Polymer Degradation Rate Research Articles

Subcritical CO2 sintering of microspheres of different polymeric materials to fabricate scaffolds for tissue engineering

The aim of this study was to use CO2 at sub-critical pressures as a tool to sinter 3D, macroporous, microsphere-based scaffolds for bone and cartilage tissue engineering. Porous scaffolds composed of ~200μm microspheres of either poly(lactic-co-glycolic acid) (PLGA) or polycaprolactone (PCL) were prepared using dense phase CO2 sintering, which were seeded with rat bone marrow mesenchymal stromal cells (rBMSCs), and exposed to either osteogenic (PLGA, PCL) or chondrogenic (PLGA) conditions for 6weeks. Under osteogenic conditions, the PLGA constructs produced over an order of magnitude more calcium than the PCL constructs, whereas the PCL constructs had far superior mechanical and structural integrity (125 times stiffer than PLGA constructs) at week 6, along with twice the cell content of the PLGA constructs. Chondrogenic cell performance was limited in PLGA constructs, perhaps as a result of the polymer degradation rate being too high. The current study represents the first long-term culture of CO2-sintered microsphere-based scaffolds, and has established important thermodynamic differences in sintering between the selected formulations of PLGA and PCL, with the former requiring adjustment of pressure only, and the latter requiring the adjustment of both pressure and temperature. Based on more straightforward sintering conditions and more favorable cell performance, PLGA may be the material of choice for microspheres in a CO2 sintering application, although a different PLGA formulation with the encapsulation of growth factors, extracellular matrix-derived nanoparticles, and/or buffers in the microspheres may be advantageous for achieving a more superior cell performance than observed here.

Effect of drug loading and laser surface melting on drug release profile from biodegradable polymer

ABSTRACTThe biodegradable polymer such as poly(l‐lactic acid) is promising in drug delivery applications because it allows for drug release in a controlled manner. In a polymer‐based drug delivery system, drug release is controlled by polymer degradation and drug loading concentration. In this study, effect of drug concentration on drug release profile is investigated through polymer crystallinity, chain mobility, and polymer degradation, as characterized by the wide‐angle X‐ray diffraction, differential scanning calorimetry, and gel permeation chromatography, respectively. The addition of drug has been shown to accelerate polymer degradation and drug release rate. With a low drug concentration, the slow polymer degradation kinetics results in an induction period of drug release, during which a limited amount of drug is released. The induction period is undesirable because it delays drug release and effectiveness. Since drug release is controlled by polymer degradation, which is a function of polymer crystallinity, laser surface melting is conducted to reduce polymer surface crystallinity and modify its degradation. The effect of laser crystallinity modification on drug release is investigated. A numerical model is also implemented based on hydrolysis and diffusion mechanisms to investigate the effects of drug loading and laser surface melting on polymer degradation and drug release process. It has been demonstrated that laser treatment shortens the induction period of drug release while keeps the release rate unmodified, as desired in drug delivery applications. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4147–4156, 2013

Uptake and Transfection with Polymeric Nanoparticles Are Dependent on Polymer End-Group Structure, but Largely Independent of Nanoparticle Physical and Chemical Properties

