Polymer-based Structures Research Articles

Exploring the in vitro and in vivo compatibility of PLA, PLA/GNP and PLA/CNT-COOH biodegradable nanocomposites: Prospects for tendon and ligament applications.

Anterior cruciate ligament (ACL) reconstructive surgeries are the most frequent orthopedic procedures in the knee. Currently, existing strategies fail in completely restoring tissue functionality and have a high failure rate associated, presenting a compelling argument towards the development of novel materials envisioning ACL reinforcement. Tendons and ligaments, in general, have a strong demand in terms of biomechanical features of developed constructs. We have previously developed polylactic acid (PLA)-based biodegradable films reinforced either with graphene nanoplatelets (PLA/GNP) or with carboxyl-functionalized carbon nanotubes (PLA/CNT-COOH). In the present study, we comparatively assessed the biological performance of PLA, PLA/GNP, and PLA/CNT-COOH by seeding human dermal fibroblasts (HFF-1) and studying cell viability and proliferation. In vivo tests were also performed by subcutaneous implantation in 6-week-old C57Bl/6 mice. Results showed that all formulations studied herein did not elicit cytotoxic responses in seeded HFF-1, supporting cell proliferation up to 3 days in culture. Moreover, animal studies indicated no physiological signs of severe inflammatory response after 1 and 2 weeks after implantation. Taken together, our results present a preliminary assessment on the compatibility of PLA reinforced with GNP and CNT-COOH nanofillers, highlighting the potential use of these carbon-based nanofillers for the fabrication of reinforced synthetic polymer-based structures for ACL reinforcement. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2182-2190, 2017.

MEMS transducers low-cost fabrication using SU-8 in a sacrificial layer-free process

We report novel low-cost and rapid fabrication technologies for the fabrication of movable polymer-based MEMS structures, electrically actuated. Using SU-8 photoresist as structural material, both ordinary and functionalized by conductive fillers (nano-Ag particles and carbon nanotubes), the novel fabrication methods provide simple processes, without the use of any sacrificial layer, for achieving suspended structures (beams and membranes) through either binary or gray-scale photolithography. Experimental validation has demonstrated high yields (over 99% for one of the process flows) in achieving electrostatically actuated microstructures with resonant frequencies in the 0.5–0.8 MHz range, with a stable dynamic behavior tested for a period longer than three months. The technology also confirms that ‘doping’ SU-8 with conductive fillers can preserve its photo-patterning capabilities, while modifying other physical properties (electrical conductivity). As the patterning of SU-8 films can take place on either rigid or flexible substrates, this low-cost technology promises a wide range of applications.

Incorporation of Copper Enhances the Anti-Ageing Property of Flame-Sprayed High-Density Polyethylene Coatings

High-density polyethylene (HDPE)-copper (Cu) composite coatings were prepared through depositing HDPE-Cu core-shell particles by flame spraying. The HDPE-Cu composite coatings and the HDPE coatings were aged in xenon lamp ageing testing chamber. The variations of chemical compositions and surface morphology of the coatings before and after the ageing testing were analyzed using infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry and ultraviolet-visible spectrophotometer. Results show that there is no chemical composition variation in the HDPE-Cu coatings. Cracks were found on the surfaces of the HDPE coatings, while the HDPE-Cu coating shows almost intact surface morphology. These results suggest that the HDPE-Cu coatings present better anti-ageing performances than the HDPE coatings. Further assessment of the function of Cu shells on the anti-ageing property reveals that Cu shells not only enhanced the absorption of the coatings to ultraviolet, but also increased their reflectivity to visible light. Additionally, the Cu shells enhanced the decomposition temperature and thermal stability of HDPE in the composite coatings. These results give bright insight into potential anti-ageing applications of the polymer-based structures.

Silicon anodes protected by a nitrogen-doped porous carbon shell for high-performance lithium-ion batteries.

