulus of 5.9-34.8 GPa, porosity of 41.06-65.01%, hardness of 115.2-182.8 VHN, and compressive strength of 81.3-218.6 MPa. There were significant differences in each type of these data among the different sintering temperatures (p<0.01). Results of this study confirmed the feasibility of fabricating porous titanium implants by 3DP: pore size and pore interconnectivity were conducive to bone cell ingrowth for implant stabilization, and the mechanical properties matched well with those of the human bone.

Novel processing of iron-manganese alloy-based biomaterials by inkjet 3-D printing

The present work provides an assessment of 3-D printed iron-manganese biodegradable scaffolds as a bone scaffold material. Iron-based alloys have been investigated due to their high strength and ability to slowly corrode. Current fabrications of Fe-based materials generate raw material which must be machined into their desired form. By using inkjet 3-D printing, a technique which generates complex, customizable parts from powders mechanically milled Fe-30Mn (wt.%) powder was directly processed into scaffolds. The 3-D printed parts maintained an open porosity of 36.3% and formed a mixed phase alloy of martensitic ε and austenitic γ phases. Electrochemical corrosion tests showed the 3-D printed Fe-Mn to desirably corrode significantly more rapidly than pure iron. The scaffolds exhibited similar tensile mechanical properties to natural bone, which may reduce the risk of stress shielding. Cell viability testing of MC3T3-E1 pre-osteoblast cells seeded directly onto the Fe-Mn scaffolds using the live/dead assay and with cells cultured in the presence of the scaffolds' degradation products demonstrated good in vitro cytocompatibility compared to tissue culture plastic. Cell infiltration into the open pores of the 3-D printed scaffolds was also observed. Based on this preliminary study, we believe that 3-D printed Fe-Mn alloy is a promising material for craniofacial biomaterial applications, and represents an opportunity for other biodegradable metals to be fabricated using this unique method.

HTCC-modified nanoclay for tissue engineering applications: a synergistic cell growth and antibacterial efficiency

This paper deals with the synthesis of a biocompatible chitosan ammonium salt N-(2-hydroxy) propyl-3-trimethylammonium chitosan chloride (HTCC) and using it in montmorillonite ion-exchange process. HTCC-modified montmorillonite (Mt) with different chemical ratios was successfully synthesized, and their characteristics have been verified by XRD and FTIR analyses. Produced samples have been evaluated in terms of antibacterial efficiency and biocompatibility (cell culture test). Antibacterial efficiency of synthesized HTCC/Mt samples has been confirmed against both gram negative bacteria (Escherichia coli) and gram positive bacteria (Staphylococcus aureus). The results disclosed that the antibacterial efficiency of HTCC-modified montmorillonite was unexpectedly even more than HTCC. This excellent synergistic effect has been referred to entrapping bacteria between the intercalated structures of HTCC-modified montmorillonite. Then HTCC on clay layers can seriously attack and damage the entrapped bacteria. An extraordinary biocompatibility, cell attachment, and cell growth even more than tissue culture polystyrene (TCPS) have been recorded in the case of this novel kind of modified clay. Due to existing concerns about serious and chronic infections after implant placement, this natural-based bioactive and antibacterial modified clay can be used in electrospun nanofibers and other polymeric implants with promising mechanical properties for tissue engineering applications.

Bone tissue engineering: recent advances and challenges

The worldwide incidence of bone disorders and conditions has trended steeply upward and is expected to double by 2020, especially in populations where aging is coupled with increased obesity and poor physical activity. Engineered bone tissue has been viewed as a potential alternative to the conventional use of bone grafts, due to their limitless supply and no disease transmission. However, bone tissue engineering practices have not proceeded to clinical practice due to several limitations or challenges. Bone tissue engineering aims to induce new functional bone regeneration via the synergistic combination of biomaterials, cells, and factor therapy. In this review, we discuss the fundamentals of bone tissue engineering, highlighting the current state of this field. Further, we review the recent advances of biomaterial and cell-based research, as well as approaches used to enhance bone regeneration. Specifically, we discuss widely investigated biomaterial scaffolds, micro- and nano-structural properties of these scaffolds, and the incorporation of biomimetic properties and/or growth factors. In addition, we examine various cellular approaches, including the use of mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), adult stem cells, induced pluripotent stem cells (iPSCs), and platelet-rich plasma (PRP), and their clinical application strengths and limitations. We conclude by overviewing the challenges that face the bone tissue engineering field, such as the lack of sufficient vascularization at the defect site, and the research aimed at functional bone tissue engineering. These challenges will drive future research in the field.

