Prevention of bioprosthetic heart valve tissue calcification by charge modification: effects of protamine binding by formaldehyde

The Use of Genipin as an Effective, Biocompatible, Anti-Inflammatory Cross-Linking Method for Nerve Guidance Conduits

A number of natural polymer biomaterial-based nerve guidance conduits (NGCs) are developed to facilitate repair of peripheral nerve injuries. Cross-linking ensures mechanical integrity and desired degradation properties of the NGCs; however, common methods such as formaldehyde are associated with cellular toxicity. Hence, there is an unmet clinical need for alternative nontoxic cross-linking agents. In this study, collagen-based NGCs with a collagen/chondroitin sulfate luminal filler are used to study the effect of cross-linking on mechanical and structural properties, degradation, biocompatibility, and immunological response. A simplified manufacturing method of genipin cross-linking is developed, by incorporating genipin into solution prior to freeze-drying the NGCs. This leads to successful cross-linking as demonstrated by higher cross-linking degree and similar tensile strength of genipin cross-linked conduits compared to formaldehyde cross-linked conduits. Genipin cross-linking also preserves NGC macro and microstructure as observed through scanning electron microscopy and spectral analysis. Most importantly, in vitro cell studies show that genipin, unlike the formaldehyde cross-linked conduits, supports the viability of Schwann cells. Moreover, genipin cross-linked conduits direct macrophages away from a pro-inflammatory and toward a pro-repair state. Overall, genipin is demonstrated to be an effective, safe, biocompatible, and anti-inflammatory alternative to formaldehyde for cross-linking clinical grade NGCs.

The design and development of a stented tissue mitral and aortic heart valve replacement for human implantation

Abstract

A study was conducted into the development of a mitral and aortic heart valve replacement that caters for patients having suffered valve damage due to stenosis or rheumatic fever. The appeal of the valve is that it is constituted from a solid frame housing pericardial tissue leaflets, and allows the patient freedom from post-operative blood-thinning medication. The valve is designed to appeal to patients in developing areas of the world, as it features a clip-in mechanism to secure the valve assembly into the sewing ring, which is stitched in independently of the frame and leaflets. Re-operative valve replacement would then be made possible when the pericardial leaflets began to calcify. Novel aspects of the design added value to the science of heart valve replacements, through the use of sintered chrome cobalt in the valve components, the insights gained into mechanical testing of pericardium, and the patient benefits offered by the complete design. Further work is planned to fatigue test the assembly, undergo animal trials and make the valve available for commercial use.

The design and development of a stented tissue mitral and aortic heart valve replacement for human implantation

A study was conducted into the development of a mitral and aortic heart valve replacement that caters for patients having suffered valve damage due to stenosis or rheumatic fever. The appeal of the valve is that it is constituted from a solid frame housing pericardial tissue leaflets, and allows the patient freedom from post-operative blood-thinning medication. The valve is designed to appeal to patients in developing areas of the world, as it features a clip-in mechanism to secure the valve assembly into the sewing ring, which is stitched in independently of the frame and leaflets. Re-operative valve replacement would then be made possible when the pericardial leaflets began to calcify. Novel aspects of the design added value to the science of heart valve replacements, through the use of sintered chrome cobalt in the valve components, the insights gained into mechanical testing of pericardium, and the patient benefits offered by the complete design. Further work is planned to fatigue test the assembly, undergo animal trials and make the valve available for commercial use.

Polyphenol-stabilized tubular elastin scaffolds for tissue engineered vascular grafts

Tissue-engineered vascular grafts require elastic, acellular porous scaffolds with controlled biodegradability and properties matching those of natural arteries. Elastin would be a desirable component for such applications, but elastin does not easily regenerate experimentally. Our approach is to develop tubular elastin scaffolds using decellularization and removal of collagen from porcine carotid arteries ( approximately 5 mm diameter) using alkaline extraction. Because elastin is susceptible to rapid degeneration after implantation, scaffolds were further treated with penta-galloyl glucose (PGG), an established polyphenolic elastin-stabilizing agent. Scaffolds were compared in vitro with detergent-decellularized arteries for structure, composition, resistance to degradation, mechanical properties, and cytotoxicity and in vivo for cell infiltration and remodeling potential. Results showed effective decellularization and almost complete collagen removal by alkaline extraction. PGG-treated elastin scaffolds proved to be resistant to elastase digestion in vitro, maintained their cylindrical shapes, showed high resistance to burst pressures, and supported growth of endothelial cells and fibroblasts. In vivo results showed that PGG treatment reduced the rate of elastin biodegradation and controlled cell infiltration but did not hamper new collagen and proteoglycan deposition and secretion of matrix-degrading proteases. Alkali-purified, PGG-treated tubular arterial elastin scaffolds exhibit many desirable properties to be recommended for clinical applications as vascular grafts.

