Symposium SM01 : Soft Materials, Sensors, Electronics, Displays and Actuators—Functional Components for Soft Machines and Robots

"Robotics" is a field with broad interest: it combines mechanical engineering, information science, and animal physiology with manufacturing, workforce development, economics, and other areas. The most highly developed classes of robots have been build based on conceptual models provided by the body-plans of animals with skeletons (humans, horses), and have made it possible to carry out tasks that humans and animals could not (for a variety of reasons). We are interested in robots based a different, simpler class of organisms (invertebrates: starfish, worms, octopi). Because these organisms, and the robots having designs stimulated by them, have no skeletons, they provide enormous opportunities in materials and polymer science, rather than primarily in mechanical engineering. This seminar will outline one approach to soft robots, and suggest problems and opportunities in this new field.

The ultimate objective of our research in the WPI Soft Robotics Laboratory is developing robots that can be commonly used in real-world environments and in close proximity to humans. Towards this objective, we identify two key requirements; elasticity and accessibility. Soft robotic systems inherently satisfy these requirements. Nature harnesses mechanical compliance to find elegant solutions for many problems facing the robotics community. Inspired greatly by biology, we envision future robotic systems to embrace mechanical compliance with bodies composed of soft and hard components as well as integrated electronic and sensory infrastructure. Our recent research adresses this challenge on theoretical modeling, design, fabrication, actuation, sensing, and control solutions for soft robotic systems.

A soft body offers safety and adaptability, which makes robots more suitable for use in a wide range of applications from human-robot interaction to search and rescue. Our approach to create flexible intelligent machines uses either soft materials or geometric arrangements of otherwise rigid elements. In the first track, we utilize fluidic actuation of elastomeric materials to generate organic deformations defined by geometry and embedded constraints in the substrate. Despite its advantages, mechanical compliance also violates many inherent assumptions in traditional robotics. Thus, a complete soft robot architecture requires new approaches to utilize accurate theoretical models that capture the nonlinear response of elastomeric materials, proprioception that provides rich sensory information while remaining flexible, and motion control under significant time delay. Our proposed solutions utilize nonlinear material models that predict motion and force output, integrated composite magnetic deformation and force sensing, and feedback control of soft actuation to address each of these issues.

In the second track, we create flexible robotic mechanisms in a cost- and time-effective manner, with the goal of building robots as easily as printing a document. We use common planar fabrication methods to create origami-inspired foldable bodies. Utilizing a hierarchical development process of foldable robotic platforms as combinations of fundamental building-blocks, we can achieve arbitrary levels of complexity and functionality. Such designs make extensive use of foldable linkage mechanisms and other kinematic modules, which introduce a set of parameters that can be optimized for relevant task specifications and the crease pattern can be modified accordingly. This new philosophy of robot development offers improved design flexibility, ease of fabrication, cost-effectiveness, and lightweight robotic bodies compared to traditional systems.

Stretchable electronics maintain their function when subjected to stress or strain and are therefore useful for enabling electronics for wearable, implantable, and other types of novel electronics. However, these devices often require power or electrical signals from non-stretchable components. One approach is to use induction to transmit these signals wirelessly, but this has limited efficiency. Another approach is to physically connect stretchable devices to flexible substrates. These connections must maintain a mechanical and electrical connection under stress arising from deformation of the stretchable substrate. The ability to create a robust electrical connection between these mechanically disparate components may enable new types of hybrid devices. In this study we present a simple method to adhere and thereby fabricate such connections. The adhesion at the interface arises from surface chemistry that forms strong, covalent bonds. The utilization of liquid metals as the conductor provides stretchable interconnects because liquid metals are both conductive and fluidic. We characterized the mechanical and electrical properties of these hybrid devices to identify the performance and limits. While we focus on silicone elastomers and liquid metals, this approach can apply to a wide variety of other methods for fabricating stretchable devices.

Rigid-bodied robots generally excel at specific tasks in structured environments, but lack the versatility and adaptability required to interact with and locomote within the natural world. To achieve maximum versatility in soft robot design, we present robotic skins that can wrap around arbitrary soft bodies to induce desired motions and deformations. Robotic skins integrate actuation and sensing into a single conformable material, and may be leveraged to create a multitude of controllable soft robots with different functions or gaits to accommodate the demands of different environments. We show that attaching the same robotic skin to a soft body in different ways, or to different soft bodies, leads to unique motions. Further, we show that combining multiple robotic skins enables complex motions and functions. We demonstrate several instances of this versatile soft robot design paradigm by creating a continuum robot, multiple locomotion robots, and a grasping end-effector, all using the same 2D robotic skins reconfigured on the surface of various 3D soft, inanimate bodies.

Magnetic iron microparticles were embedded in shape memory polymer films through solvent casting. These composites exhibit bifunctional responses because they can be actuated by both magnetic fields and light, which triggers photothermal heating and softens the polymer, thus modulating its magnetic responsiveness. Temporary shapes obtained through combined magnetic actuation and photothermal heating can be locked after switching off the light and magnetic field. When the light is subsequently turned on without the magnetic field, the device returns to its permanent shape. This concept enables the design of materials that are reconfigurable rather than merely responsive. In cantilevers, multiple locking and unlocking cycles are demonstrated. In a flower with magnetic petals, pulsing the light can be used to select which petals are lifted. Scrolls demonstrate that the permanent shape of the film can also be changed, and the scroll can be frozen in open, closed, and intermediate configurations. A bistable snapper can similarly be magnetically actuated and optically locked and unlocked. Reconfigurable grabbers can pick up and release the objects in a repeatable fashion by combining magnets to reset the grabber and light to drive release of an object. Inexpensive permanent magnets and light emitting diodes were used for actuation. Models that account for the magnetic and elastic energies can predict the observed experimental behaviors of these devices. The concept of combining shape memory, magnetic actuation, and photothermal heating significantly enhances the capabilities of magnetic actuators for soft robotics.

Shape memory alloy (SMA) is widely utilized material as an actuation source of soft actuators by embedding it in elastomeric polymers. It is known that the SMA possesses simple actuation mechanism and high power density, but also has limitations in terms of strain range and actuation speed. In this research, we designed, fabricated, and evaluated an SMA based high-speed microscale actuator. Diamond-shaped SMA frame structures with 1–1.5 μm in thickness were fabricated using a focused ion beam (FIB) milling process. The behavior of these structures under mechanical deformation and changes in thermal conditions was investigated to use these as a driving source for a high-speed microscale actuator. The diamond-shapes frame structure allows large elongation range (~40 %) compared to bulk SMA materials (~5 %) with the aid of spring-like behavior under tensile deformation. In addition, shape memory effect (SME) was triggered at these structures by applying thermal energy delivered with ultraviolet (UV) laser. The reaction force and response speed were investigated according to changes in the laser switching speed and the irradiation energy. The fast heating and cooling phenomenon caused by the scale effect allows high-speed actuation of SMA up to 1,500 Hz, which is 40 times faster than the previous result (35 Hz). We expect that the proposed actuators can contribute to the development of micro- and nanoscale soft robots, electronic equipment or medical devices.

Soft robotic systems are currently limited by the soft actuators that power them. They predominantly rely on fluidic actuators, which limit speed and efficiency. Electrically powered muscle-mimetic actuators, such as dielectric elastomer actuators (DEAs) offer high performance actuation but they come with their own challenges. Being driven by high electric fields, DEAs are prone to failure by dielectric breakdown and electrical ageing. More importantly, DEAs are hard to scale up to deliver high forces, as large areas of dielectric are required (e.g. in stack actuators), which are much more likely to experience premature electrical failure, following the Weibull distribution for dielectric breakdown.
Here an overview is presented of a new class of self-sensing, high-performance artificial muscles, termed Hydraulically Amplified Self-healing ELectrostatic (HASEL) transducers. HASEL actuators harness an electro-hydraulic mechanism to activate soft hydraulic architectures, and combine the versatility of soft fluidic actuators with the muscle-like performance and self-sensing abilities of DEAs, while simultaneously addressing critical challenges. HASEL actuators autonomously self-heal from electrical and mechanical damage, and thus feature reliable and scalable performance. Several different designs and fabrication strategies, as well as prototypical applications are introduced; a specific geometry of HASEL is shown to linearly contract upon activation with voltage, thereby closely mimicking biological muscle. These results indicate that HASEL actuators promise to enable robust and versatile actuation in next- generation soft robotic devices.

Adaptive materials that respond mechanically to a stimulus are of interest for a wide range of technologies, including soft robots, responsive optoelectronics, environmental control systems, and controlled release of pharmaceutics, pesticides or antifouling agents. Typically these materials demonstrate relatively simple mechanical responses such as shrinking, expanding, or bending. The art of origami, where localized deformation at folds is used to create complex structures and mechanisms, provides an opportunity to harness the mechanical response of adaptive materials and channel it into an engineered structure. Frequently, adaptive materials are used at a hinge to demonstrate self-folding; however integration of these materials with more complex mechanical systems, and realization of possible emergent behavior, is not well understood. Using the waterbomb base, chomper and simple combinations thereof with an environmentally sensitive adaptive material, PEDOT:PSS, we demonstrate that the placement of the adaptive material dictates whether a structure undergoes fold inversion or reversible reconfiguration, such as snapthrough between bistable states. Design and behavior is understood by modeling the response of the origami system to the application of point, linear or areal forces arising from the adaptive material’s shape, location and response to the local environment. In contrast to externally applied forces, the integration of the adaptive material results in continuous force application, even during bi-stability. Following these principles, we also demonstrate autonomous reconfiguration of complex origami structures driven by the progress of the structure through different regions in its environment.

Stimuli-responsive structures find use in many applications and areas of technology, including soft robotics, transportation, biomimetic structures, and many others. However, before realizing these applications, “hands-free” actuators and the materials they are made of must respond reliably and controllably to external stimuli. A variety of stimuli, such as light and heat, are viable options to control the geometric response of materials. Light is a convenient actuation mechanism when compared with other options, such as uniform heating or solvent exposure, due to its biocompatibility and versatility of environment. This talk will focus on planar, polymer sheets that can change shape in a controlled manner in response to a variety of light sources. In our method, we use pre-strained polystyrene sheets that shrink in-plane by ~ 55% when heated above the glass transition temperature (~ 100°C). We pattern these sheets with ink from an inkjet printer_. When exposed to light, the inked regions on the surface absorb light preferentially (relative to the ink-free regions), heat up and release strain gradually across the thickness of the sheet. This results in local out-of-plane deformation within the sheet. While previous work has utilized this approach to create hinges that lead to sharp folds, the present work distributes ink over the surface of the polymer to create positive and negative Gaussian curvature (e.g., a sphere). The degree of curvature relates directly to the ink distribution on the polymer surface as well as the aspect ratio and geometry of the starting substrates. Experimental results are qualitatively and quantitatively compared with finite element modeling results exhibiting excellent agreement. We use this approach to create complex shapes, such as spheres and grippers, within 10 seconds of sample exposure to IR light. Notably, we demonstrate grippers that can grasp objects > 24,000 times their own weight. Having the ability to produce folds and curvature from a planar sheet presents an opportunity to produce stimuli-responsive actuators, sensors, and robotic parts from thermoplastic materials.

The development of new actuatable materials that can change their size, shape, and mechanical properties in response to an external stimulus, or multiple stimuli, has gained considerable attention recently. Such functional materials can be applied towards making new functional sensors, self-healing materials, and soft robotics, to name a few examples. The use of redox-active polymers in stimuli-responsive materials is particularly attractive because they can be activated either chemically or electrochemically. My research group has recently developed a new unimolecular redox-responsive macromolecular platform consisting of up to ten 4,4′-bipyridinium (a.k.a. viologen) subunits that are tethered by water-soluble and flexible oligoethylene glycol spacers. Upon chemical reduction of the individual dicationic viologen subunits to the corresponding radical-cations (i.e., V²⁺ to V^●+), a decrease in electrostatic repulsion occurs, as well as non-covalent intramolecular recognition between the main-chain radical viologen subunits positioned along the polymer backbone; a combined process which results in the collapse of the redox-active polymer chain. Incorporation of these oligoviologens into a hydrogel network primarily made of polyethylene glycol, followed by chemical reduction, resulted in the reversible contraction of a series of hydrogels down to one-tenth of their original starting volumes, where the majority of contraction occurred during the first 25–45 minutes. My group is now exploring how this process can be controlled by other stimuli. In my presentation, I will discuss the activation of our viologen-based soft actuators using a photoredox-based mechanism, and I will highlight how these materials are capable of lifting weights over defined distances, and therefore performing work as an artificial molecular muscle.

Small-scale soft actuators and stimuli-responsive materials have been the subject of intensive research because of the potential applications as artificial muscles in soft robots. Recently, NiOOH/Ni(OH)₂ has been identified as a promising actuating material that can expand or contract as a redox reaction occurs between the oxyhydroxide and hydroxide states [1]. This material can be used as an active coating on a thin substrate to achieve a “skin-effect” actuator that flexes with large movements. However, the performance and durability of such a design are limited by the interfacial strength between the active and passive layers.

In this work, we demonstrate that, a new design of the skin-effect actuators, made by electrodepositing a few microns’ thick of the NiOOH/Ni(OH)₂ active material on top of a gold-sputtered nano/micro-porous polycarbonate membrane, can exhibit unprecedented device strains with excellent stability and durability, due to the enhanced mechanical interlocking provided by the nano/micro pores on the passive substrate. For an actuator strip 15 mm long, 3 mm wide and 12 mm thick, a bending curvature as large as 1.3mm^-1 about the width axis (i.e. a radius of curvature of 0.77 mm over the 15 mm length), with cumulative angular deflection > 1000° (i.e. 2.8 revolutions) at the free end, can be achieved in an alkaline electrolyte environment under a triggering voltage of less than 1 V. Under cyclic potential triggering, the actuation response is fast and stable. Although the intrinsic actuation strain of the NiOOH/Ni(OH)₂ material is only about 0.2%, and actually, higher strains are not desirable as the elastic limit would be exceeded, the work density is high at 27 kJ/m³, which is comparable to mammalian muscles. The present results also demonstrate that the typical largest actuation curvature has a linear relationship with the thickness or plating time of the actuating layer, in good agreement with Stoney’s theory.

