Wearable Haptics for Orthotropic Actuation Based on Perpendicularly Nested Auxetic SMA Knotting

Abstract

Smart wearable tactile systems, designed to deliver different types of touch feedback on human skin, can significantly improve engagement through diverse actuation patterns in virtual or augmented reality environments. Here, a perpendicularly nested auxetic wearable haptic interface is reported for orthotropically decoupled multimodal actuation (WHOA), capable of producing diverse tactile feedback modes with 3D sensory perception. WHOA incorporates shape memory alloy wires that are intricately knotted into an auxetic structure oriented along orthotropic dual axes. Its perpendicularly nested auxetic structure enables orthotropic actuation, allowing independent expansion and contraction along both x and y‐axes, as confirmed by force‐strain and displacement‐time performance tests. Additionally, the perylene coating provides orthogonal electrical isolation to WHOA, allowing for stripe‐specific localized actuation and enabling multiple tactile feedback modes. As an orthotropic wearable haptic interface, WHOA distinguishes between x‐axis and y‐axis directions and ultimately delivers multi‐dimensional information regarding movements in 3D space through tactile feedback. As a result, when worn on the foot or arm, WHOA naturally delivers spatiotemporal tactile information to the user, facilitating navigation and teleoperation with 3D sensory perception.

Wearable haptics for orthotropic actuation (WHOA) can be used for teleoperation of drones and robots with directional sensory perception in 3D space. The perpendicularly nested auxetic pattern with parylene coating provide orthotopically decoupled tactile feedback and wearable multimodal haptics.

1. Introduction

The recent advancements highlight the growing importance and versatility of haptic interfaces across various domains, from entertainment and education to healthcare and industry. As technology continues to evolve, haptic feedback is expected to become more sophisticated and seamlessly integrated into our daily lives, providing new opportunities for immersive and interactive experiences. These advancements have expanded the capabilities and potential uses of haptic technology in several notable applications, such as metaverse,^{[ 1} , ² , ^{3 ]} virtual, augmented, and mixed reality (VR, AR, and MR), medical training and simulations,^{[ 4} , ^{5 ]} tactile feedback in consumer electronics,^{[ 6} , ⁷ , ^{8 ]} telerobotic systems^{[ 9} , ^{10 ]} and automotive industry.^{[ 11 ]} Haptic devices enhance user experiences in various applications by adding a sense of touch and realism to digital interactions. They are especially valuable in fields, where realistic tactile feedback is essential for immersion and skill development. These haptic devices generate tactile stimuli or sensations through various means to simulate the sense of touch for users. These stimuli are designed to create a realistic and immersive experience when interacting with digital or virtual environments. The types of haptic stimuli produced by these devices include vibrations,^{[ 12} , ^{13 ]} force and pressure,^{[ 14 ]} texture and surface feedback,^{[ 15 ]} temperature changes,^{[ 16 ]} kinesthetic feedback,^{[ 17 ]} pulsation and rhythmic patterns,^{[ 18} , ^{19 ]} and spatial feedback.^{[ 20 ]} While haptic devices offer valuable tactile feedback and enhance user experiences in various applications, they also come with certain limitations and challenges in aspects of size, weight, complexity, portability, user discomfort, and limited realism.

To address these issues, various studies have focused on developing soft wearable devices. These interfaces encompass a diverse range of technologies, including soft flexible sensors,^{[ 21 ]} pneumatic actuators,^{[ 22} , ^{23 ]} soft tactile displays,^{[ 24} , ^{25 ]} soft haptic skins,^{[ 26} , ^{27 ]} and smart clothing or wristbands,^{[ 28} , ^{29 ]} which incorporate haptic feedback components. Soft haptic interfaces offer advantages such as comfort, flexibility, and safety, making them suitable for a wide range of applications where traditional rigid haptic devices are less practical. As soft haptic technology becomes more refined and integrated into various applications, it has demonstrated its potential to offer immersive and intuitive tactile experiences, particularly in areas where gentle and adaptable interactions are essential. However, despite their potential benefits, most current devices heavily lack diversity in handling directional information. Considering that the human body can sense multiple directions, conventional haptic devices fail to fully leverage the potential of our body, thereby limiting the efficiency and diversity of haptic interactions. Particularly, to convey directional information in our 3D world, several units of actuation components are needed with multiple degrees of freedom and special modes for upward and downward movement. Therefore, there is a need for a wearable haptic interface that can provide directional information in 3D space and provide ease of use with more advanced features. Such a device could actuate either vertically or horizontally to provide orthotropic actuation and convey perceptions as a multi‐directional haptic device in teleoperations or VR, AR, and MR applications.