A review of the biocompatibility of implantable devices: current challenges to overcome foreign body response

In recent years, a variety of devices (drug-eluting stents, artificial organs, biosensors, catheters, scaffolds for tissue engineering, heart valves, etc.) have been developed for implantation into patients. However, when such devices are implanted into the body, the body can react to these in a number of different ways. These reactions can result in an unexpected risk for patients. Therefore, it is important to assess and optimize the biocompatibility of implantable devices. To date, numerous strategies have been investigated to overcome body reactions induced by the implantation of devices. This review focuses on the foreign body response and the approaches that have been taken to overcome this. The biological response following device implantation and the methods for biocompatibility evaluation are summarized. Then the risks of implantable devices and the challenges to overcome these problems are introduced. Specifically, the challenges used to overcome the functional loss of glucose sensors, restenosis after stent implantation, and calcification induced by implantable devices are discussed.

Delivery and expression of pDNA embedded in collagen matrices

Collagen matrices can be used as non-viral biocompatible gene carriers for localized implantable gene therapy. Collagen matrices embedding pDNA with enhanced binding through condensing agent linkage to the matrix or to the pDNA have been formulated, and characterized in various systems. pDNA and condensed pDNA were released intact from the matrices within 1-2 days. In vitro transfection with collagen matrices containing pDNA (luciferase encoding), pDNA in liposome (LIP), and pDNA with polyethylenimine (PEI) resulted in significantly higher expression levels in comparison to naked pDNA. pDNA-LIP matrices exhibited a dose response transfection of NIH 3T3, 293, MDA-MB-231 and smooth muscle cells (SMCs) in cell cultures. Subdermal implantations of collagen-polylysine-pDNA matrices in rats resulted in significantly higher gene expression levels in comparison to non-condensed pDNA matrices. Perivascular treatment with pDNA matrix and of naked pDNA solution in balloon-injured rat carotid arteries resulted in significant expression. In conclusion, a facile method for embedding cationic formulations of pDNA in collagen matrices was developed. These bioactive matrices seem to be suitable for tissue engineering and local gene therapy strategies.

Use of bovine jugular vein to reconstruct the right ventricular outflow tract: early results

OBJECTIVE

We evaluate early results of bovine jugular vein conduits in the pulmonary outflow.

METHODS

Between April 2000 and September 2001, 31 conduits were placed in the outflow of the right ventricle. Patients who received a conduit as a staged surgical procedure were excluded (n = 3). Implantation age ranged from 0 to 21 years (median, 3.4 years). Conduit diameter ranged from 12 to 20 mm (median, 14 mm). Transthoracic echocardiography was performed at discharge and 3 months after surgery. Patients with significant pulmonary regurgitation and/or stenosis underwent cardiac catheterization.

RESULTS

Four patients died during the follow-up period. Three deaths were unrelated to the conduit. One death was related to the complete thrombosis of the conduit. At 3 months evaluation, pulmonary valve regurgitation was absent or trivial in 19, mild in 2 and severe in 3 of 24 survivors. Four patients had nonfatal conduit-related complications. A transient thrombus formation within 1 leaflet was noted postoperatively in a patient with a moderate pulmonary regurgitation. Three patients required reoperation 3 to 5.8 months after the implantation for conduit failure (mean, 4.3 months). Cardiac catheterization before replacement revealed an aneurysmal dilation of the conduit below a severe stenosis of the pulmonary bifurcation due to important neointimal proliferation.

CONCLUSIONS

Early failure of bovine jugular vein valved conduits can occur because of exaggerated intimal proliferation or thrombotic process within the conduit. Because of these complications, close echocardiographic follow-up is mandatory during the first weeks after implantation.

Inhibition of bioprosthesis calcification due to synergistic effect of Fe/Mg ions to polyethylene glycol grafted bovine pericardium

