Mineralization of hydrogels for bone regeneration

Recapitulating cranial osteogenesis with neural crest cells in 3-D microenvironments

The experimental systems that recapitulate the complexity of native tissues and enable precise control over the microenvironment are becoming essential for the pre-clinical tests of therapeutics and tissue engineering. Here, we described a strategy to develop an in vitro platform to study the developmental biology of craniofacial osteogenesis. In this study, we directly osteo-differentiated cranial neural crest cells (CNCCs) in a 3-D in vitro bioengineered microenvironment. Cells were encapsulated in the gelatin-based photo-crosslinkable hydrogel and cultured up to three weeks. We demonstrated that this platform allows efficient differentiation of p75 positive CNCCs to cells expressing osteogenic markers corresponding to the sequential developmental phases of intramembranous ossification. During the course of culture, we observed a decrease in the expression of early osteogenic marker Runx2, while the other mature osteoblast and osteocyte markers such as Osterix, Osteocalcin, Osteopontin and Bone sialoprotein increased. We analyzed the ossification of the secreted matrix with alkaline phosphatase and quantified the newly secreted hydroxyapatite. The Field Emission Scanning Electron Microscope (FESEM) images of the bioengineered hydrogel constructs revealed the native-like osteocytes, mature osteoblasts, and cranial bone tissue morphologies with canaliculus-like intercellular connections. This platform provides a broadly applicable model system to potentially study diseases involving primarily embryonic craniofacial bone disorders, where direct diagnosis and adequate animal disease models are limited.

Development of biomimetic nanocomposites as bone extracellular matrix for human osteoblastic cells

Here, we have developed biomimetic nanocomposites containing chitosan, poly(vinyl alcohol) and nano-hydroxyapatite-zinc oxide as bone extracellular matrix for human osteoblastic cells and characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction. Scanning electron microscopy images revealed interconnected macroporous structures. Moreover, in this study, the problem related to fabricating a porous composite with good mechanical strength has been resolved by incorporating 5wt% of nano-hydroxyapatite-zinc oxide into chitosan-poly(vinyl alcohol) matrix; the present composite showed high tensile strength (20.25MPa) while maintaining appreciable porosity (65.25%). These values are similar to human cancellous bone. These nanocomposites also showed superior water uptake, antimicrobial and biodegradable properties than the previously reported results. Compatibility with human blood and pH was observed, indicating nontoxicity of these materials to the human body. Moreover, proliferation of osteoblastic MG-63 cells onto the nanocomposites was also observed without having any negative effect.

In vitro and in vivo investigation of bisphosphonate-loaded hydroxyapatite particles for peri-implant bone augmentation

Locally applied bisphosphonates, such as zoledronate, have been shown in several studies to inhibit peri-implant bone resorption and recently to enhance peri-implant bone formation. Studies have also demonstrated positive effects of hydroxyapatite (HA) particles on peri-implant bone regeneration and an enhancement of the anti-resorptive effect of bisphosphonates in the presence of calcium. In the present study, both hydroxyapatite nanoparticles (nHA) and zoledronate were combined to achieve a strong reinforcing effect on peri-implant bone. The nHA-zoledronate combination was first investigated in vitro with a pre-osteoclastic cell assay (RAW 264.7) and then in vivo in a rat model of postmenopausal osteoporosis. The in vitro study confirmed that the inhibitory effect of zoledronate on murine osteoclast precursor cells was enhanced by loading the drug on nHA. For the in vivo investigation, either zoledronate-loaded or pure nHA were integrated in hyaluronic acid hydrogel. The gels were injected in screw holes that had been predrilled in rat femoral condyles before the insertion of miniature screws. Micro-CT-based dynamic histomorphometry and histology revealed an unexpected rapid mineralization of the hydrogel in vivo through formation of granules, which served as scaffold for new bone formation. The delivery of zoledronate-loaded nHA further inhibited a degradation of the mineralized hydrogel as well as a resorption of the peri-implant bone as effectively as unbound zoledronate. Hyaluronic acid with zoledronate-loaded nHA, thanks to its dual effect on inducing a rapid mineralization and preventing resorption, is a promising versatile material for bone repair and augmentation. Copyright © 2015 John Wiley & Sons, Ltd.

