Construction and Evaluation of Small-Diameter Bioartificial Arteries Based on a Combined-Mold Technology

Arterial stenosis or blockage is the leading cause of cardiovascular disease, and the common solution is to substitute the arteries by autologous veins or bypass the blood vessels physically. With the development of science and technology, arteries with diameter larger than 6 mm can be substituted by unbiodegradable polymers, such as polytetrafluoroethylene, clinically. Nevertheless, the construction of a small-diameter (less than 6 mm) artery with living cells has always been a thorny problem. In this study, a suit of combined mold was designed and forged for constructing small-diameter arterial vessels. Based on this combined mold, bioactive arterial vessels containing adipose-derived stem cells (ASCs) and different growth factors (GFs) were assembled together to mimic the inner and middle layers of the natural arteries. Before assembling, ASCs and GFs were loaded into a gelatin/alginate hydrogel. To enhance the mechanical property of the bilayer arterial vessels, polylactic–glycolic acid (PLGA) was applied on the surface of the bilayer vessels to form the outer third layer. The biocompatibility, morphology and mechanical property of the constructed triple-layer arterial vessels were characterized. The morphological results manifested that cells grow well in the gelatin/alginate hydrogels, and ASCs were differentiated into endothelial cells (ECs) and smooth muscle cells (SMCs), respectively. In addition, under the action of shear stress produced by the flow of the culture medium, cells in the hydrogels with high density were connected to each other, similar to the natural vascular endothelial tissues (i.e., endothelia). Especially, the mechanical property of the triple-layer arterial vessels can well meet the anti-stress requirements as human blood vessels. In a word, a small-diameter arterial vessel was successfully constructed through the combined mold and has a promising application prospect as a clinical small-diameter vessel graft.

Synthetic Polymers for Organ 3D Printing

Three-dimensional (3D) printing, known as the most promising approach for bioartificial organ manufacturing, has provided unprecedented versatility in delivering multi-functional cells along with other biomaterials with precise control of their locations in space. The constantly emerging 3D printing technologies are the integration results of biomaterials with other related techniques in biology, chemistry, physics, mechanics and medicine. Synthetic polymers have played a key role in supporting cellular and biomolecular (or bioactive agent) activities before, during and after the 3D printing processes. In particular, biodegradable synthetic polymers are preferable candidates for bioartificial organ manufacturing with excellent mechanical properties, tunable chemical structures, non-toxic degradation products and controllable degradation rates. In this review, we aim to cover the recent progress of synthetic polymers in organ 3D printing fields. It is structured as introducing the main approaches of 3D printing technologies, the important properties of 3D printable synthetic polymers, the successful models of bioartificial organ printing and the perspectives of synthetic polymers in vascularized and innervated organ 3D printing areas.

An Interpenetrating Alginate/Gelatin Network for Three-Dimensional (3D) Cell Cultures and Organ Bioprinting

Crosslinking is an effective way to improve the physiochemical and biochemical properties of hydrogels. In this study, we describe an interpenetrating polymer network (IPN) of alginate/gelatin hydrogels (i.e., A-G-IPN) in which cells can be encapsulated for in vitro three-dimensional (3D) cultures and organ bioprinting. A double crosslinking model, i.e., using Ca²⁺ to crosslink alginate molecules and transglutaminase (TG) to crosslink gelatin molecules, is exploited to improve the physiochemical, such as water holding capacity, hardness and structural integrity, and biochemical properties, such as cytocompatibility, of the alginate/gelatin hydrogels. For the sake of convenience, the individual ionic (i.e., only treatment with Ca²⁺) or enzymatic (i.e., only treatment with TG) crosslinked alginate/gelatin hydrogels are referred as alginate-semi-IPN (i.e., A-semi-IPN) or gelatin-semi-IPN (i.e., G-semi-IPN), respectively. Tunable physiochemical and biochemical properties of the hydrogels have been obtained by changing the crosslinking sequences and polymer concentrations. Cytocompatibilities of the obtained hydrogels are evaluated through in vitro 3D cell cultures and bioprinting. The double crosslinked A-G-IPN hydrogel is a promising candidate for a wide range of biomedical applications, including bioartificial organ manufacturing, high-throughput drug screening, and pathological mechanism analyses.