Calcification has limited the durability of bioprosthetic heart valves fabricated from glutaraldehyde pretreated porcine aortic valves or bovine pericardium (BP). The present study describes calcium antagonistic effect of polyethylene glycol grafted bovine pericardium (PEG-GABP) with Fe2+/Mg2+ delivery from a co-matrix system in rat subcutaneous model. Retrieved samples were biochemically evaluated for calcification and alkaline phosphate (AP) activity. Scanning electron micrographs of 21-day explants had shown excessive calcification with glutaraldehyde treated BP (control). However, the PEG grafting and Fe/Mg release had substantially inhibited the deposition of calcium on BP. The extractable alkaline phosphatase activity was also reduced with PEG grafting and metal ion release to BP. The extractable AP had shown peak activity at 72 h [for GATBP--250.5 +/- 1.2 nm pnp/mg protein/min enzyme activity (unit), PEG-GABP--165.2 +/- 16.6 units], but markedly reduced after 21 days (22.1 +/- 1.8 and 12.0 +/- 1.5 units, respectively). The initial high levels may be due to tissue injury via surgery, which mitigated with time. It is assumed that ferric ions may slow down or retard the calcification process by the inhibition of proper formation of hydroxy apatite while magnesium ions disrupt the growth of these crystals by replacing Ca2+. In addition it maybe hypothesized that these metal ions may inhibit the key element alkaline phosphatase, which acts as the substrate for mineralization. Hence, it is conceivable that a combination therapy via surface grafting of PEG and local delivery of low levels of ferric and magnesium ions may prevent the bioprosthesis associated calcification.

Evaluation of kangaroo aortic valved conduits in a juvenile sheep model: preliminary findings

BACKGROUND

Biological heart valve substitutes, manufactured from either porcine or bovine tissue, have been in use for more than 30 years. Despite low thrombogenicity and excellent performance, bioprosthetic heart valves tend to degenerate and calcify early in young patients because of patient and valve related factors. The aim of this study was to examine the calcification behavior of glutaraldehyde-preserved kangaroo heart valves in a juvenile sheep model.

METHODS

Porcine (n = 10) and kangaroo (n = 10) valved conduits were implanted in the descending aortic position of juvenile sheep and retrieved after 6, 8, and 12 months. Retrieved valved conduits were examined for morphological changes and calcification of the valve tissue, using Von Kossa's stain technique and atomic absorption spectrophotometry.

RESULTS

Structural valve deterioration, characterized by increased stiffness and severe calcification, occurred in 100% of the porcine conduits within 4 months. Kangaroo valve leaflets were significantly (p < 0.001) less calcified at 6 months (3.39+/-1.80 microg/mg), 8 months (5.86+/-4.57 microg/mg), and at 12 months (14.38+/-6.72 microg/mg), compared to porcine valves at 3 months (176.45+/-42.88 microg/mg ) and at 4 months (154.67+/-52.67 microg/mg ). Porcine aortic wall tissue was more calcified (118.24+/-42.86 microg/mg) than kangaroo aortic wall tissue (79.55+/-26.40 microg/mg).

CONCLUSIONS

Kangaroo heart valves calcify less than porcine heart valves. These findings suggest that a different donor valve tissue has a lower calcification potential probably due to a difference in the morphological ultrastructure. This could result in improved long-term durability of kangaroo heart valves.

Calcification resistance, biostability, and low immunogenic potential of porcine heart valves modified by dye-mediated photooxidation

The calcification potential, biostability, and immunogenic response of materials intended for long-term in vivo use, such as in heart-valve bioprostheses, are essential components of device performance. Here we explore these properties in photooxidized porcine heart valves. To study immunological sensitization, we injected tissue extracts intradermally into guinea pigs. Test and control animals received a challenge patch of the appropriate extract and were scored for dermal reactions. Neither cottonseed oil nor sodium chloride extracts of photooxidized heart-valve tissues caused any dermal inflammatory response. After implantation in the rat subcutaneous model for 90 days, the calcium content of 48-h-treated photooxidized cusp tissue [0.04 +/- 0.00 mg/g wet weight (gww)] was comparable to that of unimplanted control tissues (usually <1 mg/gww) and much lower than that of glutaraldehyde-treated controls (71 +/- 15 mg/gww). The porcine aortic wall calcium content (49 +/- 31 mg/gww) was comparable to that of glutaraldehyde-treated controls (59 +/- 8 mg/gww). Histologically, a time-dependent decrease in inflammation and vascularization with increasing photooxidation time was noted in the rat model along with an increase in the stability and organization of collagen bundles. In summary, porcine valve tissues treated by dye-mediated photooxidation were resistant to calcification, were biostable, and demonstrated a low immunogenic response, indicating potential for use in heart-valve bioprostheses.

