An in vitro and in vivo study: Tissue Engineered Scaffolds for Use in Bone Regeneration

Dispersant and Protective Roles of Amphiphilic Poly(ethylene phosphate) Block Copolymers in Polyester/Bone Mineral Composites

Composites of synthetic bone mineral substitutes (BMS) and biodegradable polyesters are of particular interest for bone surgery and orthopedics. Manufacturing of composite scaffolds commonly uses mixing of the BMS with polymer melts. Melt processing requires a high homogeneity of the mixing, and is complicated by BMS-promoted thermal degradation of polymers. In our work, poly(L-lactide) (PLLA) and poly(ε-caprolactone) (PCL) composites reinforced by commercial β-tricalcium phosphate (βTCP) or synthesized carbonated hydroxyapatite with hexagonal and plate-like crystallite shapes (hCAp and pCAp, respectively) were fabricated using injection molding. pCAp-based composites showed advanced mechanical and thermal characteristics, and the best set of mechanical characteristics was observed for the PLLA-based composite containing 25 wt% of pCAp. To achieve compatibility of polyesters and pCAp, reactive block copolymers of PLLA or PCL with poly(tert-butyl ethylene phosphate) (C1 and C2, respectively) were introduced to the composite. The formation of a polyester-b-poly(ethylene phosphoric acid) (PEPA) compatibilizer during composite preparation, followed by chemical binding of PEPA with pCAp, have been proved experimentally. The presence of 5 wt% of the compatibilizer provided deeper homogenization of the composite, resulting in a marked increase in strength and moduli as well as a more pronounced nucleation effect during isothermal crystallization. The use of C1 increased the thermal stability of the PLLA-based composite, containing 25 wt% of pCAp. In view of positive impacts of polyester-b-PEPA on composite homogeneity, mechanical characteristics, and thermal stability, polyester-b-PEPA will find application in the further development of composite materials for bone surgery and orthopedics.

Recent advances in modified poly (lactic acid) as tissue engineering materials

As an emerging science, tissue engineering and regenerative medicine focus on developing materials to replace, restore or improve organs or tissues and enhancing the cellular capacity to proliferate, migrate and differentiate into different cell types and specific tissues. Renewable resources have been used to develop new materials, resulting in attempts to produce various environmentally friendly biomaterials. Poly (lactic acid) (PLA) is a biopolymer known to be biodegradable and it is produced from the fermentation of carbohydrates. PLA can be combined with other polymers to produce new biomaterials with suitable physicochemical properties for tissue engineering applications. Here, the advances in modified PLA as tissue engineering materials are discussed in light of its drawbacks, such as biological inertness, low cell adhesion, and low degradation rate, and the efforts conducted to address these challenges toward the design of new enhanced alternative biomaterials.

Evaluation of Polymeric Particles for Modular Tissue Cultures in Developmental Engineering

Developmental engineering (DE) aims to culture mammalian cells on corresponding modular scaffolds (scale: micron to millimeter), then assemble these into functional tissues imitating natural developmental biology processes. This research intended to investigate the influences of polymeric particles on modular tissue cultures. When poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA) and polystyrene (PS) particles (diameter: 5-100 µm) were fabricated and submerged in culture medium in tissue culture plastics (TCPs) for modular tissue cultures, the majority of adjacent PMMA, some PLA but no PS particles aggregated. Human dermal fibroblasts (HDFs) could be directly seeded onto large (diameter: 30-100 µm) PMMA particles, but not small (diameter: 5-20 µm) PMMA, nor all the PLA and PS particles. During tissue cultures, HDFs migrated from the TCPs surfaces onto all the particles, while the clustered PMMA or PLA particles were colonized by HDFs into modular tissues with varying sizes. Further comparisons revealed that HDFs utilized the same cell bridging and stacking strategies to colonize single or clustered polymeric particles, and the finely controlled open pores, corners and gaps on 3D-printed PLA discs. These observed cell-scaffold interactions, which were then used to evaluate the adaptation of microcarrier-based cell expansion technologies for modular tissue manufacturing in DE.

Enhanced osteoinductive capacity of poly(lactic-co-glycolic) acid and biphasic ceramic scaffolds by embedding simvastatin

OBJECTIVES

This study evaluated the effect of embedding simvastatin (SIM) on the osteoinductive capacity of PLGA + HA/βTCP scaffolds in stem cells from human exfoliated deciduous teeth (SHED).

