Integrating Melt Electrowriting and Fused Deposition Modeling to Fabricate Hybrid Scaffolds Supportive of Accelerated Bone Regeneration.

Emerging additive manufacturing (AM) strategies can enable the engineering of hierarchal scaffold structures for guiding tissue regeneration. Here, the advantages of two AM approaches, melt electrowriting (MEW) and fused deposition modelling (FDM), are leveraged and integrated to fabricate hybrid scaffolds for large bone defect healing. MEW is used to fabricate a microfibrous core to guide bone healing, while FDM is used to fabricate a stiff outer shell for mechanical support, with constructs being coated with pro-osteogenic calcium phosphate (CaP) nano-needles. Compared to MEW scaffolds alone, hybrid scaffolds prevent soft tissue collapse into the defect region and support increased vascularization and higher levels of new bone formation 12 weeks post-implantation. In an additional group, hybrid scaffolds are also functionalized with BMP2 via binding to the CaP coating, which further accelerates healing and facilitates the complete bridging of defects after 12 weeks. Histological analyses demonstrate that such scaffolds support the formation of well-defined annular bone, with an open medullary cavity, smooth periosteal surface, and no evidence of abnormal ectopic bone formation. These results demonstrate the potential of integrating different AM approaches for the development of regenerative biomaterials, and in particular, demonstrate the enhanced bone healing outcomes possible with hybrid MEW-FDM constructs.

A Multifunctional Scaffold for Bone Infection Treatment by Delivery of microRNA Therapeutics Combined With Antimicrobial Nanoparticles.

Treating bone infections and ensuring bone repair is one of the greatest global challenges of modern orthopedics, made complex by antimicrobial resistance (AMR) risks due to long-term antibiotic treatment and debilitating large bone defects following infected tissue removal. An ideal multi-faceted solution would will eradicate bacterial infection without long-term antibiotic use, simultaneously stimulating osteogenesis and angiogenesis. Here, a multifunctional collagen-based scaffold that addresses these needs by leveraging the potential of antibiotic-free antimicrobial nanoparticles (copper-doped bioactive glass, CuBG) to combat infection without contributing to AMR in conjunction with microRNA-based gene therapy (utilizing an inhibitor of microRNA-138) to stimulate both osteogenesis and angiogenesis, is developed. CuBG scaffolds reduce the attachment of gram-positive bacteria by over 80%, showcasing antimicrobial functionality. The antagomiR-138 nanoparticles induce osteogenesis of human mesenchymal stem cells in vitro and heal a large load-bearing defect in a rat femur when delivered on the scaffold. Combining both promising technologies results in a multifunctional antagomiR-138-activated CuBG scaffold inducing hMSC-mediated osteogenesis and stimulating vasculogenesis in an in vivo chick chorioallantoic membrane model. Overall, this multifunctional scaffold catalyzes killing mechanisms in bacteria while inducing bone repair through osteogenic and angiogenic coupling, making this platform a promising multi-functional strategy for treating and repairing complex bone infections.

3D bioprinting of cartilaginous templates for large bone defect healing.

