Engineering pH-Responsive, Self-Healing Vesicle-Type Artificial Tissues with Higher-Order Cooperative Functionalities.

Multicellular organisms demonstrate a hierarchical organization where multiple cells collectively form tissues, thereby enabling higher-order cooperative functionalities beyond the capabilities of individual cells. Drawing inspiration from this biological organization, assemblies of multiple protocells are developed to create novel functional materials with emergent higher-order cooperative functionalities. This paper presents new artificial tissues derived from multiple vesicles, which serve as protocellular models. These tissues are formed and manipulated through non-covalent interactions triggered by a salt bridge. Exhibiting pH-sensitive reversible formation and destruction under neutral conditions, these artificial vesicle tissues demonstrate three distinct higher-order cooperative functionalities: transportation of large cargoes, photo-induced contractions, and enhanced survivability against external threats. The rapid assembly and disassembly of these artificial tissues in response to pH variations enable controlled mechanical task performance. Additionally, the self-healing property of these artificial tissues indicates robustness against external mechanical damage. The research suggests that these vesicles can detect specific pH environments and spontaneously assemble into artificial tissues with advanced functionalities. This leads to the possibility of developing intelligent materials with high environmental specificity, particularly for applications in soft robotics.

Artificial tissue creation under microgravity conditions: Considerations and future applications.

Traditional tissue engineering methods often fail to promote robust cell growth and differentiation, limiting the development of functioning tissues. However, the microgravity conditions created by rotating wall vessel bioreactors minimize shear stress and unload the gravitational force usually placed on cells. In a microgravity environment, cell proliferation, cell differentiation, and the 3D organization of cells are altered, potentially encouraging the formation of more biosimilar artificial tissues for certain cell types. Additionally, cells in these engineered tissues display lowered immunogenicity, pointing to the transplantation potential of tissues engineered in microgravity conditions. However, these benefits are not consistent across all cell types, and the long-term impact of microgravity on tissue development and stability remains an unanswered question. Even so, there is potential that with further research, microgravity tissue engineering will have productive clinical applications for medical and pharmaceutical purposes.

Biomaterials for extrusion-based bioprinting and biomedical applications.

Amongst the range of bioprinting technologies currently available, bioprinting by material extrusion is gaining increasing popularity due to accessibility, low cost, and the absence of energy sources, such as lasers, which may significantly damage the cells. New applications of extrusion-based bioprinting are systematically emerging in the biomedical field in relation to tissue and organ fabrication. Extrusion-based bioprinting presents a series of specific challenges in relation to achievable resolutions, accuracy and speed. Resolution and accuracy in particular are of paramount importance for the realization of microstructures (for example, vascularization) within tissues and organs. Another major theme of research is cell survival and functional preservation, as extruded bioinks have cells subjected to considerable shear stresses as they travel through the extrusion apparatus. Here, an overview of the main available extrusion-based printing technologies and related families of bioprinting materials (bioinks) is provided. The main challenges related to achieving resolution and accuracy whilst assuring cell viability and function are discussed in relation to specific application contexts in the field of tissue and organ fabrication.

Current aspects of organoid technology for biomaterial toxicity analysis.

Organoids provide us an opportunity to understand how diseases affect cellular physiology, human tissues or organs. They are indespensible tools for biomaterial toxicity analysis, drug discovery and regenerative medicine.

Bioengineering of High Cell Density Tissues with Hierarchical Vascular Networks for Ex Vivo Whole Organs.

The development of tissue-like structures such as cell sheets, spheroids, and organoids has contributed to progress in regenerative medicine. Simultaneous achievement of scale up and high cell density of these tissues is challenging because sufficient oxygen cannot be supplied to the inside of large, high cell density tissues. Here, in vitro fabrication of vessels to supply oxygen to the inside of millimeter-sized scaffold-free tissues whose cell density is ≈200 million cells mL-1 , corresponding to those of native tissues, is shown. Hierarchical vascular networks by anastomosis of capillaries and a large vessel are essential for oxygen supply, whereas a large vessel or capillary networks alone make negligible contributions to the supply. The hierarchical vascular networks are formed by a top-down approach, which offers a new option for ex vivo whole organs without decellularization and 3D-bioprinting.

Versatile Phospholipid Assemblies for Functional Synthetic Cells and Artificial Tissues.

The bottom-up construction of a synthetic cell from nonliving building blocks capable of mimicking cellular properties and behaviors helps to understand the particular biophysical properties and working mechanisms of a cell. A synthetic cell built in this way possesses defined chemical composition and structure. Since phospholipids are native biomembrane components, their assemblies are widely used to mimic cellular structures. Here, recent developments in the formation of versatile phospholipid assemblies are described, together with the applications of these assemblies for functional membranes (protein reconstituted giant unilamellar vesicles), spherical and nonspherical protoorganelles, and functional synthetic cells, as well as the high-order hierarchical structures of artificial tissues. Their biomedical applications are also briefly summarized. Finally, the challenges and future directions in the field of synthetic cells and artificial tissues based on phospholipid assemblies are proposed.

Freeform 3D printing of vascularized tissues: Challenges and strategies.

