Anisotropic Materials for Skeletal‐Muscle‐Tissue Engineering

Repair of damaged skeletal‐muscle tissue is limited by the regenerative capacity of the native tissue. Current clinical approaches are not optimal for the treatment of large volumetric skeletal‐muscle loss. As an alternative, tissue engineering represents a promising approach for the functional restoration of damaged muscle tissue. A typical tissue‐engineering process involves the design and fabrication of a scaffold that closely mimics the native skeletal‐muscle extracellular matrix (ECM), allowing organization of cells into a physiologically relevant 3D architecture. In particular, anisotropic materials that mimic the morphology of the native skeletal‐muscle ECM, can be fabricated using various biocompatible materials to guide cell alignment, elongation, proliferation, and differentiation into myotubes. Here, an overview of fundamental concepts associated with muscle‐tissue engineering and the current status of muscle‐tissue‐engineering approaches is provided. Recent advances in the development of anisotropic scaffolds with micro‐ or nanoscale features are reviewed, and how scaffold topographical, mechanical, and biochemical cues correlate to observed cellular function and phenotype development is examined. Finally, some recent developments in both the design and utility of anisotropic materials in skeletal‐muscle‐tissue engineering are highlighted, along with their potential impact on future research and clinical applications.     

Crosslinking Strategies for 3D Bioprinting of Polymeric Hydrogels

Three‐dimensional (3D) bioprinting has recently advanced as an important tool to produce viable constructs that can be used for regenerative purposes or as tissue models. To develop biomimetic and sustainable 3D constructs, several important processing aspects need to be considered, among which crosslinking is most important for achieving desirable biomechanical stability of printed structures, which is reflected in subsequent behavior and use of these constructs. In this work, crosslinking methods used in 3D bioprinting studies are reviewed, parameters that affect bioink chemistry are discussed, and the potential toward improving crosslinking outcomes and construct performance is highlighted. Furthermore, current challenges and future prospects are discussed. Due to the direct connection between crosslinking methods and properties of 3D bioprinted structures, this Review can provide a basis for developing necessary modifications to the design and manufacturing process of advanced tissue‐like constructs in future.     

A finite element model of skeletal muscles

The present paper surveys recent developments in constitutive and computational modelling of skeletal muscles, concerning mainly the generalization to two- and three-dimensional (2D, 3D) continuum deformation analysis of typical one-dimensional (1D) Hill-type muscle models. Extending our previous work in the field and recent contributions by other authors, we describe a constitutive model for skeletal muscles that incorporates all the features of the 3 typical elements (parallel elastic, series elastic and contractile elements) in Hill's muscle model. In particular the proposed incompressible transversely isotropic model incorporates: a multiplicative split of the fibre stretch into contractile and (series) elastic stretches; the possibility of energy storage in the series elastic element; the dependence of the contractile stress on the strain rate; the governing equation of activation dynamics, so that general histories of neural stimulation may be taken as input data. The resulting 2D or 3D constitutive equations are implemented as user subroutines in the large deformation finite element software package ABAQUS. Simple numerical tests are presented and discussed, as well as an example that involves passive or active deformations of a pelvic floor muscle using shell finite elements.     

3D Bioprinting: from Benches to Translational Applications

Over the last decades, the fabrication of 3D tissues has become commonplace in tissue engineering and regenerative medicine. However, conventional 3D biofabrication techniques such as scaffolding, microengineering, and fiber and cell sheet engineering are limited in their capacity to fabricate complex tissue constructs with the required precision and controllability that is needed to replicate biologically relevant tissues. To this end, 3D bioprinting offers great versatility to fabricate biomimetic, volumetric tissues that are structurally and functionally relevant. It enables precise control of the composition, spatial distribution, and architecture of resulting constructs facilitating the recapitulation of the delicate shapes and structures of targeted organs and tissues. This Review systematically covers the history of bioprinting and the most recent advances in instrumentation and methods. It then focuses on the requirements for bioinks and cells to achieve optimal fabrication of biomimetic constructs. Next, emerging evolutions and future directions of bioprinting are discussed, such as freeform, high‐resolution, multimaterial, and 4D bioprinting. Finally, the translational potential of bioprinting and bioprinted tissues of various categories are presented and the Review is concluded by exemplifying commercially available bioprinting platforms.     