Tissue reactions to epoxy-crosslinked porcine heart valves post-treated with detergents or a dicarboxylic acid

Calcification limits the long-term durability of xenograft glutaraldehyde (GA)-crosslinked heart valves. Previously, a study in rats showed that epoxy-crosslinked heart valves reduced lymphocyte reactions to the same extent as the GA-crosslinked control and induced a similar foreign-body response and calcification reaction. The present study was aimed at reducing the occurrence of calcification of epoxy-crosslinked tissue. Two modifications were carried out and their influence on cellular reactions and the extent of calcification after 8 weeks' implantation in weanling rats was evaluated. First, epoxy-crosslinked valves were post-treated with two detergents to remove cellular elements, phospholipids and small soluble proteins, known to act as nucleation sites for calcification. The second approach was to study the effect of the impaired balance between negatively and positively charged amino acids by an additional crosslinking step with a dicarboxylic acid. The detergent treatment resulted in a washed-out appearance of especially the cusp tissue. With the dicarboxylic acid, both the cusps and the walls had a limited washed-out appearance. The wall also demonstrated some detachment of the subendothelium. After implantation, both detergent and dicarboxylic acid post-treatment histologically resulted in reduced calcification at the edges of cusps and walls. However, total amounts of calcification, measured by atomic emission spectroscopy, were not significantly reduced. Data concerning the presence of lymphocytes varied slightly, but were in the same range as the GA-crosslinked control, i.e., clearly reduced compared with a noncrosslinked control. It is concluded that both the double detergent and the dicarboxylic acid post-treatment of epoxy-crosslinked heart valve tissue do not represent a sound alternative in the fabrication of heart valve bioprostheses.

Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering

Various research groups around the world are actively investigating cardiovascular prostheses of biological origin. This review article discusses the need for such bioprosthetics and the potential role for natural tissues in cardiovascular applications such as cardiac valves and vascular grafts. Upon implantation, unmodified natural materials are subject to chemical and enzymatic degradation, seriously decreasing the life of the prosthesis. Therefore, methods such as glutaraldehyde and polyepoxide crosslinking treatments and dye-mediated photooxidation have been developed to stabilize the tissue while attempting to maintain its natural mechanical properties. Also, residual cellular components in a bioprosthetic material have been associated with undesired effects, such as calcification and immunological recognition, and thus have been the motivation for various decellularization processes. The effects of these stabilization and decellularization treatments on mechanical, biological and chemical properties of treated tissues have been investigated, specifically with regard to calcification, immunogenicity, and cytotoxicity concerns. Despite significant advances in the area of cardiovascular prostheses, there has yet to be developed a completely biocompatible, long-lasting implant. However, with the recent advent of tissue engineering, the possibility of applying selective cell seeding to naturally derived bioprosthetics moves us closer to a living tissue replacement.

Model experimental system for investigation of heart valve calcification in vitro

A model system was developed for the in vitro quantitative investigation of the calcification process occurring in heart valves. The process of heart valve calcification consists of the formation of calcium phosphates at the heart valve-biological fluid interface. Calcium phosphate deposits may consist of more than one calcium phosphate mineral phase, differing with respect to their physical and chemical properties. The kinetics of the formation of hydroxyapatite, the model inorganic compound for the calcified deposits, was precisely monitored in a reactor containing supersaturated calcium phosphate solutions in which the heart valves were immersed after being treated with glutaraldehyde and mounted on special racks. The precipitation process, accompanied with proton release in the solution, was monitored by a pair of glass-saturated calomel electrodes. Upon initiation of the formation of calcium phosphate deposits, the supersaturation in the working solution was reestablished through the addition of titrant solutions made with the appropriate concentration to compensate for the ions precipitated. With this methodology, not only the rates were measured very precisely but also the nucleation capability of the various substrates could be evaluated. Moreover, it was possible to identify the formation of intermediate calcium phosphate phases formed during the calcification process. Valves previously treated with glutaraldehyde were shown to nucleate octacalcium phosphate, which at lower supersaturations converted to the thermodynamically more stable hydroxyapatite. The rates measured were found to depend on the solution supersaturation, while the apparent order of the precipitation process was found to be 1.

In vitro and in vivo effects of tetrakisphosphonates on bone resorption, tumor osteolysis, ectopic calcification, and macrophages

The biological effects of bisphosphonates in calcium-related disorders are attributed to the incorporation of the bisphosphonates in bone, enabling direct interaction with osteoclasts and/or osteoblasts. The high accumulation of bisphosphonates in bone, due to their high affinity to hydroxyapatite (HAP), is essential for mediating in vitro and in vivo activity. In this study we examined the activity of tetrakisphosphonates, molecules containing two P-C-P type bisphosphonate moieties connected by a carbon chain. The novel compounds were examined in a battery of in vitro and in vivo models including HAP formation and dissolution, ectopic calcification, bone resorption, tumor osteolysis, and of macrophage-like cells (anti- or pro-inflammatory properties). The inhibition of ectopic calcification was ranked as follows: geminal bisphosphonates > bisacylphosphonates > tetrakisphosphonates. Pamidronate, but not the tetrakisphosphonates, was an effective antiosteolytic agent. Neither DNTP (tetrasodium 1,9-dihydroxynonane 1,1,9,9-tetrakisphosphonate) nor the bisacylphosphonate, PiBP (pimeloylbisphosphonate) seem to possess strong macrophage suppressive or inductive effects and can be considered to be relatively inactive in terms of anti- or pro-inflammatory action. A significant anticalcification effect was caused by various phosphonates, such as the tetrakisphosphonates, but DNTP, a tetrakisphosphonate, was found toxic as it impeded somatic growth and bone development.

