Synthetic Elastomers Research Articles

Current trends in bio‐based elastomer materials

AbstractElastomers play an irreplaceable role in our society due to their unique properties. Natural rubber is directly obtained from plants and is widely used in tires, shoes, etc. Recently, modified natural rubbers are proposed to expand the application of natural rubber. However, these natural rubbers have a limited variety of molecular structures and may not be able to meet ever‐demanding applications. Traditional synthetic elastomers have a variety of molecular structures and their properties are used in various fields, but mainly originate from fossil resources. This review deals with bio‐based elastomers, and more specifically natural rubber and bio‐based synthetic elastomers. Based on reprocessability, bio‐based elastomers can also be divided into bio‐based chemically cross‐linked ones and thermoplastic ones. Compared to traditional fossil‐based elastomers, bio‐based ones may alleviate environmental pollution and promote the sustainable development of the elastomer industry.

Stabilizing and improving elastic bioengineered scaffolds mimicking extracellular matrix for use in wound repair and regeneration

AbstractElastic composite scaffolds are used to mimic extracellular matrix in tissue regeneration. They are composed of synthetic elastomers in favor of natural elastin. Many elastomers require thermal polymerization for fiber stabilization. If the scaffold contains protein, additional treatments are required for stabilization. These treatments may modify the structure and function of the scaffold. In the present study a protein‐elastomeric polymer composite of poly (glycerol sebacate), (PGS), silk fibroin, and type I collagen (termed PFC) was used as a model material to investigate a new method to improve stabilization of electrospun fibers. The purpose of this study was to optimize conditions in order to reduce internal flow of an elastomeric prepolymer during fiber fabrication. Co‐electrospun sacrificial poly (ethylene oxide) (PEO) was used to provide scaffold fiber stabilization during fiber formation in order to test different protocols to prevent fiber fusion. Using PEO at a 1:1 ratio with PFC followed by glutaraldehyde treatment, removal of PEO and heat treatment to polymerize PGS resulted in the most stable and consistent fiber morphology. Functionally, scaffolds had increased porosity and improved cell infiltration. The study provides an improved procedure for fabrication of composite electrospun scaffolds requiring stabilization by both chemical and thermal methods.

Topology reset, reshuffling, and reconstruction of synthetic elastomers

Polymers with three-dimensional network structures are practically important. Owing to their insoluble and infusible properties, network polymers have high thermal and chemical resistance. However, such a durable nature discourages recovery, recycling, and property modification. Hence, there has been a demand for the realization of a methodology that enables modification of the properties of network polymers anytime and anywhere. Herein, we report topology-reset, -reshuffling, and -reconstruction (termed T-rex3) for modulating the properties of poly(dimethyl siloxane) networks with reversibly photocleavable chains. The T-rex3 methodology is environmentally friendly and enables facile recovery, recycling, and modulation of viscoelasticity simply by mixing network and cyclic polymers and following photo-irradiation. With benefits such as wide range tunability and dramatic ON–OFF switchability of viscoelasticity, the T-rex3 developed here opens a new avenue for a variety of applications, such as adhesives, rubbers, in vitro tissue models, and materials for 3D modeling.

Simultaneously Toughening and Stiffening Elastomers with Octuple Hydrogen Bonding.

Current synthetic elastomers suffer from the well-known trade-off between toughness and stiffness. By a combination of multiscale experiments and atomistic simulations, a transparent unfilled elastomer with simultaneously enhanced toughness and stiffness is demonstrated. The designed elastomer comprises homogeneous networks with ultrastrong, reversible, and sacrificial octuple hydrogen bonding (HB), which evenly distribute the stress to each polymer chain during loading, thus enhancing stretchability and delaying fracture. Strong HBs and corresponding nanodomains enhance the stiffness by restricting the network mobility, and at the same time improve the toughness by dissipating energy during the transformation between different configurations. In addition, the stiffness mismatch between the hard HB domain and the soft poly(dimethylsiloxane)-rich phase promotes crack deflection and branching, which can further dissipate energy and alleviate local stress. These cooperative mechanisms endow the elastomer with both high fracture toughness (17016 J m-2 ) and high Young's modulus (14.7MPa), circumventing the trade-off between toughness and stiffness. This work is expected to impact many fields of engineering requiring elastomers with unprecedented mechanical performance.