This study presents a wearable haptic interface integrated with orthotropic fabric actuators (WHOA), developed by knotting the shape memory alloy wires into a specialized auxetic structures pattern along orthotropic dual axes. The proposed WHOA exhibits orthotropic tactile feedback with directionally localized mechanical deformations, such as expansion or compression occurring in mutually perpendicular directions. The displacement and force produced on one perpendicular side of the auxetic pattern are independent to the other side, resulting in orthotropically decoupled tactile feedback. This feature makes it highly useful for providing 3D directional perception in haptics applications. WHOA, made of knotted SMA wires, can be easily deformed from its pre‐memorized shape into a stretched state, allowing it to conform to the shape of an object, thus exhibiting a stretchable and shape‐fitting property. Additionally, the temporal programmability of the SMA allows the fabric to actuate in multi‐fashion ways, forming horizontal and vertical stripes that provide haptic feedback to the user. Noteworthy, before using the fabric actuator as a haptic wearable, a thin parylene coating is applied to ensure the electrical insulation of the SMA wires. This coating enables the fabric actuator to generate stripe‐specified diverse tactile feedback to the user skin by utilizing individual joule heating. These tactile sensations can be used in virtual feedback systems to provide 3D spatial perception. By delivering stripe‐specified cutaneous feedback, the independently controlled orthotropic actuation can be effectively utilized for spatial recognition during 3D navigation, such as guiding drones and aiding blind individuals in finding their path. Furthermore, in Table S1 (Supporting Information), various haptic actuators^{[ 30} , ³¹ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ^{42 ]} are compared alongside the WHOA, detailing key performance attributes and specifications. This table provides a clear and concise comparison, showcasing unique features and advantages of the orthotropic actuator. The key performance metrics such as accuracy, type of actuation, and the range of stimuli are highlighted, allowing for a side‐by‐side evaluation of each device. WHOA demonstrates superior performance with significant accuracy, exceeding the capabilities of several other devices, especially in terms of spatial navigation and guidance. By expanding the potential of fabric‐based actuators and their application to wearable haptic interfaces, WHOA complements efforts aimed at developing portable and comfortable wearable haptics using textiles and soft actuators.

2. Results and Discussion

2.1. Wearable Haptics for Orthotropic Actuation (WHOA)

As shown in Figure 1a, the unidirectional haptic interface is suitable for guiding vehicles moving on 2D plane. However, when it comes to synchronizing with mobiles navigating in 3D, such as drones, these interfaces lack the dimensionality to include movement along an additional axis, that is, going up and down. This is where WHOA demonstrates its superiority, by incorporating orthotropic actuation that distinguishes actuation from one axis to another. The primary structure of WHOA as shown in Figure 1b, has a nested tiling of re‐entrant networks with a preset edge angle having two opposite orthogonal auxetic SMA patterns. The edge angle of each matrix alters from ϑ to ϑ' when a mechanical influence from the outside is exerted at room temperature, producing orthotropic behavior. However, each unit lattice undergoes the shape memory effect upon heating to the deformed WHOA, causing the edge angle to return to ϑ either in x‐direction, y‐direction, or both directions simultaneously, under the provided control input. The structure, as a result, shrinks back to its pre‐memorized shape, depending on the axis along which heating is applied, leading to the independent motion of orthogonal axial stripes. Through parylene coating at the junction between two orthotropic auxetic patterns, multi‐segment multimodal haptics was realized with separately controlled actuators with multiple degree‐of‐freedoms in a multi‐array WHOA structure as shown in Figure 1c.

Figure 1.

Schematic of fabricating WHOA and its actuation behavior. a) WHOA providing better directional haptic feedback that enhances control in virtual and real environments for improved spatial perception in 3D space. b) Schematic illustration of orthotropic SMA fabric when joule heating is applied in either vertical or horizontal stripes and both stripes. c) Multi‐segment multimodal haptic device electrically isolated with parylene coating. d) 3D spatial haptic feedback of drone moving in 3D space for search and rescue operations, showing the operator's ability to perform in harsh conditions with limited visibility. e) Hands‐free 3D navigation support for visually impaired persons in ascending and descending stairs, highlighting on the compact, lightweight design of WHOA with unobtrusive assistance and spatial tactile feedback in complex environments.

