Tunable anisotropic networks for 3-D oriented neural tissue models

Biodegradable Honeycomb-Mimic Scaffolds Consisting of Nanofibrous Walls

The scaffold is a porous three-dimensional (3D) material that supports cell growth and tissue regeneration. Such 3D structures should be generated with simple techniques and nontoxic ingredients to mimic bio-environment and facilitate tissue regeneration. In this work, simple but powerful techniques are demonstrated for the fabrication of lamellar and honeycomb-mimic scaffolds with poly(L-lactic acid). The honeycomb-mimic scaffolds with tunable pore size ranging from 70 to 160 µm are fabricated by crystal needle-guided thermally induced phase separation in a directional freezing apparatus. The compressive modulus of the honeycomb-mimic scaffold is ≈4 times higher than that of scaffold with randomly oriented pore structure. The fabricated honeycomb-mimic scaffold exhibits a hierarchical structure from nanofibers to micro-/macro-tubular structures. Pre-osteoblast MC3T3-E1 cells cultured on the honeycomb-mimic nanofibrous scaffolds exhibit an enhanced osteoblastic phenotype, with elevated expression levels of osteogenic marker genes, than those on either porous lamellar scaffolds or porous scaffolds with randomly oriented pores. The advanced techniques for the fabrication of the honeycomb-mimic structure may potentially be used for a wide variety of advanced functional materials.

Magnetically anisotropic hydrogels for tissue engineering

Many soft tissues of the human body possess hierarchically anisotropic structures, exhibiting orientation-specific mechanical properties and biological functionality. Hydrogels have been proposed as promising scaffold materials for tissue engineering applications due to their water-rich composition, excellent biocompatibility, and tunable physico-chemical properties. However, conventional hydrogels with homogeneous structures often exhibit isotropic properties that differ from those of biological tissues, limiting their further application. Recently, magnetically anisotropic hydrogels containing long-range ordered magneto-structures have attracted widespread interest owing to their highly controllable assembly strategy, rapid magnetic responsiveness and remote spatiotemporal regulation. In this review, we summarize the latest progress of magnetically anisotropic hydrogels for tissue engineering. The fabrication strategy of magnetically anisotropic hydrogels from magnetic nanofillers with different dimensions is systemically introduced. Then, the effects of magnetically anisotropic cues on the physicochemical properties of hydrogels and the cellular biological behavior are discussed. And the applications of magnetically anisotropic hydrogels in the engineering of different tissues are emphasized. Finally, the current challenges and the future perspectives for magnetically anisotropic hydrogels are presented.

Anisotropic 3D scaffolds for spinal cord guided repair: Current concepts

A spinal cord injury (SCI) can be caused by unforeseen events such as a fall, a vehicle accident, a gunshot, or a malignant illness, which has a significant impact on the quality of life of the patient. Due to the limited regenerative potential of the central nervous system (CNS), SCI is one of the most daunting medical challenges of modern medicine. Great advances have been made in tissue engineering and regenerative medicine, which include the transition from two-dimensional (2D) to three-dimensional (3D) biomaterials. Combinatory treatments that use 3D scaffolds may significantly enhance the repair and regeneration of functional neural tissue. In an effort to mimic the chemical and physical properties of neural tissue, scientists are researching the development of the ideal scaffold made of synthetic and/or natural polymers. Moreover, in order to restore the architecture and function of neural networks, 3D scaffolds with anisotropic properties that replicate the native longitudinal orientation of spinal cord nerve fibres are being designed. In an effort to determine if scaffold anisotropy is a crucial property for neural tissue regeneration, this review focuses on the most current technological developments relevant to anisotropic scaffolds for SCI. Special consideration is given to the architectural characteristics of scaffolds containing axially oriented fibres, channels, and pores. By analysing neural cell behaviour in vitro and tissue integration and functional recovery in animal models of SCI, the therapeutic efficacy is evaluated for its successes and limitations.

Closed-loop vasculature network design for bioprinting large, solid tissue scaffolds

