A simplified theory of crystallisation induced by polymer chain scissions for biodegradable polyesters

An integrated mechanical degradation model to explore the mechanical response of a bioresorbable polymeric scaffold

Simulation of bioresorbable medical devices is hindered by the limitations of current material models. Useful simulations require that both the short- and long-term response must be considered; existing models are not physically-based and provide limited insight to guide performance improvements. This study presents an integrated degradation framework which couples a physically-based degradation model, which predicts changes in both crystallinity (Xc) and molecular weight (Mn), with the results of a micromechanical model, which predicts the effective properties of the semicrystalline polymer. This degradation framework is used to simulate the deployment of a bioresorbable PLLA (Poly (L-lactide) stent into a mock vessel and the subsequent mechanical response during degradation under different diffusion boundary conditions representing neointimal growth. A workflow is established in a commercial finite element code that couples both the immediate and long-term responses. Clinically relevant lumen loss is reported and used to compare different responses and the effect of neo-intimal tissue regrowth post-implantation on degradation and on the mechanical response is assessed. In addition, the effects of possible changes in Xc, which could occur during processing and stent deployment, are explored.

UV Light Degradation of Polylactic Acid Kickstarts Enzymatic Hydrolysis

Polylactic acid (PLA) and bioplastics alike have a designed degradability to avoid the environmental buildup that petroplastics have created. Yet, this designed biotic-degradation has typically been characterized in ideal conditions. This study seeks to relate the abiotic to the biotic degradation of PLA to accurately represent the degradation pathways bioplastics will encounter, supposing their improper disposal in the environment. Enzymatic hydrolysis was used to study the biodegradation of PLA with varying stages of photoaging. Utilizing a fluorescent tag to follow enzyme hydrolysis, it was determined that increasing the amount of irradiation yielded greater amounts of total enzymatic hydrolysis by proteinase K after 8 h of enzyme incubation. While photoaging of the polymers causes minimal changes in chemistry and increasing amounts of crystallinity, the trends in biotic degradation appear to primarily be driven by photoinduced reduction in molecular weight. The relationship between photoaging and enzyme hydrolysis appears to be independent of enzyme type, though commercial product degradation may be impacted by the presence of additives. Overall, this work reveals the importance of characterizing biodegradation with relevant samples that ultimately can inform optimization of production and disposal.

Fast Depolymerization of PET Bottle Mediated by Microwave Pre-Treatment and An Engineered PETase

Recycling plastics is the key to reaching a sustainable materials economy. Biocatalytic degradation of plastics shows great promise by allowing selective depolymerization of man-made materials into constituent building blocks under mild aqueous conditions. However, insoluble plastics have polymer chains that can reside in different conformations and show compact secondary structures that offer low accessibility for initiating the depolymerization reaction by enzymes. In this work, we overcome these shortcomings by microwave irradiation as a pre-treatment process to deliver powders of polyethylene terephthalate (PET) particles suitable for subsequent biotechnology-assisted plastic degradation by previously generated engineered enzymes. An optimized microwave step resulted in 1400 times higher integral of released terephthalic acid (TPA) from high-performance liquid chromatography (HPLC), compared to original untreated PET bottle. Biocatalytic plastic hydrolysis of substrates originating from PET bottles responded to 78 % yield conversion from 2 h microwave pretreatment and 1 h enzymatic reaction at 30 °C. The increase in activity stems from enhanced substrate accessibility from the microwave step, followed by the administration of designer enzymes capable of accommodating oligomers and shorter chains released in a productive conformation.

