Electrospun bioactive nanocomposite scaffolds of polycaprolactone and nanohydroxyapatite for bone tissue engineering

Hydroxyapatite nanoparticle reinforced peptide amphiphile nanomatrix enhances the osteogenic differentiation of mesenchymal stem cells by compositional ratios

In the field of bone tissue engineering, there is a need for materials that mimic the native bone extracellular matrix (ECM). This need is met through the creation of biphasic composites intended to mimic both the organic and inorganic facets of the native bone ECM. However, few studies have created composites with organic ECM analogous components capable of directing cellular behaviors and many are not fabricated in the nanoscale. Furthermore, few attempts have been made at investigating how variations of organic and inorganic components affect the osteogenic differentiation of human mesenchymal stem cells (hMSCs). To address these issues, biphasic nanomatrix composites consisting of hydroxyapatite nanoparticles (HANPs) embedded within peptide amphiphile (PA) nanofibers tailored with the RGDS cellular adhesion motif (PA-RGDS) were created at various HANP to PA-RGDS ratios. Fabrication of these biphasic nanomatrix composites was confirmed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The long-term cellularity and osteogenic differentiation of hMSCs in response to the different compositional ratios were then assessed by quantifying the timed expression of genes indicative of osteogenic differentiation, alkaline phosphatase activity, and DNA content over time. Decreased cellularity and the expression of genes over time correlated with increasing compositional ratios between HANP and PA-RGDS. The highest HANP to PA-RGDS ratio (66% HANP) exhibited the greatest improvement to the osteogenic differentiation of hMSCs. Overall, these results demonstrate that the compositional ratio of biphasic nanomatrix composites plays an important role in influencing the osteogenic differentiation of hMSCs. Based on the observations presented within this study, these biphasic nanomatrix composites show promise for future usage in bone tissue engineering applications.

Mineralization of nanohydroxyapatite on electrospun poly(l-lactic acid)/gelatin by an alternate soaking process: A biomimetic scaffold for bone regeneration

Biomimetic nanocomposite scaffolds were fabricated by electrospinning poly(l-lactic acid) and a blend of poly(L-lactic acid)/gelatin to eliminate the use of collagen. The scaffolds were mineralized via alternate soaking in calcium and phosphate solutions, whereby 66.8% nanohydroxyapatite formation was successfully induced which is similar to that of native human bone (60%). The poly(L-lactic acid)/gelatin scaffolds had uniform nanohydroxyapatite formation throughout the scaffold. The mineralization enhanced the tensile modulus and tensile strength without increasing the brittleness. The in vitro biocompatibility of scaffolds was evaluated with murine adipose tissue–derived stem cells. The scaffolds with nanohydroxyapatite aided cell attachment and promoted cell–cell interaction. The mineralization and osteocalcin expression of the murine adipose tissue–derived stem cells were maximum in the poly(L-lactic acid)/gelatin/nanohydroxyapatite scaffold. Therefore, the gelatin and nanohydroxyapatite in poly(L-lactic acid)/gelatin/nanohydroxyapatite scaffolds provided cues for the differentiation of murine adipose tissue–derived stem cells. The biochemical nature of poly(L-lactic acid)/gelatin/nanohydroxyapatite scaffold accelerated osteogenic differentiation and could be a potential candidate for bone regeneration.

Mechano-morphological studies of aligned nanofibrous scaffolds of polycaprolactone fabricated by electrospinning

