Small-diameter vascular tissue engineering

Temperature-modulated separation of vascular cells using thermoresponsive-anionic block copolymer-modified glass

Introduction

Vascular tissue engineering is a key technology in the field of regenerative medicine. In tissue engineering, the separation of vascular cells without cell modification is required, as cell modifications affect the intrinsic properties of the cells. In this study, we have developed an effective method for separating vascular cells without cell modification, using a thermoresponsive anionic block copolymer.

Methods

A thermoresponsive anionic block copolymer, poly(acrylic acid)-b-poly(N-isopropylacryl-amide) (PAAc-b-PNIPAAm), with various PNIPAAm segment lengths, was prepared in two steps: atom transfer radical polymerization and subsequent deprotection. Normal human umbilical vein endothelial cells (HUVECs), normal human dermal fibroblasts, and human aortic smooth muscle cells (SMCs) were seeded onto the prepared thermoresponsive anionic block copolymer brush-modified glass. The adhesion behavior of cells on the copolymer brush was observed at 37 °C and 20 °C.

Results

A thermoresponsive anionic block copolymer, poly(acrylic acid)-b-poly(N-isopropylacrylamide) (PAAc-b-PNIPAAm), with various PNIPAAm segment lengths was prepared. The prepared copolymer-modified glass exhibited anionic properties attributed to the bottom PAAc segment of the copolymer brush. On the PAAc-b-PNIPAAm, which had a moderate PNIPAAm length, a high adhesion ratio of HUVECs and low adhesion ratio of SMCs were observed at 37 °C. By reducing temperature from 37 °C to 20 °C, the adhered HUVECs were detached, whereas the SMCs maintained adhesion, leading to the recovery of purified HUVECs by changing the temperature.

Conclusions

The prepared thermoresponsive anionic copolymer-modified glass could be used to separate HUVECs and SMCs by changing the temperature without modifying the cell surface. Therefore, the developed cell separation method will be useful for vascular tissue engineering.

The Effects of Biomimetic Surface Topography on Vascular Cells: Implications for Vascular Conduits

Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide and represent a pressing clinical need. Vascular occlusions are the predominant cause of CVD and necessitate surgical interventions such as bypass graft surgery to replace the damaged or obstructed blood vessel with a synthetic conduit. Synthetic small-diameter vascular grafts (sSDVGs) are desired to bypass blood vessels with an inner diameter <6 mm yet have limited use due to unacceptable patency rates. The incorporation of biophysical cues such as topography onto the sSDVG biointerface can be used to mimic the cellular microenvironment and improve outcomes. In this review, the utility of surface topography in sSDVG design is discussed. First, the primary challenges that sSDVGs face and the rationale for utilizing biomimetic topography are introduced. The current literature surrounding the effects of topographical cues on vascular cell behavior in vitro is reviewed, providing insight into which features are optimal for application in sSDVGs. The results of studies that have utilized topographically-enhanced sSDVGs in vivo are evaluated. Current challenges and barriers to clinical translation are discussed. Based on the wealth of evidence detailed here, substrate topography offers enormous potential to improve the outcome of sSDVGs and provide therapeutic solutions for CVDs.

Improving the hemocompatibility of a porohyperelastic layered vascular graft using luminal reversal microflows

Vascular graft thrombosis is a long-standing clinical problem. A myriad of efforts have been devoted to reducing thrombus formation following bypass surgery. Researchers have primarily taken a chemical approach to engineer and modify surfaces, seeking to make them more suitable for blood contacting applications. Using mechanical forces and surface topology to prevent thrombus formation has recently gained more attention. In this study, we have designed a bilayered porous vascular graft capable of repelling platelets and destabilizing absorbed protein layers from the luminal surface. During systole, fluid penetrates through the graft wall and is subsequently ejected from the wall into the luminal space (Luminal Reversal Flow - LRF), pushing platelets away from the surface during diastole. In-vitro hemocompatibility tests were conducted to compare platelet deposition in high LRF grafts with low LRF grafts. Graft material properties were determined and utilized in a porohyperelastic (PHE) finite element model to computationally predict the LRF generation in each graft type. Hemocompatibility testing showed significantly lower platelet deposition values in high versus low LRF generating grafts (median±IQR = 5,708 ± 987 and 23,039 ± 3,310 platelets per mm2, respectively, p=0.032). SEM imaging of the luminal surface of both graft types confirmed the quantitative blood test results. The computational simulations of high and low LRF generating grafts resulted in LRF values of -10.06 μm/s and -2.87 μm/s, respectively. These analyses show that a 250% increase in LRF is associated with a 75.2% decrease in platelet deposition. PHE vascular grafts with high LRF have the potential to improve anti-thrombogenicity and reduce thrombus-related post-procedure complications. Additional research is required to overcome the limitations of current graft fabrication technologies that further enhance LRF generation.

Versatile Design of NO-Generating Proteolipid Nanovesicles for Alleviating Vascular Injury

Vascular injury is central to the pathogenesis and progression of cardiovascular diseases, however, fostering alternative strategies to alleviate vascular injury remains a persisting challenge. Given the central role of cell-derived nitric oxide (NO) in modulating the endogenous repair of vascular injury, NO-generating proteolipid nanovesicles (PLV-NO) are designed that recapitulate the cell-mimicking functions for vascular repair and replacement. Specifically, the proteolipid nanovesicles (PLV) are versatilely fabricated using membrane proteins derived from different types of cells, followed by the incorporation of NO-generating nanozymes capable of catalyzing endogenous donors to produce NO. Taking two vascular injury models, two types of PLV-NO are tailored to meet the individual requirements of targeted diseases using platelet membrane proteins and endothelial membrane proteins, respectively. The platelet-based PLV-NO (pPLV-NO) demonstrates its efficacy in targeted repair of a vascular endothelium injury model through systemic delivery. On the other hand, the endothelial cell (EC)-based PLV-NO (ePLV-NO) exhibits suppression of thrombosis when modified onto a locally transplanted small-diameter vascular graft (SDVG). The versatile design of PLV-NO may enable a promising therapeutic option for various vascular injury-evoked cardiovascular diseases.

Sulfated hyaluronic acid/collagen-based biomimetic hybrid nanofiber skin for diabetic wound healing: Development and preliminary evaluation

Diabetic foot ulcers (DFUs) are one of the most serious and devastating complication of diabetes, manifesting as foot ulcers and impaired wound healing in patients with diabetes mellitus. To solve this problem, sulfated hyaluronic acid (SHA)/collagen-based nanofibrous biomimetic skins was developed and used to promote the diabetic wound healing and skin remodeling. First, SHA was successfully synthetized using chemical sulfation and incorporated into collagen (COL) matrix for preparing the SHA/COL hybrid nanofiber skins. The polyurethane (PU) was added into those hybrid scaffolds to make up the insufficient mechanical properties of SHA/COL nanofibers, the morphology, surface properties and degradation rate of hybrid nanofibers, as well as cell responses upon the nanofibrous scaffolds were studied to evaluate their potential for skin reconstruction. The results demonstrated that the SHA/COL, SHA/HA/COL hybrid nanofiber skins were stimulatory of cell behaviors, including a high proliferation rate and maintaining normal phenotypes of specific cells. Notably, SHA/COL and SHA/HA/COL hybrid nanofibers exhibited a significantly accelerated wound healing and a high skin remodeling effect in diabetic mice compared with the control group. Overall, SHA/COL-based hybrid scaffolds are promising candidates as biomimetic hybrid nanofiber skin for accelerating diabetic wound healing.

Nonlinear Elasticity of Blood Vessels and Vascular Grafts

The transplantation of vascular grafts has emerged as a prevailing approach to address vascular disorders. However, the development of small-diameter vascular grafts is still in progress, as they serve in a more complicated mechanical environment than their counterparts with larger diameters. The biocompatibility and functional characteristics of small-diameter vascular grafts have been well developed; however, mismatch in mechanical properties between the vascular grafts and native arteries has not been accomplished, which might facilitate the long-term patency of small-diameter vascular grafts. From a point of view in mechanics, mimicking the nonlinear elastic mechanical behavior exhibited by natural blood vessels might be the state-of-the-art in designing vascular grafts. This review centers on elucidating the nonlinear elastic behavior of natural blood vessels and vascular grafts. The biological functionality and limitations associated with as-reported vascular grafts are meticulously reviewed and the future trajectory for fabricating biomimetic small-diameter grafts is discussed. This review might provide a different insight from the traditional design and fabrication of artificial vascular grafts.