Polyethylene-Like Blends Amenable to Abiotic Hydrolytic Degradation

Long-chain aliphatic polyester-18,18 (PE-18,18) exhibits high density polyethylene-like material properties and, as opposed to high density polyethylene (HDPE), can be recycled in a closed loop via depolymerization to monomers under mild conditions. Despite the in-chain ester groups, its high crystallinity and hydrophobicity render PE-18,18 stable toward hydrolysis even under acidic conditions for one year. Hydrolytic degradability, however, can be a desirable material property as it can serve as a universal backstop to plastic accumulation in the environment. We present an approach to render PE-18,18 hydrolytically degradable by melt blending with long-chain aliphatic poly(H-phosphonate)s (PP). The blends can be processed via common injection molding and 3D printing and exhibit HDPE-like tensile properties, namely, high stiffness (E = 750-940 MPa) and ductility (ε_tb = 330-460%) over a wide range of blend ratios (0.5-20 wt % PP content). Likewise, the orthorhombic solid-state structure and crystallinity (χ ≈ 70%) of the blends are similar to HDPE. Under aqueous conditions in phosphate-buffered media at 25 °C, the blends' PP component is hydrolyzed completely to the underlying long-chain diol and phosphorous acid within four months, as evidenced by NMR analyses. Concomitant, the PE-18,18 major blend component is partially hydrolyzed, while neat PE-18,18 is inert under identical conditions. The hydrolysis of the blend components proceeded throughout the bulk of the specimens as confirmed by gel permeation chromatography (GPC) measurements. The significant molar mass reduction upon extended immersion in water (M _n(virgin blends) ≈ 50-70 kg mol^-1; M _n(hydrolyzed blends) ≈ 7-11 kg mol^-1) resulted in embrittlement and fragmentation of the injection molded specimens. This increases the surface area and is anticipated to promote eventual mineralization by abiotic and biotic pathways of these HDPE-like polyesters in the environment.

Characterization of P(3HB) from untreated raw palm oil mill effluent using Azotobacter vinelandii ΔAvin_16040 lacking S-layer protein

The production of poly(3-hydroxybutyrate) [P(3HB)] from untreated raw palm oil mill effluent (urPOME), the first wastewater discharge from crude palm oil extraction, is discussed. The mutant strain Azotobacter vinelandii ΔAvin_16040, which lacks the S-layer protein but has a better P(3HB) synthesis capability than the wild type strain ATCC 12,837, was chosen for this study. UrPOME substrate, with high biological oxygen demand (BOD), chemical oxygen demand (COD) and suspended solids, was used without pre-treatment. DSMZ-Azotobacter medium which was devoid of laboratory sugar(s) was used as the basal medium (BaM). Initially, Azotobacter vinelandii ΔAvin_16040 generated 325.5, 1496.3, and 1465.7 mg L^-1 of P(3HB) from BaM with 20% urPOME, 2BaM with 20% urPOME and 20 g L^-1 sucrose, and 2BaM with 20% urPOME and 2 mL L^-1 glycerol, respectively. P(3HB) generation was enhanced by nearly tenfold using statistical optimization, resulting in 13.9 g L^-1. Moreover, the optimization reduced the compositions of mineral salts and sugar in the medium by 48 and 97%, respectively. The urPOME-based P(3HB) product developed a yellow coloration most possibly attributed to the aromatic phenolics content in urPOME. Despite the fact that both were synthesised by ΔAvin_16040, thin films of urPOME-based P(3HB) had superior crystallinity and tensile strength than P(3HB) produced only on sucrose. When treated with 10 and 50 kGy of electron beam irradiation, these P(3HB) scissioned to half and one-tenth of their original molecular weights, respectively, and these cleavaged products could serve as useful base units for specific polymer structure construction.

Biodegradation of poly(butylene succinate) in soil laboratory incubations assessed by stable carbon isotope labelling

Using biodegradable instead of conventional plastics in agricultural applications promises to help overcome plastic pollution of agricultural soils. However, analytical limitations impede our understanding of plastic biodegradation in soils. Utilizing stable carbon isotope (¹³C-)labelled poly(butylene succinate) (PBS), a synthetic polyester, we herein present an analytical approach to continuously quantify PBS mineralization to ¹³CO₂ during soil incubations and, thereafter, to determine non-mineralized PBS-derived ¹³C remaining in the soil. We demonstrate extensive PBS mineralization (65 % of added ¹³C) and a closed mass balance on PBS−¹³C over 425 days of incubation. Extraction of residual PBS from soils combined with kinetic modeling of the biodegradation data and results from monomer (i.e., butanediol and succinate) mineralization experiments suggest that PBS hydrolytic breakdown controlled the overall PBS biodegradation rate. Beyond PBS biodegradation in soil, the presented methodology is broadly applicable to investigate biodegradation of other biodegradable polymers in various receiving environments.

This study applies stable carbon isotope labelling to study polymer biodegradation in soils. This labelling enables accurate and precise tracking of polymer carbon during biodegradation and, thereby, provides a holistic picture of this process.

