3D printed multi-scale scaffolds with ultrafine fibers for providing excellent biocompatibility

Electrohydrodynamic Direct-Writing Micro/Nanofibrous Architectures: Principle, Materials, and Biomedical Applications

Electrohydrodynamic (EHD) direct-writing has recently gained attention as a highly promising additive manufacturing strategy for fabricating intricate micro/nanoscale architectures. This technique is particularly well-suited for mimicking the extracellular matrix (ECM) present in biological tissue, which serves a vital function in facilitating cell colonization, migration, and growth. The integration of EHD direct-writing with other techniques has been employed to enhance the biological performance of scaffolds, and significant advancements have been made in the development of tailored scaffold architectures and constituents to meet the specific requirements of various biomedical applications. Here, a comprehensive overview of EHD direct-writing is provided, including its underlying principles, demonstrated materials systems, and biomedical applications. A brief chronology of EHD direct-writing is provided, along with an examination of the observed phenomena that occur during the printing process. The impact of biomaterial selection and architectural topographic cues on biological performance is also highlighted. Finally, the major limitations associated with EHD direct-writing are discussed.

Three-Dimensional Bioprinting in Vascular Tissue Engineering and Tissue Vascularization of Cardiovascular Diseases

In the 21st century, significant progress has been made in repairing damaged materials through material engineering. However, the creation of large-scale artificial materials still faces a major challenge in achieving proper vascularization. To address this issue, researchers have turned to biomaterials and three-dimensional (3D) bioprinting techniques, which allow for the combination of multiple biomaterials with improved mechanical and biological properties that mimic natural materials. Hydrogels, known for their ability to support living cells and biological components, have played a crucial role in this research. Among the recent developments, 3D bioprinting has emerged as a promising tool for constructing hybrid scaffolds. However, there are several challenges in the field of bioprinting, including the need for nanoscale biomimicry, the formulation of hydrogel blends, and the ongoing complexity of vascularizing biomaterials, which requires further research. On a positive note, 3D bioprinting offers a solution to the vascularization problem due to its precise spatial control, scalability, and reproducibility compared with traditional fabrication methods. This paper aims at examining the recent advancements in 3D bioprinting technology for creating blood vessels, vasculature, and vascularized materials. It provides a comprehensive overview of the progress made and discusses the limitations and challenges faced in current 3D bioprinting of vascularized tissues. In addition, the paper highlights the future research directions focusing on the development of 3D bioprinting techniques and bioinks for creating functional materials.

Three-dimensional printing and decellularized-extracellular-matrix based methods for advances in artificial blood vessel fabrication: A review

Blood vessels are the tubes through which blood flows and are divided into three types: millimeter-scale arteries, veins, and capillaries as well as micrometer-scale capillaries. Arteries and veins are the conduits that carry blood, while capillaries are where blood exchanges substances with tissues. Blood vessels are mainly composed of collagen fibers, elastic fibers, glycosaminoglycans and other macromolecular substances. There are about 19 feet of blood vessels per square inch of skin in the human body, which shows how important blood vessels are to the human body. Because cardiovascular disease and vascular trauma are common in the population, a great number of researches have been carried out in recent years by simulating the structures and functions of the person's own blood vessels to create different levels of tissue-engineered blood vessels that can replace damaged blood vessels in the human body. However, due to the lack of effective oxygen and nutrient delivery mechanisms, these tissue-engineered vessels have not been used clinically. Therefore, in order to achieve better vascularization of engineered vascular tissue, researchers have widely explored the design methods of vascular systems of various sizes. In the near future, these carefully designed and constructed tissue engineered blood vessels are expected to have practical clinical applications. Exploring how to form multi-scale vascular networks and improve their compatibility with the host vascular system will be very beneficial in achieving this goal. Among them, 3D printing has the advantages of high precision and design flexibility, and the decellularized matrix retains active ingredients such as collagen, elastin, and glycosaminoglycan, while removing the immunogenic substance DNA. In this review, technologies and advances in 3D printing and decellularization-based artificial blood vessel manufacturing methods are systematically discussed. Recent examples of vascular systems designed are introduced in details, the main problems and challenges in the clinical application of vascular tissue restriction are discussed and pointed out, and the future development trends in the field of tissue engineered blood vessels are also prospected.

