Removing fresh tissue from explanted polyurethane prostheses: which approach facilitates physico-chemical analysis?

In vitro and in vivo degradation correlations for polyurethane foams with tunable degradation rates

Polyurethane foams present a tunable biomaterial platform with potential for use in a range of regenerative medicine applications. Achieving a balance between scaffold degradation rates and tissue ingrowth is vital for successful wound healing, and significant in vivo testing is required to understand these processes. Vigorous in vitro testing can minimize the number of animals that are required to gather reliable data; however, it is difficult to accurately select in vitro degradation conditions that can effectively mimic in vivo results. To that end, we performed a comprehensive in vitro assessment of the degradation of porous shape memory polyurethane foams with tunable degradation rates using varying concentrations of hydrogen peroxide to identify the medium that closely mimics measured in vivo degradation rates. Material degradation was studied over 12 weeks in vitro in 1%, 2%, or 3% hydrogen peroxide and in vivo in subcutaneous pockets in Sprague Dawley rats. We found that the in vitro degradation conditions that best predicted in vivo degradation rates varied based on the number of mechanisms by which the polymer degraded and the polymer hydrophilicity. Namely, more hydrophilic materials that degrade by both hydrolysis and oxidation require lower concentrations of hydrogen peroxide (1%) to mimic in vivo rates, while more hydrophobic scaffolds that degrade by oxidation alone require higher concentrations of hydrogen peroxide (3%) to model in vivo degradation. This information can be used to rationally select in vitro degradation conditions that accurately identify in vivo degradation rates prior to characterization in an animal model.

Analyzing material changes consistent with degradation of explanted polymeric hernia mesh related to clinical characteristics

BACKGROUND

Proposed mechanisms that potentially contribute to polypropylene mesh degradation after in vivo exposure include oxidizing species and mechanical strains induced by normal healing, tissue integration, muscle contraction, and the immediate and chronic inflammatory responses.

METHODS

This study explores these potential degradation mechanisms using 63 mesh implants retrieved from patients after a median implantation time of 24 months following hernia repair surgery (mesh explants) and analysis of multivariate associations between the material changes and clinical characteristics. Specifically, polypropylene mesh degradation was characterized in terms of material changes in surface oxidation, crystallinity and mechanical properties, and clinical characteristics included mesh placement location, medical history and mesh selection.

RESULTS

Compared to pristine control samples, subsets of mesh explants had evidence of surface oxidation, altered crystallinity, or changed mechanical properties. Using multivariate statistical approach to control for clinical characteristics, infection was a significant factor affecting changes in mesh stiffness and mesh class was a significant factor affecting polypropylene crystallinity changes.

CONCLUSIONS

Highly variable in vivo conditions expose mesh to mechanisms that alter clinical outcomes and potentially contribute to mesh degradation. These PP mesh explants after 0.5 to 13 years in vivo had measurable changes in surface chemistry, crystallinity and mechanical properties, with significant trends associated with factors of mesh placement, mesh class, and infection.

Shape memory polyurethanes with oxidation-induced degradation: In vivo and in vitro correlations for endovascular material applications

The synthesis of thermoset shape memory polymer (SMP) polyurethanes from symmetric, aliphatic alcohols and diisocyanates has previously demonstrated excellent biocompatibility in short term in vitro and in vivo studies, although long term stability has not been investigated. Here we demonstrate that while rapid oxidation occurs in these thermoset SMPs, facilitated by the incorporation of multi-functional, branching amino groups, byproduct analysis does not indicate toxicological concern for these materials. Through complex multi-step chemical reactions, chain scission begins from the amines in the monomeric repeat units, and results, ultimately, in the formation of carboxylic acids, secondary and primary amines; the degradation rate and product concentrations were confirmed using liquid chromatography mass spectrometry, in model compound studies, yielding a previously unexamined degradation mechanism for these biomaterials. The rate of degradation is dependent on the hydrogen peroxide concentration, and comparison of explanted samples reveals a much slower rate in vivo compared to the widely accepted literature in vitro real-time equivalent of 3% H2O2. Cytotoxicity studies of the material surface, and examination of the degradation product accumulations, indicate that degradation has negligible impact on cytotoxicity of these materials.