Mechanical and morphological studies of aligned nanofibrous meshes of poly(epsilon-caprolactone) (PCL) fabricated by electrospinning at different collector rotation speeds (0, 3000 and 6000 rpm) for application as bone tissue scaffolds are reported. SEM, XRD and DSC analyses were used for the morphological characterization of the nanofibers. Scaffolds have a nanofibrous morphology with fibers (majority) having a diameter in the range of 550-350 nm (depending on fiber uptake rates) and an interconnected pore structure. With the increase of collector rotation speed, the nanofibers become more aligned and oriented perpendicular to the axis of rotation. Deposition of fibers at higher fiber collection speeds has a profound effect on the morphology and mechanical properties of individual fibers and also the bulk fibrous meshes. Nanoindentation was used for the measurement of nanoscopic mechanical properties of individual fibers of the scaffolds. The hardness and Young's modulus of aligned fibers measured by nanoindentation decreased with collector rotation speeds. This reveals the difference in the local microscopic structure of the fibers deposited at higher speeds. The sequence of nanoscopic mechanical properties (hardness and modulus) of three fibers is PCL at 0 rpm > PCL at 3000 rpm > PCL at 6000 rpm. This may be explained due to the decrease in crystallinity of fibers at higher uptake rates. However, uni-axial tensile properties of (bulk) scaffolds (tensile strength and modulus) increased with increasing collector rotation speed. The average ultimate tensile strength of scaffolds (along the fiber alignment) increased from 2.21 +/- 0.23 MPa for PCL at uptake rate of zero rpm, to a value of 4.21 +/- 0.35 MPa for PCL at uptake rate of 3000 rpm and finally to 9.58 +/- 0.71 MPa for PCL at 6000 rpm. Similarly, the tensile modulus increased gradually from 6.12 +/- 0.8 MPa for PCL at uptake rate of zero rpm, to 11.93 +/- 1.22 MPa for PCL at uptake rate of 3000 rpm and to 33.20 +/- 1.98 MPa for PCL at 6000 rpm. The sequence of macroscopic mechanical properties (tensile strength and modulus) of three fibers, from highest to lowest, is PCL at 0 rpm < PCL at 3000 rpm < PCL at 6000 rpm. This is attributed to the increased fiber alignment and packing and decrease in inter-fiber pore size at higher uptake rates.

Reinforcement of electrospun membranes using nanoscale Al2O3 whiskers for improved tissue scaffolds

Poly(ε-caprolactone) (PCL) is a promising material for tissue engineering applications; however, it can be difficult to create scaffolds with the morphology, hydrophilicity, and mechanical properties necessary to support tissue growth. Typically, pure PCL scaffolds have good cellular adhesion, but somewhat low mechanical properties (elastic modulus and tensile strength). This study addresses these issues by incorporating Al(2)O(3) whiskers as reinforcements within PCL membranes generated by electrospinning. Membranes were prepared with Al(2)O(3) content ranging from 1 to 20 wt % and characterized using XRD, TEM, and SEM to determine composition and morphology. The Al(2)O(3) whiskers were well dispersed within the PCL fibers, and the membranes had a highly porous morphology. The elastic modulus was significantly improved by the well aligned whisker reinforcements as verified by tensile testing. The cell morphology and proliferation studies demonstrate Al(2)O(3) whisker reinforced PCL scaffolds maintained the good biocompatibility. These improvements demonstrate that Al(2)O(3) whisker reinforced PCL scaffolds can be considered as a biocompatible material for tissue engineering and dental applications.

Biocomposites and hybrid biomaterials based on calcium orthophosphates

The state-of-the-art of biocomposites and hybrid biomaterials based on calcium orthophosphates that are suitable for biomedical applications is presented in this review. Since these types of biomaterials offer many significant and exciting possibilities for hard tissue regeneration, this subject belongs to a rapidly expanding area of biomedical research. Through successful combinations of the desired properties of matrix materials with those of fillers (in such systems, calcium orthophosphates might play either role), innovative bone graft biomaterials can be designed. Various types of biocomposites and hybrid biomaterials based on calcium orthophosphates, either those already in use or being investigated for biomedical applications, are extensively discussed. Many different formulations, in terms of the material constituents, fabrication technologies, structural and bioactive properties as well as both in vitro and in vivo characteristics, have already been proposed. Among the others, the nanostructurally controlled biocomposites, those containing nanodimensional compounds, biomimetically fabricated formulations with collagen, chitin and/or gelatin as well as various functionally graded structures seem to be the most promising candidates for clinical applications. The specific advantages of using biocomposites and hybrid biomaterials based on calcium orthophosphates in the selected applications are highlighted. As the way from the laboratory to the hospital is a long one, and the prospective biomedical candidates have to meet many different necessities, this review also examines the critical issues and scientific challenges that require further research and development.

