# Heparin-Eluting Tissue-Engineered Bioabsorbable Vascular Grafts
## 3 Heparin Conjugation Methods
### Physical Conjugation
#### 3.1.3 Coaxial Electrospinning Technique  
Electrospinning technology is often used to create grafts that take into account a variety of important properties of bioabsorbable synthetic grafts, such as degradation rate, response to high-pressure environments (arteries), and cell infiltration rate. Among  
Figure 2: Subclinical CO2 technique for long-term sustained release of heparin. (**A**) Adaptation of subclinical CO2 exposure for TEVG. (**B**) Scanning electron microscopy (SEM) of HB-EGF on the scaffold. (**C**) Long release profile. Adopted with permission from [PERSON] et al. electrospun materials, synthetic or natural polymers such as PLLA, PCL, PGA, collagen, fibrin gel, and chitosan are frequently used [41]. Coaxial electrospinning has been widely used to produce scaffolds with heparin blends. Coaxial electrospinning (Figure 3) is an extension of regular electrospinning. It can mix different materials in different ratios to produce core and shell fibers. The grafts made by coaxial electrospinning also can cooperate with different drugs, proteins, and cells. Furthermore, coaxial electrospinning has been shown to produce scaffolds that can mimic the natural ECM in terms of fiber alignment and scaffold seamlessness [42]. Coaxial electrospinningning conjugation technology has been shown to inhibit thrombus deposition associated with heparin-induced changes in the electrified environment of the material, and at the same time, it has been shown to regulate SMC growth by adjusting the fiber and pore diameters of the electrospun fibers while creating the graft. This is considered to be superior to other physical conjugation methods [43]. In terms of materials, PCL and PLLA are used as grafting materials because of their longer degradation period and higher mechanical strength compared to other materials. In addition, poly(l-lactide-\(\alpha\)-\(\varepsilon\)-caprolactone) (P(LLA-CL)) copolymers are very popular among small-diameter arterial graft blends [25]. [PERSON] et al., compared P(LLA-CL) (50:50), endothelialized P(LLA-CL) and heparinized P(LLA-CL) grafts in a canine femoral artery model. This group used coaxial electrospinningning with two syringes pumping 6% PLLA-CL and 12% heparin solutions, respectively [26]. Figure 3: A scheme of coaxial electrospinning. Two separate syringes pump core (heparin) and shell solutions at different feeding rates to a rotating collector under working voltage. The needle tip to collector distance and collector rotating speeds can be adjusted. The patency rate at day 7 for both pure P(LLA-CL) and endothelialized P(LLA-CL) groups was 75%, while P(LLA-CL)/heparin was 100%. For pure P(LLA-CL) on day 14, the endothelialized P(LLA-CL) was 25% when P(LLA-CL)/Heparin was 50%; the patency rate for the P(LLA-CL)/Heparin group on day 30 was 25% compared to 0% for the other groups. There was no endothelialization along the graft luminal surface in the P(LLA-CL)/Heparin graft. The [PERSON] group similarly examined heparin-loaded grafts. However, they were concerned that their combination might not be sufficient to maintain patency for a longer time period, therefore they considered a blend of natural polymers. The synergistic effects of combining P(LLA-CL) with natural polymers, vascular endothelial growth factor (VEGF), and heparin encapsulation altered the graft lumen's surface and had a marked effect on surface endothelialization [23]. Other experiments using natural polymers include one recent study on a rabbit infrarenal aortic model. They compared heparin/P(LLA-CL)/collagen/elastin, P(LLA-CL)/collagen/elastin with VEGF, and ePTFE before endothelialization. The grafts were prepared using P(LLA-CL) with heparin and VEGF. Heparin and VEGF had a synergistic effect with each other for slower heparin release and longer VEGF bioactivity. The results showed that the test group maintained a higher patency than the other control groups, and the graft specimens at day 7 proved that endothelialization had occurred sufficiently by using DAPI staining [26].