Development of nonviral particles for gene delivery requires a greater understanding of the properties that enable gene delivery particles to overcome the numerous barriers to intracellular DNA delivery. Linear poly(beta-amino) esters (PBAE) have shown substantial promise for gene delivery, but the mechanism behind their effectiveness is not well quantified with respect to these barriers. In this study, we synthesized, characterized, and evaluated for gene delivery an array of linear PBAEs that differed by small changes along the backbone, side chain, and end group of the polymers. We examined particle size and surface charge, polymer molecular weight, polymer degradation rate, buffering capacity, cellular uptake, transfection, and cytotoxicity of nanoparticles formulated with these polymers. Significantly, this is the first study that has quantified how small differential structural changes to polymers of this class modulate buffering capacity and polymer degradation rate and relates these findings to gene delivery efficacy. All polymers formed positively charged (zeta potential 21-29 mV) nanosized particles (∼150 nm). The polymers hydrolytically degraded quickly in physiological conditions, with half-lives ranging from 90 min to 6 h depending on polymer structure. The PBAE buffering capacities in the relevant pH range (pH 5.1-7.4) varied from 34% to 95% protonatable amines, and on a per mass basis, PBAEs buffered 1.4-4.6 mmol of H(+)/g. When compared to 25 kDa branched polyethyleneimine (PEI), PBAEs buffer significantly fewer protons/mass, as PEI buffers 6.2 mmol of H(+)/g over the same range. However, due to the relatively low cytotoxicity of PBAEs, higher polymer mass can be used to form particles than with PEI and total buffering capacity of PBAE-based particles significantly exceeds that of PEI. Uptake into COS-7 cells ranged from 0% to 95% of cells and transfection ranged from 0% to 93% of cells, depending on the base polymer structure and the end modifications examined. Five polymers achieved higher uptake and transfection efficacy with less toxicity than branched-PEI control. Surprisingly, acrylate-terminated base polymers were dramatically less efficacious than their end-capped versions, in terms of both uptake (1-3% for acrylate, 75-94% for end-capped) and transfection efficacy (0-1% vs 20-89%), even though there are minimal differences between acrylate and end-capped polymers in terms of DNA retardation in gel electrophoresis, particle size, zeta potential, and cytotoxicity. These studies further elucidate the role of polymer structure for gene delivery and highlight that small molecule end-group modification of a linear polymer can be critical for cellular uptake in a manner that is largely independent of polymer/DNA binding, particle size, and particle surface charge.

Damage modeling of degradable polymers under bulk erosion

Unit micro-cell models with different architectures were designed to simulate the degradation process and chemical damage behavior of degradable polymers under bulk erosion. The pores in the micro-cell models were introduced to mimic the state of rapid water diffusion into the polymers under bulk erosion, while three different arrangements of pores were considered to investigate their effects on the degradation rate of different polymers with the same molecular weight. Different porosity levels were also used to study the degradation responses of the polymers having different molecular weights. A heat and mass transfer analogy was adopted to enable the analysis to be run on a general purpose finite element (FE) code. In the present work, a finite element software package ABAQUS incorporated with a user-defined material subroutine was used to perform the analysis, in which a heat transfer function was utilized to simulate Fickian mass diffusion for the polymers through analogy. With the proposed method, the effects of chemical damage on the mechanical properties of the degradable polymers under bulk erosion could be predicted and the predicted trend of the mass loss of the polymers followed experimental results obtained from the open literature. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Ionic Stabilization of the Polythiophene-Oxygen Charge-Transfer Complex.

A substantial reduction in the rate of irreversible polymer photo-oxidation was observed through the ionic stabilization of the polymer-O2 charge-transfer complex (CTC) in amorphous polythiophene thin films. Through the incorporation of anionic functionality containing mobile cations, it was found that CTC stability increases with increasing cation charge density. This results in an increased rate of electron transfer to molecular oxygen relative to photosensitization and reaction of 1O2, leading to a reduction in the overall rate of polymer degradation. UV-vis and FTIR spectroscopy were utilized to determine the identity of intermediate and irreversible photodegradation products as well as the effect of ion identity on the dominant photo-oxidation mechanism. As polymer-O2 CTCs are common in the photo-oxidation of many conjugated polymers, these results have significant implications for the ongoing effort to produce commercially viable organic electronic and photonic devices.

Partially resorbable composite bone plate with controlled degradation rate, desired mechanical properties and bioactivity

