Fabrication of fully aligned self-assembled cell-laden collagen filaments for tissue engineering via a hybrid bioprinting process

Cell-laden structures play a pivotal role in various tissue engineering applications, particularly in tissue restoration. Interactions between cells within bioprinted structures are crucial for successful tissue development and regulation of stem cell fate through intricate cell-to-cell signaling pathways. In this study, we developed a new technique that combines polyethylene glycol (PEG)-infused submerged bioprinting with a stretching procedure. This approach facilitated the generation of fully aligned collagen structures consisting of myoblasts and a low concentration (2 wt%) of collagen to efficiently encourage muscle tissue regeneration. By adjusting several processing parameters, we obtained biologically safe and mechanically stable cell-laden collagen filaments with uniaxial alignment. Notably, the cell filaments exhibited markedly elevated cellular activities compared to those exhibited by conventional bioprinted filaments, even at similar cell densities. Moreover, when we implanted structures containing adipose stem cells into mice, we observed a significantly increased level of myogenesis compared to that in normally bioprinted struts. Thus, this promising approach has the potential to revolutionize tissue engineering by fostering enhanced cellular interactions and promoting improved outcomes in regenerative medicine.

Enhanced Vascular-like Network Formation of Encapsulated HUVECs and ADSCs Coculture in Growth Factors Conjugated GelMA Hydrogels

Tissue engineering primarily aimed to alleviate the insufficiency of organ donations worldwide. Nonetheless, the survival of the engineered tissue is often compromised due to the complexity of the natural organ architectures, especially the vascular system inside the organ, which allows food-waste transfer. Thus, vascularization within the engineered tissue is of paramount importance. A critical aspect of this endeavor is the ability to replicate the intricacies of the extracellular matrix and promote the formation of functional vascular networks within engineered constructs. In this study, human adipose-derived stem cells (hADSCs) and human umbilical vein endothelial cells (HUVECs) were cocultured in different types of gelatin methacrylate (GelMA). In brief, pro-angiogenic signaling growth factors (GFs), vascular endothelial growth factor (VEGF165) and basic fibroblast growth factor (bFGF), were conjugated onto GelMA via an EDC/NHS coupling reaction. The GelMA hydrogels conjugated with VEGF165 (GelMA@VEGF165) and bFGF (GelMA@bFGF) showed marginal changes in the chemical and physical characteristics of the GelMA hydrogels. Moreover, the conjugation of these growth factors demonstrated improved cell viability and cell proliferation within the hydrogel construct. Additionally, vascular-like network formation was observed predominantly on GelMA@GrowthFactor (GelMA@GF) hydrogels, particularly on GelMA@bFGF. This study suggests that growth factor-conjugated GelMA hydrogels would be a promising biomaterial for 3D vascular tissue engineering.

The one-step fabrication of porous hASC-laden GelMa constructs using a handheld printing system

The fabrication of highly porous cell-loaded structures in tissue engineering applications has been a challenging issue because non-porous cell-laden struts can cause severe cell necrosis in the middle region owing to poor transport of nutrients and oxygen. In this study, we propose a versatile handheld 3D printer for the effective fabrication of porous cell-laden methacrylated gelatin (GelMa) with high porosity (≈97%) by air injection and a bubble-making system using mesh filters through which a mixture of air/GelMa bioink is passed. In particular, the pore size and foamability of the cell constructs could be manipulated using various processing parameters (rheological properties of GelMa, filter size and number, and air-bioink volume ratio). To demonstrate the feasibility of the cell construct as a tissue engineering substitute for muscle regeneration, in vitro cellular activities and in vivo regeneration ability of human adipose stem cells were assessed. The in vitro results demonstrated that the human adipose stem cells (hASCs) fabricated using the handheld 3D printer were alive and well-proliferated. Furthermore, the in vivo results showed that the hASCs-constructs directly printed from the handheld 3D printer showed significant restoration of functionality and efficient muscle regeneration in the volumetric muscle loss model of mice. Based on these results, the fabrication method of the porous cell-laden construct could be a promising tool for regenerating muscle tissues.

