Bioprinting of Small-Diameter Blood Vessels

There has been an increasing demand for bioengineered blood vessels for utilization in both regenerative medicine and drug screening.However,the availability of a true bioengineered vascular graft remains limited.Three-dimensional(3D)bioprinting presents a potential approach for fabricating blood vessels or vascularized tissue constructs of various architectures and sizes for transplantation and regeneration.In this review,we summarize the basic biology of different blood vessels,as well as 3D bioprinting approaches and bioink designs that have been applied to fabricate vascular and vascularized tissue constructs,with a focus on small-diameter blood vessels.

Effect of heating methods on drying quality of small-diameter birch lumbers

Following a normal low temperature drying schedule, the small-diameter Birch lumbers （ 1 000mm× 45mm ×30mm） were dried with consecution-heats or intermittent-heating, visual drying defects （bow, crook, twist, check along grain and end check） were measured, and then statistical analyses were performed. It was found that the drying quality of small-diameter Birch lumbers could be improved with intermittent-heating, but the intermittent time should be prolonged. Prolonging intermittent time helped to weaken or even avoid wood distortion and drying checks. It wash＇ t helpful in avoiding crook. The drying quality of small-diameter Birch lumbers with all kinds of drying methods reached the second class of the Chinese National Standard. The drying quality reached the first class of the Chinese National Standard with the intermittent-heating in the area of visual wood defects.

Construction of Oriented Structure in Inner Surface of Small-Diameter Artificial Blood Vessels:A Review

There is an urgent need for small-diameter artificial blood vessels in clinic.Physical,chemical and biological factors should be integrated to avoid thrombosis and intimal hyperplasia after implantation and to promote successful fabrication of small-diameter artificial blood vessels.From a physical perspective,the internal oriented structures of natural blood vessels plays an important role in guiding the directional growth of cells,improving the blood flow environment,and promoting the regeneration of vascular tissue.In this review,the effects of the oriented structures on cells,including endothelial cells(ECs),smooth muscle cells(SMCs)and stem cells,as well as the effect of the oriented structures on hemodynamics and vascular tissue remodeling and regeneration are introduced.Various forms of oriented structures(fibers,grooves,channels,etc.)and their construction methods are also reviewed.Conclusions and future perspectives are given.It is expected to give some references to relevant researches.

SMALL-DIAMETER KOREAN PINES AND THEIR ROLES INNATURAL KOREAN PINE FOREST

Korean pine population strueture in natural Korean pine forest was studied through massive field investigation and indoor analvsis. Small-diameter Korean pines origination, growth charaeter and their roles in stand were discussed from the view of population origination and tree growth. The results show that small-diameter Korean pines origination and growth havc close relations to the overstory canopy structure dynamics and play an important role in the maintenance and development of Korean pine forest. The process from small-diameter trees to dominant canopy is a selfmaintenance phase, with diffieulty, in Korean pine population. To complete this phase, it not only demands some morphological characters and physiological conditions, but severe forest strueture conditions as well. The time for complating this phase needs separation from overstory Korean pines and converge with overstory broad-leaf trees. In vertical space, it needs to fell overstory canopy or reduce the layers of overstory canopy, and in horizontal space it should be separated from Korean pines but accompanied by broad-leaf trees.

