Cartilaginous spheroid-assembly design considerations for endochondral ossification: towards robotic-driven biomanufacturing

Rapidly Degrading Hydrogels to Support Biofabrication and 3D Bioprinting Using Cartilage Microtissues

In recent years, there has been increased interest in the use of cellular spheroids, microtissues, and organoids as biological building blocks to engineer functional tissues and organs. Such microtissues are typically formed by the self-assembly of cellular aggregates and the subsequent deposition of a tissue-specific extracellular matrix (ECM). Biofabrication and 3D bioprinting strategies using microtissues may require the development of supporting hydrogels and bioinks to spatially localize such biological building blocks in 3D space and hence enable the engineering of geometrically defined tissues. Therefore, the aim of this work was to engineer scaled-up, geometrically defined cartilage grafts by combining multiple cartilage microtissues within a rapidly degrading oxidized alginate (OA) supporting hydrogel and maintaining these constructs in dynamic culture conditions. To this end, cartilage microtissues were first independently matured for either 2 or 4 days and then combined in the presence or absence of a supporting OA hydrogel. Over 6 weeks in static culture, constructs engineered using microtissues that were matured independently for 2 days generated higher amounts of glycosaminoglycans (GAGs) compared to those matured for 4 days. Histological analysis revealed intense staining for GAGs and negative staining for calcium deposits in constructs generated by using the supporting OA hydrogel. Less physical contraction was also observed in constructs generated in the presence of the supporting gel; however, the remnants of individual microtissues were more observable, suggesting that even the presence of a rapidly degrading hydrogel may delay the fusion and/or the remodeling of the individual microtissues. Dynamic culture conditions were found to modulate ECM synthesis following the OA hydrogel encapsulation. We also assessed the feasibility of 3D bioprinting of cartilage microtissues within OA based bioinks. It was observed that the microtissues remained viable after extrusion-based bioprinting and were able to fuse after 48 h, particularly when high microtissue densities were used, ultimately generating a cartilage tissue that was rich in GAGs and negative for calcium deposits. Therefore, this work supports the use of OA as a supporting hydrogel/bioink when using microtissues as biological building blocks in diverse biofabrication and 3D bioprinting platforms.

Laser-assisted bioprinting of targeted cartilaginous spheroids for high density bottom-up tissue engineering

Multicellular spheroids such as microtissues and organoids have demonstrated great potential for tissue engineering applications in recent years as these 3D cellular units enable improved cell-cell and cell-matrix interactions. Current bioprinting processes that use multicellular spheroids as building blocks have demonstrated limited control on post printing distribution of cell spheroids or moderate throughput and printing efficiency. In this work, we presented a laser-assisted bioprinting approach able to transfer multicellular spheroids as building blocks for larger tissue structures. Cartilaginous multicellular spheroids formed by human periosteum derived cells (hPDCs) were successfully bioprinted possessing high viability and the capacity to undergo chondrogenic differentiation post printing. Smaller hPDC spheroids with diameters ranging from ∼100 to 150µm were successfully bioprinted through the use of laser-induced forward transfer method (LIFT) however larger spheroids constituted a challenge. For this reason a novel alternative approach was developed termed as laser induced propulsion of mesoscopic objects (LIPMO) whereby we were able to bioprint spheroids of up to 300µm. Moreover, we combined the bioprinting process with computer aided image analysis demonstrating the capacity to 'target and shoot', through automated selection, multiple large spheroids in a single sequence. By taking advantage of target and shoot system, multilayered constructs containing high density cell spheroids were fabricated.