Silicon (Si) anodes, which are among the most promising candidates for high-energy lithium-ion batteries (LIBs), have attracted considerable attention from both academic and industrial communities. However, Si anodes usually suffer from an inherently low conductivity and extremely large volume change during the lithiation and delithiation processes, and consequently exhibit an inferior rate capability and poor cycle life. In this paper, we report new porous polymer-derived carbon coated Si nanoparticles (NPs) as the next generation anodes for LIBs to overcome these serious problems. Specifically, a porous covalent triazine framework (CTF) polymer shell was synthesized by in situ trimerization of p-benzenedinitrile in molten ZnCl2. Then, core-shell structured Si/nitrogen-doped porous carbon (Si@NPC) spheres were easily produced after high-temperature annealing. As an anode for LIBs, Si@NPC delivers a high capacity of 1390 mA h g-1 at 0.5 A g-1, stable cycle performance (107% capacity retention at 1 A g-1 for 200 cycles), and excellent rate capability of up to approximately 420 mA h g-1 at 16 A g-1. Such an exciting performance can be attributed to the ultra-stable, highly conductive, N-doped, and porous carbon shell. This work not only offers a new solution to the large volume change of Si-based anodes, but also enables the synthesis of porous polymer-based core-shell structures for energy storage and conversion.

Influence of matrix nature on the post-fire mechanical behaviour of notched polymer-based composite structures for high temperature applications

Abstract This study aims to investigate the influence of matrix nature (thermosetting – epoxy 914; and thermoplastic – Polyphenylene Sulfide PPS) on the tensile thermo-mechanical behaviour of both notched and unnotched laminates. Depending on variable prior fire-exposures, the purpose was to compare the changes on the residual tensile mechanical properties of carbon fibre reinforced epoxy 914- and PPS-based laminates subjected to stress concentration for aeronautical purposes (e.g., at service temperatures higher than glass transition temperature, Tg). With respect to the unnotched laminates, the area of the hole is an open access through the thickness for the heat flux causing more or less thermal degradation (depending on matrix nature) and resulting in variably decreasing the laminate tensile properties. Surprisingly, a low heat flux leads to virtually no decrease in the residual tensile strength of PPS-based laminates; however, in epoxy 914-based composites, the exposure to a low heat flux is more detrimental as exposure time is long. Fractography analyses were performed to investigate damage mechanisms in perforated laminates. The influence of fire-induced damages, the subsequent degradation of the mechanical properties due to fire exposure, combined with the overstresses near the hole all contribute to significantly decrease both stiffness and strength of C/epoxy notched laminates. In notched C/PPS laminates, the fire-induced degradation, resulting from the redistribution of melted PPS matrix within the fibre network, may compete with a relaxation effect in the overstressed 0° carbon fibres in the vicinity of the hole. These competing mechanisms are expected to reduce the influence of fire-exposure on the residual tensile strength of notched C/PPS laminates.

A Review of Thermal Spray Metallization of Polymer-Based Structures

A literature review on the thermal spray deposition of metals onto polymer-based structures is presented. The deposition of metals onto polymer-based structures has been developed to enhance the thermal and electrical properties of the resulting metal-polymer material system. First, the description of the thermal spray metallization processes and technologies for polymer-based materials are outlined. Then, polymer surface preparation methods and the deposition of metal bond-coats are explored. Moreover, the thermal spray process parameters that affect the properties of metal deposits on polymers are described, followed by studies on the temperature distribution within the polymer substrate during the thermal spray process. The objective of this review is devoted to testing and potential applications of thermal-sprayed metal coatings deposited onto polymer-based substrates. This review aims to summarize the state-of-the-art contributions to research on the thermal spray metallization of polymer-based materials, which has gained recent attention for potential and novel applications.

Rapid Prototyping of Chemical Microsensors Based on Molecularly Imprinted Polymers Synthesized by Two-Photon Stereolithography.

Two-photon stereolithography is used for rapid prototyping of submicrometre molecularly imprinted polymer-based 3D structures. The structures are evaluated as chemical sensing elements and their specific recognition properties for target molecules are confirmed. The 3D design capability is exploited and highlighted through the fabrication of an all-organic molecularly imprinted polymeric microelectromechanical sensor.

New approach to design of ceramic/polymer material compounds

The damage tolerance of carbon fibre-reinforced ceramic-matrix composite materials depends on their porosity and can be rather significant. Complex structures are difficult to produce. The integration of simple geometric structures of ceramic-matrix composite materials in complex polymer-based hybrid structures is a possible approach of realising those structures. These hybrid material compounds, produced in a cost-efficient way, combine the different advantages of the individual components in one hybrid material compound. In addition the individual parts can be designed to fit a specific application and the resulting forces. All these different advantages result in a significant reduction of not only the production costs and the production time, but also opens up new areas of application, such as the large-scale production of wear-resistant and chemically inert, energy dampening components for reactors or in areas of medicine. The low wettability of the ceramic component however is a disadvantage of this approach. During the course of this contribution, different C/C composite materials with a specific porosity were produced, while adjusting the resin/hardening agent-ratio, as well as the processing parameters. After the production, different penetration tests were conducted with a polymer component. The final part of the article is comprised of the microstructural analysis and the explanation of the mechanical relationships.