Clopidogrel-loaded vascular grafts prepared using digital light processing 3D printing

The leading cause of death worldwide and a significant factor in decreased quality of life are the cardiovascular diseases. Endovascular operations like angioplasty, stent placement, or atherectomy are often used in vascular surgery to either dilate a narrowed blood artery or remove a blockage. As an alternative, a vascular transplant may be utilised to replace or bypass a dysfunctional or blocked blood vessel. Despite the advancements in endovascular surgery and its popularisation over the past few decades, vascular bypass grafting remains prevalent and is considered the best option for patients in need of long-term revascularisation treatments. Consequently, the demand for synthetic vascular grafts composed of biocompatible materials persists. To address this need, biodegradable clopidogrel (CLOP)-loaded vascular grafts have been fabricated using the digital light processing (DLP) 3D printing technique. A mixture of polylactic acid-polyurethane acrylate (PLA-PUA), low molecular weight polycaprolactone (L-PCL), and CLOP was used to achieve the required mechanical and biological properties for vascular grafts. The 3D printing technology provides precise detail in terms of shape and size, which lead to the fabrication of customised vascular grafts. The fabricated vascular grafts were fully characterised using different techniques, and finally, the drug release was evaluated. Results suggested that the performed 3D-printed small-diameter vascular grafts containing the highest CLOP cargo (20% w/w) were able to provide a sustained drug release for up to 27 days. Furthermore, all the CLOP-loaded 3D-printed materials resulted in a substantial reduction of the platelet deposition across their surface compared to the blank materials containing no drug. Haemolysis percentage for all the 3D-printed samples was lower than 5%. Moreover, 3D-printed materials were able to provide a supportive environment for cellular attachment, viability, and growth. A substantial increase in cell growth was detected between the blank and drug-loaded grafts.

Fabrication of a Triple-Layer Bionic Vascular Scaffold via Hybrid Electrospinning

Tissue engineering aims to develop bionic scaffolds as alternatives to autologous vascular grafts due to their limited availability. This study introduces a novel wet-electrospinning fabrication technique to create small-diameter, uniformly aligned tubular scaffolds. By combining this innovative method with conventional electrospinning, a bionic tri-layer scaffold that mimics the zonal structure of vascular tissues is produced. The inner and outer layers consist of PCL/Gelatin and PCL/PLGA fibers, respectively, while the middle layer is crafted using PCL through Wet Vertical Magnetic Rod Electrospinning (WVMRE). The scaffold's morphology is analyzed using Scanning Electron Microscopy (SEM) to confirm its bionic structure. The mechanical properties, degradation profile, wettability, and biocompatibility of the scaffold are also characterized. To enhance hemocompatibility, the scaffold is crosslinked with heparin. The results demonstrate sufficient mechanical properties, good wettability of the inner layer, proper degradability of the inner and middle layers, and overall good biocompatibility. In conclusion, this study successfully develops a small-diameter tri-layer tubular scaffold that meets the required specifications.

Small diameter vascular grafts: progress on electrospinning matrix/stem cell blending approach

The exploration of the next-generation small diameter vascular grafts (SDVGs) will never stop until they possess high biocompatibility and patency comparable to autologous native blood vessels. Integrating biocompatible electrospinning (ES) matrices with highly bioactive stem cells (SCs) provides a rational and promising solution. ES is a simple, fast, flexible and universal technology to prepare extracellular matrix-like fibrous scaffolds in large scale, while SCs are valuable, multifunctional and favorable seed cells with special characteristics for the emerging field of cell therapy and regenerative medicine. Both ES matrices and SCs are advanced resources with medical application prospects, and the combination may share their advantages to drive the overcoming of the long-lasting hurdles in SDVG field. In this review, the advances on SDVGs based on ES matrices and SCs (including pluripotent SCs, multipotent SCs, and unipotent SCs) are sorted out, and current challenges and future prospects are discussed.

Fabrication of Vascular Grafts Using Poly(ε-Caprolactone) and Collagen-Encapsuled ADSCs for Interposition Implantation of Abdominal Aorta in Rhesus Monkeys

The production of small-diameter artificial vascular grafts continues to encounter numerous challenges, with concerns regarding the degradation rate and endothelialization being particularly critical. In this study, porous PCL scaffolds were prepared, and PCL vascular grafts were fabricated by 3D bioprinting of collagen materials containing adipose-derived mesenchymal stem cells (ADSCs) on the internal wall of the porous PCL scaffold. The PCL vascular grafts were then implanted in the abdominal aorta of Rhesus monkeys for up to 640 days to analyze the degradation of the scaffolds and regeneration of the aorta. Changes in surface morphology, mechanical properties, crystallization property, and molecular weight of porous PCL revealed a similar degradation process of PCL in PBS at pH 7.4 containing Thermomyces lanuginosus lipase and in situ in the abdominal aorta of rhesus monkeys. The contrast of in vitro and in vivo degradation provided valuable reference data for predicting in vivo degradation based on in vitro enzymatic degradation of PCL for further optimization of PCL vascular graft fabrication. Histological analysis through hematoxylin and eosin (HE) staining and fluorescence immunostaining demonstrated that the PCL vascular grafts successfully induced vascular regeneration in the abdominal aorta over the 640-day period. These findings provided valuable insights into the regeneration processes of the implanted vascular grafts. Overall, this study highlights the significant potential of PCL vascular grafts for the regeneration of small-diameter blood vessels.

Development of a Tri-Layered Vascular Construct and In Vitro Evaluation of Endothelization

Advances in the development of vascular substitutes for small-sized arteries are ongoing because the present grafts do not entirely meet the requirements of native equivalents and are suboptimal in clinical performance. This study aims to develop a tri-layered vascular construct mimicking natural tissue using polyester blends and to investigate its endothelization through in vitro studies as a potential small-caliber vascular graft. The innermost layer is obtained by dip coating as a tubular porous film with a lumen diameter of 3 mm and a pore size of ≤8 µm. Circumferentially aligned electrospun fiber (diameter 100-800 nm) with a deviation angle of 15° are deposited over the porous film forming the intermediate layer. The random electrospun fibers (diameter 100-1100 nm) deviating at different angles are wrapped as the outermost layer. The mechanical properties of the tri-layered vascular construct are determined to be 44.80 ± 14.80 MPa for Young's modulus and 4.25 ± 0.75 MPa for ultimate tensile strength. MTS and cell behavior studies show that the isolated human umbilical cord vein endothelial cells proliferate and line the lumen of the vascular substitute. The vascular construct developed, with its biomimetic architecture, mechanical features, size, and endothelization, can be tested with in vivo studies.

3D-Printed Shape Memory Poly(alkylene terephthalate) Scaffolds as Cardiovascular Stents Revealing Enhanced Endothelialization

Cardiovascular diseases are the leading cause of death and current treatments such as stents still suffer from disadvantages. Balloon expansion causes damage to the arterial wall and limited and delayed endothelialization gives rise to restenosis and thrombosis. New more performing materials that circumvent these disadvantages are required to improve the success rate of interventions. To this end, the use of a novel polymer, poly(hexamethylene terephthalate), is investigated for this application. The synthesis to obtain polymers with high molar masses up to 126.5 kg mol-1 is optimized and a thorough chemical and thermal analysis is performed. The polymers are 3D-printed into personalized cardiovascular stents using the state-of-the-art solvent-cast direct-writing technique, the potential of these stents to expand using their shape memory behavior is established, and it is shown that the stents are more resistant to compression than the poly(l-lactide) benchmark. Furthermore, the polymer's hydrolytic stability is demonstrated in an accelerated degradation study of 6 months. Finally, the stents are subjected to an in vitro biological evaluation, revealing that the polymer is non-hemolytic and supports significant endothelialization after only 7 days, demonstrating the enormous potential of these polymers to serve cardiovascular applications.

