Tubular Constructs Research Articles

Toward In Vitro Vascular Wall Models: Digital Light Processing of Acrylate‐Endcapped Urethane‐Based Polymers into Tubular Constructs

Toward In Vitro Vascular Wall Models: Digital Light Processing of Acrylate‐Endcapped Urethane‐Based Polymers into Tubular Constructs

Bio‐Inspired Design of 4D‐Printed Scaffolds Capable of Programmable Multi‐Step Transformations Toward Vascular Reconstruction

AbstractMany withered leaves or flowers spontaneously curl and transform from flattened structures into tubular constructs upon dehydration. Inspired by this phenomenon, an innovative strategy is developed to design stimuli‐responsive scaffolds that are capable of programmable transformation from flattened 2D constructs into various curled 3D tissue‐mimicking structures. Specifically, Janus‐structured scaffolds consisting of a passive layer of polycaprolactone and an active, triggered‐transforming layer of methacrylate gelatin/alginate hydrogel are 3D‐printed, which replicate the asymmetrical structure of leaves and enabled on‐demand transformation into desired curvatures and shapes through mimicking the regulation mechanism of leaves curling behavior by mesophyll matrix stiffness and vein patterns. Particularly for vascular reconstruction, multi‐step transformation scaffolds capable of primary 2D‐to‐3D transformation into tubular constructs induced by dehydration, and secondary transformation to adapt to the local intravascular geometry in vivo, are successfully developed. In addition, the oriented polycaprolactone layer of the printed scaffold can enable the adhesion, proliferation, and orientation of endothelial cells, thus demonstrating a novel strategy for the design of tissue‐engineered blood vessels. In general, this study provides an innovative design strategy for programmable biomaterials capable of shape morphing and adaptation in physiological conditions, thereby opening up a new avenue for the design of intelligent biomaterials in regenerative medicine.

Optimization of crashworthiness design of foam-filled crash boxes under oblique loading for electric vehicles

To increase the energy-absorbing capability of frontal collision management systems and improve vehicle crash safety, foam-filled crash boxes should be optimized. On the basis of a double tubular construction, a novel foam-filled crash box with a different design is developed. The energy absorption capacity, initial peak force, and deformation modes of the original and improved crash boxes were examined using impact models. As opposed to the full-filling design, it is demonstrated that the filling design may utilize less foam while increasing specific energy absorption. The stability of continuing deformation after the first buckling is determined by the foam-filled crash box. For the foam-filled crash box, a better-optimized design technique is suggested using the Taguchi method and principal component analysis (PCA). Compression tests are used to validate the design concept. Therefore, the optimal design technique of the crash box is suitable and practical for the crashworthiness design of crash boxes, considering the combined effect of significant indicators for electric vehicles.

Fabrication of a Compliant Vascular Graft Using Extrusion Printing and Electrospinning Technique

AbstractSmall‐diameter vascular grafts having biomechanical properties similar to those of native arteries for the treatment of cardiovascular diseases are still elusive. Here, a hybrid extrusion printing and electrospinning technique is presented in which a layer of electrospun nanofibers is deposited over the printed gelatin‐methacryloyl (gelMA) constructs to improve the mechanical performance of gelMA grafts. Various blends of polycaprolactone (PCL) and poly(L‐lactide‐co‐ε‐caprolactone) (PLCL) polymer solutions are used to produce electrospun nanofibers. The variation of gelMA concentration is found to have a negligible role in the overall strength of the graft. It is shown that the burst pressure and tensile strength of the fiber‐reinforced gelMA constructs are comparable to those of native human arteries. Moreover, the compliance of grafts reinforced by 100% PCL and 75/25% PCL/PLCL nanofibers are found to be similar to human muscular arteries and elastic arteries, respectively. The cytocompatibility assessment shows that gelMA presents a bioactive surface for the endothelial cells to survive and grow. Also, PCL/PLCL electrospun nanofibers offer cellular metabolic activity in the same order of magnitude as observed in the control. Therefore, this hybrid technique opens up new possibilities for the fabrication of tubular constructs in tissue engineering.

