Semicrystalline Polymer Micro/Nanostructures Formed by Droplet Evaporation of Aqueous Poly(ethylene oxide) Solutions: Effect of Solution Concentration.

Deposits formed after evaporation of sessile droplets, containing aqueous solutions of poly(ethylene oxide), on hydrophilic glass substrates were studied experimentally and mathematically as a function of the initial solution concentration. The macrostructure and micro/nanostructures of deposits were studied using stereo microscopy and atomic force microscopy. A model, based on thin-film lubrication theory, was developed to evaluate the deposit macrostructure by estimating the droplet final height. Moreover, the model was extended to evaluate the micro/nanostructure of deposits by estimating the rate of supersaturation development in connection with the driving force of crystallization. Previous studies had only described the macrostructure of poly(ethylene oxide) deposits formed after droplet evaporation, whereas the focus of our study was the deposit micro/nanostructures. Our atomic force microscopy study showed that regions close to the deposit periphery were composed of predominantly semicrystalline micro/nanostructures in the form of out-of-plane lamellae, which require a high driving force of crystallization. However, deposit central areas presented semicrystalline micro/nanostructures in the form of in-plane terraces and spirals, which require a lower driving force of crystallization. Increasing the initial concentration of solutions led to an increase in the lengths and thicknesses of the out-of-plane lamellae at the deposits' periphery and enhanced the tendency to form spirals in the central areas. Our numerical study suggested that the rate of supersaturation development and thus the driving force of crystallization increased from the center toward the periphery of droplets, and the supersaturation rate was lower for solutions with higher initial concentrations at each radius. Therefore, periphery areas of droplets with lower initial concentrations were suitable for the formation of micro/nanostructures which require higher driving forces, whereas central areas of droplets with higher initial concentration were desirable for the formation of micro/nanostructures which require lower driving forces. These numerical results were in good qualitative agreement with the experimental findings.

Exploring how changes to the steroidal core alter oleogelation capability in sterol: γ-oryzanol blends.

Oleogels based on sterols such as ß-sitosterol blended with the sterol ester γ-oryzanol are a very interesting class of systems, but there are aspects of their formation and structure that remain elusive. It has previously been shown that a methyl group on the C30 position of the sterol-ester plays an important role in gelation. This work explored the effect that having C30 methyl groups on both the sterol and the sterol-ester had on the gelation process and subsequent gel structure. Lanosterol and saponified γ-oryzanol (which was synthesized as part of this study) were identified as materials of interest, as both feature a methyl group on the C30 position of their steroidal cores. It was observed that both sterols formed gels when blended with γ-oryzanol, and also that lanosterol gelled sunflower oil without the addition of γ-oryzanol. All of these gels were significantly weaker than that formed by ß-sitosterol blended with γ-oryzanol. To explore why, molecular docking simulations along with AFM and SAXS were used to examine these gels on a broad range of length scales. The results suggest that saponified γ-oryzanol-γ-oryzanol gels have a very similar structure to that of ß-sitosterol-γ-oryzanol gels. Lanosterol-γ-oryzanol gels and pure lanosterol gel, however, form with a totally different structure facilitated by the head-to-tail stacking motif exhibited by lanosterol. These results give further evidence that relatively slight changes to the molecular structure of gelators can result in significant differences in subsequent gel properties.

Dispersive and filter loss performance of calcium carbonate nanoparticles in water for drilling fluid applications.

Adding nanoparticles in a drilling fluid can aid in the sealing of the nanopores in the borehole wall rock and the mud cake; in this way, the filtrate loss of the drilling fluid can be reduced and the borehole wall is stabilized. In this work, the spectrophotometric method was used to study the effect of dispersants on calcium carbonate nanoparticles. The best dispersion effect was achieved at cetyltrimethyl ammonium bromide (CTAB) concentration of 4 wt%, dispersing time of 45 min, pH value of 8 and stirring speed of 900 rpm. The structure analysis showed that the adsorption layer was formed on the surface of calcium carbonate nanoparticles after CTAB modification, and no new crystalline compounds appeared. Under these optimized dispersing conditions, aggregation was prevented as manifested by the dramatically decreased average particle size of calcium carbonate nanoparticles while the surface hydrophilicity and Zeta potential of calcium carbonate nanoparticles both increased. Furthermore, we showed that a drilling fluid incorporating such well dispersed calcium carbonate nanoparticles exhibit decreased filter loss and thus better performance in sealing compared to the calcium carbonate nanoparticles without dispersants.

