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A combination of experimental, theoretical and numerical studies is used to investigate the variation of elastic moduli of lignocellulosic (bamboo) fiber cell walls with moisture content (MC). Our Nanoindentation results show that the longitudinal elastic modulus initially increased to a maximum value at about 3% MC and then decreased linearly with increasing MC. In contrast, the transverse moduli decrease linearly with MC. We showed that amorphous materials in cell walls have key roles in the variation of elastic modulus with increasing MC. Elastic modulus of lignin, calculated by molecular dynamics simulations, increases initially with increasing MC, and then decreases. In contrast, elastic modulus of hemicellulose decreases constantly with MC. Below 10% MC, water molecules tend to break hydrogen bonds between polymer chains and form new hydrogen bond bridges between the polymer chains, while above 10% MC, water molecules aggregate together and create nano-droplets inside the materials. During the process of bridging, the fractional free volume of lignin decreases. The free volume reduction along with formation of hydrogen bond bridges causes a growth in elastic modulus of lignin at low MC. The constant increase of hemicellulose free volume, however, causes the aggregation of voids in the system and diminution of elastic properties.

Low temperature cracking is a prevalent mechanism of failure in hot mix asphalt (HMA) pavements. Prolonged cold temperatures can lead to premature cracking and subsequent failure of pavements. It has been previously shown that fibers as additives can improve the mechanical properties of asphalt mastic (asphalt binder and filler material passing the 75 μm sieve). However, the success of any effort to evaluate the low-temperature behavior of the fiber-reinforced asphalt mastic is based on the understanding of the fracture and toughening mechanisms. The objective of this study was to develop an asphalt mastic with enhanced resistance against cracking with the use of small amounts of fiber. Here, the effects of fiber length and content in the asphalt mastic were studied to create an asphalt mastic with enhanced fracture toughness. The Single Edge Notched Three-point Bending (SENB) test was performed at low temperatures to measure the strength and fracture toughness for polypropylene fiber-reinforced asphalt mastic. The major conclusions were that both fiber length and content improved the fracture properties of mastics significantly, and crack bridging and fiber pull-out were the two main toughening mechanisms in these materials.

Nature employs lipids to construct nanostructured membranes that self-assemble in an aqueous environment to separate the cell interior from the exterior environment. Membrane composition changes among species and according to environmental conditions, which allows organisms to occupy a wide variety of different habitats. Lipid bilayers are phase-change materials that exhibit strong thermotropic and lyotropic phase behavior in an aqueous environment, which may also cause thermal rectification. Among different types of lipids, archaeal lipids are of great interest due to their ability to withstand extreme conditions. In this paper, nonequilibrium molecular dynamics simulations were employed to study the nanostructures and thermal properties of different archaeols and to investigate thermal rectification effects in asymmetric archaeal membranes. In particular, we are interested in understanding the role of bridged phytanyl chains and cyclopentane groups in controlling the phase transition temperature and heat flow across the membrane. Our results indicate that the bridged phytanyl chains decrease the molecular packing of lipids, whereas the existence of cyclopentane rings on the tail groups increases the molecular packing by enhancing the interactions between isoprenoid chains. We found that macrocyclic archaeols have the highest thermal conductivity, whereas macrocyclic archaeols with two cyclopentane rings have the lowest. The effect of the temperature on the variation of thermal conductivity was found to be progressive. Our results further indicate that small thermal rectification effects occur in asymmetric archaeol bilayer membranes at around 25 K temperature gradient. The calculated thermal rectification factor was around 0.09 which is in the range of rectification factor obtained experimentally for nanostructures such as carbon nanotubes (0.07). Such phenomena may be of biological significance and could also be optimized for use in various engineering applications.

This paper presents a review of high temperature ceramics research for aerospace applications. Following a brief historical perspective, the paper reviews the effort to toughen ceramics for high temperature structural applications. These include: efforts to toughen zirconia-based ceramics, aluminum oxide, silicon carbide, silicon nitride, molybdenum disilicide and zirconium diborides and carbon-based composites. The development of thermal protection systems is also reviewed within the context of thermal barrier coatings (TBCs) and thermal protection systems for space vehicles. The paper concludes with a final section in which the implications of the results are then discussed for the thermostructural applications of ceramics in aerospace structures.