The Design and Fabrication of Engineered Tubular Tissue Constructs Enabled by Electrohydrodynamic Fabrication Techniques: A Review

AbstractElectrohydrodynamic processes have emerged as promising methods for fabricating polymetric fiber‐based artificial tubular tissues. Existing review articles focus on the biological applications and processing materials associated with electrohydrodynamic processes in artificial tubular constructs, while overlooking the design and fabrication of these constructs. To address this gap, this review article emphasizes the design and fabrication of tubular tissue constructs enabled by employing electrohydrodynamic processes. This article begins by presenting an overview of two electrohydrodynamic processes: solution electrospinning (SE) and melt electrowriting (MEW). It then delves into the control of the fiber diameter enabled by SE and MEW, offering insights into the manipulation of processing parameters to achieve desired fiber diameters. Additionally, the review highlights cutting‐edge strategies for electrohydrodynamic processes to create tubular structures with customized microarchitectures. This includes fiber alignment control for SE and pore morphology design for MEW. Moreover, the review covers the creation of customized macroscale tubular geometries through collector geometry design. Lastly, a comprehensive survey is presented for designing multiphasic tubular structures specifically for electrohydrodynamic techniques or in tandem with other techniques. The objective of this review is to offer a thorough understanding of the design considerations and potential applications of tubular structures fabricated by electrohydrodynamic processes.

A Melt Electrowriting Toolbox for Automated G‐Code Generation and Toolpath Correction of Flat and Tubular Constructs

A Melt Electrowriting Toolbox for Automated G‐Code Generation and Toolpath Correction of Flat and Tubular Constructs

Triaxial mechanical characterization of ultrasoft 3D support bath-based bioprinted tubular GelMA constructs

Support bath-based extrusion bioprinting is an emerging additive manufacturing paradigm that allows for the creation of geometrically complex three-dimensional tissue-mimicking constructs. Although this approach enables arbitrary geometries to be printed, preserving shape and mechanical integrity after the construct is extracted from the support bath is one of the main challenges today. In the present study, we systematically tested the effect of critical printing parameters on construct integrity and stiffness. Specifically, we varied the concentration and temperature of the bioink and support bath material, hydrogel crosslinking time, and cell density of the bioink. While previous studies on hydrogel materials quantify construct properties based on a single loading mode-such as uniaxial testing or rheological experiments, for example—we used a multiaxial mechanical testing protocol to assess the printed construct’s response in tension, compression, and shear. For each sample, we determined a single set of material parameters by simultaneously fitting all three modes in our inverse finite element framework. In support of future modeling work, we analyzed three different constitutive models with respect to their ability to match the construct’s mechanical response. We observed that GelMA concentration, temperature, and cell density have a statistically significant effect on the construct stiffness; support bath temperature and UV crosslinking time have a weak effect on construct stiffness; and support bath concentration appears to have no direct effect on the measured construct properties. Our construct stiffness were found to vary between 0.07 and 2.2 kPa depending on printing conditions, and showed noticeable tension–compression asymmetry as well as pronounced nonlinear behavior when loaded under shear.

Fabrication of heart tubes from iPSC derived cardiomyocytes and human fibrinogen by rotating mold technology

Due to its structural and functional complexity the heart imposes immense physical, physiological and electromechanical challenges on the engineering of a biological replacement. Therefore, to come closer to clinical translation, the development of a simpler biological assist device is requested. Here, we demonstrate the fabrication of tubular cardiac constructs with substantial dimensions of 6 cm in length and 11 mm in diameter by combining human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and human foreskin fibroblast (hFFs) in human fibrin employing a rotating mold technology. By centrifugal forces employed in the process a cell-dense layer was generated enabling a timely functional coupling of iPSC-CMs demonstrated by a transgenic calcium sensor, rhythmic tissue contractions, and responsiveness to electrical pacing. Adjusting the degree of remodeling as a function of hFF-content and inhibition of fibrinolysis resulted in stable tissue integrity for up to 5 weeks. The rotating mold device developed in frame of this work enabled the production of tubes with clinically relevant dimensions of up to 10 cm in length and 22 mm in diameter which—in combination with advanced bioreactor technology for controlled production of functional iPSC-derivatives—paves the way towards the clinical translation of a biological cardiac assist device.