PDMS-ZnO Piezoelectric Nanocomposites for Pressure Sensors.

The addition of piezoelectric zinc oxide (ZnO) fillers into a flexible polymer matrix has emerged as potential piezocomposite materials that can be used for applications such as energy harvesters and pressure sensors. A simple approach for the fabrication of PDMS-ZnO piezoelectric nanocomposites based on two ZnO fillers: nanoparticles (NP) and nanoflowers (NF) is presented in this paper. The effect of the ZnO fillers' geometry and size on the thermal, mechanical and piezoelectric properties is discussed. The sensors were fabricated in a sandwich-like structure using aluminium (Al) thin films as top and bottom electrodes. Piezocomposites at a concentration of 10% w/w showed good flexibility, generating a piezoelectric response under compression force. The NF piezocomposites showed the highest piezoelectric response compared to the NP piezocomposites due to their geometric connectivity. The piezoelectric compound NF generated 4.2 V while the NP generated 1.86 V under around 36 kPa pressure. The data also show that the generated voltage increases with increasing applied force regardless of the type of filler.

On the Effect of Substrate Viscoelasticity on the Evaporation Kinetics and Deposition Patterns of Nanosuspension Drops.

This study investigates the evaporation of sessile pure water and nanosuspension drops on viscoelastic polydimethylsiloxane (PDMS) films. We varied the viscoelasticity of the PDMS films by controlling the curing ratio and categorized them into three types: stiff (10:1, 20:1, 40:1), soft (60:1, 80:1), and very soft (100:1, 120:1, 140:1, 160:1). On stiff surfaces, pure water drops initially evaporate in a constant contact radius (CCR) mode, followed by a constant contact angle mode, and finally in a mixed mode of evaporation. Nanosuspension drops follow the same trend as water drops but with a difference toward the end of their lifetimes, when a short second CCR mode is observed. Complete evaporation of nanosuspension drops on stiff substrates leads to particle deposition patterns similar to a coffee ring with cracks and deposition tails. On soft surfaces, the initial spreading is followed by a pseudo-CCR mode. Complete evaporation of nanosuspension drops on soft substrates leads to deposits in the form of a uniform ring with a sharp ox-horn profile. Unexpectedly, the initial spreading is followed by a mixed mode on very soft substrates, on which wetting ridges (WRs) pulled up by the vertical component of surface tension are clearly observed in the vicinity of the contact line (CL). As the evaporation proceeds, the decreasing contact angle breaks the force balance in the horizontal direction at the CL and gives rise to a net horizontal force, which causes the CL to recede, transferring the horizontal force to the WR. Because of the viscoelastic nature of the very soft substrate, this horizontal force acting on the WR cannot be completely countered by the bulk of the substrate underneath. As a result, the WR moves horizontally in a viscous-flow way, which also enables the CL to be continuously anchored to the ridge and to recede relative to the bulk of the substrate. Consequently, a mixed mode of evaporation occurs. Complete evaporation of nanosuspension drops on very soft substrates leads to finger-like deposits.

Nonisothermal Spreading Dynamics of Self-Rewetting Droplets.

We experimentally studied the spreading dynamics of binary alcohol mixtures (and pure liquids for reference) deposited on a heated substrate in a partially wetting situation under nonisothermal conditions. We show that the spreading mechanism of an evaporating droplet exhibits a power-law growth with early-stage exponents that depend strongly and nonmonotonically on the substrate temperature. Moreover, we investigated the temporal and spatial thermal dynamics in the droplet using infrared thermography, revealing the existence of unique thermal patterns due to thermal and/or solutal instabilities, which lead to surface tension gradients, namely the Marangoni effect. Our key findings are that the temperature of the substrate drastically affects the early-stage inertial-capillary spreading regime owing to the nonmonotonic surface tension-temperature dependence of the self-rewetting liquids. At later stages of wetting, the spreading dynamics enters the viscous-capillary dominated regime, with the characteristic low kinetics mirroring the behavior of pure liquids.

Morphology of Poly(styrene- co-butadiene) Random Copolymer Thin Films and Nanostructures on a Graphite Surface.

