The inkjet technique has the capability of generating droplets in the picoliter volume range, firing thousands of times in a few seconds and printing in the noncontact manner. Since its emergence, inkjet technology has been widely utilized in the publishing industry for printing of text and pictures. As the technology developed, its applications have been expanded from two-dimensional (2D) to three-dimensional (3D) and even used to fabricate components of electronic devices. At the end of the twentieth century, researchers were aware of the potential value of this technology in life sciences and tissue engineering because its picoliter-level printing unit is suitable for depositing biological components. Currently inkjet technology has been becoming a practical tool in modern medicine serving for drug development, scaffold building, and cell depositing. In this article, we first review the history, principles and different methods of developing this technology. Next, we focus on the recent achievements of inkjet printing in the biological field. Inkjet bioprinting of generic biomaterials, biomacromolecules, DNAs, and cells and their major applications are introduced in order of increasing complexity. The current limitations/challenges and corresponding solutions of this technology are also discussed. A new concept, biopixels, is put forward with a combination of the key characteristics of inkjet printing and basic biological units to bring a comprehensive view on inkjet-based bioprinting. Finally, a roadmap of the entire 3D bioprinting is depicted at the end of this review article, clearly demonstrating the past, present, and future of 3D bioprinting and our current progress in this field.

Three-dimensional (3D) bioprinting has become a flexible tool in regenerative medicine with potential for various applications. Further development of the new 3D bioprinting field lies in suitable bioink materials with satisfied printability, mechanical integrity, and biocompatibility. Natural polymers from marine resources have been attracting increasing attention in recent years, as they are biologically active and abundant when comparing to polymers from other resources. This review focuses on research and applications of marine biomaterials for 3D bioprinting. Special attention is paid to the mechanisms, material requirements, and applications of commonly used 3D bioprinting technologies based on marine-derived resources. Commonly used marine materials for 3D bioprinting including alginate, carrageenan, chitosan, hyaluronic acid, collagen, and gelatin are also discussed, especially in regards to their advantages and applications.

The ready-to-use, structure-supporting hydrogel bioink can shorten the time for ink preparation, ensure cell dispersion, and maintain the preset shape/microstructure without additional assistance during printing. Meanwhile, ink with high permeability might facilitate uniform cell growth in biological constructs, which is beneficial to homogeneous tissue repair. Unfortunately, current bioinks are hard to meet these requirements simultaneously in a simple way. Here, based on the fast dynamic crosslinking of aldehyde hyaluronic acid (AHA)/N-carboxymethyl chitosan (CMC) and the slow stable crosslinking of gelatin (GEL)/4-arm poly(ethylene glycol) succinimidyl glutarate (PEG-SG), we present a time-sharing structure-supporting (TSHSP) hydrogel bioink with high permeability, containing 1% AHA, 0.75% CMC, 1% GEL and 0.5% PEG-SG. The TSHSP hydrogel can facilitate printing with proper viscoelastic property and self-healing behavior. By crosslinking with 4% PEG-SG for only 3 min, the integrity of the cell-laden construct can last for 21 days due to the stable internal and external GEL/PEG-SG networks, and cells manifested long-term viability and spreading morphology. Nerve-like, muscle-like, and cartilage-like in vitro constructs exhibited homogeneous cell growth and remarkable biological specificities. This work provides not only a convenient and practical bioink for tissue engineering, targeted cell therapy, but also a new direction for hydrogel bioink development.

Skin acts as an essential barrier, protecting organisms from their environment. For skin trauma caused by accidental injuries, rapid healing, personalization, and functionality are vital requirements in clinical, which are the bottlenecks hindering the translation of skin repair from benchside to bedside. Herein, we described a novel design and a proof-of-concept demonstration of an adaptive bioprinting robot to proceed rapid in situ bioprinting on a full-thickness excisional wound in mice. The threedimensional (3D) scanning and closed-loop visual system integrated in the robot and the multi-degree-of-freedom mechanism provide immediate, precise, and complete wound coverage through stereotactic bioprinting, which hits the key requirements of rapid-healing and personalization in skin repair. Combined with the robot, epidermal stem cells and skin-derived precursors isolated from neonatal mice mixed with Matrigel were directly printed into the injured area to replicate the skin structure. Excisional wounds after bioprinting showed complete wound healing and functional skin tissue regeneration that closely resembling native skin, including epidermis, dermis, blood vessels, hair follicles and sebaceous glands etc. This study provides an effective strategy for skin repair through the combination of the novel robot and a bioactive bioink, and has a promising clinical translational potential for further applications.

Proliferation of HPSCs in vitro can promote its broad clinical therapeutic use. For in vitro co-culture, interaction between the stem cell and feeder cell as well as their spatial position are essential. To imitate the natural microenvironment, a 3D engineered scaffold for CD34 + cells co-culture was established via 3D bioprinting. Herein, the concentration of hydrogel and the ratio of two kinds of cells were optimized. Flow cytometry, real time PCR and RNA-seq technology were applied to analyze the effect of the engineered scaffold on expanded cells. After 10 days co-culture with the engineered scaffold, the expansion of CD34 + CD38 − cells can reach 33.57-folds and the expansion of CD34 + CD184 + cells can reach 16.66-folds. Result of PCR and RNA-seq indicates that the CD34 + cells in 3D group exhibited a tendency of interaction with the engineered scaffold. Compared to 2D co-culture, this customizable 3D engineered scaffold can provide an original and integrated environment for HPSCs growth. Additionally, this scaffold can be modified for different cell co-culture or cell behavior study. Umbilical cord blood (UC) is considered to be an alternative hematopoietic stem cell source for curing patients with hematologic diseases by allogeneic hematopoietic cell transplantation 1. However, the limited number of transplantable hematopoietic progenitor or stem cells (HPSCs) is the major obstacle to broad clinical application 2,3. To date, human HPSCs expansion in vitro is still a challenge because of their easy differentiation during culture 4. In nature, HPSCs reside in a complex microenvironment, known as niche, containing multiple components of the extracellular matrix (ECM) and various stromal cells including mesenchymal stem cells, adipocytes, osteoblasts, endothelial cells 5,6. In order to achieve HPSCs expansion in vitro, several strategies, including adding soluble cytokines 7-9 , culturing with stromal cells 10,11 and structuring the 3D environment with different materials 12-15 were utilized. All the strategies mentioned above were attempts to mimic the natural conditions for HPSC growth. In recent years, several studies had been processed to prove the superiority of threedimensional (3D) culture systems in HPSCs proliferation and stemness maintenance 10,13,14,16-19. Although these studies made certain achievements, they also had limitations to achieve the natural niche construction due to the multi-cellular composition and complex structure. For instance, in Raic's study, a porous polyethylene glycol (PEG) hydrogel scaffold was fabricated for 3D co-culture of HPSCs and mesenchymal stem cells (MSCs), but the cells were seeded on the surface of scaffold with a top-down strategy 16. Moreover, the expansion of stromal cells in the co-culture system may causes the nutrient competition of cells against the expansion of HPSCs. To restrain the growth of stromal cells, methods like mitomycin C treatment 20 and irradiation 21 have been developed for the pre-treatment of feeder cells to restrain their growth. However, ...