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سال هفتم شماره 3 (پیاپی 24، پاییز 1396)

Polyurethanes and their counterparts, polyurethane-ureas (PUUs), a favorable group of polymers, are prepared from a wide variety of materials. PUs/PUUs offer a wide range of controllable properties and therefore, are known as good candidates for a broad range of applications. Among the very broad field of applications known for this group of polymers, biomedical devices and prosthesis, drug delivery vehicles and tissue engineering scaffolds have attracted most attention over recent decades. This study systematically reviews the current literature for biomedical applications of polyurethanes (PUs) and polyurethane-ureas. On the other hand, in addition to different qualities that are required for successful performance of a biomaterial (including biocompatibility, bio-stability and/ or biodegradability, hydrophilicity and/or hydrophobicity, adjustable mechanical properties, etc), antibacterial behavior is considered as an inevitable prerequisite in many applications. Therefore, the especial emphasis of this review paper is placed on polymers with antibacterial activity and the strategies towards preparation of antibacterial PUs and PUUs, in particular. Current strategies applied for preparation of antibacterial polyurethanes and polyurethane-ureas are reviewed. Among such approaches, adoption of nano-silver, blending with natural polymers of well-known inherent antibacterial activity such as chitosan, loading of antibacterial drugs and surface modification with antibacterial agents are discussed in details.

Linear low density polyethylene (LLDPE) is a linear polyethylene with considerable number of short branches that is produced by the copolymerization of ethylene with α-olefins such as 1-butene, 1-hexene and 1-octene. LLDPE has higher tensile strength and higher impact resistance than LDPE. It is very flexible and elongates under stress. It can be used to make thinner films, with better environmental stress cracking resistance. It is an important family of polyolefins, which has been widely used as thermoplastic polymers because of their combined stiffness and stress-crack resistance. Commonly, LLDPE is produced through a two-stage process including comonomer and copolymer formation in two separate reactors; though an alternative LLDPE preparation method that has attracted good attention among academic and industrial producers is tandem polymerization where one catalyst produces α-olefin in situ and another copolymerizes ethylene and α-olefin monomers. Compared to the commonly used two-stage processes, single-stage approach has a clear advantage in plant investment, α-olefin purification, storage, transport and reducing operational unit. Furthermore in tandem systems, the reactivity of the active sites must be well matched so that the product of one cycle does not overwhelm the overall tandem sequence. In this paper, a historical overview is provided on the developments in LLDPE production through tandem polymerization of ethylene.

The main aim in drug delivery systems, is to increase drug bioavailability at specific time and location of the body, the ability to maintain a relatively constant drug concentration in the specific period of time, adjustable release rate of the drug, the ability to deliver multiple drug substances, increasing the efficiency and reducing the side effects on the other non-diseased host tissues. Some therapeutic agents such as proteins/peptides, nucleic acids, anti-carcinogens, and other drugs, used nowadays, may have the drawbacks of low bioavailability, rapid clearance, high toxicity and very harmful side effects. The invention of drug delivery and drug carriers has created a revolution in the treatment of many diseases which is increasing in progress. Phospholipids are compounds with bipolar nature due to the presence of phosphate group, a hydrophilic head and two lipophilic branches. Phospholipids can carry hydrophilic and hydrophobic pharmaceutical compounds in a structure compatible with living cells and serve their penetration in living cells and into the target cell. Phospholipids as surface-active wetting agents cover the surface of crystals to enhance the hydrophilicity of hydrophobic drugs and higher efficiency in drug delivery. In this paper, studies are extended to different kinds of phospholipids, their structures, their sources, physical properties, their complexes with drugs, phospholipid micelles and effective factors in their selection for drug delivery systems.