Rate of polymer degradation is very important for implantable biomaterials since controlling the degradation rate may complement the biological response and affected mechanical property requirements. In spite of numerous publications on the potential use of combinations of poly lactic acid/bioactive glass fillers for degradable bone plate, little information exists on the controlling degradation rate and its effects on the other aspects such as biomechanical compatibility, bioactivity, etc. Our previous study revealed that a composite bone plate consist of poly l-lactic acid/braided bioactive glass fibers has the initial mechanical properties near to cortical bone. In this study, degradation rate and mechanical behavior of the composite bone plate were assessed, and also degradation rate was controlled by using proper manufacturing process and improving bonding between matrix and reinforcement. Moreover, bioactivity of the composite was considered before and after controlling degradation rate, because of the important role of bioactivity and ion release in healing acceleration. Results showed that degradation process of the composite could be controlled properly. Strength of the treated composite decreased only about 5% through 2 weeks and about 35% after 8 weeks while, the strength loss for the untreated composites was about 50 and 70 percent after 2 weeks and 8 weeks of degradation respectively. Although calcium-phosphate formation on the surface of the composite was postponed in the treated samples, the bioactivity of the composite remained unchanged and bone-like apatite was formed on the composite surface which is important for the application of the composite in bone tissue environment.

Development and Long‐Term In Vivo Evaluation of a Biodegradable Urethane‐Doped Polyester Elastomer

We have recently reported upon the development of crosslinked urethane-doped polyester (CUPE) network elastomers, which was motivated by the desire to overcome the drawbacks presented by crosslinked network polyesters and biodegradable polyurethanes for soft tissue engineering applications. Although the effect of the isocyanate content and post-polymerization conditions on the material structure-property relationship was examined in detail, the ability of the diol component to modulate the material properties was only studied briefly. Herein, we present a detailed report on the development of CUPE polymers synthesized using diols 4, 6, 8, 10, or 12 methylene units in length in order to investigate what role the diol component plays on the resulting material's physical properties, and assess their long-term biological performance in vivo. An increase in the diol length was shown to affect the physical properties of the CUPE polymers primarily through lowered polymeric crosslinking densities and elevated material hydrophobicity. The use of longer chain diols resulted in CUPE polymers with increased molecular weights resulting in higher tensile strength and elasticity, while also increasing the material hydrophobicity to lower bulk swelling and prolong the polymer degradation rates. Although the number of methylene units largely affected the physical properties of CUPE, the choice of diol did not affect the overall polymer cell/tissue-compatibility both in vitro and in vivo. In conclusion, we have established the diol component as an important parameter in controlling the structure-property relationship of the polymer in addition to diisocyanate concentration and post-polymerization conditions. Expanding the family of CUPE polymers increases the choices of biodegradable elastomers for tissue engineering applications.

In vitro degradation rate of apatitic calcium phosphate cement with incorporated PLGA microspheres

Calcium phosphate cements (CPCs) are frequently used as bone substitute material. Despite their superior clinical handling and excellent biocompatibility, they exhibit poor degradability, which limits bone ingrowth into the implant. Microspheres were prepared from poly( d, l-lactic-co-glycolic acid) (PLGA) and included in injectable CPCs as porogens in order to enhance its macroporosity after the polymeric microspheres had degraded. Upon degradation of the PLGA microspheres, acid is produced that enhances the dissolution rate of the CPC. However, the effect of the characteristics of PLGA microspheres on the degradation rate of CPCs has never been studied before. Therefore, the purpose of the current study was to investigate the dependence of CPC degradation on the chemical and morphological characteristics of incorporated PLGA microspheres. With respect to the chemical characteristics of the PLGA microspheres, the effects of both PLGA molecular weight (5, 17 and 44 kDa) and end-group functionalization (acid-terminated or end-capped) were studied. In addition, two types of PLGA microspheres, differing in morphology (hollow vs. dense), were tested. The results revealed that, although both chemical parameters clearly affected the polymer degradation rate when embedded as hollow microspheres in CPC, the PLGA and CPC degradation rates were mainly dependent on the end-group functionalization. Moreover, it was concluded that dense microspheres were more efficient porogens than hollow ones by increasing the CPC macroporosity during in vitro incubation. By combining all test parameters, it was concluded that dense PLGA microspheres consisting of acid-terminated PLGA of 17 kDa exhibited the highest and fastest acid-producing capacity and correspondingly the highest and fastest amount of porosity. In conclusion, the data presented here indicate that the combination of dense, acid-terminated PLGA microspheres with CPC emerges as a successful combination to achieve enhanced apatitic CPC degradation.