Application of Flame-Sprayed Coatings as Heating Elements for Polymer-Based Composite Structures

Flame-sprayed nickel-chromium-aluminum-yttrium (NiCrAlY) and nickel-chromium (NiCr) coatings were deposited on fiber-reinforced polymer composites for use as heating elements of structures that were exposed to cold environments. Electrical current was applied to the coatings to increase the surface temperature by way of Joule heating. The surface temperature profiles of the coatings were measured under free and forced convection conditions at different ambient temperatures, ranging from −25 to 23 °C. It was found that at ambient air temperatures below 0 °C, the surface temperature of the coating remained above 0 °C for both the forced and free convection conditions, and there was a nearly homogeneous temperature distribution over the coating surface. This suggests that flame-sprayed coatings could be used as heating elements to mitigate ice accretion on structures, without the presence of areas of localized high temperature.

Digital mirror devices and liquid crystal displays in maskless lithography for fabrication of polymer-based holographic structures

Polymer-based holographic and diffractive optical elements have gained increasing interest due to their potential to be used in a broad range of applications, such as illumination technology, micro-optics, and holography. We present a production process to fabricate polymer-based diffractive optical elements and holograms. The process is based on maskless lithography, which is used to fabricate optical elements in photoresist. We discuss several lab-level lithography setups based on digital mirror devices and liquid crystal devices with respect to illumination efficiency, resolution, and contrast. The entire optical setup is designed with emphasis on low-cost components, which can be easily implemented in an optical research lab. In a first step, a copy of the microstructures is replicated into optical polymeric materials by means of a soft stamp hot embossing process. The soft stamp is made from polydimethylsiloxan, which is coated onto the microstructure in the photoresist. The hot embossing process is carried out by a self-made and low-cost hot embossing machine. We present confocal topography measurements to quantify the replication accuracy of the process and demonstrate diffractive optical elements and holographic structures, which were fabricated using the process presented.

Piezoelectric P(VDF-TrFE) transducers assembled with micro injection molded polymers

A polymer-based uniaxial accelerometer with piezoelectric polyvinylidene fluoride copolymer (P(VDF-TrFE)) transducers and micro injection molded polypropylene (PP) structures has been designed, manufactured and characterized. The mass-loaded cantilever structure shows a quasi-static charge sensitivity of 0.69pC/g and a resonance frequency of 735Hz. Because of its efficient processing and its small dimensions the polymer-based sensor (polySENS) can be integrated into polymer-based compound structures in order to measure low-frequency vibrations. The focus of this contribution is on the design, processing as well as on the characterization of the polySENS accelerometer to determine the sensor properties of the accelerometer due to manufacturing particularities. Additionally, investigations regarding the applied materials and the manufacturing, in particular the mold filling have been done to qualify the novel technological approach.

Interfacial smoothening of polymer multilayers with molecules: a new approach in growing supramolecular layer structures

The development of polymer-based structures is hindered due to the difficulties in realizing well-defined modulated structures without interpenetration of charged polymers. Two types of polyelectrolyte multilayer structures, with and without embedded layers of the biomolecule, glutamic acid, are compared using infrared spectroscopy and X-ray and neutron scattering techniques. Structure characterization of polyelectrolyte multilayers focuses usually on the arrangement along the surface normal. Using specular and off-specular neutron scattering we get insight into the in- and out-of-plane arrangement of the structure. A formation of well-defined, smooth layers is observed in the structure containing the biomolecule. This reduced roughness is associated with a drastic decrease in chain mobility responsible for interpenetration. In contrast, the multilayer without the biomolecule forms an island-like structure. Thus biomolecules can interrupt the propagation of island-like structures at such interfaces which can be effectively used in achieving well-defined supramolecular layer structures out of charged polymers.

Stimuli-responsive hierarchically self-assembled 3D porous polymer-based structures with aligned pores.

We have developed a novel approach for the fabrication of self-assembled porous materials with uniaxial tubular pores. The approach is based on the use of microtubes formed by stimuli-induced rolling of polymer bilayers consisting of hydrophobic and stimuli-responsive hydrophilic polymers. Different objects, for example yeast cells, can be encapsulated inside the tubes during their rolling. The self-rolled tubes filled with the yeast cells are capable of controlled self-assembly and form a uniaxial tubular homogeneously filled scaffold. Moreover, our approach allows design of porous materials with the pores having different properties.

