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The goal of the field of haptics is to create technologies that manipulate the sense of touch. In virtual and augmented reality, haptic devices are for touch what loudspeakers and RGB displays are for hearing and vision. Haptic systems that utilize micromotors or other miniaturized mechanical devices (e.g., for vibration and pneumatic actuation) produce interesting effects, but are quite far from reproducing the feeling of real materials. They are especially deficient in recapitulating surface properties: fine texture, friction, viscoelasticity, tack, and softness. The central argument of this progress report is that in order to reproduce the feel of everyday objects, molecular control must be established over the properties of materials; ultimately, such control will enable the design of materials which can change these properties in real time. Stimuli‐responsive organic materials, such as polymers and composites, are a class of materials which can change their oxidation state, conductivity, shape, and rheological properties, and thus might be useful in future haptic technologies. Moreover, the use of such materials in research on tactile perception could help elucidate the limits of human tactile sensitivity. The work described represents the beginnings of this new area of inquiry, in which the defining approach is the marriage of materials science and psychology.

 

A type of haptic device is described that delivers two modes of stimulation simultaneously and at the same location on the skin. The two modes of stimulation are mechanical (delivered pneumatically by inflatable air pockets embedded within a silicone elastomer) and electrical (delivered by a conductive polymer). The key enabling aspect of this work is the use of a highly plasticized conductive polymer based on poly(3,4‐ethylenedioxythiphene) (PEDOT) blended with elastomeric polyurethane (PU). To fabricate the “electropneumotactile” device, the polymeric electrodes are overlaid directly on top of the elastomeric pneumatic actuator pockets. Co‐placement of the pneumatic actuators and the electrotactile electrodes is enabled by the stretchability of the PEDOT:tosylate/PU blend, allowing the electrotactiles to conform to underlying pneumatic pockets under deformation. The blend of PEDOT and PU has a Young's modulus of ≈150 MPa with little degradation in conductivity following repeated inflation of the air pockets. The ability to perceive simultaneous delivery of two sensations to the same location on the skin is supported by experiments using human subjects. These results show that participants can successfully detect the location of pneumatic stimulation and whether electrotactile stimulation is delivered (yes/no) at a rate significantly above chance (mean accuracy = 94%).

 

Polymeric arrays of microrelief structures have a range of potential applications. For example, to influence wettability, to act as biologically inspired adhesives, to resist biofouling, and to play a role in the “feel” of an object during tactile interaction. Here, we investigate the damage to micropillar arrays comprising pillars of different modulus, spacing, diameter, and aspect ratio due to the sliding of a silicone cast of a human finger. The goal is to determine the effect of these parameters on the types of damage observed, including adhesive failure and ploughing of material from the finger onto the array. Our experiments point to four principal conclusions [1]. Aspect ratio is the dominant parameter in determining survivability through its effect on the bending stiffness of micropillars [2]. All else equal, micropillars with larger diameter are less susceptible to breakage and collapse [3]. The spacing of pillars in the array largely determines which type of adhesive failure occurs in non-surviving arrays [4]. Elastic modulus plays an important role in survivability. Clear evidence of elastic recovery was seen in the more flexible polymer and this recovery led to more instances of pristine survivability where the stiffer polymer tended to ablate PDMS. We developed a simple model to describe the observed bending of micropillars, based on the quasi-static mechanics of beam-columns, that indicated they experience forces ranging from 10 −4 –10 −7 N to deflect into adhesive contact. Taken together, results obtained using our framework should inform design considerations for microstructures intended to be handled by human users. 

Progress in the field of soft devices–that is, the types of haptic, robotic, and human-machine interfaces (HRHMIs) in which elastomers play a key role–has its basis in the science of polymeric materials and chemical synthesis. However, in examining the literature, it is found that most developments have been enabled by off-the-shelf materials used either alone or as components of physical blends and composites. A greater awareness of the methods of synthetic chemistry will accelerate the capabilities of HRHMIs. Conversely, an awareness of the applications sought by engineers working in this area may spark the development of new molecular designs and synthetic methodologies by chemists. Several applications of active, stimuli-responsive polymers, which have demonstrated or shown potential use in HRHMIs are highlighted. These materials share the fact that they are products of state-of-the-art synthetic techniques. The progress report is thus organized by the chemistry by which the materials are synthesized, including controlled radical polymerization, metal-mediated cross-coupling polymerization, ring-opening polymerization, various strategies for crosslinking, and hybrid approaches. These methods can afford polymers with multiple properties (i.e., conductivity, stimuli-responsiveness, self-healing, and degradable abilities, biocompatibility, adhesiveness, and mechanical robustness) that are of great interest to scientists and engineers concerned with soft devices for human interaction. 

Haptic devices are in general more adept at mimicking the bulk properties of materials than they are at mimicking the surface properties. Herein, a haptic glove is described which is capable of producing sensations reminiscent of three types of near‐surface properties: hardness, temperature, and roughness. To accomplish this mixed mode of stimulation, three types of haptic actuators are combined: vibrotactile motors, thermoelectric devices, and electrotactile electrodes made from a stretchable conductive polymer synthesized in the laboratory. This polymer consists of a stretchable polyanion which serves as a scaffold for the polymerization of poly(3,4‐ethylenedioxythiophene). The scaffold is synthesized using controlled radical polymerization to afford material of low dispersity, relatively high conductivity, and low impedance relative to metals. The glove is equipped with flex sensors to make it possible to control a robotic hand and a hand in virtual reality (VR). In psychophysical experiments, human participants are able to discern combinations of electrotactile, vibrotactile, and thermal stimulation in VR. Participants trained to associate these sensations with roughness, hardness, and temperature have an overall accuracy of 98%, whereas untrained participants have an accuracy of 85%. Sensations can similarly be conveyed using a robotic hand equipped with sensors for pressure and temperature.