(Nanowerk Spotlight) Since the early 1990s, the quest to enhance virtual, augmented, and mixed reality experiences has driven researchers to innovate in the field of haptic interfaces. These interfaces aim to provide realistic touch feedback, a critical component for fully immersive digital environments. However, developing haptic systems that are not only lightweight and comfortable but also capable of conveying a range of tactile sensations has been a significant and ongoing challenge.
Recent soft wearable devices using shape-memory alloys, air-based pneumatics, or flexible electronics encapsulated in elastomers have made strides on the comfort front. But these existing approaches remain limited in their capacity to provide nuanced haptic signals beyond simple vibration. This restricts the breadth of information they can intuitively communicate to users. As a result, current haptics still represent a tactile bottleneck holding back the scope of virtual interactions.
Now, researchers at the Korea Advanced Institute of Science and Technology (KAIST) have reported a breakthrough smart textile that tackles these limitations. Their wearable haptic auxetic fabric (WHAF) combines superb wearability with a remarkable ability to generate both spatially and temporally complex haptic patterns. The findings are published in Advanced Materials (“Easy-To-Wear Auxetic SMA Knot-Architecture for Spatiotemporal and Multimodal Haptic Feedbacks”).
Schematic of fabricating WHAF and its structural properties. a) Schematic illustration of how the auxetic-knotted SMA fabric behaves when an external force is applied to deform the structure and is heated to return to its pre-memorized shape. b) Shape-fitting feature of the WHAF covering a sphere globe and a football having prolate spheroid shape. Scale bars: 15 mm (left) and 45 mm (right). c) Image showing the auxetic-knotted SMA fabric fabricated as an active fabric connected with passive fabric by simple sewing. The magnified image, outlined by the red dots, shows how SMA wires are interlaced and knotted to form tessellation of the re-entrant unit. The magnified image, outlined by the blue dots, shows how edges of the active fabric are stitched together with the passive fabric. d) Comparison of current flow within bare SMA wires and Parylene-coated SMA wires, both having a physical contact at one point. The image below shows how zone-specified Joule heating results with contraction of only the electrical energy applied columns. e) Schematic illustrations showing the WHAF worn to provide zone-specific tactile force feedback, or to serve as a personal exercise advisor by providing kinesthetic feedback. (Reprinted with permission by Wiley-VCH Verlag)
The WHAF’s unusually dynamic physical behaviors derive from its novel metastructure. The researchers intricately interlaced and knotted responsive shape-memory alloy wire pairs into an auxetic lattice arrangement exhibiting a re-entrant geometry. This topology enables the WHAF to fully expand or contract in three dimensions – displaying a rare negative Poisson’s ratio effect unseen in normal fabrics. Auxetic structures expand perpendicularly when stretched rather than thinning, lending excellent conformability.
“Auxetic” refers to a property of materials that behave in a counterintuitive way when stretched or compressed. In most materials, when you pull them apart (stretch them), they become thinner in the perpendicular direction, and when you compress them, they expand in the perpendicular direction. This is quantified by a property called the Poisson’s ratio, which is positive for these typical materials.
However, in auxetic materials, this behavior is reversed. When you stretch auxetic materials, they become thicker in the perpendicular direction, and when you compress them, they become thinner in the perpendicular direction. This means auxetic materials have a negative Poisson’s ratio.
This unusual property gives auxetic materials unique advantages, such as high energy absorption and fracture resistance, and can enhance other properties like shear stiffness. Due to these characteristics, auxetic materials find applications in a wide range of fields, including protective gear, medical devices, aerospace, and, as in the case of the article, advanced textiles for haptic feedback in virtual reality environments. Their ability to expand in all directions when stretched can be particularly useful in creating more comfortable, adaptable, and effective products.
Besides ensuring excellent stretchability, drapeability and shape conformability for a perfect fit against skin, the WHAF’s auxetic architecture grants it a standout talent. Taking advantage of the lattice interconnectivity, the team applied a thin insulating microfilm to separately control the heating of individual wire junctions within the knots. As a result, voltage can selectively actuate localized fabric regions. This realizes zone-specified actuation unprecedented for haptic textiles, opening up spatiotemporal stimulation possibilities.
Whereas previous shape-memory alloy smart fabrics could only exhibit total uniform contraction, the WHAF’s segmented control allows intricate patterns of compression and sweeping pressure waves. Users perceive these complex tactile sequences as discernible tactile shapes, textures and motions. For instance, the researchers demonstrated the WHAF communicating directional cues through sequential squeeze pulses around the forearm.
Additionally, modulating the voltage tunes the force output timing and stiffness in actuated regions. Combined with its excellent skin adhesion once worn, this enables the lightweight WHAF to provide readily discernible kinesthetic resistance feedback. The researchers showed that wearing the fabric on joints turned finger flexion and arm bending into perceptibly more strenuous motions.
User tests evidenced the superlative accuracy and intuitiveness of the WHAF’s extensive haptic vocabulary. Participants reliably identified directional navigational prompts and spatial patterns delivered through the fabric. The WHAF thus empowers practical applications like hands-free mobile navigation. Meanwhile, in an immersive virtual rover simulation, WHAF haptics successfully allowed users to steer around obstructive crater terrain despite blinding dust impeding vision.
The researchers suggest their adaptable auxetic smart textile constitutes a versatile multimodal haptic interface breakthrough. The WHAF’s unparalleled wearability and dynamic actuation capacities help resolve key limitations in fabrics and skins for interactive virtual realities. Its conveying of nuanced tactile sensations through readily comprehended modality-spanning cues could greatly boost intuitiveness and maneuverability when exploring digital spaces. The KAIST team’s novel metastructure paradigm for imparting advanced responsive properties onto everyday fabrics looks set to accelerate innovation around skins interfacing humans with virtual worlds.
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