Bioprinted hASC ‐laden collagen/ HA constructs with meringue‐like macro/micropores

Extrusion‐based bioprinting is one of the most effective methods for fabricating cell‐laden mesh structures. However, insufficient cellular activities within the printed cylindrical cell‐matrix blocks, inducing low cell‐to‐cell interactions due to the disturbance of the matrix hydrogel, remain to be addressed. Hence, various sacrificial materials or void‐forming methods have been used; however, most of them cannot solve the problem completely or require complicated fabricating procedures. Herein, we suggest a bioprinted cell‐laden collagen/hydroxyapatite (HA) construct comprising meringue‐like porous cell‐laden structures to enhance osteogenic activity. A porous bioink is generated using a culinary process, i.e., the whipping method, and the whipping conditions, such as the material concentration, time, and speed, are selected appropriately. The constructs fabricated using the meringue‐like bioink with MG63 cells and human adipose stem cells exhibit outstanding metabolic and osteogenic activities owing to the synergistic effects of the efficient cell‐to‐cell interactions and HA stimulation released from the porous structure. The in vitro cellular responses indicate that the meringue‐like collagen bioink for achieving an extremely porous cell‐laden construct can be a highly promising cell‐laden material for various tissue regeneration applications.

Isotropic and Anisotropic Scaffolds for Tissue Engineering: Collagen, Conventional, and Textile Fabrication Technologies and Properties

In this review article, tissue engineering and regenerative medicine are briefly explained and the importance of scaffolds is highlighted. Furthermore, the requirements of scaffolds and how they can be fulfilled by using specific biomaterials and fabrication methods are presented. Detailed insight is given into the two biopolymers chitosan and collagen. The fabrication methods are divided into two categories: isotropic and anisotropic scaffold fabrication methods. Processable biomaterials and achievable pore sizes are assigned to each method. In addition, fiber spinning methods and textile fabrication methods used to produce anisotropic scaffolds are described in detail and the advantages of anisotropic scaffolds for tissue engineering and regenerative medicine are highlighted.

Soft and Stretchable Optical Waveguide: Light Delivery and Manipulation at Complex Biointerfaces Creating Unique Windows for On-Body Sensing

Over the past few decades, optical waveguides have been increasingly used in wearable/implantable devices for on-body sensing. However, conventional optical waveguides are stiff, rigid, and brittle. A mismatch between conventional optical waveguides and complex biointerfaces makes wearable/implantable devices uncomfortable to wear and potentially unsafe. Soft and stretchable polymer optical waveguides not only inherit many advantages of conventional optical waveguides (e.g., immunity to electromagnetic interference and without electrical hazards) but also provide a new perspective for solving the mismatch between conventional optical waveguides and complex biointerfaces, which is essential for the development of light-based wearable/implantable sensors. In this review, polymer optical waveguides' unique properties, including flexibility, biocompatibility and biodegradability, porosity, and stimulus responsiveness, and their applications in the wearable/implantable field in recent years are summarized. Then, we briefly discuss the current challenges of high optical loss, unstable signal transmission, low manufacturing efficiency, and difficulty in deployment during implantation of flexible polymer optical waveguides, and propose some possible solutions to these problems.

Development of a tannic acid cross-linking process for obtaining 3D porous cell-laden collagen structure

Cell-printing is an emerging technique that enables to build a customized structure using biomaterials and living cells for various biomedical applications. In many biomaterials, alginate has been widely used for rapid gelation, low cost, and relatively high processability. However, biocompatibilities enhancing cell adhesion and proliferation were limited, so that, to overcome this problem, an outstanding alternative, collagen, has been extensively investigated. Many factors remain to be proven for cell-printing applications, such as printability, physical sustainability after printing, and applicability of in vitro cell culture. This study proposes a cell-laden collagen scaffold fabricated via cell-printing and tannic acid (TA) crosslinking process. The effects of the crosslinking agent (0-3wt% TA) in the cell-laden collagen scaffolds on physical properties and cellular activities using preosteoblasts (MC3T3-E1) were presented. Compared to the cell-laden collagen scaffold without TA crosslinking, the scaffold with TA crosslinking was significantly enhanced in mechanical properties, while reasonable cellular activities were observed. Concisely, this study introduces the possibility of a cell-printing process using collagen and TA crosslinking and in vitro cell culture for tissue regeneration.

