Rheological characterization of cell-laden alginate-gelatin hydrogels for 3D biofabrication

Arthroscopic device with bendable tip for the controlled extrusion of hydrogels on cartilage defects

Advanced tools for the in situ treatment of articular cartilage lesions are attracting a growing interest in both surgery and bioengineering communities. The interest is particularly high concerning the delivery of cell-laden hydrogels. The tools currently available in the state-of-the-art hardly find an effective compromise between treatment accuracy and invasiveness. This paper presents a novel arthroscopic device provided with a bendable tip for the controlled extrusion of cell-laden hydrogels. The device consists of a handheld extruder and a supply unit that allows the extrusion of hydrogels. The extruder is equipped with a disposable, bendable nitinol tip (diameter: 4 mm, length: 92 mm, maximum bending angle: 90°) that guarantees access to hard-to-reach areas of the joint, which are difficult to get to, with conventional arthroscopic instruments. The tip accommodates a biocompatible polymer tube that is directly connected to the cartridge containing the hydrogel, whose plunger is actuated by a volumetric or pneumatic supply unit (both tested, in this study). Three different chondrocyte-laden hydrogels (RGD-modified Vitrogel®, methacrylated gellan gum, and an alginate-gelatine blend) were considered. First, the performance of the device in terms of resolution in hydrogel delivery was assessed, finding values in the range between 4 and 102 µL, with better performance found for the pneumatic supply unit and no significant differences between straight tip and bent tip conditions. Finite element simulations suggested that the shear stresses and pressure levels generated during the extrusion process were compatible with a safe deposition of the hydrogels. Biological analyses confirmed a high chondrocyte viability over a 7-day period after the extrusion of the three cell-laden hydrogel types, with no differences between the two supply units. The arthroscopic device was finally tested ex vivo by nine orthopedic surgeons on human cadaver knees. The device allowed surgeons to easily deliver hydrogels even in hard-to-reach cartilage areas. The outcomes of a questionnaire completed by the surgeons demonstrated a high usability of the device, with an overall preference for the pneumatic supply unit. Our findings provide evidence supporting the future arthroscopic device translation in pre-clinical and clinical scenarios, dealing with osteoarticular treatments.

Fanganiello R. Design of Alginate/Gelatin Hydrogels for Biomedical Applications: Fine-Tuning Osteogenesis in Dental Pulp Stem Cells While Preserving Other Cell Behaviors

Alginate/gelatin (Alg-Gel) hydrogels have been used experimentally, associated with mesenchymal stromal/stem cells (MSCs), to guide bone tissue formation. One of the main challenges for clinical application is optimizing Alg-Gel stiffness to guide osteogenesis. In this study, we investigated how Alg-Gel stiffness could modulate the dental pulp stem cell (DPSC) attachment, morphology, proliferation, and osteogenic differentiation, identifying the optimal conditions to uncouple osteogenesis from the other cell behaviors. An array of Alg-Gel hydrogels was prepared by casting different percentages of alginate and gelatin cross-linked with 2% CaCl2. We have selected two hydrogels: one with a stiffness of 11 ± 1 kPa, referred to as "low-stiffness hydrogel", formed by 2% alginate and 8% gelatin, and the other with a stiffness of 55 ± 3 kPa, referred to as "high-stiffness hydrogel", formed by 8% alginate and 12% gelatin. Hydrogel analyses showed that the average swelling rates were 20 ± 3% for the low-stiffness hydrogels and 35 ± 2% for the high-stiffness hydrogels. The degradation percentage was 47 ± 5% and 18 ± 2% for the low- and high-stiffness hydrogels, respectively. Both hydrogel types showed homogeneous surface shape and protein (Alg-Gel) interaction with CaCl2 as assessed by physicochemical characterization. Cell culture showed good adhesion of the DPSCs to the hydrogels and proliferation. Furthermore, better osteogenic activity, determined by ALP activity and ARS staining, was obtained with high-stiffness hydrogels (8% alginate and 12% gelatin). In summary, this study confirms the possibility of characterizing and optimizing the stiffness of Alg-Gel gel to guide osteogenesis in vitro without altering the other cellular properties of DPSCs.