A finite element model of skeletal muscles

The present paper surveys recent developments in constitutive and computational modelling of skeletal muscles, concerning mainly the generalization to two- and three-dimensional (2D, 3D) continuum deformation analysis of typical one-dimensional (1D) Hill-type muscle models. Extending our previous work in the field and recent contributions by other authors, we describe a constitutive model for skeletal muscles that incorporates all the features of the 3 typical elements (parallel elastic, series elastic and contractile elements) in Hill's muscle model. In particular the proposed incompressible transversely isotropic model incorporates: a multiplicative split of the fibre stretch into contractile and (series) elastic stretches; the possibility of energy storage in the series elastic element; the dependence of the contractile stress on the strain rate; the governing equation of activation dynamics, so that general histories of neural stimulation may be taken as input data. The resulting 2D or 3D constitutive equations are implemented as user subroutines in the large deformation finite element software package ABAQUS. Simple numerical tests are presented and discussed, as well as an example that involves passive or active deformations of a pelvic floor muscle using shell finite elements.     

4D Biofabrication Using Shape‐Morphing Hydrogels

Despite the tremendous potential of bioprinting techniques toward the fabrication of highly complex biological structures and the flourishing progress in 3D bioprinting, the most critical challenge of the current approaches is the printing of hollow tubular structures. In this work, an advanced 4D biofabrication approach, based on printing of shape‐morphing biopolymer hydrogels, is developed for the fabrication of hollow self‐folding tubes with unprecedented control over their diameters and architectures at high resolution. The versatility of the approach is demonstrated by employing two different biopolymers (alginate and hyaluronic acid) and mouse bone marrow stromal cells. Harnessing the printing and postprinting parameters allows attaining average internal tube diameters as low as 20 µm, which is not yet achievable by other existing bioprinting/biofabrication approaches and is comparable to the diameters of the smallest blood vessels. The proposed 4D biofabrication process does not pose any negative effect on the viability of the printed cells, and the self‐folded hydrogel‐based tubes support cell survival for at least 7 d without any decrease in cell viability. Consequently, the presented 4D biofabrication strategy allows the production of dynamically reconfigurable architectures with tunable functionality and responsiveness, governed by the selection of suitable materials and cells.     

Human-Recombinant-Elastin-Based Bioinks for 3D Bioprinting of Vascularized Soft Tissues

Bioprinting has emerged as an advanced method for fabricating complex 3D tissues. Despite the tremendous potential of 3D bioprinting, there are several drawbacks of current bioinks and printing methodologies that limit  the ability to print elastic and highly vascularized tissues. In particular, fabrication of complex biomimetic structure that are entirely based on 3D bioprinting is still challenging primarily due to the lack of suitable bioinks with high printability, biocompatibility, biomimicry, and proper mechanical properties. To address these shortcomings, in this work the use of recombinant human tropoelastin as a highly biocompatible and elastic bioink for 3D printing of complex soft tissues is demonstrated. As proof of the concept, vascularized cardiac constructs are bioprinted and their functions are assessed in vitro and in vivo. The printed constructs demonstrate endothelium barrier function and spontaneous beating of cardiac muscle cells, which are important functions of cardiac tissue in vivo. Furthermore, the printed construct elicits minimal inflammatory responses, and is shown to be efficiently biodegraded in vivo when implanted subcutaneously in rats. Taken together, these results demonstrate the potential of the elastic bioink for printing 3D functional cardiac tissues, which can eventually be used for cardiac tissue replacement.     

Responsive biomaterials for 3D bioprinting: A review

Three-dimensional (3D) bioprinting enables a controlled deposition of cells, biomaterials, and biological compounds (i.e., bioinks) to build complex 3D biological models, biological living systems, and therapeutic products. Developing responsive biomaterials as novel bioinks has been a central focus of research in the field of bioprinting because of their controllable material properties in response to printing-induced external or internal stimuli. In this review, we highlight the most recent advances of responsive biomaterials for 3D bioprinting applications. We review commonly used stimuli-responsive biomaterials and strategies for utilizing multifunctional responsiveness to achieve desirable printability, structural formability, cell viability, and construct bioactivity for 3D bioprinting. We also summarize major bioink formulation strategies currently adopted in 3D bioprinting. We subsequently discuss several promising applications of 3D printing involving responsive biomaterials, such as bioprinting in a supporting bath, 4D bioprinting, and bioprinting new controlled drug delivery systems. Future perspectives on the design and development of novel multifunctional bioinks based on responsive biomaterials and technological innovations are also presented.     

