Increasing Degradation Time Research Articles

Temperature-sensitive biodegradable mixed star-shaped block copolymers hydrogels for an injection application

3-arm star-shaped poly(d,l-lactic-co-glycolic acid)-b-methoxy poly(ethylene glycol)(3sPLGA-mPEG) and 4sPLGA-mPEG copolymers are synthesized via the arm-first method. A facile method to obtain a temperature-sensitive physical injectable hydrogel is found using simply mixing two 4sPLGA-mPEG block copolymers aqueous solutions individually with thermogelling property (sol-gel-sol transition) or using simply mixing an aqueous sol of a 3sPLGA-mPEG block copolymer with a precipitate of a similar 3sPLGA-mPEG copolymer but with a varying PLGA/mPEG block ratio. A dramatic tuning of the sol–gel transition temperature is conveniently achieved by merely changing mix ratios. The degradation of mixed 3sPLGA-mPEG hydrogel proceeds by hydrolysis of ester bonds followed by the erosion of gel in PBS solution at body temperature for nearly 50 days. The mass loss and reduction of molecular weight are detected with increasing degradation time, and the mix ratios are found to slightly influence the degradation profiles. MTT assay and histological observations are used to examine the mixed copolymer solution. Both in vitro and in vivo results illustrate acceptable biocompatibility of our mixed materials. Collectively, our results show that the mixed star-shaped PLGA-mPEG block copolymer is a promising candidate as a novel injectable gel.

Fabrication of Poly(D,L-lactide-co-glycolide) Microspheres and Degradation Characteristics in vitro

Poly(D,L-lactide-co-glycolide) (PLGA, 75/25) microspheres were prepared by the emulsion solvent extraction/evaporation technique and their degradation behaviors in vitro at 37°C were investigated. PLGA microspheres with a smooth and nonporous surface were obtained, with a mean particle size of 114.15 μm. It was observed that mass loss and water uptake of PLGA increased with increasing degradation time; however, weight average molecular weight and the pH value of the phosphate buffered saline (PBS) solution decreased. The observed relative rates of mass loss vs. molecular weight decreases were consistent with the explanation that PLGA underwent bulk degradation rather than surface degradation. During the degradation time, the surface of the PLGA microspheres became coarse and there were many micropores that developed upon immersion in the PBS. The microspheres lost their spherical form with increasing degradation time.

Biodegradation, soft and hard tissue integration of various polyethylene glycol hydrogels: a histomorphometric study in rabbits

(i) To evaluate biodegradation, hard and soft tissue integration using various polyethylene glycol (PEG) hydrogels; (ii) to evaluate the influence of arginine-glycine-aspartic acid (RGD) on two types of PEG hydrogels. In seven rabbits, six treatment modalities were randomly applied subperiosteally on the skull: (1) a dense network PEG hydrogel (PEG1), (2) PEG1 modified with RGD (PEG1-RGD), (3) a looser network PEG hydrogel (PEG2), (4) PEG2 modified with RGD (PEG2-RGD), (5) a collagen membrane, and (6) a polylactide/polyglycolide/trimethylene carbonate membrane. The animals were sacrificed at 14 days. Histomorphometric analyses were performed on undecalcified Epon sections using a standardized region of interest. For statistical analysis, paired t-test and signed rank test were applied. PEG1 and PEG1-RGD remained intact and maintained the shape. PEG2 and PEG2-RGD completely degraded and were replaced by connective tissue and bone. The largest amount of mineralized tissue was found for PEG2-RGD (21.4%), followed by PEG 2 (9.5%). The highest percentage of residual hydrogel/membrane was observed for PEG1-RGD (55.6%), followed by PEG1 (26.7%). Modifications of the physico-chemical properties of PEG hydrogels and the addition of RGD influenced soft and hard tissue integration and biodegradation. PEG1 showed an increased degradation time and maintained the shape. The soft tissue integration was enhanced by adding an RGD sequence. A high turn-over rate and extensive bone regeneration was observed using PEG2. The addition of RGD further improved bone formation and soft tissue integration.

The Degradation Behavior of Poly(α-hydroxyacid) Composite Material Containing Porcine Cortical Bone in a Phosphate-Buffered Solution

The composite containing 10 wt% of osteoconductive porcine cortical bone (PCB) and 90 wt% of bioabsorbable poly(L-lactide) (PLLA) was manufactured. The degradation behaviors of the PLLA/PCB composite in-vitro up to 20 weeks, including water uptake, weight loss, mechanical properties, molecular weight and morphology, were investigated by soaking in phosphate-buffered solution (PBS). The results indicated that the water uptake and weight loss increased with increasing degradation time, while the mechanical properties and molecular weight decreased. There were a large number of microcracks and erosive holes on the surface and in the interior of the PLLA/PCB materials after 20 weeks. The degradation mechanism of PLLA component in the PLLA/PCB is indicative of an autocatalysis process. The degradation of PLLA complies with first-order kinetics, and the rate constant of the biodegradation process of PLLA component is 6.6 × 10−3 day−1.