Hence, WHOA itself offers enhanced wearability and versatility, along with the Velcro detachable design, facilitating effortless integration into wearable devices.^{[ 43 ]} The use of SMA, renowned for its exceptional power‐to‐weight ratio, generates significant force with minimal weight, reducing the size and volume of the actuator and enhancing efficiency by directly converting thermal energy into mechanical movement. Also, by providing directional haptic feedback during complex navigation in 3D space, the customized multimodal WHOA offers a comfortable and intuitive experience in the teleoperation of a drone moving in a 3D virtual reality environment, as illustrated in Figure 1d. Notably, the compact form factor of the WHOA allows for seamless incorporation into footwear, presenting a promising solution for enhancing navigation for visually impaired individuals. When worn under the shoes, the WHOA ensures a comfortable and unobtrusive user experience, thereby promoting natural and unrestricted movement, as shown in Figure 1e. This transformative technology holds immense potential for applications in assistive, and wearable devices, offering improved mobility and functionality to users across diverse settings. It can also function as a personal path navigator by providing tactile feedback inputs to the blind going up the stairs, as depicted in Figure 1e.

2.2. Orthotropic Behavior of WHOA

Figure S1a (Supporting Information) provides detailed explanation about how the SMA wires are wound on the aluminum mold and Figure S1b (Supporting Information) shows the dimension of the orthogonal auxetic structure pattern on the aluminum mold.^{[ 44} , ^{45 ]} Particularly, Reef knot is designed to create a symmetrical and balanced structure using SMA wires with a diameter of 0.35 mm as illustrated in Figure S1c (Supporting Information). Moreover, at least three knots are generated to ensure a stable structure at each vertex by evenly distributing forces via creating multiple turns and loops, as illustrated in Figure S1d (Supporting Information). Furthermore, the fabrication of orthogonal unit is carried out in four steps as illustrated in Figure S2a (Supporting Information). The WHOA is simple to weave into conventional textiles, allowing it to seamlessly blend active and passive elements into a sole wearable tangible fabric, as shown in Figure S2b,c (Supporting Information). Video S1 (Supporting Information) demonstrates the process of knotting and interweaving SMA wires to make the WHOA. The flexibility of auxetic structures arises from their specific geometric arrangement of the re‐entrant structure. This unique arrangement allows these structures to deform under load, allowing them to distribute stress more evenly and resist deformation in a way that is different from conventional materials. Hence, the orthotropic fabric shows high flexibility and stretchability as shown in Figure S3 (Supporting Information).

As WHOA is composed of orthogonal auxetic patterns, the negative Poisson's ratio behavior is the key advantageous characteristic derived from their unique mechanical properties. This allows WHOA to expand uniformly in all directions when subjected to tension, resulting in a structure that can adapt its highly curved complex shapes,^{[ 46 ]} as illustrated in Figure 2a,b with a mouse and a tennis ball, respectively. Orthotropic behavior characterizes the unique functionality of WHOA with distinct mechanical deformations along mutually perpendicular planes. WHOA exhibits independent variation in these orthogonal directions under electric control inputs, as demonstrated in Figure 2c. The sequences of images show the cases in which a WHOA is in pre‐memorized shape, x‐axis controlled deformation, y‐axis controlled deformation, and both axes‐controlled deformation. Each pair of SMA wires that make up the unit of WHOA is separately controlled by changing its surface electrical characteristics, which results in a variety of tactile haptic patterns. Without the spatially regulated Joule heating, the initial electrical conductivity of SMA wires inevitably results in current dispersion through physically coupled areas. This is demonstrated in Figure S4 (Supporting Information), where supplying currents, i ₁and i ₂, two separate SMA wires results in current redistribution at physically and electrically connected joints, deviating from input values. However, coating the SMA wires with a micro‐scale layer of Parylene electrically insulates their surfaces.^{[ 47} , ^{48 ]} This prevents current redistribution even at physically connected joints, enabling precise current flow only through the specified SMA wires. Consequently, as depicted in Figure S4 (Supporting Information), parylene coating enables stripe‐specified contraction, as highlighted by the contrast in thermal IR images of coated and non‐coated samples. In such a way, WHOA can provide diverse tactile feedback patterns, enhancing the VR/AR experience beyond total compression by delivering spatiotemporal variations during feedback.

