Layer-by-layer bioprinting is a logical choice for the fabrication of stratified tissues like articular cartilage. Printing of viable organ replacements, however, is dependent on bioinks with appropriate rheological and cytocompatible properties. In cartilage engineering, photocrosslinkable glycosaminoglycan-based hydrogels are chondrogenic, but alone have generally poor printing properties. By blending the thermoresponsive polymer poly(N-isopropylacrylamide) grafted hyaluronan (HA-pNIPAAM) with methacrylated hyaluronan (HAMA), high-resolution scaffolds with good viability were printed. HA-pNIPAAM provided fast gelation and immediate post-printing structural fidelity, while HAMA ensured long-term mechanical stability upon photocrosslinking. The bioink was evaluated for rheological properties, swelling behavior, printability and biocompatibility of encapsulated bovine chondrocytes. Elution of HA-pNIPAAM from the scaffold was necessary to obtain good viability. HA-pNIPAAM can therefore be used to support extrusion of a range of biopolymers which undergo tandem gelation, thereby facilitating the printing of cell-laden, stratified cartilage constructs with zonally varying composition and stiffness.

wileyonlinelibrary.comwith rapid production of patient-specifi c grafts, allowing precise control over internal and external architecture and customized mechanical properties. These techniques can be used for printing biological materials together with living cells, hence the term "bioprinting." 3D bioprinting offers researchers a unique way of depositing cell-laden biocompatible materials, the so-called bioinks, in high-resolution structures with a line thickness on the order of hundreds of microns. Due to the promise of such a technology, several commercial bioprinters have entered the market and bioinks are the subject of intense investigation. [1][2][3] Bioink formulation is often considered one of the most critical aspects of high-resolution cellular bioprinting.Cellular printing requires a bioink with two key properties, namely printability and cytocompatible crosslinking. The identifi cation of printable polymeric systems is mainly done through rheological evaluation of a material's shear thinning behavior and shear recovery. Shear thinning correlates directly with a bioink's ability to be extruded at low pressure (<3 bar), something which ensures high postprinting cell viability. [ 4 ] Shear recovery, on the other hand, relates to the ink's resistance to fl ow after printing, which ensures high fi delity of the printed structure. The presence of cells, however, greatly restricts the crosslinking options as physiologic temperature and pH need to be maintained and harsh chemicals avoided. Hydrogel bioinks can be crosslinked via covalent or physical interactions or a combination thereof. Ultraviolet light initiated crosslinking of (meth)acrylated polymers has been used most often in bioinks, but the presence of potentially toxic monomers and photoinitiators may complicate clinical translation. [5][6][7][8] Physically crosslinked gelation based on temperature, hydrophobic/hydrophilic or ionic interactions has been utilized for precrosslinking of several bioink materials including poly( N -isopropylacrylamide) conjugated hyaluronan (HA-pNIPAAm), [ 9 ] gelatin, [ 10,11 ] alginate, [ 12 ] and gellan. [ 13 ] Precrosslinking before printing or directly during deposition to stabilize the printed lines is generally followed by a fi nal crosslinking which further increases the mechanical properties and stabilizes the whole structure.For cartilage engineering applications, natural polymers from animal or plant sources including alginate, collagen, gelatin, Bioprinting is an emerging technology for the fabrication of patient-specifi c, anatomically complex tissues and organs. A novel bioink for printing cartilage grafts is developed based on two unmodifi ed FDA-compliant polysaccharides, gellan and alginate, combined with the clinical product BioCartilage (cartilage extracellular matrix particles). Cell-friendly physical gelation of the bioink occurs in the presence of cations, which are delivered by co-extrusion of a cation-loaded transient support polymer to stabilize overhanging structures. Rheological properties of...

Biofabrication techniques including threedimensional bioprinting could be used one day to fabricate living, patient-specific tissues and organs for use in regenerative medicine. Compared to traditional casting and molding methods, bioprinted structures can be much more complex, containing for example multiple materials and cell types in controlled spatial arrangement, engineered porosity, reinforcement structures and gradients in mechanical properties. With this complexity and increased function, however, comes the necessity to develop guidelines to standardize the bioprinting process, so printed grafts can safely enter the clinics. The bioink material must firstly fulfil requirements for biocompatibility and flow. Secondly, it is important to understand how process parameters affect the final mechanical properties of the printed graft. Using a gellan-alginate physically crosslinked bioink as an example, we show shear thinning and shear recovery properties which allow good printing resolution. Printed tensile specimens were used to systematically assess effect of line spacing, printing direction and crosslinking conditions. This standardized testing allowed direct comparison between this bioink and three commercially-available products. Bioprinting is a promising, yet complex fabrication method whose outcome is sensitive to a range of process parameters. This study provides the foundation for highly needed best practice guidelines for reproducible and safe bioprinted grafts.

On page 7406, M. Zenobi‐Wong and co‐workers present a bioink which allows the bioprinting of complex cell‐laden cartilaginous structures with regulatory‐compliant biomaterials. The printed grafts have tunable mechanical properties and, since they can be developed to contain extracellular matrix particles, are also tissue‐specific. Complex and overhanging structures can be printed using a co‐extruded support polymer, which also acts as a cation‐reservoir to rapidly crosslink the bioink.