Highly Transparent and Integrable Surface Texture Change Device for Localized Tactile Feedback

Soft Sensors and Actuators for Wearable Human-Machine Interfaces

Haptic human-machine interfaces (HHMIs) combine tactile sensation and haptic feedback to allow humans to interact closely with machines and robots, providing immersive experiences and convenient lifestyles. Significant progress has been made in developing wearable sensors that accurately detect physical and electrophysiological stimuli with improved softness, functionality, reliability, and selectivity. In addition, soft actuating systems have been developed to provide high-quality haptic feedback by precisely controlling force, displacement, frequency, and spatial resolution. In this Review, we discuss the latest technological advances of soft sensors and actuators for the demonstration of wearable HHMIs. We particularly focus on highlighting material and structural approaches that enable desired sensing and feedback properties necessary for effective wearable HHMIs. Furthermore, promising practical applications of current HHMI technology in various areas such as the metaverse, robotics, and user-interactive devices are discussed in detail. Finally, this Review further concludes by discussing the outlook for next-generation HHMI technology.

Frequency-dependent electrical properties of microscale self-enclosed ionic liquid enhanced soft composites

The incorporation of room temperature ionic liquids (ILs) into dielectric elastomer composites is currently generating great interest due to their potential applications in soft actuators and optical-related devices. Experiments have shown that the electrical properties of IL enhanced soft composites (ILESCs) are dependent on AC (alternating current) frequency of the electrical loading. This current work helps develop a mixed micromechanical model with the incorporation of an electric double layer (EDL) to predict the electrical properties of the ILESCs while revealing the physical mechanisms (including crowding and overscreening structures, percolation thresholds, interfacial tunneling, Maxwell-Wagner-Sillars polarization) that underpin the phenomena. Particularly, Bazant-Storey-Kornyshev (BSK) phenomenological theory is integrated into the EDL surface diffusion model for the first time to evaluate the influence of crowding and overscreening effects. The results show excellent agreement with experimental data of IL enhanced PDMS composites over the frequency range from 1 Hz to 10 GHz. Parametric analysis from the perspective of designing is conducted to explore the methods for optimization of ILESCs with high dielectric constants and frequency-dependent stability. It is found that an IL with a smaller size and aspect ratio increases the dielectric constant of the ILESCs more significantly below the interface relaxation frequency. Increasing the surface charge density of the matrix and using ILs delay the frequency-facilitated dielectric response, which is beneficial to maintain the dielectric stability of the ILESCs.

Development of a Soft Actuator from Fast Swelling Macroporous PNIPAM Gels for Smart Braille Device Applications in Haptic Technology

The development of a cost-efficient braille device is a crucial challenge in haptic technology to improve the integration of visually impaired people. Exclusion of any group threatens the proper functioning of society. Commercially available braille devices still utilize piezoelectric actuators, which are expensive and bulky. The challenge of a more adapted braille device lies in the integration of a high number of actuators─on a millimeter scale─in order to independently move a matrix of pins acting as tactile cues. Unfortunately, no actuation strategy has been adapted to tackle this challenge. In this study, we develop a soft actuator based on a thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel. We introduce macroporosity to the gel (pores of 10 to 100 μm). It overcomes the diffusion─which is the limiting kinetic factor─and accelerates the gel response time from hours for the bulk gel to seconds for the macroporous gel. We study the properties of porous gels with various porosities. We also compare a mechanically reinforced nanocomposite gel (made of PNIPAM and Laponite clay) to a "classic" gel. As a result, we develop a fast-actuating gel with high cyclic performance. We then develop a single-pin braille setup, where actuation is controlled thanks to a swift temperature control of a macroporous gel cylinder. This new strategy offers a very promising actuation technology. It offers a simple and cost-efficient alternative to the current braille devices.

A Moisture-Resistant Soft Actuator with Low Driving Voltages for Haptic Stimulations in Virtual Games

Strong and robust stimulations to human skins with low driving voltages under high moisture working conditions are desirable for wearable haptic feedback applications. Here, a soft actuator based on the "air bubble" electret structure is developed to work in high-moisture environments and produce haptic sensations to human skin with low driving voltages. Experimentally, the water soaking and drying process has been conducted repeatedly for the first time and the 20th time to test the antimoisture ability of the actuator as it recovers its output force up 90 and 65% of the initial value, respectively. The threshold voltages for sensible haptic sensations for the fingertip and palm of volunteers have been characterized as 7 and 10 V, respectively. Furthermore, a demonstration example has been designed and conducted in a virtual boxing game to generate the designated haptic sensations according to the gaming conditions with an accuracy of 98% for more than 100 tests. As such, the design principle, performance characteristic, and demonstration example in this work could inspire various applications with improved reliability for wearable haptic devices.