Confined Crystallization of Thin Plasma-Polymerized Nanocomposite Films with Maleic Anhydride and Cellulose Nanocrystals under Hydrolysis

The creation of novel surface morphologies through thin-film patterning is important from a scientific and technological viewpoint in order to control specific surface properties. The pulsed-plasma polymerization of thin nanocomposite films, including maleic anhydride (MA) and cellulose nanocrystals (CNC), may result in different metastable film morphologies that are difficult to control. Alternatively, the transformation of deposited plasma films into crystalline structures introduces unique and more stable morphologies. In this study, the structural rearrangements of plasma-polymerized (MA+CNC) nanocomposite films after controlled hydrolysis in a humid atmosphere were studied, including effects of plasma conditions (low duty cycle, variable power) and monomer composition (ratio MA/CNC) on hydrolysis stability. The progressive growth of crystalline structures with fractal dendrites was observed in confined thin films of 30 to 50 nm. The structures particularly formed on hydrophilic substrates and were not observed before on the more hydrophobic substrates, as they exist as a result of water penetration and interactions at the film/substrate interface. Furthermore, the nucleating effect and local pinning of the crystallites to the substrate near CNC positions enhanced the film stability. The chemical structures after hydrolysis were further examined through XPS, indicating esterification between the MA carboxylic acid groups and CNC surface. The hydrolysis kinetics were quantified from the conversion of anhydride groups into carboxylic moieties by FTIR analysis, indicating enhanced hydrolytic stability of p(MA+CNC) nanocomposite films relative to the pure p(MA) films.

Influence of nitric acid on biodegradation of polystyrene and low-density polyethylene by Cephalosporium species

Petroleum-based polymers are not susceptible to microorganisms because of its high molecular weight. Acid treatments convert the polymers into a more oxidized form having low molecular weight. The present in-vitro degradation study focuses on the potential of Cephalosporium species to degrade acid-treated polystyrene (PS) and low-density polyethylene (LDPE) films. A weight loss of around 12% and 13% was achieved for PS and LDPE films respectively in eight weeks of treatment with Cephalosporium species. Fourier transform infrared spectroscopy analysis showed the formation of hydroxyl and carbonyl groups in nitric acid treated PS and LDPE films, respectively. Scanning electron microscopy indicated modifications in the surface morphology of PS and LDPE films after chemical and microbial treatment. An increase in crystallinity of pre-treated polymer samples was observed after fungal treatment. The observations of present study confirmed the enzymatic deterioration and assimilation of pre-treated PS and LDPE samples by the microbial species.

Impact of Melt Processing Conditions on the Degradation of Polylactic Acid

To reduce the degradation of polylactic acid (PLA) during processing, which reduces the molecular weight of PLA and its properties, prior studies have recommended low processing temperatures. In contrast, this work investigated the impact of four factors affecting shear heating (extruder type, screw configuration, screw speed, and feed rate) on the degradation of PLA. The polylactic acid was processed using a quad screw extruder (QSE) and a comparable twin screw extruder (TSE), two screw configurations, higher screw speeds, and several feed rates. The processed PLA was characterized by its rheological, thermal, and material composition properties. In both screw configurations, the QSE (which has a greater free volume) produced 3–4 °C increases in melt temperature when the screw speed was increased from 400 rpm to 1000 rpm, whereas the temperature rise was 24–25 °C in the TSE. PLA processed at low screw speeds, however, exhibited greater reductions in molecular weight—i.e., 9% in the QSE and 7% in the TSE. Screw configurations with fewer kneading blocks, and higher feed rates in the QSE, reduced degradation of PLA. At lower processing temperatures, it was found that an increase in melt temperature and shear rate did not significantly contribute to the degradation of PLA. Reducing the residence time during processing minimized the degradation of PLA in a molten state.

Manufacturing, characterization, and degradation of a poly(lactic acid) warp-knitted spacer fabric scaffold as a candidate for tissue engineering applications