Under higher scanning frequencies of the triggering potential corresponding to actuating cycles shorter than 5 s, the motion of the actuator mimics well the muscle-like motion of a fishtail. The ultra-large deformation, low triggering voltages, and ease and low costs to fabricate, are significant merits of the present material system that justify its future developments in soft robots and other biomimetic devices.

Reference:

[1] K.W. Kwan, N.Y. Hau, S.P. Feng and A.H.W. Ngan, (2017), “Electrochemical actuation of nickel hydroxide/oxyhydroxide at sub-volt voltages”, Sensors and Actuators B: Chemical 248, 657-664.

In this study, we demonstrated a methodology for the design and simulation of self-bending bilayer microstructures using pH-sensitive hydrogels. The goal of this study is to characterize the performance of self-bending behavior, provide fundamental information for engineering design, and validate simulations of physics-based models with experiments, theoretical approach, and finite element method (FEM) simulation. The self-bending behavior of hydrogel bilayers, composed of an active layer and a passive layer, is completely reversible and allows the structure to fold and unfold without permanent deformation. The effects of design parameters on the self-bending behavior of the microstructures of hydrogel bilayers are explored by varying the extrinsic geometric variables. The study of FEM simulations verifies that the final shape of the bilayer sheet is governed by intrinsic properties, such as the elastic modulus and the swelling ratio, and extrinsic geometrical factors, including the thickness ratio of the bilayer and the aspect ratio of the structure. Self-bending flower-shaped microstructures can be realized by a simple bilayer concept. Simultaneously, programmable deformation of self-bending microstructures can be simulated, forming a basis for the design of relevant actuating models. This simple reversible actuating system is, therefore, promising for a wide range of applications, such as delivery systems, cell encapsulation, artificial tissue, and soft robotics, owing to its simplicity of fabrication, high reversibility of actuation, and biocompatibility of materials. This fundamental investigation not only provides insights into the bending of bilayer microstructures but also has important implications for responsive and intelligent soft matter.

Man-made rubbers, have excellent mechanical toughness but are inherently stiff due to topological constraints known as entanglements, which prevent polymer chains from crossing and act as crosslinks. Thus, entanglements place a physical lower bound on how soft elastomers can be made without adding liquid fillers. As such, soft materials with Young’s moduli, E<0.2MPa are composed of multiple components and are not chemically pure substances. By introducing liquid fillers to polymeric materials, the stiffness may be decreased, however this swollen material is mechanically brittle and leaks the filler material upon deformation inhibiting their use in many applications. Additionally, swelling with solvent hindering their ability to be formed or molded into structures. I will discuss the synthesis of soft, programmable elastomers using controlled polymerization techniques to fabricate triblock co-polymers with a middle block of silicone polymers in a ‘bottlebrush’ architecture which eliminates entanglements making the material soft without the necessity for solvent. The triblock polymer includes functional end blocks composed of a thermoplastic, polystyrene, which undergoes a glass transition upon cooling, allow this material to thermoset reversibly, i.e. 3D printed. I will present the synthesis and mechanical characterization of this material, with some preliminary data on high resolution 3D printing of finely detailed soft structures.

Sustainability, recently, is getting intensive attention from research communities due to critical environmental concerns. Lignin is the second most abundant renewable polymers on earth, after cellulose. However, utilization of lignin as a feedstock for value added products is very limited. We report a method to control the macromolecular structure of lignin to prepare thermo-responsive shape memory composites. Lignin was reacted with a nitrile butadiene rubber in the melt. The resulted composites exhibited excellent shape memory characteristics. The shape of materials can be programmed by external stress at selected temperatures. Physical crosslinks induced by hydrogen-bonding between plentiful hydroxyl groups of lignin and nitrile groups of rubber fix the pre-determined (temporary) shape of the materials. The chemical crosslinks within the composites recover the permanent (original) shape of the materials by applying an external stimulus, temperature. The crosslinked density and molecular rigidity of these materials were tuned by a thermal annealing process. The presence of unstable ether linkages within β-O-4’, β-5’ phenylcoumaran and β-β’ resinol structural units of lignin promoted the generation of free-radicals for crosslinking reactions. Thermal activation of the lignin based composites indicated significant improvement of the elastic work density and the shape memory characteristics. The chemistry of various lignin molecules, their corresponding thermo-responsivity and programmable shape characteristics of the lignin based composites are discussed.

Soft elastomeric surfaces with dynamically tunable dry adhesion have ample applications in transfer printing, pick-and-place handling processes, as well as climbing robots, among others. Polydimethylsiloxane (PDMS) is one of the most commonly used elastomers in soft robotics due to its manufacturability and desirable mechanical properties. Previous studies have shown that by inserting a rigid core into cylindrical PDMS posts, the dry adhesion strength of the PDMS surface to an opposing rigid surface can be significantly enhanced. In this talk, first as a natural extension to previous work, we demonstrate a strategy to dynamically tune the dry adhesion of a PDMS post by inserting a core whose rigidity can be tuned on demand. We achieve this by the use of Conductive Propylene-Based Elastomers (CPBE), a class of elastomers that can be laser patterned and whose rigidity can be dynamically tuned by resistive heating. As the core rigidity of the composite core-shell structure is altered, so does the stress distribution at the interface and hence the adhesion strength. We then propose a novel method that directly manipulates the stress distribution at the interface by embedding subsurface microfluidic channels of Low Melting Point Alloy (LMPA), whose rigidity can also be quickly tuned by resistive heating. The multi-step fabrication methods for these soft structures will be discussed. Adhesion experiments show that the adhesion change ratios can be easily above two times for both methods. We have also conducted FEA simulations to model interface crack propagation and predict adhesion strength for the composites with various geometries. Our experimental and simulation results are in good quantitative agreement.

Lightweight and flexible ultraviolet (UV) photodetectors (PDs) are of increasing interest for modern applications such as wearable devices, synthetic skins, and robotics. A strong, reliable response under low driving voltages and over a broad working range of incident light intensities is highly desired for such UV PDs. To date, most of the commercial UV PDs are based on photomultiplier tubes (PMTs) and gallium nitride p-n junction photodiodes. However, the fragile vacuum tube and the requirement of extremely high driving voltages make PMT-based UV PDs ill-suited for many applications out in the field. Inorganic photodiodes are rigid, necessitate thick active films, and require complex and expensive processing. In contrast, thin film UV PDs based on organic-inorganic nanocomposite have high sensitivity, adjustable response range, lightweight, mechanical flexibility, and solution processability, making them promising for wider applications. In this work, UV PDs based on F8T2:ZnO quantum dots (QDs) nanocomposite with different weight ratios were fabricated with the structure of ITO/PEDOT:PSS/F8T2:ZnO QDs/BCP/Al on PET substrates, where ITO serves as anode, PEDOT:PSS works as a hole transport layer, F8T2:ZnO QDs works as the active layer, BCP functions as an electron transport/hole blocking layer, and Al serves as cathode. Under dark and reverse bias, holes are blocked by BCP due to the large energy barrier of 2.8 eV between the work function of Al (-4.2 eV) and the HOMO of BCP (-7.0 eV) and electrons would be difficult to inject from ITO to the active layer due to the high energy barrier of 2.1 eV between the HOMO of PEDOT:PSS (-5.2 eV) and the LUMO of F8T2 (-3.1 eV) because more F8T2 are close to the PEDOT:PSS layer. Photomultiplication was achieved by control the distribution of ZnO QDs in the active layer. More ZnO QDs in the active layer close to the Al cathode can effectively trap electrons under UV illumination, leading to band bending, which allows hole injection from Al cathode into the active layer under reverse bias, and hole transfer through the active layer and collected by ITO anode. Devices with F8T2:ZnO weight ratio in the range of 1:10-1:3 showed a low dark current density in the low 10^-6 mA/cm² under -2 V reverse bias. A narrow EQE peak centered at 354 nm exhibited ~16000% under -3 V reverse bias with the full width at half maximum (FWHM) of 14 nm, which is due to the wavelength dependence of the light penetration depth in the active layer. Hole-only devices were fabricated to understand the device working mechanism. The devices were bended to a curvature radius of 0.7 cm for 150 times. The dark current, UV photocurrent, and photoresponse rate remained unchanged before and after bending while EQE dropped slightly. All the results demonstrated F8T2:ZnO as a promising candidate for making lightweight, flexible UV PDs with tunable photoresponse range.

In order to address the issue of ever-growing and non-recyclable electronic wastes in recent years, natural green materials including silks, polysaccharides, and cellulose papers have been employed as the substrate for modern electronics. However, none of these substrates possess sufficiently robust mechanical properties, such as accommodating large deformation under strain, to serve as a suitable substrate for next-generation stretchable-electronics devices. Here we have demonstrated a hygroscopic hydrogel that are air-stable, recyclable and stretchable. They were prepared by a simple polymer blend, which consisted of two hydrophilic polymers, PVA and PMAA. EG is used as a hygroscopic solvent and crosslinker. In contrast with conventional hydrogels, this hydrogel does not need to be rehydrated by aqueous water to maintain its high stretchability; they can absorb water molecules from the ambient environment as plasticizers to increase chain mobility and make the network softer. The hydrogel exhibited high mechanical performance with a Young’s modulus of 7.5kPa, linear region of 40%, and elongation of over 550% at break. Most importantly, the hydrogels could be hydrolyzed by water and re-casted many times. We further built on the unique advantages of the hydrogel to demonstrate a stretchable memory device that used deoxyribonucleic acid (DNA) as the memory layer on top of the hydrogel substrate. The device exhibited a write-once-read-many-times (WORM) type behavior with a memory ratio of 10⁴, retention time over 10⁵ s, and mechanical durability over 30% strain. To the best of our knowledge, this is the first report of a resistive memory successfully integrated with a stretchable and recyclable substrate.

Photothermal triggering of shape memory polymers is an appealing non-contact mode of actuation for responsive materials and soft robotics. Such photothermal triggering has already been demonstrated using Au nanospheres or nanorods, and combining them enables wavelength-selective triggering. By increasing the complexity of the materials, soft robots can be designed for advanced functions while using simple architectures. We report shape memory recovery in thermoplastic polyurethane shape memory polymers with embedded Au nanospheres and nanorods using light-emitting diodes with wavelengths of 530 nm and 860 nm matched to their surface plasmon resonances to selectively heat the nanospheres and nanorods, respectively. The concept of wavelength-selective photothermal heating of Au nanoparticles with different surface plasmon resonances could be extended to more than two wavelengths using a series of Au nanorods of different aspect ratios. A wavelength-controlled stage was designed to demonstrate mechanical coupling of the selective photothermal heating of shape memory polymer films with embedded Au nanospheres and nanorods. The sequence of illumination by light-emitting diodes of different wavelengths determines the height and tilt of a wavelength-controlled stage with Au nanospheres and Au nanorods embedded in different legs.

Amphibious and aquatic soft robotic systems have numerous applications and functionalities, such as land and underwater exploration, search-and-rescue, and shore and oceanic environmental cleaning (e.g. garbage collection), just to name a few. For such robotic systems to execute diversified modes of locomotion and tasks, they will need to have the ability to crawl, walk, and swim, as well as to grasp, fetch, sample and manipulate surrounding objects. Hierarchical fibrillar structures have been shown to be advantageous when grasping objects of different size, shape, and topography. Through the integration of such structures on the exterior of a soft robot, a great amount of conformity and continuous adhesive force can be accomplished in a controlled manner. For instance, by lining fibrillar structures in a soft cavity membrane, pneumatic systems can convex the membrane for fiber attachment and return to the initial configuration for detachment. Considering that such grippers may be utilized in diverse applications and environmental conditions (e.g. different temperature and saturation), it is vital to analyze them in such conditions. This study will focus on adhesive properties of fibrillar structures made of different materials against a wide range of temperatures in both dry and wet environments. The following materials will be considered for fabricating the fibers: polydimethylsiloxane (PDMS) and polyurethane acrylate (PUA), for their known performance in both dry and wet conditions. A friction/adhesion characterization setup will be used to examine the adhesive forces of the synthetic fibers under dry, moist, and fully saturated conditions at different temperature ranges. While fibrillar structures have been shown to conform and adhere to various surface topographies, this study will initially consider flat objects to characterize the performance of different materials. Through analyzing adhesive properties under varied environmental conditions, superior fibrillar material can be selected for a diversified set of applications. The findings will be essential in the development of amphibious and aquatic soft robotic systems.

Human definitions of “softness” are closely related to the mechanical definition of compliance. The compliance is determined by the mechanical properties (Young’s modulus) of the material as well as the geometry of the object. The compliance determines how deep a finger pushes into an object (indentation depth) and contact area between the finger and an object. There is currently little information about whether humans are sensitive to either contact area or indentation depth, and by how much. To decouple the effect of contact area from indentation depth, we prepared a set of PDMS samples that have different indentation depths, at constant contact area and vice versa. Usually the contact area and indentation depth of a bulk sample is determined by the Young's modulus. We made samples that vary in contact area, but have a constant indentation depth by taking advantage of “substrate stiffening” which occurs when films are thin (< 3 mm). In addition, we also fabricated samples that vary in indentation depth, but have constant contact area through micropatterning. Human subjects were asked to pick which PDMS sample "felt softer" from nine different combinations of indentation depth and contact area. This allowed us to quantify the respective magnitudes of indentation depth and contact area as a cue in tactile perception. We also present applications and design criterions to modulate “softness” for human-machine interfaces.