Damaged or diseased bone can be treated using autografts or a range of different bone grafting biomaterials, however limitations with such approaches has motivated increased interest in developmentally inspired bone tissue engineering (BTE) strategies that seek to recapitulate the process of endochondral ossification (EO) as a means of regenerating critically sized defects. The clinical translation of such strategies will require the engineering of scaled-up, geometrically defined hypertrophic cartilage grafts that can be rapidly vascularised and remodelled into bone in mechanically challenging defect environments. The goal of this study was to 3D bioprint mechanically reinforced cartilaginous templates and to assess their capacity to regenerate critically sized femoral bone defects. Human mesenchymal stem/stromal cells (hMSCs) were incorporated into fibrin based bioinks and bioprinted into polycaprolactone (PCL) frameworks to produce mechanically reinforced constructs. Chondrogenic priming of such hMSC laden constructs was required to support robust vascularisation and graft mineralisation in vivo following their subcutaneous implantation into nude mice. With a view towards maximising their potential to support endochondral bone regeneration, we next explored different in vitro culture regimes to produce chondrogenic and early hypertrophic engineered grafts. Following their implantation into femoral bone defects within transiently immunosuppressed rats, such bioprinted constructs were rapidly remodelled into bone in vivo, with early hypertrophic constructs supporting higher levels of vascularisation and bone formation compared to the chondrogenic constructs. Such early hypertrophic bioprinted constructs also supported higher levels of vascularisation and spatially distinct patterns of new formation compared to BMP-2 loaded collagen scaffolds (here used as a positive control). In conclusion, this study demonstrates that fibrin based bioinks support chondrogenesis of hMSCs in vitro, which enables the bioprinting of mechanically reinforced hypertrophic cartilaginous templates capable of supporting large bone defect regeneration. These results support the use of 3D bioprinting as a strategy to scale-up the engineering of developmentally inspired templates for BTE. STATEMENT OF SIGNIFICANCE: Despite the promise of developmentally inspired tissue engineering strategies for bone regeneration, there are still challenges that need to be addressed to enable clinical translation. This work reports the development and assessment (in vitro and in vivo) of a 3D bioprinting strategy to engineer mechanically-reinforced cartilaginous templates for large bone defect regeneration using human MSCs. Using distinct in vitro priming protocols, it was possible to generate cartilage grafts with altered phenotypes. More hypertrophic grafts, engineered in vitro using TGF-ß3 and BMP-2, supported higher levels of blood vessel infiltration and accelerated bone regeneration in vivo. This study also identifies some of the advantages and disadvantages of such endochondral bone TE strategies over the direct delivery of BMP-2 from collagen-based scaffolds.

Scaffold microarchitecture regulates angiogenesis and the regeneration of large bone defects.

Emerging 3D printing technologies can provide exquisite control over the external shape and internal architecture of scaffolds and tissue engineering (TE) constructs, enabling systematic studies to explore how geometric design features influence the regenerative process. Here we used fused deposition modelling (FDM) and melt electrowriting (MEW) to investigate how scaffold microarchitecture influences the healing of large bone defects. FDM was used to fabricate scaffolds with relatively large fibre diameters and low porosities, while MEW was used to fabricate scaffolds with smaller fibre diameters and higher porosities, with both scaffolds being designed to have comparable surface areas. Scaffold microarchitecture significantly influenced the healing response following implantation into critically sized femoral defects in rats, with the FDM scaffolds supporting the formation of larger bone spicules through its pores, while the MEW scaffolds supported the formation of a more round bone front during healing. After 12 weeksin vivo, both MEW and FDM scaffolds supported significantly higher levels of defect vascularisation compared to empty controls, while the MEW scaffolds supported higher levels of new bone formation. Somewhat surprisingly, this superior healing in the MEW group did not correlate with higher levels of angiogenesis, with the FDM scaffold supporting greater total vessel formation and the formation of larger vessels, while the MEW scaffold promoted the formation of a dense microvasculature with minimal evidence of larger vessels infiltrating the defect region. To conclude, the small fibre diameter, high porosity and high specific surface area of the MEW scaffold proved beneficial for osteogenesis and bone regeneration, demonstrating that changes in scaffold architecture enabled by this additive manufacturing technique can dramatically modulate angiogenesis and tissue regeneration without the need for complex exogenous growth factors. These results provide a valuable insight into the importance of 3D printed scaffold architecture when developing new bone TE strategies.

Biofabrication of Poly(glycerol sebacate) Scaffolds Functionalized with a Decellularized Bone Extracellular Matrix for Bone Tissue Engineering.

