Collagen organization in mandrel-grown vascular grafts

Nerve guidance conduit development for primary treatment of peripheral nerve transection injuries: A commercial perspective

Commercial nerve guidance conduits (NGCs) for repair of peripheral nerve discontinuities are of little use in gaps larger than 30 mm, and for smaller gaps they often fail to compete with the autografts that they are designed to replace. While recent research to develop new technologies for use in NGCs has produced many advanced designs with seemingly positive functional outcomes in animal models, these advances have not been translated into viable clinical products. While there have been many detailed reviews of the technologies available for creating NGCs, none of these have focussed on the requirements of the commercialisation process which are vital to ensure the translation of a technology from bench to clinic. Consideration of the factors essential for commercial viability, including regulatory clearance, reimbursement processes, manufacturability and scale up, and quality management early in the design process is vital in giving new technologies the best chance at achieving real-world impact. Here we have attempted to summarise the major components to consider during the development of emerging NGC technologies as a guide for those looking to develop new technology in this domain. We also examine a selection of the latest academic developments from the viewpoint of clinical translation, and discuss areas where we believe further work would be most likely to bring new NGC technologies to the clinic. STATEMENT OF SIGNIFICANCE: NGCs for peripheral nerve repairs represent an adaptable foundation with potential to incorporate modifications to improve nerve regeneration outcomes. In this review we outline the regulatory processes that functionally distinct NGCs may need to address and explore new modifications and the complications that may need to be addressed during the translation process from bench to clinic.

Stabilisation of Collagen Sponges by Glutaraldehyde Vapour Crosslinking

Glutaraldehyde is a well-recognised reagent for crosslinking and stabilising collagens and other protein-based materials, including gelatine. In some cases, however, the use of solutions can disrupt the structure of the material, for example, by causing rapid dispersion or distortions from surface interactions. An alternative approach that has been explored in a number of individual cases is the use of glutaraldehyde vapour. In this study, the effectiveness of a range of different glutaraldehyde concentrations in the reservoir providing vapour, from 5% to 25% (w/v), has been explored at incubation times from 5 h to 48 h at room temperature. These data show the effectiveness of the glutaraldehyde vapour approach for crosslinking collagen and show that materials with defined, intermediate stability could be obtained, for example, to control resorption rates in vivo.

Retention of endothelium on ovine collagen biomatrix vascular conduits under physiological shear stress

This study evaluates the adhesion of endothelial cells to 4 mm internal diameter, ovine collagen biomatrix vascular conduits. The biomatrix conduit is formed in a living animal and the wall consists of a complete, naturally produced matrix reinforced with polyester mesh. We propose that the microarchitecture of the matrix lining the lumen may promote endothelial cell attachment without pretreatment with adhesive proteins or extra cellular matrix components. Endothelial cell adhesion to the biomatrix surface was assessed by subjecting conduits seeded with ovine aortic endothelial cells (OAEC) to physiological range shear stresses of 16 and 32 dyn/cm2 in vitro. OAECs were isolated, cultured and seeded (1 x 10(6) cells/ml) by rotation onto the luminal surface of 20 cm lengths of biomatrix vascular conduits (n = 36). The seeded conduits were divided into three groups and cultured either for 24 h (n = 12), 48 h (n = 12) or 72 h (n = 12). Following culture, the conduits from each group were subjected to flow rates of either 240 ml/min (n = 6) or 480 ml/min (n = 6) with heparinized sheep blood for 1 h. Luminal surface cell cover was determined pre- and post-flow from Datura stramonium lectin labeled en face preparations. Histological analysis demonstrated that OAECs attach to the luminal surface of biomatrix conduits and form confluent monolayers within 24-48 h. Flow testing revealed that, for both flow rates and independent of the time in culture, there was no significant decrease in cell cover after flow (p = 0.13). The results support the hypothesis that a vascular conduit, engineered from a naturally formed biomatrix, provides a suitable substrate for the formation of flow resistant endothelium.

