A new method of fabricating a blend scaffold using an indirect three-dimensional printing technique

Current advances in engineering meniscal tissues: insights into 3D printing, injectable hydrogels and physical stimulation based strategies

The knee meniscus is the cushioning fibro-cartilage tissue present in between the femoral condyles and tibial plateau of the knee joint. It is largely avascular in nature and suffers from a wide range of tears and injuries caused by accidents, trauma, active lifestyle of the populace and old age of individuals. Healing of the meniscus is especially difficult due to its avascularity and hence requires invasive arthroscopic approaches such as surgical resection, suturing or implantation. Though various tissue engineering approaches are proposed for the treatment of meniscus tears, three-dimensional (3D) printing/bioprinting, injectable hydrogels and physical stimulation involving modalities are gaining forefront in the past decade. A plethora of new printing approaches such as direct light photopolymerization and volumetric printing, injectable biomaterials loaded with growth factors and physical stimulation such as low-intensity ultrasound approaches are being added to the treatment portfolio along with the contemporary tear mitigation measures. This review discusses on the necessary design considerations, approaches for 3D modeling and design practices for meniscal tear treatments within the scope of tissue engineering and regeneration. Also, the suitable materials, cell sources, growth factors, fixation and lubrication strategies, mechanical stimulation approaches, 3D printing strategies and injectable hydrogels for meniscal tear management have been elaborated. We have also summarized potential technologies and the potential framework that could be the herald of the future of meniscus tissue engineering and repair approaches.

Porous Flow Field for Next-Generation Proton Exchange Membrane Fuel Cells: Materials, Characterization, Design, and Challenges

Porous flow fields distribute fuel and oxygen for the electrochemical reactions of proton exchange membrane (PEM) fuel cells through their pore network instead of conventional flow channels. This type of flow fields has showed great promises in enhancing reactant supply, heat removal, and electrical conduction, reducing the concentration performance loss and improving operational stability for fuel cells. This review presents the research and development progress of porous flow fields with insights for next-generation PEM fuel cells of high power density (e.g., ∼9.0 kW L^-1). Materials, fabrication methods, fundamentals, and fuel cell performance associated with porous flow fields are discussed in depth. Major challenges are described and explained, along with several future directions, including separated gas/liquid flow configurations, integrated porous structure, full morphology modeling, data-driven methods, and artificial intelligence-assisted design/optimization.

Potential use of bioactive nanofibrous dural substitutes with controlled release of IGF-1 for neuroprotection after traumatic brain injury

For patients suffering from traumatic brain injury (TBI), the closure of dural defects after decompressive craniectomy is the prerequisite to restoring normal physiological functions. It is also an urgent challenge to provide a neuroprotection effect against the primary and secondary nerve damage during long-term recovery. To solve these issues, we herein develop a class of bioactive, nanofibrous dural substitutes that can long-term release insulin-like growth factor 1 (IGF-1) for improving the survival and neurite outgrowth of neural cells after TBI. Such dural substitutes were polycaprolactone (PCL) nanofibers encapsulated with hyaluronic acid methacryloyl (HAMA)/IGF-1 by blend or coaxial electrospinning techniques, achieving bioactive PCL/HAMA/IGF nanofibrous dural substitutes with different release profiles of IGF-1. The nanofibrous dural substitutes exhibited good mechanical properties and hydrophobicity, which prevent cerebrospinal fluid leakage, maintain normal intracranial pressure, and avoid external impact on the brain. We also found that the viability and neurite outgrowth of SH-SY5Y cells and primary neurons were significantly enhanced after neurite transection or oxygen and glucose deprivation treatment. Taken together, such PCL/HAMA/IGF nanofibrous dural substitutes hold promising potential to provide neuroprotection effects after primary and secondary nerve damage in TBI, which would bring significant benefits to the field of neurosurgery involving the use of artificial dura mater.