Development and performance evaluation of a novel elastic bacterial nanocellulose/polyurethane small caliber artificial blood vessels

There is an increasing demand for small-diameter blood vessels. Currently, there is no clinically available small-diameter artificial vessel. Bacterial nanocellulose (BNC) has vast potential for applications in artificial blood vessels due to its good biocompatibility. At the same time, medical polyurethane (PU) is a highly elastic polymer material widely used in artificial blood vessels. This study reports a composite small-diameter BNC/PU conduit using a non-solvent-induced phase separation method with the highly hydrophilic BNC tube as the skeleton and the hydrophobic polycarbonate PU as the filling material. The results revealed that the compliance and mechanical matching of BNC/PU tubes were higher than BNC tubes; the axial/radial mechanical strength, burst pressure, and suture strength were significantly improved; the blood compatibility and cell compatibility were also excellent. The molecular and subcutaneous embedding tests showed that the composite tubes had lighter inflammatory reactions. The results of the animal substitution experiments showed that the BNC/PU tubes kept blood flow unobstructed without tissue proliferation after implantation in rats for 9 months. Thus, the BNC/PU small-diameter vascular prosthesis had the potential for long-term patency and acted as an ideal material for small-diameter vessels.

Hybrid Materials for Vascular Applications: A Preliminary In Vitro Assessment

The production of biomedical devices able to appropriately interact with the biological environment is still a great challenge. Synthetic materials are often employed, but they fail to replicate the biological and functional properties of native tissues, leading to a variety of adverse effects. Several commercial products are based on chemically treated xenogeneic tissues: their principal drawback is due to weak mechanical stability and low durability. Recently, decellularization has been proposed to bypass the drawbacks of both synthetic and biological materials. Acellular materials can integrate with host tissues avoiding/mitigating any foreign body response, but they often lack sufficient patency and impermeability. The present paper investigates an innovative approach to the realization of hybrid materials that combine decellularized bovine pericardium with polycarbonate urethanes. These hybrid materials benefit from the superior biocompatibility of the biological tissue and the mechanical properties of the synthetic polymers. They were assessed from physicochemical, structural, mechanical, and biological points of view; their ability to promote cell growth was also investigated. The decellularized pericardium and the polymer appeared to well adhere to each other, and the two sides were distinguishable. The maximum elongation of hybrid materials was mainly affected by the pericardium, which allows for lower elongation than the polymer; this latter, in turn, influenced the maximum strength achieved. The results confirmed the promising features of hybrid materials for the production of vascular grafts able to be repopulated by circulating cells, thus, improving blood compatibility.

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.

Antioxidant and Trilayered Electrospun Small-Diameter Vascular Grafts Maintain Patency and Promote Endothelialisation in Rat Femoral Artery

Vascular grafts with a small diameter encounter inadequate patency as a result of intimal hyperplasia development. In the current study, trilayered electrospun small-diameter vascular grafts (PU-PGACL + GA) were fabricated using a poly(glycolic acid) and poly(caprolactone) blend as the middle layer and antioxidant polyurethane with gallic acid as the innermost and outermost layers. The scaffolds exhibited good biocompatibility and mechanical properties, as evidenced by their 6 MPa elastic modulus, 4 N suture retention strength, and 2500 mmHg burst pressure. Additionally, these electrospun grafts attenuated cellular oxidative stress and demonstrated minimal hemolysis (less than 1%). As a proof-of-concept, the preclinical evaluation of the grafts was carried out in the femoral artery of rodents, where the conduits demonstrated satisfactory patency. After 35 days of implantation, ultrasound imaging depicted adequate blood flow through the grafts, and the computed vessel diameter and histological staining showed no significant stenosis issue. Immunohistochemical analysis confirmed matrix deposition (38% collagen I and 16% elastin) and cell infiltration (42% for endothelial cells and 55% for smooth muscle cells) in the explanted grafts. Therefore, PU-PGACL + GA showed characteristics of a clinically relevant small-diameter vascular graft, facilitating re-endothelialization while preserving the anticoagulant properties of the synthetic blood vessels.

Macromolecular Crowding Enhances Matrix Protein Deposition in Tissue-Engineered Vascular Grafts

Successful in vitro culture of small-diameter tissue-engineered vascular grafts (TEVGs) requires rapid deposition of biomacromolecules secreted by vascular smooth muscle cells in a polyglycolic acid mesh scaffold's three-dimensional (3D) porous environment. However, common media have lower crowding conditions than in vivo tissue fluids. In addition, during the early stages of construction, most of the biomolecules secreted by the cells into the medium are lost, which negatively affects the TEVG culture process. In this study, we propose the use of macromolecular crowding (MMC) to enhance medium crowding to improve the deposition and self-assembly efficiency of major biomolecules in the early stages of TEVG culture. The addition of carrageenan significantly increased the degree of MMC in the culture medium without affecting cell viability, proliferation, and metabolic activity. Protein analysis demonstrated that the deposition of collagen types I and III and fibronectin increased significantly in the cell layers of two-dimensional and 3D smooth muscle cell cultures after the addition of a MMC agent. Collagen type I in the culture medium decreased significantly compared with that in the medium without a MMC agent. Scanning electron microscopy demonstrated that MMC agents considerably enhanced the formation of matrix protein structures during the early stages of 3D culture. Hence, MMC modifies the crowding degree of the culture medium, resulting in the rapid formation of numerous matrix proteins and fiber structures. Impact Statement Small-diameter tissue-engineered vascular grafts (TEVGs) are one of the most promising means of treating cardiovascular diseases; however, the in vitro construction of TEVGs has some limitations, such as slow deposition of extracellular matrix (ECM), long culture period, and poor mechanical properties. We hypothesized that macromolecular crowding can increase the crowding of the culture medium to construct a more bionic microenvironment, which enhances ECM deposition in the medium to the cell layer and reduces collagen loss, accelerating and enhancing TEVG culture and construction in vitro.

Enhanced hemocompatibility and rapid magnetic anastomosis of electrospun small-diameter artificial vascular grafts

Background: Small-diameter (<6 mm) artificial vascular grafts (AVGs) are urgently required in vessel reconstructive surgery but constrained by suboptimal hemocompatibility and the complexity of anastomotic procedures. This study introduces coaxial electrospinning and magnetic anastomosis techniques to improve graft performance. Methods: Bilayer poly(lactide-co-caprolactone) (PLCL) grafts were fabricated by coaxial electrospinning to encapsulate heparin in the inner layer for anticoagulation. Magnetic rings were embedded at both ends of the nanofiber conduit to construct a magnetic anastomosis small-diameter AVG. Material properties were characterized by micromorphology, fourier transform infrared (FTIR) spectra, mechanical tests, in vitro heparin release and hemocompatibility. In vivo performance was evaluated in a rabbit model of inferior vena cava replacement. Results: Coaxial electrospinning produced PLCL/heparin grafts with sustained heparin release, lower platelet adhesion, prolonged clotting times, higher Young's modulus and tensile strength versus PLCL grafts. Magnetic anastomosis was significantly faster than suturing (3.65 ± 0.83 vs. 20.32 ± 3.45 min, p < 0.001) and with higher success rate (100% vs. 80%). Furthermore, magnetic AVG had higher short-term patency (2 days: 100% vs. 60%; 7 days: 40% vs. 0%) but similar long-term occlusion as sutured grafts. Conclusion: Coaxial electrospinning improved hemocompatibility and magnetic anastomosis enhanced implantability of small-diameter AVG. Short-term patency was excellent, but further optimization of anticoagulation is needed for long-term patency. This combinatorial approach holds promise for vascular graft engineering.

