More than Just a Tug: Understanding the User Experience and Wearability of Skin Stretch Systems

Abstract

Skin stretch is a form of haptic feedback that has great potential to produce dynamic touch feedback on the body but has been neglected relative to other forms of haptics like vibration and compression. Skin stretch is unique in that it is a very natural and familiar form of touch, but it is difficult to replicate artificially through wearable technologies. Devices must interface with the skin such that they can produce the desired sensation without causing the user discomfort, produce consistent feedback across the body, and be producible in an easy to manufacture for. Current strategies have primarily focused on smaller skin stretch deployments and have relied on adhesives to anchor to the body without considering how such anchors impact the wearer. This means that the literature has failed to engage with them with the curiosity and interdisciplinary thinking required to make devices that work to address major design concerns. Moreover, the literature lacks clear qualitative feedback about non-prescriptive perceptions of skin stretch (i.e. non-guided perception of skin stretch). So, to understand how to optimize skin stretch devices for user experience and wearability this work focused on the iterative design and testing of a novel garment-based skin stretch system. The first major undertaking in this work was the development of a system that aids in addressing the gaps in our understanding of skin stretch feedback. Clear design criteria were developed to guide the design process based on existing literature on wearability and device design. The design process from initial concepts and benchtop testing to testable working prototypes is covered. The finalized system uses shape memory alloy (SMA) coil actuators, which can be attached and detached using hook and eyes. The system anchors to the body using silicone which allows for strong grip to the skin without the negative effects of adhesives. Throughout a series of tests the design was tested and refined to further improve the system. The first set of prototypes was characterized mechanically, and then deployed on real users to understand the base effects of device. This examination followed a mixed methods study design by examining functional qualities like perceived force and comfort, while also asking long form questions about how users perceive and understand the device. Through this study it was found that users can experience of an array of distinct stimuli through the use of skin stretch haptics. Users found the experience novel and enjoyable but felt the system could be stronger in power. The system design was refined based upon the feedback of the initial test as well as researcher observations. This led to key design changes to the fit, closure, and general system set up. To validate these changes to the design, the initial and refined prototypes were compared against one another. The results showed that the design, and more importantly fit, of a skin stretch device can greatly change how the stimuli is perceived, with better fit leading to more consistent haptic feedback. This was followed by a study comparing adhesive anchors to silicone anchors. It was found that users unanimously preferred silicone anchors because they were more comfortable, more sustainable for long-term use, and were less painful than adhesive anchors when doffing the device. A final perception study focused on two-point threshold and direction discrimination testing. This examination moved the analysis of the system beyond mere detection and into more complex touch perceptions. The two-point threshold results found that depending on direction of actuation the threshold distance changed, with the vertical threshold being stronger than the horizontal threshold. The direction discrimination results showed that participants were not able to easily determine the direction of a skin stretch stimuli. This suggests that further design improvements are needed to enable clear direction discrimination. In the end, this research advances the state of the art of skin stretch as a mode of haptic interaction, adds to the body of literature about garment-based haptic system design and engineering, and demonstrates the importance of finding balance between functional performance and wearability through an analysis of the wearables literature and reflects on the way forward for skin stretch haptics. The broad contributions of this work center around the novel system design as it applies to the user, the nuanced nature of user perception of skin strain stimuli, as well as clear design feedback for the development of skin stretch devices. These studies have allowed for a much clearer picture of what skin stretch is and how it could be used for the betterment of people’s lives.