**1. Introduction**

In nature, all the complicated phenomena of life, including human organs, are the result of biochemical and biophysical changes of molecules (or materials at a molecular level). For example, small organic molecules or compounds combine to form larger polymers (macromolecules or biomacromolecules). Macromolecules arrange in specific ways to form cells with organelles inside the cell membrane (Figure 1). While homogeneous cells organize into tissues, heterogeneous cells aggregate into organs with particular physiological functions. It has taken, in some cases, thousands of years to evolve from tiny organic molecules to microcells, mesotissues, and macro-organs [1–3].

At present, organ failure is the main cause of mortality all over the world. Despite the rapid development in pharmacological, interventional, and surgical therapies during the last several decades, the only cure for organ failure is allograft organ transplantation, which is seriously limited by issues, such as donor organ shortage, life-long immune rejection, and ethical conflict [4–6].

Three-dimensional (3D) organ bioprinting is the utilization of 3D printing technologies to assemble multiple cell types or stem cells/growth factors along with other biomaterials in a layer-by-layer fashion to produce bioartificial organs that maximally imitate their natural counterparts [7–9]. Traditionally, 3D printing is named rapid prototyping (RP), solid freeform fabrication (SFF), or additive manufacturing (AM) based on the dispersion–accumulation (i.e., discrete–accumulation) principle of computer-aided manufacturing (CAM) techniques. Before 3D printing, an object can be divided into numerous two-dimensional (2D) layers with a defined thickness. These 2D layers can be piled up by selectively adding the desired materials in a highly reproductive layer-by-layer manner under the instruction of computer-aided design (CAD) models [10–13]. Patient-specific organ image data, such as computerized

tomography (CT) and magnetic resonance imaging (MRI), can be easily transferred into CAD models for customized organ manufacturing with predefined geometrical shapes, biomaterial constituents, and physiological functions [14–17].

**Figure 1.** Different levels of materials (or molecules) existing in the human body, from small organic molecules or compounds to larger polymers (i.e., macromolecules or biomacromolecules), cells, tissues, organs, and systems in organisms.

The 3D organ bioprinting procedure involves changes to the properties of a series of materials at molecular, cell, tissue, and organ levels. It is an emerging new interdisciplinary field that needs cooperation of many fields of science and technology, such as biomaterials, biology, physics, chemistry, computers, mechanics, bioinformatics, and medicine (Figure 2).

**Figure 2.** Relationships of organ manufacturing, including three-dimensional (3D) bioprinting, with other pertinent fields of science and technology.

During the last 16 years, a large variety of 3D bioprinting technologies have been exploited, which has led to the emergence of fully automatic manufacturing of bioartificial organs for wide biomedical applications, such as high-throughput drug screening, controlled cell transplantation, customized organ repair/regeneration/replacement/restoration, pathological mechanism analysis, metabolism model establishment, and living tissue/organ cryopreservation [10–20]. Based on the working principles, these technologies can be classified into four major groups: inkjet-based, extrusion-based, laser-based, and their combinations (Figure 3). Each of the former three groups has advantages and disadvantages in bioartificial organ manufacturing.

**Figure 3.** Graphical description of 3D bioprinting types [10–20]. Images reproduced with permission from [10–20].

Several series of unique automatic and semiautomatic bioartificial organ manufacturing technologies have been created in my own group with the proper integration of modern high technologies, including computer, biology (e.g., cells and stem cells), biomaterials (e.g., polymers), chemistry, mechanics, and medicine. With these unique high technologies, we have solved all the bottleneck problems that have perplexed tissue engineers and other scientists for more than 6–7 decades, such as large-scale tissue/organ manufacturing [21–24], hierarchical vascular/nerve network construction with a fully endothelialized inner surface and antisuture/antistress capabilities [25–29], step-by-step adipose-derived stem cell (ASC) differentiation in a 3D construct [30–32], long-term preservation of bioartificial tissues/organs [33–35], in vitro metabolism model establishment [36,37], high-throughput drug screening [38–40], in vivo biocompatibilities of implanted biomaterials [41–43]. Several polymers have played essential and ubiquitous roles for bioartificial organ manufacturing with the incorporation of multiple cell types, stem cells/growth factors, and hierarchical vascular and neural networks with antisuture and antistress functions.

### **2. Role of Polymers in 3D Organ Bioprinting**

3D organ bioprinting is not a simple and easy engineering approach. Like building a nuclear plant, it requires intermingling of intricate architectural design, appropriate biomaterial selection, special building process, multicellular incorporation, supportive structure utilization, controllable stem cell induction, simultaneous or sequential tissue formation and maturation, multiple tissue coordination, and the means of coordinating these procedures to form large-scale living organs [25–33].

Polymers are large molecules made up of many small and identical repeating units bonded by covalent bonds [44]. The smallest repeating unit in a polymer is called a monomer, while the number of repeat units in a polymer chain is termed the degree of polymerization (DP) or chain length. Polymers can be divided into natural and synthetic groups according to their origin. Most natural polymers are water-soluble and endowed with some common biological and physiological properties, such as being pliable as soft tissues and organs, friendly for cell encapsulation and transplantation, and easy to handle and reshape. The properties of synthetic polymers depend on many factors, such as processing conditions, molecular weights, monomer distributions, chain structures (e.g., size, geometry, inter/intrabonding, and branching), and the presence of additives. In particular, linear polymers have long chains (or backbones) that contain small chemical groups on the repeating units.