Prevention of bioprosthetic heart valve calcification by ethanol preincubation. Efficacy and mechanisms

BACKGROUND

Calcification of the cusps of bioprosthetic heart valves fabricated from either glutaraldehyde cross-linked porcine aortic valves or bovine pericardium frequently causes the clinical failure of these devices. Our investigations studied ethanol pretreatment of glutaraldehyde cross-linked porcine aortic valves as a new approach to prevent cuspal calcification. The hypothesis governing this approach holds that ethanol pretreatment inhibits calcification resulting from protein structural alterations and lipid extraction.

METHODS AND RESULTS

Results demonstrated complete inhibition of calcification of glutaraldehyde-pretreated porcine bioprosthetic aortic valve cusps by 80.0% ethanol in rat subdermal implants (60-day ethanol-pretreated calcium level, 1.87 +/- 0.29 micrograms/mg tissue compared with control calcium level, 236.00 +/- 6.10 micrograms/mg tissue) and in sheep mitral valve replacements (ethanol-pretreated calcium level, 5.22 +/- 2.94 micrograms/mg tissue; control calcium level, 32.50 +/- 11.50 micrograms/mg tissue). The mechanism of ethanol inhibition may be explained by several observations: ethanol pretreatment resulted in an irreversible alteration in the amide I band noted in the infrared spectra for both purified type I collagen and glutaraldehyde cross-linked porcine aortic leaflets. Ethanol pretreatment also resulted in nearly complete extraction of leaflet cholesterol and phospholipid.

CONCLUSIONS

Ethanol pretreatment of glutaraldehyde cross-linked porcine aortic valve bioprostheses represents a highly efficacious and mechanistically based approach and may prevent calcific bioprosthetic heart valve failure.

Novel anti-calcification treatment of biological tissues by grafting of sulphonated poly(ethylene oxide)

Biological porcine tissue was modified by the direct coupling of sulphonated poly(ethylene oxide) (PEO-SO3) containing amino acid end groups after glutaraldehyde fixation. The calcification of the modified tissue [bioprosthetic tissue (BT)-PEO-SO3] and control (BT control) was investigated by in vivo rate subdermal, canine aorta-illiac shunt and right ventricle-pulmonary artery shunt implantation models. Less calcium deposition of BT-PEO-SP3 than of BT control was observed in in vivo tests. Such a reduced calcification of BT-PEO-SO3 can be explained by decreases of residual glutaraldehyde groups, a space filling effect and, therefore, improved biostability and synergistic blood-compatible effects of PEO and SO3 groups after the covalent binding of PEO-SO3 to tissue. This simple method can be a useful anti-calcification treatment for implantable tissue valves.

Calcification of polyurethanes implanted subdermally in rats is enhanced by calciphylaxis

Calcification complicates the use of the polymer polyurethane in cardiovascular implants. To date only costly experimental circulatory animal models have been useful for investigating this disease process. In this paper we report that polyurethane calcification in rat subdermal implants is enhanced by overdosing with a vitamin-D analog. The calcification-prone state, known as calciphylaxis, was induced in 4-week old rats by oral administration of a vitamin-D analog, dihydrotachysterol. We studied two commercially available polyurethanes (Biomer and Mitrathane) and two proprietary polyurethanes (PEU-2000 and PEU-100). PEU-100 is unique because it is derivatized with ethanehydroxy-bisphosphonate (EHBP) for calcification resistance. Polyurethane calcium and phosphate levels and morphological changes due to calciphylaxis were compared with those of control rat subdermal explants in 60-day studies. Increased polyurethane mineralization was observed due to calciphylaxis with 60-day rat subdermal explants of Biomer, Mitrathane, and PEU-2000 (calcium levels, respectively, 4.13 +/- 0.56, 18.61 +/- 2.73, and 3.37 +/- 0.22 microgram/mg, mean +/- standard error) as compared to control explants (calcium levels, respectively, 1.22 +/- 0.1, 12.57 +/- 0.86, and 0.20 +/- 0.86 microgram/mg). The study also demonstrated that with 60-day implants calciphylaxis had no side effects on somatic growth and serum calcium levels. Explant surface morphology of these polyurethane explants examined by scanning electron microscopy, back scattering electron imaging coupled with energy dispersive X-ray spectroscopy, and light microscopy demonstrated the presence of predominantly surface-oriented calcification. PEU-100, derivatized with 100 n.moles/ mg of EHBP, resisted calcification with explant calcium levels 0.51 +/- 0.01 (calciphylaxis) and 0.38 +/- 0.01 (control) microgram/mg. It is concluded that calciphylaxis enhances superficial polyurethane calcification in rat subdermal implants and that an EHBP-modified polyurethane resists calcification despite calciphylaxis. Rat subdermal implants using calciphylaxis may be generally useful for evaluating the calcification potential of various biomedical polymers.