Hyperbranched Polyacetals with Tunable Degradation Rates

We report the first synthesis of hyperbranched polyacetals via a melt transacetalization polymerization process. The process proceeds via the self-condensation of an AB2 type monomer carrying a hydroxyl group and a dimethylacetal unit; the continuous removal of low boiling methanol drives the equilibrium toward polymer formation. Because of the susceptibility of the acetal linkage to hydrolysis, the polymer degrades readily under mildly acidic conditions to yield the corresponding hydroxyl–aldehyde as the primary product. Furthermore, because of the unique topology of hyperbranched structures, the rate of polymer degradation was readily tuned by changing just the nature of the end-groups alone; instead of the dimethylacetal bearing monomer, longer chain dialkylacetals (dibutyl and dihexyl) monomers yielded hyperbranched polymers carrying longer alkyl groups at their molecular periphery. The highly branched topology and the relatively high volume fraction of the terminal alkyl groups resulted in a significant lowering of the ingress rates of the aqueous reagents to the loci of degradation, and consequently the degradation rates of the polymers were dramatically influenced by the hydrophobic nature of the terminal alkyl substituents. The simple synthesis and easy tunability of the degradation rates make these materials fairly attractive candidates for use as degradable scaffolds.

Controlling the degradation rate of thermoresponsive photo-cross-linked poly(organophosphazene) hydrogels with compositions of depsipeptide and PEG chain lengths

Thermosensitive and photo-cross-linkable poly(organophosphazenes) containing various amounts of isoleucine ethyl ester, AMPEG550, AMPEG750, aminoethyl methacrylate, and depsipeptide were synthesized, characterized, and were investigated for in vitro and in vivo degradation rates. The aqueous solutions of all polymers showed a sol–gel phase transition behavior against the temperature changes. The polymer degradation rate was majorly affected by the type and degree of the side group’s substitutions. The rate of degradation was initially dependent on the degree of photo-cross-linking, and in later stages on the amounts of the depsipeptide and PEG chain lengths in the polymer network. The amount of the pendant depsipeptide and chain length of the α-amino-ω-methoxy-poly(ethylene glycol) had significant impact on the polymer degradation rates. The polymers with high amount of the depsipeptide, and PEG750 showed fast degradation. These results suggest that the degradation rate of the injectable and dual cross-linkable gels can be tuned to the desired extent and may find wide utilities in various biomedical applications, where the gel strength and degradation rate are needed to be tightly regulated.

Impregnation of poly(L‐lactide‐ran‐cyclic carbonate) copolymers with useful compounds with supercritical carbon dioxide

AbstractIn this article, we cover the development of L‐lactide (L‐LA) random copolymers into which useful compounds, such as repellents and antibacterial agents, were impregnated by high concentration. Outstanding controlled release materials were developed with statistical random copolymers of L‐LA with cyclic carbonate (CC) [2,2‐dimethyltrimethylene carbonate (2,2‐DTMC) or tetramethylene carbonate (TEMC)] with tin 2‐ethyL‐hexanoate as a catalyst at 150°C (2,2‐DTMC) or 120°C (TEMC) for 24 h without solvent. The preparation of improved controlled release materials was performed with useful organic compounds with low boiling points and synthetic L‐LA random copolymers containing CCs as base materials under supercritical carbon dioxide (scCO2). Low‐boiling‐point compounds, such as d‐limonene and hinokitiol, were used. In impregnation experiments with scCO2, the amounts of low‐boiling‐point compounds increased with increasings L‐LA content. The compound content impregnated into poly(L‐lactide‐ran‐cyclic carbonate) [poly(L‐LA‐ran‐CC)] was greater than that of the experiment with poly(L‐lactide‐ran‐ε‐caprolactone) previously studied. When the enzymatic degradation of poly(L‐LA‐ran‐CC) was performed with proteinase K, copolymers with a greater L‐LA content degraded more rapidly than did copolymers with a greater CC content. In a controlled release experiment with poly(L‐lactide‐ran‐2,2‐dimethyltrimethylene carbonate) (76/24) or poly(L‐lactide‐ran‐tetramethylene carbonate) (81/19), the rate of polymer degradation and the rate of impregnated compound release were almost the same. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Flow-induced degradation of drag-reducing polymer solutions within a high-Reynolds-number turbulent boundary layer

Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms−1. Measurements were acquired up to 10.7m downstream of the leading edge, yielding downstream-distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and downstream distance. At the highest speed tested (20ms−1) no drag reduction was discernible at the first measurement location (0.56m downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (~10−3s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (~0.1s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.