Partially oxidized polyvinyl alcohol as a promising material for tissue engineering

The desired clinical outcome after implantation of engineered tissue substitutes depends strictly on the development of biodegradable scaffolds. In this study we fabricated 1% and 2% oxidized polyvinyl alcohol (PVA) hydrogels, which were considered for the first time for tissue-engineering applications. The final aim was to promote the protein release capacity and biodegradation rate of the resulting scaffolds in comparison with neat PVA. After physical crosslinking, characterization of specific properties of 1% and 2% oxidized PVA was performed. We demonstrated that mechanical properties, hydrodynamic radius of molecules, thermal characteristics and degree of crystallinity were inversely proportional to the PVA oxidation rate. On the other hand, swelling behaviour and protein release were enhanced, confirming the potential of oxidized PVA as a protein delivery system, besides being highly biodegradable. Twelve weeks after in vivo implantation in mice, the modified hydrogels did not elicit severe inflammatory reactions, showing them to be biocompatible and to degrade faster as the degree of oxidation increased. According to our results, oxidized PVA stands out as a novel biomaterial for tissue engineering that can be used to realize scaffolds with customizable mechanical behaviour, protein-loading ability and biodegradability. Copyright © 2015 John Wiley & Sons, Ltd.

Experimental and numerical measurements of adhesion energies between PHEMA and PGLYMA with hydroxyapatite crystal

Synthetic orthopaedic materials consisting of a single bioinert polymeric material do not meet the complex biological and physical requirements of scaffold-guided bone tissue repair and regeneration. Of particular interest is the design of biocompatible hydrogel-hydroxyapatite composite bone substitutes with outstanding interfacial adhesion that would warranty the ability for the composite to withstand functional loadings without exhibiting brittle fractures during the dynamic guided tissue regeneration. For this purpose, the hydroxylated side chain of chemically cross-linked poly (2-hydroxyethyl methacrylate) (pHEMA) is substitute with a carboxylated side chain to make poly (glycerol methacrylate) (pGLYMA). Here, we carry out atomistic simulations and atomic force microscopy to predict and experimentally determine the interfacial adhesion energies of pHEMA and pGLYMA with the surface of single-crystalline hydroxyapatite (HA) whiskers. Both experimental and numerical results showed that pGLYMA has stronger adhesion forces with HA and may be used for preparing a high-affinity polymer-HA composite. The high adhesive interactions between pGLYMA and HA were found to be due to strong electrostatic energies.

Development of thermosensitive hydrogels of chitosan, sodium and magnesium glycerophosphate for bone regeneration applications

Thermosensitive injectable hydrogels based on chitosan neutralized with sodium beta-glycerophosphate (Na-β-GP) have been studied as biomaterials for drug delivery and tissue regeneration. Magnesium (Mg) has been reported to stimulate adhesion and proliferation of bone forming cells. With the aim of improving the suitability of the aforementioned chitosan hydrogels as materials for bone regeneration, Mg was incorporated by partial substitution of Na-β-GP with magnesium glycerophosphate (Mg-GP). Chitosan/Na-β-GP and chitosan/Na-β-GP/Mg-GP hydrogels were also loaded with the enzyme alkaline phosphatase (ALP) which induces hydrogel mineralization. Hydrogels were characterized physicochemically with respect to mineralizability and gelation kinetics, and biologically with respect to cytocompatibility and cell adhesion. Substitution of Na-β-GP with Mg-GP did not negatively influence mineralizability. Cell biological testing showed that both chitosan/Na-β-GP and chitosan/Na-β-GP/Mg-GP hydrogels were cytocompatible towards MG63 osteoblast-like cells. Hence, chitosan/Na-β-GP/Mg-GP hydrogels can be used as an alternative to chitosan/Na-β-GP hydrogels for bone regeneration applications. However the incorporation of Mg in the hydrogels during hydrogel formation did not bring any appreciable physicochemical or biological benefit.

A new heterologous fibrin sealant as scaffold to recombinant human bone morphogenetic protein-2 (rhBMP-2) and natural latex proteins for the repair of tibial bone defects