Phenomenological Analysis of the Cured Kinetics in the Order n of the Reaction as a Function of the Temperature of an EPDM Polymer

Abstract The fabrication of rubber products involves a complex chemical process, which is invariably assisted by temperature and pressure. Obviously, the curing process specifications have a strong impact on the quality and mechanical properties of the final product. Mechanistic kinetic models and phenomenological or empirical models have been used to describe the cure behavior of rubber. The empirical models ignore the chemical details and fit the data to a mathematic functional form where the constants of the model are determined based on experimental procedures such as rheometry. In this paper, we present the phenomenological characterization of the ethylene propylene diene monomer (EPDM) synthetic elastomer by mobile die rheometer (MDR). The kinetic parameters were calculated from Kamal-Ryan, Sestak-Berggren, and the Isayev-Deng methods at different temperatures. An Arrhenius-type function for the order of reaction n is introduced to improve the fit. Finally, Sesták–Berggren and Isayev-Denǵs empirical models were compared in order to determine which best describes the behavior including the proposed expression. The graphical and analytical description of the cure kinetics for the EPDM rubber was obtained consistently. The expression proposed to describe the order of the reaction is predicted to give a better establishment of processing time. It was noted that for EPDM at higher temperatures, the increase of the rate of reaction occurs in short period of time, which could cause premature curing if the supply system is inadequate.

A well plate-based multiplexed platform for incorporation of organoids into an organ-on-a-chip system with a perfusable vasculature.

Owing to their high spatiotemporal precision and adaptability to different host cells, organ-on-a-chip systems are showing great promise in drug discovery, developmental biology studies and disease modeling. However, many current micro-engineered biomimetic systems are limited in technological application because of culture media mixing that does not allow direct incorporation of techniques from stem cell biology, such as organoids. Here, we describe a detailed alternative method to cultivate millimeter-scale functional vascularized tissues on a biofabricated platform, termed 'integrated vasculature for assessing dynamic events', that enables facile incorporation of organoid technology. Utilizing the 3D stamping technique with a synthetic polymeric elastomer, a scaffold termed 'AngioTube' is generated with a central microchannel that has the mechanical stability to support a perfusable vascular system and the self-assembly of various parenchymal tissues. We demonstrate an increase in user familiarity and content analysis by situating the scaffold on a footprint of a 96-well plate. Uniquely, the platform can be used for facile connection of two or more tissue compartments in series through a common vasculature. Built-in micropores enable the studies of cell invasion involved in both angiogenesis and metastasis. We describe how this protocol can be applied to create both vascularized cardiac and hepatic tissues, metastatic breast cancer tissue and personalized pancreatic cancer tissue through incorporation of patient-derived organoids. Platform assembly to populating the scaffold with cells of interest into perfusable functional vascularized tissue will require 12-14 d and an additional 4 d if pre-polymer and master molds are needed.

Design optimization of bridge pier bearings based on material and shape

Elastomer bearings are design efficient and economical, so these are more widely used for bridge construction. Steel bearings were predominantly used before and their performance was good with few disadvantages due to their corrosive nature and high maintenance. Hence elastomer was introduced as a matrix material for steel bearings. In present study, analysis of mechanical properties is carried by changing elastomer material. The results showed that it is better to produce strong and effective reinforced elastomer bearing using material other than natural rubber. The study compares results of bearing elastomer using silicone rubber, PTFE (Polytetrafluoroethylene) with the conventional synthetic natural rubber elastomer bearing. The effect of shape on maximum stress and deformation is also analysed. The bearing used in the case study is designed for forces applicable to metro rail viaduct. The load calculations are done using the specification provided by BMRCL (Bangalore Metro Rail Corporation Limited). The analysis for stress and deformation is performed using ANSYS. The maximum stress, strain and deformation in bearing using new materials as elastomer were found to be lesser than the bearing using neoprene elastomer bearing.

