Silicone Elastomer Matrix Research Articles

Chameleon-inspired tunable multi-layered infrared-modulating system via stretchable liquid metal microdroplets in elastomer film

This report presents liquid metal-based infrared-modulating materials and systems with multiple modes to regulate the infrared reflection. Inspired by the brightness adjustment in chameleon skin, shape-morphing liquid metal droplets in silicone elastomer (Ecoflex) matrix are used to resemble the dispersed “melanophores”. In the system, Ecoflex acts as hormone to drive the deformation of liquid metal droplets. Both total and specular reflectance-based infrared camouflage are achieved. Typically, the total and specular reflectances show change of ~44.8% and 61.2%, respectively, which are among the highest values reported for infrared camouflage. Programmable infrared encoding/decoding is explored by adjusting the concentration of liquid metal and applying areal strains. By introducing alloys with different melting points, temperature-dependent infrared painting/writing can be achieved. Furthermore, the multi-layered structure of infrared-modulating system is designed, where the liquid metal-based infrared modulating materials are integrated with an evaporated metallic film for enhanced performance of such system.

Influence of Silver Nanoparticles on Color Stability of Room-Temperature-Vulcanizing Maxillofacial Silicone Subjected to Accelerated Artificial Aging

In this in vitro study, we assessed the color stability of an A-2186 room-temperature-vulcanizing (RTV) silicone elastomer by incorporating silver nanoparticles during accelerated artificial aging. Using five intrinsic silicone pigment types, including no pigment (colorless), red, blue, mocha, and a combination of the three, we created 160 disk-shaped specimens. These were evenly distributed across 20 experimental groups, each containing 8 samples (n = 8). The specimens underwent aging for 250 and 500 h in an artificial aging chamber. A colorimeter was used to measure the values of L*a*b* according to the Commission Internationale de L’Éclairage (CIE) standards. The 50:50% perceptibility threshold (∆E* = 1.1) and acceptability threshold (∆E* = 3.0) were used in the interpretation of the recorded color differences. At the 0.05 level of significance, the one-way analysis of variance (ANOVA) and Tukey post hoc test were used in the statistical analysis. Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) showed that 0.2% AgNPs after 500 h of aging protected the silicone elastomer matrix and all characteristic bonds of the silicone elastomer. In contrast, silicone without AgNPs showed the distortion of all bonds after 500 h. Chromatic alterations (∆E* > 0) were observed in all specimen groups, surpassing the perceptible threshold (1.1 units), except for mocha, with 0.2% AgNPs after 250 h of aging, which remained below the perceptible threshold (∆E* = 0.97). All groups demonstrated ∆E* values below the acceptable threshold, except for the red color, which exhibited a highly significant color change (p = 0.000). This study determined that all specimens, including colorless silicone, underwent color changes (∆E* > 0), with red displaying a notably significant chromatic alteration. Additionally, AgNPs demonstrated substantial protection of the silicone and reduced the color change across all groups and colors, with enhanced efficacy corresponding to higher AgNP concentrations (0.2% AgNPs > 0.15% AgNPs > 0.1% AgNPs).

Design and characterization of electro-active soft actuator based on phase transition of microencapsulated silicon-ethanol composite

In this study, we demonstrate the development of a fast-response soft composite actuator fabricated by mechanical mixing of silicone elastomer (matrix) and ethanol (as phase-change fluid). Joule heating generated by electrical actuation causes a liquid-vapor transition of ethanol embedded in silicone microcapsules; provides the required pressure for mimicking the natural movements. We discuss the critical design variables to improve the temperature response, chemical composition, mechanical properties, force generated, and microstructure of soft actuator. Our soft actuator produces a high load-to-weight ratio (1000 times greater than its own) using a low-voltage source (˂20V). The stability of the soft actuator under multiple working cycles would bring the application to different robotic areas. The actuator using the mechanism presented here is easy to manufacture, automate, and recyclable. It can be used in a range of robotic applications.

Exploring Lipoic Acid-Mediated Dynamic Bottlebrush Elastomers as a New Platform for the Design of High-Performance Thermally Conductive Materials.