Tissue engineering has special interest in bone tissue aiming at future medical applications Studies have focused on recombinant human bone morphogenetic protein-2 (rhBMP-2) and natural latex proteins due to the osteogenic properties of rhBMP-2 and the angiogenic characteristic of fraction 1 protein (P-1) extracted from the rubber tree Hevea brasiliensis. Furthermore, heterologous fibrin sealant (FS) has been shown as a promising alternative in regenerative therapies. The aim of this study was to evaluate these substances for the repair of bone defects in rats. A bone defect measuring 3mm in diameter was created in the proximal metaphysis of the left tibia of 60 rats and was implanted with rhBMP-2 or P-1 in combination with a new heterologous FS derived from snake venom. The animals were divided into six groups: control (unfilled bone defect), rhBMP-2 (defect filled with 5μg rhBMP-2), P-1 (defect filled with 5μg P-1), FS (defect filled with 8μg FS), FS/rhBMP-2 (defect filled with 8μg FS and 5μg rhBMP-2), FS/P-1 (defect filled with 8μg FS and 5μg P-1). The animals were sacrificed 2 and 6 weeks after surgery. The newly formed bone projected from the margins of the original bone and exhibited trabecular morphology and a disorganized arrangement of osteocyte lacunae. Immunohistochemical analysis showed intense expression of osteocalcin in all groups. Histometric analysis revealed a significant difference in all groups after 2 weeks (p<0.05), except for the rhBMP-2 and FS/rhBMP-2 groups (p>0.05). A statistically significant difference (p<0.05) was observed in all groups after 6 weeks in relation to the volume of newly formed bone in the surgical area. In conclusion, the new heterologous fibrin sealant was found to be biocompatible and the combination with rhBMP-2 showed the highest osteogenic and osteoconductive capacity for bone healing. These findings suggest a promising application of this combination in the regeneration surgery.

Combining collagen and bioactive glasses for bone tissue engineering: a review

Collagen (COL), the most abundant protein in mammals, offers a wide range of attractive properties for biomedical applications which are the result of its biocompatibility and high affinity to water. However, due to the relative low mechanical properties of COL its applications are still limited. To tackle this disadvantage of COL, especially in the field of bone tissue engineering, COL can be combined with bioactive inorganic materials in a variety of composite systems. One of such systems is the collagen-bioactive glass (COL-BG) composite family, which is the theme of this Review. BG fillers can increase compressive strength and stiffness of COL-based structures. This article reviews the relevant literature published in the last 15 years discussing the fabrication of a variety of COL-BG composites. In vitro cell studies have demonstrated the osteogenic, odontogenic, and angiogenic potential of these composite systems, which has been confirmed by stimulating specific biochemical indicators of relevant cells. Bony integration and connective tissue vessel formation have also been studied by implantation of the composites in vivo. Areas of future research in the field of COL-BG systems, based on current challenges, and gaps in knowledge are highlighted.

3-Dimensional cell-laden nano-hydroxyapatite/protein hydrogels for bone regeneration applications

The ability to encapsulate cells in three-dimensional (3D) protein-based hydrogels is potentially of benefit for tissue engineering and regenerative medicine. However, as a result of their poor mechanical strength, protein-based hydrogels have traditionally been considered for soft tissue engineering only. Hence, in this study we tried to render these hydrogels suitable for hard tissue regeneration, simply by incorporation of bioactive nano-hydroxyapatite (HAp) into a photocrosslinkable gelatin hydrogel. Different cell types were also encapsulated in three dimensions in the resulting composites to prepare cell-laden constructs. According to the results, HAp significantly improves the stiffness of gelatin hydrogels, while it maintains their structural integrity and swelling ratio. It was also found that while the bare hydrogel (control) was completely inert in terms of bioactivity, a homogeneous 3D mineralization occurs throughout the nanocomposites after incubation in simulated body fluid. Moreover, encapsulated cells readily elongated, proliferated, and formed a 3D interconnected network with neighboring cells in the nanocomposite, showing the suitability of the nano-HAp/protein hydrogels for cellular growth in 3D. Therefore, the hydrogel nanocomposites developed in this study may be promising candidates for preparing cell-laden tissue-like structures with enhanced stiffness and increased osteoconductivity to induce bone formation in vivo.

A novel nanocomposite for bone tissue engineering based on chitosan–silk sericin/hydroxyapatite: biomimetic synthesis and its cytocompatibility

Scheme of the formation mechanism of CS–SS/HA-s and CS–SS/HA-g nanocomposites.

In vitro and in vivo evaluation of self-mineralization and biocompatibility of injectable, dual-gelling hydrogels for bone tissue engineering