A Review on the 3D Printing of Functional Structures for Medical Phantoms and Regenerated Tissue and Organ Applications

Medical models, or “phantoms,” have been widely used for medical training and for doctor-patient interactions. They are increasingly used for surgical planning, medical computational models, algorithm verification and validation, and medical devices development. Such new applications demand high-fidelity, patient-specific, tissue-mimicking medical phantoms that can not only closely emulate the geometric structures of human organs, but also possess the properties and functions of the organ structure. With the rapid advancement of three-dimensional (3D) printing and 3D bioprinting technologies, many researchers have explored the use of these additive manufacturing techniques to fabricate functional medical phantoms for various applications. This paper reviews the applications of these 3D printing and 3D bioprinting technologies for the fabrication of functional medical phantoms and bio-structures. This review specifically discusses the state of the art along with new developments and trends in 3D printed functional medical phantoms (i.e., tissue-mimicking medical phantoms, radiologically relevant medical phantoms, and physiological medical phantoms) and 3D bio-printed structures (i.e., hybrid scaffolding materials, convertible scaffolds, and integrated sensors) for regenerated tissues and organs.     

Airflow‐Assisted 3D Bioprinting of Human Heterogeneous Microspheroidal Organoids with Microfluidic Nozzle

Hydrogel microspheroids are widely used in tissue engineering, such as injection therapy and 3D cell culture, and among which, heterogeneous microspheroids are drawing much attention as a promising tool to carry multiple cell types in separated phases. However, it is still a big challenge to fabricate heterogeneous microspheroids that can reconstruct built‐up tissues' microarchitecture with excellent resolution and spatial organization in limited sizes. Here, a novel airflow‐assisted 3D bioprinting method is reported, which can print versatile spiral microarchitectures inside the microspheroids, permitting one‐step bioprinting of fascinating hydrogel structures, such as the spherical helix, rose, and saddle. A microfluidic nozzle is developed to improve the capability of intricate cell encapsulation with heterotypic contact. Complex structures, such as a rose, Tai chi pattern, and single cell line can be easily printed in spheroids. The theoretical model during printing is established and process parameters are systematically investigated. As a demonstration, a human multicellular organoid of spirally vascularized ossification is reconstructed with this method, which shows that it is a powerful tool to build mini tissues on microspheroids.     

Microfluidics‐Enabled Multimaterial Maskless Stereolithographic Bioprinting

A stereolithography‐based bioprinting platform for multimaterial fabrication of heterogeneous hydrogel constructs is presented. Dynamic patterning by a digital micromirror device, synchronized by a moving stage and a microfluidic device containing four on/off pneumatic valves, is used to create 3D constructs. The novel microfluidic device is capable of fast switching between different (cell‐loaded) hydrogel bioinks, to achieve layer‐by‐layer multimaterial bioprinting. Compared to conventional stereolithography‐based bioprinters, the system provides the unique advantage of multimaterial fabrication capability at high spatial resolution. To demonstrate the multimaterial capacity of this system, a variety of hydrogel constructs are generated, including those based on poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacryloyl (GelMA). The biocompatibility of this system is validated by introducing cell‐laden GelMA into the microfluidic device and fabricating cellularized constructs. A pattern of a PEGDA frame and three different concentrations of GelMA, loaded with vascular endothelial growth factor, are further assessed for its neovascularization potential in a rat model. The proposed system provides a robust platform for bioprinting of high‐fidelity multimaterial microstructures on demand for applications in tissue engineering, regenerative medicine, and biosensing, which are otherwise not readily achievable at high speed with conventional stereolithographic biofabrication platforms.     

Neue Methoden zur Beurteilung der Betriebsfestigkeit im Fahrzeugauslegungs‐ und ‐absicherungsprozess

Advanced durability evaluation in vehicle design and validation process The modern process of evaluation and validation conducted in the automotive industry uses experimental, metrological, and calculation‐based methods. Offering various examples, the present paper describes new developments in the determination and evaluation of operating strength, particularly in terms of virtual methods and their application in practice. The first point considered is the virtual determination of load data, the second is the improvement of calculated fatigue life. Two current examples in the development of methods are presented in this context: The first example examines the inhomogeneity of materials in calculating aluminium castings. The second example describes the approach taken in the configuration of components made of short‐fibre‐reinforced polymers, applying a new method of calculation.     