Microwave performance of photoresist-alumina microcomposites for batch fabrication of thick polymer-based dielectric structures

The goal of this paper is to investigate the electrical properties of photoresist-alumina microcomposites with different portions of ceramic content. Substrates of photoresist-alumina microcomposites are fabricated and a comprehensive analysis is performed to characterize their dielectric constant and dielectric loss tangent at microwave frequencies up to 40 GHz. To evaluate the performance of these materials for microwave applications, the properties of various lithographically fabricated antenna elements are examined and analysed based on the measured electrical properties. The experimental results show that the electrical properties of the photoresist composite are nonlinearly affected by ceramic content and also a minimum percentage of ceramic portion is required to improve the electrical properties of the photoresist composite. For instance, comparison of 0 wt% with 23 wt% SU8-alumina shows that no reduction is achieved for the dielectric loss tangent. Comparison of 38 wt% with 48 wt% SU8-alumina microcomposite shows that the dielectric loss tangent is improved from 0.03 to 0.01 and the dielectric constant is increased from 3.8 to 5.0 at 25 GHz. These improvements can result in superior performance for the photoresist-based microwave components.

Controlling Bandgap in Electroactive Polymer-Based Structures

A waveguide with a periodic structure is able to filter waves whose frequencies lie in the band-gap ranges displayed in dispersion diagrams. The lengths of these forbidden bands depend on the contrast in material and geometrical properties of parts of the system which realize the periodicity. In this paper, a novel way to control band gaps is proposed: to set the geometric characteristics of a prestretched waveguide made of soft dielectric elastomer applying an external voltage to pairs of electrodes applied with a regular pattern on the device. The technique proves to be feasible and is able to tune accurately the position of band gaps over all frequency spectrum. For the investigated system, a device able to guide flexural waves, bandgap ranges of about 100-200 Hz have been obtained over frequencies on the order of 1 kHz.

Deep x-ray lithography processing for batch fabrication of thick polymer-based antenna structures

Deep x-ray lithography is applied for the first time to fabricate polymer-based antenna structures with different portions of ceramic contents. To produce successful and viable antenna structures, three different methods are proposed using positive and negative tone resists. In the first method the structures are lithographically fabricated avoiding an intermediate molding step using SU-8 as a photosensitive resist filled with fine ceramic powder with particles in the submicron range. In the second and third methods a polymethylmethacrylate (PMMA) mold is first fabricated by x-ray lithography, and then SU-8/MMA mixed with the high ceramic powder content is injected into the mold. In these methods a final step of crosslinking for SU-8 and polymerization for MMA is also required. Optimized fabrication parameters allow the production of high quality antenna structures as thick as 2.3 mm. X-ray lithography capabilities in fabrication of antennas and other passive microwave components with special features reinforce the idea of fabricating integrated passive microwave circuits along with active circuits using this emerging technology.

Investigation of Properties of Polymer/Textile Fiber Composites

Polymer-based composite structures have advantages over other materials. The most important advantage is the higher mechanical properties obtained from the composites when supported by fiber reinforcement. The mechanical and thermal properties of fiber-reinforced composite structures are affected by the amount of fibers in the structures, orientation of the fiber and fiber length. Silk and cotton fibers are used in many fields but especially in clothing and textiles. However, there is not enough research on their usage as reinforcement fibers in composite structures. Silk fibers as a textile material have better physical and mechanic properties than other animal fibers. It is very important that the improvement of the mechanical and physical properties of the composite structures allows them to be used in many areas. From economical, technological and environmental points of view, the improved the mechanical and physical properties of polymeric materials are receiving much attention in the recent studies. In this study, various lengths (1 mm–2.5 mm and 5 mm) of waste silk and waste cotton fibers were added to high-density polyethylene (HDPE) and polypropylene (PP) polymer in the mixing ratios of (polymer:fiber) 100%:0%, 97%:3%, and 94%:6% to produce composite structures. On the other hand, known lengths (1–2.5–5 mm) of waste silk and waste cotton fibers were added to recycled polyamide-6 (PA6) and polycarbonate (PC) polymers in mixing quantities of 100%-0%, 97%-3%. A twin-screw extruder was employed for the production of composites. Tensile strength, % elongation, yield strength, elasticity modulus, Izod impact strength, melt flow index (MFI), heat deflection temperature (HDT), and Vicat softening temperature properties were determined. In order to determine the materials' thermal transition and microstructure properties, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were used. Results have shown that cotton and silk fibers behave differently than in the composite structure. Waste silk fiber composites give better mechanical properties than waste cotton fiber.

