Differences between in vitro viability and differentiation and in vivo bone-forming efficacy of human mesenchymal stem cells cultured on PCL–TCP scaffolds

Development of composite scaffolds for load-bearing segmental bone defects

The need for a suitable tissue-engineered scaffold that can be used to heal load-bearing segmental bone defects (SBDs) is both immediate and increasing. During the past 30 years, various ceramic and polymer scaffolds have been investigated for this application. More recently, while composite scaffolds built using a combination of ceramics and polymeric materials are being investigated in a greater number, very few products have progressed from laboratory benchtop studies to preclinical testing in animals. This review is based on an exhaustive literature search of various composite scaffolds designed to serve as bone regenerative therapies. We analyzed the benefits and drawbacks of different composite scaffold manufacturing techniques, the properties of commonly used ceramics and polymers, and the properties of currently investigated synthetic composite grafts. To follow, a comprehensive review of in vivo models used to test composite scaffolds in SBDs is detailed to serve as a guide to design appropriate translational studies and to identify the challenges that need to be overcome in scaffold design for successful translation. This includes selecting the animal type, determining the anatomical location within the animals, choosing the correct study duration, and finally, an overview of scaffold performance assessment.

Osteogenic differentiation of human dental pulp stem cells on β-tricalcium phosphate/poly (l-lactic acid/caprolactone) three-dimensional scaffolds

Functional tissue engineering for bone augmentation requires the appropriate combination of biomaterials, mesenchymal stem cells, and specific differentiation factors. Therefore, we investigated the morphology, attachment, viability, and proliferation of human dental pulp stem cells cultured in xeno-free conditions in human serum medium seeded on β-tricalcium phosphate/poly(l-lactic acid/caprolactone) three-dimensional biomaterial scaffold. Additionally, osteogenic inducers dexamethasone and vitamin D₃ were compared to achieve osteogenic differentiation. Dental pulp stem cells cultured in human serum medium maintained their morphology; furthermore, cells attached, remained viable, and increased in cell number within the scaffold. Alkaline phosphatase staining showed the osteogenic potential of dental pulp stem cells under the influence of osteogenic medium containing vitamin D₃ or dexamethasone within the scaffolds. Maintenance of dental pulp stem cells for 14 days in osteogenic medium containing vitamin D₃ resulted in significant increase in osteogenic markers as shown at mRNA level in comparison to osteogenic medium containing dexamethasone. The results of this study show that osteogenic medium containing vitamin D₃ osteo-induced dental pulp stem cells cultured in human serum medium within β-tricalcium phosphate/poly(l-lactic acid/caprolactone) three-dimensional biomaterial, which could be directly translated clinically.

Block copolymer of poly(ester amide) and polyesters: synthesis, characterization, and in vitro cellular response

In order to expand the properties and applications of aliphatic absorbable polyesters, a new biodegradable block copolymer family, poly(ester amide)-b-poly(ε-caprolactone) (PEA-b-PCL), was synthesized and characterized. These copolymers were synthesized by first preparing l-phenylalanine-based poly(ester amide) macroinitiators (Phe-PEAs) with free amine end groups via a solution polycondensation. The amine-terminated Phe-PEA macroinitiators were then used to initiate the ring-opening polymerization of ε-caprolactone monomer to prepare the PEA-b-PCL copolymers. The molecular weight (MW) of PEA-b-PCLs can be well controlled by adjusting the Phe-PEA MW and weight ratio of ε-caprolactone to Phe-PEA, and ranged from 7 to 50kgmol(-1). The copolymers' structure and properties were characterized by various physicochemical methods, such as nuclear magnetic resonance, gel permeation chromatography and solubility testing. The in vitro enzymatic biodegradation tests were performed to evaluate the biodegradation rate of the copolymers. The results showed that the introduction of Phe-PEA to PCL did not significantly change the degradation rate of PCL. Biological studies were conducted to assess the polymer's biological properties, like supporting the cell attachment and proliferation, and inflammation response. The results showed that the bovine aortic endothelial cells had very good attachment and proliferation performance on PEA-b-PCL coating surface. TNF-α release profiles showed that PEA-b-PCL exhibited a muted J774 macrophage inflammatory response.