Design and development of a hepatic lyo-dECM powder as a biomimetic component for 3D-printable hybrid hydrogels

Bioprinting offers new opportunities to obtain reliable 3Din vitromodels of the liver for testing new drugs and studying pathophysiological mechanisms, thanks to its main feature in controlling the spatial deposition of cell-laden hydrogels. In this context, decellularized extracellular matrix (dECM)-based hydrogels have caught more and more attention over the last years because of their characteristic to closely mimic the tissue-specific microenvironment from a biological point of view. In this work, we describe a new concept of designing dECM-based hydrogels; in particular, we set up an alternative and more practical protocol to develop a hepatic lyophilized dECM (lyo-dECM) powder as an 'off-the-shelf' and free soluble product to be incorporated as a biomimetic component in the design of 3D-printable hybrid hydrogels. To this aim, the powder was first characterized in terms of cytocompatibility on human and porcine mesenchymal stem cells (MSCs), and the optimal powder concentration (i.e. 3.75 mg ml-1) to use in the hydrogel formulation was identified. Moreover, its non-immunogenicity and capacity to reactivate the elastase enzyme potency was proved. Afterward, as a proof-of-concept, the powder was added to a sodium alginate/gelatin blend, and the so-defined multi-component hydrogel was studied from a rheological point of view, demonstrating that adding the lyo-dECM powder at the selected concentration did not alter the viscoelastic properties of the original material. Then, a printing assessment was performed with the support of computational simulations, which were useful to definea priorithe hydrogel printing parameters as window of printability and its post-printing mechanical collapse. Finally, the proposed multi-component hydrogel was bioprinted with cells inside, and its post-printing cell viability for up to 7 d was successfully demonstrated.

Gelatin/Hyaluronic Acid Photocrosslinked Double Network Hydrogel with Nano-Hydroxyapatite Composite for Potential Application in Bone Repair

The application of hydrogels in bone repair is limited due to their low mechanical strength. Simulating bone extracellular matrix, methylacrylylated gelatin (GelMA)/methylacrylylated hyaluronic acid (HAMA)/nano-hydroxyapatite(nHap) composite hydrogels were prepared by combining the double network strategy and composite of nHap in this study. The precursor solutions of the composite hydrogels were injectable due to their shear thinning property. The compressive elastic modulus of the composite hydrogel was significantly enhanced, the fracture strength of the composite hydrogel nearly reached 1 MPa, and the composite hydrogel retained its high water content at above 88%. The composite hydrogels possess good compatibility with BMSCS and have the potential to be used as injectable hydrogels for bone defect treatment.

Optimization of 3D printing and in vitro characterization of alginate/gelatin lattice and angular scaffolds for potential cardiac tissue engineering

Background: Engineering cardiac tissue that mimics the hierarchical structure of cardiac tissue remains challenging, raising the need for developing novel methods capable of creating structures with high complexity. Three-dimensional (3D)-printing techniques are among promising methods for engineering complex tissue constructs with high precision. By means of 3D printing, this study aims to develop cardiac constructs with a novel angular structure mimicking cardiac architecture from alginate (Alg) and gelatin (Gel) composite. The 3D-printing conditions were optimized and the structures were characterized in vitro, with human umbilical vein endothelial cells (HUVECs) and cardiomyocytes (H9c2 cells), for potential cardiac tissue engineering. Methods: We synthesized the composites of Alg and Gel with varying concentrations and examined their cytotoxicity with both H9c2 cells and HUVECs, as well as their printability for creating 3D structures of varying fibre orientations (angular design). The 3D-printed structures were characterized in terms of morphology by both scanning electron microscopy (SEM) and synchrotron radiation propagation-based imaging computed tomography (SR-PBI-CT), and elastic modulus, swelling percentage, and mass loss percentage as well. The cell viability studies were conducted via measuring the metabolic activity of the live cells with MTT assay and visualizing the cells with live/dead assay kit. Results: Among the examined composite groups of Alg and Gel, two combinations with ratios of 2 to 1 and 3 to 1 (termed as Alg2Gel1 and Alg3Gel1) showed the highest cell survival; they accordingly were used to fabricate two different structures: a novel angular and a conventional lattice structure. Scaffolds made of Alg3Gel1 showed higher elastic modulus, lower swelling percentage, less mass loss, and higher cell survival compared to that of Alg2Gel1. Although the viability of H9c2 cells and HUVECs on all scaffolds composed of Alg3Gel1 was above 99%, the group of the constructs with the angular design maintained significantly more viable cells compared to other investigated groups. Conclusion: The group of angular 3D-ptinted constructs has illustrated promising properties for cardiac tissue engineering by providing high cell viability for both endothelial and cardiac cells, high mechanical strength as well as appropriate swelling, and degradation properties during 21 days of incubation. Statement of Significance: 3D-printing is an emerging method to create complex constructs with high precision in a large scale. In this study, we have demonstrated that 3D-printing can be used to create compatible constructs from the composite of Alg and Gel with endothelial cells and cardiac cells. Also, we have demonstrated that these constructs are able to enhance the viability of cardiac and endothelial cells via creating a 3D structure mimicking the alignment and orientation of the fibers in the native heart.