Electro-active polymers (EAPs) are polymers that respond to an electrical stimulus (voltage or current) by changing their size or shape. They have applications in fields ranging from soft robotics and artificial muscles to electrically tunable lenses and haptic feedback devices; owing to their simple fabrication and relatively low material costs associated with them. Dielectric elastomer actuators (DEAs), which belong to the family of EAPs, are known to produce high actuation strains, and are made by sandwiching an EAP layer between two compliant electrodes. They work on the principle of Maxwell stresses, which is induction of stress due to electric field pressure from the free charges on the surface of insulating material; and the actuation performance of DEAs depend on external stimulus like applied electric fields, thickness and area of the sandwiched region; and intrinsic material properties like relative permittivity and elastic modulus. Compliancy of electrodes is also extremely critical for the performance of DEAs.

The subject of tactile perception has enjoyed widespread attention; driven by an ensemble of factors like evolution of sense of touch in robotics, tactile perception in virtual and augmented reality, and enhancing the quality of human-machine interaction for touch sensitive consumer products. DEAs have been investigated for applications in tactile and vibrotactile feedback devices via different approaches. Since the deformation produced in the active DEA layer can be small, appropriate coupling is required for out-of-plane deformation; and various approaches like hydrostatic and rigid mechanical couplings have been tried out previously. We have demonstrated a transparent device to provide unobstructed topographic texture change. We use an all transparent system based on thickness mode actuation of DEA; with the elastomeric layer, compliant electrodes and the soft passive layer as all transparent materials. Thickness mode actuation makes use of different strains produced in the active and nonactive regions of the device to produce out of plane deformations on a smooth surface; and is fabricated by coupling a soft elastomeric layer with DEA device. The device has a high actuation performance with vertical deformation of 0.155 mm and good cyclability. The overall transparency of the device is high in the visible region, with an optical transmittance of 76 percent at 550 nm. To the best of our knowledge, this is first of its kind device architecture, as well, which enables integration of DEAs onto a wide variety of substrates to enable tactile feedback.

Enhancement and optimization of the receiving input power density is a key to many applications that requires maximizing the energy input, such as energy harvesting, small signal sensing, etc. Normal incidental illumination is the most direct way to maximize the input power density when the input energy is in a form of parallel electromagnetic wave. In nature, many plants, such as sunflowers, developed phototropism to spontaneously sense and track the light source and maintain their disk to be illuminated normally to the photonic input. By tracking the sun, the sunflowers are able to efficiently raise the temperature of their disk in the morning in time to attract more visits of the pollinators. Currently there is no synthetic material system can, omni-directionally, sense, track and harvest the input emissive energy. In this work, we report a soft material system that can self-adaptively track the energy source whichever the direction of the source goes. We proposed a physically symmetric system which a neat PNIPAM hydrogel that is evenly incorporated with Au Nanoparticles as photonic absorbers for specific frequency of input light. Input photonic energy shines on the geometrically symmetric hydrogel system and induce non-symmetric temperature gradient. When the temperature of part of the hydrogel is higher than the LCST of the PNIPAM hydrogel, the gel system will automatically bend toward the light source. The bending will be terminated when the top of the hydrogel points direct to the light source and block the light from shining on the side of the gel. The start and the termination of the bending, adding together, define a spontaneous tracking functionality of the input light source. We studied the tracking system with a simple mechanical model and FEM simulation. The hydrogel system has been optimized to achieve fast and omni-directional real-time tracking in all direction. The response time of tracking is within 10s of seconds and the tracking performance covers 360 degrees of azimuthal plane. The error of tracking accuracy is better than 1%. A demonstration of an omni-directional solar vapor generator has been presented in this work by employing a phototropic hydrogel pillar array. The rate of vapor generation is comparable between the normal incidence and an angular incidence near 90 degrees to normal, indicating a significant compensation of energy harvesting in the case of the angular incidence attributed to the tracking functionality. We believe that the proposed material system can be applied in many applications that require maximizing their energy input.

We would like to be considered for the joint session with SM08: “Autonomous Hydrogels for Soft Robotics”

Sticky super-hydrophobic surfaces are widely used in transportation without loss, micro-sample analysis, and micro-reactors. However, the state-of-the-art techniques can only functionalize a specific surface and usually involve tedious procedures and severe conditions, which greatly limit their application. Herein, a series of sticky super-hydrophobicity surfaces with high water contact angle and high water adhesive force, is facilely prepared by all solution processed method based on polymerization induced phase separation between liquid crystals (LCs) and epoxy resin, which produces layers of epoxy micro-spheres (EMSs) with nano-folds on the surface of a substrate. The morphologies and size distributions of EMSs are confirmed by scanning electron microscopy. Results reveal that the obtained EMSs coated-surface exhibits high apparent contact angle of 152.0^o and high water adhesive force up to 117.6 μN. By varying the composition of the sample or preparing conditions, the sizes of the produced EMSs can be artificially regulated, and thus control the wetting properties and water adhesive behaviors. Also, the sticky super-hydrophobic surface exhibits excellent chemical stability as well as long-term durability. Water droplet transportation experiments further prove that the as-made surface can be effectively used as a mechanical hand for water transportation applications. Based on this, it is believed that the simple method proposed in this paper will pave a new way for producing a sticky super-hydrophobic surface and obtain a wide range of use.

Smart windows, with abilities of achieving energy-saving and optimizing solar energy utilization, are playing a key role in reducing the overall energy spending and increasing comfort levels for people inside the building. However, the optical modulation of smart windows is usually constricted within a limited waveband, either visible (400-800 nm) or near infrared (NIR, 800-2500 nm) region, and also they must use additional energy to maintain the colored or transparent state. Herein we report a flexible multi-responsive smart film with a widest waveband modulation covering both visible and NIR region (400-2500 nm) reported to date, by creating the compatible interface between tin doped indium oxide (ITO) nanocrystals and polar syrup containing liquid crystals with a smectic A (SmA)/chiral nematic (N*) phase transition and photo-polymerizable monomers. The transmittance of as-made smart film can be thermally changed reversibly from highly transparent (78%) to strong light-scattering (1.5%) state in the visible region, and the light-scattering state of the film also can be regulated electrically. Moreover, more than 85% of the invisible NIR light can be efficiently shielded, resulted from a well-preserved localized surface Plasmon resonance from ITO NCs. The present work represents a key step forward towards preparing optical materials with multi-functional features for applications in energy-saving smart windows.

1. Liang X, Guo S, Chen M, et al. A temperature and electric field-responsive flexible smart film with full broadband optical modulation[J]. Materials Horizons, 2017, 4(5): 878-884. (IF:10.706)
2. Liang X, Guo C, Chen M, et al. A roll-to-roll process for multi-responsive soft-matter composite films containing CsxWO3 nanorods for energy-efficient smart window applications[J]. Nanoscale Horizons, 2017, 2(6): 319-325. (IF:pending)
3. Liang X, Chen M, Guo S, et al. Dual-Band Modulation of Visible and Near-Infrared Light Transmittance in an All-Solution-Processed Hybrid Micro-Nano Composite Film[J]. ACS Applied Materials &Iinterfaces, 2017. (IF:7.506)

Here, we designed and synthesized a supramolecular material that can be self-healing. This synthesized oligomer contains many hydrogen bonds which make it easily to form 3D network. The as-prepared material is transparent and recyclable as the assemblies of the blocks crosslink the polymer chain into supramolecular material. The dissociation/association of the reversible bonds contributes to the healing properties. Furthermore, the mechanical and healing properties of the obtained supramolecular material can be changed by tuning its composition.<embed height="0" id="xunlei_com_thunder_helper_plugin_d462f475-c18e-46be-bd10-327458d045bd" type="application/thunder_download_plugin" width="0" />

Paper-based microfluidic device is widely used in analytical and clinical diagnostics and has the advantages of low-cost, free-shape patterning, and power-free fluid transport by capillary action. In particular, nitrocellulose (NC) membrane is widely used for microfluidic device due to several merits such as cost-effectiveness, outstanding transport property and immobilizability of various proteins. For patterning of fluidic channel, wax printing method is generally used because the process is simple and easy to conduct. However, wax printing requires high curing temperature which results in severe deformation of NC membrane and it is difficult to fabricate well defined patterns because it is hard to control the wax diffusion.
In this work, we developed a simple fabrication method of NC membrane fluidic channel by using single printing process without thermal treatment. We patterned the NC membrane by wet etching process using Propylene Glycol Methyl Ether Acetate (PGMEA). For screen printing, we formulated PGMEA paste by mixing with precipitated silica. Fluidic channel was completed with screen printing and drying at room temperature. Dimensions of fabricated fluidic channel were nearly same that of initially designed patterns because the barrier wall formed by dissolved residues blocked diffusion of PGMEA paste.
Based on the simple patterning method, We have successfully fabricated multiple channels and verified uniform fluid flow in the each channel. We evaluated the NC membrane fluidic channel by colorimetric assay based on enzymatic reactions. For multiple colorimetric assay, we used the multiple channels and examined the simultaneity of the sensor.

Nowadays, silicone elastomers are utilized in a wide range of applications, such as artificial muscles in the shape of dielectric elastomers, medical implants, soft robotics, microfluidic devices, and commodity products. Despite their valuable properties, silicone elastomers are due to their covalent nature thermoset polymers, thus not easily recyclable. It is a demanding challenge to turn silicone into thermoplastic, healable, and recyclable materials. For instance, introduction of hydrogen bonds is a useful technique to provide silicone elastomers with thermoplastic properties and self-healing functionalities. Here we present the synthesis of a copolymer via free radical polymerization of monomethacryloxypropyl terminated polydimethylsiloxane (PDMSMA) and 6-methyl-2-ureido-4[1H]-pyrimidone methacrylate (UPyMA). The novel copolymer possesses thermoplastic properties due to the reversible nature of the UPy self-associating dimers.^[1] In addition, self-healing properties were sought by alternative approaches. In particular, induction heating^[2] represents a novel method to externally trigger healing of polymers. This method consists of the incorporation of magnetic particles in the matrix and the exposure of the composite to an alternating magnetic field (AMF). As a consequence, the heat developed by the process activates network rearrangement. Controlled self-healing by use of an AMF generates heat rapidly and locally, and, above all, it is contactless.
In this work, we successfully report the development of a novel thermoplastic and self-healing silicone elastomer, namely P(PDMSMA-co-UPyMA). The remotely controlled healing of the damaged material was performed through exposure to an AMF of the composite elastomer with 20wt% Fe₃O₄ particle filler. Self-healing efficiency was determined by percentage of restored tensile stress and tensile strain of the healed sample compared to the native material. Scanning electron microscopy (SEM) was exploited to evaluate the morphology of samples at the healed interface. Moreover, the cross-linked copolymer was proven remouldable multiple times without showing considerable physical degradation. Hence, this material may be considered as an excellent candidate for recyclable silicone elastomers. In addition, the described self-healing approach was applied using the same conditions to another previously reported copolymer P(MEA-co-UPyMA).^[3] P(PDMSMA-co-UPyMA) and P(MEA-co-UPyMA) bear identical self-complementary motifs, although differing in the main repeating unit. Comparison of their self-healing performance under unchanged conditions aimed at assessing the versatility of the described self-healing method.

[1] F. H. Beijer, R. P. Sijbesma, H. Kooijman, A. L. Spek, E. W. Meijer, J. Am. Chem. Soc. 1998, 120, 6761-6769.
[2] C. C. Corten, M. W. Urban, Adv. Mater. 2009, 21, 5011-5015.
[3] A. Shabbir, I. Javakhishvili, S. Cerveny, S. Hvilsted, A. L. Skov, O. Hassager, N. J. Alvarez, Macromolecules 2016, 49, 3899-3910.

The scale-invariance of origami and kirigami designs provides an exciting platform for the development of microscale robotic devices. The development of microscale origami-inspired machines can aid in biological applications where it is necessary to deploy a device in a small, folded state, and have it unfold at a destination where it interacts with its environment. In developing such a device, it is necessary to have i) a robust material with low bending stiffness and ii) a remote actuation mechanism that initiates folding. For the former we use graphene, a resilient material that is atomically thin, allowing for easy bending to small radii of curvature. For the latter, the answer is not as obvious. One mechanism that has been successfully employed in microscale actuators is magnetism. Magnetic actuation offers the benefits of remote actuation while selectively affecting only magnetic materials. We explore the potential of using long-range magnetic forces to manipulate and fold graphene-based devices. We use photolithography techniques to pattern CVD grown graphene and deposit magnetic materials directly onto the graphene. Using external magnetic fields and field gradients, we manipulate and apply forces to magnetic graphene devices. We examine a range of device types to display the versatility of magnetic actuation and its applicability toward 2D-to-3D origami-inspired microfabrication.

Stretchable electroluminescent (EL) devices are essential components for future lighting and display applications. Furthermore, flexible and soft EL devices that can accommodate large mechanical strains such as bending, twisting, stretching and folding have been studied extensively. Recently, there have been some efforts to combine EL devices with soft elastomer actuators that have electroluminescent and actuating properties at the same time. However, these efforts still show complex structures and limited strain values. In this study, we demonstrate a soft actuator that can control the strain and luminance individually by using electrical stimulation. By inserting an EL layer between dielectric elastomer films, we could fabricated and soft actuator with controllable strain and electroluminescence by applying a combination of AC and DC signals. The DC signal controlled the strain of the device by the DC signal, while the AC signal component controlled the luminance of the device. The combination of AC and DC enabled simpler device structure compared to previous reports. The electrodes of the device were fabricated using hybrid structures of silver nanowires and carbon nanotubes. The transmittance of the electrodes was high enough (~88 %) to enable the observation of visible luminance with little degradation in time. The device structure fabricated in this study has much higher strain values (>100 %) than previous study (~60 %) and shows high electroluminescence values (~ 30 cd/m²). Our research is expected to be useful for next generation soft devices and actuators with EL properties.