Structural studies on bovine bioprosthetic tissues and their in vivo calcification: prevention via drug delivery

Cardiovascular calcification, the formation of calcium phosphate deposits in cardiovascular tissue, is a common end-stage phenomenon affecting a wide variety of bioprostheses. To study the process of calcification in tissue prosthetics, glutaraldehyde-treated bovine pericardium, dura mater and fascialata were implanted subcutaneously in rats and retrieved 21 days later and thereby morphological findings were correlated with biochemically determined levels of calcium. Transmission electron microscopy showed that calcification primarily involved the surface of collagen fibrils and the interfibrillar spaces. The deposition of calcium was higher with dura and fascia prostheses compared to pericardium. However, the release of Fe3+ ions from chitosan matrix had substantially inhibited the deposits of calcium in all implanted tissues. It seems that the structural and anatomical features of the tissue is one of the important factors for tissue-associated calcification. It is also confirmed that glutaraldehyde-preserved pericardium is the most suitable material for the development of cardiac prosthesis, with an appropriate drug combination therapy for prevention of pathological calcification.

Anticalcification and antiresorption effects of bisacylphosphonates

Some geminal bisphosphonates are used clinically in a number of important bone and calcium-related diseases. This work reports the anticalcification and antiresorption effects of a series of bisacylphosphonates, nongeminal compounds with varying chain lengths having oxo groups in alpha positions relative to the phosphonic functions. We compared the activity of the novel compounds to clinically used geminal bisphosphonates, and to a bisphosphonate devoid of the oxo groups. The interaction of the compounds with calcium was studied by various in vitro and in vivo models. We found that keto groups in alpha positions to the phosphonic functions render activity. The bisacylphosphonates with a shorter chain [(CH2)n, = 4, 6] were found significantly to inhibit hydroxyapatite formation and dissolution in vitro, the calcification of bioprosthetic tissue implanted subdermally in rats, and bone resorption in the intact young animal model. The various in vitro results were found to be in good correlation with the in vivo results. Structure-activity relationship studies indicate that both bisacylphosphonates and geminal bisphosphonates are active only when at least three ionizable groups are present in the molecule. The role of the keto groups is related to their contribution to chelating calcium and/or to their electron-withdrawing influence on acidity.

Validation of the shrinkage temperature of animal tissue for bioprosthetic heart valve application by differential scanning calorimetry

Shrinkage temperature is most often used to report the degree of cross-linking in glutaraldehyde-fixed animal tissue for use in bioprosthetic heart valve fabrication. Present practice utilizes the measurement of hydrothermal shrinkage observed when a sample is subjected to a temperature programme. This measurement at best gives a general indication of the efficiency of the treatment, i.e. the extent of cross-linking in the tissue. When differential scanning calorimetry has been used, the ambiguity arising from the scant reporting of the protocols used does not permit easy comparison of experimental results. This report addresses the considerations necessary to obtain optimum results in the differential scanning calorimetry experiment for the determination of shrinkage temperature in biological tissue. The shrinkage temperature of two previously unreported tissue types, porcine pericardium and equine pericardium, are provided and compared with those of bovine pericardium and porcine aortic valve leaflets.