Freeze-dried human serum albumin improves the adherence and proliferation of mesenchymal stem cells on mineralized human bone allografts

Mineralized scaffolds are widely used as bone grafts with the assumption that bone marrow derived cells colonize and remodel them. This process is slow and often unreliable so we aimed to improve the biocompatibility of bone grafts by pre-seeding them with human mesenchymal stem cells from either bone marrow or dental pulp. Under standard cell culture conditions very low number of seeded cells remained on the surface of freeze-dried human or bovine bone graft or hydroxyapatite. Coating the scaffolds with fibronectin or collagen improved seeding efficiency but the cells failed to grow on the surface until the 18th day. In contrast, human albumin was a very potent facilitator of both seeding and proliferation on allografts which was further improved by culturing in a rotating bioreactor. Electron microscopy revealed that cells do not form a monolayer but span the pores, emphasizing the importance of pore size and microstructure. Albumin coated bone chips were able to unite a rat femoral segmental defect, while uncoated ones did not. Micro-hardness measurements confirmed that albumin coating does not influence the physical characteristics of the scaffold, so it is possible to introduce albumin coating into the manufacturing process of lyophilized bone allografts.

The evaluation of ectopic bone formation induced by delivery systems for bone morphogenetic protein-9 or its derived peptide

We have earlier shown that a peptide derived from the bone morphogenetic protein-9 (pBMP-9) stimulates mouse preosteoblasts MC3T3-E1 differentiation in vitro. Here, we evaluated the effects of two delivery systems (DSs) for pBMP-9, one based on collagen and the other on chitosan. The release kinetics of BMP-9 (used as control) and pBMP-9 from these DSs were first determined in vitro by using enzyme-linked immunosorbent assay and high performance liquid chromatography assays, respectively. Micro-computerized tomography and histological analysis were then performed to study in vivo the ectopic ossification induced by both DSs containing these molecules in C57BL/6 mouse quadriceps. We found that collagen DS released in vitro about 35% of its BMP-9 within 1 h, whereas chitosan DS released 80%. The pBMP-9 was released from both DSs more slowly for up to 10 days. These release kinetics seemed to fit the Korsmeyer-Peppas model. Only chitosan DS containing BMP-9 induced strong bone formation in all mice quadriceps within 24 days. All mice quadriceps treated by pBMP-9 trapped in this DS also favored bone structures that started to mineralize. However, pBMP-9 in collagen DS failed to promote ectopic ossification within 24 days in vivo. This study highlights the importance to optimize carrier, thus improving the efficiency of pBMP-9 in vivo.

Surface modifications of magnesium alloys for biomedical applications

In recent years, research on magnesium (Mg) alloys had increased significantly for hard tissue replacement and stent application due to their outstanding advantages. Firstly, Mg alloys have mechanical properties similar to bone which avoid stress shielding. Secondly, they are biocompatible essential to the human metabolism as a factor for many enzymes. In addition, main degradation product Mg is an essential trace element for human enzymes. The most important reason is they are perfectly biodegradable in the body fluid. However, extremely high degradation rate, resulting in too rapid loss of mechanical strength in chloride containing environments limits their applications. Engineered artificial biomaterials with appropriate mechanical properties, surface chemistry, and surface topography are in a great demand. As the interaction between the cells and tissues with biomaterials at the tissue--implant interface is a surface phenomenon; surface properties play a major role in determining both the biological response to implants and the material response to the physiological condition. Therefore, the ability to modify the surface properties while preserve the bulk properties is important, and surface modification to form a hard, biocompatible and corrosion resistant modified layer have always been an interesting topic in biomaterials field. In this article, attempts are made to give an overview of the current research and development status of surface modification technologies of Mg alloys for biomedical materials research. Further, the advantages/disadvantages of the different methods and with regard to the most promising method for Mg alloys are discussed. Finally, the scientific challenges are proposed based on own research and the work of other scientists.