Vascularization is an indispensable requirement for fabricating large solid tissues and organs. The natural vasculature derived from medical imaging modalities for large tissues and organs are highly complex and convoluted. However, the present bioprinting capabilities limit the fabrication of such complex natural vascular networks. Simplified bioprinted vascular networks, on the other hand, lack the capability to sustain large solid tissues. This work proposes a generalized and adaptable numerical model to design the vasculature by utilizing the tissue/organ anatomy. Starting with processing the patient's medical images, organ structure, tissue-specific cues, and key vasculature tethers are determined. An open-source abdomen magnetic resonance image dataset was used in this work. The extracted properties and cues are then used in a mathematical model for guiding the vascular network formation comprising arterial and venous networks. Next, the generated three-dimensional networks are used to simulate the nutrient transport and consumption within the organ over time and the regions deprived of the nutrients are identified. These regions provide cues to evolve and optimize the vasculature in an iterative manner to ensure the availability of the nutrient transport throughout the bioprinted scaffolds. The mass transport of six components of cell culture media-glucose, glycine, glutamine, riboflavin, human serum albumin, and oxygen was studied within the organ with designed vasculature. As the vascular structure underwent iterations, the organ regions deprived of these key components decreased significantly highlighting the increase in structural complexity and efficacy of the designed vasculature. The numerical method presented in this work offers a valuable tool for designing vascular scaffolds to guide the cell growth and maturation of the bioprinted tissues for faster regeneration post bioprinting.

3D hydrogel models of the neurovascular unit to investigate blood-brain barrier dysfunction

The neurovascular unit (NVU), consisting of neurons, glial cells, vascular cells (endothelial cells, pericytes and vascular smooth muscle cells (VSMCs)) together with the surrounding extracellular matrix (ECM), is an important interface between the peripheral blood and the brain parenchyma. Disruption of the NVU impacts on blood-brain barrier (BBB) regulation and underlies the development and pathology of multiple neurological disorders, including stroke and Alzheimer's disease (AD). The ability to differentiate induced pluripotent stem cells (iPSCs) into the different cell types of the NVU and incorporate them into physical models provides a reverse engineering approach to generate human NVU models to study BBB function. To recapitulate the in vivo situation such NVU models must also incorporate the ECM to provide a 3D environment with appropriate mechanical and biochemical cues for the cells of the NVU. In this review, we provide an overview of the cells of the NVU and the surrounding ECM, before discussing the characteristics (stiffness, functionality and porosity) required of hydrogels to mimic the ECM when incorporated into in vitro NVU models. We summarise the approaches available to measure BBB functionality and present the techniques in use to develop robust and translatable models of the NVU, including transwell models, hydrogel models, 3D-bioprinting, microfluidic models and organoids. The incorporation of iPSCs either without or with disease-specific genetic mutations into these NVU models provides a platform in which to study normal and disease mechanisms, test BBB permeability to drugs, screen for new therapeutic targets and drugs or to design cell-based therapies.

3DICE coding matrix multidirectional macro-architecture modulates cell organization, shape, and co-cultures endothelization network

Natural extracellular matrix governs cells providing biomechanical and biofunctional outstanding properties, despite being porous and mostly made of soft materials. Among organs, specific tissues present specialized macro-architectures. For instance, hepatic lobules present radial organization, while vascular sinusoids are branched from vertical veins, providing specific biofunctional features. Therefore, it is imperative to mimic such structures while modeling tissues. So far, there is limited capability of coupling oriented macro-structures with interconnected micro-channels in programmable long-range vertical and radial sequential orientations. Herein, a three-directional ice crystal elongation (3DICE) system is presented to code geometries in cryogels. Using 3DICE, guided ice crystals growth templates vertical and radial pores through bulky cryogels. Translucent isotropic and anisotropic architectures of radial or vertical pores are fabricated with tunable mechanical response. Furthermore, 3D combinations of vertical and radial pore orientations are coded at the centimeter scale. Cell morphological response to macro-architectures is demonstrated. The formation of endothelial segments, CYP450 activity, and osteopontin expression, as liver fibrosis biomarkers, present direct response and specific cellular organization within radial, linear, and random architectures. These results unlock the potential of ice-templating demonstrating the relevance of macro-architectures to model tissues, and broad possibilities for drug testing, tissue engineering, and regenerative medicine.

Porous aligned ZnSr-doped β-TCP/silk fibroin scaffolds using ice-templating method for bone tissue engineering applications

The bone is a complex and dynamic structure subjected to constant stress and remodeling. Due to the worldwide incidence of bone disorders, tissue scaffolds and engineered bone tissues have emerged as solutions for bone grafting, which require sophisticated scaffolding architectures while keeping high mechanical performance. However, the conjugation of a bone-like scaffold architecture with efficient mechanical properties is still a critical challenge for biomedical applications. In this sense, the present study focused on the modulating the architecture of silk fibroin (SF) scaffolds crosslinked with horseradish peroxidase and mixed with zinc (Zn) and strontium (Sr)-doped β-tricalcium phosphate (ZnSr.TCP) to mimic bone structures. The ZnSr.TCP-SF hydrogels were tuned by programmable ice-templating parameters, and further freeze-dried, in order to obtain 3D scaffolds with controlled pore orientation. The results showed interconnected channels in the ZnSr.TCP-SF scaffolds that mimic the porous network of the native subchondral bone matrix. The architecture of the scaffolds was characterized by microCT, showing tunable pore size according to freezing temperatures (-196 °C: ∼80.2 ± 20.5 µm; -80 °C: ∼73.1 ± 20.5 µm; -20 °C: ∼104.7 ± 33.7 µm). The swelling ratio, weight loss, and rheological properties were also assessed, revealing efficient scaffold integrity and morphology after aqueous immersion. Thus, the ZnSr.TCP-SF scaffolds made of aligned porous structure were developed as affordable candidates for future applications in clinical osteoregeneration and in vitro bone tissue modelling.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