The development of high-performance thermally conductive interface materials is the key to unlocking the serious bottleneck of modern microelectronic technology through enhanced heat dispersion. Existing methods that utilize silicone composites rely either on loading large doses of randomly distributed thermal conductive fillers or on filling prealigned thermal conductive scaffolds with liquid silicone precursors. Both approaches suffer from several limitations in terms of physical traits and processability. We describe an alternative approach in which malleable silicone matrices, based on the dynamic cyclic disulfide nature cross-linker (α-lipoic acid), are readily prepared using ring-opening polymerization. The mechanical properties of the resultant dynamic silicone matrix are readily tunable. Stress-dependent depolymerization of the disulfide network demonstrates the ability to reprocess the silicone elastomer matrix, which allows for the fabrication of highly efficient thermal conductive composites with a 3D interconnecting, thermally conductive network (3D-graphite/MxBy composites) via in situ methods. Applications of the composites as thermal dispersion interface materials are demonstrated by LEDs and CPUs, suggesting great potential in advanced electronics.

Radiosurgery dosimetry using CaSO4:Eu OSLD film-a feasibility study.

Recent studies demonstrated that optically stimulated luminescence (OSL) systems allow the evaluation of doses for 2D mapping in a relatively fast and simple way and results show submillimeter resolution. This work presents, for the first time, an optically stimulated luminescence dosemeter (OSLD) in the form of film made with CaSO4:Eu particles embedded in a silicone elastomer matrix. The OSLD film was produced using a low-cost and relatively simple methodology. This film is reusable and the signal can be satisfactorily bleached using blue LEDs. The main dosimetric properties were evaluated using TL/OSL Risø reader with blue stimulation and Hoya U-340 filter. Investigation shows repeatability within 3% when measuring with the same film sample. Regarding the OSLD film homogeneity, nearly 12% sensitivity change was observed within the 5×5cm2 produced film. Additionally, the dose response curve shows linearity from 5 to 25Gy. Fading of the OSL signal is relatively high, about 50% in the first week and then is stable. Nevertheless, a 3×3cm2 OSLD film was successfully used to map dose distribution in radiosurgery (6MV photon beam). This work demonstrates the feasibility of 2D dosimetry using reusable OSLD films based on CaSO4:Eu.

High Permittivity Polymer Composites on the Basis of Long Single-Walled Carbon Nanotubes: The Role of the Nanotube Length.

Controlling the permittivity of dielectric composites is critical for numerous applications dealing with matter/electromagnetic radiation interaction. In this study, we have prepared polymer composites, based on a silicone elastomer matrix and Tuball carbon nanotubes (CNT) via a simple preparation procedure. The as-prepared composites demonstrated record-high dielectric permittivity both in the low-frequency range (102–107 Hz) and in the X-band (8.2–12.4 GHz), significantly exceeding the literature data for such types of composite materials at similar CNT content. Thus, with the 2 wt% filler loading, the permittivity values reach 360 at 106 Hz and >26 in the entire X-band. In similar literature, even the use of conductive polymer hosts and various highly conductive additives had not resulted in such high permittivity values. We attribute this phenomenon to specific structural features of the used Tuball nanotubes, namely their length and ability to form in the polymer matrix percolating network in the form of neuron-shaped clusters. The low cost and large production volumes of Tuball nanotubes, as well as the ease of the composite preparation procedure open the doors for production of cost-efficient, low weight and flexible composites with superior high permittivity.

An evaluation of a technique to improve the mechanical properties of maxillofacial silicone elastomers with zinc oxide nanoparticles

