Advancements in tissue engineered bone substitutes:

Inductive Materials for Regenerative Engineering

Regenerative engineering has pioneered several novel biomaterials to treat critical-sized bone injuries. However, despite significant improvement in synthetic materials research, some limitations still exist. The constraints correlated with the current grafting methods signify a treatment paradigm shift to osteoinductive regenerative engineering approaches. Because of their intrinsic potential, inductive biomaterials may represent alternative approaches to treating critical bone injuries. Osteoinductive scaffolds stimulate stem cell differentiation into the osteoblastic lineage, enhancing bone regeneration. Inductive biomaterials comprise polymers, calcium phosphate ceramics, metals, and graphene family materials. This review will assess the cellular behavior toward properties of inductive materials.

Alkali treatment facilitates functional nano-hydroxyapatite coating of 3D printed polylactic acid scaffolds

Autografting is currently the gold standard for treatment of bone defects, but has shown disadvantages in the limited volume of and donor site morbidity associated with harvested bone. Customized bone scaffolds that mimic the mechanical and biological properties of native bone are needed to augment the currently limited bone regeneration strategies. To achieve this goal, a repeated cross-hatch structure with uniform cubic pores was designed and 3D printed using polylactic acid (PLA) via fused deposition modeling (FDM). PLA surfaces were modified by wet chemical (alkali) treatment for either 1 h (1hAT) or 6 h (6hAT), followed by coating with nano-hydroxyapatite (nHA). Our hypotheses were that: (i) 6-hour (but not 1-hour) alkali treatment would enhance nHA coating, (ii) the nHA coating on the 6-hour alkali-treated surface would increase hydrophilicity and cell attachment/proliferation, and (iii) stiffness, but not effective Young's modulus, would be reduced by 6-hour alkali treatment. The effects of AT and nHA coating on scaffold morphology was observed by scanning electron microscopy and quantified using a custom MATLAB script. Chemical composition and hydrophilicity were evaluated via energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy, and water contact angle analyses, respectively. Mechanical testing and in vitro cell culture were further employed to analyze compressive properties, and cell attachment and proliferation, respectively. As expected, 6hAT led to reduced strut width and stiffness, while improving the nHA coating and hydrophilicity. Interestingly, PLA/6hAT but not PLA/6hAT/nHA demonstrated a reduction in effective modulus compared to PLA and PLA/nHA scaffolds. From in vitro experiments, the combined PLA/6hAT/nHA modification resulted in the greatest extent of cell attachment but not proliferation. These results collectively demonstrate that the PLA/6hAT/nHA scaffold exhibits properties that may prove beneficial for cancellous bone regeneration.

Bioactive poly(ε-caprolactone) microspheres with tunable open pores as microcarriers for tissue regeneration

Microparticles with porous structure can be applied as microcarriers for both cell culture and tissue regeneration. While well-controlled pore structure represents a critical challenge to be achieved. In the present study, in order to develop microcarriers for cell culture, a series of poly(ε-caprolactone) microspheres were fabricated with varied macroporous structures. Poly(ε-caprolactone) microspheres were prepared via the integration of the emulsion/solvent evaporation and particle leaching mechanisms. Particularly, by adjusting poly(ε-caprolactone) concentration and the ratio between the porogen paraffin and poly(ε-caprolactone), the microspheres with the pore size of 25.6-84.0 μm and the porosity of 57.4-75.5% were obtained. Further, the microspheres were subjected to alkaline hydrolysis, followed by surface coating with hydroxyapatite. These porous poly(ε-caprolactone) microspheres with surface modification well supported the adhesion and growth of human fibroblasts. Together, bioactive poly(ε-caprolactone) microspheres with controlled pore structure are potential to be applied in cell culture and tissue regeneration.

Promoted healing of femoral defects with in situ grown fibrous composites of hydroxyapatite and poly(DL-lactide)

Although, electrospun composite fibers have shown promise in enhancing growth, differentiation, and mineralization of osteoblasts in vitro, bone repairing capabilities have not been clarified after in vivo implantation up to now. In situ grown composites (IGC) of hydroxyapatite (HA) and poly(DL-lactide) (PDLLA) were obtained from electrospun fibers grafted with gelatin as the induction sites for HA growth. The presence and location of HA nanoparticles within electrospun fibers were proposed to affect the degradation and repairing process of femoral defects. Subcutaneous implantation of IGC led to around 90% of mass loss and 75% of molecular weight reduction during 16 weeks, which were significantly higher than those after in vitro degradation in buffer solutions. In vitro tests on MC3T3-E1 cells indicated that IGC acted as a better cell support to provide favorable conditions for cell proliferation and to stimulate the osteogenic differentiation as compared with electrospun PDLLA fibers, and blend electrospun fibrous composites. Femoral defects were created for in vivo evaluation of bone repairing, indicating that the entire defect was filled by newly formed bone with compact structure after 16 week implantation of IGC. Histological and SEM observations demonstrated a successful bridging of the critical-sized defect with rapid mineralization, continual remodeling, and abundant vasculature. The in situ grown HA nanoparticles on the surface of electrospun fibers improved the biocompatibility with defect sites, promoted the bone formation within fibrous scaffolds and enhanced the bone remodeling, indicating potentials for bone regeneration and repairing of bone defects.