Reversible Design of Dynamic Assemblies at Small Scales

Emerging bottom-up fabrication methods have enabled the assembly of synthetic colloids, microrobots, living cells, and organoids to create intricate structures with unique properties that transcend their individual components. This review provides an access point to the latest developments in externally driven assembly of synthetic and biological components. In particular, we emphasize reversibility, which enables the fabrication of multiscale systems that would not be possible under traditional techniques. Magnetic, acoustic, optical, and electric fields are the most promising methods for controlling the reversible assembly of biological and synthetic subunits since they can reprogram their assembly by switching on/off the external field or shaping these fields. We feature capabilities to dynamically actuate the assembly configuration by modulating the properties of the external stimuli, including frequency and amplitude. We describe the design principles which enable the assembly of reconfigurable structures. Finally, we foresee that the high degree of control capabilities offered by externally driven assembly will enable broad access to increasingly robust design principles towards building advanced dynamic intelligent systems.

Advances in 3D neural, vascular and neurovascular models for drug testing and regenerative medicine

Clinical trials continue to fall short regarding drugs to effectively treat brain-affecting diseases. Although there are many causes of these shortcomings, the most relevant are the inability of most therapeutic agents to cross the blood-brain barrier (BBB) and the failure to translate effects from animal models to patients. In this review, we analyze the most recent developments in BBB, neural, and neurovascular models, analyzing their impact on the drug development process by considering their quantitative and phenotypical characterization. We offer a perspective of the state-of-the-art of the models that could revolutionize the pharmaceutical industry.

Multiscale brain research on a microfluidic chip

One major challenge in current brain research is generating an integrative understanding of the brain's functions and disorders from its multiscale neuronal architectures and connectivity. Thus, innovative neurotechnology tools are urgently required for deciphering the multiscale functional and structural organizations of the brain at hierarchical scales from the molecular to the organismal level by multiple brain research initiatives launched by the European Union, United States, Australia, Canada, China, Korea, and Japan. To meet this demand, microfluidic chips (μFCs) have rapidly evolved as a trans-scale neurotechnological toolset to enable multiscale studies of the brain due to their unique advantages in flexible microstructure design, multifunctional integration, accurate microenvironment control, and capacity for automatic sample processing. Here, we review the recent progress in applying innovative μFC-based neuro-technologies to promote multiscale brain research and uniquely focus on representative applications of μFCs to address challenges in brain research at each hierarchical level. We discuss the current trend of combinational applications of μFCs with other neuro- and biotechnologies, including optogenetics, brain organoids, and 3D bioprinting, for better multiscale brain research. In addition, we offer our insights into the existing outstanding questions at each hierarchical level of brain research that could potentially be addressed by advancing microfluidic techniques. This review will serve as a timely guide for bioengineers and neuroscientists to develop and apply μFC-based neuro-technologies for promoting basic and translational brain research.

Dynamic Culture Systems and 3D Interfaces Models for Cancer Drugs Testing

The mass use of biological agents for pharmaceutical purposes started with the development and distribution of vaccines, followed by the industrial production of antibiotics. The use of dynamic systems, such as bioreactors, had been already applied in the food industry in fermentation processes and started being used for the development of pharmaceutical agents from this point on. In the last decades, the use of bioreactors and microfluidic systems has been expanded in different fields. The emergence of the tissue engineering led to the development of in vitro models cultured in dynamic systems. This is particularly relevant considering the urgent reduction of the total dependence on animal disease models that is undermining the development of novel drugs, using alternatively human-based models to make the drug discovery process more reliable. The failure out coming from animal models has been more prevalent in certain types of cancer, such as glioblastoma multiform and in high-grade metastatic cancers like bone metastasis of breast or prostatic cancer. The difficulty in obtaining novel drugs for these purposes is mostly linked to the barriers around the tumors, which these bioactive molecules have to overcome to become effective. For that reason, the individualized study of each interface is paramount and is only realistic once applying human-based samples (e.g. cells or tissues) in three-dimensions for in vitro modeling under dynamic conditions. In this chapter, the most recent approaches to model these interfaces in 3D systems will be explored, highlighting their major contributions to the field. In this section, these systems' impact on increased knowledge in relevant aspects of cancer aggressiveness as invasive or motile cellular capacity, or even resistance to chemotherapeutic agents will have particular focus. The last section of this chapter will focus on the integration of the tumor interfaces in dynamic systems, particularly its application on high-throughput drug screening. The industrial translation of such platforms will be discussed, as well as the main upcoming challenges and future perspectives.