The microarchitecture of bone tissue engineering (BTE) scaffolds has been shown to have a direct effect on the osteogenesis of mesenchymal stem cells (MSCs) and bone tissue regeneration. Poly(glycerol sebacate) (PGS) is a promising polymer that can be tailored to have specific mechanical properties, as well as be used to create microenvironments that are relevant in the context of BTE applications. In this study, we utilized PGS elastomer for the fabrication of a biocompatible and bioactive scaffold for BTE, with tissue-specific cues and a suitable microstructure for the osteogenic lineage commitment of MSCs. In order to achieve this, the PGS was functionalized with a decellularized bone (deB) extracellular matrix (ECM) (14% and 28% by weight) to enhance its osteoinductive potential. Two different pore sizes were fabricated (small: 100-150 µm and large: 250-355 µm) to determine a preferred pore size for in vitro osteogenesis. The decellularized bone ECM functionalization of the PGS not only improved initial cell attachment and osteogenesis but also enhanced the mechanical strength of the scaffold by up to 165 kPa. Furthermore, the constructs were also successfully tailored with an enhanced degradation rate/pH change and wettability. The highest bone-inserted small-pore scaffold had a 12% endpoint weight loss, and the pH was measured at around 7.14. The in vitro osteogenic differentiation of the MSCs in the PGS-deB blends revealed a better lineage commitment of the small-pore-sized and 28% (w/w) bone-inserted scaffolds, as evidenced by calcium quantification, ALP expression, and alizarin red staining. This study demonstrates a suitable pore size and amount of decellularized bone ECM for osteoinduction via precisely tailored PGS elastomer BTE scaffolds.

Development of a 3D Bioprinted Scaffold with Spatio-temporally Defined Patterns of BMP-2 and VEGF for the Regeneration of Large Bone Defects.

The local delivery of growth factors such as BMP-2 is a well-established strategy for the repair of bone defects. The limitations of such approaches clinically are well documented and can be linked to the need for supraphysiological doses and poor spatio-temporal control of growth factor release in vivo. Using bioprinting techniques, it is possible to generate implants that can deliver cytokines or growth factors with distinct spatiotemporal release profiles and patterns to enhance bone regeneration. Specifically, for bone healing, several growth factors, including vascular endothelial growth factor (VEGF) and bone morphogenic proteins (BMPs), have been shown to be expressed at different phases of the process. This protocol aims to outline how to use bioprinting strategies to deliver growth factors, both alone or in combination, to the site of injury at physiologically relevant dosages such that repair is induced without adverse effects. Here we describe: the printing parameters to generate the polymer mechanical backbone; instructions to generate the different bioinks and allow for the temporal control of both growth factors; and the printing process to develop implants with spatially defined patterns of growth factors for bone regeneration. The novelty of this protocol is the use of multiple-tool fabrication techniques to develop an implant with spatio-temporal control of growth factor delivery for bone regeneration. While the overall aim of this protocol was to develop an implant for bone regeneration, the technique can be modified and used for a variety of regenerative purposes. Graphic abstract: 3D Bioprinting Spatio-Temporally Defined Patterns of Growth Factors to Tightly Control Bone Tissue Regeneration.

3D bioprinting of prevascularised implants for the repair of critically-sized bone defects.

For 3D bioprinted tissues to be scaled-up to clinically relevant sizes, effective prevascularisation strategies are required to provide the necessary nutrients for normal metabolism and to remove associated waste by-products. The aim of this study was to develop a bioprinting strategy to engineer prevascularised tissues in vitro and to investigate the capacity of such constructs to enhance the vascularisation and regeneration of large bone defects in vivo. From a screen of different bioinks, a fibrin-based hydrogel was found to best support human umbilical vein endothelial cell (HUVEC) sprouting and the establishment of a microvessel network. When this bioink was combined with HUVECs and supporting human bone marrow stem/stromal cells (hBMSCs), these microvessel networks persisted in vitro. Furthermore, only bioprinted tissues containing both HUVECs and hBMSCs, that were first allowed to mature in vitro, supported robust blood vessel development in vivo. To assess the therapeutic utility of this bioprinting strategy, these bioinks were used to prevascularise 3D printed polycaprolactone (PCL) scaffolds, which were subsequently implanted into critically-sized femoral bone defects in rats. Micro-computed tomography (µCT) angiography revealed increased levels of vascularisation in vivo, which correlated with higher levels of new bone formation. Such prevascularised constructs could be used to enhance the vascularisation of a range of large tissue defects, forming the basis of multiple new bioprinted therapeutics. STATEMENT OF SIGNIFICANCE: This paper demonstrates a versatile 3D bioprinting technique to improve the vascularisation of tissue engineered constructs and further demonstrates how this method can be incorporated into a bone tissue engineering strategy to improve vascularisation in a rat femoral defect model.