Preparation of a novel biodegradable nanocomposite scaffold based on poly (3-hydroxybutyrate)/bioglass nanoparticles for bone tissue engineering

One of the most important challenges in composite scaffolds is pore architecture. In this study, poly (3-hydroxybutyrate) with 10% bioglass nanoparticles was prepared by the salt leaching processing technique, as a nanocomposite scaffold. The scaffolds were characterized by SEM, FTIR and DTA. The SEM images demonstrated uniformed porosities of appropriate sizes (about 250-300 microm) which are interconnected. Furthermore, higher magnification SEM images showed that the scaffold possesses less agglomeration and has rough surfaces that may improve cell attachment. In addition, the FTIR and DTA results showed favorable interaction between polymer and bioglass nanoparticles which improved interfaces in the samples. Moreover, the porosity of the scaffold was assessed, and the results demonstrated that the scaffold has uniform and high porosity in its structure (about 84%). Finally it can be concluded that this scaffold has acceptable porosity and morphologic character paving the way for further studies to be conducted from the perspective of bone tissue engineering.

Preparation and Properties of Poly(lactide-co-glycolide) (PLGA)/ Nano-Hydroxyapatite (NHA) Scaffolds by Thermally Induced Phase Separation and Rabbit MSCs Culture on Scaffolds

Biodegradable polymer/bioceramic composites scaffold can overcome the limitation of conventional ceramic bone substitutes such as brittleness and difficulty in shaping. To better mimic the mineral component and the microstructure of natural bone, novel nano-hydroxyapatite (NHA)/polymer composite scaffolds with high porosity and well-controlled pore architectures as well as high exposure of the bioactive ceramics to the scaffold surface is developed for efficient bone tissue engineering. In this article, regular and highly interconnected porous poly(lactide-co-glycolide) (PLGA)/NHA scaffolds are fabricated by thermally induced phase separation technique. The effects of solvent composition, polymer concentration, coarsening temperature, and coarsening time as well as NHA content on the micro-morphology, mechanical properties of the PLGA/NHA scaffolds are investigated. The results show that pore size of the PLGA/NHA scaffolds decrease with the increase of PLGA concentration and NHA content. The introduction of NHA greatly increase the mechanical properties and water absorption ability which greatly increase with the increase of NHA content. Mesenchymal stem cells are seeded and cultured in three-dimensional (3D) PLGA/NHA scaffolds to fabricate in vitro tissue engineering bone, which is investigated by adhesion rate, cell morphology, cell numbers, and alkaline phosphatase assay. The results display that the PLGA/NHA scaffolds exhibit significantly higher cell growth, alkaline phosphatase activity than PLGA scaffolds, especially the PLGA/NHA scaffolds with 10 wt.% NHA. The results suggest that the newly developed PLGA/NHA composite scaffolds may serve as an excellent 3D substrate for cell attachment and migration in bone tissue engineering.

Tissue Engineered Bone Formation with Polymer/Ceramic Composites by Press-and-Baking Method

A novel process was developed to fabricate polymer/ceramic composites for bone tissue engineering. The mixture of polylactic acid (PLA), calcium metaphosphate (CMP), and NaCl were compressed and subsequently heated. After dissolving the NaCl salts, porous biodegradable polymer/ceramic composite scaffolds were formed. The characteristics of the scaffolds were compared to those of scaffolds fabricated using a conventional solvent casting method, in terms of pore structure, pore size distribution, and mechanical properties. The scaffolds were seeded by osteoblasts and cultured in vitro or implanted into nude mice subcutaneously for up to 5 weeks. Cells were better grown to form tissue-like structures on CMP/PLA composites fabricated by the Press-and-Baking method. In addition, the alkaline phosphatase activity of and calcium deposition in the scaffolds explanted from mice were enhanced significantly for the scaffolds by Press-and-Baking compared to them by solvent casting. Taken together, these results suggest that CMP promote cell differentiation and proliferation via direct interaction with cells in the CMP/PLA composites. This novel PLA/CMP composite will be applicable for bone tissue engineering to support and cell differentiation and growth.

Evaluation of two biodegradable polymeric systems as substrates for bone tissue engineering

The aim of this study was to evaluate two biodegradable polymeric systems as scaffolds for bone tissue engineering. Rat bone marrow cells were seeded and cultured for 1 week on two biodegradable porous polymeric systems, one composed of poly(ethylene glycol)-terephthalate/poly(butylene terephthalate) (PEGT/PBT) and the other composed of cornstarch blended with poly(epsilon-caprolactone) (SPCL). Porous hydroxyapatite granules were used as controls. The ability of cells to proliferate and form extracellular matrix on these scaffolds was assessed by a DNA quantification assay and by scanning electron microscopy examination; their osteogenic differentiation was screened by the expression of alkaline phosphatase. In addition, the in vivo osteogenic potential of the engineered constructs was evaluated through ectopic implantation in a nude mouse model. Results revealed that cells were able to proliferate, differentiate, and form extracellular matrix on all materials tested. Moreover, all constructs induced abundant formation of bone and bone marrow after 4 weeks of implantation. The extent of osteogenesis (approximately 30% of void volume) was similar in all types of implants. However, the amount of bone marrow and the degree of bone contact were higher on HA scaffolds, indicating that the polymers still need to be modulated for higher osteoconductive capacity. Nevertheless, the findings suggest that both PEGT/PBT and SPCL systems are excellent candidates to be used as scaffolds for a cell therapy approach in the treatment of bone defects.