Three-dimensional printing of cell-laden bioink for blood vessel tissue engineering: influence of process parameters and components on cell viability

Three-dimensional (3D) bioprinting is a potential therapeutic method for tissue engineering owing to its ability to prepare cell-laden tissue constructs. The properties of bioink are crucial to accurately control the printing structure. Meanwhile, the effect of process parameters on the precise structure is not nonsignificant. We investigated the correlation between process parameters of 3D bioprinting and the structural response of κ-carrageenan-based hydrogels to explore the controllable structure, printing resolution, and cell survival rate. Small-diameter (<6 mm) gel filaments with different structures were printed by varying the shear stress of the extrusion bioprinter to simulate the natural blood vessel structure. The cell viability of the scaffold was evaluated. The in vitro culture of human umbilical vein endothelium cells (HUVECs) on the κ-carrageenan (kc) and composite gels (carrageenan/carbon nanotube and carrageenan/sodium alginate) demonstrated that the cell attachment and proliferation on composite gels were better than those on pure kc. Our results revealed that the carrageenan-based composite bioinks offer better printability, sufficient mechanical stiffness, interconnectivity, and biocompatibility. This process can facilitate precise adjustment of the pore size, porosity, and pore distribution of the hydrogel structure by optimising the printing parameters as well as realise the precise preparation of the internal structure of the 3D hydrogel-based tissue engineering scaffold. Moreover, we obtained perfused tubular filament by 3D printing at optimal process parameters.

Polymeric reinforcements for cellularized collagen-based vascular wall models: influence of the scaffold architecture on the mechanical and biological properties

A previously developed cellularized collagen-based vascular wall model showed promising results in mimicking the biological properties of a native vessel but lacked appropriate mechanical properties. In this work, we aim to improve this collagen-based model by reinforcing it using a tubular polymeric (reinforcement) scaffold. The polymeric reinforcements were fabricated exploiting commercial poly (ε-caprolactone) (PCL), a polymer already used to fabricate other FDA-approved and commercially available devices serving medical applications, through 1) solution electrospinning (SES), 2) 3D printing (3DP) and 3) melt electrowriting (MEW). The non-reinforced cellularized collagen-based model was used as a reference (COL). The effect of the scaffold's architecture on the resulting mechanical and biological properties of the reinforced collagen-based model were evaluated. SEM imaging showed the differences in scaffolds' architecture (fiber alignment, fiber diameter and pore size) at both the micro- and the macrolevel. The polymeric scaffold led to significantly improved mechanical properties for the reinforced collagen-based model (initial elastic moduli of 382.05 ± 132.01 kPa, 100.59 ± 31.15 kPa and 245.78 ± 33.54 kPa, respectively for SES, 3DP and MEW at day 7 of maturation) compared to the non-reinforced collagen-based model (16.63 ± 5.69 kPa). Moreover, on day 7, the developed collagen gels showed stresses (for strains between 20% and 55%) in the range of [5-15] kPa for COL, [80-350] kPa for SES, [20-70] kPa for 3DP and [100-190] kPa for MEW. In addition to the effect on the resulting mechanical properties, the polymeric tubes' architecture influenced cell behavior, in terms of proliferation and attachment, along with collagen gel compaction and extracellular matrix protein expression. The MEW reinforcement resulted in a collagen gel compaction similar to the COL reference, whereas 3DP and SES led to thinner and longer collagen gels. Overall, it can be concluded that 1) the selected processing technique influences the scaffolds' architecture, which in turn influences the resulting mechanical and biological properties, and 2) the incorporation of a polymeric reinforcement leads to mechanical properties closely matching those of native arteries.

Biofunctionalized Electrospun Vascular Scaffolds for Enhanced Antithrombotic Properties and In Situ Endothelialization

The development of innovative vascular substitutes has become increasingly significant due to the prevalence of vascular diseases. In this study, we designed a biofunctionalized electrospun vascular scaffold by chemically conjugating heparin molecules as an antithrombotic agent with an endothelial cell (EC)-specific antibody to promote in situ endothelialization. To optimize this biofunctionalized electrospun vascular scaffolding system, we examined various parameters, including material compositions, cross-linker concentrations, and cross-linking and conjugation processes. The findings revealed that a higher degree of heparin conjugation onto the vascular scaffold resulted in improved antithrombotic properties, as confirmed by the platelet adhesion test. Additionally, the flow chamber study demonstrated that the EC-specific antibody immobilization enhanced the scaffold's EC-capturing capability compared to a nonconjugated vascular scaffold. The optimized biofunctionalized vascular scaffolds also displayed exceptional mechanical properties, such as suture retention strength and tensile properties. Our research demonstrated that the biofunctionalized vascular scaffolds and the directed immobilization of bioactive molecules could provide the necessary elements for successful acellular vascular tissue engineering applications.

A bio-inspired nanoparticle coating for vascular healing and immunomodulatory by cGMP-PKG and NF-kappa B signaling pathways

Drug-eluting stents (DESs) implantation is an effective method to tackle in-stent restenosis (ISR), which has been considered as an efficient treatment for coronary atherosclerosis. Although fruitful results have been achieved in treating coronary artery diseases (CAD), concern has arisen regarding the long-term safety and efficacy of DESs, primarily due to adverse events such as delayed re-endothelialization, persistent inflammatory response, and late stent thrombosis (LST). Taking inspiration from the immunomodulatory functions of camouflage strategies, this study designed a bio-inspired nanoparticle-coated stent. Briefly, the platelet membrane-coated poly (lactic-co-glycolic acid)/Rapamycin nanoparticles (PNP) were sprayed onto stents, forming a homogenous nanoparticle coating. The bilayer of poly (lactic-co-glycolic acid) (PLGA) and platelet membrane works synergistically to promote the sustained-release effect of rapamycin. In vitro studies revealed that the PNP-coated surfaces promoted the competitive adhesion of endothelia cells while inhibiting smooth muscle cells. Subsequent in vivo studies demonstrated that these surfaces expedite re-endothelialization and elicit immunomodulatory effects by regulating the cGMP-PKG and NF-kappa B signaling pathways, influencing the biosynthesis cofactors and immune system signaling. The study successfully deviced a novel and biomimetic drug-eluting stent system, unraveling its detailed functions and molecular mechanism of action for enhanced vascular healing.

Effect of molar mass and alkyl chain length on the surface properties and biocompatibility of poly(alkylene terephthalate)s for potential cardiovascular applications

Cardiovascular diseases are the leading cause of death worldwide. Treatments for occluded arteries include balloon angioplasty with or without stenting and bypass grafting surgery. Poly(ethylene terephthalate) is frequently used as a vascular graft material, but its high stiffness leads to compliance mismatch with the human blood vessels, resulting in altered hemodynamics, thrombus formation and graft failure. Poly(alkylene terephthalate)s (PATs) with longer alkyl chain lengths hold great potential for improving the compliance. In this work, the effect of the polymer molar mass and the alkyl chain length on the surface roughness and wettability of spin-coated PAT films was investigated, as well as the endothelial cell adhesion and proliferation on these samples. We found that surface roughness generally increases with increasing molar mass and alkyl chain length, while no trend for the wettability could be observed. All investigated PATs are non-cytotoxic and support endothelial cell adhesion and growth. For some PATs, the endothelial cells even reorganized into a tubular-like structure, suggesting angiogenic maturation. In conclusion, this research demonstrates the biocompatibility of PATs and their potential to be applied as materials serving cardiovascular applications.

Incorporating Anticoagulant and Antiplatelet Dual Functional Groups into Thermosetting Polymer Chain for Enhancing Antithrombogenicity

In clinical practice, high-effective antithrombosis remains a challenge for blood-contacting medical devices. Inspired by the enhanced antithrombogenicity of anticoagulant and antiplatelet combination therapy, a strategy is proposed to synthesize dual-pathway antithrombotic polymers by incorporating anticoagulant and antiplatelet dual functional groups into a single thermosetting polymer chain. The synthesized polymer shows increased antithrombogenicity in vitro, with prolonged activated partial thromboplastin time (APTT) and decreased platelet adhesion. Additionally, it downregulates the expression of coagulation- and inflammation-related factors in rabbit plasma after ex vivo arteriovenous shunt assay and maintains patency of small vascular grafts for at least 6 months without thrombosis on the luminal surface after in vivo replacement of rabbit carotid artery. This work provides a new approach to producing novel antithrombotic polymers for blood-contacting medical devices.

