Thin Elastomer Research Articles

Physicochemical characterization of the metamorphosis of film-forming formulations of betamethasone-17-valerate

Following topical application of a dermatological product, the loss (by evaporation and/or absorption through the skin) of volatile excipients will alter the composition of the formulation remaining on the tissue. This so-called metamorphosis impacts the concentration of the drug in the residual vehicle, (potentially) its physical form therein and, as a result, its uptake into and subsequent permeation through the skin. This research aimed to characterise – using primarily confocal Raman microspectroscopy – the metamorphosis of film-forming formulations of betamethasone-17-valerate (at different loadings) comprised of hydroxypropyl cellulose (film-forming agent), triethyl citrate (plasticizer) and ethanol (solvent). Dissolved and crystalline drug in the films were identified separately by their different characteristic Raman frequencies (1666 cm−1 and 1659 cm−1, respectively). These Raman measurements, as well as optical imaging, confirmed corticosteroid crystallisation in the residual films left after ethanol evaporation when drug concentration exceeded the saturation limit. In vitro release tests of either sprayed or pipette-deposited films into either aqueous or ethanolic receptor solutions revealed drug release kinetics dominated by the residual film post-metamorphosis. In particular, the rate and extent of drug release depends on the concentration of dissolved drug in the residual film, which is limited by drug saturation unless supersaturation occurs. For the simple films examined here, supersaturation was not detected and the solubility limit of drug in the films was sufficient to sustain drug release at a constant flux from the saturated films through a thin silicone elastomer membrane into an aqueous receptor solution for 30 h. Flux values were ∼ 1 μg cm–2h−1 from saturated residual films independent of the amount of crystallized drug present. Flux from subsaturated films was reduced by an amount that was consistent with the lower degree of saturation.

Efficient large-deformation shell elements for the simulation of electromechanical coupling effects in incompressible dielectric elastomers

So-called electro-active polymers are not only capable of undergoing very large deformations, but they also exhibit different electromechanical coupling effects due to their dielectric or electrostrictive characteristics. In the present paper the sub-class of dielectric elastomers is studied; in particular, we focus on thin dielectric elastomer shells, which undergo large deformations under an applied electric field. We develop a framework for the modeling and subsequent efficient simulation of thin structures made from incompressible dielectric elastomers. A thermodynamically consistent phenomenological continuum model based on the free energy density is adapted imposing the kinematic assumptions of Kirchhoff-Love shells in combination with including the thickness deformation and the electric field as independent unknown fields. To make the formulation accessible to finite element simulations, existing non-linear elastic shell mixed finite elements are extended to the present hyperelastic electromechanically coupled formulation for dielectric elastomer shells. Computational results of the shell theory are compared to three-dimensional results validating the proposed modeling and numerical simulation methodology, and showing high accuracy already for coarse finite element discretizations of the shell.

Dual cross‐linking of XNBR latex with epoxy‐functional calcium silicate particles for the production of accelerator‐free medical gloves

AbstractIn this work, an innovative dual cross‐linking strategy for carboxylated nitrile butadiene rubber (XNBR) latex using epoxy‐modified calcium silicate particles is presented. In their role as dual cross‐linker, the particles are able to form covalent bonds (nucleophilic ring opening of epoxy moieties) as well as ionic cross‐links (calcium ions of the inorganic core) across the carboxylic acid groups of the rubber. To characterize the curing efficiency, thin elastomer films are prepared by using a conventional coagulant dipping process and their cross‐link densities, curing kinetics, and tensile properties are investigated as a function of the concentration of the cross‐linker and curing time. The results show that latex films containing 5 phr of epoxy‐functional particles give the highest tensile strength, which is further improved by pre‐vulcanizing the liquid latex compound at 60°C for 30 min. The latex formulations are stable over 3 days and the pre‐cured films exhibit a high resistance against gamma sterilization and subsequent accelerated aging. Moreover, sterile films do not cause any skin irritation or skin sensitization reactions, showing the high potential of epoxy‐functional particles in the production of accelerator‐free hypoallergenic gloves.