Degradation behaviors of electrospun fibrous composites of hydroxyapatite and chemically modified poly( dl-lactide)

It is essential to individually tailor the biodegradability of electrospun fibers and their composites to meet the requirements of specific application. Electrospun poly( dl-lactide) (PDLLA) fibers grafted with functional groups were obtained to induce in situ mineralization of hydroxyapatite (HA), and HA/PDLLA composites were fabricated through hot-pressing of mineralized fibers after layer-by-layer deposition. The degradation behaviors during up to 1 year incubation were clarified for functionalized PDLLA fibers, mineralized HA/PDLLA fibers and hot-pressed composites. The carboxyl and amino groups of electrospun fibers indicated enhancement and alleviation of the autocatalysis effect on the polyester hydrolysis, respectively. The distribution of HA within fiber matrices led quick and strong water absorption, and caused neutralization of the weak acid environment and alleviation of the autocatalysis effect. Due to the location of mineralized HA on the surface of functionalized fibers, significant HA loss and preferential removal of amorphous and low-crystalline apatitic phase were determined during the degradation process. The hot-pressed composites indicated dense structure, small pore size and fusion on the fiber surface, leading significantly lower degradation rate than electrospun fibers and mineralized fibers. Higher degradation rate of matrix polymers and HA loss were shown for hot-pressed composites from mineralized fibers than those from blend electrospun HA/PDLLA fibers. The obtained results should provide solid basis for further applications of functionalized PDLLA fibers, mineralized fibers and fibrous composites in biomedical areas.

A novel basalt fiber-reinforced polylactic acid composite for hard tissue repair

A basalt fiber (BF) was, for the first time, introduced into a poly(l-lactic acid) (PLLA) matrix as innovative reinforcement to fabricate composite materials for hard tissue repair. Firstly, BF/PLLA composites and pure PLLA were produced by the methods of solution blending and freeze drying. The results showed that basalt fibers can be uniformly dispersed in the PLLA matrix and significantly improve the mechanical properties and hydrophilicity of the PLLA matrix. The presence of basalt fibers may retard the polymer degradation rate and neutralize the acid degradation from PLLA. Osteoblasts were cultured in vitro to evaluate the cytocompatibility of the composite. An MTT assay revealed that osteoblasts proliferated well for 7 days and there was little difference found in their viability on both PLLA and BF/PLLA films, which was consistent with the alkaline phosphatase (ALP) activity results. A fluorescent staining observation showed that osteoblasts grew well on the composites. SEM images displayed that osteoblasts tended to grow along the fiber axis. The formation of mineralized nodules was observed on the films by Alizarin red S staining. These results suggest that the presence of basalt fibers does not noticeably affect osteoblastic behavior and the designed composites are osteoblast compatible. It is concluded that basalt fibers, as reinforcing fibers, may have promising applications in hard tissue repair.

The Life and Times of Sunscreen

Chemistry![Figure][1] CREDIT: [PHOTOS.COM][2] Nanomaterials boast a multitude of current and projected commercial uses that take advantage of their unique and beneficial properties, but the health and environmental risks they present are relatively unknown. Screening and eventual regulation are problematic because even materials of the same chemical composition have such a wide range of morphologies (including size, shape, coating, aggregation state, and crystalline phase variations) that affect their reactivity. In a demonstration of the complexity of such systems, Labille et al. examined the aging of composites of titanium dioxide (TiO2) nanoparticles commonly used in sunscreen and cosmetics. The composites—composed of several nanoparticles surrounded by a thin Al(OH)3 shell and bound in a hydrophobic polymer matrix—dispersed in water over time because of the degradation of the polymer. When aged in the presence of light, the polymer degradation rate increased, possibly due to photocatalysis by TiO2. In either case, once the polymer degraded, the remaining Al(OH)3 shell determined the stability of the composites. Stability was also sensitive to the concentration of natural organic matter; at low levels characteristic of groundwater, the composites aggregated, but higher levels characteristic of surface waters kept the composites suspended. These and similar studies raise the question of whether the nanomaterials studied and characterized in laboratory settings look or behave as they do in the environment. Environ. Pollut. 158 , 10.1016/j.envpol.2010.02.012 (2010). [1]: pending:yes [2]: http://PHOTOS.COM

