Extracellular matrix (ECM) microstructural composition regulates local cell-ECM biomechanics and fundamental fibroblast behavior: a multidimensional perspective.

The extracellular matrix (ECM) provides the principal means by which mechanical information is communicated between tissue and cellular levels of function. These mechanical signals play a central role in controlling cell fate and establishing tissue structure and function. However, little is known regarding the mechanisms by which specific structural and mechanical properties of the ECM influence its interaction with cells, especially within a tissuelike context. This lack of knowledge precludes formulation of biomimetic microenvironments for effective tissue repair and replacement. The present study determined the role of collagen fibril density in regulating local cell-ECM biomechanics and fundamental fibroblast behavior. The model system consisted of fibroblasts seeded within collagen ECMs with controlled microstructure. Confocal microscopy was used to collect multidimensional images of both ECM microstructure and specific cellular characteristics. From these images temporal changes in three-dimensional cell morphology, time- and space-dependent changes in the three-dimensional local strain state of a cell and its ECM, and spatial distribution of beta1-integrin were quantified. Results showed that fibroblasts grown within high-fibril-density ECMs had decreased length-to-height ratios, increased surface areas, and a greater number of projections. Furthermore, fibroblasts within low-fibril-density ECMs reorganized their ECM to a greater extent, and it appeared that beta1-integrin localization was related to local strain and ECM remodeling events. Finally, fibroblast proliferation was enhanced in low-fibril-density ECMs. Collectively, these results are significant because they provide new insight into how specific physical properties of a cell's ECM microenvironment contribute to tissue remodeling events in vivo and to the design and engineering of functional tissue replacements.

Small intestinal submucosa as a vascular graft: a review.

Continuing investigations of vascular graft materials suggest that unacceptable graft complications continue and that the ideal graft material has not yet been found. We have developed and tested a biologic vascular graft material, small intestine submucosa (SIS), in normal dogs. This material, when used as an autograft, allograft, or xenograft has demonstrated biocompatibility and high patency rates in aorta, carotid and femoral arteries, and superior vena cava locations. The grafts are completely endothelialized at 28 days post-implantation. At 90 days, the grafts are histologically similar to normal arteries and veins and contain a smooth muscle media and a dense fibrous connective tissue adventitia. Follow-up periods of up to 5 years found no evidence of infection, intimal hyperplasia, or aneurysmal dilation. One infection-challenge study suggested that SIS may be infection resistant, possibly because of early capillary penetration of the SIS (2 to 4 days after implantation) and delivery of body defenses to the local site. We conclude that SIS is a suitable blood interface material and is worthy of continued investigation. It may serve as a structural framework for the application of tissue engineering technologies in the development of the elusive ideal vascular graft material.

Strength over time of a resorbable bioscaffold for body wall repair in a dog model.

The change in strength over time of a biomaterial derived from the small intestinal submucosa (SIS) was determined in a dog model of body wall repair. Full-thickness body wall defects measuring 8 x 12 cm were surgically created and then repaired with a multilaminate eight-layer form of SIS in 40 dogs. Five dogs were sacrificed at each of the following time points: 1 day, 4 days, 7 days, 10 days, and 1, 3, 6, and 24 months. Ball burst tests that measured biaxial ultimate load-bearing capability were performed on the device prior to implantation and on the device/implant site at the time of sacrifice. The strength of the device at the time of implant was approximately 73 +/- 12 pounds. The strength of the implant site diminished to 40 +/- 18 pounds at 10 days, and then progressively increased to a value of 156 +/- 26 pounds at 24 months (P < 0.05). The clinical utility of a degradable biomaterial such as SIS depends on a balance between the rate of degradation and the rate of host remodeling. Naturally occurring extracellular matrix scaffolds such as SIS show rapid degradation with associated and subsequent remodeling to a tissue with strength that exceeds that of the native tissue when used as a body wall repair device.

Porosity of porcine small-intestinal submucosa for use as a vascular graft.

The porosity of a vascular graft material has been suggested as a major factor affecting the rate and degree of neovascularization of newly implanted grafts, with higher porosities generally associated with better performance. The objective of this study was to determine the water porosity of a new vascular graft material, small-intestinal submucosa (SIS), and to compare the values to those reported for other common vascular graft materials. In addition, the porosity of SIS was investigated with respect to applied pressure and applied uniaxial tension. Both rectangular, flat specimens and tubular specimens of SIS were subjected to static water pressure, and water was collected as it passed through the SIS material. SIS has a typical porosity of 0.52 mL/min.cm-2 at an applied pressure of 120 mm Hg. Although porosity appeared to be unaffected by uniaxial tension, it increased in proportion to applied pressure at a rate of 4.8 x 10(-3) mL/min.cm-2/mm Hg. These low porosity values and the past success of SIS as a vascular graft material suggest that high-porosity materials are not required for implant success.

Directional porosity of porcine small-intestinal submucosa.