STATEMENT OF SIGNIFICANCE

This paper presents an in-depth analysis on the degradation of porous, shape memory polyurethanes (SMPs), including traditional surface characterization as well as model degradation compounds with absolute quantification. This combination of techniques allows for determination of rates of degradation as well as accumulation of individual degradation products. These behaviors are used for in vivo-in vitro comparisons for determination of real time degradation rates. Previous studies have primarily been limited to surface characterization without examination of degradation products and accumulation rates. To our knowledge, our work presents a unique example where a range of material scales (atomistic-scale model compounds along with macroscopic porous SMPs) are used in conjunction with ex planted samples for calculation of degradation rates and toxicological risk.

Stent Coating Integrity of Durable and Biodegradable Coated Drug Eluting Stents

BACKGROUND

Coatings consisting of a polymer and drug are widely used in drug-eluting stents (DES) and are essential in providing programmable drug release kinetics. Among other factors, stent coating technologies can influence blood compatibility, affect acute and sub-acute healing, and potentially trigger a chronic inflammatory response.

OBJECTIVE

The aim of this study was to investigate the short-term (7 and 28 days) and long-term (90 and 180 days) coating integrity of the Xience Prime Everolimus-Eluting Stent (EES), Resolute Zotarolimus-Eluting Stent (ZES), Taxus Paclitaxel-Eluting Stent (PES), and Nobori Biolimus A9-Eluting Stent (BES) in a rabbit ilio-femoral stent model.

METHODS AND RESULTS

Stented arteries (n = 48) were harvested and the tissue surrounding the implanted stents digested away with an enzymatic solution. Results demonstrated that the majority of struts of EES were without any coating defects with a few struts showing minor defects. Similarly, for the ZES, most of the struts were without coating defects at all time points except at 180 days. The majority of PES demonstrated mostly webbing and uneven coating. In the BES group, the majority of strut coating showed polymer cracking.

CONCLUSION

Overall, the EES and ZES had fewer coating defects than the PES and BES. Coating defects, however increase over time for the ZES, whereas the percent of coating irregularities remained constant for the EES. These results provide, for the first time, a comparison of the long-term durability of these drug-eluting stent coatings in vivo.

Strut size and surface area effects on long-term in vivo degradation in computer designed poly(L-lactic acid) three-dimensional porous scaffolds

Current developments in computer-aided design (CAD) and solid free-form fabrication (SFF) techniques enable fabrication of scaffolds with precisely designed architectures and mechanical properties. The present study demonstrates the effect of precisely designed three-dimensional scaffold architectures on in vivo degradation. Specifically, three types of porous poly(L-lactic acid) (PLLA) scaffolds with variable pore sizes, strut sizes, porosities, and surface areas fabricated by indirect SFF. In addition, one experimental group of PLLA solid cylinders was fabricated. The scaffolds and cylinders were subcutaneously implanted into mice for 6, 12 and 21 weeks. The solid cylinders exhibited a faster percentage mass loss than all porous scaffolds. Among the porous scaffolds the group with the largest strut size lost percentage mass faster than the other two groups. Strong correlations between surface area and percentage mass loss were found at 12 (R(2)=0.681) and 21 (R(2)=0.671) weeks. Scaffold porosity, however, was not significantly correlated with degradation rate. Changes in molecular weight and crystallinity also resulted in changes in the chemical structures due to degradation, and the solid cylinders had faster crystallization due to more advanced degradation than the porous scaffolds. Scaffold compressive moduli decreased with degradation, but the resulting modulus was still within the lower range of human trabecular bone even after 21 weeks. The loss in compressive moduli, however, was a complex function of both degradation and the initial scaffold architecture. This study suggests that CAD and fabrication, within a given material, can significantly influence scaffold degradation profiles.

In vitro and in vivo characterisation of biodegradable polymer-based drug-eluting stent

AIMS

The objective of this study was to investigate the structural integrity and early vascular response of a polylactic acid-coated (i.e., biodegradable polymer) coronary drug-eluting stent (DES) (BioMatrix™; Biosensors International, Singapore) to three currently marketed FDA/CE- mark approved non-erodible polymer-coated DES in a porcine model.

