Inorganic Bottlebrush and Comb Polymers as a Platform for Supersoft, Solvent-Free Elastomers.

Due to their unique rheological and mechanical properties, bottlebrush polymers are inimitable components of biological and synthetic systems such as cartilage and ultrasoft elastomers. However, while their rheological properties can be precisely controlled through their macromolecular structures, the current chemical spectrum available is limited to a handful of synthetic polymers with aliphatic carbon backbones. Herein we design and synthesize a series of inorganic bottlebrush polymers based on a unique combination of polydimethylsiloxane (PDMS) and polyphosphazene (PPz) chemistry. This non-carbon-based platform allows for simple variation of the significant architectural dimensions of bottlebrush-polymer-based elastomers. Grafting PDMS to PPz and vice versa also allows us to further exploit the unique properties of these polymers combined in a single material. These novel hybrid bottlebrush polymers were cured to give supersoft, solvent-free elastomers. We systematically studied the effect of architectural parameters and chemical functionality on their rheological properties. Besides forming supersoft elastomers, the energy dissipation characteristics of the elastomers were observed to be considerably higher than those for PDMS-based elastomers. Hence this work introduces a robust synthetic platform for solvent-free supersoft elastomers with potential applications as biomimetic damping materials.

Body Temperature-Triggered Mechanical Instabilities for High-Speed Soft Robots.

Nature offers bionic inspirations for elegant applications of mechanical principles such as the concept of snap buckling, which occurs in several plants. Exploiting mechanical instabilities is the key to fast movement here. We use the snap-through and snap-back instability observed in natural rubber balloons to design an ultrafast purely mechanical elastomer actuator. Our design eliminates the need in potentially harmful stimulants, high voltages, and is safe in operation. We trigger the instability and thus the actuation by temperature changes, which bring about a liquid/gas phase transition in a suitable volatile fluid. This allows for large deformations up to 300% area expansion within response times of a few milliseconds. A few degree temperature change, readily provided by the warmth of a human hand, is sufficient to reliably trigger the actuation. Experiments are compared with the appropriate theory for a model actuator system; this provides design rules, sensitivity, and operational limitations, paving the way for applications ranging from object sorting to intimate human-machine interaction.

Hyperelastic Material Parameter Determination and Numerical Study of TPU and PDMS Dampers.

Dampers provide safety by controlling unwanted motion that is caused due to the conversion of mechanical work into another form of energy (e.g., heat). State-of-the-art materials are elastomers and include thermoplastic elastomers. For the polymer-appropriate replacement of multi-component shock absorbers comprising mounts, rods, hydraulic fluids, pneumatic devices, or electro-magnetic devices, among others, in-depth insights into the mechanical characteristics of damper materials are required. The ultimate objective is to reduce complexity by utilizing inherent material damping rather than structural (multi-component) damping properties. The objective of this work was to compare the damping behavior of different elastomeric materials including thermoplastic poly(urethane) (TPU) and silicone rubber blends (mixtures of different poly(dimethylsiloxane) (PDMS)). Therefore, the materials were hyper- and viscoelastic characterized, a finite element calculation of a ball drop test was performed, and for validation, the rebound resilience was measured experimentally. The results revealed that the material parameter determination methodology is reliable, and the data that were applied for simulation led to realistic predictions. Interestingly, the rebound resilience of the mixture of soft and hard PDMS (50:50) wt% was the highest, and the lowest values were measured for TPU.

Embedded NiTi Wires for Improved Dynamic Thermomechanical Performance of Silicone Elastomers.

The extraordinary properties of shape memory NiTi alloy are combined with the inherent viscoelastic behavior of a silicon elastomer. NiTi wires are incorporated in a silicon elastomer matrix. Benefits include features as electrical/thermal conductivity, reinforcement along with enhanced damping performance and flexibility. To gain more insight of this composite, a comprehensive dynamic thermomechanical analysis is performed and the temperature- as well as frequency-dependent storage modulus and the mechanical loss factor are obtained. The analyses are realized for the composite and single components. Moreover, the models to express the examined properties and their temperature along with the frequency dependencies are also presented.

Adherence Kinetics of a PDMS Gripper with Inherent Surface Tackiness.