Radially and axially graded multizonal bone graft substitutes targeting critical-sized bone defects from polycaprolactone/hydroxyapatite/tricalcium phosphate

Repair and regeneration of critical sized defects via the utilization of polymeric bone graft substitutes are challenges. Here, we introduce radially and axially graded multizonal bone graft substitutes fabricated from polycaprolactone (PCL), and PCL biocomposites with osteoconductive particles, that is, hydroxyapatite (HA), and β-tricalcium phosphate (TCP). The novel bone graft substitutes should provide a greater degree of freedom to the orthopedic surgeon especially for repair of critically sized bone defects. The modulus of the graft substitute could be tailored in the axial direction upon the systematic variation of the HA/TCP concentration, while in the radial direction the bone graft substitute consisted of an outer layer with high stiffness, encapsulating a softer core with greater porosity. The biocompatibility of the bone graft substitutes was investigated using in vitro culturing of human bone marrow-derived stromal cells followed by the analysis of cell proliferation and differentiation rates. The characterization of the tissue constructs included the enzymatic alkaline phosphates (ALP) activity, microcomputed tomography imaging, and polymerase chain reaction analysis involving the expressions of bone markers, that is, Runx2, ALP, collagen type I, osteopontin, and osteocalcin, overall demonstrating the differentiation of bone marrow derived stem cells (BMSCs) via osteogenic lineage and formation of mineralized bone tissue.

Stimulation of healing within a rabbit calvarial defect by a PCL/PLGA scaffold blended with TCP using solid freeform fabrication technology

The purpose of this study was to investigate the healing capacity within an 8-mm rabbit calvarial defect using a polycaprolactone (PCL)/poly(lactic-co-glycolic acid) (PLGA) scaffold blended with tri-calcium phosphate (TCP) that was constructed using solid freeform fabrication (SFF) technology. The PCL/PLGA/TCP scaffold showed a 37 % higher compressive strength and rougher surface than the PCL/PLGA scaffold. In animal experiments, new bone formation was analyzed using microcomputed tomography (micro-CT) and histological and histometric analyses. The PCL/PLGA/TCP groups had significantly greater neo-tissue areas as compared with the control groups at 4 and 8 weeks (P < 0.05). The PCL/PLGA/TCP group had significantly greater bone density as compared with the control and PCL/PLGA groups at 4 and 8 weeks (P < 0.005). The results of this study suggest that the PCL/PLGA/TCP scaffold fabricated using SFF technology is useful for recovering and enhancing new bone formation in bony defects in rabbits.

Additive manufacturing techniques for the production of tissue engineering constructs

'Additive manufacturing' (AM) refers to a class of manufacturing processes based on the building of a solid object from three-dimensional (3D) model data by joining materials, usually layer upon layer. Among the vast array of techniques developed for the production of tissue-engineering (TE) scaffolds, AM techniques are gaining great interest for their suitability in achieving complex shapes and microstructures with a high degree of automation, good accuracy and reproducibility. In addition, the possibility of rapidly producing tissue-engineered constructs meeting patient's specific requirements, in terms of tissue defect size and geometry as well as autologous biological features, makes them a powerful way of enhancing clinical routine procedures. This paper gives an extensive overview of different AM techniques classes (i.e. stereolithography, selective laser sintering, 3D printing, melt-extrusion-based techniques, solution/slurry extrusion-based techniques, and tissue and organ printing) employed for the development of tissue-engineered constructs made of different materials (i.e. polymeric, ceramic and composite, alone or in combination with bioactive agents), by highlighting their principles and technological solutions.

Leveraging "raw materials" as building blocks and bioactive signals in regenerative medicine