Soft actuators that can show mechanical deformation under various external stimuli have extensively been studied to exploit soft robotics with precise control over their motions. The key challenges for the development of practically useful soft actuators lie in large strain, high mechanical strength, durable actuation motion, and low power consumption. Polymer actuators based on ionic polymers that can be operating under a few volts have been considered as the most promising candidate for such actuators; however, the motion of such actuators is limited to “bending motion”, committed in the early stages of technologies. Herein, we report a new strategy for the development of high-performance ionic polymer actuators with various actuation motions. We employed ionic liquid-containing block copolymers having high ionic conductivity and stretchable properties. Various geometric changes of the actuators beyond typical trilaminar shape were attempted, based on comprehensive understanding of electric field distribution in the polymer layers. The resultant actuators showed great promises towards diverse actuation motions such as helical, lateral expansion/shrinkage, and self-closing behavior. Given that our actuators can be operatable only under 3 volts, it can be applicable to various biomimetic devices.

Soft, functional composites based on suspensions of nanoscale active elements embedded in a compliant matrix have potential to create a new class of mechanical, electrical and optical materials for use in actuators, stimuli-responsive materials, and structures with controlled mechanical properties. The primary limitation preventing the development of these materials is difficulty in integrating the active materials of interest with matrix materials with appropriate properties without compromising functionality. Boron Nitride Nanotubes (BNNTs), insulating analogues of carbon nanotubes, exhibit thermal, chemical, and mechanical stability, electroactive functionality, and radiation shielding ability; making them an ideal base for composites. Of particular interest are BNNT’s high strength, which may allow them to reinforce traditional soft materials, piezoelectric properties which allow them to be used as actuators, high band gap and dielectric properties which allow them to be used in optical nanostructures and anisotropic specific thermal conductivity. Unfortunately, as synthesized BNNTs take the form low density fluff that must be suspended in a matrix for use. Polydimethylsiloxane (PDMS), a silicon based organic polymer appears to be an ideal matrix material as it demonstrates high elasticity, permitting piezoelectric driven flexure, optical clarity, allowing it to be used in optical structures, and contrast in thermal and dielectric properties with BNNT allowing for development of anisotropic materials. To date, successful integration of BNNT’s and PDMS has not been realized due to the chemical incompatibility of the two materials. While the dispersion and hydrogen bonding solubility parameters of both BNNTs and PDMS are similar, the difference in polar values prevents BNNTs from dispersing in PDMS. We have overcome this limitation utilizing a co-solvent mixing procedure with tetrahydrofuran (THF) followed by simultaneous stirring and evaporation under a nitrogen purge. THF has an intermediate polar coefficient of solubility allowing the BNNTs and PDMS to begin to mix. Subsequent prolonged stirring accompanied with a slow nitrogen purge to remove THF and thicken the mixture produces a stable dispersion with varying concentrations (0.3 – 2 wt%) of BNNTs that can be cast into arbitrary forms. The resulting composites show no apparent clustering of BNNTs, which would compromise properties and demonstrate tunable, wavelength dependent optical absorption. Preliminary tension testing has yielded mixed results indicating an increase in ultimate tensile strength but decrease in Young’s modulus, opposite to what was expected with the addition of BNNTs to PDMS, further testing is being pursued to confirm and explain this effect. Future exploration of alignment and texturing of nanotubes as well as probing of electromechanical and thermal properties could yield a novel soft material composite for sensing and actuation.

Recent developments of artificial skins and wearable devices demand soft actuating materials with high flexibility and yield stress. Among these materials, dielectric elastomer actuators (DEA) show flexibility, light weight, thin structure and high mechanical strain, making them strong candidates for future smart materials. Recently, reports on imparting new properties to DEAs such as electroluminescence have been reported. However, the luminance show values around a few cd/m² which are still weak for daily applications. Here, we report an electroluminescent DEA (ELDEA) combining silver nanowire (AgNW) and carbon nanotube (CNT) hybrid electrode on one side and eutectic gallium indium (eGaIn) electrode on the other side. In particular, eGaIn material is advantageous in imparting self-healing properties to DEA electrodes. Our structure enabled transparency on one electrode and high reflectance on the other side of the electrodes. This resulted in ELDEAs with enhanced electroluminescence and strain values compared to previous reports. We expect that our ELDEAs will be useful in future smart wearable devices and artificial skins where high luminance and mechanical strains are desired at the same time.

Dielectric elastomer actuators (DEAs) are a type of electroactive polymer device that can change their shape and/or size when stimulated with high voltage. They are inherently flexible, light and inexpensive. However, the requirement for a high voltage stimulus hinders their advantages. Theoretically, decreasing the size (thickness) of DEAs can lower the voltage requirement with the downside of also decreasing the power and strain outputs. One way to compensate for the decrease in power and strain at smaller scale is to form multi-layer DEA structures using smaller DEA units that will multiply the output while keeping the voltage low.

Conventional approaches for fabricating DEAs are not suitable for fabrication of stand-alone DEAs in micro-scale. In earlier studies, some possible alternatives were introduced for fabricating small scale multi-layer DEAs relying on soft-lithography, polymer solution casting and injection molding processes. DEAs were fabricated using poly(dimethylsiloxane) (PDMS) as the dielectric material. For the conductive layers, a conductive composite made of PDMS and multi-walled carbon nanotubes (MWCNT) was used. This study evaluates the feasibility of each fabrication approach by comparing the output efficiency, repeatability, actuation force and distance of DEAs.

Three-dimensional structures capable of reversible changes in shape, i.e. 4D printed structures, may enable new generations of soft robotics, implantable medical devices, and consumer products. Here, thermally-responsive liquid crystal elastomers (LCEs) are direct-write printed into 3D structures with controlled molecular order. Molecular order is locally programmed by the shear associated with extrusion, with the order following the print path used to build the 3D object. This order controls the stimulus response. Locally, each aligned LCE filament undergoes a 40% reversible contraction along the print direction on heating. However, this combination of controlled geometry and stimulus response in 3D enables the manufacture of objects capable of shape transformations that are atypical for this class of materials. For example, porous scaffolds can be designed to undergo reversible volumetric contraction on heating despite the isochoric nature of the stimulus response in LCEs. Furthermore, we demonstrate that by printing shells with regions of both positive and negative Gaussian curvature, actuators that undergo rapid, repetitive snap-through transitions, can be realized. Finally, we will discuss expanding the use of this printing technique to a variety of liquid crystal inks, enabling fabrication of light and humidity responsive 3D structures.

Researchers in biomedical applications have long had interest in the creation of smaller, softer, safer and more intelligent robots [1]. The development of both micro-electro-mechanical system (MEMS) and nanotechnology have made great strides in building smaller robots [2]. However, these micro/nano devices are generally made of silicon or metallic materials, whose intrinsically inflexible properties restrict the transformation of microrobots and pose potential threats for medical applications to a significant degree. Current progress in developing soft and smart materials such as stimuli-responsive hydrogel and shape memory polymer enables us to devise micromachines that can perceive external environments and respond with programmability through the conventional manufacturing techniques of MEMS. Using Origami/Kirigami design principles as a framework, we can establish complex three-dimensional (3D) microstructures employing self-folding polymer films [3]. This programmable matter allows a single machine to transform into multiple folding forms.
Here, we utilize magnetic hydrogel nanocomposites as programmable matter to construct microrobots that emulate the forms, locomotion, and morphological plasticity of various microorganisms. We encode and recode the 3D forms, magnetic anisotropy, and locomotion of the flexible microrobot powered and propelled by external magnetic fields, and then observe and study its motility and maneuverability in a non-structural, heterogeneous, and dynamically changing environment. Using acquired knowledge on microrobot locomotion with various forms in different environments, we encode the morphological adaptation of a transformative microrobot. The microrobot is able to change its form autonomously to optimal locomotion in diverse environments. The coordination between sensory input and transformation output is the key to achieving adaptive locomotion. This study provides a reference for autonomous targeted therapies using smaller, softer, safer and more intelligent robots.
Reference
[1] R. Wood and C. Walsh, “Smaller , Softer , Safer , Smarter Robots,” Sci. Transl. Med., vol. 19, pp. 5–6, 2013.
[2] J. P. Whitney, P. S. Sreetharan, K. Y. Ma, and R. J. Wood, “Pop-up book MEMS,” J. Micromechanics Microengineering, vol. 21, no. 11, p. 115021, 2011.
[3] H.-W. Huang, M. S. Sakar, A. J. Petruska, S. Pane, and B. J. Nelson, “Soft micromachines with programmable motility and morphology,” Nat Commun, vol. 7, 2016.

Soft lithography is ubiquitous in the fabrication of microfluidics, which, as a technology, has enabled applications in several fields including analytical chemistry, medical diagnostics, microelectronics, and soft robotics. In most of these applications, soft lithography was employed to generate micro-channel geometries of fixed dimensions that ideally would remain invariant during operation. In soft robotics, however, micro-channels contained in soft actuating systems are inherently dynamic and the specific dimensions are expected to change and, by extension the liquid transport properties of the system. Not understanding this problem or generating ways to control it will hamper soft systems with distributed liquid transport systems. Herein, we explored the channel deformation phenomenon (using finite elemental analysis and experimental investigation) observed in elastomeric materials and applied it to the fabrication of microfluidic systems with dynamic channel geometries of predictable dimensions and morphology, thus leading to devices with “programmable” transport properties. While this approach can provide access to channel geometries that are otherwise challenging to fabricate using replica molding, it will also enable fabrication of systems with predictable deformation behavior in pneumatic actuation. We analyzed the channel profile at various states of deformation for comparison with the simulation domain by: (i) imaging channel replicates using microtomography (microCT) or optical microscope, and/or (ii) real-time monitoring of the channel deformation using optical microscopy. By linking these observations to simulation we could fabricate microchannel networks with predictable changes in geometry when subjected to various stresses. This work will provide insight into the channel deformation processes expected in soft robotic systems with embedded networks of microchannels (which may contain liquid metals, reagents, fuel sources, etc.) leading to devices with reliable/predictable transport properties throughout the deformations associated with actuation.

The locomotion of soft robotics is becoming an emerging research frontier since it is extremely crucial for designing and fabricating soft machines that can implement usual robotic functions. Despite some recent success of soft robots locomoting on horizontal and lightly-tilt surfaces, it is still a great challenge to achieve climbing of soft robots on wall or vertical surfaces due to the lack of effective switchable adhesion actuators to counter the gravity. The climbing soft robots could largely expand the horizons of soft robotics in their potential applications in intelligent surveillance, inspection, and detection. In addition, it could become more challenging if the soft machine is required to carry a certain load when climbing on multiple types of surfaces. In this talk, we propose a new octopus-inspired adhesion actuator that allows for rapid, strong and switchable adhesion upon pneumatic actuation. Rather than the conventional way of applying negative pressure to the suction actuator, we use positive pressure to actuate the switchable adhesion. The adhesion actuator demonstrates strong adhesion forces, which exhibits a strong loading capacity of 60-80 times the weight of adhesion actuator itself on vertical surfaces, as well as on a variety of surfaces: wet (slippery), dry, smooth and semi-smooth. Based on this adhesion actuator, we build a novel and simple inchworm-inspired soft robot which can walk and climb vertically on smooth and semi-smooth surfaces with great loading capability of five times its own weight. Furthermore, this soft robot can also walk and climb under water as smoothly as on the ground. We demonstrate that the bio-inspired soft robots can achieve climbing on different types of vertical smooth and semi-smooth surfaces, including dry, wet, slippery, and underwater surfaces.

Micro-aerial vehicles (MAVs) are a rapidly growing area of interest due to potential applications in surveillance, exploration, and defense. Several research groups have taken inspiration from nature in creating bird and insect-sized MAVs that use flapping-wing motions for propulsion and control [1,2]. While lithium batteries are widely used in powering larger designs, they lack sufficient energy density at small scales to deliver meaningful flight times in smaller MAVs. Harvard’s RoboBee, the smallest sub-gram MAV developed to date, uses oscillating piezoelectric actuators to achieve flight, but this design requires an electric tether to deliver power to the robot [3,4]. Here we present an alternative method of high force and high frequency actuation that takes advantage of the large volumetric energy density of hydrocarbon fuels to sustain untethered flight.

This work details the creation of a small, soft, 3D-printed combustion actuator that integrates with the RoboBee. Weighing less than 30mg, these actuators possess thin elastomeric membranes that rapidly expand and contract to mechanically drive the wings of the RoboBee. Actuation occurs through the expansion of heated gases emitted from the combustion of heptane fuel, which takes place in an adjoining chamber. The vapor pressure of the heptane drives the fuel from a storage chamber to the combustion chamber, where autoignition is triggered through an embedded resistive heating element. This combustion actuator was fabricated from an elastomeric polyurethane resin using projection stereolithography 3D-printing. Actuation frequencies upwards of 100Hz were reported during tests with a compressed air setup. The advances in this work represent a unique method of achieving untethered locomotion in microrobotics, which is an essential milestone in realizing the envisioned applications of this class of devices.

References:
[1] G. C. H. E. de Croon, K. M. E. de Clercq, R. Ruijsink, B. Remes, and C. de Wagter, “Design, Aerodynamics, and Vision-Based Control of the DelFly,” Int. J. Micro Air Veh., vol. 1, no. 2, pp. 71–97, 2009.
[2] M. Keennon, K. Klingebiel, and H. Won. “Development of the Nano Hummingbird: A Tailless Flapping Wing Micro Air Vehicle,” 50th AIAA Aerospace Sciences Meeting, 2012.
[3] K. Y. Ma, P. Chirarattananon, S. B. Fuller, and R. J. Wood, “Controlled Flight of a Biologically Inspired, Insect-Scale Robot,” Science, vol. 340, no. 6132, pp. 603-607, 2013.
[4] R. Wood, R. Nagpal, and G. Y. Wei, “Flight of the RoboBees,” Scientific American, 2013.