Caudicles in vandoid orchids: A carotenoid-based soft material with unique properties

130 years ago, Darwin observed that caudicles in vandoid orchids possess considerable elasticity and further hypothesized that their elasticity serves to improve pollination efficiency. However, there has been no study that seeks to either quantitatively backup Darwin's hypothesis or characterize this natural material for practical use. Here we show that vandoid caudicles are a novel kind of soft material with extremely high extensibility (1190%), low modulus (160 kPa) and density lower than that of water. Vandoid caudicles contain carotenoids that attach to basal polymers through noncovalent interactions. Inspired by the chemical structure of caudicles, we synthesize calcium-alginate/polyacrylamide hydrogels supplemented with carotenoids and demonstrate that their strength as well as stretchability are enhanced two-fold. Our findings identify a new carotenoid-based material system with unique properties that approach the current boundaries of the Ashby chart, demonstrating potential application of carotenoids as biocompatible reinforcing agent for hydrogels. Statement of SignificanceWe have investigated the microstructure, mechanical properties and chemical components of vandoid caudicles as an elastic plant tissue and demonstrated a bio-inspired design that can enhance the elasticity of hydrogels. Existing research on vandoid caudicles are very few and mainly focus on their phylogenetics and developmental process, and the potential application of caudicles in the field of material sciences remains unexplored. Our results showed that caudicles are more stretchable than most natural and synthetic elastomers and have a modulus similar to hydrogels. Carotenoids, an important chemical component of caudicles, can be used as supplements to hydrogels to improve their strength and stretchability.

3D printing of a biocompatible double network elastomer with digital control of mechanical properties.

The majority of 3D-printed biodegradable biomaterials are brittle, limiting their potential application to compliant tissues. Poly (glycerol sebacate) acrylate (PGSA) is a synthetic biodegradable and biocompatible elastomer, compatible with light-based 3D printing. In this work we employed digital-light-processing (DLP)-based 3D printing to create a complex PGSA network structure. Nature-inspired double network (DN) structures with two geometrically interconnected segments with different mechanical properties were printed from the same material in a single shot. Such capability has not been demonstrated by any other fabrication technique. The biocompatibility of PGSA after 3D printing was confirmed via cell-viability analysis. We used a finite element analysis (FEA) model to predict the failure of the DN structure under uniaxial tension. FEA confirmed the soft segments act as sacrificial elements while the hard segments retain structural integrity. The simulation demonstrated that the DN design absorbs 100% more energy before rupture than the network structure made by single exposure condition (SN), doubling the toughness of the overall structure. Using the FEA-informed design, a new DN structure was printed and the FEA predicted tensile test results agreed with tensile testing of the printed structure. This work demonstrated how geometrically-optimized material design can be easily and rapidly achieved by using DLP-based 3D printing, where well-defined patterns of different stiffnesses can be simultaneously formed using the same elastic biomaterial, and overall mechanical properties can be specifically optimized for different biomedical applications.

Tough Ordered Mesoporous Elastomeric Biomaterials Formed at Ambient Conditions.

Synthetic dry elastomers are randomly cross-linked polymeric networks with isotropic and unordered higher-level structural features. However, their growing use as soft-tissue biomaterials has demanded the need for an ordered and anisotropic nano-micro (or) mesoarchitecture, which is crucial for imparting specific properties such as hierarchical toughening, anisotropic mechanics, sustained drug delivery, and directed tissue growth. High processing cost, poor control in 3D, and compromised mechanical properties have made it difficult to synthesize tough and dry macroscopic elastomers with well-organized nano-microstructures. Inspired from biological design principles, we report a tough ordered mesoporous elastomer formed via bottom-up lyotropic self-assembly of noncytotoxic, polymerizable amphiphilic triblock copolymers and hydrophobic polymers. The elastomer is cross-linked using covalent cross-links and physical hydrophobic entanglements that are organized in a periodic manner at the nanoscale. This transforms into a well-ordered hexagonal arrangement of nanofibrils that are highly oriented at the micron scale, further organized as 3D macroscale objects. The ordered nano-microstructure and molecular multinetwork endows the elastomer with hierarchical toughening while possessing excellent stiffness and elongation comparable to engineering elastomers like silicone and vulcanized rubber. Processing of the elastomer is performed at ambient conditions using 3D printing and photo-cross-linking, which is fast and energy efficient and enables production of complex 3D objects with tailorable sub-millimeter features such as macroporosity. Furthermore, the periodic and amphiphilic nanostructure permits functionalization of the elastomer with secondary components such as inorganic nanoparticles or drug molecules, enabling complementary mechanical properties such as high stiffness and functional capabilities such as in localized drug delivery applications.