Statement of problemSilicone elastomer is the most used material for fabricating maxillofacial prostheses, but the material has low tensile and tear strength and insufficient elasticity. Whether the addition of zinc oxide nanoparticles will improve these properties is unclear. PurposeThe purpose of this in vitro study was to evaluate the effect of adding different concentrations of zinc oxide (ZnO) nanoparticles on the clinically critical mechanical properties of a maxillofacial silicone. Material and methodsNano-ZnO was added in concentrations of 1%, 2%, 3%, and 5% by weight to Cosmesil M511 High Temperature Vulcanization (HTV) silicone elastomer. Silicone without nano-ZnO or ethanol served as a conventional group, while silicone without nano-ZnO and with ethanol served as the control group. Tensile strength and elongation tests were done according to International Organization for Standardization (ISO) 37. A tear strength test was done according to ISO 34-1. A Shore A hardness test was done according to ISO 7619. In total 144 specimens were fabricated, with 48 specimens for each test. Field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) tests were used to assess the efficiency of the dispersion method, monitoring the particle size of nano-ZnO. Statistical analysis used 1-way ANOVA and the Tukey post hoc test (α=.05). ResultsFESEM showed a homogenous dispersion of ZnO nanoparticles within the silicone matrix. XRD and FESEM showed a reduction in cluster size of nano-ZnO after sonication. The FTIR-ATR test revealed no significant difference between the conventional and the control group (P>.05). The 3%ZnO demonstrated significantly higher tensile strength, elongation percentage, and tear strength than the control group (P<.05). Shore A hardness showed a significant decrease between the conventional and control group. Hardness increased directly and proportionally to an increase in the concentration of nanofiller. ConclusionsSonication of nano-ZnO in ethanol represented an effective and straightforward way to disperse nano-ZnO in a silicone elastomer matrix. This improved the quality of the nanocomposite without affecting the base material and without the need for a coupling agent or addition of a third material. The overall mechanical properties of the M511 maxillofacial silicone elastomer improved most with a 3%ZnO concentration.

Automated Routing of Muscle Fibers for Soft Robots

This article introduces a computational approach for routing thin artificial muscle actuators through hyperelastic soft robots, in order to achieve a desired deformation behavior. Provided with a robot design and a set of example deformations, we continuously co-optimize the routing of actuators, and their actuation, to approximate example deformations as closely as possible. We introduce a data-driven model for McKibben muscles, modeling their contraction behavior when embedded in a silicone elastomer matrix. To enable the automated routing, a differentiable hyperelastic material simulation is presented. Because standard finite elements are not differentiable at element boundaries, we implement a moving least squares formulation, making the deformation gradient twice differentiable. Our robots are fabricated in a two-step molding process, with the complex mold design steps automated. While most soft robotic designs utilize bending, we study the use of our technique in approximating twisting deformations on a bar example. To demonstrate the efficacy of our technique in soft robotic design, we show a continuum robot, a tentacle, and a four-legged walking robot.

Making something out of nothing: Enhanced flaw tolerance and rupture resistance in elastomer–void “negative” composites

Elastomers often exhibit large stretchability but are not typically designed with robust energy dissipating mechanisms. As such, many elastomers are sensitive to the presence of flaws: cracks, notches, or any other features that cause inhomogeneous deformation significantly decrease the effective stretchability. To address this issue, we have dispersed voids into a silicone elastomer matrix, thereby creating a “negative” composite that provides increased fracture resistance and stretchability in pre-cut specimens while simultaneously decreasing the weight. Experiments and simulations show that the voids locally weaken the specimen, guiding the crack along a tortuous path that ultimately dissipates more energy. We investigate two geometries in pre-cut specimens (interconnected patterns of voids and randomly distributed discrete voids), each of which more than double the energy dissipated prior to complete rupture, as compared to that of the pristine elastomer. We also demonstrate that the energy dissipated during fracture increases with the volume fraction of the voids. Overall, this work demonstrates that voids can impart increased resistance to rupture in elastomers with flaws. Since additive manufacturing processes can readily introduce/pattern voids, we expect that applications of these elastomer–void​ “composites” will only increase going forward, as will the need to understand their mechanics.

Facile Fabrication of Microfluidic Chips for 3D Hydrodynamic Focusing and Wet Spinning of Polymeric Fibers.