METHODS AND RESULTS

BioMatrix™, XIENCE V™ (Abbott Vascular, Santa Clara, CA, USA), TAXUS® Liberté™ (Boston Scientific, Natick, MA, USA), and Cypher SELECT™ (Cordis, Johnson & Johnson, Miami, FL, USA) stents were implanted in pig coronaries for seven days. Polymer integrity was assessed by scanning electron microscopy (SEM) following tissue digestion. In vitro expansion of the BioMatrix™ was also performed. SEM analysis of in vivo stents demonstrated polymer defects on the abluminal surface of all DES including polymer cracking (BioMatrix™), bridging (TAXUS Liberté™), round-small defects (Cypher SELECT™), and flaking (XIENCE V™). Histologically, the myocardium revealed no evidence of acute myocardial infarction or microscopic scarring, moreover all intramyocardial vessels were found to be patent with no evidence of emboli. In vitro results demonstrated greater BioMatrix™ polymer cracking and lifting.

CONCLUSIONS

These results illustrate the presence of polymer defects in all DES (TAXUS Liberté™, Cypher SELECT™, XIENCE V™, BioMatrix™) implanted seven-days in pigs, with absence of myocardial damage in this small number of samples. Polymer coating irregularity was greater in BioMatrix™ stent expanded in vitro as compared to in vivo, suggesting simulated benchtop deployment induces greater damage to the biodegradable polymer coating than in vivo deployment in healthy porcine coronary arteries.

Fluoropassivation and gelatin sealing of polyester arterial prostheses to skip preclotting and constrain the chronic inflammatory response

Fluoropassivation and gelatin coating have been applied to polyethylene terephthalate (PET) vascular prosthesis to combine the advantages of both polytetrafluoroethylene (PTFE) and PET materials, and to eliminate the preclotting procedure. The morphological, chemical, physical, and mechanical properties of such prostheses were investigated and compared with its original model. Fluoropassivation introduced -OCF(3), -CF(3), and -CFCF(2)- structures onto the surface of the polyester fibers. However, the surface fluorine content was only 28-32% compared to the 66% in expanded PTFE (ePTFE) grafts. The fluoropassivation decreased the hydrophilicity, slightly increased the water permeability, and marginally lowered the melting point and the crystallinity of the PET fibers. After gelatin coating, the fluoropassivated and nonfluoropassivated prostheses showed similar surface morphology and chemistry. While gelatin coating eliminated preclotting, it also renders the prostheses slightly stiffer. The original prosthesis had the highest bursting strength (275 N), with the fluoropassivated and gelatin-sealed devices showing similar bursting strength between 210 and 230 N. Fluoropassivation and gelatin coating lowered the retention strength by 23 and 30% on average, respectively. In vitro enzymatic incubation had only marginal effect on the surface fluorine content of the nongelatin-sealed prostheses. However, the gelatin-sealed ones significantly lost their surface fluorine after in vitro enzymatic incubation (by 69-85%) or in vivo 6-month implantation (by 51-60%), showing the lability of the fluoropolymer layer under the hostile biological environment.

Materials characterization of explanted mechanical heart valves and comparison to patients' clinical data

In the present study, twelve explanted mechanical heart valves (MHVs)with pyrolitic carbon tilting disc and 14 bileaflet MHVs were analyzed to investigate the effects of material properties on valve performance and patients' general health conditions. Optical and scanning electron microscopy was used to investigate material imperfections, wear patterns or damages to housing and occluder components. All analyzed tilting disc valves exhibited wear effects, particularly due to abrasion and impact to both disc and housing. Wear of pyrolitic carbon disc and housing did not influence their in vivo performance. In the bileaflet MHVs, breakaway of the pyrolitic carbon coating sometimes caused malfunctioning and required surgical retrieval of the valve. In all cases, occurrence of clinical symptoms was more likely when wear effects were located in critical areas. The study supports a correlation between the properties of the MHVs material and patients' symptoms.

