Recent advances in tissue engineering and biofabrication technology have yielded a plethora of biological tissues. Among these, engineering of bioartificial muscle stands out for its exceptional versatility and its wide range of applications. From the food industry to the technology sector and medicine, the development of this tissue has the potential to affect many different industries at once. However, to date, the biofabrication of cultured meat, biorobotic systems, and bioartificial muscle implants are still considered in isolation by individual peer groups. To establish common ground and share advances, this review outlines application-specific requirements for muscle tissue generation and provides a comprehensive overview of commonly used biofabrication strategies and current application trends. By solving the individual challenges and merging various expertise, synergetic leaps of innovation that inspire each other can be expected in all three industries in the future.

Large bone defects are commonly treated by replacement with auto- and allografts, which have substantial drawbacks including limited supply, donor site morbidity, and possible tissue rejection. This study aimed to improve bone defect treatment using a custom-made filament for tissue engineering scaffolds. The filament consists of biodegradable polylactide acid (PLA) and a varying amount (up to 20%) of osteoconductive S53P4 bioglass. By employing an innovative, additive manufacturing technique, scaffolds with optimized physico-mechanical and biological properties were produced. The scaffolds feature adjustable macro- and microporosity (200–2000 µm) with adaptable mechanical properties (83–135 MPa). Additionally, controllable calcium release kinetics (0–0.25 nMol/µL after 24 h), tunable mesenchymal stem cell (MSC) adhesion potential (after 24 h by a factor of 14), and proliferation (after 168 h by a factor of 18) were attained. Microgrooves resulting from the 3D-printing process on the surface act as a nucleus for cell aggregation, thus being a potential cell niche for spheroid formation or possible cell guidance. The scaffold design with its adjustable biomechanics and the bioglass with its antimicrobial properties are of particular importance for the preclinical translation of the results. This study comprehensibly demonstrates the potential of a 3D-printed bioglass composite scaffold for the treatment of critical-sized bone defects.

By combining a photocurable and a thermogelling hydrogel, it is possible to perform 3D freeform structuring via two-photon-polymerization and to manufacture concatenated parts without additional support structures.

Electrical stimulation enhances functionality and accelerates maturation in biofabricated tissues, which are particularly important for muscle tissue engineering applications. Accordingly, there is demand for 3D-printable electrically conductive cytocompatible scaffolds that enable patient-specific geometries and localized electrical stimulation, as well as incorporate further maturation-promoting geometrical cues. Filament-based scaffolds from fused filament fabrication could overcome current limitations in geometric freedom, size and partially cytotoxic additives. In this study, biodegradable polylactic acid (PLA)-based conductive filaments incorporating graphene nanoplatelets (GNPs) or carbon nanofibers (CNFs) were developed via melt-mixing extrusion to possibly enable the electrical functionalization of muscle scaffolds. A two-stage process combining twin-screw and single-screw extrusion was preferred to allow for higher filler incorporation. Filament morphology, printability, electrical conductivity, and cytocompatibility were systematically evaluated. Homogeneous filaments containing up to 16 wt.% GNPs or 3.6 wt.% CNFs were successfully produced and processed by fused filament fabrication into scaffold geometries supporting myoblast orientation. Electrical conductivity was measured above 16 wt.% GNPs, with up to 2.7 µS/m, with printed constructs capable of connecting a circuit. GNP-based filaments were cytocompatible, supporting myoblast attachment and elongated morphology. An adjustable electrical stimulation setup demonstrated improved muscle maturation and contractile responses of C2C12 myoblasts, highlighting biodegradable conductive filaments’ potential for electrically active muscle tissue scaffolds.

Biofabrication of hierarchical tissues with features ranging various size ranges and controllable anisotropy remains a challenge in 3D-bioprinting. To overcome this hurdle, the application of multi-functional microfibers acting as cell-instructive bioink additive, recently gained particular attention. In this work, we investigate a microfluidic spinning process for the fabrication of collagen microfibers with adjustable diameters ranging from 5 to 50 μm. The thread was collected on a rotating winder and fragmented into microfibers of defined length (60–300 μm). By integrating microfiber fragments into an agarose-hyaluronan hydrogel, fine-tuning of its viscosity range (10–1000 mPa∗s), and thus the precise control of the extruded strands’ diameter (0.3–1.4 mm) was achieved. While remaining strong shear-thinning behavior (n-value 0.6), E-modulus and yield stress were decreased in fiber-filled hydrogel, hinting at an interaction of agarose polymer chains with microfibers. Remarkably, the orientation of collagen microfibers could be directed either parallel or orthogonal to the printing path. This allows the biofabrication of hydrogel structures with adjustable domains of defined anisotropy. Finally, the fibers showed excellent biofunctionality both in 2D and 3D. Besides a high degree of alignment of individual cells along the microfiber axis (>80 % of cells), hMSCs built a dense, branched network in 3D. Moreover, PC12 and C2C12 were successfully differentiated in 2D and 3D. Specifically, neurite length was higher on smaller fiber diameters, even spanning non-adjacent clusters. Elongated, multi-nuclei myotubes were formed, indicating C2C12 differentiation. In summary, the work demonstrates the great potential of 3D-bioprinting in cross-scale organization of fragmented collagen microfibers.

A multimodal 3D-bioprinting process for fabrication of muscle precursors with high suture retention strength and biofunctionality.