Evaluation of the lipid-rich layer of reamer aspirate

The fatty layer of aspirate obtained by reaming the femoral shaft using a reamer/irrigator/aspirator (RIA) device was characterized for fatty acid content and the presence of adult stem cells. Gas chromatography analysis was performed on samples taken from multiple patients to determine and compare the fatty acid contents of aspirate lipid samples. All four patients had the same four fatty acids present in the highest percentages: oleic, palmitic, linoleic and stearic. After successful isolation from bulk material, cells isolated from this lipid-rich layer were studied to determine their osteogenic and growth potential on a clinically available ceramic bone graft substitute. The results of metabolic activity and intracellular protein assays indicated that the ceramics supported growth of the cells isolated from the aspirate fat layer, although levels of alkaline phosphatase (ALP) expression were low for cells grown on the ceramics. Cells will not transition along the osteogenic pathway when they are actively dividing, and active growth may have contributed to the lack of ALP expression in this study. Isolated cells grown on tissue culture plastic expressed significant levels of the bone marker ALP. The results of this study suggest that cells isolated from the fat layer of RIA aspirate proliferate on ceramic bone void filler and have the potential to differentiate along an osteogenic pathway. Previously considered waste, the lipid-rich fat layer of aspirate may be a source of mesenchymal stem cells that, either alone or in conjunction with currently available synthetic bone graft material, could be used to stimulate new bone growth.

Contrasts between organic participation in apatite biomineralization in brachiopod shell and vertebrate bone identified by nuclear magnetic resonance spectroscopy

Unusually for invertebrates, linguliform brachiopods employ calcium phosphate mineral in hard tissue formation, in common with the evolutionarily distant vertebrates. Using solid-state nuclear magnetic resonance spectroscopy (SSNMR) and X-ray powder diffraction, we compare the organic constitution, crystallinity and organic matrix-mineral interface of phosphatic brachiopod shells with those of vertebrate bone. In particular, the organic-mineral interfaces crucial for the stability and properties of biomineral were probed with SSNMR rotational echo double resonance (REDOR). Lingula anatina and Discinisca tenuis shell materials yield strikingly dissimilar SSNMR spectra, arguing for quite different organic constitutions. However, their fluoroapatite-like mineral is highly crystalline, unlike the poorly ordered hydroxyapatite of bone. Neither shell material shows (13)C{(31)P} REDOR effects, excluding strong physico-chemical interactions between mineral and organic matrix, unlike bone in which glycosaminoglycans and proteins are composited with mineral at sub-nanometre length scales. Differences between organic matrix of shell material from L. anatina and D. tenuis, and bone reflect evolutionary pressures from contrasting habitats and structural purposes. The absence of organic-mineral intermolecular associations in brachiopod shell argues that biomineralization follows different mechanistic pathways to bone; their details hold clues to the molecular structural evolution of phosphatic biominerals, and may provide insights into novel composite design.

The fabrication and characterization of biodegradable HA/PHBV nanoparticle-polymer composite scaffolds

This study reports the fabrication and characterization of nano-sized hydroxyapatite (HA)/poly(hydroxyabutyrate-co-hydroxyvalerate) (PHBV) polymer composite scaffolds with high porosity and controlled pore architectures. These scaffolds were prepared using a modified thermally induced phase-separation technique. This investigation focuses on the effect of fabrication conditions on the overall pore architecture of the scaffolds and the dispersion of HA nanocrystals within the composite scaffolds. The morphologies, mechanical properties and in vitro bioactivity of the composite scaffolds were investigated. It was noted that the pore architectures could be manipulated by varying phase-separation parameters. The HA particles were dispersed in the pore walls of the scaffolds and were well bonded to the polymer. The introduction of HA greatly increased the stiffness and strength, and improved the in vitro bioactivity of the scaffolds. The results suggest these newly developed nano-HA/PHBV composite scaffolds may serve as an effective three-dimensional substrate in bone tissue engineering.

Ectopic osteogenesis and hematopoiesis after implantantion of bone marrow cells seeded on HAP/PLLA scaffold