A low voltage-powered soft electromechanical stimulation patch for haptics feedback in human-machine interfaces

One grand challenge in haptic human-machine interface devices is to electromechanically stimulate sensations on the human skin wirelessly by thin and soft patches under a low driving voltage. Here, we propose a soft haptics-feedback system using highly charged, polymeric electret films with an annulus-shape bump structure to induce mechanical sensations on the fingertip of volunteers under an applied voltage range of 5-20 V. As an application demonstration, a 3 × 3 actuators array is used for transmitting patterned haptic information, such as letters of 'T', 'H', 'U' letters and numbers of '0', '1', '2'. Moreover, together with flexible lithium batteries and a flexible circuit board, an untethered stimulation patch is constructed for operations of 1 h. The analytical model, design principle, and performance characterizations can be applicable for the integration of other wearable electronics toward practical applications in the fields of AR (augmented reality), VR (virtual reality) and robotics.

Valveless microliter combustion for densely packed arrays of powerful soft actuators

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane-oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

A Microfabricated Dual Slip-Pressure Sensor with Compliant Polymer-Liquid Metal Nanocomposite for Robotic Manipulation

Conventional grippers fall behind their human counterparts as they do not have integrated sensing capabilities. Piezoresistive and capacitive sensors are popular choices because of their design and sensitivity, but they cannot measure pressure and slip simultaneously. It is imperative to measure slip and pressure concurrently. We demonstrate a dual slip-pressure sensor based on a thermal approach. The sensor comprises two concentric microfabricated heaters maintained at constant temperature. An elastic dome, with embedded liquid metal droplets, is placed on top of concentric heaters. Heat transfer between sensor and the object in contact occurs through the elastic dome. This heat transfer causes changes in the power absorbed by the sensor to maintain its temperature and allows for measurement of pressure while identifying slip events. Liquid metal droplets contribute to enhanced thermal conductivity (0.37 W/m-K) and reduced specific heat (0.86 kJ/kg-K) of the polymer without compromising on mechanical properties (Young's modulus-0.5 MPa). For pressure monitoring, sensor measures change in power ratio against increase in applied force, demonstrating a highly linear performance, with a high sensitivity of 0.0356 N^-1 (pressure only) and 0.0189 N^-1 (slip with simultaneous pressure applied). The sensor discriminates between different contact types with a 96% accuracy. Response time of the sensor (60-75 ms) matches the measured response time in human skin. The sensor does not get affected by mechanical vibrations paving way for easy integration with robotic manipulators and prosthetics.

High-k, Ultrastretchable Self-Enclosed Ionic Liquid-Elastomer Composites for Soft Robotics and Flexible Electronics

Soft robotics focuses on mimicking natural systems to produce dexterous motion. Dielectric elastomer actuators (DEAs) are an attractive option due to their large strains, high efficiencies, lightweight design, and integrability, but require high electric fields. Conventional approaches to improve DEA performance by incorporating solid fillers in the polymer matrices can increase the dielectric constant but to the detriment of mechanical properties. In the present work, we draw inspiration from soft and deformable human skin, enabled by its unique structure, which consists of a fluid-filled membrane, to create self-enclosed liquid filler (SELF)-polymer composites by mixing an ionic liquid into the elastomeric matrix. Unlike hydrogels and ionogels, the SELF-polymer composites are made from immiscible liquid fillers, selected based on interfacial interaction with the elastomer matrix, and exist as dispersed globular phases. This combination of structure and filler selection unlocks synergetic improvements in electromechanical properties-doubling of dielectric constant, 100 times decrease in Young's modulus, and ∼5 times increase in stretchability. These composites show superior thermal stability to volatile losses, combined with excellent transparency. These ultrasoft high-k composites enable a significant improvement in the actuation performance of DEAs-longitudinal strain (5 times) and areal strain (8 times)-at low applied nominal electric fields (4 V/μm). They also enable high-sensitivity capacitive pressure sensors without the need of miniaturization and microstructuring. This class of self-enclosed ionic liquid polymer composites could impact the areas of soft robotics, shape morphing, flexible electronics, and optoelectronics.