Surface micropatterning of 3D printed PCL scaffolds promotes osteogenic differentiation of BMSCs and regulates macrophage M2 polarization

Micropatterned structures on the surface of materials possessing biomimetic properties to mimic the extracellular matrix and induce cellular behaviors have been widely studied. However, it is still a major challenge to obtain internally stable and controllable micropatterned 3D scaffolds for bone repair and regeneration. In this study, 3D scaffolds with regular grating arrays using polycaprolactone (PCL) as a matrix material were prepared by combining 3D printing and soft lithography, and the effects of grating micropatterning on osteogenic differentiation of BMSCs and M1/M2 polarization of macrophages were investigated. The results showed that compared with the planar group and the 30um grating spacing group, PCL with a grating spacing of 20um significantly promoted the osteogenic differentiation of BMSCs, induced the polarization of RAW264.7 cells toward M2 type, and suppressed the expression of M1-type pro-inflammatory genes and markers. In conclusion, we successfully constructed PCL-based three-dimensional scaffolds with stable and controllable micrographs (grating arrays) inside, which possess excellent osteogenic properties and promote the formation of an immune microenvironment conducive to osteogenesis. This study is a step forward to the exploration of bone-filling materials affecting cell behavior, and makes a new contribution to the provision of high-quality materials.

Customisable Silicone Vessels and Tissue Phantoms for In Vitro Photoplethysmography Investigations into Cardiovascular Disease

Age-related vessel deterioration leads to changes in the structure and function of the heart and blood vessels, notably stiffening of vessel walls, increasing the risk of developing cardiovascular disease (CVD), which accounts for 17.9 million global deaths annually. This study describes the fabrication of custom-made silicon vessels with varying mechanical properties (arterial stiffness). The primary objective of this study was to explore how changes in silicone formulations influenced vessel properties and their correlation with features extracted from signals obtained from photoplethysmography (PPG) reflectance sensors in an in vitro setting. Through alterations in the silicone formulations, it was found that it is possible to create elastomers exhibiting an elasticity range of 0.2 MPa to 1.22 MPa. It was observed that altering vessel elasticity significantly impacted PPG signal morphology, particularly reducing amplitude with increasing vessel stiffness (p < 0.001). A p-value of 5.176 × 10-15 and 1.831 × 10-14 was reported in the red and infrared signals, respectively. It has been concluded in this study that a femoral artery can be recreated using the silicone material, with the addition of a softener to achieve the required mechanical properties. This research lays the foundation for future studies to replicate healthy and unhealthy vascular systems. Additional pathologies can be introduced by carefully adjusting the elastomer materials or incorporating geometrical features consistent with various CVDs.

Calcium phosphate coating enhances osteointegration of melt electrowritten scaffold by regulating macrophage polarization

The osteoimmune microenvironment induced by implants plays a significant role in bone regeneration. It is essential to efficiently and timely switch the macrophage phenotype from M1 to M2 for optimal bone healing. This study examined the impact of a calcium phosphate (CaP) coating on the physiochemical properties of highly ordered polycaprolactone (PCL) scaffolds fabricated using melt electrowritten (MEW). Additionally, it investigated the influence of these scaffolds on macrophage polarization and their immunomodulation on osteogenesis. The results revealed that the CaP coated PCL scaffold exhibited a rougher surface topography and higher hydrophilicity in comparison to the PCL scaffold without coating. Besides, the surface morphology of the coating and the release of Ca2+ from the CaP coating were crucial in regulating the transition of macrophages from M1 to M2 phenotypes. They might activate the PI3K/AKT and cAMP-PKA pathways, respectively, to facilitate M2 polarization. In addition, the osteoimmune microenvironment induced by CaP coated PCL could not only enhance the osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) in vitro but also promote the bone regeneration in vivo. Taken together, the CaP coating can be employed to control the phenotypic switching of macrophages, thereby creating a beneficial immunomodulatory microenvironment that promotes bone regeneration.