Development of mechanically flexible, and stretchable sensors based on electrical signals such as capacitance, voltage, and resistance, which can detect and simultaneously visualize large strain involved with human motion, is in great demand. Here, we demonstrate a highly stretchable, large strain capacitive sensor. This sensor can visualize the strain based on strain-responsive structural color (SC). Our device contains an elastomeric sensing film responsible for capacitance change upon strain in which a self-assembled block copolymer (BCP) photonic crystal (PC) film with 1-D periodic in-plane lamellae aligned parallel to the film surface is embedded for the efficient visualization of the strain. The capacitance change arises from dimension change of the elastomer film upon strain. The mechanochromic BCP PC film responds to the strain, giving rise to the SC change with strain. The initial red SC in a sensor is blue-shifted and turns blue when stretched to 100%, resulting in full colored SC alteration as a function of strain. Our BCP SC strain sensor exhibits fast strain response with multi-cycle reliability over 1000 times in both capacitance and SC change. This allows for efficient visible recognition of the strained positions by finger bending as well as poking with a sharp object.

This talk will discuss efforts to pattern and shape-reconfigure liquid metals as conductive inks for stretchable, soft, and reconfigurable electronics. Alloys of gallium are noted for their low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be patterned into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. Because it is a liquid, the metal is extremely soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomeric or gel substrates, it is possible to form soft, flexible, and conformal electrical components, stretchable antennas, and ultra-stretchable wires that maintain metallic conductivity up to ~800% strain. Thus, these materials are well-suited for soft robotics because they decouple electrical and mechanical properties. In addition to introducing the advantages of these materials for soft robotics, this talk will focus on (1) new ways to pattern the metal utilizing vacuum filling of microchannels, and (2) new ways to manipulate and reconfigure the shape of liquid metals utilizing electrochemical surfce oxidation, which lowers the interfacial tension of the metal. These advances have implications for soft machines and robots that have ultra-soft mechanical properties.

The folding and cutting of materials provides a facile means to tune mechanical and physical characteristics. Kirigami, the Japanese art of paper cutting, has demonstrated that the inclusion of cut patterns enables complex 3D structures from 2D sheets, elastic softening, and large deformations under external loading. These features have primarily been achieved through either the bending of beams defined by cuts or the rotation of hinges between beams. However, the synergistic coupling of these deformation modes has not been explored. Here we show that the inclusion of minor cuts into patterned structures defined by major cuts integrates hinge rotations with beam bending and generates significant tunability, further reducing stiffness by a factor of 30 and increasing the deformability by a factor of 2. Experimental results are validated by theoretical predictions in which the addition of minor cuts of various lengths to major cuts exhibits a similar response to varying boundary conditions and beam shapes. We present generalized equations to provide criteria for designing optimized kirigami structures in various applications. This concept of hybrid structures is demonstrated with highly stretchable conductors, which show nearly constant resistance with strains up to ~400 %, and rapid magnetoactive soft actuators, which elongates to 330 % in ~0.1 s with a maximum strain rate of 10,000 % s^-1. This work can enable substantial variations in stiffness and deformation of functional materials for applications in soft robotics, stretchable electronics, and human-machine interfaces.

With the rise of soft robotics and wearable electronics, there rises the secondary, but equally limiting issue, of dissipating the heat produced by the electronics while maintaining flexibility. Bartlett et al.¹ recently demonstrated that soft composites, consisting of liquid metal micro-droplets dispersed in a silicone matrix, can be used to attach a high-power LED to a user as well as prevent it from overheating. The integration of liquid metal micro-droplets allows for a material with a maximum thermal conductivity of 10 Wm^-1K^-1, when stretched, while being able to sustain a minimum flexibility of 200% of its original length. However, as in the case of traditional electronics, the power density of wearables is bound to increase with time, ultimately necessitating use of active liquid cooling. In fact, a liquid cooled viscoelastic actuator has been recently used in a high power compliant robotic leg prototype. However, as in thermoregulatory liquid cool garments, the tubing material used in this design has a very low thermal conductivity, of around 0.2 Wm^-1K^-1; a value far too low to effectively deal with the amount of heat that will inevitably be generated by wearable electronics.
To address this issue, in this work we introduce a high performance stretchable heat exchanger made out of the liquid metal-silicone composites. Since stretching violates most assumptions used in design of conventional heat exchangers (e.g. constant areas and cross sections, heat transfer coefficients, and flow rates), we developed a new theoretical frame for design of stretchable thermal management devices. Specifically, we will discuss time scaling and the quasi-static shape models for single stream heat exchanger undergoing axial stretching and compression. To validate this thermofluidic design framework, a prototype stretchable heat exchanger design has been built and its performance characterized. Using this data, an optimized heat exchanger design has been proposed that minimizes the required pumping pressure and maximizes heat dissipation while maintaining the essential flexibility of wearable electronics.

1. Bartlett et al. PNAS, 114, 2017.

Introducing methods for instant strong bonding between hydrogels and antagonistic materials – from soft to hard – allows us to demonstrate elastic, yet tough biomimetic devices and machines with a high level of complexity [1]. Tough hydrogels strongly attach, within seconds, to plastics, elastomers, leather, bone and metals reaching unprecedented interfacial toughness exceeding 2000 J/m². Our approach is applicable in rapid prototyping and in delicate environments inaccessible for extended curing and cross-linking. The combination of ionic hydrogels with antagonistic materials such as elastomers, polymers and metals allows to create soft electronics and hybrid machines. We demonstrate stretchable batteries for self-powered soft devices, adaptive lenses, and autonomous electronic skin for triggered drug delivery. We further introduce a new family of biodegradable hydrogels that are reversibly stretchable, are able to self-heal and are resistant to dehydration. Soft machines and robots – built from hydrogels with tuned mechanical properties – are designed to be operated in ambient conditions and degrade after use. Besides progressing stand-alone soft machines, our advances in the synthesis of biodegradable hydrogels bring bionic soft robots a step closer to nature.


[1] D. Wirthl, R. Pichler, M. Drack, G. Kettlgruber, R. Moser, R. Gerstmayr, F. Hartmann, E. Bradt, R. Kaltseis, C. M. Siket, S. E. Schausberger, S. Hild, S. Bauer, M. Kaltenbrunner, "Instant tough bonding of hydrogels for soft machines and electronics", Science Advances, 3(6), e1700053 (2017).

With the growing demand for wearable computers, there is a need for materials systems that can perform computational tasks without relying on external electrical power. Using theory and simulation, we design a materials system that “computes” by integrating the inherent behavior of self-oscillating gels undergoing the Belousov–Zhabotinsky (BZ) reaction and piezoelectric (PZ) plates. These “BZ-PZ” units are connected electrically to form a coupled oscillator network, which displays specific modes of synchronization. We exploit this attribute in employing multiple BZ-PZ networks to perform pattern matching on complex multi-dimensional data, such as colored images. By decomposing a colored image into sets of binary vectors, we use each BZ-PZ network, or “channel”, to store distinct information about the color and the shape of the image and perform the pattern matching operation. Our simulation results indicate that the multi-channel BZ-PZ device can detect subtle differences between the input and stored patterns, such as the color variation of one pixel or a small change in the shape of an object. To demonstrate a practical application, we utilize our system to process a colored Quick Response (QR) code and show its potential in cryptography and steganography.

Soft and stretchable electronics have paved the way for sensing and actuation that is intrinsically soft and exhibits properties similar to that of natural biological tissue. While these technologies are capable of undergoing extreme deformations, these soft circuits lose electronic functionality when damaged and repair is only possible in certain cases through manual intervention, the use of redundant electronics, or the application of heat. The ability for these devices to operate in complex environments is critical for progress in wearable computing, soft machines and robotics, and inflatable structures and deployables. Here, we use a liquid metal embedded elastomer (LMEE) that is composed of micron-scale droplets embedded within a soft silicone elastomer matrix. The composite is intrinsically insulating. Application of local pressure causes a local change in electrical conductivity through the formation of a percolating conductive network of liquid metal droplets with high electrical conductivity (σ = 1370 S-cm^-1). This enables circuits to both be created and subsequently reconfigured when damaged, through the autonomous, in-situ formation of new electrical pathways. The mechanically-sintered lines are experimentally characterized under uniaxial strain for different volume loadings of liquid metal. The LMEE composite is soft (E = 1.5MPa) and exhibits negligible changes in trace resistance (< 10% increase) when loaded to 50% strain. To demonstrate the use of these material as damage-resistant wiring for soft robotics, we integrate it into an electrically-powered soft quadruped. The LMEE composite is used to transmit power to shape memory alloy actuators embedded in the soft robot limbs. After damage, the power trace autonomously self-heals and there is no visual change in the gait of the soft robot. This materials architecture demonstrates an unprecedented level of robust functionality that has the potential to enable soft-matter electronics and machines to exhibit the extraordinary resilience of soft biological tissue and organisms.

Electronic skins have witnessed tremendous interest and development over the last decade¹. Functional soft, flexible and stretchable materials are crucial to the continued evolution of skin-like sensor applications in emerging soft robotics and novel human-machine interfaces. For example, ‘robots’ can don on sensor active skins to shake human hands with comfortable pressure and measure our health biometrics². Here, I will discuss our recent work in achieving force-sensitive electronic skin technologies using soft polymeric materials. Bio-inspired digitization of analog signals have also enabled us to develop an artificial mechano-receptor that optically interfaces with neurons³. These technologies would be extremely applicable to advanced health technologies for neuroprosthetics and healthcare robotics.

Furthemore, recent developments in self-healing polymeric systems have propelled the exciting notion that electronic systems can repair themselves when damaged. One method to enable self-healing of damaged interfaces is via reversible hydrogen bonding⁴. However, one potential drawback of such systems is the significant reduction in healing efficiencies in the presence of a high humidity environment due to quenching of the surface hydroxyl groups by water molecules. I will also present some of our recent work on creating printable, humidity-robust self-healable polymer composites that could be useful for next generation, soft electronic sensor skins.


1. Hammock, M. L., Chortos, A., Tee, B. C. K., Tok, J. B. H. & Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress. Adv. Mater. 25, 5997–6038 (2013).
2. Schwartz, G. et al. Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring. Nature Communications 4, 1859–8 (2013).
3. Tee, B. C. K. et al. A skin-inspired organic digital mechanoreceptor. Science 350, 313–316 (2015).
4. Tee, B. C. K., Wang, C., Allen, R. & Bao, Z. An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications. 1–8 (2012). doi:10.1038/nnano.2012.192

Progress in soft lithography, additive manufacturing, biohybrid engineering, and soft materials integration have lead to extraordinary new classes of soft-matter sensors, circuits, and actuators. These materials represent the building blocks of soft machines, robots, and bio-inspired systems that will exhibit the rich multifunctional versatility and robust adaptability of soft biological organisms. While there are key challenges in materials and manufacturing that remain to be addressed, further progress in soft robotics now depends on accomplishing a new set of goals: systems-level materials integration, untethered functionality, and robot autonomy. In this talk, I will focus on this latter set of challenges and the new fundamental questions that emerge when exploring the interface of soft multifunctional materials, rigid microelectronics, and robot mobility. In particular, I will report efforts by my lab to create an untethered soft robot capable of walking in a variety of environments, including rocky terrain and confined spaces. I’ll also present recent work on mechanically robust and self-healing electronics that can withstand extreme loading and damage. When used as internal circuit wiring within an electrically-powered soft robot, such materials enable autonomous response to tearing, puncturing, or material removal – damage modes that would be catastrophic for most other soft-bodied robots. I will close by highlighting ongoing efforts to create new computational tools for modeling the motion and surface interactions of limbed soft robots. Based on continuum mechanics, finite element analysis, and emerging techniques in computer graphics, these tools represent another critical requirement for soft robot autonomy by potentially enabling on-board computational intelligence and adaptive decision making.

Skin-like sensors require arrays of wired interconnects to detect touch or force. For these wired grids, the number of sensing units scales with the square of the number of wired leads connecting to the edges of the sensing surface, so the sensors often require a multi-layer fabrication process on flexible substrates, which increases the complexity of the manufacturing process and limits the scalability of the skin-like sensors. This work presents a scalable skin-like sensing array based on resistive networks of capacitive touch pads on metallized paper. The technique permits detecting external stimuli on a two-dimensional meshed area, while it has a single conductive layer with only two wired leads. To demonstrate the feasibility of the networks, the prototype sensors are simple to fabricate with laser-based ablation of metallized paper to vaporize conductive coating on the cellulose for patterning resistors and capacitors. With the designed patterns shown in this work, the impedance of the networks shifts to a specific range that depends on the activated sensing region. In this work, two case studies show the scalability and feasibility of the networks. One example includes a device for detecting both location and volume of applied droplets of water. Another example demonstrates that two wired leads are capable of detecting finger touch from 31 distinct buttons laid out in the format of a conventional keypad. Future applications might include wearable human-machine interfaces, skin-like force sensing array, or smart sensing embedded in civil infrastructure for detection of leaks.