Valve leaflet‐inspired elastomeric scaffolds with tunable and anisotropic mechanical properties

Fibrous scaffolds, which can mimic the elastic and anisotropic mechanical properties of native tissues, hold great promise in recapitulating the native tissue microenvironment. We previously fabricated electrospun fibrous scaffolds made of hybrid synthetic elastomers (poly(1,3‐diamino‐2‐hydroxypropane‐co‐glycerol sebacate)‐co‐poly (ethylene glycol) (APS‐co‐PEG) and polycaprolactone (PCL)) to obtain uniaxial mechanical properties similar to those of human aortic valve leaflets. However, conventional electrospinning process often yields scaffolds with random alignment, which fails to recreate the anisotropic nature of most of the soft tissues such as native heart valves. Inspired by the structure of native valve leaflet, we designed a novel valve leaflet‐inspired ring‐shaped collector to modulate the electrospun fiber alignment and studied the effect of polymer formulation (PEG amount [mole %] in APS‐co‐PEG; ratio between APS‐co‐PEG and PCL; and total polymer concentration) in tuning the biaxial mechanical properties of the fibrous scaffolds. The fibrous scaffolds collected on the ring‐shaped collector displayed anisotropic biaxial mechanical properties, suggesting that their biaxial mechanical properties are closely associated with the fiber alignment in the scaffold. Additionally, the scaffold stiffness was easily tuned by changing the composition and concentration of the polymer blend. Human valvular interstitial cells (hVICs) cultured on these anisotropic scaffolds displayed aligned morphology as instructed by the fiber alignment. Overall, we generated a library of biologically relevant fibrous scaffolds with tunable mechanical properties, which will guide the cellular alignment.

Phenomenological model for the reaction order n in the kinetics of curing an elastomer EPDM

AbstractThe precise control of curing reaction parameters allows a better crosslinking polymer. Modelling and optimization of this process require a correct kinetic of curing model. The kinetics of the crosslinking reaction is studied for the ethylene propylene diene monomer (EPDM) synthetic elastomer by mobile die rheometer (MDR). The kinetic parameters of reaction were calculated from Kamal-Ryan, Sestak-Berggren, and the Isayev-Deng methods at different temperatures. An Arrhenius-type function for the order of reaction n is introduced to improve the adjusting. Finally, a graphical and analytical description of the cure kinetics was developed. The order of reaction is predicted to better establishment of processing time. It was noted that for EPDM at higher temperatures, the increase of the rate of reaction occurs in short period of time, which could cause premature curing if the supply system is inadequate.

Mimicking "J-Shaped" and Anisotropic Stress-Strain Behavior of Human and Porcine Aorta by Fabric-Reinforced Elastomer Composites.

An ex vivo heart perfusion device preserves the donor heart in a warm beating state during transfer between extraction and implantation surgeries. One of the current challenges includes the use of rigid and noncompliant plastic tubes, which causes injuries to the heart at the junction between the tissue and the tube. The compliant and rapidly strain-stiffening mechanical property that generates a "J-shaped" stress-strain behavior is necessary for producing the Windkessel effect, which ensures continuous flow of blood through the aorta. In this study, we mimic the J-shaped and anisotropic stress-strain behavior of human aorta in synthetic elastomers to replace the problematic noncompliant plastic tube. First, we assess the mechanical properties of human (n = 1) and porcine aorta (n = 14) to quantify the nonlinear and anisotropic behavior under uniaxial tensile stress from five different regions of the aorta. Second, fabric-reinforced elastomer composites were prepared by reinforcing silicone elastomers with embedded fabrics in a trilayer geometry. The knitted structures of the fabric provide strain-stiffening as well as anisotropic mechanical properties of the resulting composite in a deterministic manner. By optimizing the combination between different elastomers and fabrics, the resulting composites matched the J-shaped and anisotropic stress-strain behavior of natural human and porcine aorta. Finally, improved analytical constitutive models based on Gent's and Mooney-Rivlin's constitutive model (to describe the elastomer matrix) combined with Holzapfel-Gasser-Ogden's model (to represent the stiffer fabrics) were developed to describe the J-shaped behavior of the natural aortas and the fabric-reinforced composites. We anticipate that the suggested fabric-reinforced silicone elastomer composite design concept can be used to develop complex soft biomaterials, as well as in emerging engineering fields such as soft robotics and microfluidics, where the Windkessel effect can be useful in regulating the flow of fluids.