3D printed grafts with gradient structures for organized vascular regeneration

Synthetic vascular grafts suitable for small-diameter arteries(<6 mm) are in great need.However,there are still no commercially available small-diameter vascular grafts(SDVGs) in clinical practice due to thrombosis and stenosis after in vivo implantation.When designing SDVGs,many studies emphasized reendothelization but ignored the importance of reconstruction of the smooth muscle layer(SML).To facilitate rapid SML regeneration,a high-resolution 3D printing method was used to create a novel bilayer SDVG with structures and mechanical properties mimicking natural arteries.Bioinspired by the collagen alignment of SML,the inner layer of the grafts had larger pore sizes and high porosity to accelerate the infiltration of cells and their circumferential alignment,which could facilitate SML reconstruction for compliance restoration and spontaneous endothelialization.The outer layer was designed to induce fibroblast recruitment by low porosity and minor pore size and provide SDVG with sufficient mechanical strength.One month after implantation,the arteries regenerated by 3D-printed grafts exhibited better pulsatility than electrospun grafts,with a compliance(8.9%) approaching that of natural arteries(11.36%) and significantly higher than that of electrospun ones(1.9%).The 3D-printed vascular demonstrated a three-layer structure more closely resembling natural arteries while electrospun grafts showed incomplete endothelium and immature SML.Our study shows the importance of SML reconstruction during vascular graft regeneration and provides an effective strategy to reconstruct blood vessels through 3D-printed structures rapidly.

Tailored endothelialization enabled by engineered endothelial cell vesicles accelerates remodeling of small-diameter vascular grafts

Current gold standard for the replacement of small-diameter blood vessel(ID<4 mm)is still to utilize the autologous vessels of patients due to the limitations of small-diameter vascular grafts(SDVG)on weak endothelialization,intimal hyperplasia and low patency.Herein,we create the SDVG with the tailored endothelialization by applying the engineered endothelial cell vesicles to camouflaging vascular grafts for the enhancement of vascular remodeling.The engineered endothelial cell vesicles were modified with azide groups(ECVs-N3)through metabolic glycoengineering to precisely link the vascular graft made of PCL-DBCO via click chemistry,and thus fabricating ECVG(ECVs-N3 modified SDVG),which assists inhibition of platelet adhesion and activation,promotion of ECs adhesion and enhancement of anti-inflammation.Furthermore,In vivo single-cell transcriptome analysis revealed that the proportion of ECs in the cell composition of ECVG surpassed that of PCL,and the tailored endothelialization enabled to convert endothelial cells(ECs)into some specific ECs clusters.One of the specific cluster,Endo_C5 cluster,was only detected in ECVG.Consequently,our study integrates the engineered membrane vesicles of ECVs-N3 from native ECs for tailored endothelialization on SDVG by circumventing the limitations of living cells,and paves a new way to construct the alternative endothelialization in vessel remodeling following injury.

Design and preparation of polyurethane-collagen/ heparin-conjugated polycaprolactone double-layer bionic small-diameter vascular graft and its preliminary animal tests

Background People recently realized that it is important for artificial vascular biodegradable graft to bionically mimic the functions of the native vessel. In order to overcome the high risk of thrombosis and keep the patency in the clinical small-diameter vascular graft （SDVG） transplantation, a double-layer bionic scaffold, which can offer anticoagulation and mechanical strength simultaneously, was designed and fabricated via electrospinning technique. Methods Heparin-conjugated polycaprolactone （hPCL） and polyurethane （PU）-collagen type I composite was used as the inner and outer layers, respectively. The porosity and the burst pressure of SDVG were evaluated. Its biocompatibility was demonstrated by the 3-（4,5-dimethyl-2-thiazol）-2,5-diphenyl-2H tetrazolium bromide （MTT） test in vitro and subcutaneous implants in vivo respectively. The grafts of diameter 2.5 mm and length 4.0 cm were implanted to replace the femoral artery in Beagle dog model. Then, angiography was performed in the Beagle dogs to investigate the patency and aneurysm of grafts at 2, 4, and 8 weeks post-transplantation. After angiography, the patent grafts were explanted for histological analysis. Results The double-layer bionic SDVG meet the clinical mechanical demand. Its good biocompatibility was proven by cytotoxicity experiment （the cell＇s relative growth rates （RGR） of PU-collagen outer layer were 102.8%, 109.2% and 103.5%, while the RGR of hPCL inner layer were 99.0%, 100.0% and 98.0%, on days 1, 3, and 5, respectively） and the subdermal implants experiment in the Beagle dog. Arteriography showed that all the implanted SDVGs were patent without any aneurismal dilatation or obvious anastomotic stenosis at the 2nd, 4th, and 8th week after the operation, except one SDVG that failed at the 2nd week. Histological analysis and SEM showed that the inner layer was covered by new endothelial-like cells. Conclusion The double-layer bionic SDVG is a promising candidate as a replacement of native small-diameter vascular graft.