Evaluation of biodegradable synthetic scaffold coated on arterial prostheses implanted in rat subcutaneous tissue

Polyester arterial prostheses impregnated with various synthetic biodegradable materials and with gelatin were implanted subcutaneously in rats for 3-180 days. The inflammation was assessed by quantifying the activity of alkaline phosphatase and by histology. The degradation of the scaffold materials was determined by scanning electron microscopy (SEM), size exclusion chromatography (SEC), and differential scanning calorimetry (DSC). The alkaline phosphatase activity induced by the polymer-impregnated grafts was similar to that induced by the non-impregnated controls during most of the post-implantation periods. Histological studies revealed that the acute inflammatory response was moderate to mild and was similar for all types of specimens, except for the gelatin-impregnated grafts that induced a severe acute inflammation during the first 2 weeks post-implantation. At 4 and 6 months, significant disintegration of the scaffold was observed, accompanied by enhanced tissue infiltration and a reactivation of the acute inflammatory phase. Linear and exponential degradation rates of the synthetic polymers were described. The relative degradation rates of the biodegradable polymers were ranked as following: PLLACL > PDLLA > PLLA > PCEL. In conclusion, biodegradable polymers may provide an option as sealant/scaffolding materials for vascular prosthesis. It is suggested that the degradation rate of the polymer scaffolding materials should be higher to achieve early healing while without inducing strong inflammation.

Porous-conductive chitosan scaffolds for tissue engineering II. in vitro and in vivo degradation

Porous-conductive chitosan scaffolds were fabricated by blending conductive polypyrrole (PPy) particles with chitosan solution and employing an improved phase separation method. In vitro and in vivo degradation behaviors of these scaffolds were investigated. In the case of in vitro degradation, an enzymatic degradation system was employed and lysozyme was used as a working enzyme. Meanwhile, the degradation products of scaffolds, glucosamine and N-acetyl-glucosamine, were also analyzed with a HPLC method. In vivo degradation of scaffolds was performed by subcutaneously implanting these scaffolds in rat for pre-scheduled time intervals. In the both cases, the weight-loss of scaffolds was monitored during the whole degradation process for evaluating the degradation of scaffolds. The changes in conductivity of scaffolds afterin vitro or in vivo degradation were also measured using a four-point technique. It was observed that the pore parameters of scaffolds themselves could significantly influence the degradation behaviors of scaffolds but the PPy content in the scaffolds seemed not to impart its effect to the degradation of scaffolds. Degradation dynamics of scaffolds and conductivity measurements indicated that these scaffolds shown fairly different behaviors in their in vitro and in vivo degradation process. According to the results obtained from in vitro and in vivo degradation of scaffolds and based on some requirements of practical tissue engineering application, it was suggested that the PPy content in the scaffold should be slightly higher than 3 wt.% but lower than 6 wt.%.

In vivo evaluation of a novel electrically conductive polypyrrole/poly(D,L-lactide) composite and polypyrrole-coated poly(D,L-lactide-co-glycolide) membranes

This study evaluated the in vivo biocompatibility and biodegradation behavior of a novel polypyrrole (PPy)/poly(D,L-lactide) (PDLLA) composite and PPy-coated poly(D,L-lactide-co-glycolide) membranes. Test membranes were implanted subcutaneously in rats for 3-120 days. The biocompatibility was assessed by quantifying the alkaline and acid phosphatase secretion, the immunohistochemical staining of the ED-2-positive macrophages, and the histology at the tissue/material interface. The degradation was investigated using scanning electron microscopy. Pure PDLLA and poly(D,L-lactide-co-glycolide) membranes were used as references, whereas expanded polytetrafluoroethylene and a commercial styrene-butadiene rubber were used as controls. The enzyme activity of the PPy-containing specimens was shown to be similar to that of the references. The histological findings were consistent with the enzymatic results, showing a mild-to-moderate acute inflammation followed by a resolution of the inflammatory response with a decrease in inflammatory cells for each biodegradable membrane. The tissue reactions to the PPy, which was either in the form of nanoparticles or surface coating, were comparable to the response to the neighboring biodegradable materials. Elevated ED-2-positive macrophage populations appeared as early as day 3 in the loose connective tissue surrounding the implants. The density of these populations was related to the degree of inflammation. Scanning electron microscopy showed that the degradation of the PPy/PDLLA composite was not affected by the presence of PPy.