Mechanical Strain Stabilizes Reconstituted Collagen Fibrils against Enzymatic Degradation by Mammalian Collagenase Matrix Metalloproteinase 8 (MMP-8)

BackgroundCollagen, a triple-helical, self-organizing protein, is the predominant structural protein in mammals. It is found in bone, ligament, tendon, cartilage, intervertebral disc, skin, blood vessel, and cornea. We have recently postulated that fibrillar collagens (and their complementary enzymes) comprise the basis of a smart structural system which appears to support the retention of molecules in fibrils which are under tensile mechanical strain. The theory suggests that the mechanisms which drive the preferential accumulation of collagen in loaded tissue operate at the molecular level and are not solely cell-driven. The concept reduces control of matrix morphology to an interaction between molecules and the most relevant, physical, and persistent signal: mechanical strain.Methodology/Principal FindingsThe investigation was carried out in an environmentally-controlled microbioreactor in which reconstituted type I collagen micronetworks were gently strained between micropipettes. The strained micronetworks were exposed to active matrix metalloproteinase 8 (MMP-8) and relative degradation rates for loaded and unloaded fibrils were tracked simultaneously using label-free differential interference contrast (DIC) imaging. It was found that applied tensile mechanical strain significantly increased degradation time of loaded fibrils compared to unloaded, paired controls. In many cases, strained fibrils were detectable long after unstrained fibrils were degraded.Conclusions/SignificanceIn this investigation we demonstrate for the first time that applied mechanical strain preferentially preserves collagen fibrils in the presence of a physiologically-important mammalian enzyme: MMP-8. These results have the potential to contribute to our understanding of many collagen matrix phenomena including development, adaptation, remodeling and disease. Additionally, tissue engineering could benefit from the ability to sculpt desired structures from physiologically compatible and mutable collagen.

Morphological changes of annealed poly‐ε‐caprolactone by enzymatic degradation with lipase

AbstractThe effects of crystallinity and temperature on enzymatic degradation of poly‐ε‐caprolactone (PCL) films and structural changes after degradation have been studied using weight loss, differential scanning calorimetry, and optical microscopy. The weight loss during the enzymatic degradation of PCL suggested that the extent of biodegradation and the rate of degradation strongly depend on the initial crystallinity. PCL films of lower crystallinity (24%) degraded much faster than films of higher crystallinity (45%). The crystallinity of low‐crystalline PCL films increased with increasing degradation time, whereas the crystallinity of high‐crystalline PCL films decreased with time. The spherulite size increased with increasing degradation time for low‐crystalline samples but decreased with time for high‐crystalline samples. These results revealed that degradation occurs first in the amorphous region where the degradation rate is much higher, and the crystalline region of the PCL film started to degrade simultaneously for those PCL with higher crystallinity. The enzymatic degradation of PCL proceeded from the free amorphous to restricted amorphous followed by lamellar edges, where PCL chains have higher mobility irrespective of hydrolysis temperature. Caproic acid was identified as the primary product formed after degradation and confirmed by proton nuclear magnetic resonance spectroscopy, suggesting that degradation occurs through the depolymerization mechanism. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 202–211, 2010

Influence of Degradation of Poly-L-lactide on Mass Loss, Mechanical Properties, and Crystallinity in Phosphate-Buffered Solution

A bulk degradation of poly-L-lactide (PLLA) rectangular bars was studied by incubating them in phosphate-buffered solution at 37°C for different periods. The characteristics of the mass, water uptake, mechanical properties, thermal behavior, and crystallinity of the PLLA samples were investigated. The results indicate that mass loss and water uptake of PLLA increase with increasing time; however, pH value and mechanical strength decrease. The melting temperature, melting enthalpy, and crystallinity of PLLA firstly increase and then decrease with increasing degradation time. There is no new diffraction peak formed with increasing degradation time, which indicates that the degradation could occur in the amorphous regions firstly and then in the crystalline regions.