Bone tissue reconstruction and reparation is a big challenge in medicine. Biocomposite materials based on hidroxyapatite are widely used in reparation of bone defects. Adult bone marrow-derived stem cells may be considered in two categories: hematopoietic stem cells (HSC) from the bone marrow and mesenchymal stem cells from the bone marrow stroma (BMSC). HSC and BMSC do not only coexist in one organ, but functionally cooperate. BMSC have a critical role in the formation of hematopoietic microenvironment (HME). The aim of this study was to investigate the interactions between bone marrow cells and biocomposites based on HAp/PLLA (hidroxyapatite/poly-L-lactide) after subcutaneous implantation in Balb/c mice. In that purpose, bone marrow cells of Balb/c mice were seeded in HAp/PLLA tubes (15 mm?1,5 mm). The HAp/PLLA tubes with BMC was subcutaneously implanted with a needle into the intrascapsular region of the mouse. Implants were extracted after 2, 6 and 12 weeks. In implants after 2 and 6 weeks we found angiogenesis, collagenogenesis and new bone. Ectopic hematopoiesis was seen in implants after 12 weeks from implantation. As a good scaffold in the role of supporting osteogenesis and hematopoiesis, biocomposites HAp/PLLA can be good bone substitute materials in the bone reparation process. These results showed that the HAp/PLLA scaffold owned biological properties comparable to natural bone.

Collagen nanofibres are a biomimetic substrate for the serum-free osteogenic differentiation of human adipose stem cells

Electrospinning has recently gained widespread attention as a process capable of producing nanoscale fibres that mimic native extracellular matrix. In this study, we compared the osteogenic differentiation behaviour of human adipose stem cells (ASCs) on a 3D nanofibre matrix of type I rat tail collagen (RTC) and a 2D RTC collagen-coated substrate, using a novel serum-free osteogenic medium. The serum-free medium significantly enhanced the numbers of proliferating cells in culture, compared to ASCs in traditional basal medium containing 10% animal serum, highlighting a potential clinical role for in vitro stem cell expansion. Osteogenic differentiation behaviour was assessed at days 7, 14 and 21 using quantitative real-time RT-PCR analysis of the osteogenic genes collagen I (Coll I), alkaline phosphatase (ALP), osteopontin (OP), osteonectin (ON), osteocalcin (OC) and core-binding factor-alpha (cbfa1). All genes were upregulated (>one-fold) in ASCs cultured on nanofibre scaffolds over 2D collagen coatings by day 21. Synthesis of mineralized extracellular matrix on the scaffolds was assessed on day 21 with Alizarin red staining. These studies demonstrate that 3D nanoscale morphology plays a critical role in regulating cell fate processes and in vitro osteogenic differentiation of ASCs under serum-free conditions.

Tissue engineering for bone defect healing: an update on a multi-component approach

The need for an interdisciplinary approach in order to establish new therapeutic strategies for the therapy of bone defects has been acknowledged by the scientific community for many years. This awareness makes itself felt when looking at the multitude of approaches--ranging from cell-based to scaffold-based strategies and also including the use of osteogenic growth factors and genetic engineering--that are currently being combined to assess their potential to develop effective concepts for the treatment of extensive loss of osseous tissue. With a strong focus on the preclinical research in this field, the goal of this review is to give an update on the multi-component approaches that are currently being investigated in tissue engineering of bone.

Tailoring Mechanical Behavior of PVA-Bioactive Glass Hybrid Foams

Porous scaffolds have been developed in many forms and materials, but few have reached the combination of adequate physical, biological and mechanical properties. In previous works hybrid foams bioactive glass polyvinyl alcohol (PVA) were prepared by the sol-gel process for application as scaffold for bone tissue engineering. We observed that synthesis parameters such as PVA hydrolysis grade, PVA solution concentration, and PVA content in the hybrids affected both synthesis results and structural characteristics of the obtained foams. A marked change in foaming behavior occurs for PVA contents around 60%. In this work we analyze the effect of different compositions and synthesis parameters on the mechanical behavior of PVA-bioative glass foams. The compression tests showed that an increase of PVA fraction changes the mechanical behavior due to different mechanisms leading to cell collapse. For hybrids with lower PVA contents (20 to 30%) the cell collapse is due to brittle crushing. For intermediate polymer content (40-60%) the contribution of plastic yielding in the plateau region increases and it becomes the predominant mechanism of cell collapse for samples with higher polymer content (70-80%).

