3D Functional Neuronal Networks in Free-Standing Bioprinted Hydrogel Constructs

Multiscale Organization of Neural Networks in a 3D Bioprinted Matrix

The efficient establishment of in vitro neural models that accurately mimic the structural and functional connectivity of neural networks is critical in neuroscience research. 3D bioprinting shows great potential for constructing sophisticated in vitro models with high freedom of design. However, mature neurons are delicate and susceptible to manipulation. Here, extrusion-based 3D bioprinting is employed to fabricate gelatin methacryloyl (GelMA)-based constructs containing embryonic day 18 (E18) rat cortical neurons, referred to as 3D neuMatrix. 3D neuMatrix displays favorable neuronal viability, with the progressive formation of a 3D brain-like neural network with local and long-range functional axon connections. Compared with 2D cultured neurons, 3D neuMatrix is more similar to the E18 cortex according to the bulk transcriptomic profile, with a recreation of cellular components in the cerebral cortex. The 3D neuMatrix is employed to establish a disease model of ischemic stroke, with a faithful recapitulation of the viability, function, and transcriptomic features of rats with middle cerebral artery occlusion/reperfusion (MCAO/R). These findings demonstrate the formation of multiscale neural circuits within 3D neuMatrix and its valuable potential in the study of neurodevelopment, disease modeling with drug screening, and in vitro intelligence.

Construction of Multicellular Neural Tissue Using Three-Dimensional Printing Technology: Cell Interaction

The study of the human nervous system remains challenging due to its inherent complexity and difficulty in obtaining original samples. Three-dimensional (3D) bioprinting is a rapidly evolving technology in the field of tissue engineering that has made significant contributions to several disciplines, including neuroscience. In order to more accurately reflect the intricate multicellular milieu of the in vivo environment, an increasing number of studies have commenced experimentation with the coprinting of diverse cell types. This article provides an overview of technical details and the application of 3D bioprinting with multiple cell types in the field of neuroscience, focusing on the challenges of coprinting and the research conducted based on multicellular printing. This review discusses cell interactions in coprinting systems, stem cell applications, the construction of brain-like organoids, the establishment of disease models, and the potential for integrating 3D bioprinting with other 3D culture techniques.

The Art of Neuroregeneration De Novo and In Situ

Neuroregeneration refers to the ability of the nervous system to repair or regenerate neural components subsequently to spinal cord and traumatic brain injuries, peripheral nerve damage, and neurodegenerative diseases. Here, we discuss lead effectors of the healing process, neural stem cells, and non-invasive physical methods, for neuroregeneration de novo and in situ.

Development of Neural Cells and Spontaneous Neural Activities in Engineered Brain-Like Constructs for Transplantation

Stem cell transplantation has demonstrated efficacy in treating neurological disorders by generating functional cells and secreting beneficial factors. However, challenges remain for current cell suspension injection therapy, including uncontrollable cell distribution, the potential for tumor formation, and limited ability to treat spatial defects. Therefore, implants with programmable cell development, tailored 3D structure, and functionalized biomaterials have the potential to both control cell distribution and reduce or heal spatial defects. Here, a biomimetic material system comprising gelatin, alginate, and fibrinogen has been developed for neural progenitor cell constructs using 3D printing. The resulting constructs exhibit excellent formability, stability, and developmental functions in vitro, as well as biocompatibility and integration into the hippocampus in vivo. The controllability, reproducibility, and material composition of the constructs show potential for use in personalized stem cell-based therapies for defective neurological disorders, neural development research, disease modeling, and organoid-derived intelligent systems.

Hyaluronic Acid-Based 3D Bioprinted Hydrogel Structure for Directed Axonal Guidance and Modeling Innervation In Vitro

Neurons form predefined connections and innervate target tissues through elongating axons, which are crucial for the development, maturation, and function of these tissues. However, innervation is often overlooked in tissue engineering (TE) applications. Here, multimaterial 3D bioprinting is used to develop a novel 3D axonal guidance structure in vitro. The approach uses the stiffness difference of acellular hyaluronic acid-based bioink printed as two alternating, parallel-aligned filaments. The structure has soft passages incorporated with guidance cues for axonal elongation while the stiff bioink acts as a structural support and contact guidance. The mechanical properties and viscosity differences of the bioinks are confirmed. Additionally, human pluripotent stem cell (hPSC) -derived neurons form a 3D neuronal network in the softer bioink supplemented with guidance cues whereas the stiffer restricts the network formation. Successful 3D multimaterial bioprinting of the axonal structure enables complete innervation by peripheral neurons via soft passages within 14 days of culture. This model provides a novel, stable, and long-term platform for studies of 3D innervation and axonal dynamics in health and disease.