In this study, we investigated the mineralization capacity and biocompatibility of injectable, dual-gelling hydrogels in a rat cranial defect as a function of hydrogel hydrophobicity from either the copolymerization of a hydrolyzable lactone ring or the hydrogel polymer content. The hydrogel system comprised a poly(N-isopropylacrylamide)-based thermogelling macromer (TGM) and a polyamidoamine crosslinker. The thermogelling macromer was copolymerized with (TGM/DBA) or without (TGM) a dimethyl-γ-butyrolactone acrylate (DBA)-containing lactone ring that modulated the lower critical solution temperature and thus, the hydrogel hydrophobicity, over time. Three hydrogel groups were examined: (1) 15wt.% TGM, (2) 15wt.% TGM/DBA, and (3) 20wt.% TGM/DBA. The hydrogels were implanted within an 8mm critical size rat cranial defect for 4 and 12weeks. Implants were harvested at each timepoint and analyzed for bone formation, hydrogel mineralization and tissue response using microcomputed tomography (microCT). Histology and fibrous capsule scoring showed a light inflammatory response at 4weeks that was mitigated by 12weeks for all groups. MicroCT scoring and bone volume quantification demonstrated a similar bone formation at 4weeks that was significantly increased for the more hydrophobic hydrogel formulations - 15wt.% TGM and 20wt.% TGM/DBA - from 4weeks to 12weeks. A complementary in vitro acellular mineralization study revealed that the hydrogels exhibited calcium binding properties in the presence of serum-containing media, which was modulated by the hydrogel hydrophobicity. The tailored mineralization capacity of these injectable, dual-gelling hydrogels with hydrolysis-dependent hydrophobicity presents an exciting property for their use in bone tissue engineering applications.

Introduction to cell-hydrogel mechanosensing

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

Biofunctionalization of ulvan scaffolds for bone tissue engineering

Photo-cross-linked ulvan scaffolds were designed with the aim to induce and support enzyme mediated formation of apatite minerals, in the absence of osteogenic growth factors. Scaffold formation with a desired geometry was investigated using chemically modified ulvan bearing radically polymerizable groups. Further bioactivity was incorporated by the use of alkaline phosphatase (ALP) induced minerals. Successful modification of UV cross-linked ulvan scaffolds was revealed by (1)H NMR. The presence of the mineral formation was evidenced by Raman spectroscopy and XRD techniques. Investigations of the morphology confirmed the homogeneous mineralization using ALP. The MC3T3 cell activity clearly showed that the mineralization of the biofunctionalized ulvan scaffolds was effective in improving the cellular activity.

Generation of composites for bone tissue-engineering applications consisting of gellan gum hydrogels mineralized with calcium and magnesium phosphate phases by enzymatic means

Mineralization of hydrogels, desirable for bone regeneration applications, may be achieved enzymatically by incorporation of alkaline phosphatase (ALP). ALP-loaded gellan gum (GG) hydrogels were mineralized by incubation in mineralization media containing calcium and/or magnesium glycerophosphate (CaGP, MgGP). Mineralization media with CaGP:MgGP concentrations 0.1:0, 0.075:0.025, 0.05:0.05, 0.025:0.075 and 0:0.1 (all values mol/dm³ , denoted A, B, C, D and E, respectively) were compared. Mineral formation was confirmed by IR and Raman, SEM, ICP-OES, XRD, TEM, SAED, TGA and increases in the the mass fraction of the hydrogel not consisting of water. Ca was incorporated into mineral to a greater extent than Mg in samples mineralized in media A-D. Mg content and amorphicity of mineral formed increased in the order A < B < C < D. Mineral formed in media A and B was calcium-deficient hydroxyapatite (CDHA). Mineral formed in medium C was a combination of CDHA and an amorphous phase. Mineral formed in medium D was an amorphous phase. Mineral formed in medium E was a combination of crystalline and amorphous MgP. Young's moduli and storage moduli decreased in dependence of mineralization medium in the order A > B > C > D, but were significantly higher for samples mineralized in medium E. The attachment and vitality of osteoblastic MC3T3-E1 cells were higher on samples mineralized in media B-E (containing Mg) than in those mineralized in medium A (not containing Mg). All samples underwent degradation and supported the adhesion of RAW 264.7 monocytic cells, and samples mineralized in media A and B supported osteoclast-like cell formation. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of physical and chemical treatments on the molecular weight and degradation of alginate-hydroxyapatite composites

Degradation of alginate remains a critical issue to allow predictable biological performance upon implantation of alginate-based materials. Therefore, the objective of the current study is to compare the effects of γ-irradiation (dry state, 20-80 kGy), partial (1 and 4%) periodate oxidation (aqueous solution), and autoclaving (dry state) on the molecular weight of alginate, as well as the degradation behavior of alginate-based composites. The results show that γ-irradiation is by far the most destructive technique characterized by strongly reduced molecular weights and rapid loss of composite integrity upon soaking in simulated body fluid. Partial periodate oxidation is less destructive as characterized by more moderate decreases in molecular weight, but the production of hydrolytically labile bonds compromises the integrity of the resulting composites. Autoclaving is shown to be a powerful tool to reduce the molecular weight of alginate in a controllable and mild manner without compromising the integrity of the resulting alginate-hydroxyapatite composites, simply by increasing the number of repetitive autoclaving cycles.