3D Printing of Aqueous Two-Phase Systems with Linear and Bottlebrush Polyelectrolytes.

We formed core-shell-like polyelectrolyte complexes (PECs) from an anionic bottlebrush polymer with poly (acrylic acid) side chains with a cationic linear poly (allylamine hydrochloride). By varying the pH, the number of side chains of the polyanionic BB polymers (Nbb), the charge density of the polyelectrolytes, and the salt concentration, the phase separation behavior and salt resistance of the complexes could be tuned by the conformation of the BBs. By combining the linear/bottlebrush polyelectrolyte complexation with all-liquid 3D printing, flow-through tubular constructs were produced that showed selective transport across the PEC membrane comprising the walls of the tubules. These tubular constructs afford a new platform for flow-through delivery systems.

3D Printing of Aqueous Two‐Phase Systems with Linear and Bottlebrush Polyelectrolytes

AbstractWe formed core–shell‐like polyelectrolyte complexes (PECs) from an anionic bottlebrush polymer with poly (acrylic acid) side chains with a cationic linear poly (allylamine hydrochloride). By varying the pH, the number of side chains of the polyanionic BB polymers (Nbb), the charge density of the polyelectrolytes, and the salt concentration, the phase separation behavior and salt resistance of the complexes could be tuned by the conformation of the BBs. By combining the linear/bottlebrush polyelectrolyte complexation with all‐liquid 3D printing, flow‐through tubular constructs were produced that showed selective transport across the PEC membrane comprising the walls of the tubules. These tubular constructs afford a new platform for flow‐through delivery systems.

The Past, Present, and Future of Tubular Melt Electrowritten Constructs to Mimic Small Diameter Blood Vessels - A Stable Process?

Melt Electrowriting (MEW) is a continuously growing manufacturing platform. Its advantage is the consistent production of micro- to nanometer fibers, that stack intricately, forming complex geometrical shapes. MEW allows tuning of the mechanical properties of constructs via the geometry of deposited fibers. Due to this, MEW can create complex mechanicsonly seen in multi-material compounds and serve as guiding structures for cellular alignment. The advantage of MEW is also shown in combination with other biotechnological manufacturing methods to create multilayered constructs that increase mechanical approximation to native tissues, biocompatibility, and cellular response. These features make MEW constructs a perfect candidate for small-diameter vascular graft structures. Recently, studies have presented fascinating results in this regard, but is this truly the direction that tubular MEW will follow or are there also other options on the horizon? This perspective will explore the origins and developments of tubular MEW and present its growing importance in the field of artificial small-diameter vascular grafts with mechanical modulation and improved biomimicry and the impact of it in convergence with other manufacturing methods and how future technologies like AI may influence its progress.

Polyhydroxybutyrate-based osteoinductive mineralized electrospun structures that mimic components and tissue interfaces of the osteon for bone tissue engineering.

Scaffolds for bone tissue engineering should enable regeneration of bone tissues with its native hierarchically organized ECM and multiple tissue interfaces. To achieve this, inspired by the structure and properties of bone osteon, we fabricated polyhydroxybutyrate-based mineralized electrospun fibrous scaffolds. After studying multiple polyhydroxybutyrate (PHB)-based fibers, we chose 7%PHB/1%Gelatin fibers (PG) to fabricate mineralized fibers that mimic mineralized collagen fibers in bone. The mineralized PG (mPG) surface had a rough, hydrophilic layer of low crystalline calcium phosphate which was biocompatible to BMSCs, induced their proliferation and was osteoinductive. Subsequently, by modulating the electrospinning process, we fabricated mPG-based novel higher order fibrous scaffolds that mimic the macroscale geometries of osteons of bone ECM. Inspired by the aligned collagen fibers in bone lamellae, we fabricated mPG scaffolds with aligned fibers that could direct anisotropic elongation of mBMSCs. Further, we fabricated electrospun mPG-based osteoinductive tubular constructs which can mimic cylindrical bone components like osteons or lamellae or be used as long bone analogues based on their dimensions. Finally, to regenerate tissue interfaces in bone, we introduced a novel bi-layered scaffold-based approach. An electrospun bi-layered tubular construct that had PG in the outer layer and 7%PHB/0.5%Polypyrrole fibers (PPy) in the inner layer was fabricated. The bi-layered tubular construct underwent preferential surface mineralization only on its outer layer. This outer mineralized layer supported osteogenesis while the inner PPy layer could support neural cell growth. Thus, the bi-layered tubular construct may be used to regenerate haversian canal in the osteons which hosts nerve fibers. Overall, the study introduced novel techniques to fabricate biomimetic structures that can regenerate components of bone osteon and its multiple tissue interfaces. The study lays foundation for the fabrication of a modular scaffold that can regenerate bone with its hierarchical structure and complex tissue interfaces.