In recent years, freeform three-dimensional (3D) printing has led to significant advances in the fabrication of artificial tissues with vascularized structures. This technique utilizes a supporting matrix that holds the extruded printing ink and ensures shape maintenance of the printed 3D constructs within the prescribed spatial precision. Since the printing nozzle can be translated omnidirectionally within the supporting matrix, freeform 3D printing is potentially applicable for the fabrication of complex 3D objects, incorporating curved, and irregular shaped vascular networks. To optimize freeform 3D printing quality and performance, the rheological properties of the printing ink and supporting matrix, and the material matching between them are of paramount importance. In this review, we shall compare conventional 3D printing and freeform 3D printing technologies for the fabrication of vascular constructs, and critically discuss their working principles and their advantages and disadvantages. We also provide the detailed material information of emerging printing inks and supporting matrices in recent freeform 3D printing studies. The accompanying challenges are further discussed, aiming to guide freeform 3D printing by the effective design and selection of the most appropriate materials/processes for the development of full-scale functional vascularized artificial tissues.

Progress in 3D bioprinting technology for tissue/organ regenerative engineering.

Escalating cases of organ shortage and donor scarcity worldwide are alarming reminders of the need for alternatives to allograft tissues. Within the last three decades, research efforts in the field of regenerative medicine and tissue engineering continue to address the unmet need for artificial tissues and organs for transplant. Work in the field has evolved to create what we consider a new field, Regenerative Engineering, defined as the Convergence of advanced materials science, stem cell science, physics, developmental biology and clinical translation towards the regeneration of complex tissues and organ systems. Included in the regenerative engineering paradigm is advanced manufacturing. Three-dimensional (3D) bioprinting is a promising and innovative biofabrication strategy to precisely position biologics, including living cells and extracellular matrix (ECM) components, in the prescribed 3D hierarchal organization to create artificial multi-cellular tissues/organs. In this review, we outline recent progress in several bioprinting technologies used to engineer scaffolds with requisite mechanical, structural, and biological complexity. We examine the process parameters affecting bioprinting and bioink-biomaterials and review notable studies on bioprinted skin, cardiac, bone, cartilage, liver, lung, neural, and pancreatic tissue. We also focus on other 3D bioprinting application areas including cancer research, drug testing, high-throughput screening (HTS), and organ-on-a-chip models. We also highlight the current challenges associated with the clinical translation of 3D bioprinting and conclude with the future perspective of bioprinting technology.

3D bioprinting of hydrogel-based biomimetic microenvironments.

Three-dimensional (3D) bioprinting is a promising technology to produce cell-laden constructs via patterning living cells, biological factors and biomaterials in a precisely controlled manner. However, it is still a challenge to fabricate human tissues/organs with biological functions for clinical application via 3D bioprinting. Several key issues should be carefully addressed to overcome this challenge, specifically the construction of biomimetic microenvironments. 3D printing has been broadly demonstrated the ability to create structures mimicking native tissues, while it also has the capability to produce biomimetic microenvironments. Therefore, this review will give an overview of the current advances in the art of building and controlling hydrogel-based biomimetic microenvironments in cells-laden 3D bioprinting, which are classified by their physical, chemical, and biological features. In the end, we will elaborate the outlook of 3D bioprinting of biomimetic microenvironment. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1695-1705, 2019.

3D Bioprinting of Artificial Tissues: Construction of Biomimetic Microstructures.

It is promising that artificial tissues/organs for clinical application can be produced via 3D bioprinting of living cells and biomaterials. The construction of microstructures biomimicking native tissues is crucially important to create artificial tissues with biological functions. For instance, the fabrication of vessel-like networks to supply cells with initial nutrient and oxygen, and the arrangement of multiple types of cells for creating lamellar/complex tissues through 3D bioprinting are widely reported. The current advances in 3D bioprinting of artificial tissues from the view of construction of biomimetic microstructures, especially the fabrication of lamellar, vascular, and complex structures are summarized. In the end, the conclusion and perspective of 3D bioprinting for clinical applications are elaborated.

Taking a deep look: modern microscopy technologies to optimize the design and functionality of biocompatible scaffolds for tissue engineering in regenerative medicine.

This review focuses on modern nonlinear optical microscopy (NLOM) methods that are increasingly being used in the field of tissue engineering (TE) to image tissue non-invasively and without labelling in depths unreached by conventional microscopy techniques. With NLOM techniques, biomaterial matrices, cultured cells and their produced extracellular matrix may be visualized with high resolution. After introducing classical imaging methodologies such as µCT, MRI, optical coherence tomography, electron microscopy and conventional microscopy two-photon fluorescence (2-PF) and second harmonic generation (SHG) imaging are described in detail (principle, power, limitations) together with their most widely used TE applications. Besides our own cell encapsulation, cell printing and collagen scaffolding systems and their NLOM imaging the most current research articles will be reviewed. These cover imaging of autofluorescence and fluorescence-labelled tissue and biomaterial structures, SHG-based quantitative morphometry of collagen I and other proteins, imaging of vascularization and online monitoring techniques in TE. Finally, some insight is given into state-of-the-art three-photon-based imaging methods (e.g. coherent anti-Stokes Raman scattering, third harmonic generation). This review provides an overview of the powerful and constantly evolving field of multiphoton microscopy, which is a powerful and indispensable tool for the development of artificial tissues in regenerative medicine and which is likely to gain importance also as a means for general diagnostic medical imaging.