Three-dimensional bioabsorbable textiles represent a novel technology for the manufacturing of tissue engineering scaffolds. In the present study, 3D bioabsorbable poly(lactic acid) (PLA) spacer fabric scaffolds are fabricated by warp-knitting and their potential for tissue engineering is explored in vitro. Changes in physical properties and mechanical performance with different heat setting treatments are assessed. To characterize the microenvironment experienced by cells in the scaffolds, yarn properties are investigated prior to, and during, hydrolytic degradation. The differences in yarn morphology, thermal properties, infrared spectra, and mechanical properties are investigated and monitored during temperature accelerated in vitro degradation tests in phosphate buffered saline (PBS) solution at 58 °C and pH 7.4 for 55 days. Yarn and textile cytocompatibility are tested to assess the effect of materials employed, manufacturing conditions, post processing and sterilization on cell viability, together with the cytocompatibility of the textile degradation products. Results show that the heat setting process can be used to modify scaffold properties, such as thickness, porosity, pore size and stiffness within the range useful for tissue regeneration. Scaffold degradation rate in physiological conditions is estimated by comparing yarn degradation data with PLA degradation data from literature. This will potentially allow the prediction of scaffold mechanical stability in the long term and thus its suitability for the remodelling of different tissues. Mouse calvaria preosteoblast MC3T3-E1 cells attachment and proliferation are observed on the scaffold over 12 days of in vitro culture by 4',6-diamidino-2-phenylindole (DAPI) fluorescent staining and DNA quantification. The present work shows the potential of spacer fabric scaffolds as a versatile and scalable scaffold fabrication technique, having the ability to create a microenvironment with appropriate physical, mechanical, and degradation properties for 3D tissue engineering. The high control and tunability of spacer fabric properties makes it a promising candidate for the regeneration of different tissues in patient-specific applications.

Biodegradation of poly(L-lactic acid) and poly(ε-caprolactone) patches by human amniotic fluid in an in-vitro simulated fetal environment

Open spina bifida or myelomeningocele (MMC) is a devastating neurologic congenital defect characterized by primary failure of neural tube closure of the spinal column during the embryologic period. Cerebrospinal fluid leak caused by the MMC spinal defect in the developing fetus can result in a constellation of encephalic anomalies that include hindbrain herniation and hydrocephalus. The exposure of extruded spinal cord to amniotic fluid also poses a significant risk for inducing partial or complete paralysis of the body parts beneath the spinal aperture by progressive spinal cord damage in-utero. A randomized trial demonstrated that prenatal repair by fetal surgery, sometimes using patches, to cover the exposed spinal cord with a watertight barrier is effective in reducing the postnatal neurologic morbidity as evidenced by decreased incidence and severity of postnatal hydrocephalus and the reduced need for ventricular-peritoneal shunting. Currently, the use of inert or collagen-based patches are associated with high costs and inadequate structural properties. Specifically, the inert patches do not degrade after implantation, causing the need for a post-natal removal surgery associated with trauma for the newborn. Our present study is aimed towards in-vitro degradation studies of a newly designed patch, which potentially can serve as a superior alternative to existing patches for MMC repair. This novel patch was fabricated by blending poly(l-lactic acid) and poly(ε-caprolactone). The 16-week degradation study in amniotic fluid was focused on tracking changes in crystallinity and mechanical properties. An additional set of designed patches was exposed to phosphate-buffered saline (PBS), as a time-paired control. Crystallinity studies indicate the progress of hydrolytic degradation of the patch in both media, with a preference to bulk erosion in phosphate buffered saline and surface erosion in amniotic fluid. Mechanical testing results establish that patch integrity is not compromised up to 16 weeks of exposure either to body fluids analog (PBS) or to amniotic fluid.

Melt-Spun, Cross-Section Modified Polycaprolactone Fibers for Use in Tendon and Ligament Tissue Engineering

Tissue Engineering is considered a promising route to address existing deficits of autografts and permanent synthetic prostheses for tendons and ligaments. However, the requirements placed on the scaffold material are manifold and include mechanical, biological and degradation-related aspects. In addition, scalable processes and FDA-approved materials should be applied to ensure the transfer into clinical practice. To accommodate these aspects, this work focuses on the high-scale fabrication of high-strength and highly oriented polycaprolactone (PCL) fibers with adjustable cross-sectional geometry and degradation kinetics applying melt spinning technology. Four different fiber cross-sections were investigated to account for potential functionalization and cell growth guidance. Mechanical properties and crystallinity were studied for a 24-week exposure to phosphate-buffered saline (PBS) at 37 °C. PCL fibers were further processed into scaffolds using multistage circular braiding with three different hierarchical structures. One structure was selected based on its morphology and scaled up in thickness to match the requirements for a human anterior cruciate ligament (ACL) replacement. Applying a broad range of draw ratios (up to DR9.25), high-strength PCL fibers with excellent tensile strength (up to 69 cN/tex) could be readily fabricated. The strength retention after 24 weeks in PBS at 37 °C was 83–93%. The following braiding procedure did not affect the scaffolds’ mechanical properties as long as the number of filaments and the braiding angle remained constant. Up-scaled PCL scaffolds resisted loads of up to 4353.88 ± 37.30 N, whilst matching the stiffness of the human ACL (111–396 N/mm). In conclusion, this work demonstrates the fabrication of highly oriented PCL fibers with excellent mechanical properties. The created fibers represent a promising building block that can be further processed into versatile textile implants for tissue engineering and regenerative medicine.