MATERIALS AND METHODS

Scaffolds were produced by PLGA solvent dissolution, addition of HA/βTCP, solvent evaporation, and leaching of sucrose particles to impart porosity. Biphasic ceramic particles (70% HA/30% βTCP) were added to the PLGA in a 1:1 (w:w) ratio. Scaffolds with SIM received 1% (w:w) of this medication. Scaffolds were synthesized in a disc-shape and sterilized by ethylene oxide. The experimental groups were (G1) PLGA + HA/βTCP and (G2) PLGA + HA/βTCP + SIM in non-osteogenic culture medium, while (G3) SHED and (G4) MC3T3-E1 in osteogenic culture medium were the positive control groups. The release profile of SIM from scaffolds was evaluated. DNA quantification assay, alkaline phosphatase activity, osteocalcin and osteonectin proteins, extracellular calcium detection, von Kossa staining, and X-ray microtomography were performed to assess the capacity of scaffolds to induce the osteogenic differentiation of SHED.

RESULTS

The release profile of SIM followed a non-liner sustained-release rate, reaching about 40% of drug release at day 28. Additionally, G2 promoted the highest osteogenic differentiation of SHED, even when compared to the positive control groups.

CONCLUSIONS

In summary, the osteoinductive capacity of poly(lactic-co-glycolic) acid and biphasic ceramic scaffolds was expressively enhanced by embedding simvastatin.

CLINICAL RELEVANCE

Bone regeneration is still a limiting factor in the success of several approaches to oral and maxillofacial surgeries, though tissue engineering using mesenchymal stem cells, scaffolds, and osteoinductive mediators might collaborate to this topic.

Comparison of two different biomaterials in the bone regeneration (15, 30 and 60 days) of critical defects in rats

PURPOSE

To evaluate and compare two types of different scaffolds in critical bone defects in rats.

METHODS

Seventy male Wistar rats (280 ± 20 grams) divided into three groups: control group (CG), untreated animals; biomaterial group 1 (BG1), animals that received the scaffold implanted hydroxyapatite (HA)/poly(lactic-co-glycolic) acid (PLGA); and biomaterial group 2 (BG2), animals that received the scaffolds HA/PLGA/Bleed. The critical bone defect was induced in the medial region of the skull calotte with the aid of an 8-mm-diameter trephine drill. The biomaterial was implanted in the form of 1.5 mm thick scaffolds, and samples were collected after 15, 30 and 60 days. Non-parametric Mann-Whitney test was used, with the significance level of 5% (p ≤ 0.05).

RESULTS

Histology revealed morphological and structural differences of the neoformed tissue between the experimental groups. Collagen-1 (Col-1) findings are consistent with the histological ones, in which BG2 presented the highest amount of fibers in its tissue matrix in all evaluated periods. In contrast, the results of receptor activator of nuclear factor kappa-Β ligand (Rank-L) immunoexpression were higher in BG2 in the periods of 30 and 60 days, indicating an increase of the degradation of the biomaterial and the remodeling activity of the bone.

CONCLUSIONS

The properties of the HA/PLGA/Bleed scaffold were superior when compared to the scaffold composed only by HA/PLGA.

A Paradigm Shift in Tissue Engineering: From a Top–Down to a Bottom–Up Strategy

Tissue engineering (TE) was initially designed to tackle clinical organ shortage problems. Although some engineered tissues have been successfully used for non-clinical applications, very few (e.g., reconstructed human skin) have been used for clinical purposes. As the current TE approach has not achieved much success regarding more broad and general clinical applications, organ shortage still remains a challenging issue. This very limited clinical application of TE can be attributed to the constraints in manufacturing fully functional tissues via the traditional top–down approach, where very limited cell types are seeded and cultured in scaffolds with equivalent sizes and morphologies as the target tissues. The newly proposed developmental engineering (DE) strategy towards the manufacture of fully functional tissues utilises a bottom–up approach to mimic developmental biology processes by implementing gradual tissue assembly alongside the growth of multiple cell types in modular scaffolds. This approach may overcome the constraints of the traditional top–down strategy as it can imitate in vivo-like tissue development processes. However, several essential issues must be considered, and more mechanistic insights of the fundamental, underpinning biological processes, such as cell–cell and cell–material interactions, are necessary. The aim of this review is to firstly introduce and compare the number of cell types, the size and morphology of the scaffolds, and the generic tissue reconstruction procedures utilised in the top–down and the bottom–up strategies; then, it will analyse their advantages, disadvantages, and challenges; and finally, it will briefly discuss the possible technologies that may overcome some of the inherent limitations of the bottom–up strategy.