Mesenchymal stem cell responses to bone-mimetic electrospun matrices composed of polycaprolactone, collagen I and nanoparticulate hydroxyapatite

The performance of biomaterials designed for bone repair depends, in part, on the ability of the material to support the adhesion and survival of mesenchymal stem cells (MSCs). In this study, a nanofibrous bone-mimicking scaffold was electrospun from a mixture of polycaprolactone (PCL), collagen I, and hydroxyapatite (HA) nanoparticles with a dry weight ratio of 50/30/20 respectively (PCL/col/HA). The cytocompatibility of this tri-component scaffold was compared with three other scaffold formulations: 100% PCL (PCL), 100% collagen I (col), and a bi-component scaffold containing 80% PCL/20% HA (PCL/HA). Scanning electron microscopy, fluorescent live cell imaging, and MTS assays showed that MSCs adhered to the PCL, PCL/HA and PCL/col/HA scaffolds, however more rapid cell spreading and significantly greater cell proliferation was observed for MSCs on the tri-component bone-mimetic scaffolds. In contrast, the col scaffolds did not support cell spreading or survival, possibly due to the low tensile modulus of this material. PCL/col/HA scaffolds adsorbed a substantially greater quantity of the adhesive proteins, fibronectin and vitronectin, than PCL or PCL/HA following in vitro exposure to serum, or placement into rat tibiae, which may have contributed to the favorable cell responses to the tri-component substrates. In addition, cells seeded onto PCL/col/HA scaffolds showed markedly increased levels of phosphorylated FAK, a marker of integrin activation and a signaling molecule known to be important for directing cell survival and osteoblastic differentiation. Collectively these results suggest that electrospun bone-mimetic matrices serve as promising degradable substrates for bone regenerative applications.

A novel spatially designed and functionally graded electrospun membrane for periodontal regeneration

A periodontal membrane with a graded structure allows tailoring of the layer properties to design a material system that will retain its physical, chemical and mechanical characteristics for a period long enough to optimize periodontal regeneration. In this work a novel functionally graded membrane (FGM) was designed and fabricated via sequential multilayer electrospinning. The FGM consists of a core layer (CL) and two functional surface layers (SLs) interfacing with bone (nano-hydroxyapatite, n-HAp) and epithelial (metronidazole, MET) tissues. The CL comprises a neat poly(DL-lactide-co-ε-caprolactone) (PLCL) layer surrounded by two composite layers composed of a protein/polymer ternary blend (PLCL:PLA:GEL). Electrospinning parameters involved in fabrication of the individual layers (i.e. neat PLCL, ternary blend, PLA:GEL+10%n-HAp and PLA:GEL+25%MET) were optimized to obtain fibrous layers free of beads. Morphology, structure and mechanical property studies were carried out on each electrospun layer. The individual fiber morphology and roughness of the functional SLs, which are the n-HAp containing and drug-incorporating layers were evaluated by atomic force microscopy. The CL structure demonstrated higher strength (8.7 MPa) and a more elastic behavior (strain at break 357%) compared with the FGM (3.5 MPa, 297%). Incorporation of n-HAp to enhance osteoconductive behavior and MET to combat periodontal pathogens led to a novel FGM that holds promise at solving the drawbacks of currently available membranes.

The performance of dental pulp stem cells on nanofibrous PCL/gelatin/nHA scaffolds

The aim of current study is to investigate the in vitro and in vivo behavior of dental pulp stem cells (DPSCs) seeded on electrospun poly(epsilon-caprolactone) (PCL)/gelatin scaffolds with or without the addition of nano-hydroxyapatite (nHA). For the in vitro evaluation, DNA content, alkaline phosphatase (ALP) activity and osteocalcin (OC) measurement showed that the scaffolds supported DPSC adhesion, proliferation, and odontoblastic differentiation. Moreover, the presence of nHA upregulated ALP activity and promoted OC expression. Real-time PCR data confirmed these results. SEM micrographs qualitatively confirmed the proliferation and mineralization characteristics of DPSCs on both scaffolds. Subsequently, both scaffolds seeded with DPSCs were subcutaneously implanted into immunocompromised nude mice. Scaffolds with nHA but without cells were implanted as control. Histological evaluation revealed that all implants were surrounded by a thin fibrous tissue capsule without any adverse effects. The cell/scaffold composites showed obvious in vivo hard tissue formation, but there was no sign of tissue ingrowth. Further, the combination of nHA in scaffolds did upregulate the expression of specific odontogenic genes. In conclusion, the incorporation of nHA in nanofibers indeed enhanced DPSCs differentiation towards an odontoblast-like phenotype in vitro and in vivo.