Flexible electronics are devices that can be bent, folded, stretched, or conformed regardless of their material composition without losing the electronic functionality. These electronics are employed in healthcare – designing stretchable electronic skins or lightweight smart sensors conformal to human body for biomonitoring and energy harvesting applications. Despite their increasing demands, only a handful of these devices have been commercialized due to the lack of novel functional materials available along with the complex fabrication mechanisms needed to process them. Unlike conventional silicon-based microelectronics manufacturing that is limited to rigid wafers, flexible electronics need to be incorporated onto plastics, paper, fibers and even biological tissues – necessitating low temperature processing. In addition, these devices need to be inexpensive and customizable according to an individual’s body needs with short manufacturing lead times. Polymers are the most common materials to be 3D printed today due to the simplicity of extruding them in molten form that quickly cools and hence solidifies. However, there is a great demand for developing methods to easily print metals. Current methods for additive manufacturing of metals tend to be prohibitively expensive, and use energy-intensive lasers at high sintering temperatures in excess of 800°C. Secondly, they need vacuum-like pressures to avoid oxidation while handling metal nanoparticles, leading to porosity in finished parts, low resolution and poor electrical conductivity, apart from having slow printing speeds. Finally, the operating procedures are impossible to integrate with various polymeric, organic, soft and biological materials. Here, we present a simple approach that utilizes low melting point gallium-based alloys that offer the electrical and thermal benefits of various metals like gallium and indium, combined with the ease of printing due to its low viscosity. Despite having high surface tension, these metals build mechanically stable structures due to the formation of a thin surface oxide. The oxide skin forms spontaneously in presence of air allowing us to direct-write planar as well as free-standing, out-of-plane conductive microstructures at room temperature, down to a resolution of ~10 microns, on-demand, using a pneumatic dispensing robot at relatively low pressures. We have demonstrated rapid prototyping of functional electronics such as flexible and stretchable antennas for defense communications, consumer-friendly electronic devices like laser pointers and inductive power coils for wireless charging of smartphones, and wearable thermoelectric generators for energy-harvesting applications. We have also exhibited the patterning of 3D multilayered microchannels with vasculature using liquid metals as a sacrificial template at room temperature that can be embedded in lab-on-a-chip devices to enable inexpensive fabrication of personalized healthcare sensors.

Liquid metal based-soft electronics have attracted attention in fields such as soft robotics, biomedical devices and wearable electronics. Methods to pattern liquid metals have been demonstrated using microchannel injection, direct writing, and microcontact printing. The above techniques are limited in scalability for mass manufacturing, resolution, and repeatability. In our previous work, we created liquid metal nanoparticle inks by sonicating bulk liquid metal in ethanol, printed these inks onto various substrates, and used mechanical sintering to coalesce the liquid metal nanoparticles into conductive paths. In this talk, we show that conductive paths are also possible using laser sintering. We characterize laser sintering of liquid metal nanoparticle films and demonstrate that this approach is compatible with soft substrates and smaller, solid-core particles, neither of which is possible using mechanical sintering. We further present the integration of the optimized laser sintering system into the scalable manufacturing of soft electronics. We investigate the laser sintering phenomenon through comparison with focused ion beam ablation and studying the effects of thermal propagation in sintered films. We study the effects of laser fluence, nanoparticle size, film thickness and substrate materials on the resistance of the sintered films. Finally, we demonstrate several representative devices to highlight the electrical stability of the fabricated circuits under flexing and the ease of manufacturing multilayer and intricately patterned circuits.

Abstract
Electronic skins (E-Skins) designed to emulate sensing properties of human skin, have been explored for a variety applications including smart prosthetics, human machine interfaces, and wearable health monitoring devices. In particular, biomimetics has led new concepts in material designing and device structure manipulation with the aim of mimicking tactile sensing features of human skin intelligently. However, most of the e-skin architectures developed so far are ultrasensitive in low pressure (0-10 kPa) regime but become almost insensitive beyond this narrow pressure sensing range, which limits their use for practical applications.
In this talk, we present a novel synthetic cellular-structured ionic polymer composite inspired from structural and mechanotransduction features of living cell and explore its potential application in piezo-capacitive e-skin device. The key innovation of our work is our material design, where ionic liquid confined silica microstructures (artificial intracellular) well dispersed in thermoplastic elastomer chains (artificial extracellular) enabling fascinating structural, mechanical, and functional properties at molecular level. Structure-derived confinement of ionic species in silica matrix under no force condition and subsequent stimulus driven squeezing-out of ions through intrinsic silica-thermoplastic elastomer nanochannels seems to emulate structural and functional features of living cell in a more pronounced way that resulted in high pressure sensitivity of the e-skin device consistently in both low-pressure (42-48 kPa^-1 (0-10 kPa)) and medium/high pressure regimes (5-7 kPa^-1 (50-100 kPa)), enabling scope of gentle touch to the object manipulation tasks in robotics. Considering both the unprecedented sensitivity and wide pressure sensing range achieved in single device architecture, the pressure sensing e-skin developed in this work is one of the highest among all reported pressure sensors to the date.
We believe that the experimental results presented here provides an empathetic solution to the low sensitivity, narrow pressure sensing range and complex device architecture related issues associated with e-skins developed so far.

Human-machine interfaces (HMIs) have been highlighted with the advent of wireless sensor networks (WSNs) and the internet of things; HMIs may require wearable/attachable electronics exhibiting stretchability, biocompatibility, and even transmittance. Furthermore, due to limited weight and volume for wearability, energy efficient and even self-powered devices are required. Here, we report practical approaches for stably self-powered and transparent attachable ionic communicators (STAICs) based on self-cleanable triboelectric nanogenerators. STAICs can be easily applied on human skin due to their softness and its chemically anchored robust layers. STAICs function as real-time WSNs for communicating between humans and machines. Surface functionalization on STAICs by (Heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane improves sensitivity of STAICs and makes them electrically and optically stable due to the self-cleaning effect without sacrificing transmittance. This research will be a foundation for potential development of attachable ionics, self-powered sensor networks, and monitoring system for biomechanical motion.

Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported. Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores, causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film’s transparency and its ability to continuously manipulate various low-surface-tension droplets from free sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.

The emergence of photo-responsive bulk materials provide a promising alternative to processing accessibility and self-healing applications. In contrast to thermal, mechanical, electrical and chemical stimuli, light-triggered transformers represent the practical optomechanical prototype in a non-invasive manner with high spatiotemporal precision. Here we report a new class of Pphotoresponsive polymer elastomers based on hexaarylbiimidozole. Several optomechanical behaviors have been demonstrated based on the photoswitching effect. First, light stimulus has been used to melting, cutting, drilling, surface patterning and remolding the PTA-HABI gel network. Secondly, the solvent-free PTA-HABI elastomer exhibits photo-enhanced self-healing properties by simply taking two pieces of elastomers tied together at room temperature. Finally, the reversible photo-actuated spring is fabricated and exhibits photoswitchable plastic strain behavior. Therefore, HABI-based polymer elastomers, including solvated gel and solvent-free elastomer, is promising as smart materials in light processing accessibility and automatic self-healing.

The rapidly emerging field of soft robotics presents a new opportunity to develop a new class of wearable assistive technology optimized for the needs of individuals with residual capacity, i.e. where only small to moderate levels of assistance is needed to improve function ability (e.g. walking, grasping) as well as augmenting human performance. Compared to traditional robotics, soft robotics offers advantages of inherent compliance and low weight; however, it also brings fundamental challenges, including sensing, calibration, actuation, efficiency and control. Our group has developed a number of prototype systems for the lower and upper extremity and the philosophy around their designs are briefly described here.

We present here the development of a physical human eye model – based on hybrid soft/rigid materials - in order to create a test bench reproducing the optical eye properties. Current literature does not cover properly the optical parameters of soft materials constituting the human eye.
For that reason, we first investigate the reflection and transmission spectrums as well as the refractive index of the different parts of a porcine eye – which is close to human one. The extraction of the optical parameters (absorption and scattering coefficients, reflection and transmission percentages of every part of the eye), allow the fabrication of a physical device mimicking a real eye. We are using materials which can model as finely as possible every part of the eye, particularly for the sclera or the iris which is often neglected. We present a soft actuated model of the iris where the aperture ranges from 1 mm to 8 mm. Finally, all the different parts are put together to obtain a device mimicking exactly the optical properties of an eye.

This presentation will describe a unique parabolic acoustic reflector with an inflatable structure, which has tunable gain and directivity. Conventional parabolic reflectors focus and amplify sound waves using metallic or plastic dishes with fixed geometries. This work aims to create a morphable reflecting surface that deforms into a concave structure to provide directional amplification of incoming acoustic waves. The deformable concave structure is a silicone elastomer (Ecoflex 00-10) that has a measured reflection coefficient of approximately 0.9 at frequencies ranging from 500 Hz to 5000 Hz. This reflective coefficient suggests these silicone-based elastomers are capable of serving as reflective substrates for advanced morphable devices to manipulate sound. For parabolic reflectors, acoustic gain and directionality depend on the level of vacuum applied to the elastomeric membranes, which affects the curvature of the paraboloid. Experiments performed in closed and open environments, along with simulations, demonstrate that the soft reflecting surface is capable of transformation into a set of desired parabolic shapes between an initial planar geometry (neutral position) and configurations with varying curvature. This type of system might find future uses for adjustable parabolic microphones and long-range communication devices.

Interest in stretchable organic semiconductors stems from its competence in depositing elastic semiconducting films onto soft substrates such as silicones or artificial skin. Some promising applications of stretchable semiconductor systems are developing flexible displays, elastic sensors, artificial electronic skins (E-skins), and then integrating them together for creating intelligent electronics, such as soft robotics. Besides stretchable electrodes, the key challenge for fabricating good E-skins and other soft circuits that achieve their full potential is to obtain robust organic semiconductor components with desired mechanical properties while retaining good electronic performance under bend and stretch with high curvatures.Three typical approaches for the stretchable semiconducting films have been engineered during the last decades. The first approach based on geometric designs. This strategy demonstrates the possibility of imparting stretchability to brittle semiconducting films while maintaining their electrical properties under certain strain. However, it is still a challenge to retain electrical performance under large strains or deformations using this approach owing to the limit of the dimensions of the semiconductors and its adhesion to the elastic substrate. The second approach is directly synthesizing new semiconductors through incorporating modified side-chains or dynamic non-covalent molecular units into polymer chains. Nevertheless, the elaborate synthesis process of the intrinsically elastic semiconductor polymers and limited stretchability because of the structure hindered their further application. The third route is blending semiconductor polymers with dissimilar insulating elastomeric polymer toward stretchable semiconductor nanocomposite via continues semiconducting network. The resulting network structure in the film will enhance the polymer chain dynamics and is necessary to achieve good charge transport. These researchers have mainly focused on this nanocomposite as a strategy to obtain the stretchable devices. The understanding and control of such network formation in semiconducting films are few studies, even though the microstructure in the semiconducting films is crucially important for fabrication of stretchable electronics.
In this study, we systematically examined the morphology of the semiconducting films with different blends and its role in the mechanical and electronic performance. We found that it is extremely beneficial to use low weight fractions of the semiconducting materials to achieve both excellent stretchability and charge-transport properties in the bicomponent semiconductor-dielectric active layers of polymer transistors. The film morphology can be controlled by adjusting the ratio of the semiconducting materials and insulating materials. Correlation between microstructure and charge-transport performance has also been investigated.

Nanomaterials offer an attractive solution to the challenges faced for low-cost printed electronics, with applications ranging from additively manufactured sensors to wearables. While many reports have presented “fully printed” thin-film transistors (TFTs) from carbon nanotubes (CNTs), they have all used non-printing techniques to form some portion of the devices, compromising the benefits of throughput and on-the-fly customization that an exclusively printed process offers. Here we report hysteresis-free carbon nanotube TFTs (CNT-TFTs) fabricated entirely using an aerosol jet printing technique; this includes the printing of all layers: semiconducting CNTs, metallic electrodes and insulating gate dielectrics, using the same printer. One of the foremost challenges to a completely printed CNT-TFT approach has been the difficulty of obtaining a robust printed dielectric. We show that, under appropriate printing conditions, a now commercially available dielectric ink can be reliably printed and yield negligible hysteresis and low threshold voltage in CNT-TFTs. Conditions were optimized for printing on both rigid and flexible substrates, including insight into the trade-offs between top- and bottom-gate device geometries. Flexible CNT-TFTs on Kapton film demonstrate minimal variations in performance for over 1000 cycles of bending tests with curvature radii down to 1 mm. New insights are also gained concerning the role of charge trapping in Si substrate-supported devices, where exposure to high substrate fields results in irreversible degradation. With all of the benefits that CNT-TFTs offer for the field of low-cost, multi-functional electronics, this result is a critical step forward as it enables a completely additive, maskless method to fully print CNT-TFTs of direct relevance for the burgeoning areas of flexible/foldable, wearable, and biointegrated electronics.

Paper-based electronics are emerging as lightweight, recyclable, and low-cost options for advanced mechanical, chemical, and electrical sensing. As one potential application of paper-based electronics, skin-like sensors may be capable of detecting touch, temperature, pressure, and humidity on synthetic or natural surfaces. Current efforts to fabricate paper-based electronic devices – papertronics – include printing, coating, and laser processing of off-the-shelf paper.
In this presentation, we describe a method for manufacturing tunable electronic composites for skin-like sensors. We apply papermaking techniques to produce highly porous composites of cellulose fiber loaded with carbon black. Detailed characterization shows their conductive performance across a range of frequencies. These composites also have a nonlinear, percolating increase in conductivity with respect to their concentration of carbon black.
For elastic deformation, our composites show piezoresistive behavior with electrical resistivity dependent on applied pressures or forces. Testing of mechanical strength also shows the effect of the embedded nanoparticles on cellulose-based composites. Furthermore, we embossed the composite paper to tune its porosity and thus enhance its conductivity. Overall, this work demonstrates a simple and scalable method to fabricate composites with tunable electromechanical properties for paper-based skin sensors. Our approach has the potential to open opportunities for prosthetics and robotics, haptic feedback, and structural health monitoring on expansive surfaces of buildings and vehicles.