Tailoring the pore design of embroidered structures by melt electrowriting to enhance the cell alignment in scaffold-based tendon reconstruction

Tissue engineering of ligaments and tendons aims to reproduce the complex and hierarchical tissue structure while meeting the biomechanical and biological requirements. For the first time, the additive manufacturing methods of embroidery technology and melt electrowriting (MEW) were combined to mimic these properties closely. The mechanical benefits of embroidered structures were paired with a superficial micro-scale structure to provide a guide pattern for directional cell growth. An evaluation of several previously reported MEW fiber architectures was performed. The designs with the highest cell orientation of primary dermal fibroblasts were then applied to embroidery structures and subsequently evaluated using human adipose-derived stem cells (AT-MSC). The addition of MEW fibers resulted in the formation of a mechanically robust layer on the embroidered scaffolds, leading to composite structures with mechanical properties comparable to those of the anterior cruciate ligament. Furthermore, the combination of embroidered and MEW structures supports a higher cell orientation of AT-MSC compared to embroidered structures alone. Collagen coating further promoted cell attachment. Thus, these investigations provide a sound basis for the fabrication of heterogeneous and hierarchical synthetic tendon and ligament substitutes.

Gelatin-Based Metamaterial Hydrogel Films with High Conformality for Ultra-Soft Tissue Monitoring

Implantable hydrogel-based bioelectronics (IHB) can precisely monitor human health and diagnose diseases. However, achieving biodegradability, biocompatibility, and high conformality with soft tissues poses significant challenges for IHB. Gelatin is the most suitable candidate for IHB since it is a collagen hydrolysate and a substantial part of the extracellular matrix found naturally in most tissues. This study used 3D printing ultrafine fiber networks with metamaterial design to embed into ultra-low elastic modulus hydrogel to create a novel gelatin-based conductive film (GCF) with mechanical programmability. The regulation of GCF nearly covers soft tissue mechanics, an elastic modulus from 20 to 420 kPa, and a Poisson's ratio from - 0.25 to 0.52. The negative Poisson's ratio promotes conformality with soft tissues to improve the efficiency of biological interfaces. The GCF can monitor heartbeat signals and respiratory rate by determining cardiac deformation due to its high conformability. Notably, the gelatin characteristics of the biodegradable GCF enable the sensor to monitor and support tissue restoration. The GCF metamaterial design offers a unique idea for bioelectronics to develop implantable sensors that integrate monitoring and tissue repair and a customized method for endowing implanted sensors to be highly conformal with soft tissues.

A bioengineered trachea-like structure improves survival in a rabbit tracheal defect model

A practical strategy for engineering a trachea-like structure that could be used to repair or replace a damaged or injured trachea is an unmet need. Here, we fabricated bioengineered cartilage (BC) rings from three-dimensionally printed fibers of poly(ɛ-caprolactone) (PCL) and rabbit chondrocytes. The extracellular matrix (ECM) secreted by the chondrocytes combined with the PCL fibers formed a "concrete-rebar structure," with ECM deposited along the PCL fibers, forming a grid similar to that of native cartilage. PCL fiber-hydrogel rings were then fabricated and alternately stacked with BC rings on silicone tubes. This trachea-like structure underwent vascularization after heterotopic transplantation into rabbits for 4 weeks. The vascularized bioengineered trachea-like structure was then orthotopically transplanted by end-to-end anastomosis to native rabbit trachea after a segment of trachea had been resected. The bioengineered trachea-like structure displayed mechanical properties similar to native rabbit trachea and transmural angiogenesis between the rings. The 8-week survival rate in transplanted rabbits was 83.3%, and the respiratory rate of these animals was similar to preoperative levels. This bioengineered trachea-like structure may have potential for treating tracheal stenosis and other tracheal injuries.