Non-enzymatic and enzymatic degradation of poly(ethylene glycol)-b-poly(ɛ-caprolactone) diblock copolymer micelles in aqueous solution

The non-enzymatic and enzymatic degradation behaviors of the monomethoxy-poly(ethylene glycol)-b-poly(ɛ-caprolactone) diblock copolymers (MPEG–PCL) micelles in aqueous solution were investigated by DLS, 1H NMR, SEC and HPLC. It is found that the degradation mechanism of MPEG–PCL micelles in aqueous solution in non-enzymatic case is quite different from that in the presence of enzyme. In non-enzymatic case, the degradation induced by acidic catalysis was not found in low pH aqueous solution but the degradation of the micelles occurred under neutral and basic conditions. The degradation of MPEG–PCL micelles first happens near the interface region of the MPEG shell and PCL core, leading to the part detachment of PEG chains. With increasing degradation time, the degradation inside the PCL core with a random scission on PCL chains occurred. Compared with non-enzymatic degradation, the enzymatic degradation of MPEG–PCL micelles is much fast and the degradation rate of MPEG–PCL micelles is proportional to either the micelles or the enzyme concentration in a certain range. Based on the micelle degradation behaviors that we observed, a possible mechanism for the enzymatic degradation of the MPEG-b-PCL micelles including PCL core erosion which results in cavitization of micellar core and micellar dissociation is proposed.

Starch–poly(ε‐caprolactone) and starch–poly(lactic acid) fibre‐mesh scaffolds for bone tissue engineering applications: structure, mechanical properties and degradation behaviour

In scaffold-based tissue engineering strategies, the successful regeneration of tissues from matrix-producing connective tissue cells or anchorage-dependent cells (e.g. osteoblasts) relies on the use of a suitable scaffold. This study describes the development and characterization of SPCL (starch with epsilon-polycaprolactone, 30:70%) and SPLA [starch with poly(lactic acid), 30:70%] fibre-meshes, aimed at application in bone tissue-engineering strategies. Scaffolds based on SPCL and SPLA were prepared from fibres obtained by melt-spinning by a fibre-bonding process. The porosity of the scaffolds was characterized by microcomputerized tomography (microCT) and scanning electron microscopy (SEM). Scaffold degradation behaviour was assessed in solutions containing hydrolytic enzymes (alpha-amylase and lipase) in physiological concentrations, in order to simulate in vivo conditions. Mechanical properties were also evaluated in compression tests. The results show that these scaffolds exhibit adequate porosity and mechanical properties to support cell adhesion and proliferation and also tissue ingrowth upon implantation of the construct. The results of the degradation studies showed that these starch-based scaffolds are susceptible to enzymatic degradation, as detected by increased weight loss (within 2 weeks, weight loss in the SPCL samples reached 20%). With increasing degradation time, the diameter of the SPCL and SPLA fibres decreases significantly, increasing the porosity and consequently the available space for cells and tissue ingrowth during implantation time. These results, in combination with previous cell culture studies showing the ability of these scaffolds to induce cell adhesion and proliferation, clearly demonstrate the potential of these scaffolds to be used in tissue engineering strategies to regenerate bone tissue defects.

Enzymatic Chain Scission Kinetics of Poly(ε-caprolactone) Monolayers

The hydrolytic and enzymatic degradation behavior of poly(epsilon-caprolactone) (PCL) is investigated using the Langmuir monolayer technique, and an improved data acquisition and data reduction procedure is presented. Hydrolytic and enzymatic monolayer degradation experiments of PCL with various molecular weights by Pseudomonas cepacia lipase have been carried out to analyze the influence of subphase pH, subphase temperature, enzyme concentration, and the packing density of polymer chains on the degradation kinetics. The enzymatic monolayer degradation results in an exponential increase in the number of dissolved degradation fragments with increasing degradation time, which confirms random chain scission to be the dominant scission mechanism. The increase in the enzymatic scission rate constant with decreasing initial average molecular weight of the polymers is assigned to the influence of the area density of polar terminal groups on the substrate-enzyme complex formation.

Study on oxidative degradation behaviors of polyesterurethane network

In this study H 2O 2/CoCl 2 system was used as an oxidative environment to investigate the in vitro degradation behavior of a crosslinked polyesterurethane network. Weight loss, water absorption, mechanical properties, swelling degree and gel content were determined as a function of degradation time. The results showed that the H 2O 2/CoCl 2 system could effectively accelerate the degradation of the polyurethanes. The samples had almost completely degraded after 84 days of incubation in a 3% H 2O 2/2% CoCl 2 solution at 37 °C. The weight loss process could be approximately divided into three major phases: the weight decreased slowly during an induction phase (1), which was followed by a phase characterized by accelerated weight loss (2), and finally by a phase showing slow linear weight loss until complete erosion (3). The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of degraded polyurethane networks showed that the polyurethane underwent chain scission of the copolyester segment chains, and the joints of copolyester and 2,2,4- and 2,4,4-trimethylhexamethylene during degradation in the oxidative environment. The glass transition temperature of degraded polyesterurethane networks decreased significantly with increasing degradation time. From these results the H 2O 2/CoCl 2 system can be used as an oxidative condition to evaluate the in vitro degradability of polyesterurethane.

