Biointeractive polymers and tissue engineered blood vessels

Enhanced growth of animal and human endothelial cells on biodegradable polymers

Biodegradable polymers such as poly(L-lactic acid) (PLLA), poly(glycolic acid) (PGA) and PGA coated with PLLA are being employed for cell transplantation and for in vivo regeneration of vascular tissue. Ingrowth and organization of fibrovascular tissue inside polymer scaffolds lead to the occlusion of the regenerated blood vessel. In order to provide regulatory mechanisms to control the development of an inner capsule, endothelialization of these materials is necessary. To achieve this, we employed a novel ammonia plasma technique to surface modified PLLA substrates. Human endothelial cell (HUVEC) and rabbit microvascular endothelial cell (RbMVEC) growth was studied on modified PLLA and control PLLA. Our studies show that modified PLLA and fibronectin (Fn)-coated modified PLLA exhibited statistically significant improvement in HUVEC and RbMVEC growth (P<0.001) when compared to PLLA and Fn-coated PLLA. Therefore, ammonia plasma treatment gives us the unique capability of modifying prosthetic biomaterials of various constructs with the eventual transplantation of mammalian cells to be used in tissue engineering or as biological implants.

Accelerated healing of Dacron grafts seeded by preclotting with autologous bone marrow blood

> Studies have suggested that bone marrow-derived cells in the circulation may have the capacity and potential to endothelialize and heal vascular graft surfaces. We have investigated whether accelerated endothelialization could be achieved for Dacron grafts seeded by preclotting with bone marrow blood (BMB). Five 8 mm x 6 cm Dacron grafts seeded and preclotted with BMB and four controls preclotted with peripheral blood were implanted in the descending thoracic aorta (DTA) of mongrel dogs for 2 and 4 weeks. Two additional BMB DTA grafts were studied for 3 months. Five pairs of BMB and control grafts (4 mm x 6 cm) were bilaterally implanted into the carotids of dogs for 1 week and five pairs for 4 weeks. All grafts remained patent. BMB seeding/preclotting was a simple, effective method to accelerate early graft endothelialization without increasing thrombogenicity. Further studies are needed before clinical application can be recommended.

Functional arteries grown in vitro

A tissue engineering approach was developed to produce arbitrary lengths of vascular graft material from smooth muscle and endothelial cells that were derived from a biopsy of vascular tissue. Bovine vessels cultured under pulsatile conditions had rupture strengths greater than 2000 millimeters of mercury, suture retention strengths of up to 90 grams, and collagen contents of up to 50 percent. Cultured vessels also showed contractile responses to pharmacological agents and contained smooth muscle cells that displayed markers of differentiation such as calponin and myosin heavy chains. Tissue-engineered arteries were implanted in miniature swine, with patency documented up to 24 days by digital angiography.

Proliferation of endothelial cells on surface-immobilized albumin-heparin conjugate loaded with basic fibroblast growth factor

Seeding of endothelial cells (ECs) on the luminal surface of small-diameter vascular grafts is a promising method to avoid occlusion of these prostheses. Immobilization of basic fibroblast growth factor (bFGF) to substrates used to coat or fill porous prostheses may enhance the formation of a confluent monolayer of ECs. Human umbilical vein endothelial cells (HUVECs) were grown on bFGF-loaded albumin-heparin conjugate bound to CO2 gas-plasma-treated polystyrene. In the order of 2-3 ng/cm2 bFGF had to be immobilized to form a confluent monolayer of HUVECs. The most prominent effect of surface-immobilized bFGF was stimulation of the proliferation shortly after seeding, resulting within 3 days in confluent cell monolayers with high density. In contrast, in cultures with 0.3 ng/mL bFGF in the medium instead of bFGF bound to the surface, it took almost a week before the cell layers reached confluency. Binding of bFGF to heparin and the biological activity of bFGF towards ECs were not influenced by the (radio-)labeling of bFGF with iodine. However, only a minor part of the bFGF used in this study displayed heparin affinity. Furthermore, degradation and multimerization of labeled bFGF in time occurred when the growth factor was stored at 20 degrees -37 degrees C. This limits the use of labeled bFGF to short-term (hours) experiments. In conclusion, bFGF loading of vascular graft surfaces through complexation of bFGF with a heparin-containing matrix probably will lead to more rapid formation of a confluent monolayer of ECs on graft surfaces upon seeding of the cells.