Auxeticity as a Mechanobiological Tool to Create Meta-Biomaterials

Mechanical and morphological design parameters, such as stiffness or porosity, play important roles in creating orthopedic implants and bone substitutes. However, we have only a limited understanding of how the microarchitecture of porous scaffolds contributes to bone regeneration. Meta-biomaterials are increasingly used to precisely engineer the internal geometry of porous scaffolds and independently tailor their mechanical properties (e.g., stiffness and Poisson's ratio). This is motivated by the rare or unprecedented properties of meta-biomaterials, such as negative Poisson's ratios (i.e., auxeticity). It is, however, not clear how these unusual properties can modulate the interactions of meta-biomaterials with living cells and whether they can facilitate bone tissue engineering under static and dynamic cell culture and mechanical loading conditions. Here, we review the recent studies investigating the effects of the Poisson's ratio on the performance of meta-biomaterials with an emphasis on the relevant mechanobiological aspects. We also highlight the state-of-the-art additive manufacturing techniques employed to create meta-biomaterials, particularly at the micrometer scale. Finally, we provide future perspectives, particularly for the design of the next generation of meta-biomaterials featuring dynamic properties (e.g., those made through 4D printing).

Electric-Field-Driven Printed 3D Highly Ordered Microstructure with Cell Feature Size Promotes the Maturation of Engineered Cardiac Tissues

Engineered cardiac tissues (ECTs) derived from human induced pluripotent stem cells (hiPSCs) are viable alternatives for cardiac repair, patient-specific disease modeling, and drug discovery. However, the immature state of ECTs limits their clinical utility. The microenvironment fabricated using 3D scaffolds can affect cell fate, and is crucial for the maturation of ECTs. Herein, the authors demonstrate an electric-field-driven (EFD) printed 3D highly ordered microstructure with cell feature size to promote the maturation of ECTs. The simulation and experimental results demonstrate that the EFD jet microscale 3D printing overcomes the jet repulsion without any prior requirements for both conductive and insulating substrates. Furthermore, the 3D highly ordered microstructures with a fiber diameter of 10-20 µm and spacing of 60-80 µm have been fabricated by maintaining a vertical jet, achieving the largest ratio of fiber diameter/spacing of 0.29. The hiPSCs-derived cardiomyocytes formed ordered ECTs with their sarcomere growth along the fiber and developed synchronous functional ECTs inside the 3D-printed scaffold with matured calcium handling compared to the 2D coverslip. Therefore, the EFD jet 3D microscale printing process facilitates the fabrication of scaffolds providing a suitable microenvironment to promote the maturation of ECTs, thereby showing great potential for cardiac tissue engineering.

Recent advances in melt electro writing for tissue engineering for 3D printing of microporous scaffolds for tissue engineering

Melt electro writing (MEW) is a high-resolution 3D printing technique that combines elements of electro-hydrodynamic fiber attraction and melts extrusion. The ability to precisely deposit micro- to nanometer strands of biocompatible polymers in a layer-by-layer fashion makes MEW a promising scaffold fabrication method for all kinds of tissue engineering applications. This review describes possibilities to optimize multi-parametric MEW processes for precise fiber deposition over multiple layers and prevent printing defects. Printing protocols for nonlinear scaffolds structures, concrete MEW scaffold pore geometries and printable biocompatible materials for MEW are introduced. The review discusses approaches to combining MEW with other fabrication techniques with the purpose to generate advanced scaffolds structures. The outlined MEW printer modifications enable customizable collector shapes or sacrificial materials for non-planar fiber deposition and nozzle adjustments allow redesigned fiber properties for specific applications. Altogether, MEW opens a new chapter of scaffold design by 3D printing.

Progress of 3D Printing Techniques for Nasal Cartilage Regeneration

Once cartilage is damaged, its self-repair capacity is very limited. The strategy of tissue engineering has brought a new idea for repairing cartilage defect and cartilage regeneration. In particular, nasal cartilage regeneration is a challenge because of the steady increase in nasal reconstruction after oncologic resection, trauma, or rhinoplasty. From this perspective, three-dimensional (3D) printing has emerged as a promising technology to address the complexity of nasal cartilage regeneration, using patient's image data and computer-aided deposition of cells and biomaterials to precisely fabricate complex, personalized tissue-engineered constructs. In this review, we summarized the major progress of three prevalent 3D printing approaches, including inkjet-based printing, extrusion-based printing and laser-assisted printing. Examples are highlighted to illustrate 3D printing for nasal cartilage regeneration, with special focus on the selection of seeded cell, scaffolds and growth factors. The purpose of this paper is to systematically review recent research about the challenges and progress and look forward to the future of 3D printing techniques for nasal cartilage regeneration.Level of Evidence III This journal requires that authors assign a level of evidence to each submission to which Evidence-Based Medicine rankings are applicable. This excludes Review Articles, Book Reviews, and manuscripts that concern Basic Science, Animal Studies, Cadaver Studies, and Experimental Studies. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors https://www.springer.com/00266 .