3D direct-write printing of water soluble micromoulds for high-resolution rapid prototyping

Direct-write printing has contributed tremendously to additive manufacturing; in particular extrusion based printing where it has extended the range of materials for 3D printing and thus enabled use across many more sectors. The printing inks for direct-write printing however, need careful synthesis and invariably undergo extensive preparation before being able to print. Hence, new ink synthesis efforts are required every time a new material is to be printed; this is particularly challenging for low storage modulus (G') materials like silicones, especially at higher resolutions (under 10 µm). Here we report the development of a precise (< 10 µm) 3D printable polymer, with which we 3D print micromoulds which are filled with standard silicones like polydimethylsiloxane (PDMS) and left to cure at room temperature. The proof of concept is demonstrated using a simple water soluble polymer as the mould material. The approach enables micrometre scale silicone structures to be prototyped with ease, away from the cleanroom.

The effect of blending poly (l-lactic acid) on in vivo performance of 3D-printed poly(l-lactide-co-caprolactone)/PLLA scaffolds

Blending poly (l-lactic acid, PLLA) with poly (l-lactide-co-caprolactone, PLCL) is an effective strategy for developing new PLCL/PLLA blend based biomaterials. However, the effect of PLLA on in vivo performance of PLCL/PLLA blends is unclear yet. To address this issue, in this study, the effect of PLLA on in vivo biodegradability and biocompatibility of 3D-printed scaffolds of PLCL/PLLA blend was investigated. Three kinds of different 3D-printed PLCL/PLLA scaffolds using different blends with different mass ratios of the polymers, were prepared and implanted subcutaneously. The shrinkage and tissue responses were monitored by ultrasonography after the implantation. 2 months post-operation, the in vivo performances of the scaffolds were investigated histologically. All scaffolds showed good biocompatibility and allowed fast tissues ingrowth, however PLCL50/PLLA50 scaffold with the highest PLLA ratio induced the thickest the fibrous capsule surrounding the scaffolds and highest inflammatory scores. Furthermore, it was found that the fine porous structures of all scaffolds were well maintained, indicating the 3D-printed scaffolds were degraded through a surface erosion but not bulk erosion way. However, different scaffolds showed different shrinkage and degradation ratios, and PLCL50/PLLA50 scaffold resulted in a significant shrinkage, while PLCL90/PLLA10 scaffold showed the better structural stability. Therefore, PLLA at blending different ratio had different effects on the in vivo performance of 3D-printed PLCL/PLLA scaffolds. Particularly, PLCL/PLLA scaffolds blending with low ratio of PLLA, such as PLCL90/PLLA10 scaffold showed better application potential in tissue engineering. Our findings provide a new insight on the rational design, constrcution and application of the 3D-printed PLCL/PLLA scaffolds.

Main Applications and Recent Research Progresses of Additive Manufacturing in Dentistry

In recent ten years, with the fast development of digital and engineering manufacturing technology, additive manufacturing has already been more and more widely used in the field of dentistry, from the first personalized surgical guides to the latest personalized restoration crowns and root implants. In particular, the bioprinting of teeth and tissue is of great potential to realize organ regeneration and finally improve the life quality. In this review paper, we firstly presented the workflow of additive manufacturing technology. Then, we summarized the main applications and recent research progresses of additive manufacturing in dentistry. Lastly, we sketched out some challenges and future directions of additive manufacturing technology in dentistry.

Development of Silk Fibroin Scaffolds by Using Indirect 3D-Bioprinting Technology

Due to the excellent biocompatibility of natural polymers, a variety of natural polymers have been widely used as biomaterials for manufacturing tissue engineered scaffolds. Despite the excellent biological activity of natural polymers, there have been obstacles in using them on their own to prepare 3D scaffolds with sufficient mechanical strength. Although multiple 3D-bioprinting technologies have recently emerged as effective manufacturing tools for scaffold preparation, scaffold preparation using only natural polymers with tunable mechanical properties is still difficult. Herein, we introduce novel scaffold fabrication methods using the natural polymer silk fibroin via indirect 3D-bioprinting technology. The developed silk fibroin scaffolds showed biocompatibility and tunable mechanical strength by changing the concentration of the silk fibroin. Furthermore, controlling the flexibility of the silk fibroin scaffolds was made possible by changing the solvent for the silk fibroin solution used to fabricate the scaffold. Consequently, silk fibroin scaffolds fabricated via our method can be considered for various applications in the bioengineering of either soft or musculoskeletal tissues.