Additive manufacturing of cell-loaded alginate enriched with alkaline phosphatase for bone tissue engineering application

Hydrogels are gaining interest as scaffolds for bone tissue regeneration due to ease of incorporation of cells and biological molecules such as enzymes. Mineralization of hydrogels, desirable for bone tissue regeneration applications, may be achieved enzymatically by incorporation of alkaline phosphatase (ALP). Additive manufacturing techniques such as bioplotting enable the layer-by-layer creation of three-dimensional hydrogel scaffolds with highly defined geometry and internal architecture. In this study, we present a novel method to produce macroporous hydrogel scaffolds in combination with cell-loaded capsule-containing struts by 3D bioplotting. This approach enables loading of the capsules and strut phases with different cells and/or bioactive substances and hence makes compartmentalization within a scaffold possible. 3D porous alginate scaffolds enriched with ALP and MG-63 osteoblast-like cells were produced by bioplotting struts of alginate which were loaded with pre-fabricated alginate capsules. Two combinations were compared, namely ALP in the struts and cells in the capsules and vice-versa. Both combinations were cytocompatible for cells and mineralization of scaffolds could be detected in both cases, according to an OsteoImage staining. ALP had no adverse effect on cytocompatibility and enhanced mitochondrial activity.

Interactions between inorganic and organic phases in bone tissue as a source of inspiration for design of novel nanocomposites

Mimicking the nanostructure of bone and understanding the interactions between the nanoscale inorganic and organic components of the extracellular bone matrix are crucial for the design of biomaterials with structural properties and a functionality similar to the natural bone tissue. Generally, these interactions involve anionic and/or cationic functional groups as present in the organic matrix, which exhibit a strong affinity for either calcium or phosphate ions from the mineral phase of bone. This study reviews the interactions between the mineral and organic extracellular matrix components in bone tissue as a source of inspiration for the design of novel nanocomposites. After providing a brief description of the various structural levels of bone and its main constituents, a concise overview is presented on the process of bone mineralization as well as the interactions between calcium phosphate (CaP) nanocrystals and the organic matrix of bone tissue. Bioinspired synthetic approaches for obtaining nanocomposites are subsequently addressed, with specific focus on chemical groups that have affinity for CaPs or are involved in stimulating and controlling mineral formation, that is, anionic functional groups, including carboxyl, phosphate, sulfate, hydroxyl, and catechol groups.

Tissue response to composite hydrogels for vertical bone augmentation in the rat

The objective of the present study was to develop a preclinical animal model for evaluating bone augmentation and to examine the level of bone augmentation induced by hydrogel composites. Design criteria outlined for the development of the animal model included rigid immobilization of bilateral implants apposed to the parietal bone of the rat, while avoiding the calvarial sutures. The animal model was evaluated through the implantation of hydrogel composites of oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles releasing bone morphogenetic protein-2 (BMP-2). The BMP-2 release profile was varied and compared to the implantation of a material control without BMP-2. Each hydrogel composite was implanted within a polypropylene cassette, which was immobilized to the calvarial bone using screws, and empty cassettes were implanted as a control. The design criteria for the animal model were realized; however, the level of bone augmentation did not vary between any of the groups after 4 weeks. Osteoclastic bone resorption occurred to a higher extent in groups releasing BMP-2, but the cause could not be elucidated. In conclusion, a promising bone augmentation model was established in the rat; however, refinement of the hydrogel composites was suggested to optimize the constructs for bone augmentation applications.

Evaluation of multifunctional polysaccharide hydrogels with varying stiffness for bone tissue engineering