Stabilization of pericardial tissue by dye-mediated photooxidation

Bovine pericardial tissue was stabilized through a dye-mediated photooxidation reaction. Shrink temperature analysis of the stabilized tissue indicated a material with similar properties to untreated pericardial tissue and unlike identical tissue treated with glutaraldehyde. Photooxidized tissue was resistant to extraction when compared with untreated tissue or control tissues treated in the absence of light or dye. Photooxidized tissue was also resistant to enzymatic digestion by pepsin and to chemical digestion by cyanogen bromide (CNBr). In contrast, untreated or control treated tissues were readily digested by these reagents. Reduction of photooxidized tissue with beta-mercaptoethanol prior to CNBr digestion partially restored susceptibility of the tissue to CNBr digestion, indicating the photooxidation of methionine residues. Soluble collagen derived from bovine pericardium was used as a model compound for the photooxidation reaction. Polyacrylamide gel electrophoresis analysis indicated the photooxidative conversion of collagen into higher molecular weight aggregates consistent with intermolecular crosslink formation. Photooxidized tissue was stable to in vivo degradation when compared with control tissue. Results presented here indicate a crosslinked pericardial tissue produced by dye-mediated photooxidation possessing properties of chemical stability, enzymatic stability, in vivo stability, and biomechanical integrity suitable for use as a biomaterial.

Posttreatment with amino compounds effective in prevention of calcification of glutaraldehyde treated pericardium

An attempt has been made to compare the role of different amino compounds in prevention of calcification of glutaraldehyde (GA)-treated pericardium implanted subcutaneously in the rat for 12 weeks. GA pretreated pericardial samples were postfixed separately with glycine, albumin, gentamicin, glycine + gentamicin, and albumin + gentamicin. Severe calcification was noticed in animals undergoing albumin or gentamicin postfixation. Mild calcification was observed in glycine and albumin + gentamicin postfixed materials whereas calcium was undetectable either chemically or morphologically in GA-pretreated pericardium postfixed with glycine + gentamicin (0.32 +/- 0.13.mg/g calcium, dry wt; von Kossa 0) at the 12th week of implantation in the rat.

In vivo biostability and calcification-resistance of surface-modified PU-PEO-SO3

To examine the biostability and calcification-resistance of polyurethanes (PUs), the surface of PU was grafted with hydrophobic perfluorodecanoic acid (PFDA) (PU-PFDA), hydrophilic polyethyleneoxide (PEO) (PU-PEO1000), and further negatively charged sulfonate groups (PU-PEO1000-SO3). An in vivo animal test was conducted by subcutaneous implantation in rats during 2, 4 and 6 months. A scanning electron microscope study demonstrated that the degree of surface cracking on explanted PUs was increased in the following order: PU-PFDA > PU > PU-PEO1000 > PU-PEO1000-SO3. In the results of energy dispersive x-ray analysis and inductively coupled plasma atomic emission spectrometry, the deposition of calcium was found abundantly, but that of phosphorus was hardly in existence in all implanted PUs, suggesting that this calcium compound is not a hydroxyapatite. The calcium contents, regardless of implantation time, were also increased in the same order (PU-PFDA > PU > PU-PEO1000 > PU-PEO1000-SO3). After 6 months implantation, no severe tissue reactions were observed and calcification almost occurred on polymer surfaces in all implants. Such superior biostability and anticalcification of PU-PEO1000-SO3 might be attributed to synergistic effects of its excellent surface smoothness, sulfonate acid (SO3-) groups, nonadhesive and mobile PEO, and the high hydrophilicity and enhanced blood compatibility. Therefore, PU-PEO1000-SO3 is promising as biostable and calcification-resistant biomaterial.

A new route of drug administration: intrauterine delivery of insulin and calcitonin

High molecular weight drugs in general, and peptides in particular, are usually delivered by parenteral route because they are poorly absorbed or degraded in the gastrointestinal tract. To optimize therapy, it is desirable to search for nonparenteral routes of administration and to deliver the drug in a controlled-release fashion. We report here on the absorption and the systemic biological effect of two peptides, insulin and calcitonin, after instillation into the uterus of the rat. Intrauterine delivery was compared to subcutaneous injections in intact and ovariectomized rats. In addition, we describe results of a preliminary study on calcitonin absorption from controlled-release matrices inserted in the rat uterus. The amount and duration of the hypoglycemic and the hypocalcemic effects induced by intrauterine delivery of insulin and calcitonin, respectively, were equivalent to those obtained after subcutaneous injections. The results were similar in intact and ovariectomized rats. It is concluded that the intrauterine administration of both insulin and calcitonin is bioequivalent to subcutaneous injection. The therapy of a number of clinically important diseases could benefit from this discovery.

Mechanical properties and histology of charge modified bioprosthetic tissue resistant to calcification

Modification of bioprosthetic heart valves tissue by covalently binding protamine sulphate, results in stable covalent links of protamine to the tissue, conferring resistance to calcification. We report here the morphological evaluation and mechanical properties (elastic modulus and ultimate tensile strength) of protamine-bound bioprosthetic tissue that have high anticalcification potential. Protamine-bound bioprosthetic tissue had significantly higher tissue modulus and ultimate tensile strength values than control tissue groups. However, the mechanical properties and tissue architecture were inferior to those of bioprosthetic tissue.