Controlled Release Carriers of Growth Factors FGF-2 and TGF<I>β</I>1: Synthesis, Characterization and Kinetic Modelling

The purpose of this work is to produce microspheres loaded with transforming growth factor beta1 TGFbeta1 and basic fibroblast growth factor FGF-2; to ensure the protein protection from degradation during the encapsulation and storage steps, to evaluate the release rate and the microspheres toxicity. The water in oil in water double emulsion technique was adapted to avoid the protein degradation during the encapsulation. The obtained microspheres were deeply characterized to evaluate their size, morphology, toxicity, the way of degradation, the protein stability and release rate. The microspheres were found to be biocompatible and the encapsulation efficiency was about 35%. It was observed that the obtained microspheres increase the shelf life of the growth factors. The diffusion coefficient was quantified using Fick's law of diffusion that was combined to an empirical equation representing the decrease in the protein stability. Such modelling helped to give indirect information about the microspheres morphology and drug distribution within the microspheres. The main conclusion consists of the formation of a higher compact polymer matrix when smaller particles are produced, which has different distinct effects: the encapsulation efficiency and the stability of the encapsulated growth factor are enhanced while both the growth factor diffusion and the polymer degradation rates decrease.

Blends of novel L‐tyrosine‐based polyurethanes and polyphosphate for potential biomedical applications

AbstractElastomeric biodegradable polyurethanes and polyphosphate have been developed using an L‐ tyrosine‐based diphenolic monomer desaminotyrosine‐tyrosine hexyl ester (DTH). Soft segments, which are polycaproloctone diol (PCL) and polyethylene glycol (PEG) have been used for the synthesis of two biodegradable L‐tyrosine polyurethanes (LTUs), which are PEG‐C‐DTH and PCL‐C‐DTH. An investigation of the physico‐chemical properties shows that these polymers have dramatically different properties. By blending LTUs with L‐tyrosine polyphosphate (LTP), we hope to produce a family of materials with a wide range of thermal, morphological, surface, and degradative properties. Examination of the blends shows a smooth surface morphology with a partially phase‐separated structure. These findings are consistent with the results obtained from thermal analysis of the blends. Hydrophilic nature of PEG imparts the PEG‐based blends (PEG‐C‐DTH/LTP) with a significantly higher surface and bulk hydrophilicity compared with the PCL‐based blends (PCL‐C‐DTH/LTP). Finally, the blends demonstrate a rapid initial hydrolytic degradation in phosphate buffered saline (PBS) followed by a significantly slower, prolonged degradation. The observed trend may occur due to the rapid hydrolytic degradation rate of the polyphosphate polymer followed by the degradation of the polyurethane component. Thus, tuning the physical properties by blending LTUs with LTP may be useful for drug delivery device and soft tissue engineering scaffold applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Interplay of anionic charge, poly(ethylene glycol), and iodinated tyrosine incorporation within tyrosine‐derived polycarbonates: Effects on vascular smooth muscle cell adhesion, proliferation, and motility