Biomimetic tri-layered small-diameter vascular grafts with decellularized extracellular matrix promoting vascular regeneration and inhibiting thrombosis with the salidroside

Small-diameter vascular grafts (SDVGs) are urgently required for clinical applications. Constructing vascular grafts mimicking the defining features of native arteries is a promising strategy. Here, we constructed a tri-layered vascular graft with a native artery decellularized extracellular matrix (dECM) mimicking the component of arteries. The porcine thoracic aorta was decellularized and milled into dECM powders from the differential layers. The intima and media dECM powders were blended with poly (L-lactide-co-caprolactone) (PLCL) as the inner and middle layers of electrospun vascular grafts, respectively. Pure PLCL was electrospun as a strengthening sheath for the outer layer. Salidroside was loaded into the inner layer of vascular grafts to inhibit thrombus formation. In vitro studies demonstrated that dECM provided a bioactive milieu for human umbilical vein endothelial cell (HUVEC) extension adhesion, proliferation, migration, and tube-forming. The in vivo studies showed that the addition of dECM could promote endothelialization, smooth muscle regeneration, and extracellular matrix deposition. The salidroside could inhibit thrombosis. Our study mimicked the component of the native artery and combined it with the advantages of synthetic polymer and dECM which provided a promising strategy for the design and construction of SDVGs.

The Influence of Textile Structure Characteristics on the Performance of Artificial Blood Vessels

Cardiovascular disease is a major threat to human health worldwide, and vascular transplantation surgery is a treatment method for this disease. Often, autologous blood vessels cannot meet the needs of surgery. However, allogeneic blood vessels have limited availability or may cause rejection reactions. Therefore, the development of biocompatible artificial blood vessels is needed to solve the problem of donor shortage. Tubular fabrics prepared by textile structures have flexible compliance, which cannot be matched by other structural blood vessels. Therefore, biomedical artificial blood vessels have been widely studied in recent decades up to the present. This article focuses on reviewing four textile methods used, at present, in the manufacture of artificial blood vessels: knitting, weaving, braiding, and electrospinning. The article mainly introduces the particular effects of different structural characteristics possessed by various textile methods on the production of artificial blood vessels, such as compliance, mechanical properties, and pore size. It was concluded that woven blood vessels possess superior mechanical properties and dimensional stability, while the knitted fabrication method facilitates excellent compliance, elasticity, and porosity of blood vessels. Additionally, the study prominently showcases the ease of rebound and compression of braided tubes, as well as the significant biological benefits of electrospinning. Moreover, moderate porosity and good mechanical strength can be achieved by changing the original structural parameters; increasing the floating warp, enlarging the braiding angle, and reducing the fiber fineness and diameter can achieve greater compliance. Furthermore, physical, chemical, or biological methods can be used to further improve the biocompatibility, antibacterial, anti-inflammatory, and endothelialization of blood vessels, thereby improving their functionality. The aim is to provide some guidance for the further development of artificial blood vessels.

Endothelium-Mimetic Surface Modification Improves Antithrombogenicity and Enhances Patency of Vascular Grafts in Rats and Pigs

We first identified thrombomodulin (TM) and endothelial nitric oxide (NO) synthase as key factors for the antithrombogenic function of the endothelium in human atherosclerotic carotid arteries. Then, recombinant TM and an engineered galactosidase responsible for the conversion of an exogenous NO prodrug were immobilized on the surface of the vascular grafts. Surface modification by TM and NO cooperatively enhanced the antithrombogenicity and patency of vascular grafts. Importantly, we found that the combination of TM and NO also promoted endothelialization, whereas it reduced adverse intimal hyperplasia, which is critical for the maintenance of vascular homeostasis, as confirmed in rat and pig models.

MCP-1-Loaded Poly(l-lactide-co-caprolactone) Fibrous Films Modulate Macrophage Polarization toward an Anti-inflammatory Phenotype and Improve Angiogenesis

Tissue engineering approaches such as the electrospinning technique can fabricate nanofibrous scaffolds which are widely used for small-diameter vascular grafting. However, foreign body reaction (FBR) and lack of endothelial coverage are still the main cause of graft failure after the implantation of nanofibrous scaffolds. Macrophage-targeting therapeutic strategies have the potential to address these issues. Here, we fabricate a monocyte chemotactic protein-1 (MCP-1)-loaded coaxial fibrous film with poly(l-lactide-co-ε-caprolactone) (PLCL/MCP-1). The PLCL/MCP-1 fibrous film can polarize macrophages toward anti-inflammatory M2 macrophages through the sustained release of MCP-1. Meanwhile, these specific functional polarization macrophages can mitigate FBR and promote angiogenesis during the remodeling of implanted fibrous films. These studies indicate that MCP-1-loaded PLCL fibers have a higher potential to modulate macrophage polarity, which provides a new strategy for small-diameter vascular graft designing.

Nanofibers with homogeneous heparin distribution and protracted release profile for vascular tissue engineering

In clinic, controlling acute coagulation after small-diameter vessel grafts transplantation is considered a primary problem. The combination of heparin with high anticoagulant efficiency and polyurethane fiber with good compliance is a good choice for vascular materials. However, blending water-soluble heparin with fat-soluble poly (ester-ether-urethane) urea elastomer (PEEUU) uniformly and preparing nanofibers tubular grafts with uniform morphology is a huge challenge. In this research, we have compounded PEEUU with optimized constant concentration of heparin by homogeneous emulsion blending, then spun into the hybrid PEEUU/heparin nanofibers tubular graft (H-PHNF) for replacing rats' abdominal aorta in situ for comprehensive performance evaluation. The in vitro results demonstrated that H-PHNF was of uniform microstructure, moderate wettability, matched mechanical properties, reliable cytocompatibility, and strongest ability to promote endothelial growth. Replacement of resected abdominal artery with the H-PHNF in rat showed that the graft was capable of homogeneous hybrid heparin and significantly promoted the stabilization of vascular smooth muscle cells (VSMCs) as well as stabilizing the blood microenvironment. This research demonstrates the H-PHNF with substantial patency, indicating their potential for vascular tissue engineering.

Nitric oxide releasing poly(vinyl alcohol)/S-nitrosated keratin film as a potential vascular graft

Nitric oxide (NO) releasing vascular graft is promising due to its merits of thromboembolism reduction and endothelialization promotion. In this study, keratin-based NO donor of S-nitrosated keratin (KSNO) was blended with poly(vinyl alcohol) (PVA) and further crosslinked with sodium trimetaphosphate (STMP) to afford PVA/KSNO biocomposite films. These films could release NO sustainably for up to 10 days, resulting in the promotion of HUVECs growth and the inhibition of HUASMCs growth. In addition, these films displayed good blood compatibility and antibacterial activity. Taken together, these films have potential applications in vascular grafts.

Precise Preparation of a Multilayer Tubular Cell Sheet with Well-Aligned Cells in Different Layers to Simulate Native Arteries

Compared with artificial vascular grafts, bottom-up tubular cell sheets (TCSs) without scaffolds have shown promise for patients with cardiovascular disease. However, TCS therapy also faces the challenges of lengthy maturation time, elaborate operation, and weak mechanical strength. In this work, a structured small-diameter vascular graft (SDVG), consisting of three layers of TCSs, with different cell types and arrangements, was fabricated using layer-by-layer assembly of naturally formed TCSs and further cell culture. To this end, a surface-patterned collagen-coated cylindrical substrate was designed for the efficient harvesting of naturally formed and well-aligned TCSs. The patterned collagen (type I) layer facilitated the adhesion and orientation of cells, and a continuous tubular cell monolayer was naturally formed after approximately 4 days in cell culture. Biocompatible near-infrared (NIR) light was used to trigger the photothermal phase transition of the collagen coated on the cylindrical substrate to dissociate the collagen layer. As a result, an intact TCS could be harvested within a few minutes. These naturally formed and well-aligned TCSs exhibited outstanding free-standing performance without rugosity, facilitating their operability and practical application. A ring tensile test showed that orientation was critical for improving the mechanical properties of TCSs. The layer-by-layer assembly of SDVGs not only is easy to manipulate and has a short preparation time but also overcomes the bottleneck of forming a hierarchically structured vascular graft. This approach shows promise for repairing damaged blood vessels.