Hydroxyapatite Nanoparticles Promote the Development of Bone Microtissues for Accelerated Bone Regeneration by Activating the FAK/Akt Pathway

Scaffold-free bone microtissues differentiated from mesenchymal stem cell (MSC) spheroids offer great potential for bottom-up bone tissue engineering as a direct supply of cells and osteogenic signals. Many biomaterials or biomolecules have been incorporated into bone microtissues to enhance their osteogenic abilities, but these materials are far from clinical approval. Here, we aimed to incorporate hydroxyapatite (HAP) nanoparticles, an essential component of bone matrix, into MSC spheroids to instruct their osteogenic differentiation into bone microtissues and further self-organization into bone organoids with a trabecular structure. Furthermore, the biological interaction between HAP nanoparticles and MSCs and the potential molecular mechanisms in the bone development of MSC spheroids were investigated by both in vitro and in vivo studies. As a result, improved cell viability and osteogenic abilities were observed for the MSC spheroids incorporated with HAP nanoparticles at a concentration of 30 μg/mL. HAP nanoparticles could promote the sequential expression of osteogenic markers (Runx2, Osterix, Sclerostin), promote the expression of bone matrix proteins (OPN, OCN, and Collagen I), promote the mineralization of the bone matrix, and thus promote the bone development of MSC spheroids. The differentiated bone microtissues could further self-organize into linear, lamellar, and spatial bone organoids with trabecular structures. More importantly, adding FAK or Akt inhibitors could decrease the level of HAP-induced osteogenic differentiation of bone microtissues. Finally, excellent new bone regeneration was achieved after injecting bone microtissues into cranial bone defect models, which could also be eliminated by the Akt inhibitor. In conclusion, HAP nanoparticles could promote the development of bone microtissues by promoting the osteogenic differentiation of MSCs and the formation and mineralization of the bone matrix via the FAK/Akt pathway. The bone microtissues could act as individual ossification centers and self-organize into macroscale bone organoids, and in this meaning, the bone microtissues could be called microscale bone organoids. Furthermore, the bone microtissues revealed excellent clinical perspectives for injectable cellular therapies for bone defects.

Mini-bones: miniaturized bone in vitro models

In bone tissue engineering (TE) and regeneration, miniaturized, (sub)millimeter-sized bone models have become a popular trend since they bring about physiological biomimicry, precise orchestration of concurrent stimuli, and compatibility with high-throughput setups and high-content imaging. They also allow efficient use of cells, reagents, materials, and energy. In this review, we describe the state of the art of miniaturized in vitro bone models, or 'mini-bones', describing these models based on their characteristics of (multi)cellularity and engineered extracellular matrix (ECM), and elaborating on miniaturization approaches and fabrication techniques. We analyze the performance of 'mini-bone' models according to their applications for studying basic bone biology or as regeneration models, disease models, and screening platforms, and provide an outlook on future trends, challenges, and opportunities.

Engineered human osteoarthritic cartilage organoids

The availability of human cell-based models capturing molecular processes of cartilage degeneration can facilitate development of disease-modifying therapies for osteoarthritis [1], a currently unmet clinical need. Here, by imposing specific inflammatory challenges upon mesenchymal stromal cells at a defined stage of chondrogenesis, we engineered a human organotypic model which recapitulates main OA pathological traits such as chondrocyte hypertrophy, cartilage matrix mineralization, enhanced catabolism and mechanical stiffening. To exemplify the utility of the model, we exposed the engineered OA cartilage organoids to factors known to attenuate pathological features, including IL-1Ra, and carried out mass spectrometry-based proteomics. We identified that IL-1Ra strongly reduced production of the transcription factor CCAAT/enhancer-binding protein beta [2] and demonstrated that inhibition of the C/EBPβ-activating kinases could revert the degradative processes. Human OA cartilage organoids thus represent a relevant tool towards the discovery of new molecular drivers of cartilage degeneration and the assessment of therapeutics targeting associated pathways.