Directed Assembly of Liquid Metal-Elastomer Conductors for Stretchable and Self-Healing Electronics

Growing interest in soft robotics, stretchable electronics, and electronic skins has created demand for soft, compliant, and stretchable electrodes and interconnects. Here, dielectrophoresis (DEP) is used to assemble, align, and sinter eutectic gallium indium (EGaIn) microdroplets in uncured poly(dimethylsiloxane) (PDMS) to form electrically conducting microwires. There are several noteworthy aspects of this approach. 1) Generally, EGaIn droplets in silicone at loadings approaching 90 wt% remain insulating and form a conductive network only when subjected to sintering. Here, DEP facilitates assembly of EGaIn droplets into conductive microwires at loadings as low as 10 wt%. 2) DEP is done in silicone for the first time, enabling the microwires to be cured in a stretchable matrix. 3) Liquid EGaIn droplets sinter during DEP to form a stretchable metallic microwire that retains its shape after curing the silicone. 4) Use of liquid metal eliminates the issue of compliance mismatch observed in soft polymers with solid fillers. 5) The silicone-EGaIn "ink" can be assembled by DEP within the crevices of severely damaged wires to create stretchable interconnects that heal the damage mechanically and electrically. The DEP process of this unique set of materials is characterized and the interesting attributes enabled by such liquid microwires are demonstrated.

Polyvinyl Alcohol-Stabilized Liquid Metal Hydrogel for Wearable Transient Epidermal Sensors

Wearable epidermal sensors are attracting growing interests in human activity monitoring and flexible touch display, but they are still limited by the poor self-healing property and the difficult dissolvable feature. Herein, we report polyvinyl alcohol (PVA)-stabilized liquid metal particles (LMPs) (PVA-LMPs) hydrogels with excellent self-healing performance and the dissolvable feature for wearable epidermal sensors, constructed by dispersing LMPs of eutectic gallium and indium into the borate-modified PVA polymer networks. Interestingly, the PVA-LMPs hydrogels exhibited excellent electrically and mechanically self-healing ability. Moreover, the PVA-LMPs hydrogel can be fabricated as epidermal sensors, which can accurately monitor the human activities. Additionally, the epidermal sensors are dissolvable, showing an attractive feature for on demand transient electronics. It is demonstrated that the hydroxyl groups of PVA can stabilize LMPs via hydrogen-bonding interactions. Furthermore, the dynamic cross-linking bonds between hydrogels and LMPs can rupture and coalesce reversibly in the hydrogel network, which endow the hydrogels with both electrically and mechanically self-healing ability. This work shows the potential of constructing next-generation multifunctional hydrogel-based epidermal sensors for human activity monitoring, wearable healthcare diagnosis, portable electronics, and robot tactile systems.

An Electroactive and Transparent Haptic Interface Utilizing Soft Elastomer Actuators with Silver Nanowire Electrodes

An electroactive and transparent haptic interface having a rectangular void pattern creates tunable surface textures by controlling the wavelength and amplitude of independent void-lines. To make an active tactile surface, the transparent haptic interface employs a silver nanowire (AgNW) electrode to be compliant with the deformed elastomer surface. Here, the dielectric elastomer is newly blended with polydimethylsiloxane and Ecoflex prepolymer to simultaneously control the mechanical stiffness and transparency. The relative resistance of the AgNW electrode on a single void line is nearly unchanged under bending test, confirming the high stretchability and conductivity of the nanowire-networked electrode. The optical transparencies are 92-85%, depending on the ratio of the Ecoflex solution. Transparency values decreas by 7 and 16% after coating with AgNWs at densities of 30 and 140 mg m^-2 , respectively. Using EP31, the void line is deformed by 90 µm under a field intensity of 13.0 V µm^-1 . The haptic surface is successfully controlled by applying voltage, which produces four different surface textures, from relatively smooth to rough feeling, depending on the distance between deformed void lines. This haptic interface can be applied to diverse display systems as an external add-on screen and will help to realize programmable surface textures in the future.