Size effect and electrical ageing of PDMS dielectric elastomer with competing failure modes

Large-scale dielectric elastomer generators dielectric elastomer generators (DEGs) such as those employed in wave energy converter projects require a significant volume of electrically stressed materials. Meanwhile, predictions of energy output from such systems are generally extrapolated from electrical and mechanical breakdown measurements performed on small scale samples, where the presence of small defects can be extremely small. This can lead to overly optimistic upscaled predictions for the performance and reliability of full-scale devices. In this study, multilayer DEGs were prepared to evaluate the dielectric breakdown strength of thin polydimethylsiloxane PDMS elastomer at different values of active areas. The results indicated the presence of two separate breakdown mechanisms resulting in an enhanced size effect and a reduced reliability for the larger samples. Electrical ageing tests were performed on three different sample geometries and the dielectric breakdown strength was found to be marginally affected by the time under stress. A Weibull competing failure model was applied to the distribution of experimental breakdowns and electrical reliability was accurately modeled over more than four decades of variation in the electrode area.

(Invited) inkjet-Printed Soft Electronic Devices and Sensors for Wearable Health Monitoring Applications

Stretchable electronic devices and sensors built on thin elastomer substrates may find a wide range of applications in wearable health monitoring devices owing to their ability to conformably adapt to human skin for long-term and comfortable wearing. In this talk, I will present our work on developing intrinsically-stretchable electronic materials that are formulated as electronic inks and patterend using a highly scalable and low-cost inkjet printing process. A variety of soft sensors including silver nanoparticle based ultrasensitive resistive strain sensor and pressure sensor, porous elastomer dielectric based capacitive pressure sensor, and multimodal sensor that can differentiate pressure, strain, and temperature stimuli have all been demonstrated on ultrathin polydimethylsiloxane (PDMS) substrate by printing. These devices have been used in applications including motion and gesture tracking, voice recognition, arterial pulse waveform and photoplethysmogram monitoring. In addition, I will also report our work on the development of a unique porous PDMS sponge electrode as a gel-free dry interface for motion-artifact tolerant recording of electrophysiological signals. The porous structure of the soft electrode significantly increases the effective contact area and lowers the electrode-skin impedance, which result in improved signal-to-noise ratio. Compared to commercial rigid electrodes, the soft and thin form factor also makes the spongy electrode well-suited for long-term recording of high quality biopotential signals including electrocardiogram (ECG) and electromyography (EMG) on ambulatory patients. As a demonstration of its clinical usage with a focus on maternal and fetal health monitoring, I will show an array of such soft electrodes integrated on a printed patch of textile-based interconnections, and used for recording and spatiotemporal mapping of the uterine contraction patterns from patients in active labor.

Pretension-Free and Self-Recoverable Coiled Artificial Muscle Fibers with Powerful Cyclic Work Capability.

Similar to natural muscle fibers, coiled artificial muscle fibers provide a straightforward contraction. However, unlike natural muscle fibers, their recovery from the contracted state to the initial state requires high stress, resulting in almost zero work during a full actuation cycle. Herein, a self-recoverable coiled artificial muscle fiber was prepared by conformally coating an elastic carbon nanotube (CNT) fiber with a very thin liquid crystal elastomer (LCE) sheath. The as-obtained muscle fiber demonstrated excellent actuation properties comprising 56.9% contractile stroke, 1522%/s contraction rate, 7.03 kW kg-1 power density, and 32,000 stable cycles. The LCE chains were helically aligned in a nematic phase, and the phase change of the LCE caused by Joule heating drove the actuation process. Moreover, the LCE/CNT fiber had a well-separated, torsionally stable, and elastic coiled structure, which permitted large contractile strokes and acted as an elastic template for external-stress-free recovery. Thus, the use of self-recoverable muscle fibers to mimic the natural muscles for object dragging, multidirectional bending, and quick striking was demonstrated.

Nanoparticle-Based Tough Polymers with Crack-Propagation Resistance.

Although thin elastomer films of polymer nanoparticles are regarded as environmentally friendly materials, the low mechanical strength of the films limits their use in various applications. In the present study, we investigated the fracture resistance of latex films composed of acrylic nanoparticles where a small quantity of a rotaxane crosslinker was introduced. In contrast to conventional nanoparticle-based elastomers, the latex films composed of the rotaxane-crosslinked nanoparticles exhibited unusual crack propagation behavior; the direction of crack propagation changed from a direction parallel to the crack to one perpendicular to the crack, resulting in an increase in tear resistance. These findings will help to broaden the scope of design of new types of tough polymers composed of environmentally friendly polymer nanoparticles.