Molecular dependencies of dynamic stiffening and strengthening through high strain rate microparticle impact of polyurethane and polyurea elastomers

This study investigates the molecular dependencies of dynamic stiffening and strengthening through comparison of high strain rate impact responses of various polyurethanes and polyureas. We use an in-house designed tabletop microimpact experimental platform—the laser-induced particle impact test—to perform high strain rate impacts and measure the corresponding material response. Dynamic mechanical analysis and differential scanning calorimetry are used to show that glass transition temperature is a useful predictor of the impact response at ambient temperatures. Meanwhile, solid-state nuclear magnetic resonance spectroscopy identifies segmental dynamics as an important determinant of the variation in both dynamic stiffening and strengthening. The impact responses of polyurethanes and polyureas both show clear dependencies on the molecular weight of the soft segment. This comparison suggests the state of intermolecular hydrogen bonding plays a key role in dynamic stiffening and strengthening. This study aims to identify the molecular dependencies of the impact response and establish a foundation for further design and testing of optimal high strain rate characteristics in synthetic elastomers.

A novel green approach for the preparation of high performance nitrile butadiene rubber-pristine graphene nanocomposites

Present article scrutinizes the effect of pristine few layers graphene incorporation into NBR matrix via a green approach in terms of mechanical properties and two different fabrication routes viz. latex casting and latex casting followed by dry rubber mixing. Large scale production of pristine few layers graphene was accomplished by means of planetary ball milling and was characterized using X-ray diffraction (XRD), Raman spectroscopy and Transmission electron microscopy (TEM) analysis. The produced graphene and procured MWCNT was then successfully incorporated (separately) into NBR latex. It was found out that at a particular filler loading of 3 phr for latex casting method, 5 phr for dry rubber mixing method and only 0.4 phr for MWCNT based samples the produced nanocomposites demonstrated a significant increment in tensile properties. However, when only the supernatant from the 3 phr graphene dispersion was used, the sample showed an improved increment (363%) in mechanical properties due to the removal of other bulk particles during settling which might otherwise act as stress concentrators inside the composite. The demand for multifunctional elastomers is increasing now a day and hence these findings where pristine graphene has been successfully and uniformly incorporated into NBR matrix (an important synthetic elastomer with huge applications) would be scientifically significant.

MAIN GOVERNING FACTORS INFLUENCING MECHANICAL PROPERTIES OF SHORT-CUT ARAMID FIBER–REINFORCED ELASTOMERS

ABSTRACT This study concerns short-cut aramid fiber reinforcement of synthetic elastomer compounds and their influence on the processability and mechanical properties. Short-fiber reinforcement of elastomers is very complex, because it depends on many mutually interacting factors: fiber concentration, fiber orientation distribution, fiber length and distribution, fiber-matrix interfacial strength, and properties of the matrix. The relationship between these influencing factors is highlighted in an S-SBR compound by design of experiments. Two 3 mm long aramid fibers were used: an epoxy-amine–coated fiber and a virgin fiber without coating. To potentially achieve a fiber–matrix interaction, the following silane coupling agents were employed: bis-(triethoxysilylpropyl)-disulfane (TESPD), bis-(triethoxysilylpropyl)-tetrasulfane (TESPT), S-3-(triethoxysilylpropyl)-octanethioate (NXT), and an alkylpolyether-mercapto-silane (Si 363), all in combination with the adhesion-activated aramid fibers and in comparison with the virgin fibers. They are compared on equimolar basis with regard to the amount of reactive ethoxy groups versus TESPD, making use of a “design of experiments” approach of the experimental setup. The outcome shows that, contrary to common assumptions, the effect of the fiber–matrix interaction is grossly overshadowed by the effects of other factors (i.e., fiber concentration and orientation) on the vulcanization system. For each mechanical property response, an optimization prediction is calculated and confirmed with an experimental run, showing, for example, a 330% potential improvement in the Young's modulus.

