Use of Materials Science to Understand Haptic Perception

Conspectus The haptic sense captures information arising from the somatosensory system─the sensor system of the body excluding the eyes, ears, nose, and tongue. That is, it captures stimuli arising from the skin (i.e., touch) and from internal structures (i.e., the musculoskeletal system and internal organs). The field of research called haptics is concerned with understanding and manipulating this sense, often using engineered technology, and usually for creating novel or realistic touch sensations. Fundamental to every tactile interaction is an interface between the skin and a material. Given that essentially all material objects are composed of or covered in organic media, we reasoned that we, as organic materials scientists, might be able to contribute to the understanding of the sense of touch by manipulating material properties on the molecular scale. Over time, our research group acquired additional skills in electrical engineering and developed strong collaborations with cognitive and behavioral scientists. With a shared curiosity about the sense of touch, we made what we believe are original contributions to the field of haptics. Our approach is guided by a paradigm consisting of four layers from which hypotheses can be generated, experiments can be designed, and whose analytical techniques may be applied. The layers are (1) material composition, (2) material properties, (3) interfacial properties between the skin and the material, and (4) human perception. For example, a material may be composed of one part silicon and two parts oxygen (material composition), which leaves the surface terminated in dipoles and thus a high surface polarizability (material properties). These dipoles may then interact with the skin with a strong van der Waals interaction and high friction (interfacial properties). This friction may lead to stick-slip behavior and could possibly be perceived as fine texture or roughness, even if the surface is smooth (perception). Another useful organizing principle is that of active vs passive touch. That is, engaging with an object with intent vs having an object brush up against one’s skin without expectation. In either case, the sensation perceived can be described as either fundamental (e.g., roughness, coldness, compliance, and slipperiness) or blended (e.g., wetness). Beginning with an example of how our approach can be used to understand active touch of a blended sensation, we show how polyacrylamide hydrogels can be tuned by adjusting both the mechanical compliance and thermal conductivity to elicit different levels of perceived wetness. We then show how a purpose-designed conductive polymer can render sensations of roughness in the context of a virtual reality simulation that operates by both passive and active modalities. Lastly, we demonstrate a form of haptic “holography” using the photoacoustic effect; that is, π-conjugated materials coated on the skin can render sensations of vibration (perceived passively) when exposed to pulsed light. Throughout this Account, we describe how control of materials can be used to elicit senses artificially for both fundamental knowledge and for the ultimate development of richer human-machine interfaces.