Enhancing cell packing in buckyballs by acoustofluidic activation

How to pack materials into well-defined volumes efficiently has been a longstanding question of interest to physicists, material scientists, and mathematicians as these materials have broad applications ranging from shipping goods in commerce to seeds in agriculture and to spheroids in tissue engineering. How many marbles or gumball candies can you pack into a jar? Although these seem to be idle questions they have been studied for centuries and have recently become of greater interest with their broadening applications in science and medicine. Here, we study a similar problem where we try to pack cells into a spherical porous buckyball structure. The experimental limitations are short of the theoretical maximum packing density due to the microscale of the structures that the cells are being packed into. We show that we can pack more cells into a confined micro-structure (buckyball cage) by employing acoustofluidic activation and their hydrodynamic effect at the bottom of a liquid-carrier chamber compared to randomly dropping cells onto these buckyballs by gravity. Although, in essence, cells would be expected to achieve a higher maximum volume fraction than marbles in a jar, given that they can squeeze and reshape and reorient their structure, the packing density of cells into the spherical buckyball cages are far from this theoretical limit. This is mainly dictated by the experimental limitations of cells washing away as well as being loaded into the chamber.

Soft Ring-Shaped Cellu-Robots with Simultaneous Locomotion in Batches

Untethered mini-robots can move single cells or aggregates to build complex constructs in confined spaces and may enable various biomedical applications such as regenerative repair in medicine and biosensing in bioengineering. However, a significant challenge is the ability to control multiple microrobots simultaneously in the same space to operate toward a common goal in a distributed operation. A locomotion strategy that can simultaneously guide the formation and operation of multiple robots in response to a common acoustic stimulus is developed. The scaffold-free cellu-robots comprise only highly packed cells and eliminate the influence of supportive materials, making them less cumbersome during locomotion. The ring shape of the cellu-robot contributes to anisotropic cellular interactions which induce radial cellular orientation. Under a single stimulus, several cellu-robots form predetermined complex structures such as bracelet-like ring-chains which transform into a single new living entity through cell-cell interactions, migration or cellular extensions between cellu-robots.

Recent Advances in Natural Gum-Based Biomaterials for Tissue Engineering and Regenerative Medicine: A Review

The engineering of tissues under a three-dimensional (3D) microenvironment is a great challenge and needs a suitable supporting biomaterial-based scaffold that may facilitate cell attachment, spreading, proliferation, migration, and differentiation for proper tissue regeneration or organ reconstruction. Polysaccharides as natural polymers promise great potential in the preparation of a three-dimensional artificial extracellular matrix (ECM) (i.e., hydrogel) via various processing methods and conditions. Natural polymers, especially gums, based upon hydrogel systems, provide similarities largely with the native ECM and excellent biological response. Here, we review the origin and physico-chemical characteristics of potentially used natural gums. In addition, various forms of scaffolds (e.g., nanofibrous, 3D printed-constructs) based on gums and their efficacy in 3D cell culture and various tissue regenerations such as bone, osteoarthritis and cartilage, skin/wound, retinal, neural, and other tissues are discussed. Finally, the advantages and limitations of natural gums are precisely described for future perspectives in tissue engineering and regenerative medicine in the concluding remarks.

Photosensitive Hydrogel Creates Favorable Biologic Niches to Promote Spinal Cord Injury Repair

Photochemistry is considered to be a promising strategy for hydrogels to mimic the complex and dynamic properties of natural extracellular matrix. However, it is seldom applied in 3D tissue engineering and regenerative medicine due to the attenuation of light. In this study, phenyl azide photchemistry and optical fiber technology are first used to localize adhesive protein on the inner surface of the nerve guidance conduit in a 3D hydrogel scaffold. In vitro coculture assay of neural stem cells (NSCs) shows that photoimmobilization of collagen significantly improves the adhesion and survival of NSCs in the conduit, and exhibits synergistic effect with the sustainable release of growth factor. After implantation in transected spinal cord, the optimized hydrogel scaffold is found to improve the locomotion recovery of rats 12 weeks after spinal cord injury (SCI). Histological analysis suggests that the designed hydrogel scaffold provides a favorable biological niche for neuronal regeneration, thus producing directional neuron tissue and promoting the repair of SCI. This study demonstrates a promising hydrogel scaffold for SCI repair and provides the first understanding of the photoimmobilization of adhesive protein in a 3D hydrogel conduit concerning its functions on spinal cord tissue restoration.