Components found within the extracellular matrix (ECM) have emerged as an essential subset of biomaterials for tissue engineering scaffolds. Collagen, glycosaminoglycans, bioceramics, and ECM-based matrices are the main categories of "raw materials" used in a wide variety of tissue engineering strategies. The advantages of raw materials include their inherent ability to create a microenvironment that contains physical, chemical, and mechanical cues similar to native tissue, which prove unmatched by synthetic biomaterials alone. Moreover, these raw materials provide a head start in the regeneration of tissues by providing building blocks to be bioresorbed and incorporated into the tissue as opposed to being biodegraded into waste products and removed. This article reviews the strategies and applications of employing raw materials as components of tissue engineering constructs. Utilizing raw materials holds the potential to provide both a scaffold and a signal, perhaps even without the addition of exogenous growth factors or cytokines. Raw materials contain endogenous proteins that may also help to improve the translational success of tissue engineering solutions to progress from laboratory bench to clinical therapies. Traditionally, the tissue engineering triad has included cells, signals, and materials. Whether raw materials represent their own new paradigm or are categorized as a bridge between signals and materials, it is clear that they have emerged as a leading strategy in regenerative medicine. The common use of raw materials in commercial products as well as their growing presence in the research community speak to their potential. However, there has heretofore not been a coordinated or organized effort to classify these approaches, and as such we recommend that the use of raw materials be introduced into the collective consciousness of our field as a recognized classification of regenerative medicine strategies.

SOX9 gene plus heparinized TGF-β 3 coated dexamethasone loaded PLGA microspheres for inducement of chondrogenesis of hMSCs

Microparticulated types of scaffolds have been widely applied in stem cell therapy and the tissue engineering field for the regeneration of wound tissues. During application of simple genes or growth factors and cell delivery vehicles, we designed a method that employs dexamethsone loaded PLGA microspheres consisting of polyplexed SOX9 genes plus heparinized TGF-β 3 on the surface of polymeric microspheres prepared using a layer-by-layer (LbL) method. The fabrication of the polyplexed SOX9 genes plus heparinized TGF-β 3 and their subsequent coating onto dexamethsone loaded PLGA microspheres represents a method for functionalization of the polymeric matrix. The use of SOX9 gene plus heparinized TGF-β 3 coated dexamethsone loaded PLGA microspheres was evaluated to determine their potential as both gene carriers and cell delivery vehicle. By adhesion of hMSCs onto SOX9 gene plus heparinized TGF-β 3 coated dexamethsone loaded PLGA microspheres, the chondrogenesis-related specific genes of collagen type II were increased 30 times comparing to control. Also, the specific extracellular matrix of glycosaminoglycan (GAG) production of hMSCs adhered onto SOX9 gene plus heparinized TGF-β 3 coated dexamethasone loaded PLGA microspheres increased more 2.5 times than control group. Not only in vitro culture but in vivo results, the specific genes of COMP, aggrecan, collagen type II, and SOX9 showed much more gene expressions such as 20, 15, 10, 8 times.

Porous scaffold architecture guides tissue formation

Critical-sized bone defect regeneration is a remaining clinical concern. Numerous scaffold-based strategies are currently being investigated to enable in vivo bone defect healing. However, a deeper understanding of how a scaffold influences the tissue formation process and how this compares to endogenous bone formation or to regular fracture healing is missing. It is hypothesized that the porous scaffold architecture can serve as a guiding substrate to enable the formation of a structured fibrous network as a prerequirement for later bone formation. An ovine, tibial, 30-mm critical-sized defect is used as a model system to better understand the effect of the scaffold architecture on cell organization, fibrous tissue, and mineralized tissue formation mechanisms in vivo. Tissue regeneration patterns within two geometrically distinct macroscopic regions of a specific scaffold design, the scaffold wall and the endosteal cavity, are compared with tissue formation in an empty defect (negative control) and with cortical bone (positive control). Histology, backscattered electron imaging, scanning small-angle X-ray scattering, and nanoindentation are used to assess the morphology of fibrous and mineralized tissue, to measure the average mineral particle thickness and the degree of alignment, and to map the local elastic indentation modulus. The scaffold proves to function as a guiding substrate to the tissue formation process. It enables the arrangement of a structured fibrous tissue across the entire defect, which acts as a secondary supporting network for cells. Mineralization can then initiate along the fibrous network, resulting in bone ingrowth into a critical-sized defect, although not in complete bridging of the defect. The fibrous network morphology, which in turn is guided by the scaffold architecture, influences the microstructure of the newly formed bone. These results allow a deeper understanding of the mode of mineral tissue formation and the way this is influenced by the scaffold architecture. © 2012 American Society for Bone and Mineral Research.

