In vitro modulation of macrophage phenotype and inhibition of polymer degradation by dexamethasone in a human macrophage/Fe/stress system

Biodegradable polymeric materials for flexible and degradable electronics

Biodegradable electronics have great potential to reduce the environmental footprint of electronic devices and to avoid secondary removal of implantable health monitors and therapeutic electronics. Benefiting from the intensive innovation on biodegradable nanomaterials, current transient electronics can realize full components’ degradability. However, design of materials with tissue-comparable flexibility, desired dielectric properties, suitable biocompatibility and programmable biodegradability will always be a challenge to explore the subtle trade-offs between these parameters. In this review, we firstly discuss the general chemical structure and degradation behavior of polymeric biodegradable materials that have been widely studied for various applications. Then, specific properties of different degradable polymer materials such as biocompatibility, biodegradability, and flexibility were compared and evaluated for real-life applications. Complex biodegradable electronics and related strategies with enhanced functionality aimed for different components including substrates, insulators, conductors and semiconductors in complex biodegradable electronics are further researched and discussed. Finally, typical applications of biodegradable electronics in sensing, therapeutic drug delivery, energy storage and integrated electronic systems are highlighted. This paper critically reviews the significant progress made in the field and highlights the future prospects.

State of the Art of Small-Diameter Vessel-Polyurethane Substitutes

Cardiovascular diseases are a severe threat to human health. Implantation of small-diameter vascular substitutes is a promising therapy in clinical operations. Polyurethane (PU) is considered one of the most suitable materials for this substitution due to its good mechanical properties, controlled biostability, and proper biocompatibility. According to biodegradability and biostability, in this review, PU small-diameter vascular substitutes are divided into two groups: biodegradable scaffolds and biostable prostheses, which are applied to the body for short- and long-term, respectively. Following this category, the degradation principles and mechanisms of different kinds of PUs are first discussed; then the chemical and physical methods for adjusting the properties and the research advances are summarized. On the basis of these discussions, the problems remaining at present are addressed, and the contour of future research and development of PU-based small-diameter vascular substitutes toward clinical applications is outlined.

Synthesis, properties and biomedical applications of hydrolytically degradable materials based on aliphatic polyesters and polycarbonates

Polyester-based polymers represent excellent candidates in synthetic biodegradable and bioabsorbable materials for medical applications owing to their tailorable properties. The use of synthetic polyesters as biomaterials offers a unique control of morphology, mechanical properties and degradation profile through monomer selection, polymer composition (i.e. copolymer vs. homopolymer, stereocomplexation etc.) and molecular weight. Within this review, the synthetic routes, degradation modes and application of aliphatic polyester- and polycarbonate-based biomaterials are discussed.

Degradability of polymers for implantable biomedical devices

Many key components of implantable medical devices are made from polymeric materials. The functions of these materials include structural support, electrical insulation, protection of other materials from the environment of the body, and biocompatibility, as well as other things such as delivery of a therapeutic drug. In such roles, the stability and integrity of the polymer, over what can be a very long period of time, is very important. For most of these functions, stability over time is desired, but in other cases, the opposite–the degradation and disappearance of the polymer over time is required. In either case, it is important to understand both the chemistry that can lead to the degradation of polymers as well as the kinetics that controls these reactions. Hydrolysis and oxidation are the two classes of reactions that lead to the breaking down of polymers. Both are discussed in detail in the context of the environmental factors that impact the utility of various polymers for medical device applications. Understanding the chemistry and kinetics allows prediction of stability as well as explanations for observations such as porosity and the unexpected behavior of polymeric composite materials in some situations. In the last part, physical degradation such interfacial delamination in composites is discussed.

In vitro biostability evaluation of polyurethane composites in acidic, basic, oxidative, and neutral solutions

New and improved properties can often be achieved by compounding two or more different but compatible materials. But, can failure possibility also be increased by such a compounding strategy? In this article, we compared the in vitro biostability of composites with that of the pure polymer. We tested three model composites in oxidative, acidic, basic, and neutral solutions. We found that oxidation degradation was much more profound in the composites than in the corresponding pure polymer. This degradation seemed to be an intrinsic property of composite materials. We also observed the well documented interfacial debonding between filler and matrix and its effects on the mechanical reinforcement of the hydrated composites. The improvements in acid and base resistance were also observed.