Tissue reaction to polypyrrole-coated polyester fabrics: an in vivo study in rats

Electrically conductive polypyrrole is very attractive for tissue engineering because of its potential to modulate cellular activities through electrical stimulation. However, its in vivo behaviors have not been fully studied. This paper investigates the in vivo biocompatibility and biostability of PPy-coated polyester fabrics. Three PPy-coated fabrics were prepared using phosphonylation (PPy-Phos), plasma activation (PPy-Plas), and plasma activation plus heparin treatment (PPy-Plas-HE). Virgin and fluoropassivated fabrics (F-PET) were controls. The specimens were implanted subcutaneously in the back of rats for 3-90 days, then harvested and processed for enzymatic, histological, and morphological analyses. A noninvasive MRI method was used to continuously monitor the inflammation. The level of acid and alkaline phosphatase showed a similar or a less intensive cellular reaction by the PPy-coated fabrics, when compared to the controls. Histology supported the enzymatic results and showed a fast collagen infiltration at 28 days for the PPy-Phos fabric. MRI reported an overall decrease of inflammation over time, with the PPy-coated fabrics showing a similar or mild inflammation in contrast to the non-coated fabrics. PPy clusters and excessive PPy laminary coating on the PPy-Plas and PPy-Plas-HE were lost with the implantation. This experiment suggests a similar in vivo biocompatibility of the PPy-coated and noncoated polyester fabrics and the importance of achieving a thin, uniform PPy coating.

Patch incorporation in diaphragmatic hernia

BACKGROUND/PURPOSE

Biomaterial insertion often is required for closure of congenital diaphragmatic hernia (CDH). The optimal biomaterial remains uncertain. This study was designed to compare a commonly used patch (polytetrafluoroethylene) with a recently available fabric, fluorinated polyester. The aim of this study was to determine the clinical performance, histological tissue-polymer interaction, bacterial adhesion, and shrinkage rates of biomaterial inserted endoscopically into a CDH lamb model.

METHODS

Polytetrafluoro-ethylene (PTFE) and fluorinated polyester (FP) were randomised for laparoscopic patch insertion into 12 lambs. All lambs (age <4 weeks) underwent 3-port laparoscopy, surgical creation of diaphragmatic hernia, and sutured patch placement. Two PTFE and 2 FP lambs were killed at 1-, 3-, and 6-month intervals postoperatively. Postmortem examination histopathology, electron microscopy, and specific bacterial broth immersion (Escherichia coil, Staphylococcus aurens, and epidermidis) were performed.

RESULTS

All 12 lambs completed the study with intact patches that were fully peritonised. One abdominal adhesion was noted in a FP lamb at 6 months. FP was comparatively easier to insert, manipulate, and suture endoscopically. Histopathology findings showed that PTFE patches created a strong peripheral foreign body reaction with dystrophic calcification, whereas FP was well incorporated with intrapatch fibroblastic activity and neovascularsation. No significant difference in resistance to bacterial adhesion of relevant organisms was noted between the materials. Graft shrinkage for FP was 7% in one direction only, evident by 3 months.

CONCLUSIONS

Fluorinated polyester has advantages in this laparoscopic lamb model. It shows rapid and sustained incorporation with intrapatch neovascularisation when compared with polytetrafluoro-ethylene's significant foreign body reaction. It was preferred for its endoscopic handling and suturing properties. The laparoscopic techniques used may contribute to the general lack of adhesions, and insufficient data are available to comment on the comparative effect of the materials on adhesion formation. No difference was demonstrated in resistance to bacterial adherence in the harvested materials.

Vascugraft polyurethane arterial prosthesis as femoro-popliteal and femoro-peroneal bypasses in humans: pathological, structural and chemical analyses of four excised grafts