Microfluidic wet spinning has gained increasing interest in recent years as an alternative to conventional wet spinning by offering higher control in fiber morphology and a gateway for the development of multi-material fibers. Conventionally, microfluidic chips used to create such fibers are fabricated by soft lithography, a method that requires both time and investment in necessary cleanroom facilities. Recently, additive manufacturing techniques were investigated for rapid and cost-efficient prototyping. However, these microfluidic devices are not yet matching the resolutions and tolerances offered by soft lithography. Herein, we report a facile and rapid method using selected arrays of hypodermic needles as templates within a silicone elastomer matrix. The produced microfluidic spinnerets display co-axially aligned circular channels. By simulation and flow experiments, we prove that these devices can maintain laminar flow conditions and achieve precise 3D hydrodynamic focusing. The devices were tested with a commercial polyurethane formulation to demonstrate that fibers with desired morphologies can be produced by varying the degree of hydrodynamic focusing. Thanks to the adaptability of this concept to different microfluidic spinneret designs—as well as to its transparency, ease of fabrication, and cost-efficient procedure—this device sets the ground for transferring microfluidic wet spinning towards industrial textile settings.

Two-dimensional radiation dosimetry based on LiMgPO4 powder embedded into silicone elastomer matrix

Two-dimensional (2D) spatial radiation dosimetry based on optically stimulated luminescence (OSL) phenomena is presented. By embedding OSL active particles (LiMgPO4) into the transparent and flexible silicone elastomer matrix (polydimethylsiloxane silicone), the favorable dosimetric properties of the OSL material are exploited towards real 2D OSL signal. Direct measurement of the 2D OSL dose distribution using prototype flat foils, irradiated with beta radiation, stimulated with blue light (470 nm) and read out using a high-sensitive CCD camera has been obtained. Basic dosimetric parameters, including repeatability, dose-response, and fading has been measured. A separate experiment was dedicated to measure the real 2D proton depth dose distribution, using unmodulated and collimated 58.8 MeV proton beam used for proton radiotherapy at the IFJ PAN.

Dispersion of Zinc Oxide Nanoparticles in Maxillofacial Silicone Elastomer by Ultrasonication: A Morphological Study

Objective: Aggregation and agglomeration of nanoparticles in silicone elastomer is a common problem that adversely affects the mechanical properties of the silicone because the aggregations act as a weak and stress concentrating point within the silicone elastomer matrix. The objective of this study was to evaluate the effect of sonication on the dispersion of ZnO nanoparticles in M-511 heat vulcanized maxillofacial silicone. &#x0D; Methods: Nano-ZnO was added in concentrations of 1%, 2%, 3%, and 5% by weight to Cosmesil M-511 heat vulcanized maxillofacial silicone elastomer, after sonication of ZnO nanoparticles in ethanol for 30 minutes at room temperature, and 1%, 2%, 3%, and 5% were added by weight without sonication of ZnO nanoparticles. Then field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) tests were used to assess the efficiency of the dispersion method and to monitor the particle size of nano-ZnO.&#x0D; Results: FESEM test showed a reduction in cluster size of nano-ZnO as a result of sonication. XRD and FESEM showed a homogenous dispersion of ZnO nanoparticles within the silicone matrix.&#x0D; Conclusions: Based on the results of this morphological study, sonication of nano-ZnO in ethanol represented an effective and easy way to disperse nano-ZnO in a silicone elastomer matrix. This led to a superior quality nanocomposite without affecting the base material and without the need for a coupling agent or addition of any third material.

ANTIFUNGAL EFFECT OF PHOTOCATALYTIC NANO -TITANIUM DIOXIDE INCORPORATED IN SILICONE ELASTOMER:

INTRODUCTION: Titanium dioxide nanoparticles may improve the antifungal properties of silicone elastomer prosthetic materials. OBJECTIVES: To evaluate the photocatalytic antifungal activity of silicone elastomer incorporated with titanium dioxide nanoparticles against Candida albicans. MATERIALS AND METHODS Titanium dioxide nano-particles (Aeroxide® TiO2 P25, Sigma Aldrich, USA) were characterized using xray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), uv-vis spectrophotometeric analysis, methylene blue degradation efficiency test and direct contact test. Sixty six disc shaped specimens (6 mm diameter x 1mm thickness) of silicone elastomer (SE) (MED-4210, Factor II, USA) were included and divided into 6 groups based on the concentration of nano-TiO2 in silicone elastomer (0%, 2%, 4%, 6%, 8%, 10%) (W/W). Impregnated culture test was used to evaluate the antifungal activity of all SE samples against candida albicans as colonies forming units (CFUs) of treated aqueous suspension of candida albicans were measured at 0, 30, 60, 90 and 120 min follow‑up under UVA light. The dispersion of TiO2 within SE matrix was analysed by Laser scanning microscopy (LSM). RESULTS: Data were analysed using ANOVA test and Tuckey's Post Hoc test. Nano-TiO2 characterization tests confirmed the crystalline structure (85% anatase and 15% rutile), shape (irregular faceted), size (22nm), absorption spectrum (320-400nm) and the photocatalytic antifungal ability of the as-purchased Aeroxide TiO2 P25. Planktonic cultures of candida albicans exposed to SE/TiO2 samples reduced candida albicans count by 4% - 99.99%, depending on the nanoparticles concentration and follow-up time. The candida albicans counts did not decrease in group I without TiO2. LSM of SE specimens showed finer dispersion of TiO2 within SE matrix in group II, III and IV than that in group V and VI. CONCLUSIONS: Incorporation of TiO2 P25 within SE (MED-4210) shows promising inhibitory activity against candida albicans.

Microstructural design for enhanced shape memory behavior of 4D printed composites based on carbon nanotube/polylactic acid filament

Four-dimensional (4D) printing technology, combining 3D printing with stimuli-responsive materials, has attracted extensive attention for smart additive manufacturing. This work focuses on microstructural design, shape recovery behavior and recovery force characterization of 4D printed angle-ply laminated and rectangular braided preforms and their silicone elastomer matrix composites. The angle-ply laminated and rectangular braided preforms were printed using polylactic acid (PLA) and carbon nanotube reinforced PLA (CNT/PLA) based shape memory polymer (SMP) filaments. The X-ray micro-computed tomography, a high resolution and nondestructive imaging technique, was employed to characterize the microstructures of 4D printed specimens and their post-bending damage morphology. The effects of microstructure, CNT filler and silicone elastomer matrix on shape recovery behavior, recovery force and flexural property were studied. The infilling of CNT enabled earlier initiation of specimen shape recovery process, improved the recovery force up to 144% and enhanced the flexural load and modulus. The infusion of silicone matrix increased the final shape recovery ratio to 97.3%, improved the recovery force and enhanced flexural property. The knowledge gained in this research will benefit future design optimization for smarter, faster and stronger actuators.

Measurements and identification of smart magnetomechanical elastomer composite materials properties in shear mode

Magnetorheological elastomer composite materials (MRE) are a new class of intelligent active materials composed of ferromagnetic particles, of micrometric size, dispersed in a silicone elastomer matrix, which exhibit variable stiffness and damping properties which are modifiable under the application of an external magnetic field. Currently, these devices are primarily used in automotive and building applications, but they can easily be adapted to meet the requirements of aerospace applications. The development process and experimental characterization needed to evaluate the active control performance of this material have been made. This is done by characterizing the mechanical properties as a function of the magnetic field, as a function of the excitation frequency and as a function of the different percentage of ferromagnetic particles loading. An example of application of this material in aeronautics is also presented.

Magneto-sensitive bistable soft actuators: Experiments, simulations, and applications

Bistable structures featuring two stable states have been widely applied in designing fast and high-force-output actuators under various types of stimuli, such as mechanical force, swelling, thermal expansion, and so on. In this paper, we designed a magneto-actuated mechanism to realize the reversible shape transition between two curved stable configurations of a buckled beam using magneto actuation. The beam is composed of a silicone elastomer matrix with embedded micro-sized iron particles. The magnetic response of these iron particles endows the composite beam with the ability to snap from one stable shape to the other when the magnitude of the surrounding magnetic field exceeds the threshold value. By separately analyzing the electric-magnetic field and the magnetic-mechanical field, we formulate a simple and efficient computational method to numerically predict the critical current on the onset of snap-through. The computational and experimental critical currents show good agreement for different material and geometrical parameters, including the thickness of the beams, iron particle mixing ratios of the material, and the distances of the beam to the electromagnet. The proof-of-concept design is demonstrated to be efficient in the application of a magneto-responsive soft switch and a catapult for ejecting small objects, providing new insights into designing contactless, low-voltage-actuated bistable structures.