Close-to-native bone repair via tissue-engineered endochondral ossification approaches

In order to solve the clinical challenges related to bone grafting, several tissue engineering (TE) strategies have been proposed to repair critical-sized defects. Generally, the classical TE approaches are designed to promote bone repair via intramembranous ossification. Although promising, strategies that direct the osteogenic differentiation of mesenchymal stem/stromal cells are usually characterized by a lack of functional vascular supply, often resulting in necrotic cores. A less explored alternative is engineering bone constructs through a cartilage-mediated approach, resembling the embryological process of endochondral ossification. The remodeling of an intermediary hypertrophic cartilaginous template triggers vascular invasion and bone tissue deposition. Thus, employing this knowledge can be a promising direction for the next generation of bone TE constructs. This review highlights the most recent biomimetic strategies for applying endochondral ossification in bone TE while discussing the plethora of cell types, culture conditions, and biomaterials essential to promote a successful bone regeneration process.

Correlation of the regenerative potential of dermal fibroblasts in 2D culture with the biological properties of fibroblast-derived tissue spheroids

In situ 3D bioprinting is a new emerging therapeutic modality for treating human skin diseases. The tissue spheroids have been previously suggested as a powerful tool in rapidly expanding bioprinting technology. It has been demonstrated that the regenerative potential of human dermal fibroblasts could be quantitatively evaluated in 2D cell culture and confirmed after implantation in vivo. However, the development of unbiassed quantitative criteria of the regenerative potential of 3D tissue spheroids in vitro before their in situ bioprinting remains to be investigated. Here it has been demonstrated for the first time that specific correlations exist between the regenerative potential of human dermal fibroblasts cultured in vitro as 2D cell monolayer with biological properties of 3D tissue spheroids fabricated from these fibroblasts. In vitro assessment of biological properties included diameter, spreading and fusion kinetics, and biomechanical properties of 3D tissue spheroids. This comprehensive characterization could be used to predict tissue spheroids' regenerative potential in vivo.

A platform for automated and label-free monitoring of morphological features and kinetics of spheroid fusion

Spheroids are widely applied as building blocks for biofabrication of living tissues, where they exhibit spontaneous fusion toward an integrated structure upon contact. Tissue fusion is a fundamental biological process, but due to a lack of automated monitoring systems, the in-depth characterization of this process is still limited. Therefore, a quantitative high-throughput platform was developed to semi-automatically select doublet candidates and automatically monitor their fusion kinetics. Spheroids with varying degrees of chondrogenic maturation (days 1, 7, 14, and 21) were produced from two different cell pools, and their fusion kinetics were analyzed via the following steps: (1) by applying a novel spheroid seeding approach, the background noise was decreased due to the removal of cell debris while a sufficient number of doublets were still generated. (2) The doublet candidates were semi-automatically selected, thereby reducing the time and effort spent on manual selection. This was achieved by automatic detection of the microwells and building a random forest classifier, obtaining average accuracies, sensitivities, and precisions ranging from 95.0% to 97.4%, from 51.5% to 92.0%, and from 66.7% to 83.9%, respectively. (3) A software tool was developed to automatically extract morphological features such as the doublet area, roundness, contact length, and intersphere angle. For all data sets, the segmentation procedure obtained average sensitivities and precisions ranging from 96.8% to 98.1% and from 97.7% to 98.8%, respectively. Moreover, the average relative errors for the doublet area and contact length ranged from 1.23% to 2.26% and from 2.30% to 4.66%, respectively, while the average absolute errors for the doublet roundness and intersphere angle ranged from 0.0083 to 0.0135 and from 10.70 to 13.44°, respectively. (4) The data of both cell pools were analyzed, and an exponential model was used to extract kinetic parameters from the time-series data of the doublet roundness. For both cell pools, the technology was able to characterize the fusion rate and quality in an automated manner and allowed us to demonstrate that an increased chondrogenic maturity was linked with a decreased fusion rate. The platform is also applicable to other spheroid types, enabling an increased understanding of tissue fusion. Finally, our approach to study spheroid fusion over time will aid in the design of controlled fabrication of “assembloids” and bottom-up biofabrication of living tissues using spheroids.