Dual polymer networks: a new strategy in expanding the repertoire of hydrogels for biomedical applications.

Inspired by the double network hydrogel systems we report the formulation of dual networks, which expands the repertoire of this class of materials for potential biomedical applications. The tough dual network hydrogels were designed through sequential interpenetrating polymer formation, applying green chemistry and low-cost methods, devoid of any initiator-activator complexes that may pose risks in biomedical applications. The dual networks were synthesized in two steps, firstly the water soluble poly(vinyl alcohol) was subjected to cryogelation that formed the first network, which was then expanded by intrusion of a dilute solution of sodium alginate and complexed with a solution of calcium chloride under ambient conditions and further freeze-thawed. These hydrogels are flexible, ductile and porous with the ability to absorb and retain fluids as well as possess the versatility to easily incorporate biological molecules/drugs/antibiotics to be applied in tissue matrices or drug delivery systems. The dual network hydrogels can be tailored to have varying mechanical properties, shapes, size, thickness and particularly can be made physically porous if required, to suit the users intended application.

Combinatorial design of calcium meta phosphate poly(vinyl alcohol) bone-like biocomposites.

The incidence of degenerative diseases and the ageing population have added to the growing demand for bone grafts. Although autologous bone continues to be the gold standard, limited yield and potential morbidity of the donor site pose considerable challenges. Currently, clinically used synthetic grafts based on calcium phosphates are mechanically brittle and not compliant hence composite scaffolds are expected to be provide viable solutions. In this study we report composites of calcium meta phosphate-poly (vinyl alcohol) with tunable mechanical properties, low swelling and excellent biocompatibility. The elastomeric nature of the composites resist brittle fracture and the scaffolds can be easily shaped to the bone defect by the surgeon. Testing on bone plug shaped specimens of the scaffolds, exhibited superior mechanical properties compared to currently commercially available bone plugs with additional advantages being the ability to increase porosity without compromising properties in compression and degree of swelling, which make these composites promising synthetic alternatives for bone grafts and bone tissue engineering.

Hydrogel Composite Magnetic Scaffolds: Toward Cell-Free In Situ Bone Tissue Engineering.

Reconstruction of critical sized bone defects in the oral and maxillofacial region continues to be clinically challenging despite the significant development of osteo-regenerative materials. Among 3D biomaterials, hydrogels and hydrogel composites have been explored for bone regeneration, however, their inferior clinical performance in comparison to autografts is mainly attributed to variable rates of degradation and lack of vascularization. In this study, we report hydrogel composite magnetic scaffolds formed from calcium carbonate, poly(vinyl alcohol) (PVA), and magnetic nanoparticles (MNPs), using PVA as matrix and calcium carbonate particles in vaterite phase as filler, to enhance the cross-linking of matrix and porosity with MNPs that can target and regulate cell signaling pathways to control cell behavior and improve the osteogenic and angiogenic potential. The physical and mechanical properties were evaluated, and cytocompatibility was investigated by culturing human osteoblast-like cells onto the scaffolds. The vaterite phase due to its higher solubility in comparison to calcium phosphates, combined with the freezing-thawing process of PVA, yielded porous scaffolds that exhibited adequate thermal stability, favorable water-absorbing capacity, excellent mineralization ability, and cytocompatibility. An increasing concentration from 1, 3, and 6 wt % MNPs in the scaffolds showed a statistically significant increase in compressive strength and modulus of the dry specimens that exhibited brittle fracture. However, the hydrated specimens were compressible and showed a slight decrease in compressive strength with 6% MNPs, although this value was higher compared to that of the scaffolds with no MNPs.

In vitro and in vivo optimization of impaction allografting by demineralization and addition of rh-OP-1.

Impaction allografting is a bone tissue engineering technique currently used in lower limb reconstruction orthopedic surgery. Our hypothesis was that biological optimization can be achieved by demineralization and addition of osteogenic protein-1 (OP-1) to the allograft. The objective of our in vitro study was to evaluate human mesenchymal stem cell (MSC) proliferation (Alamar Blue assay, titrated thymidine assay, total DNA Hoechst 33258, and scanning electron microscopy) and osteogenic differentiation (alkaline phosphatase assay) in two types of impacted carrier, namely, demineralized bone matrix (DBM) and insoluble collagenous bone matrix (ICBM), with or without OP-1. The objective in vivo was to compare the osteogenic potential of impacted DBM with or without OP-1, with that of impacted fresh frozen allograft (FFA), again with or without OP-1. DBM + OP-1 optimized osteoinduction and significantly improved (p < 0.05) proliferation and differentiation in comparison to the majority of all other graft preparation in vitro. In addition, DBM + OP-1 was significantly superior, with regard to osteogenesis, compared to the impacted FFA alone (p < 0.001), FFA + OP-1 (p = 0.01) and DBM alone (p = 0.02) in vivo. We propose that partial demineralization and addition of OP-1 provides a good method for improving the osteoinductive properties of fresh allograft currently used in the impaction grafting technique.

Optimizing HAPEX topography influences osteoblast response.