Bone marrow-derived heparan sulfate potentiates the osteogenic activity of bone morphogenetic protein-2 (BMP-2)

Lowering the efficacious dose of bone morphogenetic protein-2 (BMP-2) for the repair of critical-sized bone defects is highly desirable, as supra-physiological amounts of BMP-2 have an increased risk of side effects and a greater economic burden for the healthcare system. To address this need, we explored the use of heparan sulfate (HS), a structural analog of heparin, to enhance BMP-2 activity. We demonstrate that HS isolated from a bone marrow stromal cell line (HS-5) and heparin each enhances BMP-2-induced osteogenesis in C2C12 myoblasts through increased ALP activity and osteocalcin mRNA expression. Commercially available HS variants from porcine kidney and bovine lung do not generate effects as great as HS5. Heparin and HS5 influence BMP-2 activity by (i) prolonging BMP-2 half-life, (ii) reducing interactions between BMP-2 with its antagonist noggin, and (iii) modulating BMP2 distribution on the cell surface. Importantly, long-term supplementation of HS5 but not heparin greatly enhances BMP-2-induced bone formation in vitro and in vivo. These results show that bone marrow-derived HS effectively supports bone formation, and suggest its applicability in bone repair by selectively facilitating the delivery and bioavailability of BMP-2.

Heparan sulfate enhances the self-renewal and therapeutic potential of mesenchymal stem cells from human adult bone marrow

Insufficient cell number hampers therapies utilizing adult human mesenchymal stem cells (hMSCs) and current ex vivo expansion strategies lead to a loss of multipotentiality. Here we show that supplementation with an embryonic form of heparan sulfate (HS-2) can both increase the initial recovery of hMSCs from bone marrow aspirates and increase their ex vivo expansion by up to 13-fold. HS-2 acts to amplify a subpopulation of hMSCs harboring longer telomeres and increased expression of the MSC surface marker stromal precursor antigen-1. Gene expression profiling revealed that hMSCs cultured in HS-2 possess a distinct signature that reflects their enhanced multipotentiality and improved bone-forming ability when transplanted into critical-sized bone defects. Thus, HS-2 offers a novel means for decreasing the expansion time necessary for obtaining therapeutic numbers of multipotent hMSCs without the addition of exogenous growth factors that compromise stem cell fate.

SEM sample preparation for cells on 3D scaffolds by freeze-drying and HMDS

Common dehydration methods of cells on biomaterials for scanning electron microscopy (SEM) include air drying, hexamethyldisilazane (HMDS) or tetramethysilane (TMS) treatment and critical point drying (CPD). On the other side, freeze-drying has been widely employed in dehydrating biological samples and also in preparing porous biomaterial scaffolds but not in preparing cells on three-dimensional (3D) biomaterials for SEM examination. In this study, we compare cells on porous hydroxyapatite (HA) prepared by air drying, HMDS and freeze-drying. The effects of fixation and using phosphate buffered saline (PBS) in the fixation were also assessed on three porous calcium phosphate (CaP) materials, namely, HA, α-tricalcium phosphate (α-TCP) and β-tricalcium phosphate (β-TCP) samples. There is no significant difference in samples prepared by HMDS treatment and freeze-drying viewed at low magnification. Besides, it is better not to use phosphate buffer in the fixation step for CaP materials to avoid undesirable spontaneous precipitation of CaPs. On the other hand, fewer exchanges of liquids are required for freeze-drying and hence chemical fixation may not be absolutely required for samples prepared by freeze-drying. Other technical details of the preparation were also investigated and discussed. This study suggests both HMDS and freeze-drying can be employed to dehydrate cells on 3D scaffolds for SEM examination.

Unitary bioresorbable cage/core bone graft substitutes for spinal arthrodesis coextruded from polycaprolactone biocomposites

A unitary bioresorbable cage/core bone graft substitute consisting of a stiff cage and a softer core with interconnected porosity is offered for spinal arthrodesis. Polycaprolactone, PCL, was used as the matrix and hydroxyapatite, HA, and β-tricalcium phosphate, TCP, were used in the formulation of the cage layer to impart modulus increase and osteoconductivity while the core consisted solely of PCL. The crystallinity, biodegradation rate (under accelerated conditions) and mechanical properties, i.e., the uniaxial compression, relaxation modulus upon step compression and cyclic compressive fatigue properties, of the co-extruded cage/core bone graft substitutes could be manipulated by changes in the concentration of HA/TCP in the cage layer. The cyclic fatigue behavior of the cage/core bone graft substitutes were also compared to the behavior of bovine vertebral cancellous bone characterized under similar testing conditions. The biocompatibility of the cage/core bone graft substitutes were assessed via in vitro culturing of human bone marrow derived stromal cells, BMSCs. The cell proliferation rates, time dependencies of the alkaline phosphates (ALP) activity and the expressions of bone markers, i.e., Runx2, ALP, collagen type I, osteopontin and osteocalcin, and the collected μ-CT images demonstrated the differentiation of BMSCs via osteogenic lineage and formation of mineralized bone tissue to indicate the biocompatibility of the cage/core bone graft substitutes.