Figure 2.

Orthotropic actuation behavior of WHOA. a) Image showing the shape adaptation of the computer mouse by WHOA as top & side views illustrate the shape adaptive feature. b) Image showing the shape adaptation of the tennis ball by WHOA as top & side views illustrate the shape adaptive feature. c) Orthotropic behavior of WHOA by controlling it either in x‐axis or in y‐axis. d) Experimental images of WHOA exhibiting actuation in x‐direction by operating only vertical stripes using joule heating showing thermal IR image. e) Displacement‐time plot illustrating orthotropic behaviors, clearly showing negligible deformation along y‐axis. f) Plot between force and prescribed strain. g) Experimental images of WHOA exhibiting actuation in y‐direction by operating only horizontal stripes using joule heating showing thermal IR image. h) Displacement–time plot illustrating orthotropic behavior, clearly showing negligible deformation along x‐axis. i) Plot between force and prescribed strain.

To elucidate the orthotropic nature, a comprehensive force analysis using finite element modeling (FEM) has been conducted, as illustrated in Figure S5a (Supporting Information). Through the detailed analysis of stress, deformation, and displacement profiles, the insights of the orthotropic behavior inherent within the structure are described in Figure S5b–d (Supporting Information). Meanwhile, Figure S6 (Supporting Information) demonstrates the orthotropic motion under mechanical stretching of the single unit actuator, which pairs the auxetic structures. Due to negative Poisson's ratio of unit lattice,^{[ 49 ]} the mechanical stretching along the x‐direction occurs solely in the x‐axis without affecting the y‐axis, as shown in Figure S6b (Supporting Information). Similarly, in y‐axis, the negative Poisson's ratio of the orthogonal side unit lattice allows motion along the y‐axis without affecting the other axis, as illustrated in Figure S6c (Supporting Information). Figure S7 (Supporting Information) illustrates the schematic of orthotropic actuation, where the nested auxetic architecture exhibits actuation independently in either the x‐direction or y‐direction, demonstrating orthotropically decoupled behavior. This can be additionally observed in real‐time from Video S2 (Supporting Information).

The orthotropic actuation response of a WHOA is further investigated by taking thermal IR images during stripe‐specified Joule heating. Each line is subjected to an electrical current for 2.5 s of heating and 9 s of cooling. Real images taken following the input conditions clearly demonstrate orthotropic actuation behavior along both x and y‐axes, as depicted in Figure 2d,g, respectively. The time‐displacement response was analyzed to assess the performance of the orthogonal motion and their interrelation. These findings are represented in the plots shown in Figure 2e,h for the horizontal and vertical axes, respectively. These plots elucidate a substantial difference in the motion of the perpendicular axes, demonstrating that while the x‐axis is in motion, the y‐axis remains relatively static with negligible small displacement. Furthermore, a detailed exploration of the force generated upon actuation was conducted by extending the structure along both axes, ranging from 0% to 100% strain, at a fixed current of 4.0 A for 8 s to one axis stripe‐lines, visually represented in Figure 2f,i. The bar graphs illustrate the resultant force measured at prescribed strains, allowing a comprehensive comparison of the forces exerted along the x‐ and y‐axes. These graphical representations effectively showcase the distinctive orthotropic behavior exhibited by the perpendicular axial forces. They demonstrate how these forces operate independently under different mechanical loads, emphasizing mechanical response to the applied strain along each axis.

2.3. Orthotropic Actuation of WHOA

The orthotropic actuation of WHOA is evaluated under tensile force along the x and y‐axes individually. Each sample was subjected to repetitive deformation cycles over time under constant tension of 1.25 N. As a result, consistent orthotropic behavior was observed across all cyclic actuation of x‐axis (blue line) whereas y‐axis (red line) is at rest, as illustrated in Figure 3a. Conversely, Figure 3b shows consecutive orthotropic behavior along the y‐axis while the x‐axis remains unaffected. Nevertheless, when both axes are actuated simultaneously, displacement along both axes is measured at a stable and repetitive range, as illustrated in Figure 3c. The consistency underscores the robustness and reliability of the actuator response to tension, suggesting a stable mechanical performance over successive cycles. The observed homogeneity in orthotropic behavior across multiple cycles proves structural integrity and predictable mechanical responses under repetitive actuation conditions.