Navigating Neurotoxicity and Safety Assessment of Nanocarriers for Brain Delivery: Strategies and Insights

Nanomedicine, an area that uses nanomaterials for theragnostic purposes, is advancing rapidly, particularly in the detection and treatment of neurodegenerative diseases. The design of nanocarriers can be optimized to enhance drug bioavailability and targeting to specific organs, improving therapeutic outcomes. However, clinical translation hinges on biocompatibility and safety. Nanocarriers can cross the blood-brain barrier (BBB), potentially causing neurotoxic effects through mechanisms such as oxidative stress, DNA damage, and neuroinflammation. Concerns about their accumulation and persistence in the brain make it imperative to carry out a nanotoxicological risk assessment. Generally, this involves identifying exposure sources and routes, characterizing physicochemical properties, and conducting cytotoxicity assays both in vitro and in vivo. The lack of a specialized regulatory framework creates substantial gaps, making it challenging to translate findings across development stages. Additionally, there is a pressing need for innovative testing methods due to constraints on animal use and the demand for high-throughput screening. This review examines the mechanisms of nanocarrier-induced neurotoxicity and the challenges in risk assessment, highlighting the impact of physicochemical properties and the advantages and limitations of current neurotoxicity evaluation models. Future perspectives are also discussed. Additional guidance is crucial to improve the safety of nanomaterials and reduce associated uncertainty. STATEMENT OF SIGNIFICANCE: Nanocarriers show tremendous potential for theragnostic purposes in neurological diseases, enhancing drug targeting to the brain, and improving biodistribution and pharmacokinetics. However, their neurotoxicity is still a major field to be explored, with only 5% of nanotechnology-related publications addressing this matter. This review focuses on the issue of neurotoxicity and safety assessment of nanocarriers for brain delivery. Neurotoxicity-relevant exposure sources, routes, and molecular mechanisms, along with the impact of the physicochemical properties of nanomaterials, are comprehensively described. Moreover, the different experimental models used for neurotoxicity evaluation are explored at length, including their main advantages and limitations. To conclude, we discuss current challenges and future perspectives for a better understanding of risk assessment of nanocarriers for neurobiomedical applications.

Gel-Based Suspension Medium Used in 3D Bioprinting for Constructing Tissue/Organ Analogs

Constructing tissue/organ analogs with natural structures and cell types in vitro offers a valuable strategy for the in situ repair of damaged tissues/organs. Three-dimensional (3D) bioprinting is a flexible method for fabricating these analogs. However, extrusion-based 3D bioprinting faces the challenge of balancing the use of soft bioinks with the need for high-fidelity geometric shapes. To address these challenges, recent advancements have introduced various suspension mediums based on gelatin, agarose, and gellan gum microgels. The emergence of these gel-based suspension mediums has significantly advanced the fabrication of tissue/organ constructs using 3D bioprinting. They effectively stabilize and support soft bioinks, enabling the formation of complex spatial geometries. Moreover, they provide a stable, cell-friendly environment that maximizes cell viability during the printing process. This minireview will summarize the properties, preparation methods, and potential applications of gel-based suspension mediums in constructing tissue/organ analogs, while also addressing current challenges and providing an outlook on the future of 3D bioprinting.

Engineering complex tissue-like microenvironments with biomaterials and biofabrication

Advances in tissue engineering for both system modeling and organ regeneration depend on embracing and recapitulating the target tissue's functional and structural complexity. Microenvironmental features such as anisotropy, heterogeneity, and other biochemical and mechanical spatiotemporal cues are essential in regulating tissue development and function. Novel biofabrication strategies and innovative biomaterial design have emerged as promising tools to better reproduce such features. These facilitate a transition towards high-fidelity biomimetic structures, offering opportunities for a deeper understanding of tissue function and the development of superior therapies. In this review, we explore some of the key structural and compositional aspects of tissues, lay out how to achieve similar outcomes with current fabrication strategies, and identify the main challenges and promising avenues for future research.

Electrophysiological features of cortical 3D networks are deeply modulated by scaffold properties

Three-dimensionality (3D) was proven essential for developing reliable models for different anatomical compartments and many diseases. However, the neuronal compartment still poses a great challenge as we still do not understand precisely how the brain computes information and how the complex chain of neuronal events can generate conscious behavior. Therefore, a comprehensive model of neuronal tissue has not yet been found. The present work was conceived in this framework: we aimed to contribute to what must be a collective effort by filling in some information on possible 3D strategies to pursue. We compared directly different kinds of scaffolds (i.e., PDMS sponges, thermally crosslinked hydrogels, and glass microbeads) in their effect on neuronal network activity recorded using micro-electrode arrays. While the overall rate of spiking activity remained consistent, the type of scaffold had a notable impact on bursting dynamics. The frequency, density of bursts, and occurrence of random spikes were all affected. The examination of inter-burst intervals revealed distinct burst generation patterns unique to different scaffold types. Network burst propagation unveiled divergent trends among configurations. Notably, it showed the most differences, underlying that functional variations may arise from a different 3D spatial organization. This evidence suggests that not all 3D neuronal constructs can sustain the same level of richness of activity. Furthermore, we commented on the reproducibility, efficacy, and scalability of the methods, where the beads still offer superior performances. By comparing different 3D scaffolds, our results move toward understanding the best strategies to develop functional 3D neuronal units for reliable pre-clinical studies.