Damage mechanisms in elastomeric foam composites: Multiscale X-ray computed tomography and finite element analyses

To understand the mechanisms of damage initiation and propagation in glass microballoon (GMB)/silicone elastomer matrix syntactic foam composites, multiscale X-ray computed tomography images obtained during in situ compression were analyzed using quantitative image processing and digital volume correlation (DVC) techniques. High-resolution tomograms revealed that both the initiation and propagation of GMB collapse depended on the local clustering of GMBs, leading to the formation of pseudo-crush bands in the composite. This resulted in prominent bands of axial strain variation that grew to several millimeters in length as revealed by DVC. A new mechanism based on stress redistribution around collapsed GMBs is proposed to explain the damage propagation, which is supported by finite element analysis and successfully captures the effects of cluster orientation on the observed damage.

Textile-Based, Interdigital, Capacitive, Soft-Strain Sensor for Wearable Applications.

The electronic textile area has gained considerable attention due to its implementation of wearable devices, and soft sensors are the main components of these systems. In this paper, a new sensor design is presented to create stretchable, capacitance-based strain sensors for human motion tracking. This involves the use of stretchable, conductive-knit fabric within the silicone elastomer matrix, as interdigitated electrodes. While conductive fabric creates a secure conductive network for electrodes, a silicone-based matrix provides encapsulation and dimensional-stability to the structure. During the benchtop characterization, sensors show linear output, i.e., R2 = 0.997, with high response time, i.e., 50 ms, and high resolution, i.e., 1.36%. Finally, movement of the knee joint during the different scenarios was successfully recorded.

Shape memory behavior and recovery force of 4D printed textile functional composites

Four-dimensional (4D) printing of multi-directionally reinforced preforms has tremendous potential for the development of next generation functional composites by using the capabilities of 3D printing technology, 3D textile preform design, and polymer shape memory behavior. This work demonstrates the shape memory behavior and recovery force of 4D printed circular braided tube preforms and their silicone elastomer matrix composites. The preforms were printed by fused deposition modeling using the shape memory polymer (SMP), polylactic acid (PLA). The effects of braiding angle, tube wall thickness, and shape recovery temperature on the shape memory behavior of 4D printed tube preforms and their silicone elastomer matrix composites have been characterized. Measurements of shape recovery forces of the preform and composite were conducted using dynamic mechanical analysis (DMA). The braided microstructural parameters and shape recovery temperature have a significant effect on the preform shape memory behavior. The introduction of the silicone elastomer matrix greatly enhances the shape recovery force, shape recovery ratio, as well as radial compressive failure load of the 4D printed preform/silicone elastomer matrix composite. Building on these results, a potential application for 4D printed textile functional composites is presented.

Self-lubricating icephobic elastomer coating (SLIC) for ultralow ice adhesion with enhanced durability

A durable, self-lubricating icephobic elastomer coating (SLIC) was realized by systematic optimization of silicone oil infusion levels within a robust, weather-resistant silicone elastomer matrix for ultra-low ice adhesion, targeting high durability levels for a variety of applications operating in harsh icing environments such as in aviation. This coating combines multiple properties that synergistically enhance the ice release effect, i.e., hydrophobicity, low surface roughness, coating elasticity and lubrication enabled interfacial slippage. Supercooled drop impact icing experiments on rotating fan blades at rpms of same order of magnitude as commercial aircraft engines showed that the SLIC repeatedly reduced more ice on the coating than other commercial hydrophobic and superhydrophobic surfaces. In addition, the SLIC displayed long-term self-replenishing lubrication – after repeated mechanical abrasion of the surface, lubricants stored within the bulk of the coating could continuously migrate to the surface to replace the lost lubricants, thus promoting low ice adhesion. Durability tests also showed that the SLIC could withstand long-term micron-sized droplet impact at 25 m/s and acceleration loading up to 276 g without coating degradation. This study is the first to demonstrate a lubricated, self-replenishing coating material capable of repeated ice removal from the coating surface in harsh environmental conditions.