Regulation of smooth muscle cell adhesion, proliferation, and motility on biomaterials is critical to the performance of blood-contacting implants and vascular tissue engineering scaffolds. The goal of this study was to examine the underlying substrate-smooth muscle cell response relations, using a selection of polymers representative of an expansive library of multifunctional, tyrosine-derived polycarbonates. Three chemical components within the polymer structure were selectively varied through copolymerization: (1) the content of iodinated tyrosine to achieve X-ray visibility; (2) the content of poly(ethylene glycol) (PEG) to decrease protein adsorption and cell adhesivity; and (3) the content of desaminotyrosyl-tyrosine (DT), which regulates the rate of polymer degradation. Using quartz crystal microbalance with dissipation, we quantified differential serum protein adsorption behavior because of the chemical components DT, iodinated tyrosine, and PEG: increased PEG content within the polymer structure progressively decreased protein adsorption but the simultaneous presence of both DT and iodinated tyrosine reversed the effects of PEG. The complex interplay of these components was next tested on the adhesion, proliferation, and motility behavior cultured human aortic smooth muscle cells. The incorporation of PEG into the polymer reduced cell attachment, which was reversed in the presence of iodinated tyrosine. Further, we found that as little as 10% DT content was sufficient to negate the PEG effect in polymers containing iodinated tyrosine, whereas in non-iodinated polymers, the PEG effect on cell attachment was reversed. Cross-functional analysis of motility and proliferation revealed divergent substrate chemistry related cell response regimes. For instance, within the series of polymers containing both iodinated tyrosine and 10% of DT, increasing PEG levels lowered smooth muscle cell motility without a change in the rate of cell proliferation. In contrast, for non-iodinated tyrosine and 10% of DT, increasing PEG levels increased cell proliferation significantly while reducing cell motility. Clearly, the polycarbonate polymer library offers a sensitive platform to modulate cell adhesion, proliferation, and motility responses, which, in turn, may have implications for controlling vascular remodeling in vivo. Additionally, our data suggests unique biorelevant properties following the incorporation of iodinated subunits in a polymeric biomaterial as a potential platform for X-ray visible devices.

Ultrasonic degradation of poly(acrylic acid)

AbstractThe ultrasonic degradation of poly(acrylic acid), a water‐soluble polymer, was studied in the presence of persulfates at different temperatures in binary solvent mixtures of methanol and water. The degraded samples were analyzed by gel permeation chromatography for the time evolution of the molecular weight distributions. A continuous distribution kinetics model based on midpoint chain scission was developed, and the degradation rate coefficients were determined. The decline in the rate of degradation of poly(acrylic acid) with increasing temperature and with an increment in the methanol content in the binary solvent mixture of methanol and water was attributed to the increased vapor pressure of the solutions. The experimental data showed an augmentation of the degradation rate of the polymer with increasing oxidizing agent (persulfate) concentrations. Different concentrations of three persulfates—potassium persulfate, ammonium persulfate, and sodium persulfate—were used. It was found that the ratio of the polymer degradation rate coefficient to the dissociation rate constant of the persulfate was constant. This implies that the ultrasonic degradation rate of poly(acrylic acid) can be determined a priori in the presence of any initiator. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

A Library of L-Tyrosine-Derived Biodegradable Polyarylates for Potential Biomaterial Applications, Part I: Synthesis, Characterization and Accelerated Hydrolytic Degradation

A combinatorial library of biodegradable polyarylates derived from L-tyrosine was synthesized and characterized. These polyarylates are A–B-type co-polymers consisting of a cyclic dipeptide and a diacid. General structure–property correlations were established by comparing aryl diacid co-polymers and aliphatic diacid co-polymers. The synthesized polymers were characterized by FT-IR, 1H-NMR, 13C-NMR for their chemical structure, by DSC and TGA for their thermal characteristics and by GPC for their molecular weight distribution. The T g of polymers decreased and water absorption increased with increasing number of methylene groups in the polymer backbone. Using a cyclic peptide derived from L-tyrosine as co-monomer we obtained optimum bioactivity and biocompatibility. Combinatorial approaches of designing material increased effectively the number of available degradable polymers which can be used in different biomaterials applications. General structure–property correlation makes polymers' properties varied in a predictable and systematic fashion. Accelerated hydrolytic degradation studies of polyarylates were performed at 70°C in acid and alkali medium. The degradation rates of polymers were in accordance with their water absorption. The degradation rates of samples in acid medium were lower than those in alkali medium.