Peptoid-containing collagen mimetics with cell binding activity

Collagen mimetic peptides containing the peptoid residue Nleu (Goodman Bhumralkar, Jefferson, Kwak, Locardi. Biopolymers 1998;47:127-142) were tested for interactions with epithelial cells and fibroblasts. Molecules containing the sequence Gly-Pro-Nleu with a minimum of nine repeats showed cell binding activity. The activity of these molecules appeared to be conformationally sensitive, with the triple-helical form being preferred. When immobilized on a surface, the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence stimulated the attachment and growth of corneal epithelial cells and fibroblasts and the migration of epithelial tissue. The peptide sequence KDGEA inhibited cell attachment to the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence, suggesting that cell binding to this collagen mimetic involves the alpha2beta1 heterodimer integrin receptor. Interestingly, peptides containing the sequence (GlyNleu-Pro-)(10)-NH(2) did not have cell binding activity. The discovery that triple-helical peptides containing the Gly-Pro-Nleu sequences interact with cells opens up new opportunities in the design of collagen mimetic biomaterials.

Effects of mesh modification on the structure of a mandrel-grown biosynthetic vascular prosthesis

Mandrel-grown, mesh-reinforced vascular prostheses require adequate tissue coverage of the mesh for effective clinical function, particularly in low blood flow situations. Development of the ovine collagen-based Omniflowtrade mark vascular prosthesis has shown that the extent of this tissue cover is dependent on the interactions of the mandrel and the mesh with the sheep host. In the present study, the effects of chemical changes to the mesh have been examined. These data indicate that certain treatments of the mesh, particularly collagen or heparin, lead to increased tissue coverage while the number of sheep cells present and the ultrastructure of the resulting vessel remain unchanged.

Evaluation of a collagen-based biosynthetic material for the repair of abdominal wall defects

A collagen tissue polymer composite manufactured in sheep and prepared in two different forms (wet and dry) was compared to polypropylene mesh and to a control group for effectiveness in the repair of an abdominal wall defect in a rabbit model. The wet and dry patches were shown to differ significantly in their pore size. The wet material was shown to retain its natural porosity and promoted neovascularization, tissue integration, cellular infiltration, and neomatrix formation compared to the dry collagen-polymer patch. This material was superior to the polypropylene mesh implant, which was associated with significant adhesions. The appearance of type VI collagen was the earliest sign of new cell infiltration and neomatrix formation within the implant. New deposition of type VI collagen was apparent throughout the thickness of the implant within 4 weeks, followed by type III collagen accumulation. Decreased porosity of the collagen component in the dry patches resulted in a totally nonintegrated implant. This induced a foreign-body capsule with minimal cellular tissue infiltration and no deposition of collagen types VI and III within the implant.

Analysis of 274 Omniflow Vascular Prostheses implanted over an eight-year period

BACKGROUND

Polytetrafluoroethylene (PTFE), dacron, and, more recently, collagen prostheses are finding increasing use for femoropopliteal reconstruction when a suitable vein is not available. The main factors to be considered when choosing a prosthesis are patency, susceptibility to infection and formation of aneurysms.

METHODS

Sheep collagen prostheses were implanted on 274 occasions in the femoropopliteal or crural regions.

RESULTS

The patency rate for supragenual bypass after 3 years was 61.9% with good vascular periphery and 44% with poor vascular periphery. If the prosthesis extended below the knee, the patency rate was 55.4% with good and 35.3% with unfavourable vascular periphery. Patency for the femorocrural bypass was 28.7% after 2 years. The rate of infection was 0% and an aneurysm occurred in three patients (1.1%).

CONCLUSIONS

With this low infection rate and very slight danger of aneurysm, the long-term results suggest that the ovine collagen prosthesis can be recommended for use when no suitable vein is available.

Clinical use of omniflow vascular graft as arteriovenous bridging graft for hemodialysis