Skin‐Mountable Vibrotactile Stimulator Based on Laterally Multilayered Dielectric Elastomer Actuators (Adv. Funct. Mater. 23/2023)

Dielectric Elastomer Actuators In article number 2213589, Kilwon Cho and co-workers report a flexible and skin-mountable vibrotactile-stimulator based on thin lateral dielectric elastomer actuators. The actuator shows a significant actuation at low operating voltage because of a laterally aligned dielectric multilayer structure with short dielectric distance and a soft elastomer/ionic liquid composite. The vibrotactile-stimulator covers a broad perception frequency/amplitude range of human skin.

Crosslinking ofXNBRlatex with multifunctional epoxides: An energy‐efficient curing strategy for the fabrication of accelerator‐free rubber gloves

AbstractHerein, we pursue an efficient curing strategy using multifunctional epoxides, which undergo a nucleophilic ring opening reaction across the free COOH groups of a carboxylated nitrile butadiene rubber (XNBR). Following a prevulcanization protocol, we cure the rubber in the latex phase at 60°C. Thin films are then prepared by using a conventional coagulant dipping process, in which the final cure degree of the elastomer is reached during drying (postvulcanization). In a comprehensive approach, cure kinetics, tensile properties, and crosslink density of the thin elastomer films are studied as a function of the functionality and concentration of the applied crosslinker. The results clearly show that films with high tensile strength (39 MPa) are obtained without prevulcanization. Stability tests reveal that the shelf life of the latex formulations is limited to less than 1 day due to hydrolysis reactions of the epoxy groups of the water‐soluble crosslinker. However, the storage stability can be extended to several days if nonwater‐soluble epoxy crosslinkers are applied. The decent shelf life together with the fast cure rates and excellent mechanical properties make epoxy crosslinking a promising curing strategy for preparing dipped rubber gloves without hyperallergenic accelerators used in classical sulfur vulcanization.

Fiber pumps for wearable fluidic systems

Incorporating pressurized fluidic circuits into textiles can enable muscular support, thermoregulation, and haptic feedback in a convenient wearable form factor. However, conventional rigid pumps, with their associated noise and vibration, are unsuitable for most wearables. We report fluidic pumps in the form of stretchable fibers. This allows pressure sources to be integrated directly into textiles, enabling untethered wearable fluidics. Our pumps consist of continuous helical electrodes embedded within the walls of thin elastomer tubing and generate pressure silently through charge-injection electrohydrodynamics. Each meter of fiber generates 100 kilopascals of pressure, and flow rates approaching 55 milliliters per minute are possible, which is equivalent to a power density of 15 watts per kilogram. The benefits in design freedom are considerable, which we illustrate with demonstrations of wearable haptics, mechanically active fabrics, and thermoregulatory textiles.

Skin‐Mountable Vibrotactile Stimulator Based on Laterally Multilayered Dielectric Elastomer Actuators

AbstractSkin‐stimulation technology has attracted intense attention for virtual/augmented reality applications and tactile‐feedback systems. However, bulky, heavy, and stiff characteristics of existing skin‐stimulating devices limit their wearability and comfort, thus disturbing the immersive experience of users. This study presents a new type of thin and lightweight dielectric elastomer actuator for developing a skin‐mountable vibrotactile stimulator. A new methodology is suggested to enhance the operating efficiency of dielectric elastomer actuators based on a laterally aligned dielectric multilayer structure (≈900 layer) with short dielectric distance (≈10 µm) and a soft elastomer/ionic liquid composite with low modulus and high dielectric constant. With the improved structural/material properties, the flexible actuator exhibits high displacements at low operating voltage (<200 V) over a wide frequency range (≈800 Hz). Therefore, the finger‐band type vibrotactile stimulator based on the laterally multilayered dielectric elastomer actuators can exert indentations that have the ability of stimulating all mechanoreceptors in human skin over the full perception frequency/amplitude range. In addition, the actuator shows a high electromechanical stability for long‐term operation due to time‐efficient and precise fabrication process using sophisticated photolithography and secondary sputtering. Therefore, this vibrotactile stimulator shows high promise for use in tactile‐assistive devices, tactile communications, haptic feedback, and beyond.