Biostability and biological performance of a PDMS-based polyurethane for controlled drug release

Polymers have been used to deliver therapeutic agents in a range of medical devices with drug eluting stents being the most widespread current application. Although polymers enable controlled release of a therapeutic agent, the polymeric surface has been reported to provide suboptimal biocompatibility and haemocompatibility and it has been suggested that currently used polymers may be at least partly responsible for the late adverse events observed in intravascular stent systems. In this study, the biostability and biological performance of a siloxane-based polyurethane elastomer (E2A) demonstrating excellent long-term biostability in the unloaded state was investigated following incorporation of a therapeutic agent. After implantation in an ovine model for 6 months, samples were assessed using SEM and ATR-FTIR to determine changes in the surface chemical structure and morphology of the materials and tensile testing was used to examine changes in bulk characteristics. Biological response was assessed using in vitro cytotoxicity testing and histological analysis. Results indicated that incorporation of 25mg/g dexamethasone acetate (DexA) into the siloxane-based polyurethane resulted in no significant difference in the biostability and biocompatibility of the material. Some level of cytotoxic potential was exhibited which was believed to result from residual DexA leaching from samples during the extraction process. These findings suggest that E2A is a potential candidate for a delivery vehicle of therapeutic agents in implantable drug delivery applications.

Implant Degradation and Poor Healing After Endovascular Repair of Abdominal Aortic Aneurysms: An Analysis of Explanted Stent-Grafts

PURPOSE

To study explanted stent-grafts to achieve a better understanding of the mechanisms of failure after endovascular treatment of abdominal aortic aneurysms (AAA).

METHODS

Twelve stent-grafts were harvested at autopsy (n=3) or during surgical conversion (n=9). Device alterations were investigated by macroscopic examination, radiography, and surface analysis techniques. Healing around the implants was studied via histology and immunohistochemistry, with particular attention to the stent-graft/tissue interface.

RESULTS

Degradation was more important with Vanguard stent-grafts (off the market) than with AneuRx and Talent stent-grafts, but rupture of nitinol wires and poor surface finish in Talent stent-grafts raise concern about their corrosion resistance and long-term stability. Poor healing was observed around stent-grafts even after several years of implantation, with absence of vascular smooth muscle cells, fibroblasts, and collagen formation. In addition to the well-known foreign body reaction around the graft, numerous polymorphonuclear cells characteristic of the first step of healing were present in tissues around stent-grafts retrieved at surgical conversion. Factors explaining the lack of tissue organization around stent-grafts are discussed.

CONCLUSION

The long-term stability of implants remains a concern and requires more transparency from manufacturers regarding the surface properties of their devices. Lack of neointima formation impairs biological fixation of the implant to the vessel wall, leading to possible endoleaks and migration. New-generation stent-grafts promoting biological fixation should be developed to improve clinical outcomes of this minimally invasive treatment.

Effect of processing route and acetone pre-treatment on the biostability of pellethane materials used in medical device applications

Thermoplastic polyurethanes, such as Pellethane 2363 80A (Pel80A) and Pellethane 2363 55D (Pel55D) are widely used in the medical device industry because of their biological and mechanical properties. However, premature failure in such devices has been observed and attributed to environmental stress cracking (ESC). The current work investigates the possibility of reducing ESC via bulk morphology manipulation. This can be achieved through various processing routes such as solvent-casting (SC) and hot-press quenching (HPQ). The effect of stress on the bulk morphology of Pel55D and Pel80A was evaluated using small-angle X-ray scattering (SAXS) in conjunction with tensile testing. SC samples exhibited greater phase separation compared with HPQ samples. Alignment of hard segment domains became apparent around the point of yield. Onset of ESC with respect to SC and HPQ routines was determined using the Zhao-Stokes glass-wool test with optical (OM) and environment scanning electron microscopy (ESEM). Improvement in biostability of Pel80A was found in HPQ samples compared to those that were SC. A secondary objective of this work was to investigate the effect of acetone pre-treatment on surface morphology. High resolution imaging of acetone treated and untreated SC Pel80A showed significant differences in surface morphology.

Understanding the biodegradation of polyurethanes: from classical implants to tissue engineering materials

After almost half a century of use in the health field, polyurethanes (PUs) remain one of the most popular group of biomaterials applied for medical devices. Their popularity has been sustained as a direct result of their segmented block copolymeric character, which endows them with a wide range of versatility in terms of tailoring their physical properties, blood and tissue compatibility, and more recently their biodegradation character. While they became recognized in the 1970s and 1980s as the blood contacting material of choice in a wide range of cardiovascular devices their application in long-term implants fell under scrutiny with the failure of pacemaker leads and breast implant coatings containing PUs in the late 1980s. During the next decade PUs became extensively researched for their relative sensitivity to biodegradation and the desire to further understand the biological mechanisms for in vivo biodegradation. The advent of molecular biology into mainstream biomedical engineering permitted the probing of molecular pathways leading to the biodegradation of these materials. Knowledge gained throughout the 1990s has not only yielded novel PUs that contribute to the enhancement of biostability for in vivo long-term applications, but has also been translated to form a new class of bioresorbable materials with all the versatility of PUs in terms of physical properties but now with a more integrative nature in terms of biocompatibility. The current review will briefly survey the literature, which initially identified the problem of PU degradation in vivo and the subsequent studies that have led to the field's further understanding of the biological processes mediating the breakdown. An overview of research emerging on PUs sought for use in combination (drug + polymer) products and tissue regeneration applications will then be presented.

