Impact of Cell Loading of Recombinant Spider Silk Based Bioinks on Gelation and Printability

Only kosmotrope anions trigger fibrillization of the recombinant core spidroin eADF4(C16) from Araneus diadematus

Recombinant core spidroin eADF4(C16) has received increasing attention due to its ability to form micro- and nano-structured scaffolds, which are based on nanofibrils with great potential for biomedical and biotechnological applications. Phosphate anions have been demonstrated to trigger the eADF4(C16) self-assembly into cross-beta fibrils. In the present work, we systematically addressed the effect of nine sodium anions, namely SO4 2- , HPO4 2- (Pi), F- , Cl- , Br- , NO3 - , I- , SCN- , and ClO4 - from the Hofmeister series on the in vitro self-assembly kinetics of eADF4(C16). We show that besides the phosphate anions, only kosmotropic anions such as sulfate and fluoride can initiate the eADF4(C16) fibril formation. Global analysis of the self-assembly kinetics, utilizing the platform AmyloFit, showed the nucleation-based mechanism with a major role of secondary nucleation, surprisingly independent of the type of the kosmotropic anion. The rate constant of the fibril elongation in mixtures of phosphate anions with other studied anions correlated with their kosmotropic or chaotropic position in the Hofmeister series. Our findings suggest an important role of anion hydration in the eADF4(C16) fibrillization process.

Rheological and Biological Impact of Printable PCL-Fibers as Reinforcing Fillers in Cell-Laden Spider-Silk Bio-Inks

The development of bio-inks capable of being 3D-printed into cell-containing bio-fabricates with sufficient shape fidelity is highly demanding. Structural integrity and favorable mechanical properties can be achieved by applying high polymer concentrations in hydrogels. Unfortunately, this often comes at the expense of cell performance since cells may become entrapped in the dense matrix. This drawback can be addressed by incorporating fibers as reinforcing fillers that strengthen the overall bio-ink structure and provide a second hierarchical micro-structure to which cells can adhere and align, resulting in enhanced cell activity. In this work, the potential impact of collagen-coated short polycaprolactone-fibers on cells after being printed in a hydrogel is systematically studied. The matrix is composed of eADF4(C16), a recombinant spider silk protein that is cytocompatible but non-adhesive for cells. Consequently, the impact of fibers could be exclusively examined, excluding secondary effects induced by the matrix. Applying this model system, a significant impact of such fillers on rheology and cell behavior is observed. Strikingly, it could be shown that fibers reduce cell viability upon printing but subsequently promote cell performance in the printed construct, emphasizing the need to distinguish between in-print and post-print impact of fillers in bio-inks.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

Flow Simulation and Gradient Printing of Fluorapatite- and Cell-Loaded Recombinant Spider Silk Hydrogels

Hierarchical structures are abundant in almost all tissues of the human body. Therefore, it is highly important for tissue engineering approaches to mimic such structures if a gain of function of the new tissue is intended. Here, the hierarchical structures of the so-called enthesis, a gradient tissue located between tendon and bone, were in focus. Bridging the mechanical properties from soft to hard secures a perfect force transmission from the muscle to the skeleton upon locomotion. This study aimed at a novel method of bioprinting to generate gradient biomaterial constructs with a focus on the evaluation of the gradient printing process. First, a numerical approach was used to simulate gradient formation by computational flow as a prerequisite for experimental bioprinting of gradients. Then, hydrogels were printed in a single cartridge printing set-up to transfer the findings to biomedically relevant materials. First, composites of recombinant spider silk hydrogels with fluorapatite rods were used to generate mineralized gradients. Then, fibroblasts were encapsulated in the recombinant spider silk-fluorapatite hydrogels and gradually printed using unloaded spider silk hydrogels as the second component. Thereby, adjustable gradient features were achieved, and multimaterial constructs were generated. The process is suitable for the generation of gradient materials, e.g., for tissue engineering applications such as at the tendon/bone interface.

Recombinant Spider Silk Bioinks for Continuous Protein Release by Encapsulated Producer Cells

Targeted therapies using biopharmaceuticals are of growing clinical importance in disease treatment. Currently, there are several limitations of protein-based therapeutics (biologicals), including suboptimal biodistribution, lack of stability, and systemic side effects. A promising approach to overcoming these limitations could be a therapeutic cell-loaded 3D construct consisting of a suitable matrix component that harbors producer cells continuously secreting the biological of interest. Here, the recombinant spider silk proteins eADF4(C16), eADF4(C16)-RGD, and eADF4(C16)-RGE have been processed together with HEK293 producer cells stably secreting the highly traceable reporter biological TNFR2-Fc-GpL, a fusion protein consisting of the extracellular domain of TNFR2, the Fc domain of human IgG1, and the luciferase of Gaussia princeps as a reporter domain. eADF4(C16) and eADF4(C16)-RGD hydrogels provide structural and mechanical support, promote HEK293 cell growth, and allow fusion protein production by the latter. Bioink-captured HEK293 producer cells continuously release functional TNFR2-Fc-GpL over 14 days. Thus, the combination of biocompatible, printable spider silk bioinks with drug-producing cells is promising for generating implantable 3D constructs for continuous targeted therapy.

DNA Functionalized Spider Silk Nanohydrogels for Specific Cell Attachment and Patterning

Nucleated protein self-assembly of an azido modified spider silk protein was employed in the preparation of nanofibrillar networks with hydrogel-like properties immobilized on coatings of the same protein. Formation of the networks in a mild aqueous environment resulted in thicknesses between 2 and 60 nm, which were controlled only by the protein concentration. Incorporated azido groups in the protein were used to "click" short nucleic acid sequences onto the nanofibrils, which were accessible to specific hybridization-based modifications, as proved by fluorescently labeled DNA complements. A lipid modifier was used for efficient incorporation of DNA into the membrane of nonadherent Jurkat cells. Based on the complementarity of the nucleic acids, highly specific DNA-assisted immobilization of the cells on the nanohydrogels with tunable cell densities was possible. Addressability of the DNA cell-to-surface anchor was demonstrated with a competitive oligonucleotide probe, resulting in a rapid release of 75-95% of cells. In addition, we developed a photolithography-based patterning of arbitrarily shaped microwells, which served to spatially define the formation of the nanohydrogels. After detaching the photoresist and PEG-blocking of the surface, DNA-assisted immobilization of the Jurkat cells on the nanohydrogel microstructures was achieved with high fidelity.