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Development and mechanical characterization of decellularized scaffolds for an active aortic graft

Decellularized porcine aortas are proposed as scaffolds for revolutionary active aortic grafts. A change in the static and dynamic mechanical properties, associated with the microstructure of elastin and collagen fibers, corresponds to alteration in the cyclic expansion and perfusion, in addition to possible graft damage. Therefore, the present study thoroughly investigates the mechanical response of the decellularized scaffolds of human and porcine origin to static and dynamic mechanical loads. The responses of the native human and porcine aortas are also compared; this is unavailable in the literature. Because the aorta is subjected to pulsatile blood pressure, dynamical responses to cyclic loads and their associated viscoelastic properties are particularly relevant for advanced graft design. In parallel, this study examines the microstructure of the decellularized aorta. The resulting data are compared to the analogous data obtained for the native human and porcine tissues. The results indicate that by using an optimized decellularization protocol - based on sodium dodecyl sulfate (SDS) and DNase - that minimizes mechanical and structural changes of the tissue, layered scaffolds with static and dynamic properties very similar to natural human aortas are obtained. In particular, a decellularized porcine aorta is non-inferior to a decellularized human aorta. STATEMENT OF SIGNIFICANCE: About 55,000 patients undergo abdominal aortic aneurysm repair annually in the USA. The currently implanted grafts present a large mechanical mismatch with the native tissue. This increases the pulsatile nature of the blood flow with negative consequences to the organ perfusion. For this reason, biomimetic and mechanically compatible grafts for aortic repair are urgently needed and they can be obtained through tissue engineering. In this study, scaffolds from porcine and human aortas are obtained from an optimized decellularization protocol. They are accurately compared to the native tissue and present the ideal static and dynamic mechanical properties for developing innovative aortic grafts.

Current biofabrication methods for vascular tissue engineering and an introduction to biological textiles

Cardiovascular diseases are the leading cause of mortality in the world and encompass several important pathologies, including atherosclerosis. In the cases of severe vessel occlusion, surgical intervention using bypass grafts may be required. Synthetic vascular grafts provide poor patency for small-diameter applications (< 6 mm) but are widely used for hemodialysis access and, with success, larger vessel repairs. In very small vessels, such as coronary arteries, synthetics outcomes are unacceptable, leading to the exclusive use of autologous (native) vessels despite their limited availability and, sometimes, quality. Consequently, there is a clear clinical need for a small-diameter vascular graft that can provide outcomes similar to native vessels. Many tissue-engineering approaches have been developed to offer native-like tissues with the appropriate mechanical and biological properties in order to overcome the limitations of synthetic and autologous grafts. This review overviews current scaffold-based and scaffold-free approaches developed to biofabricate tissue-engineered vascular grafts (TEVGs) with an introduction to the biological textile approaches. Indeed, these assembly methods show a reduced production time compared to processes that require long bioreactor-based maturation steps. Another advantage of the textile-inspired approaches is that they can provide better directional and regional control of the TEVG mechanical properties.

Poly(octamethylene citrate) Modified with Glutathione as a Promising Material for Vascular Tissue Engineering

One of the major goals of vascular tissue engineering is to develop much-needed materials that are suitable for use in small-diameter vascular grafts. Poly(1,8-octamethylene citrate) can be considered for manufacturing small blood vessel substitutes, as recent studies have demonstrated that this material is cytocompatible with adipose tissue-derived stem cells (ASCs) and favors their adhesion and viability. The work presented here is focused on modifying this polymer with glutathione (GSH) in order to provide it with antioxidant properties, which are believed to reduce oxidative stress in blood vessels. Cross-linked poly(1,8-octamethylene citrate) (cPOC) was therefore prepared by polycondensation of citric acid and 1,8-octanediol at a 2:3 molar ratio of the reagents, followed by in-bulk modification with 0.4, 0.8, 4 or 8 wt.% of GSH and curing at 80 °C for 10 days. The chemical structure of the obtained samples was examined by FTIR-ATR spectroscopy, which confirmed the presence of GSH in the modified cPOC. The addition of GSH increased the water drop contact angle of the material surface and lowered the surface free energy values. The cytocompatibility of the modified cPOC was evaluated in direct contact with vascular smooth-muscle cells (VSMCs) and ASCs. The cell number, the cell spreading area and the cell aspect ratio were measured. The antioxidant potential of GSH-modified cPOC was measured by a free radical scavenging assay. The results of our investigation indicate the potential of cPOC modified with 0.4 and 0.8 wt.% of GSH to produce small-diameter blood vessels, as the material was found to: (i) have antioxidant properties, (ii) support VSMC and ASC viability and growth and (iii) provide an environment suitable for the initiation of cell differentiation.

Construction of Rapid Extracellular Matrix-Deposited Small-Diameter Vascular Grafts Induced by Hypoxia in a Bioreactor

Cardiovascular disease has become one of the most globally prevalent diseases, and autologous or vascular graft transplantation has been the main treatment for the end stage of the disease. However, there are no commercialized small-diameter vascular graft (SDVG) products available. The design of SDVGs is promising in the future, and SDVG preparation using an in vitro bioreactor is a favorable method, but it faces the problem of long-term culture of >8 weeks. Herein, we used different oxygen (O₂) concentrations and mechanical stimulation to induce greater secretion of extracellular matrix (ECM) from cells in vitro to rapidly prepare SDVGs. Culturing with 2% O₂ significantly increased the production of the ECM components and growth factors of human dermal fibroblasts (hDFs). To accelerate the formation of ECM, hDFs were seeded on a polycaprolactone (PCL) scaffold and cultured in a flow culture bioreactor with 2% O₂ for only 3 weeks. After orthotopic transplantation in rat abdominal aorta, the cultured SDVGs (PCL-decellularized ECM) showed excellent endothelialization and smooth muscle regeneration. The vascular grafts cultured with hypoxia and mechanical stimulation could accelerate the reconstruction speed and obtain an improved therapeutic effect and thereby provide a new research direction for improving the production and supply of SDVGs.

Densified collagen tubular grafts for human tissue replacement and disease modelling applications

There is a significant need across multiple indications for an off-the-shelf bioengineered tubular graft which fulfils the mechanical and biological requirements for implantation and function but does not necessarily require cells for manufacture or deployment. Herein, we present a tissue-like tubular construct using a cell-free, materials-based method of manufacture, utilizing densified collagen hydrogel. Our tubular grafts are seamless, mechanically strong, customizable in terms of lumen diameter and wall thickness, and display a uniform fibril density across the wall thickness and along the tube length. While the method enables acellular grafts to be generated rapidly, inexpensively, and to a wide range of specifications, the cell-compatible densification process also enables a high density of cells to be incorporated uniformly into the walls of the tubes, which we show can be maintained under perfusion culture. Additionally, the method enables tubes consisting of distinct cell domains with cellular configurations at the boundaries which may be useful for modelling aortic disease. Further, we demonstrate additional steps which allow for luminal surface patterning. These results highlight the universality of this approach and its potential for developing the next generation of bioengineered grafts.

Optimizing the Porohyperelastic Response of a Layered Compliance Matched Vascular Graft to Promote Luminal Self-Cleaning

Thrombosis and intimal hyperplasia have remained the major failure mechanisms of small-diameter vascular grafts used in bypass procedures. While most efforts to reduce thrombogenicity have used a biochemical surface modification approach, the use of local mechanical phenomena to aid in this goal has received somewhat less attention. In this work, the mechanical, fluid transport, and geometrical properties of a layered and porous vascular graft are optimized within a porohyperelastic finite element framework to maximize self-cleaning via luminal reversal fluid velocity (into the lumen). This is expected to repel platelets as well as inhibit the formation of and/or destabilize adsorbed protein layers thereby reducing thrombogenic potential. A particle swarm optimization algorithm was utilized to maximize luminal reversal fluid velocity while also compliance matching our graft to a target artery (rat aorta). The maximum achievable luminal reversal fluid velocity was approximately 246 μm/s without simultaneously optimizing for host compliance. Simultaneous optimization of reversal flow and compliance resulted in a luminal reversal fluid velocity of 59 μm/s. Results indicate that a thick highly permeable compressible inner layer and a thin low permeability incompressible outer layer promote intraluminal reversal fluid velocity. Future research is needed to determine the feasibility of fabricating such a layered and optimized graft and verify its ability to improve hemocompatibility.