An understanding of the properties of multifunctional materials is important for the design of devices for biomedical applications. In this paper, a combination of experiments and models was used to study the mechanical and hyperthermic properties of magnetic nanoparticles (MNP)-filled PDMS composites for biomedical applications. These are studied as a function of the weight of MNP, γ-Fe2O3. The results showed the effects on mechanical behavior, and specific losses in a magnetic field. The measured Young's moduli are in good agreement with the moduli predicted from the Bergström-Boybce model. Specific losses calculated from magnetic measurements are used to predict the thermal dose under in-vivo conditions. The implications of the results were discussed for potential applications in biomedical devices.

This paper is focused on understanding the role of combustive environment on thermal fatigue of Nextel™ 720/alumina CMCs. The role of pre-existing flaws in thermal monotonic and fatigue failure of the CMCs is also computationally and experimentally investigated. A notch (hole) was fabricated on the samples to study the effect of defects on the mechanical behavior of CMCs in combustion environment. Fatigue life in the combustion condition for notched samples was lower by an order of magnitude in comparison to the unnotched samples in combustion environment and notched samples in isothermal furnace results across the range of applied stress. The different fatigue performance is attributed to the thermal gradient stresses and increased rate of oxidation due to a high moisture level in the combustion rig test condition. The former will be further verified using finite element analysis and the latter from finite element analysis and microscopic analysis of the fracture surfaces.

Concrete is the most used material in the world. It is also one of the most versatile yet complex materials that humans have used for construction. However, an important weakness of concrete (cement-based composites) is its low tensile properties. Therefore, over the past 30 years many studies were focused on improving its tensile properties using a variety of physical and chemical methods. One of the most successful attempts is to use polymer fibers in the structure of concrete to obtain a composite with high tensile strength and ductility. The advantages of polymer fiber as reinforcing material in concrete, both with regard to reducing environmental pollution and the positive effects on a country's economy, are beyond dispute. However, a thorough understanding of the mechanical behavior of fiber-reinforced concrete requires a knowledge of fiber/matrix interfaces at the nanoscale. In this study, a combination of atomistic simulations and experimental techniques has been used to study the nanostructure of fiber/matrix interfaces. A new model for calcium-silicate-hydrate (C-S-H)/fiber interfaces is also proposed on the basis of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. Finally, the adhesion energy between the C-S-H gel and three different polymeric fibers (poly(vinyl alcohol), nylon-6, and polypropylene) were numerically studied at the atomistic level because adhesion plays a key role in the design of ductile fiber-reinforced composites. The mechanisms of adhesion as a function of the nanostructure of fiber/matrix interfaces are further studied and discussed. It is observed that the functional group in the structure of polymer macromolecule affects the adhesion energy primarily by changing the C/S ratio of the C-S-H at the interface and by absorbing additional positive ions in the C-S-H structure.

Cement is one of the most consumed materials in the world. The cement industry is responsible for a large portion of global carbon dioxide emissions. Cement production and therefore carbon ...

Given their amphiphilic nature and chemical structure, phospholipids exhibit a strong thermotropic and lyotropic phase behaviour in an aqueous environment. Around the phase transition temperature, phospholipids transform from a gel-like state to a fluid crystalline structure. In this transition, many key characteristics of the lipid bilayers such as structure and thermal properties alter. In this study, we employed atomistic simulation techniques to study the structure and underlying mechanisms of heat transfer in dipalmitoylphosphatidylcholine (DPPC) lipid bilayers around the fluid–gel phase transformation. To investigate this phenomenon, we performed non-equilibrium molecular dynamics simulations for a range of different temperature gradients. The results show that the thermal properties of the DPPC bilayer are highly dependent on the temperature gradient. Higher temperature gradients cause an increase in the thermal conductivity of the DPPC lipid bilayer. We also found that the thermal conductivity of DPPC is lowest at the transition temperature whereby one lipid leaflet is in the gel phase and the other is in the liquid crystalline phase. This is essentially related to a growth in thermal resistance between the two leaflets of lipid at the transition temperature. These results provide significant new insights into developing new thermal insulation for engineering applications.

