Introduction

The regeneration of musculoskeletal tissues such as articular cartilage and meniscus remains a significant challenge. This can be attributed, at least in part, to our inability to engineer grafts that mimic the spatial structure and composition of the native tissue. Additive (bio)manufacturing strategies are impacting on diverse areas of healthcare, including the development of regenerative scaffolds and the in vitro engineering of biological tissues. In particular, the ability to spatially and temporally pattern cells, regulatory factors and supporting biomaterials using emerging bioprinting platforms is transforming our ability to engineer functional cartilaginous tissues. Not only can bioprinting enable the engineering of anatomically complex grafts, but also the biofabrication of tissues mimicking the spatial composition of articular cartilage and meniscus. Using multiple-tool biofabrication strategies, it is also possible to reinforce such biological grafts to produce composites with mechanical properties approaching that of native tissues. Alternatively, 3D printing can be used to produce structures to help guide the development of engineered grafts, enabling the fabrication of tissues with biomimetic collagen networks critical to the biomechanical functionality of soft tissues. This talk will describe two examples from our lab where different biofabrication and 3D (bio)printing strategies are combined to engineer functional cartilage and osteochondral grafts.

Content

The first example will describe how we have used melt-electrowriting (MEW) to produce arrays of polymeric structures whose function is to orient the growth of cellular aggregates spontaneously generated within these structures, and to provide tensile reinforcement to the resulting tissues. Inkjeting was used to deposit defined numbers of cells into MEW structures, which self-assembled into an organized array of spheroids within hours, ultimately generating a hybrid tissue that was hyaline-like in composition. Structurally, the engineered cartilage mimicked the histotypical organization observed in skeletally immature synovial joints. This biofabrication framework was then used to generate scaled-up (6mm × 6mm) cartilage implants containing over 3,500 cellular aggregates in under 15 minutes. After 8 weeks in culture, a 50-fold increase in the compressive properties of these MEW reinforced tissues were observed, while the tensile properties were still dominated by the polymer network, resulting in a composite construct demonstrating tension-compression nonlinearity mimetic of the native tissue. Helium ion microscopy further demonstrated the development of an arcading collagen network within the engineered tissue. This hybrid bioprinting strategy provides a versatile and scalable approach to engineer cartilage biomimetic grafts for biological joint resurfacing.

Modular biofabrication strategies that use microtissues or organoids as biological building blocks represent another promising approach for engineering replacement tissues and organs at scale. The second part of this talk will describe the development of such a biofabrication strategy to engineer osteochondral tissues by spatially localising phenotypically distinct cartilage microtissue populations within an instructive 3D printed polymer framework. We first identified that immature cartilage microtissues could spontaneously fuse to form a homogenous macrotissue, and that the fusion and quality of this macrotissue was a function of microtissue cellular density. Next, 3D printed polymeric frameworks were used to further guide microtissue fusion and the subsequent self-organisation process, resulting in the development of a macroscale tissue with a zonal collagen organisation analogous to the structure seen in native articular cartilage. Hypertrophic cartilage microtissues were also engineered as bone precursor tissues, which were then spatially localised below phenotypically stable cartilage microtissues to engineer osteochondral grafts. Implantation of these engineered grafts into critically-sized caprine osteochondral defects resulted in effective stabilisation of the defect and aided in restoring a more normal articular surface after 6 months. This work presents an effective method for engineering developmentally inspired osteochondral implants at a clinically relevant scale.

Acknowledgments

This publication was developed with the financial support of Science Foundation Ireland (SFI) under grant number 12/RC/2278 and 17/SP/4721. This research is co-funded by the European Regional Development Fund and SFI under Ireland’s European Structural and Investment Fund. This research has been co-funded by Johnson & Johnson 3D Printing Innovation & Customer Solutions, Johnson & Johnson Services Inc.