Long-bone critical-size defects treated with tissue-engineered grafts: a study on sheep

Standardized particulate bone constructs, obtained by expanding autologous mesenchymal stem cells (MSCs) onto coral granules in vitro, were transplanted into long-bone, critical-size defects in sheep. Control experiments were also performed in which autologous bone grafts were implanted. Defect cavities were lined with a preformed vascularized membrane (induced by temporarily inserting a cement spacer for 6 weeks prior to bone construct implantation), which served as a mold keeping the engineered bone granules in place. Radiographic, histological, and computed tomographic tests performed 6 months later showed that the osteogenic abilities of the engineered construct and autograft were significantly greater than those of coral scaffold alone. No significant differences were found between the amount of newly formed bone in defects filled with coral/MSCs and those filled with autograft, yet radiological scores differed significantly between the two groups (21% and 100% healed cortices, respectively). The present study on a clinically relevant animal model provides the first evidence that standardized particulate bone constructs can be used to repair large bone defects and that their osteogenic ability approaches that of bone autograft, the bone repair benchmark. By proving feasibility, the present study makes possible the treatment of segmental bone losses with bone constructs engineered from granules, a process which is much simpler than preparing customized massive constructs using computer-assisted techniques. Important parameters, such as the rate of scaffold resorption and the number of MSCs to be seeded on the scaffolds, need to be optimized before reaching pertinent definitive conclusions.

Gelatin-based microcarriers as embryonic stem cell delivery system in bone tissue engineering: an in-vitro study

Mouse embryonic stem cells were cultured on commercially available biodegradable macroporous microcarriers. A culture period of 1-2 weeks was needed to colonize the microcarriers. Embryonic stem cells retained their pluripotency for up to 14 days when cultured in medium supplemented with leukemia inhibitory factor. Replacing this medium by differentiation medium for 2 weeks initiated osteogenic differentiation. Encapsulation of the cell-loaded microcarriers in photopolymerizable polymers (methacrylate-endcapped poly-D,L-lactide-co-caprolactone), triacetin/hydroxyethylmethacrylate (HEMA) as solvent and with/without gelatin as porogen, resulted in a homogeneous distribution of the microcarriers in the polymer. As observed by transmission electron microscopy, viability of the cells was optimal when gelatin was omitted and when using triacetin instead of HEMA.

Carbon Nanotubes as Scaffolds for Cell Culture and Effect on Cellular Functions

To investigate the dependence of biocompatibility of carbon materials on crystal structure with the aim of developing biomedical applications, single-(SW) and multi-walled (MW) carbon nanotubes (CNTs) were employed as scaffolds for cell culture and compared with graphite (GP). SaOS2 cells were used to investigate the properties and response of osteoblast-like cells. Polycarbonate membranes (PC) coated with CNTs by vacuum filtration formed a meshwork nanostructure. Cells grown on CNTs greatly extended in all directions. In terms of cell proliferation, alkaline phosphatase (ALP).activity, and protein adsorption on the substrates, CNTs showed better results than PC and GP. SW showed the best cell proliferation and total ALP. These favorable results might be attributed to the structure of CNTs and the affinity of CNTs toward proteins, thereby suggesting that CNTs could be potential scaffold materials for cell culture.

Macroporous hydrogels based on 2-hydroxyethyl methacrylate. Part 5: hydrolytically degradable materials

Macroporous hydrogels based on 2-hydroxyethyl methacrylate, 2-ethoxyethyl methacrylate and N-(2-hydroxypropyl)methacrylamide, methacrylic acid and [2-(methacryloyloxy)ethyl]trimethylammonium chloride crosslinked with N,O-dimethacryloylhydroxylamine were prepared. Hydrogels were degraded in a buffer of pH 7.4. Completely water-soluble polymers were obtained over time periods ranging from 2 to 40 days. The process of degradation was followed gravimetrically and by optical and electron microscopy. In vivo biological tests with hydrogels based on copolymers of 2-ethoxyethyl methacrylate/N-(2-hydroxypropyl)methacrylamide were performed.

Die Überbrückung von posttraumatischen Knochendefekten

Bony defects as a result of injury or disease can be caused by a variety of conditions such as acute injury, fall fractures in osteoporotic patients or tumours and congenital malformations of the musculoskeletal system which necessitate the resection of affected parts of the bone. This results in a multitude of defects concerning localisation and specificity as well as a number of conditions involving both hard and soft tissue structures and various situations of different patients. A reasonable classification of defects which is relevant for practical purposes includes four basic types: defects of the spine, metaphyseal defects as well as partial and complete diaphyseal defects of long bones. A variety of options exists for the treatment of these conditions. The aim of all efforts is to reinstall the integrity of affected structures long-lastingly and dependably and at the same time guarantee the normal function of joints involved. In addition to classical treatment strategies which involve the use of autogenous and allogenous corticocancellous bone grafts a great number of bone substitute materials can also be used. Further options lie in complex reconstructive methods such as the transport of whole segments or the transplantation of vascularised bone grafts. The field of new regenerative strategies including tissue engineering as well as stem cell and gene therapy holds great promise for the future. The aim of this review is to derive a ranking from the evaluation of biological and mechanical characteristics for the treatment of posttraumatic defects.