Recent Advances in Three-Dimensional Stem Cell Culture Systems and Applications

Cell culture is one of the most core and fundamental techniques employed in the fields of biology and medicine. At present, although the two-dimensional cell culture method is commonly used in vitro, it is quite different from the cell growth microenvironment in vivo. In recent years, the limitations of two-dimensional culture and the advantages of three-dimensional culture have increasingly attracted more and more attentions. Compared to two-dimensional culture, three-dimensional culture system is better to realistically simulate the local microenvironment of cells, promote the exchange of information among cells and the extracellular matrix (ECM), and retain the original biological characteristics of stem cells. In this review, we first present three-dimensional cell culture methods from two aspects: a scaffold-free culture system and a scaffold-based culture system. The culture method and cell characterizations will be summarized. Then the application of three-dimensional cell culture system is further explored, such as in the fields of drug screening, organoids and assembloids. Finally, the directions for future research of three-dimensional cell culture are stated briefly.

Mesenchymal stem cells loaded on 3D-printed gradient poly(ε-caprolactone)/methacrylated alginate composite scaffolds for cartilage tissue engineering

Cartilage has limited self-repair ability due to its avascular, alymphatic and aneural features. The combination of three-dimensional (3D) printing and tissue engineering provides an up-and-coming approach to address this issue. Here, we designed and fabricated a tri-layered (superficial layer (SL), middle layer (ML) and deep layer (DL)) stratified scaffold, inspired by the architecture of collagen fibers in native cartilage tissue. The scaffold was composed of 3D printed depth-dependent gradient poly(ε-caprolactone) (PCL) impregnated with methacrylated alginate (ALMA), and its morphological analysis and mechanical properties were tested. To prove the feasibility of the composite scaffolds for cartilage regeneration, the viability, proliferation, collagen deposition and chondrogenic differentiation of embedded rat bone marrow mesenchymal stem cells (BMSCs) in the scaffolds were assessed by Live/dead assay, CCK-8, DNA content, cell morphology, immunofluorescence and real-time reverse transcription polymerase chain reaction. BMSCs-loaded gradient PCL/ALMA scaffolds showed excellent cell survival, cell proliferation, cell morphology, collagen II deposition and hopeful chondrogenic differentiation compared with three individual-layer scaffolds. Hence, our study demonstrates the potential use of the gradient PCL/ALMA construct for enhanced cartilage tissue engineering.

Boosting in vitro cartilage tissue engineering through the fabrication of polycaprolactone-gelatin 3D scaffolds with specific depth-dependent fiber alignments and mechanical stimulation

Due to the limited self-healing ability of natural cartilage, several tissue engineering strategies have been explored to develop functional replacements. Still, most of these approaches do not attempt to recreate in vitro the anisotropic organization of its extracellular matrix, which is essential for a suitable load-bearing function. In this work, different depth-dependent alignments of polycaprolactone-gelatin electrospun fibers were assembled into three-dimensional scaffold architectures to assess variations on chondrocyte response under static, unconfined compressed and perfused culture conditions. The in vitro results confirmed that not only the 3D scaffolds specific depth-dependent fiber alignments potentiated chondrocyte proliferation and migration towards the fibrous systems, but also the mechanical stimulation protocols applied were able to enhance significantly cell metabolic activity and extracellular matrix deposition, respectively.

3D Cell Printing of Tissue/Organ-Mimicking Constructs for Therapeutic and Drug Testing Applications

The development of artificial tissue/organs with the functional maturity of their native equivalents is one of the long-awaited panaceas for the medical and pharmaceutical industries. Advanced 3D cell-printing technology and various functional bioinks are promising technologies in the field of tissue engineering that have enabled the fabrication of complex 3D living tissue/organs. Various requirements for these tissues, including a complex and large-volume structure, tissue-specific microenvironments, and functional vasculatures, have been addressed to develop engineered tissue/organs with native relevance. Functional tissue/organ constructs have been developed that satisfy such criteria and may facilitate both in vivo replenishment of damaged tissue and the development of reliable in vitro testing platforms for drug development. This review describes key developments in technologies and materials for engineering 3D cell-printed constructs for therapeutic and drug testing applications.

