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Coronary artery stenting
Published in Debabrata Mukherjee, Eric R. Bates, Marco Roffi, Richard A. Lange, David J. Moliterno, Nadia M. Whitehead, Cardiovascular Catheterization and Intervention, 2017
Raffaele Piccolo, Stephan Windecker
Polymers have been pivotal for the development of local drug delivery, and in particular DES. Polymeric materials act as a drug reservoir and allow for controlled drug release over time. The drug may be dissolved either in a reservoir surrounded by a polymer film or within a polymeric matrix. Controlled drug release can occur by diffusion, chemical reaction, or solvent activation. Biodegradable polymers allow drug release by both drug diffusion and matrix degradation, whereas nondegradable polymers enable drug release by particle dissolution.24 Early efforts to identify suitable polymers for stent coating were characterized by exuberant inflammatory and thrombotic responses, resulting in excessive neointimal hyperplasia and arterial occlusion.25 These adverse effects have been attributed in part to inappropriate polymer degradation and the molecular weight of the compounds. More recently, a wide variety of biocompatible polymers, some of which trigger no or minimal inflammatory response, have been developed as carriers for DES. Furthermore, some stents have only an abluminal polymer coating (asymmetric coating) (Figure 32.4). The investigation of new drugs for local delivery therefore mandates addressing not only the drug itself but also the biocompatibility of the polymeric carrier.
-Glutamic Acid): Efficient Carrier of Cancer Therapeutics and Diagnostics
Published in Mansoor M. Amiji, Nanotechnology for Cancer Therapy, 2006
Guodong Zhang, Edward F. Jackson, Sidney Wallace, Chun Li
In an effort to increase the rate of polymer degradation, Lu et al.84 prepared PG–cystamine– [Gd(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)] (molecular weight of PG, 50,000 Da) where the metal chelator DOTA was conjugated with PG using a disulfide bond. They showed that glutathione and other endogenous sulfhydryl-containing biomolecules could exchange their free SH group with an S–S bond in PG–cystamine–[Gd(III)–DOTA], resulting in rapid release of Gd(III)–DOTA from the polymer and subsequent clearance of Gd-containing species from the body. Use of PG–cystamine–[Gd(III)–DOTA] produced significant blood-pool contrast enhancement on MRI scans of the heart and blood vessels in nude mice bearing OVCAR-3 human ovarian carcinoma xenograft when compared with use of small-molecular-weight contrast agents.
Elements of Polymer Science
Published in E. Desmond Goddard, James V. Gruber, Principles of Polymer Science and Technology in Cosmetics and Personal Care, 1999
E. Desmond Goddard, James V. Gruber
Esters, amides, or nitriles pendant groups can be converted to the corresponding carboxylic acids. Poly(carboxylic acids) may be dehydrated upon heating to poly(carboxylic acid anhydrides). Phenyl pendant groups, such as those in polystyrene, undergo the characteristic reactions of aromatic rings, such as alkylation, halogenation, or sulfonation. In all cases, competing polymer degradation may take place. It is important to assess whether the molecular weight of the polymer is affected by the chemical transformation. DISTRIBUTION OF MOLECULAR WEIGHTS
Synthetic biodegradable polyesters for implantable controlled-release devices
Published in Expert Opinion on Drug Delivery, 2022
Jinal U. Pothupitiya, Christy Zheng, W. Mark Saltzman
Polymer degradation in implants occurs through a combination of events that include hydrolysis, oxidation, enzymatic degradation, and physical degradation. For most polyesters, physical changes to the matrix occur due to the penetration of water, which can cause matrix swelling. Water penetration induces hydrolysis of the polyester backbones, leading to polymer degradation and drug release. The pH of the implant environment influences the rate of polymer degradation and therefore the drug release profile. In general, strongly acidic and alkaline environments enhance the degradation process of polyesters [79,102]. For most aliphatic polyesters, such as PLA, PCL, and polyhydroxyalkanoates (PHA), polymer degradation can also be influenced by enzymatic hydrolysis by esterases [103]. The extent of enzymatic activity on the polyesters greatly depends on substrate specificity [104,105], flexibility of polymer chains [106], and hydrophilicity of the aliphatic polymer [107,108]. Additionally, polymer inherent properties such as pH sensitivity, molecular weight, polymer architecture, crystallinity, and hydrophilicity influence polymer degradation significantly [2], often by controlling the spatial or temporal pattern of water uptake.
An overview of PLGA in-situ forming implants based on solvent exchange technique: effect of formulation components and characterization
Published in Pharmaceutical Development and Technology, 2021
Tarek Metwally Ibrahim, Nagia Ahmed El-Megrab, Hanan Mohammed El-Nahas
Polymer degradation is the process of hydrolytic cleavage of polymeric chains that are cut into oligomers and then into monomers. The prerequisite for polymeric erosion is the degradation of polymeric implant mass. This can be induced by releasing the water-soluble degradation products from the PLGA matrix after the hydrolysis process (Vhora et al. 2021). Lactic and glycolic acids are examples of monomeric end products that are easily eliminated by the Krebs cycle (Elsawy et al. 2017). Moreover, the water can penetrate the polymer rapidly than the polymer bonds are degraded. Therefore, the polymer is hydrolyzed over the whole matrix homogeneously forming the degradation products, and then the bulk erosion occurs. These degradation products contain carboxylic chain ends that can autocatalyze the ester bond hydrolysis and then accelerate the degradation of the whole polymeric matrix (Kamaly et al. 2016). Amini-Fazl (2021) studied the relationship between the pH, amount of released monomers and amount of paclitaxel released during the degradation stage of PLGA-ISFI systems. The pH of the system was reduced as a result of acidic products released during the degradation, consequently increasing the drug release.
The differences between surface degradation and bulk degradation of FEM on the prediction of the degradation time for poly (lactic-co-glycolic acid) stent
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2022
Xianda Yang, Weirong Zhang, Jie Yao, Anqiang Sun, Yuanming Gao, Meng Guo, Yubo Fan
Previous studies have shown that the external environment, where the polymer degradation occurs, influences the degradation process greatly (Deng and Uhrich 2002; Deng et al. 2005; Zolnik and Burgess 2007; Deng et al. 2008). Mechanical loads in the external environment are found to be a critical parameter affecting the degradation process (Fan et al. 2008; Yang et al. 2008; Li et al. 2010; Chu et al. 2016, 2017). It has been demonstrated that loading could accelerate the degradation rate of poly (glycolide-co-l-lactide) (PDLLA) (Fan et al. 2008). In particular, dynamic loading results in a significant degradation rate than that under static loading (Yang et al. 2008). Moreover, different thresholds of tensile loading result in different degradation rates (Guo et al. 2016). Limited theoretical studies have included the effect of mechanical loading on BDS degradation. The inclusion of mechanical load as a parameter in the degradation model is necessary to address the deficiency of previous models and to improve the reliability of theoretical prediction for BDS degradation. The small size, structure complexity, and high cost to manufacture BDS limit the use of experiments to study the degradation behavior of BDS. Theoretical models and especially finite element method (FEM) have been extensively used for structural analysis and demonstrated to be an efficient tool to analyze the BDS degradation process (Soares et al. 2008; Winzer et al. 2008; Fan et al. 2017; Gao et al. 2019). The degradation mechanism of PLA co-polymer is shown to be hydrolysis, a bulk degradation mode is commonly used in FEM to describe the degradation behavior of polymers (Vieira et al. 2014; Pauck and Reddy 2015).