Quality Monitoring of Thermally Degraded Stationary Gas Turbine Blade Material by Surface Wave Technique

For the quality monitoring and reliable application of stationary gas turbine blade (vane), near surface damages in the isothermally degraded vein material (i.e., cobalt based superalloy) were characterized by ultrasonic surface wave technique. Surface wave velocity and attenuation were measured for the artificially degraded specimens at 1100°C, together with microstructural analysis and micro-hardness measurement. Surface wave velocity increased with thermal degradation time, which was attributed to the increasing depletion of solute chromium near the surface. Strong frequency dependence of surface wave velocity was observed in the specimens with surface depletion layer. Attenuation coefficient of surface wave increased with increasing degradation time. The potential of ultrasonic surface wave technique to assess near surface damages in vein material was discussed with an emphasis on the relationship between the microstructural damage and the governing principles of ultrasonic response.

Phase Separation and Hydrothermal Degradation of 3 mol% Y2O3-ZrO2 Ceramics

Phase separation and hydrothermal degradation in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (3Y-TZP) were studied by X-ray diffractometry. The Rietveld analysis showed that crystal structure of sintered body remarkably changes at high temperature. The sintered body is phase separated into high and low yttrium region with increasing sintering temperature. Both regions are assigned to the tetragonal phase. The high yttrium region can be analyzed using tetragonal phase model with c/a≒1. The amount of monoclinic phase induced by hydrothermal degradation increased with increasing degradation time. Aging kinetics of tetragonal to monoclinic phase transition was analyzed with Johnson-Mehl-Avrami equation. Fraction of transformable phase evaluated with the equation is in good agreement with the fraction of low yttrium region determined with the Rietveld analysis. The quantitative analyses clearly show that the low yttrium region selectively transforms into monoclinic phase under the hydrothermal condition, while the high yttrium region does not transform. Transformable fraction during the hydrothermal condition in 3Y-TZP ceramics is governed by the amount of low yttrium region. The formation of low yttrium region due to phase separation is responsible for hydrothermal degradation in sintered body.

GFRP/ステンレス鋼接着継手の環境劣化評価(S16-3 接着・ろう付,S16 締結・接合部のプロセスと信頼性評価)

The environmental degradation of GFRP/stainless-steel adhesive joints exposed to high temperatures and high humidity was studied. After, adhesive joints were degraded, fatigue tests were carried out on the joints at room temperature. The results showed that short-term exposure to high temperature slightly in creased the fatigue strength of the adhesive joints. However, the joints' fatigue strengths degraded under high temperatures and high humidity decreased with increasing degradation time. It is clear that the effect of moisture on adhesive joints is very large. Furthermore, an evaluation method for degradation durability of adhesive joints was developed. The relationship between degradation temperature and degradation time was calculated. The retention ratio of fatigue strength on adhesive joints was evaluated using arrhenius plots.

Enzymatic degradation behavior of porous silk fibroin sheets

We investigated the degradation behavior of porous silk fibroin sheets by in vitro enzymatic experiments with α-chymotrypsin, collagenase IA, and protease XIV. With 1.0 U/ml protease XIV, 70% of a silk fibroin sheet was degraded within 15 days at 37°C. When the fibroin sheet was exposed to collagenase IA, the amount of Silk II crystalline structure in the sheets decreased slightly, and a small amount of Silk I crystalline structure was formed. When protease XIV was used, almost all Silk II disappeared, but the crystallinity increased overall because the amount of Silk I increased. During digestion with protease XIV, the pore size of the fibroin sheets increased with increasing degradation time, until the sheets finally collapsed and became totally shapeless. The average molecular weight of the products after degradation with the three enzymes followed the order protease XIV < collagenase IA < α-chymotrypsin. More than 50% of the products resulting from degradation with protease XIV were free amino acids.

Effects of in vitro degradation on properties of poly(DL-lactide-co-glycolide) pertinent to its biological performance.