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Cancer cell migration on straight, wavy, loop and grid microfibre patterns

Cell migration plays an important role in physiological and pathological processes where the fibrillar morphology of extracellular matrices (ECM) could regulate the migration dynamics. To mimic the morphological characteristics of fibrillar matrix structures, low-voltage continuous electrospinning was adapted to construct straight, wavy, looped and gridded fibre patterns made of polystyrene (of fibre diameter ca. 3 μm). Cells were free to explore their different shapes in response to the directly-adhered fibre, as well as to the neighbouring patterns. For all the patterns studied, analysing cellular migration dynamics of MDA-MB-231 (a highly migratory breast cancer cell line) demonstrated two interesting findings: first, although cells dynamically adjust their shapes and migration trajectories in response to different fibrillar environments, their average step speed is minimally affected by the fibre global pattern; secondly, a switch in behaviour was observed when the pattern features approach the upper limit of the cell body's minor axis, reflecting that cells' ability to divert from an existing fibre track is limited by the size along the cell body's minor axis. It is therefore concluded that the upper limit of cell body's minor axis might act as a guide for the design of microfibre patterns for different purposes of cell migration.

3D bioprinting: current status and trends—a guide to the literature and industrial practice

The multidisciplinary research field of bioprinting combines additive manufacturing, biology and material sciences to create bioconstructs with three-dimensional architectures mimicking natural living tissues. The high interest in the possibility of reproducing biological tissues and organs is further boosted by the ever-increasing need for personalized medicine, thus allowing bioprinting to establish itself in the field of biomedical research, and attracting extensive research efforts from companies, universities, and research institutes alike. In this context, this paper proposes a scientometric analysis and critical review of the current literature and the industrial landscape of bioprinting to provide a clear overview of its fast-changing and complex position. The scientific literature and patenting results for 2000–2020 are reviewed and critically analyzed by retrieving 9314 scientific papers and 309 international patents in order to draw a picture of the scientific and industrial landscape in terms of top research countries, institutions, journals, authors and topics, and identifying the technology hubs worldwide. This review paper thus offers a guide to researchers interested in this field or to those who simply want to understand the emerging trends in additive manufacturing and 3D bioprinting.

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Electrohydrodynamic jet 3D printing in biomedical applications

Electrohydrodynamic Jet 3D Printing (e-jetting) is a promising technique developed from electrospinning, which enables precise fiber deposition in a layer-by-layer fashion with customized designs. Several studies have verified that e-jetted scaffolds were able to support cell attachment, proliferation, and extracellular matrix formation, as well as cell infiltration into the scaffold due to the well-defined pores. Besides, e-jetting has also been combined with other techniques to incorporate biomaterials (e.g., hydrogels and cell spheroids) that could not be e-jetted, to promote the biological performance of the scaffold. In the recent decade, applying e-jetting in the fabrication of tissue-engineered scaffolds has drawn a lot of interest. Moreover, efforts have been put to develop varied scaffolds for some specific biomedical applications such as cartilage, tendon, and blood vessel, which exhibited superior mechanical properties and promoted cell behaviors including cellular alignment and differentiation. This review article also provides the reader with some crucial considerations and major limitations of e-jetting, such as scaffold design, printability of large-scale constructs, applicable biomaterials, and cell behaviors. Overall, this review article expounds on perspectives in the context of development and biomedical applications of this technique. STATEMENT OF SIGNIFICANCE: E-jetting technique is able to produce fibers with diameter in micrometer scale, which has been considered as a promising 3D printing technique. This technique has shown promise for regeneration of tissue engineered scaffolds with well-defined structures, which has been reported to apply in regeneration of different tissue types. The superior controllability of the process endows the feasibility of constructing multi-scale scaffolds with great biological mimicry and cellular infiltration. The incorporation of other biomaterials into the e-jetted networks further reinforces the scope of applications as compared to e-jetted scaffolds only. There is no doubt that e-jetting will be a great tool for tissue engineered scaffolding, and this review article will give overall perspectives in this topic.