Small-intestinal submucosa (SIS) has been shown to be a promising biomaterial for vascular graft applications. This study examines the directionality property of SIS porosity using 35 SIS specimens from 13 pigs. In addition, the effects of the weight of the donor pig, pre-conditioning of 13 additional SIS specimens, and the duration of the test of five additional SIS specimens on such porosity are reported. The porosity from serosal to mucosal direction was found to be four times greater than the porosity in the opposite direction. The weight of the donor pig was not found to be an important factor in SIS porosity. Preconditioning served to increase the average serosal porosity index at 120 mm Hg static water pressure from 2.99 to 8.33 mL/(min cm2). The porosity in the mucosal direction was not affected by preconditioning. Porosity in both directions decreased with increasing test duration. The directionality property of SIS porosity may be an important factor in its success as a vascular graft. The term 'porosity' is used throughout this article, but current standards also refer to the term 'permeability' to describe the passage of liquid through a vascular graft.

Mechanical properties of xenogeneic small-intestinal submucosa when used as an aortic graft in the dog.

Small-intestinal submucosa (SIS) has been shown to induce tissue remodelling in vivo when used as a vascular graft. The present study investigated in physical and mechanical properties of remodeled aortic grafts derived from xenogeneic SIS material. Eight infrarenal aortic grafts were implanted in mongrel dogs. The grafts were explanted at 1 or 2 months and tested for compliance and hoop mechanical properties. The morphologic changes within the grafts were also characterized. The remodeling process produced graft structures which were significantly stronger than both the normal artery (P = .012) and the original SIS graft (P = .0001), and the compliance of these structures was one third that of normal artery and similar to the original SIS grafts. The remodeled grafts were > 10 times the thickness of the implanted SIS. Immunohistochemical analysis of remodeled tissues suggest that the SIS material was degraded and resorbed over time. The remodeling process transformed a material which was physically and mechanically quite different from normal aorta into a blood conduit which had the physical and mechanical properties needed to function in this mammalian arterial system.

Elastic modulus of prepared canine jejunum, a new vascular graft material.

The submucosal connective tissue of the jejunum has been shown to be suitable for use as a vascular graft in preliminary dog studies. To partially characterize the mechanical properties of this new graft material, longitudinal stress (sigma)-strain (epsilon)-data were obtained on 13 specimens of canine jejunum, stripped of its mucosal and external smooth-muscle layers. The ratio of stress to strain is the modulus of elasticity (E). It was found that the stress sigma-strain epsilon-data fitted the expression sigma = K epsilon alpha very well. For a typical specimen sigma = 2.69 x 10(6) epsilon 2.33. The modulus of elasticity (E = sigma 1-1/alpha K1/alpha) was found to increase with increasing stress, ranging from about 2,000 to 9,000 mmHg. For the average specimen E = 573 sigma 0.57, where sigma is in mmHg, (1 mmHg = 133.3 Pascals).

Multilaminate resorbable biomedical device under biaxial loading.

The design and test of a multilaminate sheet developed for a hernia repair application is presented. As biomaterial applications become more complex, characterization of uniaxial properties becomes insufficient and biaxial testing becomes necessary. A measure of the in-plane biaxial strength of the device is inferred from a ball burst test. The results of this test for different thicknesses of the device are correlated with the uniaxial strength of the material. A biaxial test such as the ball burst test is more indicative of the properties of a planar material than would be a uniaxial test. The interactions in the biaxial mode of failure are of value and can be related back to a classical uniaxial tensile test from the ball burst test. The material used in this study to fabricate the device was a resorbable biomaterial called small intestinal submucosa (SIS). The effects of rehydration on the stiffness and associated ball burst properties of the SIS device were also measured. It is shown that at a rehydration time of 5 min from a reference dry state, steady-state mechanical properties are reached.

The use of xenogeneic small intestinal submucosa as a biomaterial for Achilles tendon repair in a dog model.

A study was conducted to evaluate the tissue response to a xenogeneic biomaterial when this material was used to repair an experimentally induced Achilles tendon defect in the dog. Twenty dogs had a 1.5 cm segmental defect of the Achilles tendon created surgically which was then repaired with acellular connective tissue derived from porcine small intestinal submucosa (SIS). The animals were sacrificed at 1, 2, 4, 8, 12, 16, 24, and 48 weeks and the neotendons examined for uniaxial longitudinal tensile strength, morphologic appearance, hydroxyproline (collagen) content, and disappearance of the originally implanted SIS material over time. The contralateral normal Achilles tendons served as controls as did four additional dogs that had a 1.5 cm segmental Achilles tendon defect created surgically without subsequent surgical repair with SIS. Results showed the SIS remodeled neotendons to be stronger than the musculotendinous origin or the boney insertion (> 1000 N) by 12 weeks after surgery and to consist of organized collagen-rich connective tissue similar to the contralateral normal tendons. The four dogs in which no SIS was implanted showed inferior strength at the comparable time points of 4, 8, 12, and 16 weeks. Immunohistochemical studies suggest that the SIS biomaterial becomes degraded within the first eight weeks and serves as a temporary scaffold around which the body deposits appropriate and organized connective tissue. SIS is a promising biomaterial worthy of further investigation for orthopedic soft tissue applications.