Strategies and progress in synthetic textile fiber biodegradability

Abstract

The serious issue of textile waste accumulation has raised attention on biodegradability as a possible route to support sustainable consumption of textile fibers. However, synthetic textile fibers that dominate the market, especially poly(ethylene terephthalate) (PET), resist biological degradation, creating environmental and waste management challenges. Because pure natural fibers, like cotton, both perform well for consumer textiles and generally meet certain standardized biodegradability criteria, inspiration from the mechanisms involved in natural biodegradability are leading to new discoveries and developments in biologically accelerated textile waste remediation for both natural and synthetic fibers. The objective of this review is to present a multidisciplinary perspective on the essential bio-chemo-physical requirements for textile materials to undergo biodegradation, taking into consideration the impact of environmental or waste management process conditions on biodegradability outcomes. Strategies and recent progress in enhancing synthetic textile fiber biodegradability are reviewed, with emphasis on performance and biodegradability behavior of poly(lactic acid) (PLA) as an alternative biobased, biodegradable apparel textile fiber, and on biological strategies for addressing PET waste, including industrial enzymatic hydrolysis to generate recyclable monomers. Notably, while pure PET fibers do not biodegrade within the timeline of any standardized conditions, recent developments with process intensification and engineered enzymes show that higher enzymatic recycling efficiency for PET polymer has been achieved compared to cellulosic materials. Furthermore, combined with alternative waste management practices, such as composting, anaerobic digestion and biocatalyzed industrial reprocessing, the development of synthetic/natural fiber blends and other strategies are creating opportunities for new biodegradable and recyclable textile fibers.

Article Highlights

How do the printing parameters of fused filament fabrication and structural voids influence the degradation of biodegradable devices?

Fused Filament Fabrication (FFF), a commonly used additive manufacturing technology, is now employed widely in biomedical fields for fabricating geometrically complex biodegradable devices. Structural voids arising from the printing process exist within the objects manufactured by FFF. This paper reveals the underlying mechanism of how the printing parameters and voids affect the degradation behaviours of devices made of biodegradable polyesters. It was found that both voids and internal architecture (layer height, for instance) affect the degradation rate by interacting with the reaction-diffusion process. Large suppression of the degradation rate was found when auto-catalytic hydrolysis and diffusion are significant. Degradation rate reduced in an approximately logarithmic manner as void size increased. The extent this effect depended on the strength of auto-catalytic hydrolysis and diffusion, void size and overall device size. The internal architecture of FFF products (regulated by printing parameters) influences the degradation rate by altering the diffusion speed of acid catalysts (regulated by diffusion path length). Both void size and internal architecture should be considered in fabricating biodegradable devices using FFF. STATEMENT OF SIGNIFICANCE: A geometric model that relates printing parameters with voids of FFF is developed to characterise the structure of FFF components. Such a model, when coupled with a degradation model, offers end-to-end simulation capability (e.g. from printing parameters to degradation rate) for predicting degradation properties. The model is validated against the in vitro degradation data obtained in this study. To our knowledge, the impact of printing parameters and voids on degradation is investigated here for the first time. It is found that both the void size and the internal architecture determined by the printing parameters play an essential role in regulating degradation behaviours.

Enhanced Mechanical Stability and Biodegradability of Ti-Infiltrated Polylactide

Biodegradable polymers have been often used in place of conventional nondegradable polymers for industrial and medical applications. In particular, polylactide (PLA) has been regarded as a popular ecofriendly plastic and has many advantages like good biocompatibility and processability. Yet, it still has some drawbacks in mechanical properties. Here, we prepared Ti-infiltrated PLA by mimicking the gelatinous jaw of a seaworm whose mechanical properties are toggled up and down by the tiny amount of metal ions, expecting to prepare a new type of alternative. Ti induced significant chemical and microstructural changes in the PLA, which led to a notable improvement in the mechanical properties as compared to the neat PLA. The Ti-infiltrated PLA exhibited high resistance to rapid degradation. More importantly, the toxicity assessment demonstrated that the resulting PLA is still biocompatible and nontoxic. Consequently, we proved that the Ti-infiltrated PLA has high mechanical properties comparable to conventional nondegradable polymers and good biocompatibility as well as delayed biodegradability. We anticipate the current Ti-infiltrated PLA to be an ecofriendly replacement of some conventional plastics, which helps preserve a green environment.