The use of hydrogels for bone regeneration has been limited due to their inherent low modulus to support cell adhesion and proliferation as well as their susceptibility to bacterial infections at the wound site. To overcome these limitations, we evaluated multifunctional polysaccharide hydrogels of varying stiffness to obtain the optimum stiffness at which the gels (1) induce proliferation of human dermal fibroblasts, human umbilical vascular endothelial cells (HUVECs), and murine preosteoblasts (MC3T3-E1), (2) induce osteoblast differentiation and mineralization, and (3) exhibit an antibacterial activity. Rheological studies demonstrated that the stiffness of hydrogels made of a polysaccharide blend of methylcellulose, chitosan, and agarose was increased by crosslinking the chitosan component to different extents with increasing amounts of genipin. The gelation time decreased (from 210 to 60 min) with increasing genipin concentrations. Proliferation of HUVECs decreased by 10.7 times with increasing gel stiffness, in contrast to fibroblasts and osteoblasts, where it increased with gel stiffness by 6.37 and 7.8 times, respectively. At day 14 up to day 24, osteoblast expression of differentiation markers-osteocalcin, osteopontin-and early mineralization marker-alkaline phosphatase, were significantly enhanced in the 0.5% (w/v) crosslinked gel, which also demonstrated enhanced mineralization by day 25. The antibacterial efficacy of the hydrogels decreased with the increasing degree of crosslinking as demonstrated by biofilm formation experiments, but gels crosslinked with 0.5% (w/v) genipin still demonstrated significant bacterial inhibition. Based on these results, gels crosslinked with 0.5% (w/v) genipin, where 33% of available groups on chitosan were crosslinked, exhibited a stiffness of 502±64.5 Pa and demonstrated the optimal characteristics to support bone regeneration.

The role of small molecules in musculoskeletal regeneration

The uses of bone morphogenetic proteins and parathyroid hormone therapeutics are fraught with several fundamental problems, such as cost, protein stability, immunogenicity, contamination and supraphysiological dosage. These downsides may effectively limit their more universal use. Therefore, there is a clear need for alternative forms of biofactors to obviate the drawbacks of protein-based inductive factors for bone repair and regeneration. Our group has studied small molecules with the capacity to regulate osteoblast differentiation and mineralization because their inherent physical properties minimize limitations observed in protein growth factors. For instance, in general, small molecule inducers are usually more stable, highly soluble, nonimmunogenic, more affordable and require lower dosages. Small molecules with the ability to induce osteoblastic differentiation may represent the next generation of bone regenerative medicine. This review describes efforts to develop small molecule-based biofactors for induction, paying specific attention to their novel roles in bone regeneration.

Subcutaneous tissue response and osteogenic performance of calcium phosphate nanoparticle-enriched hydrogels in the tibial medullary cavity of guinea pigs

In the current study, oligo(poly(ethylene glycol) fumarate) (OPF)-based hydrogels were tested for the first time as injectable bone substitute materials. The primary feature of the material design was the incorporation of calcium phosphate (CaP) nanoparticles within the polymeric matrix in order to compare the soft tissue response and bone-forming capacity of plain OPF hydrogels with CaP-enriched OPF hydrogel composites. To that end, pre-set scaffolds were implanted subcutaneously, whereas flowable polymeric precursor solutions were injected in a tibial ablation model in guinea pigs. After 8 weeks of implantation, histological and histomorphometrical evaluation of the subcutaneous scaffolds confirmed the biocompatibility of both types of hydrogels. Nevertheless, OPF hydrogels presented a loose structure, massive cellular infiltration and extensive material degradation compared to OPF-CaP hydrogels that were more compact. Microcomputed tomography and histological and histomorphometrical analyses showed comparable amounts of new trabecular bone in all tibias and some material remnants in the medial and distal regions. Particularly, highly calcified areas were observed in the distal region of OPF-CaP-treated tibias, which indicate a heterogeneous distribution of the mineral phase throughout the hydrogel matrix. This phenomenon can be attributed to either hindered gelation under highly perfused in vivo conditions or a faster degradation rate of the polymeric hydrogel matrix compared to the nanostructured mineral phase, resulting in loss of entrapment of the CaP nanoparticles and subsequent sedimentation.

Dual setting α-tricalcium phosphate cements

An extension of the application of calcium phosphate cements (CPC) to load-bearing defects, e.g. in vertebroplasty, would require less brittle cements with an increased fracture toughness. Here we report the modification of CPC made of alpha-tricalcium phosphate (α-TCP) with 2-hydroxyethylmethacrylate (HEMA), which is polymerised during setting to obtain a mechanically stable polymer-ceramic composite with interpenetrating organic and inorganic networks. The cement liquid was modified by the addition of 30-70 % HEMA and ammoniumpersulfate/tetramethylethylendiamine as initiator. Modification of α-TCP cement paste with HEMA decreased the setting time from 14 min to 3-8 min depending on the initiator concentration. The 4-point bending strength was increased from 9 MPa to more than 14 MPa when using 50 % HEMA, while the bending modulus decreased from 18 GPa to approx. 4 GPa. The addition of ≥50 % HEMA reduced the brittle fracture behaviour of the cements and resulted in an increase of the work of fracture by more than an order of magnitude. X-ray diffraction analyses revealed that the degree of transformation of α-TCP to calcium deficient hydroxyapatite was lower for polymer modified cements (82 % for polymer free cement and 55 % for 70 % HEMA) after 24 h setting, while the polymerisation of HEMA in the cement liquid was quantitative according to FT-IR spectroscopy. This work demonstrated the feasibility of producing fracture resistant dual-setting calcium phosphate cements by adding water soluble polymerisable monomers to the liquid cement phase, which may be suitable for an application in load-bearing bone defects.

