Myrthe A. Plaisier

The sense of touch is underutilized in our digital lives, especially when compared to the prevalence and complexity of information transfer through our visual and auditory interactions with devices; the impact of this is profound as touch plays a major role in our perception of our world, and its omission in our commonly used devices limits our accessibility to the digital space, particularly for blind and visually impaired users. Fortunately, the stimuli‐responsive shape‐morphing properties of liquid crystal elastomers (LCEs) promise a solution: in our work, we mechanically program an LCE bilayer coating to achieve durable, high‐amplitude switchable surface protrusions, or tactile pixels, which we assemble into a digital platform for generating tactile images and haptic feedback. We further characterize and describe our tactile pixel design by microscopy techniques and simulations that use the neo‐classical theory of rubber elasticity. We demonstrate safe and effective electronically addressable activation of individual LCE pixels within seconds, accompanied by a psychophysical evaluation of tactile line orientation discrimination in which the performance of our LCE tactile pixels is comparable to swell paper, a standard tactile graphic medium. Thereby, we establish LCEs as a contender for the next‐generation dynamic tactile displays.

Relief based tactile refreshable displays that are currently on the market are composed of pixel-based elements. While current devices mainly focus on making text accessible by raising pixels, some are capable of displaying tactile images as well. The dimensions of the pixels used in these devices are often based on Braille dimensions, which may not be optimal for image recognition. This study systematically investigates how pixel diameter, spacing between pixels, and whether the pixels are equidistant or not affect perceptual performance in recognizing pixelated tactile images. In Experiment 1, we varied pixel diameter (0.5 mm, 1.0 mm, 1.5 mm) and edge-to-edge spacing (0.5 mm, 1.0 mm) across equidistant grids, and found that smaller edge-to-edge spacing significantly improved recognition, with a stronger effect for larger pixels. In Experiment 2, we compared equidistant and non-equidistant (Braille-based) pixel configurations. We created a fair comparison by keeping the total number of pixels in the grid constant across conditions. Recognition rates were significantly higher for equidistant configurations. Exploratory analysis of the confusion matrices revealed that images with curved lines or oblique features were misidentified more often, especially under low-resolution and non-equidistant conditions. These findings highlight the limitations of Braille-based configurations for image rendering and offer design guidelines for future tactile displays that aim to support both text and images.

In this survey, we give an overview of hands-free haptic devices specifically designed for navigation guidance while walking. We present and discuss the devices by body part, namely devices for the arm, foot and leg, back, belly and shoulders, waist and finally the head. Although the majority of the experimental tests were successful in terms of reaching the target while being guided by the device, the experimental requirements were wide-ranging. The distances to be covered ranged from just a few meters to more than a kilometer, and while some of the devices worked autonomously, others required the experimenter to act as Wizard of Oz. To compare the usefulness and potential of these devices, we created a table in which we rated several relevant aspects such as autonomy, conspicuity and compactness. Major conclusions are that outdoor devices have the highest technology readiness level, because these allow autonomous navigation through GPS, and that the most compact devices still require the action of an experimenter. Unfortunately, none of the hands-free devices are at a level of readiness where they could be useful to people with visual impairments. The most important factor that should be improved is localization accuracy, which should be high and available at all times.

Vibrotactile feedback can be built into clothing such as vests. This means that often vibrotactile information is presented to the back. It is known that the back has a relatively low spatial acuity. Spatial acuity varies across different limbs and sometimes with different locations on a limb. These known anisotropies suggest that there might be systematic variations in vibrotactile spatial acuity for different areas of the back and also for different orientations (i.e. horizontal vs. vertical). Here we systematically measured spatial acuity in four areas of the back for both horizontal and vertical orientations. The results show no significant differences in spatial acuity for the back areas that were tested. Spatial acuity was, however, higher in the horizontal direction than in the vertical direction by roughly a factor of two. This means that when designing vibrotactile displays for the back the tactor density can be lower in the vertical direction than in the horizontal direction and density should be constant for different areas of the back.

In this study, we were interested in the question whether vibrotactile thresholds on the back, in terms of the Just Noticeable Difference (JND), scale with back length. Although there exists only indirect physiological evidence, it could be assumed that the density of mechanoreceptors is lower if back size is larger. As a consequence, the JND would increase with back length. We measured psychophysical curves for 40 male participants with back lengths in the range of 36 to 55 cm. Nine equally spaced vibration motors were placed in vertical direction on their back, 2 cm left of the spine. For each stimulus pair, participants had to decide whether the second vibration was above or below the first vibration. It was found that the slope of the fit of the JND as a function of back length was not significantly different from zero, so contrary to our expectation, we did not find an influence of back length on JND. This means that when customizing a wearable haptic device for the back, measuring back length is not the way to go.