A model of polymer degradation and erosion for finite element analysis of bioresorbable implants

Finite element analysis is a powerful tool for the design of bioresorbable medical implants made of aliphatic polyesters such as bioresorbable vascular scaffolds. However polymer erosion has been traditionally modelled using empirical rules rather than differential equations. The rule-based models are difficult to implement in a finite element analysis. Consequently, these models have been limited to simple geometries such as plates or spheres. This paper presents a set of differential equations that govern the hydrolytic chain scission and bulk erosion of bioresorbable implants where polymer erosion is modelled using a differential equation instead of empirical rules. These differential equations can be conveniently solved using a commercial finite element package to calculate the molecular weight and mass loss as functions of time for bioresorbable implant made of aliphatic polyesters. A case study of Absorb bioresorbable vascular scaffolds (BVSs) is presented using data obtained from the literature, where 98 Absorb BVSs were implanted in 40 porcine coronary arteries. It is demonstrated that the finite element model can fit the data of both molecular weight and mass loss as functions of time to an accuracy of approximately 5%. The finite element model and the back-calculated model parameters can be used to design future implants that degrade in a controlled pattern with required mechanical performance.

The Effect of Natural Additives on the Composting Properties of Aliphatic Polyesters

Compounds of plant origin are used with polymers as functional additives. However, these substances often have biological (antimicrobial) activity. The bactericidal and fungicidal properties of natural additives can affect the composting process of biodegradable polymers. The scientific novelty of the manuscript is the investigation of the effect of the addition of herbal antimicrobial functional substances on the composting process of green polymers. The aim of the study is to analyze composting processes of biodegradable polymers polylactide (PLA) and polyhydroxyalkanoate (PHA) containing β-carotene, juglone, morin, and curcumin. As part of the research, six-month composting of materials was performed. At time intervals of one month, the weight loss of samples, surface energy, colour change, mechanical properties, and carbonyl indices (based on FTIR spectroscopy) of composted materials were examined. The research results showed that the addition of selected plant substances slightly slowed down the process of polymer composting. Slower degradation of samples with plant additives was confirmed by the results of mechanical strength tests and the analysis of changes in carbonyl index (CI). The CI analysis showed that PLA and PHA containing a natural additive degrade a month later than reference samples. However, PLA and PHA polyesters with β-carotene, juglone, morin, and curcumin were still very biodegradable.

Conversion of Waste Plastic to Oils for Tribological Applications

Plastics are widely used owing to their light weight, easy production, and low cost. Even though plastics find application in different fields of industries and households, they do not degrade easily. If plastics are not disposed of appropriately, it has been shown that they cause widespread environmental pollution, which poses risks to human health. Recycling waste plastics has been an alternative to mitigating plastic pollution, which usually requires high labour costs and produces contaminated water during processing. If plastic recycling will contribute to the development of tribological products like lubricating oils, it is a safer alternative to disposing of plastics in the environment. In order to understand the tribological use of plastics by recycling, the present study reviews different techniques that can be employed to transform waste plastics into petroleum-based oils. The viscosity, density, and friction of pyrolyzed waste plastic oils are investigated and compared with commercial lubricants to assess their potential lubrication applications. The segregation processes, catalytic isomerization dewaxing, and Fischer–Tropsch method to recycle waste plastics are also reviewed to provide an insight into the methods to transform pyrolyzed waste plastic into lubricants.