Advanced Polymers for Three-Dimensional (3D) Organ Bioprinting

Three-dimensional (3D) organ bioprinting is an attractive scientific area with huge commercial profit, which could solve all the serious bottleneck problems for allograft transplantation, high-throughput drug screening, and pathological analysis. Integrating multiple heterogeneous adult cell types and/or stem cells along with other biomaterials (e.g., polymers, bioactive agents, or biomolecules) to make 3D constructs functional is one of the core issues for 3D bioprinting of bioartificial organs. Both natural and synthetic polymers play essential and ubiquitous roles for hierarchical vascular and neural network formation in 3D printed constructs based on their specific physical, chemical, biological, and physiological properties. In this article, several advanced polymers with excellent biocompatibility, biodegradability, 3D printability, and structural stability are reviewed. The challenges and perspectives of polymers for rapid manufacturing of complex organs, such as the liver, heart, kidney, lung, breast, and brain, are outlined.

3D Bioprinting of Adipose-Derived Stem Cells for Organ Manufacturing

Organ manufacturing is an attractive high-tech research field which can solve the serious donor shortage problems for allograft organ transplantation, high throughput drug screening, and energy metabolism model establishment. How to integrate heterogeneous cell types along with other biomaterials to form bioartificial organs is one of the kernel issues for organ manufacturing. At present, three-dimensional (3D) bioprinting of adipose-derives stem cell (ADSC) containing hydrogels has shown the most bright futures with respect to overcoming all the difficult problems encountered by tissue engineers over the last several decades. In this chapter, we briefly introduce the 3D ADSC bioprinting technologies for organ manufacturing, especially for the branched vascular network construction.

Direct-write and sacrifice-based techniques for vasculatures

Fabricating biomimetic vasculatures is considered one of the greatest challenges in tissue regeneration due to their complex structures across various length scales. Many strategies have been investigated on how to fabricate tissue-engineering vasculatures (TEVs), including vascular-like and vascularized structures that can replace their native counterparts. The advancement of additive manufacturing (AM) technologies has enabled a wide range of fabrication techniques that can directly-write TEVs with complex and delicate structures. Meanwhile, sacrifice-based techniques, which rely on the removal of encapsulated sacrificial templates to form desired cavity-like structures, have also been widely studied. This review will specifically focus on the two most promising methods in these recently developed technologies, which are the direct-write method and the sacrifice-based method. The performance, advantages, and shortcomings of each technique are analyzed and compared. In the discussion, we list current challenges in this field and present our vision of next-generation TEVs technologies. Perspectives on future research in this field are given at the end.

Bioartificial Organ Manufacturing Technologies

Bioartificial organ manufacturing technologies are a series of enabling techniques that can be used to produce human organs based on bionic principles. During the last ten years, significant progress has been achieved in the development of various organ manufacturing technologies. According to the degree of automation, organ manufacturing technologies can be divided into three main groups: (1) fully automated; (2) semi-automated; (3) handworked (or handmade); each has the advantages and disadvantages for bioartificial organ manufacturing. One of the most promising bioartificial organ manufacturing technologies is to use combined multi-nozzle three-dimensional printing techniques to automatically assemble personal cells along with other biomaterials to build exclusive organ substitutes for defective/failed human organs. This is the first time that advanced bioartificial organ manufacturing technologies have been reviewed. These technologies hold the promise to greatly improve the quality of health and average lifespan of human beings in the near future.

Natural Polymers for Organ 3D Bioprinting

Three-dimensional (3D) bioprinting, known as a promising technology for bioartificial organ manufacturing, has provided unprecedented versatility to manipulate cells and other biomaterials with precise control their locations in space. Over the last decade, a number of 3D bioprinting technologies have been explored. Natural polymers have played a central role in supporting the cellular and biomolecular activities before, during and after the 3D bioprinting processes. These polymers have been widely used as effective cell-loading hydrogels for homogeneous/heterogeneous tissue/organ formation, hierarchical vascular/neural/lymphatic network construction, as well as multiple biological/biochemial/physiological/biomedical/pathological functionality realization. This review aims to cover recent progress in natural polymers for bioartificial organ 3D bioprinting. It is structured as introducing the important properties of 3D printable natural polymers, successful models of 3D tissue/organ construction and typical technologies for bioartificial organ 3D bioprinting.