Electro‐Assisted Bioprinting of Low‐Concentration GelMA Microdroplets

Low‐concentration gelatin methacryloyl (GelMA) has excellent biocompatibility to cell‐laden structures. However, it is still a big challenge to stably fabricate organoids (even microdroplets) using this material due to its extremely low viscosity. Here, a promising electro‐assisted bioprinting method is developed, which can print low‐concentration pure GelMA microdroplets with low cost, low cell damage, and high efficiency. With the help of electrostatic attraction, uniform GelMA microdroplets measuring about 100 μm are rapidly printed. Due to the application of lower external forces to separate the droplets, cell damage during printing is negligible, which often happens in piezoelectric or thermal inkjet bioprinting. Different printing states and effects of printing parameters (voltages, gas pressure, nozzle size, etc.) on microdroplet diameter are also investigated. The fundamental properties of low‐concentration GelMA microspheres are subsequently studied. The results show that the printed microspheres with 5% w/v GelMA can provide a suitable microenvironment for laden bone marrow stem cells. Finally, it is demonstrated that the printed microdroplets can be used in building microspheroidal organoids, in drug controlled release, and in 3D bioprinting as biobricks. This method shows great potential use in cell therapy, drug delivery, and organoid building.     

Visible Light-Induced 3D Bioprinting Technologies and Corresponding Bioink Materials for Tissue Engineering: A Review

Three-dimensional (3D) bioprinting based on traditional 3D printing is an emerging technology that is used to precisely assemble biocompatible materials and cells or bioactive factors into advanced tissue engineering solutions. Similar technology, particularly photo-cured bioprinting strategies, plays an important role in the field of tissue engineering research. The successful implementation of 3D bioprinting is based on the properties of photopolymerized materials. Photocrosslinkable hydrogel is an attractive biomaterial that is polymerized rapidly and enables process control in space and time. Photopolymerization is frequently initiated by ultraviolet (UV) or visible light. However, UV light may cause cell damage and thereby, affect cell viability. Thus, visible light is considered to be more biocompatible than UV light for bioprinting. In this review, we provide an overview of photo curing-based bioprinting technologies, and describe a visible light crosslinkable bioink, including its crosslinking mechanisms, types of visible light initiator, and biomedical applications. We also discuss existing challenges and prospects of visible light-induced 3D bioprinting devices and hydrogels in biomedical areas.     

The Next Frontier of 3D Bioprinting: Bioactive Materials Functionalized by Bacteria

3D bioprinting has become a flexible technical means used in many fields. Currently, research on 3D bioprinting is mainly focused on the use of mammalian cells to print organ and tissue models, which has greatly promoted progress in the fields of tissue engineering, regenerative medicine, and pharmaceuticals. In recent years, bacterial bioprinting has gradually become a rapidly developing research fields, with a wide range of potential applications in basic research, biomedicine, bioremediation, and other field. Here, this works reviews new research on bacterial bioprinting, and discuss its future research direction.     

High Throughput Screening of Cell Mechanical Response Using a Stretchable 3D Cellular Microarray Platform

Cells in vivo are constantly subjected to multiple microenvironmental mechanical stimuli that regulate cell function. Although 2D cell responses to the mechanical stimulation have been established, these methods lack relevance as physiological cell microenvironments are in 3D. Moreover, the existing platforms developed for studying the cell responses to mechanical cues in 3D either offer low‐throughput, involve complex fabrication, or do not allow combinatorial analysis of multiple cues. Considering this, a stretchable high‐throughput (HT) 3D cell microarray platform is presented that can apply dynamic mechanical strain to cells encapsulated in arrayed 3D microgels. The platform uses inkjet‐bioprinting technique for printing cell‐laden gelatin methacrylate (GelMA) microgel array on an elastic composite substrate that is periodically stretched. The developed platform is highly biocompatible and transfers the applied strain from the stretched substrate to the cells. The HT analysis is conducted to analyze cell mechano‐responses throughout the printed microgel array. Also, the combinatorial analysis of distinct cell behaviors is conducted for different GelMA microenvironmental stiffnesses in addition to the dynamic stretch. Considering its throughput and flexibility, the developed platform can readily be scaled up to introduce a wide range of microenvironmental cues and to screen the cell responses in a HT way.     