Cytotoxic reaction and TNF-alpha response of macrophages to polyurethane particles

Their unique mechanical and biological properties make polyurethanes (PUs) ideal materials for many implantable devices. However, uncertain long-term biostability in the human physiological environment limits their extensive clinical applications. Chronic inflammatory response associated with macrophage activation has been suggested as a prime factor; although the mechanism of macrophage activation in response to biomaterial surfaces and debris is still unknown. The overall objective of this work was to study the response of macrophages to PU materials in vitro by measuring cell viability and activity. The studies were carried out using phagocytozable-size PU particles from three types of commercially-available PUs: Pellethane 2363 80ABA (PL); Tecothane TT2065 (TC65); and Tecothane TT2085 (TC85). These polymers posess the same generic composition but differ in the length of hard and soft segments, as revealed by the FTIR and NMR studies. The results showed that PU particles affected both viability and activity of J774 macrophages. The percentage of mortality ranged from 1 to 15% with 10-100 microg ml(-1) of particles after 24 and 48 h incubation. These three types of particles induced different mortality on the macrophages. Specifically, the mortality with PL particles was 1-4% (p > 0.05), while the mortality with TC85 particles was 2-10% (p < 0.05) and 4-15% with TC65 (p < 0.05). Conversely, these particles also affected cell proliferation. Cell numbers increased by 132 and 167% after 24 and 48 h incubation, respectively, without particles, whereas the cell numbers increased only 46 and 78% with TC65, 66 and 105% with TC85, and 67 and 110% with PL in the presence of 100 microg ml(-1) of particles for the respective incubation times. PU particles also increased TNF-alpha release from macrophage. After having been incubated for 24 h with 100 microg ml(-1) particles of TC65, TC85, and PL, macrophages release TNF-alpha 7.4, 5.2, and 4.1 times more than the control. In conclusion, PU particles had cytotoxic effects on J774 macrophage at high concentrations. The order of macrophage response for three types of particles was TC65 > TC85 > PL. PU particles' effect on macrophage viability and activity depends on the concentration of particles and their chemical composition, especially on the ratio of hard to soft segments.

The effect of oxidation on the enzyme-catalyzed hydrolytic biodegradation of poly(urethane)s

Although the biodegradation of polyurethanes (PU) by oxidative and hydrolytic agents has been studied extensively, few investigations have reported on the combination of their effects. Since neutrophils (PMN) arrive at an implanted device first and release HOCl, followed by monocyte-derived macrophages (MDM) which have potent esterase activities and oxidants of their own, the combined effect of oxidative and hydrolytic degradation on radiolabeled polycarbonate-polyurethanes (PCNU)s was investigated and compared to that of a polyester-PU (PESU) and a polyether-PU (PEU). The PCNUs were synthesized with PCN (MW = 1,000), and butanediol (14C-BD) and one of two diisocyanates, hexane-1,6-diisocyanate (14C-HDI) or methylene bis-p-phenyl diisocyanate (MDI). The PESU and PEU were synthesized using toluene-diisocyanate (14C-TDI), with polycaprolactone and polytetramethylene oxide as soft segments respectively, and ethylene diamine as the chain extender. The effect of pre-treatment with 0.1 mM HOC1 for 1 week on the HDI-based PCNUs and both TDI-based PUs resulted in a significant inhibition of radiolabel release (RR) elicited by cholesterol esterase (CE), when compared to buffer alone, whereas the MDI-based PCNU showed a small but significant increase. When PMN were activated on the HDI-based PCNU surface with phorbol myristate acetate (PMA), HOCl was released for 3 h, and was almost completely abolished by sodium azide (AZ). Simultaneously, the PMN-elicited RR, shown previously to be due to the esterolytic cleavage by serine proteases, was inhibited approximately 75% by PMA-activation of the cells, but significantly increased relative to the latter when AZ was added. Both in vitro oxidation by HOCl and the release of HOCI by PMN were associated with the inhibition of RR and suggest perturbations between oxidative and hydrolytic mechanisms of biodegradation.