From April 1992 to November 1995, 109 bridging arteriovenous fistulas were performed in 89 patients for hemodialysis at National Taiwan University Hospital. These included 61 Omniflow vascular grafts (OVG) in 47 patients and 48 GoreTex polytetrafluoroethylene (PTFE) grafts in 42 patients. There were 36 male and 53 female patients, whose ages ranged from 24 to 84 years with a mean of 59 +/- 2 years. The operation was performed under lidocaine local anesthesia with antibiotic prophylaxis. The anatomic configurations included 50 (45.9%) forearm loop grafts, 19 (17.4%) forearm straight grafts, and 40 (36.7%) thigh loop grafts. Hemodialysis via the bridging graft was started 2 weeks after the operation. The patients received hemodialysis 3 times a week or more when indicated. No patients suffered from limb loss. Pseudoaneurysm and graft infection requiring operation occurred 0.96% and 1.92% per patient year, respectively, in the OVG group, and pseudoaneurysm and graft infection both occurred 2.38% per patient in the PTFE group. In the OVG group, the thigh loop graft had better patency than the forearm straight graft. The forearm loop graft had a better patency rate than the forearm straight graft. In the OVG group, the 1 to 4 year patency rate was 77 +/- 6 % 58 +/- 8%, 48 +/- 9%, and 34 +/- 13%, respectively, whereas in the PTFE group, the 1 to 4 year patency rate was 69 +/- 7%, 50 +/- 9%, 43 +/- 10%, and 26 +/- 15%, respectively. Therefore, the Omniflow is a good alternative bridging graft for hemodialysis.

Collagen organization in an oriented fibrous capsule

X-ray diffraction and polarized light microscopy studies were carried out on a fibrous capsule to examine the collagen organization produced in a granuloma response. The capsule had been formed around a silicone tube covered with polyester mesh following implantation in a sheep. The fibrous capsule has previously been shown to include a substantial amount of type III collagen, in addition to type I collagen. Whereas granulomas generally show little order, the sample in the present study has a high degree of orientation of the collagen fibrils along the tube direction. X-ray diffraction patterns of the capsule showed an axial D period near 65 nm, a value similar to that of other type III collagen-containing tissues. This axial period is less than the 67 nm period seen in tendon and bone, which contain only type I collagen. The decreased D period is consistent with an increase in the tilting of the collagen molecules with respect to the fibril axis.

Structural analysis of a collagen--polyester composite vascular prosthesis

The Omniflow Vascular Prosthesis is a collagen--polyester composite which has been used successfully for peripheral vascular replacement. In this study, we have examined the distribution of the various connective tissue components and the ultrastructural organisation of these in order to understand and allow improvement of its functional properties. Using immunohistology with specific monoclonal antibodies, types I and III collagens were found to be the major components throughout the prosthesis. Type VI collagen was also present but was mainly associated with cells, particularly around the polyester mesh and silicone interfaces. While elastin was absent, two elastic tissue microfibrillar proteins were present uniformly throughout the structure. Ultrastructurally, clear differences existed between the local environments of the inner surface, which had formed around the silicone mandrel, the polyester mesh within the prosthesis, and the outer collagenous tissue which formed the central wall. At the inner surface, the amount of collagen was less and the orientation of these fibres was not well defined. The collagen fibrils in the polyester region were smaller than those of the main wall, which were well ordered and orientated along the axis of the device.

Development of an ovine collagen-based composite biosynthetic vascular prosthesis

The search for an ideal vascular prosthesis to bypass peripheral vascular obstructive lesions is necessary where autologous tissues are either unavailable or unsuitable. This paper will outline the development and use of vascular conduits, principally of biological origin. The clinical benefits and limitations of these materials are discussed. The development of a composite biosynthetic prosthesis (Omniflow¿) is described, together with the testing methods used to determine and predict its suitability for use as an arterial substitute. The ovine biosynthetic prosthesis has significantly improved surface and mural properties over previous attempts at producing prostheses for vascular reconstruction. Immunohistological studies on samples recovered from dogs after 4 years show that the original ovine collagen is still present after 4 years, and it is further augmented by the deposition of new, host-derived connective tissue.

Osteogenic capacity of collagen in repair of established periodontal defects

Periodontal bone defects were established in four dogs, with one proximal lesion and one furcation lesion in each quadrant. These defects were treated with the implantation of collagen membranes, collagen sponge or a combination of membrane and sponge, inserted between the mucoperiosteal flaps and the bone defects. Control sites were treated in a similar surgical manner to the experimental sites, but no collagen was inserted. Substantial amounts of new bone formed in those cases treated with the collagen products, especially those treated with the membrane either with or without the sponge. The membranes limited the infiltration of small round cells, whereas in the control sites, inflammatory cells infiltrated to the bone surface. New connective tissue attachment was established in experimental situations, especially with the use of the membranes alone or in conjunction with sponge.