3D bioprinting spatiotemporally defined patterns of growth factors to tightly control tissue regeneration.

Therapeutic growth factor delivery typically requires supraphysiological dosages, which can cause undesirable off-target effects. The aim of this study was to 3D bioprint implants containing spatiotemporally defined patterns of growth factors optimized for coupled angiogenesis and osteogenesis. Using nanoparticle functionalized bioinks, it was possible to print implants with distinct growth factor patterns and release profiles spanning from days to weeks. The extent of angiogenesis in vivo depended on the spatial presentation of vascular endothelial growth factor (VEGF). Higher levels of vessel invasion were observed in implants containing a spatial gradient of VEGF compared to those homogenously loaded with the same total amount of protein. Printed implants containing a gradient of VEGF, coupled with spatially defined BMP-2 localization and release kinetics, accelerated large bone defect healing with little heterotopic bone formation. This demonstrates the potential of growth factor printing, a putative point of care therapy, for tightly controlled tissue regeneration.

Nano-particle mediated M2 macrophage polarization enhances bone formation and MSC osteogenesis in an IL-10 dependent manner.

Engineering a pro-regenerative immune response following scaffold implantation is integral to functional tissue regeneration. The immune response to implanted biomaterials is determined by multiple factors, including biophysical cues such as material stiffness, topography and particle size. In this study we developed an immune modulating scaffold for bone defect healing containing bone mimetic nano hydroxyapatite particles (BMnP). We first demonstrate that, in contrast to commercially available micron-sized hydroxyapatite particles, in-house generated BMnP preferentially polarize human macrophages towards an M2 phenotype, activate the transcription factor cMaf and specifically enhance production of the anti-inflammatory cytokine, IL-10. Furthermore, nano-particle treated macrophages enhance mesenchymal stem cell (MSC) osteogenesis in vitro and this occurs in an IL-10 dependent manner, demonstrating a direct pro-osteogenic role for this cytokine. BMnPs were also capable of driving pro-angiogenic responses in human macrophages and HUVECs. Characterization of immune cell subsets following incorporation of functionalized scaffolds into a rat femoral defect model revealed a similar profile, with micron-sized hydroxyapatite functionalized scaffolds eliciting pro-inflammatory responses characterized by infiltrating T cells and elevated expression of M1 macrophages markers compared to BMnP functionalized scaffolds which promoted M2 macrophage polarization, tissue vascularization and increased bone volume. Taken together these results demonstrate that nano-sized Hydroxyapatite has immunomodulatory potential and is capable of directing anti-inflammatory innate immune-mediated responses that are associated with tissue repair and regeneration.

Tissue-specific extracellular matrix scaffolds for the regeneration of spatially complex musculoskeletal tissues.

Biological scaffolds generated from tissue-derived extracellular matrix (ECM) are commonly used clinically for soft tissue regeneration. Such biomaterials can enhance tissue-specific differentiation of adult stem cells, suggesting that structuring different ECMs into multi-layered scaffolds can form the basis of new strategies for regenerating damaged interfacial tissues such as the osteochondral unit. In this study, mass spectrometry is used to demonstrate that growth plate (GP) and articular cartilage (AC) ECMs contain a unique array of regulatory proteins that may be particularly suited to bone and cartilage repair respectively. Applying a novel iterative freeze-drying method, porous bi-phasic scaffolds composed of GP ECM overlaid by AC ECM are fabricated, which are capable of spatially directing stem cell differentiation in vitro, promoting the development of graded tissues transitioning from calcified cartilage to hyaline-like cartilage. Evaluating repair 12-months post-implantation into critically-sized caprine osteochondral defects reveals that these scaffolds promote regeneration in a manner distinct to commercial control-scaffolds. The GP layer supports endochondral bone formation, while the AC layer stimulates the formation of an overlying layer of hyaline cartilage with a collagen fiber architecture better recapitulating the native tissue. These findings support the use of a bi-layered, tissue-specific ECM derived scaffolds for regenerating spatially complex musculoskeletal tissues.

3D printed microchannel networks to direct vascularisation during endochondral bone repair.

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.