2, Institute of Materials Research and Engineering, Singapore, , Singapore
3, University of California, San Diego, San Diego, California, United States
With the increasing prevalence of machines, future breakthroughs to enhance the intuitiveness of human-machine interfaces requires further development of haptics. One of the greatest barriers faced in haptic devices is their lack of conformability to the body. Therefore, the development of soft and conformable actuators is necessary to simulate fundamental modes of touch such as temperature, texture, contact pressure and stiffness. Our work to develop haptic devices using organic materials bridges the gap between haptics and soft actuators. Haptic perception falls under two categories – tactile and kinesthetic. This work focuses on the actuation of kinesthetic stimuli which provides information on the shape and stiffness of objects in virtual spaces. Devices capable of providing kinesthetic perception through force feedback include gloves driven by motors , pneumatics  and magnetorheological fluids . However, these devices are bulky, which impedes freedom of motion, and are non-conformable to the hand, which deteriorates the perception of kinesthetic stimuli. To address these issues, this work utilizes the thermomechanical transitions of a polymer with glass transition temperature (Tg) around body temperature to form a variable stiffness glove. Variable stiffness (VS) materials have been developed for soft robotic applications such as fibers , and composites using elastomers as scaffolds for VS materials . We have developed a VS fabric by coating a soft fabric “scaffold” with a VS polymer. This design allows the polymer to be highly conformable to the body, which has yet to be demonstrated in the field of haptics.
The variable stiffness fabric was prepared by coating Spandex fibers with poly(butyl methacrylate) (PBMA), a thermoplastic with Tg of 45°C. By coating the bottom and sides of fingers of a Spandex glove with PBMA, and modulating the temperature of the composite with thermoelectric elements, we found that human subjects could perceive changes in stiffness of the kinesthetic glove. Response times reported varied inversely to the power of the thermoelectrics. Finite element analysis of a finger further verified that stress is localized at finger joints, one of the locations on the body with highest density of kinesthetic receptors. Furthermore, highest stresses occur around the curvature of the fingers, reinforcing the necessity of conformal coverage of the finger by the kinesthetic device. The development of a soft and conformable kinesthetic glove creates a foundation for further research into soft haptic devices for more intuitive human-machine interfaces.
 C. S. Tzafestas. IEEE Transactions on Systems, Man, and Cybernetics - Part A: Systems and Humans, 2003
 P. Polygerinos et al. Robotics and Autonomous Systems, 2015
 S. H. Winter and M. Bouzit. IEEE Transactions on Neural Systems and Rehabilitation Engineering, 2007
 A. Tonazzini et al. Advanced Materials, 2016
 I. M. Van Meerbeek et al. Advanced Materials, 2016