Development of an electrospun nano-apatite/PCL composite membrane for GTR/GBR application

In dental practice, membranes are used as a barrier to prevent soft tissue ingrowth and create space for slowly regenerating periodontal and bony tissues. The aim of this study was to develop a biodegradable membrane system which can be used for guided tissue or bone regeneration. Three types of composite fibrous membranes based on nano-apatite (nAp) and poly(epsilon-caprolactone) (PCL) were made by electrospinning, i.e. n0 (nAp:PCL=0:100), n25 (nAp:PCL=25:100) and n50 (nAp:PCL=50:100) with average fiber diameters ranging from 320 to 430 nm. Their structural, mechanical, chemical and biological properties were evaluated. Tensile test revealed that n25 had the highest strength and toughness, indicating there is an optimal ratio of nAp to polymer for mechanical reinforcement. Subsequently, a simulated body fluid immersion test confirmed that the presence of nAp enhanced the bioactive behavior of the membranes. Finally, an in vitro osteoblast cell study showed that all membranes supported proliferation, but the presence of nAp facilitated an early cell differentiation. This study demonstrated that an electrospun membrane incorporating nAp is strong, enhances bioactivity and supports osteoblast-like cell proliferation and differentiation. The membrane system can be used as a prototype for the further development of an optimal membrane for clinical use.

A hybrid biomimetic scaffold composed of electrospun polycaprolactone nanofibers and self-assembled peptide amphiphile nanofibers

Nanofibrous electrospun poly (epsilon-caprolactone) (ePCL) scaffolds have inherent structural advantages, but lack of bioactivity has limited their usefulness in biomedical applications. Thus, here we report the development of a hybrid, nanostructured, extracellular matrix (ECM) mimicking scaffold by a combination of ePCL nanofibers and self-assembled peptide amphiphile (PA) nanofibers. The PAs have ECM mimicking characteristics including a cell adhesive ligand (RGDS) and matrix metalloproteinase-2 (MMP-2) mediated degradable sites. Transmission electron microscope imaging verified successful PA self-assembly into nanofibers (diameters of 8-10 nm) using a solvent evaporation method. This evaporation method was then used to successfully coat PAs onto ePCL nanofibers (diameters of 300-400 nm), to develop hybrid, bioactive scaffolds. Scanning electron microscope characterization showed that the PA coatings did not interfere with the porous ePCL nanofiber network. Human mesenchymal stem cells (hMSCs) were seeded onto the hybrid scaffolds to evaluate their bioactivity. Significantly greater attachment and spreading of hMSCs were observed on ePCL nanofibers coated with PA-RGDS as compared to ePCL nanofibers coated with PA-S (no cell adhesive ligand) and uncoated ePCL nanofibers. Overall, this novel strategy presents a new solution to overcome the current bioactivity challenges of electrospun scaffolds and combines the unique characteristics of ePCL nanofibers and self-assembled PA nanofibers to provide an ECM mimicking environment. This has great potential to be applied to many different electrospun scaffolds for various biomedical applications.

Problem of hydroxyapatite dispersion in polymer matrices: a review

This review summarizes recent work on manufacturing biocomposites suitable for bone tissue engineering. There is a great need to engineer multi-phase (i.e. composite) materials that combine the advantages exhibited by each component of the material, with a structure and composition similar to that of natural bone. The discussion concentrates on the preparation of nanocomposites containing hydroxyapatite particles (one of the most widely used bioceramics materials) with polymer matrices. Special attention is paid to the preparation of nanocomposites with individual (non-aggregated) nanoparticles because this is a key problem in nanotechnology industrialization. Controlling the mixing between so two dissimilar phases is a critical challenge in the design of these inorganic-organic systems. Several approaches that may be applied to overcome this problem will be described in this review.