Figure 3.

Structural characteristics and design parameters of WHOA. a) Orthotropic behavior of WHOA by taking cyclic responses of displacement–time plot under tension when x‐axis is in motion while the y‐axis is at rest. b) Orthotropic behavior of WHOA by taking cyclic responses of displacement‐time plot under tension when y‐axis is in motion while the x‐axis is at rest. c) Displacement–time plot illustrating the cyclic response of displacements of both x‐and y‐axis motion at the same time under tension. d) Relationship between normal force per unit mass and radius of the curvature of the surface. e) Effect of normalized axial force of actuated axis with respect to non‐actuated axis under the condition of different scalability levels. f) Effect of axial force by changing in the unit density of WHOA. g) Investigation of dynamic behavior of WHOA by computing energy density according to changes of unit size. h) Axial force of actuation in x‐axis as time goes. i) Orthotropic behavior of WHOA by actuating it for 100 cycles under tension, while keeping the y‐axis at rest.

In wearable haptic devices, the magnitude of normal force applied to the skin directly affects its performance in terms of perceptivity. Moreover, the normal force generated by the actuation of SMA is influenced by its initial deformation when worn on a curved body surface. Therefore, the relationship between normal force and radius of curvature is investigated, revealing that the normal force decreases as the radius of curvature increases, as illustrated in Figure 3d. Additionally, compared with the work done by skin drag,^{[ 50 ]} a decrease in drag was observed as the radius of curvature decreased. These findings are significant in the context of orthotropic actuators, as they provide insights into the mechanical behavior and performance characteristics of structures subjected to varying curvatures. In addition, the effect of normalized force on various scaled patterns is compared. Specifically, the patterns were varied in terms of the size of the unit lattice and the number of units. Unit 2 having side length of 20 mm exhibits greater force compared to the others, as shown in Figure 3e. This observation is significant for the design criteria of auxetic patterns, as it underscores the differences in force distribution in structures with varying patterns and scales. Subsequently, the unit density of the orthogonal auxetic pattern was examined in relation to the normalized axial force, as shown in Figure 3f. The result reveals a notable trend wherein increasing unit density corresponded to higher axial force due to areal ratio between immovable part (rectangular core part) and actuator parts (side auxetic parts) in the auxetic unit cell. This observation summarizes the influence of pattern density on force distribution. Moreover, the analogy follows the same trend in terms of energy density as the pattern density increases as illustrated in Figure 3g. However, Figures S8 and S9 (Supporting Information) show that regardless of variance in scales or pattern density, the orthotropic actuation behavior is maintained under all conditions. The insights gained from these investigations can serve as important guidelines for designing high‐performance orthotropic actuators for practical applications.

The magnitude of axial force generated over time can be specifically controlled with varying input currents (from 2.0 A to 4.0 A), as shown in Figure 3h. As the current increases, the magnitude of the force also increases, and the force reaches its stabilized level more rapidly. Additionally, when subjected to multiple actuation cycles, WHOA consistently generates highly repeatable and stable force responses for more than 100 cycles, as convincingly depicted in Figure 3i. This advantageous characteristic highlights the capability of WHOA to consistently provide repeated levels of tactile feedback when used as a wearable haptic interface. By strategically isolating the actuated sections, WHOA generates compression exclusively in the middle zone while maintaining the integrity of the surrounding regions. Video S3 (Supporting Information) demonstrates the durability and reliability of a sample, showcasing its ability to undergo multiple shape deformations without loss of performance. Moreover, the WHOA features superior structural stability against external harsh stimuli, allowing it to quickly recover its original structure and maintain stability even under significant deformation. For example, when subjected to severe plastic deformation by hammering, it restores to its pre‐memorized shape upon heating (Figure S10, Supporting Information).

