RESOURCES

Abstract
Patients suffering from large bone defects are in urgent need of suitable bone replacements. Besides biocompatibility, such replacements need to mimic the 3D architecture of bone and match chemical, mechanical and biological properties, ideally promoting ossification. As natural bone mainly contains collagen type I and carbonate hydroxyapatite, a 3D-printable biomaterial consisting of methacrylated gelatin (GelMA) and nanohydroxyapatite (nHAp) would be beneficial to mimic the composition and shape of natural bone. So far, such nanocomposite hydrogels (NCH) suffered from unsatisfactory rheological properties making them unsuitable for extrusion-based 3D printing with high structural fidelity. In this study, we introduce a novel GelMA/nHAp NCH composition, incorporating the rheological modifier carbomer to improve rheological properties and addressing the challenge of calcium cations released from nHAp that hinder GelMA gelation. Leveraging its shear-thinning and self-healing properties, the NCH ink retains its shape and forms cohesive structures after deposition, which can be permanently stabilized by subsequent UV crosslinking. Consequently, the NCH enables the printing of 3D structures with high shape fidelity in all dimensions, including the z-direction, allowing the fabrication of highly macroporous constructs. Both the uncured and the UV crosslinked NCH behave like a viscoelastic solid, with G′> G″ at deformations up to 100–200 %. After UV crosslinking, the NCH can, depending on the GelMA concentration, reach storage moduli of approximately 10 to over 100 kPa and a mean Young’s Modulus of about 70 kPa. The printed scaffolds permit not only cell survival but also osteogenic differentiation, highlighting their potential for bone tissue engineering.

Abstract
Melt electrowriting (MEW) onto a rotating cylindrical mandrel enables the fabrication of tubular scaffolds for tissue engineering, such as vascular grafts, with microstructures that support cellular ingrowth and customizable biomechanical properties. However, these scaffolds exhibit a systematic deviation of deposited fibers from the planned design, previously unreported in the existing literature. Unlike the known deviations in planar scaffolds, this deviation affects a wider range of designs, including meandering toolpaths, where it can result in pronounced alternating fiber spacing. Since this deviation often exceeds 100 µm and most biologically relevant structures are significantly smaller, it can compromise scaffold integrity, rendering the product unsuitable for clinical use. This study investigates the origin of this deviation using a novel automated optical scanning system consisting of a custom microscope integrated into a four-axis bioprinter. High-resolution images of entire tubular scaffolds are captured to precisely measure fiber deviation. Besides this empirical approach, a mathematical model was developed based on simple geometric considerations to predict deviation from jet and printing parameters, which closely matches experimental measurements. Finally, four toolpath strategies that avoid the alternating fiber spacing were evaluated. Some strategies reduce mean fiber spacing variation to ± 4 µm, facilitating the fabrication of highly homogeneous porous structures.

Abstract
Thanks to its natural complexity and functionality, decellularized extracellular matrix (dECM) serves as an excellent foundation for creating highly cell-compatible bioinks and bioresins. This enables the bioprinted cells to thrive in an environment that closely mimics their native ECM composition and offers customizable biomechanical properties. To formulate dECM bioinks and bioresins, one must first pulverize and/or solubilize the dECM into non-crosslinked fragments which can then be chemically modified as needed. In bioprinting, the solubilized dECM-derived material is typically deposited and/or crosslinked in a layer-by-layer fashion to build 3D hydrogel structures. Since the introduction of the first liver-derived dECM-based bioinks, a wide variety of decellularized tissue have been employed in bioprinting, including kidney, heart, cartilage, and adipose tissue among others. This review aims to summarize the critical steps involved in tissue-derived dECM bioprinting, starting from the decellularization of the ECM to the standardized formulation of bioinks and bioresins, ultimately leading to the reproducible bioprinting of tissue constructs. Notably, this discussion also covers photocrosslinkable dECM bioresins, which are particularly attractive due to their ability to provide precise spatiotemporal control over the gelation in bioprinting. Both in extrusion printing and vat photopolymerization, there is a need for more standardized protocols to fully harness the unique properties of dECM-derived materials. In addition to mammalian tissues, the most recent bioprinting approaches involve the use of microbial extracellular polymeric substances in bioprinting of bacteria. This presents similar challenges as those encountered in mammalian cell printing and represents a fascinating frontier in bioprinting technology.