Properties of Recycled Polycarbonate/Waste Silk and Cotton Fiber Polymer Composites

Polymer-based composite structures have advantages over many other materials. The most important advantage is the higher mechanical properties obtained from the composites when supported by fiber reinforcement. The mechanical and thermal properties of fiber-reinforced composite structures are affected by the amount of fibers in the structures, orientation of the fibers and fiber length. Silk and cotton fibers are used in many fields but especially in clothing and textiles. However, there is not enough research on their usage as reinforcement fibers in composite structures. Silk fibers as a textile material have better physical and mechanical properties than other animal fibers. The improvement of the mechanical and physical properties of the composite structures allows them to be used in many areas. From economical, technological and environmental points of view, the improvement of mechanical and physical properties of polymeric materials are receiving much attention in recent studies. In this study, different application areas were chosen to evaluate the waste silk and waste cotton rather than classic textile applications. Waste silk and cotton and recycled polycarbonate polymer were mixed and as a result composite structures were obtained. Silk and cotton waste fiber dimensions were in between 1 mm, 2.5 mm and 5 mm. The recycled PC/silk and cotton wastes were mixed in the rates of 97%/3%. Mixtures were prepared by twin-screw extruder. Tensile strength, % elongation, yield strength, elasticity modulus, Izod impact strength, melt flow index (MFI), heat deflection temperature (HDT) and Vicat softening temperature properties were determined. To determine the materials' thermal transition and microstructure properties, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were used.

BioMEMS Technologies for Regenerative Medicine

AbstractThe emergence of BioMEMS fabrication technologies such as soft lithography, micromolding and assembly of 3D structures, and biodegradable microfluidics, are already making significant contributions to the field of regenerative medicine. Over the past decade, BioMEMS have evolved from early silicon laboratory devices to polymer-based structures and even biodegradable constructs suitable for a range ofex vivoandin vivoapplications. These systems are still in the early stages of development, but the long-term potential of the technology promises to enable breakthroughs in health care challenges ranging from the systemic toxicity of drugs to the organ shortage.Ex vivosystems for organ assist applications are emerging for the liver, kidney and lung, and the precision and scalability of BioMEMS fabrication techniques offer the promise of dramatic improvements in device performance and patient outcomes.Ultimately, the greatest benefit from BioMEMS technologies will be realized in applications for implantable devices and systems. Principal advantages include the extreme levels of achievable miniaturization, integration of multiple functions such as delivery, sensing and closed loop control, and the ability of precision microscale and nanoscale features to reproduce the cellular microenvironment to sustain long-term functionality of engineered tissues. Drug delivery systems based on BioMEMS technologies are enabling local, programmable control over drug concentrations and pharmacokinetics for a broad spectrum of conditions and target organs. BioMEMS fabrication methods are also being applied to the development of engineered tissues for applications such as wound healing, microvascular networks and bioartificial organs. Here we review recent progress in BioMEMS-based drug delivery systems, engineered tissue constructs and organ assist devices for a range ofex vivoandin vivoapplications in regenerative medicine.

Design of pull-out stresses for prestrained SMA wire/polymer hybrid composites

Shape memory alloy (SMA) materials possess many unique mechanical, thermal and thermal-mechanical properties that govern their temperature-dependent shape memory, superelastic and pseudoelastic effects. SMA actuators, in the forms of wire or strip embedded into polymer-based composite structures are able to control the structures' shape and stiffness in order to accommodate any environment impacts. Due to an intrinsic shape memory effect (SME) of prestrained SMA wires, additional recovery forces, which are able to alternate the dynamic and buckling properties of the structures, are generated upon heating. This heat is usually induced by a means of externally applied current. To fully use the benefit of the SME in SMA composite structures, understanding the interfacial bonding behaviour and stress transfer properties between the embedded SMA wires and matrix is crucial. In this paper, the wire pull-out properties of prestrained SMA wire/polymer composites subjected to different temperatures were experimentally studied. The results indicated that the critical debonding stress, that initiates fully debond at a bond interface between the SMA wire and matrix increased with increasing the externally applied current, and thus changing the phase condition in the wire. This phenomenon was due to the SME of the prestrained wires and can be interpreted by using a newly developed phase–stress–displacement diagram.