Scaffolding Strategies for Tissue Engineering and Regenerative Medicine Applications

During the past two decades, tissue engineering and the regenerative medicine field have invested in the regeneration and reconstruction of pathologically altered tissues, such as cartilage, bone, skin, heart valves, nerves and tendons, and many others. The 3D structured scaffolds and hydrogels alone or combined with bioactive molecules or genes and cells are able to guide the development of functional engineered tissues, and provide mechanical support during in vivo implantation. Naturally derived and synthetic polymers, bioresorbable inorganic materials, and respective hybrids, and decellularized tissue have been considered as scaffolding biomaterials, owing to their boosted structural, mechanical, and biological properties. A diversity of biomaterials, current treatment strategies, and emergent technologies used for 3D scaffolds and hydrogel processing, and the tissue-specific considerations for scaffolding for Tissue engineering (TE) purposes are herein highlighted and discussed in depth. The newest procedures focusing on the 3D behavior and multi-cellular interactions of native tissues for further use for in vitro model processing are also outlined. Completed and ongoing preclinical research trials for TE applications using scaffolds and hydrogels, challenges, and future prospects of research in the regenerative medicine field are also presented.

Quantitative ultrasound imaging of cell-laden hydrogels and printed constructs

In the present work we have revisited the application of quantitative ultrasound imaging (QUI) to cellular hydrogels, by using the reference phantom method (RPM) in combination with a local attenuation compensation algorithm. The investigated biological samples consisted of cell-laden collagen hydrogels with PC12 neural cells. These cell-laden hydrogels were used to calibrate the integrated backscattering coefficient (IBC) as a function of cell density, which was then used to generate parametric images of local cell density. The image resolution used for QUI and its impact on the relative IBC error was also investigated. Another important contribution of our work was the monitoring of PC12 cell proliferation. The cell number estimates obtained via the calibrated IBC compared well with data obtained using a conventional quantitative method, the MTS assay. Evaluation of spectral changes as a function of culture time also provided additional information on the cell cluster size, which was found to be in close agreement with that observed by microscopy. Last but not least, we also applied QUI on a 3D printed cellular construct in order to illustrate its capabilities for the evaluation of bioprinted structures. STATEMENT OF SIGNIFICANCE: While there is intensive research in the areas of polymer science, biology, and 3D bio-printing, there exists a gap in available characterisation tools for the non-destructive inspection of biological constructs in the three-dimensional domain, on the macroscopic scale, and with fast data acquisition times. Quantitative ultrasound imaging is a suitable characterization technique for providing essential information on the development of tissue engineered constructs. These results provide a detailed and comprehensive guide on the capabilities and limitations of the technique.

3-D geometry and irregular connectivity dictate neuronal firing in frequency domain and synchronization

The replication of the complex structure and three dimensional (3-D) interconnectivity of neurons in the brain is a great challenge. A few 3-D neuronal patterning approaches have been developed to mimic the cell distribution in the brain but none have demonstrated the relationship between 3-D neuron patterning and network connectivity. Here, we used photolithographic crosslinking to fabricate in vitro 3-D neuronal structures with distinct sizes, shapes or interconnectivities, i.e., milli-blocks, micro-stripes, separated micro-blocks and connected micro-blocks, which have spatial confinement from "Z" dimension to "XYZ" dimension. During a 4-week culture period, the 3-D neuronal system has shown high cell viability, axonal, dendritic, synaptic growth and neural network activity of cortical neurons. We further studied the calcium oscillation of neurons in different 3-D patterns and used signal processing both in Fast Fourier Transform (FFT) and time domain (TD) to model the fluorescent signal variation. We observed that the firing frequency decreased as the spatial confinement in 3-D system increased. Besides, the neuronal synchronization significantly decreased by irregularly connecting micro-blocks, indicating that network connectivity can be adjusted by changing the linking conditions of 3-D gels. Earlier works showed the importance of 3-D culture over 2-D in terms of cell growth. Here, we showed that not only 3-D geometry over 2-D culture matters, but also the spatial organization of cells in 3-D dictates the neuronal firing frequency and synchronicity.