Improved Oxidative Biostability of Porous Shape Memory Polymers by Substituting Triethanolamine for Glycerol

While many aromatic polyurethane systems suffer from poor hydrolytic stability, more recently proposed aliphatic systems are oxidatively-labile. The use of the renewable monomer glycerol as a more oxidatively-resistant moiety for inclusion in shape memory polymers (SMPs) is demonstrated here. Glycerol-containing SMPs and the amino alcohol control compositions are compared, with accelerated degradation testing displaying increased stability (time to complete mass loss) as a result of the inclusion of glycerol without sacrificing the shape memory, thermal transitions, or the ultralow density achieved with the control compositions. Gravimetric analysis in accelerated oxidative solution indicates that the control will undergo complete mass loss by approximately 18 days, while lower concentrations of glycerol will degrade fully by 30 days and higher concentrations will possess approximately 40% mass at the same time. In real time degradation analysis, high concentrations of glycerol SMPs have 96% mass remaining at 8 months with 88% gel fraction remaining that that time, compared to less than 50% mass for the control samples with 5% gelation. Mechanically, low glycerol-containing SMPs were not robust enough for testing at three months, while high glycerol concentrations displayed increased elastic moduli (133% of virgin materials) and 18% decreased strain to failure. The role of the secondary alcohol, as well as isocyanates, is presented as being a crucial component in controlling degradation; a free secondary alcohol can more rapidly undergo oxidation or dehydration to ultimately yield carboxylic acids, aldehydes, carbon dioxide, and alkenes. Understanding these pathways will improve the utility of medical devices through more precise control of property loss and patient risk management through reduced degradation.

Modeling of Biodegradable Polyesters With Applications to Coronary Stents

The interest in biodegradable polymers for clinical and biomedical engineering applications has seen a dramatic increase in the last 10 years. Recent innovations include bioresorbable polymeric stents (BPS), which are temporary vascular scaffolds designed to restore patency and provide short-term support to a blocked blood vessel, before becoming naturally resorbed over time. BPS offer possibilities to overcome the long-term complications often observed with the permanent metallic stents, well established in the treatment of coronary and peripheral artery disease. From the perspective of designing next generation BPS, the bulk degradation behavior of the polymer material adds considerable complications. Computational modeling offers an efficient framework to predict and provide understanding into the behavior of medical devices and implants. Current computational modeling techniques for the degradation of BPS are either phenomenologically or physically based. In this work, a physically based polymer degradation model is implemented into a number of different computational frameworks to investigate the degradation of a number of polymeric structures. A thermal analogy is presented to implement the degradation model into the commercially available finite-element code, abaqus/standard. This approach is then applied to the degradation of BPS, and the effects of material, boundary condition, and design on the degradation rates of the stents are examined. The results indicate that there is a notable difference in the molecular weight trends predicted for the different materials and boundary condition assumptions investigated, with autocatalysis emerging as a dominant mechanism controlling the degradation behavior. Insights into the scaffolding ability of the various BPS examined are then obtained using a suggested general relationship between Young's modulus and molecular weight.

A multi-scale method for modeling degradation of bioresorbable polyesters

A multi-scale model using the cellular automata (CA) and kinetic Monte Carlo (KMC) methods is presented to simulate the degradation process of bioresorbable polyesters such as polylactide (PLA), polyglycolide (PGA) and their copolymers. The model considers the underlying chemical and physical events such as polymer chain scission, oligomer production, crystallization induced by polymer chain scissions, oligomer diffusion and microstructure evolution due to erosion of the small chains. A macroscopic device is discretized into an array of mesoscopic cells. Each cellular lattice is assumed to be made of one polymer chain, which undergoes hydrolysis reaction. The polymer chain scission is modeled using a kinetic Monte Carlo method. Oligomer production, chain crystallization and formation of cavities due to polymer collapse are also modeled on the cellular lattice. Oligomer diffusion is modeled by using Fick's laws at the macroscopic scale. The diffusion coefficient is taken as dependent on the porosity caused by the formation of the cavities. The interactions among the microscopic hydrolysis reaction, mesoscopic formation of cavities and macroscopic diffusion are taken into account. The proposed method forms Multi Scale Cellular Monte Carlo Automata (MS-CMCA). The three-scale approach consists of continuous method and discrete method to deal with certainty problem with underlying stochastic phenomenon. Demonstration examples are provided which show that the model can fit with experimental data in the literature very well.

STATEMENT OF SIGNIFICANCE

The original work in this paper is a multi-scale method (including micro scale, mesoscopic scale, macro scale and their coupling) for modeling degradation of bioresorbable polyesters and provides understanding to the process of degradation of biodegradable polymers. The result denotes the solution is reliable. As we know, there have no papers recently to implement three scales modeling and its coupling. There is a two-scale model of amorphous polyester degradation described by Han and Pan (Acta Biomaterialia 2011), our model accounts for effects of re-crystallization to explain the degradation process from three scales and takes into account of copolymers. From our model, the molecular weight distribution with time, chain number with time, degree of crystallinity with time, the evolution of polymer inner shape, weight loss with time (which is found from calculation that both oligomer diffusion and small molecules solution work to the weight loss) can be obtained from the calculation of the three scale model.