Bioluminescent imaging: Emerging technology for non-invasive imaging of bone tissue engineering

Bone tissue engineering is a multidisciplinary research area in which new strategies are developed to treat patients with large bone defects as occurring during e.g. hip revisions, upon trauma or in spinal fusions. In vivo evaluation of bone formation in animal models is highly relevant for graft evaluation but is time-consuming, invasive and difficult to quantify. As a consequence, most in vivo studies ignore the dynamic nature of bone regeneration and the molecular processes underlying it. In vivo bioluminescent imaging (BLI) is a relatively young research field with great potential that may overcome these problems. BLI encompasses non-invasive imaging of luciferase gene activity using cooled charge coupled device cameras in luciferase transgenic animals or in grafted, luciferase transgenic cells. The imaging procedure is technically simple and quantifiable. Because luciferase expression can be put under the control of tissue-specific regulatory elements, BLI allows non-invasive imaging of processes highly relevant to bone tissue engineering like differentiation, apoptosis, vasculogenesis and inflammation. In this review, we describe the basic principle of BLI and discuss transgenic animals and constructs currently available for application in bone tissue engineering. Furthermore, we reflect on technical developments that will make BLI even more promising for future application in bone tissue engineering research.

Hydroxyapatite scaffolds for bone tissue engineering made by 3D printing

Nowadays, there is a significant need for synthetic bone replacement materials used in bone tissue engineering (BTE). Rapid prototyping and especially 3D printing is a suitable technique to create custom implants based on medical data sets. 3D printing allows to fabricate scaffolds based on Hydroxyapatite with complex internal structures and high resolution. To determine the in vitro behaviour of cells cultivated on the scaffolds, we designed a special test-part. MC3T3-E1 cells were seeded on the scaffolds and cultivated under static and dynamic setups. Histological evaluation was carried out to characterise the cell ingrowth. In summary, the dynamic cultivation method lead to a stronger population compared to the static cultivation method. The cells proliferated deep into the structure forming close contact to Hydroxyapatite granules.

Hydroxyapatite scaffolds for bone tissue engineering made by 3D printing

Nowadays, there is a significant need for synthetic bone replacement materials used in bone tissue engineering (BTE). Rapid prototyping and especially 3D printing is a suitable technique to create custom implants based on medical data sets. 3D printing allows to fabricate scaffolds based on Hydroxyapatite with complex internal structures and high resolution. To determine the in vitro behaviour of cells cultivated on the scaffolds, we designed a special test-part. MC3T3-E1 cells were seeded on the scaffolds and cultivated under static and dynamic setups. Histological evaluation was carried out to characterise the cell ingrowth. In summary, the dynamic cultivation method lead to a stronger population compared to the static cultivation method. The cells proliferated deep into the structure forming close contact to Hydroxyapatite granules.

Tissue engineering

PURPOSE OF REVIEW

Regenerative medicine holds promise for the restoration of tissues and organs damaged by wear, trauma, neoplasm, or congenital deformity. Tissue engineering combines the disciplines of cell biology and biomedical engineering to effect the design and maturation of various tissues. Despite progress in some areas of tissue regeneration, there has not been significant translation to clinical practice. This article reviews the present understanding of and advances in regenerative medicine, as well as describing limitations in current techniques and areas that need further development. A discussion of the state of the art in the regeneration of skin, cartilage, bone, adipose tissue, and neural tissue is included.

RECENT FINDINGS

Differences between extracorporeal and in-vitro tissue engineering are discussed, as well as tissue engineering principles, including the use of bioactive scaffolds, progenitor cells and stem cells, the need for cellular and tissue patterning, microcirculation development, and the use of external stimuli for differentiation. Much needs to be learned about progenitor cell biology, cell-cell interactions, cellular interactions with the extracellular matrix, and about the cues needed for differentiation of functional tissues.

SUMMARY

The current limitations in regenerative medicine techniques and the gaps in current knowledge of cellular biology and tissue development represent significant research opportunities in tissue engineering.