Damage and fiber misalignment of woven fabrics during discontinuous polymer processing remain challenging. To overcome these obstacles, a promising switchable elastomeric adherence gripper is introduced here. The inherent surface tackiness is utilized for picking and placing large sheets. Due to the elastomer's viscoelastic material behavior, the surface properties depend on loading speed and temperature. Different peeling speeds result in different adherence strength of an interface between the gripper and the substrate. This feature was studied in a carefully designed experimental test set-up including dynamic thermomechanical, as well as dynamic mechanical compression analyses, and adherence tests. Special emphases were given to the analyses of the applicability as well as the limitation of the viscoelastic gripper and the empirically modeling of the gripper's pulling speed-dependent adherence characteristic. Two formulations of poly(dimethylsiloxane) (PDMS) with different hardnesses were prepared and analyzed in terms of their applicability as gripper. The main insights of the analyses are that the frequency dependency of the loss factor tanδ is of particular importance for the application along with the inherent surface tackiness and the low sensitivity of the storage modulus to pulling speed variations. The PDMS-soft material formulation exhibits the ideal material behavior for an adhesive gripper. Its tanδ varies within the application relevant loading speeds between 0.1 and 0.55; while the PDMS-hard formulation reveals a narrower tanδ range between 0.09 and 0.19. Furthermore, an empirical model of the pulling speed-dependent strain energy release rate G(v) was derived based on the experimental data of the viscoelastic characterizations and the probe tack tests. The proposed model can be utilized to predict the maximum mass (weight-force) of an object that can be lifted by the gripper.

From Playroom to Lab: Tough Stretchable Electronics Analyzed with a Tabletop Tensile Tester Made from Toy-Bricks.

Toy bricks are an ideal platform for the cost-effective rapid prototyping of a tabletop tensile tester with measurement accuracy on par with expensive, commercially available laboratory equipment. Here, a tester is presented that is not only a versatile demonstration device in mechanics, electronics, and physics education and an eye-catcher on exhibitions, but also a powerful tool for stretchable electronics research. Following the "open-source movement" the build-up of the tester is described and all the details for easy reproduction are disclosed. A a new design of highly conformable all-elastomer based graded rigid island printed circuit boards is developed. Tough bonded to this elastomer substrate are imperceptible electronic foils bearing conductors and off-the-shelf microelectronics, paving the way for next generation smart electronic appliances.

An imperceptible plastic electronic wrap.

Extremely compliant sub-2-µm sensor films enable temperature mapping on complex 3D objects, like integrated circuits on printed circuit boards, food packages, and on human skin. In their stretchable form, these metal films withstand strains up to 275%. This imperceptible electronic foil technology platform offers new avenues for the design of complex, hybrid rigid-island stretchable-interconnect electronic devices such as RGB light-emitting diode (LED) strips that can be stretched and twisted without impairing their function.

25th anniversary article: A soft future: from robots and sensor skin to energy harvesters.

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.

An ultra-lightweight design for imperceptible plastic electronics.

Electronic devices have advanced from their heavy, bulky origins to become smart, mobile appliances. Nevertheless, they remain rigid, which precludes their intimate integration into everyday life. Flexible, textile and stretchable electronics are emerging research areas and may yield mainstream technologies. Rollable and unbreakable backplanes with amorphous silicon field-effect transistors on steel substrates only 3 µm thick have been demonstrated. On polymer substrates, bending radii of 0.1 mm have been achieved in flexible electronic devices. Concurrently, the need for compliant electronics that can not only be flexed but also conform to three-dimensional shapes has emerged. Approaches include the transfer of ultrathin polyimide layers encapsulating silicon CMOS circuits onto pre-stretched elastomers, the use of conductive elastomers integrated with organic field-effect transistors (OFETs) on polyimide islands, and fabrication of OFETs and gold interconnects on elastic substrates to realize pressure, temperature and optical sensors. Here we present a platform that makes electronics both virtually unbreakable and imperceptible. Fabricated directly on ultrathin (1 µm) polymer foils, our electronic circuits are light (3 g m(-2)) and ultraflexible and conform to their ambient, dynamic environment. Organic transistors with an ultra-dense oxide gate dielectric a few nanometres thick formed at room temperature enable sophisticated large-area electronic foils with unprecedented mechanical and environmental stability: they withstand repeated bending to radii of 5 µm and less, can be crumpled like paper, accommodate stretching up to 230% on prestrained elastomers, and can be operated at high temperatures and in aqueous environments. Because manufacturing costs of organic electronics are potentially low, imperceptible electronic foils may be as common in the future as plastic wrap is today. Applications include matrix-addressed tactile sensor foils for health care and monitoring, thin-film heaters, temperature and infrared sensors, displays, and organic solar cells.

Zinc oxide nanowire rigid platforms on elastomeric substrates.