Electrospun composite poly(L-lactic acid)/tricalcium phosphate scaffolds induce proliferation and osteogenic differentiation of human adipose-derived stem cells

Development of tissue-engineered bone constructs has recently focused on the use of electrospun composite scaffolds seeded with stem cells from various source tissues. In this study, we fabricated electrospun composite scaffolds consisting of beta-tricalcium phosphate (TCP) crystals and poly(L-lactic acid) (PLA) at varying loading levels of TCP (0, 5, 10, 20 wt%) and assessed the composite scaffolds' material properties and ability to induce proliferation and osteogenic differentiation of human adipose-derived stem cells (hASCs) in the presence of osteogenic differentiating medium. The electrospun scaffolds all exhibited a nonwoven structure with an interconnected porous network. With the addition of TCP, the fiber diameter increased with each treatment ranging from 503.39 +/- 20.31 nm for 0 wt% TCP to 1267.36 +/- 59.03 nm for 20 wt% TCP. Tensile properties of the composite scaffolds were assessed and the overall tensile strength of the neat scaffold (0 wt% TCP) was 847 +/- 89.43 kPA; the addition of TCP significantly decreased this value to an average of 350.83 +/- 38.57 kPa. As the electrospun composite scaffolds degraded in vitro, TCP was released into the medium with the largest release occurring within the first 6 days. Human ASCs were able to adhere, proliferate and osteogenically differentiate on all scaffold combinations. DNA content increased in a temporal manner for each scaffold over 18 days in culture although for the day 12 timepoint, the 10 wt% TCP scaffold induced the greatest hASC proliferation. Endogenous alkaline phosphatase activity was enhanced on the composite PLA/TCP scaffolds compared to the PLA control particularly by day 18. It was noted that at the highest TCP loading levels of 10 and 20 wt%, there was a dramatic increase in the amount of cell-mediated mineralization compared to the 5 wt% TCP and the neat PLA scaffold. This work suggests that local environment cues provided by the biochemical nature of the scaffold can accelerate the overall osteogenic differentiation of hASCs and encourage rapid ossification.

Nanostructured Biocomposite Scaffolds Based on Collagen Coelectrospun with Nanohydroxyapatite

Nanofibrous biocomposite scaffolds of type I collagen and nanohydroxyapatite (nanoHA) of varying compositions (wt %) were prepared by electrostatic cospinning. The scaffolds were characterized for structure and morphology by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The scaffolds have a porous nanofibrous morphology with random fibers in the range of 500-700 nm diameters, depending on the composition. FT-IR and XRD showed the presence of nanoHA in the fibers. The surface roughness and diameter of the fibers increased with the presence of nanoHA in biocomposite fiber as evident from AFM images. Tensile testing and nanoindendation were used for the mechanical characterization. The pure collagen fibrous matrix (without nanoHA) showed a tensile strength of 1.68 +/- 0.10 MPa and a modulus of 6.21 +/- 0.8 MPa with a strain to failure value of 55 +/- 10%. As the nanoHA content in the randomly oriented collagen nanofibers increased to 10%, the ultimate strength increased to 5 +/- 0.5 MPa and the modulus increased to 230 +/- 30 MPa. The increase in tensile modulus may be attributed to an increase in rigidity over the pure polymer when the hydroxyapatite is added and/or the resulting strong adhesion between the two materials. The vapor phase chemical crosslinking of collagens using glutaraldehyde further increased the mechanical properties as evident from nanoindentation results. A combination of nanofibrous collagen and nanohydroxyapatite that mimics the nanoscale features of the extra cellular matrix could be promising for application as scaffolds for hard tissue regeneration, especially in low or nonload bearing areas.

Biomimetic nanocomposites for bone graft applications

Allograft bone, dematerialized bone matrix and calcium-based synthetic materials have long been used as bone graft substitutes. First-generation bone graft substitutes as stand-alone graft substitutes have not developed as hoped. It remains a great challenge to design an ideal bone graft that emulates nature’s own structures or functions. To further improve the performance of such bone graft substitutes, scientists are investigating biomimetic processes to incorporate the desirable nano-features into the next generation of biomaterials. In this regard, nanostructured biomaterials less than 100 nm in at least one dimension, in particular nanocomposites, are perceived to be beneficial and potentially ideal for bone applications, owing to their nanoscale functional characteristics that facilitate bone cell growth and subsequent tissue formation. In fact, bone itself is a nanocomposite system with a complex hierarchical structure. This review reports the impact of biomimetically derived nanocomposite biomaterials for use in bone applications and provides possible suggestions for future research and development.