Manufacturing Process for Multilayer Dielectric Elastomer Transducers Based on Sheet-to-Sheet Lamination and Contactless Electrode Application

Dielectric elastomer transducers (DETs) consist of thin elastomer films and compliant conductive electrodes on each side. Several DE-based systems, e.g., DE actuators, loudspeakers or sensors, have great potential in industrial applications. Different manufacturing processes for DE-based transducers have been realized so far. An alternative manufacturing process, based on sheet-to-sheet lamination of pre-fabricated elastomer films, appears promising for the manufacturing of large numbers of DETs, as, on the one hand, the commercially available elastomer films produced are characterized by homogeneous and reproducible properties and, on the other hand, the curing time during processing can be avoided. However, the handling of thin elastomer films and a controlled film transfer onto a surface is a challenge, as pre-stretch and wrinkles can easily occur. The presented sheet-to-sheet lamination mechanism facilitates controllable film handling well, whereby a high number of DE layers can be achieved. This paper focuses on the developed, automated film lamination of pre-cut film sheets and the electrode application via a jetting system, representing core components of the corresponding overall manufacturing process of DE laminates. The process realization on a laboratory scale is presented and manufactured DET specimens investigated for validation purposes.

Effects of stripe domains on the loading-unloading hysteretic behavior of monodomain nematic elastomers

The stress-director coupling of monodomain nematic elastomers may lead to soft deformation and the appearance of stripe domains under uniaxial tension. Based on a continuum mechanical model and finite element method, we investigated the loading-unloading hysteretic behavior of thin monodomain nematic elastomer sheets. Under orthogonal uniaxial loading tests, stripe domains appeared in the center of the specimens after the critical strain was reached. Furthermore, a stress peak was observed, which was followed by a stress plateau. During unloading, these stripe domains did not disappear, and the unloading curve was significantly lower than the loading curve, resulting in large stress-strain hysteresis and unloading residual strain. Additionally, the presence of stripe domains led to an increased strain rate correlation in stress-strain curves, while significantly extending the period of strain recovery without unloading constraints.

Soft elastomer coatings for ionogels

Ionogels are stretchable ionic conductors with unique advantages, such as non-volatility, thermal, chemical, and electrochemical stability. Nevertheless, the exchange of solvent between ionogels and the external environment hinders their applications. Ionogel has the problem of ionic liquid (IL) leakage, or the ionogel absorbs moisture from the atmosphere. Here we develop a strategy to coat thin elastomers on ionogels. Silanes are added into the precursor of ionogels and elastomers to form a strong interface adhesion between the two polymer networks. The elastomer coating can prevent ionogels from absorbing moisture or leaking ionic liquid, while does not change their transparency, stretchability, and conductivity. We establish a procedure to quantify the loss of ionic liquid from ionogels, with the calibrated relation of conductivity/volume ratio. We find that the elastomer coatings work well because of their low diffusivity to ionic liquid on the order of 10−16m2/s. The coating strategy would be universal to various ionogels and elastomers, thus we expect its application in soft robots and stretchable devices made of ionogels.

Electromechanical stability of wrinkled dielectric elastomers

Thin dielectric elastomers with compliant electrodes can experience two types of instability: wrinkling, which occurs to relax in-plane compressive stresses through out-of-plane deformations, and electromechanical instability, a sudden thinning deformation that often preludes to dielectric breakdown. In this work we study electromechanical instability of a wrinkled dielectric membrane. The theory, which is based on an averaged description of wrinkles through tension field theory, provides a modification of the so-called “Hessian method” to account for the presence of wrinkles. These findings are consistent with available experimental data, showing that wrinkles in dielectric membranes can be “stable”, in the sense that they can be maintained and restrained, until when electromechanical instability takes place in the wrinkled membrane. We finally provide examples involving both homogeneous and inhomogeneous deformations, showing that the hierarchy of wrinkling and electromechanical instability can be tuned by suitable choices of the electro-mechanical controls.

In-plane dual-electrode triboelectric nanogenerator based on differential surface functionalization

Stretchable triboelectric nanogenerators (s-TENGs) are promising power sources for self-powered wearable electronics. Conventional single-electrode s-TENGs require an external ground electrode to form a closed circuit. We propose an in-plane dual-electrode s-TENG based on differential surface functionalization to eliminate the external ground electrode. The s-TENG comprises fully stretchable materials such as carbon nanotube thin films and elastomers. The top surface of the elastomer was functionalized to produce a difference in electronegativity between the two electrode regions. A high output power density of 3.5 W m−2 was obtained by tapping the two electrode regions by hand.

Measurement of Film-Elastomer Interface Adhesion by Continuous Buckling.