Fibrin Hydrogels for Endothelialized Liver Tissue Engineering with a Predesigned Vascular Network

The design and manufacture of a branched vascular network is essential for bioartificial organ implantation, which provides nutrients and removes metabolites for multi-cellular tissues. In the present study, we present a technology to manufacture endothelialized liver tissues using a fibrin hydrogel and a rotational combined mold. Both hepatocytes and adipose-derived stem cells (ADSCs) encapsulated in a fibrin hydrogel were assembled into a spindle construct with a predesigned multi-branched vascular network. An external overcoat of poly(dl-lactic-co-glycolic acid) was used to increase the mechanical properties of the construct as well as to act as an impervious and isolating membrane around the construct. Cell survivability reached 100% in the construct after 6 days of in vitro culture. ADSCs in the spindle construct were engaged into endothelial cells/tissues using a cocktail growth factor engagement approach. Mechanical property comparison and permeability evaluation tests all indicated that this was a viable complex organ containing more than two heterogeneous tissue types and a functional vascular network. It is, therefore, the first time an implantable bioartificial liver, i.e., endothelialized liver tissue, along with a hierarchical vascular network, has been created.

Gelatin-Based Hydrogels for Organ 3D Bioprinting

Three-dimensional (3D) bioprinting is a family of enabling technologies that can be used to manufacture human organs with predefined hierarchical structures, material constituents and physiological functions. The main objective of these technologies is to produce high-throughput and/or customized organ substitutes (or bioartificial organs) with heterogeneous cell types or stem cells along with other biomaterials that are able to repair, replace or restore the defect/failure counterparts. Gelatin-based hydrogels, such as gelatin/fibrinogen, gelatin/hyaluronan and gelatin/alginate/fibrinogen, have unique features in organ 3D bioprinting technologies. This article is an overview of the intrinsic/extrinsic properties of the gelatin-based hydrogels in organ 3D bioprinting areas with advanced technologies, theories and principles. The state of the art of the physical/chemical crosslinking methods of the gelatin-based hydrogels being used to overcome the weak mechanical properties is highlighted. A multicellular model made from adipose-derived stem cell proliferation and differentiation in the predefined 3D constructs is emphasized. Multi-nozzle extrusion-based organ 3D bioprinting technologies have the distinguished potential to eventually manufacture implantable bioartificial organs for purposes such as customized organ restoration, high-throughput drug screening and metabolic syndrome model establishment.

3D Bioprinting Technologies for Hard Tissue and Organ Engineering

Hard tissues and organs, including the bones, teeth and cartilage, are the most extensively exploited and rapidly developed areas in regenerative medicine field. One prominent character of hard tissues and organs is that their extracellular matrices mineralize to withstand weight and pressure. Over the last two decades, a wide variety of 3D printing technologies have been adapted to hard tissue and organ engineering. These 3D printing technologies have been defined as 3D bioprinting. Especially for hard organ regeneration, a series of new theories, strategies and protocols have been proposed. Some of the technologies have been applied in medical therapies with some successes. Each of the technologies has pros and cons in hard tissue and organ engineering. In this review, we summarize the advantages and disadvantages of the historical available innovative 3D bioprinting technologies for used as special tools for hard tissue and organ engineering.

Biodegradable Polymers and Stem Cells for Bioprinting

It is imperative to develop organ manufacturing technologies based on the high organ failure mortality and serious donor shortage problems. As an emerging and promising technology, bioprinting has attracted more and more attention with its super precision, easy reproduction, fast manipulation and advantages in many hot research areas, such as tissue engineering, organ manufacturing, and drug screening. Basically, bioprinting technology consists of inkjet bioprinting, laser-based bioprinting and extrusion-based bioprinting techniques. Biodegradable polymers and stem cells are common printing inks. In the printed constructs, biodegradable polymers are usually used as support scaffolds, while stem cells can be engaged to differentiate into different cell/tissue types. The integration of biodegradable polymers and stem cells with the bioprinting techniques has provided huge opportunities for modern science and technologies, including tissue repair, organ transplantation and energy metabolism.