A self-assembly and cellular migration based fabrication of high-density 3D tubular constructs of barrier forming membranes.

Three-dimensional (3D) in vitro models, superior in simulating physiological conditions compared to 2D models, offer intricate cell-cell and cell-ECM interactions with diverse signaling cues like fluid shear stress and growth factor gradients. Yet, developing 3D tissue barrier models, specifically perfusable luminal structures with dense, multicellular constructs maintained for extended durations with oxygen and nutrients, remains a technical challenge. Here, we describe a molding-based approach for the fabrication of free-standing, perfusable, high cellular density tissue constructs using a self-assembly and migration process to form functional barriers. This technique utilizes a polytetrafluoroethylene (PTFE)-coated stainless-steel wire, held by stainless steel needles, as a template for a perfusable channel within an elongated PDMS well. Upon adding a bio-ink mix of cells and collagen, it self-assembles into a high cell density layer conformally around the wire. Removing the wire reveals a hollow construct, connectable to an inlet and outlet for perfusion. This scalable method allows creating varied dimensions and multicellular configurations. Notably, post-assembly, cells such as human umbilical vein endothelial cells (HUVECs) migrate to the surface and form functional barriers with adherens junctions. Permeability tests and fluorescence imaging confirm that these constructs closely mimic in vivo endothelial barrier permeability, exhibiting the lowest permeability among all in vitro models in the literature. Unlike traditional methods involving uneven post-seeding of endothelial cells leading to subpar barriers, our approach is a straightforward alternative for fabricating complex perfusable 3D tissue constructs and effective tissue barriers for use in various applications, including tissue engineering, drug screening, and disease modeling.

Optimization of foam-filled crash-box under axial loading for pure electric vehicle

To increase the energy-absorbing capability of frontal collision management systems and raise vehicle crash safety, foam-filled crash boxes should be optimized. On the basis of a double tubular construction, a novel foam-filled crash box with different design is developed. The energy absorption capacity, initial peak force and deformation modes of the original and improved crash boxes were examined using impact models. As opposed to the full-filling design, it is demonstrated that the filling design may utilize less foam while increasing specific energy absorption. The stability of the continuing deformation after the first buckling is determined by the foam filled crash-box combined. For the foam-filled crash box, a better optimized design technique is suggested using Radial Basic Function and Non-dominated Sorting GA II. Compression tests are used to validate the design concept. Therefore, the optimal design technique of the crash box is suitable and practical for the crashworthiness design of crash boxes, taking consideration the combined effect of significant indicators for electric vehicle.

Advances in Fabrication Technologies for the Development of Next-Generation Cardiovascular Stents.

Coronary artery disease is the most prevalent cardiovascular disease, claiming millions of lives annually around the world. The current treatment includes surgically inserting a tubular construct, called a stent, inside arteries to restore blood flow. However, due to lack of patient-specific design, the commercial products cannot be used with different vessel anatomies. In this review, we have summarized the drawbacks in existing commercial metal stents which face problems of restenosis and inflammatory responses, owing to the development of neointimal hyperplasia. Further, we have highlighted the fabrication of stents using biodegradable polymers, which can circumvent most of the existing limitations. In this regard, we elaborated on the utilization of new fabrication methodologies based on additive manufacturing such as three-dimensional printing to design patient-specific stents. Finally, we have discussed the functionalization of these stent surfaces with suitable bioactive molecules which can prove to enhance their properties in preventing thrombosis and better healing of injured blood vessel lining.