Hydrogels for Large-Scale Expansion of Stem Cells

Stem cells demonstrate considerable promise for various preclinical and clinical applications, including drug screening, disease treatments, and regenerative medicine. Producing high-quality and large amounts of stem cells is in demand for these applications. Despite challenges, as hydrogel-based cell culture technology has developed, tremendous progress has been made in stem cell expansion and directed differentiation. Hydrogels are soft materials with abundant water. Many hydrogel properties, including biodegradability, mechanical strength, and porosity, have been shown to play essential roles in regulating stem cell proliferation and differentiation. The biochemical and physical properties of hydrogels can be specifically tailored to mimic the native microenvironment that various stem cells reside in vivo. A few hydrogel-based systems have been developed for successful stem cell cultures and expansion in vitro. In this review, we summarize various types of hydrogels that have been designed to effectively enhance the proliferation of hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs), respectively. According to each stem cell type's preference, we also discuss strategies for fabricating hydrogels with biochemical and mechanical cues and other characteristics representing microenvironments of stem cells in vivo. STATEMENT OF SIGNIFICANCE: In this review article we summarize current progress on the construction of hydrogel systems for the culture and expansion of various stem cells, including hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs). The Significance includes: (1) Provide detailed discussion on the stem cell niches that should be considered for stem cell in vitro expansion. (2) Summarize various strategies to construct hydrogels that can largely recapture the microenvironment of native stem cells. (3) Suggest a few future directions that can be implemented to improve current in vitro stem cell expansion systems.

Highly Ordered 3D Tissue Engineering Scaffolds as a Versatile Culture Platform for Nerve Cells Growth

Tissue engineering scaffolds provide an encouraging alternative for nerve injuries due to their biological support for nerve cell growth, which can be used for neuronal repair. Nerve cells have been reported to be mostly cultured on 2D scaffolds that cannot mimic the native extracellular matrix. Herein, highly ordered 3D scaffolds are fabricated for nerve cell culture by melt electrospinning writing, the microstructures and geometries of the scaffolds could be well modulated. An effective strategy for scaffold surface modification to promote nerve cell growth is proposed. The effects of scaffolds with different surface modifications, viz., plasma treatment, single poly-D-lysine (PDL) coating after plasma treatment, single laminin (LM) coating after plasma treatment, double PDL and LM coatings after plasma treatment, on PC12 cell growth are evaluated. Experiments show the scaffold modified with double PDL and LM coatings after plasma treatment facilitated the growth of PC12 cells most effectively, indicating the synergistic effect of PDL and LM on the growth of nerve cells. This is the first systematic and quantitative study of the effects of different scaffold surface modifications on nerve cell growth. The above results provide a versatile culture platform for growing nerve cells, and for recovery from peripheral nerve injury.

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.

Effect of Dual Pore Size Architecture on In Vitro Osteogenic Differentiation in Additively Manufactured Hierarchical Scaffolds

The combination of macro- and microporosity is a potent manner of enhancing osteogenic potential, but the biological events leading to this increase in osteogenesis are not well understood. In this study, we investigated the effect of a dual pore size scaffold on the physical and biological properties, with the hypothesis that cell condensation is the determining factor for enhanced osteogenic differentiation. To this end, a hierarchical scaffold possessing a dual (large and small) pore size was fabricated by combining two additive manufacturing techniques: melt electrospinning writing (MEW) and fused deposition modeling (FDM). The scaffolds showed a mechanical stiffness of 23.2 ± 1.5 MPa similar to the FDM control scaffold, while the hybrid revealed an increased specific surface area of 1.4 ± 0.1 m²/g. The scaffold was cultured with primary human osteoblasts for 28 days, which showed enhanced cell adhesion and proliferation. The hierarchical structure was also beneficial for in vitro alkaline phosphate activity and mineralization and showed an increased expression of osteogenic protein and genes. Mesenchymal condensation markers related to osteoblastic differentiation (CDH2, RhoA, Rac1, and Cdc42) were upregulated in the hybrid construct, demonstrating that the MEW membrane provided an environment more suitable for the recapitulation of cell condensation, which in turn leads to higher osteogenic differentiation. In summary, this study demonstrated that the hierarchical scaffold developed in this paper leads to a significant improvement in the scaffold properties such as increased specific surface area, initial cell adhesion, cell proliferation, and in vitro osteogenesis.