Protein adsorption and receptor-ligand binding are two key first steps in the cell adhesion process. The ability to predetermine the success or failure of these processes would significantly advance the field of tissue engineering. This study examines fibronectin adsorption on functionalized electrospun polycaprolactone (PCL) scaffolds and determines the affinity of cell receptors for the adsorbed fibronectin onto each scaffold. After determining the affinity values, model plots were developed for each scaffold type based on the amount of fibronectin on the surface of the scaffold. The ability to theoretically predict the level of cell binding to a tissue engineering scaffold would significantly impact the decision of what scaffold type to use for a specific application. Results show that aminated PCL scaffolds adsorb significantly more protein than hydrolyzed PCL scaffolds; however, greater α5β1–fibronectin binding occurs on the hydrolyzed scaffolds. This is attributed to a higher affinity between the receptors on the cell and the fibronectin adsorbed onto hydrolyzed scaffolds compared with aminated scaffolds., POLYM. ENG. SCI., 54:2587–2594, 2014. © 2013 Society of Plastics Engineers

Designing scaffolds to modulate protein adsorption is a key to building advanced scaffolds for tissue regeneration. Protein adsorption to tissue engineering scaffolds is critical in early cell attachment, survival, and eventual proliferation. The goal of this study is to examine the effect of functionalization on fibronectin adsorption to electrospun polycaprolactone (PCL) scaffolds through experimentation using fluorescently labeled fibronectin and to couple this experimental data with analysis of interaction energies obtained through molecular dynamics (MD) simulations to develop a better understanding of the adsorption process. This study is the first to analyze and compare experimental and MD simulation results of fibronectin adsorption on functionalized electrospun PCL scaffolds. Electrospun nanofiber PCL scaffolds were treated with either 1 N NaOH (hydrolyzed) or 46% hexamethylenediamine (HMD) (aminated) solution to be compared with untreated (control) scaffolds. We found that aminated PCL scaffolds experimentally adsorbed more fibronectin than control scaffolds, whereas hydrolyzed scaffolds showed decreased adsorption. MD simulations carried out with NVT ensemble at a temperature of 310 K indicated a higher work of adhesion for both functionalized scaffolds over control. Also, the simulations revealed different conformations of fibronectin on each scaffold type after adsorption, with the arginine-glycine-aspartic acid sequence appearing most accessible on the aminated scaffolds. This suggests that functionalization affects not only the quantity of protein that will adsorb on a scaffold but how it attaches as well, which could affect subsequent cell attachment.

This paper describes the development and validation of a one-dimensional, large-strain consolidation-coupled contaminant transport model. The RCM-XPORT2 model uses the CS2 large-strain consolidation approach coupled with reactive advective/dispersive solute transport through a deforming saturated porous media, capped with a reactive sequestering geomaterial known as a reactive core mat (RCM). Validation of the model is presented by simulating consolidation-coupled contaminant transport experiments performed on naphthalene-spiked sediment from the Neponset River in Milton, Massachusetts. Finally, the paper presents a hypothetical case study of pre- and post-RCM application polychlorinated biphenyl (PCB) fate and transport in the Lower Neponset River using RCM-XPORT2 and U.S. EPA Water Quality Analysis Simulation Program surface water quality model. The Neponset River case study evaluates the long-term effect of sediment RCM capping on reducing the water column PCB concentration.

For early-stage breast cancers mastectomy is an aggressive form of treatment. Therefore, there is a need for new treatment strategies that can enhance the use of lumpectomy by eliminating residual cancer cells with limited side effects to reduce local recurrence. Although, various radiotherapy-based methods have been developed, residual cells are found in 20–55% of the time at the first operation. Furthermore, some current treatment methods result in poor cosmesis. For the last decade, the authors have been exploring the use of polymeric composite materials in single and multi-modal implantable biomedical devices for post-operative treatment of breast cancer. In this paper, the concept and working principles of the devices, as well as selected results from experimental and numerical investigations, are presented. The results show the potential of the biomedical implants for cancer treatment.

This paper presents the results of a combined experimental and theoretical study of the fracture of marble/adhesive interfaces that are relevant to the historic preservation and the conservation of marble artifacts. Thermoplastic and thermosetting adhesives are considered in this study as well as combinations of the two applied in sequence. The mode mixity dependence of interfacial fracture toughness is measured using Brazil-nut specimens. This dependence is also explained using a crack-shielding model that accounts for the crack profiles and the contact between mating asperities.