Adsorption of baboon fibrinogen and the adhesion of platelets to a thin film polymer deposited by radio-frequency glow discharge of allylamine

Platelet adhesion under static and flow conditions from a washed platelet suspension containing albumin to a polymer deposited by radio-frequency glow discharge of allylamine vapour on a poly(ethylene terephthalate) substrate was measured. Electron spectroscopy for chemical analysis was used to characterize the surface. Fibrinogen adsorption from a series of dilute plasma solutions to radio-frequency glow discharge/allylamine, measured using 125I radiolabelled baboon fibrinogen, increased with decreasing plasma dilution to a level much higher than that previously observed on polyurethanes. Elutability by sodium dodecyl sulphate of fibrinogen adsorbed from dilute plasma also increased with increasing plasma concentration, but fibrinogen preadsorbed from plasma became non-elutable when surfaces were stored in buffer for 5 d before contact with sodium dodecyl sulphate. Platelet adhesion to substrates which had been pre-adsorbed with dilute plasma was measured using baboon platelets radiolabelled with 111In. Adhesion greatly decreased as the plasma concentration used for preadsorption increased, suggesting that non-specific platelet binding to the bare surface occurs when protein coverage is incomplete. Non-specific platelet binding was inhibited to varying degrees by preadsorption of different proteins to the surface. Platelet adhesion to surfaces preadsorbed with dilute (1.0%) baboon and human plasmas lacking fibrinogen (i.e. serum, heat-defibrinogenated plasma and congenitally afibrinogenemic plasma) was diminished compared with normal plasma. Addition of exogenous fibrinogen to the deficient plasma partially restored platelet adhesion to normal levels. Adhesion to surfaces preadsorbed with human plasma deficient in von Willebrand factor was comparable to that observed with normal plasma. The plasma preadsorption studies with fibrinogen deficient media suggested that adsorbed fibrinogen is necessary for platelet adhesion to the radio-frequency glow discharge/allylamine substrate at high protein coverage. However, since adhesion was greatly reduced when the plasma preadsorbed substrate was stored in buffer before platelet contact, the conformation of adsorbed fibrinogen is also important in mediating platelet adhesion to radio-frequency glow discharge.

Bisacylphosphonates inhibit hydroxyapatite formation and dissolution in vitro and dystrophic calcification in vivo

Some geminal bisphosphonates are used clinically for a number of important bone/calcium related diseases; however, side effects and lack of selectivity impede their wide use. This work reports the synthesis and evaluation of bisacylphosphonates (e.g., adipoyl- and suberoylbisphosphonate). These compounds were found to inhibit significantly hydroxyapatite formation and dissolution in vitro and the calcification of bioprosthetic tissue implanted subdermally in rats. These are the first instances of nongeminal bisphosphonates [P-(C)n-P, n greater than or equal to 2] that have been reported to be active in calcium-related disorders. The reported bisacylphosphonates possess apparent lower toxicity, and their calcium complexes/salts have improved solubility properties. Therefore, they are of potential importance for clinical applications.

Cardiovascular implant calcification: a survey and update

Calcification of cardiovascular prosthetic implants is a common and important problem. This review provides an update based upon the Conference on Cardiovascular Implant Calcification held as part of the 13th World Congress of the International Society for Heart Research, 1989. A variety of cardiovascular prostheses are affected clinically by calcification, including bioprosthetic heart valves, aortic homografts and trileaflet polymeric valve prostheses. In addition, experimental studies have demonstrated calcification of artificial heart devices in ventricular assist systems in long-term calf studies. The pathophysiology of this disease process is incompletely understood. A common element between the various types of cardiovascular implant calcification is the localization of calcific deposits to devitalized cells and membranous debris. Prevention of cardiovascular implant calcification by either biomaterial modifications or regional drug therapy (controlled release) is being investigated.

Development of a new in vitro model for studying implantable polyurethane calcification

The objective of this study was to reproduce mineralization of polymeric substrate in an extracirculatory environment which would facilitate investigation of the calcification mechanism in implantable biomaterials and methods of prevention. Calcification was examined on polyurethane films incubated in metastable solutions of calcium phosphate and the role of strain, serum and polymer porosity was examined. Validation of the model was evaluated by examining the calcification of both highly calcifiable biomaterial (bioprosthetic tissue) and a non-calcifiable biomaterial (charge-modified tissue and polyurethane containing anticalcification agent). It is concluded that the developed model is adequately sensitive to diagnose biomaterials' propensity to calcify and could serve as a pre-screening method to examine calcification mechanism and methods of prevention.