Measurement of interfacial properties between thin films and elastomers is investigated. As a prototype, the interface adhesion between a graphite nanoflake and an elastic polymer is determined by topography imaging of the induced graphite buckles using atomic force microscopy. A theoretical analysis is carried out to establish the relationship among interface adhesion, elastic strain energy, and buckle surface area. The strain energy of the graphite is obtained by employing an elastic plate deflection theory. To introduce the buckles, different methods are applied, including thermal contraction, bending, and stretching, and different substrate materials, namely, polydimethylsiloxane and polystyrene, are used. The uncertainty in measuring the interface adhesion is discussed. These investigations provide a promising approach to characterize the interfacial properties of multilayer samples.

Printed and Laser-Scribed Stretchable Conductors on Thin Elastomers for Soft and Wearable Electronics

As printed electronics is evolving toward applications in biosensing and wearables, the need for novel routes to fabricate flat, lightweight, stretchable conductors is increasing in importance but still represents a challenge, limiting the actual adoption of ultrathin wearable devices in real scenarios. A suitable strategy for creating soft yet robust and stretchable interconnections in the aforementioned technological applications is to use print-related techniques to pattern conductors on top of elastomer substrates. In this study, some thin elastomeric sheets—two forms of medical grade thermoplastic polyurethanes and a medical grade silicone—are considered as suitable substrates. Their mechanical, surface, and moisture barrier properties—relevant for their application in soft and wearable electronics—are first investigated. Various approaches are tested to pattern conductors, based on screen printing of 1) conducting polymer [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)] or 2) stretchable Ag ink and 3) laser scribing of laser-induced graphene (LIG). The electromechanical properties of these materials are investigated by means of tensile testing and concurrent electrical measurements up to a maximum strain of 100%. Performance of the different stretchable conductors is compared and rationalized, evidencing the differences in onset and propagation of failure. LIG conductors embedded into MPU have shown the best compromise in terms of electromechanical performance for the envisioned application. LIG/MPU showed full recovery of initial resistance after multiple stretching up to 30% strain and recovery of functionality even after 100% stretch. These have been then used in a proof-of-concept application as connectors for a wearable tattoo biosensor, providing a stable and lightweight connection for external wiring.

Investigation And Simulation Of Dielectric Elastomer Actuators Used In Artificial Muscle Applications

Dielectric Elastomer Actuator (DEA) consists of a thin dielectric elastomer membrane sandwiched between two electrode layers. When low current high voltage is applied to the two conductive layers, opposite loads occur on the surface which tends to pull one another. This voltage application causes thinning in width and expansion in surface area. DEAs are the favourite subject of research due to their low cost advantages, fast response, high energy density, wide deformation and softness. Due to the rigidity of the electric motors and the metal components of the robot, soft-acting robots using DEA are preferred to perform complex tasks instead of conventional robots. Robots with DEA have higher flexibility and better adaptability. Therefore, soft robots are popular topics in robotics research. DEAs are the best candidate materials for next-generation soft robot actuators and artificial muscles. In this study, simulation of robotic system has been realized by using DEAs calculation methods. Simulation results were compared with the data obtained from the application. This study will be the source of future studies on the subject. In the simulation, Matlab 2016 student and Labview Home and Students programs were used.

Fully Polymeric Domes as High-Stroke Biasing System for Soft Dielectric Elastomer Actuators.

The availability of compliant actuators is essential for the development of soft robotic systems. Dielectric elastomers (DEs) represent a class of smart actuators which has gained a significant popularity in soft robotics, due to their unique mix of large deformation (>100%), lightweight, fast response, and low cost. A DE consists of a thin elastomer membrane coated with flexible electrodes on both sides. When a high voltage is applied to the electrodes, the membrane undergoes a controllable mechanical deformation. In order to produce a significant actuation stroke, a DE membrane must be coupled with a mechanical biasing system. Commonly used spring-like bias elements, however, are generally made of rigid materials such as steel, and thus they do not meet the compliance requirements of soft robotic applications. To overcome this issue, in this paper we propose a novel type of compliant mechanism as biasing elements for DE actuators, namely a three-dimensional polymeric dome. When properly designed, such types of mechanisms exhibit a region of negative stiffness in their force-displacement behavior. This feature, in combination with the intrinsic softness of the polymeric material, ensures large actuation strokes as well as compliance compatibility with soft robots. After presenting the novel biasing concept, the overall soft actuator design, manufacturing, and assembly are discussed. Finally, experimental characterization is conducted, and the suitability for soft robotic applications is assessed.