Zinc oxide nanostructured thin films are transparent semiconducting ceramics increasingly used in a wide range of integrated devices. This paper outlines a simple strategy to integrate arrays of zinc oxide nanostructured thin films on elastomeric substrates using templated patterning. The arrays are robust to large uniaxial strains (up to 20% strain), do not fracture, and maintain electrical functionality. The integration of brittle nanostructured semiconducting materials on elastomeric substrates opens promising routes for the manufacture of deformable and stretchable electronics.

Microstructured silicone substrate for printable and stretchable metallic films.

Stretchable electronics (i.e., hybrid inorganic or organic circuits integrated on elastomeric substrates) rely on elastic wiring. We present a technique for fabricating reversibly stretchable metallic films by printing silver-based ink onto microstructured silicone substrates. The wetting and pinning of the ink on the elastomer surface is adjusted and optimized by varying the geometry of micropillar arrays patterned on the silicone substrate. The resulting films exhibit high electrical conductivity (∼11 000 S/cm) and can stretch reversibly to 20% strain over 1000 times without failing electrically. The stretchability of the ≥200 nm thick metallic film relies on engineered strain relief in the printed film on patterned PDMS.

Elastic components for prosthetic skin.

We have developed two fundamental components to manufacture a prosthetic skin: a stretchable pressure sensor formed of piezoelectric elastomer/ferroelectret multilayer sandwiched between stretchable electrodes and stretchable thin-film transistors. The components are prepared and embedded in silicone rubber, a polymer, which mimics the mechanical compliance of human skin. We demonstrate the stretchability of the sensory unit. In both relaxed and stretched states, the soft sensor skin transduces kPa pressures into electrical currents in the µA range.

User-friendly, miniature biosensor flow cell for fragile high fundamental frequency quartz crystal resonators.

For the application of high fundamental frequency (HFF) quartz crystal resonators as ultra sensitive acoustic biosensors, a tailor-made quartz crystal microbalance (QCM) flow cell has been fabricated and tested. The cell permits an equally fast and easy installation and replacement of small and fragile HFF sensors. Usability and simple fabrication are two central features of the HFF-QCM flow cell. Mechanical, thermal, electrical and chemical requirements are considered. The design of the cell combines these, partially contradictory, requirements within a simple device. Central design concepts are discussed and a brief description of the fabrication, with a special focus on the preparation of crucial parts, is provided. For test measurements, the cell was equipped with a standard 50 MHz HFF resonator which had been surface-functionalised with a self-assembled monolayer of 1-octadecanethiol. The reliable performance is demonstrated with two types of experiments: the real time monitoring of phospholipid monolayer formation and its removal with detergent, as well as step-wise growth of a protein multilayer system by an alternating immobilisation of streptavidin and biotinylated immunoglobulin G.

Store depletion-activated CaT1 currents in rat basophilic leukemia mast cells are inhibited by 2-aminoethoxydiphenyl borate. Evidence for a regulatory component that controls activation of both CaT1 and CRAC (Ca(2+) release-activated Ca(2+) channel) channels.

The intestinal Ca(2+) transport protein CaT1 encoded by TRPV6 has been reported (Yue, L., Peng, J. B., Hediger, M. A., and Clapham, D. E. (2001) Nature 410, 705-709) to be all or a part of the Ca(2+) release-activated Ca(2+) channel (CRAC). The major characteristic of CRAC is its activation following store depletion. We expressed CaT1 in HEK293 cells and rat basophilic leukemia (RBL) mast cells and measured whole-cell currents by the patch clamp technique. In HEK293 cells, the expression of CaT1 consistently yielded a constitutively active current, the size of which was strongly dependent on the holding potential and duration of voltage ramps. In CaT1-expressing RBL cells, the current was either activated by store depletion or was constitutively active at a higher current density. CaT1 currents could be clearly distinguished from endogenous CRAC by their typical current-voltage relationship in divalent free solution. 2-aminoethoxydiphenyl borate (2-APB), which is considered a blocker of CRAC, was tested for its inhibitory effect on both cell types expressing CaT1. Endogenous CRAC as well as store-dependent CaT1-derived currents of RBL cells were largely blocked by 75 microm 2-APB, whereas constitutively active CaT1 currents in both RBL and HEK293 cells were slightly potentiated. These results indicate that despite the difference in the permeation properties of CRAC and CaT1 channels, the latter are similarly able to form store depletion-activated conductances in RBL mast cells that are inhibited by 2-APB.