Water soluble polymers as mobilitycontrol agents in the oil recovery process have attracted great attention in recent years. The use of polymers leads to an improved mobility in the oil reservoir by increasing the viscosity of the injected fluid (water) and by reducing the permeability through adsorption of the polymer chains on the surface of the rock which can improve sweep efficiency during enhanced oil recovery processes. Partially hydrolyzed polyacrylamide (HPAM) is the most widely used polymer to date for polymer flooding in EOR. The limitations of HPAM include, among others, the low resistance toward the presence of salts. Salts will lead to a significant reduction of solution viscosity and even the precipitation upon interaction with divalent ions. In addition to HPMA, biopolymers such as xanthan gum have been used for EOR. The drawbacks of biopolymers originate from high cost, high susceptibility to biodegradation and potential for injectivity problems due to cellular debris remaining from the manufacturing process. To address these problems, hydrophobically modified polymers (HMPs) have been widely studied as flooding agents in EOR over the past two decades.In this review, some examples of synthetic polymers, biopolymers and especially HMPs for enhanced oil recovery applications are discussed along with their limitations.

In recent decades, the "GelMA" hydrogels as one of the biocompatible and biodegradable biomaterials are introduced in various applications of biomedical engineering. GelMA results from direct reaction of gelatin and methacrylic anhydride which has specific biological and physical properties making it suitable for the design and engineering of scaffolds, creating micro or nanoscale polymer nanocomposites, cell signaling, designing drug delivery systems, biosensors, and gene transfer or other biomedical engineering applications. GelMA forms cross-linked hydrogel by exposure to ultra-violet radiation. Various techniques could be applied in designing and manufacturing of GelMA in micro size, such as photopatterning, micromolding, self-assembly phenomenon, microfluidic, bioprinting, fibers and fabrics weaving. Three-dimensional structures and scaffolds based on GelMA hydrogel could be designed to mimic the structure of the natural tissue, used in tissue engineering and regeneration medicine. However, in this case, there are some challenges such as different length scales, making copies of capillary hollow microcapillaries, angiogenic production in micro size scale and limitations in oxygen-carrying through centimeter dimension, need to be investigated further. Using the combined methods of fabrication and exact investigations on the effect of process parameters and introduction of new additives could be the part of the solution. GelMA capabilities for use in various manufacturing methods, besides, its physical flexibility, mechanical and biological properties are promising for future biomedical applications and producing self-assembled organs with different types of cells.

In patients suffering from peripheral arterial disease; where vessels narrow and/or lose their efficiency and kidney failure; where hemodialysis is performed through an arteriovenous (AV) fistula that connects an artery to a vein, to purify blood, three types of surgical treatment; namely angioplasty, endarterectomy, and bypass grafting are vigorously considered. In cases like acute artery stenosis and multi-focal stenosis, a bypass is generally used. In addition, burns can damage blood vessels and cause fluid loss. This may result in low blood volume (hypovolemia) and in this case a bypass graft surgery is inevitable. However, in some cases, patients lack appropriate vessels for autologous grafting (autologous grafting includes grafting of a tissue from one site to another site of the same body). Furthermore, in autologous transplantation, a patient undergoes two surgeries simultaneously. In this respect, researchers have focused on designing artificial blood vessels as vascular implants. A class of materials that is highly regarded promising is polyurethanes, due to a number of outstanding properties including blood compatibility, biocompatibility, and most importantly, capability to tailor desirable properties. This report focuses on application of polyurethanes as artificial blood vessels while the impact of key parameters such as design of the polyurethane backbone, surface modification, and bulk modification, on the polymer key properties including: toxicity, endothelialization, and platelets adhesion are reviewed

In the recent decades, many articles have been published on the development of lignin and mostly related to the chemical modification of lignin.The direct use of lignin often leads to the synthesis and production of low-value added materials. Hence, chemical modification of lignin not only increases its activity in the reactions but also represents a fine dispersion in the polymer materials. Chemical structure of lignin contains phenyl propane units, originating from three aromatic alcohol precursors of p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The main uses of lignin can be classified into two different methods. In the first method, without chemical modification, lignin is directly incorporated into a polymer matrix to give new or improved properties. In the second method, a large range of polymer materials is synthesized via chemical modification of the lignin. Chemical modification of the lignin has been studied frequently for various purposes. Lignin modification through reaction with formaldehyde, epichlorohydrin, phenol, propylene oxide, propylene carbonate, polyethylene glycol and synthesis of lignin-based copolymers are among reactions studied in the literature. However, this article has been restricted to the discussion on chemical modification of lignin and its use as a source of monomer in the polymerization process.

