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 mechanics only 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.

Fabric-Enhanced Vascular Graft with Hierarchical Structure for Promoting the Regeneration of Vascular Tissue

Natural blood vessels have completed functions, including elasticity, compliance, and excellent antithrombotic properties because of their mature structure. To replace damaged blood vessels, vascular grafts should perform these functions by simulating the natural vascular structures. Although the structures of natural blood vessels are thoroughly explored, constructing a small-diameter vascular graft that matches the mechanical and biological properties of natural blood vessels remains a challenge. A hierarchical vascular graft is fabricated by Electrospinning, Braiding, and Thermally induced phase separation (EBT) processes, which could simulate the structure of natural blood vessels. The internal electrospun structure facilitates the adhesion of endothelial cells, thereby accelerating endothelialization. The intermediate PLGA fabric exhibits excellent mechanical properties, which allow it to maintain its shape during long-term transplantation and prevent graft expansion. The external macroporous structure is beneficial for cell growth and infiltration. Blood vessel remodeling aims to combine a structure that promotes tissue regeneration with anti-inflammatory materials. The results in vitro demonstrated that it EBT vascular graft (EBTVG) has matched the mechanical properties, reliable cytocompatibility, and the strongest endothelialization in situ. The results in vitro and replacement of the resected artery in vivo suggest that the EBTVG combines different structural advantages with biomechanical properties and reliable biocompatibility, significantly promoting the stabilization and regeneration of vascular endothelial cells and vascular smooth muscle cells, as well as stabilizing the blood microenvironment.

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 ultrafast, light-based technique sculpts cell-laden hydrogel bioresins into 3D 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, 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 in the volumetric printing process, high-resolution bioprinted structures are successfully achieved. Tensile, burst, and bending mechanical properties of printed tubes are 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 are obtained by building trilayered cell-laden vessels, and features (valves, branches, fenestrations) that can 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.

Advances in the development of tubular structures using extrusion-based 3D cell-printing technology for vascular tissue regenerative applications

Until recent, there are no ideal small diameter vascular grafts available on the market. Most of the commercialized vascular grafts are used for medium to large-sized blood vessels. As a solution, vascular tissue engineering has been introduced and shown promising outcomes. Despite these optimistic results, there are limitations to commercialization. This review will cover the need for extrusion-based 3D cell-printing technique capable of mimicking the natural structure of the blood vessel. First, we will highlight the physiological structure of the blood vessel as well as the requirements for an ideal vascular graft. Then, the essential factors of 3D cell-printing including bioink, and cell-printing system will be discussed. Afterwards, we will mention their applications in the fabrication of tissue engineered vascular grafts. Finally, conclusions and future perspectives will be discussed.

Review of Polymeric Biomimetic Small-Diameter Vascular Grafts to Tackle Intimal Hyperplasia

Small-diameter artificial vascular grafts (SDAVG) are used to bypass blood flow in arterial occlusive diseases such as coronary heart or peripheral arterial disease. However, SDAVGs are plagued by restenosis after a short while due to thrombosis and the thickening of the neointimal wall known as intimal hyperplasia (IH). The specific causes of IH have not yet been deduced; however, thrombosis formation due to bioincompatibility as well as a mismatch between the biomechanical properties of the SDAVG and the native artery has been attributed to its initiation. The main challenges that have been faced in fabricating SDAVGs are facilitating rapid re-endothelialization of the luminal surface of the SDAVG and replicating the complex viscoelastic behavior of the arteries. Recent strategies to combat IH formation have been mostly based on imitating the natural structure and function of the native artery (biomimicry). Thus, most recently, developed grafts contain a multilayered structure with a designated function for each layer. This paper reviews the current polymeric, biomimetic SDAVGs in preventing the formation of IH. The materials used in fabrication, challenges, and strategies employed to tackle IH are summarized and discussed, and we focus on the multilayered structure of current SDAVGs. Additionally, the future aspects in this area are pointed out for researchers to consider in their endeavor.