Surface modification of a polycarbonate-urethane using a vitamin-E-derivatized fluoroalkyl surface modifier

Fluorinated surface-modifying macromolecules (SMMs) have been previously reported on and shown to limit the hydrolytic degradation of polyurethanes. The SMM molecules achieve this effect by allowing for the selective migration of terminal fluorinated groups to the polymer's surface, which may then shield more hydrolytically-sensitive groups in the base polyurethane backbone. A further extension of the SMM concept would be to utilize the migration of the fluorine tails to simultaneously deliver biologically active moieties to the surface. This study explored the synthesis and characterization of a vitamin-E (natural anti-oxidant) coupled surface modifier, as a model for the bioactive SMM concept. The SMM was synthesized using lysine diisocyanate (LDI), polycarbonate diol (PCN), and a fluoroalcohol. By derivatizing the LDI pendant ester, vitamin E was coupled to the SMM. The vitamin-E SMM was physically characterized using gel-permeation chromatography (GPC) and its anti-oxidant activity was assessed in the presence of 0.1 mM NaOCl. Polymer degradation experiments were carried out using 10 mM NaOCl incubation solutions, and the relative material breakdown was assessed using GPC and scanning electron microscopy (SEM). The results indicate that while the fluoro-component reduced damage of the PU, the bioactive component achieved a further deactivating effect. A similar action may also be effective against superoxide anions generated by human macrophages.

Hydrolytic degradation of poly(carbonate)-urethanes by monocyte-derived macrophages

Polycarbonate (PCN)-based polyurethanes (PCNU) are rapidly becoming the chosen polyurethane (PU) for long-term implantation since they have shown decreased susceptibility to oxidation. However, monocyte-derived macrophages (MDM), the cell implicated in biodegradation, also contain hydrolytic activities. Hence, in this study, an activated human MDM cell system was used to assess the biostability of a PCNU, synthesized with 14C-hexane diisocyanate (HDI) and butanediol (BD), previously shown to be susceptible to hydrolysis by cholesterol esterase (CE). Monocytes, isolated from whole blood and cultured for 14 days on polystyrene (PS) to mature MDM, were gently trypsinized and seeded onto 14C-PCNU. Radiolabel release and esterase activity, as measured with p-nitrophenylbutyrate, increased for almost 2 weeks. At 1 week, the increase in radiolabel release and esterase activity were diminished by more than 50% when the protein synthesis inhibitor, cycloheximide, or the serine esterase/protease inhibitor, phenylmethylsulfonylfluoride was added to the medium. This strongly suggests that in part, it was MDM esterase activity which contributed to the PU degradation. In an effort to simulate the potential combination of oxidative and hydrolytic activities of inflammatory cells. 14C-PCNU was exposed to HOCl and then CE. Interestingly, the release of radiolabeled products by CE was significantly inhibited by the pre-treatment of PCNU with HOCl. The results of this study show that while the co-existing roles of oxidation and hydrolysis in the biodegradation of PCNUs remains to be elucidated, a clear relationship is drawn for PCNU degradation to the hydrolytic degradative activities which increase in MDM during differentiation from monocytes, and during activation in the chronic phase of the inflammatory response.

Model systems to assess the destructive potential of human neutrophils and monocyte-derived macrophages during the acute and chronic phases of inflammation

Isolated cell systems of human neutrophils (PMNs) and monocyte-derived macrophages (MDMs) were used to compare the destructive potential of these cells during the acute and chronic phases of inflammation, respectively. The contrast in the damage to poly(urethane)s (PUs) was monitored by measuring radiolabel release elicited from a (14)C-polyester-urea-urethane (PEUU) during incubation with both cell types. Human PMN were seeded onto polymer-coated glass slips and both radiolabel release as well as serine protease activity [assayed with N-benzyloxycarbonyl lysine thiobenzyl ester (BLT)] were measured 18 h later. Human monocytes were cultured on polystyrene tissue culture plates for 14 days, trypsinized, and seeded onto the polymer-coated glass slips; then, radiolabel release and esterase activity [assayed with p-nitrophenylbutyrate (PNB)] were measured after 18 h. Coverslips with MDM were also incubated for an additional 2 weeks. At 18 h postincubation with the PEUU, MDM elicited 25 times more radiolabel release per 10(6) cells than PMN at 18 h and continued to increase more than sevenfold over the 18-h value during the subsequent 14-day period. The BLT activity in PMN did not increase significantly during the 18-h incubation period, whereas the PNB activity in MDM increased more than fourfold. The MDM, but not the PMN elicited radiolabel release, was inhibited by the protein synthesis inhibitor cycloheximide, as was the increase in PNB activity. The data provide evidence for a hydrolytic role for MDM and, to a lesser extent PMN, in the biodegradation of implanted materials. The full implication of the release of polymer-derived chemical agents from this hydrolytic cleavage of the implanted biomaterials, on the propagation of the inflammatory response, remains to be elucidated.