2.4. WHOA for Wearable Footwear Haptics

Figure 4a shows schematic illustration of the WHOA, depicting how the horizontal (red) and vertical (blue) stripes are aligned when worn on the foot. The schematic describes the spatial arrangement of the ergonomic WHOA on the curved surface of the instep of a foot. Figure 4b shows thermal IR images of WHOA under independently controlled stripe‐specified actuation. The actuation of lines 1, 2, 4, and 5 clearly shows stripe‐specified heating solely to the x‐axis (lines 1 and 2) or the y‐axis (lines 4 and 5). Upon successfully achieving independent control of each column and row in the WHOA, the isometric force generated through various input current levels for both horizontal and vertical stripes was measured and plotted, as displayed in Figure S11 (Supporting Information). The isometric force measurements were conducted by pre‐stretching the structure with a 40% strain and securing one end to a load cell. Heat was then supplied to a single column of the WHOA (comprising 3 × 3 stripes per column and row) by applying different current values until the force reached 0.61 N for all cases, and the SMA's surface temperature approached ≈65 °C. Further insight into the control of the input voltage/current supplied for varying heating rate comparisons is summarized in Figure S12 (Supporting Information). Evidently, a direct relationship exists where higher electrical energy input leads to a correspondingly higher temperature in the SMA.

Figure 4.

WHOA as a wearable footwear haptic device. a) A schematic illustration showing placement of each column and row in WHOA when worn on right foot. b) Experimental images of WHOA exhibiting horizontal and vertical stripe actuations using joule heating, showing thermal IR images. c) Displacement–time plot illustrating that displacement of x‐axis is independent to the other axis. d) Investigation of difference in temperature of skin and actuated patterns under actuation. e) Accuracy of pattern recognition through confusion matrix analysis. f) Noticeability plot exploring perception variations across different pattern stimuli. g) Image illustrating examination of user comfort across diverse pattern perceptions.

Prior to exploring how users perceive conveyed tactile patterns, an examination of temperature changes and the pressure applied to the skin was conducted to identify the skin sensation responsible for feedback perception. Pressure and heat were chosen as the primary stimuli generated during WHOA actuation. Figure 4c illustrates the normalized signal intensity for six distinct lines from Line 1 to Line 6 across various conditions or commands: Down, UP, Left, Blocked, Forward, and Stop. The y‐axis represents the normalized signal intensity, while the x‐axis lists the specific conditions. Each line demonstrates unique response patterns to these conditions. For instance, Line 1 exhibits high intensity during the Downward condition, whereas Line 2 shows a significant peak for the Upward condition. Line 3 indicates notably the Left condition, and Line 4 shows a distinct rise in intensity at the Stop condition. Line 5 reaches its maximum during the Forward condition, while Line 6 has peaks at the Right condition. The shaded areas highlight specific conditions for easier correlation with signal variances. The thermal IR images above each condition provide a visual representation of the respective states, aiding in the interpretation of the signal responses. The graph effectively illustrates the varying behaviors of each signal line under different commands, which are crucial for understanding their operational characteristics in the given context. Moreover, to verify that the heating of the SMA for actuation has minimal effect on the user's skin, the temperature changes of both the SMA and the user's skin were monitored simultaneously during consecutive SMA actuation cycles (3 s of heating and 10 s of cooling, with a total cycle duration of 13 s). The comparison effectively illustrates that the heat dissipates during each cooling cycle, allowing the skin to restore its temperature without being significantly affected, as exemplified in Figure 4d.

Afterward, a user test experiment was conducted to evaluate the effectiveness of the WHOA by testing it on 20 subjects, comprising five females and fifteen males aged between 23 and 48 years. Human subject tests for the cognition test were conducted with approval given by KAIST IRB (KH2024‐175). Each participant wore a footwear haptic device on their right foot, and they were instructed to maintain their posture throughout the experiment while being given the freedom to rest as needed. The experiment consisted of multiple trials, with each trial involving the participant experiencing different tactile feedback on their right foot based on independently actuated horizontal and vertical stripes. Following each trial, participants were asked to press a buzzer upon sensing any stimuli to measure their reaction time. They were then prompted to describe the sensations they experienced before classifying them using a predefined described image as Line 1 to Line 6. The participants were then asked the following questions about their experience: “Did you feel that you could readily experience the sensation and perceive the direction coordinates?”, “Did the experience feel as though it was moving quickly or slowly?”, “To what extent did each stripe feel noticeable?”, “How comfortable was the feeling?”. Participants were also asked to rate their level of confidence in their responses on a 5‐point Likert scale for classification, direction, and mobility.