Strategies for controlled delivery of growth factors and cells for bone regeneration

The controlled delivery of growth factors and cells within biomaterial carriers can enhance and accelerate functional bone formation. The carrier system can be designed with pre-programmed release kinetics to deliver bioactive molecules in a localized, spatiotemporal manner most similar to the natural wound healing process. The carrier can also act as an extracellular matrix-mimicking substrate for promoting osteoprogenitor cellular infiltration and proliferation for integrative tissue repair. This review discusses the role of various regenerative factors involved in bone healing and their appropriate combinations with different delivery systems for augmenting bone regeneration. The general requirements of protein, cell and gene therapy are described, with elaboration on how the selection of materials, configurations and processing affects growth factor and cell delivery and regenerative efficacy in both in vitro and in vivo applications for bone tissue engineering.

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.

Biomaterials-Based Strategies for the Engineering of Mechanically Active Soft Tissues

Load-bearing, mechanically active tissues are routinely subjected to non-linear mechanical deformations. Consequently, these tissues exhibit complex mechanical properties and unique tissue organizations. Successful engineering of mechanically active tissues relies on the integration of the mechanical sensing mechanism found in the native tissues into polymeric scaffolds. Intelligent biomaterials that closely mimic the structural organizations and multi-scale responsiveness of the natural extracellular matrices (ECM), when strategically combined with multipotent cells and dynamic culture devices that generate physiologically relevant physical forces, will lead to the creation of artificial tissues that are mechanically robust and biologically functional.

Biomimetic three-dimensional nanocrystalline hydroxyapatite and magnetically synthesized single-walled carbon nanotube chitosan nanocomposite for bone regeneration

Background

Many shortcomings exist in the traditional methods of treating bone defects, such as donor tissue shortages for autografts and disease transmission for allografts. The objective of this study was to design a novel three-dimensional nanostructured bone substitute based on magnetically synthesized single-walled carbon nanotubes (SWCNT), biomimetic hydrothermally treated nanocrystalline hydroxyapatite, and a biocompatible hydrogel (chitosan). Both nanocrystalline hydroxyapatite and SWCNT have a biomimetic nanostructure, excellent osteoconductivity, and high potential to improve the load-bearing capacity of hydrogels.

Methods

Specifically, three-dimensional porous chitosan scaffolds with different concentrations of nanocrystalline hydroxyapatite and SWCNT were created to support the growth of human osteoblasts (bone-forming cells) using a lyophilization procedure. Two types of SWCNT were synthesized in an arc discharge with a magnetic field (B-SWCNT) and without a magnetic field (N-SWCNT) for improving bone regeneration.

Results

Nanocomposites containing magnetically synthesized B-SWCNT had superior cytocompatibility properties when compared with nonmagnetically synthesized N-SWCNT. B-SWCNT have much smaller diameters and are twice as long as their nonmagnetically prepared counterparts, indicating that the dimensions of carbon nanotubes can have a substantial effect on osteoblast attachment.

Conclusion

This study demonstrated that a chitosan nanocomposite with both B-SWCNT and 20% nanocrystalline hydroxyapatite could achieve a higher osteoblast density when compared with the other experimental groups, thus making this nanocomposite promising for further exploration for bone regeneration.

Exogenous mineralization of cell-seeded and unseeded collagen-chitosan hydrogels using modified culture medium

Induced biomineralization of materials has been employed as a strategy to increase integration with host tissue, and more recently as a method to control cell function in tissue engineering. However, mineralization is typically performed in the absence of cells, since hypertonic solutions that lack the nutrients and culture components required for the maintenance of cell viability are often used. In the present study, we exposed fibroblast-seeded three-dimensional collagen-chitosan hydrogels to a defined culture medium modified to have specific concentrations of ions involved in biomineralization. The modified medium caused a significant increase in calcium deposition in collagen-chitosan gels, relative to constructs incubated in a standard medium, though serum supplementation attenuated mineral deposition. Collagen-chitosan constructs became opaque over 3 days of mineralization in modified Dulbecco's modified Eagle medium (DMEM), in contrast to translucent control gels incubated in standard DMEM. Histological staining confirmed increased levels of mineral in the treated constructs. Rheological characterization showed that both the storage and loss moduli increased significantly in mineralized materials. Mineralization of fibroblast-seeded constructs resulted in decreased cell viability and proliferation rate over 3 days of incubation in modified medium, but the cell population remained over 75% viable and regained its proliferative potential after rescue in standard culture medium. The ability to mineralize protein matrices in the presence of cells could be useful in creating mechanically stable tissue constructs, as well as to study the effects of the tissue microenvironment on cell function.