Antiviral activity of multifunctional composite based on TiO2-modified hydroxyapatite

An antiviral activity of TiO₂-modified hydroxyapatite composite (HA/TiO₂) had been investigated. The HA/TiO₂ composite (HA50:Ti50) was prepared by a solid state reaction method followed by calcination at 650 °C for 2 h. Phase formations and morphologies of the obtained HA/TiO₂ composite powders were determined using XRD and SEM. XRD result confirmed that HA/TiO₂ composite was successfully prepared. SEM revealed small crystals of anatase TiO₂ embedded in larger HA crystals. A strong antiviral activity against H1N1 Influenza A Virus was observed at 0.5 mg/ml concentration of the composite under the UV irradiation for 60 min. It showed the highest rate of reducing virus titer approximately more than 2 log/h. Results obtained from this study indicated that HA/TiO₂ composite could be a promising material to be used as antimicrobial filtration applications such as in face masks.

Osseointegration of porous apatite-wollastonite and poly(lactic acid) composite structures created using 3D printing techniques

A novel apatite-wollastonite/poly(lactic acid) (AW/PLA) composite structure, which matches cortical and cancellous bone properties has been produced and evaluated in vitro and in vivo. The composites structure has been produced using an innovative combination of 3D printed polymer and ceramic macrostructures, thermally bonded to create a hybrid composite structure. In vitro cell assays demonstrated that the AW structure alone, PLA structure alone, and AW/PLA composite were all biocompatible, with the AW structure supporting the proliferation and osteogenic differentiation of rat bone marrow stromal cells. Within a rat calvarial defect model the AW material showed excellent osseointegration with the formation of new bone, and vascularisation of the porous AW structure, both when the AW was implanted alone and when it was part of the AW/PLA composite structure. However, the AW/PLA structure showed the largest amount of the newly formed bone in vivo, an effect which is considered to be a result of the presence of the osteoinductive AW structure stimulating bone growth in the larger pores of the adjacent PLA structure. The layered AW/PLA structure showed no signs of delamination in any of the in vitro or in vivo studies, a result which is attributed to good initial bonding between polymer and ceramic, slow resorption rates of the two materials, and excellent osseointegration. It is concluded that macro-scale composites offer an alternative route to the fabrication of bioactive bone implants which can provide a match to both cortical and cancellous bone properties over millimetre length scales.

Enhanced cell proliferation and osteogenic differentiation in electrospun PLGA/hydroxyapatite nanofibre scaffolds incorporated with graphene oxide

One of the goals of bone tissue engineering is to mimic native ECM in architecture and function, creating scaffolds with excellent biocompatibility, osteoinductive ability and mechanical properties. The aim of this study was to fabricate nanofibrous matrices by electrospinning a blend of poly (L-lactic-co-glycolic acid) (PLGA), hydroxyapatite (HA), and grapheme oxide (GO) as a favourable platform for bone tissue engineering. The morphology, biocompatibility, mechanical properties, and biological activity of all nanofibrous matrices were compared. The data indicate that the hydrophilicity and protein adsorption rate of the fabricated matrices were significantly increased by blending with a small amount of HA and GO. Furthermore, GO significantly boosted the tensile strength of the nanofibrous matrices, and the PLGA/GO/HA nanofibrous matrices can serve as mechanically stable scaffolds for cell growth. For further test in vitro, MC3T3-E1 cells were cultured on the PLGA/HA/GO nanofbrous matrices to observe various cellular activities and cell mineralization. The results indicated that the PLGA/GO/HA nanofibrous matrices significantly enhanced adhesion, and proliferation in MCET3-E1 cells and functionally promoted alkaline phosphatase (ALP) activity, the osteogenesis-related gene expression and mineral deposition. Therefore, the PLGA/HA/GO composite nanofibres are excellent and versatile scaffolds for applications in bone tissue regeneration.

