Introduction and review of literature
1.2 Review of literature
1.2.5 Silk biomaterials for vascular tissue engineering applications .1 Silk: varieties and molecular structure
1.2.5.3 An update on pre-clinical implications of silk-based TEVGs
which could pre-determine the immune response by means of fabricating ‘immune-informed’
tissue-engineered grafts.
Highly porous grafts although showed better remodeling but their compromised strength, which potentially reduced their in vivo feasibility.
Only low molecular weight SF grafts were mechanically stable till six months.
Plaiting and winding of silk fibers
Sprague- Dawley rats
Abdominal aorta
No observed acute thrombosis for silk grafts.
Silk graft showed superior patency rates than PTFE grafts over 1 year.
Silk grafts showed early arrangement of intimal and medial layer. 12-week explants revealed almost confluent endothelium.
Organization of medial layer was assisted by bone marrow derived cells.
Presence of vasa vasorum like capillaries were detected.
No sign of aneurysm formation.
Macrophage assisted graft degradation over extended time.
[189]
Braiding and winding of natural and recombinant silk fibers
Sprague- Dawley rats
Abdominal aorta
Excellent patency (85%).
Signs of neo-tissue formation along the lumen.
Presence of organized SMC layer and mature endothelium.
Recombinant grafts (modified to express collagen active sites) showed faster transanastomotic ingrowth of CD31 positive cells compared with control grafts comprised of domestic BM silk fibers.
[190]
Transgenic SF braiding grafts
Sprague- Dawley rats
Abdominal aorta
Signs of thrombus formation in occluded grafts.
No sign of intimal hyperplasia.
Superior endothelialization in central portion of transgenic SF grafts than wild type.
[191]
Double-
raschel knitted silk TEVGs
Sprague- Dawley rats
Abdominal aorta
Tunable elasticity and thickness of scaffold wall by manipulating knitting pattern.
Suitable patency and no sign of intimal hyperplasia.
PGDE crosslinking and aqueous SF coating endow adequate permeability comparable with medical grade PTFE grafts.
No significant difference in tissue ingrowth after 2 and 8 weeks, signifying slower degradation of scaffold.
[192]
Fibroin sponge coated double-
raschel knitted silk TEVGs
Beagle dogs
Common carotid artery
No acute thrombosis.
Sonography observation of 1-year implant suggested signs of intimal plaque formation at the middle and proximal anastomotic portion of the graft.
Sponge layer was completely replaced with fibrous tissue in 1-year explants.
Minimal detection of foreign body giant cells, substantiating immune compatibility.
[193]
Fibroin sponge coated double-
raschel knitted silk TEVGs
Sprague- Dawley rats
Abdominal aorta
Silk grafts coated with lower SF concentration (1-2.5% w/v) sponge facilitate constructive remodeling and prevent intimal hyperplasia/stenosis.
Significantly improved tissue infiltration in 1-2.5% SF coated grafts.
Complete degradation of sponge and replacement with host tissue.
[194]
Double-
raschel knitted TEVGs (SF and PE splice grafts)
Beagle dogs
Abdominal aorta
SF coating assist in rapid endothelialization and prevent medial thickening.
Thrombus formation in gelatin coated grafts.
SF coated grafts had thinner luminal layer compared with gelatin coated ones.
Higher ECs observed in SF coated silk fiber grafts.
[195]
Elastin modified double-
raschel knitted SF grafts
Rats Abdominal aorta
Elastin coating facilitates ECs’ attachment while prevent platelets adhesion.
Very good patency.
No sign of excessive immune response or fibrous capsule formation.
Rapid endothelialization within 2 weeks of implantation.
[196]
Plaited silk fibers and cocoon filaments coated with SF solution
Beagle dogs
Carotid artery
Dismal patency possibly due to lost mechanical strength by virtue of degradation.
Limited remodeling in large animals.
3-months data revealed superior endothelialization of silk graft compared with PTFE ones.
[197]
Braided SF threads
C57BL/6 mice
Right carotid artery
~13% (4/30) patency after 6 months.
2-week explants showed presence of SMA and CD31 positive cells in neointimal.
Complete endothelialization at 4 weeks.
Significant presence of collagen and elastin fibers after 4 weeks.
Signs of SF degradation at 6 months.
[140]
Silk fabric core layer embedded in a porous
scaffold
Rabbits Common carotid artery
No sign of stenosis and aneurysmal dilation.
Complete endothelialization in 3 months.
Blood flow profile was comparable with native artery.
Organized medial layer comprising of SMCs substantiating constructive graft remodeling.
[198, 199]
Double-
raschel knitted silk TEVGs
Sprague- Dawley rats
Inferior vena cava
95% patency for silk grafts, 80% patency for ePTFE grafts over 4 weeks.
Full lumen coverage by ECs in 4 weeks.
Podoplanin-positive mesothelial cells were traced on the outer surface of graft.
[200]
Electrospun silk TEVG
Lewis rats
Abdominal aorta
No sign of thrombus formation.
Good short-term patency.
Infiltration of host vascular cells observed in explant.
[201]
Newly developed elastic lamina along the graft lumen.
Notable presence of vasa vasorum at the outer scaffold surface.
Electrospun silk TEVGs
Sprague- Dawley rats
Abdominal aorta
Graft survival at 24 weeks: 95% (20/21) silk grafts, 73% (16/22) ePTFE grafts.
Rapid endothelialization in 6 weeks.
Regression of neointimal was also observed.
Constructive remodeling over time in terms of new ECM deposition, minimal number of inflammatory cells and reduction in granulation tissue.
[202]
Tri-layered SF/PCL composite electrospun TEVGs
Sprague- Dawley rats
Right carotid artery
Good patency.
Significant infiltration of host vascular cells.
[203]
Bi-layered SF/PLCL composite electrospun TEVGs
Rabbit Carotid artery
Good patency.
Graft remodeling and infiltration of host vascular cells.
Loss of mechanical strength over time.
[204]
Electrospun silk TEVGs (aqueous and HFP based)
Sprague- Dawley rats
Abdominal aorta
HFP grafts performed better than aqueous grafts.
No sign of aneurysm formation.
Neointimal regression over time.
HFP grafts facilitated faster contractile phenotype transition of SMCs.
Increased ECM production in HFP grafts.
[205]
Tri-layered silk TEVG (inner/outer electrospun layer, an intermediate woven layer)
Minipig and Sheep
Carotid artery
The study identified that sheep would be suitable option for a long-term pre-clinical trial.
Grafts were easy to handle during surgery and showed adequate suture retention.
Pre-seeding of cells is not required for SilkGraft.
[154]
Composite electrospun
Rabbit Left carotid artery
Graft degradation and neo-tissue formation.
[206]
PLCL grafts coated with silk/heparin
Remarkable patency.
No sign of intimal thickening.
Minimal inflammation resulting from biomaterial degradation.
Discontinuous endothelialization after 2 months.
Composite silk-
polyurethane (Silkothane) graft for haemodialysis
Sheep Arterioven ous shunt
Implantation between external jugular vein and common carotid artery.
No graft related complications.
8/9 sheep: 100% primary patency.
Signs of initial inflammation and infiltration of ECs.
Overall 100% patency.
[207]
While prior literature suggests the predominant use of Bombyx mori silk for vascular tissue engineering applications, present thesis work explores the non-mulberry silk varieties. In addition, various innovative approaches are demonstrated to improve the in situ remodeling and clinical feasibility of silk-based TEVGs.