Fabrication and Characterization of the Core-Shell Structure of Poly(3-Hydroxybutyrate-4-Hydroxybutyrate) Nanofiber Scaffolds

Tissue engineering scaffolds with nanofibrous structures provide positive support for cell proliferation and differentiation in biomedical fields. These scaffolds are widely used for defective tissue repair and drug delivery. However, the degradation performance and mechanical properties of scaffolds are often unsatisfactory. Here, we successfully prepared a novel poly(3-hydroxybutyrate-4-hydroxybutyrate)/polypyrrole (P34HB-PPy) core-shell nanofiber structure scaffold with electrospinning and in situ surface polymerization technology. The obtained composite scaffold showed good mechanical properties, hydrophilicity, and thermal stability based on the universal material testing machine, contact angle measuring system, thermogravimetric analyzer, and other methods. The results of the in vitro bone marrow-derived mesenchymal stem cells (BMSCs) culture showed that the P34HB-PPy composite scaffold effectively mimicked the extracellular matrix (ECM) and exhibited good cell retention and proliferative capacity. More importantly, P34HB is a controllable degradable polyester material, and its degradation product 3-hydroxybutyric acid (3-HB) is an energy metabolite that can promote cell growth and proliferation. These results strongly support the application potential of P34HB-PPy composite scaffolds in biomedical fields, such as tissue engineering and soft tissue repair.

3D printing of tissue engineering scaffolds: a focus on vascular regeneration

Tissue engineering is an emerging means for resolving the problems of tissue repair and organ replacement in regenerative medicine. Insufficient supply of nutrients and oxygen to cells in large-scale tissues has led to the demand to prepare blood vessels. Scaffold-based tissue engineering approaches are effective methods to form new blood vessel tissues. The demand for blood vessels prompts systematic research on fabrication strategies of vascular scaffolds for tissue engineering. Recent advances in 3D printing have facilitated fabrication of vascular scaffolds, contributing to broad prospects for tissue vascularization. This review presents state of the art on modeling methods, print materials and preparation processes for fabrication of vascular scaffolds, and discusses the advantages and application fields of each method. Specially, significance and importance of scaffold-based tissue engineering for vascular regeneration are emphasized. Print materials and preparation processes are discussed in detail. And a focus is placed on preparation processes based on 3D printing technologies and traditional manufacturing technologies including casting, electrospinning, and Lego-like construction. And related studies are exemplified. Transformation of vascular scaffolds to clinical application is discussed. Also, four trends of 3D printing of tissue engineering vascular scaffolds are presented, including machine learning, near-infrared photopolymerization, 4D printing, and combination of self-assembly and 3D printing-based methods.

High-resolution electrohydrodynamic bioprinting: a new biofabrication strategy for biomimetic micro/nanoscale architectures and living tissue constructs

Electrohydrodynamic (EHD) printing is a newly emerging additive manufacturing strategy for the controlled fabrication of three-dimensional (3D) micro/nanoscale architectures. This unique superiority makes it particularly suitable for the biofabrication of artificial tissue analogs with biomimetic structural organizations similar to the scales of native extracellular matrix (ECM) or living cells, which shows great potentials to precisely regulate cellular behaviors and tissue regeneration. Here the state-of-the-art advancements of high-resolution EHD bioprinting were reviewed mainly including melt-based and solution-based processes for the fabrication of micro/nanoscale fibrous scaffolds and living tissues constructs. The related printing materials, innovations on structure design and printing processes, functionalization of the resultant architectures as well as their effects on the mechanical and biological properties of the EHD-printed structures were introduced and analyzed. The recent explorations on the EHD cell printing for high-resolution cell-laden microgel patterning and 3D construct fabrication were highlighted. The major challenges as well as possible solutions to translate EHD bioprinting into a mature and prevalent biofabrication strategy were finally discussed.