A cell-printing approach for obtaining hASC-laden scaffolds by using a collagen/polyphenol bioink

In the cell-printing process, bioink has been considered as an extremely important component for successful fabrication of macroscale cell-laden structures. Bioink should be non-toxic, biocompatible, and printable. To date, alginate has been widely used as a whole or partial component of bioink because it is non-toxic to embedded cells and even it can provide good printability with rapid gelation under calcium ions. However, alginate bioinks do not possess cell-activating ability. To overcome the shortcomings of alginate-based bioinks, a new collagen bioink, which was mixed with human adipose stem cells (hASCs) and crosslinked with a polyphenol (tannic acid), was proposed. The feasibility of the bioink was demonstrated using several in vitro assessments for comparison of the macroscale porous cell-laden collagen/polyphenol structure containing the hASCs with the conventional alginate-based cell-laden structure. The levels of the metabolic activity, including the cell viability and cell proliferation, of the cell-laden collagen structure were significantly higher than those of the control (alginate-based cell-laden structure). The results show that the newly designed bioink and cell-laden structure are potentially new outstanding components for regeneration of various tissues.

New strategy for enhancing in situ cell viability of cell-printing process via piezoelectric transducer-assisted three-dimensional printing

Tissue engineering has become one of the great applications of three-dimensional cell printing because of the possibility of fabricating complex cell-laden scaffolds. Three typical methods (inkjet, micro-extrusion, and laser-assisted bio-printing) have been used to fabricate structures. Of these, micro-extrusion is a comparatively easy method, but has some drawbacks such as low in situ cell viability after fabricating cell-laden structures because of the high wall shear stress in micro-sized nozzles. To overcome this shortcoming, we suggest an innovative cell printing method, which is assisted by a piezoelectric transducer (PZT). The PZT assistance in the dispensing process enhances the printing efficiency and cell viability by decreasing the wall shear stress within a nozzle because the PZT effect can lower the shear viscosity of the bioink via micro-scale vibration. In this study, 5 wt% cell-laden alginate was used as a bioink, and various PZT conditions (frequencies up to ∼400 Hz and amplitudes up to ∼40.5 μm) were simultaneously applied to the cell-printing process to examine the effectiveness of the PZT. The PZT-assisted cell-printing method was found to be highly effective in direct cell printing and could achieve cell-laden structures with high in situ cell viability.

Phage as versatile nanoink for printing 3-D cell-laden scaffolds

Bioprinting is an emerging technology for producing tissue-mimetic 3-D structures using cell-containing hydrogels (bioink). Various synthetic and natural hydrogels with key characteristics, including biocompatibility, biodegradability, printability and crosslinkability, have been employed as ink materials in bioprinting. Choosing the right cell-containing "bioink" material is the most essential step for fabricating 3-D constructs with a controlled mechanical and biochemical microenvironment that can lead to successful tissue regeneration and repair. Here, we demonstrate that the genetically engineered M13 phage holds great potential for use as a versatile nanoink for printing 3-D cell-laden matrices. In particular, M13 phages displaying integrin-binding (GRGDS) and calcium-binding (DDYD) domains on their surface were blended with alginate to successfully form Ca(2+)-crosslinked hydrogels. Furthermore, 3-D cell-laden scaffolds with high cell viability were generated after optimizing the printing process. The MC3T3-E1 cells within these scaffolds showed enhanced proliferation and differentiation rates that increased proportionally with the concentration of phages in the 3-D matrices compared with the rates of cells in pure alginate scaffolds.