Shape memory polymers been developed in the past years as a valid alternative to more traditional shape-memory materials. Shape memory polymers belong to a class of very smart materials that have the ability to remember their original shape. This advanced functionality makes shape memory polymer suitable and promising materials for diverse technological applications, including sensors and actuators particularly including the fabrication of smart biomedical devices. The polymers deform into a temporary shape and returns to its original shape by external environmental stimuli such as chemicals, temperature or pH. Therefore, a temperature-sensitive shape memory polymer is one that undergoes a structural change at a certain temperature called the transition temperature. A change in shape caused by a change in temperature is called a thermally-induced shape memory effect. Shape-memory research was initially founded on the thermally-induced dual-shape effect. This concept has been extended to other stimuli by either indirect thermal actuation or direct actuation by addressing stimuli-sensitive groups on the molecular level. This paper is intended to serve as a brief review of key concepts associated with shape memory material. This review describes the fundamental aspects of molecular design of suitable polymer architectures, tailored programming and recovery processes, with the focus being on the structure of thermally sensitive shape memory polymers.

Recently, polymers have shown great potential in biological science. Natural polymers including polysaccharides with bacterial, animal and fungal sources are a good candidate to mimic structure of biological materials such as extracellular matrices. Therefore, they have gained much attentionin therapeutic and tissue engineering applications. Polysaccharides among all natural polymers show promising potential for preparation of nanostructured carriers. Nowadays, nanofilms, nanoparticles, and nanofibers are well known carriers for drug delivery and tissue engineering applications. Gum tragacanth is a natural polymer and a complex carbohydrate including polysaccharide structure. It comprises excellent physical, chemical and biological properties such as thermal and mechanical behavior; biodegradability, biocompatibility, and antimicrobial effect on wound healing and burn infections. Previously, raw gum tragacanth used to be applied locally as a superabsorbent hydrogels, antibacterial nanocapsuls, and mucilage for would healing treatment and deep wound scar. In recent years nanofibers have shown potential in tissue scaffold and mats for delivery of therapeutic drugs. Researchers have developed methods to engineer nanostructured fibers and tuned physical parameters such as diameter, tensile modulus, and degradation properties. In this review some of the recent works on gum tragacanth nanofibers and incorporation with other polymeric materials are discussed. Antimicrobial and would healing characteristics of gum tragacanth is being highlighted.

During the last decades, multinuclear catalysts have captured the researcher's attention and also have shown impressive progress. The multinuclear catalysts such as metallocene-based complexes can be mono (homo) or multi (hetero) metallic(s). Albeit, reason and mechanism of the results from olefin polymerization and copolymerization in the presence of these catalysts are explained suggestively but a number of practical-theoretical studies confirmed cooperative effects between centers in relation to the structures. Among this class of catalysts, the sort of center and linkage are the main factors in comparison to their mononuclear analogs. Totally, the sort of catalyst center (active site), steric hindrance, electronic effects and inter-center distances can cause a variation in performance and catalyst behavior, producing polyolefin with different microstructure and properties. Moreover, such designed multinuclear structures exhibit the unprecedented levels of polyolefin branching, enhanced enchainment selectivity for functional and unfunctional α-olefin comonomers, enhanced polyolefin tacticity and molecular weight, modified chain transfer kinetics and LLDPE synthesis with a single binuclear catalyst and ethylene. In the following, diverse results of multinuclear metallocene-based catalysts are reported such as increasing or decreasing catalyst activity, average molecular weight, molecular weight distribution, stereospecific index, selectivity, content and type of branching or even maintaining catalytic and polymeric characteristics.