Ideal Three‐Dimensional Electrode Structures for Electrochemical Energy Storage

Three‐dimensional electrodes offer great advantages, such as enhanced ion and electron transport, increased material loading per unit substrate area, and improved mechanical stability upon repeated charge–discharge. The origin of these advantages is discussed and the criteria for ideal 3D electrode structure are outlined. One of the common features of ideal 3D electrodes is the use of a 3D carbon‐ or metal‐based porous framework as the structural backbone and current collector. The synthesis methods of these 3D frameworks and their composites with redox‐active materials are summarized, including transition metal oxides and conducting polymers. The structural characteristics and electrochemical performances are also reviewed. Synthesis of composite 3D electrodes is divided into two types — template‐assisted and template‐free methods — depending on whether a pre‐made template is required. The advantages and drawbacks of both strategies are discussed.     

Progress in 3D Printing of Carbon Materials for Energy‐Related Applications

The additive‐manufacturing (AM) technique, known as three‐dimensional (3D) printing, has attracted much attention in industry and academia in recent years. 3D printing has been developed for a variety of applications. Printable inks are the most important component for 3D printing, and are related to the materials, the printing method, and the structures of the final 3D‐printed products. Carbon materials, due to their good chemical stability and versatile nanostructure, have been widely used in 3D printing for different applications. Good inks are mainly based on volatile solutions having carbon materials as fillers such as graphene oxide (GO), carbon nanotubes (CNT), carbon blacks, and solvent, as well as polymers and other additives. Studies of carbon materials in 3D printing, especially GO‐based materials, have been extensively reported for energy‐related applications. In these circumstances, understanding the very recent developments of 3D‐printed carbon materials and their extended applications to address energy‐related challenges and bring new concepts for material designs are becoming urgent and important. Here, recent developments in 3D printing of emerging devices for energy‐related applications are reviewed, including energy‐storage applications, electronic circuits, and thermal‐energy applications at high temperature. To close, a conclusion and outlook are provided, pointing out future designs and developments of 3D‐printing technology based on carbon materials for energy‐related applications and beyond.     

Recent Advances in Engineering Bioinks for 3D Bioprinting

The 3D bioprinting can controllably deposit bioink containing cells and fabricate complex bionic tissue structures in a fast and scalable way, which is expected to completely change the scenario of clinical organ transplantation. Bioprinting holds broad application prospect in tissue engineering, life sciences, and clinical medicine. In the process of 3D bioprinting, bioink, as the carrier of cells and bioactive substances, influences cell activity and accuracy of organ structure after printing. To better understand and design bioink, in this review, the concept, development, and basic composition of bioink are introduced, while focusing on the advantages and disadvantages of various biomaterials, and the use of common cells and biomolecules that constitute bioink. In addition, the properties and applications of various stimuli-responsive smart materials for 4D bioprinting are mentioned. The challenges and development trends of bioink are also summarized.     

Sociotechnical alignment in biomedicine: The 3D bioprinting market beyond technology convergence

The nature of hybrid technologies has been frequently interpreted with the concept of technology convergence. However, this concept tends to highlight only technical aspects of technology and market evolution. In order to provide a more comprehensive picture, the concept of sociotechnical alignment is explored here.The field of 3D bioprinting (the production of biological structures with automated, computer-controlled bioprinters) is focused on here. In the emergent global bioprinting market, companies have relied on three core technologies (tissue engineering, additive manufacturing, and software development) and continue to receive inputs from other technologies.On the biological side, bioprinting has benefited from new approaches such as the use of induced pluripotent stem cells. On the engineering side, it has been possible to use relatively cheap technologies such as open-source processing Arduino boards. On the software side, the proliferation of open source packages has strengthened the possibilities of bioprinting. The combination between these and other technology fringes involves a process of sociotechnical alignment whereby technical, scientific, and political issues are always at play.As a result, different companies have been able to realize different market strategies, having varied geographical reach. However, the first movements towards extensive globalization can also be noticed. In this way, the current diversity of the bioprinting market may be jeopardized in the years to come.