Here, we report the piezoresistive behavior of ultra-soft, flexible and biocompatible carbon nanostructures (CNS) based polydimethylsiloxane (PDMS) nanocomposites under monotonic and cyclic loadings. CNS used here comprise MWCNTs that are branched, cross-linked and share common walls. The piezoresistive gauge factors obtained are 32, 15 and 7 for 0.05 wt. %, 0.1 wt. % and 0.2 wt. % of CNS loading, respectively. These gauge factors are higher than that of the conventional metallic sensors, which exhibit gauge factor of around 2. The nanocomposites remained conductive under cyclic loading and showed good piezoresistive sensitivity even after 100 cycles at both relatively low (5%) and high (30%) strain amplitudes. It is demonstrated that the strain sensing ability of the CNS/PDMS can be tuned, in terms of linearity and stretchability (from 25 % to 50 %) by varying the wt. % of CNS in PDMS matrix according to application requirements. To further elucidate the piezoresistive behavior of the CNS/PDMS composites, a simple theoretical model is developed relating the mechanical deformation to the piezoresistive response of the nanocomposites under uniaxial stretch. This study demonstrates the potential of CNS/PDMS nanocomposites in personal health monitoring owing to its strain-sensing capability.

With the increasing prevalence of machines, future breakthroughs to enhance the intuitiveness of human-machine interfaces requires further development of haptics. One of the greatest barriers faced in haptic devices is their lack of conformability to the body. Therefore, the development of soft and conformable actuators is necessary to simulate fundamental modes of touch such as temperature, texture, contact pressure and stiffness. Our work to develop haptic devices using organic materials bridges the gap between haptics and soft actuators. Haptic perception falls under two categories – tactile and kinesthetic. This work focuses on the actuation of kinesthetic stimuli which provides information on the shape and stiffness of objects in virtual spaces. Devices capable of providing kinesthetic perception through force feedback include gloves driven by motors [1], pneumatics [2] and magnetorheological fluids [3]. However, these devices are bulky, which impedes freedom of motion, and are non-conformable to the hand, which deteriorates the perception of kinesthetic stimuli. To address these issues, this work utilizes the thermomechanical transitions of a polymer with glass transition temperature (Tg) around body temperature to form a variable stiffness glove. Variable stiffness (VS) materials have been developed for soft robotic applications such as fibers [4], and composites using elastomers as scaffolds for VS materials [5]. We have developed a VS fabric by coating a soft fabric “scaffold” with a VS polymer. This design allows the polymer to be highly conformable to the body, which has yet to be demonstrated in the field of haptics.
The variable stiffness fabric was prepared by coating Spandex fibers with poly(butyl methacrylate) (PBMA), a thermoplastic with Tg of 45°C. By coating the bottom and sides of fingers of a Spandex glove with PBMA, and modulating the temperature of the composite with thermoelectric elements, we found that human subjects could perceive changes in stiffness of the kinesthetic glove. Response times reported varied inversely to the power of the thermoelectrics. Finite element analysis of a finger further verified that stress is localized at finger joints, one of the locations on the body with highest density of kinesthetic receptors. Furthermore, highest stresses occur around the curvature of the fingers, reinforcing the necessity of conformal coverage of the finger by the kinesthetic device. The development of a soft and conformable kinesthetic glove creates a foundation for further research into soft haptic devices for more intuitive human-machine interfaces.

[1] C. S. Tzafestas. IEEE Transactions on Systems, Man, and Cybernetics - Part A: Systems and Humans, 2003
[2] P. Polygerinos et al. Robotics and Autonomous Systems, 2015
[3] S. H. Winter and M. Bouzit. IEEE Transactions on Neural Systems and Rehabilitation Engineering, 2007
[4] A. Tonazzini et al. Advanced Materials, 2016
[5] I. M. Van Meerbeek et al. Advanced Materials, 2016

Assembly of 3D architectures with programmable shape configurations has important applications in broad areas. However, previously assembled 3D architectures are either not reversible or free-standing, limiting their applications and/or functions. We demonstrate a novel reversible self-assembly of soft 3D architectures actuated by the self-folding of smart polymer substrates with programmed geometries. The substrates can reversibly respond to external stimuli to actuate planar origami/kirigami films for self-assembly of 3D architectures and they become parts of final 3D architectures, empowering their reversibility and freedom. The self-assembly process is controlled by origami and kirigami design patterned by direct laser writing. A variety of 3D architectures like “flower”, “rainbow”, “sunglasses”, “box”, “ptramid”, “grating”, and “armchair”, are assembled. The reported self-assembly also shows wide applicability to various soft materials including epoxy, polyimide, laser-induced graphene, and metal films. A micro light-emitting diode (µ-LED) and a flex sensor are integrated with the self-assembled 3D architectures, indictaing the potential applications in soft robotics, wearable electronics, bioelectronics, and microelectromechanical systems (MEMS).

Responsive materials with functions of forming three-dimensional (3D) origami and/or kirigami structures have a broad range of applications in bioelectronics, metamaterials, microrobotics, and microelectromechanical (MEMS) systems. To realize such functions, building blocks of actuating components usually possess localized inhomogeneity so that they respond differently to external stimuli. Previous fabrication strategies lie in localizing nonswellable or less-swellable guest components in their swellable host polymers to reduce swelling ability. Herein, inspired by ice plant seed capsules, we report an opposite strategy of implanting swellable guest medium inside nonswellable host polymers to locally enhance the swelling inhomogeneity. Specifically, we adopted a skinning effect induced surface polymerization combined with direct laser writing to control gradient of swellable cyclopentanone (CP) in both vertical and lateral directions of the nonswellable SU-8. For the first time, the laser direct writing was used as a novel strategy for patterning programmable polymer gel films. Upon stimulation of organic solvents, the dual-gradient gel films designed by origami or kirigami principles exhibit reversible 3D shape transformation. Molecular dynamics (MD) simulation illustrates that CP greatly enhances diffusion rates of stimulus solvent molecules in the SU-8 matrix, which offers the driving force for the programmable response. Furthermore, this bioinspired strategy offers unique capabilities in fabricating responsive devices such as a soft gripper and a locomotive robot, paving new routes to many other responsive polymers.

Challenges associated with mechanical fracture of electrical conductors has hindered the realization of truly flexible high performance wearable electronics. Here, transparent healable electrodes have been developed and examined to alleviate these problems. The composite electrode features a layer of interconnecting AgNWs network on a polyurethane film modified with Diels–Alder adducts (PU-DA). Surface modification using hydrophilic molecules improved adhesion of the AgNWs network and resulted in mechanically robust flexible electrodes with a figure of merit sheet resistance of 13.3 Ω/sq and 77% transmittance at 550 nm. Transparent and flexible healable heaters (TFHH) with good mechanical and thermal stability were fabricated using these electrodes for potential applications in thermochromics, electrically driven displays and defrosters. The PU-DA TFHHs exhibited high Joule heating temperatures of 102 °C with a low operation voltage (6 V), fast thermal response (150 s) and enhanced robustness to endure large repeated mechanical strain for over 500 bending cycles with small variance in resistance (<10%). After deliberate damage by a knife cut, the electrodes healed and recovered back to its original conductivity via a simple heat treatment at 120 °C. Uniquely, the healing process can also be triggered by utilising electrical power.

Most applications that require rotational positioning and high torque generation for mechanical performance are often non-compact with a complex design that is not ideal for nanotechnological applications. Twist-spun carbon nanotube (CNT) yarn can serve as an actuation for high-performance motion systems like artificial muscles that require torsional rotation in addition to bending and contraction and micromechanical devices. Their nanoscale dimension and aspect ratio is attractive for torsional sensing. Torsional acceleration in CNT yarn can be driven in both directions for conversion of mechanical energy to electrical energy. This can find application in sensors that generate electrical signals through applied torsional rotation.
The effect of twist on the elasticity, yarn strength, strain to failure and piezoresistivity is discussed. CNT yarns with low twist angle (10-20^o) was produced to compare with a medium to high twisted yarns (37-45^o). The lowly twisted CNT yarn had a lower breaking strength and elongation at rupture but higher rigidity while the highly twisted CNT yarn showed higher ultimate breaking strength and higher strain to failure but lower stiffness modulus. The decrease in electrical resistance upon application of torsional loading to the CNT yarns demonstrate that applied twist increases fiber compaction, resulting in increased electrical contact between nanotubes and a negative piezoresistance. The piezoresistive response of highly twisted yarn is however lower than that of the low twisted thread leading to conclusion of twist being of mechanical advantage due to its effect on intertube slippage and less effective in strain sensing applications.

Self-powered sensors with high sensitivity and low-cost fabrication are promising for a wide range of applications, however, the stretchability of the devices has been a long-standing challenge. Here we present a newly-designed hyper-stretchable self-powered sensor that can withstand strain as large as 300% by fabricating piezoelectric poly[vinylidene fluoride] (PVDF) micro/nanofibers into a self-similar structure through helix-electrohydrodynamic direct-writing and self-organized buckling. Helix-electrohydrodynamic direct-writing is able to deposite serpentine micro/nanofibers with diverse geometries in a low-cost, large-scale and additive manner. Self-organized buckling utilizes the driven force from the prestrained elastomer to assemble serpentine fibers into ultra-stretchable self-similar architecture. Quantitative analysis provides detailed insights into the establishment of serpentine design and printing rules for self-similar fibers. The hyper-stretchable self-powered sensors have shown repeatable and consistent electrical outputs with high sensitivity. Meanwhile, such sensors can simultaneously measure different physical quantities, such as stretching, pressure and impulse rate, which is attractive for wearable electronics applications.

Microfluidic devices have shown considerable promise in a wide range of applications from medical screening to portable energy sources. Leveraging the successes of microfluidic technology, Micro fuel cells (μFCs) have been attracting much attention as a leading candidate for prospective portable power sources and battery replacements. Their ability to create an efficient and clean source of energy combined with the ease of portability makes μFCs available to meet the needs of various portable electronic applications in the future. To this purpose, making more efficient devices with cost effective processes and materials is crucial.
Many efforts on fuel cell miniaturization are focused on silicon-based techniques as silicon is the most common substrate in MEMS technology. However, combining silicon devices with polymeric fuel cells at mm or sub-mm scale presents many challenges, none of which have been solved in a completely satisfactory manner. Furthermore, in recent years due to material property issues associated with PDMS such as bulk absorption of small molecules and evaporation through the device, there has been a tendency towards the employment of thermoplastics for microfluidic systems. The importance of micro-structures of polymers for micro fuel cell fabrication is enormous, particularly when considered as a low-cost alternative to the silicon- or glass-based MEMS technologies, for disposable small electronic gadgets.
In this study, an air-breathing micro fuel cell with direct hydrogen flow through porous anode electrode is realized and reported. The performance of the microdevice with embedded flow channels and electrodes is characterized in ambient conditions. To achieve this goal, we implement a SU-8 microchannel stamp to transfer a pattern into a Nafion 1110 membrane by hot embossing. Nafion 212 thermally seals the whole device as a blanket top layer. The fabrication process of microchannels and micro fuel cell construction is evaluated with optical and scanning electron microscopy (SEM) step by step. Variations of hydrogen feed rate on performance were investigated. The shared-anode characteristic design of the double-sided cell is further studied by separate polarization and electrochemical impedance spectroscopy (EIS) measurements taken from each side and compared with data for the complete cell. The maximum power density per superficial (footprint) area is 68.4 mWcm^-2 for the shared anode stack. In the present micro fuel cell architecture, the external package is constructed of Nafion polymer, which offers a thin, light and flexible energy source with low cost of manufacturing. The device performance offers a high volumetric and gravimetric energy density for portable applications.

We present a printed paper actuator as a low cost, reversible and electrical actuation and sensing method. This is a novel but easily accessible enabling technology that expands upon the library of actuation-sensing materials in HCI. By integrating three physical phenomena, including the bilayer bending actuation, the shape memory effect of the thermoplastic and the current-driven joule heating via conductive printing filament, we developed the actuator by simply printing a single layer conductive Polylactide (PLA) on a piece of copy paper via a desktop fused deposition modeling (FDM) 3D printer. This paper describes the fabrication process, the material mechanism, and the transformation primitives, followed by the electronic sensing and control methods. A software tool that assists the design, simulation and printing toolpath generation is introduced. Finally, we explored applications under four contexts: robotics, interactive art, entertainment and home environment.

In order to simulate body movements in the field of health care and soft robotics, studies on compliant stretchable strain sensors capable of converting mechanical strain into electrical signals have been actively investigated recently. In addition to sensing performance, there is a growing demand for the stability of sensors required for commercialization of compliant sensors such as linearity, hysteresis, and mechanical reliability. Capacitive strain sensor among various strain sensors is advantageous to obtain the stability of compliant strain sensor such as signal linearity and hysteresis aspects. However, application of capacitive strain sensors are still limited in that they could not have gauge factor more than 1 due to the isotropic deformation shape of incompressible dielectrics.
This study proposed a highly sensitive capacitive type strain sensor with robust mechanical reliability by using an elastomeric material having an anisotropic deformation shape. An appropriate two-dimensional mesoscale structural material was applied as a composite material to impart the strain anisotropy of the elastic dielectric. Through the architected design, highly stretchable capacitive-type strain sensor with an improved gauge factor of 3.21 was realized. In addition, the developed sensor could achieve 100 % stretchability, and excellent cyclic durability over 5,000 cycles under 30 % tensile strain with no mechanical degradation. Furthermore, our developed sensor maintained a large stiffness in the thickness direction that can suppress the deformation of the dielectric due to the pressure, thereby distinguished stretch signals clearly. Our research is advantageous in that it has developed materials with non-existed mechanical properties through architectural design of structural materials, and expected to open up a broader material table in the field of strain sensor engineering.