The performance of heparin modified poly(ε-caprolactone) small diameter tissue engineering vascular graft in canine-A long-term pilot experiment in vivo

Long-term in vivo observation in large animal model is critical for evaluating the potential of small diameter tissue engineering vascular graft (SDTEVG) in clinical application, but is rarely reported. In this study, a SDTEVG is fabricated by the electrospinning of poly(ε-caprolactone) and subsequent heparin modification. SDTEVG is implanted into canine's abdominal aorta for 511 days in order to investigate its clinical feasibility. An active and robust remodeling process was characterized by a confluent endothelium, macrophage infiltrate, extracellular matrix deposition and remodeling on the explanted graft. The immunohistochemical and immunofluorescence analysis further exhibit the regeneration of endothelium and smooth muscle layer on tunica intima and tunica media, respectively. Thus, long-term follow-up reveals viable neovessel formation beyond graft degradation. Furthermore, the von Kossa staining exhibits no occurrence of calcification. However, although no TEVG failure or rupture happens during the follow-up, the aneurysm is found by both Doppler ultrasonic and gross observation. Consequently, as-prepared TEVG shows promising potential in vascular tissue engineering if it can be appropriately strengthened to prevent the occurrence of aneurysm.

Designed and fabrication of triple-layered vascular scaffold with microchannels

Currently, one of the best preparation strategies for the triple-layered vascular scaffold is to imitate the three-layer structure of natural blood vessels to achieve the biofunctional characteristics of vascular transplantation. Here, we developed a combinatorial method to fabricate triple-layered vascular scaffold (TVS) by using electrospinning and coaxial 3 D printing. First, Polycaprolactone-collagen (PCL-Col) was applied to prepared the inner layer of TVS by electrospinning. Second, egg white/sodium alginate (EW/SA) blend hydrogel was extruded to form hollow filaments by coaxial 3 D printing and crosslinking mechanism, which enwound around the surface of the inner layer in a circumferential direction as the intermediate layer of TVS. Finally, electrospun PCL-Col nanofibers were wrapped on the surface of hydrogel layer as the outer layer of TVS. The morphological characterization and mechanical strength of the fabricated TVS were measured. Compared with natural blood vessels, results shown that ultimate tensile stress (UTS), strain to failure (STF), the estimated burst strength and the suture retention strength (SRS) of TVS were superior. Also, the fabricated TVS exhibits good hydrophilicity and excellent flexibility. Moreover, the biocompatibility of TVS was investigated through human umbilical vein endothelial cells (HUVECs), the results demonstrated that cells can successfully attach the surface of graft and maintain high viability. In summary, all of results demonstrated that this method could fabricate a novel triple-layered vascular scaffold, possessing appropriate mechanical properties and good biological properties, which has the potential to be used in tissue engineered vascular grafts applications.

Construction and application of textile-based tissue engineering scaffolds: a review

Tissue engineering (TE) provides a practicable method for tissue and organ repair or substitution. As the most important component of TE, a scaffold plays a critical role in providing a growing environment for cell proliferation and functional differentiation as well as good mechanical support. And the restorative effects are greatly dependent upon the nature of the scaffold including the composition, morphology, structure, and mechanical performance. Medical textiles have been widely employed in the clinic for a long time and are being extensively investigated as TE scaffolds. However, unfortunately, the advantages of textile technology cannot be fully exploited in tissue regeneration due to the ignoring of the diversity of fabric structures. Therefore, this review focuses on textile-based scaffolds, emphasizing the significance of the fabric design and the resultant characteristics of cell behavior and extracellular matrix reconstruction. The structure and mechanical behavior of the fabrics constructed by various textile techniques for different tissue repairs are summarized. Furthermore, the prospect of structural design in the TE scaffold preparation was anticipated, including profiled fibers and some unique and complex textile structures. Hopefully, the readers of this review would appreciate the importance of structural design of the scaffold and the usefulness of textile-based TE scaffolds in tissue regeneration.

Acellular Small-Diameter Tissue-Engineered Vascular Grafts

Tissue-engineered vascular grafts (TEVGs) are considered one of the most effective means of fabricating vascular grafts. However, for small-diameter TEVGs, there are ongoing issues regarding long-term patency and limitations related to long-term in vitro culture and immune reactions. The use of acellular TEVG is a more convincing method, which can achieve in situ blood vessel regeneration and better meet clinical needs. This review focuses on the current state of acellular TEVGs based on scaffolds and gives a summary of the methodologies and in vitro/in vivo test results related to acellular TEVGs obtained in recent years. Various strategies for improving the properties of acellular TEVGs are also discussed.