Pump-Less Platform Enables Long-Term Recirculating Perfusion of 3D Printed Tubular Tissues.

The direction and pattern of fluid flow affect vascular structure and function, in which vessel-lining endothelial cells exhibit variable cellular morphologies and vessel remodeling by mechanosensing. To recapitulate this microenvironment, some approaches have been reported to successfully apply unidirectional flow on endothelial cells in organ-on-a-chip systems. However, these platforms encounter drawbacks such as the dependency on pumps or confinement to closed microfluidic channels. These constraints impede their synergy with advanced biofabrication techniques like 3D bioprinting, thereby curtailing the potential to introduce greater complexity into engineered tissues. Herein, a pumpless recirculating platform (UniPlate) that enables unidirectional media recirculation through 3D printed tubular tissues, is demonstrated.The device is made of polystyrene via injection molding in combination with 3D printed sacrifical gelatin templates. Tubular blood vessels with unidirectional perfusion are firstly engineered. Then the design is expanded to incorporate duo-recirculating flow for culturing vascularized renal proximal tubules with glucose reabsorption function. In addition to media recirculation, human monocyte recirculation in engineered blood vessels is also demonstrated for over 24h, with minimal loss of cells, cell viability, and inflammatory activation. UniPlate can be a valuable tool to more precisely control the cellular microenvironment of organ-on-a-chip systems for drug discovery.

Combined light-cured and sacrificial hydrogels for fabrication of small-diameter bionic vessels by 3D bioprinting.

Bionic grafts can replace autologous tissue through tissue engineering in cases of cardiovascular disease. However, small-diameter vessel grafts remain challenging to precellularize. Bionic small-diameter vessels with endothelial and smooth muscle cells (SMCs) manufactured with a novel approach. A 1-mm-diameter bionic blood vessel was constructed by combining light-cured hydrogel gelatin-methacryloyl (GelMA) with sacrificial hydrogel Pluronic F127. Mechanical properties of GelMA (Young's modulus and tensile stress) were tested. Cell viability and proliferation were detected using Live/dead staining and CCK-8 assays, respectively. The histology and function of the vessels were observed using hematoxylin and eosin and immunofluorescence staining. GelMA and Pluronic were printed together using extrusion. The temporary Pluronic support was removed by cooling during GelMA crosslinking, yielding a hollow tubular construct. A bionic bilayer vascular structure was fabricated by loading SMCs into the GelMA bioink, followed by perfusion with endothelial cells. In the structure, both cell types maintained good cell viability. The vessel showed good histological morphology and function. Using light-cured and sacrificial hydrogels, we formed a small ca bionic vessel with a small caliber containing SMCs and endothelial cells, demonstrating an innovative approach for construction of bionic vascular tissues.

Volumetric Printing across Melt Electrowritten Scaffolds Fabricates Multi-Material Living Constructs with Tunable Architecture and Mechanics.

Major challenges in biofabrication revolve around capturing the complex, hierarchical composition of native tissues. However, individual 3D printing techniques have limited capacity to produce composite biomaterials with multi-scale resolution. Volumetric bioprinting recently emerged as a paradigm-shift in biofabrication. This ultra-fast, light-based technique sculpts cell-laden hydrogel bioresins into three-dimensional structures in a layerless fashion, providing enhanced design freedom over conventional bioprinting. However, it yields prints with low mechanical stability, since soft, cell-friendly hydrogels are used. Herein, for the first time, the possibility to converge volumetric bioprinting with melt electrowriting, which excels at patterning microfibers, is shown for the fabrication of tubular hydrogel-based composites with enhanced mechanical behavior. Despite including non-transparent melt electrowritten scaffolds into the volumetric printing process, high-resolution bioprinted structures were successfully achieved. Tensile, burst and bending mechanical properties of printed tubes were tuned altering the electrowritten mesh design, resulting in complex, multi-material tubular constructs with customizable, anisotropic geometries that better mimic intricate biological tubular structures. As a proof-of-concept, engineered tubular structures were obtained by building tri-layered cell-laden vessels, and features (valves, branches, fenestrations) that could be rapidly printed using this hybrid approach. This multi-technology convergence offers a new toolbox for manufacturing hierarchical and mechanically tunable multi-material living structures. This article is protected by copyright. All rights reserved.