Poly(DL-lactide-co-glycolide) (PLGA) is a widely used biodegradable polymer. In this study, effects of in vitro degradation on its dynamic mechanical properties, thermal behavior, molecular weight, fluid uptake and dimension stability were investigated. The in vitro experiment was conducted at 37 degrees C in phosphate buffer solution with pH=7.4 in a dynamic incubator. The dynamic mechanical experiments were performed on rectangular specimens under three point bending. For an aging period of 6 weeks, the dynamic mechanical properties were found to decrease significantly with the increased time. The absorbed fluid had a significant effect on polymer storage modulus. The molecular weight decreased with aging time, and was found to be different at the center and the surface, being higher at the center in the beginning and lower thereafter. The polydispersity did not change significantly with degradation. The fluid uptake and specimen geometric dimension increased proportionally with aging time. The glass transition and thermal decomposition temperatures decreased with the increased degradation time.

Changes in Composition of Hydrolyzed Poly(butylene adipate-co-caproamide) Characterized by Pyrolysis-GC-MS, 1H-NMR and FTIR

The changes in composition of an abiotically degraded polyester-amide, poly(butylene adipate-co-caproamide), in an aqueous environment at 37°C; 60°C and 80°C were investigated. The changes in polymer composition were studied using pyrolysis-GC-MS, 1H-NMR, FTIR, and size exclusion chromatography, and degradation products were analyzed by solid-phase extraction and subsequent GC-MS. During the degradation of PEA in an aqueous environment, the main degradation products were 6-aminohexanoic acid, 1-hydroxydodecanoic acid-6-one-5-oxo and dimers. After longer degradation times and at higher degradation temperatures, 1-hydroxydodecanoic acid-6-one-5-oxo was cleaved further into hexanedioic acid and 1,4-butanediol. At 80°C, the degradation was fastest with the largest weight loss due to dissolution of oligomer. At 80°C, the amide content in the copolymer increased with increasing degradation time. We suggest that there is a two-step degradation mechanism with the formation of linear degradation products, where the hydrolysis of ester bonds is favored over that of amide bonds.

Crystallite Size Analysis of Pressure-Crystallized Polyethylene by Nitric Acid Etching and Gel-Permeation Chromatography. II. Folded-Chain Crystals and Ordinary Extended-Chain Crystals

Statistical analysis of average lamellar thickness and its size distribution in the molecular chain direction has been made on a variety of folded-chain crystals and ordinary extended-chain crystals of linear polyethylene which were crystallized under high pressures of 106 MPa and 305 MPa. This was done by fuming nitric acid degradation followed by gel-permeation chromatography (GPC) measurement of molecular weight distribution of the degraded samples. The observed GPC curves were corrected for instrumental spreading using the Chang–Huang’s method of correction. Several new experimental facts became clear. First, the existence of folds was confirmed in the ordinary extended-chain crystals formed at 305 MPa. Second, the average lamellar thickness of the defect-free crystalline core of folded-chain crystals was often found to be approximately one tenth the average thickness of ordinary extended-chain crystals in the same sample. Third, in ordinary extended-chain crystals with average lamellar thicknesses of several hundred nanometers, it was observed that the average crystal thickness decreased markedly with increasing degradation time, compared with the folded-chain crystals and that the chain-thinning effect tended to be become greater with an increase in average crystal thickness. This suggests strongly that ordinary extended-chain crystals have surface structures easily accessible to nitric acid etching. Since the crystal thickness of ordinary extended-chain crystals is comparable to the number-average molecular weight of the original material, crystalline lamellae will be terminated predominantly in the form of extended-chains. It may be reasonable to assume extended-chain crystals with gradually disordered, long surface layers from about 20 to 50 nanometers as a possible model for ordinary extended-chain crystals.

GPC analysis of degraded annealed single crystals

AbstractAnnealed polyethylene single crystals have been degraded with fuming nitric acid, and the molecular weight distribution of the fragments determined by using a gel‐permeation chromatograph. Peaks due to chain folding were observed in these distributions as for unannealed single crystals. The peaks moved to lower molecular weight with increasing degradation time. Comparison of the lowest molecular weight peak length after a given degradation time with the low‐angle x‐ray periodicity before degradation gave information about a disordered surface layer. The thickness of this layer at early states of degradation was dependent solely on annealing temperature, though changes in the layer must have occurred with annealing time, since there was an increase in reaction rate with annealing time. At higher degradation states, the thickness of the layer was dependent solely on the original low‐angle periodicity. This has been related to the depth at which some folds are buried beneath the lamellar surfaces. The relevance of these observations to the structure of annealed single crystals is discussed.