A constitutive law for degrading bioresorbable polymers

This paper presents a constitutive law that predicts the changes in elastic moduli, Poisson's ratio and ultimate tensile strength of bioresorbable polymers due to biodegradation. During biodegradation, long polymer chains are cleaved by hydrolysis reaction. For semi-crystalline polymers, the chain scissions also lead to crystallisation. Treating each scission as a cavity and each new crystal as a solid inclusion, a degrading semi-crystalline polymer can be modelled as a continuum solid containing randomly distributed cavities and crystal inclusions. The effective elastic properties of a degrading polymer are calculated using existing theories for such solid and the tensile strength of the degrading polymer is predicted using scaling relations that were developed for porous materials. The theoretical model for elastic properties and the scaling law for strength form a complete constitutive relation for the degrading polymers. It is shown that the constitutive law can capture the trend of the experimental data in the literature for a range of biodegradable polymers fairly well.

Anaerobic digestion of aliphatic polyesters

Anaerobic processes for the treatment of plastic materials waste represent versatile and effective approach in environmental protection and solid waste management. In this work, anaerobic biodegradability of model aliphatic polyesters, poly(L-lactic acid) (PLA), and poly(ɛ-caprolactone) (PCL), in the form of powder and melt-pressed films with varying molar mass, was studied. Biogas production was explored in batch laboratory trials at 55 ± 1°C under a nitrogen atmosphere. The inoculum used was thermophilic digested sludge (total solids concentration of 2.9%) from operating digesters at the Central Waste Water Treatment Plant in Prague, Czech Republic. Methanogenic biodegradation of PCLs typically yielded from 54 to 60% of the theoretical biogas yield. The biodegradability of PLAs achieved from 56 to 84% of the theoretical value. High biogas yield (up to 677 mL/g TS) with high methane content (more than 60%), comparable with conventionally processed materials, confirmed the potential of polyester samples for anaerobic treatment in the case of their exploitation in agriculture or as a packaging material in the food industry.

Degradation mechanisms of bioresorbable polyesters. Part 1. Effects of random scission, end scission and autocatalysis

A mathematical model was developed to relate the degradation trend of bioresorbable polymers to different underlying hydrolysis mechanisms, including noncatalytic random scission, autocatalytic random scission, noncatalytic end scission or autocatalytic end scission. The effect of each mechanism on molecular weight degradation and potential mass loss was analysed. A simple scheme was developed to identify the most likely hydrolysis mechanism based on experimental data. The scheme was first demonstrated using case studies, then used to evaluate data collected from 31 publications in the literature to identify the dominant hydrolysis mechanisms for typical biodegradable polymers. The analysis showed that most of the experimental data indicates autocatalytic hydrolysis, as expected. However, the study shows that the existing understanding on whether random or end scission controls degradation is inappropriate. It was revealed that pure end scission cannot explain the observed trend in molecular weight reduction because end scission would be too slow to reduce the average molecular weight. On the other hand, pure random scission cannot explain the observed trend in mass loss because too few oligomers would be available to diffuse out of a device. It is concluded that the chain ends are more susceptible to cleavage, which produces most of the oligomers leading to mass loss. However, it is random scission that dominates the reduction in molecular weight.

Degradation mechanisms of bioresorbable polyesters. Part 2. Effects of initial molecular weight and residual monomer

This paper presents an understanding of how initial molecular weight and initial monomer fraction affect the degradation of bioresorbable polymers in terms of the underlying hydrolysis mechanisms. A mathematical model was used to analyse the effects of initial molecular weight for various hydrolysis mechanisms including noncatalytic random scission, autocatalytic random scission, noncatalytic end scission or autocatalytic end scission. Different behaviours were identified to relate initial molecular weight to the molecular weight half-life and to the time until the onset of mass loss. The behaviours were validated by fitting the model to experimental data for molecular weight reduction and mass loss of samples with different initial molecular weights. Several publications that consider initial molecular weight were reviewed. The effect of residual monomer on degradation was also analysed, and shown to accelerate the reduction of molecular weight and mass loss. An inverse square root law relationship was found between molecular weight half-life and initial monomer fraction for autocatalytic hydrolysis. The relationship was tested by fitting the model to experimental data with various residual monomer contents.