We studied the morphology of poly(styrene- co-butadiene) random copolymers on a graphite surface. Polymer solutions were spin coated onto graphite, at various concentrations and molecular weights. The polymer films and nanostructures were imaged using atomic force microscopy. Above the overlap concentration, thin films formed. However, total wetting did not occur, despite the polymers being well above their Tg. Instead, dewetting was observed, suggesting the films were in a state of metastable equilibrium. At lower concentrations, the polymers formed networks, nanoislands, and nanoribbons. Ordered nanopatterns were observed on the surface; the polymers orientated themselves due to π-π stacking interactions reflecting the crystalline structure of the graphite. At the lowest concentration, this ordering was very pronounced. At higher concentrations, it was less defined but still statistically significant. Higher degrees of ordering were observed with poly(styrene- co-butadiene) than polystyrene and polybutadiene homopolymers as the copolymer's aromatic rings are distributed along a flexible chain, which maximizes π-π stacking. At the two lowest concentrations, the size of the nanoislands and nanoribbons remained similar with varying molecular weight. However, at higher concentrations, the polymer network features were largest at the lowest molecular weight, indicating that in this case, a large proportion of shorter chains stay on top of the adsorbed ones. The contact angles of the polymer nanostructures remained mostly constant with size, which is due to the strong polymer/graphite adhesion dominating over line tension and entropic effects.

Molecular Interactions behind the Self-Assembly and Microstructure of Mixed Sterol Organogels.

In this work, we have employed docking and atomistic molecular dynamics (MD) simulations supported by complementary experiments using atomic force microscopy, rheology, and spectroscopy to investigate the self-assembled structure of ß-sitosterol and γ-oryzanol molecules into cylindrical tubules in a nonaqueous solvent. Docking models of several phytosterols, including sitosterol, with oryzanol and other sterol esters demonstrate that for systems to form tubules, the phytosterol sterane group must be stacked in a wedge shape with the ester sterane group and a hydrogen bond must form between the hydroxyl group of the phytosterol and the carbonyl group of the ester. MD of the self-assembled structure were initiated with the molecules in a roughly cylindrical configuration, as suggested from previous experimental studies, and the configurations were found to be stable during 50 ns simulations. We performed MD simulations of two tubules in proximity to better understand the aggregation of these fibrils and how the fibrils interact in order to stick together. We found that an interfibril network of noncovalent bonds, in particular van der Waals and π-π contacts, which is formed between the ferulic acid groups of oryzanol through the hydroxyl, methoxy, and aromatic groups, is responsible for the surface-to-surface interactions between fibrils; an observation supported by molecular spectroscopy. We believe that these interactions are of primary importance in creating a strong organogel network.

Microstructure of ß-Sitosterol:γ-Oryzanol Edible Organogels.

Rheology and atomic force microscopy (AFM) were employed to examine the microstructure of ß-sitosterol:γ-oryzanol organogels in sunflower oil. Using time-resolved rheology, we followed gel formation, paying specific attention to the fibril aggregation process, which had not been studied in detail previously for this system. Using AFM, we observed gel structures directly and obtained detailed information on the gel structure, far exceeding previous studies. Our analysis suggests that though gels are formed by the self-assembly and aggregation of one-dimensional fibrils, the manner in which these fibrils aggregate into ribbons results in complex structures of higher dimensionality. We emphasize that it is a surprise to find ribbons and not twisted strands. Comparing AFM images of 10% w/w and 20% w/w gelator systems, we observed differences in the degree of branching which are consistent with the rheology. We also observed the individual self-assembled fibrils which make up these gels with much greater clarity than in previous microscopy studies, and the fibril diameters of ∼9.8 nm we measured agree excellently with those obtained from existing small-angle neutron scattering data. These results provide new insight into the structure and formation kinetics of this important organogel system.

Influence of Local Heating on Marangoni Flows and Evaporation Kinetics of Pure Water Drops.

The effect of localized heating on the evaporation of pure sessile water drops was probed experimentally by a combination of infrared thermography and optical imaging. In particular, we studied the effect of three different heating powers and two different locations, directly below the center and edge of the drop. In all cases, four distinct stages were identified according to the emerging thermal patterns. In particular, depending on heating location, recirculating vortices emerge that either remain pinned or move azimuthally within the drop. Eventually, these vortices oscillate in different modes depending on heating location. Infrared data allowed extraction of temperature distribution on each drop surface. In turn, the flow velocity in each case was calculated and was found to be higher for edge heating, due to the one-directional nature of the heating. Additionally, calculation of the dimensionless Marangoni and Rayleigh numbers yielded the prevalence of Marangoni convection. Heating the water drops also affected the evaporation kinetics by promoting the "stick-slip" regime. Moreover, both the total number of depinning events and the pinning strength were found to be highly dependent on heating location. Lastly, we report a higher than predicted relationship between evaporation rate and heating temperature, due to the added influence of the recirculating flows on temperature distribution and hence evaporation flux.