The results obtained from the experiment provided valuable insights into the performance and usability of WHOA as a wearable haptic device. Using a confusion matrix, the accuracy of perception from actuating each stripe line in the WHOA was summarized, as illustrated in Figure 4e. Figure S13a (Supporting Information) summarizes the overall confusion matrix, indicating the accuracy in correctly identifying the true sensations from perceived feelings and highlighting the high precision of the tactile feedback mechanism provided by the WHOA. Figure 4f indicates the noticeability ratings which imply the effectiveness of the tactile feedback provided by WHOA. Moreover, comfort ratings reflected the overall satisfaction and high comfort levels experienced by participants during the experiment, as depicted in Figure 4g. Furthermore, data summarizing reaction time offered insights into the responsiveness of participants to tactile stimuli, demonstrating the efficiency of the orthotropic fabric actuator in eliciting prompt responses (Figure S13b, Supporting Information), Overall, these findings underscore the importance and potential applications of orthotropic fabric actuators in enhancing user experience and interaction in various domains, including wearable technology, assistive devices, and virtual reality systems.

2.4.1. WHOA for Navigation of Visually Impaired Individuals Using Tactile Feedback

The WHOA is seamlessly integrated into specialized footwear designed for haptic navigation assistance, particularly catering to the needs of visually impaired individuals. Our footwear solution incorporates crosswise Velcro fastenings, facilitating the attachment of the orthotropic fabric actuator onto the foot as shown in Figure 5a. The control of WHOA is carried out using a custom control circuit, comprising a microcontroller, gate switches, and a portable power supply. Figure S14 (Supporting Information) summarizes the detailed configuration of the custom control circuit. The strategic utilization of horizontally and vertically controlled stripes within WHOA enables the creation of a rich 3D perception, providing users with tactile cues spanning directional awareness from right to left, top to bottom, forward to backward, and downward to upward, as illustrated in Figure 5b. Moreover, the compact design of the WHOA enables it to deliver feedback even in the confined space inside a shoe when worn on the foot, without causing fatigue to the wearer due to its lightweight. Additionally, by receiving feedback through the feet rather than the hands, users can enjoy a hands‐free experience, allowing their hands to remain free. This setup provides intuitive directional cues through the feet, helping users easily perceive the direction in which they need to move next. Figure 5c shows examples of cases where the WHOA is used as a personal navigation aid for visually impaired individuals, including scenarios that guide users in moving up and down stairs through 3D directional perception.

Figure 5.

Application of WHOA for blind person navigation using tactile feedback. a) Images showing the footwear made up of WHOA wear on dorsal part of foot, using passive fabric and Velcro strap attachment on all four sides and making 3D perception for user based on independently actuated vertical and horizontal stripes. b) Image illustrating schematic of blind person wearing WHOA on foot. c) Images showing real‐time operation of WHOA. When right side is blocked and left side has a way to go, then WHOA will guide the person to move left using tactile sensation. Specially, WHOA can guide the blind to go upward and downward in complex 3D pathways such as stairs.

The significance of our independent orthotropic actuation lies in its ability to provide nuanced haptic feedback to users, thereby enhancing their spatial orientation and navigation capabilities. By leveraging the unique properties of orthotropic materials and the versatility of haptic technology, WHOA opens avenues for tailored sensory experiences that transcend traditional tactile interfaces. This approach not only addresses the specific challenges faced by visually impaired individuals but also holds promise for broader applications in wearable technology and human‐computer interaction. Through this research, the aim is to advance the frontier of haptic navigation solutions in the sense of spatial recognition as described in Figure 5c, ultimately fostering greater independence and empowerment for users in their daily lives.