Following positive results obtained in in vitro studies and in vivo implantations in animals, a clinical trial using the Vascugraft polyurethane arterial prosthesis as a below-knee substitute was undertaken in 15 patients. Eight grafts became occluded during the first year, and segments from four of them were explanted and made available for pathological, structural and chemical investigations. The implantation periods ranged from 21 to 358 days. Failures were associated with kinking (one case), possible anastomotic mismatch between the graft and the artery (one case), and poor run-off (two cases). No organized collagenous internal encapsulation was noted; however, endothelial-like cells were observed at the anastomotic site of one graft. No significant structural degradation of the prostheses was observed in those grafts implanted for 21, 38 and 46 days. Some deteriorations in the fibrous structure were observed on the external surface of the prosthesis implanted for 358 days. High-resolution carbon C1s analysis by ESCA demonstrated a 60 to 80% decrease in carbonate content on the surface of all explanted prostheses. Chemical analyses of each polyurethane graft by IR, SEC and DSC revealed no significant chemical changes. The clinical performance of the Vascugraft prosthesis for below-knee implantation proved to be no more impressive than that of expanded polytetrafluorethylene, the currently accepted reference. The decision by B. Braun Melsungen AG to end this program is therefore to be regarded as highly professional.

Chemical and morphological analysis of explanted polyurethane vascular prostheses: the challenge of removing fixed adhering tissue

During in vivo experiments to evaluate the biocompatibility and biostability of alternative biomaterials, the ideal protocol for the handling and preservation of the explanted material is often compromised in order to meet the needs of both the pathologist and the materials scientist. Explants surrounded by tissue are often fixed in formalin or glutaraldehyde to facilitate later pathological and histological analysis, but the subsequent removal of such fixed tissue from thermally sensitive and less chemically stable polymers, such as polyurethanes, poses major problems for the materials scientist, who does not wish to modify the chemical, physical or morphological characteristics of the underlying biomaterial. The present study has attempted to find a solution to this problem by exposing virgin specimens of the microporous polyurethane Vascugraft vascular prosthesis to six different cleaning conditions, all known to be effective in removing fixed tissue. These conditions included the use of 20% aqueous potassium hydroxide solution for 48 h at room temperature, 5% sodium bicarbonate solution for 5 min at the boil, and 9, 10, 11 and 12N hydrochloric acid for 48 h at room temperature. The appearance and chemical properties of the virgin and treated specimens were compared using electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, gel permeation chromatography for molecular weight and differential scanning calorimetry techniques. The use of temperatures close to the boil resulted in the formation of a translucent, rubbery material with gross changes in the microporous and microfibrous structure. The strongly acidic and alkaline conditions caused a loss in the surface carbonate group content. In addition, 12N hydrochloric acid reduced the molecular weight and urethane content. Consequently, 9N hydrochloric acid is recommended as the cleaning agent of choice for removing fixed tissue from this type of microporous polyurethane. Control experiments on virgin material should also be included in any cleaning protocol.

In vivo performance of the polyesterurethane Vascugraft prosthesis implanted as a thoraco-abdominal bypass in dogs: an exploratory study

Among the various prototype vascular prostheses that have been developed over recent years as small vessel substitutes, the Vascugraft polyurethane device produced by Braun-Melsungen AG has a number of attractive features. As well as having high mechanical compliance similar to that of the arterial tree, it has been manufactured from a specially synthesized poly(ester urethane) with improved biostability and its microfibrous structure provides a highly porous wall with open communicating pores. With a view to evaluating the in vivo biofunctionality and biostability of this prosthesis in the dog, 10 mm diameter grafts were implanted as thoraco-abdominal bypasses for prescheduled periods of 1 months and 12 months, and their performance monitored in terms of gross morphology, histology and the measurement of the chemical and physical properties of the explanted and cleaned specimens. Both grafts were patent at retrieval. Each had a smooth and glistening flow surface without organized mural thrombi and showed the development of a thin collagenous internal capsule with the presence of endothelial-like cells. Both grafts were well encapsulated externally and revealed a small distal bend or kink which is frequently observed by any thoraco-abdominal bypass in dogs. The fresh explanted prostheses were cleaned by a new enzyme treatment which provided specimens for microscopic, mechanical and thermal analyses, as well as studies of the surface and bulk chemistry. By comparing the results from the explanted and cleaned material with those of the virgin prosthesis, we have observed some deterioration in the integrity of the microfibrous structure, some loss in mechanical performance, marginal changes in molecular weight, and an apparent microphase separation of the hard and soft segment domains at a depth of a few microns. While the biofunctionality of a 10 mm calibre device has been demonstrated, additional in vivo studies are recommended to assess the biofunctionality at different diameters and the biostability over longer periods of implantation.