Soft dielectric elastomers usually suffer from low electrical breakdown strength or electromechanical instability [1,2]. However, to realize very large actuation strains high voltages are usually required if the film thickness is within the micrometer scale. From the high voltage cable industry voltage stabilization of the utilized materials is very common, and excellent results are achieved e.g. from thermoplastic polyethylene (PE) for which the electrical breakdown strength has been improved by more than 50% by addition of minute amounts of so-called aromatic voltage stabilizers [3]. For silicone elastomers various studies have shown that voltage stabilization is indeed possible [4] but not to a comparable extent to that of the PE material. PE is a thermoplastic material and therefore the electrical breakdown may be very different for that of silicone elastomers. In order to voltage stabilize silicone elastomers various studies are conducted to identify the breakdown patterns which may have different origins, such as thermal or electronic [5]. Finally, various approaches to voltage stabilize dielectric elastomers are discussed.


References
[1] P. Brochu, Q. Pei, Macromol. Rapid Commun. 2010, 31, 10.
[2] F. B. Madsen, A. E. Daugaard, S. Hvilsted, A. L. Skov, Macromol. Rapid Commun. 2016, 37, 378.
[3] V. Englund, R. Huuva, S. M. Gubanski, T. Hjertberg, Polym. Degrad. Stab. 2009, 94, 823.
[4] A. H. A Razak, A. L. Skov, RSC Adv. 2017, 7, 468.
[5] L. Yu, F. B. Madsen, A. L. Skov, Int. J. Smart Nano Mater., DOI: 10.1080/19475411.2017.1376358.

Proton conduction of polyoxometalate has been received considerable interest because of their unique structural properties. The fabrication of hybrid polymer films of polypyrrole–polyoxometalate was reported with their resistive-type humidity sensing performance. The hybrid polymer nano films with different thickness were elaborately prepared via co-electrodeposition of free pyrrole monomers with metal oxide clusters. The sensing response/recovery time of sample with the thickness of 59 nm exhibited 1.9 s and 1.1 s at 98% relatively humidity and the sensing range is 11%~98% relatively humidity. In polypyrrole chain, the outstanding sensing could be ascribed synergistic effect of the proton acid doping structure and the oxidation doping structure. The humidity sensor based on nanocomposite was found to be repeatable even after two months remaining well-operated response and recovery time.

Spiropyrans (SPs) are one of the photochromic materials, which changes their colors through cis-trans isomerization by external stimuli such as UV-vis light, metal ions, and thermal energy. Introducing different functional groups to SPs is an important factor in adjusting their sensing properties. We found that SPs could be easily modified with a triazole linkage made by Cu-catalyzed alkyne-azide cycloaddition (CuAAC) from an alkyne-substituted SP. We synthesized a spiropyran derivative with the 1,2,3-triazole unit that showed selectivity toward Cu²⁺ ion forming the metal-SP complex with a 3:2 ratio. With calix[4]arene-conjugated SPs, we found very interesting “cooperative bindings” of metal ions to SP-triazole-calix[4]arene, the second binding to which was more facilitated than the first binding. By introducing ethylene glycol units to SP through the triazole, the functionalized SPs showed improved solubility in aqueous environments, which showed the enhanced ability of cyanide sensing.

It is well known that Strain gauges are commonly used for Non-destructive Testing (NDT) and Structural Health Monitoring (SHM) in various fields such as automotive, aerospace, robot and electronic industries. To monitor the structural health exactly, the strain gauge must be transformed conformally on the 3D surface of the structure. However, the conventional 2 dimensional strain gauges cannot be conformally transformed to free-form 3 dimensional surfaces.
In this work, we have fabricated a strain gauge directly on curved surfaces using 3D printing technique. We have formulated shear-thinning conductive inks to prevent inks from flowing down during the printing process. After that, we uniformly printed the inks along curved surfaces of the structure by controlling the distance between a dispensing nozzle and curved surfaces, printing speed and applied pressure. The printed pattern was locally sintered by using 808 nm diode laser in order to prevent damages of the base structure during sintering process. To investigate the feasibility of the strain gauge as a sensor to measure deformation of a structure, we evaluated changes in resistance of the strain gauge depending on the strain applied to the structure.

Dielectric elastomers find more and more uses but nevertheless the fundamentals behind the electrical breakdown of these thin and elastic films are still not fully understood and elucidated. Dielectric breakdown strength measurement is one of the most common methods to evaluate stability of polymers in an electric field. This breakdown test has been extensively used over many years and is still gaining on importance, due to an increasing demand on novel polymeric materials applied in high electric fields, such as: dielectric or transport layers, modern devices or flexible electronics[1].
There are only few theoretical models that assess the physical processes occurring during a breakdown phenomenon, for example: the hole-induced breakdown model, the electron-trapping breakdown model, the resonant-tunneling-induced breakdown model and the filamentary model [2]. All these theories consider movements of electrons from electrodes to polymer film samples. Other theory is the so-called electro-mechanical model [3]. It implies, that polymer films are not always smooth, and when an electric field is applied, the force gets bigger at the thinnest spot of the film. For that reason the film sample starts to deform and when electric strength is reached at the thinnest spot, breakdown occurs [3]. This is also referred to as electro-mechanical instability (EMI) and has been extensively studied by modelling [4]–[6].
In this work a high-speed camera is used in order to capture macroscopic processes taking place during the dielectric breakdown to verify if the time-scale and behavior of the electrical breakdown can elucidate the underlying behavior.

References
[1] V. A. Zakrevski and N. T. Sudar, ‘Electrical Breakdown of Thin Polymer Films’, Phys. Solid State, vol. 47, no. 5, p. 961, 2005.
[2] A. Belkin, A. Bezryadin, L. Hendren, and A. Hubler, ‘Recovery of Alumina Nanocapacitors after High Voltage Breakdown’, Sci. Rep., vol. 7, no. 1, p. 932, 2017.
[3] J. Blok and D. G. Legrand, ‘Dielectric breakdown of polymer films’, J. Appl. Phys., vol. 40, no. 1, pp. 288–293, 1969.
[4] X. Zhao and Z. Suo, ‘Theory of dielectric elastomers capable of giant deformation of actuation’, Phys. Rev. Lett., vol. 104, no. 17, pp. 1–4, 2010.
[5] J. Huang, T. Lu, J. Zhu, D. R. Clarke, and Z. Suo, ‘Large, uni-directional actuation in dielectric elastomers achieved by fiber stiffening’, Appl. Phys. Lett., vol. 100, no. 21, 2012.
[6] X. Zhou et al., ‘Electrical breakdown and ultrahigh electrical energy density in poly(vinylidene fluoride-hexafluoropropylene) copolymer’, Appl. Phys. Lett., vol. 94, no. 16, pp. 92–95, 2009.

Transparent silicone resin is commonly used in optical bonding processes which attaches the touch panel directly to the LCD, filling the small air gap between the front of the display and the back of the touch panel. The goal of optical bonding is to improve the performance of the display by reducing the amount of reflected ambient light under outdoor environments.¹ Silicone resin is a flexible material and excellent in transparency, adhesion reliability, and durability to ensure higher quality of display images in terms of brightness and contrast. On the fact that silicon nanoparticles (SiNPs) is easily oxidized under atmosphere, it is expected that SiNPs can be used as an oxygen-getter in the silicone resin and a gas barrier creating a tortuous path in film. Thus we studied the introduction of SiNPs into silicone resin and their gas barrier properties, improving the resistance to oxygen transmission. First, SiNPs with halide functionalities were prepared from metal silicide, reacted with alkenyl magnesium halide to give alkenyl functionalized SiNPs, and then underwent the hydrosilylation with silicone containing Si-H in the presence of Pt catalyst to give SiNPs-imbedded silicone materials. These materials can be utilized themselves as optical clear resin or additives in commercial silicone resin for the display. The sheets of silicone resin with SiNPs (4-10 nm diameter) or films in which less than 1 wt% of SiNPs are dispersed were made and tested for gas (especially dioxygen) permeability. These film with SiNPs could be reduced about half the gas permeability of traditional silicone resin film (<280 barrier for O₂; high light transmission: >99% in visible region). It may be expected that such SiNPs-imbedded silicone materials are particularly effective for incorporation into LCD module or the like which are susceptible to degradation from oxygen and/or moisture. Ongoing the study, we will present the synthesis and property of optically clear silicone resin with SiNP in details.

Reference
1. D. Lu, J. Wang, C. Li, J. Yuan, J. Sawanobori,J. Lin, A. Litke, M. Levandoski, “Liquid Optically Clear Adhesives for Display Applications” in 2012 International Conference on Electronic Packaging Technology & High Density Packaging, 13-16 Aug 2012, Guilin, Guangxi, China.

Force-measuring touch sensors are widely used in the fields of automotive, industrial, and medical applications. Recently, capacitive-type touch sensors have attracted considerable attention in these fields because these sensors are well known to have low temperature sensitivity, low power consumption, and a robust structure. Recently, there have been many researches on flexible capacitive-type touch sensors for curved surfaces while having these advantages. However, the main target is to apply it to a rigid curved surface which is not deformed. Most of these sensors have calibration problems because their initial values and sensitivities change depending on the curvature of the surface. In recent years, it is also necessary to apply force to surfaces that are fluidly actuated in the field of soft robotics or well-deformed surface (e.g., biological surface, artificial skin). In this case, the ideal touch sensor measures the force up to tens of kPa without changing the initial value and sensitivity according to the applied surface curvature. However, there is not much research on bending-insensitive touch sensors at present, and studies on existing bending-insensitive touch sensors have small force measurement ranges (~ 2 kPa).
In this paper, studies were conducted to develop a bending-insensitive capacitive-type touch sensor capable of measuring a relatively wide range of forces (up to tens of kPa). The sensor consists of top and bottom electrode layers, and there is a dielectric layer in between. The bottom electrode is made of patterned FPCB and the top electrode is made of stretchable electrode, so it can be easily extended to bending. The study contents are as follows: 1) We fabricated two types of stretchable top electrodes applicable to our devices by using AgNW and AgNW/PEDOT: PSS hybrid materials, which are typical materials for stretchable electrodes, to fabricate electrodes with little change in resistance according to the stretch. The thickness of the electrodes is determined by coated number using mayer rod method. Through resistance change analysis according to stretch, we can confirm that fabricated electrodes using AgNW/PEDOT:PSS hybrid materials have very small initial sheet resistance (~0.8 Ω/sq) and gauge factor of about 3. 2) We analyzed the effect of resistance change caused by stretching of the top electrode on capacitance change of the sensor while maintaining the distance between top and bottom electrodes constant. 3) Finally, depending on the geometry of the structures forming the dielectric layer (e.g., height and shape of the structures, and distance between structures), the distance between the top and the bottom electrodes is determined during bending, which causes a change in the capacitance of the sensor. Through the bending tests, we propose a dielectric layer structure that has the smallest change.

Human skin is a stretchable sensor, which has inspired the development of mimics, with certain level of sophistication, to achieve wearable electronics. This rapidly growing field has currently received tremendous attention. Traditionally, “electronic skin” is generally considered as a stretchable sheet with sensitive sensors for various stimuli, including deformation, pressure, light and temperature. The sensors provide signal through stretchable electrical conductors like gold nanomeshes, silver nanowires and etc., which transmit signals via electrons. These “electronic skins” are limited in specific applications, such as biocompatibility in biometric sensor and transparency in tunable optics, although they meet the basic requirement of conductivity and stretchability. On the other side, the human skin report signals via ions. In this project, we explore the “ionic skin”, which has the potential in the development of a new type of sensory sheet that is highly stretchable, transparent, and biocompatible. The hydrogels, polymeric networks swollen with water, are highly stretchable and transparent ionic conductors. With combination of carrageenan and polyacrylamide, the double network hydrogel achieves extremely high stretchability, excellent transparency, and biocompatibility. The sodium chloride in water make the hydrogel as an excellent ion conductor. These double network ionic hydrogels are successfully fabricated as a strain sensor to monitor human motion. In addition, a highly stretchable, transparent ionic touch panel is also achieved.

Achieving biological softness and flexibility has been a critical challenge for bio-electronic materials inareas of implantable bionic interfaces and wearable devices. Unlike inherently rigid electronic materi-als, bioengineered interfaces or scaffolds require stretchable softness as well as functional porosity forselective ionic transports. Toward this goal of providing electrically conducting biomaterials, we reportultrathin single-walled carbon nanotube (SWNT) layer coating on biodegradable polycaprolactone (PCL)membranes via layer-by-layer (LBL) assembly technique. The resulting membranes have unique, hier-archically structured 3D network of conductive paths around asymmetric nano-/micro-pores, providingsoftness as well as stretchable, anisotropic electrical conductivity. The strain sensors embedded on themembrane also demonstrate bending directional sensitivity as well as piezoresistivity with tunable GaugeFactor (GF) in the ranges of 5–13 with stretchability of up to 100%. Furthermore, these composite mem-branes are biocompatible as evident by neuronal cell attachment tests with in vitro PC12 cell lines. Asfollows, this newly developed implantable multifunctional membrane has considerable potential forapplications in bioengineered devices such as mechano-sensitive artificial interfaces and skins.

Wireless implantable technology has the potential of revolutionizing the medical field, facilitating the use of complex circuitry in soft, miniaturized packages. One enabling component of wireless implants is a Schottky diode rectifier based on a semiconductor suitable for thin-film applications. Among the available flexible semiconductors, amorphous Indium-Gallium-Zinc-Oxide (IGZO) has shown versatile properties, with its high electron mobility >10 cm²/Vs and low fabrication temperature < 200 Celsius. In this study, we present platinum (Pt)-IGZO Schottky contact diodes fabricated on a thermoset thiol-ene/acrylate shape memory polymer (SMP) substrate. The mechanical properties of the polymer are tailored to enhance biocompatibility and adhesion between metal gate contact and SMP substrate. The resulting devices can therefore be stiff enough to facilitate device implantation and then soften in vivo to approach internal body tissue moduli. Based on a preliminary study on glass substrates, UV-ozone treatment on the Schottky metal surface maximizes rectification ratios to ~10⁶ by adjusting treatment to 1