Hydrogels are 3D hydrophilic networks of polymers that can absorb and retain large amounts of water and gel under certain physical (e.g., thermosensitive), chemical (e.g., covalent bonding), or biochemical (e.g., enzymatic) conditions. The physical and chemical properties of hydrogels can be designed for specific biomedical applications by selecting proper polymer components, inorganic solvents, and gelation protocols [45–47]. Compared with other states of polymers, hydrogels can provide a benign and stable environment for living cells to grow, migrate, aggregate, proliferate, and differentiate inside. The integration of hydrogels with 3D bioprinting technologies has offered numerous attractive features for complex organ manufacturing [48–50].

During the 3D organ bioprinting process, different polymers have different roles and functions. Most natural polymers, which are employed as the main components of bioinks, have the following roles: (1) provide cells and bioactive agents with support such as accommodation; (2) build vascular, neural, and lymphatic networks as semipermeatable substrates for nutrient, gas (e.g., oxygen), metabolite, and biosignal exchange; (3) guide homogeneous and heterogeneous histogenesis and organogenesis in a predefined way; and (4) promote tissue and organ maturation under specific biochemical and biophysical conditions. Meanwhile, most synthetic polymers are applied for the following functions: (1) improve multicellular handling or allotting in space to mimic their natural counterparts; (2) enhance the mechanical properties of vascular and neural networks with antisuture and antistress capabilities; (3) commit (or complete) extra functions, such as sacrificing supports and protecting covers.

Globally, the literature has reported on a number of polymers for use in 3D bioprinting. These are summarized in Figure 4 [51–74]. These polymers need to meet several basic requirements for 3D printing of bioartificial organs and subsequently clinical applications (Figure 5): (1) biocompatible (i.e., nontoxic or no obvious toxicity, no or low immunological reaction); (2) bioprintable using 3D bioprinters; (3) biostable or crosslinkable with strong enough mechanical properties; (4) biodegradable (in particular, the biodegradation rate should match the new tissue/organ generation speed); (5) suturable with host vascular and nerve networks; (6) permeable for nutrients and gases; (7) biostorable before being printed; and (8) sterilizable.

**Figure 4.** Polymers that have been used for tissue and 3D organ printing.

**Figure 5.** Basic requirements for selecting a polymer for 3D bioprinting of bioartificial organs.

In the following sections, seven normal polymers that have been frequently employed in 3D organ bioprinting with excellent biocompatibility, biodegradability, biostability, and bioprintability are individually analyzed according to their natural or synthetic properties.

### **3. Natural Polymers for 3D Organ Bioprinting**

Natural polymers are widely existent in animal, plant, and microbe tissues as the main components of extracellular matrices (ECMs) or decellularized extracellular matrices (dECMs). These polymers include (1) proteins, such as collagen I–V, elastin, keratin, actin, tubulin, and myosin; (2) polysaccharides, such as chitin, alginate, and starch; (3) glycoproteins, such as mucin, lectin, miraculin, transferrin, and nectin; (4) proteoglycans, such as decorin, syndecan, versican, betaglycan, lumican, and fibromodulin. Compared with synthetic polymers, most natural polymers dissolve in inorganic solvents, such as water, phosphate bu ffer saline (PBS), and Dulbecco's modified Eagle medium (DMEM), which are cell-friendly. Few of the natural polymer solutions or hydrogels can be used directly as cell-loading matrices for 3D organ bioprinting.

Due to the special physical, chemical, and biological properties, most natural polymer solutions or hydrogels cannot be printed alone with a sol–gel transformation taking place during the 3D printing processes [25–33]. These polymers are often used as additives for several theromsensitive or chemical crosslinkable polymer (e.g., gelatin, agar/agarose, and alginate) solutions or hydrogels for 3D bioprinting. The 3D bioprintability of natural polymers are mainly determined by the molecular weight, viscosity, hydrophilicity, and crosslinkability of the polymer solutions or hydrogels. The 3D bioprinting accuracy of the cell-laden natural polymer solutions or hydrogels depends largely on the polymer concentration, viscoelasticity, gelation speed, and shear thinning behavior.

The main advantages of natural polymers for 3D organ bioprinting is that they can entrap viable cells and bioactive agents before printing, protect cells and bioactive agents during 3D printing, and form semipermeable substrates after 3D printing. Before 3D printing, cells and bioactive agents are normally embedded in natural polymer solutions or hydrogels. During 3D printing, the natural polymer chains can safeguard cells from printing stress and provide cells with predesigned 3D milieus similar to those in a native organ. After 3D printing, the polymer chains can be physically/chemically/enzymatically crosslinked to form semipermeable substrates. These semipermeable substrates are permeable to nutrients, gases (e.g., oxygen), and metabolites of cells.

At present, the most frequently used natural polymers for 3D organ bioprinting are collagen, gelatin, alginate, fibrinogen, starch, hyaluronan, chitosan, silk, dextran, agar (or agarose), and matrigel (or dECM). Among these polymers, alginate, gelatin, fibrinogen, and dECM are the most promising candidates for 3D organ bioprinting.