Condensation heat transfer characteristics of vapor flow in vertical small-diameter tube with variable wall temperature

To explore the condensation characteristics of vapor flow inside vertical small-diameter tubes, the classical Nusselt theory is revised and an analytical model with variable tube wall temperature is established by considering the effect of surface tension exerted by condensate film bending as well as the effect of shear stress on vapor-liquid interface. The effects of various factors including tube wall temperature and gravityon flow condensation in small-diameter tubes are analyzed theoretically to show the heat transfer characteristics. Comparison with the experimental data indicates that the proposed analytical model is fit to reveal the fundamental characteristics of flow condensation heat transfer in vertical small-diameter tube.

Small-diameter vertical shafts constructed in the shallow space of steep mountainous areas

Japan is a mountainous country comprising several islands,in which mountains occupy 70%of the entire land.Therefore,numerous transmission line towers have been constructed in the shallow space of steep mountainous areas.In such construction,monorails and/or cableways are generally used to transport materials and equipment to the construction site instead of using a construction road,as the former method is more economically viable.However,with this method,drill rigs or vertical shaft sinking machines cannot be transported to the site.Four small-diameter vertical shafts of 2.5 or 3.0 m as the foundation for high-voltage transmission line towers in mountainous areas are traditionally constructed manually in Japan.Over the past two decades,however,dangerous and poor environmental conditions for workers regarding the manual construction of these small-diameter vertical shafts have become a major problem.Meanwhile,owing to the poor environmental conditions of small-diameter vertical shafts to be constructed in the shallow space of steep mountainous areas,a decrease has occurred in the number of young workers entering these projects.Namely,the construction of transmission line towers with using the manually traditional small-diameter vertical shafts is becoming difficult in Japan from abovementioned problems.Hence development of a new technology to solve the problems is necessary for Japan’s economic growth.With this knowledge,the authors have developed a new small-diameter vertical shaft construction system,in which workers do not have to enter the vertical shaft during construction,as machines are used instead.The applicability of the proposed system was confirmed by means of the construction of three actual vertical shafts at two construction sites,as well as in factory and field tests.The applicability and details of the final proposed system are summarized in this paper.

Bilayer vascular grafts with on-demand NO and H_(2)S release capabilities

Nitric oxide(NO)and hydrogen sulfide(H_(2)S)gasotransmitters exhibit potential therapeutic effects in the car-diovascular system.Herein,biomimicking multilayer structures of biological blood vessels,bilayer smalldiameter vascular grafts(SDVGs)with on-demand NO and H_(2)S release capabilities,were designed and fabri-cated.The keratin-based H_(2)S donor(KTC)with good biocompatibility and high stability was first synthesized and then electrospun with poly(L-lactide-co-caprolactone)(PLCL)to be used as the outer layer of grafts.The elec-trospun poly(ε-caprolactone)(PCL)mats were aminolyzed and further chelated with copper(II)ions to construct glutathione peroxidase(GPx)-like structural surfaces for the catalytic generation of NO,which acted as the inner layer of grafts.The on-demand release of NO and H_(2)S selectively and synergistically promoted the proliferation and migration of human umbilical vein endothelial cells(HUVECs)while inhibiting the proliferation and migration of human umbilical artery smooth muscle cells(HUASMCs).Dual releases of NO and H_(2)S gaso-transmitters could enhance their respective production,resulting in enhanced promotion of HUVECs and inhi-bition of HUASMCs owing to their combined actions.In addition,the bilayer grafts were conducive to forming endothelial cell layers under flow shear stress.In rat abdominal aorta replacement models,the grafts remained patency for 6 months.These grafts were capable of facilitating rapid endothelialization and alleviating neo-intimal hyperplasia without obvious injury,inflammation,or thrombosis.More importantly,the grafts were expected to avoid calcification with the degradation of the grafts.Taken together,these bilayer grafts will be greatly promising candidates for SDVGs with rapid endothelialization and anti-calcification properties.