Strontium-doped hydroxyapatite polysaccharide materials effect on ectopic bone formation

Previous studies performed using polysaccharide-based matrices supplemented with hydroxyapatite (HA) particles showed their ability to form in subcutaneous and intramuscular sites a mineralized and osteoid tissue. Our objectives are to optimize the HA content in the matrix and to test the combination of HA with strontium (Sr-HA) to increase the matrix bioactivity. First, non-doped Sr-HA powders were combined to the matrix at three different ratios and were implanted subcutaneously for 2 and 4 weeks. Interestingly, matrices showed radiolucent properties before implantation. Quantitative analysis of micro-CT data evidenced a significant increase of mineralized tissue formed ectopically with time of implantation and allowed us to select the best ratio of HA to polysaccharides of 30% (w/w). Then, two Sr-substitution of 8% and 50% were incorporated in the HA powders (8Sr-HA and 50Sr-HA). Both Sr-HA were chemically characterized and dispersed in matrices. In vitro studies performed with human mesenchymal stem cells (MSCs) demonstrated the absence of cytotoxicity of the Sr-doped matrices whatever the amount of incorporated Sr. They also supported osteoblastic differentiation and activated the expression of one late osteoblastic marker involved in the mineralization process i.e. osteopontin. In vivo, subcutaneous implantation of these Sr-doped matrices induced osteoid tissue and blood vessels formation.

The performance of bone tissue engineering scaffolds in in vivo animal models: A systematic review

Bone tissue engineering is an excellent alternative for the regeneration of large bone defects caused by trauma or bone pathologies. Scaffolds, stem cells, and bioactive molecules are the three key components of bone regeneration. Although a wide range of biomaterials of various compositions and structures has been proposed in the literature, these materials are rarely used in clinical applications. Therefore, more standardized studies are required to design scaffolds that enable better bone regeneration and are suitable for clinical use. The aim of this systematic review was to compare the performance of scaffolds used in preclinical animal studies to determine which class of materials has achieved a higher rate of bone neoformation (osteoinduction and osteoconduction). The selected studies were divided into three groups according to the following experimental models: studies that used subcutaneous models, bone defects in calvaria, and bone defects in long bones. Despite the large number of parameters in the included studies, we generally concluded that biomaterials containing calcium phosphates had important osteoinductive effects and were essential for better performance of the materials. Furthermore, natural polymers generally had better performance than synthetic polymers did, especially when the materials were associated with stem cells. The combination of materials from different classes was the most promising strategy for bone tissue regeneration.

Modelling the nanomechanical response of a micro particle-matrix system for nanoindentation tests

A lot of experimental, numerical simulation and analytical modelling work has been done on how the substrate affects the measured hardness and elastic modulus of the coating/substrate system for nanoindentation tests. Little work has been done on the elastic-plastic behaviour of micro particle-matrix systems. Clifford et al have proposed an empirical model to describe the spatially dependent composite modulus during nanoindentation tests for linear elastic particles embedded in a linear elastic matrix. However, no such models have been developed for elastic-plastic composites. In this study, finite element simulations were used to determine the elastic modulus and hardness of hard particles embedded in a soft matrix and vice versa. An extended Clifford model has been developed to determine the elastic modulus and hardness for elastic-plastic composites with various particle shapes and volume fractions.

Monolithic calcium phosphate/poly(lactic acid) composite versus calcium phosphate-coated poly(lactic acid) for support of osteogenic differentiation of human mesenchymal stromal cells

Calcium phosphates (CaPs), extensively used synthetic bone graft substitutes, are often combined with other materials with the aim to overcome issues related to poor mechanical properties of most CaP ceramics. Thin ceramic coatings on metallic implants and polymer-ceramic composites are examples of such hybrid materials. Both the properties of the CaP used and the method of incorporation into a hybrid structure are determinant for the bioactivity of the final construct. In the present study, a monolithic composite comprising nano-sized CaP and poly(lactic acid) (PLA) and a CaP-coated PLA were comparatively investigated for their ability to support proliferation and osteogenic differentiation of bone marrow-derived human mesenchymal stromal cells (hMSCs). Both, the PLA/CaP composite, produced using physical mixing and extrusion and CaP-coated PLA, resulting from a biomimetic coating process at near-physiological conditions, supported proliferation of hMSCs with highest rates at PLA/CaP composite. Enzymatic alkaline phosphatase activity as well as the mRNA expression of bone morphogenetic protein-2, osteopontin and osteocalcin were higher on the composite and coated polymer as compared to the PLA control, while no significant differences were observed between the two methods of combining CaP and PLA. The results of this study confirmed the importance of CaP in osteogenic differentiation while the exact properties and the method of incorporation into the hybrid material played a less prominent role.