High-intensity focused ultrasound-induced mechanochemical transduction in synthetic elastomers

While study in the field of polymer mechanochemistry has yielded mechanophores that perform various chemical reactions in response to mechanical stimuli, there is not yet a triggering method compatible with biological systems. Applications such as using mechanoluminescence to generate localized photon flux in vivo for optogenetics would greatly benefit from such an approach. Here we introduce a method of triggering mechanophores by using high-intensity focused ultrasound (HIFU) as a remote energy source to drive the spatially and temporally resolved mechanical-to-chemical transduction of mechanoresponsive polymers. A HIFU setup capable of controlling the excitation pressure, spatial location, and duration of exposure is employed to activate mechanochemical reactions in a cross-linked elastomeric polymer in a noninvasive fashion. One reaction is the chromogenic isomerization of a naphthopyran mechanophore embedded in a polydimethylsiloxane (PDMS) network. Under HIFU irradiation evidence of the mechanochemical transduction is the observation of a reversible color change as expected for the isomerization. The elastomer exhibits this distinguishable color change at the focal spot, depending on ultrasonic exposure conditions. A second reaction is the demonstration that HIFU irradiation successfully triggers a luminescent dioxetane, resulting in localized generation of visible blue light at the focal spot. In contrast to conventional stimuli such as UV light, heat, and uniaxial compression/tension testing, HIFU irradiation provides spatiotemporal control of the mechanochemical activation through targeted but noninvasive ultrasonic energy deposition. Targeted, remote light generation is potentially useful in biomedical applications such as optogenetics where a light source is used to trigger a cellular response.

A study of saliva lubrication using a compliant oral mimic

Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions.PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's OSi(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds.Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlation coefficient = 0.87). This demonstrates a powerful method to study mouthfeel mechanisms.

Alginate Materials and Dental Impression Technique: A Current State of the Art and Application to Dental Practice.

Hydrocolloids were the first elastic materials to be used in the dental field. Elastic impression materials include reversible (agar-agar), irreversible (alginate) hydrocolloids and synthetic elastomers (polysulfides, polyethers, silicones). They reproduce an imprint faithfully, providing details of a high definition despite the presence of undercuts. With the removal of the impression, being particularly rich in water, the imprints can deform but later adapt to the original shape due to the elastic properties they possess. The advantages of using alginate include the low cost, a better tolerability on the part of the patient, the ease of manipulation, the short time needed for execution, the instrumentation and the very simple execution technique and possibility of detecting a detailed impression (even in the presence of undercuts) in a single step. A comprehensive review of the current literature was conducted according to the PRISMA guidelines by accessing the NCBI PubMed database. Authors conducted a search of articles in written in English published from 2008 to 2018. All the relevant studies were included in the search with respect to the characteristics and evolution of new marine derived materials. Much progress has been made in the search for new marine derived materials. Conventional impression materials are different, and especially with the advent of digital technology, they have been suffering from a decline in research attention over the last few years. However, this type of impression material, alginates (derived from marine algae), have the advantage of being among the most used in the dental medical field.

Engineering and manipulation of a mevalonate pathway in Escherichia coli for isoprene production.

Isoprene is a useful phytochemical with high commercial values in many industrial applications including synthetic rubber, elastomers, isoprenoid medicines, and fossil fuel. Currently, isoprene is on large scale produced from petrochemical sources. An efficient biological process for isoprene production utilizing renewable feedstocks would be an important direction of research due to the fossil raw material depletion and air pollution. In this study, we introduced the mevalonate (MVA) pathway genes/acetoacetyl-coenzyme A thiolase (mvaE) and MVA synthase (mvaS) from Enterococcus faecalis (E. faecalis); MVA kinase (mvk) derived from Methanosarcina mazei (M. mazei); and phosphomevalonate kinase (pmk), diphosphomevalonate decarboxylase (mvaD), and isopentenyl diphosphate isomerase (idi) from Streptococcus pneumoniae (S. pneumoniae) to accelerate dimethylallyl diphosphate (DMAPP) accumulation in Escherichia coli (E. coli). Together with a codon-optimized isoprene synthase (ispS) from Populus alba (P. alba), E. coli strain succeeded in formation of isoprene. We then manipulated the heterologous MVA pathway for high-level production of isoprene, by controlling the gene expression levels of the MVA pathway genes. We engineered four E. coli strains which showed different gene expression levels and different isoprene productivities, and we also characterized them with quantitative real-time PCR and metabolite analysis. To further improve the isoprene titers and release the toxicity to cells, we developed the extraction fermentation by adding dodecane in cultures. Finally, strain BL2T7P1TrcP harboring balanced gene expression system produced 587 ± 47mg/L isoprene, with a 5.2-fold titer improvement in comparison with strain BL7CT7P. This work indicated that a balanced metabolic flux played a significant role to improve the isoprene production via MVA pathway.