Functional Evaluation of Primary Renal Cell/Biomaterial Neo-Kidney Augment Prototypes for Renal Tissue Engineering

Development of a tissue-engineered neo-kidney augment (NKA) requires evaluation of defined, therapeutically relevant cell and cell/biomaterial composites (NKA constructs) for regenerative potential in mammalian kidney. Previous work identified primary renal cell populations that extended survival and improved renal function in a rodent model of chronic kidney disease (CKD). This study extends that work toward the goal of developing NKA by (i) screening in vivo inflammatory and fibrotic responses to acellular biomaterials delivered to healthy rodent renal parenchyma, (ii) evaluating the functionality of renal cell/biomaterial combinations in vitro, (iii) generating NKA constructs by combining therapeutically relevant cell populations with biocompatible biomaterial, and (iv) evaluating in vivo neokidney tissue development in response to NKA constructs delivered to healthy rodent renal parenchyma. Gelatin and hyaluronic acid (HA)-based hydrogels elicited the least inflammatory and fibrotic responses in renal parenchyma relative to polycaprolactone (PCL) and poly(lactic- co-glycolic acid) (PLGA) beads or particles and were associated with neovascularization and cellular infiltration by 4 weeks postimplantation. Renal cell populations seeded onto gelatin or HA-based hydrogels were viable and maintained a tubular epithelial functional phenotype during an in vitro maturation of 3 days as measured by transcriptomic, proteomic, secretomic, and confocal immunofluorescence assays. In vivo delivery of cell-seeded NKA constructs (bioactive renal cells + gelatin hydrogels) to healthy rodent renal parenchyma elicited neokidney tissue formation at 1 week postimplantation. To investigate a potential mechanism by which NKA constructs could impact a disease state, the effect of conditioned media on TGF-β signaling pathways related to tubulo-interstitial fibrosis associated with CKD progression was evaluated. Conditioned medium was observed to attenuate TGF-β-induced epithelial–mesenchymal transition (EMT) in vitro in a human proximal tubular cell line (HK2).

Resveratrol effect on osteogenic differentiation of rat and human adipose derived stem cells in a 3-D culture environment

The goal of this study was to investigate the effect of resveratrol treatment on the osteogenic potential of human and rat adipose derived stem cells in a 3-D culture environment. Adipose derived stem cells (ADSCs) have been widely studied and have shown promise as a potential source of osteogenic progenitor cells. Previous work had investigated the effect of 25 μM resveratrol on the osteogenic differentiation of rat ADSCs in a 3-D environment and found that pre-treating cells for one passage prior to seeding on the scaffold yielded significantly more mineralization than untreated cells. We first sought to investigate whether this result was also observable with human ADSCs and found that the human cells did not respond to 25 μM resveratrol in a positive manner suggesting a species specific difference in resveratrol dosage. Therefore, we next investigated multiple doses at or below 25 μM resveratrol for both rat and human ADSCs. We found that doses below 25 μM caused significantly more mineralization than 0 (untreated) and 25 μM treated cells in a 3-D culture environment. Further, we observed species differences in the total amount of mineralized matrix, as well as the mean mineral density suggesting that the nature of mineralization of the extracellular matrix was different between species. Histological examination of the scaffolds showed that the human cell constructs remain highly cellular in nature with small pockets of mineralization, while rat cell constructs showed much larger and more mature mineralized nodules. Taken together, we demonstrate dose dependent differences in the mineralization response of human and rat ADSCs to resveratrol treatment, suggesting that in vitro pre-conditioning of 3D adipose-derived stem cell constructs may be an effective strategy to promote osteogenic differentiation prior to implantation.