Host response to tissue engineered devices

The two main components of a tissue engineered device are the transplanted cells and the biomaterial, creating a device for the restoration or modification of tissue or organ function. The implantation of polymer/cell constructs combines concepts of biomaterials and cell transplantation. The interconnections between the host responses to the biomaterial and transplanted cells determines the biocompatibility of the device. This review describes the inflammatory response to the biomaterial component and immune response towards transplanted cells. Emphasis is on how the presence of the transplanted cell construct affects the host response. The inflammatory response towards a biomaterial can impact the immune response towards transplanted cells and vice versa. Immune rejection is the most important host response towards the cellular component of tissue engineered devices containing allogeneic, xenogeneic or immunogenic ex vivo manipulated autologous cells. The immune mechanisms towards allografts and xenografts are outlined to provide a basis for the mechanistic hypotheses of the immune response towards encapsulated cells, with antigen shedding and the indirect pathway of antigen presentation predominating. A review of experimental evidence illustrates examples of the inflammatory response towards biodegradable polymer scaffold materials, examples of devices appropriately integrated as assessed morphologically with the host for various applications including bone, nerve, and skin regeneration, and of the immune response towards encapsulated allogeneic and xenogeneic cells.

Advances in tissue engineering of blood vessels and other tissues

Tissue engineering is a new and rapidly expanding field, in which techniques are being developed for culturing a variety of tissues both in vitro and in vivo using polymer 'scaffolds' to support tissue growth. Polymer scaffolds used in tissue engineering are generally biodegradable, often involving compounds which are already approved for human implantation. In some cases, these polymers may be chemically modified to exhibit selective cell adhesion properties, which enhance cell attachment and subsequent tissue growth. Many cell types have been successfully cultured on these scaffolds, including smooth muscle cells, endothelial cells, hepatocytes and chondrocytes. Tissue engineering holds the potential for the in vitro development of autologous or allogeneic transplantable vascular conduits. Each year in the USA, there are approximately 1.4 million procedures performed which require arterial prostheses. Most of these procedures are in small calibre (< 6 mm) vessels, for which synthetic graft materials are not generally suitable. While autologous venous or arterial vessels are generally used, not all patients possess adequate conduit for revascularization. Tubular scaffolds have been specially designed for culturing small calibre arteries in vitro. Bovine aortic vascular cells were seeded and cultured on these polymer scaffolds, and grown under conditions of pulsatile pressure and intra-luminal flow. To minimize contamination during the weeks of tissue culture required to produce an arterial prosthesis, a sterile incubator system was developed. Preliminary studies have achieved good cell densities of both smooth muscle cells and endothelial cells on biodegradable polymer scaffolds.

Fibrin glue containing fibroblast growth factor type 1 and heparin decreases platelet deposition

BACKGROUND

The early success rates of endarterectomy and angioplasty are influenced by the thrombogenicity of the deendothelialized surface. We previously reported decreased platelet deposition after 30 and 120 minutes and after 28 days on expanded polytetrafluoroethylene (ePTFE) grafts coated with fibrin glue (FG) containing fibroblast growth factor type 1 (FGF-1) and heparin in canine aortoiliac bypass grafts when compared with control uncoated grafts. The FG/FGF-1/heparin coating has been shown to enhance spontaneous endothelialization at 28 days in canine ePTFE bypass grafts. The current study evaluates the thrombogenicity of this FG/FGF-1/heparin suspension applied to a balloon de-endothelialization model of endarterectomy in canine carotid arteries.

METHODS

Nine dogs underwent bilateral, deendothelialization balloon injury to 6-cm segments of their carotid arteries. Fibrin glue (fibrinogen 32.1 mg/mL + thrombin 0.32 U/mL) containing FGF-1 (11 ng/mL) and heparin (250 U/mL) was applied to the luminal surface of one carotid artery in each dog. Both femoral arteries were circumferentially dissected but not balloon injured; one femoral artery was clamped for the same period as the carotid arteries. In the 6 acute dogs, 10 minutes prior to the restitution of flow in both carotid arteries and one femoral artery, 4 to 8 x 10(9) (111)In-labelled autologous platelets were injected intravenously. Four-cm segments of both carotid and femoral arteries were excised after 15 or 120 minutes of circulation (n = 3/time/artery, 24 arteries). In the 3 chronic dogs, the radiolabelled platelets were injected 30 days after carotid injury. The carotid and femoral vessels were then excised after 120 minutes of perfusion. Radioactive platelet deposition was quantitated by gamma counting.

RESULTS

After 2 hours, the injured carotid arteries demonstrated significantly more platelet deposition than either uninjured femoral artery controls (P < 0.001). There was also a significant 45.2% decrease (P = 0.008) in platelet deposition on the balloon injured carotid arteries treated with FG/FGF-1/heparin when compared with balloon injured carotid arteries alone. At 30 days there was an insignificant trend toward decreased thrombogenicity in the FG/FGF-1/heparin treated injured carotids.

CONCLUSION

Surface coating with FG/FGF-1/heparin significantly decreases platelet deposition on balloon injured canine carotid arteries after 2 hours of perfusion and may be clinically applicable in endarterectomy and angioplasty procedures. The long-term induction of reendothelialization of arterial surfaces by this technique is under investigation.