2.4.2. WHOA as Cutaneous Feedback in 3D Space Navigation

The addition of tactile feedback along the orthotropic axis creates an additional dimension for user interaction. This feature is particularly beneficial when the WHOA is synchronized with drones navigating in 3D. As a comprehensive demonstration of an advanced drone navigation system using the WHOA, a VR environment was predesigned to simulate a scenario where a drone is navigated with assistance from the WHOA during rescue missions in a building on fire. Mapping between tactile modes and motions of drone is shown in Figure 6a. Figure 6b shows a schematic illustrating the configuration in which a rescue operator is equipped with a VR headset and a WHOA attached to the foot. The spatial data collected by the drone are transferred to the operator through tactile feedback, enabling them to perceive and navigate through the visually disturbed environment. This is particularly beneficial in scenarios where visibility is significantly reduced by smoke or debris, as it enables the operator to “feel” their surroundings without relying on visual cues. Diverse commands such as moving left, right, upward, downward, forward, backward, or detecting obstructions ahead trigger a distinct haptic pattern communicated to the operator.

Figure 6.

Cutaneous feedback of drone for safe navigation in 3D space. a) Mapping between tactile modes and motions of drone. b) Images showing rescue person controlling VR drone in case of burning building, such that WHOA is on foot to give the haptic sensation in a blocked view situation due to fire smoke. c) IR thermal camera images showing 3D spatial mapping of direction coordinates of the drone movement. d) VR‐based simulation images showing the motion of drone in cases of going forward, left, and up, in such a way the vision is blocked by fire smoke.

Figure 6c,d demonstrates the practical application of these navigation aids in a VR simulation environment and the concept illustration is shown in Video S4 (Supporting Information). The WHOA guides several potential paths that the drone might take, such as moving forward, turning left, or ascending, through tactile stimuli on a foot. Each path is adjusted based on real‐time environmental feedback, including the presence or absence of fire and smoke. These simulated images not only reflect the drone's current status and planned movements but also aid in planning and executing safe navigation strategies through hazardous areas. By integrating these sensory inputs into the operator's decision‐making process, users intuitively understand the real‐time condition of the drone, and control with confidence, thereby improving the safety and efficiency of rescue operations in visibility‐constrained situations. In addition, Figure S15 (Supporting Information) illustrates how small and lightweight the WHOA can be at varying scales. Also, as reported in Figure S16 (Supporting Information), WHOA does not produce acoustic noise under operation.

3. Conclusion

This work suggests a novel approach with the creation of a smart wearable haptic interface imbued with orthotropically decoupled actuators: linking SMA wires with an auxetic‐architecture structure in order to construct an inventive orthotropic fabric actuator that can give 3D perception. As a wearable haptic interface, WHOA can be easily and directly worn on curved body parts with great comfort by fastening the orthotropic fabric actuator with a Velcro strap. Furthermore, due to its layered orthogonal auxetic structure with a negative Poisson's ratio, WHOA can move in the form of both vertical and horizontal stripes. Particularly, WHOA is the sole wearable actuator that can produce 3D stripe‐specified stimuli, allowing it to communicate dimensionally varied tactile patterns when utilized as a wearable haptic interface. Simply by wearing the lightweight WHOA on the foot, users can intuitively sense spatial and temporal data with exact zone accuracy during hands‐free navigation, benefiting both visually impaired individuals and drone teleoperation in virtual reality. The novel design of WHOA not only enhances the flexibility and adaptability of wearable haptic interfaces but also opens the door to more immersive and precise tactile feedback experiences. WHOA capability to deliver localized, directional stimuli adds a new dimension to haptic communication, making it highly suitable for applications requiring detailed spatial awareness. However, its lightweight nature further contributes to its practicality for long‐term use.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

M.K. and S.O. contributed equally to this work. T.‐E. S. contributed in parylene coating; M.K. and S.O. wrote the manuscript and performed conceptualization; M.K. performed to design, computations, fabrication, experiment, and demo; W.J., M.M., and D.S. helped with figure draft and revision; Y.Y performed the structural analysis; J.‐H.R. contributed to configuring demonstrations; I‐K.O. supervised and managed the collaborations, revision, and writing of the manuscript. All authors discussed the results and commented on the paper.

Supporting information

Supporting Information

Supplemental Video 1

Supplemental Video 2

Supplemental Video 3

Supplemental Video 4

Khan M., Oh S., Song T.‐E., Ji W., Mahato M., Yang Y., Saatchi D., Ali S. S., Roh J., Yun D., Ryu J.‐H., Oh I.‐K., Wearable Haptics for Orthotropic Actuation Based on Perpendicularly Nested Auxetic SMA Knotting. Adv. Mater. 2025, 37, 2411353. 10.1002/adma.202411353

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.