Anti-Sca-1 antibody-functionalized vascular grafts improve vascular regeneration via selective capture of endogenous vascular stem/progenitor cells

Small-diameter vascular grafts fabricated from synthetic biodegradable polymers exhibit beneficial mechanical properties but often face poor regenerative potential.Different tissue engineering approaches have been employed to improve tissue regeneration in vascular grafts,but there remains a requirement for a new generation of synthetic grafts that can orchestrate the host response to achieve robust vascular regeneration.Vascular stem/progenitor cells(SPCs)are mostly found in quiescent niches but can be activated in response to injury and participate in endothelium and smooth muscle regeneration during neo-artery formation.Here,we developed a functional vascular graft by surface immobilization of stem cell antigen-1(Sca-1)antibody on an electrospun poly(ε-caprolactone)graft(PCL-Sca-1 Ab).PCL-Sca-1 Ab promoted capture and retainment of Sca-1+SPCs in vitro.In rat abdominal aorta replacement models,PCL-Sca-1 Ab stimulated in vivo recruitment of Sca-1+SPCs,and drove SPCs differentiation towards vascular cell lineages.The origin of infiltrated Sca-1+SPCs was further investigated using a bone marrow transplantation mouse model,which revealed that Sca-1+SPCs originating from the resident tissues and bone marrow contributed to rapid vascular regeneration of vascular grafts.Our data indicated that PCL-Sca-1 Ab vascular grafts may serve as a useful strategy to develop next generation cell-free vascular grafts.

Soft nanofiber modified micropatterned substrates enhance nativelike endothelium maturation via CXCR4/calcium-mediated actin cytoskeleton assembly

Regeneration and maturation of native-like endothelium is crucial for material-guided small-diameter vascular regeneration.Although parallel-microgroove-patterned(micropatterned)substrates are capable of promoting endothelial regeneration with native-like endothelial cell(EC)alignment,their unbefitting high-stiffness acutely inhibits cell–matrix interaction and endothelial maturation.Given that the sufficient softness of nanofibers allows cells to deform the local matrix architecture to satisfy cell survival and functional requirements,in this study,an effective strategy of decorating micropatterned substrate with soft nanofibers was exploited to enhance cell–matrix interaction for engineering healthy endothelium.Results demonstrated that the micropatterned nanofibrous membranes were successfully obtained with high-resolution parallel microgrooves(groove width:~15μm;groove depth:~5μm)and adequate softness(bulk modulus:2.27±0.18 MPa).This particular substrate markedly accelerated the formation and maturation of confluent native-like endothelium by synchronously increasing cell–cell and cell–matrix interactions.Transcriptome analysis revealed that compared with smooth features,the microgrooved pattern was likely to promote endothelial remodeling via integrinα5-mediated microtubule disassembly and type I interleukin 1 receptormediated signaling pathways,whereas the nanofibrous pattern was likely to guide endothelial regeneration via integrinα5β8-guided actin cytoskeleton remodeling.Nevertheless,endowing micropatterned substrate with soft nanofibers was demonstrated to accelerate endothelial maturation via chemokine(C-X-C motif)receptor 4/calcium-mediated actin cytoskeleton assembly.Furthermore,numerical simulation results of hemodynamics indicated the positive impact of the micropatterned nanofibers on maintaining stable hemodynamics.Summarily,our current work supports an affirmation that the micropatterned nanofibrous substrates can significantly promote regeneration and maturation of native-like endothelium,which provides an innovative method for constructing vascular grafts with functional endothelium.