An atomic force microscopy (AFM) method was developed to quantify the adhesion forces between and cohesive forces within the layers of a drug-eluting stent (DES). Surface pairs representing both the individual components and the complete chemistry of each layer within the DES were prepared. As a model, the CYPHER Sirolimus-eluting Coronary Stent was studied. This DES consists of a stainless steel stent substrate, a parylene C primer layer, and a drug-eluting layer that contains poly(ethylene-co-vinyl acetate), poly(n-butyl methacrylate), and sirolimus (rapamycin). Coated AFM tips and two-dimensional substrates or coupons, which act as surrogates to the CYPHER Stent, were prepared and characterized. The force-displacement measurements were conducted to evaluate the adhesion between the middle parylene C layer and the 316L stainless steel substrate, the adhesion between the parylene C layer and the outer drug-eluting layer, and the cohesion between the three constituents of the drug-eluting layer. The average adhesion forces between the parylene C to drug layer varied from 88 to 167 nN, and the drug layer-to-drug layer interactions were between 194 and 486 nN within the model CYPHER Stent coating. All the adhesion forces measured were larger than those observed for gold-gold interactions, which yielded a pull of force of 19 nN (Zong et al., J Appl Phys 2006;100:104313-104323).

Despite offering many advantages over traditional rigid actuators, soft pneumatic actuators suffer from a lack of comprehensive, computationally efficient models and precise embedded control schemes without bulky flow-control valves and extensive computer hardware. In this article, we consider an inexpensive and reliable soft linear actuator, called the reverse pneumatic artificial muscle (rPAM), which consists of silicone rubber that is radially constrained by symmetrical double-helix threading. We describe analytical and numerical static models of this actuator, and compare their performance against experimental results. To study the application of rPAMs to operate underlying kinematic linkage skeletons, we consider a single degree-of-freedom revolute joint that is driven antagonistically by two of these actuators. An analytical model is then derived, and its accuracy in predicting the static joint angle as a function of input pressures is presented. Using this analytical model, we perform dynamic characterization of this system. Finally, we propose a sliding-mode controller, and a sliding mode controller augmented by a feed-forward term to modulate miniature solenoid valves that control air flow to each actuator. Experiments show that both controllers function well, while the feed-forward term improves the performance of the controller following dynamic trajectories.

Bamboo, a fast-growing grass, has a higher strength-to-weight ratio than steel and concrete. The unique properties of bamboo come from the natural composite structure of fibers that consists mainly of cellulose microfibrils in a matrix of intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Here, we have used atomistic simulations to study the mechanical properties of and adhesive interactions between the materials in bamboo fibers. With this aim, we have developed molecular models of lignin, hemicellulose and LCC structures to study the elastic moduli and the adhesion energies between these materials and cellulose microfibril faces. Good agreement was observed between the simulation results and experimental data. It was also shown that the hemicellulose model has stronger mechanical properties than lignin while lignin exhibits greater tendency to adhere to cellulose microfibrils. The study suggests that the abundance of hydrogen bonds in hemicellulose chains is responsible for improving the mechanical behavior of LCC. The strong van der Waals forces between lignin molecules and cellulose microfibril is responsible for higher adhesion energy between LCC and cellulose microfibrils. We also found out that the amorphous regions of cellulose microfibrils are the weakest interfaces in bamboo fibrils. Hence, they determine the fibril strength.

Recently, hydrogels have been employed in a variety of engineering applications as promising materials, since their porous structure and hydrophilicity enables them to absorb a large amount of water. Atomistic simulations lead to a better understanding of their properties at nanoscale, especially mechanical properties. In this study, hydrogel is studied using a molecular dynamics (MD) framework, considering condensed-phased optimized molecular potential (COMPASS) as the force field. Polyethylene glycol diglycidyl ether (PEDGE) and poly-oxy-alkylene-amines (Jeffamine) are the epoxy and curing agent used for hydrogels, and a novel cross-linking method is applied. Radial Distribution Functions (RDFs) show that the cross-links are the hydrophilic part of hydrogel. RDFs and mechanical properties are reported for different water amounts. The results show that an increase in water content leads to a decrease in elastic modulus of the hydrogel.

In this paper, molecular dynamic simulation was used to study the effect of water on the equilibrated structure and mechanical properties of cross-linked hydrogel at multiple scales. The hydrogel consisted of Polyethylene glycol diglycidyl ether (PEGDGE) as epoxy and the Jeffamine, poly-oxy-alkylene-amines, as curing agent. The results for systems with various water contents indicated that the cross-links were more hydrophilic within the hydrogel structure. Effects of cross-linking on the transport properties were also investigated by computing diffusion coefficients of water molecules. A new Coarse-Grained (CG) scheme for hydrogels is proposed, and validated by comparing the transport properties with the all-atom method, demonstrating the capability of the model to capture the correct dynamic evolution of the system. The all-atom model of the hydrogel was mapped to the CG model using the MARTINI force field. This method resulted in a more realistic representation of the stiffness of the system, compared to the previous experimental studies in the literature. The variation of the stiffness of the hydrogel as a function of the water content showed that 40% water content is the optimal value for mechanical performance of the hydrogel.