Poly(styrene-co-butadiene) random copolymer thin films and nanostructures on a mica surface: morphology and contact angles of nanodroplets.

The self-assembly of poly(styrene-co-butadiene) random copolymers on mica surfaces was studied by varying solution concentrations and polymer molecular weights. Toluene solutions of the poly(styrene-co-butadiene) samples were spin coated onto a mica surface and the resulting polymer morphology was investigated by atomic force microscopy. At higher concentrations, thin films formed with varying thicknesses; some dewetting was observed which depended on the molecular weight. Total dewetting did not occur despite the polymer's low glass transition temperature. Instead, partial dewetting was observed suggesting that the polymer was in a metastable equilibrium state. At lower concentrations, spherical cap shaped nanodroplets formed with varying sizes from single polymer chains to aggregates containing millions of chains. As the molecular weight was increased, fewer aggregates were observed on the surface, albeit with larger sizes resulting from increased solution viscosities and more chain entanglements at higher molecular weights. The contact angles of the nanodroplets were shown to be size dependent. A minimum contact angle occurs for droplets with radii of 100-250 nm at each molecular weight. Droplets smaller than 100 nm showed a sharp increase in contact angle; attributed to an increase in the elastic modulus of the droplets, in addition, to a positive line tension value. Droplets larger than 250 nm also showed an increased contact angle due to surface heterogeneities which cannot be avoided for larger droplets. This increase in contact angle plateaus as the droplet size reaches the macroscopic scale.

"Biodrop" Evaporation and Ring-Stain Deposits: The Significance of DNA Length.

Small sessile drops of water containing either long or short strands of DNA ("biodrops") were deposited on silicon substrates and allowed to evaporate. Initially, the triple line (TL) of both types of droplet remained pinned but later receded. The TL recession mode continued at constant speed until almost the end of drop lifetime for the biodrops with short DNA strands, whereas those containing long DNA strands entered a regime of significantly lower TL recession. We propose a tentative explanation of our observations based on free energy barriers to unpinning and increases in the viscosity of the base liquid due to the presence of DNA molecules. In addition, the structure of DNA deposits after evaporation was investigated by AFM. DNA self-assembly in a series of perpendicular and parallel orientations was observed near the contact line for the long-strand DNA, while, with the short-stranded DNA, smoother ring-stains with some nanostructuring but no striations were evident. At the interior of the deposits, dendritic and faceted crystals were formed from short and long strands, respectively, due to diffusion and nucleation limited processes, respectively. We suggest that the above results related to the biodrop drying and nanostructuring are indicative of the importance of DNA length, i.e., longer DNA chains consisting of linearly bonded shorter, rod-like DNA strands.

Effect of Poly(ethylene oxide) Molecular Weight on the Pinning and Pillar Formation of Evaporating Sessile Droplets: The Role of the Interface.

We report on the drying process of sessile droplets of aqueous poly(ethylene oxide) (PEO) solutions studied by contact angle analysis. Liquid samples were prepared with the same initial concentration of four different molecular weights, Mw, of PEO. Droplets with initial volumes of between 1 and 5 µL were left to evaporate while temperature, pressure, and relative humidity were kept constant. Residues were formed with either a disklike puddle or a distinctive tall conical pillar shape. The latter occurred following a four-stage deposition process: pinned drying, during which the contact line is stationary; pseudodewetting, where the receding contact line is induced by precipitation; bootstrap building, during which the liquid droplet is lifted on freshly precipitated solid; and late drying. Contact angle analysis allowed us to monitor all stages during drying and consider transitions between stages for different molecular weights. We illustrate the mechanisms taking place during the crucial stages of pinning and depinning, revealing the effect of adhesion and contact line friction for high molecular weights and its influence on the final morphology of the dried PEO solute. To this end, we performed PEO solution droplet evaporation on PEO and PTFE films demonstrating the importance of interfacial interaction phenomena. We show that the formation of disklike puddles for high molecular weights on glass is associated with continuous droplet contact line pinning. This results from the strong adhesion due to the interdigitation of the loops and tails of a polymer layer (adsorbed on glass during evaporation) with the polymer gel network inside the droplet that forms as water evaporates.

thin films of poly(isoprene-b-ethylene oxide) diblock copolymers on mica: an atomic force microscopy study.