HAPEX (hydroxyapatite-reinforced polyethylene composite) is a second-generation orthopedic biomaterial designed as a bone analog material, which has found clinical success. The use of topography in cell engineering has been shown to affect cell attachment and subsequent response. Thus, by combining bioactivity and enhancing osteoblast response to the implant surface, improved tissue repair and implant life span may be achieved. In this study a primary human osteoblast-like cell model has been used to study the influence of surface topography and chemistry produced by three different production methods. Scanning electron microscopy, fluorescence microscopy, and confocal scanning laser microscopy have been used to study cell adhesion; tritiated thymidine uptake has been used to observe cell proliferation; and the reverse transcriptase-polymerase chain reaction and biochemical methods have been used to study phenotypic expression. Transmission electron microscopy has also been used to look at more long-term morphology. The results show that topography significantly influences cell response, and may be a means of enhancing bone apposition on HAPEX.

In vitro proliferation and differentiation of human mesenchymal stem cells on hydroxyapatite versus human demineralised bone matrix with and without osteogenic protein-1.

BACKGROUND: Allografting is currently used in lower limb reconstruction surgery. Demineralised bone matrix (DBM) is more osteoinductive compared with allografts but lacks mechanical strength. Osteogenic protein-1 (OP-1) can improve the osteoinductivity of the allograft, however recent reports indicate significant allograft resorption when it is combined with OP-1. OBJECTIVES: Our hypothesis was that hydroxyapatite (HA) with human-mesenchymal stem cells (h-MSCs) and OP-1 (HA+h-MSCs+OP-1) has similar osteoinductive properties to human-DBM+h-MSCs. The objective was to evaluate h-MSC proliferation (by tritiated thymidine incorporation, total DNA Hoechst 33258 and scanning electron microscopy) and osteogenic differentiation (from alkaline phosphatase activity) in human demineralised bone matrix (h-DBM) and HA, with or without OP-1. RESULTS: H-MSC proliferation on HA+OP-1 was significantly higher compared with that on HA at all time points (p < 0.05) and compared with DBM alone [day 1, (198.4 vs 95.4) p = 0.042; day 14 (286.1 vs 119.9), p < 0.001]. H-MSC proliferation was higher in DBM+OP-1, at all time points compared with HA+OP-1 but the difference was not statistically significant (p > 0.05). H-MSC differentiation was significantly higher in HA+OP-1 compared with HA (p < 0.05) but not significantly different from diffferentiation on DBM alone (p > 0.05). Differentiation was significantly higher on DBM+OP-1 at all time points compared with HA (p < 0.05) and with HA+OP-1 [day 1, (21.1 vs 10.1) (p = 0.03); day 7 (39.4 vs 7.1) (p < 0.01); day 14 (40.2 vs 14.4) (p < 0.001)]. CONCLUSIONS: When HA+h-MSCs is combined with OP-1 in vitro its osteogenic potential is similar to that of DBM+h-MSCs alone which may be adequate for non-weight-bearing applications. Mechanical testing however is of great importance for weight-bearing applications and the in vivo testing of the composite graft HA+h-MSCs+OP-1 vs DBM+h-MSCs is recommended.

A novel ex vivo culture system for studying bone repair.

Repair of large bony defects still remains a challenge for surgeons. Hydroxyapatite (HA) is well known for its biocompatibility and osseoconduction properties in the osseous environment. In this study the biofunctionality of a newly developed scaffold comprising of collagen and HA, with variable macropores was examined. The biological response was evaluated using primary human osteoblast cells (HOBs). Cell infiltration, proliferation and differentiation were assessed. The results showed that HOBs were able to migrate from the collagen into the HA pores with greater cell migration and infiltration observed in those scaffolds with larger pores. Furthermore, it was shown that Alkaline Phosphatase, a differentiation marker for HOBs was enhanced as the average macropore size increased. This in vitro model provides a more relevant method of testing the biofunctionality and migration ability of cells at a trauma site following implantation in bone and cartilage.

Enhancing the osteoinductive properties of hydroxyapatite by the addition of human mesenchymal stem cells, and recombinant human osteogenic protein-1 (BMP-7) in vitro.

Hydroxyapatite (HA) has been widely used as a bone graft substitute. In this study, we investigated whether the addition of osteogenic protein-1 (OP-1) further enhanced the weak osteoinductive properties of hydroxyapatite when loaded with human mesenchymal stem cells (h-MSCs). Over a 14 day period, cell proliferation in both groups was assessed qualitatively using SEM and quantitatively using alamar blue assay. Cell differentiation was also evaluated by measurement of ALP activity, which was expressed against total DNA. HA/MSC loaded with OP-1 demonstrated a statistically significant increase (p<0.001) in cell proliferation at all time points in comparison to unloaded samples. ALP activity per DNA was also significantly enhanced (p<0.001) in loaded samples when compared to unloaded controls. SEM demonstrated increased cellular attachment and proliferation into HA pores at all time points in the loaded samples. Our study suggests that the osteoinductive potential of HA can be improved in vitro by the combined incorporation of MSCs and OP-1.