Design and Implementation of a Textile-Based Wearable Balance Belt1 (2015)

The high risk and cost associated with falling in both elderly patients and those suffering from chronic impairment from dizziness or balance disorders demonstrate the need for an effective balance prosthesis, a device used to reduce imbalance and dizziness in patients with abnormal vestibular function [1]. This need has resulted in investigations of two primary types of balance prosthetics, implantable and nonimplantable. Due to surgical risks associated with an implantable device there is a clear need for further development of a nonimplantable device. Wearable devices capable of providing sensory feedback about balance have been shown to be effective in compensating for vestibular impairment [2] However, for a wearable device to be as effective as an implantable device in the patient’s daily life, it must take a form that is acceptable for long-term use: comfortable, unobtrusive, and aesthetically pleasing.We have previously developed a wearable balance prosthesis in the form of a belt that uses vibrotactile feedback as a sensory substitute. This prosthetic has shown positive results and has reduced the risk of falling in severe patients and the amount of trunk sway in moderate patients [2]. While the technology has been shown to be effective, further development of its form is needed. In its current incarnation as a clinical device, its comfort and fit render it impractical for long-term use by an average consumer. Here, we describe the development of a consumer prototype device that is soft, low profile, and conforms to consumer-focused aesthetics. Further, we discuss our approach to development for manufacturability.The methods employed in designing the consumer prototype of our wearable balance belt focused on overcoming the challenges of integrating the necessary hardware and wiring into a soft device, the fabric belt. The wearable balance belt works by using vibrotactile feedback from four tactors (EAI C-2) placed around the waist, in dorsal, ventral, and lateral positions. An inertial measurement unit (IMU) comprising triaxial accelerometers and gyroscopes is placed on the dorsal waist to sense body position and sway. If the user’s body position is detected at outside the acceptable range the belt vibrates signaling the user to self-correct.To support this process the belt requires, on top of tactors and the IMU, a microprocessor and electronics on a main printed circuit board (PCB), batteries, and a user interface (containing two switches) that allows the user to both reset the belt and turn it on and off. The two lithium-ion batteries allow for 10 hr of usage.For the purposes of repair and device assembly, it was necessary that the hardware and wiring be removable. The belt hardware components and their physical dimensions (maximum dimension in each direction) are described in Table 1.Our user-centric design requirements included:Our approach to component layout followed a process similar to Gemperle et al. [3], taking into account the available body real estate and the sizes of our components. The largest component (main processing PCB) was supported with a rigid backplane for durability and placed on the dorsal waist, the location of the largest planar body area (Fig. 1). The battery pack was segmented with a flexible fabric joint and placed next to the central processing unit to minimize the length of wiring required and to utilize the dorsal waist. The user interface was placed left of the center-front of the belt for easy reachability.The belt design comprises a series of layers—the layup. Layup design required the consideration of two key variables: protection of the hardware (and protection of the wearer from rigid corners of the hardware) and development of a smooth surface profile. In addition, material choices required consideration of the user’s comfort, in terms of hand, feel, and moisture-management. To protect the hardware and smooth out the surface geometry of the device components, layers of a three-dimensional knit spacer fabric were cut to shape, filling in voids in the component surfaces and cushioning the rigid components. Spacer fabrics are engineered to provide cushioning as well as airflow, through their relatively low bulk density and open knit structure.These layers were housed within a shell of spandex knit fabric, providing extensibility as well as comfort. Finally, a surface layer of wicking Lycra knit fabric with a rim of elastic binding was applied to provide moisture-management and a tapered edge. Figure 2 illustrates the entire textile layup.Components were attached to the textile layup through mechanical stabilization within fitted pockets. Stitching between fabric layers creates “pocket” spaces, and an overlap designed into the Lycra knit inner shell allows the component to be inserted and removed, along with its protective spacer layers.For the tactors, to ensure accurate tactor placement across the range of user waist sizes, adjustability was necessary. Further, to prevent tactor movement from being dampened by the textile housing, a stand-off between the spacer layer and the tactor was needed. Tactors were placed within an opening in the spacer layer and affixed in place with a hook-and-loop attachment. Elastic welt openings in the wicking outer-layer allow users to access each tactor for adjustment purposes (Fig. 3).To enable wiring insertion and removal, a wire tunnel was created in the Lycra inner shell by selectively stitching an overlap on the superior edge of the textile structure (Fig. 3).To fit a wider range of waist sizes, the belt is both extensible and adjustable. Although the rigid components do not stretch, the fabric between them allows 4 in. of extension. Elastic conductors were used for all integrated wiring (iStretch, Minnesota Wire, St. Paul, MN). Finally, the belt closure uses a custom-designed buckle and adjustable hook-and-loop closure (Fig. 5) to allow 4 in. of additional adjustability, allowing each belt size to effectively fit a range of 8 in. of waist circumferences.Finally, the overall visual appearance of the design was developed to reference athletic gear and functional wearable devices. Style lines were integrated at the back of the belt to provide directional cues to the user about device placement and orientation.Figures 4 and 5 show the final prototype of the balance belt created using the methods previously discussed.The resulting product provides a more consumer-friendly version of the balance belt, as compared to the clinical device (Fig. 6). Device components are housed in a soft, breathable structure made entirely of textiles. The current prototype is designed for easy integration and removal of hardware components, allowing the device hardware and the textile-supporting belt to be manufactured separately and integrated in final stages of production. Further, the removable outer shell is washable for easy maintenance. Next steps include adding additional sizes to fit the extremes of the population and assessing belt performance with user testing.