3D bioprinting and its innovative approach for biomedical applications

3D bioprinting or additive manufacturing is an emerging innovative technology revolutionizing the field of biomedical applications by combining engineering, manufacturing, art, education, and medicine. This process involved incorporating the cells with biocompatible materials to design the required tissue or organ model in situ for various in vivo applications. Conventional 3D printing is involved in constructing the model without incorporating any living components, thereby limiting its use in several recent biological applications. However, this uses additional biological complexities, including material choice, cell types, and their growth and differentiation factors. This state-of-the-art technology consciously summarizes different methods used in bioprinting and their importance and setbacks. It also elaborates on the concept of bioinks and their utility. Biomedical applications such as cancer therapy, tissue engineering, bone regeneration, and wound healing involving 3D printing have gained much attention in recent years. This article aims to provide a comprehensive review of all the aspects associated with 3D bioprinting, from material selection, technology, and fabrication to applications in the biomedical fields. Attempts have been made to highlight each element in detail, along with the associated available reports from recent literature. This review focuses on providing a single platform for cancer and tissue engineering applications associated with 3D bioprinting in the biomedical field.

Vascular Grafts: Technology Success/Technology Failure

Vascular prostheses (grafts) are widely used for hemodialysis blood access, trauma repair, aneurism repair, and cardiovascular reconstruction. However, smaller-diameter (≤4 mm) grafts that would be valuable for many reconstructions have not been achieved to date, although hundreds of papers on small-diameter vascular grafts have been published. This perspective article presents a hypothesis that may open new research avenues for the development of small-diameter vascular grafts. A historical review of the vascular graft literature and specific types of vascular grafts is presented focusing on observations important to the hypothesis to be presented. Considerations in critically reviewing the vascular graft literature are discussed. The hypothesis that perhaps the "biocompatible biomaterials" comprising our vascular grafts-biomaterials that generate dense, nonvascularized collagenous capsules upon implantation-may not be all that biocompatible is presented. Examples of materials that heal with tissue reconstruction and vascularity, in contrast to the fibrotic encapsulation, are offered. Such prohealing materials may lead the way to a new generation of vascular grafts suitable for small-diameter reconstructions.

Application of decellularized vascular matrix in small-diameter vascular grafts

Coronary artery bypass grafting (CABG) remains the most common procedure used in cardiovascular surgery for the treatment of severe coronary atherosclerotic heart disease. In coronary artery bypass grafting, small-diameter vascular grafts can potentially replace the vessels of the patient. The complete retention of the extracellular matrix, superior biocompatibility, and non-immunogenicity of the decellularized vascular matrix are unique advantages of small-diameter tissue-engineered vascular grafts. However, after vascular implantation, the decellularized vascular matrix is also subject to thrombosis and neoplastic endothelial hyperplasia, the two major problems that hinder its clinical application. The keys to improving the long-term patency of the decellularized matrix as vascular grafts include facilitating early endothelialization and avoiding intravascular thrombosis. This review article sequentially introduces six aspects of the decellularized vascular matrix as follows: design criteria of vascular grafts, components of the decellularized vascular matrix, the changing sources of the decellularized vascular matrix, the advantages and shortcomings of decellularization technologies, modification methods and the commercialization progress as well as the application prospects in small-diameter vascular grafts.

Construction of spider silk protein small-caliber tissue engineering vascular grafts based on dynamic culture and its performance evaluation

Tissue engineering is an alternative method for preparing small-caliber (<6 mm) vascular grafts. Dynamic mechanical conditioning is being researched as a method to improve mechanical properties of tissue engineered blood vessels. This method attempts to induce unique reaction in implanted cells that regenerate the matrix around them, thereby improving the overall mechanical stability of the grafts. In this study, we used a bioreactor to seed endothelial cells and smooth muscle cells into the inner and outer layers of the electrospun spider silk protein scaffold respectively to construct vascular grafts. The cell proliferation, mechanical properties, blood compatibility and other indicators of the vascular grafts were characterized in vitro. Furthermore, the vascular grafts were implanted in Sprague Dawley rats, and the vascular grafts' patency, extracellular matrix formation, and inflammatory response were evaluated in vivo. We aimed to construct spider silk protein vascular grafts with the potential for in vivo implantation by using a pulsating flow bioreactor. The results showed that, when compared with the static culture condition, the dynamic culture condition improved cell proliferation on vascular scaffolds and enhanced mechanical function of vascular scaffolds. In vivo experiments also showed that the dynamic culture of vascular grafts was more beneficial for the extracellular matrix deposition and anti-thrombogenesis, as well as reducing the inflammatory response of vascular grafts. In conclusion, dynamic mechanical conditioning aid in the resolution of challenges impeding the application of electrospun scaffolds and have the potential to construct small-caliber blood vessels with regenerative function for cardiovascular tissue repair.

Design, Fabrication, and Application of Mini-Scaffolds for Cell Components in Tissue Engineering

The concept of "lockyballs" or interlockable mini-scaffolds fabricated by two-photon polymerization from biodegradable polymers for the encagement of tissue spheroids and their delivery into the desired location in the human body has been recently introduced. In order to improve control of delivery, positioning, and assembly of mini-scaffolds with tissue spheroids inside, they must be functionalized. This review describes the design, fabrication, and functionalization of mini-scaffolds as well as perspectives on their application in tissue engineering for precisely controlled cell and mini-tissue delivery and patterning. The development of functionalized mini-scaffolds advances the original concept of "lockyballs" and opens exciting new prospectives for mini-scaffolds' applications in tissue engineering and regenerative medicine and their eventual clinical translation.

Development of Innovative Biomaterials and Devices for the Treatment of Cardiovascular Diseases

Cardiovascular diseases have become the leading cause of death worldwide. The increasing burden of cardiovascular diseases has become a major public health problem and how to carry out efficient and reliable treatment of cardiovascular diseases has become an urgent global problem to be solved. Recently, implantable biomaterials and devices, especially minimally invasive interventional ones, such as vascular stents, artificial heart valves, bioprosthetic cardiac occluders, artificial graft cardiac patches, atrial shunts, and injectable hydrogels against heart failure, have become the most effective means in the treatment of cardiovascular diseases. Herein, an overview of the challenges and research frontier of innovative biomaterials and devices for the treatment of cardiovascular diseases is provided, and their future development directions are discussed.

Potential of a Composite Conduit with Bacterial Nanocellulose and Fish Gelatin for Application as Small-Diameter Artificial Blood Vessel

Bacterial nanocellulose (BNC) has received great attention for application as an artificial blood vessel material. However, many results showed that pristine BNC could not perfectly meet all the demands of blood vessels, especially for rapid endothelialization. In order to improve the properties of small-caliber vessels, different concentrations of fish gelatin (Gel) were deposited into the 3D network tubes and their properties were explored. The BNC/Gel composite tubes were treated with glutaraldehyde to crosslink BNC and fish gelatin. Compared with pristine BNC tubes, the BNC/Gel tubes had a certain improvement in mechanical properties. In vitro cell culture demonstrated that the human endothelial cells (HUVECs) and human smooth muscle cells (HSMCs) planted on the internal walls of BNC/Gel tubes showed better adhesion, higher proliferation and differentiation potential, and a better anticoagulation property, as compared to the cells cultured on pristine BNC tubes. Whole-blood coagulation experiments showed that the BNC/Gel tube had better properties than the BNC tube, and the hemolysis rate of all samples was less than 1.0%, satisfying the international standards for medical materials. An increase in the content of fish gelatin also increased the mechanical properties and the biocompatibility of small-caliber vessels. Considering the properties of BNC/Gel tubes, 1.0 wt/v% was selected as the most appropriate concentration of fish gelatin for a composite.