The structural behavior of three amphiphilic semicrystalline poly(isoprene-b-ethylene oxide) block copolymers (PI-b-PEO) with different PEO volume fraction (f(PEO) = 0.32, 0.49, and 0.66), spin-coated on freshly cleaved mica surfaces from aqueous solutions, was investigated by atomic force microscopy. We focus on the dependence of the resulting thin film nanostructures on the molecular characteristics (f(PEO) and molecular weight) and the adsorbed amount. The nanostructures obtained immediately after spin-coating were robust and remained unchanged after annealing and/or aging. The PEO affinity for the highly hydrophilic mica and the tendency of the hydrophobic and low surface energy PI to dewet and be at the free interface caused the soft PI-b-PEO micelles to collapse leading to the formation of 2D dendritic networks over mica. We show that, for all three polymers, the dendritic monolayer thickness can be predicted by a model consisting of a PEO crystallized layer (directly on top of mica) of the same thickness in all cases and a PI brush layer on top. In thicker areas, polymer material self-assembled into conelike multilamellar bilayers on top of the monolayer and oriented parallel to the substrate for both symmetric and asymmetric diblock copolymers with the lowest f(PEO). We compare the lateral morphology of the films and discuss the thickness heterogeneity, which results from the coupling and competition of crystallization kinetics, phase separation, and wetting/dewetting phenomena highlighting the role of the two blocks to inhibit or enhance certain morphologies. We show that the deviation of the f(PEO) = 0.32 thin film from its bulk phase structure (cylinders in hexagonal lattice) continues for several lamellar bilayers away from the substrate. For the asymmetric PI-b-PEO polymer with the higher PEO volume fraction (f(PEO) = 0.66) and higher APT, laterally extensive stacks of flat-on lamellar crystallites formed on the surface demonstrating the crucial role of the PEO crystallization.

Atomic Force Microscopy of Phytosterol Based Edible Oleogels.

This work reviews the use of atomic force microscopy (AFM) as a tool to investigate oleogels of edible triglyceride oils. Specific attention is given to those oleogels based on phytosterols and their esters, a class of material the authors have studied extensively. This work consists of a summary of the role of AFM in imaging edible oleogels, including the processing and preparation steps required to obtain high-quality AFM images of them. Finally, there is a comparison between AFM and other techniques that may be used to obtain structural information from oleogel samples. The aim of this review is to provide a useful introduction and summary of the technique for researchers in the fields of gels and food sciences looking to perform AFM measurements on edible oleogels.

Thermomechanical Properties of Virgin and Recycled Polypropylene-High-Density Polyethylene Blends.

This paper provides evidence and discusses the variability in the thermomechanical behaviour of virgin and recycled polypropylene/high-density polyethylene blends without the addition of other components, which is sparse in the literature. Understanding the performance variability in recycled polymer blends is of critical importance in order to facilitate the re-entering of recycled materials to the consumer market and, thus, contribute towards a circular economy. This is an area that requires further research due to the inhomogeneity of recycled materials. Therefore, the thermal and mechanical properties of virgin and recycled polypropylene/high-density polyethylene blends were investigated systematically. Differential scanning calorimetry concludes that both the recycled and virgin blends are immiscible. Generally, recycled blends have lower overall crystallinity and melting temperatures compared with virgin blends while, remarkably, their crystallisation temperatures are compared favourably. Dynamical mechanical analysis showed little variation in the storage modulus of recycled and virgin blends. However, the alpha and beta relaxation temperatures are lower in recycled blends due to structural deterioration. Deterioration in the thermal and mechanical properties of recycled blends is thought to be caused by the presence of contaminants and structural degradation during reprocessing, resulting in shorter polymeric chains and the formation of imperfect crystallites. The tensile properties of recycled blends are also affected by the recycling process. The Young's modulus and yield strength of the recycled blends are inferior to those of virgin blends due to the deterioration during the recycling process. However, the elongation at break of the recycled blends is higher compared with the virgin blends, possibly due to the plasticity effect of the low-molecular-weight chain fragments.

A vertical additive-lathe printing system for the fabrication of tubular constructs using gelatin methacryloyl hydrogel.