Using stainless steel 316L for drug-eluting stents needs specific surface finishing due to corrosion phenomena that take place on the metal surface upon prolonged contact with human tissue. Poly (o-chloro-p-xylylene) (Parylene C) is one of the inert and biocompatible materials that are used for 316L coating with γ - methacryloxypropyltrimethoxysilane as an adhesion promoter. In this study, a combination of atomic force microscopy experiments and contact theories have been used to quantify the work of adhesion between parylene C/316L and silane added parylene C/316L. An atomistic simulation has been used, first, to investigate and compare the adhesion at the room temperature with the experiments and then, to investigate the effect of aqueous environment with higher temperature, inside the body, on the adhesion between layers in the structure of drug eluting stent. The simulation results of simplified model for 316L are in good agreement with the experimental results and suggest that the week affiliation between this polymer and 316L is mainly due to Van der Waals interactions. The effect of temperature on the adhesion is found to be regressive and as the water molecules permeate the polymer the adhesion decreases. They also imply that the effect of silane on the adhesion between parylene C and steel is modest.

In biological hydrogels, the gel matrix is usually reinforced with micro- or nano-fibers and the resulting composite is tough and strong. In contrast, synthetic hydrogels are weak and brittle, although they are highly elastic. Other than in food, the main structural application of hydrogels is as soft contact lenses. The developing interest in soft tissue engineering has exposed a need for strong synthetic hydrogels to act as scaffolds for tissue growth.In this work a new class of hydrogels based on fiber reinforced hydrogel composites with a cartilage-like structure is designed. A 3D rapid prototyping technique was used to form crossed “logpiles” of elastic fibers that are then impregnated with an epoxy-based hydrogel in order to form a fiber-reinforced gel structure. The fibrous construct improves the strength, modulus and toughness of the hydrogel and also constrains the swelling. By altering the construct geometry and studying the effect on mechanical properties we will develop the understanding needed to design strong hydrogels for biomedical devices and soft machines.

We describe a new class of hydrogels based on fiber reinforced hydrogel composites with a cartilage-like structure. In analogy to the spinning of a spider web, a pultrusion system is developed to spin micron -diameter fibers from polymer solution in order to build three dimensional patterned fibrous structures. Impregnating the fibrous construct with epoxy -amine hydrogel forms fiber -reinforced hydrogel composites. The fibrous construct improves the strength, modulus and toughness of the hydrogel and also const rains the swelling. By altering the construct geometry (fiber diameter, density and polymer solution) and studying the effect on mechanical properties we will develop the understanding needed to design strong hydrogels for biomedical devices and soft machines.

This paper presents an implantable polymer/metal alloy thin film structure for localized post-operative treatment of breast cancer. A combination of experiments and models is used to study the temperature changes due to Joule heating by patterned metallic thin films embedded in poly-dimethylsiloxane. The heat conduction within the device and the surrounding normal/cancerous breast tissue is modeled with three-dimensional finite element method (FEM). The FEM simulations are used to explore the potential effects of device geometry and Joule heating on the temperature distribution and lesion (thermal dose). The FEM model is validated using a gel model that mimics biological media. The predictions are also compared to prior results from in vitro studies and relevant in vivo studies in the literature. The implications of the results are discussed for the potential application of polymer/metal thin film structures in hyperthermic treatment of cancer.

This paper evaluates the effect of volume change due to consolidation on one-dimensional transport of contaminants. The rate of consolidation and excess pore pressure dissipation result in a transient, nonlinear advective component of transport through clay. Consolidation experiments were conducted on kaolinite samples prepared at 50% water content. Bromide, a non reactive tracer, was mixed with the upper half of the kaolinite sample at concentration of 960 mg/L. Flux of bromide as a result of consolidation was monitored in the water surrounding the kaolinite sample. The results show continuous flow of bromide as a result of water drainage by consolidation. The bromide mass flux compared well with consolidation rate and ceased at the end of the consolidation process. A formulation for the simultaneous consolidation and diffusion of solutes in clays, solved by finite difference, is also presented. A hypothetical case is solved to demonstrate the effect of consolidation under single and double drainage on contaminant transport and breakthrough. The results show that consolidation in doubly drained clay impacts concentration profiles, but does not significantly impact breakthrough of the diffusive flux. Consolidation under single drainage conditions significantly impacts the diffusional flux.