RESOURCES

Abstract
Self-assembling bioinks offer the possibility to biofabricate with molecular precision, hierarchical control, and biofunctionality. For this to become a reality with widespread impact, it is essential to engineer these ink systems ensuring reproducibility and providing suitable standardization. We have reported a self-assembling bioink based on disorder-to-order transitions of an elastin-like recombinamer (ELR) to co-assemble with graphene oxide (GO). Here, we establish reproducible processes, optimize printing parameters for its use as a bioink, describe new advantages that the self-assembling bioink can provide, and demonstrate how to fabricate novel structures with physiological relevance. We fabricate capillary-like structures with resolutions down to ∼10 µm in diameter and ∼2 µm thick tube walls and use both experimental and finite element analysis to characterize the printing conditions, underlying interfacial diffusion-reaction mechanism of assembly, printing fidelity, and material porosity and permeability. We demonstrate the capacity to modulate the pore size and tune the permeability of the resulting structures with and without human umbilical vascular endothelial cells. Finally, the potential of the ELR-GO bioink to enable supramolecular fabrication of biomimetic structures was demonstrated by printing tubes exhibiting walls with progressively different structure and permeability.

Abstract
Biofabrication using well-matched cell/materials systems provides unprecedented opportunities for dealing with human health issues where disease or injury overtake the body’s native regenerative abilities. Such opportunities can be enhanced through the development of biomaterials with cues that appropriately influence embedded cells into forming functional tissues and organs. In this context, biomaterials’ reliance on rigid biofabrication techniques needs to support the incorporation of a hierarchical mimicry of local and bulk biological cues that mimic the key functional components of native extracellular matrix. Advances in synthetic self-assembling peptide biomaterials promise to produce reproducible mimics of tissue-specific structures and may go some way in overcoming batch inconsistency issues of naturally sourced materials. Recent work in this area has demonstrated biofabrication with self-assembling peptide biomaterials with unique biofabrication technologies to support structural fidelity upon 3D patterning. The use of synthetic self-assembling peptide biomaterials is a growing field that has demonstrated applicability in dermal, intestinal, muscle, cancer and stem cell tissue engineering.

Abstract
The field of bioprinting has shown a tremendous development in recent years, focusing on the development of advanced in vitro models and on regeneration approaches. In this scope, the lack of suitable biomaterials that can be efficiently formulated as printable bioinks, while supporting specific cellular events, is currently considered as one of the main limitations in the field. Indeed, extracellular matrix (ECM)-derived biomaterials formulated to enable printability and support cellular response, for instance via integrin binding, are eagerly awaited in the field of bioprinting. Several bioactive laminin sequences, including peptides such as YIGSR and IKVAV, have been identified to promote endothelial cell attachment and/or neurite outgrowth and guidance, respectively. Here, we show the development of two distinct bioinks, designed to foster vasculogenesis or neurogenesis, based on methacrylated collagen and hyaluronic acid (CollMA and HAMA, respectively), both relevant ECM-derived polymers, and on their combination with cysteine-flanked laminin-derived peptides. Using this strategy, it was possible to optimize the bioink printability, by tuning CollMA and HAMA concentration and ratio, and modulate their bioactivity, through adjustments in the cell-active peptide sequence spatial density, without compromising cell viability. We demonstrated that cell-specific bioinks could be customized for the bioprinting of both human umbilical vein cord endothelial cells (HUVECs) or adult rat sensory neurons from the dorsal root ganglia, and could stimulate both vasculogenesis and neurite outgrowth, respectively. This approach holds great potential as it can be tailored to other cellular models, due to its inherent capacity to accommodate different peptide compositions and to generate complex peptide mixtures and/or gradients.

Abstract
Abstract The introduction of 3D bioprinting to fabricate living constructs with tailored architecture has provided a new paradigm for biofabrication, with the potential to overcome several drawbacks of conventional scaffold-based tissue regeneration strategies. Hydrogel-based materials are suitable candidates regarding cell biocompatibility but often display poor mechanical properties. Self-assembling peptides are a promising source of biomaterials to be used as 3D scaffolds based on their similarity to extracellular matrices (structurally and mechanically). In this study, an advanced bioink for biofabrication is presented based on the optimization of a RAD16-I-based biomaterial. The strategy followed to build 3D predefined structures by 3D printing is based on an enhancement of bioink viscosity by adding methylcellulose (MC) to a RAD16-I solution. The resultant constructs display high shape fidelity and stability and embedded human mesenchymal stem cells present high viability after 7 days of culture. Moreover, cells are also able to differentiate to the adipogenic lineage, suggesting the suitability of this novel biomaterial for soft tissue engineering applications.

Abstract
Abstract Bioprinting of 3D cell-laden constructs with well-defined architectures and controlled spatial distribution of cells is gaining importance in the field of Tissue Engineering. New 3D tissue models are being developed to study the complex cellular interactions that take place during both tissue development and in the regeneration of damaged and/or diseased tissues. Despite advances in 3D printing technologies, suitable hydrogels or 'bioinks' with enhanced printability and cell viability are lacking. Here we report a study on the 3D bioprinting of a novel group of self-assembling peptide-based hydrogels. Our results demonstrate the ability of the system to print well-defined 3D cell laden constructs with variable stiffness and improved structural integrity, whilst providing a cell-friendly extracellular matrix “like” microenvironment. Biological assays reveal that mammary epithelial cells remain viable after 7 days of in vitro culture, independent of the hydrogel stiffness.