Facilitating the mineralization of oligo(poly(ethylene glycol) fumarate) hydrogel by incorporation of hydroxyapatite nanoparticles

Exploring strategies to induce the mineralization of hydrogels is an important step toward the development of hydrogel-based materials for bone regeneration. In the current study, the effect of incorporating hydroxyapatite (HA) nanoparticles on the mineralization capacity of an inert poly(ethylene glycol) (PEG)-based hydrogel was investigated. HA nanoparticles were either directly loaded into oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel or loaded into commonly used gelatin microsphere porogens that were subsequently integrated in the OPF matrix. Mineralization of composites after immersion of the samples in simulated body fluid up to 28 days was assessed. In contrast to the blank OPF hydrogel, the HA-containing constructs strongly mineralized such that the average rate of calcium uptake by the material was enhanced by orders of magnitude. The mineral formed was observed to be apatitic and needle shaped. The presented method allows modification of inert PEG-based hydrogels into bioactive biomaterials for applications in bone regeneration.

Engineered microenvironments for self-renewal and musculoskeletal differentiation of stem cells

Stem cells hold great promise for therapies aimed at regenerating damaged tissue, drug screening and studying in vitro models of human disease. However, many challenges remain before these applications can become a reality. One such challenge is developing chemically defined and scalable culture conditions for derivation and expansion of clinically viable human pluripotent stem cells, as well as controlling their differentiation with high specificity. Interaction of stem cells with their extracellular microenvironment plays an important role in determining their differentiation commitment and functions. Regenerative medicine approaches integrating cell-matrix and cell-cell interactions, and soluble factors could lead to development of robust microenvironments to control various cellular responses. Indeed, several of these recent developments have provided significant insight into the design of microenvironments that can elicit the targeted cellular response. In this article, we will focus on some of these developments with an emphasis on matrix-mediated expansion of human pluripotent stem cells while maintaining their pluripotency. We will also discuss the role of matrix-based cues and cell-cell interactions in the form of soluble signals in directing stem cell differentiation into musculoskeletal lineages.

Biological interactions of graphene-family nanomaterials: an interdisciplinary review

Graphene is a single-atom thick, two-dimensional sheet of hexagonally arranged carbon atoms isolated from its three-dimensional parent material, graphite. Related materials include few-layer-graphene (FLG), ultrathin graphite, graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanosheets (GNS). This review proposes a systematic nomenclature for this set of Graphene-Family Nanomaterials (GFNs) and discusses specific materials properties relevant for biomolecular and cellular interactions. We discuss several unique modes of interaction between GFNs and nucleic acids, lipid bilayers, and conjugated small molecule drugs and dyes. Some GFNs are produced as dry powders using thermal exfoliation, and in these cases, inhalation is a likely route of human exposure. Some GFNs have aerodynamic sizes that can lead to inhalation and substantial deposition in the human respiratory tract, which may impair lung defense and clearance leading to the formation of granulomas and lung fibrosis. The limited literature on in vitro toxicity suggests that GFNs can be either benign or toxic to cells, and it is hypothesized that the biological response will vary across the material family depending on layer number, lateral size, stiffness, hydrophobicity, surface functionalization, and dose. Generation of reactive oxygen species (ROS) in target cells is a potential mechanism for toxicity, although the extremely high hydrophobic surface area of some GFNs may also lead to significant interactions with membrane lipids leading to direct physical toxicity or adsorption of biological molecules leading to indirect toxicity. Limited in vivo studies demonstrate systemic biodistribution and biopersistence of GFNs following intravenous delivery. Similar to other smooth, continuous, biopersistent implants or foreign bodies, GFNs have the potential to induce foreign body tumors. Long-term adverse health impacts must be considered in the design of GFNs for drug delivery, tissue engineering, and fluorescence-based biomolecular sensing. Future research is needed to explore fundamental biological responses to GFNs including systematic assessment of the physical and chemical material properties related to toxicity. Complete materials characterization and mechanistic toxicity studies are essential for safer design and manufacturing of GFNs in order to optimize biological applications with minimal risks for environmental health and safety.