Needle-like ion-doped hydroxyapatite crystals influence osteogenic properties of PCL composite scaffolds

Surface topography and chemistry both play a crucial role on influencing cell response in 3D porous scaffolds in terms of osteogenesis. Inorganic materials with peculiar morphology and chemical functionalities may be proficiently used to improve scaffold properties-in the bulk and along pore surface-promoting in vitro and in vivo osseous tissue in-growth. The present study is aimed at investigating how bone regenerative properties of composite scaffolds made of poly(Ɛ-caprolactone) (PCL) can be augmented by the peculiar properties of Mg(2+) ion doped hydroxyapatite (dHA) crystals, mainly emphasizing the role of crystal shape on cell activities mediated by microstructural properties. At the first stage, the study of mechanical response by crossing experimental compression tests and theoretical simulation via empirical models, allow recognizing a significant contribution of dHA shape factor on scaffold elastic moduli variation as a function of the relative volume fraction. Secondly, the peculiar needle-like shape of dHA crystals also influences microscopic (i.e. crystallinity, adhesion forces) and macroscopic (i.e. roughness) properties with relevant effects on biological response of the composite scaffold: differential scanning calorimetry (DSC) analyses clearly indicate a reduction of crystallization heat-from 66.75 to 43.05 J g(-1)-while atomic force microscopy (AFM) ones show a significant increase of roughness-from (78.15  ±  32.71) to (136.13  ±  63.21) nm-and of pull-off forces-from 33.7% to 48.7%. Accordingly, experimental studies with MG-63 osteoblast-like cells show a more efficient in vitro secretion of alkaline phosphatase (ALP) and collagen I and a more copious in vivo formation of new bone trabeculae, thus suggesting a relevant role of dHA to support the main mechanisms involved in bone regeneration.

Improvement of the fracture toughness of hydroxyapatite (HAp) by incorporation of carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and nylon

The potential of improving the fracture toughness of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and polymerized ε-caprolactam (nylon) was studied. A series of HAp samples with CfSWCNTs concentrations varying from 0 to 1.5 wt.%, without, and with nylon addition was prepared. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were used to characterize the samples. The three point bending test was applied to measure the fracture toughness of the composites. A reproducible value of 3.6±0.3 MPa.√m was found for samples containing 1 wt.% CfSWCNTs and nylon. This value is in the range of the cortical bone fracture toughness. Increase of the CfSWCNTs content results to decrease of the fracture toughness, and formation of secondary phases.

Interfacial and biological properties of the gradient coating on polyamide substrate for bone substitute

Fabrication of bioactive and mechanical matched bone substitutes is crucial for clinical application in bone defects repair. In this study, nano-hydroxyapatite/polyamide (nHA/PA) composite was coated on injection-moulded PA by a chemical corrosion and phase-inversion technique. The shear strength, gradient composition and pore structure of the bioactive coating were characterized. Osteoblast-like MG63 cells were cultured on pure PA and composite-coated PA samples. The cells' adhesion, spread and proliferation were determined using MTT assay and microscopy. The results confirm that the samples with the nHA/PA composite coating have better cytocompatibility and have no negative effects on cells. To investigate the in vivo biocompatibility, both pure PA and composite-coated PA cylinders were implanted in the trochlea of rabbit femurs and studied histologically, and the bonding ability with bone were determined using push-out tests. The results show that composite-coated implants exhibit better biocompatibility and the shear strength of the composite-coated implants with host bone at 12 weeks can reach 3.49±0.42 MPa, which is significantly higher than that of pure PA implants. These results indicate that composite-coated PA implants have excellent biocompatibility and bonding abilities with host bone and they have the potential to be applied in repair of bone defects.

Substituted hydroxyapatites with antibacterial properties

Reconstructive surgery is presently struggling with the problem of infections located within implantation biomaterials. Of course, the best antibacterial protection is antibiotic therapy. However, oral antibiotic therapy is sometimes ineffective, while administering an antibiotic at the location of infection is often associated with an unfavourable ratio of dosage efficiency and toxic effect. Thus, the present study aims to find a new factor which may improve antibacterial activity while also presenting low toxicity to the human cells. Such factors are usually implemented along with the implant itself and may be an integral part of it. Many recent studies have focused on inorganic factors, such as metal nanoparticles, salts, and metal oxides. The advantages of inorganic factors include the ease with which they can be combined with ceramic and polymeric biomaterials. The following review focuses on hydroxyapatites substituted with ions with antibacterial properties. It considers materials that have already been applied in regenerative medicine (e.g., hydroxyapatites with silver ions) and those that are only at the preliminary stage of research and which could potentially be used in implantology or dentistry. We present methods for the synthesis of modified apatites and the antibacterial mechanisms of various ions as well as their antibacterial efficiency.