CARD11: Polymeric Endoluminal Paving for Prevention of Paravalvular Leak in Transcatheter Aortic Valve Replacement

Background: Calcific aortic valve disease and associated aortic stenosis is increasingly common, with prevalence across the age spectrum. Transcatheter aortic valve replacement (TAVR) has emerged as a life-saving, non-surgical alternative for replacement of diseased aortic valves, restoring non-obstructive blood flow from the heart. While effective, current TAVR technology remains limited by adaptability to patient-specific anatomy, leading to incomplete deployment and non-conformal fit resulting in paravalvular leak. Polymeric endoluminal paving was previously developed as a methodology allowing deployment of polymeric materials on tissue surfaces, serving as structural materials and barriers. Here we adapt the paving methodology to fill gaps and defects resulting from incomplete TAVR valve deployment to limit paravalvular leak. Specifically, we assess the efficacy pf polymeric paving in vitro as a means of gap filling and leak reduction in valves deployed in 3-D physical models of severely stenosed aortic valves. We hypothesize that applied polymeric biomaterials will fill and conform to irregular aortic valve geometries acting as a sealant to reduce paravalvular leak. Methods: Polymeric hydrogels were prepared from ABA block copolymer macromers composed of diacrylate terminated by PLA-b-PEG-b-PLA. Hydrogel networks were prepared by swelling these macromers, following the addition of free radical initiators. Hollow tubular constructs were formed via melt-processing in PDMS molds to fit as a “sleeve” around deployed PolyV® nitinol stent (Fig 1A). The sealing efficacy of polymers was examined using a silicone replica of CAVD aortic root with deployed stent + polymer construct (Fig 1B). A rubber stopper was used to fully occlude aortic lumen and restrict flow to paravalvular areas for quantitative analysis. To test sealing against hydrostatic pressure, valve construct was connected to Tygon tubing and water was added at equivalent 10 mmHg increments at 5-minute intervals. Volume from paravalvular flow was measured at each step. Experiments were performed with N = 4 polymeric sleeves. Results: We found significant reduction in paravalvular leakage with polymer sleeves compared to no polymer at all pressures tested. Figure 1C shows the mean ± standard deviation volume of water leaked over 5 minutes of corresponding hydrostatic pressure. No leakage was detected up to 60mmHg for all polymer sleeves tested. At approximately 85-90 mmHg, the rubber stopper failed, however the polymer sleeve remained intact. Conclusions: Polymeric paving utilized with TAVR is a promising combinative therapy for preventing paravalvular leakage. Our results indicate that potential polymeric biomaterials are capable of conforming to non-physiologic aortic valve morphologies, filling and sealing paravalvular gaps, while withstanding physiologic trans-aortic pressures. With further translation, polymer paving can be useful in improving TAVR outcomes.

Experimental investigation of thermoset composite laminates manufactured with a novel sheet-winding compression-molding (SWCM) process

The carbon fiber-reinforced plastic (CFRP) tubular structure is suitable for aircraft and aerospace applications requiring strong axial strength and moderate thermal expansion. However, utilizing CFRP tubular constructions necessitates the addition of end-fitting components, which introduces new problems, such as adhesion. This study presented a novel two-step sheet-winding compression-molding (SWCM) process for fabricating integral thermosetting CFRP tubular-bar structure elements, which could be further drilled into a tubular-lug structural element. The influence of the proposed SWCM technique on the curing of the epoxy prepregs is demonstrated to be minor. The mechanical characteristics of SWCM specimens are much superior to those of control autoclave samples. The investigation of morphology provides a discussion and assumptions regarding the enhancement mechanism. It is considered that the advantage of SWCM specimens is that the particular wrapped edge suppresses the effect of free-edge delamination. This study provides a valuable guideline for designing and fabricating tubular-lug structural components.