Electrospinning of bioactive polycaprolactone-gelatin nanofibres with increased pore size for cartilage tissue engineering applications

Polycaprolactone (PCL) electrospun scaffolds have been widely investigated for cartilage repair application. However, their hydrophobicity and small pore size has been known to prevent cell attachment, proliferation and migration. Here, PCL was blended with gelatin (GEL) combining the favorable biological properties of GEL with the good mechanical performance of the former. Also, polyethylene glycol (PEG) particles were introduced during the electrospinning of the polymers blend by simultaneous electrospraying. These particles were subsequently removed resulting in fibrous scaffolds with enlarged pore size. PCL, GEL and PEG scaffolds formulations were developed and extensively structural and biologically characterized. GEL incorporation on the PCL scaffolds led to a considerably improved cell attachment and proliferation. A substantial pore size and interconnectivity increase was obtained, allowing cell infiltration through the porogenic scaffolds. All together these results suggest that this combined approach may provide a potentially clinically viable strategy for cartilage regeneration.

Layer-by-layer coated porous 3D printed hydroxyapatite composite scaffolds for controlled drug delivery

Interconnected porous scaffolds are widely used in the applications of tissue repair and regeneration. Sustained local delivery of drugs and growth factors around the implanted scaffolds could accelerate the growth of cells and contribute to the regeneration of damaged tissues. In this study, porous hydroxyapatite composite scaffolds were prepared through 3D bio-printing for bone tissue engineering and were subsequently coated with chitosan and sodium hyaluronate by layer-by-layer (LBL) deposition. It was found that the LBL coating on the porous scaffolds could reduce the swelling ratio of scaffolds in size and increase the compressive strength by about 70%. The degradation rate of the scaffolds slowed down due to the LBL coating. Rhodamine B (RHB) and bovine serum albumin (BSA) were chosen as model drugs in order to understand the loading and release behaviors of the scaffolds. Small RHB molecules could penetrate deep into the LBL coated scaffolds and released a little slower than that without coating. Meanwhile, large BSA molecules showed faster release rate compared to that without coating. In addition, there was no significant cytotoxicity effect of these composite scaffolds towards MC-3T3E1 cells and the scaffolds provided proper conditions for cell adhesion and proliferation, indicating that the printed hydroxyapatite composite scaffolds exhibit a great potential in hard tissue engineering as a sustained delivery system.

Gelatin-polysaccharide composite scaffolds for 3D cell culture and tissue engineering: Towards natural therapeutics

Gelatin is a promising material as scaffold with therapeutic and regenerative characteristics due to its chemical similarities to the extracellular matrix (ECM) in the native tissues, biocompatibility, biodegradability, low antigenicity, cost-effectiveness, abundance, and accessible functional groups that allow facile chemical modifications with other biomaterials or biomolecules. Despite the advantages of gelatin, poor mechanical properties, sensitivity to enzymatic degradation, high viscosity, and reduced solubility in concentrated aqueous media have limited its applications and encouraged the development of gelatin-based composite hydrogels. The drawbacks of gelatin may be surmounted by synergistically combining it with a wide range of polysaccharides. The addition of polysaccharides to gelatin is advantageous in mimicking the ECM, which largely contains proteoglycans or glycoproteins. Moreover, gelatin-polysaccharide biomaterials benefit from mechanical resilience, high stability, low thermal expansion, improved hydrophilicity, biocompatibility, antimicrobial and anti-inflammatory properties, and wound healing potential. Here, we discuss how combining gelatin and polysaccharides provides a promising approach for developing superior therapeutic biomaterials. We review gelatin-polysaccharides scaffolds and their applications in cell culture and tissue engineering, providing an outlook for the future of this family of biomaterials as advanced natural therapeutics.