STATEMENT OF SIGNIFICANCE

Bioprinting is an emerging technology for producing tissue-mimetic 3-D structures using cell-containing hydrogels called bioink. Choosing the right bioink is essential for fabricating 3-D structures with controlled mechanical and biochemical properties which lead to successful tissue regeneration. Therefore, there is a growing demand for a new bioink material that can be designed from molecular level. Here, we demonstrate that genetically engineered M13 phage holds great potential for use as versatile bioink. The phage-based bioink benefits from its replicability, self-assembling property, and tunable molecular design and enables bioprinted scaffolds to exhibit improved cell viability, proliferation and differentiation. This study opens the door for the development of genetically tunable nanofibrous bioink materials which closely mimic natural structural proteins in the extracellular matrix.

Mechanically reinforced cell-laden scaffolds formed using alginate-based bioink printed onto the surface of a PCL/alginate mesh structure for regeneration of hard tissue

Cell-printing technology has provided a new paradigm for biofabrication, with potential to overcome several shortcomings of conventional scaffold-based tissue regeneration strategies via controlled delivery of various cell types in well-defined target regions. Here we describe a cell-printing method to obtain mechanically reinforced multi-layered cell-embedded scaffolds, formed of micron-scale poly(ε-caprolactone) (PCL)/alginate struts coated with alginate-based bioink. To compare the physical and cellular activities, we used a scaffold composed of pure alginate (without cells) coated PCL/alginate struts as a control. We systematically varied the ratio of alginate cross-linking agent, and determined the optimal cell-coating conditions to form the PCL/alginate struts. Following fabrication of the cell (MG63)-laden PCL/alginate scaffold, the bioactivity was evaluated in vitro. The laden cells exhibited a substantially more developed cytoskeleton compared with those on a control scaffold consisting of the same material composition. Based on these results, the printed cells exhibited a significantly more homogenous distribution within the scaffold compared with the control. Cell proliferation was determined via MTT assays at 1, 3, 7, and 14 days of culture, and the proliferation of the cell-printed scaffold was substantially in excess (∼2.4-fold) of that on the control. Furthermore, the osteogenic activity such as ALP was measured, and the cell-laden scaffold exhibited significantly greater activity (∼3.2-fold) compared with the control scaffold.

Combined additive manufacturing approaches in tissue engineering

Advances introduced by additive manufacturing (AM) have significantly improved the control over the microarchitecture of scaffolds for tissue engineering. This has led to the flourishing of research works addressing the optimization of AM scaffolds microarchitecture to optimally trade-off between conflicting requirements (e.g. mechanical stiffness and porosity level). A fascinating trend concerns the integration of AM with other scaffold fabrication methods (i.e. "combined" AM), leading to hybrid architectures with complementary structural features. Although this innovative approach is still at its beginning, significant results have been achieved in terms of improved biological response to the scaffold, especially targeting the regeneration of complex tissues. This review paper reports the state of the art in the field of combined AM, posing the accent on recent trends, challenges, and future perspectives.

A novel cell-printing method and its application to hepatogenic differentiation of human adipose stem cell-embedded mesh structures

We report a cell-dispensing technique, using a core-shell nozzle and an absorbent dispensing stage to form cell-embedded struts. In the shell of the nozzle, a cross-linking agent flowed continuously onto the surface of the dispensed bioink in the core nozzle, so that the bioink struts were rapidly gelled, and any remnant cross-linking solution during the process was rapidly absorbed into the working stage, resulting in high cell-viability in the bioink strut and stable formation of a three-dimensional mesh structure. The cell-printing conditions were optimized by manipulating the process conditions to obtain high mechanical stability and high cell viability. The cell density was 1 × 10(7) mL(-1), which was achieved using a 3-wt% solution of alginate in phosphate-buffered saline, a mass fraction of 1.2 wt% of CaCl2 flowing in the shell nozzle with a fixed flow rate of 0.08 mL min(-1), and a translation velocity of the printing nozzle of 10 mm s(-1). To demonstrate the applicability of the technique, preosteoblasts and human adipose stem cells (hASCs) were used to obtain cell-laden structures with multi-layer porous mesh structures. The fabricated cell-laden mesh structures exhibited reasonable initial cell viabilities for preosteoblasts (93%) and hASCs (92%), and hepatogenic differentiation of hASC was successfully achieved.