Reproducing both the mechanical and biological performance of native blood vessels remains an ongoing challenge in vascular tissue engineering. Additive-lathe printing offers an attractive method of fabricating long tubular constructs as a potential vascular graft for the treatment of cardiovascular diseases. Printing hydrogels onto rotating horizontal mandrels often leads to sagging, resulting in poor and variable mechanical properties. In this study, an additive-lathe printing system with a vertical mandrel to fabricate tubular constructs is presented. Various concentrations of gelatin methacryloyl (gelMA) hydrogel were used to print grafts on the rotating mandrel in a helical pattern. The printing parameters were selected to achieve the bonding of consecutive gelMA filaments to improve the quality of the printed graft. The hydrogel filaments were fused properly under the action of gravity on the vertical mandrel. Thus, the vertical additive-lathe printing system was used to print uniform wall thickness grafts, eliminating the hydrogel sagging problem. Tensile testing performed in both circumferential and longitudinal direction revealed that the anisotropic properties of printed gelMA constructs were similar to those observed in the native blood vessels. In addition, no leakage was detected through the walls of the gelMA grafts during burst pressure measurement. Therefore, the current printing setup could be utilized to print vascular grafts for the treatment of cardiovascular diseases.

Elastic modulus of a polymer nanodroplet: theory and experiment.

We redevelop a theoretical model that, in conjunction with atomic force microscopy (AFM), can be used as a noninvasive method for determination of the elastic modulus of a polymer nanodroplet residing on a flat, rigid substrate. The model is a continuum theory that combines surface and elasticity theories for prediction of the droplet's elastic modulus, given experimental measurement of its adsorbed height. Utilization of AFM-measured heights for relevant droplets reported in the literature and from our own experiments illustrated the following: the significance of both surface and elasticity effects in determining a polymer droplet's spreading behavior; the extent of a continuum theory's validity as one approaches the nanoscale; and a droplet size effect on the elastic modulus.

Nanomechanical properties of phospholipid microbubbles.

This study uses atomic force microscopy (AFM) force-deformation (F-Δ) curves to investigate for the first time the Young's modulus of a phospholipid microbubble (MB) ultrasound contrast agent. The stiffness of the MBs was calculated from the gradient of the F-Δ curves, and the Young's modulus of the MB shell was calculated by employing two different mechanical models based on the Reissner and elastic membrane theories. We found that the relatively soft phospholipid-based MBs behave inherently differently to stiffer, polymer-based MBs [Glynos, E.; Koutsos, V.; McDicken, W. N.; Moran, C. M.; Pye, S. D.; Ross, J. A.; Sboros, V. Langmuir2009, 25 (13), 7514-7522] and that elastic membrane theory is the most appropriate of the models tested for evaluating the Young's modulus of the phospholipid shell, agreeing with values available for living cell membranes, supported lipid bilayers, and synthetic phospholipid vesicles. Furthermore, we show that AFM F-Δ curves in combination with a suitable mechanical model can assess the shell properties of phospholipid MBs. The "effective" Young's modulus of the whole bubble was also calculated by analysis using Hertz theory. This analysis yielded values which are in agreement with results from studies which used Hertz theory to analyze similar systems such as cells.

Recent advancements in the bioprinting of vascular grafts.

Recent advancements in the bioinks and three-dimensional (3D) bioprinting methods used to fabricate vascular constructs are summarized herein. Critical biomechanical properties required to fabricate an ideal vascular graft are highlighted, as well as various testing methods have been outlined to evaluate the bio-fabricated grafts as per the Food and Drug Administration (FDA) and International Organization for Standardization (ISO) guidelines. Occlusive artery disease and cardiovascular disease are the major causes of death globally. These diseases are caused by the blockage in the arteries, which results in a decreased blood flow to the tissues of major organs in the body, such as the heart. Bypass surgery is often performed using a vascular graft to re-route the blood flow. Autologous grafts represent a gold standard for such bypass surgeries; however, these grafts may be unavailable due to the previous harvesting or possess a poor quality. Synthetic grafts serve well for medium to large-sized vessels, but they fail when used to replace small-diameter vessels, generally smaller than 6 mm. Various tissue engineering approaches have been used to address the urgent need for vascular graft that can withstand hemodynamic blood pressure and has the ability to grow and remodel. Among these approaches, 3D bioprinting offers an attractive solution to construct patient-specific vessel grafts with layered biomimetic structures.