Nasal Reconstruction Using a Customized Three-Dimensional-Printed Stent for Congenital Arhinia: Three-Year Follow-up

A male Mongolian child with a complete congenital absence of both nose and nasal passage had a poor survival prognosis due to respiratory distress. To enable his survival, a new nose capable of conferring respiratory function was constructed. Following reconstructive surgery, an absence of mucoepithelium in the nasal passage can lead to rhinostenosis. To avoid this complication, a custom-made nasal silicone stent was created using three-dimensional (3D) printing technology in conjunction with the patient's computed tomography data. The stent was implanted for 2 months to maintain the shape and size of the nasal passage. At 2 months after stent implantation, the mucoepithelium tissue in the passage had successfully regenerated with no immune reaction. Three years after stent removal, respiratory function, nasal passage structure, and external nose shape were maintained without additional medical care. These results indicate the successful nasal reconstruction in an arhinia patient using a customized, 3D-printed nasal stent. Laryngoscope, 129:582-585, 2019.

Pore shape and size dependence on cell growth into electrospun fiber scaffolds for tissue engineering: 2D and 3D analyses using SEM and FIB-SEM tomography

Electrospun nanofibers have ability to boost cell proliferation in tissue engineered scaffolds as their structure remind cells extra cellular matrix of the native tissue. The complex architecture and network of nanofibrous scaffolds requires advanced characterization methods to understand interrelationship between cells and nanofibers. In our study, we used complementary 2D and 3D analyses of electrospun polylactide-co-glycolide acid (PLGA) scaffolds in two configurations: aligned and randomly oriented nanofibers. Sizes of pores and fibers, pores shapes and porosity, before and after cell culture, were verified by imaging with scanning electron microscopy (SEM) and combination of focus ion beam (FIB) and SEM to obtain 3D reconstructions of samples. Using FIB-SEM tomography for 3D reconstructions and 2D analyses, a unique set of data allowing understanding cell proliferation mechanism into the electrospun scaffolds, was delivered. Critically, the proliferation of cells into nanofibers network depends mainly on the pore shape and pores interconnections, which allow deep integration between cells and nanofibers. The proliferation of cells inside the network of fibers is much limited for aligned fibers comparing to randomly oriented fibers. For random fibers cells have easier way to integrate inside the scaffold as the circularity of pores and their sizes are larger than for aligned scaffolds. The complex architecture of electrospun scaffolds requires appropriate, for tissue engineering needs, cell seeding and culture methods, to maximize tissue growth in vitro environment.

Three-Dimensional Printing of Tissue/Organ Analogues Containing Living Cells

The technical advances of three-dimensional (3D) printing in the field of tissue engineering have enabled the creation of 3D living tissue/organ analogues. Diverse 3D tissue/organ printing techniques with computer-aided systems have been developed and used to dispose living cells together with biomaterials and supporting biochemicals as pre-designed 3D tissue/organ models. Furthermore, recent advances in bio-inks, which are printable hydrogels with living cell encapsulation, have greatly enhanced the versatility of 3D tissue/organ printing. Here, we introduce 3D tissue/organ printing techniques and biomaterials that have been developed and widely used thus far. We also review a variety of applications in an attempt to repair or replace the damaged or defective tissue/organ, and develop the in vitro tissue/organ models. The potential challenges are finally discussed from the technical perspective of 3D tissue/organ printing.

Computer-aided multiple-head 3D printing system for printing of heterogeneous organ/tissue constructs

Recently, much attention has focused on replacement or/and enhancement of biological tissues via the use of cell-laden hydrogel scaffolds with an architecture that mimics the tissue matrix, and with the desired three-dimensional (3D) external geometry. However, mimicking the heterogeneous tissues that most organs and tissues are formed of is challenging. Although multiple-head 3D printing systems have been proposed for fabricating heterogeneous cell-laden hydrogel scaffolds, to date only the simple exterior form has been realized. Here we describe a computer-aided design and manufacturing (CAD/CAM) system for this application. We aim to develop an algorithm to enable easy, intuitive design and fabrication of a heterogeneous cell-laden hydrogel scaffolds with a free-form 3D geometry. The printing paths of the scaffold are automatically generated from the 3D CAD model, and the scaffold is then printed by dispensing four materials; i.e., a frame, two kinds of cell-laden hydrogel and a support. We demonstrated printing of heterogeneous tissue models formed of hydrogel scaffolds using this approach, including the outer ear, kidney and tooth tissue. These results indicate that this approach is particularly promising for tissue engineering and 3D printing applications to regenerate heterogeneous organs and tissues with tailored geometries to treat specific defects or injuries.