Biological small-calibre tissue engineered blood vessels developed by electrospinning and in-body tissue architecture

There are no suitable methods to develop the small-calibre tissue-engineered blood vessels (TEBVs) that can be widely used in the clinic. In this study, we developed a new method that combines electrospinning and in-body tissue architecture(iBTA) to develop small-calibre TEBVs. Electrospinning imparted mechanical properties to the TEBVs, and the iBTA imparted biological properties to the TEBVs. The hybrid fibres of PLCL (poly(L-lactic-co-ε-caprolactone) and PU (Polyurethane) were obtained by electrospinning, and the fibre scaffolds were then implanted subcutaneously in the abdominal area of the rabbit (as an in vivo bioreactor). The biotubes were harvested after four weeks. The mechanical properties of the biotubes were most similar to those of the native rabbit aorta. Biotubes and the PLCL/PU vascular scaffolds were implanted into the rabbit carotid artery. The biotube exhibited a better patency rate and certain remodelling ability in the rabbit model, which indicated the potential use of this hybridization method to develop small-calibre TEBVs. Sketch map of developing the biotube. The vascular scaffolds were prepared by electrospinning (A). Silicone tube was used as the core, and the vascular scaffold was used as the shell (B). The vascular scaffold and silicone tube were implanted subcutaneously in the abdominal area of the rabbit (C). The biotube was extruded from the silicone tube after 4 weeks ofembedding (D). The biotube was implanted for the rabbit carotid artery (E).

An Investigation of the Constructional Design Components Affecting the Mechanical Response and Cellular Activity of Electrospun Vascular Grafts

Cardiovascular disease is anticipated to remain the leading cause of death globally. Due to the current problems connected with using autologous arteries for bypass surgery, researchers are developing tissue-engineered vascular grafts (TEVGs). The major goal of vascular tissue engineering is to construct prostheses that closely resemble native blood vessels in terms of morphological, mechanical, and biological features so that these scaffolds can satisfy the functional requirements of the native tissue. In this setting, morphology and cellular investigation are usually prioritized, while mechanical qualities are generally addressed superficially. However, producing grafts with good mechanical properties similar to native vessels is crucial for enhancing the clinical performance of vascular grafts, exposing physiological forces, and preventing graft failure caused by intimal hyperplasia, thrombosis, aneurysm, blood leakage, and occlusion. The scaffold's design and composition play a significant role in determining its mechanical characteristics, including suturability, compliance, tensile strength, burst pressure, and blood permeability. Electrospun prostheses offer various models that can be customized to resemble the extracellular matrix. This review aims to provide a comprehensive and comparative review of recent studies on the mechanical properties of fibrous vascular grafts, emphasizing the influence of structural parameters on mechanical behavior. Additionally, this review provides an overview of permeability and cell growth in electrospun membranes for vascular grafts. This work intends to shed light on the design parameters required to maintain the mechanical stability of vascular grafts placed in the body to produce a temporary backbone and to be biodegraded when necessary, allowing an autologous vessel to take its place.

Construction of enzyme-laden vascular scaffolds based on hyaluronic acid oligosaccharides-modified collagen nanofibers for antithrombosis and in-situ endothelialization of tissue-engineered blood vessels

The current use of synthetic grafts often yields low patency in the reconstruction of small-diameter blood vessels owing to the deposition of thrombi and imperfect coverage of the endothelium on the graft lumen. Therefore, the design of vascular scaffolds with antithrombotic performance and endothelialization is greatly required. Herein, we developed an enzyme-laden scaffold based on hyaluronic acid oligosaccharides-modified collagen nanofibers (labeled HA-COL) to improve the anti-platelet capacity and endothelialization of vascular grafts. In this study, HA-COL nanofibers not only encouraged the endothelialization of vascular scaffolds, but acted as an antiplatelet enzyme-laden platform. Apyrase (Apy) and 5'-nucleotidase (5'-NT) were covalently grafted onto the nanofibers, which in turn converted the platelet-sensitive substance: adenosine diphosphate (ADP) into adenosine monophosphate (AMP) and adenosine, thereby, improving the antithrombotic performance of the scaffolds. Notably, the catalytic end-product: adenosine would work in coordination with HA-COL to synergistically enhance the endothelialization of the vascular scaffolds. The results demonstrated that the enzyme-laden scaffolds maintained catalytic performance, reduced platelet adhesion and aggregation, and guaranteed higher patency after 1-month in situ transplantation. Moreover, these scaffolds showed optimal cytocompatibility, tissue compatibility, scaffold biodegradability and tissue regenerative capability during in vivo implantation. Overall, these engineered vascular scaffolds demonstrated their capacity for endothelialization and antithrombotic performance, suggesting their potential for small-diameter vascular tissue engineering applications. STATEMENT OF SIGNIFICANCE: Considering the critical problems in small-diameter vascular reconstruction, the enzyme-laden vascular scaffolds were prepared for improving in-situ endothelialization and antithrombotic performances of artificial blood vessels. The electrospun HA-COL nanofibers were used as the main matrix materials, which provided favorable structural templates for the regeneration of vasculature and functioned as a platform for the loading of enzymes. The enzyme-laden scaffolds with the biomimetic cascading reaction would convert ADP into adenosine, thereby, decreasing the sensitivity of platelets and improving the antithrombotic performance of tissue-engineered blood vessels (TEBVs). The nanofibrous scaffolds exhibited optimal cytocompatibility, tissue compatibility and regenerative capability, working together with catalytic products of dual-enzyme reaction that would synergistically contribute to TEBVs endothelialization. This study provides a new method for the improvement of in-situ endothelialization of small-diameter TEBVs while qualified with antithrombotic performance.

Amphiphilic and fatigue-resistant organohydrogels for small-diameter vascular grafts

Hydrogels are used in vascular tissue engineering because of their good biocompatibility. However, most natural hydrogels exhibit high swelling ratio, poor mechanical stability, and low durability, which are key limitations for wider applications. Amphiphilic and fatigue-resistant organohydrogels were fabricated here via the click chemical reaction of unsaturated functional microbial polyhydroxyalkanoates and polyethylene glycol diacrylate and a combination of two-dimensional material graphdiyne. These organohydrogels were maintained stable in body fluids over time, and their tensile moduli remained unchanged after more than 2000 cycles of cyclic stretching. The tubular scaffolds presented good biocompatibility and perfusion in vitro. After transplantation in vivo, the vascular grafts exhibited obvious cell infiltration and tissue regeneration, having a higher patency rate than the control group in 3 months. This fabrication method provides a strategy of improving and promoting the application of organohydrogels as implant materials for small-diameter vascular graft.

Negative In Vivo Results Despite Promising In Vitro Data With a Coated Compliant Electrospun Polyurethane Vascular Graft

INTRODUCTION

There is a growing need for small-diameter (<6 mm) off-the-shelf synthetic vascular conduits for different surgical bypass procedures, with actual synthetic conduits showing unacceptable thrombosis rates. The goal of this study was to build vascular grafts with better compliance than standard synthetic conduits and with an inner layer stimulating endothelialization while remaining antithrombogenic.

METHODS

Tubular vascular conduits made of a scaffold of polyurethane/polycaprolactone combined with a bioactive coating based on chondroitin sulfate (CS) were created using electrospinning and plasma polymerization. In vitro testing followed by a comparative in vivo trial in a sheep model as bilateral carotid bypasses was performed to assess the conduits' performance compared to the actual standard.

RESULTS

In vitro, the novel small-diameter (5 mm) electrospun vascular grafts coated with chondroitin sulfate (CS) showed 10 times more compliance compared to commercial expanded polytetrafluoroethylene (ePTFE) conduits while maintaining adequate suturability, burst pressure profiles, and structural stability over time. The subsequent in vivo trial was terminated after electrospun vascular grafts coated with CS showed to be inferior compared to their expanded polytetrafluoroethylene counterparts.

CONCLUSIONS

The inability of the experimental conduits to perform well in vivo despite promising in vitro results may be related to the low porosity of the grafts and the lack of rapid endothelialization despite the presence of the CS coating. Further research is warranted to explore ways to improve electrospun polyurethane/polycaprolactone scaffold in order to make it prone to transmural endothelialization while being resistant to strenuous conditions.